We explore the crystallization of a high surface area imine-linked two-dimensional covalent organic framework (2D COF). The growth process reveals rapid initial formation of an amorphous network that subsequently crystallizes into the layered 2D network. The metastable amorphous polymer may be isolated and resubjected to growth conditions to form the COF. These experiments provide the first mechanistic insight into the mechanism of imine-linked 2D COF formation, which is distinct from that of boronate-ester linked COFs.

Covalent organic frameworks (COFs) are two- or three-dimensional (2D or 3D) polymer networks with designed topology and chemical functionality, permanent porosity, and high surface areas. These features are potentially useful for a broad range of applications, including catalysis, optoelectronics, and energy storage devices. But current COF syntheses offer poor control over the material’s morphology and final form, generally providing insoluble and unprocessable microcrystalline powder aggregates. COF polymerizations are often performed under conditions in which the monomers are only partially soluble in the reaction solvent, and this heterogeneity has hindered understanding of their polymerization or crystallization processes. Here we report homogeneous polymerization conditions for boronate ester-linked, 2D COFs that inhibit crystallite precipitation, resulting in stable colloidal suspensions of 2D COF nanoparticles. The hexagonal, layered structures of the colloids are confirmed by small-angle and wide-angle X-ray scattering, and kinetic characterization provides insight into the growth process. The colloid size is modulated by solvent conditions, and the technique is demonstrated for four 2D boronate ester-linked COFs. The diameter of individual COF nanoparticles in solution is monitored and quantified during COF growth and stabilization at elevated temperature using in situ variable-temperature liquid cell transmission electron microscopy imaging, a new characterization technique that complements conventional bulk scattering techniques. Solution casting of the colloids yields a free-standing transparent COF film with retained crystallinity and porosity, as well as preferential crystallite orientation. Collectively this structural control provides new opportunities for understanding COF formation and designing morphologies for device applications.

The ability to accurately and quantitatively characterize structure−mechanochemical activity relationships is important for informing the fundamental understanding of mechanochemical reactivity and, in turn, the successful advancement of the rapidly growing field of polymer mechanochemistry. Ultrasound-induced mechanical activation of polymers remains one of the most general methods for studying mechanophore reactivity; however, the activation rates of scissile mechanophores are still routinely deduced from changes in polymer size using gel permeation chromatography (GPC) that indirectly report on mechanophore activation with questionable accuracy. Here, the rates of ultrasound-induced mechanochemical activation of two distinct scissile and fluorogenic mechanophores are measured using photoluminescence spectroscopy and compared directly to rates determined using various methods for analyzing chain scission kinetics from GPC measurements. This systematic study confirms that the conventional method for analyzing chain scission kinetics is inaccurate and that it provides a misleading picture of mechanophore activity. Instead, time-dependent changes in the GPC refractive index response closely reproduce the rates of mechanophore activation determined spectroscopically. These results expand on prior work by providing a systematic evaluation of the methods used to characterize mechanophore activation kinetics and emphasize the need for a unified approach to kinetic analysis in the field of polymer mechanochemistry. Moreover, analysis of mechanophore activation efficiency reveals an important insight into the consequences of molecular weight dispersity on the characterization of mechanophore reactivity.

Despite recent advances in polymer mechanochemistry, a more complete understanding of the factors that dictate the ultrasound-induced mechanochemical activation efficiency of mechanophores is necessary. Here, we examine how the identity of a mechanophore, and hence its unique force-coupled reactivity, affects the competition between mechanophore activation and nonspecific polymer backbone scission. Polymers incorporating distinct mechanophores but with putatively similar “chain-centeredness” exhibit widely different mechanochemical activation efficiencies. Furthermore, we employ mechanophores that can be orthogonally cleaved following ultrasonication using heat or light to report on the degree of nonspecific backbone scission that occurs for different mechanophore-containing polymers subjected to ultrasound-induced mechanical force. Our results illustrate that the identity of the mechanophore as well as its position in the polymer chain are inextricably important parameters that together control the selectivity of mechanophore activation during ultrasonication.

