Studies into the effects of hydrogen-bonding on excited state dynamics

Abstract

Hydrogen bonding (H-bonding) has a significant effect on the channelling of energy with a molecule. This thesis describes the development of a Transient Electronic Absorption Spectroscopy (TEAS) experiment at Southampton with the aim of further investigating this effect. The photodynamics of acetic acid (AA) and trifluoroacetic acid (TFA) dimers, held together by H-bonds, were also measured using Time-Resolved Infrared (TRIR) and TEAS measurements performed at the Central Laser Facility.

The photodynamics of the AA dimer are observed using a UV pump pulse with mid-IR and UV/Vis probe pulses. UV excitation at 200 nm leads to the population of the S2 state. This rapidly relaxes, over <1 ps, to the S1 state, causing a “buckling” of one of the AA monomers. The “buckled” AA conformation then relaxes to the vibrationally-hot ground state which subsequently undergoes vibrational cooling.

Substitution of methyl hydrogen atoms with fluorine atoms, to form TFA from AA, caused a significant weakening to the strength of the H-bonds holding the monomers together. UV excitation, at 200 nm, caused an initial breaking of these H-bonds in TFA. Over 1.5 ps, we see reformation of the H-bonds in the same "buckled" structure the AA adopted. This was all observed using in the TRIR measurements performed at the Central Laser Facility.

With the goal of further exploring the effect of H-bonding on excited state dynamics, a TEAS experiment was developed in Southampton. This entailed the planning and construction of a beam path, exploring the use of a liquid wire guided gravity jet as a sample delivery system and coordinating the experiment using LabVIEW. Finally, to test the TEAS experimental setup, new measurements were performed on Rhodamine-6G. Excitation at 343 nm yields the population of an excited state which relaxes to the S1 state over hundreds of femtoseconds. This S1 state has a lifetime which far exceeds the 167 ps pump-probe window which can be observed with the Southampton experiment.

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Identifiers

ORCID for Russell Minns: orcid.org/0000-0001-6775-2977

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