J. CHEM. SOC. FARADAY TRANS., 1994, 90(18), 2845-2847 FARADAY COMMUNICATIONS Intramolecular Excimer Formation in 1,5=Di(S=anthryl)=n=pentane T. A. Smith, G. D. Scholes, G. 0.Turner and K. P. Ghiggino Photophysics Laboratory, School of Chemistry, The University of Melbourne, Parkville , Victoria, 3052,A ustra lia Steady-state absorption and time-resolved fluorescence studies of the bichromophore 1,5-di(9-anthryl)-n- pentane (A5A) in isopentane (IP)-methylcyclohexane (MCH) (3 : 1) are reported. It is demonstrated that in this solvent a significant proportion of the ground-state bichromophores exist in close, sandwich-like conformations which are able to form excimers directly upon excitation. Excimer formation occurring between an electronically excited and a ground-state chromophore is of both theoreti- cal and practical interest. Excimers are believed to play a role in some photochemical rea~tionsl-~and the process of excimer formation has also been used to probe microviscosity and structure in a variety of molecular Bichromophoric molecules, in which the interacting chromo- phores are covalently linked, offer a unique avenue for inves- tigation of the kinetics of intramolecular excimer formation and decay, and studies on these model systems are relevant to the formulation and interpretation of kinetic schemes for excited-state dynamics of aromatic polymers.Previous time- resolved fluorescence studies on intramolecular excimer for- mation in several bichromophoric molecules have found complex kinetic behaviour variously suggested to arise from multiple excimer' and/or monomer' species and transient kinetic effects.' The absorption and emission character- istics of the bichromophoric molecule, 1,5-di(9-anthryl)-n- pentane (A5A) are now reported.It is shown using corrobo- rating absorption spectral evidence that, in an IP-MCH solvent mixture, a significant proportion of the ground-state bichromophores exist in a close, sandwich-like conformation from which the molecules are able to form excimers instanta- neously. The fluorescence spectrum of a dilute, degassed solution of A5A in an IP-MCH solvent mixture (3 : 1) consists of a structured monomer (anthracene-like) emission with maxima at CQ. 390, 415, 445 and 480 nm and a broad, structureless excimer emission with a maximum at ca.520 nm. Fluores- cence decays collected as described previ~usly'~ from the monomer [IM(t)] and excimer [I,(t)] emission regions of A5A in this solvent following excitation at ca. 310 nm can be fit quite satisfactorily using a double-exponential decay function [eqn. (1) and (2)] with the two decay constants Iu1= 1/2.63 ns-' and A, = 1/65.22 ns-', and the corresponding normal- ized pre-exponential factors, A,, = 98.52, A,, = 1.48 (for emission at 390 nm) and A,, = 23.60 and A,, = 76.40 (for emission at 510 nm). The observed double-exponential behaviour is in contrast to that displayed by some other bichromophoric molecules whose fluorescence decay profiles have been found to require more than two exponentialterm^.'^'^' The A5A decays, however, do not follow a simple Birks'-type excimer kinetic ~cheme'~.'' since the ratio of the two pre-exponential factors [A,, and A,, in eqn.(2)] is not unity, as expected for uniformly diffusing interacting chromo- phores. IM(t)= All exp(-A,t) + A,, exp(-1,t) (1) Zachariasse et al.' have investigated in considerable detail the intramolecular excimer formation in 2,4-di(2-pyrenyl) pentane (2DPP) in toluene using both NMR and time-correlated single-photon counting (TCSPC) fluorescence decay techniques. As far as their TCSPC experiments were concerned, these authors reported essentially double-exponential fluorescence decay behaviour (after accounting for ruc-2DPP impurity in the meso-2DPP), with the excimer decay pre-exponential factor ratio deviating slightly from unity. This was interpreted as a failure of Birks' kinetic scheme owing to the presence (and excitation) of some chromophores in the so-called 'preformed' excimer confor- mation (ground-state conformations lending themselves to effectively instantaneous excimer formation).In other words, these authors argued that the boundary condition in the deri- vation of Birks' scheme, [D*],=o = 0, may not be valid for 2DPP. Similarly, we have investigated the possibility that the deviation from Birks' kinetic scheme displayed by A5A may also be due to the excitation of some preformed dimer con- formation. In the case of A5A, the deviation from unity of the ratio of AZ1 and A,, is far more marked than the data reported by Zachariasse et al.' The kinetic scheme can be summarised by Fig.1. The solu- tion of the corresponding rate equations gives the following expressions [eqn. (3)-(6)] for the pre-exponential factors, Aij, appearing in eqn. (1) and (2). n =fc D* Fig. 1 Kinetic scheme proposed for A5A (3) (4) where: [M*], and [D*l0 are the concentrations of excited monomer and dimer at t = 0 (ie. immediately following excitation), X = k,, + k, + k, and Y = kFD + kID+ kMD. From these equations, an expression can readily be obtained which quantifies the ratio of concentrations of ‘preformed’ dimer sites to ‘monomer’ sites. (7) Using eqn. (7) and the pre-exponential factors obtained from the fluorescence decay analysis given above, a dimer/ monomer ratio of ca.53% is obtained. If such a large