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 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.