Improving the Switching Capacity of Glyco-Self-Assembled Monolayers on Au(111)

A Journal of

Accepted Article

Title: Improving the Switching Capacity of Glyco-SAMs on Au(111)

Authors: Felix Tuczek, Thisbe Lindhorst, Ellen Fast, Alexander

Schlimm, Irene Lautenschläger, Kai Uwe Clausen, Thomas

Strunskus, and Carina Spormann

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To be cited as: Chem. Eur. J. 10.1002/chem.201903644

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Improving the Switching Capacity of Glyco-SAMs on Au(111)

Ellen Fast^[a]#, Alexander Schlimm^[b]#, Irene Lautenschläger^[b], Kai Uwe Clausen^[b], Thomas Strunskus^[c],

Carina Spormann^[a], Thisbe K. Lindhorst^[a]*, Felix Tuczek^[b]*

Abstract: Self-assembled monolayers (SAMs) decorated with

photoisomerizable azobenzene glycosides are useful tools to

investigate the effect of ligand orientation on carbohydrate

recognition. However, photoswitching of SAMs between two specific

states is characterized by a limited capacity. The goal of this study is

the improvement of photoswitchable azobenzene glyco-SAMs.

Different concepts, in particular self-dilution and rigid biaryl

backbones, were investigated. The required SH-functionalized

azobenzene glycoconjugates were synthesized following a modular

approach, the respective glyco-SAMs were fabricated on Au(111)

and their photoswitching properties extensively investigated with a

powerful set of methods (IRRAS, XPS and NEXAFS). Indeed, the

combination of tailor-made biaryl-azobenzene glycosides and

suitable diluter molecules led to photoswitchable glyco-SAMs with a

significantly enhanced and unprecedented switching capacity.

projecting from the bacterial surface, called fimbriae (or pili).^[3]

Among the most important bacterial fimbriae are so-called

type 1 fimbriae, which mediate adhesion to terminal α-D-

mannopyranoside components of the glycocalyx through the

type 1-fimbrial lectin FimH.^[4]

Many principles of the highly complex supramolecular

interactions leading to cell adhesion are still not well understood.

Therefore, model systems, in particular so-called glyco-SAMs

(self-assembled monolayers),^[5]were used to mimic the

glycocalyx and to allow the investigation and interrogation of

carbohydrate-protein interactions on surfaces. As a first step into

this direction, we prepared glyco-SAMs derived from molecules

composed of an azobenzene moiety, an alkanethiol linker and a

mannoside head group and investigated them by XPS (X-ray

photoelectron spectroscopy), NEXAFS (near-edge X-ray

absorption fine structure) and IRRAS (infrared reflection-

absorption spectroscopy).^[6]Here, photoisomerization of the

azobenzene N=N double bond between two isomeric states

(trans and cis) allows the reversible reorientation of a sugar

ligand, if the molecule is properly adsorbed on a surface. Indeed,

when conjugated to an alkanethiol chain, azobenzene glycosides

self-assemble to monolayers on gold (Au(111)). Utilizing the

intensity change of the C(aryl)-O(mannoside) stretching band

upon cis/trans-isomerization (E/Z isomerization), we were in fact

able to monitor a reversible, photoinduced switching of the

orientation of the head group. Nevertheless, the observed

intensity change was small (about 4 %). More recently, we

showed the importance of ligand orientation in carbohydrate-

specific bacterial adhesion using a SAM of azobenzene

glycosides containing OEG (oligoethylene) moieties.^[7]When we

applied this system to bacterial adhesion, the adhesion of type

1-fimbriated E. coli was greatly reduced in the cis- in comparison

to the trans-state. Remarkably, photoswitching of azobenzene

glycoconjugates also alters bacterial adhesion on cell surfaces.^[8]

Regarding the actual performance of photoswitchable glyco-

SAMs, it is important to note that the trans→cis isomerization of

the azobenzene moiety is associated with a large spatial change

and, therefore, the molecules on the surface require enough free

volume to undergo the isomerization. This, however, is in direct

conflict with the nature of most SAMs which consist of densely

packed molecules.^[9]Correspondingly, in our first study

employing glycoazobenzene alkanethiols without an OEG group,

less bulky alkanethiols were needed as diluter molecules to

achieve trans→cis isomerization of the glycoazobenzenes in

SAMs (Figure 1, a).^[6]On the other hand, when we switched

bacterial adhesion, the employed SAMs containing OEG

moieties and no further diluter molecules were used for the

fabrication of the photoswitchable SAMs.^[7]The exact influence

of the OEG groups on the switching properties of the respective

glyco-SAMs is unclear.

Introduction

Many biological processes such as cell signalling, cell

recognition, or cell adhesion are mediated by molecular

interactions occurring at the cell surface, which is covered by a

layer of diverse glycoconjugates, called the glycocalyx.^[1]

Therefore, the elucidation of the mechanisms, which underlie

carbohydrate recognition, is a key to our understanding of cell

surface biology. Carbohydrates are ligands of a class of proteins,

called lectins,^[2]which apparently govern much of cell-cell

interactions by specific carbohydrate-lectin interactions. For

example, many Enterobacteriaceae such as Escherichia coli (E.

coli) accomplish firm adhesion to the surface of their host cells

by lectins, which are constituents of adhesive organelles

# These authors contributed equally to this paper.

[a]

Ellen Fast, Carina Spormann, Thisbe K. Lindhorst

Otto Diels Institute of Organic Chemistry

Christian-Albrechts-University Kiel

Otto-Hahn-Platz 4, 24118 Kiel

tklind@oc.uni-kiel.de

[b]

Alexander Schlimm, Irene Lautenschläger, Kai Uwe Clausen, Felix

Tuczek

Institute of Inorganic Chemistry

Christian-Albrechts-University Kiel

Max-Eyth Straße 2, 24118 Kiel

ftuczek@ac.uni-kiel.de

[c]

Thomas Strunskus

Institute for Materials Science – Multicomponent Materials

Christian-Albrechts-University Kiel

Kaisertr. 2, 24143

Supporting information for this article is given via a link at the end of

the document.

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mannoside was selected as photoswitchable carbohydrate

ligand, as in our earlier work.^[6,7]Regarding the self-diluting

approach, the 6-position of the mannopyranoside ring forms an

ideal attachment point for a bulky protecting group. For the

fabrication of rigid SAMs, aryl-aryl cross coupling reactions were

employed to conjugate azobenzene α-D-mannoside to a rigid

backbone. The chemical composition and the integrity of the

prepared SAMs were determined by XPS, and their switching

capacities were investigated using a combination of IRRAS and

NEXAFS.

Results and Discussion

1. Synthesis

Figure 1. Three different approaches of photoswitchable glyco-SAMs,

fabricated from azobenzene glycoconjugates: a) Cis→trans isomerization is

facilitated by diluter molecules; b) concept of self-dilution; c) photoswitching of

rigid (biphenyl-containing) SAMs. The trans (E)-state of the azobenzene hinge

is shown in blue, the cis (Z)-state in red, the green sugar residue symbolizes

α-D-mannopyranoside, and the grey oval a bulky protecting group.

For the fabrication of self-diluting SAMs (cf. Figure 1, b)

glycoazobenzene alkanethiols are required, carrying a bulky

protecting group which can be cleaved under mild conditions on

the SAM surface. The primary hydroxy group of the sugar moiety

is a practical position for the installation of a bulky moiety as it is

amenable to a regioselective reaction. In first attempts, a trityl

group was selected for the modification of 6-OH, however, it

caused problems in later steps of the synthesis. Then, a tert-

butyldiphenylsilyl (TBDPS) ether was chosen for the protection

of the 6-position of the sugar, which can be cleaved under mild

acidic conditions or by fluoride ions. This approach works very

well even with free D-mannose, where the regioselective

silylation of the 6-hydroxy group and a subsequent acetylation

step, in order to protect the secondary OH-groups, furnished the

mannose tetraacetate 2 in 74 % over 2 steps (Scheme 1). This

derivative was then converted into a mannosyl donor, first by

selective deprotection of the anomeric position employing

ethylenediamine in a mixture with acetic acid^[17]and secondly by

In order to further improve the properties of photoswitchable

glyco-SAMs, it is essential to understand the dependence of the

switching capacity (i.e., the fraction of molecules undergoing

photoinduced cis/trans-isomerization) on the physicochemical

properties of the head groups and the underlying SAM. Key

parameters in this regard are the rigidity of the chains contained

in the molecules forming the SAM, their lateral density as well as

their intermolecular interactions, and the free volume required for

the switching of the head groups.^[10,11]A first approach to

providing sufficient free volume for photoswitching was based on

the application of non-planar substrates like nanoparticles.

Because of the surface curvature of these particles, the

adsorbed molecules have significantly more free volume than on

the base-promoted addition of the reducing sugar

3 to

a

flat surface.^[10,12]Other concepts also allow effective

trichloroacetonitrile to yield the O-mannosyl trichloroacetimidate

4,^[18]carrying the requested bulky protecting group at C-6.

In the following mannosylation step, the hydroxy azobenzene

derivative 5 served as glycosyl acceptor. It carries the aliphatic

6-acetylthio-hexanyl linker, which is required for the fabrication

of SAMs. It was obtained by selective nucleophilic substitution of

6-acetylthio-1-hexanesulfonate with dihydroxyazobenzene (cf.

