Visible-light organic photocatalysis for latent radical-initiated polymerization via 2e⁻/1H⁺ transfers: initiation with parallels to photosynthesis.
We report the latent production of free radicals from energy stored in a redox potential through a 2e(-)/1H(+) transfer process, analogous to energy harvesting in photosynthesis, using visible-light organic photoredox catalysis (photocatalysis) of methylene blue chromophore with a sacrificial sterically hindered amine reductant and an onium salt oxidant. This enables light-initiated free-radical polymerization to continue over extended time intervals (hours) in the dark after brief (seconds) low-intensity illumination and beyond the spatial reach of light by diffusion of the metastable leuco-methylene blue photoproduct. The present organic photoredox catalysis system functions via a 2e(-)/1H(+) shuttle mechanism, as opposed to the 1e(-) transfer process typical of organometallic-based and conventional organic multicomponent photoinitiator formulations. This prevents immediate formation of open-shell (radical) intermediates from the amine upon light absorption and enables the "storage" of light-energy without spontaneous initiation of the polymerization. Latent energy release and radical production are then controlled by the subsequent light-independent reaction (analogous to the Calvin cycle) between leuco-methylene blue and the onium salt oxidant that is responsible for regeneration of the organic methylene blue photocatalyst. This robust approach for photocatalysis-based energy harvesting and extended release in the dark enables temporally controlled redox initiation of polymer syntheses under low-intensity short exposure conditions and permits visible-light-mediated synthesis of polymers at least 1 order of magnitude thicker than achievable with conventional photoinitiated formulations and irradiation regimes.
Department of Chemical and Biological Engineering, University of Colorado Boulder , 3415 Colorado Ave., Boulder, Colorado 80303, United States.
Alan
A
Aguirre-Soto
Chern-Hooi
CH
Lim
Albert T
AT
Hwang
Charles B
CB
Musgrave
Jeffrey W
JW
Stansbury
D002384:Catalysis; D004583:Electrons; D005609:Free Radicals / Q000737:chemistry; D006859:Hydrogen / Q000737:chemistry*; D008027:Light; D008751:Methylene Blue / Q000031:analogs & derivatives / Q000737:chemistry*; D015394:Molecular Structure; D009861:Onium Compounds / Q000737:chemistry*; D009942:Organometallic Compounds / Q000737:chemistry*; D010084:Oxidation-Reduction; D055668:Photochemical Processes; D017319:Photosensitizing Agents / Q000737:chemistry*; D058105:Polymerization
D005609:Free Radicals; D009861:Onium Compounds; D009942:Organometallic Compounds; D017319:Photosensitizing Agents; D006859:Hydrogen; D008751:Methylene Blue