Ultrafast Dynamics and Strong Two-Photon Absorption Properties of Nonplanar β-Functionalized “Push–Pull” Copper Corroles with a Mixed Substituent Pattern

Abstract

A new series of nonplanar and unsymmetrically β-functionalized "push-pull" copper corroles, CuTPC(CHO)R₇ [R = H, Br, Ph, Me, or 2-thienyl (Th)], were synthesized and characterized to elucidate the effect of β-functionalization and nonplanarity on the photophysical, redox, and nonlinear optical (NLO) properties on the corrole ring. The synthetic route to unsymmetrically β-octasubstituted copper corroles includes bromination of CuTPC(CHO) to get CuTPC(CHO)Br₇ in 80% yield, which was further subjected to the Pd-catalyzed Suzuki reaction. CuTPC(CHO)Br₇ exhibited a large red shift in the Soret band (Δλ_max = 35-40 nm) and both the Q bands (Δλ_max = 10-50 nm), as compared to CuTPC and CuTPC(CHO). CuTPC(CHO)Br₇ was 510 and 290 mV anodically shifted in the first oxidation and the first reduction compared to CuTPC owing to the strong -I effect of CHO and Br groups. Density functional theory studies revealed that all the β-octasubstituted copper corroles exhibited highly nonplanar saddle-shape conformation of the corrole ring. Very high torsional saddling was observed for CuTPC(CHO)Th₇ (79-83°) than that for CuTPC (49-53°), even larger than that for CuTPCBr₈ (67-70°). Femtosecond laser-induced third-order NLO studies from these copper corroles showed strong two-photon absorption cross-sections (0.48-6.98 × 10⁴ GM) and self-focusing-type positive nonlinear refraction behavior. The observed structure-dependent two-photon absorption coefficients (β) are in the range of ∼2.7-20.9 × 10^-12 m/W, and the <i>n</i>₂ values are in the range of ∼0.64-6.45 × 10^-18 m²/W. The present results may facilitate a new window for these copper corroles in nonlinear optical devices, femtosecond optical limiters, and many other ultrafast photonic applications.