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 = 1,1'42,6-pyridinediyIbis (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 DFr 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 (LET)-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 (MLCT)-M-3 -> (MC)-M-3 barrier. The first luminescence index is Delta E, the difference between the e(g)* and lowest energy pi* PDOS bands. The second luminescence index is d x pi, the product of the amount of it character in the t(2g) band with the amount of ruthenium d character in the 1 pi* band. These luminescence measures are intended as qualitative rather than quantitative predictors. Low values of Delta E and high values of d x pi are shown to correlate with lack of luminescence for the five compounds studied in this paper, while high values of Delta E and low values of d x pi correlate well with luminescence. (C) 2013 Elsevier B.V. All rights reserved.