Synthesis and photocatalytic activity of ultrathin two-dimensional porphyrin nanodisks via covalent organic framework exfoliation

Impact Factor:

6.581

DOI number:

10.1038/s42004-019-0158-8

Journal:

Commun. Chem.

Abstract:

Two-dimensional (2D) materials, such as chalcogenides, graphene1, and organic polymers2,3,4,5, have recently received significant attention due to their unique electronic and photochemical properties. In order to obtain these 2D materials in bulk, the exfoliation of layered materials in the liquid phase is one of the simplest synthetic procedures6,7,8. Meanwhile, covalent organic frameworks (COFs)9,10,11 have also recently been employed as starting materials for exfoliation into multiply-layered 2D polymers in the liquid phase12,13. For example, Zamora et al. reported a sonication procedure for the exfoliation of COFs into nano-sized layered structures14,15. More recently, Dichtel et al. reported that polar solvents, such as dioxane, water, and N,N-dimethylformamide (DMF), exfoliate COFs16. However, most of these previous studies showed that the exfoliated COFs, comprised of 1,3,5-triphenylbenzene unit, still possess structures with several layers. It is still unclear whether or not it is possible to exfoliate COFs composed of π-conjugated molecules such as porphyrins for the synthesis of 2D polymer nanodisks. Feng and Wang et al. reported the exfoliation of anthraquinone COFs using a ball milling method to produce 5 nm-thick redox active COFs as cathodes for lithium-ion battery17. Banerjee et al. reported the novel method to exfoliate COFs using a Diels-Alder reaction of N-hexylmaleimide and anthracene in COFs to exfoliate into the nanosheets12. Consequently, the further development of exfoliation methods is necessary to realize the rational syntheses of sub-nanometer- thick 2D π-conjugated ultrathin polymer nanodisks18,19. Porphyrins are unique molecules in terms of their photochemical properties and structural diversities7,20. Not surprisingly, they have been employed as building blocks for COFs and have demonstrated carrier-transport as well as electro-catalytic properties10,21,22. Despite this, less is still known about the possible exfoliation of porphyrin COFs and their rational design applications. In the present study, we report an exfoliation method for COFs composed of porphyrin molecules through the incorporation of metal ions and the use of axial ligands to disrupt the π-π stacking of the porphyrin COFs via steric hindrance. We observe the clear formation of uniform nanometer-thick disk-like dispersed macro-molecular structures using copper as the metal ion and 4-ethylpyridine as the axial ligand. Furthermore, we describe the photocatalytic activity of these structures for H2 generation under irradiation with visible and near-infrared (NIR) light23,24,25,26,27,28,29. Results Exfoliation design in porphyrin covalent organic framework In order to exfoliate the COFs into two-dimensional (2D) polymer layers (referred as to exfoliated nanodisks in this study), we proposed that the coordination of axial ligands at the porphyrin centers of the COFs would disrupt the π-π interactions between the 2D-COF layers, resulting exfoliation (Fig. 1a). In this study, we employed the stable porphyrin-containing COFs (DhaTph) reported by Banerjee et al30. due to its chemical stability in solvents, a consequence of intramolecular hydrogen bonding. As axial ligands for exfoliation, we chose pyridines (i.e., pyridine size: 3.8 Å) to disrupt a π-π stacking (∼4 Å) between DhaTph layers. The DhaTph was built using the 5,10,15,20-tetrakis(4-aminophenyl)-21H,23H-porphyrin (Tph, 1) unit, and the 2,5-dihydroxyterephthalaldehyde (Dha, 2) linker unit. The DhaTph was synthesized through the formation of the Schiff-base between the amine (−NH2) of 1 and the aldehydes of 2 in the presence of acetic acid (CH3COOH), as previously described30. Detailed synthesis conditions and the chemical structures of the building blocks are shown in Supplementary Fig. 1. As reported by Banerjee et al. and Jiang et al., DhaTph is chemically stable due to the intramolecular hydrogen bonding within its planer surfaces

Co-author:

Kota Nomura, Mingshan Zhu, Xinxi Li, Jiawei Xue, Tetsuro Majima

First Author:

Zeyu Fan

Correspondence Author:

Yasuko Osakada

Volume:

2

Issue:

1

Page Number:

55

ISSN No.:

1359-7345

Translation or Not:

no

Date of Publication:

2019-05-17

Links to published journals:

https://www.nature.com/articles/s42004-019-0158-8