Coordination Nanosheets Based on TerpyridineâZinc(II) Complexes

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Coordination Nanosheets Based on TerpyridineZinc(II) Complexes: As Photoactive Host Materials Takamasa Tsukamoto, Kenji Takada, Ryota Sakamoto, Ryota Matsuoka, Ryojun Toyoda, Hiroaki Maeda, Toshiki Yagi, Michihiro Nishikawa, Naoaki Shinjo, Shuntaro Amano, Tadashi Iokawa, Narutaka Ishibashi, Tsugumi Oi, Koshiro Kanayama, Rina Kinugawa, Yoichiro Koda, Toshiyuki Komura, Shuhei Nakajima, Ryota Fukuyama, Nobuyuki Fuse, Makoto Mizui, Masashi Miyasaki, Yutaro Yamashita, Kuni Yamada, Wenxuan Zhang, Ruo-Cheng Han, Wenyu Liu, Taro Tsubomura, and Hiroshi Nishihara J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b12810 • Publication Date (Web): 21 Mar 2017 Downloaded from http://pubs.acs.org on March 28, 2017

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Journal of the American Chemical Society

Miyasaki, Masashi; The University of Tokyo, Department of Chemistry, Graduate School of Science Yamashita, Yutaro; The University of Tokyo, Department of Chemistry, Graduate School of Science Yamada, Kuni; The University of Tokyo, Department of Chemistry, Graduate School of Science Zhang, Wenxuan; The University of Tokyo, Department of Chemistry, Graduate School of Science Han, Ruo-Cheng; University of Science and Technology of China, National Laboratory for Physical Sciences at Microscale Liu, Wenyu; The University of Tokyo, Department of Chemistry, Graduate School of Science Tsubomura, Taro; Seikei University, Department of Materials and Life Science Nishihara, Hiroshi; The University of Tokyo, Department of Chemistry, School of Science

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Coordination Nanosheets Based on Terpyridine-Zinc(II) Complexes: As Photoactive Host Materials Takamasa Tsukamoto,†,‡ Kenji Takada,† Ryota Sakamoto,†,§,* Ryota Matsuoka,† Ryojun Toyoda,† Hiroaki Maeda,† Toshiki Yagi,† Michihiro Nishikawa,∆ Naoaki Shinjo,† Shuntaro Amano,† Tadashi Iokawa,† Narutaka Ishibashi,† Tsugumi Oi,† Koshiro Kanayama,† Rina Kinugawa,† Yoichiro Koda,† Toshiyuki Komura,† Shuhei Nakajima,† Ryota Fukuyama,† Nobuyuki Fuse,† Makoto Mizui,† Masashi Miyasaki,† Yutaro Yamashita,† Kuni Yamada,† Wenxuan Zhang,† Ruocheng Han,† Wenyu Liu,† Taro Tsubomura,∆ and Hiroshi Nishihara†,* †

Department of Chemistry, Graduate School of Science, The University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo 113-

0033, Japan ‡

Japan Society for the Promotion of Science (JSPS), Ichibancho, Chiyoda-ku, Tokyo 102-8471, Japan

§

JST-PRESTO, 4-1-8, Honcho, Kawaguchi, Saitama 332-0012, Japan

∆

Department of Materials and Life Science, Seikei University, 3-3-1 Kichijoji-kitamachi, Musashino-shi, Tokyo 180-8633,

