Two photon absorption properties of 1,3,5-tricyano-2,4,6-tris(styryl)benzene derivatives

Two-photon absorption (TPA) properties of 1,3,5-tricyano-2,4,6-tris(styryl)benzene derivatives have been investigated. Comparison of the absorption and fluorescence spectra reveals that these compounds show large Stokes shifts, which increase gradually as the conjugation length increases. One-photon absorption and excitation spectra are similar except that the latter exhibit several peaks near λ_max. It is also found that the one- and two-photon-induced fluorescence excitation spectra are quite similar, which indicates that the one- and two-photon allowed-excited states are the same. The peak TPA cross section values (δ_max) measured with nanosecond pulses by the two-photon-induced fluorescence method are in the range (50-2620) × 10^-50 cm⁴ s/photon. The δ_max value increases as the donor strength and conjugation length increase. A linear relationship is observed between δ_max and β, and this δ-β relationship is found to serve as a useful synthetic strategy for the design of novel TPA dyes with the octupolar structure.