Singlet and Triplet Contributions to the Excited-State Activities of Dihydrophenazine, Phenoxazine, and Phenothiazine Organocatalysts Used in Atom Transfer Radical Polymerization

Aditi Bhattacherjee*, Mahima Sneha*, Luke J Lewis-Borrell, Giordano Amoruso, Thomas A A Oliver, Jasper L Tyler, Ian P. Clark, Andrew J Orr-Ewing*

*Corresponding author for this work

Research output: Contribution to journalArticle (Academic Journal)peer-review

Abstract

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 spectroscopies. 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 (S1). The S1-state lifetimes 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 our results with previously reported O-ATRP performances of the various photoredox catalysis shows that high triplet-state quantum yields are not a pre-requisite for controlling polymer dispersity. For example, the 5,10-di(4-cyanophenyl)-5,10-dihydrophenazine photocatalyst, shown previously to exert good polymerization control, possesses the shortest S1-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 behaviour of organic photoredox catalysts in O-ATRP reactions.
Original languageEnglish
Pages (from-to)3613 - 3627
Number of pages15
JournalJournal of the American Chemical Society
Volume143
Issue number9
Early online date25 Feb 2021
DOIs
Publication statusPublished - 10 Mar 2021

Cite this