Porphyrin covalently functionalized MoS₂ nanosheets: “Click” synthesis and tunable nonlinear absorption

Functionalized MoS₂ nanosheets are of significant academic and commercial interest due to their use in electronic and photonic applications as transistors, electrocatalysts, electrodes, and nonlinear optical materials. However, MoS₂ nanosheets exhibit low reactivity and poor dispersibility in common solvents, which significantly hinders their functionalization with optoelectronic groups and their downstream applications. Herein, a porphyrin covalently functionalized MoS₂ nanohybrid (MoS₂-Por) is synthesized by a mild thiol−ene “click” reaction. Optical studies demonstrate that a dispersion of the MoS₂-Por nanohybrid in N,N-dimethylformamide (DMF) exhibits reverse saturable absorption (RSA) behavior to femtosecond pulsed laser irradiation at 800 nm, in contrast to the saturable absorption (SA) displayed by MoS₂ nanosheets. This significant switch likely stems from enhanced two-photon absorption (TPA) in the MoS₂ nanosheets, which may be ascribed to efficient charge transfer in the excited-state of the MoS₂-Por nanohybrid. Polymethyl methacrylate (PMMA) films MoS₂-Por/PMMA exhibit a low optical limiting threshold of 1.2 mJ cm^-2 (18.3 GW cm^-2), indicating great potential as an optical limiter. The MoS₂-Por nanohybrid also exhibits stronger self-focusing than MoS₂ nanosheets towards femtosecond pulses at 800 nm due to the increased free-carries concentration arising from the improved TPA. These observations demonstrate that the MoS₂-Por nanohybrid is an excellent candidate for optical limiting in the infrared region towards femtosecond pulses, and shed new light on the construction of covalently functionalized MoS₂ nanohybrids for tuning nonlinear optical properties and broadening application scope.