J . Phys. Chem. 1986, 90, 3880-3882
3880
Dynamics of Aromatic Hydrocarbon Cation-Tetracyanoethyiene Anion Geminate Ion Pairs in Acetonitrile Sohrtion with Implications to the Mechanism of the Strongly Exothermic Charge Separation Reaction in the Excited Singlet State Noboru Mataga,* Yu Kanda, and Tadashi Okada Department of Chemistry, Faculty of Engineering Science, Osaka University, Toyonaka, Osaka 560, Japan (Received: April 15, 1986; In Final Form: May 28, 1986)
The mechanism of the strongly exothermic electron-transfer reaction has been investigated in the case of excited singlet aromatic hydrocarbon-tetracyanoethylene system in acetonitrile by measuring behaviors of the ion pairs produced in the course of fluorescence quenching with picosecond laser photolysis. The nonfluorescent CT complex formation as a mechanism for the lack of an "inverted region" has been shown improbable. Moreover, measurements of the charge recombination (CR) rate of the aromatic hydrocarbon cation-tetracyanoethylene anion geminate pairs have provided useful data, with their small energy gaps between the ion pair and ground state, for a "normal region" of the energy gap dependence of the CR reaction.
Introduction Numerous investigations' on the fluorescence quenching reaction due to the charge separation (CS) in polar solvents, A* B A*-B AS*7.-BS*f, indicate that the bimolecular quenching rate constant k depends strongly upon the energy gap AGet between the (A*...B\ state and (As"-Bs'*) state around the zero energy gap. Namely k, shows a steep rise around the zero energy gap toward the downhill side. However, k, is a constant, diffusion-controlled value even in the very strongly downhill region2 These results show that the observation of the so-called "inverted regionn3 in the photoinduced C S reaction in polar solvents is difficult. Several possibilities were suggested to interpret the lack of an "inverted region" in the observed k,-AGet relation. One possibility involves the participation of excited when -Ace, becomes electronic states of the ion pair too large, keeping the Franck-Condon factor of the electron transfer (ET) process large.2b*4q5However, this interpretation depends on the nature of the individual quencher and fluorescer molecules and does not seem to be general enough to cover wide range of fluorescer-quencher systems. Another interpretation assumes the formation of a nonfluorescent charge-transfer (CT) (A'.Bf)s complex in the course of quenching, A* + B deactivation to the ground and/or local triplet state or dissociation into ions. Namely, a complex formation by diffusion-controlled collision will take place, leading to the lack of the "inverted region".6 Of course, it is well-known that fluorescent exciplex formation can take place in many systems in nonpolar solvents' and there is a possibility that a nonfluorescent complex is formed in solvents of intermediate polarity in addition to the very weakly fluorescent exciplex.'.' We are discussing here the case of photoinduced ET in strongly polar solvents like acetonitrile. There is a third possibility suggested recently for this problem as follows? Although photoinduced E T occurs by very weak interaction without complex formation, a much higher frequency of solvent reorientational phonon around the charged solute than that surrounding the neutral solute will result in a large Franck-Condon factor even in the strongly exothermic region.
+
- -
-
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( 1 ) See,for example: (a) Mataga, N.; Ottolenghi, M. In Molecular Associations; Foster, R., Ed.; Academic: New York, 1979; Vol. 2, p 1. (b) Mataga, N. In Molecular Interactions; Ratajczak, H., Orville-Thomas, W. J., Eds.; Wiley: Chichester, 1981; Vol. 2, p 509. (2) Rehm, D.; Weller, A. (a) Ber. Bunsen-Ges. Phys. Chem. 1%9,73,834; (b) Isr. J. Chem. 1970, 8, 259. (3) Marcus, R. A. (a) J. Chem. Phys. 1956,24,966; (b) Annu. Rev. Phys. Chem. 1964, IS, 155. (4) Mataga, N. Bull. Chem. SOC.Jpn. 1970, 43, 3523. (5) Beitz, J. V.; Miller, J. R. J. Chem. Phys. 1979, 71, 4579. (6) Masuhara, H.; Mataga, N. Acc. Chem. Res. 1981, 14, 312. (7) Mataga, N. Pure Appl. Chem. 1984, 56, 1255. (8) Kakitani, T.; Mataga, N. (a) J. Phys. Chem. 1985,89,8; (b) Chem. Phys. 1985,93,381; (c) J. Phys. Chem. 1985,89,4752; (d) J. Phys. Chem. 1986, 90, 993.
