Singlet and Triplet Contributions to the Excited-State Activities of Dihydrophenazine, Phenoxazine, and Phenothiazine Organocatalysts Used in Atom Transfer Radical Polymerization
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-ar...
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| Vydané v: | Journal of the American Chemical Society Ročník 143; číslo 9; s. 3613 |
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| Hlavní autori: | , , , , , , , |
| Médium: | Journal Article |
| Jazyk: | English |
| Vydavateľské údaje: |
United States
10.03.2021
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| ISSN: | 1520-5126, 1520-5126 |
| On-line prístup: | Zistit podrobnosti o prístupe |
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| Shrnutí: | 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
,
-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
). The S
-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
-state lifetime (135 ps in DMF and 180 ps in
,
-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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| Bibliografia: | ObjectType-Article-1 SourceType-Scholarly Journals-1 ObjectType-Feature-2 content type line 23 |
| ISSN: | 1520-5126 1520-5126 |
| DOI: | 10.1021/jacs.1c00279 |