class="head no_bottom_margin" id="sec1title">IntroductionThe design and fabrication of supramolecular polymers and materials by precise "bottom-up" self-assembly of building blocks has been an appealing and vital theme for chemists owing to the distinct dynamic properties of supramolecular materials (, , , , , , , , , , ). Many efforts have been realized in the constructions of sophisticated supramolecular architectures by elaborating the structural design of building blocks and controlling the dimensions, sizes, and manners of the further self-assembly to form diversified supramolecular materials exhibiting the superiority and functionality (, , , , href="#bib64" rid="bib64" class=" bibr popnode">Zhang et al., 2018c, href="#bib65" rid="bib65" class=" bibr popnode">Zhang et al., 2018d, href="#bib55" rid="bib55" class=" bibr popnode">Xing et al., 2018, href="#bib25" rid="bib25" class=" bibr popnode">Ji et al., 2018, href="#bib45" rid="bib45" class=" bibr popnode">Tian et al., 2014, href="#bib43" rid="bib43" class=" bibr popnode">Tao et al., 2019). Meanwhile, man-made 2D organic materials have also attracted much attention of chemists since the rise of graphene (href="#bib10" rid="bib10" class=" bibr popnode">Colson and Dichtel, 2013, href="#bib68" rid="bib68" class=" bibr popnode">Zhuang et al., 2015). Although amounts of 2D metal/covalent organic frameworks have been built and fabricated by different kinds of synthetic methodologies and libraries (href="#bib50" rid="bib50" class=" bibr popnode">Wang et al., 2018a, href="#bib51" rid="bib51" class=" bibr popnode">Wang et al., 2018b, href="#bib52" rid="bib52" class=" bibr popnode">Wang et al., 2018c, href="#bib53" rid="bib53" class=" bibr popnode">Wang et al., 2019, href="#bib3" rid="bib3" class=" bibr popnode">Baek et al., 2013, href="#bib34" rid="bib34" class=" bibr popnode">Matsumoto et al., 2018, href="#bib54" rid="bib54" class=" bibr popnode">Xiao et al., 2018), the design and construction of 2D supramolecular assemblies are still under fledging stage, especially those exhibiting large-area, ultra-thin, free-standing, water-soluble features (href="#bib61" rid="bib61" class=" bibr popnode">Zhang et al., 2013, href="#bib36" rid="bib36" class=" bibr popnode">Pfeffermann et al., 2015, href="#bib60" rid="bib60" class=" bibr popnode">Yue et al., 2016).A reasonable strategy to fabricate 2D supramolecular assemblies involves the design of rigid and multi-branched monomers, which would define the assemblies growth in a highly ordered direction, resulting in rigid supramolecular frameworks (href="#bib15" rid="bib15" class=" bibr popnode">Dong et al., 2018). However, the strictly rigid structure mostly inhibited the large-area polymer growth because of the vertical packing tendency of rigid structures and the lack of flexibility, which is significant to allow the adaptive interactions among small-molecular-weight 2D assemblies with different edge shapes. Some groups strive to overcome this issue by interfacial self-assembly strategy (href="#bib36" rid="bib36" class=" bibr popnode">Pfeffermann et al., 2015, href="#bib15" rid="bib15" class=" bibr popnode">Dong et al., 2018), however, requiring special processing technique. Flexible hyperbranched monomers are good examples to form large-size supramolecular assemblies (href="#bib24" rid="bib24" class=" bibr popnode">Huang and Gibson, 2004, href="#bib17" rid="bib17" class=" bibr popnode">Fernández et al., 2008, href="#bib66" rid="bib66" class=" bibr popnode">Zhou et al., 2010, href="#bib12" rid="bib12" class=" bibr popnode">Dong et al., 2011a, href="#bib13" rid="bib13" class=" bibr popnode">Dong et al., 2011b, href="#bib14" rid="bib14" class=" bibr popnode">Dong et al., 2014, href="#bib42" rid="bib42" class=" bibr popnode">Tao et al., 2012, href="#bib16" rid="bib16" class=" bibr popnode">Fang et al., 2013, href="#bib48" rid="bib48" class=" bibr popnode">Wang et al., 2014) but remain a challenging issue how to precisely construct the dimensions of the assemblies. In many cases, such flexible hyperbranched building blocks tend to self-assemble into spheres or particles because of the flexibility-induced surface curving (href="#bib12" rid="bib12" class=" bibr popnode">Dong et al., 2011a, href="#bib13" rid="bib13" class=" bibr popnode">Dong et al., 2011b, href="#bib21" rid="bib21" class=" bibr popnode">Groombridge et al., 2017, href="#bib46" rid="bib46" class=" bibr popnode">Tian et al., 2017, href="#bib11" rid="bib11" class=" bibr popnode">Datta et al., 2018, href="#bib31" rid="bib31" class=" bibr popnode">Liu et al., 2018). Hence, it is still a fundamental