0022-36S4/86/2090-3880$01.50/0
Although the first two interpretations were proposed many years ago, clear-cut experimental verification for those mechanisms in solution is difficult task. In the present investigation, we examined the second possibility for the case of the extremely exothermic CS reactions of several aromatic hydrocarbon (AH)-tetracyanoethylene (TCNE) systems in acetonitrile solution, by directly observing the formation of geminate ion pairs in the encounter between excited A H and TCNE and their charge recombination (CR) decay and dissociation into free ion by means of picosecond laser photolysis and transient absorption as well as transient photocurrent measurements. Results of these measurements are discussed in the following in comparison with the case of direct excitation of the ground-state C T complex. The extremely large exothermicity in the photoinduced CS reaction of the AH-TCNE-acetonitrile systems leads to a much smaller energy gap (-AGip) between the ion pair state and the neutral ground state compared with the other aromatic hydrocarbon-acceptor or -donor systems examined before. Therefore, the comparison of the CR rate constants of AHs'+-TCN&ion pairs with those ion pairs with a larger energy gap (-AGip) will be very interesting and important from the theoretical viewpoint3** of the energy gap dependence of the ET reactions in the C R process in strongly polar solutions. Discussion will be given along this line.
Experimental Section Time-resolved transient absorption spectra in the picosecond to several nanosecond regime were measured by means of a microcomputer-controlled mode-locked Nd3+-YAG laser photolysis ~ y s t e m . ~The third harmonic was used for the excitation of AH, and the excitation of the ground-state C T complex was made with the second harmonic. Time-resolved transient absorption spectra in the several tens of nanoseconds to microsecond regime were measured with an excimer laser photolysis system.1° The methods for the picosecond laser-induced transient photoconductivity measurements were very similar to those reported before," except that the third harmonic of the mode-locked Nd3+-YAG laser was used as the exciting pulse. Pyrene (Py) was purified by repeated recrystallization from ethanol and subsequent vacuum sublimation. Perylene (Per) was chromatographed on alumina and silica gel and recrystallized from ethanol. 9,lO-Diphenylanthracene (DPA) was Wako scintillation grade and used without further purification. 1,12-Benzperylene (BPer) was guaranteed reagent grade reagent of Nakarai and used without further purification. Guaranteed reagent grade TCNE (9) (a) Masuhara, H.; Ikeda, N.; Miyasaka, H.; Mataga, N . J . Specfrosc. SOC.Jpn. 1982, 31, 19. (b) Miyasaka, H.; Masuhara, H.; Mataga, N. Laser Chem. 1983, 1, 357. (IO) Hirata, Y.; Mataga, N . (a) J . Phys. Chern. 1985, 89, 4031; (b) J . Spectrosc. SOC.Jpn. 1985, 34, 104. (11) Hirata, Y.; Kanda, Y . ;Mataga, N. J . Phys. Chem. 1983, 87, 1659.
0 1986 American Chemical Society
Letters
The Journal of Physical Chemistry, Vol. 90, No. 17, 1986 3881
tB
60ps
I
loops
"- I
0
400ps
a
O.'t 1 ns
400
500
600
700
A / nm
400
500
600
700 hlnm
Figure 1. Picosecond time-resolved transient absorption spectra of the AH-TCNE system in acetonitrile solution excited with the third harmonic of the YAG laser. The delay times from the exciting pulse are indicated in the figure. AH: (A) pyrene; (B) 1.12-benzperylene. [TCNE] 0.3 M, [pyrene] = 2.7 X lo-' M, and [1,12-bezperylene] = 1.8 X lo4 M.
