Exploring Excited State Proton Transfer Dynamics upon Ultraviolet Excitation

Area of Science:

• Physical Chemistry
• Spectroscopy
• Quantum Mechanics

Background:

• Excited state intramolecular proton transfer (ESIPT) is a crucial photophysical process.
• Ultrafast dynamics in ESIPT systems are not fully understood, especially upon excitation to higher energy states.

Purpose of the Study:

• To investigate ESIPT dynamics in salicylaldehyde azine (SAA) and 1,5-dihydroxyanthraquinone (DHAQ) upon excitation to higher excited states (S_n).
• To delineate the ultrafast photophysics and understand the influence of excitation energy on ESIPT.

Main Methods:

• Transient absorption spectroscopy with sub-30 fs pulses and broadband visible probing.
• Steady-state measurements and Time-Dependent Density-Functional Theory (TD-DFT) calculations.

Main Results:

• In SAA, ESIPT occurred in ~30 fs, with wavelength-dependent coherent vibrational beats influenced by the excited state.
• In DHAQ, ESIPT showed a primary time-constant of ~85 fs and a slower component of 9 ps.
• The ESIPT rate was invariant to excitation energy, but product yield and coherent oscillations varied.

Conclusions:

• Ultrafast ESIPT rates are largely independent of excitation energy.
• Higher energy excitation can alter product yields and vibrational coherences due to alternative decay pathways.