propor- tion were in this configuration, the ground-state interchromophore interaction must be strong and possibly observable in the absorption spectrum through resonance (molecular exciton) splittings. The information we require about any ground-state interactions may be obtained from either the ‘B, or the ‘La anthracene absorption bands, however, since any spectral features will be far more obvious in the ‘B, band owing to its larger oscillator strength, it is this band on which we have concentrated. The electronic absorption spectrum of A5A in the IP-MCH mixture is shown in Fig. 2(a) and exhibits an intense band above ca.42 OOO cm-in addition to the normal anthracene absorption band at ca. 39000 cm-’ [cf: the absorption spectrum of 9- methyl anthracene in 3 : 1 isopentane :methylcyclohexane shown in Fig. 2(b)]. There is also evidence of a weak absorb- ance by A5A at ca. 38000 cm-l. These spectral features of A5A are consistent with exciton splitting of the anthracene absorption band, indicative of a strong interaction between ground-state chromophores. Furthermore, while a broad, unstructured absorption band, indicative of interactions cor- responding to a range of ensemble-averaged conformations might be expected, we observe a clearly defined splitting into wavenumber/cm -’ Fig. 2 Electronic absorption spectra (a) A5A and (b) 9-methylanthracene in 3 : 1 IP-MCH and (c) A5A in ethanol J. CHEM.SOC. FARADAY TRANS., 1994, VOL.90 two bands. The absorption spectrum thus provides convinc- ing additional evidence for the existence, in this solvent mixture, of two distinct ground-state absorbing conformers assigned to be a ground-state (dimer) conformation in addi- tion to the normal, weakly interacting, extended monomer conformation of anthracene chromophores. The relative intensities of the two bands of the exciton-split doublet (above ca. 42000 cm- and below ca. 38 OOO cm-’) also provide some structural information about the dimer. The observation that the intense absorption band corre-sponding to an allowed transition (cf.the weaker, red-shifted band) is blue-shifted relative to the monomer band leads us to conclude that in the dimer the anthracene chromophores are arranged in a sandwich conformation with long axes close to parallel.l9 Furthermore, it is evident that the ground-state dimer exists as this single distinct sandwich-conformation. The area under each of the two bands in the absorption spectrum may also be used to make a quantitative estimate of the dimer/monomer ratio using eqn. (8): Cdimcr Adimer Ip~~nomcr12~t;o”omer On -Aymer I p$mer 12 dimer (8) POnomer YO” where c is the concentration of the absorbing species, A is proportional to the oscillator strength for the transition, v is the absorption band maximum (in cm-’)and p is the dipole transition moment. For this dimer geometry, p:Fmerx &p~~nomcr The oscillator strengths for the two bands .20 were calculated by integrating from 41 670 to 46730 cm-’ and from 37 880 to 41 490 cm- which, from eqn.(8), gives a dimer/monomer ratio of 1.59, which is significantly higher than that predicted from the time-resolved fluorescence results. The discrepancy between the two values may be attributable to (i) possible contribution by o-bond absorption particularly at higher absorption energies influencing the oscillator strength estimates and (ii) the assumption that the monomer and dimer species have equal fluorescence quantum yields for the analysis leading to eqn. (7). Neverthe- less, the absorption data provide qualitative support for the conclusions drawn from the fluorescence analysis, namely that the deviation from Birks’ kinetics may be attributable to the direct excitation of some preformed dimer configuration. For comparison, the absorption spectrum of A5A in ethanol is shown in Fig. 2(c).It is clear that the appearance of the high-energy absorption band is strongly dependent upon solvent. The 3 : 1 IP-MCH mixture is not expected to be as good a solvent for the anthracene moieties as ethanol, so molecules may be forced to adopt a more contracted configu- ration with the anthracene groups close together. In the better solvent, ethanol, the favoured ground-state configu- ration is more extended, thus decreasing the proportion of preformed dimer conformers. A5A is thus shown to be a rare example of a bichromophoric system which shows double-exponential fluorescence decay behaviour, although the decay kinetics cannot be described by Birks’ excimer scheme.The corrobo- rating evidence obtained from steady-state absorption and time-resolved fluorescence techniques indicate that A5A in an IP-MCH (3 : 1) solvent mixture is a convincing example of a molecule exhibiting two distinct ground-state absorbing con- formers. These results emphasise the important role molecu- lar conformation and solvent play in intramolecular excimer formation kinetics in bichromophoric molecules. T.A.S. acknowledges the support of the Ernst and Grace Matthei Fellowship, University of Melbourne. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 2847 References 12 J.M. G. Martinho and M. A. Winnik, J. Chem. Phys., 1987,91, 1 V. Yakhot, M. D. Cohen and Z. Ludmer, Adv. Photochem., 1979, 11,489. 13 3640. K. P. Ghiggino, T. A. Smith and G. J. Wilson, J. Mod. Opt., 2 J. 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