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

Abstract

The electronic properties of a series of eight copper chalcogenide clusters including [Cu₁₂S₆(dpppt)₄] (dpppt = Ph₂P­(CH₂)₅PPh₂), [Cu₁₂Se₆(dppo)₄] (dppo = Ph₂P­(CH₂)₈PPh₂), [Cu₁₂S₆(dppf)₄] (dppf = Ph₂PCpFeCpPPh₂), [Cu₁₂S₆(PPh₂Et)₈], [Cu₁₂S₆(PEt₃)₈], [Cu₂₄S₁₂(PEt₂Ph)₁₂], [Cu₂₀S₁₀(PPh₃)₈], and [Cu₂₀S₁₀(P^<i>t</i>Bu₃)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 ∼2.5 eV, i.e., below ∼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 ∼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 ∼820 nm, whereas the oblate cluster [Cu₂₀S₁₀(P^<i>t</i>Bu₃)₈] displays a relatively weak orange emission at ∼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