Department of Chemistry, Purdue University, West Lafayette, Indiana 47907-2084, USA.
J Am Chem Soc. 2010 Feb 10;132(5):1611-20. doi: 10.1021/ja908103u.
The excited-state dynamics of phenylvinylacetylene (1-phenyl-1-buten-3-yne, PVA) have been studied using laser-induced fluorescence spectroscopy, ultraviolet depletion spectroscopy, and the newly developed method of ultraviolet population transfer spectroscopy. Both isomers of PVA (E and Z) show a substantial loss in fluorescence intensity as a function of excitation energy. This loss in fluorescence was shown to be due to the turn-on of a nonradiative process by comparison of the laser-induced fluorescence spectrum to the ultraviolet depletion spectrum of each isomer, with a threshold 600 cm(-1) above the electronic origin in Z-PVA and 1000 cm(-1) above the electronic origin in E-PVA. Ab initio and density functional theory calculations have been used to show that the most likely source of the nonradiative process is from the interaction of the pi pi* state with a close lying pi sigma* state whose minimum energy structure is bent along the terminal CCH group. Ultraviolet population transfer spectroscopy has been used to probe the extent to which excited-state isomerization is facilitated by the interaction with the pi sigma* state. In ultraviolet population transfer spectroscopy, each isomer was selectively excited to vibronic levels in the S(1) state with energies above and below the threshold for fluorescence quenching. The ultraviolet-excited populations are then recooled to the zero point levels using a reaction tube designed to constrain the supersonic expansion and increase the collision cooling capacity of the expansion. The new isomeric distribution was detected in a downstream position using resonant-2-photon ionization spectroscopy. From these spectra, relative isomerization quantum yields were calculated as a function of excitation energy. While the fluorescence quantum yield drops by a factor of 50-100, the isomerization quantum yields remain essentially constant, implying that the nonradiative process does not directly involve isomerization. On this basis, we postulate that isomerization occurs on the ground-state potential energy surface after internal conversion. In these experiments, the isomerization to naphthalene was not observed, implying a competition between isomerization and cooling on the ground-state potential energy surface.
苯乙烯乙炔（1-苯基-1-丁烯-3-炔，PVA）的激发态动力学已通过激光诱导荧光光谱、紫外光消耗光谱以及新开发的紫外光布居转移光谱法进行了研究。PVA 的两种异构体（E 和 Z）都表现出荧光强度随激发能的显著降低。通过将每种异构体的激光诱导荧光光谱与紫外光消耗光谱进行比较，证明这种荧光损失是由于非辐射过程的开启所致，Z-PVA 中的非辐射过程在电子起源上方 600cm(-1)，而 E-PVA 中的非辐射过程在电子起源上方 1000cm(-1)。从头算和密度泛函理论计算表明，最有可能的非辐射过程源是来自于ππ态与紧邻的πσ态的相互作用，而πσ态的最低能量结构是沿着末端 CCH 基团弯曲的。紫外光布居转移光谱法已被用于研究与πσ态的相互作用促进激发态异构化的程度。在紫外光布居转移光谱法中，每个异构体都被选择性地激发到 S(1)态的振动态，这些振动态的能量高于或低于荧光猝灭的阈值。然后，使用设计用于约束超声速膨胀并增加膨胀碰撞冷却能力的反应管将紫外激发的布居冷却到零点水平。在下游位置使用共振-2 光子电离光谱法检测新的异构体分布。从这些光谱中，计算了作为激发能量函数的相对异构化量子产率。虽然荧光量子产率下降了 50-100 倍，但异构化量子产率基本保持不变，这意味着非辐射过程不直接涉及异构化。在此基础上，我们假设异构化发生在内部转换后的基态势能面上。在这些实验中，未观察到萘的异构化，这意味着基态势能面上的异构化和冷却之间存在竞争。
