2 research outputs found
Tailoring the Properties of Surface-Immobilized Azobenzenes by Monolayer Dilution and Surface Curvature
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 <i>cis</i> to the original <i>trans</i> state within a short time
scale. These observations indicate cooperativity in the photoswitching
process of mixed SAMs
Formation of Carboxy- and Amide-Terminated Alkyl Monolayers on Silicon(111) Investigated by ATR-FTIR, XPS, and X‑ray Scattering: Construction of Photoswitchable Surfaces
We
have prepared high-quality, densely packed, self-assembled monolayers
(SAMs) of carboxy-terminated alkyl chains on Si(111). The samples
were made by thermal grafting of methyl undec-10-enoate under an inert
atmosphere and subsequent cleavage of the ester functionality to disclose
the carboxylic acid end-group. X-ray photoelectron spectroscopy (XPS)
and grazing incidence X-ray diffraction (GIXD) indicate a surface
coverage of about 50% of the initially H-terminated sites. In agreement,
GIXD implies a rectangular unit mesh of 6.65 and 7.68 Å side
lengths, containing two molecules in a regular zigzag-like substitution
pattern for the ester- and carboxy-terminated monolayer. Hydrolysis
of the remaining H–Si(111) bonds at the surface furnished HO–Si(111)
groups according to XPS and attenuated total reflection Fourier-transform
infrared spectroscopy (ATR-FTIR) studies. The amide-terminated alkyl
SAM on Si(111) assembled in a 2-(6-chloro-1<i>H</i>-benzotriazol-1-yl)-1,1,3,3-tetramethyluronium
hexafluorophosphate (HCTU)-mediated one-pot coupling reaction under
an inert atmosphere, whereby the active ester forms in situ prior
to the reaction with an amino-functionalized photoswitchable fulgimide.
ATR-FTIR and XPS studies of the fulgimide samples revealed closely
covered amide-terminated SAMs. Reversible photoswitching of the headgroup
was read out by applying XPS, ATR-FTIR, and difference absorption
spectra in the mid-IR. In XPS, we observed a reversible breathing
of the amide/imide C1s and N1s signals of the fulgimide. The results
demonstrate the general suitability of HCTU as a reagent for amide
couplings to carboxy-terminated alkyl SAMs and the on-chip functionalization
toward photoswitchable Si(111) surfaces