Nature of the 'dark' 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 pipi* state (1 B-1(1u) in D-2h) and the dark pisigma* state (1 (1)A(u) in C-2h). The state switch from the linear pipi* state to the bent pisigma(-) 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 pipi*/pisigma* intersection for phenylacetylene and benzonitrile, suggesting that the state switch may be a phenomenon of common occurrence in aromatic molecules containing CdropC and CdropN groups. (C) 2004 Elsevier B.V. All rights reserved.