Institute of Chemistry and the Farkas Center for Light Induced Processes, The Edmond Safra Campus, The Hebrew University of Jerusalem, Jerusalem, Israel.
J Phys Chem A. 2012 Nov 26;116(46):11111-7. doi: 10.1021/jp304325x. Epub 2012 Aug 31.
A new route for the nonradiative decay of photoexcited, H-bonded, nitrogen-containing, heterocyclic dimers is offered and exemplified by a study of the pyrazole dimer. In some of these systems the N(3s) Rydberg state is the lowest excited singlet state. This state is formed by direct light absorption or by nonradiative transition from the allowed ππ* state. An isomer of this Rydberg state is formed by H atom transfer to the other component of the dimer. The newly formed H-bonded radical pair is composed of two radicals (a H-adduct of pyrazole, a heterocyclic analogue of the NH(4) radical) and the pyrazolium π-radical. It is calculated to have a shallow local minimum and is the lowest point on the PES of the H-pyrazole/pyrazolium radical pair. This species can cross back to the ground state of the original dimer through a relatively small energy gap and compete with the H-atom loss channel, known for the monomer. In both Rydberg dimers, an electron occupies a Rydberg orbital centered mostly on one of the two components of the dimer. This Rydberg Center Shift (RCS) mechanism, proposed earlier (Zilberg, S.; Kahan, A.; Haas, Y. Phys. Chem. Chem. Phys. 2012, 14, 8836), leads to deactivation of the electronically excited dimer while keeping it intact. It, thus, may explain the high photostability of the pyrazole dimer as well as other heterocyclic dimers.
为光激发的氢键合含氮杂环二聚体的非辐射衰变提供了一条新途径，并以吡唑二聚体的研究为例加以说明。在这些体系中的一些中，N(3s)Rydberg 态是最低的激发单线态。该态是通过直接光吸收或从允许的ππ*态的非辐射跃迁形成的。该 Rydberg 态的异构体是通过 H 原子转移到二聚体的另一个组成部分形成的。新形成的氢键自由基对由两个自由基（吡唑的 H-加合物，NH(4)自由基的杂环类似物）和吡唑鎓π-自由基组成。它被计算为具有浅的局部最小值，并且是 H-吡唑/吡唑鎓自由基对的 PES 上的最低点。该物种可以通过相对较小的能量间隙返回到原始二聚体的基态，并与单体中已知的 H 原子损失通道竞争。在两个 Rydberg 二聚体中，一个电子占据一个以二聚体的两个组成部分之一为中心的 Rydberg 轨道。这种 Rydberg Center Shift (RCS) 机制（Zilberg, S.; Kahan, A.; Haas, Y. Phys. Chem. Chem. Phys. 2012, 14, 8836），导致电子激发的二聚体失活，同时保持其完整。因此，它可以解释吡唑二聚体以及其他杂环二聚体的高光稳定性。
