Introduction

Organic-inorganic hybrid materials have attracted attention, since desired functions can be tailored by organic motifs, while their organic assembled structures can be immobilized by inorganic frameworks.1 For immobilization, silicate nanochannels of mesoporous silica have often been used, where certain performances of organic motifs can be enhanced due to site isolation or long-range molecular ordering.2 Since the pioneering work by us3a and other groups3b, 3c in 2001, we have demonstrated so far that, by using functional amphiphiles as templates for the sol-gel synthesis of organic/silica nanocomposites, a variety of organic functionalities can be incorporated into silicate nanochannels. On the other hand, we have also reported that hydrophobic trinuclear dendritic pyrazolate complexes of Au(I), Ag(I), and Cu(I), self-assembled into columnar aggregates through metallophilic interactions, forming photoluminescent superhelical fiber4a and phosphorescent organogels4b, which are applicable to the fabrication of rewritable phosphorescent papers.4c

My research subject focuses on utilization of an amphiphilic version of the cylindrical assembly of trinuclear Au(I) pyrazolate complex as a template for the sol-gel synthesis of novel phosphorescent mesostructured silica composites. New functional organic-inorganic hybrid materials, such as 1) self-repairable optoelectronic nanostructured materials and 2) metal ion permeation in congested hexagonal silicate nanochannels, could be developed by using these organometallic/silica nanocomposites.

Results and Discussion

1.Encouraged Self-Repairing of 1D Molecular Assembly in Mesoporous Silica by a 'Nanoscopic Inorganic Template'

Mesostructured silica film composite with a hexagonal geometry Au3Pz3/Silicahex (Figure 1b), was successfully synthesized by using the amphiphilic trinuclear Au(I) pyrazolate complex Au3Pz3 (Figure 1a) as a template for the sol-gel synthesis with tetrabutoxysilane (TBOS). Typically, an acidic aqueous EtOH solution (EtOH, 29 mg, 0.6 mmol; HCl, 12 N, 0.15 mg, 1.5 x 10-3 mmol; H2O, 6 mg, 0.3 mmol) of a mixture of Au3Pz3 (5 mg, 1.25 x 10-3 mmol) and TBOS (24 mg, 74.9 x 10-3 mmol) was kept for 12 h at room temperature ([Au3Pz3]/[TBOS]/[EtOH]/[HCl]/[H2O] = 1/60/504/1.2/266), where partial oligomerization of TBOS took place. The resulting viscous solution (20 ・L) was spin-coated on a quartz plate at 3000 rpm for 15 s, affording a colorless transparent film (Figure 1b, inset), which was dried in air for 24 h at room temperature and then additional 12 h at 40 °C. The powdered sample of Au3Pz3/Silicahex obtained by scratching the spin-coated film displayed X-ray diffraction (XRD) peaks at 2θ = 2.16°, 3.70°, and 4.30°, which can be indexed as d100, d110, and d200, respectively, of a hexagonal structure with an interpore distance of 4.1 nm (Figure 2a (B)). The absence of the peak due to d110 for the as-prepared film of Au3Pz3/Silicahex suggests that the c-axis of the hexagonal unit cell is oriented parallel to the quartz surface (Figure 2a (A)). Transmission electron microscopy (TEM) of the cross-section of the spin-coated film allowed for visualization of the hexagonal geometry of the silicate framework (Figure 3). Noteworthy, when the molar ratio of TBOS to Au3Pz3 was changed from 60:1 to 20:1 (8 mg, 25 x 10-3 mmol), a lamellar structure with an interlayer distance of 3.7 nm formed (Figures 2b (A) and B) of the spin-coated film (Au3Pz3/Silicalam).

