REINVESTIGATION OF THE ABSORBING AND EMITTING CHARGE-TRANSFER EXCITED-STATES OF [CU(NN)2]+ SYSTEMS

The absorption and emission spectra of a series of [Cu(NN)2]+ systems, where NN denotes a heteroaromatic, chelating ligand derived from 1,10-phenanthroline or 2,2'-bipyridine, have been investigated. Detailed group theoretical assignments of the relevant low-lying charge-transfer states are presented. For the first time, a charge-transfer absorption band has been identified with a polarization perpendicular to the axis joining the metal and the ligand centers (the z axis). One of the most novel properties of the complexes is that they exhibit thermally activated emission. Data presented include absorption and emission polarization measurements from glycerin solutions over the temperature range 90-175 K as well as data from a 4:1 (v/v) ethanol-methanol solution at 90 K. The emission can be attributed to two different excited states in thermal communication, and both components of the emission are z polarized. The results are inconsistent with a model that assigns the emissions to spin-orbit components derived from a triplet excited state (Parker, W.L.; Crosby, G.A.J. Phys. Chem. 1989, 93, 5692-5696), but they are in accord with a previous proposal which assigns the thermally activated emission to a singlet state (Kirchhoff, J. R., et al. Inorg. Chem. 1983, 22, 2380-2384).