5062
J . Phys. Chem. 1984,88, 5062-5064
Laser Photolysis Studies on the Rate Constants of Electron-Transfer Reactions of Aromatic Molecules in Solution A. Tsuchida, M. Yamatnoto,* and Y. Nishijima* Department of Polymer Chemistry, Kyoto University, Yoshida, Sakyo- ku, Kyoto, Japan 606 (Received: February 8, 1984)
Absolute rate constants for electron-transferreactions of aromatic molecules in solution at room temperature have been studied by the nanosecond laser photolysis method. Holes or electrons of photochemically produced radical cations or anions were transferred to second aromatic compounds. The decay of the ion radical donor and the rise in the amount of the ion radical acceptor for each donor and acceptor pair were observed at their absorption maxima, and the simulation of this rise and decay gave electron transfer rate constants. The relationship between these rate constants and the reaction free energy change showed that the electron transfer of highly exothermic reactions proceeds with a diffusion-controlled rate constant, which suggests that electron transfer occurs through some loose complex in solution.
Introduction
Electron transfer (ET) is one of the most fundamental processes in chemistry and many reports in this field have been presented from e~perimentall-~ and t h e ~ r e t i c a l ~standpoints. -~ For the fluorescence quenching of organic molecules by ET, Rehm and Weller (RW) determined quenching rate constants in the range of +5 to -60 kcal/mol free energy change.' They showed that the exothermic ET reactions are nearly diffusioncontrolled processes. Hitherto, many systematic experiments have been carried out and most of them have shown the tendency that E T reactions in solution proceed with diffusion-controlled rate constants even in the highly exothermic r e g i ~ n . ~ . ~ On the other hand, many theoretical treatments of ET reactions have been In particular, the Marcus theory has been widely applied in many studies to explain experimental data, because other models have some limitations for use in practical cases. The Marcus theory is applicable to the adiabatic outersphere E T reactions, and the point is that it predicts the decrease in the transfer rate constants in the highly exothermic region. This region is called the "Marcus inverted region" and is caused by the Franck-Condon factor. The results of experiments so far conducted are not in full agreement with this theory. In a few systems, evidence for a decrease of the rate constants has been Beitz and Miller especially observed the decrease of transfer rates by about five orders of magnitude.' However, their experiments were made in rigid glass at 77 K, and such a marked decrease has not been found to occur in solution at room temp e r a t ~ r e .In ~ solution, there is a high possibility that some complexes such as exciplex participate as a transient state in ET reactions. Recently, Weller proposed that exciplex formation may provide a fast route for the ET process in solution.I0 Exciplexes (1) (a) Rehm, D.; Weller, A. Eer. Eunsenges. Phys. Chem. 1969, 73,834. (b) Rehm, D.; Weller, A. Isr. J. Chem. 1970, 8, 259. (c) Schomburg, H.; Staerk, H.; Weller, A. Chem. Phys. Lett. 1973, 22, 1. (2) For example: Ballardini, R.; Varani, G.; Indelli, M. T.; Scandola, F.; Balzani, V. J. Am. Chem. SOC.1978, 100, 7219. (3) Creutz, C.; Sutin, N. J . Am. Chem. SOC.1977, 99, 241. (4) Wallace, W. L.; Bard, A. J. J. Phys. Chem. 1979, 83, 1350. (5) (a) Frank, A. J.; Gratzel, M.; Henglein, A,; Janata, E. Eer. Bunsenges. Phys. Chem. 1976,80,541. (b) Scheerer, R.; Gratzel, M. J . Am. Chem. SOC. 1977, 99, 865. (6) Arai, S.; Grev, D. A.; Dorfman. L. M. J. Chem. Phys. 1967,46, 2512. (7) For other references see: (a) Beitz, J. V.; Miller, J. R. J . Chem. Phys. 1979, 71, 4579. (b) Scandola, F.; Balzani, V. J . Am. Chem. Soc. 1979, 101, 6140. (8) (a) Marcus, R. A. J . Chem. Phys. 1956, 24, 966. (b) Marcus, R. A. Annu. Rev. Phys. Chem. 1964, 15, 155. (9) Albery, W. J. Annu. Rev. Phys. Chem. 1980, 31, 227. (10) (a) Weller, A. Z . Phys. Chem. (Weisbaden) 1982, 130, 129. (b) Weller, A. In 'Light-Induced Charge Separation in Biology and Chemistry";
Gerischer, H., Katz, J. J., Eds.; Verlag Chemie: Weinheim, West Germany, 1979; p 131.