On exposure to 254 nm UV light with a hand-held lamp, Au3Pz3/Silicahex with a hexagonal geometry emitted in red (Figure 1b, inset) with a luminescence center (・em) at 693 nm (Figure 4b). Monitoring the emission at 693 nm provided an excitation spectrum with a peak maximum at 276 nm (Figure 4a), indicating that this red luminescence has a very large Stokes shift (Δ・ = 417 nm). Also noteworthy is a long lifetime (・20 = 7.8・s) of this luminescence, evaluated at 20 °C by means of transient emission spectroscopy upon laser excitation at 266 nm. These spectral features are characteristic of phosphorescing events from metal-centered triplet excited states modified with a Au(I)-Au(I) metallophilic interaction. Quite analogous phosphorescence properties were observed for a bulk material of assembled Au3Pz3 (・em = 690 nm, ・20 = 6.3 ・s) and silica composite Au3Pz3/Silicalam with a lamellar geometry (・em = 690 nm,・20 = 6.7 ・s), indicating that the photochemical properties of assembled Au3Pz3 in the solid state are essentially maintained to the hybridization with silica.

The bulk material of assembled Au3Pz3 became less emissive upon heating (Figure 5a, red). For example, when this material was stepwise heated from 20 to 40, 80, 120 and 140 °C, its phosphorescence was progressively less intensified to 41, 20, 12 and 9%, respectively, of the original intensity (Figure 5a, red). Likewise, a thin film of mesostructured silica composite Au3Pz3/Silicahex with a hexagonal geometry showed lower emission capabilities at higher temperatures (Figure 5a, gray). However, this nanocomposite material showed a much smaller temperature dependency than bulk Au3Pz3. For example, upon heating to 80 and 120 °C, Au3Pz3/Silicahex retained even 77 and 56% of its original luminescence intensity, respectively (Figure 5a, gray). Such a heat-resistant phosphorescing capability is quite advantageous for many practical applications. Although the light-emitting capability of silica composite Au3Pz3/Silicalam with a lamellar geometry showed a better heat resistivity (Figure 5a, green) than that of bulk Au3Pz3 (Figure 5a, red), is was obviously inferior to that of Au3Pz3/Silicahex adopting a hexagonal geometry (Figure 5a, gray).

If the observed luminescence loss upon heating is induced only by thermal quenching of the photoexcited state, lowering the temperature could coincidentally result in complete restoration of the original luminescence intensity. However, all of these materials, on cooling from 140 to 20 °C, displayed only a partial recovery of the luminescence intensity (Figure 5a, blue bar area): 9 ・ 35% for Au3Pz3 (red), 35 ・ 56% for Au3Pz3/Silicahex (gray), and 15・ 25% for Au3Pz3/Silicalam (green). Therefore, it is likely that the luminescence loss upon heating is more or less caused by a thermally induced structural damage of cylindrical assembly of Au3Pz3.

For the contrasting heat-resistivities of the luminescence properties of Au3Pz3/Silicahex and Au3Pz3/Silicalam, I wondered if the hexagonal and lamellar silicate frameworks inherently possess different thermal stabilities from one another. Upon calcination of Au3Pz3/Silicahex at 450 °C for 3 h, the calcined film of Au3Pz3/Silicahex still showed a diffraction pattern consistent with a hexagonal geometry (Figure 2a (C)), whilst the observed 2・ values (2.90・ and 5.00・) indicate reduction of the interpore distance from 4.1 to 2.9 nm. In sharp contrast, Au3Pz3/Silicalam was completely disrupted to lose its lamellar structure (Figure 2b (C)). To my surprise, such a structural disruption also took place even when Au3Pz3/Silicalam was heated to 140 °C, where the XRD profile became totally silent within only 10 min (Figure 2b (D)). This observation suggests the crashed silicate layers hamper reconstruction of the elaborate molecular geometry for light emission.