SI). The Lewis acid-promoted reaction of 4 with 5 under standard

conditions^[19]gave the desired mannoside 6 in excellent yield and

as sole α-anomer. Treatment of the protected mannoside 6 with

sodium methanolate in methanol to liberate both the OH-groups

and the thiol group gave 7 in 90 %. When this thiol is freshly

prepared as for the fabrication of SAMs, no disulphide oxidation

products are present according to NMR and MS analysis. In

order to test the feasibility of the silyl ether cleavage, 7 was

submitted to standard deprotection conditions using TBAF

(Scheme 1) to quantitatively deliver the OH-free mannoside 8

(for ¹H-NMR cf. SI).

photoisomerization on flat surfaces, such as the platform

approach,^[13,14]where the size of a ring system which is adsorbed

to a gold surface determines the intermolecular distances of the

photoswitchable molecules mounted on this platform. This

approach can be turned up-side down using bulky protecting

groups, which provide space between the chemisorbed

molecules during SAM formation and can be cleaved on the

surface after the adsorption process. Lahann et al. showed that

such SAMs are stable even after the deprotection step.^[15]This

approach can be termed a self-diluting process (Figure 1, b). On

the contrary, SAMs can also be fabricated from molecules based

on rigid biphenyl backbones leading to especially densely

packed monolayers due to strong intermolecular  interactions

(Figure 1, c). Counterintuitively, such SAMs, consisting of

azobenzene-biphenyl thiols for example, show excellent

switching properties on surfaces.^[16]Pace et al. attributed these

results to a cooperative character of the switching process.^[16]

Herein, we adapt both of these approaches towards effective

photoisomerization of azobenzene-containing SAMs (i.e., Figure

1 b and c)^[15,16]to the design of new azobenzene glyco-SAMs, in

order to investigate whether we find an improved switching

behavior. Because of its biological importance, azobenzene α-D-

In view of the high yields and selectivities of the described

reactions, the thiohexyl-modified azobenzene mannoside 7

appears as an ideal molecule for testing the concept of self-

dilution to facilitate photoisomerization of SAMs (see below).

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Scheme 1. Synthesis of the 6-O-silylated mannoside 7 for fabrication of self-diluting glyco-SAMs. Reagents and conditions: a) TBDPS-Cl, DMAP, pyridine, 16 h, rt;

b) Ac₂O, pyridine, 16 h, rt; c) H₂N(CH₂)₂NH₂, H₃CCO₂H, THF, rt, 16 h; d) Cl₃CCN, DBU, CH₂Cl₂, 0°C → rt, 2 h; e) BF₃·OEt₂, CH₂Cl₂, 0°C → rt, 2 h; f) MeOH, NaOMe,

rt, 1 h; g) THF, TBAF, 4 h rt. TBDPS: tert-butyldiphenylsilyl; DMAP: 4-dimethylaminopyridine; DBU: 1,8-diazabicyclo[5.4.0]undec-7-ene; TBAF: tetrabutylammonium

fluoride.

For the fabrication of rigid SAMs, the molecules depicted in

Figure 2 were targeted. The two mannosides differ in the

arrangement of the biphenyl and the azo moiety relative to the

sugar head group. A third azobenzene derivative was required

as less bulky, though photoswitchable diluter molecule (see

below). Figure 2 exemplifies the employed modular synthetic

approach, where in each case complementary aryl iodides and

The photoswitchable mannoside 14 (Scheme 2) was selected as

first target molecule for the rigid SAM approach. For its synthesis,

commercially available iodo hydroxyazobenzene 9 was reacted

with dimethylthiocarbamoyl chloride and DABCO as non

nucleophilic base to give 10 in very good 82 % yield. In the

following Newman-Kwart rearrangement, the S-thiocarbamate

11 was formed under solvent-free conditions at 200 °C in almost

quantitative yield (97 %).

arylboronic esters could be cross-coupled in

a Suzuki

reaction.^[20]Furthermore, it was necessary to select an

appropriate method to furnish the free arylthiol functional groups.

It turned out that the Newman-Kwart rearrangement could be

successfully employed, in which O-thiocarbamates are thermally

rearranged to S-thiocarbamates by intramolecular aryl migration.

The latter can be readily hydrolysed under basic conditions to

deliver the required thiophenols.

Scheme 2. Synthesis of the rigid p-mannosyloxyphenyl-azobenzene derivative

14. Reagents and conditions: a) DABCO, dimethylthiocarbamoyl chloride,

DMF, 5 h, 70 °C; b) 200 °C, 3 h; c), K₂CO₃, TBABr, Pd(PPh₃)₄, toluene, H₂O,

4 h, 80 °C; d), KOH, MeOH, 16 h, rt. DABCO: 1,4-diazabicyclo[2.2.2]octane;

TBABr: tetrabutylammonium bromide.

Figure 2. A modular synthesis delivers the target molecules (right) for the

fabrication of rigid SAMs, employing the respective aryl iodides and arylboronic

esters (boxed on the left) in Suzuki cross-coupling. S-Thiocarbamates were

derived from the respective O-thiocarbamates by Newman-Kwart

rearrangement and served as precursors of the required aromatic thiols.

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In the penultimate step of the synthesis, the iodoazobenzene

11 was subjected to a Suzuki coupling with the literature-known

boronic ester 12^[21]in order to introduce the α-D-

mannopyranoside head group. This palladium-catalysed

reaction was performed in a biphasic solvent mixture with

tetrabutylammonium bromide (TBABr) as phase-transfer catalyst

and delivered the protected mannoside 13 in a moderate yield of

32 %. The final deprotection step under basic conditions

removed the O-acetyl protecting groups and delivered the free

arylthiol at the same time to furnish the target molecule 14.

As an alternative to 14 exhibiting a biphenyl spacer “above”

the N=N group, mannoside 20 was synthesized, where the

biphenyl spacer is positioned “below” the N=N moiety. To this

end, the S-thiocarbamate 15^[22]and the mannosyloxy-

azobenzeneboronate ester 16 or 17, respectively, were

submitted to the Suzuki reaction to yield the desired cross-

coupling products, 18 or 19, respectively, in fair yields. The

boronates carrying different sugar protecting groups were

obtained from the respective literature-known aryl iodides in high

yield (cf. SI). Here, the chosen synthetic route provides an

advantageous flexibility with respect to the variation of the sugar

head group.

diluter molecule (Scheme 4). Included into SAMs, it can provide

space for the mannose head groups but at the same time it can

exhibit π-π-interactions in the backbone of the SAM. Because

we were not satisfied with the yields obtained in the Suzuki cross-

coupling reactions, we tried various catalysts to improve the

yields of this reaction. For the synthesis of 24, bis(2-amino-4,6-

dihydroxypyrimidine)palladium(II)diacetate was synthesized

from palladium acetate^[23]and used for the coupling of 22 and

23.^[24]Indeed, this gave the biphenyl azobenzene 24 almost

quantitively.^[25]

Unfortunately, the same catalyst and reaction conditions did

not lead to improvement of the yields for the mannoside

analogues (cf. Schemes 2 and 3). After reaction of 24 with

dimethylthiocarbamoyl chloride and sodium hydride, the O-

thiocarbamate 25 was obtained and subsequently heated for 15

minutes at 270°C in a Kugelrohr distillation apparatus to give the

rearranged S-thiocarbamate 26 in a good yield. In this case, the

rearrangement step was sensitive as longer reaction times

resulted in low yields or even decomposition. As before, the free

SH-group was furnished with an excess of KOH to give the diluter

molecule 27, to be compared with commercially available 4-

biphenylthiol (28) (see below).

The Newman-Kwart rearrangement delivers the target

mannoside 20 with the free arylthiol group starting from the O-

acetylated analogue 18. The O-allylated derivative 19 leads to

21 under the same reaction conditions (Scheme 3). We were

interested to investigate the influence of the sugar protecting

groups on the photoswitchability of SAMS fabricated form these

molecules. The OH-unprotected mannosides are rather

hydrophilic in comparison to their protected analogues, where O-

acetylation leads to more electron-deficient glycosides in

contrary to their O-allylated analogues.

Scheme 4. Diluter molecules 27 and 28 for fabrication of mixed SAMs.

Reagents and conditions for the synthesis of 27: a) bis(2-amino-4,6-

dihydroxypyrimidine)palladium(II)diacetate, MeOH, Na₂HPO₄, 2 h, 60 °C; b)

NaH, dimethylthiocarbamoyl chloride, DMF, 24 h, 98 °C, c), 270 °C, 15 min; d)

KOH, MeOH, 2 h 70 °C.

Prior to SAM fabrication and the investigation of their

photoswitchability, the switching properties of the synthesized

compounds were investigated in homogeneous solution (cf. SI).

The photostationary states (PSS) and the half-lives associated

with the thermal cis→trans relaxation were determined by NMR

spectroscopy (cf. Table S12). As the investigated azobenzene

derivatives are soluble in different solvents, the comparability of

the results is somewhat limited, but clearly all tested compounds,

7, 8, 14, 20, 21, and 27, can reversibly be photoswitched

between their cis- and trans-forms in homogeneous solution. The

determined half-lives of thermal cis→trans relaxation are in a

usual range (12 to 15 h), with the exception of 20 and 27 which

exhibit faster cis→trans relaxation (half-lives < 2 h).

Scheme 3. Synthesis of the rigid p-mannosyloxy-p’-phenyl-azobenzene

derivatives 20 and 21. Reagents and conditions: a) Pd(PPh₃)₄, K₂CO₃, TBABr,

toluene, H₂O, 16 h, 90°C; b) KOH, MeOH, 1-2 h, reflux. TBABr:

tetrabutylammonium bromide.

