J. Phys. Chem. 1995,99, 13062-13064
13062
Formation of Extremely Long-Lived Charge-Separated State following Photoinduced Electron Transfer in Poly(N-vinylcarbazole) Coadsorbed with 1,2,4,5Tetracyanobenzene on a Macroreticular Resin Shoji Kotani, Hiroshi Miyasaka," and Akira Itaya* Department of Polymer Science and Engineering, Kyoto Institute of Technology, Matsugasaki, Kyoto 606, Japan Received: May 4, 1995; In Final Form: July 13, 1995@
An extremely long-lived (>>8 h) charge-separated species was observed following photoinduced electron transfer in poly(N-vinylcarbazole) coadsorbed with 1,2,4,.5-tetracyanobenzene as an electron acceptor on a macroreticular resin, Amberlite XAD-8, at room temperature. The formation and deactivation of the longlived charge-separated states were discussed from the viewpoints of the effective hole transfer via the carbazolyl groups along the polymer chain to result in the ion pair with rather large interionic distance and the longdistance back electron transfer in the ion pair.
Introduction
+c H-c
One of the aims of the investigations on the photoinduced electron-transfer (ET) proce~sesl-~ is to access the molecular systems such as the photosynthetic reaction center in plants, where the long-lived charge-separated (CS) state is attained by the charge shift reactions in polychromophoric systems, and the electron released by the photoinduced charge separation is utilized efficiently for the subsequent chemical reactions. For the initial step of the aim, many efforts have been devoted to attain the long-lived CS state by developing covalently linked molecular assemblies that provide a closer mimic of complicated chromophore compositions and three-dimensional arrangements of such photosynthetic pigment^.^.^ According to these works and theoretical investigations, it has been deduced that an increase in the interionic distance, which results in the decrease of the electronic tunneling matrix element for charge recombination (CR), is a key process to generate long-lived CS states. If we concentrate our attention to produce the long-lived CS states, it seems intriguing to utilize aromatic vinylpolymers as a component, since some of these macromolecules have photoconductive properties attributable to the sequential chargeshift reactions among the pendant aromatic group^.^ Hence, it seems possible to generate the long-lived CS state after the photoinduced CS, in the condition where rather long interionic distance attained by the charge shift reaction is fixed in these macromolecular systems. Along this line, we have investigated the photoinduced ET and its related processes of one of typical photoconductive polymers, poly(N-vinylcarbazole),coadsorbed with an electron acceptor on macroreticular resins. In the present heterogeneous system, the CS state was observed even at 8 h after the excitation at room temperature. On the basis of the experimental results, the mechanisms of the generation and the deactivation of this long-lived ionic species will be discussed.
Experimental Section Transient diffuse reflectance absorption spectra were measured by using a microcomputer-controlled nanosecond laser photolysis systemEwith an excimer laser (351 nm, 20 ns fwhm, 11 d l c m 2 ) as an excitation source. Pulsed Xe lamps (200 ps and 1 ps fwhm) were used as monitoring beams for the measurements of the time profiles of the absorption intensity and transient spectra, respectively. The absorption intensity @
Abstract published in Advance ACS Absrrucrs, August 15, 1995.
0022-365419512099-13062$09.0010
H?
t
& NcxxcN PVCz
CN
NC
TCNB I
I
7 H1 CHI -C-C
I
-0-R-0-C 11
cn20 CHI-C-C I
-0-CH,
nz
-C -C Hs
11 I 0 CH, I
11
cn20 cn,-c -c-0-c I 11 cn*o
H,
CH,-C-C-O-cn,
I 11 ynzO
XAD-8 Figure 1. Molecular structures of compounds used.
