J. Phys. Chem. 1992,96, 5614-5617
5614 (6) (7) (8) 1851. (9)
Berr, S. S. J . Phys. Chem. 1987, 91, 4760. Hayter, J. B.; Penfold, J. Mol. Phys. 1981, 42, 109. Hayter, J. B.; Penfold, J. J . Chem. Soc., Faraday Trans. 1 1981, 7 7 , Hansen, J.-P.; Hayter, J. B. Mol. Phys. 1982, 46, 651.
(10) Reiss-Husson, F.; Luzatti, V. J . Phys. Chem. 1964, 68, 3504. (11) Ulmius, J.; Lindman, 8.;Lindbloom, G.; Drakenburg, T. J . Colloid Inferfuce Sci. 1978, 65, 88. (12) Bruins, E. M. Proc. Acad. Amsterdam 1932, 35, 107. ( 1 3) Gamboa, C.; Sepulveda, L.; Soto, R. J . Phys. Chem. 1981,85, 1429.
Photochemical Switching of Ionic Conductivity in Composite Films Containing a Crowned Spirobenzopyran Keiichi Kimura,' Takashi Yamashita, and Masaaki Yokoyama Chemical Process Engineering, Faculty of Engineering, Osaka University, Yamada-oka, Suita, Osaka 565, Japan (Received: January 21, 1992; In Final Form: March 13, 1992)
Spirobenzopyran derivatives incorporating a monoazacrown ether moiety at the 8-position, Le., crowned spirobenzopyrans, have been applied to composite films that undergo photoinduced switching of ionic conductivity, taking advantage of the photochemical cation-bindingability changes of the crowned spirobenzopyrans. In plasticized poly(viny1 chloride) films containing LiClO, and a crowned spirobenzopyran,which show bi-ionic conducting behavior, isomerization of the crowned spirobenzopyran to its corresponding merocyanine form proceeded under UV-irradiated or dark conditions. This depressed the Li+ conduction in the film, thus decreasing its ionic conductivity by about a half. Subsequent visible-light irradiation allowed isomerization back to the spiropyran form, restoring the ionic conductivity to the initial value. Much more significant ionic-conductivity switching with about 20-fold changes was realized in composite films of lithium poly(perfluorosulfonate)/oligooxyethylene diacetatelcrowned spirobenzopyran, which are single-ionic conductors, by alternatively turning on and off the visible light.
Considerable attention has been focused on organic ion conductors or polymer electrolytes.',* One main goal for designing organic ion conductors is high electrical conductivity, but there are very few organic ion conductors whose ionic conductivities can be switched by external stimuli. We have already studied the thermo- and photoresponse in ionic conductivities of poly(viny1 chloride) (PVC) composite films containing LiC104 and an azobenzene liquid crystaL3 The azobenzene liquid crystal isomerizes reversibly from its trans to cis forms by photoirradiation and thereby experiences drastic phase transitions. For instance, UV-light irradiation on the composite film at room temperature caused distinct phase transitions from the crystal or liquid crystal to the isotropic liquid states, drastically enhancing ion mobility and, therefore, ionic conductivity in the film. Subsequent visible-light irradiation again diminished the ionic conductivity. It was thus found that ionic conductivity switching is photochemically feasible with the composite films, which are useful for electrostatic printings4 Spirobenzopyran derivatives generally isomerize to their corresponding zwitterionic merocyanine forms upon illumination with UV light and vice versa with visible light. Spirobenzopyran derivatives such as 1, where a monoazacrown ether moiety is incorporated to the &position as a cation-binding site, can act as photochemical controllers of cation binding in s o l ~ t i o n .When ~ NO'
