Evidence for the TICT mediated nonradiative deexcitation process for the excited coumarin-1 dye in high polarity protic solvents

Photophysical properties of coumarin-1 (C1) dye in different protic solvents have been investigated using steady-state and time-resolved fluorescence measurements. Correlation of the Stokes' shifts (Δν¯) with the solvent polarity (Δf) suggests the intramolecular charge transfer (ICT) character for the dye fluorescent state. Fluorescence quantum yields (Φf) and lifetimes (τf) of the dye show an abrupt reduction in high polarity solvents having Δf >∼0.28. In these solvents τf is seen to be strongly temperature dependent, though it is temperature independent in solvents with Δf <∼0.28. It is inferred that in high polarity protic solvents there is a participation of an additional nonradiative decay process via the involvement of twisted intramolecular charge transfer (TICT) state. Unlike present results, no involvement of TICT state was observed even in strongly polar aprotic solvent like acetonitrile. It is indicated that the intermolecular hydrogen bonding of the dye with protic solvents in addition with the solvent polarity helps in the stabilization of the TICT state for C1 dye. Unlike most TICT molecules, the activation barrier (ΔEa) for the TICT mediated nonradiative process for C1 dye is seen to increase with solvent polarity. This is rationalized on the basis of the assumption that the TICT to ground state conversion is the activation-controlled rate-determining step for the present system than the usual ICT to TICT conversion as encountered for most other TICT molecules.