Indolic squaraines as two-photon absorbing dyes in the visible region: X-ray structure, electrochemical, and nonlinear optical characterization

X-ray structure, electrochemical, and nonlinear optical characterization of indolic squaraines as Two-Photon absorbing (TPA) dyes in the visible region was investigated. Squaraines are the condensation product of electron rich molecules, usually activated arenes,π-excessive heterocycles and heteromorphic anhydrobases, with squaric acid. Squaraines were synthesized by refluxing the appropriate indole precursor and squaric acid in a BuOH/toluene mixture under a Dean-Stark trap for a required period of time. Cyclic voltammetry experiments were performed to probe molecular energy levels and the extent of intramolecular donor-acceptor interactions. It was reported that the design and synthesis of π-extended squaraines exhibiting very large TPA cross section efficiencies localized in the NIR region, with the peak performances above 1μm.