Photophysical
properties of coumarin-1 (C1) dye in different protic solvents have
been investigated using steady-state and timeresolved fluorescence
measurements. Correlation of the Stokes shifts (Δv-) with the solvent polarity (Δf) suggests the intramolecular charge transfer (ICT) character for the dye fluorescent state. Fluorescence quantum yields (Φf) and lifetimes (tf) of the dye show an abrupt reduction in high polarity solvents having Δf
>~0.28. In these solvents sf 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.