(AA) was calculated by the following equation:
AA(t) = (I, - Ze)/Z0 (1) where l o and le are intensities of the diffuse reflected light without and with the excitation laser light. Poly(N-vinylcarbazole)(PVCz) was prepared by the cationic polymerization9 (Mn = 1.04 x lo5, MwIMn= 1.60). 1,2,4,5Tetracyanobenzene (TCNB, Wako GR grade) was recrystallized from ethanol and sublimated before use. Acrylic macroreticular resin, Amberlite XAD-8, (Rohm & Haus, mean pore diameter = 225 A, grain size = 250-850 pm), was washed with NaOH(aq) solution, HCl(aq) solution, deionized water, THF, and methanol and dried before use. The structure of XAD-8 is shown in Figure 1, where R is the alkyl chain (-CH2- or -CHz-CHz-). PVCz and TCNB were adsorbed on XAD-8 in the following manner. A 1,2-dichloroethanesolution of PVCz ([Cz] = 0.02 M) and TCNB (0.013 M), containing purified XAD-8 (1.5 g/10 mL) was kept at 30 "C for more than 24 h, followed by filtration through a glass filter and drying in a vacuum. The pore radius of the macroreticular resin is almost the same as the radius of the gyration of the polymer (ca. 120 A) in 1,2-dichloroethane solution. Hence, each polymer chain seems to be separately 0 1995 American Chemical Society
Letters
J. Phys. Chem., Vol. 99, No. 35, 1995 13063
0.151
a
0.1
Q
0.05
400
600 Wavelength I nm
800
Figure 2. Transient absorption spectra of PVCz-TCNB on XAD-8 system at 296 K, excited with a nanosecond 351 nm laser pulse.
adsorbed on XAD-8. Even if more than one chain is adsorbed on one pore, it should be remarked that the photoinduced ET reactions such as CS, hole transfer (HT), and CR take place in a finite space. All the measurements were performed under vacuum at 296 or 195 K.
Results Figure 2 shows transient absorption spectra of PVCz-TCNB adsorbed on XAD-8 system at 296 K. Each of the spectra shows two absorption maxima at 465 and ca. 770 nm. On the basis of the positions of the absorption maxima and their spectral band shapes, the former absorption band and the latter one can be safely assigned to the TCNB anion (TCNB-)'O-I2 and the cation of carbazolyl moieties (Czf) in PVCZ,'*-'~respectively. According to our preliminary picosecond laser photolysis studies,I6 it was observed that the ionic species appeared within a few nanoseconds after the excitation. Hence, the charge separation in the excited singlet state is responsible for the production of the ionic species. It should be remarked here that the ionic species was observed even at 8 h after the excitation. Since such a long-lived ionic species was observed also by the steady-state irradiation with ca. 350 nm light of Xe lamp, the production of the long-lived ionic species is not attributable to multiphoton ionization process. The relative ratio between the absorption signals of the cation and anion is identical during the decay process, indicating that the disappearance of the ionic species is due to the CR process. The temperature dependence of the decay process of Cz+ and TCNB- in the several hours time region is shown in Figure 3, where the absorption intensities at 465 nm were plotted against the logarithmic time. The decay profiles will be discussed in later part. Although the decrease in the temperature increases the lifetime of the CS state to some extent, the temperature effect on the CR process in this time region is very small, indicating that the CR process has rather small activation energy. Discussion As mentioned in the Introduction, it is deduced, from theoretical and experimental studies on ET, that an increase in the interionic distance resulting in the decrease of the electronic tunneling matrix element for CR is indispensable for the production of such a long-lived charge-separated (CS) state. Although the translational diffusion of ions cannot take place in the present adsorbed systems, a hole-transfer process through carbazolyl (Cz) groups along the polymer chain may occur; the hole conduction is the origin of the photocurrent in PVCz films.'
O'
'
'
"102
'
"""'103 ' ' """'104 ' Time 1 sec
'
A
Figure 3. Temperature dependence of the time profile of the CS state in PVC-TCNB on XAD-8 system, monitored at TCNB- absorption. Solid lines are to guide the eye.