In order to modify SAMs formed from the p-mannosyloxy-p’-

phenyl-azobenzene derivatives 20 as mixed SAMs, the

azobenzene derivative 27 was prepared as photoswitchable

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one doublet at 161.9 eV and 163.1 eV (Figure 3, c). The binding

energy and the splitting of 1.2 eV are characteristic for a thiolate

moiety.^[28–30]The presence of this species proves the covalent

attachment of the molecules to the gold surface. The Si 2p region

shows one doublet (101.0 eV and 101.7 eV) as well, which can

be assigned to the silicon atom of the protection group (Figure 3,

d).

Further information on the constitution of the SAM of 7 on

Au(111) is provided by NEXAFS recorded at the carbon and

nitrogen K-edges (see SI, Figure S6). The nitrogen K-edge

NEXAFS shows a characteristic resonance at 399.1 eV. The

energy of this feature corresponds well to the N 1s-to-*-

transition reported in the literature for azobenzene units.^[27]

Angular-dependent measurements (SI, Fig. S6) show that the

intensity of this resonance increases from the 30° the 90°

spectrum. This allows determining the orientation of the molecule

on the surface.^[27]

2. Surface spectroscopic investigation of the self-

diluting glyco-SAMs

First, the concept of self-diluting SAMs was evaluated. To this

end, the azobenzene mannoside 7 carrying the bulky TBDPS

protecting group at C-6 was employed. A SAM of this molecule

was deposited on Au(111) by immersing a gold substrate into a

solution of 7 in methanol. The resulting monolayer was

investigated using a range of surface-spectroscopic and surface-

analytical methods. For comparison, a SAM of the azobenzene

mannoside 8, the deprotected analogue of 7, was also prepared

and investigated by the same methods.

X-ray photoelectron spectroscopy and near-edge X-ray

absorption fine structure spectroscopy

In order to check the integrity and purity of the adsorbed

molecular layer of 7, X-ray photoelectron spectroscopy (XPS)

was employed. The resulting spectra are shown in Figure 3. The

C 1s region contains four different signals (Figure 3, a). The main

component at 284.7 eV (red) corresponds to the carbon atoms

bound to other carbon, hydrogen, silicon and sulfur atoms. The

second species (blue) at 285.3 eV is associated with carbon

atoms bound to nitrogen atoms. The signals at higher binding

energies (286.4 eV and 288.0 eV), finally, correspond to carbon

atoms bound to oxygen. Thereby the green species is assigned

to the carbon atoms bound to only one oxygen and the orange

species to the anomeric carbon bound to two oxygens.

Importantly, the relative contributions of the different species fit

exactly to the ratio (75:5:18:2) derived from the chemical

composition of the azobenzene mannoside.

Figure 4. Definition of the NEXAFS and IRRAS transition dipole moments

(TDM) and angles for the determination of the molecular orientation. The two

different orientations A and B of the TDM of the C_aryl-O stretching vibration in

the cis-isomer are shown. The average over all angles of the rotation around

the molecule axis is referred to as orientation C.

The tilt angle α of the N 1s-to-* transition dipole moment

relative to the surface normal amounts to 58±2°. Since, for the

trans-azobenzene this transition dipole moment is perpendicular

to the molecular axis, the azobenzene shows a molecular tilt

angle β of 32 ±2° to the surface normal, respectively (90-

see Figure 4. This is in good agreement with the tilt angles of

other alkyl-SAMs in the literature.^[31]The carbon K-edge

NEXAFS exhibits several resonances. The most prominent π*-

resonance at 285.5 eV shows a weak angular dependence, its

intensity decreasing to lower angles of incidence. This result is

in accordance with the N-NEXAFS data (see SI, Figure S6).

Figure 3. X-ray photoelectron spectroscopy data of compound 7 adsorbed on

Au(111). Four different regions are shown: C 1s (a), N 1s (b), S 2p (c), and Si

2p (d).

In the N 1s region only one signal at 399.5 eV is observed,

which corresponds to the nitrogen atoms of the azo group

(Figure 3, b).^[6,26,27]The S 2p region, on the other hand, shows

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Figure 5. Vibrational data of compound 7 and 8. a) Deprotection of the 7-SAM with KF leads to the 7-SAM^deprot. b) This display shows the reversibility of the

cis/trans-isomerization of the 7-SAM^deprotby means of the C_aryl-O-stretch intensity after irradiation with light of 365 nm or 440 nm c) The measured bulk IR of

compound 7 before and after the deprotection (8) and the IRRA spectra of compound 7 in a monolayer on Au(111) before and after the cleavage of the TBDPS

group (7-SAM^deprot). d) Comparison of the switching behavior before (7-SAM, grey) and after (7-SAM^deprot, black) the cleavage of the TBDPS group.

the surface IR spectrum because of the surface selection rule.^[32]

Infrared reflection absorption spectroscopy

The C_aryl-O vibration thus is well suited to monitor the cis/trans-

isomerization of the glyco-SAMs adsorbed on Au(111) with

IRRAS.^[6]

The SAM of 7 (Figure 5, a) and its switching behavior were

further investigated employing infrared-reflection-absorption-

spectroscopy (IRRAS). Intensities of around 10^-3absorbance

units are compatible with the formation of a monolayer on

Au(111) (Figure 5, c).^[14]Notably, the different relative intensities

in the measured bulk and surface IR spectrum, in particular in

the regions around 1600 and 1200 cm^-1, are caused by the

surface selection rule.^[32]Such differences are a first hint for a

well-organized monolayer and a preferred orientation of the

molecules on the surface.

The measured IR spectra were compared to calculated

bulk IR spectra in order to designate the bands to specific

vibrational modes. The IRRA spectrum of the 7-SAM shows

several significant bands in the fingerprint region. The bands

around 1600 cm^-1correspond to C_aryl-C_arylstretching vibrations of

the azobenzene unit. A combination of the N=N stretching and

aromatic C-C stretching vibration is located at 1498 cm^-1. The

most prominent vibration at 1247 cm^-1can be assigned to the

C_aryl-O stretching vibration of the glycosidic oxygen. Importantly,

the transition dipole moment of this vibrational mode is parallel

to the axis of the azobenzene unit in the trans-state. Therefore,

the transition dipole moment of this vibration shows the same

orientation with respect to surface normal as the azobenzene

unit in the SAM. Moreover, the orientation of this vibration

changes during the isomerization of the azobenzene N=N double

bond. This, in turn, directly influences the intensity of the band in

In order to detect the isomerization process with high

sensitivity PM-IRRAS (polarization-modulated-IRRAS) in the

spectral range of the C_aryl-O vibration was applied. Due to the

presence of a small shoulder in the investigated IR band (C_aryl-O

stretching vibration) at lower wavenumbers, the band was fitted

with two Gaussians to determine the intensity of the pure C_aryl-O

band (see SI, Figure S14-S19). The second band corresponds

to a C-H bending vibration within the mannose moiety. This

vibration is a combination of different vibrational modes.

Moreover, the mannose moiety can rotate around the anomeric

C-O bond. For this reason, the intensity of this band is barely

affected by the isomerization process and can be considered as

constant. The change of the C_aryl-O bond orientation was

calculated by geometry optimizations of the cis- and trans-isomer

(see SI, Figure S22). For the cis-isomer a CNNC angle (C_aryl

-

N_azo=N_azo-C_aryl) of 69.4° was obtained while it is 179.9° for the

trans-isomer. Thus CNNC angle change γ amounts to 110.5°

during the isomerization reaction (cf Figure 4), a result which fits

well to data of pure azobenzenes reported in the literature.^[33]

Nevertheless, only small changes (ΔI_exp.=2 %) of the C_aryl-O

stretch intensity are observed upon irradiation of 7-SAM

adsorbed on gold with light of 365 nm (Figure 5, d; see SI, Figure

S16).

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Taking the tilt angle β determined by NEXAFS into account,

the angle φ between the transition dipole moment of the C_aryl-O

stretching vibration and the surface normal can be determined.

However, whereas for trans φ_trans=β, there are several possibili-

ties for cis; i.e., the azo group can isomerize towards the surface

(“into” the SAM, A) or into a configuration more parallel to the

surface (“onto” the SAM, B; φ(A)_cis=180-γ-β, φ(B)_cis=γ-β; Figure

4). Moreover, an equal distribution of all transition-dipole orienta-

tions within these two extremes; i.e., within a corresponding cone

around the molecular axis, is theoretically possible (Figure 4, C).

In this case, a rotational averaging over all possible azimuthal

angles has to be performed (see Experimental section). Although,

due to steric reasons, a preferential orientation of the head group

(e.g., B) might exist in the cis-configuration of the investigated

glyco-SAMs, this question cannot be decided without further

information. In view of this situation we thus compare ΔI_expvalues

based on IRRAS data with ΔI_theovalues obtained from orientation

A (head group towards the surface), B (head group approx.

parallel to the surface) and C (rotational average) in Table 1.

Based on C, an intensity decrease of ΔI_theo=71 % is predicted if

all molecules in the 7-SAM switch to the cis-isomer. The

observed intensity change (ΔI_exp=2 %) thus would indicate that

only 3% of molecules switch to the cis-isomer (Table 1). Taking

the steric demand of the TBDPS group into account, this result

is consistent with previous studies^[6]on pure glyco-SAMs.

In order to increase the free volume for the trans→cis

isomerization of surface-adsorbed compound 7 the protection

group was cleaved off by immersing the functionalized substrate

into a solution of potassium fluoride in methanol for two days

(Figure 5, a). Resulting 7-SAM^deprot(after on-surface cleavage of

the TBDPS group) was again investigated by XPS, NEXAFS and

IRRAS to evaluate its switching behavior. Except for small

changes in intensity, the surface IR spectrum, shown in Figure 5

(c), exhibits no large differences between the protected and

deprotected SAM of compound 7 on Au(111). This result is

compatible with the similarity of the bulk spectrum of 7 and the

bulk spectrum of 8 obtained from deprotection of 7 in

homogeneous solution (Figure 5, c).

