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J. Phys. Chem. 1991, 95, 509-5 11 an intermediate that is susceptible to oxygen quenching, as shown in Scheme I. The ionization potential in zeolite can be expressed as
1, = Ig + P+ +
v,
where I , is the ionization potential in zeolite, I g is the gas-phase ionization potential (7.55 eV for pyrene), P+ is the polarization energy of the cation in the medium (e.g., zeolite in this case), and the V, is the energy state of the ionized electron.24 P+ can be calculated from the Born charging expression as follows:
P+ = (e2/2r)(1 - 1/e)
(2)
where the e is the charge of the electron and r is the interaction distance of the ion and the surrounding media. Normally, P+ is around -1.5 to -2.0 eV for a typical c (=2.0); therefore, the difference between gas-phase ionization potential and ionization potential in medium Is - Iz as in eq 1) is around 3.0-3.5 eV or larger if c is larger than 2. In fact, single-photon ionization of 3-aminoperylene with a green light (530 nm, 2.34 eV) had been reported in NaLS micelles system.26 Kasai reported the formation of NO'+ and NO2'- ion pair in NO-treated and our early studies on the pyrene fluorescence III/I ratio less than unity" all support a rather polar environment inside the zeolite X and Y. This polar environment may be the cause of the single-photon ionization of pyrene we observed. In other words, it may be the result of a large value of the dielectric constant inside the zeolite X and Y, the active site in particular. Although Richardson observed the ESR signal of arene radical cation in which may be caused by pro(24) This value is typically -1.5 eV in water.2s Presumably, it does not change much in zeolite particularly in fully hydrated samples. (25) Barker, G. C.; Bottura, G.; Cloke, G.; Gardner, A. W.; Williams, M. J. Ekcrroanal. Chem. Inrerfac. Elecrrochem. 1974, 50, 323. (26) Thomas, J. K.;Picuilo, P. J. Am. Chem. Soc. 1978,100,3239-3240. (27) Kasai, P. H.; Bishop, Jr., R. J. J . Am. Chem. SOC. 1972, 94, 5560-5566.
tonation from the zeolite framework (Bronsted acid site, for example); we, however, did not detect any trace of pyrene cation radical in the zeolite samples studied here in the absence of the laser excitation by a UV-vis spectrometer. The active site for the single-photon ionization of pyrene could be a radical promoter (or electron acceptor) site proposed by Rabo et al. in KY29and recently by Shih in ZSM-5.30 This electron acceptor site could react with arene to form a ground-state electron-transfer complex as arene + EA site arene'+.-EA'Although we did not resolve the spectrum of this ground-state electron-transfer complex by UV-vis spectroscopy, it might be very similar to that of the ground state of either anthracene or pyrene. Excitation with the 337-nm laser could photolyze this complex and create the ionized electron and arene cation radical in the alkali-metal-cation-exchanged zeolite X or Y.
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Conclusion We have observed a series of alkali-metal ionic clusters inside the zeolites X and Y through photoionization of anthracene- or pyrene-impregnated samples. Both mono- and biphotonic ionization of pyrene were observed in these zeolites. The active site that provides the single-photon ionization process may be the electron-acceptor site a t which a ground-state charge-transfer complex between pyrene and the zeolite framework could be formed. Acknowledgment. We thank the National Science Foundation (Grant CHE-89-11906) for support of this work. We also thank Dr. D. E. Vaughan at Exxon Research Center for a helpful discussion on the properties of cation-exchange zeolites, Dr. X. Liu for commenting on the origin of the active sites, and R. T. Gajek at UOP for providing the Na-exchanged zeolite Y. (28) Richardson, J. T. J . Caral. 196'1.9, 172-177. (29) Rabo, J. A.; Poutsma, M.L. Ado. Chem. 1971, 102, 284. (30) Shih, S.J. Carol. 1983, 79, 390-395.
