Pressure-Induced Solid-State Polymerization of Optically-Tunable Diphenyl-Substituted Diacetylene

Solid-state topochemical polymerization (SSTP) requires well-defined geometries and space symmetries between the starting monomers and resulting polymer, and diacetylenes are excellent precursors, reacting through a 1,4-addition mechanism. The hydrocarbon molecule 1,4-diphenyl-1,3-butadiyne (DPB) has a four-carbon chain with alternating triple/single bonds, capped on each end with a phenyl group, i.e. centrosymmetric with unsaturated π-bonding characteristics. To fully realize its potential for photocatalytic applications, improved control over the assembly process is desirable to form well-ordered poly(diphenylbutadiyne) (PDPB). Here, it is shown that with increasing pressure, DPB undergoes a series of solid-state chemical reactions while maintaining crystalline order related to the starting monomeric structure. Quenchable PDPB compounds begin forming at ca. 5 GPa, which exhibit optically-tunable absorbance and photoluminescence that is controllable through the extent of compression. Above ca. 15 GPa, the system transforms into a nonhexagonally-packed crystalline array with mixed sp2/sp3 character. These stepwise changes with compression are irreversible in nature, as observed by in situ diffraction and spectroscopic methods. For the first time, the simple SSTP synthesis route allows well-aligned DPB molecules to directly transform into a PDPB material via self-assembly solely through pressure generation within a diamond anvil cell without the traditional use of catalysts, temperature, radiation, templates, or solvents.