화학공학소재연구정보센터
Journal of the American Chemical Society, Vol.118, No.38, 9042-9051, 1996
Donor-Acceptor Interaction and Photochemistry of Polymethylene-Linked Bichromophores in Solution
The ground-state and the excited-state spectroscopic properties of four series of polymethylene-linked anthracene-dialkylaniline bichromophores were compared as a probe to the relationship between energetics and distance in photoinduced electron transfer (PET). The results demonstrate that, when the energy level of the charge transfer (CT) stale is lowered below that of the localized excited state by appropriate substituents, there is a strong electron-donor-acceptor (EDA) interaction in the ground state which is absent in other bichromophores. Absorption and fluorescence excitation studies revealed that there is an unusually 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 this 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 solvent. 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 emission remained essentially unchanged. The risetime of the 605 nm emission 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 emission may be derived from the trans conformer of A-2, which is the major 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 orbital 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.