Light-Induced Stacking of Metal-Free 2,2’-Bipyridine Derivatives and Polymers

Photochromic bipyridines typically appear in literature in the form of N,N’-disubstituted-4,4’-bipyridines, known as viologens. Following irradiation with UV light, a viologen undergoes one-electron transfer to form a blue-colored radical cation detectable by electron paramagnetic resonance (EPR) spectroscopy. Metal complexes with 2,2’-bipyridine ligands are also known to form photosensitizers. In this work, we investigated a new photo-responsive mechanism of metal-free 2,2’-bipyridines and its applications in photo-switchable polymers. The 2,2’-bipyridine derivatives synthesized in this study undergoes color change from light yellow to magenta upon UV irradiation with a new absorption peak at 550 nm. EPR spectroscopy did not show any detectable free radicals in the colored form, indicating a photochromic pathway that is different from viologens. Reversibility of the photochromism is highly tunable by changing the solvent environments and polymer structures when 2,2’-bipyridine derivatives are incorporated into polymer backbones. For example, when polyethylene glycol is copolymerized with the 2,2’-bipyridine derivatives, the semi-crystalline product appears to be magenta after UV irradiation. The colored state stays stable after removal of UV light. Heating and cooling without UV convert it back to the white semicrystalline solid, which also remains stable until being exposed to UV again. In this talk, we will discuss the tunability and reversibility of the photo-responsiveness of 2,2’-bipyridine derivatives and their polymers which are uniquely different from known photochromophores. The photochromism is attributed to the UV-induced staking of bipyridines, representing a new class of molecular photo-responses that can be adapted to various photo-switchable materials.