ntrinsic" Response of Polymer Liquid Crystals in Photochemical Phase Transition Tomiki Ikeda,* Takeo Sasaki, and Haeng-Boo Kim Photochemical Process Division, Research Laboratory of Resources Utilization. Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 227, Japan (Received: August 17, 1990)
Time-resolved measurements were performed on the photochemically induced isothermal phase transition of polymer liquid crystals (PLC) with mesogenic side chains of phenyl benzoate (PAPB3) and cyanobiphenyl (PACB3) under conditions wherein the photochemical reaction of the doped photoresponsive molecule (4-butyl-4'-methoxyazobenzene,BMAB) was completed within -10 ns, and the subsequent phase transition of the matrix PLC from nematic (N) to isotropic (I) state was followed by time-resolved measurements of the birefringence of the system. Formation of a sufficient amount of the cis isomer of BMAB with a single pulse of a laser lowered the N-I phase transition temperature of the mixture, inducing the N-I phase transition of PLCs isothermally in a time range of -200 ms. This time range is comparable to that of low molecular weight liquid crystals, indicating that suppression in mobility of mesogens in PLCs does not affect significantly the thermodynamically controlled process.
Introduction Photochemical phase transition is an isothermal phase transition triggered by a photochemical reaction of photoresponsive molecules doped in the matrix liquid crystals (LC). There have been many reports on the photochemical phase transition of LCs in which 'To whom correspondence should be addressed.
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such photoresponsive guest molecules as azobenzene and spiropyran derivatives were used.'+ In the azobenzene derivative/LC ( I ) Haas, W. E.; Nelson, K.F.; Adams, J. E.; Dir, G. A. J. Electrochem. Sor. 121. ... 1914. . - - - , 1667. --(2) Ogura, K.;Hirabayashi, H.; Uejima, A,; Nakamura, K.Jpn. J. Appl. Phys. 1982, 21, 969. (3) Attard, G.; Williams, G. Narure 1987, 326, 544.
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0 1991 American Chemical Society
510 The Journal of Physical Chemistry, Vol. 95, No. 2, 1991
Letters
Time ( m s )
WE3
C~H&N=N@CH~
Pm3
BMAB
HOAB
Figure 1. Structures of PLCs and photoresponsive dopants used in this study.
mixtures, the trans forms of the azobenzenes are favorable for the stabilization of the LC phase because of their rodlike shape, while the cis forms act as an ‘impurity” to the system; thus, photoisomerization from the trans form to the cis form causes the lowering of the LC to isotropic (I) phase transition temperature (Tc) of the mixture, and when the TC of the mixture is reduced below the irradiation temperature, the phase transition is induced isothermally.6 This process is essentially different from the ‘heat-mode” process, where lasers are usually employed as a heat source and the irradiated sites undergo the phase transition due to rise in temperature above TC.’* The photochemical phase transition may be regarded as a thermodynamically controlled process. The photochemical phase transition is composed of two processes: the photochemical reaction of the doped molecules and the phase transition of the matrix LCs. Under the steady-state irradiation, the two processes occur simultaneously: gradual decrease of Tc by accumulation of, for example, the cis form of azobenzene guests. It is, therefore, very difficult to explore the time course of the second process alone. Our previous studies revealed that the time required for the completion of the photochemical phase transition is longer by about 1 order of magnitude in PLCs than in low molecular weight LCs under the steady-state irradiation when evaluated under the same condition, i.e., the same photoresponsive dopant, the same mesogen, and the same irradiation condition.6g-h This result was interpreted in terms of suppressed mobility of mesogens in PLCs. In the present study, we evaluated the intrinsic response of PLCs in the photochemical phase transition by means of a pulsed laser so as to produce a sufficient amount of the cis form of doped 4-butyl-4’-methoxyazobenzene (BMAB) within 10 ns, a time scale that is infinitesimal in comparison with the second process of the phase transition; thus, TC of the system was lowered instantaneously (- 10 ns) below the irradiation temperature, and the relaxation of the matrix PLCs from nematic (N) to I state
