PHOTOLUMINESCENCE PROPERTIES OF THE STRUCTURALLY ANALOGOUS TETRANUCLEAR COPPER(I) CLUSTERS CU4X4(DPMP)4 (X = I, BR, CL DPMP = 2-(DIPHENYLMETHYL)PYRIDINE)

Reported are the photophysical properties of the copper(I) halide clusters Cu4X4(dPMP)4 with the bulky organic base 2-(diphenylmethyl)pyridine (X = I, Br, Cl) which have been shown by previous crystal structure determinations to have ''cubane''-like Cu4X4 cores with very similar Cu-Cu distances (2.87-3.01 angstrom). As solids at 77 K, these compounds each exhibit a strong, relatively long-lived emission band [lambda(max)em = 467, 487, and 505 nm; tau = 18.1, 10.0, and 10.0 mus (for X = I, Br, and Cl, respectively)], which is attributed to radiative decay from a halide to ligand charge transfer (XLCT) excited state. At higher temperatures, a weak shoulder appeared in the emission spectra for the Cu4I4(dpmp)4 and Cu4Br4(dpmp)4 solids (at approximately 570 and approximately 626 nm, respectively), and the relative intensities of these shoulders could be fit to the function A exp(-DELTAE/RT), with DELTAE = 990 +/- 370 cm-1 for the appearance of this red emission in Cu4Br4(dpmP)4. The excited state (ES) lifetimes observed for these shoulders were identical to those observed for the still dominant higher energy bands. In analogy to earlier results with the copper(I) iodide cluster Cu4I4(PY)4, this longer wavelength band was attributed to emission from a Cu4X4 core centered ES of mixed halide-to-copper charge transfer (XMCT) and metal centered ''d-s'' orbital parentage. The nature of such transitions in Cu4X4L4 clusters is discussed with respect to the Cu-Cu distances in the ground state structures.