Photoexcitation Dynamics of 4-Aminopthalimide in Solution Investigated Using Femtosecond Time-Resolved Infrared Spectroscopy

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 S₁ and S₂ states, respectively. The excited 4-AP in the S₁ state relaxed to the ground state, while 4-AP in the S₂ state relaxed via the S₁ state without the occurrence of ESIPT. The enol form of 4-AP (Enol 4-AP) in the S₁ state was calculated to be ~10 kcal/mol higher in energy than the keto form in the S₁ state, indicating that keto-to-enol tautomerization was endergonic, ultimately resulting in no observable ESIPT for 4-AP in the S₁ state. Upon the excitation of 4-AP to the S₂ state, the transition to Enol-4-AP in the S₁ state was found to be exergonic; however, ESIPT must compete with an internal conversion from the S₂ to the S₁ state. The internal S₂ → S₁ conversion was significantly faster than the solvent-assisted ESIPT, resulting in a negligible ESIPT for the 4-AP excited to the S₂ 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.