Two-dimensionally extended porphyrin tapes: Synthesis and shape-dependent two-photon absorption properties

We report the synthesis and characterization of L- and T-shaped porphyrin tapes as extensible structural motifs of two-dimensionally extended porphyrin tapes. The two-photon absorption (TPA) cross-section values (σ ⁽²⁾ for L- and T-shaped porphyrin tapes as well as those for linear trimeric and tetrameric porphyrin tapes were measured by an open-aperture Z-scan method at 2300 nm, a wavelength at which the one-photon absorption contribution is either zero or almost negligible. Under these conditions, the σ⁽²⁾ values for the linear porphyrin tape trimer and tetramer were determined to be 18500 and 41200 GM, respectively. The σ ⁽²⁾ value for the L-shaped trimer was determined to be 8700 GM, which is only half that of the linear trimer, whereas the σ⁽²⁾ value for the Tshaped tetramer was measured to be 35700 GM. These results clearly indicate the dependence of the TPA crosssection on the molecular shape, which underscores the importance of directionality in the jt-conjugation pathway for the enhancement of TPA crosssection.