Trans-A2B2 type meso-5-Nitrothien-2-yl Porphyrins: Scalable Synthesis, Spectroscopic and Physicochemical Investigations

Abstract Trans -A 2 B 2 type porphyrins featuring 5-nitrothien-2-yl group at the meso- 5,15 positions of tetrapyrrolic macrocycles have been synthesized in a two-step process via a modified dipyrromethane approach. In the initial stage of the synthesis, the requisite dipyrromethane building blocks were efficiently synthesized through the acid-catalyzed condensation of the corresponding aldehyde with pyrrole using trifluoroacetic acid (TFA) as a catalyst. The second stage comprises the acidic condensation of substituted dipyrromethane with 5-nitrothiophene-2-carboxaldehyde/4-nitrobenzaldehyde in chloroform using TFA as a catalyst under the N 2 atmosphere, followed by DDQ oxidation. To elucidate the influence of the meso -substituent (thienyl versus phenyl) on the electronic properties, the structural, photophysical, and redox characteristics were evaluated using 1 H NMR, UV-Vis spectroscopy, and electrochemical analysis. The extent of electron delocalization, driven by the near-coplanarity of the thien-2-yl moieties with the central porphyrin π-core, was probed using electronic spectroscopy and cyclic voltammetry. The anodic shift of the redox potentials and the bathochromic shift in the electronic absorption spectra of the trans -A 2 B 2 type porphyrins bearing 5-nitrothien-2-yl groups at the meso-5,15-positions indicate strong electronic communication between the central porphyrin π-system and the peripheral substituents.