Polytriazoles with Aggregation-Induced Emission Characteristics: Synthesis by Click Polymerization and Application as Explosive Chemosensors

Abstract

The copper-catalyzed 1,3-dipolar cycloaddition of alkynes with azides is a typical example of “click” reaction. Since the reaction enjoys the advantages of high efficiency and regioselectivity and requires mild reaction conditions and simple purification procedures, it has become a versatile synthetic tool with applicability in diverse areas including bioconjugation and surface modification. The click reaction has also been utilized in polymer science with emphasis on the functionalization of preformed polymers through postpolymerization approaches. The effort of developing the reaction into a new polymerization technique, however, has met with only limited success. The polymerization reactions of arylene diazides and arylene diynes catalyzed by copper(I) species were sluggish, taking as long as 7-10 days to finish. The products often precipitated from the reaction mixtures even at the oligomer stage or became insoluble in common organic solvents after purification, unless very long alkyl chains, such as n-dodecyl groups, were attached to the arylene rings. The prepared polytrizoles were nonluminescent in the solid state, although their dilute solution emitted UV light, suggesting that the polymer emission has been quenched by the aggregate formation. If the polytriazoles are to be utilized as light-emitting materials, this issue must be properly tackled because luminophores are commonly used as solid films in their practical applications. We have recently discovered an intriguing phenomenon of aggregation-induced emission (AIE): a series of nonemissive molecules such as tetraphenylethene (TPE) and hexaphenylsilole as well as their derivatives are induced to emit efficiently by aggregate formation. The AIE effect greatly boosts the fluorescence quantum yields (ΦF) of the molecules by up to 3 orders of magnitude, turning them from faint luminophores into strong emitters. Thanks to their unique AIE characteristics, the molecules have been found to serve as chemosensors, bioprobes, stimuli-responsive nanomaterials, and active layers in the construction of efficient organic light-emitting diodes. Among the AIE luminophores, the TPE system has received much attention because of its facile preparation, ready functionalization, good photostability, and high photoluminescence (PL) efficiency. For practical applications, these low molecular weight luminophores have to be fabricated into solid films by expensive techniques such as vacuum vapor deposition processes, which are not well suited to the manufacture of large-area, flat-panel devices. One way to surmount this processing disadvantage is to synthesize high molecular weight polymers, which can be readily fabricated into large-area films by simple macroscopic processing techniques such as spin coating and static casting. However, polymers with efficient light emissions in the aggregate or solid state are rare because aggregation of the polymer chains commonly quenches light emission. In this paper, we report a group of new TPE-containing polytriazoles (P3) synthesized from the click polymerization of diyne (1) with diazides (2; Scheme 1). The light emission of the polymers is dramatically enhanced, instead of being quenched, by aggregate formation. The diyne and diazide monomers, namely 1,4-bis(propargyloxy)benzene (1), 1,2-bis[4-(azidohexyloxy)phenyl]-1,2-diphenylethene (2a), and 1,2-bis[(4-azidomethyl)phenyl]-1,2-diphenylethene (2b), were synthesized according to Scheme S1. The reactions proceeded smoothly, and the desired monomers were obtained in good yields. The monomer structures were confirmed by spectroscopic analyses (see Supporting Information for detailed characterization data). We attempted to transform the monomers to their polymers by 1,3-dipolar polycycloaddition. We have previously succeeded in the catalyst-free polycycloaddition of bis(aroylacetylene)s with diazides by simple heating. No high molecular weight polymers, however, were obtained when 1 and 2 were refluxed in THF for 110 h (Table 1, no. 1). This is due to the low reactivity of diyne 1 because it contains no election-withdrawing groups. Although the monomers could be polymerized by