Excited-state dipole moments of some Coumarin dyes from a solvatochromic method using the solvent polarity parameter, E N T

Abstract

Excited-state dipole moments of a set of Coumarin dyes extensively used in laser applications have been determined for the first time, employing a solvatochromic procedure utilising the microscopic solvent polarity parameter, E^N _T. Our results are expected to be quite reliable in view of the fact that the correlation of the solvatochromic Stokes shifts is superior to that obtained using bulk solvent polarity functions. The experimentally determined dipole-moment changes are in fair agreement with recent semi-empirical computational predictions. It is shown that the discrepancies observed in some cases can be overcome using more appropriate parametrisation in the calculation. The insight provided by the present measurements into the nature of the emitting state is mentioned.