| Abstract | 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 πδ* state (1 ¹Au in C₂h). The state switch from the linear ππ* state to the bent πδ* 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*/πδ* 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. |
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