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J. Phys. Chem. 1983, 87,1680-1682
Monophotonic Ionization through a Long-Lived Ion Pair: N,N,N',N'-Tetramethyl-p-phenylenediamine in Acetonitrile Solution Yoshlnori Hlrata" and Noboru Mataga' Department of Chemistry, Faculty of €ngineering Science, Osaka University, Toyonaka, Osaka 560, Japan (Received: March 2, 1983)
The monophotonic ionization of N,N,",N'-tetramethyl-p-phenylenediamine (TMPD) has been studied by means of transient absorption and transient photoconductivity measurements in acetonitrile solution. It has been concluded that the photoionization of TMPD is similar to that of 2,7-bis(dimethylamino)-4,5,9,1O-tetrahydropyrene in acetonitrile solution. The formation and decay of the bound ion-pair state which is formed from the fluorescent state of TMPD and gives a quite similar absorption spectrum to the dissociated TMPD cation radical has been observed directly. The decay of the bound ion-pair state has been found to obey the reciprocal square root time dependence in the submicro to tens of microsecond time region.
Introduction Recently we demonstrated the importance of the solvent-solute exciplex interaction in the ionization process of some aromatic diamines with low oxidation potentials in acetonitrile (ACN) solution excited with near-UV photons. In the system of 2,7-bis(dimethylamin0)-4,5,9,10tetrahydropyrene (BDATP) in acetonitrile solution, the ion pair with solvent, BDATP+...(ACN)2-, which is formed from the fluorescent state of BDATP (rf= 2.3 ns), dissociates into separate ions and gives photoconductivity with a rise time of 9 ns.' On the other hand, N,N,N',N'tetramethylbenzidine (TMB) shows a quite slow rise in the photocurrent (4.6 f 0.5 ps) which agrees well with the lifetime of the fast decay component (4.2 ps) of the TMB cation absorbance.2 In this case, the observed ion pair is considered to be TMB+-.(ACN);, where (ACN),- is an anion radical with higher degrees of aggregation.'V2 It is highly probable that ion pairs are formed as an intermediate in the photoionization process of aromatic amines in acetonitrile solution, since a slow rise in the photoconductivity is observed also in some other system^.^ Photoionization through an ion pair with a rather long lifetime does not seem to be a special phenomenon but a rather general one in acetonitrile solution. N,NJV'JV'-Tetramethyl-p-phenylenediamine (TMPD) has a low oxidation potential of 0.16 V vs. SCE4 and can be ionized by irradiation with rather low-energy photons. Extensive studies concerned with the photoionization of TMPD both in polar and nonpolar solvents have been performed by many researcher^."^ In alcoholic solvents, TMPD is ionized biphotonically with excitation at 347 nm and the second-photon-absorbing state which is populated from the Franck-Condon state of the system is considered to be the "semi-ionized" state.5 However, a detailed physical picture of the "semi-ionized" state has never been documented yet, although some authors explained it as a charge transfer to solvent (CTTS) state.8 (1) Hirata, Y.; Mataga, N.; Sakata, Y.; Misumi, S. J.Phys. Chem. 1982, 86, 1508; Ibid., in press. (2) Hirata, Y.; Takimoto, M.; Mataga, N. Chem. Phys. Lett., in press. (3) Nogami, T.; Mizuhara, T.; Kobayashi, N.; Aoki, M.; Akashi, T.; Shirota, Y.; Mikawa, H.; Sumitani, M. Bull. Chem. SOC. Jpn. 1981,54,
1559. Hirata, Y.; Mataga, N. unpublished results. (4) Rehm, D.; Weller, A. 2. Phys. Chem. (Frankfurt am Main) 1970, 6'9, 183. (5) Potashnik, R.; Ottolenghi, M.; Bensasson, R. J. Phys. Chem. 1969, 73, 1912. (6) Richards, J. T.; Thomas, J. K. Trans. Faraday SOC.1970,66, 621. (7) Choi. H. T.; Sethi, D. S.; Braun, C. L. J . Chem. Phys. 1982, 77,
