Density-functional study of luminescence in polypyridine ruthenium complexes

Abstract 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 3 MLCT →  3 MC barrier. The first luminescence index is Δ E , 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 t 2 g band with the amount of ruthenium d character in the 1 π * band. These luminescence measures are intended as qualitative rather than quantitative predictors. Low values of Δ E 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 Δ E and low values of d  ×  π correlate well with luminescence.