1. Introduction

Photocrystallography is regarded as the ultimate method which enables the three dimensional structural determination of molecules in their photo-activated states. Novel photosensitive materials, such as chalcogenides, photochromic organic compounds and charge transfer materials, are interest of their application to optical data storage. One of the photo-active materials: spin crossover (SCO) complexes; have received increasing interest since their potential for the molecular switchable application. The well known LIESST (Light Induced Excited Spin State Trapping) phenomena are evident via an extension of the Fe-N bond length by~0.2 A which corresponds to the electronic transition 1A1 (LS) →5T2 (HS), which the pronounced structural transformation allows us to gain insight into the relationship of Fe-N bonding interactions and magnetic properties based on accurate charge density studies.

The recent discovered SCO complexes; t-[Fe(abpt)2(NCS)2] (abpt=4-amino-3,5-bis(pyridin-2-yl) 1,2,4-triazole) polymorph C and D, both have two unique Fe atoms in their respective lattices, but only one Fe undergoes the spin transition from high spin (HS) to low spin (LS) and the other stays at HS. Consequently, this type site-selective spin transition may cause unusually concerted structural transition. In general, the LIESST effect would result in the elongation of Fe-N bond length and a substantial increase on the distortion of Fe octahedral coordination geometry due to the spin transition (LS → HS) of Fe ion. The previous discovery of thermal-induced commensurate modulation in polymorph C urged us to find out the relationship between structure and magnetic properties. Thus, in attempt to understand the LIESST effect toward this site-selective spin transition compound, the structures and bonding nature of polymorph C are investigated at 25 K both before and after the light irradiation.

In the case of polymorph D, the preliminary SQUID magnetic measurement and infrared (IR) spectroscopic studies indicate that the photo-reaction of D involves not only conventional LIESST, but also another photo-reaction. Another photo-reaction product is able to trap metastable HS until warming up to 110 K even after turning off irradiation, which is precedent among SCO complexes. Thus, to fully understand the relationship between molecular structures and physical properties in this fascinating complex, detailed X-ray structural analysis is essential. Single crystal structures of polymorph D before irradiation (ground-state, GS), under irradiation (metastable-state 1, MS1), and after irradiation (metastable-state 2, MS2) are investigated by X-ray single crystal diffraction at 25 K. Moreover, the MEM/electrostatic potential are also studied to clarify the chemical bonding nature such as metal-ligand coordination bonding and intermolecular interaction.

The present study is aimed to uncover the structural phase transition associated with site-selective LIESST phenomena in SCO complexes t-[Fe(abpt)2(NCS)2] polymorph C and D by X-ray diffraction using synchrotron radiation. Hopefully, combined with complementary measurements, such as IR spectra and SQUID magnetic measurement, we can obtain comprehensive understanding of these interesting photo-reactions.

2. Experimental and Analysis

2.1 Synchrotron X-ray Structural Determination

The center of this thesis is the photo-induced X-ray diffraction structural determination and charge density analysis. The study of photo-induced processes in single crystals offers a unique possibility for elucidating detailed single-crystal-to-single-crystal changes at atomic level. To achieve accurate photo-induced structural determination, one of the key points is to make sure the whole single crystal is under laser irradiation without crystal fracture. The excitation wavelength, 532nm, was chosen at the tail part of the absorption band of 500nm from UV-Vis spectra. The key on the in-situ photoexcitation studies may be the size of the crystal; the smaller the crystal is, the more stable the metastable state seems to be. Under this circumstance, using synchrotron radiation to obtain high resolution data certainly helps a great deal. Thus, the in-situ photo-induced X-ray diffraction experiments were performed at BL02B1 in SPring-8 facility, equipped with a Rigaku large cylindrical IP detector.

2.3 MEM charge density/electrostatic potential

The MEM charge density enables us to construct the fine structural model from X-ray diffraction data up to charge density level. As the electrostatic potential is an important function for the study of the chemical bonding nature; considerable attention has been paid to its derivation from the X-ray measurements. In this study, the electrostatic potential is evaluated using MEM charge density as an efficient approach to visualize the bonding nature inside materials, especially intermolecular hydrogen bonding.

