AbtractIn this work, we report the mechanistic origins of the triplet excited state of carbazole-cyanobenzene donoracceptor (D-A) fluorophores in EnT-based photocatalytic reactions and demonstrate the key factors that control the accessibility of the 3 LE (locally excited triplet state) and 3 CT (charge-transfer triplet state) via a combined photochemical and transient absorption spectroscopic study. We found that the energy order between 1 CT (charge transfer singlet state) and 3 LE dictates the accessibility of 3 LE/ 3 CT for EnT, which can be effectively engineered by varying solvent polarity and D-A character to depopulate 3 LE and facilitate EnT from the chemically more tunable 3 CT state for photosensitization. Following the above design principle, a new D-A fluorophore with strong D-A character and weak redox potential is identified, which exhibits high efficiency for Ni(II)-catalyzed cross-coupling of carboxylic acids and aryl halides with a wide substrate scope and high selectivity. Our results not only provide key fundamental insight on the EnT mechanism of D-A fluorophores but also establish its wide utility in EnT-mediated photocatalytic reactions.
A facile Zn-induced reductive homocoupling reaction was used to synthesize azo-linked porous organic frameworks (azo-POFs) from easily accessed nitro monomers.
We
report a facile synthesis of carbazolic porous organic frameworks
(Cz-POFs) via FeCl3 promoted oxidative polymerization.
Using bulky, dendritic building blocks with high connectivity, the
porosity of Cz-POFs was significantly enhanced. Specifically, Cz-POF-1
and Cz-POF-3 show high surface areas of 2065 and 1927 m2 g–1, respectively. These surface areas are 3.1
and 2.1 times larger than those of Cz-POF-2 and Cz-POF-4 constructed
from less branched building blocks, respectively. At 1 bar and 273
K, Cz-POF-3 exhibits the highest CO2 uptake (21.0 wt %)
and CH4 uptake (2.54 wt %), while Cz-POF-1 has the highest
H2 uptake (2.24 wt %) at 77 K. These values are among the
highest reported for porous organic polymers. In addition, Cz-POFs
exhibit good ideal CO2/N2 selectivities (19–37)
and CO2/CH4 selectivities (4.4–7.1) at
273 K, showing great promise for gas storage and separation applications.
We report a facile approach to fine tune the redox potentials of π-conjugated porous organic frameworks (POFs) by copolymerizing carbazolic electron donor (D) and electron acceptor (A) based comonomers at different ratios. The resulting carbazolic copolymers (CzCPs) exhibit a wide range of redox potentials that are comparable to common transition-metal complexes and are used in the stepwise photocatalytic degradation of lignin β-O-4 models. With the strongest oxidative capability, CzCP100 (D:A = 0:100) exhibits the highest efficiency for the oxidation of benzylic β-O-4 alcohols, while the highly reductive CzCP33 (D:A = 66:33) gives the highest yield for the reductive cleavage of β-O-4 ketones. CzCPs also exhibit excellent stability and recyclability and represent a class of promising heterogeneous photocatalysts for the production of fine chemicals from sustainable lignocellulosic biomass.
We describe a metal-free, photocatalytic hydrodefluorination (HDF) of polyfluoroarenes (FA) using pyrene-based photocatalysts (Py). The weak "π-hole-π" interaction between Py and FA promotes the electron transfer against unfavorable energetics (ΔG up to 0.63 eV) and initiates the subsequent HDF. The steric hindrance of Py and FA largely dictates the HDF reaction rate, pointing to an inner-sphere electron transfer pathway. This work highlights the importance of the size and shape of the photocatalyst and the substrate in controlling the electron transfer mechanism and rates as well as the overall photocatalytic processes.
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