Photochemistry and Photophysics of Coordination Compounds

90 N. Armaroli et al.
Fig. 20 Ligand 11 2,9-di-tert-butyl-1,10-phenanthroline
served large Stokes shift (over 5000 cm –1 )hadattributedittothelowest
3 MLCT excited state [91], likewise the popular family of octahedral Ru(II)polypyridines
[6]. McMillin and coworkers, instead, suggested that emission
of [Cu(NN)2] + compoundsarisesfromtwoMLCTexcitedstatesinthermal
equilibrium, i.e. a singlet ( 1 MLCT)andatriplet( 3 MLCT) [92]. The energy
gap between these states was found to be about 1500–2000 cm –1 and, at room
temperature, the population of the lower lying 3 MLCT level exceeds that of
1 MLCT. At 77 K where the excited state population is largely frozen in the
triplet, the emission band is red-shifted and much weaker compared to room
temperature, a rather unusual trend. Recent studies have confirmed and refined
this rationale [81, 85, 89].
A few years ago our group discussed detailed temperature-dependent luminescence
studies of a series of [Cu(NN)2] + complexes of 2,9-disubstituted
phenanthroline ligands [85]. The above-described two-level model, which implies
red-shift and intensity decrease of the emission band upon temperature
lowering,isalwaysobeyedexceptwhenlongalkylchainsareutilizedassubstituents
of the phenanthroline chelating agent. In this case the “regular”
trend is obeyed only until the matrix remains fluid (T>150 K) but, when the
matrix becomes rigid (T