J O U R N A L O F C H E M I S T R Y Materials Photophysical characterization of dilute solutions and ordered thin films of alkyl-substituted polyfluorenes Julie Teetsov and Marye Anne Fox*† Department of Chemistry, University of Texas at Austin, Austin, TX 78712, USA Received 9th April 1999, Accepted 23rd June 1999 Absorbance and fluorescence spectra of dilute solutions and thin films of polyfluorene rigid-rod polymers 1–3 bearing two hexyl, octyl, or dodecyl groups at the 9-position define the eVect of polymer chain interactions on excited state relaxation.Film morphology is controlled by annealing of 250 nm thick films. Under these conditions, the degree of interchain interaction follows the degree of thermotropic liquid crystalline ordering which is, in turn, a function of the length of the attached alkyl substituents.Alkyl substituents also aVect the solubility of these polymeric liquid crystals in organic solvents; low solubility favors strong ground state aggregation, as is evidenced by a red-shifted absorption band. In the annealed films, aggregate and excimer formation is evidenced by a broadening of the absorbance band, an increase in the intensity of the low energy emission, the appearance of new long-lived fluorescent species, and structure-dependent changes in observed fluorescence quantum yields.molecules with extended p-systems are known to adopt even 1. Introduction more planar geometries in the excited state, thus facilitating There is great interest in the fundamental optoelectronic aggregation through p-stacking.11 Because both ground state properties of rod-like conjugated polymers because of their aggregates23,25 and excimers21,26 can enhance non-radiative utility in light-emitting devices,1 lasers,2 thin film transistors,3 decay in solution and in the solid state, aggregation is often and polarizers.4 Polyfluorenes,5–13 poly(phenyleneethynyl- considered undesirable in light-emitting devices. Nonetheless, enes),4,12,14 poly( p-phenylenes),15 and derivatives of poly- recent work by Grell et al.11 with poly(dioctylfluorene) suggests phenylene16,17 and poly( p-phenylenevinylenes)18 fit into a class that fluorescence eYciency may, in fact, increase in the solid of rigid-rod blue light-emitting polymers.19 The extended con- state compared with that observed in homogeneous solution. jugation that is characteristic of these families enhances charge According to Bradley et al.,8 polyfluorenes form clusters in delocalization by virtue of the greater molecular planarity solvents with poor solvating characteristics and in thin films attained along their rigid backbones.This electronic delocaliz- prepared either from poor solvents or by spin-coating from ation also influences a number of the polymers’ physical good solvents while the support is cooled to -77 °C and properties including their band gaps, absorption coeYcients, warmed slowly to induce thermodynamically controlled aggreand emission quantum yields.2 gation.However, the eVect of aggregation on observable Rigid–rod polymers often also form stable liquid crystalline fluorescence lifetimes and quantum yields is still unclear phases and self-organize in the solid state either upon heating because evidence for a correlation of either excimer or aggre- (thermotropic) or upon adding solvent ( lyotropic).Studies of gate formation with the observed fluorescence eYciency is very the absorption and fluorescence4,14,20–22 of several rigid-rod sample-dependent.27 polymers show that appended alkyl side chains can aVect both Because the correlation between local order and emission liquid crystallinity and interpolymer interactions.Little is eYciency remains ambiguous, we sought to study interchain known, however, about the correlation between the shifts in interactions of liquid crystalline 9,9-dialkylpolyfluorenes 1–3 molecular packing and trends in the eYciency of light emission.with polymer lengths that significantly exceed the eVective In fact, one important reason for studying a family of closely p-conjugation length. We are particularly related polymers diVering only in the length of the attached alkyl chains is to examine whether rigid-rod polymers aligned over large areas can display high degrees of dichroic absorption and emission.With optimal mechanical alignment, interpolymer interactions may serve to enhance the observed dichroic ratio because of the enhanced on-axis dipole moment of the aggregate array compared with that of the individual molecule or a randomly dispersed film. For example, Grell interested in observing the influence of the alkyl chain length et al.11 have shown an increase in the dichroic ratios from 7 on spectral trends and excited state lifetimes within the series, to 20 upon inducing aggregation in their polymer films.hoping that we may discern design features that will assist in It is also known that interchain p,p* interactions can preparation of new high eYciency LEDs. In this study, we influence the net observable fluorescence eYciencies in this describe aggregate and excimer formation in a series of dialkylseries through both ground state aggregation and excimer ated polyfluorenes 1–3 in solution and as thin films.By formation. Ground state aggregates formed between two or determining trends in fluorescence lifetimes and fluorescence more polymer chains are evidenced by a broadening of the quantum yields, we establish that the length of the linear alkyl absorption spectra23 or by the appearance of new absorption side chain does indeed play an important role in controlling bands.8 Excimer formation, i.e., complexation between an interchain interactions.In addition, we show that the rigidity excited state of a molecule and the same species in the ground of these polymeric liquid crystals, as related to their eVective state, is particularly favorable in rigid-rod polymers24 because conjugation length, is influenced by the alkyl substituents and that it is therefore possible to control attainable local order in pristine and annealed liquid crystalline thin films by syntheti- †Present address: OYce of the Chancellor, Box 7001/Holladay Hall, North Carolina State University, Raleigh, NC 27695–7001, USA.cally modifying the length of the attached chain. J. Mater. Chem., 1999, 9, 2117–2122 2117355 nm laser pulses) and sample emission were focused into 2. Experimental and out of the integrating sphere using fiber optics. All spectra 2.1. Instrumentation were corrected for the monochromator and photomultiplier tube (PMT) spectral response.Incident radiant energy was Fourier-transform infrared spectra were obtained as KBr converted to a specified number of photons by multiplying the pellets on a Nicolet 510P spectrometer. Gel permeation chromobserved emission intensity by wavelength. The observed atography (GPC) on Waters Styragel columns with THF as emission intensity was obtained by integrating over all emission eluent gave peaks that were calibrated against commercial wavelengths in the additive spectrum, where each wavelength’s polystyrene standards.(Although GPC calibration against intensity was obtained from an average of 50 decays. Laser polystyrene standards is a routine procedure for estimating drift introduced an approx. 6% relative standard deviation molecular weights of rigid-rod polymers, some recent work (measured over the duration of each sample measurement). has shown these values to be roughly a factor of 3 larger than Each reported quantum yield is an average of 2–3 sample those attained by light scattering.11 Here, however, the estimeasurements with an estimated 10% relative standard mated molecular weights agree roughly with those expected deviation.from the additive extinction coeYcients of the appended fluorenyl groups.) Liquid crystalline phase transitions were determined with a 3. Results and discussion Perkin Elmer Series 7 UNIX diVerential scanning calorimeter 3.1. Polymer properties (DSC). Thermal decomposition was determined with a Perkin Elmer Series 7 Thermal Gravimetric Analyzer (TGA).As established by polarizing light microscopy, polymers 1–3 Crystalline transitions for bulk and thin films were identified exist as polycrystalline yellow flakes whose molecular weights with an Olympus polarizing light microscope (PLM) equipped were characterized by GPC (Table 1), displaying regions of with a camera attachment and hot and cold stages. order with dimensions exceeding one micron.Polymers 1–3 readily dissolved (100 mg mL-1) in toluene, chloroform, and 2.2. Materials THF to yield clear violet solutions. The polymers could be dissolved in dichloromethane, cyclohexane, and n-heptane only Samples 1–3 were a gift from Drs Edward Woo and Michael upon heating. The resulting clear violet solution quickly turned Inbasekaran from Central & New Businesses R & D, Dow translucent yellow upon cooling, with the formation of a gel Chemical Company, Midland, Michigan.All other materials or precipitation of the polymer. The yellow color is assigned (Aldrich) were used as received. to a polymer aggregate8 which shows a new absorption band at 437 nm (see below). The appearance of the 437 nm band in 2.3. Thin film preparation poor solvents results from a mismatch of Hildebrandt para- Thin films were prepared with a Specialty Coating Systems meters.11 In contrast to polymers 1 and 3, at room constant Inc.Spin Coater, model P6204-A. Polymer solutions (3 wt.%) temperature and a 10 mg mL-1 concentration, polymer 2 began prepared in toluene were placed on 1 cm glass squares. The to form aggregates in toluene and tetrahydrofuran, as evisubstrate was rotated at a 500 rpm ramping speed for 1 s, denced by the appearance of the characteristic 437 nm peak. followed by 3000 rpm for 30 s, producing 250±30 nm thick These two latter solvents have better matched Hildebrandt films, as determined by a Tencor Instruments Alpha Step 100 parameters.profilometer. Films were used as spun or were annealed under Bulk thermal properties of 1–3 were determined with both Ar at either 160 or 250 °C for 2 h before being cooled at DSC and PLM (Table 2).These techniques complement each 30 °Cmin-1 to preserve the order of the polymer’s liquid other because DSC is the more sensitive thermal technique, crystalline phase. Near-field optical microscopy results show but does not allow for visual observation of bulk polymer or order at 200 nm in pristine films of 3 to a greater extent than thin film morphology.In pristine films not previously subjected in 2, with essentially complete disorder observed with 1. After to heating cycles, an endothermic crystalline-to-nematic A annealing, films of 1 show the greatest order, followed by liquid crystalline phase transition followed by a second endothose of 2 and 3.Table 1 Molecular weights of the poly(dialkylfluorene)s 2.4. Transient and steady-state spectroscopy Polymer Mn a±500 PDb±1 nc±2 Absorbance spectra were recorded on a Shimadzu spectrometer. Fluorescence measurements were made with an 1 17 000 4.1 51 SLM Aminco SPF 500 fluorimeter. Temperature-dependent 2 24 000 4.4 62 measurements were performed with a water-blanketed quartz 3 21 000 2.6 42 cuvette.Time-resolved fluorescence lifetimes were obtained at aNumber averaged molecular weights (Mn). bPolydispersities (PD) are the Center for Fast Kinetics Research by time-correlated determined by GPC vs. polystyrene standards in THF, where PD= single-photon counting using a mode-locked, synchronously Mn/Mw (number averaged/weight averaged molecular weight).cThe pumped, cavity-dumped dye laser (l=300 and 340 nm, number of monomer repeat units (n) is calculated from the Mn. 3 mJ pulse-1).28 Emissive photons were collected at 90° with respect to the excitation beam (300 nm) and were passed through a monochromator to a Hamamatsu Model R2809U Table 2 Thermal properties of the poly(dialkylfluorene)sa micro-channel plate.Data analysis was conducted after decon- Temp./±2 °C (DH/±0.01 J g-1) volution of the instrument response function (fwhm ~80 ps). All solutions were purged with N2 for 5 min before the Polymer SP N(A) N(B) I measurement and had an optical density (OD)=0.1 at 340 nm. 1 94 162–213 (3.69) 222–246 (3.67) 290–300 2.5. Fluorescence quantum yields 2 72 80–103 (11.63) 108–157 (11.05) 278–283 3 47 62–77 (0.58) 83–116 (4.57) 116–118 Solution quantum yields were measured relative to 9,10- aDetermined by diVerential scanning calorimetery and polarized light diphenylanthracene (W=0.9 in cyclohexane)29 using an SLM microscopy of pristine polymers without thermal pretreatment.SP Aminco SPF 500 fluorimeter. A Lab Sphere 10 cm integrating and I refer to softening and melting (isotropic) point.N(A) and N(B) sphere was used to obtain absolute quantum yields following refer to nematic A and nematic B liquid crystalline phases. the method of de Mello et al.30 Laser excitation (10 mW 2118 J. Mater. Chem., 1999, 9, 2117–2122thermic nematic A to nematic B liquid crystalline transition is heating cycle takes place without permanent (covalent) crosslinking or decomposition.Similarly, a bulk sample of 2 was observed in 1–3, Table 2. These phases are quasi-reversible, with the distribution among phases depending on cooling rates. heated to 250 °C, dissolved in toluene, and spun to produce a typical film. The absence of the 437 nm peak under these The materials can be recycled among these phases without evidence of degradation.7 Films of 1–3 annealed to a tempera- conditions shows that this peak is due to aggregation, rather than to decomposition of the film.Polymer films were thus