Multi-state emission properties and the inherent mechanism of D–A–D type asymmetric organic boron complexes

Abstract

Fluorescence emission properties in the single molecule, crystalline and amorphous state are key factors determining the application fields and scope of luminogens. However, the luminescence efficiency of luminogens is usually inconsistent in these three existing forms. To explore multi-state emission properties and the inherent mechanism of luminogens, and obtain rare mechanical force-induced luminescent enhancement and multicolor switching, three D–A–D (Donor–Acceptor–Donor)-type organic boron complexes containing tetraphenylethylene (TPE) and different aromatic amine units are designed and synthesized. TPEDKBF₂DMeA, TPEDKBF₂Ca and TPEDKBF₂DBeA respectively exhibit aggregation caused quenching (ACQ), AIE and dual-state emission (DSE) fluorescence characters. The orange crystals (Oc) of TPEDKBF₂Ca show a twisted molecular configuration and head-to-tail J-type stacking mode, while the red crystals (Rc) of TPEDKBF₂Ca form scarce super unit cells with the help of two face-to-face antiparallel dimers and the other four single molecules. TPEDKBF₂DBeA adopts an antiparallel head-to-head J-type stacking mode, leading to π–π stacking interactions. The faint fluorescence emission of TPEDKBF₂DMeA is attributed to parallel J-type stacking based on theoretical calculations. Additionally, TPEDKBF₂Ca, TPEDKBF₂DBeA and TPEDKBF₂DMeA in turn display reversible multi-color switching, blue-shifted fluorescence emission and remarkable luminescent enhancement, and negligible wavelength shift. Detailed single crystal analyses and theoretical calculations indicate that the molecular stacking mode is the critical factor affecting the optical and mechanochromic properties. Meanwhile, this work demonstrates that aryl amine substituents can tune the multistate emission properties of organic fluorophores, which provides a deep understanding for molecular design of multi-state emitters and mechanochromic materials.