0022-3654/84/2088-5062$01.50/0
TABLE I: Half-Wave Potentials for One-Electron Oxidation and Reduction, and Absorotion Spectra Data of Donors and Acceotors Ell2 vs. ~max, 4 0 3 , compd Ag/Agf, V nm M-' cm-' + 1.06 443 11.4 PY ECZ +0.82 780 9.4 DMT +0.36 470 10.4
DMASt TCNB p-DCNB
+0.28 -1.97" -2.1 1
DMTP a
640 457.5 430 530
-1.02'
8.9
10.0 6.5 12.3
Reference 12
have stronger interaction between donors and acceptors than encounter complexes, and may give a bypass route to the direct ET process. Then, the possibility that ET takes place via exciplexes cannot be ruled out in the experiments on fluorescence quenching. Therefore, studies of ET reactions in which exciplex formation is inhibited may provide important i n f o r m a t i ~ n . ~ We have studied the ET reaction from a neutral molecule to a cation radical, and also from an anion radical to a neutral molecule in acetonitrile solvent at room temperature by the nanosecond ruby laser photolysis method (347-nm excitation) and examined the relationship between E T rate constants and the reaction free energy change. Experimental Section
Method. The E T reaction between a photoexcited electron donor 1 (D1*) and an electron acceptor 1 (A,) in polar solvents gives a cation radical and an anion radical: D1*
+ A1
--*
D1'.
+ AI-.
(1)
The role of reaction 1 is to produce free D1+-and AI--in solution, and this process is completed immediately after the excitation by a laser pulse (14-11s pulse width). Then, a third solute in the or solution, a donor 2 (D2) or an acceptor 2 (A2) quenches D1+A,-., and forms D2+. or A2--, respectively: D1+. Ai-.
+ D2 + A2
---*
+ Dz+* A1 + A,. D1
(2) (3)
By properly adjusting the concentration (Table 11), one may observe the decay of DI+-or Al-., with the corresponding rise of D2+. or A2--, respectively. Chemicals. Acetonitrile (MeCN) was refluxed over PzOs several times and was fractionally distilled. 1-Chlorobutane, 2-chlorobutane, isopentane, and 2-methyltetrahydrofuran for y-ray irradiation were dried on molecular sieves and then distilled prior to use. Commercially available N-ethylcarbazole (ECZ), pyrene (Py), 1,4-dicyanobenzene (p-DCNB), 1,2-dicyanobenzene (o0 1984 American Chemical Society
il
ET Reactions of Aromatic Molecules in Solution
The Journal of Physical Chemistry, Vol. 88, No. 21, 1984 5063
TABLE I1 ET Reaction Systemsa and ET Rate Constants no.
[Dlltbmol/L
[All, mol/L
1 2 3 4 5
ECZ, 3.2 x 10-4 PY, 1.3 x 10-3 ECZ, 2.7 x 10-4 ECZ, 2.8 X lo4 DMASt, 6.3 X
DMTP, 1.0 X o-DCNB, 5.0 X DMTP, 1.0 X DMTP, 1.0 X lov2 o-DCNB, 5.0 X
6
ECZ, 3.1
p-DCNB, 2.0 X
X lo4
P21, mol/L
[A2],mol/L TCNB, 5.0 x 10-5
DMASt, 3.1 X DMASt, 3.4 X p-DCNB, 4.0 X DMT, 2.0 X DMTP, 4.0 x 10-4
AG: kcal/mol
kr, M-I s-I
-25.1 -18.0 -12.5 -3.23 -1.84, 1 .84d 3.23
1.6 X 10l0 2.0 x 1010 1.3 X 10'O
1.0 x 1010 5.6 x 109, 3.5 x 109d