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(4) (a) Eich, M.; Wendorff, J. H.; Reck, B.; Ringsdorf, H. Makromol. Chem. Rapid CMmun. 1987,8,59. (b) Eich, M.; Wendorff, J. H. Ibid. 1987, 8, 461. (5) McArdle, C. B. Liquid Crysrals 1987,2, 573. (6) (a) Tazuke, S.;Kurihara, S.;Ikeda, T. Chem. Lett. 1987, 91 1. (b) Kurihara, S.;Jkeda, T.; Tazukc, S.Jpn. J . Appl. Phys. 1988,27, L1791. (c) Ikeda. T.; Itakura, H.; Lee, C. H.; Winnik, F. M.; Tazukc, S.Macromolecules 1988. 21, 3536. (d) Ikeda, T.; Horiuchi, S.; Karanjit, D. B.; Kurihara, S.; Tazuke, S. /bid. 1990, 23, 36. (e) Ikeda. T.; Horiuchi, S.; Karanjit, D. B.; Kurihara, S.;Tazuke, S. Ibid 1990, 23, 42. (f) Ikeda, T.; Miyamoto, T.; Kurihara. S.;Tsukada, M.; Tazuke, S.Mol. Crysr. Liq. Crysr. 1990, 1828, 357. (g) Ikeda, T.; Miyamoto, T.; Kurihara, S.;Tsukada, M.; Tazuke, S.Ibid. 1990, 1824 373. (h) Ikeda, T.; Kurihara, S.; Karanjit, D. B.; Tazuke, S. Macromolecules 1990, 23, 3938. (7) Shibaev, V. P.;Kostromin, S. G.; Plate, N. A.; Ivanov, S.A.; Vetrov, V. Yu.; Yakovlev, 1. A. Polym. Commun. 1983,24, 364. ( 8 ) (a) Coles, H. J.; Simon, R. Mol. Crysr. Liq. Crysr. 1984,102,43. (b) Cola, H.J.; Simon, R. Polymer 1985, 26, 1801. (9) Sasaki, A. Mol. Cryst. Liq. Crysr. 1986,139, 103.
Time ( m s ) Figure 2. Time-resolved measurements of birefringence: (A) I, HOAB/PAPB3; 2, BMAB/PAPB3. (B) BMAB/PACB3. Pulse irradiation was performed at Td = 0.999.
was followed by timeresolved measurements of birefringence. We found that the N-I phase transition of PLCs occurred isothermally in a time range of -200 ms, which is comparable to that of low molecular weight LCs.
Experimental Section Figure 1 shows the structures of PLCs used in this study, poly(4-methoxyphenyl 4-(acryloyloxy)propoxybenzoate) (PAPB3)6dand poly( 4-cyano-4’-( 3-acryloyloxypropoxy)biphenyl) (PACB3),6hand the photoresponsive dopants, BMAB6’ and 4hydroxyazobenzene (HOAB). The physical properties of the PLCs are as follows: the molecular weight determined by gel permeation chromatography is 3600 (PAPB3) and 5000 (PACB3); the glass transition temperature determined by differential scanning calorimetry (DSC) is 32 (PAPB3) and 35 OC (PACB3); the N-I phase transition temperature (TNI) detected by polarizing micra~copyis 62 (PAPB3) and 71 OC (PACB3). Polymer films (film thickness, -20 pm) containing 3 mol 8 of the photoresponsive dopant were prepared by casting the polymer solution in chloroform on to glass plates. After they were dried completely under reduced pressure, the films were annealed in a thermostat at temperatures where the PLC films showed a liquid-crystalline phase. The polymer films were then examined for their LC phase behavior with an Olympus Model BHSP polarizing microscope equipped with a Mettler hot-stage model FP-80 and FP-82. The polymer films thus prepared were placed in a thermostated block between two crossed polarizers and irradiated with a Spectron HL-21 Nd:YAG laser (the third harmonic, 355 nm; 12 mJ/pulse; pulse width, 10 ns fwhm). The photochemically induced phase transition behavior wm followed by monitoring the intensity of the linearly polarized light at 633 nm from a He-Ne laser transmitted through a pair of crossed polarizers with a Hamamatsu R-928 photomultiplier and recorded with an Iwatsu TS-8123 storage scope.
