Two Colour Photoflow Chemistry for Macromolecular Design.

We report a photochemical flow setup that exploits λ-orthogonal reactions using two different colours of light (λ1 =350 nm and λ2 =410 nm) in sequential on-line irradiation steps. Critically, both photochemically reactive units (a visible-light reactive chalcone and a UV-activated photo-caged diene) are present in the reaction mixture. We demonstrate the power of two colour photoflow by the wavelength-selective end group modification of photo-caged polymer end groups and the subsequent polymer ring closure driven by a [2+2] cycloaddition. Importantly, we evidence that the high energy gate does not induce the visible light reaction of the chalcone, which attests the true λ-orthogonal nature of the flow reaction system. For the first time, this study opens the realm of photoflow reactions to λ-orthogonal photochemistry.