Difluoroboron β-diketonate (BF 2 bdk) compounds show environment-sensitive optical properties in solution, aggregation-induced emission (AIE) and multi-stimuli responsive fluorescence switching in the solid state. Here, a series of 4-azepane-substituted β-diketone (bdk) ligands ( L-H , L-OMe , L-Br ) and their corresponding difluoroboron dyes ( D-H , D-OMe , D-Br ) were synthesized, and various responsive fluorescence properties of the compounds were studied, including solvatochromism, viscochromism, AIE, mechanochromic luminescence (ML) and halochromism. Compared to the β-diketones, the boron complexes exhibited higher extinction coefficients but lower quantum yields, and red-shifted absorption and emission in CH 2 Cl 2 . Computational studies showed that intramolecular charge transfer (ICT) dominated rather than π–π* transitions in all the compounds regardless of boron coordination. In solution, all the bdk ligands and boron dyes showed red-shifted emission in more polar solvents and increased fluorescence intensity in more viscous media. Upon aggregation, the emission of the β-diketones was quenched, however, the boronated dyes showed increased emission, indicative of AIE. Solid-state emission properties, ML and halochromism, were investigated on spin cast films. For ML, smearing caused a bathochromic emission shift for L-Br , and powder X-ray diffraction (XRD) patterns showed that the “as spun” and thermally annealed states were more crystalline and the smeared state was amorphous. No obvious ML emission shift was observed for L-H or L-OMe , and the boronated dyes were not mechano-active. Trifluoroacetic acid (TFA) and triethylamine (TEA) vapors were used to study halochromism. Large hypsochromic emission shifts were observed for all the compounds after exposure to TFA vapor, and reversible fluorescence switching was achieved using the acid/base pair.
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Controlling solid-state optical properties of stimuli responsive dimethylamino-substituted dibenzoylmethane materials
The mechanochromic luminescent (ML) properties of boron coordinated β-diketones (BF 2 bdks) have been widely studied, however the stimuli responsive properties of uncoordinated β-diketones (bdks) are less known. While bdk dyes show promising properties, including high contrast ML, rapid room temperature self-erasure, solvatochromism, and aggregation induced emission (AIE), previously reported dyes exhibit emission over a narrow range of wavelengths (∼420–500 nm). To tune luminescence over a broader color range, dimethylamino (DMA) substituted β-diketones were synthesized with a series of electron donating and withdrawing substituents. Solvatochromism, aggregation induced emission (AIE), ML, and thermochromism were investigated. Most dyes showed positive solvatochromic shifts and were responsive to mechanical and thermal stimuli. The solid-state emission wavelengths correlated with the electron withdrawing strength of the para substituent and ranged from blue to orange (488–578 nm). Structural and thermal characterization was performed by powder X-ray diffraction (XRD) and differential scanning calorimetry (DSC) respectively. These data indicate that stimuli responsive properties are the result of a crystalline to amorphous phase transition. Additionally, the aggregation induced emission (AIE) properties of iodo and cyano substituted dyes were measured in THF/H 2 O solutions. Both showed intense emission resulting from aggregation. The sensitivity of these dyes toward matrix polarity was further investigated by fabricating polystyrene films with increasing concentrations of dye and camphoric anhydride, a non-emissive polar dopant. A red-shift in emission was observed in these films which demonstrated that matrix effects can be used to tune the emission of DMA-substituted diketones.
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- NSF-PAR ID:
- 10098659
- Date Published:
- Journal Name:
- Materials Chemistry Frontiers
- Volume:
- 1
- Issue:
- 9
- ISSN:
- 2052-1537
- Page Range / eLocation ID:
- 1804 to 1817
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Emissive β‐diketones (bdks) and difluoroboron complexes (BF2bdks) show multi‐stimuli responsive luminescence in both solution and the solid state. A series of bdk ligands and boron coordinated dyes were synthesized with different cyclic amine substituents in the 4‐position to explore ring size effects on various luminescent properties, including solvatochromism, viscochromism, aggregation‐induced emission (AIE), mechanochromic luminescence (ML) and halochromism. Red‐shifted absorption and emission were observed in CH2Cl2for both bdk ligands and boron dyes with increasing substituent ring size. The compounds displayed bathochromic emission in more polar solvents, and higher fluorescence intensity in more viscous media. The AIE compounds exhibited enhanced emission when aggregated. For solid‐state properties, a large emission wavelength shift was shown for the piperidine substituted bdk after melt quenching on weighing paper. Large blue‐shifted emissions were observed in all the boron dye spin cast films after trifluoroacetic acid vapor annealing, and the original emissions were partially recovered after triethylamine vapor treatment.
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/c7qm00157f
