Picosecond spectral relaxation of curcumin excited state in a binary solvent mixture of toluene and methanol.

Picosecond spectral relaxation of the excited state of curcumin in a binary solvent mixture of toluene and MeOH (or MeOH-d4) is reported with an instrument time resolution of ∼40 ps. With increasing mole fraction of MeOH (MeOH-d4) the fluorescence intensity and lifetime of curcumin increase to a maximum at a MeOH (MeOH-d4) mole fraction of 0.14 (0.40) and then decrease. In addition, fluorescence decays taken at the red edge of the emission spectrum started to show measurable rise times (170 to 30 ps), the magnitude of which decreased gradually with increasing alcohol mole fraction. This is attributed to the modulation of the nonradiative rates associated with the excited-state intermolecular H(D) bonding between the pigment and the polar protic solvent. As a consequence, the solvation times in the binary mixture were observed to slow down considerably (20-40 times) at certain solvent compositions compared to neat MeOH. The fact that three Gaussian components are needed to adequately represent the steady-state emission spectra and two isosbestic points are observed in the time-resolved area normalized emission (TRANE) spectra of the pigment suggests the existence of at least three species in the excited state. The observed results are rationalized with a scheme where ground state of the pigment exists in free and H-bonded (intermolecular) state. Optical excitation results in a mixture of these species in the excited state and the observed spectral relaxation correspond to the conversion of these two species in to a third species where dipolar solvation and intermolecular H-bonding have been optimized.