Lang Lab

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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 distanceinphotoinducedelectrontransfer(PET). Theresultsdemonstratethat,whentheenergylevelofthecharge- transfer (CT) state is lowered below that of the localized excited state by appropriate substituents, there is a strong electron-donor-acceptor(EDA)interactioninthegroundstatewhichisabsentinotherbichromophores. 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 (τf) and the lifetime (Φf) of the two emissions are dependent on the viscosity of the alkane solvent. The Φf and the τf of the 490 nm emission increased when the solvent viscosity was 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. TheresultssuggestthatthetwoCTemissionsmaybederivedfromthetwodifferentconformationsofthe CT state. The 490 nm emission may be derived from the trans conformer of A-2, which is the major conformer at equilibrium. TheEDAinteractioninthetransconformerofA-2maybetreatedasatypeofthrough-σ-bondinteraction viathespacerbetweenthenorbitaloftheanilinonitrogenandtheπorbitaloftheanthrylgroup. Thedirectexcitation of A-2 from its EDA ground state to the CT state indicates that PET may occur in a σ-linked bichromophoric system upon light absorption, thus reaching the limiting rate of a photochemical process.