Rational design of isostructural 2D porphyrin-based covalent organic frameworks for tunable photocatalytic hydrogen evolution.

Covalent organic frameworks have recently gained increasing attention in photocatalytic hydrogen generation from water. However, their structure-property-activity relationship, which should be beneficial for the structural design, is still far-away explored. Herein, we report the designed synthesis of four isostructural porphyrinic two-dimensional covalent organic frameworks (MPor-DETH-COF, M = H2, Co, Ni, Zn) and their photocatalytic activity in hydrogen generation. Our results clearly show that all four covalent organic frameworks adopt AA stacking structures, with high crystallinity and large surface area. Interestingly, the incorporation of different transition metals into the porphyrin rings can rationally tune the photocatalytic hydrogen evolution rate of corresponding covalent organic frameworks, with the order of CoPor-DETH-COF < H2Por-DETH-COF < NiPor-DETH-COF < ZnPor-DETH-COF. Based on the detailed experiments and calculations, this tunable performance can be mainly explained by their tailored charge-carrier dynamics via molecular engineering. This study not only represents a simple and effective way for efficient tuning of the photocatalytic hydrogen evolution activities of covalent organic frameworks at molecular level, but also provides valuable insight on the structure design of covalent organic frameworks for better photocatalysis.

Three-Dimensional Covalent Organic Frameworks: From Topology Design to Applications.

ConspectusCovalent organic frameworks (COFs) represent a novel type of crystalline porous polymers with potential applications in many areas. Considering their covalent connectivity in different dimensions, COFs are classified as two-dimensional (2D) layered structures or three-dimensional (3D) networks. In particular, 3D COFs have gained increasing attention recently because of their remarkably large surface areas (>5000 m2/g), hierarchical nanopores and numerous open sites. However, it has been proven to be a major challenge to construct 3D COFs, as the main driving force for their synthesis comes from the formation of covalent bonds. In addition, there are several stones on the roads blocking the development of 3D COFs. First, the successful topology design strategies of 3D COFs have been limited to [4 + 2] or [4 + 3] condensation reactions of the tetrahedral molecules with linear or triangular building blocks in the first decade, which led to only three available topologies (ctn, bor, and dia) and strongly restricted the incorporation of some important functional units. Next, as it is very challenging to obtain large-size single crystals of 3D COFs and the same building blocks may yield many possible structures that are quite difficult to identify from simulations, their structure determination has been considered a major issue. Last, the building blocks utilized to synthesize 3D COFs are very limited, which further affects their functionalization and applications. Therefore, since it was first announced in 2007, research studies regarding 3D COFs have been underexplored for many years, and very few examples have been reported.To confront these obstacles in 3D COFs, we started contributing to this field in 2016. Considering that many interesting quadrilateral molecules (e.g., pyrene and porphyrin) cannot be easily derivatized into linear or triangular motifs, we developed a novel topology design strategy to construct 3D COFs via [4 + 4] condensation reactions of tetrahedral and quadrilateral building blocks. After many trials, we found that this is a general synthetic strategy to build 3D COFs with the new pts topology. In addition, we explored the structure determination of polycrystalline 3D COFs prepared by our developed strategy via a 3D electron diffraction technique. Moreover, we expanded the toolbox of molecular building blocks for creating 3D COFs and successfully demonstrated the functionalization of 3D COFs with characteristic properties and applications. In this Account, we summarize our above ongoing research contributions, including (i) a novel topology design strategy for the synthesis of 3D COFs; (ii) attempts to determine the crystal structure of polycrystalline 3D COFs with atomic resolution; and (iii) the diversification of building blocks and applications of functionalized 3D COFs. Overall, our studies not only offer a new paradigm of expansion in the topology design strategy and building block families of 3D COFs, but also provide an idea of future opportunities for relevant researchers in this field.

Self-Assembled Extended π-Systems for Sensing and Security Applications.