6027. ( 8 ) Ottolenghi, M. C h ~ mPhys. . Lett. 1971, 12, 339
0022-365418312087-1680$01 SO10
In this work, we have investigated the monophotonic ionization of TMPD in acetonitrile solution by using picosecond and microsecond transient absorption spectroscopy as well as picosecond laser-induced transient photoconductivity measurements. The results of this study shows clearly that the photoionization mechanism of TMPD is rather similar to that of BDATP in acetonitrile solution. TMPD ionizes from its fluorescent state ( T ~= 1.2 ns) and produces a bound ion pair, the absorption spectrum of which cannot be distinguished from that of the dissociated TMPD radical cation. The decay of the ion pair has been measured for the time range from submicrosecond to tens of microsecond by observing the absorbance of the TMPD cation band which decreases proportionally to reciprocal square root of time, t-'iz. Such a time dependence of the decay curve is well-known for geminate ion-pair rec~mbination.~
Experimental Section Picosecond transient absorption spectra were measured by using the mode-locked Nd3+:YAGlaser photolysis and spectroscopy system, the details of which are given elsewhere.'O An N2 laser photolysis system1' was used for the microsecond transient absorption spectral measurements. Fluorescence lifetimes were determined by exciting the sample with the second harmonic of the mode-locked ruby laser and by observing the decay curve with a high-speed multichannel plate photomultiplier (HTV R-l194UX)-fast storage oscilloscope (Tektronix 7834-7A19-7B80)combination. The experimental setup for transient photoconductivity measurements was similar to the one reported before' and a fast preamplifier (HP 8447D) connected to a fast storage oscilloscope was used. TMPD was obtained from its dihydrochloride (GR grade) by dissolving in water and precipitating out free amine with addition of ammonia. The precipitate was purified by recrystallization from n-hexane and subsequent sublimation in vacuo. GR-grade acetonitrile was refluxed over calcium hydride and carefully distilled. The samples were prepared in quartz cells of 1-cm optical path and the absorbance of samples at the excitation wavelength was adjusted to 1.0-2.0 and 0.7-1.2 for the absorption and photocurrent measurements, respectively. (9) Warman, J. M.; Infelta, P. P.; de Haas, M. P.; Hummel, A. Can. J. Chem. 1977,55,2249. Tagawa, S.; Tabata, Y.; Kobayashi. H.; Washio, M. Radiat. Phys. Chem. 1982, 19, 193. (10) Masuhara, H.; Ikeda, N.; Miyasaka, H.; Mataga, N. J. Spectrosc. Soc. J p n . 1982, 31, 19. (11) Yasoshima, S.; Masuhara, H.; Mataga, N.; Suzaki, H.; Uchida, T.; Minami, S. J. Spectrosc. Soc. J p n . 1981, 30, 93.
0 1983 American Chemical Society
The Journal of Physical Chemistry, Vol. 87, No. 10, 1983
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Flgure 2. Microsecond transient absorption spectra of TMPD in acetonitrile solution. Delay times are shown in the figure.
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Figure 1. Time-resolved absorption spectra of TMPD in acetonitrile solution. Delay times after the excitation pulse are indicated in the figure. The broken line at 33 ps shows the S, S, absorption spectrum in isooctane solution. Spectrum a is obtained by exciting the sample with an attenuated laser pulse.
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All samples were degassed by repeated freezepumpthaw cycles and all measurements were performed a t room temperature.
