J. Phys. Chem. 1983, 87, 1659-1662
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Picosecond Laser Photolysis and Transient Photocurrent Studies of the Ionic Dissociation Mechanism of Heteroexclmers: Pyrene-N,N-Dimethylaniline and Pyrene-p -Dicyanobenzene Systems in Polar Solvents Yoshlnori Hlrata, Yu Kanda, and Noboru Mataga' Department of Chemistty, FacuW of Englneering Science, Osaka University, Toyonaka, Osaka 560, Japan (Received: January 25, 1983)
The ionic photodissociationmechanism of pyrene-N,N-dimethylanilineand pyrene-p-dicyanobenzene systems in various polar solvents has been studied by picosecond laser photolysis and transient photoconductivity measurements. The ionic dissociation process from the geminate ion pair and its solvent dependence have been directly observed with photocurrent measurements for the first time. On the basis of these results, a model for the ionic photodissociation of typical heteroexcimer systems has been proposed, which is in agreement also with some results of recent studies of the magnetic field effect upon photochemically produced geminate pairs.
Introduction It is well-known that charge transfer (CT) in the excited state and the heteroexcimer formation processes are affected strongly by the solvent polarity and, moreover, not only the heteroexcimer formation process itself but also the electronic and geometrical structures of heteroexcimers seem to be controlled by the solvent polarity. In a medium of considerable polarity, formation of solvated ion radicals becomes important as a nonradiative deactivation path of some typical heteroexcimer systems as well as excited CT complexes because of their strongly polar nature. Details of the mechanism of this process are important as a basis for elucidating photochemical and photobiological primary processes. In fluorescence studies of heteroexcimers, it was recognized that, upon increasing solvent polarity, both the fluorescence quantum yield and the decay time of typical heteroexcimers such as pyrene-DMA (N,N-dimethylaniline) and anthracene-DEA (N,N-diethylaniline) decrease, with fluorescence yield decreasing more rapidly with increasing solvent polarity than does the corresponding fluorescence decay time.lV2 The following interpretations were deemed possible for these observations. (a) The heteroexcimer state may be considered as a resonance hybrid of the CT configuration (A-D+) mixed with the LE (locally excited) configuration (A*D) or (AD*). The solvation of the heteroexcimer by polar solvents will oppose the electronic delocalization between A and D since the solvation energy increases with increasing charge separation. Moreover, the solvation may induce more or less the change of the heteroexcimer geometrical structure leading to a further decrease of the electronic delocalization interaction between A and D. Therefore, the heteroexcimer electronic structure may become more polar with increasing polarity of the solvent and this would lead to a decrease in the radiative transition probability. The radiationless transition probability, however, will increase with solvent polarity because the energy gap between relevant electronic states becomes smaller in a more polar solvent, and, furthermore, because of the formation of nonfluorescent solvated ion pairs in polar solvents.' (b) There is competition between the formation processes of the nonfluorescent solvated ion pair, (&-.-D,+), (1) Mataga, N.; Okada, T.; Yamamoto, N. Chem. Phys. Lett. 1967,1, 119. (2) Knibbe, H.; Rollig, K.; Schiifer, F. P.; Weller, A. J . Chem. Phys. 1967,47, 1184. 0022-3654103l2007-I 659$01.50/0
and the fluorescent heteroexcimer or relaxed fluorescent state, (A-D+)*,in the encounter complex. It is assumed (A-D+l,"
(A****D)
I*i
k ( A ,+ -**D, )
A
+
As-
:0
that kip and ki are increased in polar solvents while kc is not, resulting in a larger decrease in the fluorescence yield than the fluorescence lifetime.2 According to interpretation a, it is possible that there exist various heteroexcimer states which differ in energy as well as in structure depending upon the solvation, and a statistical average of the system distributed over these states may be observed in a polar solution. By the same token, there may be various solvated ion-pair states. There would be rapid interconversions between heteroexcimers with different solvations and different structures (A-D+),* e (A-D+),* 2
...
