Key Laboratory of Applied Surface and Colloid Chemistry of Ministry of Education, Key Laboratory for Macromolecular Science of Shaanxi Province, School of Chemistry & Chemical Engineering, Shaanxi Normal University, Xi'an, Shaanxi 710119, People's Republic of China.
J Phys Chem A. 2020 Aug 13;124(32):6411-6419. doi: 10.1021/acs.jpca.0c03525. Epub 2020 Aug 4.
Photochromic phenylhydrazones are one of the most promising candidates for a photoswitchable fluorescent probe with potential applications in various fields, but mechanistic understanding of the origin of this unique behavior is limited. In this work, we explored the emission nature and switching mechanism of a model phenylhydrazone-based fluorescent photoswitch, DMA-PHA, by means of TD-DFT and CASPT2 calculations. The fluorescence-emitting configuration of DMA-PHA does not involve an excited-state intramolecular proton transfer process since the resonance effect between the DMA group and the rest part of the molecule in the excited state strengthens the hydrogen bond and thus stabilizes the emissive state. The light-induced fluorescence toggling results from ↔ interconversion driven by an out-of-plane C═N bond torsion and assisted by a N-N single bond rotation, which in total lead to a charge separation process losing the fluorescence activity. The N-N bond rotation in phenylhydrazone further enhances the competitive nonradiative decay and reduces the photoisomerization yields. The theoretical results will provide the guidance for the rational design of novel and improved photoswitchable fluorescent probes with desired performances.
光致变色苯腙是最有前途的光致变色荧光探针候选物之一，具有在各个领域的潜在应用，但对这种独特行为的起源的机械理解是有限的。在这项工作中，我们通过 TD-DFT 和 CASPT2 计算，研究了模型苯腙基荧光光开关 DMA-PHA 的发射性质和开关机制。DMA-PHA 的荧光发射构象不涉及激发态分子内质子转移过程，因为在激发态下 DMA 基团和分子其余部分之间的共振效应增强了氢键，从而稳定了发射态。光诱导荧光的切换是由面外 C=N 键扭转驱动的↔互变引起的，并得到 N-N 单键旋转的辅助，这总共导致了电荷分离过程失去荧光活性。苯腙中的 N-N 键旋转进一步增强了竞争性非辐射衰减并降低了光致异构化产率。理论结果将为设计具有所需性能的新型和改进的光致变色荧光探针提供指导。
