Dual-Mode Nonlinear Optical Response in Metal–Organic Chromophores for Optical Limiting

This study reports the synthesis, photophysical characterization, and near-infrared (near-IR) optical limiting (OL) response of a two-photon absorbing chromophore, 1, and two metal–organic chromophores, Ru-1 and Ir-1. Chromophore 1 was designed by connecting triphenylamine units to the 5,5′-positions of a 2,2′-bipyridine core through alkyne linkages, forming a centrosymmetric, quadripolar donor–acceptor–donor (D-A-D) architecture known to promote strong two-photon absorption (2PA). The metal–organic chromophores Ru-1 and Ir-1 were designed to exploit the high spin–orbit coupling of Ru(II) and Ir(III), enabling efficient population of long-lived triplet states that can undergo triplet excited-state absorption (ESA). The excited states of all target molecules were investigated by using femtosecond and nanosecond transient absorption spectroscopy. The optical limiting performance of the chromophores in solution toward 5 ns pulses was evaluated by using an open aperture, nonlinear transmission set up. Ru-1 and Ir-1 display strong OL responses to 5 ns pulses at 1064 nm, arising from two distinct nonlinear pathways: 2PA and ESA. These findings provide valuable insight into the molecular design strategies required to achieve dual-mechanism optical limiting materials suitable for practical applications.