No abstract

Developing effective synthetic strategies as well as enriching functionalities for sp2‐carbon‐linked covalent organic frameworks (COFs) still remains a challenge. Now, taking advantage of a variant of Knoevenagel condensation, a new fully conjugated COF (g‐C34N6‐COF) linked by unsubstituted C=C bonds was synthesized. Integrating 3,5‐dicyano‐2,4,6‐trimethylpyridine and 1,3,5‐triazine units into the molecular framework leads to the enhanced π‐electron communication and electrochemical activity. This COF shows uniform nanofibrous morphology. By assembling it with carbon nanotubes, a flexible thin‐film electrode for a micro‐supercapacitor (MSC) can be easily obtained. The resultant COF‐based MSC shows an areal capacitance of up to 15.2 mF cm−2, a high energy density of up to 7.3 mWh cm−3, and remarkable rate capability. These values are among the highest for state‐of‐the‐art MSCs. Moreover, this device exhibits excellent flexibility and integration capability.

The polarity of a semiconducting molecule affects its intrinsic photophysical properties, which can be tuned by varying the molecular geometry. Herein, we developed a D3h‐symmetric tricyanomesitylene as a new monomer which could be reticulated into a vinylene‐linked covalent organic framework (g‐C54N6‐COF) via Knoevenagel condensation with another D3h‐symmetric monomer 2,4,6‐tris(4′‐formyl‐biphenyl‐4‐yl)‐1,3,5‐triazine. Replacing tricyanomesitylene with a C2v‐symmetric 3,5‐dicyano‐2,4,6‐trimethylpyridine gave a less‐symmetric vinylene‐linked COF (g‐C52N6‐COF). The octupolar conjugated characters of g‐C54N6‐COF were reflected in its scarce solvatochromic effects either in ground or excited states, and endowed it with more promising semiconducting behavior as compared with g‐C52N6‐COF, such as enhanced light‐harvesting and excellent photo‐induced charge generation and separation. Along with the matched energy level, g‐C54N6‐COF enabled the two‐half reactions of photocatalytic water splitting with an average O2 evolution rate of 51.0 μmol h−1 g−1 and H2 evolution rate of 2518.9 μmol h−1 g−1. Such values are among the highest of state‐of‐the‐art COF photocatalysts.

The photodissociation of formic acid has been studied experimentally and theoretically. Ab initio calculations were performed to study the dissociative profiles of five reaction channels on the S 0 , S 1 , and T 1 potential energy surfaces. The vibrationally excited nascent products were detected using a time-resolved Fourier transform infrared spectrometer after laser photolysis at 248 or 193 nm. In the 248 nm photolysis, the HCOOH molecule was first excited to the S 1 state, but it was found that the dissociation takes place on the S 0 surface after internal conversion. The products of the vibrationally excited CO, CO 2 (v 3) and H 2 O(v 1) were detected. During the dissociation process the vibrationally energized molecule is geometrically memorized and dynamically controlled, with the yield preference of CO and H 2 O over that of CO 2 and H 2. The ratio of CO(vу1)/CO 2 (v у1) is estimated as Ͻ7.5. Vibrationally excited CO (v) and CO 2 (v 3) are also found in the 193 nm photolysis but the CO/CO 2 ratio increases to 11. Most of the dissociation is thought to occur on the S 0 state. At this wavelength another dissociation channel which produces OH and HCO radicals on S 1 surface has been identified. The dissociation is unlikely to occur on the T 1 surface, as the energy barriers are fairly high.