There would be also interconversions between solvated ion pairs with different structures, Le., various solvent-shared ion pairs or geminate ion pairs
Exchange between the heteroexcimer and the solventshared ion pairs would be also possible. It is very difficult in either case to discriminate between various heteroexcimers, solvated ion pairs, and dissociated ions in polar solvents by means of transient absorption spectroscopy because all these species show spectra very similar to those of ion radicals. On the other hand, laser-induced transient photoconductivity measurement has the advantage that it can discriminate between solventshared or geminate ion pairs and dissociated ions, and it has a much higher sensitivity to detect charged species than the transient absorption measurement, as was demonstrated in our previous work^.^-^ However, the time (3) Taniguchi, Y.; Nishina, Y.; Mataga, N. Bull. Chem. SOC.Jpn. 1972, 45, 764.
(4)Taniguchi, Y.; Mataga, N. Chem. Phys. Lett. 1972,13, 596. (5) Masuhara, H.; Shimada, M.; Tsujino, N.; Mataga, N. Bull. Chem. SOC.Jpn. 1971,44, 3310. (6)M.asuhara, H.;Hino, T.; Mataga, N. J. Phys. Chem. 1975, 79,994. (7)Hino, T.; Akazawa, H.; Masuhara, H.; Mataga, N. J . Phys. Chem. 1976, 80,33.
0 1983 American Chemical Soclety
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The Journal of Physical Chemistry, Vol. 87,No. 10, 1983
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TABLE I: Photocurrent Rise Times and Heteroexcimer Fluoroescence Decay Times of the Pyrene-DMA System in Various Solvents solvent
37.5 (20 "C) 20.7 ( 2 5 "C)
methyl e t h y l ketone pyridine
1 8 . 5 ( 2 0 "C) 12.3 ( 2 1 "C)
tetrahydrofuran methanol ethanol 2-propanol (1
Ea
acetonitrile acetone
Solvent dielectric constant.
7.58 ( 2 5 "C) 3 2 . 7 ( 2 5 "C) 24.55 ( 2 5 " C ) 19.92 ( 2 5 "C)
[ DMA 1/M
rrion/ns
0.30 0.10 0.35 0.30
rfHE/ns
52 6.3 i 0.4 7.0 i 0 . 5 1 2 . 7 i 0.9 24.1 i 0.7 24.5 * 1 . 5 23.8 i 1.4 24.4 t 1.8 31.9 i 2.3 b
0.085 0.15 0.30 0.90 1.36 0.51 0.1 5 0.29 0.25
8 30 b
- 2.3
6.8 i 0.4 7.7 f 0.2 14.1 i 0 7 32.0 f 0.7 33.0 i 0.3 33.2 i 0 . 5 37.8 ? 0.2 43.0 i 0 . 3 87.0 i 0.7 10.3 i 0.1 3 8 . 5 i 0.2 6 9 . 5 t 0.3
Photocurrent signal was lower t h a n t h e limit of the sensitivity of t h e detection system
TABLE 11: Photocurrent Rise Times and Fluorescence Decay Times of the Pyrene-DCNB System in Various Solvents solvent pyridine 1,2-dichloroethane o-dichlorobenzene dichloromethane tetrahydrofuran acetonitrile Solvent dielectric constant.
Ea
12.4 (21 "C) 10.36 ( 2 5 ° C ) 9 . 9 3 ( 2 5 "C) 8 . 9 3 ( 2 5 "C) 7 . 5 8 ( 2 5 "C) 37.5 ( 2 0 "C)
[ DCNB]/M
Trion/ns
0.058 0.05 0.03 0.05 0.042 8.44 x 10-4 2.34 x 10-3
5.5 t 16.4 i 28.3 i 18.1 t 46.0 k 52.1 i 24.9 i
Decay time of t h e heteroexcimer fluorescence.
resolution of our previous transient photoconductivity measurement using a Q-switched ruby laser for excitation was low, so that any accurate direct observation of the dissociation process from the geminate ion pair was difficult. In the present work, we have observed directly the dissociation process from the geminate ion pair for the first time in the case of typical heteroexcimer systems in various polar solvents by means of picosecond laser-induced photocurrent measurements. On the basis of this result, the dynamic behaviors of heteroexcimers and solvated ion pairs have been elucidated.
