Tunneling reactions of biphenyl anion radical with electron acceptor organic molecules

Electron tunneling between organic species was first detected, by direct kinetic experiments, for reactions of the biphenyl anion radical with naphthalene and pyrene [11] and triphenylethylene [12], As is known, upon irradiating vitreous solutions containing biphenyl or pyrene, Py, these acceptors react with electrons to form Ph2 and Py with characteristic optical spectra [13]. Ph2 particles have been found [11] to enter into the electron exchange reactions at 77 K with naphthalene, Nh, and pyrene molecules in mixtures of ethyl alcohol and diethyl ether (2 1). 

During the y-radiolysis of vitreous solutions containing only biphenyl (0.1 M) or only pyrene (0.02 M), the yield of Ph2 and Py- at 77K is high enough for them to be recorded at an irradiation dose of 1019 eV cm-3. At 77 K these particles have been observed to decay spontaneously (Fig. 5), evidently, due to proton transfer from alcohol molecules (the most probable process in the case of Ph2 anion radicals [14]) or to recombination with counterions formed during radiolysis. Naphthalene and pyrene additives to solutions of Ph2 essentially accelerate the decay of the Ph2 anion radical at 77 K which is naturally accounted for by electron transfer from Ph2 to Nh and Py. In agreement with this conclusion the decay of Py in the presence of Ph2 is slower than its spontaneous decay in the absence of Ph2.  

Electron transfer from the Ph2 anion radical to Nh and Py most likely proceeds via the tunneling mechanism since the diffusion of aromatic molecules at 77 K is negligible, even in less rigid methylcyclohexane glass [15, 16]. A fall in temperature down to 4.2 K results in a complete cessation of Ph2 decay both in the absence of additives and in the presence of Nh. Thus, the reaction of the Ph2 anion radical with Nh appears to be a process of activated electron tunneling. The reaction kinetics has been quantitatively studied only for the electron transfer from Ph2 to Nh because, in the case of 

such a study is hindered by an overlapping of the absorption spectra of particles involved in the reaction. 

From the kinetic curves of Ph2 decay in the absence and presence of Nh, assuming the spontaneous decay and the decay via the reaction with Nh to be independent, one can extract the kinetic curve for the reaction of Ph2 with Nh. Such kinetic curves obtained for several concentrations of naphthalene, N, are depicted in Fig. 5. The experimental points obtained for different values of N, when plotted in the coordinates N 1 log [n(t)/n(t0)] vs. log , are seen to lie around the same straight line which corresponds to the kinetic law in eqn. (26) of Chap. 4 for electron tunneling reactions. The distance of tunneling for the reaction of Ph2 with Nh at 77 K estimated by the same method as that used for inorganic anion radicals (see above) proved to be 21A at t = 103s. 

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