ture just above the second nematic liquid crystalline transition and then cooled rapidly to preserve liquid crystalline order assumed to be stable within the heating range (25–250 °C) of our experiments.produce arrays with enhanced crystalline areas, consistent with the observations of Grell et al.7 The softening and melting of polymers 1–3 are not clearly observed by DSC, but softening 3.2. Spectral characterization and melting ranges are reported from PLM hot stage obser- Solution phase absorbance spectra of 1–3 are similar in vation. Long alkyl chains have been shown to decrease the bandwidth, absorption maximum, and intensity in toluene and liquid crystalline phase transition temperature in rigid–rod THF.Absorbance spectra of 1–3 at concentrations greater polymers by increasing entropy by accessing a larger number than 10 mg mL-1 or in poor solvents such as n-heptane or of contributing conformations.31 Consistent with this gencyclohexane show the same band at 437 nm as was observed eralization, our DSC data (Table 2) show that the temperature in the heated films, Fig. 2. Because this peak is reversible and of the liquid crystalline phase transition is lowest for 3, dependent on concentration, temperature, and solvent, it is followed by 2 and 1. also most likely a result of polymer aggregation.8,11 The degree PLM was also used to characterize thin films (250 nm thick) of aggregation (as established by the intensity of the 437 nm at room temperature.The films of 1–3 are considered amorphband) is greatest in 2, followed by 1 and 3. This suggests that ous because of the absence of birefringence under crossed the octyl chain length may provide the optimal supramolecular polarizers in their pristine states (before annealing). In conordering in this series of polyfluorenes in solution.trast, annealed films displayed high birefringence under these Absorbance spectra of pristine films of 1–3 are also similar conditions because of the order of their thermotropic liquid in lmax and intensity (Fig. 2B), with absorption maxima similar crystalline states. The liquid crystalline states of 1–3 are to those observed in solution, Fig. 2A. These films lack evi- characterized as nematic because of the characteristic Schlieren dence of the 437 nm aggregate band, although the bandwidth texture32 observed by PLM (Fig. 1). This texture results from is broadened from that observed in solution. Annealing of lamella (made of polymer chains) which form around the films causes further broadening, Fig. 2C, and induces the nucleation points at defects on the surface of the glass appearance of the absorption at 437 nm. A similar low energy substrate.33,34 absorption band has been described in the aggregate produced Polyfluorene thin films have excellent chemical stability in in films of ladder-type poly( p-phenylene)23 and poly(pheny- the presence of oxygen, moisture, and light as evidenced by leneethylene),14 as well as in poly(dioctylfluorene) films cooled the absence of new spectral bands in the IR.For example, to -78 °C and slowly heated to ambient temperatures to there is no evidence for the presence of carbonyl absorbinduce aggregation.11 Spectral broadening was attributed to a ances which might have been introduced by possible photobroader distribution of eVective conjugation lengths conse- oxidation.13 Bliznyuk et al.have observed, for example, that quent to polymer p,p* interactions in the ground state end-capped polyfluorenes form fluorenones, by a process which aggregates.23 is impossible here by virtue of the bisalkylation at the 9- Fluorescence spectra of 1–3 in solution are also similar position.In addition, transient emission spectra exhibit in bandwidth, the wavelength of emission, and intensity unchanged kinetic profiles in both pristine and annealed films, (Table 3), implying that the lengths of the appended alkyl even after more than one week of exposure to ambient chains do not appreciably change the excited state surface of laboratory conditions. Nor did thermal gravimetric analysis the polymer in solution.Emission from pristine films of 1–3 (TGA) show any change in mass until above 400 °C for 1–3. is red-shifted from their solution phase fluorescence maxima, The polymer films 1–3, however, do show a new absorption with the greatest shift being observed in 3. Annealing further band at 437 nm upon heating which is not present in dilute red-shifts the emission of films of 2 and 3, but not of 1.Fig. 3 solution or in a pristine film. When a film of 2 that was spin cast from toluene and heated to 250 °C to induce the 437 nm peak was redissolved in toluene, its solution spectra did not display the 437 nm peak. Nor did the redissolved polymer show any change in molecular weight by GPC. Thus, the Fig. 2 Absorbance spectra of 2: A) as a 10-4 M solution in THF; B) Fig. 1 Polarized light micrograph (magnified 800×) showing charac- as a 250 nm pristine (amorphous) film; C) film (B) annealed to 160 °C. A is normalized to B and C at 440 nm for ease of comparison of teristic nematic liquid crystalline Schlieren texture of a 250 nm film of 1 annealed at 250 °C (scale bar equals 5 mm). intensity of the long wavelength aggregate band.J. Mater. Chem., 1999, 9, 2117–2122 2119Table 3 Red-shifted emission of poly(dialkylfluorene) films films suggests that planarization is already suYcient to achieve maximum eVective conjugation length beyond which annealing Solutiona Pristine filmb Annealed filmb has no further eVect. Polymer lmax/nm of 2nd vibronic peak emission The increased fluorescence eYciency observed at lower energies in films of 1–3 has been assigned to both aggregate 1 439 447 448 2 438 446 455 and excimer emission, an assignment made in corroboration 3 439 451 456 of assertions of Grell et al.8 and Bliznyuk.13 In going from solution to a pristine film to the annealed film, enhancement aAs 10-7 M THF.bAs 250 nm thick films of polymer spun from toluene onto glass and measured as prepared (pristine) or heated of the low energy transition is greater in 1 than in 3 or 2.This (annealed) to 160 or 250 °C. Excitation at 365 nm. observation implies that greater insulation of the polymer backbone by alkyl chains longer than six carbons does not aVect the formation of excimer. Similarly, the larger red-shifts and lower energy of emission observed in films than in solution and in annealed films than in pristine films suggest that the polymer’s ability to adopt a more planar excited state in films is facilitated by greater interchain interaction.The variation in the red-shifted emission observed in going from homogeneous solution to a pristine film to an annealed film of 1–3 suggests that alkyl chain length significantly alters the supramolecular packing and thus the energy of emission (greater eVective conjugation as a result of the extended rigid conformation and/or more favorable excimer formation).The mechanism of excimer formation is unclear. Longer alkyl chains induce greater order in pristine films, as evidenced by the greatest red-shift and the largest fractional contribution of the long-lived emitting species in 3. 3.3. Fluorescence lifetimes Fig. 3 Fluorescence spectra of 2: A) as a 10-6 M solution in THF, Fluorescence decays collected at 440 nm for 1–3 in THF were O.D.=0.1 at lexc=355 nm under atmospheric conditions; B) as a fitted to a single exponential to give approximately 550 ps 250 nm pristine (amorphous) film; and C) film (B) annealed to 160 °C. lifetimes, Table 4.These lifetimes are independent of excitation and collection wavelength. Given an absence of any intermolshows the red-shifted emission observed upon going from ecular interactions in dilute solutions of 1–3 in THF, the solution to a pristine film to an annealed film of 2. emissive species is assigned as an S1,S0 singlet exciton. When In going from solution to the thin film, 1–3 show a significant the same measurement is made in n-heptane (to induce aggrered- shifted emission, with 3 exhibiting the greatest shift gation), a transient emitting at 440 nm exhibited a lifetime of (Table 3).This suggests that the longer alkyl chains enhance 430 ps (93%) with a small contribution from a new 700 ps interchain interactions, which leads to a more planar extended species.Two longer lived species, 2 ns (7%) and 8 ns (4%), excited state for 3. Annealing films of 2 and 3 causes a further were also observed at 550 nm that were not present in solutions red-shift, but no further shift in the annealed films of 1, which of similar concentration in THF (Table 4). These new longer suggests that interchain interactions induce a red-shift from lived transients at 550 nm are assigned to excimer and aggrethe maxima observed in pristine films of 2 and 3.gate emission respectively,27 and similar lifetimes were also Fig. 4 shows the eVect of annealing on the spectrum of a observed in annealed films of 1–3 in the same spectral pristine film of 1 where no further red-shifts are observed in region, Table 5. going from pristine-to-annealed films, but where increased In films of 1–3, the lifetime of the singlet exciton is reduced fluorescence eYciency is observed at longer wavelengths.The to between 200 and 300 ps at 440 nm because of an increase absence of further red-shifts in 1 from pristine-to-annealed in possible non-radiative decay pathways. These include energy transfer to aggregate and excimer states and access to a greater number of contacts with the chain ends where exciton quenching is thought to be more eYcient.35 At 550 nm, fluorescence decays of pristine films of 1 and 3, respectively, gave three single exponential lifetimes, respectively: 300 and 400 ps, 1 and 3 ns, and 10 and 12 ns, whereas films of 2 gave only two single exponential lifetimes of approximately 700 ps and 9 ns.Upon annealing past the second nematic phase of 1–3, a decrease in the fluorescence intensity from the picosecondlived species is observed, with a greater fraction of the fluorescence emanating from the longer-lived nanosecond species (Table 5). These lifetimes and fractional contributions were not constant over wavelength in the measured range of 500–600 nm.At present, it is not possible to unambiguously assign the 1–3 and 10–12 ns species at 550 nm in 1–3, but they are thought to result from excimer and aggregate, respectively. Decays were collected at both 300 and 340 nm excitation for pristine and annealed films of 1–3 with similar results. Because 2 forms aggregate most readily in solution and Fig. 4 Fluorescence spectra of 250 nm film of 1 with an O.D.=0.5 at because this aggregation process is enhanced in films, aggregate lexc=355 nm under atmospheric conditions: A) as a pristine (amorphformation likely dominates the interchain interactions, at the ous) film; B) film annealed to 180 °C; and C) annealed to 250 °C.Fluorescence intensities of spectra are normalized at 440 nm. expense of excimer formation.Thus, the absence of the 1–3 ns 2120 J. Mater. Chem., 1999, 9, 2117–2122Table 4 Poly(dialkylfluorene) solution fluorescence lifetimes and fluorescence quantum yields Polymer tfl/ns (% composition),±0.03 ns lemission/nm x2 Wfl b±0.01 1a 0.56 (100) 440 1.09 0.64 2a 0.52 (100) 440 1.07 0.69 2c 0.43 (93), 0.72 (7) 440 1.49 0.51 2c 0.51 (89), 1.93 (7), 8.35 (4) 550 1.06 3a 0.55 (100) 440 1.19 0.65 aLifetimes determined with excitation at 300 nm in 10-7 M THF.bQuantum yields (Wfl) determined in cyclohexane relative to 9,10-diphenylanthracene with excitation at 365 nm.33 cLifetimes determined in 10-7 M n-heptane to induce aggregation. Table 5 Poly(dialkylfluorene) film fluorescence lifetimesa and quantum yieldsb tfl/±0.03 ns tfl/±0.3 ns Annealing (% contribution) (% contribution) Wfl±0.1 temp./°C lobs=440 nm x2 lobs=550 nm x2 (%) 1 25 0.27 (100) 5.64 0.3 (94), 1.3 (5), 10.2 (1) 32.86 15.5 250 0.18 (100) 87.9 0.2 (54), 3.0 (19), 11.8 (27) 2.83 10.0 2 25 0.29 (100) 4.56 0.7 (91), 9.1 (9) 3.68 17.0 160 0.21 (100) 1.70 0.7 (86), 9.8 (14) 11.5 25.5 3 25 0.32 (93), 0.77 (7) 1.14 0.4 (88), 3.2 (6), 12.3 (6) 1.40 15.5 160 0.31 (96), 0.81 (4) 1.09 0.4 (77), 3.2 (7), 11.6 (16) 1.99 13.5 Measurements with 250 nm films on glass.aLifetimes (tfl) measured using single photon counting23 with lexc=340 nm with 250 nm films. bQuantum yields (Wfl) measured using integrating sphere technique25 where each value is an average of 2–3 measurements with approx. 