Vapochromic films of pi-conjugated polymers based on coordination and desorption at hypervalent tin(iv)-fused azobenzene compounds

We report the synthesis and vapochromic behaviors of film materials consisting of hypervalent tin-containing pi-conjugated polymers. We prepared copolymers with brominated tin-fused azobenzenes and modified fluorene having tetraethylene glycol as a side chain. The synthesized polymers showed good film-formability and high affinity with coordinating solvent molecules such as dimethyl sulfoxide (DMSO). In particular, we discovered distinct color changes from blue to purple when exposed to DMSO vapor. It was revealed that color changes should originate from reversible alteration of the coordination-number between five and six of hypervalent tin(iv) in the azobenzene compounds involved in the main-chain conjugation. Moreover, we also observed that binding constants between tin and coordinating solvents could be influenced by two substitutions on the tin atom and subsequently modulated responsivity of vapochromism in films by altering the type of substituent. Furthermore, the color-change behaviors can be estimated by quantum calculations with density functional theory. We demonstrate not only that hypervalent tin can work as a switching unit for modulating the electronic structures of pi-conjugated polymers triggered by solvent coordination but also that vapochromic behaviors in films can be predicted by estimating the affinity between hypervalent tin and solvent molecules with theoretical calculations.