I noticed that the bulk material of assembled Au3Pz3, when allowed to stand at 20 °C after being heated to 140 °C, gradually retrieves the luminescence intensity (Figure 5a, red) due to a dynamic nature of the metallophilic interaction. However, this event was subsided in 4 h, to furnish only a 35 ・ 54% recovery of its original intensity (Figure 5b, red). To my surprise, in the case of mesostructured silica composite Au3Pz3/Silicahex, where the luminescent cylindrical assembly of Au3Pz3 is confined in the silicate nanochannels, perfect restoration of the phosphorescence intensity took place (Figure 5a, gray). Under identical conditions to those for bulk Au3Pz3, the phosphorescence of Au3Pz3/Silicahex was recovered only in the initial 3 h from 56 to 80% of its original intensity and then further to 100% after 5 h (Figure 5b, gray). This autonomous recovery indicates the occurrence of complete reconstruction of cylindrically assembled Au3Pz3 in the confined silicate nanochannels from heat-induced structural damages (Figure 6). The self-repairing nature, thus observed for Au3Pz3/Silicahex (Figure 5b, gray) can be referred to as 'nanoscopic template effect' (Figure 6). Of further interest, silica composite Au3Pz3/Silicalam with a lamellar geometry, in contrast, displayed a very poor self-restoration profile at 20 °C (Figure 5a, green), allowing, even after 24 h, only 35% recovery of its original luminescence intensity (Figure 5b, green). Namely, the lamellar silicate gives rise to a negative effect on the reconstruction of the luminescent geometry.

2.Metal Ion Permeation in Congested Nanochannels: Exposure Effect of Ag+ on Phosphorescent Properties of a Gold Pyrazolate Complex Immobilized in the Nanoscopic Channels of Mesoporous Silica

When mesostructured silica film composite with a hexagonal geometry Au3Pz3/Silicahex (Figure 1b) was dipped at 20 °C into THF solutions of 100 ・M AgOTf, the resulting material Ag@Au3Pz3/Silicahex (Figure 7) upon excitation at 276 nm showed new emission band at 486 nm (Figures 8a and 8b) characteristic of a Au(I)-Ag(I) heterometallic interaction,5b in addition to the less-intensified original phosphorescence band at 693 nm. On exposure to 254 nm UV light with a hand-held lamp, Ag@Au3Pz3/Silicahex emitted in green (Figure 7, inset) with a luminescence center at 486 nm (Figures 8a and 8b). The decrease in the intensity of 693 nm-emission band subsided at 18% of the original intensity in 60 min when concentration of AgOTf was 100 ・M (Figure 8a, inset), while 10 ・M AgOTf (Figure 8b, inset) resulted in smaller and slower spectral changes (41% of the original intensity in 120 min). These observations suggest incorporation of Ag+ into open pore of the hexagonal silicate nanochannels of Au3Pz3/Silicahex including the cylindrical assembly of Au3Pz3.

Ag+ and cylindrically assembled trinuclear Au(I) complex can form a sandwich adduct in solid state.5 In soft materials, the columnar assembly of Au(I) or Cu(I) pyrazolate complexes afforded reversible phosphorescent color switching properties in response to external stimuli such as heat and Ag+.4b I then attempted the sol-gel synthesis of mesoporous silica with Au3Pz3 as a template in the presence of Ag+ (Figure 9). A typical procedure for the fabrication of mesostructured silica Ag@Au3Pz3/Silicahex is as follows: a THF solution of AgOTf (2.5・L) was added to Au3Pz3 (10 mg, 2.5 x 10-3 mmol) in THF (200 ・L), and the mixture was evaporated to leave a green residue, which was then dissolved in an acidic aqueous EtOH solution (EtOH, 58 mg, 1.2 mmol; HNO3, 16 N, 0.44 mg, 4.6 x 10-3 mmol; H2O, 12 mg, 0.6 mmol). TBOS (48 mg, 150 x 10-3 mmol) was added to this solution, and the mixture was kept for 12 h at room temperature, where partial oligomerization of TBOS took place. The resulting viscous solution (20・L) was spin-coated on a quartz plate at 3000 rpm for 15 s, affording a colorless transparent film, which was dried in air for 24 h at room temperature. According to the increment in the molar ratio of AgOTf to Au3Pz3 in the sol-gel synthesis, the ratio of the emission intensity at 486 nm due to Au(I)-Ag(I) to that at 693 nm due to Au(I)-Au(I) of Au3Pz3/Silicahex was progressively increased (Figure 10). It is noteworthy that the amounts of Ag+ taken into the film of Au3Pz3/Silicahex by dipping into THF solutions of 100 and 10・M AgOTf for 60 min are estimated to be 11 and 9% (corresponding to the atomic ratios of Au to Ag = 2.8 : 1 and 3.5 : 1), respectively, based on the plots in Figure 10.

