Electronic Nature of Nonlinear Optical Properties of a Symmetrical Two-Photon Absorbing Fluorene Derivative: Experimental Study and Theoretical Modeling

A comprehensive experimental and theoretical study of linear photophysical properties, such as excited-state relaxation, two-photon absorption, and stimulated emission spectra of the symmetrical fluorene derivative 2,2′-((1E,1′E)-(9,9-diethyl-9H-fluorene-2,7-diyl)bis(ethene-2,1-diyl))bis(1-methyl-1H-pyrrole) (1), is presented. The steady-state absorption, fluorescence, excitation, and excitation anisotropy spectra of 1 in organic solvents of different polarities are investigated experimentally and modeled. The fluorescence solvatochromism of 1 suggests the occurrence of symmetry breaking in the first excited state. The nature of fast relaxation processes in the excited state of 1, with the characteristic times of several picoseconds, is investigated by transient absorption femtosecond pump-probe spectroscopy. The spectral properties of 1 are satisfactorily described by an essential-state model that, accounting for electron-vibration coupling and for polar solvation, addresses spectroscopic features not only in terms of band position and intensities but also in terms of band shapes. Specifically, we present the first calculation of frequency-resolved two-photon-excited fluorescence anisotropy spectra. Our results demonstrate that electron-vibration coupling and polar solvation quite naturally explain the puzzling experimental observation of large deviations of the anisotropies from the values expected on the basis of the relative orientation of the molecular transition dipole moments.