10% relative standard deviation. species in 2 (rather than in 1 and 3) suggests that the 9–12 ns not of 2 or 3, suggests that shorter alkyl chains allow for better interpolymer interaction, thus reducing the observed species in 1–3 can be assigned to aggregates and that 2 already fluorescence eYciency.The unexpected increase in fluorescence has an optimal configuration that inhibits excimer formation. quantum yield for annealed films of 2 suggests that the 1–3 ns- In films of 1 and 3 where both aggregate and excimer are lived species present in 1 and 3 (but not in 2) may be present, the total contributions from both of the two longresponsible for the eYcient fluorescence quenching.Thus, an lived (ns) species are 46% in 1 and 23% in 3. Thus, shorter optimal alkyl chain length produces the highest fluorescence alkyl chains likely permit better supramolecular packing of 1, eYciency in these alkyl-substituted polyfluorene films.which in turn produces greater interchain communication. The long lived species observed in films of 1–3 and the increased fractional contribution of these species as the films are annealed 4. Conclusions are evidence of enhanced interchain interactions resulting from The degree of interpolymer interactions in 9,9-dialkylated the nematic ordering of the alkylated polymer chains.polyfluorenes in solution can be controlled by choice of solvent and by the length of the alkyl chain. The degree of interchain 3.4. Fluorescence quantum yields interaction in solution is greatest for 2, followed by 1 and 3, The dependence of fluorescence quantum yields on supramol- as evidenced by the relative intensities of a new 437 nm ecular packing was probed by measuring the changes in aggregate band, even in good solvents.Pristine films of 3 show fluorescence quantum yields observed upon moving from greater red-shifts in emission bands from solution maxima solution to film and upon increasing local ordering of the thin than do those of 1 or 2. Annealing produces further red-shifts films through annealing.The fluorescence quantum yields of in 2 and 3, but not in 1. Thus, longer alkyl chains force an 1–3 in cyclohexane ranged between 51 and 69%, dependent extended ground state conformation, the planarity of which on polymer concentration, Table 4. Thus, aggregation induces in turn enhances intermolecular aggregation in solution.a lowering of the quantum yield by approximately 20%. Polyfluorenes bearing longer alkyl chains will thus exhibit Pristine films of 1–3 have identical quantum yields of enhanced on-axis dipole moments and increased dichroic approximately 16%, Table 5.36,37 This dramatic decrease from ratios. the observed solution quantum yields suggests that stronger Films of 1–3 show increased ordering upon annealing, as interactions between chains in the films drastically lowers evidenced by formation of an oriented nematic liquid crystal- fluorescence quantum yields.This is similar to the value line phase that shows a characteristic Schlieren texture. reported by Redecker et al.12 (0.50) for the fluorescence Although the exact identity of the ns-lived species cannot be quantum yield, where the value was quite dependent on film exactly determined with this data, the contribution of this morphology.Upon annealing (past their second nematic species is greatest in 1, implying that greater excited state phase) to induce greater order, films of 3 show no change in interchain communication is possible with the polyfluorenes the observed fluorescence quantum yield, whereas films of 2 bearing the shorter alkyl chains.This packing, in turn, leads show a slight increase. When films of 1 were annealed past to enhanced excimer formation. The fluorescence quantum their second nematic phase transition temperature, the quan- yields in these polymers decrease from 51–69% in solution to tum yields decreased slightly from 15.5 to 10%.The phase approximately 3% as pristine films, implying substantial fluotransition temperatures vary between 1–3, so it was possible rescence quenching in the as-deposited thin films of 1–3 as a that changes in quantum yield might be due to temperature function of packing. The fluorescence quantum yield of 1 dependent fluorescence impurity quenching rather than to decreases slightly upon annealing, whereas those of 2 and 3 supramolecular packing induced by liquid crystalline phase increase and remain the same, respectively.This suggests that transition. However, control experiments in which 2 and 3 are the shorter alkyl chains enhance interchain packing interannealed at the same transition temperature as 1 showed the actions, with predictable consequences for fluorescence eYciency in these films.same results. Thus, the decrease in quantum yield of 1, but J. Mater. Chem., 1999, 9, 2117–2122 212116 S. Tasch, A. Niko, G. Leising and U. Scherf, Appl. Phys. Lett., Acknowledgements 1996, 68, 1090. 17 Y. Yang, Q. Pei and A. J. Heeger, J. Appl. Phys., 1996, 79, 934. We gratefully acknowledge Drs Edward P. Woo and Michael 18 A. W.Grice, A. Tajbakhsh, P. L. Burn and D. Bradley, Adv. Inbaskeran at Dow Chemical for samples of polyfluorenes 1–3 Mater., 1997, 9, 1174. and for useful discussions of this work. We acknowledge 19 (a) G. Wegner, D. Neher, M. Remmer, V. Cimrova and Donald O’Connor of the Center for Fast Kinetics at the M. Schulze, Mater. Res. Soc. Symp. Proc., 1996, 413, 23; A. Kraft, A. C. Grimsdale and A.B. Holmes, Angew. Chem., Int. Ed., 1998, University of Texas at Austin for assistance with the time- 37, 402. resolved fluorescence measurements. This work was supported 20 C. Weder, J. M. Wagner and M. S. Wrighton, Mater. Res. Soc. by the Texas Advanced Research Program and the Robert A. Symp. Proc., 1996, 413, 77. Welch Foundation. 21 C.Weder and M. S. Wrighton, Macromolecules, 1996, 29, 5157. 22 J. W. Blatchford, S. W. Jessen, L. B. Lin, T. L. Gustafson, D. K. Fu, H. L. Wang, T. M. Swager, A. G. MacDiarmid and A. J. Epstein, Phys. Rev. B, 1996, 54, 9180. References 23 U. Lemmer, S. Heun, R. F. Mahrt, U. Scherf, M. Hopmeier, U. Siegner, E. O. Gobel, K. Mu� llen and H. Ba�ssler, Chem. Phys. 1 J. H. Burroughes, R. H. Friend, D. D. C. Bradley, A. R. Brown, Lett., 1995, 240, 373.R. N. Marks, K. ay, P. L. Burn and A. B. Holmes, Nature, 24 S. A. Jenekhe and J. A. Osaheni, Science, 1994, 265, 765. 1990, 347, 539. 25 T. Pauck, R. Hennig, M. Perner, U. Lemmer, U. Siegner, R. F. 2 A. Heeger, F. Hide, B. Schwartz and M. A. 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Mater., 1997, 9, 798. 32 L. C. Sawyer and D. T. Grubb, Polymer Microscopy, Chapman & 8 D. D. C. Bradley, M. Grell, X. Long, H. Mellor and A. Grice, Hall, London, 1996. Soc. Phot. Int. Eng., 1997, 3145, 254. 33 N. H. Hartshorne, Optical Properties of Liquid Crystals: Physico- 9 Q. Pei and Y.Yang, J. Am. Chem. Soc., 1996, 118, 7416. Chemical Properties and Methods of Investigation, Horwood, 10 Y. Ohmori, M. Uchida, K. Muro and K. Yoshino, Jpn. J. Appl. Chichester, 1974, Vol. 2. Phys., Part 2, 1991, 30, L1941. 34 B. A.Wood and E. L. Thomas, Nature, 1986, 324, 655. 11 M. Grell, D. D. C. Bradley, X. Long, T. Chamberlain, M. 35 C.Weder and M. S. Wrighton, Macromolecules, 1995, 29, 5157.Inbasekaran, E. P. Woo and M. Soliman, Acta Polym., 1998, 36 In measuring quantum yields for thin films, variations in refractive index, surface roughness, and film thickness can introduce incon- 49, 439. sistency into the measurement of fluorescence quantum yields.31 12 M. Redecker, D. D. C. Bradley, M. Inbasekaran and E. P. Woo, Therefore, an integrating sphere was employed to measure the Appl.Phys. Lett., 1998, 73, 1565. quantum yields for the thin films. 13 V. N. Bliznyuk, S. A. Carter, J. C. Scott, G. Klarmer, R. D. Miller 37 N. C. Greenham, I. D. W. Samuel, G. R. Hayes, R. T. Phillips, and D. C. Miller, Macromolecules, 1999, 32, 361. Y. A. R. R. Kessener, S. C. Morratti, A. B. Holmes, R. H. Friend, 14 C. E. Halkyard, M.E. Rampey, L. Kloppenburg, S. L. Studer- Chem. Phys. Lett., 1995, 241, 89. Martinez and H. F. Bunz, Macromolecules, 1998, 31, 8655. 15 G. Grem, B. Keditzky, D. Ullrich and G. Leising, Adv. Mater., 1992, 4, 36. Paper 9/02829C 2122 J. Mater. Chem., 1999, 9, 2117–2122 J O U R N A L O F C H E M I S T R Y Materials Photophysical characterization of dilute solutions and ordered thin films of alkyl-substituted polyfluorenes Julie Teetsov and Marye Anne Fox*† Department of Chemistry, University of Texas at Austin, Austin, TX 78712, USA Received 9th April 1999, Accepted 23rd June 1999 Absorbance and fluorescence spectra of dilute solutions and thin films of polyfluorene rigid-rod polymers 1–3 bearing two hexyl, octyl, or dodecyl groups at the 9-position define the eVect of polymer chain interactions on excited state relaxation.Film morphology is controlled by annealing of 250 nm thick films. Under these conditions, the degree of interchain interaction follows the degree of thermotropic liquid crystalline ordering which is, in turn, a function of the length of the attached alkyl substituents. Alkyl substituents also aVect the solubility of these polymeric liquid crystals in organic solvents; low solubility favors strong ground state aggregation, as is evidenced by a red-shifted absorption band.In the annealed films, aggregate and excimer formation is evidenced by a broadening of the absorbance band, an increase in the intensity of the low energy emission, the appearance of new long-lived fluorescent species, and structure-dependent changes in observed fluorescence quantum yields.molecules with extended p-systems are known to adopt even 1. Introduction more planar geometries in the excited state, thus facilitating There is great interest in the fundamental optoelectronic aggregation through p-stacking.11 Because both ground state properties of rod-like conjugated polymers because of their aggregates23,25 and excimers21,26 can enhance non-radiative utility in light-emitting devices,1 lasers,2 thin film transistors,3 decay in solution and in the solid state, aggregation is often and polarizers.4 Polyfluorenes,5–13 poly(phenyleneethynyl- considered undesirable in light-emitting devices.Nonetheless, enes),4,12,14 poly( p-phenylenes),15 and derivatives of poly- recent work by Grell et al.11 with poly(dioctylfluorene) suggests phenylene16,17 and poly( p-phenylenevinylenes)18 fit into a class that fluorescence eYciency may, in fact, increase in the solid of rigid-rod blue light-emitting polymers.19 The extended con- state compared with that observed in homogeneous solution.jugation that is characteristic of these families enhances charge According to Bradley et al.,8 polyfluorenes form clusters in delocalization by virtue of the greater molecular planarity solvents with poor solvating characteristics and in thin films attained along their rigid backbones.This electronic delocaliz- prepared either from poor solvents or by spin-coating from ation also influences a number of the polymers’ physical good solvents while the support is cooled to -77 °C and properties including their band gaps, absorption coeYcients, warmed slowly to induce thermodynamically controlled aggreand emission quantum yields.2 gation.However, the eVect of aggregation on observable Rigid–rod polymers often also form stable liquid crystalline fluorescence lifetimes and quantum yields is still unclear phases and self-organize in the solid state either upon heating because evidence for a correlation of either excimer or aggre- (thermotropic) or upon adding solvent ( lyotropic).Studies of gate formation with the observed fluorescence eYciency is very the absorption and fluorescence4,14,20–22 of several rigid-rod sample-dependent.27 polymers show that appended alkyl side chains can aVect both Because the correlation between local order and emission liquid crystallinity and interpolymer interactions. Little is eYciency remains ambiguous, we sought to study interchain known, however, about the correlation between the shifts in interactions of liquid crystalline 9,9-dialkylpolyfluorenes 1–3 molecular packing and trends in the eYciency of light emission.with polymer lengths that significantly exceed the eVective In fact, one important reason for studying a family of closely p-conjugation length.We are particularly related polymers diVering only in the length of the attached alkyl chains is to examine whether rigid-rod polymers aligned over large areas can display high degrees of dichroic absorption and emission. With optimal mechanical alignment, interpolymer interactions may serve to enhance the observed dichroic ratio because of the enhanced on-axis dipole moment of the aggregate array compared with that of the individual molecule or a randomly dispersed film.For example, Grell interested in observing the influence of the alkyl chain length et al.11 have shown an increase in the dichroic ratios from 7 on spectral trends and excited state lifetimes within the series, to 20 upon inducing aggregation in their polymer films.hoping that we may discern design features that will assist in It is also known that interchain p,p* interactions can preparation of new high eYciency LEDs. In this study, we influence the net observable fluorescence eYciencies in this describe aggregate and excimer formation in a series of dialkylseries through both ground state aggregation and excimer ated polyfluorenes 1–3 in solution and as thin films. By formation.Ground e aggregates formed between two or determining trends in fluorescence lifetimes and fluorescence more polymer chains are evidenced by a broadening of the quantum yields, we establish that the length of the linear alkyl absorption spectra23 or by the appearance of new absorption side chain does indeed play an important role in controlling bands.8 Excimer formation, i.e., complexation between an interchain interactions.In addition, we show that the rigidity excited state of a molecule and the same species in the ground of these polymeric liquid crystals, as related to their eVective state, is particularly favorable in rigid-rod polymers24 because conjugation length, is influenced by the alkyl substituents and that it is therefore possible to control attainable local order in pristine and annealed liquid crystalline thin films by syntheti- †Present address: OYce of the Chancellor, Box 7001/Holladay Hall, North Carolina State University, Raleigh, NC 27695–7001, USA.cally modifying the length of the attached chain.J. Mater. Chem., 1999, 9, 2117–2122 2117355 nm laser pulses) and sample emission were focused into 2. Experimental and out of the integrating sphere using fiber optics. All spectra 2.1. Instrumentation were corrected for the monochromator and photomultiplier tube (PMT) spectral response. Incident radiant energy was Fourier-transform infrared spectra were obtained as KBr converted to a specified number of photons by multiplying the pellets on a Nicolet 510P spectrometer.Gel permeation chromobserved emission intensity by wavelength. The observed atography (GPC) on Waters Styragel columns with THF as emission intensity was obtained by integrating over all emission eluent gave peaks that were calibrated against commercial wavelengths in the additive spectrum, where each wavelength’s polystyrene standards. (Although GPC calibration against intensity was obtained from an average of 50 decays.Laser polystyrene standards is a routine procedure for estimating drift introduced an approx. 