Experimental Section The methods for the picosecond laser-induced transient photoconductivity measurements were very similar to those reported before?JO In one experimental setting, the rise time of the detection system was about 7 ns, and it was less than 2 ns in another setting. Fluorescence lifetimes were determined by exciting the sample with the second harmonic of a picosecond ruby laser and by observing the fluorescence decay curve with a high-speed microchannel plate photomultiplier (HTV R-l194UX)-fast storage oscilloscope (Tektronix 7834-7A19-7B80) combinati~n.~J~ Pyrene and p-dicyanobenzenewere purified by repeated recrystallization from ethanol and subsequent vacuum sublimation. DMA was refluxed with acetic anhydride, washed with water, dried over potassium hydroxide, distilled under reduced pressure, and stored under vacuum. GR grade acetonitrile and pyridine as well as spectrograde 1,2-dichloroethane and dichloromethane were refluxed with calcium hydride and distilled. Spectrograde tetrahydrofuran was refluxed with lithium aluminum hydride (8) Hino, T.; Masuhara, H.; Mataga, N. Bull. Chem. SOC.Jpn. 1976, 49,394. (9) Hirata, Y.; Mataga, N.; Sakata, Y.; Misumi, S. J.Phys. Chem. 1982, 86,1508. (10) Hirata, Y.;Mataga, N.; Sakata, Y.; Misumi, S. J.Phys. Chem., in press.
rflns
0.3
0.8 1.5 1.7 1.6 1.4 1.0
5.7 i 19.3 t 47.0 r 20.9 i 56.2 i 51.7 ?r 24.5 ?r
O.lU 0.2b 0.4b 0.3b 0.4b 0.7' 0.2c
Decay time of t h e pyrene fluorescence.
Dhotocur rent
HE fluorescence
t
( n s )
Figure 1. Rise curves of the photocurrent and decay curves of the heteroexcimer fluorescence of the pyrene-DMA system in (1) acetone, (2) methyl ethyl ketone, and (3) pyridine solutions. [Pyrene] = 4 X lo-' M, [DMA] = (1) 0.35, (2) 0.3, and (3) 0.3 M.
and distilled. o-Dichlorobenzene was passed through a column of alumina and distilled. GR grade acetone and methyl ethyl ketone as well as spectrograde methanol, ethanol, and 2-propanol were dried over molecular sieve 4A and distilled. All sample solutions were deaerated by repeated freeze-pump-thaw cycles. The pyrene solution and DMA were deaerated separately and DMA was added to the pyrene solution by distillation in a vacuum line. All measurements were made at room temperature (25 "C).