Mechanochromism is one of the most widely developed areas in the quickly emerging field of polymer mechanochemistry. Stress-sensitive molecules called mechanophores are designed to undergo productive chemical transformations in response to mechanical force including changes in color that are useful for sensing and patterning. A variety of mechanochromic mechanophores have been developed, but modulating the photophysical properties of the mechanically generated dyes generally requires the independent preparation of discrete derivatives. Here we introduce a mechanophore platform enabling mechanically gated multicolor chromogenic reactivity. The mechanophore is based on an activated furan precursor to donoracceptor Stenhouse adducts (DASAs) masked as a hetero-Diels-Alder adduct. Mechanochemical activation of the mechanophore unveils the DASA precursor and subsequent reaction with a secondary amine generates an intensely colored DASA photoswitch. Critically, the color and photochemical properties of the DASA are controlled by the identity of the amine and thus a single mechanophore can be differentiated post-activation to produce a wide range of functionally diverse DASA compounds. We highlight the unique reactivity of this system by establishing the concept of mechanochemical multicolor soft lithography whereby a complex multicolor composite image is printed into a mechanochemically active elastomer through an iterative process of localized compression and reaction with different amines. Our results demonstrate the first example of multicolor pattern reproduction from a single mechanophore, empowering the fabrication of complex stimuli-responsive materials and paving the way for applications in patterning, sensing, and encryption.

Understanding structure–mechanochemical reactivity relationships is important for informing the rational design of new stimuli-responsive polymers. To this end, establishing accurate reaction kinetics for mechanophore activation is a key objective. Here, we validate an initial rates method that enables the accurate and rapid determination of rate constants for ultrasound-induced mechanochemical transformations. Experimental reaction profiles are well-aligned with theoretical models, which support that the initial rates method effectively deconvolutes the kinetics of specific mechanophore activation from the competitive process of nonspecific chain scission.

During the past two decades, our understanding of mechanochemical reactivity has advanced considerably. Nevertheless, an incomplete knowledge of structure–activity relationships and the principles that govern mechanochemical transformations limits molecular design. The experimental development of mechanophores has thus benefited from simple computational tools like CoGEF, from which quantitative metrics like rupture force can be extracted to estimate reactivity. Furan–maleimide (FM) and anthracene–maleimide (AM) Diels–Alder adducts are widely studied mechanophores that undergo retro-Diels–Alder reactions upon mechanical activation in polymers. Despite possessing significantly different thermal stability, similar rupture forces predicted by CoGEF calculations suggest that these compounds exhibit similar mechanochemical reactivity. Here, we directly probe the relative mechanochemical reactivity of FM and AM adducts through competitive activation experiments. Ultrasound-induced mechanochemical activation of bis-adduct mechanophores comprising covalently tethered FM and AM subunits reveals pronounced selectivityas high as ∼13:1for reaction of the FM adduct compared to the AM adduct. Computational models provide insight into the greater reactivity of the FM mechanophore, indicating a more efficient mechanochemical coupling for the FM adduct compared to the AM adduct. The methodology employed here to directly interrogate the relative reactivity of two different mechanophores using a tethered bis-adduct configuration may be useful for other systems where more common sonication-based approaches are limited by poor sensitivity.

Stress-sensitive molecules called mechanophores undergo productive chemical transformations in response to mechanical force. A variety of mechanochromic mechanophores, which change color in response to stress, have been developed, but modulating the properties of the dyes generally requires the independent preparation of discrete derivatives. Here we introduce a mechanophore platform enabling mechanically gated multicolor chromogenic reactivity. The mechanophore is based on an activated furan precursor to donor–acceptor Stenhouse adducts (DASAs) masked as a hetero-Diels–Alder adduct. Mechanochemical activation of the mechanophore unveils the DASA precursor and subsequent reaction with a secondary amine generates an intensely colored DASA. Critically, the properties of the DASA are controlled by the amine and thus a single mechanophore can be differentiated post-activation to produce a wide range of functionally diverse DASAs. We highlight this system by establishing the concept of mechanochemical multicolor soft lithography whereby a complex multicolor composite image is printed into a mechanochemically active elastomer through an iterative process of localized compression followed by reaction with different amines.