Enhanced two-photon absorption of sandwich-like coordination complexes based on squaraine and metallomacrocycle derivatives

Covalent or noncovalent multi-chromophore structures have expanded the discovery of appreciable nonlinear optical (NLO) materials. We present here a report of how drastic changes in two-photon absorption (2PA) coefficients can enhance from slight changes of electronic delocalization through coordination interaction. Experimental results illustrate that 2PA is the dominant phenomenon contributing to the NLO properties of the complexes recorded in the range of 750-1550 nm by open aperture Z-scan method. The values of degenerate 2PA cross sections (62PA) can be enhanced obviously by coordination interactions assisted by steric effect and intermolecular charge transfer, not showing a simply sum of the nonlinear parameters of the starting components. Importantly, the complexes feature high 62PA values up to 12,700 GM at 1400 nm in the second nearinfrared region. Both complexes with large 2PA coefficients and multiple excitation wavelengths may offer a novel avenue for creating useful nonlinear absorbing materials.