Nondegenerate two-photon absorption properties of a newly synthesized carbazole derivative

Abstract

A two-color two-photon laser scanning microscope (2c2pLSM) has been developed for fluorescence imaging in tissues through the nondegenerate two-photon excitation of fluorescent proteins. However, fluorescent probes have not been widely designed and applied to 2c2pLSM. Here, we have synthesized a new carbazole derivative 3,6-bis(pyridin-4-ylethynyl)-9-methyl-carbazole (BEMC), and investigated its nondegenerate two-photon absorption (ND-TPA) properties. Nondegenerate two-photon excited fluorescence (ND-TPEF) and ND-TPA cross-sections excited by a constant pump wavelength of 800 nm and several probe wavelengths from 485 nm to 650 nm were studied based on the femtosecond pump–probe technique. The fluorescence intensity is linearly dependent on each laser intensity of the two beams, and the dependence of ND-TPEF intensity on time delay between the two lasers is in good agreement with the cross-correlation of the pulse widths of the two lasers. Compared with the corresponding degenerate two-photon absorption (D-TPA) cross-sections for transition to the same excited state via the D-TPA pathway, the ND-TPA cross-sections exhibit at least twofold larger values than the D-TPA case, which is attributed to the effect of intermediate state resonance enhancement (ISRE). This study indicates that the newly synthesized carbazole derivative would serve as a promising fluorescent probe in a two-color two-photon laser scanning microscope (2c2pLSM).