Realization of highly photoresponsive azobenzene-functionalized monolayers

Abstract

We report the successful fabrication of azobenzene-functionalized self-assembled monolayers (SAMs) exhibiting high and reversible photoswitching between trans and cis states on a flat gold surface. Azobenzene thiols (MeSH and EtSH) containing meta and/or ortho substituents were chosen based on the occupied area per molecule as well as intermolecular interactions between the azobenzene aromatic rings (formation of H-aggregates). Theoretical predictions of the geometrical structures were performed to clarify the correlation between the molecular structure and photoisomerization characteristics in monolayer systems. The striking difference in absorption spectra of a trans-EtSH SAM and a cis-EtSH SAM by alternating UV and visible light irradiation was in good agreement with that in their contact angles for water, strongly indicating that the structural changes were controlled by light wavelength. By contrast, despite there being sufficient free space for each MeSH molecule, the strong tendency of the planar azobenzene units to generate H-aggregates even during cis-MeSH SAM formation lessened the trans-to-cis photoisomerization yield in a monolayer.

UV/Vis Spectroscopy Studies of the Photoisomerization Kinetics in Self-Assembled Azobenzene-Containing Adlayers

“…In a density functional the- In contrast, the π-π * band observed in the difference spectra is not or only slightly blueshifted for the azobenzene-thiol SAMs. This is in agreement with previous studies of azobenzene-and azopyridine-thiols by the groups of Russell and Hara, where very similar shifts were found 39,48 , but at variance with the recent work of Weinelt and coworkers, which reported much larger shifts for a CF 3 -azobenzene-thiol SAM, namely 22 nm on Au films of 26 nm thickness and even more than 50 nm on thick Au(111) films on mica 42,46 .…”

“…Despite the resulting strong non-linear background, the π-π * transition can be clearly resolved, demonstrating that these SAMs of short-chain thiol bound azobenzene compounds can exhibit pronounced photoswitching. Very similar spectroscopic data, albeit with larger absorption changes, were obtained in the UV/VIS studies of azobenzene thiol derivative monolayers by the group of Hara21,39 . For better comparison of the spectroscopic…”

“…For multiple reasons UV/VIS spectroscopy is particularly suited for obtaining more insight into the factors affecting photoswitching in molecular adlayers: First, the strong absorbance of azobenzene allows sensitive detection of submonolayer amounts of these species, as demonstrated in previous UV/VIS studies 28,[36][37][38][39] . Second, it enables direct comparison of the behavior of the molecules in the SAM with that in gas phase or solution, for which it is a standard technique for probing photoisomerization.…”

Molecular Motions in Functional Self-Assembled Nanostructures

“…They all aim to laterally space photo-sensitive units from each other to enable a complete and reversible isomerization. Examples of these strategies include co-adsorption of photo-sensitive and inert building blocks [154,155], the use of bulky tripods as anchoring headgroups [156,157], adsorption on porous network [158,159], and addition of lateral group within building blocks to increase the occupied area per molecule [160,161]. …”

Dynamic simulations of stimuli-responsive switching of azobenzene derivatives in self-assembled monolayers: reactive rotation potential and switching functions

“…[30] Especially, orderly packed azobenzene derivatives, such as thiolated azobenzene are often used for surface decoration to build molecular devices and functional materials. [31][32][33][34][35][36][37] All these applications are benefited from the reversible conversion between two isomers, cis and trans, under the external stimuli. Because of steric hindrance of hydrogen atoms on the two benzene rings, the trans isomer is always more energetically favoured than the cis isomer.…”