Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Color Tuning of Mechanochromic Luminescent β‑Diketones via Boron Coordination and Donor-Acceptor Effects Tristan Butler, Meng Zhuang, and Cassandra L. Fraser* Department of Chemistry, University of Virginia, McCormick Road, Charlottesville, Virginia 22904, United States
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ABSTRACT: Owing to their facile synthesis and desirable optical properties, β-diketones (bdks) are attractive scaffolds for the production of new mechanochromic luminescent (ML) materials. However, ML-active bdk dyes that can access full color luminescent states (i.e., red, green, and blue) have yet to be realized. To synthesize such a set of bdk emitters, two strategies were used to red-shift the emission: substitution with varying donor and acceptor groups; and coordination of BF2 to the βdiketonate moiety. Additionally, 3,4,5-trimethoxy-substituted phenyl rings were used to disrupt the molecular packing to promote ML and enable aggregation-induced emission (AIE). Here, a series of trimethoxy-substituted bdks were prepared via Claisen condensation, each para-substituted with either F, I, or CN groups. Their boronated complexes were synthesized via reaction with boron trifluoride diethyl etherate. Optimized geometries generated from density functional theory calculations indicated that the dye para-substituted methoxy groups were directed out of the molecular plane, potentially leading to loose solid-state packing. The AIE studies using tetrahydrofuran/H2O solutions indicated AIE activity in both bdk dyes and boron complexes. High-contrast ML was also observed for each dye investigated. Furthermore, this set of dyes exhibited emissions crossing the entire visible spectrum, thereby demonstrating a strategy for color tuning and promoting ML and AIE in bdk systems.
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INTRODUCTION
strategy is needed to produce ML-active bdk dyes that encompass the entire visible spectrum. Recently, a correlation between loose molecular packing in emissive solids and mechanically produced optical responses has been observed.11−13 The weak interactions in the crystalline phases of these materials may be easily overcome via mechanical stimulus, leading to ML emission.14 Therefore, designing solid-state emissive materials that are sterically encumbered, such as the twisted tetraphenyl ethylene derivatives developed by Tang et al., has proven to be an effective strategy for inducing ML.13 Similarly, Reddy et al. showed that cyan-emissive crystals of BF2-coordinated avobenzone contain slip planes, which may assist in the formation of mechanically produced emissive species with redshifted emission.15 Recently, rapidly self-erasing ML was detected for the 3,4,5-trimethoxy-substituted diketone (gbmOMe).16 As evidenced by the single crystal of this dye, the loose packing pattern was due in part to the para-methoxy group, which was forced out of plane through unfavorable steric interactions with the neighboring methoxy substituents. Therefore, we hypothesize that the incorporation of 3,4,5-
Mechanochromic luminescent (ML) dyes change emission wavelength in response to a mechanical perturbation.1 These emerging stimuli-responsive materials have been utilized in wide-ranging applications including optical storage, security inks, and sensing materials.2,3 In particular, dyes that incorporate β-diketone (bdk) scaffolds have garnered attention owing to their facile synthesis from a diverse pool of commercially available starting materials, which allows for the ready modulation of optical properties.4−7 Furthermore, coordination to the β-diketonate moiety allows for further optical property adjustment.8 For example, BF2 complexation was used to red-shift the emission of dinaphthoylmethane (dnm) derivatives, resulting in green-to-orange ML.9 However, red emission of dyes in the smeared (SM) state was not achieved. Previously, we utilized donor and acceptor effects to tune emission in the crystalline and amorphous states of dimethylamino (DMA)-substituted dibenzoylmethane (dbm) bdks.10 Though a wide range of colors was achieved after melt quenching, this method was insufficient to produce redemissive materials after thermal and mechanical perturbation. Additionally, DMA dyes generally exhibited rather small mechanochromic shifts (Δλ < 25 nm) compared to other ML-active bdk and BF2 bdk materials. Therefore, a different © XXXX American Chemical Society
Received: May 18, 2018 Revised: July 16, 2018
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DOI: 10.1021/acs.jpcc.8b04762 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C trimethoxy phenyl rings may be an effective strategy for producing loosely packed solids with ML capabilities. To achieve ML dyes with full-range blue-to-red emission color tunability, a series of 3,4,5-trimethoxy-substituted diketones was targeted (Scheme 1). Fluoro, iodo, and cyano
at 7.27 ppm were used to reference spectra, and the coupling constants were reported in Hertz. The mass spectra were recorded with a Micromass Q-TOF Ultima spectrometer, using electrospray ionization (ESI) mass spectrometry (MS)/ MS techniques. A Hewlett-Packard 8452A diode array UV−vis spectrophotometer was used to collect the absorption spectra. The steady-state emission spectra were measured using a HORIBA Fluorolog-3 model FL3-22 spectrofluorometer (double-grating excitation and double-grating emission monochromator). The fluorescence quantum yields, ΦF, in CH2Cl2 were calculated versus a standard of dilute quinine sulfate solution in 0.1M H2SO4 using a previously described method19 and the following values: ΦF (quinine sulfate in 0.1M H2SO4) = 0.54,20 nD21 0.1M H2SO4 = 1.33, and nD21 CH2Cl2 = 1.424. Optically dilute CH2Cl2 solutions of all samples were prepared in 1 cm path length quartz cuvettes with absorbances