6% relative standard deviation molecular weights of rigid-rod polymers, some recent work (measured over the duration of each sample measurement).has shown these values to be roughly a factor of 3 larger than Each reported quantum yield is an average of 2–3 sample those attained by light scattering.11 Here, however, the estimeasurements with an estimated 10% relative standard mated molecular weights agree roughly with those expected deviation. from the additive extinction coeYcients of the appended fluorenyl groups.) Liquid crystalline phase transitions were determined with a 3.Results and discussion Perkin Elmer Series 7 UNIX diVerential scanning calorimeter 3.1. Polymer properties (DSC). Thermal decomposition was determined with a Perkin Elmer Series 7 Thermal Gravimetric Analyzer (TGA). As established by polarizing light microscopy, polymers 1–3 Crystalline transitions for bulk and thin films were identified exist as polycrystalline yellow flakes whose molecular weights with an Olympus polarizing light microscope (PLM) equipped were characterized by GPC (Table 1), displaying regions of with a camera attachment and hot and cold stages.order with dimensions exceeding one micron. Polymers 1–3 readily dissolved (100 mg mL-1) in toluene, chloroform, and 2.2.Materials THF to yield clear violet solutions. The polymers could be dissolved in dichloromethane, cyclohexane, and n-heptane only Samples 1–3 were a gift from Drs Edward Woo and Michael upon heating. The resulting clear violet solution quickly turned Inbasekaran from Central & New Businesses R & D, Dow translucent yellow upon cooling, with the formation of a gel Chemical Company, Midland, Michigan. All other materials or precipitation of the polymer.The yellow color is assigned (Aldrich) were used as received. to a polymer aggregate8 which shows a new absorption band at 437 nm (see below). The appearance of the 437 nm band in 2.3. Thin film preparation poor solvents results from a mismatch of Hildebrandt para- Thin films were prepared with a Specialty Coating Systems meters.11 In contrast to polymers 1 and 3, at room constant Inc.Spin Coater, model P6204-A. Polymer solutions (3 wt.%) temperature and a 10 mg mL-1 concentration, polymer 2 began prepared in toluene were placed on 1 cm glass squares. The to form aggregates in toluene and tetrahydrofuran, as evisubstrate was rotated at a 500 rpm ramping speed for 1 s, denced by the appearance of the characteristic 437 nm peak.followed by 3000 rpm for 30 s, producing 250±30 nm thick These two latter solvents have better matched Hildebrandt films, as determined by a Tencor Instruments Alpha Step 100 parameters. profilometer. Films were used as spun or were annealed under Bulk thermal properties of 1–3 were determined with both Ar at either 160 or 250 °C for 2 h before being cooled at DSC and PLM (Table 2).These techniques complement each 30 °Cmin-1 to preserve the order of the polymer’s liquid other because DSC is the more sensitive thermal technique, crystalline phase. Near-field optical microscopy results show but does not allow for visual observation of bulk polymer or order at 200 nm in pristine films of 3 to a greater extent than thin film morphology.In pristine films not previously subjected in 2, with essentially complete disorder observed with 1. After to heating cycles, an endothermic crystalline-to-nematic A annealing, films of 1 show the greatest order, followed by liquid crystalline phase transition followed by a second endothose of 2 and 3. Table 1 Molecular weights of the poly(dialkylfluorene)s 2.4.Transient and steady-state spectroscopy Polymer Mn a±500 PDb±1 nc±2 Absorbance spectra were recorded on a Shimadzu spectrometer. Fluorescence measurements were made with an 1 17 000 4.1 51 SLM Aminco SPF 500 fluorimeter. Temperature-dependent 2 24 000 4.4 62 measurements were performed with a water-blanketed quartz 3 21 000 2.6 42 cuvette. Time-resolved fluorescence lifetimes were obtained at aNumber averaged molecular weights (Mn). bPolydispersities (PD) are the Center for Fast Kinetics Research by time-correlated determined by GPC vs.polystyrene standards in THF, where PD= single-photon counting using a mode-locked, synchronously Mn/Mw (number averaged/weight averaged molecular weight). cThe pumped, cavity-dumped dye laser (l=300 and 340 nm, number of monomer repeat units (n) is calculated from the Mn. 3 mJ pulse-1).28 Emissive photons were collected at 90° with respect to the excitation beam (300 nm) and were passed through a monochromator to a Hamamatsu Model R2809U Table 2 Thermal properties of the poly(dialkylfluorene)sa micro-channel plate. Data analysis was conducted after decon- Temp./±2 °C (DH/±0.01 J g-1) volution of the instrument response function (fwhm ~80 ps).All solutions were purged with N2 for 5 min before the Polymer SP N(A) N(B) I measurement and had an optical density (OD)=0.1 at 340 nm. 1 94 162–213 (3.69) 222–246 (3.67) 290–300 2.5. Fluorescence quantum yields 2 72 80–103 (11.63) 108–157 (11.05) 278–283 3 47 62–77 (0.58) 83–116 (4.57) 116–118 Solution quantum yields were measured relative to 9,10- aDetermined by diVerential scanning calorimetery and polarized light diphenylanthracene (W=0.9 in cyclohexane)29 using an SLM microscopy of pristine polymers without thermal pretreatment.SP Aminco SPF 500 fluorimeter. A Lab Sphere 10 cm integrating and I refer to softening and melting (isotropic) point. N(A) and N(B) sphere was used to obtain absolute quantum yields following refer to nematic A and nematic B liquid crystalline phases.the method of de Mello et al.30 Laser excitation (10 mW 2118 J. Mater. Chem., 1999, 9, 2117–2122thermic nematic A to nematic B liquid crystalline transition is heating cycle takes place without permanent (covalent) crosslinking or decomposition. Similarly, a bulk sample of 2 was observed in 1–3, Table 2.These phases are quasi-reversible, with the distribution among phases depending on cooling rates. heated to 250 °C, dissolved in toluene, and spun to produce a typical film. The absence of the 437 nm peak under these The materials can be recycled among these phases without evidence of degradation.7 Films of 1–3 annealed to a tempera- conditions shows that this peak is due to aggregation, rather than to decomposition of the film.Polymer films were thus ture just above the second nematic liquid crystalline transition and then cooled rapidly to preserve liquid crystalline order assumed to be stable within the heating range (25–250 °C) of our experiments. produce arrays with enhanced crystalline areas, consistent with the observations of Grell et al.7 The softening and melting of polymers 1–3 are not clearly observed by DSC, but softening 3.2.Spectral characterization and melting ranges are reported from PLM hot stage obser- Solution phase absorbance spectra of 1–3 are similar in vation. Long alkyl chains have been shown to decrease the bandwidth, absorption maximum, and intensity in toluene and liquid crystalline phase transition temperature in rigid–rod THF. Absorbance spectra of 1–3 at concentrations greater polymers by increasing entropy by accessing a larger number than 10 mg mL-1 or in poor solvents such as n-heptane or of contributing conformations.31 Consistent with this gencyclohexane show the same band at 437 nm as was observed eralization, our DSC data (Table 2) show that the temperature in the heated films, Fig. 2. Because this peak is reversible and of the liquid crystalline phase transition is lowest for 3, dependent on concentration, temperature, and solvent, it is followed by 2 and 1. also most likely a result of polymer aggregation.8,11 The degree PLM was also used to characterize thin films (250 nm thick) of aggregation (as established by the intensity of the 437 nm at room temperature.The films of 1–3 are considered amorphband) is greatest in 2, followed by 1 and 3. This suggests that ous because of the absence of birefringence under crossed the octyl chain length may provide the optimal supramolecular polarizers in their pristine states (before annealing). In conordering in this series of polyfluorenes in solution. trast, annealed films displayed high birefringence under these Absorbance spectra of pristine films of 1–3 are also similar conditions because of the order of their thermotropic liquid in lmax and intensity (Fig. 2B), with absorption maxima similar crystalline states. The liquid crystalline states of 1–3 are to those observed in solution, Fig. 2A. These films lack evi- characterized as nematic because of the characteristic Schlieren dence of the 437 nm aggregate band, although the bandwidth texture32 observed by PLM (Fig. 1). This texture results from is broadened from that observed in solution. Annealing of lamella (made of polymer chains) which form around the films causes further broadening, Fig. 2C, and induces the nucleation points at defects on the surface of the glass appearance of the absorption at 437 nm.A similar low energy substrate.33,34 absorption band has been described in the aggregate produced Polyfluorene thin films have excellent chemical stability in in films of ladder-type poly( p-phenylene)23 and poly(pheny- the presence of oxygen, moisture, and light as evidenced by leneethylene),14 as well as in poly(dioctylfluorene) films cooled the absence of new spectral bands in the IR.For example, to -78 °C and slowly heated to ambient temperatures to there is no evidence for the presence of carbonyl absorbinduce aggregation.11 Spectral broadening was attributed to a ances which might have been introduced by possible photobroader distribution of eVective conjugation lengths conse- oxidation.13 Bliznyuk et al. have observed, for example, that quent to polymer p,p* interactions in the ground state end-capped polyfluorenes form fluorenones, by a process which aggregates.23 is impossible here by virtue of the bisalkylation at the 9- Fluorescence spectra of 1–3 in solution are also similar position.In addition, transient emission spectra exhibit in bandwidth, the wavelength of emission, and intensity unchanged kinetic profiles in both pristine and annealed films, (Table 3), implying that the lengths of the appended alkyl even after more than one week of exposure to ambient chains do not appreciably change the excited state surface of laboratory conditions.Nor did thermal gravimetric analysis the polymer in solution. Emission from pristine films of 1–3 (TGA) show any change in mass until above 400 °C for 1–3.is red-shifted from their solution phase fluorescence maxima, The polymer films 1–3, however, do show a new absorption with the greatest shift being observed in 3. Annealing further band at 437 nm upon heating which is not present in dilute red-shifts the emission of films of 2 and 3, but not of 1. Fig. 3 solution or in a pristine film. When a film of 2 that was spin cast from toluene and heated to 250 °C to induce the 437 nm peak was redissolved in toluene, its solution spectra did not display the 437 nm peak.Nor did the redissolved polymer show any change in molecular weight by GPC. Thus, the Fig. 2 Absorbance spectra of 2: A) as a 10-4 M solution in THF; B) Fig. 1 Polarized light micrograph (magnified 800×) showing charac- as a 250 nm pristine (amorphous) film; C) film (B) annealed to 160 °C.A is normalized to B and C at 440 nm for ease of comparison of teristic nematic liquid crystalline Schlieren texture of a 250 nm film of 1 annealed at 250 °C (scale bar equals 5 mm). intensity of the long wavelength aggregate band. J. Mater. Chem., 1999, 9, 2117–2122 2119Table 3 Red-shifted emission of poly(dialkylfluorene) films films suggests that planarization is already suYcient to achieve maximum eVective conjugation length beyond which annealing Solutiona Pristine filmb Annealed filmb has no further eVect.Polymer lmax/nm of 2nd vibronic peak emission The increased fluorescence eYciency observed at lower energies in films of 1–3 has been assigned to both aggregate 1 439 447 448 2 438 446 455 and excimer emission, an assignment made in corroboration 3 439 451 456 of assertions of Grell et al.8 and Bliznyuk.13 In going from solution to a pristine film to the annealed film, enhancement aAs 10-7 M THF.bAs 250 nm thick films of polymer spun from toluene onto glass and measured as prepared (pristine) or heated of the low energy transition is greater in 1 than in 3 or 2.This (annealed) to 160 or 250 °C. Excitation at 365 nm. observation implies that greater insulation of the polymer backbone by alkyl chains longer than six carbons does not aVect the formation of excimer. Similarly, the larger red-shifts and lower energy of emission observed in films than in solution and in annealed films than in pristine films suggest that the polymer’s ability to adopt a more planar excited state in films is facilitated by greater interchain interaction.The variation in the red-shifted emission observed in going from homogeneous solution to a pristine film to an annealed film of 1–3 suggests that alkyl chain length significantly alters the supramolecular packing and thus the energy of emission (greater eVective conjugation as a result of the extended rigid conformation and/or more favorable excimer formation).The mechanism of excimer formation is unclear. Longer alkyl chains induce greater order in pristine films, as evidenced by the greatest red-shift and the largest fractional contribution of the long-lived emitting species in 3. 