The excitation spectrum of the film of Au3Pz3/Silicahex for 693 nm emission showed a peak maximum at 276 nm, which was less-intensified upon dipping the film into THF solutions of AgOTf (Figure 8a). On the other hand, the excitation spectra for 486 nm emission showed a peak maximum at 276 nm with a small peak at 390 nm, where the intensity of these two peaks are simultaneously intensified upon dipping. Upon excitation at 396 nm, the resulting Ag@Au3Pz3/Silicahex displayed a single luminescence center at 486 nm, while excitation at 276 nm results in emissions at both 486 and 693 nm. It should be noted here that comparison of the emission spectra after dipping for 30 and 60 min show that the emission due to Au(I)-Au(I) metallophilic interaction is mostly lost in both spectra while that due to Au(I)-Ag(I) heterometallic interaction drastically intensified from 30 to 60 min. This observation suggests that the emission due to Au(I)-Au(I) metallophilic interaction (693 nm) is quenched in part in Ag@Au3Pz3/Silicahex, and that the new emission at 486 nm was intensified upon conversion of the Au(I)-Au(I) metallophilic interaction to the Au(I)-Ag(I) heterometallic interaction. Namely, upon photoexcitation at 276 nm, an energy transfer from the excited Au(I)-Au(I) species to Au(I)-Ag(I) species likely takes place in Ag@Au3Pz3/Silicahex.

X-ray photoelectron spectroscopy (XPS) of Au3Pz3/Silicahex film allows us to evaluate the ratio of Au to Ag from the intensities of Au 4f (84.3 and 88.1 eV) and Ag 3d (368.1 and 374.1 eV) XPS peaks. Actually, after dipping into a THF solution of 100・M AgOTf for 60 min, the atomic ratio of Au to Ag was evaluated to be 3:7, at surface. Depth profiling experiments using an argon ion etcher with a 500 V accelerated voltage showed that, after sputtering for 5, 15, 30 and 75 s, the atomic ratios of Au to Ag changed to 2:3, 3:2, 2.3:1 and 3:1, respectively (Figures 11a and 11b). After sputtering for 115 s and 195 s, the peaks due to Ag 3d and Au 4f were sequentially disappeared, respectively. These results strongly suggest that the color change observed upon dipping the film of Au3Pz3/Silicahex into a THF solution of Ag+ does not originate from the adsorption of Ag+ on the surface, but is due to permeation of Ag+ into the nanochannel of Au3Pz3/Silicahex to form heterometallic composite Ag@Au3Pz3/Silicahex (Figure 1).

Conclusions

I have demonstrated the first successful fabrication of organic-inorganic nanostructured composites using mesoporous silica with a hexagonal geometry as a framework and a luminescent cylindrically assembled metal pyrazolate complex as a template for the sol-gel synthesis. New functional organic-inorganic hybrid materials can be developed by using these phosphorescent nanocomposites such as 1) self-repairable optoelectronic nanomaterials that recovers from structural damages caused by heating due to 'nanoscopic template effect' and 2) exposure effect on phosphorescent properties from red to green due to metal ion permeation in congested hexagonal silicate nanochannels. These works may shed light on a general issue of how one can ensure the high reliability of molecular devices for long-term operation and the high sensitivity of composite materials for metal ion and gas sensors.

Figure 1. a) Synthesis and molecular structure of Au3Pz3. b) Sol-gel synthesis of mesostructured silica Au3Pz3/Silicahex with a hexagonal geometry templated by columnarly assembled Au3Pz3. Photographs were taken at room temperature under room light (lower left) and on exposure to a 254 nm ultraviolet light (lower right).