Results and Discussion Examples of the laser-induced photocurrent rise curves and heteroexcimer fluorescence decay curves for pyreneDMA and pyrene-DCNB systems in several polar aprotic solvents are indicated in Figures 1and 2, respectively. The heteroexcimer fluorescence decay curves and the photocurrent rise curves can be reproduced approximately by simple exponential functions, If(t)0: exp(-t/TfHE)and i(t)
The Journal of Physical Chemistty, Vol. 87, No. 10, 1983 1601
Letters HE fluorescence
photocurrent
of the solvated ion pair as well as the heteroexcimer states of these two systems are very similar. The most striking fact revealed by the present investigation is that dissociation from the geminate ion pairs with lifetimes depending upon the nature of the solvent has been observed directly for the first time in various solvents and, moreover, the lifetime of the geminate ion pair is rather close to but a little smaller than the lifetime of the heteroexcimer in the same solvent throughout the results indicated in Tables I and I1 for the pyrene-DMA and pyrene-DCNB systems.l’ When the DMA concentration is very high in a pyridine solution, both lifetimes of the geminate ion pair and the heteroexcimer are slightly increased owing to the decrease of the solvent polarity (the dielectric constant of DMA is 4.9 at 20 “C). The fact that the TfHE value is close to the 7,ionvalue in all systems examined here suggests strongly that the heteroexcimer state and the solvated ion-pair state are closely interrelated, which seems to be quite reasonable also in view of the recent results of a study of the magnetic field effect upon the heteroexcimer fluorescence of the pyrene-NJV-diethylaniline (DEA) system in polar solvents.12 The heteroexcimer fluorescence yield of the pyrene-DEA system in alcohol solvents was slightly increased under a magnetic field and the relative increase was maximum at intermediate solvent polarity ( 6 ~ 2 25).12 : This result may be explained by assuming the interconversion between the fluorescent heteroexcimer and the nonfluorescentgeminate ion pair where the change of spin multiplicity by hyperfine interaction can take place. In less polar solvents, the concentration of ion pairs will be diminished, while in strongly polar solvents the dissociation of the geminate ion pair into free ions becomes easier and conversion of the ion pair into the heteroexcimer becomes more difficult. Both of these effects will diminish the relative magnetic field effect on the heteroexcimer fluorescence yield.12 The above results seem to suggest a “two-state model”
I.
2
3
0
200
LOO 0 t ( n s )
200
4oc
Flgure 2. Rise curves of the photocurrent and decay curves of the heteroexcimer fluorescence of the pyrene-DCNB system In (1) pyridine, (2) odichlorobenzene, and (3) tetrahydrofuran Solutions. [Pyrene] = 4 X lo-’ M, [DCNB] = (1) 0.058, (2) 0.03, and (3) 0.042 M. a (1- exp(-t/TIion)),respectively. Values of T~~ and T? in various solvents at several different concentrations of added quenchers are shown in Tables I and 11. For the pyrene-DMA system, the pyrene fluorescence is completely quenched before the rise of the photocurrent and the decay of the heteroexcimer fluorescence, owing to the diffusion-controlled quenching with high DMA concentration. Because of the limitation in its solubility, the concentration of DCNB in various solvents was not as high. However, in such aprotic polar solvents as pyridine, odichlorobenzene, and tetrahydrofuran, etc., where the solubility of DCNB was relatively high, the quenching of the pyrene fluorescence by DCNB was much faster than the rise of the photocurrent and the decay of the heteroexcimer fluorescence. In Table 11, results of measurements on the pyreneDCNB system in acetonitrile are given together with those in less polar solvents. In acetonitrile solution where no heteroexcimer fluorescence can be observed, we have examined the relation between the rise of the photocurrent and the decay of the pyrene fluorescence by using low concentrations of DCNB. The photocurrent rise time agrees exactly with the pyrene fluorescence decay time as one can see from Table 11, which means that the photocurrent rise time and the pyrene fluorescence decay time are determined by a diffusion-controlled quenching process, and the lifetime of the geminate ion pair formed after ~ ~Tf electron transfer is much shorter than the T , and values. As indicated