Hydrogen‐Bond and Supramolecular‐Contact Mediated Fluorescence Enhancement of Electrochromic Azomethines

Abstract

An electronic push-pull fluorophore consisting of an intrinsically fluorescent central fluorene capped with two diaminophenyl groups was prepared. An aminothiophene was conjugated to the two flanking diphenylamines through a fluorescent quenching azomethine bond. X-ray crystallographic analysis confirmed that the fluorophore formed multiple intermolecular supramolecular bonds. It formed two hydrogen bonds involving a terminal amine, resulting in an antiparallel supramolecular dimer. Hydrogen bonding was also confirmed by FTIR and NMR spectroscopic analyses, and further validated theoretically by DFT calculations. Intrinsic fluorescence quenching modes could be reduced by intermolecular supramolecular contacts. These contacts could be engaged at high concentrations and in thin films, resulting in fluorescence enhancement. The fluorescence of the fluorophore could also be restored to an intensity similar to its azomethine-free counterpart with the addition of water in >50 % v/v in tetrahydrofuran (THF), dimethyl sulfoxide (DMSO), and acetonitrile. The fluorophore also exhibited reversible oxidation and its color could be switched between yellow and blue when oxidized. Reversible electrochemically mediated fluorescence turn-off on turn-on was also possible.

Symmetrical and unsymmetrical azomethines with thiophene core: structure–properties investigations

“…A linear correlation between the chemical shift to higher frequencies together with increasing concentrations was noticed. This behavior confirms the presence of concentration-induced hydrogen bonding [18].…”

“…8) except for AzTh-2b, and AzTh-4b, which were non-emissive due to quenching effect typically observed in solid state [11]. As it was shown, amine groups in unsymmetrical imines can create hydrogen bonds, which should improve the luminescence [18]. It was found that all unsymmetrical imines in film exhibited luminescence, probably due to H-bonds formation.…”

“…Encouraged by the results presented in Skene et al [18] and in our work [19], we synthesized a series of symmetrical azomethines as well as comparable unsymmetrical ones, end-capped with the amine group. It is expected that the presence of the amine group in unsymmetrical imines can allow hydrogen bond formation for enhancing fluorescence [18]. In this paper, the relationship between chemical structures of the substituent attached to imine bonds and potential application properties are presented.…”

“…It was found that primary end-amine groups in imines can form hydrogen bonds, which may enhance fluorescence [18]. The hydrogen bonds may occur between the hydrogen atom and more electroactive atoms, and thus, in the case of synthesized unsymmetrical azomethines, they can appear between the ester and the amine groups.…”

“…Azomethines with thiophene structure can be prepared from diamino-thiophene-3,4-dicarboxylic acid diethyl ester (DAT) and various aldehydes [16]. Encouraged by the results presented in Skene et al [18] and in our work [19], we synthesized a series of symmetrical azomethines as well as comparable unsymmetrical ones, end-capped with the amine group. It is expected that the presence of the amine group in unsymmetrical imines can allow hydrogen bond formation for enhancing fluorescence [18].…”