J O U R N A L O F C H E M I S T R Y Materials Photochromic reaction in a molecular glass as a novel host matrix: the 4-dimethylaminoazobenzene–4,4¾,4-tris[3- methylphenyl(phenyl )amino]triphenylamine system Kazuyuki Moriwaki, Mitsushi Kusumoto, Keiichi Akamatsu, Hideyuki Nakano and Yasuhiko Shirota* Department of Applied Chemistry, Faculty of Engineering, Osaka University, Yamadaoka, Suita, Osaka 565–0871, Japan.E-mail: shirota@ap.chem.eng.osaka-u.ac.jp Received 22nd July 1998, Accepted 16th September 1998 For the purposes of clarifying the properties of a molecular glass as a novel host matrix and gaining information on the microstructure of the molecular glass, the photochromic behavior of 4-dimethylaminoazobenzene (DAAB) in a novel molecular glass of 4,4¾,4-tris[3-methylphenyl(phenyl )amino]triphenylamine (m-MTDATA) was investigated, and compared with its behavior in a polystyrene glass matrix and a benzene solution. It was found that the fraction of the photoisomerized cis-isomer of DAAB at the photostationary state is smaller in the m-MTDATA glass matrix than in the polystyrene matrix and the benzene solution, and that the apparent initial rate constant for the backward cisAtrans thermal isomerization of DAAB is much larger in the m-MTDATA glass than in the polystyrene matrix and the benzene solution. These results suggest that the average size of local free volume in the molecular glass of m-MTDATA is smaller than that in the polystyrene glass.Amorphous glasses serve as excellent host matrices because of matrices in comparison with those of amorphous polymers.their excellent film formation, transparency, isotropic proper- In addition, studies of photochromic reactions in the molecular ties, and homogeneous properties owing to the absence of glass are expected to provide information on the microstructure grain boundaries. Inorganic sol-gel glasses have been widely of the molecular glass as the host matrix. used as host matrices, embedding a variety of functional In the present study, we have investigated photochromic materials.Organic polymers have also been widely used as reactions in a molecular glass of 4,4¾,4-tris[3-methylphematrices for embedding functional materials. For example, nyl(phenyl )amino]triphenylamine (m-MTDATA) in order to molecularly-doped polymer systems where photoconducting elucidate the properties of the molecular glass as a novel host materials are dispersed in an amorphous polymer such as matrix and to gain information on the microstructure of the polycarbonate have been put to practical use as photoreceptors molecular glass.An azobenzene derivative was chosen as a in electrophotography. Photochromic reactions of low molecu- photochromic compound because azobenzenes have been most lar-weight organic compounds dispersed in a polymer matrix extensively studied.Since the m-MTDATA film shows an have also been studied extensively, and microenvironmental electronic absorption band in the near UV region, we selected eVects on photochromic reactions have been discussed in terms 4-dimethylaminoazobenzene (DAAB) as a photochromic of their molecular motions associated with glass transition and probe compound, which shows an absorption spectral change the free volume of host polymer matrices.1–11 due to trans<cis isomerization in the visible region.The In contrast to polymers, low molecular-weight organic photoisomerization behavior and the kinetics for the backward compounds tend to crystallize readily.We have been making cisAtrans thermal isomerization of DAAB in the molecular studies of the creation of low molecular-weight organic com- glass of m-MTDATA are investigated in the temperature pounds that readily form stable amorphous glasses above region below and above the Tg. The results are discussed in room temperature, which we refer to as ‘amorphous molecular comparison with those in a polystyrene matrix and a benzene materials’ or ‘molecular glasses’, considering the following solution.Preliminary results have been reported as a