In contrast to the vibrational spectra, the XPS data (see SI,

Figure S1) clearly reflect the successful deprotection reaction on

the surface; i.e., the signal in the Si 2p region becomes

significantly smaller. These data suggest that most of the TBDPS

groups were in fact cleaved by the treatment of the functionalized

substrate with potassium fluoride. At the same time the SAM

stayed intact according to the S 2p spectrum, which even after

on-SAM deprotection (see SI, Figure S1) still prominently

exhibits the thiolate species. The orientation of 7-SAM^deprotwas

again elucidated by NEXAFS. Remarkably, after the on-surface

deprotection the tilt angle β still amounts to 33±2° (ΔI_theo=69 %,

based on orientation C), very similar to the original value (see SI,

Figure S12). This underscores the conformational stability of 7-

SAM, which maintains its original orientation even after cleavage

of the TBDPS groups on SAM.

After deprotection, the intensity change due to the trans/cis-

isomerization increases significantly (ΔI_exp=5 %; Figure 5, b and

d). Based on the rotationally averaged cis-configuration C, this

intensity change indicates that 7 % of the molecules on the

surface switch to the cis-isomer, corresponding to an increase of

the switching capability of more than 100 % with respect to the

original 7-SAM. The performance of the self-diluting SAM thus is

superior to results of alkyl-based glyco-SAMs, the maximum

switching capacity of which (using a diluter compound) was

4 %.^[6]In Figure 5 (b, d) it is also shown that the cis/trans-

isomerization is fully reversible. By alternating irradiation with

light of 440 nm and 365 nm it was possible to switch the glyco-

SAM back and forth with a constant intensity change of ΔI_exp=5 %

(see SI, Figure S17). Interestingly, if compound

7 was

deprotected in solution and subsequently deposited on the

surface, the resulting 8-SAM (deprotection in solution before

surface deposition) showed only very weak changes (ΔI_exp.<1 %)

in the intensity of the C_aryl-O-vibration after irradiation with light

of 365 nm (see SI, Figure S20). This indicates that the

improvement of the switching behavior is only observed if the

TBDPS protection group is cleaved on the surface and

underlines the feasibility of the self-dilution concept towards

photoswitchable SAMs.

Table 1. Comparison of the switching behavior of the investigated glyco-SAMs. β is the tilt angle with respect to the surface normal. The angle φ describes the

orientation of the transition dipole moment of the C-O_arylstretching vibration in the cis-isomer with respect to the surface normal. A and B describe the two

extrema from Figure 4. The value ΔI_theocorresponds to the expected maximum intensity change of the C-O_arylstretching vibration considering β and the spatial

change during the isomerization in dependence of the orientation of the cis-isomer within the glyco-SAM (A: head group downwards; B: head group upwards; C:

average). ΔI_expis the experimentally (IRRAS) obtained intensity change. By means of these values the switching capacity was calculated.

SAM

Tilt angle (β)

φ(A)_cis

φ(B)_cisΔI_theo

ΔI_theo

Experimental Switching

of capacity

(ΔI_exp

ΔI_theo

Reversibility

based on based on based on change

orientation

A

orientation

B

averaged

orientation

C

intensity

/

(ΔI_exp

)

based on C

3 %

7-SAM

7-SAM^deprot

32±2°

33±2°

---

38±2°

37±2°

---

79±2°

78±2°

---

14 %

11 %

---

94 %

---

71 %

69 %

---

2 %

yes

---

5 %

7 %

8-SAM

<1 %

27 %

38 %

18 %

9 %

---

20-SAM

23±6°

22±2°

24±3°

24±5°

29±3°

47±6°

48±2°

45±3°

45±5°

40±3°

88±6°

89±2°

87±3°

87±5°

81±3°

45 %

47 %

41 %

24 %

99 %

97 %

80 %

78 %

73 %

34 %

48 %

23 %

12 %

7 %

no

20,27-SAM

20,28-SAM

21-SAM

yes

no

14-SAM

5 %

[

^deprot] = on-surface cleavage of the TBDPS group.

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Figure 6. XP spectra and normalized NEXAFS spectra at different angles of 20-SAM adsorbed on Au(111). The C 1s (a), S 2p (b) and N 1s (c) are shown. The

baseline is shown in black, the measured data are represented by black dots and the fitted sum spectrum is illustrated by the magenta line. The C K-edge spectrum

(d) and the N K-edge spectrum (e) are shown. The difference spectra (90°-30°) are shown in red.

SAM and furthermore proves the covalent attachment of the

molecule to the surface. Also the N 1s spectrum shows only one

component (100 %, blue) at 399.6 eV which corresponds to the

3. Surface spectroscopic investigation of rigid glyco-

azo moiety of the 20-SAM (Figure 6, c).^[6,26,27]In conclusion, the

SAMs

XP spectra confirm the presence of a monolayer of compound

20 (20-SAM) in high purity.

For the fabrication of rigid glyco-SAMs, the azobenzene

mannosides 14, 20, and 21 were employed. They were

Near-edge X-ray absorption fine structure spectroscopy

deposited on Au(111) from a methanol/acetone (95:5) solution,

and the resulting monolayers on Au(111) were investigated with

XPS, NEXAFS and IRRAS.

NEXAFS spectra were measured at the carbon and nitrogen K-

edges for monolayers of azobenzene mannoside 20 on Au(111)

single crystals. The carbon K-edge spectrum, shown in Figure 6

(d), exhibits a prominent resonance at 285.4 eV which can be

assigned to a 1s  * (LUMO) transition. This resonance reveals

a decreasing intensity from 90° to 30°. The second * resonance

is located at 287.4 eV and shows almost no angular dependence,

just as the third * resonance at 298.7 eV. At higher energies

(293.8 eV) several broadened * resonances can be observed.

The nitrogen K-edge NEXAFS in Figure 6 (e) reveals one

prominent resonance at 399.0 eV. The energy of this feature

again corresponds to the N 1s-to-*-transition reported in

literature for azobenzene units.^[27]The difference spectrum (red)

shows that this transition exhibits a marked angular dependence.

The tilt angle β of the compound 20 with respect to the surface

normal amounts to 23 ±6° (see SI, Figure S12) which fits well to

results of other biphenyl based SAMs in literature.^[34]This result

is in accordance with the carbon K-edge NEXAFS data.

X-ray photoelectron spectroscopy

The XPS data of the 20-SAM are shown in Figure 6. The C 1s

spectrum (Figure 6, a) contains three different signals. The main

component at 284.6 eV (59 %, red) can be assigned to the

aromatic carbon atoms bound to each other or to sulfur. A

smaller signal at 285.5 eV (7 %, blue) corresponds to the carbon

atoms bound to nitrogen. The third species (29 %, green) at

286.5 eV can be associated with carbon atoms bound to oxygen.

The anomeric carbon atom which is bound to two oxygen atoms

contributes to a fourth signal at 288.0 eV (5 %, orange). Again,

the relative fractions of the different species fit well to the

chemical composition (62:8:25:5) of compound 20. The S 2p

spectrum (Figure 6, b) exhibits only one doublet at 162.0 eV and

163.3 eV (100 %, red), corresponding to a thiolate species.^[28–30]

Importantly, there is no disulfide or other sulfur containing

impurity detectable. This clearly reflects the high purity of the

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Figure 7. a) Schematic illustration of compound 20 in a monolayer on Au(111); b) Measured bulk IR of 20 (black) and 27 (grey) and IRRA spectra of the 20-SAM

(blue) and the 20,27-SAM (marine blue) in comparison with the computed bulk IR (red) of compound 20. c) PM-IRRA spectra in the C_aryl-O region of the 20-SAM

before (grey) and after irradiation with light of 365 nm (red), followed by irradiation at 440 nm (purple); spectrum after thermal relaxation blue; d) Intensity of the

C_aryl-O stretching vibration after irradiation with light of 365 nm, followed by irradiation at 440 nm and thermal relaxation for the 20-SAM.

with a flexible backbone (see above). Based on the rotationally

Infrared reflection absorption spectroscopy

averaged cis-configuration C, a decrease of ΔI_theo=80 % of the

C_aryl-O intensity is predicted. From the observed intensity

decrease it thus can be concluded that more than one third of the

molecules (34 %) in the 20-SAM switch to the cis-isomer (Table

1). Remarkably, the switching capacity of 20-SAM thus exceeds

that obtained for SAMs formed of compounds 7 and 8 by almost

one order of magnitude.

In order to switch cis-20-SAM back to the trans-isomer, the

sample was irradiated with light of 440 nm wavelength.

Surprisingly, no increase of the C_aryl-O stretch intensity was

observed (Figure 7, c and d). Thus, it was concluded that it is not

possible to photochemically switch the SAM back to the trans-

configuration. However, after leaving the sample for 48 h at room

temperature and under N₂-atmosphere a slight increase (+ 8 %

intensity) of the C_aryl-O band could be observed, which was

attributed to a thermal relaxation process. This small amount of

trans-isomer, in turn, could be switched back to the cis-state by

irradiation with light of 365 nm.

