Pi* orbital system of alternating phenyl and ethynyl groups: measurements and calculations

The temporary anion states of a series of alternating phenyl-ethynyl compounds are studied by means of electron transmission spectroscopy. Calculations of the virtual orbital energies of these compounds are computed with ab initio HF methods as well as DFT, and excellent correlations with the experimental vertical attachment energies are obtained. Scaled orbital energies for long-chain molecules are used to predict the vertical attachment energies of these compounds. In the absence of scaling, HOMO-LUMO gaps computed by DFT are found to be in substantial disagreement with gas-phase data. Such discrepancies may cause significant errors in theoretical studies of molecular conductance.