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Results and Discussion Typical examples of the change in transmittance of the He-Ne laser light passed through the pair of the ctossed polarizers between which the sample was placed, I,, are given in Figure 2 as a function of time after single pulse irradiation of the laser. In (A), PAPB3 was used as the matrix PLC and two kinds of dopants were used, BMAB and HOAB, and in (B), PACB3 was used as the matrix PLC. The pulse irradiation was performed at a reduced temperature T,, (=T/TNI)of 0.999. It is clearly seen that I, for the BMAB/PAPB3 (A) and BMAB/PACB3 mixtures (B) decreased with time and finally became 0, which is ascribed to the complete loss of birefringence. This means that the N-I phase transition of the BMAB/PLC mixtures was induced photochemically after a sufficient amount of the cis form was produced with a single pulse of the laser (-30% degree of isomerization of BMAB) and the isothermal phase transition was completed within -200 ms.
511
J . Phys. Chem. 1991, 95,511-515 The phase transition was also confirmed by polarizing microscopy. Namely, before the pulse irradiation, both BMAB/PAPB3 and BMAB/PACB3 mixtures showed the N phase at Tred= 0.999 and after a single pulse irradiation, the irradiated sites became isotropic. It is worth mentioning here that the time required for the photoisomerization of BMAB in PLCs was quite short, and no decay of the trans isomer of BMAB on pulse irradiation could be observed with our apparatus, which possesses a IO-ns time resolution; thus, the formation of the cis form can be expressed by a 6 function with an infinitesimal time width in comparison with the phase transition of the whole system. Contrary to the behavior of the BMAB/PAPB3 mixture, no phase transition was observed for the HOAB/PAPB3 and HOAB/PACB3 mixitures under the same condition, although a change in I, was observed as shown in A. Nonoccurrence of the phase transition for the HOAB/PLC mixtures after a single pulse irradiation was also confirmed by polarizing microscopy. HOAB has electronic properties similar to those of BMAB [&(ethanol) = 348 nm;, ,e = -28000 (BMAB) and -26000 (HOAB)] but is not isomerized on photoirradiation. These results clearly demonstrate that the N-I phase transition observed for the BMAB/PLC mixtures resulted from the photochemical reaction of the dopant. From a thermodynamic point of view, the pulse irradiation to produce the cis isomer of BMAB within 10 ns is equivalent to
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setting the state of the system from an N state to an I state across the N-I phase boundary in the phase diagramlo within 10 ns; thus, immediately after the pulse irradiation, the system is in a nonequilibrium N state. We observed the relaxation process from the nonequilibrium N state to an equilibrium I state by means of the time-resolved measurements of birefringence of the system. It is generally believed that the mobility of mesogens in PLCs is suppressed owing to covalent attachment to the main chain of the polymers even in side-chain PLCs. In fact, the response of the PLCs to an external electric field, for example, has been reported to be slower than that of low molecular weight LCs." However, the fact that the relaxation from nonequilibrium N state to the equilibrium I state occurred in -200 ms in PLCs, which is comparable to that of the low molecular weight LCs such as cyanobiphenyls,I2 indicates that the suppression of mesogens in PLCs does not affect the thermodynamically controlled process significantly. In other words, the N-I phase transition in PLCs may require the movement of mesogens only to a small extent. (IO) The phase diagram piepared for the BMAB/PAPB3 (BMAB, 3 mol 5%) indicates that TNIof the mixture decreasts with increasing the cis content, but the decrease is not linear with the concentration of the cis isomer. (1 1) Plate, N . A,; Shibaev, V. P. Comb-Shaped Polymers and Liquid Crystals; Plenum Press: New York, 1987. (12) Kurihara, S.;Ikeda. T.; Sasaki, T.; Kim, H.-B.; Tazuke, S.J. Chem. SOC.,Chem. Commun., in press.
