Enhancement of triplet stability in benzene by substituents with triple bonds.

Excitation of phenylacetylene (PA) and benzonitrile to their lowest singlet states in a molecular beam has previously been shown to immediately (only during the 8 ns laser pulse) result in long-lived species with low ionization potentials (Hofstein, J.; Xu, H.; Sears, T.; Johnson, P.M. J. Phys. Chem. A 2008, 112, 1195-1201). Using the fragmentation of ions produced by photoionization at various times after initial excitation as a diagnostic for molecular geometry evolution, the long-lived species in phenylacetylene is shown to be a PA state (most likely a triplet) rather than an isomer. Delayed fluorescence and a delayed photoelectron signal indicative of S1 are also seen, indicating a singlet-triplet mixing process that is not quite in the statistical-coupling limit and is parallel to the long-lived species channel. Electronic structure calculations indicate that the lowest triplet state of phenylacetylene is nonplanar with the ethynyl group bent in a trans-configuration out of the plane of the ring. The substituent π-electrons are significantly conjugated into the ring, resulting in a tendency toward a quinoidal structure, which may be related to the unusual excited state stability. These molecules constitute the first members of a new class of excited state behaviors.