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 pi-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 sp(2)/sp(3) 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.