in the tables the pyrene-DMA system shows heteroexcimer fluorescence in addition to photoconductivity, even in very polar solvents such as acetone, methanol, and acetonitrile, while in tetrahydrofuran and 2-propanol, observation of the photoconductivity was not possible. On the other hand, it was possible to observe the rise of photocurrent even in the latter solvents for the pyrene-DCNB system, while this system does not show heteroexcimer fluorescence in very polar aprotic solvents such as acetone and acetonitrile, and also in various alcohols. The last result is in marked contrast to the fact that heteroexcimer fluorescence of the pyrene-DMA system in an alcohol is much stronger than in an aprotic solvent with approximately the same dielectric constant. All of these results indicate the marked difference between the pyrene-DMA and the pyrene-DCNB systems with respect to the ionic photodissociation as well as to the nature of the heteroexcimer state, although the energies
where (A-D+)*is the fluorescent heteroexcimer, ($-...D,+) the solvated ion pair (geminate ion pair), and (A; + D,+) the dissociated ion radicals. From this reaction scheme, we easily obtain the following equations: If@)= Al exp(-IAllt) + - 4 2 ex~(-lA2lt) i(t) = Al’(l - exp(-IXllt)) + A2’(1 - exp(-IA21t)) (2) ~
Al’ = (k4(kO +
- IAlI)/IAlIk2)Al
+ k l - IA21)/IA21k21A2 However, observed I f ( t )and i(t) can be reproduced approximately by I f ( t ) 0: exp(-t/TfHE) and i(t) CI: (1 =
(k4(k0
exp(-t/Trion)),respectively. Moreover, the T~~~~value is a little smaller than the rfHEvalue in all cases examined here. Therefore, the above simplified model cannot explain the observed result. We propose here a tentative model which is in accord with the present observation. The fluorescent heteroex(11) The 7,’” values of the pyreneDCNl3 system in 1,2-dichloroethane and dichloromethane solutions determined previously by using a Qswitched ruby laser for excitation were considerably longers than those given in Table 11, although the Tf values of the heteroexcimer are in approximate agreement. Some unknown chemical reaction might be responsible for the long ?T value but final conclusion about this long value is not very clear at present. (12) Petrov, N. Kh.; Shushin, A. I.; Frankevich, E. L. Chem. Phys. Lett. 1981, 82,339.
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The Journal of Physical Chemistry, Vol. 87, No. IO, 1983
Letters
cidated by the recent studies of the magnetic field efcimers and nonfluorescent solvated ion pairs (geminate ion fect.12J8-20 However, the picosecond laser-induced phopairs) in polar solvents may be formed competitively, depending upon the solvent polarity, from the nonrelaxed tocurrent measurement developed here can provide additional useful information for elucidating the photoCT state immediately after electron transfer. This is chemical charge transfer and ionic dissociation phenomena analogous to interpertation b in the Introduction for the by directly observing the dissociation process of the gemdifferent solvent dependence of the heteroexcimer inate pair. fluorescence yield and lifetime. Similar mechanism were On the other hand, the quantum yield of the dissociated proposed also in relation to photochemical electron transfer ion radical formation may be controlled by the backand ionic dissociation studiesa4J3 However, in order to electron transfer deactivation processes first in the nonexplain the fact that the 7,iOn value is a little smaller than relaxed CT state and next in the geminate ion pairs and the TfHE value as observed in the present investigation the heteroexcimers. According to our preliminary studies while there is an interconversion between the heteroexby means of picosecond laser photolysis and transient cimer and the geminate ion pair as revealed by the study absorption measurements with a mode-locked Nd3+:YAG of the magnetic field effect upon heteroexcimer fluoreslaser on the pyrene-DMA system,21i22 the ion absorbance cence, we must invoke multiple heteroexcimers and mulin acetone a t the delay time of 500 ps where all excited tiple solvated ion pairs as discussed in the Introduction pyrene was quenched by diffusion-controlled reaction in interpretation a and assume that the main part of the ([DMA] 0.5 M) was about 0.9 of that in acetonitrile solvated ion pairs with the largest statistical weight may solution. This result seems to suggest the importance of not be directly combined with the main part of the hetdeactivation in the second stage a t least for the pyreneeroexcimers but there may be intervening heteroexcimer DMA system, since the yield of the dissociated free ion and solvated ion-pair states. The reason the value is radicals in acetone determined by means of the nanosea little smaller than the 7fHEvalue is not very clear at the cond laser-induced photocurrent measurement was about present stage of the investigation. In addition to disso0.68 of that in acetonitrile s ~ l u t i o n Measurements .