One- and Two-Photon Fluorescence Anisotropy of Selected Fluorene Derivatives.

Abstract The steady-state excitation anisotropy spectra of fluorene derivatives were measured in viscous solvents, under the one- and two-photon excitation, over a broad spectral range (UV–Visible). The orientation of their absorption transition moments for the first, S 0 ?S 1 , and second, S 0 ?S 2 , excited states were determined. It was shown experimentally that a decrease in the angle between S 0 ?S 1 and S 0 ?S 2 transitions corresponded to an increased value of two-photon absorption (2PA) cross section for these molecules. Two-photon excitation anisotropy was nearly constant over the spectral region investigated (in contrast to one-photon excitation anisotropy spectra) and can be roughly explained by a simple model of 2PA based on the single intermediate state approximation. For comparison, the same trend in two-photon excitation anisotropy was observed for Rhodamine B inglycerol. [ABSTRACT FROM AUTHOR]