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was recrystallized from monochlorobenzene and sublimated several times under vacuum. Solvents were purified by standard method and carefully dried. Solutions for measurement were carefully deaerated.
Results and Discussion Some examples of time-resolved transient absorption spectra of AH-TCNE systems in acetonitrile solutions are shown in Figure 1, where one can recognize clearly the absorption band of the A H cation radical and, though it is not very prominent, the absorption band due to the TCNE anion radical. Since the picosecond continuum used for monitoring the time-resolved absorption spectra is very weak around 400 nm, the observed results become rather noisy there. Nevertheless, one can see clearly the TCNE anion band around 420 nm in addition to the BPer'+ band at 513 nm in Figure 1B, and it is superposed on the shoulder of the Py" band with a peak at 445 nm in the case of Figure 1A. We have observed both absorption bands due to AH" and TCNE'- ions also in the several tens of nanoseconds to microsecond regime by means of excimer laser photolysis and transient absorption spectral measurements. The absorption bands of BPer" and Py" as well as other ion radicals measured in the present work are in agreement with those in previous reports where radical ions were in the ground state, being produced by chemical reactions in the ground ~ t a t e . l * ? ' ~ y-ray radiolysis at low temperature^,'^ or microsecond flash-induced photochemical electron transfer in polar solutions.15 Therefore, the transient ion radicals measured in the present work are in the ground state. Even if ion radicals are in the excited electronic state immediately after electron transfer at encounter, they must undergo ultrafast relaxation to the ground state, because we have observed the same ion radical absorption band throughout the ten picosecond to microsecond regime. It should be noted here that, under the present experimental condition, the formation of the ground-state C T complex is negligible or its extent is very small and we are exciting almost exclusively uncomplexed AH. The excited singlet A H undergoes E T at encounter with TCNE. At any rate, almost all excited singlet AH molecules are converted to ion radicals already around the 100-ps delay time. The absorption spectra shown in Figure 1 clearly decay in the 100 ps-1 ns regime, converging to approximately constant values of absorbance. As an example, the time profile of the absorbance in the case of the BPer-TCNE system is shown in Figure 2. (12) Aalbersberg, W. J.; Hoytink, G. J.; Mackor, E. L.;Weijland, W. P. J . Chem. SOC.1959, 3049. (13) Murata, Y . ;Mataga, N. Bull. Chem. SOC.Jpn. 1971, 44, 354. (14) (a) Shida, T.; Iwata, S.J. Am. Chem. Sm. 1973,95,3473. (b) Shida, T.; Harnill, W. J . Chem. Phys. 1966, 44,4372. (15) Grellmann, K. H.; Watkins, A. R.;Weller, A. J . Lumin. 1970, 1 / 2 , 678.
