Tailoring the Properties of Surface-Immobilized Azobenzenes by Monolayer Dilution and Surface Curvature

Abstract

Photoswitching in densely packed azobenzene self-assembled monolayers (SAMs) is strongly affected by steric constraints and excitonic coupling between neighboring chromophores. Therefore, control of the chromophore density is essential for enhancing and manipulating the photoisomerization yield. We systematically compare two methods to achieve this goal: First, we assemble monocomponent azobenzene-alkanethiolate SAMs on gold nanoparticles of varying size. Second, we form mixed SAMs of azobenzene-alkanethiolates and "dummy" alkanethiolates on planar substrates. Both methods lead to a gradual decrease of the chromophore density and enable efficient photoswitching with low-power light sources. X-ray spectroscopy reveals that coadsorption from solution yields mixtures with tunable composition. The orientation of the chromophores with respect to the surface normal changes from a tilted to an upright position with increasing azobenzene density. For both systems, optical spectroscopy reveals a pronounced excitonic shift that increases with the chromophore density. In spite of exciting the optical transition of the monomer, the main spectral change in mixed SAMs occurs in the excitonic band. In addition, the photoisomerization yield decreases only slightly by increasing the azobenzene-alkanethiolate density, and we observed photoswitching even with minor dilutions. Unlike in solution, azobenzene in the planar SAM can be switched back almost completely by optical excitation from the cis to the original trans state within a short time scale. These observations indicate cooperativity in the photoswitching process of mixed SAMs.

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

“…According to a very recent study by Weinelt and coworkers for similar (but longer chain) azobenzene-thiols the TDM of the π-π * transition is oriented at an angle of ≈ 30 • with respect to the surface normal 46 . The ∆Abs values calculated using these parameters are by a factor of 4-7 larger than those measured experimentally (see Table 1).…”

Optical properties of azobenzene-functionalized self-assembled monolayers: Intermolecular coupling and many-body interactions

“…For a dilution up to 20% of a densely-packed azobenzene SAM the average tilt angle of the aromatic plane is about 45 • . 21 In our first-principles calculations, azobenzenefunctionalized SAMs are modeled by a two-dimensional crystal structure, where the alkyl chains and the gold surface are not considered. This choice is motivated by the fact that the alkyl chains do not contribute significantly to the optical absorption at photon energies below 6 eV, and that the azobenzene moieties are expected to be decoupled from the gold substrate by the alkyl chains.…”

“…A SAM is prepared by immersing a gold substrate into a methanolic solution of the thiols for 20 hours, as detailed in Ref. 21. Depending on the relative concentrations of Az11 and C12 in solution, SAMs with different chromphore densities are obtained.…”

“…Additional experimental details are reported in Ref. 21. For a direct comparison between theoretical and experimental data, we calculate DR spectra of azobenzene-functionalized SAMs on gold by employing the LayerOptics code.…”

“…13,14 A number of experimental strategies have been suggested to overcome this problem, such as replacing the aliphatic linker by an aromatic one 15 or even bulkier groups, [16][17][18] or diluting the azobenzene moieties in densely-packed alkanethiolate SAMs. 14,[19][20][21][22] From a theoretical viewpoint the issue of hindered photo-isomerization in densely-packed azobenzenefunctionalized SAMs has been addressed by a few works, mainly focused on the excitonic coupling between the chromophores, 23,24 as well as on the effects of a metal substrate. 25 While these investigations have significantly contributed to describe the excited-state properties of azobenzene derivatives, a very recent publication elucidates the role of defects in the photo-isomerization of such SAMs.…”