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)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(PtBu3)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(PtBu3)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.