Solvent Effect and Two-Photon Optical Properties of Triphenylamine-Based Donor-Acceptor Fluorophores

In this work we present a systematic investigation on the optical properties of two triphenylamine (TPA)-based donor-acceptor fluorophores: TPA-PA (phenylaldehyde) and TPA-BMO ((Z)-4-benzylidene-2-methyloxazol-5(4H)-one). The two compounds are dissolved in nine different organic solvents as dilute solutions in order to analyze the effect of solvent on their linear and nonlinear optical properties. For each compound under one-photon excitation, its fluorescence emission spectrum red-shifts more than 160 nm as the solvent polarity increases from hexane to MeCN, while the fluorescence quantum efficiency and lifetime reach maximum magnitudes in solvents with medium polarity. The quantum efficiency reaches as high as 0.72 in dioxane for TPA-PA and 0.69 in Et2O for TPA-BMO, respectively. These TPA-PA and TPA-BMO solutions are also strongly emissive upon appropriate two photon excitation, with fluorescence emission spectra identical to those under corresponding one-photon excitation. The maximum two-photon absorption cross sections are similar to 160 GM (Goeppert-Mayer units) and 250 GM for TPA-PA and TPA-BMO, respectively, regardless of the solvent identity. Particularly, for TPA-BMO solutions in strongly polar solvents, dual fluorescence peaks are observed in steady state, and distinct relaxation dynamics are detected in fluorescence decays for the two emission peaks. These dual fluorescence emission spectra and dynamics could be interpreted as signs of charge-transfer state formation.