Summary: J. Phys. Chem. 1990, 94, 2959-2963
Dynamic Stokes Shift in Coumarin: Is It Only Relaxation?
Physical Science Laboratory, DCRT, and Laboratory of Chemical Physics, NIDDK, National Institutes of
Health, Bethesda, Maryland 20892 (Received: July 31, 1989)
Picosecond fluorescence spectra of coumarin 153 in solution show transient line narrowing. The peak`s frequencyshift and
its area decay are nonexponentialin time. A biexponential fit reveals a correlation between the fast exponents for the shift
and area decay at different temperatures. A plot of shift as a function of area is temperature independent at short times.
These findings are consistent with an inhomogeneous mechanism for the early part of the dynamic Stokes shift. Several
suggested experiments may help distinguish between inhomogeneous and relaxation mechanisms.
Dynamics of solvation is a rapidly growing field, as manifested
by a number of recent review articles.'-2The simplest ultrafast
solvation process occurs when a rigid dye molecule is electronically
excited in a polar solvent. The excited molecule typically has a
much larger dipole moment. When its fluorescence spectrum is
followed as a function of time, a red shift is observed.'-8 Initially,
its peak is close to that of the absorption spectrum and with time
approaches the maximum of the steady-state fluorescence spec-