Results and Discussion Figure 1shows the picosecond time-resolved absorption spectra of TMPD in acetonitrile solution. A broad absorption spectrum extending over the whole spectral range of 400-760 nm is observed 33 ps after the excitation pulse. The shape of the spectrum except for the weak structure a t 550-650 nm is similar to that measured in an isooctane solution shown by the broken line in Figure 1. The absorption maxima observed at 570 and 620 nm are in good agreement with those of the well-known TMPD cation radical, Wurster’s blue.12 When the excitation intensity was reduced (Figure la), the peaks a t 33 ps almost disappeared. Therefore they are due to the TMPD cation radical formed by biphotonic ionization. With an increase in the delay time, the broad absorption decreases in intensity and is gradually replaced by a structured absorption band with peaks a t 570 and 620 nm. The lifetime of the broad band measured a t 750 nm is 1.3 ns and agrees well with the fluorescence lifetime of TMPD, 1.2 ns in acetonitrile solution. Therefore, the broad absorption spectrum observed at shorter delay times can be assigned unambiguously to the S, S1transition of TMPD, while the structured absorption spectrum is very similar to that of the TMPD cation. The latter spectrum can be assigned to the TMPD cation produced from the fluorescence state of TMPD just as in the case of BDATP in acetonitri1e.l Although it is known that the T, T1 absorption band of TMPD also appears around 600 nm,I3 we have confirmed that the triplet yield of TMPD in acetonitrile solution is very low so that the contribution of the T, TI absorption in 500-650-nm region can be neglected. T, absorption of Namely, only the very weak T,
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(12) Albrecht, A. C.Acc. Chem. Res. 1970,3, 238. (13) Labhart, H.; Heinzelman, W. “Organic Molecular Photophysics”; Birks, J. B., Ed.; Wiley-Interscience: London, 1973; Vol. 1, p 330.
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Figure 3. (a) The time dependence of the TMPD cation absorbance in acetonitrile solution measured at 615 nm. (b) A plot of the absorbance vs. t-’’2.
naphthalene which act as a triplet energy acceptor for TMPD, is observed in the microsecond transient absorption spectrum of the TMPD/naphthalene/ACN system excited with a 337-nm nanosecond pulse. Figure 2 shows the transient absorption spectra of TMPD in acetonitrile solution at several delay times. The absorption spectrum which is assigned to the TMPD cation reduces its intensity with increasing delay time, while the spectral shape does not change. Figure 3a shows the decay curve for the absorbance of the TMPD cation measured at 610 nm in acetonitrile solution. A plot of the absorbance against t-1/2is also shown in Figure 3b, which gives a good linear relation. The decay curve does not obey the homogeneous recombination kinetics of ions. The decay of the cation or anion radical reciprocally proportional to t1/2is known in several systems studied by pulse radiolysis and is explained by geminate ion-pair rec~mbination.~ Although geminate ion-pair recombination is considered to be a rapid process in polar solvents because of short Onsager length, we have found here quite long-lived ion pairs in acetonitrile solution. We have confirmed also that the ion-pair yield and the decay curve are not affected by air saturation of the sample solution. Therefore, the ion pairs do not react with or are not scavenged by dissolved oxygen. The dependence of the absorbance of the TMPD cation upon the excitation intensity was approximately first order, which indicates that the ion pair is formed mainly via a monophotonic process. On the other hand, the photoconductivity shows a 1.5-order dependence on the excita-
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The Journal of Physical chemistry, Vol. 87, No. 10, 1983
tion intensity, which suggests the formation of a dissociated ion due to the biphotonic proces~.'~The observed rise time of the photocurrent was limited by the response time of the apparatus (about 2 ns). As reported in a previous paper,l the decay curve of the photocurrent measured for TMPD in acetonitrile solution consists of a faster component due to the reaction, (ACNI2- (ACN);, and slow and complex component due to recombination of ions. However, the slow rise in the photoconductivity associated with the decay of the ion pair was not observed. The slow rise component of the photoconductivity seems to be masked by the superposing decay component in the same time region due to biphotonic ionization. By eliminating the biphotonic ionization with reduced excitation intensity, we can observe the slow rise of the photoconductivity, since the straight line in Figure 3b has nonzero intercept which means that the dissociation yield from the geminate pair is not zero. The yield @(t)of the ion pairs which have not yet recombined with each other at time t is given by15
