The structural diversity of three-dimensional (3D) covalent organic frameworks (COFs) are limited as there are only a few choices of building units with multiple symmetrically distributed connection sites. To date, 4 and 6-connected stereoscopic nodes with T d , D 3h , D 3d and C 3 symmetries have been mostly reported, delivering limited 3D topologies. We propose an efficient approach to expand the 3D COF repertoire by introducing a high-valency quadrangular prism (D 4h ) stereoscopic node with a connectivity of eight, based on which two isoreticular 3D imine-linked COFs can be created. Low-dose electron microscopy allows the direct visualization of their 2-fold interpenetrated bcu networks. These 3D COFs are endowed with unique pore architectures and strong molecular binding sites, and exhibit excellent performance in separating C 2 H 2 /CO 2 and C 2 H 2 /CH 4 gas pairs. The introduction of highvalency stereoscopic nodes would lead to an outburst of new topologies for 3D COFs.
A novel approach for the synthesis of a variety of polysubstituted trans-2,3-dihydropyrroles from a wide range of chalcones and β-enamine ketones (esters) via iodine-promoted tandem Michael/cyclization sequence has been developed, affording the desired products in moderate to excellent yields. This methodology is a highly efficient, convenient way to access functionalized 2,3-dihydropyrroles from readily accessible substrates under mild reaction conditions.
Covalent organic frameworks (COFs)
have attracted much attention
in porous materials because of their structural tunability and stability.
However, the problems of low transmittance and large light scattering
due to the difficulty of powder phase processing have severely limited
their practical applications in third-order nonlinear optics (NLO).
Herein, we report the first successful synthesis of a series of donor–acceptor
(D–A) COFs films (thickness of ∼100 nm) with uniform
transparency and narrow optical band gap by the solvothermal and solid–liquid
interface methods for third-order NLO applications. Most importantly,
the designed PT–PA film exhibits excellent NLO performance,
including a normalized transmission value of ∼2.55 at the beam
focus, a modulation depth (A
s) of 139%,
and a high nonlinear absorption coefficient (β) of 1.83 ×
106 cm GW–1 at the pulse energy of 1
μJ, which is superior to most recently reported third-order
NLO materials. The excellent third-order NLO properties are mainly
attributed to the homogeneous transparency of the COFs films and the
extended-ordered conjugated structure and narrow optical band gap
of the COFs films, which enhance the charge transfer in their structures
and thus yield excellent third-order NLO behavior. This work provides
an option for the development of high-performance NLO optical devices
and opens a new path for the future development of COFs films in the
optical fields.
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