Tuning Topochemical Polymerization of Diacetylenes: A Joint Synthetic, Structural, Photophysical, and Theoretical Study of a Series of Analogues of a Known Reactive Monomer, 1,6-Bis(diphenylamino)-2,4-hexadiyne (THD)

Several analogues of 1,6-bis(diphenylamino)-2,4-hexadiyne (THD) have been prepared and their thermal and photochemical solid-state polymerization reactivity assessed. The compounds involved in this investigation are 1,6-bis(dibenzylamino)-2,4-hexadiyne (1), 1,6-bis(di-p-tolylamino)-2,4-hexadiyne (2), 1,6-bis(dipentafluorophenylamino)-2,4-hexadiyne (3), 1,6-bis((N-pentafluorophenyl)phenylamino)-2,4-hexadiyne (4), 1,12-bis(diphenylamino)-5,7-dodecadiyne (5), and 1,10-bis(diphenylamino)-4,6-decadiyne (6). Compound 6 is the only diacetylene that showed measurable polymerization reactivity, and this behavior was rationalized on the basis of X-ray crystallographic results. Yet, diyne 6 is much less reactive than THD, so the preparation of weakly polymerized single crystals of 6 proved to be possible and the study of the spectroscopic properties of diluted poly-6 chains feasible. Unlike poly-THD, poly-6 is quite soluble in organic solvents such as chloroform and THF, so determination by SEC of the molecular weight of this polydiacetylene (FDA) could be achieved, M-n = 150 000-300 000 Da. The polymeric chains generated by gamma-ray irradiation of single crystals of 6 are highly luminescent, which lends strong support to poly-6 being a red phase FDA similarly to poly-THD. The transition energy of the exciton for poly-6, about 2.4 eV, is much higher than that of poly-THD, 2.171 eV at 16 K. This difference was rationalized on the basis of dissimilarities in the C-1=C-4 center dot center dot center dot C1=C4 dihedral angle between the two polymers. The structure of poly-6 in its monomer matrix was obtained by DFT calculations and, based on this structure, the C-1=C-4 center dot center dot center dot C-1=C-4 dihedral angle of the FDA chain found to be 140 degrees. The C-1=C-4 center dot center dot center dot C-1=C-4 dihedral angle in poly-THD, based on the known X-ray crystal structure of this polymer, is 167 degrees. Thus, poly-6 appears to be a new structural type of PDAs; it is also a good model for the study of the relationship that exists between chain conformation and electronic structure.