Importance of molecular size on the dynamics of solvent relaxation

The time evolution of the Stokes shift of the twisted intramolecular charge-transfer emission from (dimethylamino)benzonitrile (DMABN) and (diethylamino)benzonitrile (DEABN) is examined in 1-propanol solution. Over the temperature range from -10 to -507deg;C the Stokes shift correlation function, C(t), is nonexponential with an average relaxation time, 〈τ_s〉, different from that predicted by theories that model the solvent as a dielectric continuum. In addition, 〈τ_s〉 for DMABN is faster than for DEABN, clearly showing that the solvent relaxation measured by C(t) is dependent on the size of the solute. The data are compared with recent mean spherical approximation (MSA) models for ion and dipole solvation that take into account the relative sizes of the solvent and solute. The ion-MSA theory provides a reasonable fit to the data; however, the dipole-MSA theory significantly underestimates the time scale of the solvent relaxation. Comparison with related studies on the solvation of Coumarin 153 (C153) in 1-propanol reveals that 〈τ_s〉 decreases in the order C153 > DEABN > DMABN, opposite to that predicted by the MSA models.