UC Santa Barbara

A number of the d10 and s2 metal clusters are brightly luminescent, some with strongly medium dependent and temperature dependent spectral profiles. Such emissions are often from excited states assigned as having a mixed d-s/XMCT character, the key feature being enhanced electronic population in metal-metal bonding orbitals in these states as evidenced by the large Stokes shifts between excitation and emission maxima. The observation of such emission bands is strongly favored by relatively short metal-metal distances in the clusters. This behavior parallels that of the d8-d8 dimers held by bridging ligands in cofacial configurations for which excitation leads to substantially increased bonding between metal centers. With π-unsaturated ligands L, emissions from XLCT excited states are often observed, and the luminescence thermochromism of such complexes is the result of different temperature and medium rigidity responses of these two types of excited states. The poor coupling between d-s/XMCT and XLCT states in clusters such as Cu4I4-(py)4 can be attributed to high barrier heights for curve crossing owing to different distortion trajectories once the molecule is prepared in one of the respective excited states. Ongoing studies in these laboratories continue to survey different luminative d10 and s2 cluster systems and to apply an arsenal of synthetic, theoretical, photophysical, and photochemical tools to extend our quantitative understanding of the relevant excited states. © 1993, American Chemical Society. All rights reserved.