The described results indicate a high thermal stability of cis-

20-SAM on Au(111). Since similar SAMs devoid of a sugar head

group show reversible cis/trans-isomerization,^[16]we assume

intermolecular interactions within the SAM as a possible origin for

this observation. Hydrogen bonds of the D-mannose head group

of the SAM, could stabilize the cis-isomer on the surface and

prevent cis→trans back isomerization. In order to check such an

effect of the free hydroxy groups of the sugar ring, the O-allylated

analogue of 20, the mannoside 21 was employed and

Besides XPS and NEXAFS, IRRAS was employed to investigate

compound 20 adsorbed as a monolayer on Au(111), in particular

its switching properties. Intensities of around 10^-3absorbance

units again reflect the formation of a monolayer on Au(111)

(Figure 7, a).^[14]The measured bulk IR and IRRA spectra look very

similar to the calculated bulk IR spectrum. With the help of the

calculated spectrum the bands appearing in the measured

spectra can be assigned to specific vibrational modes. The

vibrational mode at 1600 cm^-1in the IRRA spectrum corresponds

to C_aryl-C_arylstretching vibrations of the biphenyl unit. In the IRRA

spectrum the bands at 1499 cm^-1and 1477 cm^-1exhibit a different

intensity ratio than in the bulk IR due to the surface selection

rule.^[32]These vibrations can be assigned to N=N stretching and

aromatic C-C stretching vibrations, respectively.

At 1242 cm^-1in the surface IR and at 1231 cm^-1in the bulk

IR spectrum the very prominent C_aryl-O stretching vibration can be

observed (Figure 7, b). In analogy to 7-SAM described above this

vibration is well suited to monitor the cis/trans-isomerization of 20-

SAM with PM-IRRAS.^[6]The first measurement (grey in Figure 7c)

shows the spectrum of the un-irradiated sample. After irradiation

with light of 365 nm the intensity of the C_aryl-O vibration decreases

by ΔI_exp=27 % (red) (Figure 7d). This reflects the trans→cis

isomerization of the 20-SAM and the corresponding orientational

change of the transition dipole moment of the C_aryl-O stretch. The

possible orientations of the C_aryl-O stretch in the cis-isomer were

determined in analogy to the azobenzene mannosides 7 and 8

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Figure 8. a) Schematic illustration of the 21-SAM. b) PM-IRRA spectra of the C_aryl-O region of the 21-SAM before and after irradiation with light of 365 nm and

440nm. c) The changes of the intensities of the C_aryl-O stretching vibration after irradiation with light of 365 nm or 440 nm for the 21-SAM (black) in comparison with

the data of the 20,27-SAM (grey) d) Schematic illustration of the 20,27-SAM. e) PM-IRRA spectra of the C_aryl-O region of the 20,27-SAM before and after irradiation

with light of 365 nm and 440nm. f) The changes of the intensities of the C_aryl-O stretching vibration after irradiation with light of 365 nm or 440 nm for the 20,27-SAM

(black) in comparison with the data of the pure 20-SAM (grey).g) Schematic illustration of the 20,28-SAM. h) PM-IRRA spectra of the C_aryl-O region of the 20,28-

SAM before and after irradiation with light of 365 nm and 440nm. i) The changes of the intensities of the C_aryl-O stretching vibration after irradiation with light of 365

nm or 440 nm for the 20,28-SAM (black) in comparison with the data of the 20,27-SAM (grey).

deposited % and for comparison, 8-SAM which showed almost no

trasn/cis switching on surface.on a Au(111) surface from a 1 mM

methanol/acetone (95:5) solution. The obtained film was

investigated by XPS, NEXAFS and IRRAS (see SI, Figure S4 &

S10 & S19), providing evidence for a highly pure and well-oriented

monolayer of 21 on Au(111). The tilt angle β of the molecules

within the 21-SAM was found to be 24±5° (ΔI_theo.=78 % based on

the orientationally averaged cis-configuration C), quite similar to

parent 20-SAM. The switching properties of the 21-SAM were

elucidated with PM-IRRAS; the resulting spectra are shown in

Figure 8 (a-c). By irradiation of the sample with light of 365 nm the

change of intensity is noticeably lower than for the original 20-

SAM.

From the described results we conclude that the free OH-

groups groups of the mannoside head groups of 20-SAM stabilize

the cis-isomer on the surface via intermolecular hydrogen bonds,

preventing the molecules to be switched back to the trans-state

by irradiation with 440 nm light. Otherwise, when the hydroxy

groups are protected, stabilization of the cis-isomer by H-bonds is

no longer possible and the switching process becomes reversible.

On the other hand, the steric demand of the head group is

drastically increased by four allyl protection groups which lowers

intensity of the C_aryl-O stretching vibration decreases by ΔI_exp=9 %. the free volume within the SAM. For this reason the switching

Again, this intensity change can be attributed to the trans/cis-

isomerization of compound 21 adsorbed on the surface. After

irradiation with light of 440 nm the intensity rises again. The

switching capacity calculated on the basis of ΔI_expand ΔI_theo

amounts to 12 %. Consequently, this process is reversible for 21-

SAM (Figure 8, a-c), in contrast to parent 20-SAM. However, the

capacity of the 21-SAM is distinctly lowered in comparison to the

20-SAM.

In addition to the rigid p-mannosyloxy-p’-phenyl-azobenzene

derivative 20 and its protected analogue 21, the isomeric

glycoazobenzene derivative 14 was tested, where the

azobenzene moiety is shifted one phenyl group away from the

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sugar ring, leading to

a

p-mannosyloxyphenyl-azobenzene

process is fully reversible. Thus, by dilution of 20-SAM with

compound 27 a reversible cis/trans isomerization of this glyco-

SAM is enabled.

structural motive (cf Scheme 2). A monolayer of this molecule was

deposited on gold and investigated by XPS, NEXAFS and IRRAS

as before (see SI, Figure S5 & S11 & S21). The tilt angle of the

14-SAM with respect to the surface normal was determined to

β=29±3° (ΔI_theo=73 %, based on configuration C). Upon irradiation

with light of 365 nm and 440 nm a reversible change of ΔI_exp=5 %

could be observed for the C_aryl-O stretching vibration intensity,

corresponding to a switching capacity of 7 % for 14-SAM. The

much lower value compared to 20-SAM (34 %) and 21-SAM

(12 %) can be explained by the large free volume required for the

reorientation of the mannosyloxybiphenyl moiety “above” the azo

group as compared to the shorter mannosylphenyl portion which

terminates 20-SAM. This result demonstrates that the biphenyl

group should be placed below (and not above) the azo function to

obtain a satisfactory switching behavior of the respective glyco-

SAM on a Au(111) surface.

In a final step, a different diluter compound was employed

than in the 20,27-SAM, in order to check the influence of a

comprised azobenzene unit (as in 27) on the switching properties

of a mixed glyco-SAM. To this end, 4-biphenylthiol (28, cf.

Scheme 4) was employed as non photoswitchable diluter

molecule, and a 20,28-SAM on Au(111) was prepared. The

sample was deposited from a 1 mM solution of compounds 20

and 28 (1:4) in a methanol-acetone mixture (95:5).The XPS,

NEXAFS and IRRAS data prove that the SAM is of high purity and

that the composition of the 20,28-SAM is again 1:1 (see SI; Figure

S3 & S9). The orientation of the 20,28-SAM was determined by

NEXAFS. With 24 ±3° (ΔI_theo=78 %, based on orientation C) the

tilt angle is again very similar to that of 20-SAM (see SI, Figure

S12). In Figure 8 (g-i) the PM-IRRAS data of the 20,28-SAM are

shown. By irradiation of the sample with light of 365 nm the

intensity of the C_aryl-O stretching vibration decreases by

ΔI_exp=18 %. Even though this process is reversible, the switching

behavior is significantly worse than for the pure 20-SAM and

mixed 20,27-SAM. Thus, we conclude that the azobenzene

moiety is an essential part of the diluter molecule to obtain

efficient and reversible switching properties of the respective

glyco-SAM.

Investigation of mixed rigid SAMs

In order to increase the lateral distance between the head groups

and reduce their intermolecular interactions, 20-SAM was diluted

with the azobenzene derivative 27 lacking the mannose unit.

Mixed SAMs were fabricated by immersing the substrate in a

solution containing both compounds, the diluter molecule 27 and

20 in a ratio of 4:1, which was found optimal in our earlier studies

with diluted SAMs.^[6]Moreover, it is known that the ratio of

components assembled in a SAM is not necessarily the same as

in the solution used for immersion of the gold wafer.^[10]

4. Lectin binding tests

To check the purity, composition and orientation of the mixed

20,27-SAM, XPS and NEXAFS measurements were conducted

(see SI, Figure S2 & S8). The composition in the C 1s XP

spectrum reflects a 1:1-ratio of compound 20 and 27 in the

surface-adsorbed SAM. This agrees with the observation that the

intensity of the C_aryl-O stretching vibration in the surface IR

spectrum is reduced by about 50 % (Figure 7a). The NEXAFS

data of the 20,27-SAM and the pure 20-SAM are almost equal,

because the most intense NEXAFS resonances originate from the

benzene and azo units of the molecules. This suggests that the

dilution of the SAM does not influence the orientation of the

molecules. The tilt angle β of the 20,27-SAM with regard to the

surface normal determined by NEXAFS is 22 ±4° (ΔI_theo=80 %,

based on orientation C). The IRRA spectrum of the mixed 20,27-

SAM in Figure 7 (a) still shows the same vibrations like in the pure

20-SAM. This is reasonable, since the structure of diluter

molecule 27 is very similar to that of its glycosylated analogue 20.

The C_aryl-O stretching vibration is still very prominent, but less

intense compared to the pure 20-SAM. In addition, at 1215 cm^-1

a shoulder is observable which can be assigned to a C-N

stretching vibration combined with a C_aryl-C_arylstretching vibration

of the azobenzene in the diluter molecules.

