Synthesis, Spectral and Electrochemical Studies of β-Trisubstituted Porphyrins and Monobenzoporphyrins

Meso-Tetraarylporphyrins having electron-withdrawing substituents viz. nitro, formyl, acyl, or bromo groups is the key precursor to expand the chemistry of β-functionalized porphyrins.1 These β-functionalized porphyrins have been utilized in nonlinear optics, anion sensing, gas storage, photodynamic therapy (PDT), and dye-sensitized solar cells (DSSC).2 Modulating the degree of π-conjugation and introducing suitable donor-acceptor substituents at β-positions result red-shifted electronic spectral features, low HOMO-LUMO gap and high ground state dipole moment which are essentials for enhanced nonlinear optical behavior. Benzoporphyrin derivatives are currently under investigation as photosensitizers for PDT.3 Herein, we synthesized unsymmetrically functionalized trisubstituted porphyrins and monobenzoporphyrins in one step and characterized by various spectroscopic techniques viz. UV-vis, fluorescence, NMR, MALDI-TOF mass spectrometry and electrochemical studies. The synthesized trisubstituted porphyrins and monobenzoporphyrins were significantly red-shifted as compared to their precursor porphyrins. Further, these porphyrins exhibited a large anodic shift in reduction and cathodic shift in oxidation potentials due to extended π-conjugation and electron-withdrawing β-substituents. In this presentation, we will present the facile synthesis, photophysical and intriguing electrochemical redox properties of π-extended porphyrins. Figure 1. Molecular structures, absorption spectral profile and cyclic voltammograms of synthesized porphyrins. References: Moura, N. M. M. et al, J. Porphyrins Phthalocyanines 15, 2011, 652-658. (a) Senge, M. O. et al, Eur. J. Org. Chem. 2011, 5797-5816. (b) Sankar, M. et al, Chem. Commun. 41, 2012, 6481-6483. (c) Kumar, R. et al, Inorg. Chem., 53, 2014, 12706-12719. (d) Higashino, T. et al, Dalton Trans. 44, 2015, 448-463. (e) Sankar, M. et al, ACS Appl. Energy Mater. 1, 2018, 2793-2801. (f) Dar, T. A. et al, Green Chem. 21, 2019, 1757-1768. (a) de-Torres, M. et al, Chem. Commun. 51, 2015, 2855-2858. (b) Zhang, X. et al, ACS Nano 12, 2018, 4630-4640. (c) Grover, N. et al, Inorg. Chem. 58, 2019, 2514-2522. Figure 1