Investigating Functional Organic Molecules in Solution Using Ultrafast Spectroscopy

Student thesis: Doctoral ThesisDoctor of Philosophy (PhD)

Abstract

The outcomes of photochemical processes are affected by the solvation environments of the photoactive solute molecules. A comprehensive understanding of how inter- and intramolecular interactions are mediated by solvents is necessary when assessing the suitability of functional, photochemically active molecules in a variety of applications. Here, combination of spectroscopic techniques is used to investigate the photochemical dynamics of functional organic molecules in a variety of solvents on timescales spanning eight orders of magnitude from sub-picosecond upwards.

Organic photocatalysts (OPCs) are molecules of interest in photoredox chemistry as alternatives to metal-based photocatalysts. Fluorescence emission measurements reveal dual lifetimes of the S1 states for each of the five candidate OPCs, with example fluorescence lifetimes of (5.18 ± 0.01) ns and (6.22 ± 1.27) μs for a benzophenone derivative containing two carbazole units, 2Cz-BP in DCM. Dual emission is representative of thermally activated delayed fluorescence (TADF) emitters. Two classes of OPC candidates exhibit TADF emission independent of the choice of solvent, whereas emission by a third class of OPC candidates depends on the molecular environment, exhibiting enhanced TADF emission in aggregates compared to monomers which is representative of aggregation-induced emission luminogens (AIEgens).

Two nitroaromatic phototriggered compounds (PTs) based on ortho-nitrobenzene chromophores are studied in mixed dimethyl sulfoxide (DMSO) and water solutions. It is revealed that the PTs follow two significant photochemical pathways after excitation using UV-light on timescales of (1.93 ± 0.7) ps and (13.9 ± 1.2) ps respectively. These pathways are either excited state intramolecular hydrogen transfer (ESIHT) within the singlet manifold, followed by a direct S1 to S0 internal conversion (IC) to repopulate the ground electronic state, or intersystem crossing (ISC) to populate excited triplet states. In mixed DMSO/ H2O solvents, different micro-solvation environments result in two S1 lifetimes for DMNB-Ser, 1.95 ps and (10.1 ± 1.2) ps.
Date of Award1 Oct 2024
Original languageEnglish
Awarding Institution
  • University of Bristol
SupervisorAndrew J Orr-Ewing (Supervisor) & Natalie Fey (Supervisor)

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