3. Results and Discussions

3.1 The photo-induced commensurate modulated structure in polymorph C

The structure of C at LIESST-state is obtained at 25 K after illuminating the crystal with 532nm laser. In addition to Bragg peaks of the ground-state, the reflection pattern reveals clearly peaks present at l + 1/3, l + 2/3 along c* axis as depicted in Fig. 3. A photo-induced commensurate modulated structure is thus detected with a modulated vector, q=1/3 c*. The main difference between the structures at ground and LIESST-states is the two additional molecules designated as Fe12 and Fe22 in the asymmetric unit of the LIESST-state, which can be generated from their non-modulated structure, but with a slight shift of 0.34 and 0.23 A respectively for Fe1 and Fe2 site. This would give rise to a symmetry breaking and the formation of the modulated structure. Further MEM charge density study reveals the Fe-N coordinated bonding nature related to different spin states. Regarding topological charge density ρ(r) as a function of Fe-N bond lengths, on going site-selective spin transition from the LS (Fe1 site) to the HS (Fe11 and Fe12 sites) state, the ρ(r) values at the Fe-N bond critical points decrease, which clearly show the site-selective spin transition in charge density level.

3.2 The photo-induced linkage isomerism in polymorph D

The structures of polymorph D before irradiation (ground-state, GS), under irradiation (metastable-state 1, MS1), and after irradiation (metastable-state 2, MS2) were investigated by synchrotron X-ray diffraction at 25 K. This photo-reaction involves both conventional LIESST and photo-induced linkage isomerization. The linkage isomer (LI) molecules are brought by the transformation from isothiocyanato (Fe-NCS) to thiocyanato (Fe-SCN) in the MS1. When it converts to MS2 in light-off environment, the metastable HS molecules relax to LS state, but the LI molecules remain as elucidating in Fig. 4. Furthermore, several reflections only observed with linkage isomer state but unobserved in normal ground-state were monitored through increasing temperature from 25 K MS2. These reflections which only appear with linkage isomer molecules begin weaker through warming up to 100 K, and disappear at 110 K. The full data was collected at this state (110 K) and got pure FeN6 octahedral coordination which proves the linkage isomer molecules disappear through increasing temperature to 110 K, consistent with the SQUID magnetic measurement results. Moreover, the MEM charge density/electrostatic potential are studied to clarify the intermolecular interaction in GS, MS1, and MS2. It could be concluded that the intermolecular hydrogen bonding plays an important role in stabilizing the metastable state structures.

4. Conclusion

The thesis presents the photocrystallography study of site-selective spin crossover complexes t-[Fe(abpt)2(NCS)2] polymorph C and D using single crystal X-ray diffraction. With the help of synchrotron radiation X-ray, the in-situ photo-induced diffraction experiment can be achieved successfully. (1) Polymorph C reveals photo-induced commensurate modulation in crystal structure associated with site-selective spin transition. The Fe-N coordinated bonding strength related to different spin states is studied using MEM charge density with high resolution X-ray diffraction data. Concerning to charge density ρ(r) as a function of Fe-N bond lengths, it indicates that the charge density ρ(r) values in BCPs are inverse proportional to the bond lengths. (2) Polymorph D reveals intriguing photo-induced linkage isomerization which is brought by the transformation from isothiocyanato (Fe-NCS) to thiocyanato (Fe-SCN). The structural determination in the metastable states is successfully achieved, which give the direct evidence for the precedent existence of linkage isomerization in SCO complexes. The electrostatic potential could be accurately evaluated using MEM charge density. It indicates that the intermolecular hydrogen bonding plays an important role in stabilizing the precedent metastable state structure.

Fig 1. A brief layout of photo-induced experiment at BL02B1/SPring-8.

Fig 2. Using MEM to visualize the charge density.

Fig 3. (Left) Single crystal diffraction image at b*c* plane at 25 K: left for the ground-state, right for the LIESST-state. Satellite (o) appear at l + 1/3, l + 2/3 at the LIESST-state. (Right) Packing diagram of polymorph C left for ground-state; right for LIESST-state.

Fig 4. Scheme of photoreaction of polymorph D (for photosensitive Fe site).

