J. Phys. Chem. 1984, 88, 3971-3974 ions resulted in higher S parameters than the low-spin ions (see Table I). These results suggest that the ratio of the positrons annihilated as a component of p-Ps is higher in solutions of high-spin ions than in the presence of the low-spin ions. It is evidence of the converter property of transition-metal ions.
3971
Acknowledgment. The authors are indebted to Dr. B. Molnfir and Mr. F. Gazdgcska for help in the sample preparation and to Mrs. A. Simon for cooperation in the computer evaluation. Registry No. FeCI,, 7758-94-3;CoCI,, 7646-79-9; NiCI2,7718-54-9; FeSO,,, 7720-78-7; Co(NH,),CI20H, 90530-57-7; K,[Ni(CN),], 14220-17-8;positron, 12585-85-2;positronium, 12585-87-4.
Absorption Spectra of Radical Ions of Polymers Having Carbazolyl Chromophores Hiroshi Masuhara,* Kazuhito Yamamoto, Naoto Tamai, Kazushige Inoue, and Noboru Mataga Department of Chemistry, Faculty of Engineering Science, Osaka University, Toyonaka, Osaka 560, Japan (Received: July 7, 1983; In Final Form: March 8, 1984)
The fluorescent states of poly(N-vinylcarbazole) with different degrees of polymerization, poly [2-(N-carbazolyl)ethyl vinyl ether], and polyurethanes having a 1,2-trans-dicarbazolylcyclobutanegroup were quenched in DMF by an electron donor or acceptor. Absorption spectra of the polymer ions produced were measured by using a N2 gas laser photolysis system. The poly(N-vinylcarbazole) cation showed a spectrum which could be reproduced by a superposition of the bands of the sandwich dimer cation, the second dimer cation with a partial overlap between two carbazolyl groups, and the third dimer cation with another geometry. On the other hand, other polymers gave almost the same spectrum as that of the third dimer cation. The spectrum of polymer anions was identical with that of the reference monomer anion. These results indicate that the produced charge in the polymer is not delocalized over chromophores but trapped in some sites.
Introduction Electron-transfer quenching of an aromatic hydrocarbon in the fluorescent state by an electron acceptor (or donor) leads to the prompt formation of an aromatic hydrocarbon cation (or anion) and quencher anion (or cation) in polar solvents. This process is called ionic photodissociation and its fundamental aspects of typical donor-acceptor systems have been investigated in detail by the nanosecond laser photolysis method.' For a polymer having carbazolyl chromophores (dimethyl terephthalate (DMTP) pairs), solvation to free ions of the polymer cation and the DMTP anion Ionic photodissociation yield and decay occurs dynamics of the ion radicals produced are different from those of the corresponding reference monomer systems. These results have been revealed by analyzing the transient absorption spectral data of the DMTP anion, since the absorption spectral shapes of the polymer cations are different from that of the monomer model compound, the N-ethylcarbazole cation, and differ from one another. Therefore, the nature of electronic structure of polymer ions should be clarified in order to elucidate the dynamics of polymer ions in more detail. Until now, Masuhara et aL2-*and Lachish et a1.9310have found that the spectral shape of polymer cations is broad compared to that of the monomer cation, suggesting a new electronic structure characteristic of the polymer cation. Namely, there is a possibility that the hole or electron is delocalized over a number of chromophores, although no report has been given concerning this problem as far as we know. From this viewpoint, absorption spectra of radical ions of poly-(N-vinylcarbazole) with different degrees of polymerization (from 4 to 1 loo), poly[2-(N-carbazo1yl)ethyl vinyl ether], and polyurethanes having a 1,2-trans-dicarbazolylcyclobutane group have been studied in the present work. Experimental Section Molecular structures and abbreviations of polymers and their reference compounds are given in Figure 1. These are the same as used bef~re.'I-'~The molecular weight and number average were estimated by gel-permeation chromatography (GPC) calibrated for polystyrene. A Toyo Soda HLC-802 UR liquid chromatograph was used and the solvent was tetrahydrofuran. Present address: Department of Polymer Science and Engineering, Kyoto Institute of Technology, Matsugasaki, Kyoto 606, Japan.