Actually, the typical lifetime of the CS state between Nethylcarbazole (monomer model compound of PVCz) and TCNB is less than a few nanosecondsl2Sl6in the case where the effective hole transfer cannot take place in solutions or in adsorbed systems. Hence, the long-lived ionic species observed in Figure 2 is attributable to the ion pair whose interionic distance is increased via the hole-transfer process along the polymer chain. Although the hole transfer increasing the interionic distance is an indispensable process, it should be noted that such a longlived CS state formation has never been observed for PVCz and electron-acceptor systems in solution, films, powders, or other adsorbed systems.I2-l6 When Vycor glass was used as an adsorbent (in the condition where each polymer chain could be adsorbed separately as in the present case), the CS state produced via the photoinduced ET between PVCz and TCNB completely diminished in submilliseconds time regions. l 6 By integrating the present results with those performed previously, it may be concluded that not only the increase in the interionic distance but also the trapping processes of the hole to fix rather long interionic distance are necessary for the formation of such a long-lived CS states, especially in a finite space. Actually, the small temperature dependence of the CR in the hours time region (Figure 3) suggests that the longdistance electron tunneling is responsible for the CR process of the ion pair even at room temperature. To analyze the time profiles of the CS state in the present system, we have undertaken the simulation on the basis of the long-distance electron transfer (Figure 4). This figure covers the time region from ca. lo-' to lo4 s. The solid line was a curve calculated on the basis of the following equations: W t )=
Em,)exp[-k(r,)tI
(2)
I
k( rJ = k, exp [-(r, - rJa]
(3)
P(ri) = Po exp [-r i p ] (4) Here AA(t) is the absorption intensity at time t and k(ri) is the distance-dependent CR rate constant. In the calculation, a and kQ were set to be 1 8, and lo9 s-l. Although the attenuating factor, a,in the present system contains important information, it is rather difficult to determine it at the present stage of the investigation. Hence, we used the average value for the intermolecular electron transfer processes in frozen media. The kQ value of lo9 s-l is the CR rate constant of the ion pair
13064 J. Phys. Chem., Vol. 99, No. 35, 1995
Letters
0.1
a
Q
0.05
O Y i P
1oo Time I sec
105
Figure 4. Time profiles of the CS state of PVCz-TCNB on XAD-8 system, monitored at TCNB- absorption at 296 K. A solid line is the calculated result based on eqs 2-4 (see text).
state between N-ethylcarbazole and TCNB in nonpolar solutions. I 2 P(ri) is the population of the CS states after the hole is trapped at the appropriate site with the interionic distance, ri. When the hole migration takes place along the polymer chain after the initial charge separation between A and DOas in A-*D,*D,*D,. *DTZ and the detrap process is ignored, the number of the hole trapped at D,is in proportional with p ( 1 - P)~.Here p is the probability for a certain D acting as the trap site. On the assumption that the polymer is regarded as a one-dimensional chain, the interionic distance between A- and D,+ scales ir,, ro. Here r,, is the averaged distance between two nearest Cz groups in PVCz and ro is the distance between A and Do. It should be noted that the actual polymer is not a one-dimensional chain but in fractal dimensions. However, the interionic distance of the ion pair undergoing the CR within several second to hours regions is rather small ( i < ca. 8-10). Hence we used the above simple one-dimension approximation. In the actual analysis, we used this p as a parameter and other values were fixed. The present p value corresponds to the case where 22% of Cz groups act as the trap site (p = 0.22). Although the model is rather simple and, hence, the interpretation concerning each parameter should be the same in the qualitative manner at the present stage, the calculated curve reproduces the experimental results fairly well for a wide time region (more than 10 orders of the time span). The deviation of the calculated curve from the experimental result in the early stage after the excitation might be interpreted as due to the CR between TCNB- and Cz+ which has not yet been trapped a n d or due to the presence of more than one ion pair in a finite space to result in cross recombination. At any rate, this result also supports the mechanisms that the formation of the ion pair with rather large interionic distance fixed by the tapping process is responsible for such a long-lived CS state. Finally, we briefly discuss the origin of the trap site. In aromatic vinyl polymers, it is well-known that various dimer cations are effectively produced, and the hole is trapped at the dimer cation site^.'^-'^-'^ Since the ratio of syndiotactic to isotactic sequences in the polymer affects the distribution of the dimer cation sites,I9 we have investigated the time profiles of the CS states in PVCz with different ratio of these sequences. However, the CR time profile in the time region on and after
+
ca. millisecond region was not affected by the ratio of the tacticity of the polymer. Hence, the deep trap site for the hole was not simply ascribable to the dimer cation sites. Rather, it may be attributed to the Cz groups adsorbed in the vicinity of the polar ester groups, by comparing the present results with those in porous Vycor glass systems. Moreover, we are now investigating the dynamics behaviors in the adsorbent system similar to XAD-8, where the polar ester group exists but the approach of the Cz groups to it is hindered by a bulky long alkyl chain around the ester groups. According to our preliminary results, such a long-lived CR state was not observedi6 in this system. This result also suggests strongly that the "solvated" Cz moieties in polar environments play an important role as a trap site in the production of the long-lived CS state in the present system. At any rate, the present result may provide the potential applicability for the control of the electron transfer rate by the heterogeneous microenvironments, especially the difference in the polarity in the vicinity of the ions. More detailed investigations covering wider time regions as well as the precise temperature dependence are now going on, results of which will be published shortly.
