論文 / 著書情報
Article / Book Information
題目(和文)
Title(English)

Ultrafast photoinduced dynamics of the electron donor-acceptor type
metal-organic framework

著者(和文)

BANUSamiranAuthor(English)

Samiran Banu

出典(和文)

学位:博士(理学),学位授与機関:東京工業大学,
報告番号:甲第12666号,
授与年月日:2024年3月26日,
学位の種別:課程博士,
審査員:腰原 伸也,沖本 洋一,大島 康裕,河野 正規,谷口 耕治

Citation(English)

Degree:Doctor (Science),
Conferring organization: Tokyo Institute of Technology,
Report number:甲第12666号,
Conferred date:2024/3/26,
Degree Type:Course doctor,Examiner:,,,,

学位種別(和文)

博士論文

Category(English)

Doctoral Thesis

種別(和文)

論文要旨

Type(English)

Summary

Powered by T2R2 (Tokyo Institute Research Repository)

（博士課程）Doctoral Program論 文 要 旨ＴＨＥＳＩＳ ＳＵＭＭＡＲＹ

系・コース：
Department of, Graduate major in

学生氏名：
Student’s Name

Chemistry系
コース

BANU SAMIRAN

申請学位（専攻分野）：

博士

Academic Degree Requested

Doctor of

審査員主査：
Chief Examiner（ScienceShinya Koshihara（腰原伸也）

要旨（英文 800 語程度）
Thesis Summary （approx.800 English Words ）The photo-control of macroscopic physical properties in the solid state is widely known as photoinduced
phase transition (PIPT). Ultrafast switching phenomena caused by PIPT have been particularly importantbecause of their potential applications in optical data storage, fast-switching devices, sensors, etc. (researchbackground, Chapter 1). Electron donor–acceptor (DA)-type metal–organic frameworks (MOFs) withvalence instability are promising molecular materials for the design of photo-responsive andelectronic/magnetic functional materials. In the present study, as a target and realistic material for PIPT,
we mainly focused on the DA-type (where D = donor and A = acceptor) layered MOF system,(NPr4)2[Fe2(Cl2An)3], which exhibits two-step CT phase transition (Tc = 317 and 354 K). The details of thematerial are discussed in Chapter 2. Chapter 3 describes the experimental methodology and instruments
used to conduct our desired experiments. In Chapter 4, the static optical spectra of (NPr4)2[Fe2(Cl2An)3] forboth E || chain and E⊥ chain have been reported. Herein, we observed two absorption peaks at around 0.8
and 2.6 eV. The peak at around 0.8 eV is attributed to the CT transition between the Fe and Cl2An ions.
In comparison, the peak at around 2.6 eV is attributed to an intramolecular transition in the Cl2An ions.
Here, we observed highly efficient photoinduced CT dynamics in the DA-type layered MOF:(NPr4)2[Fe2(Cl2An)3] by using time-resolved spectroscopy with a pulse width of 90-fs at room temperature(RT: 300 K) discussed in Chapter 5. The photoinduced spectral changes are similar to the thermally induced
spectral change at around 2.6 eV, suggesting photoinduced valence change of the Cl 2An ions. However, weobserved a considerable difference in the spectral shapes in the CT transition and mid-IR regions. We found
two unexpected new photoinduced absorption bands from the spectra analysis, one at the higher-energy
side of the main CT band and the other in the mid-IR energy range—just immediately after photoexcitation.
The observed spectral change strongly implies the disappearance of the local inversion center on the Cl 2Anion instantly upon photoexcitation, causing an ultrafast change in the lattice structure due to the softeningof rigid bonds. This has never been realized in a thermal phase transition. These findings demonstrate thata new electronic state with a unique lattice structure—i.e., a photoinduced hidden state—appears in thisMOF system at ultra-high speed (within 110 fs) upon photoexcitation at RT. Early-stage dynamics for theformation of structurally modulated photoinduced hidden state, discussed in the previous section, is still
unclear, and we thought of a way to visualize them in this study. Herein, we used pump-probe spectroscopy
with nearly single-cycle optical, infrared pulses (pulse width ∼6 fs) to investigate early-stage dynamics forphotoinduced structurally modulated state formation (details are in Chapter 6). After the initial large spectralchange due to the photoinduced CT at Δt = 0 fs, there is an apparent spectral change in the relaxation）

process (redshift of zero-crossing point) from 0 to 50 fs, suggesting sequential structural change
corresponding to the "hidden state" we observed. Meanwhile, there is no further spectral change up to atleast 500 fs. In addition, we observed coherent oscillations in the time evolution of ΔR/R. According to theanalysis of the coherent oscillations , we found the IR active and Raman inactive mode in the thermal
equilibrium phases ∼494 cm-1 becomes Raman active in the photoinduced state at least after 50 fs, which
implies a photoinduced symmetry breaking of this material. Hence, we assumed that the formation timescaleof the hidden state from the initial purely electronic excited state must be ∼50 fs. In the previous chapters,
we experimentally demonstrated an efficient photoinduced CT conversion between Fe and Cl 2An ions
(Cl2An3- + Fe3+→ Cl2An2- + Fe2+) at RT. But now we intend to observe the photoinduced reverse CT (Fe2++ Cl2An2-→ Fe3+ + Cl2An3-), i.e., conversion of HT phase to LT phase. To realize such thought, we performed
a time-resolve experiment by exciting the intramolecular transition peak (∼2.6 eV) with E || chain for pumpand probe pulses in the high-temperature phase (380 K). The photoinduced ΔR/R spectra can be well
reproduced by simulated ΔR/R spectrum in the CT energy range, suggesting a successful observation ofphotoinduced reverse CT between the Fe and Cl2An ions. In addition to the above result, we observed athreshold behavior for realizing reverse CT for the pump excitation fluence, i.e., Iex > 1.0 mJ/cm2, suggesting
the cooperativity in reverse CT. The detailed discussion is in Chapter 7.
In conclusion, we can mention that the present study can be considered a first step toward developing highspeed photo-responsive MOFs with magnetic nature.

備考 : 論文要旨は、和文 2000 字と英文 300 語を 1 部ずつ提出するか、もしくは英文 800 語を 1 部提出してください。
Note : Thesis Summary should be submitted in either a copy of 2000 Japanese Characters and 300 Words (Engｌish) or 1copy of 800Words (English).
注意：論文要旨は、東工大リサーチリポジトリ（T2R2）にてインターネット公表されますので、公表可能な範囲の内容で作成してください。
Attention: Thesis Summary will be published on Tokyo Tech Research Repository Website (T2R2).