TIME-RESOLVED EPR STUDY OF PHOTOEXCITED TRIPLET-STATE FORMATION IN ELECTRON-DONOR-SUBSTITUTED ACRIDINIUM IONS

The 10-methylacridinium ion displays an emission associated with a cha rge-shifted (CSH) species when substituted in the 9-position with a su bstituent having a relatively low ionization potential (naphthyl, biph enyl). Flash photolysis and time-resolved EPR (TREPR) measurements sho w that photoexcitation of these donor-acceptor systems generates an ac ridinium-localized excited (LE) triplet state. While values of zero-fi eld 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 o f 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 mech anism generates a unique dependence of isc spin selectivity on molecul ar 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 ch arge-transfer (TICT) states.