H. Vanwilligen et al., TIME-RESOLVED EPR STUDY OF PHOTOEXCITED TRIPLET-STATE FORMATION IN ELECTRON-DONOR-SUBSTITUTED ACRIDINIUM IONS, Journal of physical chemistry, 100(9), 1996, pp. 3312-3316
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.