Excited State Interactions in Electropolymerized Thin Films of Ru^II, Os^II, and Zn^II Polypyridyl Complexes

Abstract

Emission from thin films of poly[Ru(vbpy)3](PF6)2 (vbpy = 4-methyl-4‘-vinyl-2,2‘-bipyridine) prepared by reductive electropolymerization on ITO (Sn:In2O3) is dominated by time-dependent effects and dramatically reduced excited state lifetimes compared to related monomers. The origins of these effects have been investigated by photophysical measurements on a series of copolymeric films containing mixtures of the ZnII, RuII, and OsII forms of poly[M(vbpy)3](PF6)2. In copolymeric RuII and OsII films, facile energy transfer occurs from RuII to OsII. Dilution of RuII by ZnII in copoly-χ[Ru(vbpy)3](PF6)2,(1 − χ)[Zn(vbpy)3](PF6)2 (χ = mole fraction of RuII) mixtures is accompanied by an increase in emission energy and average excited state lifetime. These results point to a quenching mechanism in poly[Ru(vbpy)3](PF6)2 arising from facile energy transfer to low-energy, short-lived “trap sites”. The trap sites may be created by electronic coupling between special pairs of adjacent complexes present in small amounts. The decrease in excited state lifetime at the trap sites is predicted qualitatively by the energy gap law for nonradiative decay.