~~~ of ciation to free ions, the ion pair will undergo deactivation the ion yields and their time dependences in the few to to the ground state due to back-electron transfer from the several ten of nanosecond regions are now going on in this acceptor anion to the donor cation. This deactivation laboratory. process of the ion pair seems to be faster than that of the Another important problem in relation to the photoheteroexcimer, in view of the fact that the decay time of chemical CT and ionic dissociation was the comparison of the “loose heteroexcimer” strongly solvated in polar solvent the behaviors of the heteroexcimer systems and the excited is much shorter than that of the “compact heteroexcimer” EDA complexes stable in the ground state.23 According in the same solvent, as it is observed for the intramolecular to our transient absorption measurement with picosecond heteroexcimer system ~-(CH~)~NC~H~(CH~)~-l-pyrenyl.~~,’~ laser photolysis method, 1,2,4,5tetracyanobenzene Such a difference between the ion pair and the heteroex(TCNB)-toluene EDA complex shows ion-like transient cimer state might contribute to the observed difference absorption spectra in polar solvents, the absorbance of between rrionand 7fHE. which decays and converges to a constant value according It should be noted here that the existence of multiple ion-pair states in polar solutions is frequently demonto, A ( t ) B + C exp(-t/T), where B and C are constants independent of t. This time-dependent behavior presumstrated by quantum chemical as well as statistical meably represents the competition between the dissociation chanical cal~ulations.’~J~ The existence of multiple hetto free ions and deactivation to the ground state from the eroexcimer and solvated ion-pair states and interconversion excited CT state. The 7 values were, for example, 320 ps between them will give multiexponential functions for the in acetonitrile, 600 ps in acetone, and 3 ns in 1,2-diheteroexcimer fluorescence and the photocurrent. c h l o r ~ e t h a n e . ~Whether ~ , ~ ~ the observed spectra before If(t) a C A ; exp(-Xit) conversion to a constant value involves the geminate ion 1 pair which are in equilibrium with the excited singlet state of the complex or not is not very clear at the present stage i ( t ) a CAi’{l- exp(-Xi*)) (3) 1 of investigation. More detailed studies and comparison with the heteroexcimer systems are necessary which are However, the overwhelming term in each function will give now going on in this laboratory. the approximate single exponential decay or rise. Registry No. Pyrene, 129-00-0;Nfl-dimethylaniline, 121-69-7; The behaviors of the geminate ion pair of the heterp-dicyanobenzene, 623-26-7. oexcimer systems in polar solvents have been much elu-
-
7P
(13) Iwa, P.; Steiner, U. E.; Vogelmann, E.; Kramer, H. E. A. J. Phys. Chem. 1982,815,1277. (14) Migita, M.; Okada, T.; Mataga, N.; Sakata, Y.; Misumi, S.; Nakashima, N.;Yoshihara K.; Bull. Chem. SOC.Jpn. 1981,54, 3304. (15) Okada, T.; Migita, M.; Mataga, N.; Sakata, Y.; Misumi, S. J . Am. Chem. SOC.1981,103, 4715. (16) Salem, L. “Electrons in Chemical Reactions: First Principle”; Willey: New York, 1982; pp 241-2. (17) Levesque, D.; Weis, J. J.; Patey, G. N. J. Chem. Phys. 1980,72, 1887.
(18) Schulten, K.; Staerk, H.; Weller, A.; Werner, H.-J.; Nickel, B. Z. Phys. Chem. (Frankfurt am Main)1973,101, 37. (19)Michel-Beyerle, M. E.; Haberkorn, R.; Bube, W.; Steffons, E.; Schroder, H.; Neusser, H. J.; Schlag, E. W.; Seidlich, H. Chem. Phys. 1976, 17, 139. (20) Werner, H.-J.; Staerk, H.; Weller, A. J . Chem. Phys. 1978,68, 2419. (21) Takigawa, T.; Okada, T.; Mataga, N. unpublished results. (22) Mataga, N. Int. J. Radiat. Phys. Chem., in press. (23) Mataga, N.; Murata, Y. J. Am. Chem. SOC.1969,91, 3144. (24) Uemiya, T.; Miyasaka, H.; Masuhara, H.; Mataga, N., to be submitted for publication.