0022-3654/84/2088-397 1$01.50/0
The ratio of weight-average to number-average weights of PVCz-4, -17, -42, -84, -230, -400, and -1100 are 1.40, 1.99, 1.36, 1.22, 1.31, 5.40, and 1.78, respectively. N,N-Dimethylformamide (DMF) (Dotite Spectrosol) was purified by fractional distillation under reduced N 2 pressure. DMTP (Nakarai GR) was recrystallized four times from ethanol. 1,4-Diazabicycl0[2.2.2]octane (Dabco, Tokyo Kasei) was purified by repeated recrystallization from the mixed solvent of acetone and n-hexane. These quencher molecules are not excited by a N2 gas laser. The transient absorption spectra were measured by adjusting the ground-state absorbance at 337 nm to 1.4 or 1.7 at a path length of 1 cm. This corresponds to an ca. 5 X loy4M carbazole unit. The concentrations of DMTP and Dabco were set to 0.25 and 1.0 M, respectively, so that more than 97% of the fluorescence was quenched. The transient absorption spectral measurements were performed by using a microcomputer-controlled N2 gas laser photolysis system.14 The spectra were obtained by averaging over 20 shots. (1) (a) H. Masuhara and N. Mataga, Acc. Chem. Res., 14, 312 (1981), and papers cited therein. (b) N. Mataga, Radiat. Phys. Chem., 21, 83 (1983); Y. Hirata, Y. Kanda, and N. Mataga, J . Phys. Chem., 87, 1659 (1983). (2) H. Masuhara, S. Ohwada, N. Mataga, A. Itaya, K. Okamoto, and S . Kusabayashi, Chem. Phys. Left.,59, 188 (1978). (3) H. Masuhara, S.Ohwada, N. Mataga, A. Itaya, K. Okamoto, and S . Kusabayashi, Kobunshi Ronbunshu, 31, 275 (1980). (4) H. Masuhara, S. Ohwada, N. Mataga, A. Itaya, K. Okamoto, and S . Kusabayashi, J . Phys. Chem., 84, 2363 (1980). (5) H. Masuhara, S. Ohwada, K. Yamamoto, N. Mataga, A. Itaya, K. Okamoto, and S . Kusabayashi, Chem. Phys. Lett., 70, 276 (1980). (6) H. Masuhara, H. Shioyama, N. Mataga, T. Inoue, N. Kitamura, T. Tanabe, and S . Tazuke, Macromolecules, 14, 1738 (1981). (7) H. Masuhara and N. Mataga, J. Lumin. 24/25, 511 (1981): Invited paper for the International Conference on Luminescence, West Berlin, July 20-24, 1981. (8) H. Masuhara, K. Yamamoto, N. Tamai, K. Inoue, and N. Mataga, unpublished result. (9) U. Lachish, D. J. Williams, and R. W. Anderson, Chem. Phys. Lett., 65, 574 (1979). (10) U. Lachish, R. W. Anderson, and D. J. Williams, Macromolecules, 13, 1143 (1980). (11) K. Okamoto, M. Ozeki, A. Itaya, S. Kusabayashi, and H. Mikawa, Bull. Chem. SOC.Jpn., 48, 1362 (1975). (12) A. Itaya, K. Okamoto, and S . Kusabayashi, Bull. Chem. SOC.Jpn., 49, 2082 (1976). (13) S. Tazuke, T. Inoue, T. Tanabe, S. Hirota, and S . Saito, J . Polym. Sci., Polym. Lett. Ed., 19, 11 (1981). (14) S. Yasoshima, H. Masuhara, N. Mataga, H. Suzaki, T. Uchida, and S . Minami, J . Spectrosc. Sot. Jpn., 30, 93 (1981).
0 1984 American Chemical Society
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Masuhara et al.
The Journal of Physical Chemistry, Vol. 88, No. 18, 1984 700 y
EtCz
0.1
5
a3
F I B
wCH20CNHXNHCjr Pu-f-n X=-(CH2)6-
4
(a)
0.1
DCzB
tOCH2@Q
800 nm
PU-11-n
rn 4
0
2
0
g d
=' -CH2&HZ-
Figure 1. Molecular structures and abbreviations for the polymers and their reference compounds. Notation n is the mean degree of polymerization.
4
0.1
700
800 nm
Figure 3. Absorption spectra of the polymer cation in DMF at 700 ns after excitation: (a) PVCz-4 + DMTP system; (b) PVCz-42 + DMTP system; (c) PVCz-1100 + DMTP system. 0.2
8OOnm
01
C
e
%
n
Q
0.2
(C)
0.1 f
700
(b)
000n m
Figure 2. Absorption spectra of cation of the reference compounds in DMF (a) EtCz + DMTP system at 600 ns after excitation; (b) DCzP + DMTP system at 600 ns; (c) DCzB + DMTP system at 400 ns.
