PHOTOPHYSICAL STUDIES IN SOLUTION OF THE TETRANUCLEAR COPPER(I) CLUSTERS CU4I4L4 (L = PYRIDINE OR SUBSTITUTED PYRIDINE)

Reported are the emission and excitation spectra and emission lifetimes of a series of tetranuclear copper(I) clusters of the type Cu4I4L4 (L = pyridine, substituted pyridine, or a saturated amine) in solution and in the solid state. These materials are bright, relatively long-lived emitters in room-temperature solution which display two emission bands, an intense lower energy (LE) band (lambda-max = 690 nm, tau(LE) = 11.1-mu-s for Cu4I4(py)4, I, in 293 K toluene), and a much less intense higher energy (HE) band (lambda-max = 480 nm, tau(LE) = 0.45-mu-s for I, in 293 K toluene). Comparisons of substituent and solvent effects as well as the observation that this band is not seen for L = saturated amine have led to the conclusion that the HE emission is from an excited state either metal-to-ligand charge transfer (MLCT) or iodide-to-ligand charge transfer (XLCT) in character. The LE band is assigned as emission from a cluster-centered (CC) excited state, possibly an iodide-to-metal charge transfer. The two emissions show different excitation spectra as well as different lifetime and relative intensity profiles as the temperature is lowered. At lower T (e.g., 77 K)< the emission spectra of the well-characterized solids display properties similar to the solutions of the same compounds in frozen solutions. The rates of selective quenching of the HE excited state by Lewis bases as well as by energy transfer (at room temperature) are reported as are the quenching rates of both states by O2. The independent behavior of the lifetimes and excitation spectra of the HE and LE emissions for various Cu4I4L4 all indicate that the excited states responsible are essentially uncoupled.