本論文は、Feを錯体金属とするスピンクロスオーバー錯体[Fe(abpt)2(NCS)2]の分子パッキングの異なる特徴的な4種類の多形について、温度変化や光照射といった外部刺激による磁気的性質の変化をともなうFeの低スピン(LS)状態から高スピン(HS)状態への転移の違いと分子パッキング構造の違いの相関を、放射光を用いた単結晶X線構造解析により明らかにし、分子の協力的相互作用がもたらす分子サイト選択的なスピン転移や結合異性体が発現することを見出し、この物質の光照射効果の多様性とその構造起源を明らかにしたものである。特に、高輝度放射光X線を用い、光誘起構造相転移の様子を、従来の原子配列ではなく、電子密度での構造情報により、光誘起構造相転移を支配する分子間の協力的相互作用を、原子や分子の結合形態を可視化することにより議論し、光照射効果によってもたらされる準安定構造と結合異性体の振る舞いとの関係についても詳細に明らかにしている。論文は5章よりなる。

第1章は序論である。ここでは、本論文の研究対象であるスピンクロスオーバー錯体の光誘起現象の研究、本研究の研究手法である光照射効果による構造物性研究であるPhotocrystallography、について、従来の研究を紹介しながら概観し、本研究の目的、本論文の構成について述べている。遷移金属錯体のひとつであるスピンクロスオーバー錯体は温度や光によって遷移金属の電子状態が変化し、磁気的性質や光学的性質が変化する物質である。本研究で対象とするFeのスピンクロスオーバー錯体、trans-[Fe(abpt)2(NCS)2]は、分子のパッキングの違いにより、A~Dの4種類の多形が合成された物質である。いずれも、熱的なスピンクロスオーバー転移が異なる転移温度で起こることが、磁気感受率の測定から明らかにされている。特に、CタイプとDタイプは、2段階のスピンクロスオーバー転移を示し、錯体分子間に複数の相互作用が存在することを示唆している。本論文ではCタイプとDタイプのスピンクロスオーバー転移に着目し、光誘起の構造変化を、電子密度レベルで構造可視化し、その複雑な分子間の協力相互作用を直接観察することで、分子パッキングと光誘起のLS-HS転移の相関を解明することを目的としている。

第2章は実験法とデータ解析法の原理についての記述である。X線回折法の基礎、X線回折データを用いた電子密度解析の手法へのマキシマムエントロピー法(MEM)の応用、得られた電子密度から静電ポテンシャルを可視化する方法について概説している。本研究で行った、放射光を用いた単結晶X線回折データの実験の手順と使用した実験装置について、特に光誘起現象のその場観測の方法についても詳細に記述している。

第3章では、Cタイプの多形について、光誘起のスピンクロスオーバー転移により、コメンシュレイト構造へ相転移が起こり、その起源が、2種類のFe錯体分子のサイト選択的なスピンクロスオーバー転移にあることを解明している。本章では原子配列モデル、実験に用いた試料の作成方法、UV-VIS分光、並びにX線回折データ解析、熱的に誘起されたスピン励起状態TIESSTの構造と光誘起されたスピン励起状態LIESSTの構造を原子レベルと電子密度レベルの双方からから明らかにしている。

第4章では、Dタイプの多形について光誘起構造相転移について明らかにしている。このDタイプの光照射効果について、36%の分子はHS状態にスピンクロスオーバー転移し、残りが結合異性体へと転移する準安定状態が存在することを25Kで見出した。さらには、光照射をやめると、HS状態はLS状態へと転移し、結合異性体はそのままである第2の準安定状態へ転移することも明らかにした。この第2の準安定状態は約110Kまで保持されることもわかった。これらの複雑なスピンクロスオーバー転移の起源が、水素結合を介した分子間の協力的相互作用であることを、電子密度から得られた静電ポテンシャルによって明らかにした。

第5章は、総括である。

なお、本論文の第2、3、4章は、許朝富、王瑜、加藤健一、杉本邦久、金延恩、高田昌樹、等との共同研究であるが、論文提出者が主体となって測定および解析を行ったもので、論文提出者の寄与が十分であると判断する。

以上、本論文は、分子パッキングの異なる多形に特徴的な、光誘起スピンクロスオーバー転移現象について、電子密度レベルでの構造研究から、錯体分子間の協力的相互作用との関連を精緻に明らかにした。そして、分子パッキングに基づくスピンクロスオーバー錯体分子の光誘起材料設計の可能性を示した点で、物質科学の発展に寄与するところが大きく、よって博士(科学)の学位を授与できると認める。