Department of Chemistry and Chemistry Institute for Functional Materials, Pusan National University, Busan 46241, Republic of Korea.
Int J Mol Sci. 2024 Oct 14;25(20):11038. doi: 10.3390/ijms252011038.
Excited-state intramolecular proton transfer (ESIPT) reactions are crucial in photoresponsive materials and fluorescent markers. The fluorescent compound 4-aminophthalimide (4-AP) has been reported to exhibit solvent-assisted ESIPT in protic solvents, such as methanol, wherein the solvent interacts with 4-AP to form a six-membered hydrogen-bonded ring that is strengthened upon excitation. Although the controversial observation of ESIPT in 4-AP has been extensively studied, the molecular mechanism has yet to be fully explored. In this study, femtosecond infrared spectroscopy was used to investigate the dynamics of 4-AP in methanol and acetonitrile after excitation at 350 and 300 nm, which promoted 4-AP to the S and S states, respectively. The excited 4-AP in the S state relaxed to the ground state, while 4-AP in the S state relaxed via the S state without the occurrence of ESIPT. The enol form of 4-AP (Enol 4-AP) in the S state was calculated to be ~10 kcal/mol higher in energy than the keto form in the S state, indicating that keto-to-enol tautomerization was endergonic, ultimately resulting in no observable ESIPT for 4-AP in the S state. Upon the excitation of 4-AP to the S state, the transition to Enol-4-AP in the S state was found to be exergonic; however, ESIPT must compete with an internal conversion from the S to the S state. The internal S → S conversion was significantly faster than the solvent-assisted ESIPT, resulting in a negligible ESIPT for the 4-AP excited to the S state. The detailed excitation dynamics of 4-AP clearly reveal the molecular mechanism underlying its negligible ESIPT, despite the fact that it forms a favorable structure for solvent-assisted ESIPT.
激发态分子内质子转移（ESIPT）反应在光响应材料和荧光标记物中至关重要。荧光化合物 4-氨基邻苯二甲酰亚胺（4-AP）已被报道在质子溶剂中表现出溶剂辅助的 ESIPT，其中溶剂与 4-AP 相互作用形成一个六元氢键环，在激发时增强。尽管 4-AP 中 ESIPT 的有争议观察已经得到了广泛研究，但分子机制尚未得到充分探索。在这项研究中，飞秒红外光谱用于研究在 350nm 和 300nm 激发后，4-AP 在甲醇和乙腈中的动力学，这分别促进了 4-AP 到 S 和 S 态。处于 S 态的激发 4-AP 弛豫到基态，而 S 态的 4-AP 通过 S 态弛豫而不发生 ESIPT。处于 S 态的 4-AP 的烯醇形式（Enol 4-AP）的能量比 S 态的酮形式高约 10kcal/mol，表明酮-烯醇互变异构是吸热的，最终导致在 S 态的 4-AP 中没有可观察到的 ESIPT。当 4-AP 被激发到 S 态时，发现向 S 态的 Enol-4-AP 的跃迁是放热的；然而，ESIPT 必须与从 S 到 S 态的内部转换竞争。S → S 的内部转换速度明显快于溶剂辅助的 ESIPT，导致激发到 S 态的 4-AP 中 ESIPT 可以忽略不计。4-AP 的详细激发动力学清楚地揭示了其微小 ESIPT 的分子机制，尽管它形成了有利于溶剂辅助 ESIPT 的结构。