aspects. Amorphous molecular materials are expected to con- communication.37 stitute a novel family of organic functional materials that exhibit glass-transition phenomena usually associated with Experimental polymers.Creating such amorphous molecular materials is of interest and significance not only for technological applications Materials but also from a scientific viewpoint, opening up a new field of organic materials science that deals with ‘molecular glasses’. m-MTDATA was prepared according to the method described We have created several novel families of molecular glasses in our previous paper.12 trans-DAAB was obtained commerwith relatively high glass-transition temperatures (Tgs), which cially (Tokyo Chemical Industry, Co., Ltd.) and purified by include 4,4¾,4-tris(diphenylamino)triphenylamine (TDATA) sublimation (at ca. 80 °C and at 0.15 mmHg). Polystyrene was and its derivatives,12–16 substituted 1,3,5-tris(diphenylamino)- commercially available (Wako Pure Chem.Ind., Ltd.) and benzenes (TDABs),17–24 4,4¾,4-tris(diphenylamino)triphenyl- purified by repeated reprecipitation from benzene into benzene (TDAPB) and its derivatives,25 and others.26–31 They methanol (Mw=1.9×105, Mw/Mn=1.7). readily form uniform amorphous films by vacuum deposition, spin coating or solvent casting and have found successful Preparation of glass samples for DSC measurement application as materials for organic electroluminescent Glass samples for the determination of Tg by diVerential devices.14–16,32–36 Like polymers, molecular glasses are also scanning calorimetry (DSC) were prepared as follows.expected to function as novel host matrices for embedding Appropriate amounts of m-MTDATA and trans-DAAB with functional materials such as photochromic materials.It is of interest to elucidate the properties of molecular glasses as host molar ratios of m-MTDATA5DAAB=351, 551, 1051 and J. Mater. Chem., 1998, 8, 2671–2676 2671Fig. 1 DSC curve of a glass obtained by cooling the melt of a mixture of m-MTDATA and DAAB with a molar ratio of m- MTDATA5DAAB=351. Results and discussion Glass formation of DAAB—m-MTDATA mixtures It was found that the mixtures of varying amounts of m-MTDATA and DAAB form homogeneous, amorphous glasses, as confirmed by DSC, X-ray diVraction, and polarizing microscopy.Fig. 1 shows a DSC curve of a glass obtained by cooling the melt of the mixture of a molar ratio of m- MTDATA5trans-DAAB=351. When the sample was heated, a glass-transition phenomenon was observed at 60 °C, which was 15 °C lower than the Tg of m-MTDATA itself.The result that only one glass-transition phenomenon was observed suggests that the DAAB molecules are homogeneously dispersed in the m-MTDATA glass matrix. On further heating, an N N N N H3C CH3 CH3 N N N H3C H3C N N N H3C H3C m-MTDATA trans-DAAB cis-DAAB hn D exothermic peak due to crystallization of both m-MTDATA and DAAB was observed at ca. 129 °C, followed by a broad endothermic peak due to melting at around 193 °C. The Xray diVraction (XRD) pattern of a glass with a molar ratio 10051, were placed in a glass sample tube, and then the tube of m-MTDATA5trans-DAAB=351 is shown in Fig. 2. No was sealed oV. After the sample melted on heating in the appreciable diVerence in the XRD pattern was observed among sample tube, it was allowed to cool to room temperature to the glasses with varying molar ratios of m-MTDATA and give a homogeneous glass of m-MTDATA containing DAAB.DAAB. Table 1 lists the Tgs of the glasses of the mixtures of Photochromic reaction in amorphous films Amorphous films of m-MTDATA containing trans-DAAB were prepared on a transparent glass substrate by spin coating from a benzene solution containing appropriate amounts of m-MTDATA and DAAB.The film was dried overnight under reduced pressure before use. The resulting film was confirmed to be amorphous by X-ray diVraction and polarizing microscopy. Polystyrene films containing DAAB were also prepared by spin coating from a benzene solution. Photoisomerization of DAAB in the amorphous