# Exploring Excited State Proton Transfer Dynamics upon Ultraviolet Excitation

## TL;DR

This study explores how proton transfer happens in molecules when excited by ultraviolet light, using advanced techniques to measure fast reactions.

## Contribution

The paper presents direct measurements of ultrafast proton transfer dynamics in two molecules using UV excitation and transient absorption.

## Key findings

- ESIPT in SAA occurs ultrafast (∼30 fs), with coherent vibrational beats showing wavelength dependence.
- In DHAQ, ESIPT is observed for the first time with a time-constant of ∼85 fs and a slower 9 ps component.
- Product yield and coherent oscillations vary due to alternative decay pathways despite consistent ESIPT rates.

## Abstract

Excited state intramolecular proton transfer (ESIPT)
has been investigated
in two prototypical systemssalicylaldehyde azine (SAA) and
1,5-dihydroxyanthraquinone (DHAQ)using transient absorption
spectroscopy upon ultraviolet excitation into the less studied higher
excited (S
n
) manifold. Excitation with
sub-30 fs pulses and broadband visible probing has allowed for direct
measurement of the ESIPT rate. In conjunction with steady-state measurements
and TD-DFT calculations, a complete delineation of the ultrafast photophysics
has been carried out. In SAA, ESIPT remains ultrafast (∼30
fs), consistent with previous S1 excitation studies. Coherent
vibrational beats maps reveal significant wavelength dependence, however.
Theoretical analysis suggests that the observed modes and their intensities
in coherent vibrational spectra are modulated by the nature of the
electronically excited state. In DHAQ, the first direct observation
of ESIPT presents a time-constant of ∼85 fs, and a slower component
of 9 ps, akin to previous reports on double-proton transfer systems.
Collectively, the results suggest that while the ultrafast ESIPT rate
remains largely invariant vis-à-vis the excitation energy,
the product yield, as well as accompanying coherent oscillations,
may be substantively altered, owing to the existence of alternative
decay pathways.