The 10-methylacridinium ion displays an emission associated with a charge-shifted (CSH) species when substituted in the 9-position with a substituent having a relatively low ionization potential (naphthyl, biphenyl). Flash photolysis and time-resolved EPR (TREPR) measurements show that photoexcitation of these donor−acceptor systems generates an acridinium-localized excited (LE) triplet state. While values of zero-field splitting parameters are virtually unaffected by the nature of the substituent, spin polarization patterns observed in the TREPR spectra display a striking dependence on substituent as well as orientation of donor ring system. Flash photolysis and TREPR data show that the LE triplet state is formed from the CSH singlet state. In these directly linked donor−acceptor molecules, in which the aromatic rings are near perpendicular because of steric hindrance, CSH singlet → LE triplet intersystem crossing (isc) is driven by spin−orbit coupling. This mechanism generates a unique dependence of isc spin selectivity on molecular structure which in one case even results in a temperature-dependent spin polarization pattern. The results demonstrate that TREPR can be a valuable source of information on molecules with twisted internal charge-transfer (TICT) states.

Photoprocesses in benzonitrile solutions of C60 and chloranil (CA) have been studied by complementary techniques of nanosecond laser photolysis and Fourier transform EPR. Direct oxidation of 3C60 by CA is slow (k = (2.0 ± 0.3) × 107 M-1 s-1), consistent with the high oxidation potential of 3C60. However, the formation rate and yield of CA- are much increased by addition of perylene (Pe) or tritolylamine (TTA) via the fast reactions 3C60 + Pe → C60 + 3Pe, followed by 3Pe + CA → Pe+ + CA-, or 3C60 + TTA → C60 - + TTA+, followed by C60 - + CA → C60 + CA-. These reactions utilize the broad absorption and initial high triplet yield of C60, as well as the low oxidation potential of 3Pe or high reduction potential of 3C60, to catalyze efficient formation of CA- and enhance separation of radicals. Triplet C60 also reacts with Pe by electron transfer, forming Pe+ and C60 - with rate one-third that of energy transfer. However, the CA- formed in the Pe-catalyzed reaction is strongly spin-polarized, indicating that it is formed primarily via the 3Pe pathway. The extinction coefficient of C60 - at 1080 nm is measured (ε = 18 300 ± 1100 M-1 cm-1) using the TTA reaction.

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