Synthesis, Electrochemical and Photochemical Studies on π‐Extended Mono‐ β ‐Functionalized Porphyrin Dyads

Abstract A series of π‐conjugated mono‐ β ‐functionalized donor−acceptor porphyrin dyads ( 1 – 4 ) with various donor groups (phenyl, naphthyl, anthracenyl and pyrenyl) and their metal complexes (M=Zn( 1 a – 4 a ), Cu( 1 b – 4 b ), Co( 1 c – 4 c ) and Ni( 1 d – 4 d )) were synthesized and characterized by various spectroscopic and electrochemical techniques. A modified one‐step Horner‐Wadsworth‐Emmons reaction was performed on monoformyl porphyrin to get ethenyl‐linked mono‐ β ‐porphyrin dyads in good to excellent yields (70–90 %). Red‐shifted absorption and emission spectra were observed as compared to unsubstituted pristine tetraphenylporphyrins due to π‐extended donors. Time‐resolved fluorescence studies confirmed the effective intramolecular Förster energy transfer from the donor moiety to the porphyrin core in anthracene‐ and pyrene‐appended porphyrin dyads. DFT and TD‐DFT optimization signified that the orientation of the donor and acceptor plays a vital role in energy transfer as co‐planarity of the donor with the porphyrin core increases the energy transfer efficiency. The distribution of electron density on HOMOs and LUMOs indicated an excitation energy transfer (EET) mechanism from the donor moiety to the porphyrin acceptor core. All porphyrin dyads exhibited a cathodic shift in their oxidation potential suggesting facile oxidation of the porphyrin core due to π‐extension and the presence of electron‐donating moieties. Finally, femtosecond transient absorption spectral studies were performed to secure evidence of excitation transfer and kinetic information of the energy transfer event in the dyads.