Photocontrolled Polymerization of Aryl Grignards for Pi-Conjugated Polymer Synthesis

Woods, Eliot Freshwater

doi:https://doi.org/10.21985/n2-9ft8-5p26

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Photocontrolled Polymerization of Aryl Grignards for Pi-Conjugated Polymer Synthesis

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Part I:Current approaches to synthesize π-conjugated polymers are dominated by thermally driven, transition-metal-mediated reactions. Herein we7 show that electron-deficient Grignard monomers readily polymerize under visible-light irradiation at room temperature in the absence of a catalyst. The product distribution can be tuned by the wavelength of irradiation based on the absorption of the polymer. Conversion studies are consistent with an uncontrolled chain-growth process; correspondingly, chain extension produces all-conjugated n-type block copolymers. Preliminary results demonstrate that the polymerization can be expanded to donor–acceptor alternating copolymers. An experimental and computational investigation into the mechanism of this photopolymerization is then discussed. Spectroscopic studies performed in situ and after quenching reveal that the propagating chain is a radical anion with halide end groups. DFT calculations for model oligomers suggest a Mg-templated SRN1-type coupling, in which Grignard monomer coordination to the radical anion chain avoids the formation of free sp2 radicals and enables C–C bond formation with very low barriers. Light plays an unusual role in the reaction, photoexciting the radical anion chain to shift electron density to the termini and thus favor productive monomer binding. Part II:Metallophotoredox catalysis combines the well-established mechanisms of transition-metal-catalyzed cross-coupling reactions with one-electron redox manipulations enabled by light. In most cases, a transition metal or organic dye serves as the photoredox catalyst while a ground-state Pd or Ni catalyst performs the organometallic steps. Cross-coupling mechanisms that rely on direct photoexcitation of a light-absorbing substrate have the potential to access distinct mechanisms and deliver unique selectivity based on substrate photophysics. In this report, we describe a photoinduced, Ni-catalyzed Suzuki–Miyaura cross coupling reaction that selectively functionalizes BODIPY chromophores, a common class of tunable, bright, photostable fluorophores used in imaging, photocatalysis, triplet photosensitization, and photouncaging. Using a bis-iodo BODIPY substrate, the selectivity for mono-vs. bis-arylation was found to be governed by a remote substituent that subtly alters the excited-state properties of the substrate. Consistent with a substrate photoexcitation mechanism, high chemoselectivity is also observed in mixtures of chromophores with distinct excited-state properties. This reaction is compatible with a variety of substituted BODIPY chromophores and boronic acids and esters, enabling the rapid synthesis of unsymmetrically-substituted chromophores.

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