Picosecond dynamics of twisted internal charge transfer phenomena. The role of the solvent

To probe the role of the solvent in intramolecular charge transfer processes, and in particular, the origin of the well-known dual fluorescence phenomena of p-dimethylamino benzonitrile (DMABM), picosecond laser studies in mixed polar/nonpolar solutions were undertaken. The anomalous long wavelength emission is attributed to a complex formed between excited DMABN and butanol with a rate constant of (9.7±1.5)×108 M−1 s−1. The dominant stabilization of the twisted intramolecular charge transfer state is therefore concluded to be due to a short range specific interaction with a polar solvent molecule. A secondary solvent effect arises from a further stabilization of the complex by long range polarization interactions with solvent molecules. Evidence on the existence of ground state complexes between DMABN and butanol are also presented. Excitation of these ground state complexes leads to the rapid formation of the excited state complexes in 30 ps, which we have interpreted to be the time required for the complex to relax intramolecularly, presumably a rotational motion along the C–N bond of DMABN, to achieve the final twisted charge transfer geometry.