Rotational Currents as a Measure of Excited-State Dipole Moments Steven S. Brown and Charles L. Braun* Department of Chemistry, Dartmouth College, Hanover, New Hampshire 03755 (Received: August 24, 1990; In Final Form: December 4, 1990)
We report photocurrent transients arising from the pulsed laser excitation of the dipolar first excited singlet state SIof truw4-(dimethylamino)-4'-nitrostilbene (DMANS) in toluene solution. The currents arise from rotational reorientation of DMANS dipoles with respect to the axis of an applied electric field. The method appears to offer a simple and general approach to the measurement of the change in dipole moment upon electronic excitation of a molecule.
Introduction Photoinduced electron transfer, both intramolecular and intermolecular, is of substantial current interest.'" Progress has been stimulated by attempts to mimic the rapid electron transfers that occur in the early stages of photosynthesis. In studies of covalently linked electron donors and acceptors, transient absorption and fluorescence techniques have predominated, but measurement of excited-state dipole moments as charge separation evolves in time has often been of great We report what we believe to be a new and versatile technique for the time-resolved ( I ) Closs, G. L.; Miller, J. R. Science 1988, 240, 440. (2) Wasielewski, M. R.; Niemczyk, M. P.; Svec, W. A.; Bradley Pewitt, E. J . Am. Chem. Soc. 1985. 107, 5562. (3) Hermant, R. M.;Bakker, N. A. C.; Scherer, T.; Krijnen, B.; Verhocven, J. W. J . Am. Chem. Soc. 1990.112, 1214. (4) Gust, D.; Moore, T. A.; Moore, A. L.; Lee, S.-J.;Bittersmann, E.; Luttrull, D. K.; Rehms, A. A.; Degraziano, J. M.; Ma, X. C.; Gao, F.; &Iford, R. E.; Trier, T. T. Science 1990. 248, 199. ( 5 ) Smit, K. J.; Warman, J. M.; de Haas, M. P.; Paddon-Row, M. N.; Oliver, A. M. Chem. Phys. Lcrr. 1988,152, 177. Warman, J. M.; de Haas, M. P.; Oevering. H.; Verhoeven, J. W.; Paddon-Row, M. N.; Oliver, A. M.; Hush, N . S.Chem. Phys. Lerf. 1986, 128. 1214. (6) Paddon-Row, M. N.; Oliver, A. M..; Warman, J. M.; Smit, K. J.; de Haas, M. P.; Owering, H.; Verhoeven, J. W. J . Phys. Chem. 1988, 92,6958. (7) de Haas, M. P.; Warman, J. M. Chem. Phys. 1982, 73, 35.
measurement of the difference in dipole moment between two states of a molecule or molecular array. Transient direct current (dc) photoconductivity techniques can be used in a variety of applications, for example, in measurements of charge mobilities or time-resolved geminate pair decay kinetics! In general, those experiments rely on charge carrier separation and drift between electrodes to generate a displacement current in an external circuit. We report here the use of fast dc photocurrent measurements to observe transient changes in intramolecular charge separation. If a molecule that is free to rotate experiences a change in its charge distribution on going from its ground state to an excited electronic state, it will tend to have, on average, different orientations in each of those states with respect to an axis defined by an applied electric field. Since the change in average dipole orientation constitutes a net movement of charge in the field direction, it should be possible to observe such rotational reorientations as transient currents produced as the molecule is excited and subsequently relaxes. The magnitude and time dependence of the currents can then be used to determine excited-state dipole moments so long as other photophysical properties of the molecule are known. ~~
(8) Scott, T. W.; Braun, C. L. Chem. Phys. Lett. 1986, 127, 501.
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