Fluorene-based π-conjugated oligomers for efficient three-photon excited photoluminescence and lasing

A novel series of diphenylamino- and 1,2,4-triazole-end-capped, fluorene-based, π-conjugated oligomers that includes extended oligofluorenes and oligothienylfluorenes has been synthesized by means of the palladiumcatalyzed Suzuki cross-coupling of 9,9dibutyl-7-(diphenylamino)-2- fluorenylboronic acid and the corresponding 1,2,4,-triazole-based aryl halide as a key step. It was demonstrated that efficient two- and three-photon excited photoluminescence and lasing in the blue region are obtained by pumping near-infrared femtosecond lasers on these materials. Although the absorption and emission maxima of the highly fluorescent and extended oligofluorenes reach a saturation limit, there exists an effective conjugation length for an optimum three-photon absorption cross section in the homologous oligofluorene series. On the other hand, the multiphoton excited emission spectrum and lasing wavelength can easily be modified or tuned by an incorporation of thienyl unit(s) into the fluorene-based π-conjugated core with which exceptionally large three-photon absorption cross sections up to 3.59 x 10^-77cm⁶s ² in the femtosecond regime have been obtained, thereby highlighting the potential of this series of photonic materials. The optimized full width at half-maximum of the cavityless three-photon upconverted blue lasing spectra are sharply narrowed to approximately 6 nm with an efficiency of up to 0.013%.