Subject Number : S-16-NM-0019
Support Type : 機器利用
Proposal Title (English) : Chiral organization or chirality transformation of conjugated polymers based on spiro-carbon strategy
Username (English) : Jinjia XU
Affiliation (English) : Graduate School of Pure and Applied Sciences, University of Tsukuba

1. Summary
Highly twisted but conjugated molecules have been attracting many attentions. In my current project undertaken, I described a straightforward approach to synthesize the highly strained 9,9’-bifluorenylidene cyclic dimer (CBF) from 9,9’-bifluorenylidene (BF) via typical Yamamoto coupling reaction conditions. Sequential Yamamoto coupling reactions potentially allowed us to obtain well-defined and highly twisted but conjugated 9,9’-bifluorenylidene ladder tetramer (LBF). We proposed that those highly twisted molecules might have unique photophysical and electronic properties and furnish with biradical characters.

Scheme1. Computer generated model of BF monomer, CBF and LBF.

2. Experimental
NMR, UV/Vis Absorption and Circular dichroic (CD) spectroscopies, Thermogravimetric analysis (TGA), Differential scanning calorimetry (DSC), Electron Paramagnetic Resonance (ESR) and Electron Paramagnetic Resonance (EPR)

3. Results and Discussion

The 9,9’-bifluorenylidene (9,9–BF) monomer (BF) bearing two bromo substituents on each fluorine unit was obtained under the treatment with Lawesson’s reagent in toluene by homo-dimerization of precursor, which was synthesized by Suzuki-Miyaura coupling reaction. Unexpectedly, CBF was successfully obtained through Yamamoto coupling reaction and further purified by silica-gel column chromatography using CHCl3 and Hexane (1:1) as eluent. Compounds BF and CBF both exhibit good solubility in common organic solvents, such as dichloromethane (CH2Cl2), chloroform (CHCl3), tetrahydrofuran, and toluene and fully characterized by NMR, MALDI/TOF-MS.
The optical and photophysical properties of BF and CBF were systematically investigated. Compounds BF and CBF exhibit intense absorption in the visible range, indicating that the –electrons are delocalized over the whole molecules despite their distorted structures. CBF exhibited red color in diluted CHCl3 solution with the absorption maximum at 479 nm together with a shoulder like absorption band at 452 nm. The optical bandgaps of BF and CBF estimated from the onset of absorption are comparably determined to be 2.35 eV.
The 9,9’-bifluorenylidene derivatives were demonstrated to have ability to accept two electrons, probably favored by release of steric hindrance from proximal aromatic rings and development of a 14- electron system. The cyclic voltammogram (CV) and differential pulse voltammogram (DPV) measurements of BF and CBF were performed under deaerated conditions in CH2Cl2 containing 100 mM TBAPF6 at 25C. The BF showed clear reversible two steps one-electron reduction and oxidation processes, indicating the possible formation of stable radical anionic intermediates. The first reduction wave can be attributed to the formation of the radical anion while the second reduction wave is corresponding to the formation of dianion. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) for BF were calculated to be –5.40 eV and –3.68 eV. In contrast, the quasi-reversible oxidation waves and the first reduction of CBF were reversibly obtained while the second reduction process was irreversible. Interestingly, two closed reduction peaks were observed in DPV measurement at –1.33 eV and –1.37 eV in the first reduction process, suggesting that each 9,9–BF within CBF can be stepwise one electron reduced. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) for CBF were calculated to be –5.35 eV and –3.56 eV. The thermal behavior of BF and CBF was determined by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The TGA analyses showed that BF has relatively high decomposition temperature up to 318°C with 2% weight loss, indicating good thermal stability compared with 193°C observed for CBF, probably because of the presence of more octyloxy chains. The thermal transition processes were studied by DSC. Characterization by DSC reveals no phase transitions between 25°C and 200°C for CBF, presumedly due to the existence of enantiomeric mixture namely (P, P) and (M, M), yielding CBF more amorphous than crystalline. In stark contrast, BF showed the broadening melting point at around 153°C in the second heating process, probably attributable to multi-conformations from BF. Surprisingly, in the cooling process, one sharp peak at 150°C in additional to one small peak at 154°C was observed, thus we infer that multiple phase transitions occur sequentially by alternating the conformation of BF.
In conclusion, we successfully designed and synthesized three highly twisted but fully conjugated monomer, cyclic dimer and expected ladder tetramer based on 9,9’-bifluorenylidene building block. The bending conformation with a hoop shape would not only become a tool for an enhancement of solubility but also tuning the HOMO-LUMO bandgap as well as modulating the photophysical and redox properties. The present findings we report here would provide a promising approach toward the preparation of three dimensional -conjugated molecules with unique structures.

4. Others
Deep acknowledgements to JSPS research fellowship for financial support by a Research Fellowship for Young Scientists.
Coauthors: Prof. Masayuki Takeuchi and Dr. Atsuro Takai
Platform staffs kindly provided instructions of how to use instruments.
5. Publication/Presentation
(1) “Multiple Emissions from Indenofluorenedione in Solution and Polymer Films”
J. Xu, A. Takai, M. Takeuchi,* RSC Adv., 2016, 6, 80867-80871. DOI:10.1039/c6ra17765d

(2) “Red-Green-Blue Trichromophoric Nanoparticles with Dual Fluorescence Resonance Energy Transfer: Highly Sensitive Fluorogenic Response Toward Polyanions”
J. Xu, A. Takai, M. Takeuchi,* Chem. Eur. J., 2016, 22, 13014-13018. DOI:10.1002/chem.201602759

6. Patent
N/A