0.1
A normal excitation intensity was ca. 2 X 10l6photons cm-2. The time resolution was 300 ns which was determined by a transient memory Kawasaki Electronica M-50E. Results and Discussion Absorption Spectra of Polymer Cations. Electron-transfer quenching of the carbazole chromophore in the fluorescent state by DMTP leads to the prompt formation of a carbazole cation and a DMTP anion in polar solvents.*-* This ionic photodissociation process was confirmed not only for the model compounds but also for the polymers. The transient absorption spectrum in the wavelength region shorter than 600 nm is common to all systems investigated here and identical with that of the DMTP anion. On the other hand, the spectrum in the longer wavelength region is ascribed to cations of the carbazolyl chromophore. The spectra of the model compounds are shown in Figure 2. The EtCz cation has a peak at 780 nm and a shoulder at 700 nm, which is similar to the spectrum measured with a y-irradiation method.15 In the case of DCzP, this spectral shape is slightly modified and (15) S. Tagawa, S. Arai, M . Imamura, Y. Tabata, and K. Oshima, Macromolecules, 7, 262 (1974); A. Kira, private communication.
7
700
800nm
Figure 4. Absorption spectra of the polymer cation in DMF: (a) PCzEVE-33 + DMTP system at 300 ns after excitation;(b) PU-1-44 + DMTP system at 300 ns; (c) PU-11-25 + DMTP system at 300 ns. a small blue shift is observed. The intensity of this band is almost the same as that of the EtCz cation. The spectrum of the DCzB cation is broad and structureless, although its peak position is the same as that of the EtCz cation. Some absorption spectra of the PVCz-n cations are given in Figure 3. The absorption peak is at ca. 770 nm and no shoulder was detected near 700 nm. Their peak positions are near that of DCzP, while these bands are broader than that of the latter. It is worth noting that this characteristic is common to all PVCz cations with different n values and holds even for the PVCz-4 system. The spectra of the PCzEVE-33, PU-1-44, and PU-11-25 cations are given in Figure 4. They are very broad and show a
Radical Ions of Polymer with Carbazolyl Chromophores
400
The Journal of Physical Chemistry, Vol. 88, No. 18, 1984 3973
500 nm 1
A
B
700
I :
I 400
500 nm
Figure 5. Absorption spectra of the polymer anion in DMF at 2 ps after excitation: (a) PVCz-42 + Dabco system; (b) PVCz-1100 + Dabco system; ( c ) PCzEVE-33 + Dabco system.
peak at 780 nm, which is different from the spectrum of the PVCz-n cation but similar to that of the DCzB cation. Absorption Spectra of Polymer Anions. Electron transfer from quencher to carbazole occurs in systems containing Dabco, which results in the formation of the carbazole anion and the Dabco cation. The absorption band of the Dabco cation extends from 400 to 600 nm, but the molar extinction coefficient is small even at its maximum wavelength (ca. 1000 M-’ cm-I). Since the maximum concentration of the ion radicals produced is estimated to be less than - 5 X low5M, the maximum absorbance due to the Dabco cation is 0.025 under the laser photolysis condition of a path length of 0.5 cm. Therefore, its contribution to the present transient absorption spectra is considered to be negligible, and the observed spectrum can be ascribed to the carbazole anion. The measured spectrum is common to all model compounds and polymers examined here, and some examples are shown in Figure 5. These are identical with the reference absorption spectrum of the carbazole monomer anion. Absorption Spectra and Geometrical Structure of the Reference Dimer Ions. Absorption spectra of the carbazole cation differ from one another, indicating that the intramolecular interaction between the cationic and neutral chromophores is very sensitive to their relative geometrical structure. The absorption spectrum of the DCzB cation is broad and structureless and is different from that of the EtCz cation. Since the two carbazolyl groups of DCzB are in a form with a very small interchromophore overlap, a long-range electrostatic interaction between both ?r-electronic systems or molecular distortion characteristic of the cationic state may be responsible for the present spectrum. On the other hand, the shape of the absorption band of the DCzP cation is almost similar to that of the EtCz cation except for a blue shift of the peak position. Although extended and folded conformations should be considered in this case, the contribution of the EtCz cation band at 780 nm was hardly detected. Therefore, this spectrum may be due to a sandwich structure, namely, the hole is considered to be delocalized over two chromophores with parallel geometry. This consideration is supported and supplemented by our recent studies on meso-(dl)-2,4-di(N-carbazolyl)pentane (m-DCzPe) and ruc-(dd,ll)-2,4-di(N-carbazolyl)pentane (r-DCzPe) cations whose spectra are cited Figure 6.16 The shape of the absorption band of the m-DCzPe cation is like that of the DCzP and EtCz cations, and the peak position shifts in the order of EtCz > m-DCzPe > (16) H. Masuhara, N. Tamai, N. Mataga, F. C. De Schryver, and J. Vandendriessche, J . Am. Chem. SOC.,105, 7256 (1983).
