A highly fluorescent covalent organic framework as a hydrogen chloride sensor: roles of Schiff base bonding and π-stacking

Abstract

In this paper we report the extremely crystalline structures, high thermal stabilities, and strong fluorescence emissions of covalent organic frameworks (COFs) based on linked carbazole units. We have synthesized three stable luminescent carbazole-linked COFs, namely, BCTB–PD, BCTA–TP, and BCTB–BCTA, through Schiff base condensations of 4,4′,4′′,4′′′-([9,9′-bicarbazole]-3,3′,6,6′-tetrayl)tetrabenzaldehyde (BCTB-4CHO) with p-phenylenediamine (PD), of 4,4′,4′′,4′′′-([9,9′-bicarbazole]-3,3′,6,6′-tetrayl)tetraaniline (BCTA-4NH₂) with terephthalaldehyde (TP), and of BCTB-4CHO with BCTA-4NH₂, respectively. These COFs had large Brunauer–Emmett–Teller surface areas (up to 2212 m² g⁻¹) and outstanding thermal stabilities (decomposition temperatures of up to 566 °C). Interestingly, the intramolecular charge transfer (ICT) and fluorescence properties of these COFs were strongly influenced by their types of Schiff base bonding (BCTB-4CHN or BCTA-4NCH) and the degrees of π-stacking between their COF layers. For example, ICT from the electron-donating carbazole group to the acceptor through the Schiff base units of the type BCTB-4CHN and increasing the π-stacking distance enhanced the fluorescence emission from the COF. Moreover, BCTB–BCTA, the most fluorescent of our three COFs, functioned as a fluorescent chemosensor for HCl in solution, with outstanding sensitivity and a rapid response.