Toda Keijiro, Hirose Yoshihiro, Kazuma Emiko, Kim Yousoo, Taketsugu Tetsuya, Iwasa Takeshi
Graduate School of Chemical Sciences and Engineering, Hokkaido University, Sapporo 060-0810, Japan.
School of Interdisciplinary Mathematical Science, Meiji University, 4-21-1 Nakano, Nakano-ku, Tokyo 164-8525, Japan.
J Phys Chem A. 2022 Jul 7;126(26):4191-4198. doi: 10.1021/acs.jpca.2c02354. Epub 2022 Jun 27.
The optical near field refers to a localized light field near a surface that can induce photochemical phenomena such as dipole-forbidden transitions. Recently, the photodissociation of the S-S bond of dimethyl disulfide (DMDS) was investigated using a scanning tunneling microscope with far- and near-field light. This reaction is thought to be initiated by the lowest-energy highest occupied molecular orbital (HOMO) to lowest unoccupied molecular orbital (LUMO) transition of the DMDS molecule under far-field light. In near-field light, photodissociation proceeds at lower photon energies than in far-field light. To gain insight into the underlying mechanism, we theoretically investigated the excited states of DMDS adsorbed on Cu and Ag surfaces modeled by a tetrahedral 20-atom cluster. The frontier orbitals of the molecule were delocalized by the interaction with the metal, resulting in narrowing of the HOMO-LUMO gap energy. The excited-state distribution was analyzed using the Mulliken population analysis, decomposing molecular orbitals into metal and DMDS fragments. The excited states of the intra-DMDS transitions were found over a wider energy range, but at low energies, their oscillator strengths were negligible, which is consistent with the experimental results. Sparse modeling analysis showed that typical electronic transitions differed between the higher and lower excited states. If these low-lying excited states are efficiently excited by near-field light with different selection rules, the S-S bond dissociation reaction can proceed.
光学近场是指靠近表面的局部光场，它能引发诸如偶极禁戒跃迁等光化学现象。最近，利用具有远场和近场光的扫描隧道显微镜研究了二甲基二硫醚（DMDS）的S-S键的光解离。该反应被认为是由远场光下DMDS分子的最低能量最高占据分子轨道（HOMO）到最低未占据分子轨道（LUMO）的跃迁引发的。在近场光中，光解离在比远场光更低的光子能量下进行。为了深入了解其潜在机制，我们从理论上研究了吸附在由20个原子的四面体簇模拟的Cu和Ag表面上的DMDS的激发态。分子的前沿轨道通过与金属的相互作用而离域，导致HOMO-LUMO能隙变窄。使用Mulliken布居分析来分析激发态分布，将分子轨道分解为金属和DMDS片段。发现DMDS内部跃迁的激发态分布在更宽的能量范围内，但在低能量下，它们的振子强度可以忽略不计，这与实验结果一致。稀疏建模分析表明，典型的电子跃迁在较高和较低激发态之间存在差异。如果这些低激发态被具有不同选择规则的近场光有效激发，S-S键解离反应就可以进行。