800 nm Figure 6. (A) Reference absorption spectra of r-DCzPe (a) and mDCzPe (b) cations. See ref lb. (B) Measured absorption spectrum of PVCz-42 cation (O), the simulated one (--) with r-DCzPe and mDCzPe cations, and simulated one (. . .) with r-DCzPe and DCzB cations. DCzP by about 10 nm. Since only the sandwich structure is considered to be energetically possible as a dimer form in mDCzPe, this structure seems to hardly modify the spectral shape of the EtCz cation. An interesting contrast was given by the r-DCzPe cation in which a geometrical structure with a partial overlap between two carbazolyl groups is energetically probable. In this case, the absorption band is affected to a great extent and its peak is observed at about 710 nm. Three kinds of carbazole dimer cation are summarized here, and we propose naming these dimer cations as follows according to the nomenclature of the carbazole sandwich and second ex~ i m e r s . ’ * ~ ’The ~ ~ ~m-DCzPe * and DCzP cations with the sandwich structure are denoted as sandwich dimer cations, while the rDCzPe cation which may have a geometrical structure with a partial overlap between two chromophores is the “second dimer cation”. The DCzB cation whose absorption spectrum is different from that of the above two dimer cations is named as the “third dimer cation”. While the spectrum of the dimer cation is sensitive to the relative geometrical position of the two carbazolyl groups, the absorption spectrum of the dicarbazolyl anion is the same as that of the EtCz anion, This indicates that no interaction is effective between the anionic and neutral chromophores. Electronic Structure of Polymer Ions. The absorption spectra of PCzEVE-33, PU-1-44, and PU-11-25 cations are similar to that of the DCzB cation and not to that of the m-DCzPe and r-DCzPe cations. This indicates that the hole is stabilized in one DCzB unit as the third dimer cation and that no conformation giving the other dimer cations, as a consequence of an interaction between two DCzB units, is possible. The absorption spectrum of the PVCz cation is broad and structureless and almost independent of the n value. This spectral shape is different from that of all monomer and dimer model compounds and characteristic of the polymer. One possible explanation is that the hole is delocalized over a number of chromophores or trapped in a site produced only in polymers with high n values. This idea was substantiated by the fact that the S , Soabsorption and fluorescence spectra of these polymers in T H F change from PVCz-4 via PVCz-17 to PVCz-42, while no further change was detected by increasing the n value to PVCz-1 +-
(17) G. E. Johnson, J . Chem. Phys., 62, 4597 (1975). (18) F. C. De Schryver, J. Vandendriessche, S . Toppet, K. De Meyer, and N. Boens, Macromolecules, 15, 406 (1982).
(19) H. Masuhara, N. Tamai, and N. Mataga, to be submitted for publication.
3974
J. Phys. Chem. 1984, 88, 3974-3976
Namely, a degree of polymerization of about 40 was deemed to be a minimum unit for leading to the electronic structure characteristic of the neutral PVCz. However, the present cation absorption spectrum is common to all PVCz-n systems including PVCz-4, which indicates that the hole in the polymer is interacting with no more than a few chromophores. Since there may be configurational and conformational distributions even for PVCz-4, another and more probable explanation is based on the assumption that the positive charge is localized in a dimer cation. This means that the measured spectrum is a superposition of absorption bands of the different dimer cations summarized above, since the spectrum is not identical with that of the sandwich, the second, or the third dimer cation. Although the sandwich and second dimers are responsible for excimer emissions of P V C Z , ~ ~the , ” spectrum of their cations cannot be completely reproduced by the overlap of the bands of the m-DCzPe and r-DCzPe cations. One example of good fitting cases is given in Figure 6, suggesting that a component with absorption above 800 nm should be added. This may be realized only by considering the contribution of the DCzB cation, but the combination of the latter cation with the r-DCzPe or the m-DCzPe cation does not give the simulated spectrum similar to the measured one. Therefore, it is considered that three different dimer cations are involved and their statistical average constitutes the spectrum of the polymer cation. The formation of sandwich and second dimer sites is characteristic of PVCz and has been confirmed by fluorescence and N M R data,’2Js while the importance of another geometrical structure of two carbazolyl groups whose model is DCzB is demonstrated here for the first time.