Molecules and materials derived from self-assembled extended π-systems have strong and reversible optical properties, which can be modulated with external stimuli such as temperature, mechanical stress, ions, the polarity of the medium, and so on. In many cases, absorption and emission responses of self-assembled supramolecular π-systems are manifested several times higher when compared with the individual molecular building blocks. These properties of molecular assemblies encourage scientists to have a deeper understanding of their design to explore them for suitable optoelectronic applications. Therefore, it is important to bring in highly responsive optical features in π-systems, for which it is necessary to modify their structures by varying the conjugation length and by introducing donor-acceptor functional groups. Using noncovalent forces, π-systems can be put together to form assemblies of different shapes and sizes with varied optical band gaps through controlling intermolecular electronic interactions. In addition, using directional forces, it is possible to bring anisotropy to the self-assembled nanostructures, facilitating efficient exciton migration, resulting in the modulation of optical and electron-transport properties. In this Account, we mainly summarize our findings with optically tunable self-assemblies of extended π-systems such as p-phenylenevinylenes (PVs), p-phenyleneethynylenes (PEs), and diketopyrrolopyrroles (DPPs) as different stimuli-responsive platforms to develop sensors and security materials. We start with how PV self-assemblies and their coassemblies with appropriate electron-deficient systems can be used for the sensing of analytes in contact mode or in the vapor phase. For example, whereas the PV having electron-deficient terminal groups has high sensitivity toward trinitrotoluene (TNT) in contact mode, the supercoiled fibers formed by the coassembly of self-sorted stacks of C3-symmetrical PV and C3-symmetrical electron-deficient perylene bisimide are capable of sensing vapors of nitrobenzene and o-toluidine. The power of different functional groups in combination with PVs has been further illustrated by attaching CO2-sensitive tertiary amine moieties to a cyano-substituted PV, which allowed the bimodal detection of CO2 using fluorescence and Raman spectroscopy. Interestingly, the functionalization of PVs with terminal amide groups and chiral alkoxy side chains provided a mechanochromic system that allows self-erasable imaging. Whereas PVs exhibit quenching of fluorescence in most cases during self-assembly, PE derivatives exhibit aggregation-induced emission. This property of PEs has been exploited for the development of stimuli-responsive security materials, especially for currency and documents. For instance, the blue fluorescence of a PE attached to hydrophilic oxyethylene side chains coated on a filter paper upon contact with water changes to cyan emission due to the change in the molecular packing. Interestingly, the molecular packing of a Bodipy-attached PE-based gelator allowed a stress-induced change in the emission behavior, resulting in strong near-infrared (NIR) emission upon the application of mechanical stress or gelation. Finally, the use of DPP-based π-systems for the development of NIR transparent optical filters that block UV-vis light and their security- and forensic-related applications are described. These selected examples of the π-system self-assemblies provide an idea of the current status and future opportunities for scientists interested in this field of self-assembly and soft materials research.

Supramolecular Surface Charge Regulation in Ionic Covalent Organic Nanosheets: Reversible Exfoliation and Controlled Bacterial Growth.

Poor control on the exfoliation of covalent organic frameworks (COFs) remains a disadvantage for their application as two-dimensional nanosheets. An equally important problem is the reversible control at the available surface charges on COFs. Herein, a strategy for the reversible exfoliation, re-stacking, and surface-charge control of a propidium iodide based ionic covalent organic framework, PI-TFP, using cucurbit[7]uril (CB[7]) induced molecular recognition, is reported. The surface charge on PI-TFP facilitates its initial self-exfoliation. However, complexation with CB[7] resulted in re-stacking with concomitant decrease in zeta potential from +28±3.0 to +0.004±0.003 mV. Addition of 1-adamantylamine hydrochloride (AD) facilitates decomplexation of PI-TFP from CB[7], resulting in exfoliation and an increase in zeta potential to +24±3.0 mV. Such control on the exfoliation, re-stacking, and the associated regulation of the surface charge in PI-TFP was exploited for controlling bacterial growth. Thus, the activity of E. coli and S. aureus bacteria obtained with the self-exfoliated PI-TFP could be reversibly controlled by the CB[7]/AD pair.

Bimodal detection of carbon dioxide using fluorescent molecular aggregates.

Cyano-substituted p-phenylenevinylene (R-1) aggregates exhibiting fluorescence and Raman spectroscopic responses towards CO2 are described. The aggregation-induced emission (AIE) as well as the aggregation-enhanced Raman scattering (AERS) of R-1 in aqueous conditions was reduced in the presence of a small amount of CO2, which enabled its easy and fast bimodal detection in different analytical samples.

Supramolecular Reassembly of Self-Exfoliated Ionic Covalent Organic Nanosheets for Label-Free Detection of Double-Stranded DNA.

Ionic covalent organic nanosheets (iCONs), a member of the two-dimensional (2D) nanomaterials family, offer a unique functional platform for a wide range of applications. Herein, we explore the potential of an ethidium bromide (EB)-based covalent organic framework (EB-TFP) that self-exfoliates in water resulting in 2D ionic covalent organic nanosheets (EB-TFP-iCONs) for the selective detection of double-stranded DNA (dsDNA). In an aqueous medium, the self-exfoliated EB-TFP-iCONs reassemble in the presence of dsDNA resulting in hybrid EB-TFP-iCONs-DNA crystalline nanosheets with enhanced fluorescence at 600 nm. Detailed steady-state and time-resolved emission studies revealed that the reassembly phenomenon was highly selective for dsDNA when compared to single-stranded DNA (ssDNA), which allowed us to use the EB-TFP-iCONs as a 2D fluorescent platform for the label-free detection of complementary DNA strands.