Luminescence in Phosphine-Stabilized Copper Chalcogenide Cluster Molecules-A Comparative Study

The electronic properties of a series of eight copper chalcogenide clusters including [Cu12S6(dpppt)(4)] (dpppt = Ph2P(CH2)(5)PPh2), [Cu12Se6(dppo)(4)] (dppo = Ph2P(CH2)(8)PPh2), [Cu12S6(dppf)(4)] (dppf = Ph2PCpFeCpPPh2), [Cu12S6(PPh2Et)(8)], [Cu12S6(PEt3)(8)], [Cu24S12(PEt2Ph)(12)], [Cu20S10(PPh3)(8)], and [Cu20S10((PBu3)-Bu-t)(8)] were investigated by absorption and photoluminescence (PL) spectroscopy as well as time-dependent density functional theory calculations. Major features of the experimental electronic absorption spectra are generally well-reproduced by the spectra simulated from the calculated singlet transitions. Visualization of the nonrelaxed difference densities indicates that for all compounds transitions at higher energies (above similar to 2.5 eV, i.e., below similar to 495 nm) predominantly involve excitations of electrons from orbitals of the cluster core to ligand orbitals. Conversely, the natures of the lower-energy transitions are found to be highly sensitive to the specifics of the ligand surface. Bright red PL (centered at similar to 650-700 nm) in the solid state at ambient temperature is found for complexes with all 'Cu12S6' (E = S, Se) cores as well as the dimeric 'Cu24S12', although in [Cu12S6(dppf)(4)], the PL appears to be efficiently quenched by the ferrocenyl groups. Of the two isomeric 'Cu20S10' complexes the prolate cluster [Cu20S10(PPh3)(8)] shows a broad emission that is centered at similar to 820 nm, whereas the oblate cluster [Cu20S10((PBu3)-Bu-t)(8)] displays a relatively weak orange emission at similar to 575 nm. The emission of all complexes decays on the time scale of a few microseconds at ambient temperature. A very high photostability is quantitatively estimated for the representative complex [Cu12S6(dpppt)(4)] under anaerobic conditions.