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](PF6)2 (M = Ru, Fe, Zn), [Ru(tpy)(Fctpy)](PF6)2 (tpy = 2,2‘:6‘,6‘ ‘-terpyridine), and [Ru(pytpy)2](PF6)2 were prepared and characterized. Cyclic voltammetric analysis indicated RuIII/II and ferrocenium/ferrocene redox couples near expected potentials (RuIII/II ∼1.3 V and ferrocenium/ferrocene ∼0.6 V vs Ag/AgCl). In addition to dominant πtpy → πtpy* UV absorptions near 240 and 280 nm and dπRu → πtpy* MLCT absorptions around 480 nm, the complexes [Ru(Fctpy)2](PF6)2 and [Ru(tpy)(Fctpy)](PF6)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*tpyRu)1] transition. For [Ru(Fctpy)2](PF6)2 and [Ru(tpy)(Fctpy)](PF6)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.