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.
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
Molecular Devices
Feringa, B. L. et. Al. Nature, 2005, 437, 1337.
Stoddart, J. F. et. al. Angew. Chem., Int. Ed. 2000, 30, 3348
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.
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.
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
Previous Work
Xanethene-based cyclic
azobenzene dimer
Tamoaki, N. et. al. Org. Lett. 2004, 6, 2595.
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.
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%
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.
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.
NMR Spectral Changes
After irradiation
1(t,t)
80%
1(c,c)
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.
Absorption Spectra and Photochemical Isomerization
366 nm
366 nm
436 nm
2(t)
326 nm
395 nm
436 nm
2(c)
1(t,t)
1(c,c)
320 nm 365 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.
Cyclic Azobenzene Dimers
Figure 10. Isokinetic plot for ΔＨ‡ - ΔＳ‡ of the cis-trans thermal isomerization of cyclic azobenzene dimers.
Absorption Spectra and Photochemical Isomerization
366 nm
366 nm
436 nm
2(t)
436 nm
2(c)
1(t,t)
1(c,c)
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.
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.
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.
Thermal Isomerization
Inversion Mechanism
Figure 10. Isokinetic plot for ΔＨ‡ - ΔＳ‡ of the cis-trans thermal isomerization of cyclic azobenzene dimers.
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.

Photopharmacology: Optical Control of Activity

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