3.3. Fluorescence lifetimes Fig. 3 Fluorescence spectra of 2: A) as a 10-6 M solution in THF, Fluorescence decays collected at 440 nm for 1–3 in THF were O.D.=0.1 at lexc=355 nm under atmospheric conditions; B) as a fitted to a single exponential to give approximately 550 ps 250 nm pristine (amorphous) film; and C) film (B) annealed to 160 °C.lifetimes, Table 4. These lifetimes are independent of excitation and collection wavelength. Given an absence of any intermolshows the red-shifted emission observed upon going from ecular interactions in dilute solutions of 1–3 in THF, the solution to a pristine film to an annealed film of 2.emissive species is assigned as an S1,S0 singlet exciton. When In going from solution to the thin film, 1–3 show a significant the same measurement is made in n-heptane (to induce aggrered- shifted emission, with 3 exhibiting the greatest shift gation), a transient emitting at 440 nm exhibited a lifetime of (Table 3).This suggests that the longer alkyl chains enhance 430 ps (93%) with a small contribution from a new 700 ps interchain interactions, which leads to a more planar extended species. Two longer lived species, 2 ns (7%) and 8 ns (4%), excited state for 3. Annealing films of 2 and 3 causes a further were also observed at 550 nm that were not present in solutions red-shift, but no further shift in the annealed films of 1, which of similar concentration in THF (Table 4).These new longer suggests that interchain interactions induce a red-shift from lived transients at 550 nm are assigned to excimer and aggrethe maxima observed in pristine films of 2 and 3.gate emission respectively,27 and similar lifetimes were also Fig. 4 shows the eVect of annealing on the spectrum of a observed in annealed films of 1–3 in the same spectral pristine film of 1 where no further red-shifts are observed in region, Table 5. going from pristine-to-annealed films, but where increased In films of 1–3, the lifetime of the singlet exciton is reduced fluorescence eYciency is observed at longer wavelengths.The to between 200 and 300 ps at 440 nm because of an increase absence of further red-shifts in 1 from pristine-to-annealed in possible non-radiative decay pathways. These include energy transfer to aggregate and excimer states and access to a greater number of contacts with the chain ends where exciton quenching is thought to be more eYcient.35 At 550 nm, fluorescence decays of pristine films of 1 and 3, respectively, gave three single exponential lifetimes, respectively: 300 and 400 ps, 1 and 3 ns, and 10 and 12 ns, whereas films of 2 gave only two single exponential lifetimes of approximately 700 ps and 9 ns.Upon annealing past the second nematic phase of 1–3, a decrease in the fluorescence intensity from the picosecondlived species is observed, with a greater fraction of the fluorescence emanating from the longer-lived nanosecond species (Table 5).These lifetimes and fractional contributions were not constant over wavelength in the measured range of 500–600 nm. At present, it is not possible to unambiguously assign the 1–3 and 10–12 ns species at 550 nm in 1–3, but they are thought to result from excimer and aggregate, respectively.Decays were collected at both 300 and 340 nm excitation for pristine and annealed films of 1–3 with similar results. Because 2 forms aggregate most readily in solution and Fig. 4 Fluorescence spectra of 250 nm film of 1 with an O.D.=0.5 at because this aggregation process is enhanced in films, aggregate lexc=355 nm under atmospheric conditions: A) as a pristine (amorphformation likely dominates the interchain interactions, at the ous) film; B) film annealed to 180 °C; and C) annealed to 250 °C.Fluorescence intensities of spectra are normalized at 440 nm. expense of excimer formation. Thus, the absence of the 1–3 ns 2120 J. Mater. Chem., 1999, 9, 2117–2122Table 4 Poly(dialkylfluorene) solution fluorescence lifetimes and fluorescence quantum yields Polymer tfl/ns (% composition),±0.03 ns lemission/nm x2 Wfl b±0.01 1a 0.56 (100) 440 1.09 0.64 2a 0.52 (100) 440 1.07 0.69 2c 0.43 (93), 0.72 (7) 440 1.49 0.51 2c 0.51 (89), 1.93 (7), 8.35 (4) 550 1.06 3a 0.55 (100) 440 1.19 0.65 aLifetimes determined with excitation at 300 nm in 10-7 M THF.bQuantum yields (Wfl) determined in cyclohexane relative to 9,10-diphenylanthracene with excitation at 365 nm.33 cLifetimes determined in 10-7 M n-heptane to induce aggregation.Table 5 Poly(dialkylfluorene) film fluorescence lifetimesa and quantum yieldsb tfl/±0.03 ns tfl/±0.3 ns Annealing (% contribution) (% contribution) Wfl±0.1 temp./°C lobs=440 nm x2 lobs=550 nm x2 (%) 1 25 0.27 (100) 5.64 0.3 (94), 1.3 (5), 10.2 (1) 32.86 15.5 250 0.18 (100) 87.9 0.2 (54), 3.0 (19), 11.8 (27) 2.83 10.0 2 25 0.29 (100) 4.56 0.7 (91), 9.1 (9) 3.68 17.0 160 0.21 (100) 1.70 0.7 (86), 9.8 (14) 11.5 25.5 3 25 0.32 (93), 0.77 (7) 1.14 0.4 (88), 3.2 (6), 12.3 (6) 1.40 15.5 160 0.31 (96), 0.81 (4) 1.09 0.4 (77), 3.2 (7), 11.6 (16) 1.99 13.5 Measurements with 250 nm films on glass. aLifetimes (tfl) measured using single photon counting23 with lexc=340 nm with 250 nm films.bQuantum yields (Wfl) measured using integrating sphere technique25 where each value is an average of 2–3 measurements with approx. 10% relative standard deviation. species in 2 (rather than in 1 and 3) suggests that the 9–12 ns not of 2 or 3, suggests that shorter alkyl chains allow for better interpolymer interaction, thus reducing the observed species in 1–3 can be assigned to aggregates and that 2 already fluorescence eYciency.The unexpected increase in fluorescence has an optimal configuration that inhibits excimer formation. quantum yield for annealed films of 2 suggests that the 1–3 ns- In films of 1 and 3 where both aggregate and excimer are lived species present in 1 and 3 (but not in 2) may be present, the total contributions from both of the two longresponsible for the eYcient fluorescence quenching.Thus, an lived (ns) species are 46% in 1 and 23% in 3. Thus, shorter optimal alkyl chain length produces the highest fluorescence alkyl chains likely permit better supramolecular packing of 1, eYciency in these alkyl-substituted polyfluorene films.which in turn produces greater interchain communication. The long lived species observed in films of 1–3 and the increased fractional contribution of these species as the films are annealed 4. Conclusions are evidence of enhanced interchain interactions resulting from The degree of interpolymer interactions in 9,9-dialkylated the nematic ordering of the alkylated polymer chains.polyfluorenes in solution can be controlled by choice of solvent and by the length of the alkyl chain. The degree of interchain 3.4. Fluorescence quantum yields interaction in solution is greatest for 2, followed by 1 and 3, The dependence of fluorescence quantum yields on supramol- as evidenced by the relative intensities of a new 437 nm ecular packing was probed by measuring the changes in aggregate band, even in good solvents.Pristine films of 3 show fluorescence quantum yields observed upon moving from greater red-shifts in emission bands from solution maxima solution to film and upon increasing local ordering of the thin than do those of 1 or 2. Annealing produces further red-shifts films through annealing. The fluorescence quantum yields of in 2 and 3, but not in 1.Thus, longer alkyl chains force an 1–3 in cyclohexane ranged between 51 and 69%, dependent extended ground state conformation, the planarity of which on polymer concentration, Table 4. Thus, aggregation induces in turn enhances intermolecular aggregation in solution. a lowering of the quantum yield by approximately 20%. Polyfluorenes bearing longer alkyl chains will thus exhibit Pristine films of 1–3 have identical quantum yields of enhanced on-axis dipole moments and increased dichroic approximately 16%, Table 5.36,37 This dramatic decrease from ratios.the observed solution quantum yields suggests that stronger Films of 1–3 show increased ordering upon annealing, as interactions between chains in the films drastically lowers evidenced by formation of an oriented nematic liquid crystal- fluorescence quantum yields.This is similar to the value line phase that shows a characteristic Schlieren texture. reported by Redecker et al.12 (0.50) for the fluorescence Although the exact identity of the ns-lived species cannot be quantum yield, where the value was quite dependent on film exactly determined with this data, the contribution of this morphology.Upon annealing (past their second nematic species is greatest in 1, implying that greater excited state phase) to induce greater order, films of 3 show no change in interchain communication is possible with the polyfluorenes the observed fluorescence quantum yield, whereas films of 2 bearing the shorter alkyl chains. This packing, in turn, leads show a slight increase.When films of 1 were annealed past to enhanced excimer formation. The fluorescence quantum their second nematic phase transition temperature, the quan- yields in these polymers decrease from 51–69% in solution to tum yields decreased slightly from 15.5 to 10%. The phase approximately 3% as pristine films, implying substantial fluotransition temperatures vary between 1–3, so it was possible rescence quenching in the as-deposited thin films of 1–3 as a that changes in quantum yield might be due to temperature function of packing.The fluorescence quantum yield of 1 dependent fluorescence impurity quenching rather than to decreases slightly upon annealing, whereas those of 2 and 3 supramolecular packing induced by liquid crystalline phase increase and remain the same, respectively.This suggests that transition. However, control experiments in which 2 and 3 are the shorter alkyl chains enhance interchain packing interannealed at the same transition temperature as 1 showed the actions, with predictable consequences for fluorescence eYciency in these films. same results. Thus, the decrease in quantum yield of 1, but J.Mater. Chem., 1999, 9, 2117–2122 212116 S. Tasch, A. Niko, G. Leising and U. Scherf, Appl. Phys. Lett., Acknowledgements 1996, 68, 1090. 17 Y. Yang, Q. Pei and A. J. Heeger, J. Appl. Phys., 1996, 79, 934. We gratefully acknowledge Drs Edward P. Woo and Michael 18 A. W. Grice, A. Tajbakhsh, P. L. Burn and D. Bradley, Adv. Inbaskeran at Dow Chemical for samples of polyfluorenes 1–3 Mater., 1997, 9, 1174.and for useful discussions of this work. We acknowledge 19 (a) G. Wegner, D. Neher, M. Remmer, V. Cimrova and Donald O’Connor of the Center for Fast Kinetics at the M. Schulze, Mater. Res. Soc. Symp. Proc., 1996, 413, 23; A. Kraft, A. C. Grimsdale and A. B. Holmes, Angew. Chem., Int. Ed., 1998, University of Texas at Austin for assistance with the time- 37, 402.resolved fluorescence measurements. This work was supported 20 C. Weder, J. M. Wagner and M. S. Wrighton, Mater. Res. Soc. by the Texas Advanced Research Program and the Robert A. Symp. Proc., 1996, 413, 77. Welch Foundation. 21 C.Weder and M. S. Wrighton, Macromolecules, 1996, 29, 5157. 22 J. W. Blatchford, S. W. Jessen, L. B. Lin, T. L. Gustafson, D. K. Fu, H.L. Wang, T. M. Swager, A. G. MacDiarmid and A. J. Epstein, Phys. Rev. B, 1996, 54, 9180. References 23 U. Lemmer, S. Heun, R. F. Mahrt, U. Scherf, M. Hopmeier, U. Siegner, E. O. Gobel, K. Mu� llen and H. Ba�ssler, Chem. Phys. 1 J. H. Burroughes, R. H. Friend, D. D. C. Bradley, A. R. Brown, Lett., 1995, 240, 373. R. N. Marks, K. Mackay, P. L. Burn and A. B. Holmes, Nature, 24 S.A. Jenekhe and J. A. Osaheni, Science, 1994, 265, 765. 1990, 347, 539. 25 T. Pauck, R. Hennig, M. Perner, U. Lemmer, U. Siegner, R. F. 2 A. Heeger, F. Hide, B. Schwartz and M. A. Diaz-Garcia, Chem. Mahrt, U. Scherf, K. Mu� llen, H. Ba�ssler and E. O. Gobel, Chem. Phys. Lett., 1996, 256, 424. Phys. Lett., 1995, 244, 171. 3 H. Sirringhaus, N. Tessler and R. H. Friend, Science, 1998, 280, 26 I.D. W. Samuel, G. Rumbles, C. J. Collison, B. Crystall, S. C. 1741. Moratti and A. B. Holmes, Synth. Met., 1996, 76, 15. 4 C. Weder, C. Sarwa, A. Montali, C. Bastiaansen and P. Smith, 27 E. Conwell, Trends Polym. Sci., 1997, 5, 218. Science, 1998, 279, 835. 28 D. V. O’Conner and D. Phillips, Time-Correlated Single Photon 5 M. Fukuda, K. Sawada and K. Yoshino, Jpn. J. Appl.Phys., 1989, Counting, Academic Press, London, 1984. 28, L1433. 29 D. Eaton, Pure Appl. Chem., 1988, 60, 1107. 6 M. Fukuda, K. Sawda and K. Yoshino, J. Polym. Sci., Part A: 30 J. C. de Mello, F. H. Wittmann and R. H. Friend, Adv. Mater., Polym. Chem., 1993, 31, 2465. 1997, 9, 230. 7 M. Grell, D. D. C. Bradley, M. Inbasekaran and E. F. Woo, Adv. 31 M. BalauV, Macromolecules, 1986, 19, 1366.Mater., 1997, 9, 798. 32 L. C. Sawyer and D. T. Grubb, Polymer Microscopy, Chapman & 8 D. D. C. Bradley, M. Grell, X. Long, H. Mellor and A. Grice, Hall, London, 1996. Soc. Phot. Int. Eng., 1997, 3145, 254. 