β-substituted donor-acceptor porphyrins: Synthesis, energy transfer and electrochemical redox properties

Abstract

β-substituted D-A porphyrins have been synthesized via Suzuki coupling reaction and characterized by various spectroscopic techniques. Two different electron donating moieties i.e. 4-(carbazol-9-ylmethyl)phenyl and 4-(phenothiazin-10-ylmethyl)phenyl were appended at antipodal β-positions of electron acceptor moiety i.e. H2TPP(NO2). The absorption and emission data suggested an interaction between the donor and acceptor subunits. An efficient energy transfer (78–88%) from β-substituents to macrocyclic core was supported by steady state and time resolved fluorescence measurements. Density functional theory (DFT) and time dependent-density functional theory (TD-DFT) studies support the existence of intramolecular energy and charge transfer from β-donor substituents to macrocyclic core in the dyads. Förster energy transfer studies revealed that the nature of energy transfer was mainly through bond (χTB = 0.71–0.83) in nature. The basic nature of porphyrin core was elucidated by very high protonation constants (logβ2 = 9.59–10.45). The first and second oxidation potentials of 1, 2 and their Zn(II) complexes (1a and 2a) were cathodically shifted as compared to H2TPP(NO2) and ZnTPP(NO2) due to effective electron donating nature of β-substituents which led to lower HOMO-LUMO gap.