Excited singlet state kinetics on a subnanosecond time-scale

Ultrashort-lived excited states of aminophthalimides in fluid solution

The Sn-->S0 ultraviolet fluorescence spectra (270-380 nm) of 4-aminophthalimide (4AP) and its N-methyl derivative 4-amino-N-methylphthalimide (4ANMP) are reported, following the absorption of two laser pulses. In polar but non-hydrogen-bonding solvents, both molecules exhibit a principal emission maximum near 290 nm, whereas solutions in hydrogen-bonding solvents display two prominent emission bands, near 300 and 350 nm. The relative intensities of these bands depend on solvent type and, in a pump-probe experiment, on the wavelengths and temporal spacing of two ultrashort pulses. Experiments covering the range approximately 0.1-100 ps showed evidence for at least two distinct ultrashort relaxation processes, the rates of which depend on solvent. Fluorescence upconversion experiments at < 0.2 ps resolution have shown that the longer-duration process correlates with the fluorescence Stokes shift, and provide evidence that the solvent-dependent shift of the S1-->S0 fluorescence spectrum is reflected in the fluorescence quantum efficiency of a nearby electronic state.