Linear and Nonlinear Optical Properties of Quadrupolar Bithiophenes and Cyclopentadithiophenes as Fluorescent Oxygen Photosensitizers

Abstract

The linear and nonlinear optical properties of two quadrupolar bithiophenes and two quadrupolar cyclopentadithiophenes have been investigated. At the 5,5′ positions of the central bi/dithiophene units, the molecules possess 1,4-phenylalkynyl groups that bear either electron-donating (NPh2) or electron-withdrawing (SO2CF3) groups. The optical properties were experimentally studied and modelled via quantum chemistry computations of key configurations and conformations. All the compounds show good light harvesting efficiency due to their strong absorption in the visible range. These fluorescent compounds are also good two-photon absorbers in the NIR range that can photosensitize oxygen in toluene. DFT calculations reveal that the mixtures of conformers in a solution show similar linear optical properties. TD-DFT calculations reproduce the experimental spectroscopic data fairly well, including vibronic couplings in the fluorescence spectra. The lowest excited state for two-photon absorption corresponds to the S2 state. The roles of the SO2CF3 and NPh2 terminal groups on the nonlinear response were analyzed for possible bio-oriented applications, with the cyclopentadithiophenes showing the most promising figures of merit.