Nature of the `darkʹ state in diphenylacetylene and related molecules: state switch from the linear ππ∗ state to the bent πσ∗ state

Time-dependent DFT (TD-DFT), CIS, and CASSCF calculations of the excited singlet states of diphenylacetylene demonstrate the existence of a low energy crossing between the initially excited ππ∗ state (1 1B1u in D2h) and the dark πσ∗ state (1 1Au in C2h). The state switch from the linear ππ∗ state to the bent πσ∗ state accounts for all the unusual photophysical and spectroscopic properties of DPA, including an abrupt fluorescence break-off, and the appearance of the temperature/viscosity dependent transient absorption at 700 nm. The calculations also yield the ππ∗/πσ∗ intersection for phenylacetylene and benzonitrile, suggesting that the state switch may be a phenomenon of common occurrence in aromatic molecules containing CC and CN groups.