Synthesis, characterization, and optical properties of two-photon-absorbing octupolar molecule with an s-triazine core

The octupolar molecule with s-triazine as the π-electron deficient core, N-ethylcarbazole groups as the electron-donating end-groups, 2,4,6-tris[2-( N-ethyl-3-carbazole)carboxethenyl]-1,3,5- s-triazine(III), was prepared. The assemblies of III, 2,4-dimethyl-6-[2-( N-ethyl-3-carbazole)carboxethenyl]-1,3,5- s-triazine(I), and 2-methyl-4,6-bis [2-( N-ethyl-3-carbazole)carboxethenyl]-1,3,5- s-triazine(II), were also prepared. The linear and nonlinear spectra of these compounds regularly vary: (1) the spectral positions ( λ_max values) of the linear absorption, the one-photon excited fluorescence (OPEF) and the two-photon excited fluorescence (2PEF) show regular red-shifts when the branch number increases from I to III; (2) the OPEF and the 2PEF intensities are all increased significantly when going from I to III. Excited by femtosecond laser at 800 nm wavelength, the fluorescence intensities of II, III show a quadratic dependence on the pump intensity, suggesting that the emission originates from a 2PA process. Two-photon absorption (2PA) cross-section of II, III is 397 GM and 994 GW, respectively. The value of σ₂/MW of II, III varies with the ratio of 1.0:1.8.