Abstract
Transient electronic and vibrational absorption spectroscopy (TEAS and TVAS, respectively) has been applied to the study of solution phase reactive mixtures relevant to synthetic methodology used in polymerisation and small molecule synthesis.The photochemical dynamics of three classes of organic photoredox catalysts (OPCs) employed in organocatalysed atom transfer radical polymerisation (O-ATRP) are studied. In total nine catalysts were selected for study with structures that vary around the N-aryl and core- substitution of dihydrophenazine, phenoxazine and phenothiazine scaffolds, each with varying propensities for control of polymerization outcomes. Experiments for both lifetime and photoinduced electron transfer measurements were recorded in three solvents of differing polarity: N’N-dimethylformamide (DMF), dichloromethane (DCM), and toluene.
S1-state lifetimes are reported and range from 130 ps to 40 ns with considerable dependence on the photocatalyst structure and the solvent. Competition between ground-electronic state recovery and intersystem crossing controls triplet state populations and is a minor pathway in the dihydrophenazine derivatives but is of greater importance for phenoxazine and phenothiazine catalysts. Comparison of these results with previously reported O-ATRP performances of the various photoredox catalysts shows that high triplet-state quantum yields are not a pre-requisite for controlling polymer dispersity. The results call for a re-evaluation of the excited state properties of most significance in governing the photocatalytic behaviour of organic photoredox catalysts in O-ATRP reactions.
Spectroscopic signatures of the OPC excited states, electron acceptors and products of photoinduced electron transfer are tracked over sub-picosecond to nanosecond and microsecond time-intervals. Trends in bimolecular electron transfer rate coefficients are rationalized using a modified Marcus-Savéant theory of dissociative electron transfer and show that the Gibbs energy change is the major determinant of electron transfer rates in OPCs relevant for ATRP.
TVAS is applied to a recently reported reaction involving the addition of an electron-deficient alkyl radical to the strained σ‐bond of a bicyclo[1.1.0]butyl boronate complex to form a cyclobutyl boronic ester. The previously proposed single electron transfer mechanism does not adequately account for the observed spectral and kinetic data. Instead, iodine atom transfer is shown to be the preferred pathway for this reaction and is likely to be operative for other reactions of this type.
| Date of Award | 2 Dec 2021 |
|---|---|
| Original language | English |
| Awarding Institution |
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| Supervisor | Varinder K Aggarwal (Supervisor) & Andrew J Orr-Ewing (Supervisor) |
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