Excited state absorption spectra of the tetranuclear cuprous iodide cluster Cu 4I 4(py) 4 and related species

Abstract

Nanosecond flash photolysis studies have been carried out on the tetrameric clusters Cu 4I 4L 4 (L = pyridine (py), 4- t-butyl pyridine (4-Bu t-py)), 4-phenyl pyridine (Phpy) or piperidine (pip)). A broad transient absorption band in the visible region was observed for each of the clusters with aromatic heterocycle ligands upon excitation at 337 or 355 nm but no excited state absorption (ESA) was observed when L is the saturated amine piperidine. The ESA band maxima are dependent upon the nature of the pyridine ring substituent, occurring at 542 nm, 565 run, and 532 nm for L = py, Phpy, and 4-Bu t-py), respectively. These ESA bands are modestly solvent-sensitive, shifting to lower energy in solvents of lower polarity. The microsecond lifetimes of the ESA bands are comparable to those observed for emission from the triplet cluster centered excited states (3CC) previously assigned for these compounds (K.R. Kyle, C.K. Ryu, J.A. DiBenedetto and PC. Ford, J. Am. Chem. Soc., 113 (1991) 2954–2965). Consideration of this data and examination of the energetics of the states involved suggests that the transition responsible for the excited state absorption occurs from the 3CC state to a higher energy triplet halide-to-ligand charge transfer ( 3XLCT) state.