The influence of ligand localized excited states on the photophysics of second row and third row transition metal terpyridyl complexes: Recent examples and a case study

Abstract

The photophysical behavior of Ru(II) and Os(II) diimine complexes having complex aromatic hydrocarbon diimine ligands has received considerable attention as systems exhibiting intramolecular energy transfer to yield excited states with lifetimes much longer than the parent diimine complexes. Here we present a focused discussion of the photophysical behavior of transition metal complexes with modified terpyridyl ligands. The overview includes, as an example of approaches used to evaluate such systems, spectroscopic studies of a pair of Ru(II) mono- and bis-terpyridyl complexes modified with vinylpyrene (Pyr-v-tpy) to have ligand localized excited states that are equal to or lower than the energy of the known MLCT state of the parent complexes, [Ru(Mpt)2]2+ and [Ru(Mpt)(dien)]2+ (Mpt=4'-tolyl-2,2':6',2″-terpyridine, dien=diethylenetriamine). The common observation is that the presence of Pyr-v-tpy serves to lengthen the excited state lifetime of the complex through interaction of MLCT and ligand localized (IL) states. For [Ru(Pyr-v-tpy)2]2+ the excited state lifetime increases by a factor of more than 104 relative to [Ru(Mpt)2]2+. For [Ru(Pyr-v-tpy)(dien)]2+, the 3IL state is close in energy to the MLCT state of the parent [Ru(Mpt)(dien)]2+ and, while the transient absorption spectrum is significantly perturbed relative to [Ru(Mpt)(dien)]2+, the excited state decay rate changes by only a factor of four. The long-lived excited state is formed in less than a ps, indicating strong coupling of the MLCT and ligand localized manifolds.