Intra/Intermolecular Cross-linking/De-cross-linking Of Triarylimidazole Substituted Vinyl Polymer And Its Photophysical Properties

Hexaarylbiimidazole(HABI), a dimer of 2,4,5-triarylimidazole, can be prepared from 2,4,5-triarylimidazole with potassium ferricyanide under basic conditions. HABI possesses photochromic and piezochromic properties with irradiation and on grinding. The photo and piezochromism of the HABI is due to reversible dissociation of the dimer into a 2,4,5-triphenylimidazolyl radical which forms by a reversible reaction. HABI can readily cleave under mild conditions, such as visible light irradiation and pressure. Due to these unique properties, we introduced triarylimidazole into the polymer structure followed by intra/intermolecular cross- linking. The as-corss-linked polymer also shows photochromic and piezochromic properties. Compared to normal cross- linked thermoset polymers, the cross-linked, triarylimidazole-containing vinyl polymer can be recycled due to its reversible cross-linkage. This paper mainly focuses on:(1) Well-defined poly(DVT) with moderate molecular weight was prepared by reversible addition-fragmentation chain transfer(RAFT) polymerization of monomer 1-(4-(4,5-diphenyl-1H-imidazol-2- yl)phenyl)-4-vinyl-1H-1,2,3-triazole(DVT) using AIBN as the initiator. The obtained poly(DVT) was characterized by 1H NMR and FT-IR spectroscopy. The poly(DVT) was then reacted with potassium ferricyanide under basic conditions, yielding cross- linked poly(DVT). In addition, the reaction can be controlled in intramolecular or intermolecular cross- linking by adjusting the concentration of triarylimidazolyl moieties in the reaction system. Besides, the crosslinked polymer can undergo de-cross- linking by visible light irradiation. The products of entire cross- linking and de-cross- linking process was characterized by 1H NMR, FT-IR and GPC. The thermal properties of the cross- linked polymer were evaluated by DSC and TG. It was found that the decomposition temperature of cross-linked poly(DVT) increased and the glass transition behavior became blurred.(2) The photophysical properties of cross-linked polymer have been studied. We focused on the influence of visible light upon UV absorption and fluorescence spectra of cross- linked polymer. The UV absorption peak belonged to arylimidazole moieties appeared again after irradiation. At the same time, a new absorption peak relating to phenanthroimidazole group emerged and the intensity gradually improved. In addition, the fluorescence intensity of poly(DVT) was enhanced at the initial stage and declined soon with the extension of irradiation time.(3) We characterized the photochromic behavior of intramolecular cross-linked poly(DVT) and cross-linked DVT by transient absorption spectra. The half-time of photochromic behavior of intramolecular cross- linked poly(DVT) is 19.4 μs, while the half- time of cross-linked DVT is 15.4 μs. Besides, the speed of returning to the initial color of intramolecular cross- linked poly(DVT) is slower than cross-linked DVT.