Synthesis of triazole bridged unsymmetrical porphyrin dyads and porphyrin-ferrocene conjugates

Abstract

A simple method has been used to synthesize four porphyrin azides with cores such as N₄, N₃S, N₂SO and N₂S₂ in 60–90 % yields by treating the corresponding aminoporphyrins with tert-butyl nitrite (tBuONO) and azidotrimethylsilane (TMSN₃) in THF/CH₃) CN under mild reaction conditions. The hitherto unknown aminoporphyrins with heteroatom cores were synthesized from their corresponding nitroporphyrins by standard SnCl₂ /HCl reduction. The azidoporphyrins were used to synthesize six triazole-bridged unsymmetrical porphyrin dyads containing two different types of porphyrin sub-units as well as five triazole-bridged porphyrin–ferrocene conjugates under Cu^I-catalyzed “click” reaction conditions. Various Cu^I-catalyzed reaction conditions were studied and the best yields of triazole-bridged dyads and conjugates were obtained with CuI/DIPEA in THF/CH₃ CN at room temperature for overnight. The unsymmetrical porphyrin dyads and porphyrin–ferrocene conjugates were characterized by various spectroscopic and electrochemical techniques. In unsymmetrical porphyrin dyads, the NMR, absorption and electrochemical studies indicate a weak interaction between the two porphyrin sub-units. However, preliminary photophysical studies support an efficient singlet-singlet energy transfer from one porphyrin unit to another in five unsymmetrical dyads reported here. In porphyrin–ferrocene conjugates, the fluorescence of porphyrin was quenched significantly due to photo-induced electron transfer from ferrocene to porphyrin.