33 N. H. Hartshorne, Optical Properties of Liquid Crystals: Physico- 9 Q. Pei and Y. Yang, J. Am. Chem. Soc., 1996, 118, 7416. Chemical Properties and Methods of Investigation, Horwood, 10 Y.Ohmori, M. Uchida, K. Muro and K. Yoshino, Jpn. J. Appl. Chichester, 1974, Vol. 2. Phys., Part 2, 1991, 30, L1941. 34 B. A.Wood and E. L. Thomas, Nature, 1986, 324, 655. 11 M. Grell, D. D. C. Bradley, X. Long, T. Chamberlain, M. 35 C.Weder and M. S. Wrighton, Macromolecules, 1995, 29, 5157. Inbasekaran, E. P. Woo and M. Soliman, Acta Polym., 1998, 36 In measuring quantum yields for thin films, variations in refractive index, surface roughness, and film thickness can introduce incon- 49, 439.sistency into the measurement of fluorescence quantum yields.31 12 M. Redecker, D. D. C. Bradley, M. Inbasekaran and E. P. Woo, Therefore, an integrating sphere was employed to measure the Appl. Phys. Lett., 1998, 73, 1565. quantum yields for the thin films. 13 V. N. Bliznyuk, S. A. Carter, J. C. Scott, G. Klarmer, R. D. Miller 37 N. C. Greenham, I. D. W. Samuel, G. R. Hayes, R. T. Phillips, and D. C. Miller, Macromolecules, 1999, 32, 361. Y. A. R. R. Kessener, S. C. Morratti, A. B. Holmes, R. H. Friend, 14 C. E. Halkyard, M. E. Rampey, L. Kloppenburg, S. L. Studer- Chem. Phys. Lett., 1995, 241, 89. Martinez and H. F.Bunz, Macromolecules, 1998, 31, 8655. 15 G. Grem, B. Keditzky, D. Ullrich and G. Leising, Adv. Mater., 1992, 4, 36. Paper 9/02829C 2122 J. Mater. Chem., 1999, 9, 2117–2122 J O U R N A L O F C H E M I S T R Y Materials Photophysical characterization of dilute solutions and ordered thin films of alkyl-substituted polyfluorenes Julie Teetsov and Marye Anne Fox*† Department of Chemistry, University of Texas at Austin, Austin, TX 78712, USA Received 9th April 1999, Accepted 23rd June 1999 Absorbance and fluorescence spectra of dilute solutions and thin films of polyfluorene rigid-rod polymers 1–3 bearing two hexyl, octyl, or dodecyl groups at the 9-position define the eVect of polymer chain interactions on excited state relaxation.Film morphology is controlled by annealing of 250 nm thick films.Under these conditions, the degree of interchain interaction follows the degree of thermotropic liquid crystalline ordering which is, in turn, a function of the length of the attached alkyl substituents. Alkyl substituents also aVect the solubility of these polymeric liquid crystals in organic solvents; low solubility favors strong ground state aggregation, as is evidenced by a red-shifted absorption band.In the annealed films, aggregate and excimer formation is evidenced by a broadening of the absorbance band, an increase in the intensity of the low energy emission, the appearance of new long-lived fluorescent species, and structure-dependent changes in observed fluorescence quantum yields. molecules with extended p-systems are known to adopt even 1.Introduction more planar geometries in the excited state, thus facilitating There is great interest in the fundamental optoelectronic aggregation through p-stacking.11 Because both ground state properties of rod-like conjugated polymers because of their aggregates23,25 and excimers21,26 can enhance non-radiative utility in light-emitting devices,1 lasers,2 thin film transistors,3 decay in solution and in the solid state, aggregation is often and polarizers.4 Polyfluorenes,5–13 poly(phenyleneethynyl- considered undesirable in light-emitting devices.Nonetheless, enes),4,12,14 poly( p-phenylenes),15 and derivatives of poly- recent work by Grell et al.11 with poly(dioctylfluorene) suggests phenylene16,17 and poly( p-phenylenevinylenes)18 fit into a class that fluorescence eYciency may, in fact, increase in the solid of rigid-rod blue light-emitting polymers.19 The extended con- state compared with that observed in homogeneous solution.jugation characteristic of these families enhances charge According to Bradley et al.,8 polyfluorenes form clusters in delocalization by virtue of the greater molecular planarity solvents with poor solvating characteristics and in thin films attained along their rigid backbones.This electronic delocaliz- prepared either from poor solvents or by spin-coating from ation also influences a number of the polymers’ physical good solvents while the support is cooled to -77 °C and properties including their band gaps, absorption coeYcients, warmed slowly to induce thermodynamically controlled aggreand emission quantum yields.2 gation.However, the eVect of aggregation on observable Rigid–rod polymers often also form stable liquid crystalline fluorescence lifetimes and quantum yields is still unclear phases and self-organize in the solid state either upon heating because evidence for a correlation of either excimer or aggre- (thermotropic) or upon adding solvent ( lyotropic).Studies of gate formation with the observed fluorescence eYciency is very the absorption and fluorescence4,14,20–22 of several rigid-rod sample-dependent.27 polymers show that appended alkyl side chains can aVect both Because the correlation between local order and emission liquid crystallinity and interpolymer interactions.Little is eYciency remains ambiguous, we sought to study interchain known, however, about the correlation between the shifts in interactions of liquid crystalline 9,9-dialkylpolyfluorenes 1–3 molecular packing and trends in the eYciency of light emission. with polymer lengths that significantly exceed the eVective In fact, one important reason for studying a family of closely p-conjugation length.We are particularly related polymers diVering only in the length of the attached alkyl chains is to examine whether rigid-rod polymers aligned over large areas can display high degrees of dichroic absorption and emission. With optimal mechanical alignment, interpolymer interactions may serve to enhance the observed dichroic ratio because of the enhanced on-axis dipole moment of the aggregate array compared with that of the individual molecule or a randomly dispersed film.For example, Grell interested in observing the influence of the alkyl chain length et al.11 have shown an increase in the dichroic ratios from 7 on spectral trends and excited state lifetimes within the series, to 20 upon inducing aggregation in their polymer films. hoping that we may discern design features that will assist in It is also known that interchain p,p* interactions can preparation of new high eYciency LEDs. In this study, we influence the net observable fluorescence eYciencies in this describe aggregate and excimer formation in a series of dialkylseries through both ground state aggregation and excimer ated polyfluorenes 1–3 in solution and as thin films.By formation. Ground state aggregates formed between two or determining trends in fluorescence lifetimes and fluorescence more polymer chains are evidenced by a broadening of the quantum yields, we establish that the length of the linear alkyl absorption spectra23 or by the appearance of new absorption side chain does indeed play an important role in controlling bands.8 Excimer formation, i.e., complexation between an interchain interactions. In addition, we show that the rigidity excited state of a molecule and the same species in the ground of these polymeric liquid crystals, as related to their eVective state, is particularly favorable in rigid-rod polymers24 because conjugation length, is influenced by the alkyl substituents and that it is therefore possible to control attainable local order in pristine and annealed liquid crystalline thin films by syntheti- †Present address: OYce of the Chancellor, Box 7001/Holladay Hall, North Carolina State University, Raleigh, NC 27695–7001, USA.cally modifying the length of the attached chain. J. Mater. Chem., 1999, 9, 2117–2122 2117355 nm laser pulses) and sample emission were focused into 2.Experimental and out of the integrating sphere using fiber optics. All spectra 2.1. Instrumentation were corrected for the monochromator and photomultiplier tube (PMT) spectral response. Incident radiant energy was Fourier-transform infrared spectra were obtained as KBr converted to a specified number of photons by multiplying the pellets on a Nicolet 510P spectrometer.Gel permeation chromobserved emission intensity by wavelength. The observed atography (GPC) on Waters Styragel columns with THF as emission intensity was obtained by integrating over all emission eluent gave peaks that were calibrated against commercial wavelengths in the additive spectrum, where each wavelength’s polystyrene standards. (Although GPC calibration against intensity was obtained from an average of 50 decays.Laser polystyrene standards is a routine procedure for estimating drift introduced an approx. 6% relative standard deviation molecular weights of rigid-rod polymers, some recent work (measured over the duration of each sample measurement). has shown these values to be roughly a factor of 3 larger than Each reported quantum yield is an average of 2–3 sample those attained by light scattering.11 Here, however, the estimeasurements with an estimated 10% relative standard mated molecular weights agree roughly with those expected deviation.from the additive extinction coeYcients of the appended fluorenyl groups.) Liquid crystalline phase transitions were determined with a 3. Results and discussion Perkin Elmer Series 7 UNIX diVerential scanning calorimeter 3.1.Polymer properties (DSC). Thermal decomposition was determined with a Perkin Elmer Series 7 Thermal Gravimetric Analyzer (TGA). As established by polarizing light microscopy, polymers 1–3 Crystalline transitions for bulk and thin films were identified exist as polycrystalline yellow flakes whose molecular weights with an Olympus polarizing light microscope (PLM) equipped were characterized by GPC (Table 1), displaying regions of with a camera attachment and hot and cold stages.order with dimensions exceeding one micron. Polymers 1–3 readily dissolved (100 mg mL-1) in toluene, chloroform, and 2.2. Materials THF to yield clear violet solutions. The polymers could be dissolved in dichloromethane, cyclohexane, and n-heptane only Samples 1–3 were a gift from Drs Edward Woo and Michael upon heating.The resulting clear violet solution quickly turned Inbasekaran from Central & New Businesses R & D, Dow translucent yellow upon cooling, with the formation of a gel Chemical Company, Midland, Michigan. All other materials or precipitation of the polymer. The yellow color is assigned (Aldrich) were used as received.to a polymer aggregate8 which shows a new absorption band at 437 nm (see below). The appearance of the 437 nm band in 2.3. Thin film preparation poor solvents results from a mismatch of Hildebrandt para- Thin films were prepared with a Specialty Coating Systems meters.11 In contrast to polymers 1 and 3, at room constant Inc.Spin Coater, model P6204-A. Polymer solutions (3 wt.%) temperature and a 10 mg mL-1 concentration, polymer 2 began prepared in toluene were placed on 1 cm glass squares. The to form aggregates in toluene and tetrahydrofuran, as evisubstrate was rotated at a 500 rpm ramping speed for 1 s, denced by the appearance of the characteristic 437 nm peak. followed by 3000 rpm for 30 s, producing 250±30 nm thick These two latter solvents have better matched Hildebrandt films, as determined by a Tencor Instruments Alpha Step 100 parameters.profilometer. Films were used as spun or were annealed under Bulk thermal properties of 1–3 were determined with both Ar at either 160 or 250 °C for 2 h before being cooled at DSC and PLM (Table 2). These techniques complement each 30 °Cmin-1 to preserve the order of the polymer’s liquid other because DSC is the more sensitive thermal technique, crystalline phase.Near-field optical microscopy results show but does not allow for visual observation of bulk polymer or order at 200 nm in pristine films of 3 to a greater extent than thin film morphology. In pristine films not previously subjected in 2, with essentially complete disorder observed with 1.After to heating cycles, an endothermic crystalline-to-nematic A annealing, films of 1 show the greatest order, followed by liquid crystalline phase transition followed by a second endothose of 2 and 3. Table 1 Molecular weights of the poly(dialkylfluorene)s 2.4. Transient and steady-state spectroscopy Polymer Mn a±500 PDb±1 nc±2 Absorbance spectra were recorded on a Shimadzu spectrometer.Fluorescence measurements were made with an 1 17 000 4.1 51 SLM Aminco SPF 500 fluorimeter. Temperature-dependent 2 24 000 4.4 62 measurements were performed with a water-blanketed quartz 3 21 000 2.6 42 cuvette. Time-resolved fluorescence lifetimes were obtained at aNumber averaged molecular weights (Mn). bPolydispersities (PD) are the Center for Fast Kinetics Research by time-correlated determined by GPC vs.polystyrene standards in THF, where PD= single-photon counting using a mode-locked, synchronously Mn/Mw (number averaged/weight averaged molecular weight). cThe pumped, cavity-dumped dye laser (l=300 and 340 nm, number of monomer repeat units (n) is calculated from the Mn. 3 mJ pulse-1).28 Emissive photons were collected at 90° with respect to the excitation beam (300 nm) and were passed through a monochromator to a Hamamatsu Model R2809U Table 2 Thermal properties of the poly(dialkylfluorene)sa micro-channel plate.Data analysis was conducted after decon- Temp./±2 °C (DH/±0.01 J g-1) volution of the instrument response function (fwhm ~80 ps). All solutions were purged with N2 for 5 min before the Polymer SP N(A) N(B) I measurement and had an optical density (OD)=0.1 at 340 nm. 1 94 162–213 (3.69) 222–246 (3.67) 290–300 2.5. Fluorescence quantum yields 2 72 80–103 (11.63) 108–157 (11.05) 278–283 3 47 62–77 (0.58) 83–116 (4.57) 116–118 Solution quantum yields were measured relative to 9,10- aDetermined by diVerential scanning calorimetery and polarized light diphenylanthracene (W=0.9 in cyclohexane)29 using an SLM microscopy of pristine polymers without thermal pretreatment.SP Aminco SPF 500 fluorimeter. A Lab Sphere 10 cm integrating and I refer to softening and melting (isotropic) point. N(A) and N(B) sphere was used to obtain absolute quantum yields following refer to nematic A and nematic B liquid crystalline phases. the method of de Mello et al.30 Laser excitation (10 mW 2118 J.Mater. Chem., 1999, 9, 2117–2122355 nm laser pulses) and sample emission were focused into 2. Experimental and out of the integrating sphere using fiber optics. All spectra 2.1. Instrumentation were corrected for the monochromator and photomultiplier tube (PMT) spectral response. Incident radiant energy was Fourier-transform infrared spectra were obtained as KBr converted to a specified number of photons by multiplying the pellets on a Nicolet 510P spectrometer. Gel permeation chromobserved emission intensity by wavelength.The observed atography (GPC) on Waters Styragel columns with THF as emission intensity was obtained by integrating over all emission eluent gave peaks that were calibrated against commercial wavelengths in the additive spectrum, where each wavelength’s polystyrene standards.