Department of Chemistry, Stony Brook University, Stony Brook, New York 11794-3400, United States.
J Phys Chem A. 2013 Aug 22;117(33):7786-93. doi: 10.1021/jp403727f. Epub 2013 Aug 13.
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.
先前的研究表明，在分子束中激发苯乙炔（PA）和苯甲腈使其处于最低单线态会立即（仅在 8 ns 激光脉冲期间）产生具有低电离势的长寿命物种（Hofstein，J.；Xu，H.；Sears，T.；Johnson，P.M. J. Phys. Chem. A 2008，112，1195-1201）。通过在初始激发后的不同时间通过光电离产生的离子的碎裂作为分子几何演化的诊断，表明苯乙炔中的长寿命物种是 PA 态（很可能是三重态）而不是异构体。还观察到延迟荧光和表明 S1 的延迟光电子信号，表明存在单重态-三重态混合过程，该过程不完全处于统计耦合极限内，并且与长寿命物种通道平行。电子结构计算表明，苯乙炔的最低三重态是非平面的，乙炔基以反式构型弯曲在环平面之外。取代基 π 电子显著地共轭到环中，导致醌式结构的趋势，这可能与不寻常的激发态稳定性有关。这些分子构成了一类新的激发态行为的第一个成员。
