Two-photon Upconversion Fluorescence Property in a Star-shaped Pyridine Chromophore

The two-photon absorption and the two-photon-excited fluorescence spectra of a star-shaped pyridine chromophore, 2,5-bis{4-{4-[N, N-bis(4-pyridylethenyl)phenylamino styryl}phenyl}-1,3,4-oxadiazole (PyPASPO), were measured via a femtosecond Ti;sapphire laser at 800 nm. Its two-photon absorption cross-sections obtained by nonlinear transmission method is 412.5 GM in the solution of CH(2)Cl(2) and 288.8 GM in the solution of THF. The linear absorption, one-photon fluorescence, fluorescence lifetime, and excitation-emission-fluorescence intensity 3D spectrum were investigated. When pumped with 800 nm femtosecond laser irradiation, chromophore PyPASPO shows strong two-photon excited green fluorescence at 571 nm in CH(2)Cl(2) and at 525 nm in THF, respectively. In CH(2)Cl(2) the maximum peaks of single-photon-excited fluorescence is at 532 nm and the fluorescence lifetime is 1.24 ns. The electrochemical properties was explored by cyclic voltammetry. The HOMO energy level is -5.21 eV and the LUMO energy level is -2.92 eV. Enhancing the charge transfer extent was adapted to increase the two-photon absorption and upconversion fluorescence emission of the star-shaped pyridine chromophore. Compound PyPASPO having four pyridine and two triphenylamine groups at the oxadiazole core not only shows largest 2PA cross-sections at 800 nm but also has strong the two-photon excited frequency upconversion emission in the green region. This work opens broad perspectives for new molecular design of 2PA-organic materials.