Chapter Open for the Excited-State Intramolecular Thiol Proton Transfer in the Room-Temperature Solution.

Area of Science:

• Photochemistry
• Supramolecular Chemistry
• Organic Chemistry

Background:

• Excited-state intramolecular proton transfer (ESIPT) is a crucial photophysical process.
• ESIPT typically involves O-H or N-H transfer, but S-H transfer is less explored.
• Understanding ESIPT mechanisms is vital for designing novel photoresponsive materials.

Purpose of the Study:

• To experimentally demonstrate excited-state intramolecular proton transfer (ESIPT) involving a thiol proton.
• To investigate the photophysical properties of 3-thiolflavone derivatives, specifically 3NTF.
• To elucidate the mechanism of ESIPT in sulfur-containing compounds at room temperature.

Main Methods:

• Synthesis of 3-thiolflavone derivatives, including 3NTF and its methylated counterpart 3MeNTF.
• Steady-state absorption and emission spectroscopy in cyclohexane.
• Time-resolved fluorescence spectroscopy.
• Computational chemistry approaches (DFT).

Main Results:

• Experimental observation of thiol-type ESIPT in 3NTF, evidenced by a large Stokes-shifted emission (12,230 cm⁻¹) at 710 nm.
• Comparison with 3MeNTF, which lacks the thiol proton and shows normal emission at 472 nm.
• Ultra-fast ESIPT rate (<180 fs) and a tautomer emission lifetime of 120 ps for 3NTF.
• Computational studies confirm thermally favorable ESIPT but reveal non-radiative decay pathways for non-emissive derivatives.

Conclusions:

• Unambiguous experimental evidence for thiol-type ESIPT in solution at room temperature.
• The diethylamino group in 3NTF promotes ππ* character in the excited state, enabling prominent tautomer emission.
• This work expands the scope of ESIPT chemistry to include non-canonical sulfur hydrogen bonds.