Molecularly-Defined Macrocycles Containing Azobenzene Main-Chain Oligomers:Synthesis And Investigation On Its Properties

The synthesis and properties of macrocyclic structures have attracted continuous attention since that the cyclic topology effect may offer unique performance. The precision synthesis of molecularly-defined macrocycles is doubtlessly a prerequisite to precisely explore the structure-property relationship. In addition, the stepwise chain-growth approach is undoubtedly the primary choice to investigate the chain-length dependent properties and the cyclic topology effects. Azobenzene is a kind of reversible photoisomerization chromophore, which is widely applied in optical materials, photo-controlling biomolecules and self-organized nanostructures. Recently, researchers found that the incorporation of azobenzenes into macrocyclic scaffolds, azobenzenophanes, revealed a novel aspect of this interesting photoswitch. Therefore, the precision synthesis of molecularly-defined azobenzene macrocycles and the modular synthetic strategy is of great significance.The influence of terminal group decreases with increasing linear polymer molecular weight. Therefore, distinguish of cyclic polymers and their linear precursor is much signficance with low molecular weight. In this work, a series of molecularly-defined linear and cyclic oligomers containing azobenzene moieties in main chain were prepared successfully, based on click chemistry and a stepwise chain-growth strategy. Then, photoresponse performances and fluorescence behavior of the cyclic oligomers were investigated compared with the corresponded linear precursors via the combination of experimental and calculated approaches. The detailed researches were summarized as the following:1. A series of molecularly-defined linear and cyclic oligomers containing alternant azobenzene and TEG moieties in main chain, were successfully prepared by click chemistry and stepwise chain growth approach. Some investigations have been evaluated by the comparison of resultant linear and cyclic products.（1） Firstly, a functionalized A-B type monomer bearing an azide and protected trimethylsilyl（TMS） TMS-alkynyl, TMS-Azo-TEG-N3, was designed and synthesized. Then, a series of well-defined linear oligomers,（Azo-TEG）n-Br from n = 1 to n = 6, were prepared via iterative CuAAC coupling reaction between（Azo-TEG）n-Br and monomer. Then, l-(Azo_n+1-TEG_n)（n = 1-6） were obtained by the Williamson reaction between（AzoTEG）n-Br and azobenzene-containing unit（HAAzo）. Finally, the intramolecular cyclization of l-(Azo_n+1-TEG_n) were carried out to yield cyclic ones, c-(Azo_n+1-TEG_n) via Glaser coupling reaction under highly dilute condition. The successful preparation of l-(Azo_n+1-TEG_n) and c-(Azo_n+1-TEG_n)（n = 1-6） and the efficiency of the stepwise chain-growth strategy were verified through comprehensive characterizations including SEC, MALDITOF MS, FT-IR measurement and 1H NMR.（2） The photoisomerization and fluorescence emission of cyclic polymers were investigated compared with the linear counterparts. The restricted conformation imposed by the cyclic topology induced an effect on the photosensitive behavior of azobenzene. c-(Azo_n+1-TEG_n) displayed an obviously faster trans-to-cis photo-isomerization of azobenzene, a blue shift for π-π* transition band corresponding to trans-azobenzene and a fluorescent enhancement compared with those of linear precursors. Furthermore, the calculated results by using density functional theory（DFT） and employing time-dependent DFT（TDDFT） with the three-parameter hybrid functional（B3LYP） agree with the experimental ones basically.2. Three kind of monomer with different end groups were designed and synthesized. The successful preparation of monnmer, linear oligomer and cyclic oligomer were verified through comprehensive characterizations including MALDI-TOF MS, FT-IR measurement and 1H NMR.