Enhanced optical limiting and nonlinear absorption properties of azoarene-appended phosphorus (V) tetratolylporphyrins

Abstract

Optical limiting performance, third-order nonlinearity X⁽³⁾, and nonlinear absorption properties have been investigated in a new class of azoarene phosphorus (V) porphyrins with charge transfer (CT) states. The introduction of axial azoarene groups into the phosphorus porphyrin structure is found to reduce the limiting threshold by a factor of 2 and lead to a rise in the second hyperpolarizability by 1 order of magnitude in the picosecond time regime and by 2 orders of magnitude in the nanosecond regime. The experimental data show reverse saturation of absorption in the nanosecond time regime and a saturation of the nonlinear absorption above a fluence of 0.5 J/cm² in the picosecond regime. The presence of the CT state reduces saturation of excited-state absorption (ESA) in the S₁ → S_n transition through the S₁ → CT transition. Faster CT → T₁ transition increases the ESA from T₁ → T_n states in the nanosecond regime. A self-consistent theoretical analysis based on rate equations is used to estimate the high-lying excited-state lifetimes and absorption cross sections from the experimental results.