Biphenyl derivatives with enhanced nonlinear absorptivities for optical limiting applications

Abstract

Novel conjugated chromophores were designed and investigated for optical power limitation based on multiphoton absorption processes. Their design is based on the push-push functionalization of a semi-rigid elongated system derived from the extension of biphenyl cores. Biphenyl moieties with tunable twist angle were examined. Phenylene-vinylene rods were selected as connecting spacers between the core and the electroactive end groups to ensure effective electronic conjugation while maintaining suitable transparency. These derivatives combine wide linear transparency and enhanced nonlinear absorptivities in the visible range. Pump-probe Kerr ellipsometry indicates large excited-state absorption cross-sections (with typical σ_e values of 5 10^-16 cm²) while nanosecond nonlinear transmission measurements and optical limitation experiments reveal very strong nonlinear absorption that can be fitted by a three-photon absorption process (leading to α₃ values up to 18000 cm³ GW^-2). Such behavior results from a sequential multiphoton process involving excited-state absorption subsequent to two-photon excitation (with typical σ₂ values of 5 10^-20 cm⁴ GW^-1). Both the linear transparency, the photostability and the nonlinear absorption spectral characteristics of these derivatives can be tuned by playing on the biphenyl twist angle. As a result, chromophores combining good linear transparency and enhanced nonlinear absorptivities in the visible range have been obtained.