Japan

ABSTRACT: Photoluminescent coordination nanosheets (CONASHs) comprising three-way terpyridine (tpy) ligands and zinc(II) ions are created by allowing the two constitutive components to react with each other at a liquid/liquid interface. Taking advantage of bottom-up CONASHs, or flexibility in organic ligand design and coordination modes, we demonstrate the diversity of the tpy-zinc(II) CONASH in structures and photofunctions. A combination of 1,3,5-tris[4-(2,2′:6′,2″terpyridyl)phenyl]benzene (1) and Zn(BF4)2 affords a cationic CONASH featuring the bis(tpy)Zn complex motif (1-Zn), while substitution of the zinc source with ZnSO4 realizes a charge-neutral CONASH with the [Zn2(-O2SO2)2(tpy)2] motif [1-Zn2(SO4)2]. The difference stems from the use of non-coordinating (BF4−) or coordinating and bridging (SO42−) anions. The change in the coordination mode alters the luminescence (480 nm blue in 1-Zn; 552 nm yellow in 1-Zn2(SO4)2). The photophysical property also differs in that 1-Zn2(SO4)2 shows solvatoluminochromism, whereas 1-Zn does not. Photoluminescence is also modulated by the tpy ligand structure. 2-Zn contains triarylamine-centered terpyridine ligand 2 and features the bis(tpy)Zn motif; its emission is substantially red-shifted (590 nm orange) compared with that of 1-Zn. CONASHs 1-Zn and 2-Zn possess cationic nanosheet frameworks with counter anions (BF4−), and thereby feature anion exchange capacities. Indeed, anionic xanthene dyes were taken up by these nanosheets, which undergo quasi-quantitative exciton migration from the host CONASH. This series of studies shows tpy-zinc(II) CONASHs as promising potential photofunctional nanomaterials.

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Introduction Bottom-up-type nanosheets, two-dimensional polymers constructed by bond formations among molecular, ionic, and atomic components, have gained global interest.1–6 Coordination nanosheets (CONASHs) are a class of two-dimensional materials featuring metal complex motifs.7–19 The vast number of combinations possible between organic ligand molecules and metal ions promises to allow fine tuning of the chemical and physical properties of CONASHs. We first demonstrated redox-modulated electrical conductivity as a useful functionality of CONASHs containing the bis(dithiolene)metal(II) complex motif.7–10 Derivatives of bis(dithiolene)metal(II) CONASHs have also shown electrocatalytic activities.11–13 Furthermore, we reported a bis(dipyrrinato)zinc(II) CONASH featuring photoelectric conversion ability.14,15 2,2′:6′,2″-Terpyridine (tpy) spontaneously forms coordination bonds with various metal ions, a property exploited in the construction of supramolecular coordination compounds or polymers.20–22 This characteristic may be applied in the construction of nanosheets. Schlüter and coworkers synthesized single-layer bis(tpy)metal(II) CONASHs at an air/water interface,16–18 and we realized multi-layer electrochromic bis(tpy)iron(II) and cobalt(II) CONASHs with applicability as the active layers for a solidified electrochromic device.19 As a next step in research into tpy-metal CONASHs, the present article examines their photofunctionality, employing zinc(II) as the metal center, because tpy-zinc(II) complexes feature luminescence.23 Schlüter and coworkers mentioned the emission from a monolayered bis(tpy)zinc(II) CONASH, but its detailed photophysical properties and possible applications are yet to be reported.18 Taking advantage of the flexibility in ligand design and coordination modes offered by bottom-up CONASHs, we prepared three luminescent tpy-zinc(II) CONASHs (Figure 1). The basic structure is 2



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shown by 1-Zn, which comprises 1,3,5-tris[4-(2,2′;6′,2″-terpyridyl)phenyl]benzene (1)18 and features the bis(tpy)zinc(II) complex motif (Figure 1a,b). Ligand 1 can also be used in another type of CONASH; the dinuclear [Zn2(-O2SO2)2(tpy)2] complex motif24–27 is first installed in a CONASH to form 1Zn2(SO4)2 (Figure 1d). We also modified the ligand molecule to give it a triarylamine center (tris[4-(4′2,2′:6′,2″-terpyridyl)phenyl]amine, 2), which was then used to synthesize CONASH 2-Zn (Figure 1a,c). The liquid/liquid interfacial synthesis allowed us to produce the multi-layer CONASH with a large sheet size (cm order). The three CONASHs were all emissive, with a characteristic luminochromic phenomenon shown by 1-Zn2(SO4)2. CONASHs 1-Zn and 2-Zn possessed exchangeable counter anions for charge neutrality. Together with their nanopore spaces, we demonstrated the replacement of the counter anions with emissive guest anionic molecules; the resulting efficient energy migration shows that the CONASH framework can act as a light-harvesting host material.