films of m-MTDATA and polystyrene was carried out by irradiation Fig. 2 X-Ray diVraction pattern of a glass obtained by cooling the with 400 nm light with a bandwidth of 10 nm from a 500 W melt of a mixture of m-MTDATA and DAAB with a molar ratio of Xenon lamp (UXL-500D, USHIO) through an interference m-MTDATA5DAAB=351. filter (IF-S 400, Vacuum Optics Co.) and an optical fiber.Photochromic reactions were analyzed from the change in the Table 1 Glass-transition temperatures (Tgs) of the glasses of mixtures electronic absorption spectra. of m-MTDATA and DAAB Sample Tg/ °C Apparatus m-MTDATA 75 m-MTDATA5DAAB=10051a 71 Tg values were determined by DSC with a Seiko DSC220C. m-MTDATA5DAAB=1051a 63 Electronic absorption spectral change were measured with a m-MTDATA5DAAB=551a 60 Hitachi U-3200 spectrophotometer.The sample was kept at a m-MTDATA5DAAB=351a 60 constant temperature by using a temperature controller aMolar ratio. (TM-105, Toho Electronics Inc.) with a tungsten heater. 2672 J. Mater. Chem., 1998, 8, 2671–2676Table 2 The cis-fraction (Y) of DAAB at the photostationary state upon irradiation with 400 nm light at 30 °C in m-MTDATA films Reaction system Y 10051-m-MTDATA film 0.26 1051-m-MTDATA film 0.48 551-m-MTDATA film 0.54 351-m-MTDATA film 0.54 than those for the polystyrene film and the benzene solution indicates that a number of trans-DAAB molecules remain unchanged in the m-MTDATA amorphous film, probably because the local free volume around the remaining trans- DAAB molecules is not large enough to allow the isomerization of DAAB from its trans-form to the cis-form in the m- MTDATA glass matrix.It is suggested that the average size of local free volume in the m-MTDATA glass is smaller than that in the polystyrene glass. The increase in the Y value with increasing concentration of DAAB in the m-MTDATA glass matrix indicates that the addition of DAAB into the m- MTDATA glass causes a change in the microstructure of the Fig. 3 Electronic absorption spectral change of 351-m-MTDATA film: glass, e.g. the size and distribution of the local free volume. It (a) before irradiation; (b)–(e) irradiated with 400 nm light for (b) 2, is suggested that the fraction of the local free volume that (c) 5, (d ) 10 and (e) longer than 30 min (photostationary state).allows the photoisomerization of DAAB increases with Electronic absorption spectra of ( f ) an amorphous film of increasing concentration of DAAB. m-MTDATA alone and (g) DAAB in a benzene solution Next, the backward cisAtrans thermal isomerization of (1.0×10-5 mol dm-3). DAAB in the m-MTDATA glass was examined. The ratio of the concentration of cis-DAAB at the initial state and at time m-MTDATA and DAAB.It was found that the Tg decreases t was determined from the absorbance at 410 nm according to as the concentration of DAAB increases. eqn. (2), Photochromism of DAAB in the m-MTDATA glass matrix [cis]0 [cis]t = A0-A2 At-A2 (2) The amorphous film with a molar ratio of m- MTDATA5DAAB=n51 prepared on a transparent glass sub- where [cis]0 and [cis]t represent the concentration of cis-DAAB strate by spin coating from a benzene solution is hereafter at the initial state and at time t, respectively, and A0, A2, and referred to as the n51-m-MTDATA film (n=3, 5, 10, 100).At are the absorbances of the film at 410 nm at the initial and The reversible transAcis and cisAtrans photoisomerizations infinite time, and at a time t, respectively. If the reaction and thermal isomerizations of DAAB took place in the n51- follows first-order kinetics, the plots of ln([cis]0/[cis]t) vs.time m-MTDATA amorphous film. Fig. 3 shows the electronic will be linear and the slope of the straight line represents the absorption spectral change for the 351-m-MTDATA film rate constant for the reaction. Fig. 4 shows the first-order together with the electronic absorption spectra of an amorph- plots for the cisAtrans thermal isomerization of the 351- and ous film of m-MTDATA alone, prepared on a glass substrate 10051-m-MTDATA films at 30 °C after the