Solvent dependence of ultrafast ground state recovery of the triphenylmethane dyes, brilliant green and malachite green

We have studied femtosecond ground state recovery dynamics of the triphenyl methane dyes brilliant green (BG) and malachite green (MG) by pump-probe sp ectroscopy at the center wavelength of 635 nm with a time resolution of 33 fs. The ultrafast recovery of the ground state bleach was highly nonexponen tial and depended on the solvent viscosity, although all time constants wer e shorter than the solvation times obtained from other measurements. We obs erved a plateau or a rise component in the signal, which indicates an inter mediate state. The rise time showed a viscosity dependence, even in the ult rafast time domain. It should be noted that the decay times were always lon ger for BG than MG, while the rise time did not show a solute dependence. T he torsional motion of the amino-substituted phenyl group may be involved i n the ultrafast process to the intermediate state, but lack of a solute dep endence indicates that only a small conformational change is involved. (C) 1999 Elsevier Science B.V. All rights reserved.