Electronic Properties of a New Two-Photon Absorbing Fluorene Derivative: The Role of Hartree-Fock Exchange in the Density Functional Theory Design of Improved Nonlinear Chromophores

One- and two-photon absorption (2PA) properties of a new fluorene derivative with diphenylamino and 2-(2\u27-hydroxyphenyl)benzothiazole substituents were investigated theoretically using the density functional theory approach with different types of functionals and experimentally by two-photon induced fluorescence methodology. The effect of the exchange-correlation functional choice oil the description of 2PA properties of fluorenes was analyzed. The best agreement of the experimental and theoretical 2PA spectra was obtained with a custom modification of the hybrid M05 functional that included 35% of Hartree-Fock exchange. This functional is recommended for reliable prediction of nonlinear optical properties of conjugated molecules. A chemical modification of the studied compound is suggested to increase the 2PA cross section