Article pubs.acs.org/Organometallics
Photoresponsive Organogelator: Utilization of Boron(III) Diketonate as a Building Block To Construct Multiresponsive Materials Cheok-Lam Wong, Chun-Ting Poon, and Vivian Wing-Wah Yam* Institute of Molecular Functional Materials [Areas of Excellence Scheme, University Grants Committee (Hong Kong)] and Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong, People’s Republic of China S Supporting Information *
ABSTRACT: A class of dithienylethene-containing boron(III) diketonates with long alkoxy chains, together with their corresponding β-diketone ligands, has been prepared and characterized. The photophysics, electrochemistry, and photochromism of the compounds have been investigated. Some of them exhibit gelation behavior, which has been explored by variable-temperature UV−vis absorption studies, and the morphology of the gels has been studied by electron microscopy. In addition, the organogels have been found to be photoresponsive with color changes under photoexcitation, leading to the construction of multiresponsive boron(III) diketonate-based materials.
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systems11 and/or coordination of various metal centers.12 Specifically, the unique absorption spectral properties of the open form and the closed form and the extended π-conjugated structures after photocyclization make dithienylethenes promising candidates for constructing functional organogels. The preparation of photoresponsive organogels based on this moiety has been demonstrated.13 In the meanwhile, the study of materials based on a boron(III)-containing architecture has been of increasing interest owing to their conceivable applications in optoelectronics and anion sensing.14 Notably, boron diketonate derivatives with four-coordinated boron centers have been of great interest owing to their rich photophysics and their various applications in emissive materials,15 chemosensors,16 and supramolecular chemistry.17 Although the feasibility of utilizing the boron(III) diketonate unit to facilitate the gelation process has been demonstrated,18 an attempt to use this moiety as a versatile building block to develop an organogelator with photoswitchable behavior has not yet been reported. With our continuing interest in designing functional materials based on the boron(III)-containing architecture19 and our success in demonstrating the photoresponsive behavior of dithienylethene-containing boron(III) diketonates,20 we believe that through judicious molecular design and by taking advantages of the ease in modifying the molecular structures of the boron(III) diketonate unit, thermo- and light-responsive soft materials can be readily obtained. Herein, the synthesis and characterization
INTRODUCTION The study of low-molecular-mass organogelators has been gaining growing attention over the past few decades. It is motivated not only by the numerous potential applications of gels but also by the fact that these systems exhibit fascinating properties with respect to their controllable and spontaneous self-assembly behavior.1 By taking advantage of the reversible self-assembly process, organogelators have been made readily tunable and sensitive to microenvironmental changes so as to act as promising molecular building blocks.2 The self-assembly of the molecules of organogelators is driven by the highly tunable noncovalent interactions, including π−π interactions, hydrophobic−hydrophobic interactions, and hydrogen bonding, making them advantageous over the traditional polymer gels that are linked by covalent bonds.3 Moreover, slight variations in the molecular structures can lead to morphological changes and different sizes of the resulting nanostructures, as well as the behaviors of the gel formed. In particular, organogelators based on photoresponsive functionalities are of great interest owing to their readily tunable gelation properties by photoirradiation as an external stimulus4a,c and their potential applications have been demonstrated.4b,d,e Such changes resulting from photoinduced processes can be realized by integrating different photosensitive units such as azobenzene,5 stilbene,6 spiropyran,7 and spirooxazine8,9 into the backbone of the organogelator molecule. Among various types of photochromic moieties, dithienylethenes have been receiving ongoing attention, which has benefited from their oustanding thermal stability and photoreversibility as well as their fast photoswitching rate,10 and continuous efforts have been made to modify their photophysics and their photochromic properties via incorporation of different aromatic © XXXX American Chemical Society
Special Issue: Tailoring the Optoelectronic Properties of Organometallic Compounds with Main Group Elements Received: April 11, 2017
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DOI: 10.1021/acs.organomet.7b00274 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics
Table 1. Photophysical Data of L1−L3 and 1-BF2−3-BF2 in Open Forma
of a series of photochromic dithienylethene-containing boron(III) diketonates with long alkoxy chains will be described. Some of the compounds are found to form gels in organic solvents, and photoinduced color changes of the organogels have been observed upon light irradiation.
compound
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L1 L2 L3 1-BF2 2-BF2 3-BF2
RESULTS AND DISCUSSION Synthesis and Characterization. The synthesis of the dithienylethene-containing β-diketones L1−L3 is achieved by Claisen condensation between 1-[4,5-bis(2,5-dimethylthiophen-3-yl)thiophene-2-yl]ethanones and methyl 3,4,5-tris(alkoxy)benzoates with various alkoxy chain lengths, as shown in Scheme 1. Subsequent reaction of the β-diketones
absorption λabs/nm (ε × 10−3/ dm3 mol−1 cm−1) 348 344 346 373 373 373
sh sh sh sh sh sh
(12.4), 400 (33.0) (11.1), 400 (30.3) (11.9), 400 (33.1) (12.9), 463 (56.5) (12.6), 463 (54.2) (8.4), 462 (40.3)
emissionb,c λem/nm (τ0/ns) 459 458 460 522 519 521
(
