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Electron energy-loss spectroscopy (EELS) is used to determine the energies of low-lying singlet and triplet states of the title compound. The EELS measurements give a splitting of 0.45 eV between the two lowest π f π* singlet states, as confirmed by optical absorption measurements, but do not give a discernable splitting between the corresponding triplet states. The experimental results are interpreted with the aid of ab initio electronic structure calculations using the CIS and CASPT2 methods. The calculations are consistent with the experiments, giving a much larger splitting between the π f π* singlet as compared to the π f π* triplet states. They also reveal that the splitting between the two lowest π f π* triplet states is dominated by through-bond (TB) interactions, whereas that between the corresponding singlet states includes also a significant contribution from the dipole-dipole coupling mechanism.

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Journal Of The American Chemical Society


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