Results and discussion Synthesis and photophysical properties of luminescent CONASH 1-Zn. First, the three-fold symmetric tpy ligand 1 was used to synthesize CONASH 1-Zn (Figure 1a,b). The CONASH framework is cationic, and thus has an accompanying counter anion BF4−. The synthesis employed a liquid/liquid interfacial coordination reaction reported by our group (Figure 2a).7–10,14,15,19 A dichloromethane solution of ligand 1 (1.0 × 10−4 mol L−1) and aqueous Zn(BF4)2 (1.3 × 10−2 mol L−1) were layered, and the reaction system was kept calm. After 5 d, the generation of 1-Zn at the interface was confirmed as a colorless film that was deposited onto substrates such as silicon and quartz for characterization. The nanosheet morphology of 1-Zn was confirmed by transmission electron microscopy (TEM) and scanning electron microscopy (SEM) images, which show a sheet and layer structure (Figure 2b,c). The main framework of 3


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1-Zn consisted of C, N, and Zn, while B and F composed the counter anion. Energy dispersive X-ray spectrometry with SEM (SEM/EDS) mapping (Figure 2d) detected these elements to be spread evenly on the nanosheet, except for B, whose peak locates near the intense C peak, hindering its precise detection. Atomic force microscopy (AFM) imaging of 1-Zn on a silicon substrate showed a flat texture with a thickness of approximately 65 nm (Figure 2e). X-ray photoelectron spectroscopy (XPS) focusing on the Zn 2p3/2, N 1s, B 1s, and F 1s core levels detected a signal for each element (Figure 2f), giving calculated ratios of Zn:N:B:F as 1 : 5.88  0.08 : 1.97  0.05 : 8.18 0.13. These values are in good agreement with those estimated from the molecular structure of 1-Zn (Zn:N:B:F = 1 : 6 : 2 : 8), considering the error accompanying XPS. The coordination of the tpy moiety with the zinc(II) ion shifted the N 1s peak to a higher binding energy (399.4 eV for 1-Zn; 397.5 eV for 1).19 Neither X-ray nor electron diffraction was observed for 1-Zn, probably because of disorder in the location of the counter anion (BF4−) or in the stacking of the nanosheet layers. 1-Zn on a quartz substrate appears colorless and transparent under ambient light, because its absorption spectrum has a solitary peak in the ultraviolet (UV) region (maximum at 340 nm, Figure 2g,i). However, it emits blue photoluminescence (PL) upon illumination with 365 nm UV light (Figure 2g). Figure 2g also displays a large CONASH film size (~1 cm). A fluorescence micrograph of lucent 1-Zn shows it to feature a large sheet (>500 μm on one side) without conspicuous defects (Figure 2h). This is consistent with luminescence spectroscopy results featuring a signal with a maximum at 480 nm (Figure 2j). The fluorescence quantum yield (Φf) and fluorescence lifetime (τf) were determined to be 10% and 2.4 ns, respectively, upon excitation at 340 nm under air at room temperature. The lifetime suggests that the emission from 1-Zn may be singlet fluorescence. The corresponding mononuclear complex M1-Zn also showed similar absorption and emission spectra in dichloromethane, with maximal absorption and PL wavelengths of 340 and 465 nm, respectively (Figure 4



