DONOR-ACCEPTOR INTERACTION AND PHOTOCHEMISTRY OF POLYMETHYLENE-LINKEDBICHROMOPHORES IN SOLUTION

The ground-state and the excited-state spectroscopic properties of fou r series of polymethylene-linked anthracene-dialkylaniline bichromopho res were compared as a probe to the relationship between energetics an d distance in photoinduced electron transfer (PET). The results demons trate that, when the energy level of the charge transfer (CT) stale is lowered below that of the localized excited state by appropriate subs tituents, there is a strong electron-donor-acceptor (EDA) interaction in the ground state which is absent in other bichromophores. Absorptio n and fluorescence excitation studies revealed that there is an unusua lly strong EDA interaction in the ground state of A-2 which is absent in other members in the A series. When A-2 is excited directly into th is EDA absorption, it exhibits two CT emissions, one at 490 nm and the other at 605 nm. The quantum yield (tau(f)) and the lifetime (Phi(f)) of the two emissions are dependent on the viscosity of the alkane sol vent. The Phi(f) and the tau(f) of the 490 nm emission increased when the solvent viscosity tvas increased; however, those of the 605 nm emi ssion remained essentially unchanged. The risetime of the 605 nm emiss ion is 420 ps, but that of the 490 nm emission is instrument-function limiting. The results suggest that the two CT emissions may be derived from the two different conformations of the CT state. The 490 nm emis sion may be derived from the trans conformer of A-2, which is the majo r conformer at equilibrium. The EDA interaction in the trans conformer of A-2 may be treated as a type of through-sigma-bond interaction via the spacer between the n orbital of the anilino nitrogen and the pi o rbital of the anthryl group. The direct excitation of A-2 from its EDA ground state to the CT state indicates that PET may occur in a sigma- linked bichromophoric system upon light absorption, thus reaching the limiting rate of a photochemical process.