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% = exp(-rc/ro) where afjis the yield of dissociated ions; rc is the Onsager length; ro is the initial separation between the cation and anion; D is the sum of the diffusion coefficients of the cation and anion. The value of rc/(7rD)'/2 can be deduced from the ratio of the slope and intercept of the straight line in Figure 3b. The Onsager length rc is given by rc = e2/(ckr) (2) where 6 is the dielectric constant of solvent and r, = 1.5 x m for acetonitrile. Using these values, we have obtained a quite small value of 8.0 X cmz/s for D. However, since the mobility of (ACN),- is similar to that of the biphenyl anion radical in acetonitrile,' this D value is almost five orders of magnitude smaller than expected. This discrepancy suggests that the observed recombination is due to a slower partially diffusion-controlled process, because eq 1 is derived by assuming the usual diffusion control. In other word, the reactivity of recombination at the encounter collision of the present ion pair is small. Even if the reactivity a t the encounter collision is very small, it can be shown16that the time dependence of the (14) The apparent 1.5-order dependence of the photoconductivity upon the excitation intensity at 347 nm can be most probably ascribed to the inner-filter effect due to the absorption of exciting light by the excited TMPD molecules as well as the produced cations. (15) Sano, H.; Tachiya, M. J. Chem. Phys. 1979, 71,1276. (16) Tachiya, M. 'Abstracts of International Symposium on the Fast Processes in Radiation Chemistry"; Tokai-mura: Japan, 1982; p 42, Int. J. Radiat. Phys. Chem., in press.
Letters
survival probability of the ion pair is proportional to t-lj2 except that the proportionality coefficient is a function of a parameter representing the reactivity. On the basis of the above results, some discussions will be given in the following on the nature of the observed ion pair. Although the dominant ion pair present in the microsecond time region is TMPD+-(ACN),-, TMPD+-. (ACN12- should be formed in the early stage of the photoionization from the relaxed S1 state in view of our previous results on the slow ionization of BDATP.' TMPD+-.(ACN)2-may be converted to TMPD+.-(ACN); within tens of nanoseconds without dissociation into free ions.' The geminate ion-pair TMPD+-s(ACN)~may be formed at first also in the biphotonic ionization. Contrary to the case of monophotonic ionization where no rapid rise in the photoconductivity was observed, its rise time for biphotonic ionization was too short to be measured. This means that recombination as well as dissociation into free ions in the geminate pair is very rapid in the case of biphotonic ionization. Accordingly, the nature of the initial ion pair, TMPD+-s(ACN)~-, in monophotonic ionization from the relaxed S1 state seems to be quite different from that in biphotonic ionization. It is possible that the initial separation ro between the cation and anion is larger in biphotonic ionization than in monophotonic ionization. However, according to eq 1, a similar decay for the geminate ion pair should be observed in both cases, although the dissociated ion yield may be higher in the case of biphotonic ionization. This prediction does not agree with experimental results. The above results indicate strongly that the ion pair produced from the relaxed S1 state of TMPD is not the so-called geminate ion pair but the bound ion pair with a definite structure formed by a solvent-solute exciplex type of interaction. This bound ion pair does not show any rapid deactivation to the ground state but only a slow dissociation to free ions via the geminate ion pair. In such a case, it is possible to show that the ion pair decays according to t-'i2.l6 It should be noted here that the bound ion pair observed in the present work can act as the intermediate state for biphotonic ionization of TMPD when excited by a light pulse with such a large width as conventional flash or giant pulse laser. The "semi-ionized'' state may be most probably identified as the bound ion pair established in this work. More detailed picosecond spectroscopy and faster response photoconductivity studies on the photoionization processes of aromatic diamines are now going on in this laboratory. Acknowledgment. This work was supported in part by a Grant-in-Aid for Special Project Research on Photobiology from the Japanese Ministry of Education, Science and Culture to N.M.