(Although GPC calibration against intensity was obtained from an average of 50 decays. Laser polystyrene standards is a routine procedure for estimating drift introduced an approx. 6% relative standard deviation molecular weights of rigid-rod polymers, some recent work (measured over the duration of each sample measurement).has shown these values to be roughly a factor of 3 larger than Each reported quantum yield is an average of 2–3 sample those attained by light scattering.11 Here, however, the estimeasurements with an estimated 10% relative standard mated molecular weights agree roughly with those expected deviation. from the additive extinction coeYcients of the appended fluorenyl groups.) Liquid crystalline phase transitions were determined with a 3.Results and discussion Perkin Elmer Series 7 UNIX diVerential scanning calorimeter 3.1. Polymer properties (DSC). Thermal decomposition was determined with a Perkin Elmer Series 7 Thermal Gravimetric Analyzer (TGA). As established by polarizing light microscopy, polymers 1–3 Crystalline transitions for bulk and thin films were identified exist as polycrystalline yellow flakes whose molecular weights with an Olympus polarizing light microscope (PLM) equipped were characterized by GPC (Table 1), displaying regions of with a camera attachment and hot and cold stages.order with dimensions exceeding one micron. Polymers 1–3 readily dissolved (100 mg mL-1) in toluene, chloroform, and 2.2.Materials THF to yield clear violet solutions. The polymers could be dissolved in dichloromethane, cyclohexane, and n-heptane only Samples 1–3 were a gift from Drs Edward Woo and Michael upon heating. The resulting clear violet solution quickly turned Inbasekaran from Central & New Businesses R & D, Dow translucent yellow upon cooling, with the formation of a gel Chemical Company, Midland, Michigan.All other materials or precipitation of the polymer. The yellow color is assigned (Aldrich) were used as received. to a polymer aggregate8 which shows a new absorption band at 437 nm (see below). The appearance of the 437 nm band in 2.3. Thin film preparation poor solvents results from a mismatch of Hildebrandt para- Thin films were prepared with a Specialty Coating Systems meters.11 In contrast to polymers 1 and 3, at room constant Inc. Spin Coater, model P6204-A.Polymer solutions (3 wt.%) temperature and a 10 mg mL-1 concentration, polymer 2 began prepared in toluene were placed on 1 cm glass squares. The to form aggregates in toluene and tetrahydrofuran, as evisubstrate was rotated at a 500 rpm ramping speed for 1 s, denced by the appearance of the characteristic 437 nm peak.followed by 3000 rpm for 30 s, producing 250±30 nm thick These two latter solvents have better matched Hildebrandt films, as determined by a Tencor Instruments Alpha Step 100 parameters. profilometer. Films were used as spun or were annealed under Bulk thermal properties of 1–3 were determined with both Ar at either 160 or 250 °C for 2 h before being cooled at DSC and PLM (Table 2).These techniques complement each 30 °Cmin-1 to preserve the order of the polymer’s liquid other because DSC is the more sensitive thermal technique, crystalline phase. Near-field optical microscopy results show but does not allow for visual observation of bulk polymer or order at 200 nm in pristine films of 3 to a greater extent than thin film morphology. In pristine films not previously subjected in 2, with essentially complete disorder observed with 1.After to heating cycles, an endothermic crystalline-to-nematic A annealing, films of 1 show the greatest order, followed by liquid crystalline phase transition followed by a second endothose of 2 and 3. Table 1 Molecular weights of the poly(dialkylfluorene)s 2.4.Transient and steady-state spectroscopy Polymer Mn a±500 PDb±1 nc±2 Absorbance spectra were recorded on a Shimadzu spectrometer. Fluorescence measurements were made with an 1 17 000 4.1 51 SLM Aminco SPF 500 fluorimeter. Temperature-dependent 2 24 000 4.4 62 measurements were performed with a water-blanketed quartz 3 21 000 2.6 42 cuvette. Time-resolved fluorescence lifetimes were obtained at aNumber averaged molecular weights (Mn).bPolydispersities (PD) are the Center for Fast Kinetics Research by time-correlated determined by GPC vs. polystyrene standards in THF, where PD= single-photon counting using a mode-locked, synchronously Mn/Mw (number averaged/weight averaged molecular weight). cThe pumped, cavity-dumped dye laser (l=300 and 340 nm, number of monomer repeat units (n) is calculated from the Mn. 3 mJ pulse-1).28 Emissive photons were collected at 90° with respect to the excitation beam (300 nm) and were passed through a monochromator to a Hamamatsu Model R2809U Table 2 Thermal properties of the poly(dialkylfluorene)sa micro-channel plate. Data analysis was conducted after decon- Temp./±2 °C (DH/±0.01 J g-1) volution of the instrument response function (fwhm ~80 ps). All solutions were purged with N2 for 5 min before the Polymer SP N(A) N(B) I measurement and had an optical density (OD)=0.1 at 340 nm. 1 94 162–213 (3.69) 222–246 (3.67) 290–300 2.5. Fluorescence quantum yields 2 72 80–103 (11.63) 108–157 (11.05) 278–283 3 47 62–77 (0.58) 83–116 (4.57) 116–118 Solution quantum yields were measured relative to 9,10- aDetermined by diVerential scanning calorimetery and polarized light diphenylanthracene (W=0.9 in cyclohexane)29 using an SLM microscopy of pristine polymers without thermal pretreatment.SP Aminco SPF 500 fluorimeter. A Lab Sphere 10 cm integrating and I refer to softening and melting (isotropic) point. N(A) and N(B) sphere was used to obtain absolute quantum yields following refer to nematic A and nematic B liquid crystalline phases.the method of de Mello et al.30 Laser excitation (10 mW 2118 J. Mater. Chem., 1999, 9, 2117–2122355 nm laser pulses) and sample emission were focused into 2. Experimental and out of the integrating sphere using fiber optics. All spectra 2.1. Instrumentation were corrected for the monochromator and photomultiplier tube (PMT) spectral response.Incident radiant energy was Fourier-transform infrared spectra were obtained as KBr converted to a specified number of photons by multiplying the pellets on a Nicolet 510P spectrometer. Gel permeation chromobserved emission intensity by wavelength. The observed atography (GPC) on Waters Styragel columns with THF as emission intensity was obtained by integrating over all emission eluent gave peaks that were calibrated against commercial wavelengths in the additive spectrum, where each wavelength’s polystyrene standards.(Although GPC calibration against intensity was obtained from an average of 50 decays. Laser polystyrene standards is a routine procedure for estimating drift introduced an approx. 6% relative standard deviation molecular weights of rigid-rod polymers, some recent work (measured over the duration of each sample measurement). has shown these values to be roughly a factor of 3 larger than Each reported quantum yield is an average of 2–3 sample those attained by light scattering.11 Here, however, the estimeasurements with an estimated 10% relative standard mated molecular weights agree roughly with those expected deviation.from the additive extinction coeYcients of the appended fluorenyl groups.) Liquid crystalline phase transitions were determined with a 3. Results and discussion Perkin Elmer Series 7 UNIX diVerential scanning calorimeter 3.1. Polymer properties (DSC). Thermal decomposition was determined with a Perkin Elmer Series 7 Thermal Gravimetric Analyzer (TGA). As established by polarizing light microscopy, polymers 1–3 Crystalline transitions for bulk and thin films were identified exist as polycrystalline yellow flakes whose molecular weights with an Olympus polarizing light microscope (PLM) equipped were characterized by GPC (Table 1), displaying regions of with a camera attachment and hot and cold stages.order with dimensions exceeding one micron. Polymers 1–3 readily dissolved (100 mg mL-1) in toluene, chloroform, and 2.2. Materials THF to yield clear violet solutions. The polymers could be dissolved in dichloromethane, cyclohexane, and n-heptane only Samples 1–3 were a gift from Drs Edward Woo and Michael upon heating. The resulting clear violet solution quickly turned Inbasekaran from Central & New Businesses R & D, Dow translucent yellow upon cooling, with the formation of a gel Chemical Company, Midland, Michigan.All other materials or precipitation of the polymer. The yellow color is assigned (Aldrich) were used as received. to a polymer aggregate8 which shows a new absorption band at 437 nm (see below). The appearance of the 437 nm band in 2.3.Thin film preparation poor solvents results from a mismatch of Hildebrandt para- Thin films were prepared with a Specialty Coating Systems meters.11 In contrast to polymers 1 and 3, at room constant Inc. Spin Coater, model P6204-A. Polymer solutions (3 wt.%) temperature and a 10 mg mL-1 concentration, polymer 2 began prepared in toluene were placed on 1 cm glass squares.The to form aggregates in toluene and tetrahydrofuran, as evisubstrate was rotated at a 500 rpm ramping speed for 1 s, denced by the appearance of the characteristic 437 nm peak. followed by 3000 rpm for 30 s, producing 250±30 nm thick These two latter solvents have better matched Hildebrandt films, as determined by a Tencor Instruments Alpha Step 100 parameters. profilometer. Films were used as spun or were annealed under Bulk thermal properties of 1–3 were determined with both Ar at either 160 or 250 °C for 2 h before being cooled at DSC and PLM (Table 2).These techniques complement each 30 °Cmin-1 to preserve the order of the polymer’s liquid other because DSC is the more sensitive thermal technique, crystalline phase. Near-field optical microscopy results show but does not allow for visual observation of bulk polymer or order at 200 nm in pristine films of 3 to a greater extent than thin film morphology. In pristine films not previously subjected in 2, with essentially complete disorder observed with 1.After to heating cycles, an endothermic crystalline-to-nematic A annealing, films of 1 show the greatest order, followed by liquid crystalline phase transition followed by a second endothose of 2 and 3. Table 1 Molecular weights of the poly(dialkylfluorene)s 2.4.Transient and steady-state spectroscopy Polymer Mn a±500 PDb±1 nc±2 Absorbance spectra were recorded on a Shimadzu spectrometer. Fluorescence measurements were made with an 1 17 000 4.1 51 SLM Aminco SPF 500 fluorimeter. Temperature-dependent 2 24 000 4.4 62 measurements were performed with a water-blanketed quartz 3 21 000 2.6 42 cuvette.Time-resolved fluorescence lifetimes were obtained at aNumber averaged molecular weights (Mn). bPolydispersities (PD) are the Center for Fast Kinetics Research by time-correlated determined by GPC vs. polystyrene standards in THF, where PD= single-photon counting using a mode-locked, synchronously Mn/Mw (number averaged/weight averaged molecular weight).cThe pumped, cavity-dumped dye laser (l=300 and 340 nm, number of monomer repeat units (n) is calculated from the Mn. 3 mJ pulse-1).28 Emissive photons were collected at 90° with respect to the excitation beam (300 nm) and were passed through a monochromator to a Hamamatsu Model R2809U Table 2 Thermal properties of the poly(dialkylfluorene)sa micro-channel plate.Data analysis was conducted after decon- Temp./±2 °C (DH/±0.01 J g-1) volution of the instrument response function (fwhm ~80 ps). All solutions were purged with N2 for 5 min before the Polymer SP N(A) N(B) I measurement and had an optical density (OD)=0.1 at 340 nm. 1 94 162–213 (3.69) 222–246 (3.67) 290–300 2.5. Fluorescence quantum yields 2 72 80–103 (11.63) 108–157 (11.05) 278–283 3 47 62–77 (0.58) 83–116 (4.57) 116–118 Solution quantum yields were measured relative to 9,10- aDetermined by diVerential scanning calorimetery and polarized light diphenylanthracene (W=0.9 in cyclohexane)29 using an SLM microscopy of pristine polymers without thermal pretreatment. SP Aminco SPF 500 fluorimeter.A Lab Sphere 10 cm integrating and I refer to softening and melting (isotropic) point.N(A) and N(B) sphere was used to obtain absolute quantum yields following refer to nematic A and nematic B liquid crystalline phases. the method of de Mello et al.30 Laser excitation (10 mW 2118 J. Mater. Chem., 1999, 9, 2117–2122355 nm laser pulses) and sample emission were focused into 2. Experimental and out of the integrating sphere using fiber optics.All spectra 2.1. Instrumentation were corrected for the monochromator and photomultiplier tube (PMT) spectral response. Incident radiant energy was Fourier-transform infrared spectra were obtained as KBr converted to a specified number of photons by multiplying the pellets on a Nicolet 510P spectrometer. Gel permeation chromobserved emission intensity by wavelength.The observed atography (GPC) on Waters Styragel columns with THF as emission intensity was obtained by integrating over all emission eluent gave peaks that were calibrated against commercial wavelengths in the additive spectrum, where each wavelength’s polystyrene standards. (Although GPC calibration against intensity was obtained from an average of 50 decays.Laser polystyrene standards is a routine procedure for estimating drift introduced an approx. 