Optical power limiting properties of two-photon absorbing fluorene and dithienothiophene-based chromophores

We have synthesized efficient two-photon absorbing fluorene and dithienothiophene (DTT) derivatives with electron donors (D) or electron acceptors (A) which are attached symmetrically or asymmetrically at the both end of conjugated links, forming D-pi-D and D-pi-A structures. The two-photon absorption (TPA) cross-section values (sigma) of these chromophores were evaluated by the nonlinear transmission measurements using 8 ns laser pulse, and also by the two-photon-induced fluorescence method with 80 fs pulse laser. The sigma values obtained were larger by the nanosecond pulse laser than that by the femtosecond laser. This result suggests that the relative contribution of the excited state absorption becomes more manifested when longer time scale pulses are used. The strong nonlinear absorption including TPA of all these materials induced an efficient optical power limiting (OPL) activity. The trend in OPL behavior agreed well with the size of the TPA intensity.