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Mechanochromic Luminescence and Aggregation Induced Emission of Dinaphthoylmethane β‑Diketones and Their Boronated Counterparts Tristan Butler, William A. Morris, Jelena Samonina-Kosicka, and Cassandra L. Fraser* Department of Chemistry, University of Virginia, McCormick Road, Charlottesville, Virginia 22904, United States S Supporting Information *
ABSTRACT: Mechanochromic luminescence has been observed in many boron coordinated β-diketonate (BF2bdk) complexes. Recently, it was shown that the metal-free methoxy-substituted dinaphthoylmethane β-diketone (dnmOMe) also displayed aggregation induced emission (AIE), solvatochromism, and high contrast mechanochromic luminescence (ML) that recovered rapidly at room temperature. In order to understand how substituents and boron coordination affect solution and solid-state optical properties, a series of methoxy- and bromo-substituted derivatives (dnm, dnmOMe, dnmBr, and dnmBrOMe) and their corresponding boron complexes (BF2dnm, BF2dnmOMe, BF2dnmBr, and BF2dnmBrOMe) were synthesized and their AIE, ML, and room temperature recovery properties were compared. All boron complexes exhibited red-shifted absorption and emission, in addition to larger solution and solid-state quantum yields than βdiketones. While AIE studies show increased emission for dnmOMe and dnmBrOMe, the emission of corresponding boron complexes diminished upon aggregation. However, boron complexes were still strongly emissive in the solid state. ML properties were investigated using spin-cast films. Smearing resulted in the appearance of blue-green emission in ligands and a color change from green to yellow-orange in boron complexes. Bromide substituted derivatives showed increased room temperature recovery times compared to other dnm ligands, and boron complexes show only partial recovery over several days. KEYWORDS: mechanochromic luminescence, aggregation induced emission, β-diketone, boron complex, solvatochromism
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H···O hydrogen bonding interactions.26 Additionally, Kato and co-workers fabricated water-soluble mechanoresponsive micelles that change emission wavelength when vortexed in water due to the formation of different molecular assemblies.27 Thus, it is important to consider not only how substituents may modify the optical properties of individual dyes but also how the supramolecular structure is impacted upon substitution. Many applications require efficient solid-state emission;28 however, luminophores that are emissive in solution are often dark in the solid state due to aggregation caused quenching (ACQ). Using a hexaphenylsilole (HPS) system, Tang and coworkers showed that some materials could become emissive upon aggregation through the restriction of molecular motions (RIM),29−31 thereby circumventing the ACQ issue.30,32 Using this technique, many different compounds have been developed that show aggregation-induced emission (AIE) or aggregationinduced enhanced emission (AIEE). These include triarylamine,28 tetraphenylethene, silole, and cyanostilbene-based materials, which have found use in OLEDs,33 optical devices,34
INTRODUCTION The development of stimuli responsive materials has garnered increasing attention in the material science community due to their potential for sensors, memory storage, displays, next generation lighting sources, and other applications.1−3 Mechanochromic luminescent (ML) dyes are unique types of stimuli responsive materials that change emission wavelength in response to external stimuli such as hydrostatic pressure or shear force.4 In some cases, spectral changes vary depending on the type of force being applied.5 ML compounds show promise for luminescent data storage and security technologies.6 There are many diverse types of ML materials including metal,7−9 organic,10 polymer,1,11−13 and liquid crystalline compounds.14 Often, these systems are designed with tunable donor (D) and acceptor (A) motifs, allowing for modification of optical properties.15,16 Recently, Chujo and co-workers reported stimuli responsive boron coordinated diiminate materials showing dynamic aggregation induced emission (AIE) properties that can be modulated through incorporation of donor and acceptor substituents.17 In the solid state, optical properties are also controlled by intermolecular interactions16,18−22 and crystal packing.15,23−25 For example, cyanostilbene-based materials have shown emission switching due to the sliding of molecular sheets, controlled via C−H···N and C− © XXXX American Chemical Society
Received: October 12, 2015 Accepted: December 9, 2015
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DOI: 10.1021/acsami.5b09688 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces and biomedical applications.35,36 Many of these materials also exhibit ML.37 Difluoroboron coordinated β-diketonate (BF2bdk) materials display impressive optical properties including high extinction coefficients,38 room temperature phosphorescence in poly(lactic acid) matrices,39 and efficient solid-state emission,40,41 as well as ML behavior.40−47 Additionally, BF2bdk ML materials can self-erase at room temperature and can withstand multiple cycles of smearing and annealing without degradation of ML properties. Optical properties of these boron ML materials can also be modulated through alkyl chain effects44 and arene substitution.40 Furthermore, heavy atom substitution results in mechanochromic luminescence quenching (MLQ) where emission becomes dark upon smearing due to enhanced intersystem crossing to the oxygen sensitive triplet state.45 In BF2bdk systems, ML has been attributed to the formation of face-to-face H-aggregates upon mechanical perturbation that act as low energy emitters in the smeared state.48 In an attempt to achieve BF2bdks with more red-shifted emission than dibenzoylmethane systems, we synthesized a series of dinaphthoylmethane ligands. In a preliminary communication, we reported solvatochromism, AIE, and high contrast ML with rapid room temperature recovery for a methoxy-substituted dinaphthoylmethane ligand (dnmOMe) even without boron coordination.46 In this study, we explore how BF2 coordination affects ML and AIE properties for a series of dinaphthoylmethane (dnm) β-diketonate systems. Additionally, substituent effects were probed via a series of methoxy- and bromo-substituted dnm compounds and their corresponding boronated complexes. The parent compound difluoroboron dnm (BF2dnm) has been previously synthesized and shown to have a high extinction coefficient, in addition to two photon absorption; however, the ML and AIE properties were not reported.38 In this study, methoxy substitution is used as a means of assessing electron donation effects on ML, whereas bromide, a σ withdrawing, π donating heavy atom substituent,43,45 allows for different kinds of ML modulation. Bromide can also disrupt molecular packing, as has been demonstrated in phenothiazine-based benzoxazole systems that show different ML mechanisms depending on bromo substitution.49 Each compound was characterized using absorption and emission spectroscopy, differential scanning calorimetry (DSC), powder X-ray diffraction (XRD), and atomic force microscopy (AFM) techniques.
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measurements were performed with a NanoLED-370 (λex = 369 nm) excitation source and a DataStation Hub as the SPC controller. Lifetime data were analyzed with DataStation v2.4 software from Horiba Jobin Yvon. Fluorescence quantum yields, φF, in CH2Cl2 were calculated versus a dilute anthracene solution in ethanol as a standard using a previously described method51 and the following values: φF anthracene in ethanol = 0.27,52 nD53 ethanol = 1.36, and nD52 CH2Cl2 = 1.424. Optically dilute CH2Cl2 solutions of all samples were prepared in 1 cm path length quartz cuvettes with absorbances
