Communication pubs.acs.org/Organometallics
Molecular Shape Recognition by Using a Switchable Luminescent Nonporous Molecular Crystal Hiroaki Imoto,† Susumu Tanaka,† Takuji Kato,† Takashi Yumura,‡ Seiji Watase,§ Kimihiro Matsukawa,†,§ and Kensuke Naka*,† †
Faculty of Molecular Chemistry and Engineering and ‡Faculty of Materials Science and Engineering, Graduate School of Science and Technology, Kyoto Institute of Technology, Goshokaido-cho, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan § Osaka Municipal Technical Research Institute, 1-6-50 Morinomiya, Joto-ku, Osaka 536-8553, Japan S Supporting Information *
ABSTRACT: A nonporous molecular crystal (NMC) has been developed for use as a host material. The NMC comprises a platinum dibromide complex with coordinating 9phenyl-9-arsafluorene ligands. The NMC also contains chlorobenzene (PhCl) molecules, which quench emissions from the platinum complex. On exposure to the vapors of several volatile organic compounds (VOC), the encapsulated PhCl was released. With the loss of PhCl, the crystal once again showed the intrinsic luminescence of the platinum complex. Furthermore, the NMC can recognize the molecular shape of VOCs; consequently, VOCs having small minimum diameters turn on emission, but large VOCs do not cause the release of the included PhCl quencher, and emission remains switched off. Interestingly, a wide range of VOCs (e.g., alcohol, ether, haloalkane, and alkane) could be used, implying that the shape recognition ability of the present NMC system is polarity independent. This is the first example of NMCs having widespread molecular shape recognition properties.
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the molecular structure of the crystal.10d As another example, platinum complexes with arsenic ligands form several crystal polymorphs, each having different, structure-dependent luminescent behavior.10e Recently, we have reported that platinum dihalide (bromide and iodide) complexes with 9-phenyl-9-arsafluorene show intense emission in the solid state (Chart 1).11,12 Single crystals of the platinum dibromide complex 1 contained solvent molecules as a consequence of the flexible arsenic complex backbones. Following subsequent investigation, we have found
onporous molecular crystals (NMCs) can act as host materials to a variety of small molecules because of their “softer” crystal structures1,2 despite the absence of any channels, which are typically necessary for traditional host materials: i.e., metal−organic frameworks (MOFs)3 and covalent−organic frameworks (COFs).4 NMC hosts are often composed of bowl-shaped molecules such as calixarenes, cryptophanes, and pincer complexes.5−8 This is because, although the molecules are isolated and not connected by pores, the space provided by the shape of these molecules allows the inclusion of guest molecules. Reports of NMCs being used for gas storage,5,6 solvent absorption,7 and reaction fields8 have been made. However, molecular recognition is still challenging, even though it is one of the central goals of supramolecular chemistry. In molecular recognition systems, luminescence is often used as the detecting signal, and the switching on and off of luminescence yields information about changes in the molecular environment.9 However, luminescent NMC host materials that have the ability to recognize molecular shapes have not been reported. In the course of our studies of luminescent transition-metal complexes with arsenic ligands, we have gained practical evidence that arsenic ligands have sufficient conformational flexibility to produce a variety of local minimum energy structures.10 For example, a platinum complex with arsenic ligands shows vapochromic luminescence on small changes to © XXXX American Chemical Society
Chart 1. Chemical Structures of 9-Phenyl-9-arsafluorene and Its Platinum Dibromide Complex 1
Received: August 1, 2016
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DOI: 10.1021/acs.organomet.6b00614 Organometallics XXXX, XXX, XXX−XXX
Communication
Organometallics
endothermic peak at 120−155 °C (Figure S8b), corresponding to the results of the TGA measurements. The space containing a PhCl molecule in 1·PhCl was slightly larger than that containing a CH2Cl2 molecule in 1·CH2Cl2, because of the relatively large size of PhCl (Figure S2c in the Supporting Information).13 The calculated conformational stabilities of 1· PhCl and 1·CH2Cl 2 are similar,13 implying that 1 is conformationally flexible and adjusts to the size of the solvents. Because of this conformational flexibility, 1 acts as a host material despite the absence of bowl-shaped structures, which conventional NMC hosts possess.5−8 There were few differences between the molecular conformations of 1·CH2Cl2 and 1·PhCl. Nevertheless, 1· CH2Cl2 showed intense emission (λ = 626 nm, Φ = 0.26),12 whereas 1·PhCl showed no emission (Φ < 0.01), at room temperature (Figure S3 in the Supporting Information). The distance between the PhCl molecule and the benzene ring at the 9-position of the ligand was approximately 2.5 Å, which is sufficiently close to allow electron transfer. Thus, quenching must have occurred through an encounter-type exciplex.14 Actually, theoretical calculations suggest that the molecular orbitals of 1 and the PhCl molecule overlap (for example, HOMO-6 and LUMO+4, as shown in Figure S4 in the Supporting Information).13 We therefore concluded that PhCl quenched the luminescence of 1. Interestingly, the PhCl molecules could be reversibly added to and released from the crystal matrices, leading to the on/off luminescence switching of 1 (Figure 2). The nonemissive crystal 1·PhCl (Φ < 0.01) was transformed into 1·CH2Cl2(vap) upon exposure to CH2Cl2 vapors for 6 h at 25 °C (Figure S1 in the Supporting Information), and the crystal became emissive again (Φ = 0.26). In contrast, when the emissive crystal 1· CH2Cl2 (Φ = 0.26) was immersed in PhCl, a new crystal was
that the luminescence of 1 can be quenched by inclusion of chlorobenzene (PhCl). PhCl is used as the recrystallization solvent and is subsequently retained in the crystals of 1. Interestingly, the orange emission of 1 was observed again after exposure of the crystals to dichloromethane (CH2Cl2) vapors. This result suggests that this NMC, consisting of the luminescent complex 1, is a good candidate for a luminescent host that can act as a molecular recognition material. These results motivated us to explore the relationship between the identity of the vapors used and the luminescence properties of the resultant crystals. In this work, NMCs containing the PhCl quencher were exposed to the vapors of various volatile organic compounds (VOCs), allowing screening of these compounds. The results of these experiments reveal that the emission switching depends on the molecular shape of VOCs such as alcohols, ethers, haloalkanes, and alkanes. This is the first study on versatile molecular shape recognition by luminescent NMCs. The platinum dibromide complex 1 was prepared following our previously published method.12 Recrystallization of 1 from hot PhCl gave crystalline 1·PhCl. For comparison, 1·CH2Cl2 was prepared by recrystallization from CH2Cl2 and methanol (MeOH). X-ray crystallography revealed the structures of these crystals (Figure 1a and Table S3 in the Supporting
Figure 1. (a) ORTEP drawing of 1·PhCl (ellipsoids at 50% probability), in which a PhCl molecule is disordered. Hydrogen atoms are omitted for clarity. (b) PhCl molecule (space-filling model) surrounded by eight molecules of 1 (capped-stick model).
Information). The crystalline matrices of 1·PhCl and 1· CH2Cl2 contained the solvent molecules PhCl and CH2Cl2, respectively. Regarding the packing structures, there are no significant intermolecular interactions such as Pt−Pt bonds, hydrogen bonds, or π−π and CH−π interactions. Thus, the main interactive forces between the molecules in the crystals are van der Waals forces. In addition, investigation of the molecular packing shows that a PhCl molecule is surrounded by eight molecules of 1, and thus, 1·PhCl constitutes an NMC (Figure 1b). Under vacuum (