reaction sysby spin coating from a benzene solution, and trans-DAAB in tem has reached the photostationary state by irradiation a benzene solution.Upon irradiation with 400 nm light, the with 400 nm light.The results for a benzene solution absorbance of the film at around 410 nm decreased due to the photoisomerization of trans-DAAB to the cis-form. When irradiation was stopped after the reaction system had reached a photostationary state, the absorption spectrum of the film gradually began to recover to give the original one due to the backward thermal isomerization of cis-DAAB to the transform.The m-MTDATA film was stable to 400 nm light irradiation under the same conditions. The cis-fraction (Y) of DAAB in the n51-m-MTDATA film at the photostationary state can be determined from eqn. (1), Y=Aet+neh et-ec BAAint-Apss Aint B (1) where et, ec and eh are the molar extinction coeYcients of trans-DAAB (et=2.9×104 M-1 cm-1 at 405 nm),38 cis-DAAB (ec=2.0×103 M-1 cm-1 at 405 nm),38 and the m-MTDATA host matrix (eh=6.2×102 M-1 cm-1 at 405 nm), and Aint and Apss are the absorbances of the film at the initial and photostationary states, respectively.Table 2 shows the Y values for the n51-m-MTDATA films at 30 °C. It was found that the cisfraction Y of DAAB in the m-MTDATA glass is smaller than Fig. 4 First-order plots for the cisAtrans thermal isomerization of those (ca. 0.85) for a polystyrene glass and a benzene solution DAAB in (a) 351-m-MTDATA film, (b) 10051-m-MTDATA film and and that the Y value increases when the concentration of (c) 351-m-MTDATA film prepared under irradiation of 400 nm light, DAAB increases in the DAAB—m-MTDATA system. The (d) polystyrene film and (e) benzene solution.Solid lines are fitting curves based on eqn. (3) for (a) and eqn. (4) for (b), (d) and (e). result that the Y value for the m-MTDATA glass is smaller J. Mater. Chem., 1998, 8, 2671–2676 2673Table 3 Kinetic parameters based on eqn. (4) for the thermal (3.3×10-4 mol dm-3) and a polystyrene glass matrix containcisAtrans isomerization of DAAB at 30 °C ing the same DAAB concentration as that in the 10051-m- MTDATA film (0.28 wt%) are also shown in Fig. 4. It was k1/min-1 f1 k2/min-1 f2 found that the cisAtrans thermal isomerization of DAAB in the m-MTDATA and polystyrene films did not follow first- 10051-m-MTDATA film 0.083 0.54 0.003 0.46 Polystyrene matrix 0.062 0.12 0.003 0.88 order kinetics and that the apparent rate constant for the Benzene solutiona — — 0.003 1.00 cisAtrans thermal isomerization in the m-MTDATA glass is initially much larger than those in the polystyrene matrix and aSingle component.the benzene solution, gradually approaching the same value as that in the solution. This result suggests that there exist cisisomers trapped in strained conformations in the host matrix, large as that of the slower component.The rate constant of which go back faster to the trans-isomers than the structurally the slower component for the 10051-m-MTDATA and polyrelaxed cis-isomers. In order to verify this idea, the 351-m- styrene films was found to be the same as for solution. It is MTDATA film was prepared by spin coating under 400 nm suggested that the faster and the slower components are light irradiation and the backward cisAtrans thermal isomeriz- attributed to the reactions of the strained cis-isomer and the ation was examined. As a result, the first-order plot for the structurally relaxed one, respectively. It is noteworthy that the 351-m-MTDATA film prepared under irradiation showed less fraction of the faster component ( f1) is considerably larger deviation from the linear plot [Fig. 4(c)] than the film prepared for the 10051-m-MTDATA film ( f1=0.54) than for the polyin the dark [Fig. 4(a)]. This result indicates that the amount styrene matrix film ( f1=0.12). This leads to a larger apparent of the strained cis-DAAB in the film prepared under irradiation rate constant at the initial stage for the 10051-m-MTDATA is smaller than that prepared in the dark.Similar phenomena film than