Synthesis of 2-nitro-3-(pyrrol-1-yl)-5,10,15,20-tetraarylporphyrins via a Clauson-Kaas reaction and the study of their electronic properties

Abstract

A facile synthetic methodology for the construction of novel porphyrin building blocks, 2-nitro-3-(pyrrol-1-yl)-5,10,15,20-tetraarylporphyrins, has been developed. The target β,β′-disubstituted porphyrins were prepared in good to excellent yields through Clauson-Kaas reaction of 2-amino-3-nitro-5,10,15,20-tetraarylporphyrins with 2,5-dimethoxytetrahydrofuran in the presence of acetic acid in toluene at 115 °C. After successful spectroscopic characterization, some of these representative porphyrin macrocycles have been evaluated for their photophysical and electrochemical properties by using UV-Vis, fluorescence spectroscopy and cyclic voltammetry, respectively. Interestingly, the electronic absorption spectra of these porphyrins displayed broadened and red shifted Soret and Q bands possibly due to the extended conjugation. In addition, S₂ emission was observed along with the characteristic S₁ emission band in the fluorescence spectra of newly prepared free-base, Zn and Mg porphyrins.