Covalent Organic Frameworks Enabling Site Isolation of Viologen-Derived Electron-Transfer Mediators for Stable Photocatalytic Hydrogen Evolution.

Electron transfer is the rate-limiting step in photocatalytic water splitting. Viologen and its derivatives are able to act as electron-transfer mediators (ETMs) to facilitate the rapid electron transfer from photosensitizers to active sites. Nevertheless, the electron-transfer ability often suffers from the formation of a stable dipole structure through the coupling between cationic-radical-containing viologen-derived ETMs, by which the electron-transfer process becomes restricted. Herein, cyclic diquats, a kind of viologen-derived ETM, are integrated into a 2,2'-bipyridine-based covalent organic framework (COF) through a post-quaternization reaction. The content and distribution of embedded diquat-ETMs are elaborately controlled, leading to the favorable site-isolated arrangement. The resulting materials integrate the photosensitizing units and ETMs into one system, exhibiting the enhanced hydrogen evolution rate (34600 µmol h-1 g-1 ) and sustained performances when compared to a single-module COF and a COF/ETM mixture. The integration strategy applied in a 2D COF platform promotes the consecutive electron transfer in photochemical processes through the multi-component cooperation.

Stable Radical Cation-Containing Covalent Organic Frameworks Exhibiting Remarkable Structure-Enhanced Photothermal Conversion.

The production of a radical cation-containing covalent organic framework (COF) has been accomplished by sequential in situ reactions, quaternization, and one-electron reduction of the 2,2'-bipyridine-based COFs. The acid-catalyzed COF formation enables the cis configuration of 2,2'-bipyridyl moieties in the structure, of which the stability arises from the eclipsed stacking of the two-dimensional layered structure. The postfunctionalization generates cyclic alkylated diquats as the sole products from the controlled quaternization. The reduction of diquat cations on the COF skeletons results in a large number of radical cations, which delocalize and uniaxially stack on top of one another by virtue of interlayered π-electronic couplings. The absorption of the near-infrared (NIR) region exhibited by the cationic radical COF is remarkably high owing to the intercharge transfer across the π-coupling interlayers. Also, the long-range array of extended and planar frameworks in such a COF leads to the extra stability of the radical cations against external stresses. The structure-enhanced performance of the COF material is witnessed with photothermal conversion efficiencies of as high as 63.8 and 55.2% when exposed to 808 and 1064 nm lasers, respectively. Further PEG modification on such a COF allows photoacoustic imaging and photothermal therapy in vivo under NIR light illumination to be manifested.

PEG-stabilized coaxial stacking of two-dimensional covalent organic frameworks for enhanced photocatalytic hydrogen evolution.

Two-dimensional covalent organic frameworks (2D COFs) featuring periodic frameworks, extended π-conjugation and layered stacking structures, have emerged as a promising class of materials for photocatalytic hydrogen evolution. Nevertheless, the layer-by-layer assembly in 2D COFs is not stable during the photocatalytic cycling in water, causing disordered stacking and declined activity. Here, we report an innovative strategy to stabilize the ordered arrangement of layered structures in 2D COFs for hydrogen evolution. Polyethylene glycol is filled up in the mesopore channels of a ß-ketoenamine-linked COF containing benzothiadiazole moiety. This unique feature suppresses the dislocation of neighbouring layers and retains the columnar π-orbital arrays to facilitate free charge transport. The hydrogen evolution rate is therefore remarkably promoted under visible irradiation compared with that of the pristine COF. This study provides a general post-functionalization strategy for 2D COFs to enhance photocatalytic performances.

Orthogonal functionalization of alternating polyesters: selective patterning of (AB) n sequences.

Precision functionalized polyesters, with defined monomer sequences, are prepared using an orthogonal post-polymerization strategy. These polyesters can be synthesized from bio-derived monomers and are targeted to degrade, by hydrolysis processes, to biocompatible diols and diacids; the new structures enabled by this methodology would be very difficult to synthesize by alternative strategies. A series of 9 well-defined highly alternating AB-type copolyesters, containing terminal and internal alkene functionalities, are synthesized in high conversions by the ring-opening copolymerization of epoxides and cyclic anhydrides. Firstly, the polyesters are functionalized by a selective hydroboration-oxidation reaction to exclusively and quantitatively hydroxylate the terminal alkenes, leaving the alternating internal alkenes unreacted. Subsequently, the internal alkenes are quantitatively transformed into carboxylic acid, amine, alkyl and oligo-ether groups, by thiol-ene reactions, to afford AB polyesters with alternating functional substituents. Three polyesters showing alternating hydrophilic/hydrophobic side-chain sequences self-assemble in solution to form nanostructures that are characterized using transmission electron microscopy and dynamic light scattering methods (R h = 100-300 nm). The selective patterning methodology provides facile, efficient and orthogonal functionalization of alternating polyesters with near-quantitative (AB) n repeat sequences. The method is expected to be generalizable to other polymers and provides access to completely new AB alternating structures with the potential to exploit ligand multi-valency and adjacency to enhance properties.

Synthesis and rac-lactide ring-opening polymerisation studies of new alkaline earth tetrahydroborate complexes.

Reaction of K[HC(C(Me)NAr')(2)] (Ar' = 2,6-C(6)H(3)(i)Pr(2)) with Mg(BH(4))(2) afforded the pseudo four-coordinate tetrahydroborate complex Mg{HC(C(Me)NAr')(2)}(BH(4))(THF) (1). The corresponding reaction of Ca(BH(4))(2)(THF)(2) or Sr(BH(4))(2)(THF)(2) gave the pseudo five-coordinate analogues M{HC(C(Me)NAr')(2)}(BH(4))(THF)(2) (M = Ca (2) or Sr (3)). All three compounds 1-3 have been structurally characterised. According to X-ray crystallography and IR spectroscopy all possess κ(3)-bound BH(4) ligands in the solid state. This coordination mode is also maintained in THF solution for 2 and 3, whereas complex 1 appears to form a bis(THF) complex containing a κ(2)-bound BH(4). Reaction of 1 with K[HC(P(Ph(2))NAr')(2)](THF)(2) formed Mg{HC(P(Ph(2))NAr')(2)}(BH(4))(THF)(2) (4) possessing a κ(3)-bound BH(4) ligand in both the solid state and solution. Compounds 1, 2 and 4 are highly active for the ring-opening polymerisation of ε-caprolactone forming dihydroxytelechelic PCL. Compound 1 is also extremely active for the ROP of rac-lactide forming highly heterotactic PLA with good agreement between predicted and measured M(n), in accord with previous studies of alkoxide and amide initiators based on this metal and ligand class. Compounds 2 and 4 were less productive and gave PLA with poorer control of M(n) and negligible heterotactic enrichment. MALDI-ToF MS analysis of the PLA formed with all three catalysts showed a mixture of both -CH(Me)CHO and -CH(Me)CH(2)OH termini arising from the M-BH(4) initiating groups.