6% relative standard deviation molecular weights of rigid-rod polymers, some recent work (measured over the duration of each sample measurement). has shown these values to be roughly a factor of 3 larger than Each reported quantum yield is an average of 2–3 sample those attained by light scattering.11 Here, however, the estimeasurements with an estimated 10% relative standard mated molecular weights agree roughly with those expected deviation.from the additive extinction coeYcients of the appended fluorenyl groups.) Liquid crystalline phase transitions were determined with a 3. Results and discussion Perkin Elmer Series 7 UNIX diVerential scanning calorimeter 3.1.Polymer properties (DSC). Thermal decomposition was determined with a Perkin Elmer Series 7 Thermal Gravimetric Analyzer (TGA). As established by polarizing light microscopy, polymers 1–3 Crystalline transitions for bulk and thin films were identified exist as polycrystalline yellow flakes whose molecular weights with an Olympus polarizing light microscope (PLM) equipped were characterized by GPC (Table 1), displaying regions of with a camera attachment and hot and cold stages.order with dimensions exceeding one micron. Polymers 1–3 readily dissolved (100 mg mL-1) in toluene, chloroform, and 2.2. Materials THF to yield clear violet solutions. The polymers could be dissolved in dichloromethane, cyclohexane, and n-heptane only Samples 1–3 were a gift from Drs Edward Woo and Michael upon heating.The resulting clear violet solution quickly turned Inbasekaran from Central & New Businesses R & D, Dow translucent yellow upon cooling, with the formation of a gel Chemical Company, Midland, Michigan. All other materials or precipitation of the polymer. The yellow color is assigned (Aldrich) were used as received.to a polymer aggregate8 which shows a new absorption band at 437 nm (see below). The appearance of the 437 nm band in 2.3. Thin film preparation poor solvents results from a mismatch of Hildebrandt para- Thin films were prepared with a Specialty Coating Systems meters.11 In contrast to polymers 1 and 3, at room constant Inc. Spin Coater, model P6204-A. Polymer solutions (3 wt.%) temperature and a 10 mg mL-1 concentration, polymer 2 began prepared in toluene were placed on 1 cm glass squares.The to form aggregates in toluene and tetrahydrofuran, as evisubstrate was rotated at a 500 rpm ramping speed for 1 s, denced by the appearance of the characteristic 437 nm peak. followed by 3000 rpm for 30 s, producing 250±30 nm thick These two latter solvents have better matched Hildebrandt films, as determined by a Tencor Instruments Alpha Step 100 parameters. profilometer.Films were used as spun or were annealed under Bulk thermal properties of 1–3 were determined with both Ar at either 160 or 250 °C for 2 h before being cooled at DSC and PLM (Table 2). These techniques complement each 30 °Cmin-1 to preserve the order of the polymer’s liquid other because DSC is the more sensitive thermal technique, crystalline phase.Near-field optical microscopy results show but does not allow for visual observation of bulk polymer or order at 200 nm in pristine films of 3 to a greater extent than thin film morphology. In pristine films not previously subjected in 2, with essentially complete disorder observed with 1. After to heating cycles, an endothermic crystalline-to-nematic A annealing, films of 1 show the greatest order, followed by liquid crystalline phase transition followed by a second endothose of 2 and 3.Table 1 Molecular weights of the poly(dialkylfluorene)s 2.4. Transient and steady-state spectroscopy Polymer Mn a±500 PDb±1 nc±2 Absorbance spectra were recorded on a Shimadzu spectrometer.Fluorescence measurements were made with an 1 17 000 4.1 51 SLM Aminco SPF 500 fluorimeter. Temperature-dependent 2 24 000 4.4 62 measurements were performed with a water-blanketed quartz 3 21 000 2.6 42 cuvette. Time-resolved fluorescence lifetimes were obtained at aNumber averaged molecular weights (Mn). bPolydispersities (PD) are the Center for Fast Kinetics Research by time-correlated determined by GPC vs.polystyrene standards in THF, where PD= single-photon counting using a mode-locked, synchronously Mn/Mw (number averaged/weight averaged molecular weight). cThe pumped, cavity-dumped dye laser (l=300 and 340 nm, number of monomer repeat units (n) is calculated from the Mn. 3 mJ pulse-1).28 Emissive photons were collected at 90° with respect to the excitation beam (300 nm) and were passed through a monochromator to a Hamamatsu Model R2809U Table 2 Thermal properties of the poly(dialkylfluorene)sa micro-channel plate.Data analysis was conducted after decon- Temp./±2 °C (DH/±0.01 J g-1) volution of the instrument response function (fwhm ~80 ps). All solutions were purged with N2 for 5 min before the Polymer SP N(A) N(B) I measurement and had an optical density (OD)=0.1 at 340 nm. 1 94 162–213 (3.69) 222–246 (3.67) 290–300 2.5. Fluorescence quantum yields 2 72 80–103 (11.63) 108–157 (11.05) 278–283 3 47 62–77 (0.58) 83–116 (4.57) 116–118 Solution quantum yields were measured relative to 9,10- aDetermined by diVerential scanning calorimetery and polarized light diphenylanthracene (W=0.9 in cyclohexane)29 using an SLM microscopy of pristine polymers without thermal pretreatment. SP Aminco SPF 500 fluorimeter.A Lab Sphere 10 cm integrating and I refer to softening and melting (isotropic) point. N(A) and N(B) sphere was used to obtain absolute quantum yields following refer to nematic A and nematic B liquid crystalline phases. the method of de Mello et al.30 Laser excitation (10 mW 2118 J.Mater. Chem., 1999, 9, 2117–2122355 nm laser pulses) and sample emission were focused into 2. Experimental and out of the integrating sphere using fiber optics. All spectra 2.1. Instrumentation were corrected for the monochromator and photomultiplier tube (PMT) spectral response. Incident radiant energy was Fourier-transform infrared spectra were obtained as KBr converted to a specified number of photons by multiplying the pellets on a Nicolet 510P spectrometer.Gel permeation chromobserved emission intensity by wavelength. The observed atography (GPC) on Waters Styragel columns with THF as emission intensity was obtained by integrating over all emission eluent gave peaks that were calibrated against commercial wavelengths in the additive spectrum, where each wavelength’s polystyrene standards.(Although GPC calibration against intensity was obtained from an average of 50 decays. Laser polystyrene standards is a routine procedure for estimating drift introduced an approx. 6% relative standard deviation molecular weights of rigid-rod polymers, some recent work (measured over the duration of each sample measurement). has shown these values to be roughly a factor of 3 larger than Each reported quantum yield is an average of 2–3 sample those attained by light scattering.11 Here, however, the estimeasurements with an estimated 10% relative standard mated molecular weights agree roughly with those expected deviation.from the additive extinction coeYcients of the appended fluorenyl groups.) Liquid crystalline phase transitions were determined with a 3.Results and discussion Perkin Elmer Series 7 UNIX diVerential scanning calorimeter 3.1. Polymer properties (DSC). Thermal decomposition was determined with a Perkin Elmer Series 7 Thermal Gravimetric Analyzer (TGA). As established by polarizing light microscopy, polymers 1–3 Crystalline transitions for bulk and thin films were identified exist as polycrystalline yellow flakes whose molecular weights with an Olympus polarizing light microscope (PLM) equipped were characterized by GPC (Table 1), displaying regions of with a camera attachment and hot and cold stages. order with dimensions exceeding one micron. Polymers 1–3 readily dissolved (100 mg mL-1) in toluene, chloroform, and 2.2. Materials THF to yield clear violet solutions. The polymers could be dissolved in dichloromethane, cyclohexane, and n-heptane only Samples 1–3 were a gift from Drs Edward Woo and Michael upon heating. The resulting clear violet solution quickly turned Inbasekaran from Central & New Businesses R & D, Dow translucent yellow upon cooling, with the formation of a gel Chemical Company, Midland, Michigan. All other materials or precipitation of the polymer. The yellow color is assigned (Aldrich) were used as received. to a polymer aggregate8 which shows a new absorption band at 437 nm (see below). The appearance of the 437 nm band in 2.3. Thin film preparation poor solvents results from a mismatch of Hildebrandt para- Thin films were prepared with a Specialty Coating Systems meters.11 In contrast to polymers 1 and 3, at room constant Inc. Spin Coater, model P6204-A. Polymer solutions (3 wt.%) temperature and a 10 mg mL-1 concentration, polymer 2 began prepared in toluene were placed on 1 cm glass squares. The to form aggregates in toluene and tetrahydrofuran, as evisubstrate was rotated at a 500 rpm ramping speed for 1 s, denced by the appearance of the characteristic 437 nm peak. followed by 3000 rpm for 30 s, producing 250±30 nm thick These two latter solvents have better matched Hildebrandt films, as determined by a Tencor Instruments Alpha Step 100 parameters. profilometer. Films were used as spun or were annealed under Bulk thermal properties of 1–3 were determined with both Ar at either 160 or 250 °C for 2 h before being cooled at DSC and PLM (Table 2). These techniques complement each 30 °Cmin-1 to preserve the order of the polymer’s liquid other because DSC is the more sensitive thermal technique, crystalline phase. Near-field optical microscopy results show but does not allow for visual observation of bulk polymer or order at 200 nm in pristine films of 3 to a greater extent than thin film morphology. In pristine films not previously subjected in 2, with essentially complete disorder observed with 1. After to heating cycles, an endothermic crystalline-to-nematic A annealing, films of 1 show the greatest order, followed by liquid crystalline phase transition followed by a second endothose of 2 and 3. Table 1 Molecular weights of the poly(dialkylfluorene)s 2.4. Transient and steady-state spectroscopy Polymer Mn a±500 PDb±1 nc±2 Absorbance spectra were recorded on a Shimadzu spectrometer. Fluorescence measurements were made with an 1 17 000 4.1 51 SLM Aminco SPF 500 fluorimeter. Temperature-dependent 2 24 000 4.4 62 measurements were performed with a water-blanketed quartz 3 21 000 2.6 42 cuvette. Time-resolved fluorescence lifetimes were obtained at aNumber averaged molecular weights (Mn). bPolydispersities (PD) are the Center for Fast Kinetics Research by time-correlated determined by GPC vs. polystyrene standards in THF, where PD= single-photon counting using a mode-locked, synchronously Mn/Mw (number averaged/weight averaged molecular weight). cThe pumped, cavity-dumped dye laser (l=300 and 340 nm, number of monomer repeat units (n) is calculated from the Mn. 3 mJ pulse-1).28 Emissive photons were collected at 90° with respect to the excitation beam (300 nm) and were passed through a monochromator to a Hamamatsu Model R2809U Table 2 Thermal properties of the poly(dialkylfluorene)sa micro-channel plate. Data analysis was conducted after decon- Temp./±2 °C (DH/±0.01 J g-1) volution of the instrument response function (fwhm ~80 ps). All solutions were purged with N2 for 5 min before the Polymer SP N(A) N(B) I measurement and had an optical density (OD)=0.1 at 340 nm. 1 94 162–213 (3.69) 222–246 (3.67) 290–300 2.5. Fluorescence quantum yields 2 72 80–103 (11.63) 108–157 (11.05) 278–283 3 47 62–77 (0.58) 83–116 (4.57) 116–118 Solution quantum yields were measured relative to 9,10- aDetermined by diVerential scanning calorimetery and polarized light diphenylanthracene (W=0.9 in cyclohexane)29 using an SLM microscopy of pristine polymers without thermal pretreatment. SP Aminco SPF 500 fluorimeter. A Lab Sphere 10 cm integrating and I refer to softening and melting (isotropic) point. N(A) and N(B) sphere was used to obtain absolute quantum yields following refer to nematic A and nematic B liquid crystalline phases. the method of de Mello et al.30 Laser excitation (10 mW 2118 J. Mater. Chem., 1999, 9, 2117–2122