期刊
JOURNAL OF THE AMERICAN CHEMICAL SOCIETY
卷 143, 期 9, 页码 3613-3627出版社
AMER CHEMICAL SOC
DOI: 10.1021/jacs.1c00279
关键词
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资金
- EPSRC [EP/R012695/1, EP/L015366/1, EP/N509619/1]
- ERC [CAPRI 290966]
- Marie Sklodowska-Curie fellowship [MARCUS 793799]
- University of Bristol
- Royal Society University Research Fellowships [UF1402310, URF\R\201007]
- EPSRC [EP/R012695/1] Funding Source: UKRI
This study investigates the photochemical dynamics of organic photoredox catalysts in O-ATRP, revealing different excited-state lifetimes and triplet-state quantum yields for catalysts with varying propensities for controlling polymerization outcomes. The results suggest that high triplet-state quantum yields are not necessary for controlling polymer dispersity in O-ATRP reactions, highlighting the importance of re-evaluating excited-state properties in governing the photocatalytic behavior of organic photoredox catalysts.
The photochemical dynamics of three classes of organic photoredox catalysts employed in organocatalyzed atom-transfer radical polymerization (O-ATRP) are studied using time-resolved optical transient absorption and fluorescence spectroscopy. The nine catalysts selected for study are examples of N-aryl and core-substituted dihydrophenazine, phenoxazine and phenothiazine compounds with varying propensities for control of polymerization outcomes. Excited singlet-state lifetimes extracted from the spectroscopic measurements are reported in N,N-dimethylformamide (DMF), dichloromethane (DCM), and toluene. Ultrafast (<200 fs to 3 ps) electronic relaxation of the photocatalysts after photoexcitation at near-UV wavelengths (318-390 nm) populates the first singlet excited state (S-1). The S-1-state lifetimes range from 130 ps to 40 ns with a considerable dependence on the photocatalyst structure and the solvent. The 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. A comparison of our results with previously reported O-ATRP performances of the various photoredox catalysts shows that high triplet-state quantum yields are not a prerequisite for controlling polymer dispersity. For example, the photocatalyst 5,10-bis(4-cyanophenyl)-5,10-dihydrophenazine, shown previously to exert good polymerization control, possesses the shortest S-1-state lifetime (135 ps in DMF and 180 ps in N,N-dimethylacetamide) among the nine examples reported here and a negligible triplet-state quantum yield. The results call for a re-evaluation of the excited-state properties of most significance in governing the photocatalytic behavior of organic photoredox catalysts in O-ATRP reactions.
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