Spectroscopic and Photophysical Properties of Complexes of 4‘-Ferrocenyl-2,2‘:6‘,2‘ ‘-terpyridine and Related Ligands

Abstract

4‘-(Ferrocenyl)-2,2‘:6‘,2‘ ‘-terpyridine (Fctpy) and 4‘-(4-pyridyl)-2,2‘:6‘,2‘ ‘-terpyridine (pytpy) were prepared from the corresponding ferrocene- and pyridinecarboxaldehyle and 2-acetylpyridine using the Krohnke synthetic methodology. Metal complexes, [M(Fctpy) 2 ](PF 6 ) 2 (M = Ru, Fe, Zn), [Ru(tpy)(Fctpy)](PF 6 ) 2 (tpy = 2,2‘:6‘,6‘ ‘-terpyridine), and [Ru(pytpy) 2 ](PF 6 ) 2 were prepared and characterized. Cyclic voltammetric analysis indicated Ru III/II and ferrocenium/ferrocene redox couples near expected potentials (Ru III/II ∼1.3 V and ferrocenium/ferrocene ∼0.6 V vs Ag/AgCl). In addition to dominant π tpy → π tpy * UV absorptions near 240 and 280 nm and d π Ru → π tpy * MLCT absorptions around 480 nm, the complexes [Ru(Fctpy) 2 ](PF 6 ) 2 and [Ru(tpy)(Fctpy)](PF 6 ) 2 exhibit an unusual absorption band around 530 nm. Resonance Raman measurements indicate that this band is due to a 1 [(d(π) Fc ) 6 ] → 1 [(d(π) Fc ) 5 (π* tpy Ru ) 1 ] transition. For [Ru(Fctpy) 2 ](PF 6 ) 2 and [Ru(tpy)(Fctpy)](PF 6 ) 2, excited-state emission and lifetime measurements indicated an upper-limit emission quantum yield of 0.003 and an upper-limit emission lifetime of 0.025 μs. The influence of the ferrocenyl site on excited-state decay is discussed, and an excited-state energy level diagram is proposed.