-\ r k i 4
O O ' CH,
OI OCH,
a crowned spirobenzopyran isomerizes to its corresponding merocyanine form in the presence of a metal ion such as Li+ under UV-light-irradiated or dark conditions, the metal ion complexed by its crown ether moiety interacts intramolecularly with a phenolate anion in the merocyanine isomer. Thus, in the crowned merocyanine, the metal ion can be bound more tightly than the corresponding spiropyran isomer due to a kind of additional binding site effect. Visible light brings about the isomerization back to the spiropyran form, attenuating its cation-binding abilities. 0022-365419212096-5614$03.00/0
The photoinduced change in the cation-binding ability of crowned spirobenzopyrans in solution opens up the possibility for photochemically controlling ionic conductivity of films containing a crowned spirobenzopyran as the key material. We recently communicated the photoresponse of ion-conducting composite films made from plasticized poly(viny1 chloride) (PVC), LiC104, and crowned spirobenzopyran lS6 We have also extended the photochemical ionic-conductivity control driven by the crowned spiroknzopyran isomerization to a single-ionic conducting system. Reported herein are the details of the photoinduced ionic-conductivity switching of crowned-spirobenzopyran-containing composite films and its mechanistic study. Experimental Section Materials. Crowned spirobenzopyrans 1-3 and an acyclic analog 4 were synthesized as reported el~ewhere.~,'PVC (average polymerization degree of 1020) was purified by repeated reprecipitation. Poly(perfluorosu1fonic acid) (Nation) (PPFS) was received as a 5 wt % alcohol solution from Aldrich and its lithium salt was obtained by neutralization with a lithium methoxide methanol solution. 2-Ethylhexyl sebacate (DOS) was purified by distillation in vacuo. Oligooxyethylene diacetate (average molecular weight of 500) (OOEAc) was prepared by treating oligooxyethylene (average molecular weight of 400) with excess acetic anhydride (70 "C, 1 day) and then purified by alumina chromatography. The lithium salts are of analytical reagent grade. Tetrahydrofuran (THF) and N,N-dimethylformamide (DMF) were distilled over Na metal and calcium hydride, respectively. Composite Film Fabrication. PVC-based composite films for measurements of ionic conductivity and absorption spectrum were prepared on indium-tin-oxide-coated (ITO) glasses (2 X 2.5 cm) by spin coating from THF, and then dried overnight at 40 OC. Typically, 100 pL of a T H F solution [60 mg PVC (51.9 wt %), 50 mg of DOS (43.3 wt %), 0.5 mg of LiC10, (0.4 wt %), and 5 mg of crowned spirobenzopyran (4.4 wt %) dissolved in 0.7 mL THF] was used for each spin coating, allowing a film thickness of about 4 pm. PVC composite films of 50-60-pm thickness for isothermal transient ionic current measurements were cast on an I T 0 glass substrate from 1 mL of a T H F solution with the same composition. PPFS-based composite films for measurements of ionic conductivity and isothermal transient ionic current were 0 1992 American Chemical Society
The Journal of Physical Chemistry, Vol. 96, No. 13, 1992 5615
Ionic Conductivity in Composite Films
a
400
500
600
VIS
700
Wavelength / nm Figure 1. Photoisomerization of crowned spirobenzopyran in composite film of PVC/DOS/LiCIO,/I: (a) under dark conditions; (b) on visible-light irradiation for 2 min; (c) on UV-light irradiation for 1 min after the visible-light irradiation; (d) on turning off the light and then heating at IO OC.
