Probing the chemical structure of monolayer covalent-organic frameworks grown via Schiff-base condensation reactions

Abstract

Two-dimensional covalent-organic frameworks (2D-COFs) on surfaces offer a facile route to new 2D materials. Schiff-base condensation reactions have proven to be an effective fabrication route for such materials. We present scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) studies of porphyrin 2D-COFs grown at a solid-vapour interface. XPS shows that covalent links between porphyrins consist of a mixture of imines and hemiaminals, a non-conjugated intermediate in the Schiff-base condensation reaction. These results demonstrate that environmental conditions during growth can have an important impact on the chemical composition of Schiff-base 2D-COFs.

Synthesis of C−C Bonded Two‐Dimensional Conjugated Covalent Organic Framework Films by Suzuki Polymerization on a Liquid–Liquid Interface

“…[21] Furthermore,t he high-resolution XPS spectrum of N1si sc omposed of two peaks (399.7 and 399.1 eV) with an intensity ratio of 1:1, which can be assigned to NH and N = C, respectively (Fig-ure 7e). [22] Both intensity ratios accord with their predicted values (24:3:1 and 1:1, respectively). Thecharacteristic Sband (435 nm) and Qb ands (525, 570, and 669 nm) of porphyrin units in the UV/Vis spectrum confirms their presence in the film (solid curve in Figure 7f).…”

Synthesis of C−C Bonded Two‐Dimensional Conjugated Covalent Organic Framework Films by Suzuki Polymerization on a Liquid–Liquid Interface

“…Thep eak-increasing of phenyl (1560 cm À1 )a nd the porphyrin skeleton (1388 cm À1 ) demonstrates the enlargement of the conjugated system after polymerization (Figure 7b). [22] Both intensity ratios accord with their predicted values (24:3:1 and 1:1, respectively). Thehigh-resolution XPS spectrum of C1scan be divided to three peaks (284.4, 286.2, and 287.8 eV) with an intensity ratio of 24:3.2:1, which can be assigned to C=C, CÀ N, and C=N, respectively (Figure 7d).…”

Imine‐Based Architectures at Surfaces and Interfaces: From Self‐Assembly to Dynamic Covalent Chemistry in 2D

“…[105] However,S TM imaging revealed the distortion of the ÀOH-functionalized 15 + 29 2D COF,w hich was ascribed to the furtherr eaction of imine with the ÀOH groups activated by thermalannealing (140 8C). Highly ordered bi-component 2D COFs with ah oneycomb structurec an also be fabricatedt hrough the self-limiting solidvapor interface reaction method, [53,89,109] in which the coupling reactioni st ailored to take place at the solid-vapor interface by introducing one precursor by means of vaporization to the surfacep reloaded with the other precursor.T he condensation between monomers 6 and 38 or 15 and 27 was investigated by Liu and co-workers. [89] Initially,t he aldehyde( 6 or 15)i sa pplied onto as ubstrate by drop-casting, and then the amine (38 or 27)i si ntroduced.…”

“…The aforementioned properties in combination witht heir structuralf eatures (planar geometry) [67,[114][115][116][117] make porphyrins as uperior candidate for use in the generation of 2D COFs. [88,109,118] Sun and co-workersi nvestigated the on-surface reactionb etween 5,10,15,20-meso-tetra(4-aminophenyl)porphine (42)a nd three dialdehydes, that is, 6, 7,a nd 14 on aH OPG surface. [118] The synthesis of 2D COF 7 + 42 was conducted by applying a mixture of monomers onto the surfacea nd subsequenta nnealing at2 00 8Cf or 30 minutes.S TM investigationr evealed the existence of small 2D COF domains that feature as quare lattice and do not showa ny preferentialo rientation along the major symmetry axeso ft he graphite surface.…”