Highly luminescent Cu(I)-phenanthroline complexes in rigid matrix and temperature dependence of the photophysical properties

We synthesized new [Cu(NN)(2)](+)-type complexes where NN = 2-5 and denotes a 2,9-disubstituted-1,10-phenanthroline ligand (related complexes of 1 and 6 ligands are used for reference purposes). For 2, 3, and 4 the ligand substituents are long alkyl-type fragments. whereas in 5 a phenyl ring is directly attached to the chelating unit. At 298 K the four complexes display relatively intense metal-to-ligand-charge-transfer (MLCT) emission bands with maxima around 720 nm, Phi (em) approximate to 1 x 10(-3) and tau >100 ns in deaerated CH2Cl2. The emission behavior at 77 K in a CH2Cl2/MeOH matrix is quite different for complexes of alkyl- (2-4) versus phenyl-substituted (5) ligands. The former exhibit very intense emission bands centered around 642 nm and hypsochromically shifted with respect to 298 K, whereas the luminescence band of [Cu(5)(2)](+) is faint and shifted toward the infrared side. These results prompted us to study in detail the temperature dependence of luminescence properties of [Cu(2)(2)](+) and [Cu(5)(2)]+ in the 300-96 K range. For both complexes the excited state lifetimes increase monotonically by decreasing temperatures, and the trend is well described by an Arrhenius-type treatment involving two equilibrated MLCT excited levels. The emission bands show a similar behavior for the two compounds (intensity decrease and red-shift) only in the 300-120 K range, when the solvent is fluid, In the frozen regime (T less than or equal to 120 K), the emission intensity of [Cu(5)(2)](+) continues to drop, whereas that of [Cu(2)(2)](+) exhibits a dramatic intensity increase. We interpret this different behavior in terms of structural factors, suggesting that long alkyl-chains in the 2,9-phenanthroline positions are optimal to prevent significant ground- and excited-state distortions in rigid matrix. We show that our results do not contradict current models describing the photophysics of [Cu(NN)(2)](+) but, instead, bring further evidence to support their validity. They also suggest guidelines for the design of Cu(I)-phenanthroline complexes showing optimized luminescence performances both in fluid and in rigid matrix, an elusive goal for over two decades.