Synthesis and photophysical studies of covalently linked porphyrin-21-thiaporphyrin dyads

The synthesis and photophysical properties of four covalently linked unsymmetrical porphyrin dyads containing two different porphyrin cores such as N₄ and N₃S are reported. The covalently linked dyads were prepared by the coupling of appropriate porphyrin having ethynylphenyl functional group at meso-position with porphyrin having iodophenyl or bromo functional group at meso-position under mild palladium coupling conditions. The photophysical study indicated an intramolecular singlet-singlet energy transfer from N₄/ZnN₄ porphyrin sub-unit to N₃S porphyrin sub-unit in all four dyads with an efficiency of energy transfer process was typically ?97%. To probe the role of linker in through bond electronic communication between the two porphyrin sub-units in dyads, the linker was varied from diphenylethyne to phenylethyne and the study revealed that the energy transfer rates and efficiencies were much higher for phenylethyne-bridged porphyrin dyads compared with diphenylethyne-bridged porphyrin dyads.