In order to investigate the switching behavior of the mixed

20,27-SAM, PM-IRRAS was employed in the spectra region of

the C_aryl-O stretch. After irradiation with 365 nm the intensity of the

C_aryl-O stretching mode decreases in the surface IR spectrum by

ΔI_exp.=38 % due to the trans/cis-isomerization and the associated

change in orientation with respect to the surface (see SI, Figure

S15). This corresponds to a switching capacity of 48 %, even

larger than observed for pure 20-SAM; i.e., nearly the half of the

molecules (48 %) in the 20,27-SAM have been switched from the

trans to the cis state. Moreover, as shown in Figure 8 (d-f), this

For a first biological evaluation of the effect of trans/cis

isomerization of the new glyco-SAMs on carbohydrate recognition,

the fluorescein-labeled lectin Concanavalin A (ConA-FITC) was

used. For this well-known lectin, a strong specificity for α-D-

mannosyl residues is known and hence it is suited for binding

studies with the herein prepared SAMs. Three different

monolayers were selected for the tests, the pure 20-SAM, with a

switching capacity of 34 % (cf. Table 1), its diluted version, 20,27-

SAM, which performed best among all investigated SAMs with a

switching capacity of 48 %, and in addition 8-SAM was tested,

which showed almost no cis/trans isomerization on surface. The

respective SAMs were subdivided into two halves and irradiated

with light of 365 nm (to effect trans→cis isomerization) or 485 nm

wavelengths (to effect cis→trans isomerization). Then the wafers

were incubated with ConA-FITC and after washing, fluorescence

of bound lectin was read out (cf. supporting information, Fig. S 84).

Strikingly, lectin binding to 20-SAM was reduced by 28 % after

irradiation with light of 365 nm, whereas 20,27-SAM after the

same trans→cis isomerization step even showed 33 % reduced

lectin binding compared to the trans-state of the SAM. These data

parallel the determined switching capacities of the SAMs (34 %

and 48 %) but also show that additional effects other than

trans→cis isomerization add to carbohydrate recognition on a

surface. This finding is not unexpected and might be interpretable

in context of the tetrameric nature of ConA at pH higher than 6.

Very much in line with the spectroscopic investigation, irradiation

of 8-SAM showed so significant effect on lectin binding. In future

experiments, cis/trans isomerization of the so far available glyco-

SAMs has to be investigated in different biological systems to

advance our understanding of the biological effect of

carbohydrate ligand orientation on surfaces.

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Conclusions

further improve the photoswitchability of cell adhesion compared

to already established systems^[6,7]and to advance our

understanding of the underlying effects.

The switching capacity of glyco-SAMs is of fundamental

importance for their biological function. Therefore, it has been our

goal to design glycoazobenzene thiols for the fabrication of glyco-

SAMs on Au(111) with high switching capacity. To this end, it was

important to form stable SAMs and concomitantly provide enough

free volume for the reversible cis/trans photoisomerization. We

have varied central molecular parameters, in particular, protecting

groups on the sugar ring and backbone rigidity of the immobilized

molecules to investigate their influence on the physicochemical

properties of the respective glyco-SAMs; in particular, their

switching behavior.

Experimental Section

General procedures

Chemicals were purchased from Sigma-Aldrich, Acros Organics,

or TCI and were used without further purification. Moisture

sensitive reactions were carried out in dry glassware and under

positive

pressure

of

nitrogen.

Before

adding

tetrakis(triphenylphospine)palladium(0), the reaction mixtures

were degassed over three freeze-pump-thaw (F-P-T) cycles.

Otherwise the flask with the reaction mixture was sonicated in an

ultrasonic bath for 10 minutes. Thin layer chromatography was

performed on silica gel plates (GF, 254, Merck). Visualization was

achieved by UV and/or vanillin 10 % sulfuric acid in ethanol

followed by heat treatment. Flash chromatography was performed

on silica gel 60 (Merck, 230-400 mesh, particle size 0.040-0.063

nm) by using distilled solvents. Optical rotations were measured

with a Perkin-Elmer 241 polarimeter (sodium D-line: 589 nm,

length of cell: 1 dm) in the noted solvents. ¹H, ¹³C, ¹¹B NMR

spectra were recorded with Bruker DRX-500 and AV-600

spectrometers. Chemical shifts were reported relative to internal

tetramethylsilane or to the residual proton of the NMR solvent. All

NMR spectra of the E-isomers of the azobenzene derivatives

were recorded after they were kept for 16 h in the dark at 40 °C.

IR spectra were measured with a Perkin Elmer FT-IR Paragon

1000 (ATR) or a Perkin Elmer Lambda 650 and were reported in

cm-1. Elemental Analyses were measured on a EuroEA 3000

from Euro Vektor and a vario MICRO cube from Elementar. The

thermal cis/trans-isomerization was measured on a Bruker ARX

300 spectrometer.

Six new azobenzene mannosides were synthesized and

characterized in bulk material and in solution, where all synthetic

compounds show reversible cis/trans isomerization. Afterwards

they were deposited on a Au(111) surface and the composition

and packing density of the formed SAMs spectroscopically

characterized with a combination of IRRAS and NEXAFS. XPS

data were used to determine the chemical composition of the

SAMs and proved the high purity of the investigated SAMs. It was

also possible to verify the deprotection “on SAM” by the Si 2p

region of XP spectrum. The molecular orientation of the SAMs

with respect to the surface was determined by NEXAFS

spectroscopy. For the alkyl SAMs a tilt angle β regarding to the

surface normal between 32°-33° was obtained; for the rigid SAMs

the tilt angle β was found between 22° and 24°. Both results fit

well to previous studies in the literature.

The switching behavior of the glyco-SAMs was investigated

with the help of IRRAS. Based on the intensity change of the C_aryl

-

O stretching vibration upon cis/trans-isomerization and the

orientation of the molecules obtained by NEXAFS, the switching

capacities of the different glyco-SAMs were determined. The

approach of self-dilution within the glyco-SAM leads to an

improvement of the inherently low switching capacities by more

than 100 % (7-SAM: 3 %; 7-SAM^deprot: 7 %). Much higher

switching capacities could be obtained using SAMs with rigid

spacers. In pure 20-SAM 34 % of the molecules switched from

trans- to the cis-isomer due to irradiation with light of 365 nm.

However, 20-SAM could not switched back to trans-state by

irradiation with light of 440 nm. Here, only a slow thermal

relaxation could be observed. By protecting the OH-groups of the

mannose moiety the reversibility of the cis/trans-isomerization

was restored (21-SAM), but under loss of switching capacity. In

contrast, diluting the rigid SAMs was found to both improve the

switching capacity and enable reversible switching. In 20,27-SAM

almost half of the molecules undergo a reversible trans/cis

isomerization (48 %). These switching properties are

unprecedented and mark a significant progress in the field of

photoswitchable glyco-SAMs. It was found to be necessary that

the diluter compound also contains an azobenzene, at the same

position as in the glyco compound. Otherwise the switching

capacity decreases sharply (20,28-SAM; 23 %). First lectin

assays with ConA showed that the observed differences in lectin

binding parallel with the switching capacities of the SAMs but that

the determined lectin binding differences are smaller than the

cis/trans ratios of the investigated SAMs.

Gold Substrates

Glass substrates with a 50 Å titanium adlayer and a 200 nm

evaporated gold film from EMF corporation (Ithaca, NY) were

used for IRRAS measurements. XPS and NEXAFS

measurements were made with sputtered Au(111) single crystals.

Preparation of Monolayers

The monolayers of 7 and 8 were prepared by immersing Au(111)

substrates in 0.5 mM solutions of the respective compound in

methanol at room temperature. The monolayers of 14, 20, 21, 27

and 28 as well as the respective mixed SAMs were prepared by

immersing Au(111) substrates in 0.5 mM solutions of the

respective compound in methanol/acetone (95:5) at room

temperature. After 48 h of immersion the sample was removed

from the solution, rinsed with methanol, acetone and dried in a

stream of nitrogen gas.

IRRAS

The surface adsorbed molecules were investigated by using a

Bruker VERTEX 70 FT-IR spectrometer equipped with a

Polarization Modulation Accessory (PMA) 50 unit (Bruker Optik

GmbH, Ettlingen, Germany). This instrument allows recording

IRRAS and PM-IRRAS data with a spectral range from 4000 down

to 800 cm^-1. IRRAS data were collected with a liquid nitrogen

cooled MCT detector in a horizontal reflection unit for grazing

In conclusion, this study provides new and detailed

information about different approaches towards photoswitchable

glyco-SAMs. Eventually, these surfaces have to be employed to

10.1002/chem.201903644

Chemistry - A European Journal

FULL PAPER for Chem. Eur. J.

incidence (Bruker A518). The sample chamber was purged with

dry nitrogen before and during measurements. A deuterated

hexadecane-thiol SAM on Au(111) was used as a reference for

the background spectrum for conventional IRRA spectra. Each

spectrum contains 2048 averaged spectra. A p-polarized beam at

an incident angle of 80° to the surface normal was used for

measurements. All spectra were recorded with 4 cm^-1resolution.

PM-IRRAS data of the switching experiments were collected with

the PMA 50 accessory using a liquid nitrogen-cooled MCT

detector. The PEM maximum efficiency was set for the half-wave

at 1750 cm^-1for analysis of the area from 1400 cm^-1to 1100 cm^-

¹. All spectra were recorded with 4 cm^-1resolution.

analyzer. The energy resolution E/DE of the beamline with 150

mm slits is 800. XP survey spectra were secured at 700 eV photon

energy using an analyzer pass energy of 100 eV, whereas for the

C 1s, Si 2p, and S 2p spectra the photon energy used was 350

eV with pass energy of 50 eV. For N 1s spectra the photon energy

was at 500 eV with pass energy of 50 eV.

