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

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, S₀S₁, and second, S₀S₂, excited states were determined. It was shown experimentally that a decrease in the angle between S₀S₁and S₀S₂ 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.