Photoinduced Hinge-Like Molecular Motion: Xanthene-Based Cyclic Azobenzene Dimers

1. Photoinduced Hinge-Like Molecular Motion:Xanthene-Based Cyclic Azobenzene Dimers S. Anitha Nagamani, Yasuo Norikane, and Nobuyuki Tamaoki* J. Org. Chem.2005, 70 9304–9313 Tomoki KATO TOBE Lab.

2. Contents • Introduction Molecular Devices Hinge-Like Molecules Components of the Hinge-Like Motion Previous Work • Results and Discussion Synthesis X–ray Crystal Analysis NMR Spectral Change Absorption Spectra and Photochemical Isomerization Thermal Isomerization • Conclusion

3. Molecular Devices Stoddart, J. F. et. al. Angew. Chem., Int. Ed. 2000, 30, 3348 Feringa, B. L. et. Al. Nature, 2005, 437, 1337. These devices driven by chemical, electrochemical, or photochemical forces. Great potential in molecular scale information processing Sauvage, J. P. Acc. Chem. Res. 1998, 31, 611.

4. Hinge-Like Molecules Hinge (蝶番) Samson, M. S. P. et. al. J. Mol. Biol. 2002, 323, 951. Frevious hinge-like molecules: thermal operated system Williams, D. J. et al. J. Am. Chem. Soc. 1992, 114, 6330. Russell, R. A. et. al. Chem.sEur. J. 2001, 15, 3406.

5. Component of the Hinge-Like Motion: Azobenzene Hinge (蝶番) Samson, M. S. P. et. al. J. Mol. Biol. 2002, 323, 951. Azobenzene: Photoresponsive system trans-azobenzene cis-azobenzene

6. Purpose of This Study (i) Obtain the crystal structures of the trans―trans and cis―cis isomers of the cyclic azobenzene dimer. (ii) Compare the different properties of the cyclic azobenzene dimer and its precursor. Previous Work Xanethene-based cyclic azobenzene dimer Tamoaki, N. et. al. Org. Lett. 2004, 6, 2595.

7. Synthesis Conditions: (i) n-BuLi, dry DMF・THF, rt, 73%; (ii) Jones reagent, acetone, rt, 95%; (iii) diphenylphosphoradine, TEA, toluene, 85 ºC, 87%; (iv) KOH, EtOH, 77ºC, 98%; (v) MnO2, benzene, 80ºC, 52%; (vi) t–BuOK, t–BuOH, DMSO, rt, 17%

8. X–ray Crystal Analysis of Precursor 2 (a) (b) Figure 1. 2. Chemical structure (a) and crystal structure of 2(t) (left)and 2(c) (right)with displacement ellipsoids shown at the 50% probability level with the dotted line showing the H–bonding (b). Table 1. Torsion Angle and Bond Angle of Azobenzene Units in Hinge Molecule and its Precursor.

9. X–ray Crystal Analysis of 1 Figure 3. 4. Chemical structure (a) and crystal structure of 1(t,t) conformer A (left) and 1(c,c) (right)with displacement ellipsoids shown at the 50% probability level (b). Table 1. Torsion Angle and Bond Angle of Azobenzene Units in Hinge Molecule and its Precursor.

10. NMR Spectral Changes After irradiation 1(t,t) 1(c,c) 80% 20% Before irradiation Figure 5. 1H NMR spectra of 1(t,t) in 1,1,2,2-tetrachloroethane-d2 before (a) and after (b) irradiation. For labeling, see the structure of 1(t,t) and 1(c,c) given beside the spectra.

11. 366 nm 366 nm 436 nm 436 nm 2(t) 1(t,t) 1(c,c) 2(c) Absorption Spectra and Photochemical Isomerization 326 nm 395 nm 365 nm 320 nm Figure 6. 7. Changes in the absorption spectra of 2(t) (left) and 1(t,t) (right) in toluene upon irradiation at (a) 366 nm: a, 0 min; b, 20 s; c, 40 s; d, 1 min; e, 1.5 min; and f, 2 min. (b) 436 nm: f, 0 min; g, 10 s; h, 20 s; and I, 30 s. (c) Absorption spectra of pure 2(c) (left) and 1(c,c) (right) measured by a photodiode array detector attached to a HPLC system. (d) Absorption changes observed at 326 nm after alternating the irradiations at 366 nm (3 min) and 436 nm (1 min) over 16 complete cycles.

12. Cyclic Azobenzene Dimers Figure 10. Isokinetic plot for ΔＨ‡ - ΔＳ‡ of the cis-trans thermal isomerization of cyclic azobenzene dimers.

13. 366 nm 366 nm 436 nm 436 nm 2(t) 1(t,t) 1(c,c) 2(c) Absorption Spectra and Photochemical Isomerization Figure 6. 7. Changes in the absorption spectra of 2(t) (left) and 1(t,t) (right) in toluene upon irradiation at (a) 366 nm: a, 0 min; b, 20 s; c, 40 s; d, 1 min; e, 1.5 min; and f, 2 min. (b) 436 nm: f, 0 min; g, 10 s; h, 20 s; and I, 30 s. (c) Absorption spectra of pure 2(c) (left) and 1(c,c) (right) measured by a photodiode array detector attached to a HPLC system. (d) Absorption changes observed at 326 nm after alternating the irradiations at 366 nm (3 min) and 436 nm (1 min) over 16 complete cycles.

14. Thermal Isomerization Figure 8. Comparison of the rate constants of the different transition for 1 and 2. Table 2. Thermodynamic Properties for the Different Transition of the 1 and 2.

15. Thermal Isomerization slow fast Figure 9. Energy diagram of 1 estimated from the experiment and theoretical calculation. Structures shown were obtained from RHF/6-31G calculations: (a) values calculated theoretically and (b) experimental values.

16. Thermal Isomerization Inversion Mechanism Figure 10. Isokinetic plot for ΔＨ‡ - ΔＳ‡ of the cis-trans thermal isomerization of cyclic azobenzene dimers.

17. Conclusion • The authors have provided unambiguous proof for the photoinduced hinge-like molecular motion using the X-ray crystal structures of 1(t,t) (open) and 1(c,c) (closed) states. • The two azo linkages in 1(t,t) cooperatively isomerize to 1(c,c) via a short-lived 1(t,c) isomer. • The thermal isomerization of 1(t,c)-1(t,t) was found to be 10 00 000 times faster than 1(c,c)-1(t,c). • The lifetime of 1(c,c) was estimated to be 6.43 years. • The photochemical and thermal behavior of 2 is similar to several 2,2’-disubstituted azobenzenes. • Quantum chemical calculations predict a relatively small energy difference between 1(t,c) and 1(c,c). • The isokinetic plot revealed that the thermal isomerization of 2(c)-2(t) and 1(c,c)-1(t,c) followed inversion mechanisms, whereas the transition 1(t,c)-1(t,t) proceeded by a different mechanism. • The cyclic structure restricts the rotation of the phenyl units around the azo linkages, thus regulating the motion of the molecular device.