obtained on I T 0 glasses by casting 100 pL of a DMF solution [typically 23 mg of lithium poly(perfluorosu1fonate) (PPFS-Li) (57.9 wt %), 6.7 mg of OOEAc (16.9 wt %), and 10 mg of crowned spirobenzopyran (25.2%) in 0.5 mL of DMF]. The DMF was gradually evaporated under a pressure of about 2.7 kPa at 40 OC for 3 h and then at 60 OC for 2 days under a highly-reduced pressure to afford a film with about 10-pm thickness. Similarly, PPFS-based composite films for absorption spectra were prepared by spin coating from 100 pL of the above-mentioned DMF solution, the film thickness being about l pm. Gold was evaporated on both types of the composite films for ionic-conductivity measurements as a disk electrode (4.7 mm diameter, about 10 nm thickness). The composite films thus obtained were kept in a desiccator and were dried again at 60 "C in vacuo for 1 h before the measurements. Measurements. The ac impedance of composite films was measured using a Solartron 1253 gain-phase analyzer and a Keithley current amplifier. The details of the measurements are as previously reportedS3 Ionic conductivities were calculated by the ColeCole plots method. Photoirradiation of composite films was achieved from their I T 0 side. Unless otherwise noted, UV (300-400 nm) and visible (>490 nm) lights were obtained by passing light of a 500-W xenon lamp through Toshiba UV-D36 and V-Y50 color filters, respectively. Isothermal transient ionic currents were measured in a manner similar to a procedure previously reported,*s9using a cell setup for ionic-conductivity measurements and a Keithley 6 17 programmable electrometer controlled and data-processed by a microcomputer through a GP-IB board. After an appropriate direct-current voltage was applied across a composite film for 3 h using two Pt electrodes, the polarity was quickly switched and the transient ionic current (I) was then monitored every second. Absorption spectra of the composite films on the I T 0 glass substrate were measured using a JASCO 660 spectrophotometer with uncoated I T 0 glass as the reference. In the case of photoirradiation, the measurement was run immediately after photoirradiation.
Results and Discussion Bi-ionic Conducting System. As a key material for the photochemical control of ionic conductivity, a crowned spirobenzopyran was first applied to a bi-ionic conducting system, in which both the cation and anion are able to participate in the ion conduction. The bi-ionic conducting system adopted here is DOSplasticized PVC film incorporating LiC104 as the ion-conducting species. Crowned spirobenzopyran carrying a monoaza- 12crown-4 moiety at the tt-position, 1, was used as the crowned spirobenzopyran, due to its great difference in the Li+-binding ability between the spiropyran and merocyanine forms in soluti~n.~ In an ion-conducting composite film of the PVC/DOS/LiClO,/l system, crowned spirobenzopyran 1 isomerizes substantially to its corresponding merocyanine form even under dark conditions,
Time / min Figure 2. Photoinduced ionic-conductivity changes in composite film of PVC/DOS/LiClO,/l on alternating irradiation of visible and UV light.
SCHEME I dark(UV) NO,
+
Lit X '
VIS
as depicted in the absorption spectrum of the composite film (Figure 1). Visible-light irradiation promoted isomerization to its spiropyran form, decreasing the absorption at 510 nm. UV light then allowed significant isomerization back to the merocyanine form. Even under dark conditions after the visible-light irradiation, the isomerization back to the merocyanine form proceeded gradually at room temperature, and rapidly at 70 OC, with the initial absorbance at 510 nm being almost restored. Thus, the photoisomerization of the crowned spirobenzopyran is quite reversible in the composite film. Figure 2 shows a typical ionic-conductivity photoresponse for the PVC-based ion-conducting composite film during alternating irradiation by visible and UV light. Obviously, the photoinduced ionieconductivity changes were synchronized with the photoisomerization of the crowned spirobenzopyran. Similar ionic-conductivity changes in the PVC/ DOS/LiC104/1 composite film were observed with alternating turning on and off of visible light, which increased and decreased its ionic conductivity, respectively. The photoirradiation system with turning on and off of visible light is more desirable for practical applications of photochemically-switchable ion-conducting systems than the system using alternating irradiation of UV and visible light, because the use of UV light may accelerate deterioration of the film components. It is thus probable that the merocyanine isomer of 1, which strongly binds Li+ due to the formation of an intramolecular phenolate-interacting Li+ complex, retards Li+ conduction in the film as illustrated in Scheme I. Visible-light-induced isomerization to the corresponding spiropyran form releases some Li+, thus promoting Li+ conduction. Figure 3 depicts Arrhenius plots of ionic conductivity in the PVC/DOS/LiC104/1 composite film under dark and visible-light-irradiated conditions, i.e., under merocyanine-rich ( S O % ) and spiropyran-rich (>70%) conditions, respectively. The linearity in the plots implies that the ion conduction in the PVC film is hardly affected by the segmental motion of the PVC. The activation energy under merocyanine-rich conditions (0.46 eV) is a little greater than that under spiropyran-rich conditions (0.45 eV), which reflects the more difficult ion conduction under merocyanine-rich conditions. Since not only Li+ but also its counteranion takes part in ion conduction in the bi-ionic conducting composite films, the type of counteranion may affect their ion-conducting behavior. So, other Li+ salts such as LiCl and LiCF3S03,besides LiC104, were also tested as ion-conducting species. A similar ionic-conductivity change profile seen in the LiClOpxntaining composite film was also observed in the LiCF3S03-and LiC1-containing composite
Kimura et al. -7. h 7
6
.
