Synthesis, spectroscopy and photochemistry of dyads and triads with porphyrins and bis(terpyridine)ruthenium(II) complex

Abstract

A bis(terpyridine)ruthenium(II) complex ([Ru] 2+ ) was covalently connected via a floppy - OCH 2 CH 2 O - spacer to the free-base porphyrin (H) or zinc(II) porphyrin (Zn) or both, to obtain dyads ([HRu] 2+ , [ZnRu] 2+ ) and triads ([HRuH] 2+ , [ZnRuH] 2+ , [ZnRuZn] 2+ ). These compounds have been fully characterized by MALDI, UV-vis, 1 H NMR (1D and 1 H - 1 H COSY) spectroscopies, and also by the cyclic and differential pulse voltammetric techniques. Absorption spectroscopy of these newly synthesized compounds shows that significant exciton coupling exists in non-polar solvents (cyclohexane and toluene) between the porphyrin ring and the bis(terpyridine)ruthenium(II) complex. Upon excitation within the Soret band of [HRu] 2+ /[HRuH] 2+ , free-base porphyrin fluorescence was found to be strongly quenched in non-polar and weakly quenched in polar solvents, probably due to ‘singlet-triplet’ energy transfer from the free-base porphyrin to the [Ru] 2+ complex. Whereas, in [ZnRu] 2+ /[ZnRuZn] 2+ , zinc(II) porphyrin fluorescence was quantitatively and reasonably quenched in non-polar and polar solvents, respectively by mainly electron transfer from the zinc(II) porphyrin to the [Ru] 2+ complex. The solvent plays a crucial role in the photophysical properties of these compounds, since the energy of the triplet metal-to-ligand charge-transfer ( 3 MLCT) excited state is influenced by the polarity of the medium. Finally, [ZnRuH] 2+ exhibits the combined fluorescence properties of [HRu] 2+ and [ZnRu] 2+ but the observed additional quenching in non-polar solvents for the zinc(II) porphyrin component is explained by energy transfer from the zinc(II) porphyrin to the free-base porphyrin and/or the bis(terpyridine)ruthenium(II) complex.