Electronic structure and spectroscopy of cadmium thiolate clusters

The singly excited configuration interaction (CIS) approximation and the CNDO model Hamiltonian have been employed to calculate the optical spectra of [Cd(SPh)₄]^2-, [Cd₂(SPh)₆]^2-, and [Cd₄(SPh)₁₀]^2- and the artificial defect models [Cd₂(SPh)₅]^- and [Cd₄(SPh)₉]^- (Ph = phenyl). The electronic transitions have been characterized by evaluating changes in orbital electron population between ground and excited states; this allows clear assignment of the nature of the optical transitions. The simulated spectra are able to reproduce well the main features of experimental optical transition bands. The lowest energy bands for the first three compounds are due to both intraligand and ligand-to-metal charge transfer transitions. The terminal thiolate levels lie insignificantly higher in energy than the bridging ones. Both experimental and simulated spectra show a blue shift with increasing sizes, directly opposed to simple predictions based on quantum confinement. This can be explained by the Jahn-Teller effect. Incomplete capping of a Cd atom creates a low-lying state that acts as an electron trap in an optical transition. Transitions involving this state have small oscillator strengths and produce a long tail on the red side of the spectrum. Analysis of changes in electron population shows that these red tails can be attributed to charge transfer transitions from the thiolates to the defect Cd, and thus significant dipole moment changes result. The self-consistent reaction field model has also been used to study the effects of solvent on optical spectra; no important changes in calculated spectra are obtained.