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

A bis(terpyridine)ruthenium(II) complex ([Ru]²⁺) was covalently connected via a floppy -OCH₂CH₂O- 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, ¹H NMR (1D and ¹H- ¹H 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 (³MLCT) 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.