I
I
I
1
2
Delay 3 Time 4 (ns 1 5
I
I
Figure 2. Time profile of the absorbance of the 1,12-benzperylene cation radical at 5 13 nm. The absorbance of the TCNE anion radical at 420
nm also shows similar decay. Insert: semilogarithmic plot of the absorbance obtained by subtracting the constant value of absorbance at long delay time from the observed decay curve. TABLE I: Charge Recombination Rate Constant k, of Geminate Ion Pairs in Acetonitrile Solution'
D
A
TCNE TCNE TCNE TCNE
perylene*' 1,12-benzperylene*
-AGip/eV
8.2 X l o B b
0.55c
1.47 X
0.78c 0.89c O.9Oc
9,10-diphenylanthracene* 1.73
pyrene* pyrene* 1-cyanopyrene* DMA pyrene* p-DCNB PMDA~
k,ls-'
2.5 1.5
2.6
X logb X X 1O1OC X 1098
3.3 x 107*
1.7f
2.788 2.80~
'An asterisk denotes that the molecule is excited in the ET reaction at the encounter. bPresent work. CReference2b. dPyromellitic dianhydride. eReference 22. /Reference 2b and: Peover, M. E. Trans. Faraday SOC.1962, 58, 2370. *Iwahara, Y.; Kanda, Y.; Okada, T.; Mataga, N., to be published. hReference 7. When the constant value a t long delay time is subtracted from the decay curve, the absorbance decay can be represented approximately by a single exponential. These results indicate that the lifetime T of the ion pair and the yield 1) of the dissociated ion radical formation are given respectively by T = (kd + k,,)-I and q = kdT, assuming the following reaction scheme is well established in the case of the ionic photodissociation of exciplex systems in strongly polar ~ o l v e n t s . ~ ~ ~ J ~ - ' ~
1) was obtained by taking the ratio of the absorbance due to the dissociated ion and the initial value estimated by extrapolating the ion absorbance to t = 0. The values of T and q have been obtained to be 200 ps and 0.51 for Py, 330 ps and 0.42for DPA, 300 ps and 0.56 for BPer, and 700 ps and 0.43for Per donors, respectively. From these values of T and q , we have estimated k, and kd. We have confirmed for the present systems that the rise time of the photocurrent is shorter than 1 ns (the limit of the time resolution of our photocurrent detection system) in accordance with the results in Figure 2 and the reaction scheme of eq 1. We have confirmed also that the decay of the photocurrent takes place in the microsecond regime, which follows the bimolecular decay law l/(photocurrent) 0: kt. In correspondence with this photocurrent behavior, we have confirmed that the decay of the ion absorbance due to bimolecular recombination occurs in the microsecond regime. These results for recombination reactions of radical ions support the validity of eq 1. The k, value shows a systematic dependence upon -AGip as indicated in Table I. The k, value increases with an increase of the -AGip value. However, kd is about 2 X lo9 s-' on the average and does not show such a systematic dependence upon the free
(16) Knibbe, H.; Rehrn, D.; Weller, A. Ber. Bunsen-Ges. Phys. Chem. 1968, 72, 257. (17) Schlten, K.; Staerk, H.; Weller, A,; Werner, H.-J.;Nickel,B. Z . Phys. Chem. N . F. 1976, 101, 371.
3882 The Journal of Physical Chemistry, Vol. 90, No. 17, 1986
Y
I
9l-(E,
400
500
600 Alnm
Figwe 3. Transient absorption spectral measurements by exciting the ground-state pyrene-TCNE complex with the second harmonic of the YAG laser: (A) acetonitrile solution, [pyrene] = 7.5 X lW3 M, [TCNE] = 0.53 M; (B) dichloromethane solution, [pyrene] = 8.5 X lo-’ M, [TCNE] = 0.1 M.
energy of the ion pair state. This kd value is rather close to that calculated by the kinetic diffusion theoryl8for acetonitrilesolution.6 In order to compare the behaviors of the ET state produced by exciting the ground-state CT complex with the above results of ion pairs produced by the encounter, we have excited the Py-TCNE complex at the CT absorption band with the second harmonic (530 nm) of the picosecond YAG laser pulse. As indicated in Figure 3, it is difficult to observe the absorption band of the ET state by the present picosecond apparatus owing to the ultrafast deactivation to the ground state. Although we recognized a slight absorbance around 450 nm in dichloromethane solution at 0 ps, it disappeared completely at 33 ps, and, moreover, such observation in acetonitrile was difficult. These results indicate that the lifetime of the photoinduced singlet ET state is shorter than a few picoseconds. Therefore, if the nonfluorescent CT complex formation is the dominant mechanism for the strongly exothermic fluorescence quenching reaction of the AH-TCNE system in acetonitrile, one should not observe the formation of geminate ion pairs with a lifetime longer than a few picoseconds. Actually, however, the present experiments have confirmed unambiguously that geminate ion pairs with lifetimes of 100 ps-1 11s are formed in the bimolecular fluorescence quenching reaction. Accordingly, the second interpretation in the Introduction for the lack of an inverted region in the k,-AG,, relation is improbable for the fluorescence quenching reaction of AH-TCNE system in acetonitrile. It should be noted here that the above results indicate the existence of different kinds of ion pairs: ion pairs that undergo rapid CR deactivation and other ones with a relatively long life. We have confirmed the existance of several kinds of ion pairs also in the case of the typical exciplex systems like pyrene-p-DCNB and pyrene-DMA in polar solvent^.^^' 1~19920
(18) Eigen, M. 2. Phys. Chem. N . F. 1954, 1, 176.
