Strong fluorescence following two-photon excitation in fully conjugated oxadiazole-based oligoaryl

Highly fluorescent oxadiazole-based chromophore with alternating alkoxy benzene and thiophene moieties has been designed and synthesized, and large two-photon absorption (TPA) cross section in near infrared (NIR) region was performed. DFT calculation predicted fully π-conjugation among molecular skeleton and minor behavior of intra-molecular charge transfer (ICT). The solvent dependent emission spectra also suggested the occurrence of ICT was trivial, although oxadiazole was electron deficient and alkoxy benzene was electron rich. On the other hand, the resulting chromophore exhibited fluorescence quantum yield up to 75% because of the incorporation of oxadiazole. The quadratic dependence of emission intensity on input excitation power indicated TPA mechanism under irradiation of NIR laser. Based on two-photon excited fluorescence method, the designed fluorophore exhibited the largest TPA cross section (620 GM) at 850 nm. The cooperative phenomena of large TPA response and strong fluorescence output in this designed molecule might ensure a potential use for application of two-photon excited light-emitting materials.