question of whether the 2D supramolecular assemblies with large area can be achieved by the direct solution-phase growth strategies rather than by the self-assembly on interfaces/surfaces.Meanwhile, one of the representative features of supramolecular assemblies involves the capability of stimuli-responsive materials owing to the unique dynamic nature. Numerous supramolecular assemblies, such as zero-dimensional vesicle/micelle (href="#bib18" rid="bib18" class=" bibr popnode">Gaitzsch et al., 2016, href="#bib50" rid="bib50" class=" bibr popnode">Wang et al., 2018a, href="#bib51" rid="bib51" class=" bibr popnode">Wang et al., 2018b, href="#bib52" rid="bib52" class=" bibr popnode">Wang et al., 2018c, href="#bib19" rid="bib19" class=" bibr popnode">Gao et al., 2018, href="#bib8" rid="bib8" class=" bibr popnode">Chen et al., 2018, href="#bib23" rid="bib23" class=" bibr popnode">Hu et al., 2018), one-dimensional fibers/tubes (href="#bib22" rid="bib22" class=" bibr popnode">Hendricks et al., 2017, href="#bib9" rid="bib9" class=" bibr popnode">Cohen et al., 2018, href="#bib56" rid="bib56" class=" bibr popnode">Yagai et al., 2019), and three-dimensional gels (href="#bib2" rid="bib2" class=" bibr popnode">Appel et al., 2012, href="#bib26" rid="bib26" class=" bibr popnode">Jones and Steed, 2016, href="#bib47" rid="bib47" class=" bibr popnode">Voorhaar and Hoogenboom, 2016), have been proved to be talented in many potential applications. However, the alterability and stimuli-responsive behavior of 2D supramolecular assemblies have not been exploited yet. Hence, our motivation in these issues locates on following two original hypotheses: (1) whether we can enable the structural rigidity and flexibility in a single 2D supramolecular assembly; (2) what unprecedented properties and functions can be brought in this rigid and flexible 2D supramolecular assemblies. Herein, we report a rationally designed 2D supramolecular assembly to demonstrate the above-mentioned proposals, achieving the direct aqueous self-assembly to form 2D supramolecular assemblies that integrate large area (up to 1000 μm2), nano-sized thickness, water-solubility, and stimuli-induced alterability.In our design, a semi-rigid tri-branch compound trans->1 was constructed (href="/pmc/articles/PMC6660589/figure/fig1/" target="figure" class="fig-table-link figpopup" rid-figpopup="fig1" rid-ob="ob-fig1" co-legend-rid="lgnd_fig1">Figure 1). The unique Y-type structure bears two viologen units and a single azobenzene unit, which can bind together in the cavity of macrocycle cucurbit[8]uril (CB[8]) to form high-affinity ternary host-guest complex (href="#bib6" rid="bib6" class=" bibr popnode">Barrow et al., 2015, href="#bib4" rid="bib4" class=" bibr popnode">Del Barrio et al., 2013, href="#bib44" rid="bib44" class=" bibr popnode">Tian et al., 2012, href="#bib35" rid="bib35" class=" bibr popnode">Pazos et al., 2019). Notably, the viologen-terminated aromatic part bears a rigid backbone with a fixed angle of 120°, whereas the azobenzene-terminated linker is flexible. This semi-rigid design is expected to enable a dendrimer-like supramolecular self-assembly in aqueous solution. We expect that the rigid 120° aromatic backbone could support sufficient space for the tubular macrocycle CB[8] and hence inhibit the steric-hindrance-caused low polymerization degree of the supramolecular assemblies. Meanwhile, the simultaneous presence of soft glycol linker provides flexibility to the resulting supramolecular assemblies. This design is distinct from the previously reported strictly rigid supramolecular frameworks, in which the polymer skeleton is rigid and stable. It is expected that our "semi-rigid" design could generate a unique large-sized supramolecular assembly that simultaneously exhibits the capability of dynamic stimuli responsiveness and adaptiveness.href="/pmc/articles/PMC6660589/figure/fig1/" target="figure" rid-figpopup="fig1" rid-ob="ob-fig1">class="inline_block ts_canvas" href="/core/lw/2.0/html/tileshop_pmc/tileshop_pmc_inline.html?title=Click%20on%20image%20to%20zoom&p=PMC3&id=6660589_gr1.jpg" target="tileshopwindow">target="object" href="/pmc/articles/PMC6660589/figure/fig1/?report=objectonly">Open in a separate windowclass="figpopup" href="/pmc/articles/PMC6660589/figure/fig1/" target="figure" rid-figpopup="fig1" rid-ob="ob-fig1">Figure 1Schematic Illustration and Molecular StructureThe molecular structure of trans->1 and the schematic representation of the supramolecular assemblies of trans->1 and CB[8]. The backbone of the final assemblies is simplified for clear presentation.
展开▼