All spectra were acquired at normal electron emission. For

determination of the relative composition of the adlayers, the XP

spectra were energy-corrected using the Au 4f_7/2line at a binding

energy of 84.0 eV as reference. Background correction was

performed using a combination of a Shirley and a linear

background for all signals. Peak fitting was performed using the

program CASA XPS. The maximum deviation of fwhm of the fitted

Gaussian within a spectrum was set to 0.2 eV. The fitting

parameter are shown in Tables S1-S7 in the Supp. Inf. All spectra

were smoothed with the SG Quadratic method (CASA XPS,

smoothing width = 5) in order to improve the signal-to-noise-ratio.

Care has been taken to ensure that this process does not alter

the statement of the spectrum.

Processing of IRRAS and PM-IRRAS data was carried out

using the OPUS software Version 6.5 (Bruker, Germany).

Baseline correction of the resulting IRRAS data was performed by

the rubber band method in an interactive mode. PM-IRRAS data

were processed by the implicit removal of the Bessel function

through manual baseline correction. For the trans/cis-

isomerization of different compounds adsorbed on Au(111) the

prepared samples were irradiated within the spectrometer using

a LED (Nichia NC4U133(T), peak wavelength: 365 (±9) nm or 440

(±5) nm, 1 LEDs, power dissipation: 12 W, luminous flux: 10 lm,

distance ~5 cm). The sample was irradiated with the respective

wavelength for 10 min.

Due to the small shoulder in the investigated IR band (C_aryl-O

stretching vibration) at lower wavenumbers, the band was fitted

with two Gaussians to determine the intensity of the C_aryl-O bond

(see SI, Figure S14-S19). The second band corresponds to a C-

H bending vibration within the mannose moiety. As this vibration

is a combination of different vibrations and the mannose moiety

can rotate around the C_aryl-O bond, the orientation of this vibration

is only barley affected by the isomerization process and was thus

neglected.

To correct for the photon flux of the NEXAFS measurements,

all spectra were divided by the spectrum obtained for a freshly

sputtered clean gold substrate and then edge-step normalized

(using the average intensities for the C K-edge between 275 ±0.5

eV and 320 ± 0.5 eV and for the N K-edge between 395 ±0.5 eV

and 420 ±0.5 eV as pre- and post-edge). The normalized spectra

were fitted employing a step function for the absorption edge and

Gaussians for the π* and σ* resonances in order to determine the

intensity I of specific resonances. In all spectra, the width of the

step function was set to 0.2 eV. The series of spectra of a specific

sample measured at different angles of incidence were fitted with

the same parameter set, i.e., the energies of the resonances were

allowed to vary in maximum by 0.2 eV and the half-widths at full

maximum by 0.3 eV, as in agreement with the estimated

experimental resolution. For determination of the orientation of

the molecular orbitals, the angular dependence of the intensities

I of the π* resonances were finally fitted to model functions for the

angular dependence (see SI, Figure S12):^[36]

In order to calculate the expected intensity change ΔI_theo

different orientations of the cis-isomer have to be taken into

account, as the molecules within the SAM can rotate around their

molecule axis. The orientations A (head group downwards) and B

(head group upwards) were calculated based on the tilt angle β

which was obtained from NEXAFS data. Using equation (1) and

(2) the expected intensity change of the C_aryl-O stretch in the

surface IR spectrum could be calculated:

퐼 = 퐴 [푃푐표푠²휃 (1 − ³푠푖푛²훽) + ¹푠푖푛²훽]

(4)

2

with the specific amplitudes of the resonances (A), the degree of

polarization (P = 0.91), the angle of incidence (θ), and the tilt angle

of the transition dipole moment of the molecule with respect to the

surface normal (β). The specified errors for the tilt angles come

from the fit.

(

푐표푠

)

∆퐼_푡ℎ푒표= 퐼_{푡푟푎푛푠}1 − 푅_푡ℎ푒표

(1)

2

휑

2

푐푖푠

푅_푡ℎ푒표

=

(2)

푐표푠

훽

Since a distribution of orientations of the head group instead

one specific orientation can be assumed within the glyco-SAM,

another orientation was introduced called C. Orientation C

describes an averaging over all possible angles of the transition

dipole moment of the C_aryl-O stretch. The averaged intensity of the

cis-isomer was calculated according equation (3):^[35]

Synthesis and analytical data of target molecules 6, 7, 13, 14,

and 18-21 (for all data cf. SI)

1

2

̅̅̅̅

퐼_푐푖푠

(

)(

(

)

=

[

3푐표푠 훽 − 1 3푐표푠 180 − 훾 − 1 + 1]

(3)

3

2

The angle γ corresponds to the angle between the molecule

axis and the transition dipole moment of the C_aryl-O stretch in the

cis-isomer.

XPS & NEXAFS

The XPS and NEXAFS measurements were performed at the

BESSY II synchrotron radiation facility using the PREVAC

endstation at the beamline HE-SGM. The experimental station is

equipped with a hemispherical VG Scienta R3000 photoelectron

Figure 9. Compound numbering as used for the assignment of NMR

spectroscopic data.

10.1002/chem.201903644

Chemistry - A European Journal

FULL PAPER for Chem. Eur. J.

(E)-p -[p ’-(6-(Acetylthio)hexoxy)phenylazo]phenyl 2,3,4,-tri-O -

acetyl-6-O -tert -butyldiphenylsilyl-α -D -mannopyranoside (6):

The mannosyl donor 4 (337 mg, 905 µmol) and the azobenzene

derivative 5 (124 mg, 333 µmol) were dissolved in dry

dichloromethane (4 mL) and the solution was cooled to 0 °C and

BF₃-etherate (0.17 mL, 1.36 mmol) added. The reaction mixture

was warmed to room temperature and stirred for 2 h. Then, it was

quenched by the addition of sat. aq. NaHCO₃(4 mL), the phases

were separated, and the organic phase was dried over MgSO₄, it

was filtered and the filtrate was concentrated under reduced

pressure. After column chromatography (toluene/ethyl acetate,

9:1) the product was obtained as an orange solid (258 mg, 287

µmol, 86 %). R_f= 0.40 (toluene/ethyl acetate, 9:1); m.p. 139 °C;

[α]²⁰D +108.5 (c = 0.73, ethyl acetate); ¹H-NMR (500 MHz, CDCl₃):

δ = 7.89-7.81 (m, 4 H, H-9, H-11, H-14, H-18), 7.67-7.62 (m, 4 H,

TBDPS-aryl-H_ortho), 7.44-7.34 (m, 6 H, TBDPS-aryl-H_meta, -H_para),

7.20-7.17 (m, 2 H, H-15, H-17), 7.00-6.97 (m, 2 H, H-8, H-12),

5.60 (d, ³J_1,2= 1.8 Hz, 1 H, H-1), 5.56 (m, 2 H, H-3, H-4), 5.45 (dd,

³J_1,2= 1.9 Hz, ³J_2,3= 3.2 Hz, 1 H, H-2), 4.03 (t, J = 6.5 Hz, 2 H, H-

(C-13), 146.9 (C-10), 135.6 (TBDPS-aryl-C_ortho), 135.5 (TBDPS-

aryl-C_ortho), 132.7 (TBDPS-aryl-C_ispo), 132.7 (TBDPS-aryl-C_ispo),

130.0 (TBDPS-aryl-C_para), 127.8 (TBDPS-aryl-C_meta), 127.8

(TBDPS-aryl-C_para), 124.5 (C-14, C-18), 124.2 (C-9, C-11), 116.6

(C-15, C-17), 114.7 (C-8, C-12), 97.6 (C-1), 71.5 (C-5), 71.4 (C-

3), 70.1 (C-2), 70.0 (C-4), 68.1 (C-19), 64.8 (C-6), 39.0 (C-24),

29.1 (C-20), 29.1 (C-23), 28.2 (C-22), 26.8 (C(CH₃)₃), 25.7 (C-21),

19.2 (C(CH₃)₃) ppm; MS (ESI): m/z: calcd. for C₄₀H₅₀N₂O₇SSi:

730.3125 [M]; found 730.3097; ε (ethyl acetate) = 19235 ± 74.4 L

× mol^-1cm^-1.

(E)-p-[p’-(N,N-Dimethyl-S-thiocarbamoyl)phenylazo]biphenyl

2,3,4,6 tetra-O-acetyl-α-D-mannopyranoside (13): The boronic

ester 12 (100 mg, 243 µmol), idoazobenzene 9, potassium

carbonate (102 mg, 738 µmol) and TBABr were dissolved in

water/toluene (1 : 2.5, 14 mL). The reaction mixture was

degassed and after addition of

a catalytic amount of

tetrakis(triphenylphospine)palladium(0), the reaction mixture was

stirred at 80 °C for 4 h and then at room temperature overnight.

The reaction mixture was diluted with ethyl acetate (30 mL) and

brine (20 mL). The organic phase was dried over MgSO₄, it was

filtered and the filtrate concentrated under reduced pressure.

