Photo-induced energy and electron transfer in carboxylic acid functionalized bis(4′-tert-butylbiphenyl-4-yl)aniline (BBA)-substituted A3B zinc porphyrins

Abstract

Three bis(4′-tert-butylbiphenyl-4-yl)aniline (BBA)-substituted A3B zinc porphyrins containing BBA attached to the three meso-positions of the porphyrin macrocycle via varied spacers i.e., directly connected, phenyl, and ethoxy phenyl, and carboxylic acid functional group at the other meso-position, were synthesized and fully characterized using 1H, 13C NMR, MALDI-TOF, HRMS (ESI+), UV-visible and fluorescence techniques. The steady-state absorption spectrum of Tetrad-1 revealed bathochromically shifted peaks indicating notable interactions between BBA and ZnP moieties while the interactions were gradually decreased in the case of Tetrad-2 and Tetrad-3 due to the introduction of the phenyl and ethoxy phenyl spacer between the chromophores. Steady-state fluorescence studies have revealed that the selective excitation of the BBA moiety at 352 nm in the tetrads resulted in the quenched BBA emission at 395 nm followed by the appearance of the porphyrin emission at 618 or 605 nm indicating the occurrence of photo-induced energy transfer (PEnT) from singlet excited BBA to ZnP i.e., 1(BBA)3*-ZnP-CO2H →\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\to$$\\end{document} (BBA)3-1ZnP*-CO2H, with the efficiencies of 28-68%. The comparison of the first oxidation potential and E0-0 of BBA and the first reduction potential of ZnP revealed the possibility of photo-induced electron transfer (PET) from the 1BBA* to ZnP, in addition to the PEnT, and the component was calculated to be 27-70%. The density functional theory (DFT) studies have shown that, in these tetrads, HOMO is mainly concentrated on BBA moieties while LUMO on the porphyrin macrocycle and the carboxylic acid functional group indicating that, once anchored onto TiO2 nanoparticles, these tetrads can be utilized as the sensitizers for DSSCs. Three BBA-functionalized zinc porphyrins, Tetrads 1-3, attached to the three meso-positions of the porphyrin macrocycle via three different spacers and a carboxylic acid functional group at the other meso-position were synthesized and the structural integrity is fully characterized. Selective photo-excitation of the BBA moiety in these tetrads lead to photo-induced energy transfer (PEnT) from 1BBA* to ZnP thus generating 1ZnP*. Based on the fluorescence quenching efficiencies and % PEnT, the possibility of the photo-induced electron transfer (PET) from the 1BBA* to ZnP is also explored. Computational studies provided insight into the frontier energy levels of the tetrads and the possibility to use them as the sensitizers for DSSC applications.