1. a Department of Electronic Chemistry, Tokyo Institute of Technology,
COMMUNICATION | ChemComm Light-driven molecular switches in azobenzene self-assembled monolayers: effect of molecular structure on reversible photoisomerization and stable cis state Mina Han,*a Daisuke Ishikawa,a Takumu Honda, a Eisuke Itob and Masahiko Hara*ab
a Department of Electronic Chemistry, Tokyo Institute of Technology,
4259 Nagatsuta, Midori-ku, Yokohama , Japan.
Fax: ; Tel:
b Flucto-Order Functions Research Team, RIKEN-HYU Collaboration
Research Center, RIKEN Advanced Science Institute, 2-1 Hirosawa,
Wako, Saitama , Japan

2. Azobenzene- Photochromism.
trans cis

3. It has been reported that densely packed azobenzene thiol self assembled monolayers (SAMs) are photochemically unreactive on flat gold surfaces.
The effects of free volume on the trans-to-cis photoconversion, the effects of molecular structure on the lifetime of cis-azobenzene in combination with reversible photoisomerization on solid surfaces have not yet been reported.

4. A single-component azobenzene substituted with sterically bulky ethyl groups at the ortho positions with respect to the azo group (Et-SH),which exhibits excellent reversible photoswitching properties as well as slow thermal cis-to-trans isomerization over a one-day period in SAMs. A comparison with Me-SH containing methyl group at the meta position has also been made to clarify the influence of free volume and molecular structure on both the trans-to-cis photoisomerizability and the lifetime of the cis form.

6. To examine reversible changes in the molecular conformation and the resulting spectroscopic features of azobenzene monolayers under UV and visible light irradiation, UV-vis absorption spectroscopy measurements were employed. (Fig. 2a).
Fig. 2 (a) UV-vis absorption spectral changes of Et-SH SAMs on a gold surface after alternating UV and visible light irradiation. Inset: Changes in absorbance at lmax obtained after each photoswitching process. (b) Difference spectra obtained after alternating UV and visible light irradiation in SAMs and in dichloromethane solution.
Difference spectra in SAMs obtained after alternating UV and visible light irradiation are in good agreement with those in solution (Fig. 2b), obviously indicative of excellent reversible nature of photoswitching between the trans and cis states on a gold surface.

7. In the case of UV-exposed Et-SH SAMs, slow thermal cis-to-trans isomerization proceeded over a one-day period (Fig. 3). These results clearly reflect that cis-azobenzene of ortho-diethylated Et-SH exhibits high stability even on a gold surface.
Fig. 3 Changes in (At - Aap) of Et-SH (triangles) and Me-SH (circles) SAMs as a function of thermal cis-to-trans isomerization time after UV light irradiation. Aap and At correspond to absorbance at Λmax of as-prepared SAMs and after dark incubation for time (t), respectively.

8. The Contact Angle measurements of as-deposited Et-SH SAMs for water on a flat gold substrate was 94±11°. Upon UV light irradiation, the CA decreased to 90±11°. Et-SH SAMs showed reversible photoswitching of surface wettability under UV and visible light irradiation.
Fig. 4 Reversible switching of contact angle for water of photoresponsive
Et-SH monolayer by UV and visible light irradiation.

9. Clarification of factors related to controlling the trans-to-cis photoisomerizability and stability of the　cis　form　in　azobenzene　monolayers: The occupied area for unsubstituted azobenzene is 0.24 nm It is enlarged up to: For　Et-SH　is　0.55nm For　Me-SH　is　0.50nm2 The critical free volume required for trans-to-cis photoisomerization in densely packed monolayers is 0.45nm2 per alkyl azobenzeneunit Both the low trans-to-cis conversion and fast thermal cis-to-trans isomerization of Me-SH SAMs is not primarily due to a lack of free volume, but rather due to molecular structure. The unsubstituted trans-azobenzene adopts a planar conformation, and that methyl substituents at the meta positions( Me-SH) have little influence on the planar structure of azobenzene. The substitution of two ethyl groups at the ortho positions(Et-SH) leads to a large distortion of the phenyl ring from coplanarity. Consequently, strong intermolecular interaction between azobenzene aromatic rings is suppressed and free volume is efficiently procured in Et-SH monolayers. Furthermore, two ethyl groups at the ortho positions would restrict large-scale distortion of the azo group for thermal cis-to-trans isomerization, thus resulting in stability of the cis state.

10. Conclusion: ~ The ortho-diethylated azobenzene molecule exhibits excellent reversible photoswitching. ~ The lifetime of the cis form is an important factor in controlling the trans-to-cis photoisomerizability. ~ Current work provides a strategy for developing new types of photoreactive surfaces potentially applicable to optical data storage and photoswitching devices.