Letters Another important problem in relation to the present results is the energy gap dependence of the CR rate of the geminate ion pairs. In Table I, the present experimental results are shown together with those of several other systems in acetonitrile solution. With an increase of the -AGip value, k,,increases at first and, with a further increase of -AGipr k, decreases. This result indicates the bell-shaped energy gap dependence of the CR rate constant. Although it is not included in Table I, k, of the etioporphyrin cation-toluquinone anion geminate pair was measured to be >lo” for -AGip = 1.46 eV in acetone ~ o l u t i o n . ~This * ~ ~-AGip value lies between those of the PMDA-pyrene and TCNE-pyrene systems in acetonitrile and does not contradict the results in Table I. It should be noted here that the PMDA-pyrene system can form also two kinds of ion pairs in acetonitrile, though their difference is not so extreme as in the case of the TCNE-pyrene system. Namely, the geminate ion pair formed by exciting the ground-state complex at the CT band with the 530-nm pulse of the picosecond YAG laser has a lifetime about 10 ps, while the lifetime of the ion pair produced by an encounter between S1 state pyrene and PMDA has been estimated to be ca. 50 PS.’~ The CR rate constant given in Table I for the PMDA-pyrene system is the value for the long-life geminate pair, which seems to have a more loose structure than the short-life one. The bell-shaped energy gap dependence of the ET rate was experimentally demonstrated by Miller et alaz3for the intramoA’--SpD, in lecular charge-shift type of reaction, A-SpD’2-methyltetrahydrofuran solution, where Sp is a rigid saturated hydrocarbon spacer, by means of the pulse radiolysis method. Such an experimental observation has been difficult for the photoinduced CS reaction in polar solutions, as discussed in the Introduction; i.e., the rate constant shows a steep rise around the zero energy gap toward the downhill side but no decrease even in the strongly downhill region. On the other hand, it has been observed that the CR rate constant k, of the geminate ion pair produced by photoinduced CS shows a rather strong dependence upon the energy gap A G i p . 7 ~ 8 d ~ 1 9Nevertheless, ~m2~4 the observed results of the CR reaction of singlet geminate pairs were only for the “inverted region”; i.e., k, -decreased with an increase of -AGi,.7,8d,19,20,22.24 In‘iiew of the above situation, the present results on the energy gap dependence of the CR reaction of AHs’+.-TCN&‘ ion pair will be very important, since they provide the data for “normal region” of the CR process in the geminate ion pairs, in addition to those for “inverted region”.
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Acknowledgment. N.M. acknowledges support by the Grant-in-Aid (No. 5843004, No. 59045097,and No. 60040059) from the Japanese Ministry of Education, Science and Culture. (19) Mataga, N. J . Mol. Struct. 1986, 135, 279. (20) Mataga, N.; Okada, T.; Kanda, Y.;Shioyama, H. Tetrahedron, in press. (21) Mataga, N.; Karen, A.; Okada, T.;Nishitani, S.;Sakata, Y . ;Misumi, S.J . Phys. Chem. 1984,88,4650. (22) Details will be given in: Mataga, N.; Shioyama, H.; Kanda, Y . J. Phys. Chem., submitted. (23) Miller, J. R.; Calcaterra, L. T.; Closs, G. L.J . Am. Chem. Soc. 1984, 106. 3047. (24) Wasielewski, M. R.; Niemczyk, M. P.; Svec, W. A.; Pewitt, E. B. J . Am. Chem. SOC.1985, 107, 1080.