In contrast with the above results on polymer cations, anion radicals of the present polymer and model compounds give the same absorption spectrum as that of the monomer anion. The electron transferred to these compounds is stabilized as a monomer anion and no interaction between the anion and the neutral chromophore was observed even in the polymer.
Summary On the basis of transient absorption spectra, it is concluded that the hole (or electron) in the polymer is trapped as some dimer cations (or monomer anion) and not delocalized over chromophores. The statistical average contribution of the three kinds of dimer cation determines the absorption spectrum of polymer cations which is ascribed to the configurational and conformational structures of polymers in solution.
Acknowledgment. PVCz-n, PCzEVE-33, and DCzP were kindly supplied by Profs. A. Itaya, K. Okamoto, and S. Kusabayashi (Yamaguchi University). Thanks are due to Prof. S. Tazuke and Dr. T. Inoue (Tokyo Institute of Technology) for their gift of PU-1-44, PU-11-25, and DCzB. H.M. thanks from the Japanese Ministry of Education, Science and Culture for the Grant-in-Aid (454125). Registry No. PVCz-n,25067-59-8; PCzEVE-n,53807-87-7; PU-I-n, 77090-85-8;PU-11-n,77090-86-9:EtCz, 86-28-2;DCzP, 25837-66-5: DCzB, 1484-96-4;(rrans-9,9’-( 1,2-cyclobutanediyl)dicarbazole-3,3’-dimethanol).(hexamethyleneisocyanate) (copolymer), 771 10-00-0: (trans9,9’-(1,2-cyclobutanediyl)dicarbazole-3,3’-dimethanol)~(m-phenylenedimethyleneisocyanate) (copolymer),79675-91-5.
Picosecond Kinetlcs by Exchange Broadening In the Infrared and Raman. 3. CH,CN*IBr Benyamin Cohen and Shmuel Weiss* Department of Chemistry, Ben-Gurion University, Beer-Sheva 841 05, Israel (Received: September 6, 1983; In Final Form: January 23, 1984)
Spectra of the v2 and v3 + u4 bands of CH3CN in the presence of IBr and in CC14solution were measured over a range of temperatures. The spectra could be analyzed to reveal the kinetics of the CH$N + IBr CH,CN-IBr reaction, in the picosecond range. Association and dissociation rate constants were determined and, from them, activation energies and entropies.
Some time ago we reported on the reaction between IBr and benzene in an inert solvent.’ The kinetics of that reaction became apparent through its effect on the line shape of the IBr vibrational band. At low temperatures two bands, one corresponding to the vibration of IBr bound to benzene (forming a charge transfer complex) and one corresponding to free IBr, are observed. As the temperature is raised these two bands broaden and coalesce in much the same way as that observed for exchange reactions in N M R and ESR. In this way it is possible to study reactions in the picosecond range (1 cm-I corresponds to 5 X s) for ground-state molecules by a simple technique. The spectral region in which the IBr vibration occurs, 230-280 cm-I, is, however, an inconvenient one. The benzene spectrum, on the other hand, is too rich for a convenient, reasonably wellseparated band to be found that will lend itself to a kinetic study. The simplest solution, therefore, to the problem of finding a suitable test case for the study of charge transfer complex association-dissociation kinetics was to change the donor. It was found that acetonitrile forms a complex with IBr the association-dis(1) B. Cohen and S. Weiss, J . Chem. Phys., 72,6804 (1980).
0022-3654/84/2088-3974$01.50/0
sociation kinetics of which are in the picosecond range and that the spectrum of this molecule includes bands suitable for our purpose and in a convenient spectral range. Spectra were taken on a Perkin-Elmer 225 infrared spectrometer with a resolution of 0.6-0.7 cm-I. Samples were contained in a RIIC variable-temperature cell. The temperature range in this study was -5 to +40 ‘C. Acetonitrile was purified by reflux over P205,treatment with NaHCO,, and subsequent distillation. Acetonitrile and IBr were dissolved in CC14 at concentrations of 0.1 to 0.4 M for IBr and 0.1 to 0.2 M for CH,CN. Fresh solutions were always used to avoid formation of products resulting from slow reactions between the reactants. The optical path was 1 mm. Results are reported as absorbances. Spectra of the u2 and u3 + v4 bands of acetonitrile in CCl, solution and in the presence of IBr are reproduced in Figures 1 and 2. The v2 band at 2253 cm-’ was chosen for study, because it involves the C N vibration which should be affected the most by complexation, since it is the C N group which is responsible for the formation of the complex with IBr. The v, + v4 combination band at 2289 cm-’ contains the C-C stretch v4 (v, is a CH, deformation) which should also be affected by complexation and 0 1984 American Chemical Society
