Semicrystalline Poly(azobenzene) with Negative Photochromism for Photochemical Melting of Thick Samples

Photomechanical effects based on photochemical transformations often suffer from challenges of restricted light penetration due to strong light attenuation, limiting their operation to micrometer-scale materials with relatively inefficient bending deformation modes. While recent reports have established semicrystalline poly(azobenzene)s as a promising class of photochemically responsive materials, strong absorption of UV light required to drive trans to cis isomerization has limited switching to within several micrometers of the surface. Negative photochromism offers a possible route to circumvent this limitation, as reversible photobleaching of the ground state absorption upon switching extends the light penetration. Herein, we report the synthesis and characterization of P(Cx-dFdC-azo), a negative photochromic semicrystalline poly(azobenzene) incorporating ortho-difluoro, dichloro-substituted azo photoswitches. These polymers exhibit near-quantitative bidirectional photoisomerization─92% conversion to cis under 617 nm irradiation and 96% conversion to trans under 405 nm irradiation, along with reversible photomelting and crystallization. Photorheological studies demonstrate a significant increase over which photomelting can be achieved, from <10 μm for conventional poly(azobenzene) to at least 300 μm for P(Cx-dFdC-azo), highlighting the potential of these materials for photochemically responsive materials with macroscopic dimensions.