for the polystyrene film. These results indicate that have also been reported for polymer film systems.3,8 the ratio of the number of the strained cis-isomers to the relaxed cis-isomers at the photostationary state is much larger Kinetic analysis of the cisAtrans thermal isomerization of for the 10051-m-MTDATA film than for the polystyrene film. DAAB in m-MTDATA films The reason why the cisAtrans thermal isomerization of DAAB both in the 10051-m-MTDATA film and in the polystyrene Since the cisAtrans thermal isomerization did not follow film can be analyzed in terms of the first-order kinetics for a simple first-order kinetics, the Kohlrausch–Williams–Watts two component system rather than the Gaussian model is that (KWW) function ([cis]t/[cis]0=exp[-(t/t)b]),39 the Gaussian both the 10051-m-MTDATA film and the polystyrene film Model,40 and first-order kinetics for a two component system involve the reaction of the relaxed cis-isomer to the extent of were applied.As a result, the cisAtrans thermal isomerization 46% and 88%, respectively. of DAAB in the 351-m-MTDATA film was successfully ana- The larger apparent rate constant for the cisAtrans thermal lyzed by the Gaussian Model, which assumes a Gaussian isomerization in the m-MTDATA film relative to the poly- distribution of the free energy of activation.In terms of this styrene film also suggests that the average size of the local free model, the ratio of the concentration of the cis-isomer at a volume in the m-MTDATA glass is smaller than that in the time t to that at the initial time, [cis]t/[cis]0, is given by polystyrene glass.eqn. (3), The results of a larger cis-fraction at the photostationary state and a larger ratio of the strained cis-isomer to the relaxed [cis]t [cis]0 = 1 p1/2 P+2 -2 exp(-x2)exp[-kAVt exp(cx)]dx (3) one for the 351-m-MTDATA film relative to the 10051-m- MTDATA film are ascribed to the diVerence in the size and where x is the stochastic variable of a Gaussian distribution, distribution of the local free volume between the 351-m- kAV is the mean rate constant, and c is the spread of the MTDATA and 10051-m-MTDATA films.It is thought that Gaussian distribution of the free energy of activation. The free trans-DAAB can be isomerized to cis-DAAB when the local energy of activation is represented as DG‡=DG‡AV-cxRT, free volume around the molecule is larger than V0; however, where DG‡AV is the mean free energy of activation.photogenerated cis-DAAB is trapped in a strained confor- The cisAtrans thermal isomerization of the 351-mmation in the host matrix when the local free volume is smaller MTDATA film was found to fit eqn.(3) with the parameters than V1. The comparison of the cis-isomer fraction (Y) at the of kAV=0.034 min-1 and c=1.8. The value of kAV is much photostationary state and the ratio of the strained cis-isomer larger than the first-order rate constant (0.003 min-1) of the to the relaxed one between the 351- and 10051-m-MTDATA cisAtrans thermal isomerization of DAAB in benzene solution.films suggests that the fraction of the free volume larger than These results suggest that most cis-isomers photochemically V0 is larger for the 351-m-MTDATA film than for the 10051-m- generated in the 351-m-MTDATA film take strained confor- MTDATA film, but that the size of the local free volume is mations and their free energies are subject to the Gaussian smaller than V1 for the 351-m-MTDATA film, whereas a distribution.local free volume larger than V1 is available for the 10051- In the case of the 10051-m-MTDATA film, the cisAtrans m-MTDATA film. thermal isomerization of DAAB could not be analyzed by the Gaussian Model but instead could be analyzed by first-order Temperature dependence of the kinetic parameters for the kinetics for a two component system [eqn.(4)], 10051-m-MTDATA film [cis]t [cis]0 =f1 exp(-k1t)+f2 exp(-k2t) (4) The temperature dependence of the kinetic parameters for the cisAtrans thermal isomerization were investigated with regard to the 10051-m-MTDATA film, the reaction of which was where fi and ki are the fractions and the rate constants for the faster (i=1) and the slower (i=2) components, respectively. analyzed by first-order kinetics for a two component system according to eqn.