Khadka Dipendra, Jayasinghe-Arachchige Vindi M, Prabhakar Rajeev, Ramamurthy Vaidhyanathan
Department of Chemistry, University of Miami, Coral Gables, FL, 33124, USA.
Photochem Photobiol Sci. 2023 Dec;22(12):2781-2798. doi: 10.1007/s43630-023-00486-2. Epub 2023 Oct 16.
Relative to isotropic organic solvent medium, the structure and conformation of a reactant molecule in an organized and confining medium are often different. In addition, because of the rigidity of the immediate environment, the reacting molecule have a little freedom to undergo large changes even upon gaining energy or modifications in the electronic structure. These alterations give rise to differences in the photochemistry of a molecular and supramolecular species. In this study, one such example is presented. α-Alkyl dibenzylketones upon excitation in isotropic solvents give products via Norrish type I and type II reactions that are independent of the chain length of the alkyl substituent. On the other hand, when these molecules are enclosed within an organic capsule of volume ~ 550 Å, they give products that are strikingly dependent on the length of the α-alkyl substitution. These previously reported experimental observations are rationalized based on the structures generated by molecular modeling (docking and molecular dynamics (MD) simulations). It is shown that MD simulations that are utilized extensively in biologically important macromolecules can also be useful to understand the excited state behavior of reactive molecules that are part of supramolecular assemblies. These simulations can provide structural information of the reactant molecule and the surroundings complementing that with the one obtained from 1 and 2D NMR experiments. MD simulated structures of seven α-alkyl dibenzylketones encapsulated within the octa acid capsule provide a clear understanding of their unique behavior in this restricted medium. Because of the rigidity of the medium, these structures although generated in the ground state can rationalize the photochemical behavior of the molecules in the excited state.
相对于各向同性的有机溶剂介质，反应物分子在有序且受限的介质中的结构和构象通常有所不同。此外，由于紧邻环境的刚性，即使反应物分子获得能量或电子结构发生改变，其进行大幅变化的自由度也很小。这些改变导致分子和超分子物种的光化学产生差异。在本研究中，给出了这样一个例子。α-烷基二苄基酮在各向同性溶剂中激发时，通过Norrish I型和II型反应生成产物，这些反应与烷基取代基的链长无关。另一方面，当这些分子被封装在体积约为550 Å的有机胶囊中时，它们生成的产物显著依赖于α-烷基取代的长度。基于分子建模（对接和分子动力学（MD）模拟）生成的结构，对这些先前报道的实验观察结果进行了合理化解释。结果表明，在生物学上重要的大分子中广泛使用的MD模拟，对于理解超分子组装体中反应性分子的激发态行为也可能是有用的。这些模拟可以提供反应物分子及其周围环境的结构信息，与从一维和二维核磁共振实验获得的信息相互补充。封装在八酸胶囊中的七种α-烷基二苄基酮的MD模拟结构，清楚地解释了它们在这种受限介质中的独特行为。由于介质的刚性，这些虽然是在基态生成的结构，却能够合理化分子在激发态的光化学行为。
