Emergence and Development of Artificial Molecular Brake

1. Emergence and Development of Artificial Molecular Brake Yang, J. -S. et al. Org. Lett. 2008, 10, 2279. J. Org. Chem. 2006, 71, 844. Tobe lab. Kazuhiro Ikuta

2. Contents ・Introduction Molecular Machines Molecular Brake Purpose of This Work ・Results and Discussion Synthesis of Molecular Brake 1 Results of NMR Spectra Results of DFT Calculations ・Summary

3. Molecular Machines Examples of molecular machines ピンセット Molecular motor Molecular tweezers Molecular switch Irie, M. et al. Chem. Commun.2005, 3895. Lehn, J. –M. et al. J. Am.Chem. Soc.2004, 126, 6637. Feringa, B. L. et al. J. Org.Chem.2005, 3, 4071. Molecular shuttle Stoddart, J. F. et al. Acc. Chem.Res.19978 31, 405.

4. Molecular Brake The first example of molecular brake Desired rotary motion ・operating at room temperature ⇒applicable to machines ・photocontrollable system ⇒clean reaction Kelly, T. R. et al. J. Am. Chem. Soc. 1994, 116, 3657. Metal ion

5. Purpose of This Work To date, an effective room-temperature photocontrollable molecular brake has yet to be demonstrated. Purpose Synthesis and characterization of a room-temperature light-driven molecular brake. Image of molecular brake compound in this work

6. Synthesis of Molecular Brake 1 Synthesis of compound 5 1) H3PO2 aq. THF SnCl2 NH2OHHCl HCl CH2Cl2 THF 2) Me3CONO 1) DIBAL-H CH2Cl2 K2CO3, KI CuCN NBS C8H17Br acetone DMF, 80 ゜C NMP, 200 ゜C 2)HCl 5 Yang, J. -S et al. J. Org. Chem. 2006, 71, 844.

7. Results of NMR Spectra Only one set of signals for trans-1 ⇒free rotation about the Cvinyl-Caryl single bonds Two sets of signals for pentiptycene group ⇒rotation of the rotator is slower than the NMR time scale • 1H and (b) 13C NMR spectra of trans-1 and cis-1 in DMSO-d6 at 298 K (500 MHz).

8. VT NMR Spectra-(1) Rotational barriers and rates for the pentiptycene rotator in cis-1 is obtained from VT (variable-temperature) NMR. 融合 coalescence cis-1 Pentiptycene peripheral phenylene (blade) region of the (a) experimental proton and (b) carbon and (c) simulated carbon VTNMR spectra of cis-1 (9 and 60 mM for proton and carbon, respectively, DMSO-d6, 500 MHz). A coalescence temperature (Tc) near 348 K is found for protons H3 and H3’, corresponding to an energy barrier of ΔG‡(348K) = 16.9 ± 0.2 kcal mol-1. Hoever the multiplicity of proton signals in the phenylene blades of pentiptycene rotator ⇒VT 13C NMR was carried out (b) and simulated (c).

9. VT NMR Spectra-(2) cis-1 Pentiptycene peripheral phenylene (blade) region of the (a) experimental proton and (b) carbon and (c) simulated carbon VTNMR spectra of cis-1 (9 and 60 mM for proton and carbon, respectively, DMSO-d6, 500 MHz). The results suggest that the rotation is nearly blocked at 298 K, and the rate constant (k) for interconversion between the two isoenergetic conformers of cis-1 is only 6 s-1. The activation parameters were obtained by Arrhenius and Eyring plots. Ea[a]ΔH‡[a] ΔS‡[b]ΔG‡298 K[a]ΔG‡348 K[a] 14.8 ± 0.5 14.1 ± 0.5 -7.6 ± 1.4 16.4 [a] kcal mol-1 [b] cal K-1 mol-1 16.8 Rotational barrier is mainly due to an enthalpic factor.

10. DFT Calculations-(1) The calculation results are justified by the good agreement of the calculated (16.75 kcal mol-1) and the NMR-determined ΔG‡value (16.4 kcal mol-1) at 298 K. U-shaped cavity V-shaped cavity DFT-derived structures for cis-1 : (a) the optimized conformation and (b) the transition structure along the pentiptycene rotation coordinate. cis-1 Brake moiety (dinitrophenyl group) is located at theU-shaped cavities. ⇒V-shaped cavitiesare inaccessible to the brake moiety as a result of severe steric interactions with H1, H1’, and bridgehead hydrogen atom.

11. DFT Calculations-(2) The rotational barriers for the pentiptycene rotator in trans-1 and the brake moiety in cis-1could not evaluated because of their low energy (decoalescence of the signals could not be observed even at 183 K in CD2Cl2). DFT calculations have been applied to retrieve the corresponding information for that in trans-1 (4.45 kcal mol-1) and the brake rotation in cis-1 (6.85 kcal mol-1). With a calculated ΔG‡ value differing by12.3 kcal mol-1 (※) for the pentiptycene rotation in trans-1 versus cis-1 at 298 K, the difference in rotation rate is inthe order of 109. (※) ΔG‡cis is 16.75 kcal mol-1 krot 1 : ～109

12. Photoswitching between trans-1 and cis-1 Ratio of [trans]/[cis]*) Wavelength (nm) trans→ cis 25/75 306 45/55 cis→ trans 254 *) photostationary states 光定常状態 Absorption spectra of trans-1 (curve a) and cis-1 (curved) and their photostationary states irradiated with alternating 306- (curves c) and 254-nm (curves b) UV light irradiation in dichloromethane. Inset shows the changes in absorbance at 322 nm starting from trans-1 (10 μM) for 7 switching cycles Photoswitching between the two photostationary states is quite robust. robust : 強固である

13. Conclusion The pentiptycene-derived stilbene 1 has been prepared and investigated as a photocontrollable molecular brake that functions at room temperature. Both experimental and computational results reveal that at 298 K rotation of the four-bladed pentiptycene (the rotator) is “free” in trans-1 but is nearly blocked in cis-1. The brake-on (cis-1) and brake-off (trans-1 ) states differ by a rotation rate of ～109-fold and can be interconverted through the ethylene trans-cis photoisomerization reactions.

14. Our Work-Rotaxane Molecular Brake

15. RWTH Aachen 11.07.2008 A Shuttling and a Rocking Molecular Machines with Reversible Brake Function Keiji Hirose and Yoshito TobeGraduate School of Engineering Science, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan

16. A Molecular Machine with Reversible Brake Function Machines at molecular level are … in perpetual Brownian motion. These motions have to be stopped effectively. Our reversible brake systems worksquantitatively in response to external photochemical and thermal stimuli. The rate of shuttling and rocking motion are proved to be reduced to less than 1% by reducing the size of ring component.