Figure 2. X-ray diffraction patterns of a) Au3Pz3/Silicahex and b) Au3Pz3/Silicalam. (A) as-synthesized spin-coated films and (B) powder samples obtained by scratching off the spin-coated films, (C) spin-coated films after calcination at 450 °C for 3 h, and (D) a spin-coated film (Au3Pz3/Silicalam) after heating at 140 °C for 10 min (blue curve).

Figure 3. TEM micrograph of an as-synthesized spin-coated film of Au3Pz3/Silicahex (cross section).

Figure 4. a) Excitation (・em = 693 nm) and b) emission (・ext = 276 nm) spectra of Au3Pz3/Silicahex at 20 °C.

Figure 5. Changes in a), b) emission intensities (normalized) at 693 nm (・ext = 276 nm) of Au3Pz3/Silicahex (gray), Au3Pz3/Silicalam (green), and bulk Au3Pz3 (red) on a) stepwise heating from 20 to 140 °C followed by natural cooling from 140 to 20 °C and b) being kept at 20 °C for 24 h after natural cooling from 140 °C.

Figure 6. Self-repairing capabilities of columnarly assembled Au3Pz3 immobilized within hexagonal silicate nanochannels (Au3Pz3/Silicahex).

Figure 7. Phosphorescence color change of the film of Au3Pz3/Silicahex caused by dipping into a THF solution of 100・M AgOTf for 60 min at 20 °C. Photographs were taken on exposure to ultraviolet light a 254 nm.

Figure 8. Emission (・ext = 276 nm) spectral changes of a film of Au3Pz3/Silicahex at 20 °C before and after dipping into a THF solution of a) 100 and b) 10 ・M AgOTf. c) Changes in emission intensities (・ext = 276 nm) of a film of Au3Pz3/Silicahex due to Au(I)-Ag(I) and Au(I)-Au(I) metallophilic interactions at 486 and 693 nm, respectively, after dipping into THF solutions of 100 (blue) and 10 ・M (green) AgOTf.

Figure 9. Schematic illustration of the sol-gel synthesis of hexagonal mesostructured silica Ag@Au3Pz3/Silicahex templated by 1D columnar assembly of Au3Pz3 in the presence of Ag+.

Figure 10. Changes in relative emission intensities ratio (・ext = 276 nm) of Au(I)-Ag(I) (・em = 486 nm) and Au(I)-Au(I) (・em = 693 nm) for a film of Ag@Au3Pz3/Silicahex prepared by the sol-gel synthesis with different molar ratios AgOTf to Au3Pz3 (0.0, 1.0, 2.0, 4.0, 6.0, 8.0, 10.0, and 16.7%)

Figure 11. Changes in XPS spectra of Au3Pz3/Silicahex after dipping into a THF solution of 100 ・M AgOTf for 60 min upon sputtering for 5, 15, 30, 55, 75, 115, 155, and 195 s. a) Au 4f and b) Ag 3d regions.

第11族元素を初めとする遷移金属元素は、金属-金属結合を含む発光性の多核錯体を与えることが知られている。多くの場合、その発光は長寿命のリン光であることから、さまざまな機能性材料への応用が期待されている。しかしながら、一般的に金属-金属相互作用の結合エネルギーは水素結合程度であるため解離しやすく、多くの場合、分子が固定された結晶状態でのみその発光特性が観測される。これは、デバイス等への応用を考える上では大きな制約となる。一方、無機物の作るナノ構造中に機能性有機物を導入した有機無機ナノ複合体は、無機物のもつ耐久性と有機物の機能とを併せ持つナノ材料として注目を集めている。特に、界面活性剤を鋳型として合成するメソ構造シリカは、異方性を有する有機無機ナノ複合体を簡便に得られるため、その応用の可能性は極めて高い。ヘキサゴナル構造を有するメソ構造シリカでは、機能性有機物で満たされた1次元のシリカナノチャネルが同一方向に向いており、さまざまな電子材料・光学材料への応用が期待される。本論文では、金ピラゾール三核錯体が金属-金属結合を介して形成するカラム状錯体をメソ構造シリカのナノチャネルに導入することにより、安定性、自己修復性および刺激応答性を兼ね備えた発光性有機無機ナノ複合体が合成できることを示し、さらにその特性について述べている。内容は、以下の3章から成っている。

