Aggregation-induced emission characteristics and distinct fluorescent responses to external pressure stimuli based on dumbbell D-π-A-π-D cyanostyrene derivatives
Introduction
In recent years, the response of luminescent materials to ambient stimuli has attracted broad attention owing to their wide applications, such as light [1], temperature [2], chemical ions [3], force [4], electronic [5] and magnetic field [6]. Among them, organic molecules in response to mechanical force have been widely developed [7]. The emission color and intensity of these materials strongly depend on the molecular arrangements, which are determined by the intramolecular conformation [8,9], intermolecular interactions [10,11], and π-stacking models [12]. This feature is called mechanofluorochromism (MFC). Moreover, these changes could be recovered via some external stimuli such as heating, fuming, acids and bases [13]. Thus far, various functional molecules, such as 3,6-bis(aryl)-1,4-diketo-pyrrolo[3,4-c]pyrroles (DPPs) [14], tetraphenylethene [15,16], triphenylacrylonitrile [17,18], o-carborane [19], anthracene [20,21], difluoroboron β-diketonate (BF2-dk) [22,23], phenanthroimidazoles [24], cyano-ethylene [25] and pyranoxanthene [26] derivatives, which show emitting wavelength changes under mechanical pressure stimuli. Generally, such compounds can induce loose molecular packing in solid state due to their non-planar conformation. The loose packing is usually damaged, resulting in mechanofluorochromism [27].
Up to now, the most reported MFC materials show red-shifted emission after external mechanical stimuli. The MFC mechanism is usually due to the phase transfer from the crystalline state to amorphous state. There are also a few molecules that show blue-shifted mechanochromism. For example, Yang and co-workers found that a β-diketone boron dyes exhibited grinding-induced blue-shifted emission [28]. Shi’s group reported that a triphenylamine functionalized benzothiazole derivatives showed blue-shifted mechanochromism because of manipulating the intermolecular interaction and molecular packing mode in organic crystal [29]. However, structurally similar molecules that exhibit distinct fluorescent responses to external pressure are less found and reported [30]. The development of such materials is very important to expand the variety of MFC materials, in-depth understanding the mechanism and expanding applications. In this work, two dumbbell D-π-A-π-D type cyanostyrene derivatives with two cyano units as acceptors and two carbazole (PBCAN) or triphenylamine (PBTAN) have been synthesized via classical Knoevenagel condensation reaction (Scheme 1). Optical properties indicated that two molecules showed aggregation-induced emission characteristics. Moreover, it was found that two compounds exhibited distinct fluorescent response upon the treatment of grinding. It was noteworthy that PBCAN showed unusual blue-shifted emission, and PBTAN exhibited red-shifted emission under mechanical force stimuli. This special feature is very interesting. XRD patterns experiments confirmed that the presence of phase transition between crystalline and amorphous states. Blue-shifted mechanochromism of PBCAN might be originated from initial close molecular packing. After that, by comparing the absorption spectra of PBTAN in different solid states, we deemed that the change in aggregated state was responsible for red-shifted mechanofluorochromism. This work can help us to develop more structurally similar molecules with different MFC behaviors.
Section snippets
General information
Nuclear magnetic resonance (NMR) spectra (1H and 13C) were recorded at room temperature using a Bruker 400 MHz Advance instrument at 400 MHz and 100 MHz by using CDCl3 as the solvent. Elemental analyses (C, H and N) were taken on a PerkinElmer 240C elemental analyzer. IR spectra were measured with a thermo nicolet IS50 FT-IR spectrometer by the incorporation of samples in KBr disks. High resolution mass spectrometry (HRMS) was measured with an Bruker solariX 70 FT-MS. The absorption spectra in
Molecular structures and synthetic routes of PBCAN and PBTAN
The molecular structures and synthetic routes for cyanostyrene derivatives PBCAN and PBTAN are depicted in Scheme 1. Firstly, the key starting materials 9-octyl-9H-carbazole-3-carbaldehyde 1 and 4-(diphenylamino)benzaldehyde 2 were synthesized according to reported procedure in the literature [31]. PBCAN was prepared through classical Knoevenagel condensation reaction between 1 and 1,4-phenylenediacetonitrile 3 in a molar weigh radio of 2/1 catalyzed by tetrabutylammonium hydroxide in ethanol,
Conclusion
In summary, two dumbbell cyanostyrene derivatives functionalized with carbazole (PBCAN) and triphenylamine (PBTAN) were designed and successfully synthesized by the Knoevenagel condensation reaction. The quantum chemical calculations and solvent-dependent optical spectra illustrated that two molecules showed a D-π-A-π-D structure, in which carbazole or triphenylamine unit acted as an electron donor and cyano group was used as an electron acceptor. It was found that both of them exhibited unique
Supplementary material
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Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was financially supported by the open fund of the Department of Chemistry of Qingdao University of Science and Technology (QUSTHX201914).
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