Molecular Engineering of Bandgaps in Covalent Organic Frameworks

Abstract

Two-dimensional (2D) covalent organic frameworks (COFs) are an emerging class of porous materials with potential for wide-ranging applications. Intense research efforts have been directed at tuning the structure and topology of COF, however the bandgap engineering of COF has received less attention, although it is a necessary step for developing the material for photovoltaic or photonic applications. Herein, we have developed an approach to narrow the bandgap of COFs by pairing triphenylamine and salicylideneaniline building units to construct an eclipsed stacked 2D COF. The ordered porous structure of 2D COF facilitates a unique moisture-triggered tautomerism. The combination of donor−acceptor charge transfer and tautomerization in the salicyclidineaniline unit imparts a large bandgap narrowing for the COF and turns it color to black. The synthesized COF with donor−acceptor dyad exhibits excellent nonlinear optical properties according to open aperture Z-scan measurements with 532 nm nanosecond laser pulses.

Imine‐Based Covalent Organic Frameworks as Photocatalysts for Metal Free Oxidation Processes under Visible Light Conditions

“…Then, the synthesis of crystalline materials 1 a-c was carried out under solvothermal conditions [24,[51][52][53] (left, Figure 2), obtaining crystalline materials that were confirmed by powder XRD analysis (left, Figure 2) and also BET measurements (from 409 to 684 m 2 /g, see S.I. Direct Kubelka-Munk plots revealed the following band gaps (see the Supporting Information): E gap = 2.52 eV (492 nm) for laminar-COF 1 b (which is in good agreement with the previously reported value), [54] E gap = 2.82 eV (440 nm) for spherical-COF 1 a and E gap = 2.68 eV (463 nm) for 3D-COF 1 c. For the corresponding crystalline materials, very similar band gaps were found: E gap = 2.53 eV (491 nm) for crystalline 1 c, E gap = 2.85 eV (435 nm) for crystalline 1 a and E gap = 2.90 eV (427 nm) for crystalline 1 b. [24,47,50,53] According to a recent report, the bandgap of laminar iminebased COFs can be tuned through structural design.…”

“…The chemical identity of the materials synthesized was confirmed by elemental analysis, Fourier-Transform Infrared spectroscopy and CP-MAS-13 C-NMR, being all the results consistent with previously reported data [24,47,50] (see S.I.). [54] Therefore, we measured the UV-vis absorption (diffuse reflectance) and emission spectra of the amorphous and crystalline materials 1 a-c ( Figure 3). ), which are in agreement with previous reported materials.…”

Nonlinear Optical Switching in Regioregular Porphyrin Covalent Organic Frameworks

“…By comparison, in the case of Por-COF-ZnCu and Por-COF-ZnNi excited-state absorption leads to characteristic RSA behavior. [11,[16][17][18][19][20] Theswitching behavior from SA to RSA is highly interesting in view of optical switching applications. [14] In Figure 3, the red lines are the best theoretical fits to the experimental data at different input intensities.F rom the fit, we deduced the nonlinear absorption coefficient (b), ground-state absorption cross section (s o ), first excited state (s 1 ), and second excited-state absorption cross sections (s 2 ) for Por-COF-HH, Por-COF-ZnCu, and Por-COF-ZnNi (Table S1, Supporting Information).…”

Covalent Organic Frameworks: Chemical Approaches to Designer Structures and Built‐In Functions

“…This requires an elaborate design of semiconductors with suitable band structures for harvesting light as well as to appreciate molecular mechanism for charge separation and collection. The π arrays of COFs harvest visible light, while their designability enables fine‐tuning of the band structure to achieve the desired orbital levels . Azine‐linked COFs (Figure a) are interesting because adding one more nitrogen atom to the central aryl knot increases the H 2 evolution rate fourfold .…”