Density-functional study of luminescence in polypyridine ruthenium complexes

A density-functional theory (DFT) study of five ruthenium complexes has been carried out with the goal of gaining deeper insight into factors governing luminescence lifetimes. The five compounds are [Ru(bpy)3]2+ (1), [Ru(L1)2]2+ (2), [Ru(tpy)2]2+ (3), [Ru(L1)(tpy)]2+ (4), and [Ru(L2)2]2+ (5), where bpy = 2,2′-bipyridine, tpy = 2,2′;6′,2″-terpyridine, L1 = 1,1′-[2,6-pyridinediylbis(methylene)]bis[3-methylimidazolium] hexafluorophosphate and L2 = 1,1′-[2,6-pyridinediylbis(methylene)]bis[3-methylbenziimidazolium]. Experimental work, including the synthesis and photophysical properties of 5 is also reported in the context of this study. Gas phase geometries optimized using X-ray crystallography geometries as start geometries were found to be close to the start geometries. Gas phase absorption spectra calculated using time-dependent DFT were found to be in good agreement with spectra measured in solution. A partial density of states (PDOS) analysis of the molecular orbitals shows that it is possible to recover a ligand field theory (LFT)-like picture. On the basis of this PDOS-derived LFT-like picture we propose two orbital-based luminescence indices, both motivated by the idea that luminescence quenching results from a low 3MLCT → 3MC barrier. The first luminescence index is ΔE, the difference between the e g * and lowest energy π* PDOS bands. The second luminescence index is d × π, the product of the amount of π character in the t2g band with the amount of ruthenium d character in the 1π* band. These luminescence measures are intended as qualitative rather than quantitative predictors. Low values of ΔE and high values of d × π are shown to correlate with lack of luminescence for the five compounds studied in this paper, while high values of ΔE and low values of d × π correlate well with luminescence.