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S1a,b in Supporting Information). DFT calculations indicate that the HOMO of M1-Zn is dominated by the π orbital of the 1,3,5-triphenylbenzene moiety, whereas the π* orbital of the tpy moiety is responsible for the LUMO (Figure S1f). Therefore, the PL of 1-Zn may be assignable to an intramolecular charge transfer (ICT) transition22 from the triphenylbenzene to the tpy parts. Synthesis and photophysical properties of 2-Zn. Another type of tpy ligand with three-fold symmetry, 2, was synthesized (Figure 1a) and used to make nanosheet 2-Zn (Figure 1c) in order to demonstrate that the PL properties of tpy-zinc(II) CONASHs may be modified by the ligand molecule. The three-fold symmetry of ligand 2 originates from a triarylamine core that serves as an electron donor.28,29 In combination with electron-accepting tpy, the ICT absorption and emission of the nanosheet should show bathochromic shifts. 2-Zn was also synthesized by the liquid/liquid interfacial technique using an aqueous solution of Zn(BF4)2 (1.3 × 10−2 mol L−1) and a dichloromethane solution of ligand 2 (1.0 × 10−4 mol L−1) (Figure 3a). After holding the reaction for 4 d, CONASH 2-Zn appeared at the liquid/liquid interface as a thin yellow film, which was transferred onto a flat substrate. The nanosheet appearance of 2-Zn was confirmed by TEM and SEM (Figure 3b,c), while SEM/EDS mapping revealed the main elements (C, N, Zn, and F) to be scattered uniformly (with B hidden by the intense signal from C, as explained for 1-Zn) (Figure 3d). The AFM image of 2-Zn on a silicon substrate shows a flat surface of ca. 65 nm thickness, similar to that of 1-Zn (Figure 3e). XPS quantified the element abundance ratios as Zn/N/B/F = 1 : 6.65  0.09 : 2.06  0.05 : 7.87  0.09 (Figure 3f), in good agreement with the expected ratios of 1 : 6.7 : 2 : 8. Neither X-ray nor electron diffraction was observed for 2-Zn, probably because of disorder in the location of the counter anion (BF4−) or in the stacking of the nanosheet layers. 2-Zn on a quartz substrate appeared yellow, and thus absorbed visible light (Figure 3g): its absorption spectrum features a band at 440 nm (Figure 3i), which is in contrast to the transparent 1-Zn (Figure 2g). 5


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2-Zn emitted orange PL upon 365 nm UV excitation (Figure 3g). Fluorescence optical microscopy showed it to have large sheet sizes (>300 μm on one side) with defectless surfaces shining orange (Figure 3h). The PL spectrum features an emission band with a maximum at 590 nm (Figure 3j), indicating that the introduction of triarylamine caused 2-Zn to undergo a bathochromic shift of 110 nm (3900 cm−1) in its luminescence compared with 1-Zn. The Φf and τf values of 2-Zn were 6% and 2.4 ns, respectively, upon 435 nm excitation in air at room temperature. The corresponding mononuclear complex M2-Zn (Figure S1a) gave absorption and emission spectra similar to those of 2-Zn (Figure S1d,e), with maximal wavelengths of 450 and 590 nm. DFT calculation revealed the HOMO and LUMO in M2-Zn to be dominated by the n orbital of the triphenylamine moiety and the π* orbital of the terpyridine moiety, respectively (Figure S1f). Therefore, the PL of 2-Zn is assignable to the singlet ICT transition, the same as that of 1-Zn. However, the difference between the two CONASHs lies in the nature of the HOMO: the electron donating triphenylamine moiety destabilizes the HOMO of 2-Zn (Figure S1f), which leads to the red-shifted emission. Thus, the PL properties of the tpy-zinc(II) CONASH may be tuned by the ligand molecule. Synthesis and photophysical properties of 1-Zn2(SO4)2. The coordination mode of the metal complex motif can be altered to bring rich variations in CONASHs. This section introduces 1-Zn2(SO4)2 (Figure 1d), which comprises ligand 1 and zinc(II) ions, but possesses a 2D framework different from that of 1-Zn. Tpy-zinc(II) complexes undergo anion-dependent coordination structural changes. The use of anions with coordination ability, such as Cl−, SO42−, and CH3COO−, results in the formation of pentacoordinated mono-tpy zinc complexes, [Zn(tpy)X2].23–27,30–33 This contrasts with the use of noncoordinating anions such as BF4− and PF6−, which form the [Zn(tpy)2]2+ motif as in 1-Zn and 2-Zn. Among the coordinating anions, SO42− can expand the [Zn(tpy)X2] motif to supramolecular coordination 6



