Meso-meso phenyl bridged unsymmetrical porphyrin dyads: synthesis, spectral, electrochemical and photophysical properties

Abstract

Seven phenyl bridged meso-meso unsymmetrical porphyrin dyads containing two different porphyrins either two types of heteroporphyrins or one porphyrin and one heteroporphyrin were synthesized by coupling of readily accessible appropriate mono-meso porphyrin boronate/heteroporphyrinboronate with meso-bromoheteroporphyrin in the presence of Cs₂CO₃/Pd(PPh}₃)₄ in toluene/DMF at 80°C for 4 h. The dyads are freely soluble in common organic solvents and were characterized by mass, NMR, absorption, electrochemical and fluorescence techniques. The NMR, absorption and electrochemical studies indicated that the two porphyrin sub-units in dyads interact weakly and they retain their characteristic features. In six out of seven dyads, the free-base porphyrin with N₄ core or its Zn(II) derivative possess singlet state energy level at higher energy hence acts as energy donor and the heteroporphyrin sub-unit with N₃S, N₂SO and N₂S₂ cores having singlet state energy level at lower energy acts as energy acceptor. Our preliminary photophysical studies on six unsymmetrical porphyrin dyads indicated a possibility of energy transfer at singlet state from donor porphyrin sub-unit to acceptor porphyrin sub-unit on selective excitation of donor porphyrin sub-unit. To gain the structural information and to demonstrate possible electronic interaction between the donor and acceptor porphyrin sub-units, DFT calculations were carried out on dyads 1, 3 and 5 by adopting B3LYP hybrid functional with Gaussian atomic basis functions. The theoretical studies predicted a considerable modification of electronic energy levels in these dyads with the change of porphyrin core in acceptor sub-unit. Calculations also indicate most efficient donor → acceptor energy transfer in case of dyad 5 supporting the experimental results.