cn
.-0
.& u 3
-8-
U C 0
G 0,
0 -
-9 0
40
120
80
Time /min Figure 5. Photoinduced ionic-conductivitychanges in composite film of PPFS-Li/OOEAc/l on turning on and off visible light while being heated at 70 O C under dark conditions.
q “------I -
v
-7
-00,
-8I(
0 400
500
600
1000
700
Wavelength / nm Figure 4. Photoisomerization of crowned spirobenzopyran in composite film of PPFS-Li/OOEAc/l: (a) under dark conditions; (b) on visiblelight irradiation for 20 min; (c) on heating at 70 O C after the visible-light irradiation. systems, showing ionic-conductivity changes of no more than 2-fold. Also, variation of the Li+ salt concentration in PVC/ DOS/1 composite films hardly augmented the magnitude of the photoinduced ionic-conductivity change (about 2-fold). Siugk-Ionic Condudng System. It is of much interest to apply a crowned spirobenzopyran to a single-ionic conducting system, where only a cation participates in ion conduction, for fabrication of an ion-conducting system whose ionic conductivity can be switched effectively by photoirradiation. Since a cation transference number of nearly 1 is attainable in the single-ionic conducting system, drastic photoinduced changes in the ionic conductivity can be expected, based on the mechanism in Scheme I. As single-ionic conducting films, composite films of PPFS-Li and OOEAc were selected. The photoresponse of an ion-conducting composite film that consists of PPFS-Li, OOEAc, and crowned spirobenzopyran 1 was first investigated. In this single-ionic conducting composite film, photoisomerization of the crowned spirobenzopyran also occurs rather efficiently (Figure 4). Under dark conditions, crowned spirobenzopyran 1is almost completely isomerized to its corresponding merocyanine form in the PPFS-Li/OOEAc/l composite film. Visible-light irradiation caused a smooth isomerization to the spiropyran form. Turning off the visible light allowed isomerization back to the merocyanine form gradually at room temperature and immediately by heating at 70 OC. The isomerization back to the merocyanine form, however, scarcely proceeded on irradiating with W light (300-400 nm) which was mainly employed in this work. Conceivably, the shorter wavelength region of the “UV light” promotes the isomerization to merocyanine form, but the longer wavelength region prevents the isomerization. Although a much shorter wavelength UV light (70%) conditions. The drift mobility ( p ) of Li+ in the composite film, which can be calculated from the transient time, was 2.8 X cm2 s-l V-I under dark conditions. The mobility was increased to 3.0 X cm2 s-l V-' for visible-light irradiation. Turning off the visible light restored the ion mobility to 2.7 X lo-" cm2 s-I V-'. In general, ionic conductivity ( u ) can be expressed as u = qnp, where q, n, and p stand for ionic charge, carrier density, and ion mobility, re-
5617
spectively. Taking into consideration that the ionic conductivity is about 20-fold greater under visible-light-irradiated conditions than under dark conditions, while the ion mobility is about 10-fold greater in the former than in the latter, the photoinduced ionicconductivity change for the composite film of PPFS-Li/OOEAc/ 1 is governed by a change in the Li+ mobility rather than in the carrier density. Temperature dependence of ionic conductivity in the singleionic conducting composite film is shown in Figure 7. The Arrhenius plots deviate from straight lines to some extent a t higher temperatures under both dark and visiblelight-irradiated conditions. The curved profiles are typical for polymer electrolytes.I0 In the PPFS-Li/OOEAc/l composite f h , therefore, segmental motion of PPFS-Li may be important for Li+ conduction. The higher activation energy under dark conditions than under visiblelight-irradiated conditions indicates the easier Li+ conduction in the latter case. In summary, the crowned spirobenzopyran, 1, has realized significant photoinduced switching of ionic conductivity in the Li+-conducting composite films, especially in the single-ionic conducting systems.