序論では、まず有機無機ナノ複合体の一般的な分類およびその構造特性について概観している。さらに、その中で、メソ構造シリカに焦点を絞ってその特性について詳述し、シリカナノチャネル内に機能性官能基を導入する手法についてまとめている。さらに、界面活性剤にあらかじめ機能性官能基を導入し、これを鋳型としてメソ構造シリカを得る手法についてその利点を解説し、この手法を複合体調製のために採用した根拠を示している。一方、金属-金属結合を含む多核錯体の特性について概観し、その中から、金、銅等のピラゾール三核錯体が金属-金属結合生成を介して形成する発光性のカラム状集積体についてその特性を解説し、その安定化にメソ構造シリカを用いる意義について述べている。

第二章では、金ピラゾール三核錯体を鋳型としたメソ構造シリカ複合体の合成とその性質について報告している。まず、金ピラゾール三核錯体に両親媒性を持たせるためにピラゾール環上にトリエチレングリコール鎖を導入した錯体の合成法およびその同定について述べている。さらに、この錯体を鋳型としたヘキサゴナルメソ構造シリカの合成について述べ、得られた複合体が276 nmの紫外光照射により693 nm付近でカラム状集積体に特徴的なリン光を示したことから、目的どおりシリカナノチャネル内に金-金結合を含むカラム状集積体が導入されたと結論づけている。続いて、得られた金ピラゾール三核錯体/シリカ複合体の熱安定性がバルクの金ピラゾール三核錯体と比較して著しく向上することを、温度可変条件下での発光スペクトルにより示している。さらに、加熱により低下した複合体の発光強度が20℃に下げてもすぐには回復しないものの、冷却後のサンプルを20℃で放置しておくと徐々に発光挙動が回復し、最終的に加熱前と全く同じ強度にまで自己修復するという興味深い現象について述べている。これまで、いくつかの自己修復性材料が報告されてきたが、それらはバルクの物性の回復を観測したものであり、本例はナノ空間内での分子レベルでの自己修復現象を観測した初めての例にあたり、その意義は極めて大きい。

第三章では第二章で得た金ピラゾール三核錯体/シリカ複合体が、銀イオンの添加によりその発光特性を大きく変化させる現象について報告している。金ピラゾール三核錯体/シリカ複合体はスピンコート法により、ガラス基板の上で透明なフィルムとして調製することができる。こうして得たフィルムを銀イオンを含む溶液に浸したところ、276 nmの紫外光照射により得られる発光波長が銀イオン導入前の693 nmから導入後には486 nmへと大きく変化することを見いだしている。この現象を詳細に解析し、銀イオンがシリカナノチャネル内に浸潤し、その結果、金ピラゾール三核錯体と銀イオンとのヘテロ金属錯体が形成し、これが発光色の変化をもたらしたと結論づけている。さらに、ラメラ構造を有する金ピラゾール三核錯体/シリカ複合体を用いて同様の実験を行ったところ、シリカ内の金ピラゾール三核錯体が溶液に溶け出してしまったことから、錯体の安定化におけるヘキサゴナルカラム構造の重要性を明らかにしている。これらの結果は金ピラゾール三核錯体/シリカ複合体中に金属イオンが浸透できることを示しており、その発光特性の変化から、金属センサーへの応用可能性があることを示している。

以上、本論文では、金ピラゾール三核錯体/シリカ複合体が優れた安定性、さらには自己修復性を示すことを明らかにし、さらに、その安定性を利用して銀イオンのような他種の金属イオンとの多核錯体を調製し、様々な発光特性を与えることができることを示している。本研究は、発光性錯体の応用可能性を飛躍的に広げることは間違いなく、今後の発展に寄与するところが大きい。よって本論文は博士(工学)の学位請求論文として合格と認められる。