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

Area of Science:

• Photochemistry
• Molecular Spectroscopy
• Materials Science

Background:

• Excited-state intramolecular proton transfer (ESIPT) is vital for photoresponsive materials and fluorescent probes.
• 4-aminophthalimide (4-AP) has been suggested to undergo solvent-assisted ESIPT, but the mechanism remains debated.
• Understanding ESIPT mechanisms is crucial for designing advanced functional materials.

Purpose of the Study:

• To elucidate the molecular mechanism of ESIPT in 4-aminophthalimide (4-AP) in methanol and acetonitrile.
• To investigate the dynamics of 4-AP in the S1 and S2 excited states using femtosecond infrared spectroscopy.
• To determine the factors influencing the occurrence or absence of ESIPT in 4-AP.

Main Methods:

• Femtosecond infrared spectroscopy was employed to probe the ultrafast dynamics of 4-AP.
• Excitation was performed at 350 nm (S1 state) and 300 nm (S2 state).
• Computational analysis was used to determine the relative energies of tautomeric forms in excited states.

Main Results:

• Excitation to the S1 state led to relaxation to the ground state without ESIPT, as keto-to-enol tautomerization was endergonic.
• Excitation to the S2 state resulted in relaxation via the S1 state, also without significant ESIPT.
• While the S2 to enol-4-AP transition in S1 is exergonic, rapid S2 → S1 internal conversion outcompeted the ESIPT pathway.

Conclusions:

• The study reveals that 4-AP exhibits negligible ESIPT due to competing ultrafast internal conversion pathways.
• Despite forming a favorable hydrogen-bonded structure, the excited-state dynamics do not support significant ESIPT in 4-AP.
• These findings clarify the molecular mechanism behind the controversial ESIPT observations in 4-AP.