References and Notes (1) Armand, M. Solid Srare Ionics 1983, 9/10, 745. (2) Ratner, R. A.; Shriver, D. F. Chem. Rev. 1988, 88, 109. (3) Kimura, K.; Suzuki, T.; Yokoyama, M.J. Phys. Chem. 1990,946090. (4) Kimura, K.; Suzuki, T.; Yokoyama, M. J . Chem. Soc., Chem. Commun. 1989, 1570. ( 5 ) Kimura, K.; Yamashita, T.; Yokoyama, M. J . Chem. Soc., Perkin Trans. 2 1992,613. (6) Kimura, K.; Yamashita, T.; Yokoyama, M.Chem. Lrrr. 1991, 965. (7) Tailor, L. D.; Nicholson, J.; Davis, R. B. Tetmhedronk r r . 1967,1585, (E) Greeuw, G.; Hoenders, B. J. J . Appl. Phys. 1984, 55, 3371. (9) Watanabe, M.; Rikukawa, M.; Sanui, K.; Ogata, N. J. Appl. Phys. 1985, 58, 736. (10) Williams, M. L.; Landel, R. F.; Ferry, J. D. J . Am. Chem. Soc. 1955, 77, 3701.
Investigation of Laser- Induced Acoustlc Phonons In Poly(p-phenylenevlnylene) Uniaxially Stretched Films Yiping Cui, D.Narayana Rao, and Paras N.Prasad* Photonics Research Laboratory, Department of Chemistry, State University of New York at Buffalo, Buffalo, New York 1421 4 (Received: January 22, 1992)
Elastic moduli of a 101 uniaxially stretched film of poly-pphenylene vinylene are determined using the technique of laser-induced acoustic phonon in a degenerate four-wave mixing phase conjugate geometry. The four-wave mixing signal obtained as a function of film rotation provides acoustic speed as a function of orientation. In order to explain the experimental observation, a theoretical description is presented to describe the acoustic wave generated by laser-induced transient grating for an arbitrary propagation direction in a medium with any symmetry. This theoretical description also provides the dependence of the acoustic wave on the polarization, the propagation direction, and the pulse width of the laser pulses. The theoretical analysis of our experimental result, using the general Christoffel equation, yields the complete elastic modulus tensor, including both the longitudinal and the shear components. The value of the elastic modulus along the draw direction is 46.6 GPa. The analysis also shows a novel feature of mode jump, not reported previously. We observe that within a certain range of angles between the acoustic propagation direction and the draw direction one kind of acoustic mode (quasi-longitudinal) is generated but for another set of angles the mode jumps to become quasi-transverse in nature. The theoretical description presented here also explains this mode-jump behavior.
Introduction
The laser-induced transient grating technique has been used to study a wide variety of dynamic processes'-* in polymers, plasmas, dye solutions and semiconductors. It has also been used as a technique to excite and to detect acoustic phonons."' This technique, pioneered by Fayer and -workersg and called by them as laser-induced phonon spectroscopy or LIPS, has been used to study elastic moduli of materials,I0 anisotropy of mechanical
properties of polymer interaction of phonons, and overtone absorption of the molecules." Acoustic waveguide modes in ultrathin solid have also been observed.20 The mechanism of laser-induced phonon in a transparent material is different from that in absorptive materials. In absorptive materials, the acoustic waves are produced by an instantaneous thermal expansion of the material, which is induced by an optical absorption followed by a rapid nonradiative relax-
0022-36S4/92/2096-5617$03.00/0@ 1992 American Chemical Society