(4). Fig. 5 shows the first-order plots for the Likewise, the cisAtrans thermal isomerization of DAAB in the polystyrene film was analyzed by first-order kinetics for a cisAtrans thermal isomerization of the 10051-m-MTDATA film at various temperatures below Tg. The apparent rate two component system. Table 3 lists the kinetic parameters for the reactions in the 10051-m-MTDATA film, the polystyrene constant of the reaction increased with rising temperature. The kinetic parameters in eqn.(4) obtained for the 10051-m- film, and the benzene solution at 30 °C. The rate constant of the faster component for the 10051-m-MTDATA film and the MTDATA film are summarized in Table 4. The results show that the values of f1 and f2 are almost constant irrespective of polystyrene film was found to be ca. 27 and ca. 21 times as 2674 J. Mater. Chem., 1998, 8, 2671–2676isomerization of the 10051-m-MTDATA supercooled liquid film in the temperature region above Tg, at 75 and 77 °C, was found to follow simple first-order kinetics, as shown in Table 4 and Fig. 6.43 The rate constant is comparable to that predicted from the Arrhenius plots for the slower reaction component below Tg.This result suggests that the photogenerated cisisomer can take a relaxed conformation in the supercooled liquid state, probably because molecular motion is not restricted in the supercooled liquid state. Summary The photochromic behavior of DAAB dispersed in a molecular glass of m-MTDATA was investigated in order to elucidate the properties of the molecular glass as a novel host matrix and to gain information on the microstructure of the molecular glass.The m-MTDATA glass was found to function as a host matrix, homogeneously embedding the photochromic molecule DAAB. It was shown that the fraction of the photoisomerized Fig. 5 First-order plots for the cisAtrans thermal isomerization of cis-isomer at the photostationary state in the m-MTDATA DAAB in 10051-m-MTDATA film at various temperatures: (a) 30, matrix is smaller than that in the polystyrene matrix and that (b) 40, (c) 50 and (d) 60° C.Solid lines are fitting curves based on eqn. (4). the apparent rate constant at the initial stage for the backward cisAtrans thermal isomerization of DAAB in the m-MTDATA glass matrix is considerably larger than those in a polystyrene Table 4 Kinetic parameters based on eqn.(4) for the cisAtrans matrix and in a benzene solution. These results indicate that thermal isomerization of DAAB in 10051-m-MTDATA film at various temperatures there is a large diVerence in the size and distribution of local free volume between the molecular glass of m-MTDATA and T/ °C k1/min-1 f1 k2/min-1 f2 the polystyrene matrix.That is, the local free volume in the molecular glass of m-MTDATA is suggested to be smaller 30 0.083 0.54 0.003 0.46 than that in the polystyrene glass. The present study presents 40 0.11 0.54 0.011 0.46 the first example of photochromic behavior in a molecular 50 0.16 0.54 0.029 0.46 60 0.37 0.53 0.061 0.47 glass as a novel host matrix and can be extended to other 75a — — 0.346 1.00 photochromic compounds and other molecular glasses to gain 77a — — 0.382 1.00 further information on the properties of molecular glasses as aSingle component.a novel class of host matrix and on the microstructure of molecular glasses. the temperature, whereas the rate constants k1 and k2 increase References with rising temperature. Fig. 6 shows the Arrhenius plots for both k1 and k2. The activation energies for the reactions of 1 Photochromism, Molecules and Systems, ed. H. Du� rr and the faster and the slower components were determined to be H. Bouas-Laurant, Elsevier, 1990. 41 and 84 kJ mol-1, respectively. The activation energy for 2 Applied Photochromic Polymer Systems, ed.C. B. 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