Acknowledgment. The authors sincerely thank Prof. N. Mataga at Laser Institute of Technology and Prof. T. Kakitani and Dr. A. Yoshimori at Nagoya University for their helpful discussions. The present work was partly supported by Grantin-Aid from the Ministry of Education, Science and Culture of Japan to H.M. (05640570 and 06640652) and A.I. (PriorityArea-Research, Photoreaction Dynamics, 06239107). References and Notes (1) Marcus, R. A,; Sutin, N. Biochim. Biophys. Acta 1985, 811, 265. (2) (a) Mataga, N. In Electron Transfer in Inorganic, Organic and Biological Systems; Bolton, J. R., Mataga, N., McLenden, G., Eds.; Advances in Chemistry Series 228; American Chemical Society: Washington, DC, 1991; Chapter 6. (b) Mataga, N.; Miyasaka, H. frog. React. Kinet. 1994, 19, 317. (3) Rips, I., Klafter, J., Jortner, J. In Photochemical Energy Conversion; Norris, J. R., Meisel, D., Eds.; Elsevier: New York, 1988; p 1. (4) Barbara, P. F.; Jarzeba, W. Adv. Photochem. 1990, 15, 1. (5) Wasielewski, M. R. Chem. Rev. 1992, 92, 435 and references therein. (6) Maruyama, K.; Osuka, A.; Mataga, N. Pure Appl. Chem. 1994, 66, 867. (7) Mort, J.; Pfister, G. In Electronic Properties of Polymers; Mort, J., Pfister, G., Eds.; Wiley-Interscience: New York, 1982; Chapter 6 and references therein. (8) Koshioka, M.; Mizuma, H.; Imagi, K.; Ikeda, N.; Fukumura, H.; Masuhara, H.; Kryschi, C. Bull. Chem. SOC. Jpn. 1990, 63, 3495. (9) Itaya, A.; Okamoto, K.: Kusabayashi, S. Polym. J . 1985, 17, 557. (IO) Masuhara, H.; Mataga, N. Chem. Phys. Lett. 1970, 6 , 608. (11) Miyasaka, H.: Ojima, S.; Mataga, N. J . Phys. Chem. 1989, 93, 3380. (12) Miyasaka, H.; Moriyama, T.; Kotani, S.; Muneyasu, R.; Itaya, A. Chem. Phys. Lett. 1994, 225, 315. (13) Masuhara, H.; Yamamoto, N.; Tamai, N.; Inoue, K.: Mataga, N. J . Phys. Chem. 1984, 88, 3971. (14) Itaya, A.; Yamada, T.; Masuhara, H. Chem. Phys. Lett. 1990, 174, 145. (15) (a) Masuhara, H.; Itaya, A. In Dynamics and Mechanism of Photoinduced Electron Transfer and Its Related Phenomena; Mataga, N., Okada, T., Masuhara, H., Eds.; Elsevier: Amsterdam, 1992; p 363. (b) Ueda, T.: Fujisawa, R.; Fukumura, H.; Itaya, A,; Masuhara, H. J . Phys. Chem. 1995, 99, 3629. (16) Makino, S.; Kotani, S.; Miyasaka, H.; Itaya, A,, manuscript in preparation. (17) Sutin, N. In Electron Transfer in Inorganic, Organic and Biological Systems; Bolton, J. R., Mataga, N., McLenden, G., Eds.; Advances in Chemistry Series 228; American Chemical Society: Washington, DC, 1991; Chapter 3, and references therein. (18) Yamamoto, M.; Tsujii, Y . ;Tsuchida, A. Chem. Phys. Lett. 1989, 154, 559. (19) The ratio of syndiotactic to isotactic sequences in PVCz synthesized by the radical polymerization was 3: I , while that by the cationic polymerization was 1:l (Okamoto, K.; Yamada, M.; Itaya, A,: Kimura, T.; Kusabayashi, S. Macromolecules 1976, 9, 645).
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