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polymers, because SO42− complexes have multiple coordinating points that allow it to bridge metal ions. In fact, there is no report that achieved the [Zn(tpy)2](SO4) type of complexes: instead, the dinuclear [Zn2(-O2SO2)2(tpy)2] complex motif was reported exclusively.24–27 1-Zn2(SO4)2 was prepared by the liquid/liquid interfacial coordination reaction (Figure 4a) of a dichloromethane solution of ligand 1 (1.0 × 10−4 mol L−1) and an aqueous solution of ZnSO4 (5.0 × 10−3 mol L−1). Leaving the interface still for 3 d led to the formation of 1-Zn2(SO4)2 at the liquid/liquid interface as a yellow film (Figure 4a). The framework of 1-Zn2(SO4)2 is charge-neutral, and thus, unlike 1-Zn and 2-Zn, it holds no counter anion. After being deposited on flat substrates, 1-Zn2(SO4)2 was subjected to spectroscopy and micrography. SEM observations revealed that 1-Zn2(SO4)2 featured a layered sheet-like structure with a lateral size of about several hundred micrometers (Figure 4b). TEM revealed that a part of 1-Zn2(SO4)2 contained small flakes of flat films (Figure 4c). SEM/EDS mapping (Figure 4d) showed the uniform distributions of C, N, Zn, S, and O in 1-Zn2(SO4)2, confirming its homogeneity. AFM showed the morphology of 1Zn2(SO4)2 (Figure 4e): the height image and cross-sectional analysis suggest that 1-Zn2(SO4)2 had a sheet morphology with a thickness of 300 nm. The coordination of the tpy group to the zinc ions was confirmed by spectroscopic methods. XPS results for 1-Zn2(SO4)2, focusing on the N 1s, Zn 2p, and S 2p core levels, are shown in Figure 4f. The N 1s peak of 1-Zn2(SO4)2 locates at 399.4 eV, which is shifted to a higher binding energy compared with that of free ligand 1 (397.5 eV).19 On the other hand, the S 2p peak shows up at 167.7 eV, which possesses a low binding energy for sulfate (typically ca. 169 eV):34,35 we attribute this to the characteristic Zn-O-SO2-O-Zn bridging structure. The elemental ratios calculated from the peak intensities are 1 : 3.07  0.07 : 0.85  0.03 for Zn/N/S. If 1-Zn2(SO4)2 were based on the bis(tpy)zinc(II) complex motif, the ratio should be 1 : 6 : 1, which is markedly different from the experimental value of close to 1 : 3 : 1 that corresponds to the [Zn2(-O2SO2)2(tpy)2] motif, as shown in 7


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Figure 1d. The XPS result, however, indicates the existence of SO4 defects. Neither X-ray nor electron diffraction was observed for 1-Zn2(SO4)2, probably because of the SO4 defect or disorder in the stacking of the nanosheet layers. The formation of the [Zn2(-O2SO2)2(tpy)2] motif in the nanosheet is further supported by the IR spectroscopy for nanosheet 1-Zn2(SO4)2, Zn2(-O2SO2)2(tpy)2, and [Fe(tpy)2](SO4) focusing on the SO42- vibrations (Figure S2). Zn2(-O2SO2)2(tpy)2 is a model complex molecule for nanosheet 1-Zn2(SO4)2, whereas [Fe(tpy)2](SO4) is a comparative reference molecule that possesses uncoordinating SO42- anion. A strong peak at ca. 1120 cm−1 was observed commonly in the IR spectra: it is derived from the 3° mode of SO42-.36 1-Zn2(SO4)2 and Zn2(-O2SO2)2(tpy)2 gave quite similar spectra: two additional peaks were observed at around 1010 and 1250 cm−1. These additional peaks were not seen in [Fe(tpy)2](SO4). The difference is caused by the change in the symmetry of SO42-, which is reduced from Td to C2v by the bridging coordination in 1-Zn2(SO4)2 and Zn2(O2SO2)2(tpy)2.24 Overall, the XPS and IR results suggest the formation of a nanosheet comprising the [Zn2(-O2SO2)2(tpy)2] motif. The photophysical properties of 1-Zn2(SO4)2 were examined next. UV-vis spectroscopy (Figure 4i) showed a broad absorption in the UV region (

Coordination Nanosheets Based on TerpyridineâZinc(II) Complexes

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