Push–pull octaphenylporphyrins with mixed substituents pattern: Synthesis, redox, ultrafast dynamics and nonlinear optical studies

Abstract

A new family of β-functionalized octaphenylporphyrins, MOPP(NO2)Br3 (M = 2H, ZnII, NiII, or CuII, and OPP = octaphenylporphyrin) has been synthesized possessing unsymmetrical substitutions of bromo and nitro groups and the effect of asymmetrical substitutions on the redox, spectral, ultrafast dynamics, and nonlinear optical properties of the porphyrin macrocycles are investigated. Routes for the mixed functionalized OPPs begin with the bromination of NiOPP(NO2), leading to the formation of NiOPP(NO2)Br3 and H2OPPBr4. These ‘push–pull’ OPPs exhibited nonplanar conformation, red-shift in absorption spectra, tunable redox properties, and very high ground state dipole moments due to nonplanarity and asymmetric β-substitutions. All synthesized porphyrins exhibited a bathochromic shift (λmax = 23–38 nm) in the B band and λmax = 21–83 nm in the last Q band in comparison to MOPP due to attachment of the bromo and nitro groups on its peripheral positions. In the cyclic voltammograms, the first oxidation and reduction potential showed an anodic shift compared to its parent porphyrins due to the presence of strong electron acceptor bromo and nitro groups, which are present on the porphyrin periphery. These β-functionalized octaphenylporphyrins have shown excellent nonlinear optical response. The observed structure-dependent nonlinear refractive index (n2) values are in the range of ∼ 0.37–1.68 × 10−19 m2 W−1, and two-photon absorption coefficient (β) values are in the range of ∼ 0.19–4.56 × 10−12 m W−1. The transient absorption studies reveal that the decay dynamics of ZnOPP(NO2)Br3 exhibited long-lived τ1 component, while NiOPP(NO2)Br3 exhibited long-lived τ2 component than other OPP derivatives.