Column chromatography (cyclohexane /ethyl acetate, 2:1) gave

the desired product as an orange solid (43.1 mg, 60.9 µmol,

25 %). R_f= 0.27 (cyclohexane/ethyl acetate, 1:1); m.p. 182 °C;

3

19), 3.92 (ddd, J_4,5= 9.3 Hz, ³J_5,6a= 4.5 Hz, J_5,6b= 1.9 Hz, 1 H,

3

H-5), 3.76 (dd, J_5,6a= 4.6 Hz, ²J_6a,6b= 11.6 Hz, 2 H, H-6a), 3.67

3

2

(dd, J_5,6b= 2.0 Hz, J_6a,6b= 11.6 Hz, 2 H, H-6b), 2.90 (t, J = 7.3

Hz, 2 H, H-24), 2.33 (s, 3 H, SCOCH₃), 2.18, 2.05, 1.92, (each s,

each 3 H; 3 OAc), 1.85-1.79 (m, 2 H, H-20), 1.65-1.59 (dt, J = 14.7

Hz, J = 7.4 Hz, 2 H, H-23), 1.53-1.42 (m, 4 H, H-21, H-22), 1.03

(s, 9 H, C(CH₃)₃) ppm; ¹³C-NMR (126 MHz, CDCl₃): δ = 196.6

(SCOCH₃), 170.2, 170.1, 169.5 (3 COCH₃), 161.4 (C-16), 157.5

(C-7), 148.3 (C-13), 146.9 (C-10), 135.8 (TBDPS-aryl-C_ortho),

135.6 (TBDPS-aryl-C_ortho), 133.3 (TBDPS-aryl-C_ipso), 133.0

(TBDPS-aryl-C_ipso), 129.7 (TBDPS-aryl-C_para), 129.7 (TBDPS-

aryl-C_para), 127.7 (TBDPS-aryl-C_meta), 127.6 (TBDPS-aryl-C_meta),

124.5 (C-14, C-18), 124.2 (C-9, C-11), 116.7 (C-15, C-17), 114.7

(C-8, C-12), 95.7 (C-1), 72.0 (C-5), 69.6 (C-2), 69.2 (C-3), 68.1

(C-19), 65.9 (C-4), 62.3 (C-6), 30.7 (SCOCH₃), 29.4 (C-23), 29.0

(C-19), 29.0 (C-20), 28.5 (C-22), 26.6 (C(CH₃)₃), 25.6 (C-21), 20.8,

20.8, 20.6 (3 COCH₃), 19.2 (C(CH₃)₃) ppm; IR (ATR): ñ = 2932,

2857, 1752, 1213, 1106, 702, 502 cm^-1; MS (ESI): m/z: calcd. for

C₄₈H₅₈N₂O₁₁SSi: 899.3603 [M]; found 899.3589.

1

[α]²⁰D +61.3 (c = 0.91, CH₂Cl₂); H-NMR (500 MHz, CDCl₃) δ =

8.0-7.98 (m, 2 H, H-14, H-18), 7.96-7.93 (m, 2 H, H-15, H-17),

7.71-7.69 (m, 2 H, H-20, H-24), 7.67-7.60 (m, 4 H, H-9, H-11, H-

21, H-23), 7.21-7.18 (m, 2 H, H-8, H-12), 5.61-5.59 (m, 2 H, H-1,

H-3), 5.49 (dd, ³J_1,2= 1.8 Hz, ³J_2,3= 3.5 Hz, 1 H, H-2), 5.40 (dd~t,

3

³J_3,4= ³J_4,5= 10.0 Hz, 1 H, H-4), 4.31 (dd, J_5,6a= 5.0 Hz, ²J_6a,6b

=

3

12.0 Hz, 1 H, H-6a), 3.61 (dd, J_5,6b= 1.8 Hz, ²J_6a,6b= 10.9 Hz, 1

H, H-6b), 4.15-4.09 (m, 2 H, H-5, H-6b), 3.12, 3.06 (each s, each

3 H, 2 CH₃), 2.22, 2.07, 2.05, 2.05 (each s, each 3 H, 4 OAc) ppm;

¹³C-NMR (126 MHz, CDCl₃): δ = 170.5, 170.0, 170.0, 169.7 (4

COCH₃), 166.2 (SCO), 155.6 (C-7), 152.7 (C-10), 151.6 (C-13),

143.1 (C-16), 136.1 (C-21, C-23), 131.5 (C-19), 131.2 (C-22),

128.4 (C-9, C-11), 127.4 (C-20, C-24), 123.6 (C-14, C-18), 123.1

(C-15, C-17), 116.9 (C-8, C-12), 95.8 (C-1), 69.4 (C-2), 69.3 (C-

5), 68.9 (C-3), 65.9 (C-4), 62.1 (C-6), 37.0 (CH₃), 20.9, 20.7, 20.7

20.7 (4 COCH₃) ppm; IR (ATR): ñ = 2937, 1745, 1214, 1034, 826

cm^-1; MS (ESI): m/z: calcd for C₃₅H₃₇O₁₁N₃S + Na⁺: 703.2041 [M];

found 730.2026; EA: calcd. for C₃₅H₃₇O₁₁N₃S: C, 59.40; H, 5.27;

N, 5.94; S, 4.53 %, found: C, 58.57; H, 4.903; N, 6.21; S. 4.75 %.

(E)-p -[p ’-(6-(Thio)hexoxy)phenylazo]phenyl-6-O -tert -

butyldiphenylsilyl-α -D -mannopyranoside (7): The protected

mannoside 6 (250 mg, 278 µmol) was dissolved in dry methanol

(2 mL) and sodium methanolate solution (20 μL, 5.4 M in

methanol) was added. After stirring for 3 h at room temperature,

the solution was neutralized with Amberlite IR120 ion exchange

resin. The resin was filtered off and the crude was concentrated

under reduced pressure. After filtration over silica, the product

(E)-p-[p’-Mercaptophenylazo]biphenyl-α-D-

mannopyranoside (14): The thiocarbamate 13 (16.7 mg, 23.5

μmol) suspended in methanol (1 mL) and potassium hydroxide

solution (0.2 mL, 4.3 M, in MeOH) was added. The reaction

solution was stirred for 1 h. Then, the same amount of potassium

hydroxide solution (0.2 mL, 4.3 M, in MeOH) was added and the

reaction solution was stirred for 16 h at room temperature. It was

neutralized with 2 M HCl and concentrated under reduced

pressure. Potassium chloride was precipitated by addition of

acetone and filtered off. The solvent was removed under reduced

pressure and the product was obtained as an orange solid (7.3

mg, 68 %). [α]²⁰D +29.3 (c = 0.02, MeOH); ¹H-NMR (500 MHz,

DMSO-d₆) δ = 7.88-7.94 (m, 4 H, H-15, H-17, H-20, H-24), 7.88-

7.87 (m, 2 H, H-15, H-17), 7.81-7.79 (m, 2 H, H-21, H-23), 7.74-

7.72 (m, 2 H, H-9, H-11), 7.22-7.12 (m, 2 H, H-8, H-12), 5.47 (d,

was obtained as an orange solid (200 mg, 273 µmol, 90 %). [α]²⁰

D

+20.0 (c = 0.6, CH₂Cl₂); ¹H-NMR (500 MHz, CDCl₃): δ = 7.87-7.84

(m, 2 H, H-14, H-18), 7.82-7.78 (m, 2 H, H-9, H-11), 7.64-7.61 (m,

4 H, TBDPS-aryl-H_ortho), 7.43-7.30 (m, 6 H, TBDPS-aryl-H_meta, -

H_para), 7.12-7.07 (m, 2 H, H-8, H-12), 6.99-6.96 (m, 2 H, H-15, H-

17), 5.58 (d, ³J_1,2= 1.3 Hz, 1 H, H-1), 4.15 (s, 1 H, H-2), 4.08 (dd,

³J_2,3= 2.9 Hz, ³J_3,4= 9.3 Hz, 1 H, H-3), 4.04-3.96 (m, 3 H, H-4, H-

19), 3.90 (d, ³J_6,5= 4.9 Hz, 2 H, H-6), 3.71 (ddd~dt, ³J_5,6= 5.0 Hz,

³J_4,5= 9.8 Hz, 1 H, H-5), 3.14 (s, 1 H, OH), 3.07 (s, 1 H, OH), 3.07

(s, 1 H, OH), 2.81 (s, 1 H, OH), 2.73-2.70 (m, 2 H, H-24), 1.86-

1.81 (m, 2 H, H-20), 1.77-1.72 (m, 2 H, H-23), 1.68 (s, 1 H, SH),

1.54-1.48 (m, 4 H, H-21, H-22), 1.03 (s, 3 H, C(CH₃)₃) ppm;

¹³C-NMR (126 MHz, CDCl₃): δ = 161.3 (C-16), 157.7 (C-7), 148.2

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FULL PAPER for Chem. Eur. J.

³J_1,2= 1.3Hz, 1 H, H-1), 3.87 (m, 1 H, H-3), 3.71 (m, 1 H, H-2) ppm.

¹³C-NMR (126 MHz, DMSO-d₆): δ = 156.5 (C-10), 151.1 (C-22),

150.5 (C-13), 142.7 (C-16), 138.9 (C-19), 132.3 (C-7), 127.9 (C-

9, C-11), 127.6 (C-21, C-23), 127.0 (C-14, C-18), 123.5 (C-20,

C24), 123.2 (C-15, C-17), 117.1 (C-8, C-12), 98.6 (C-1), 74.9 (C-

5), 70.5 (C-3), 69.9 (C-2), 66.5 (C-4), 60.0 (C-6) ppm; IR (ATR): ñ

=1589, 1131, 1001, 825, 557 cm^-1; MS (ESI): m/z: calcd. for

C₂₄H₃₄O₆N₂S + H⁺: 469.1428 [M]; found 469.1423, ε (DMSO) =

3756.39 ± 36.0 L × mol^-1cm^-1.

OCH), 4.08 (m, 1 H, OCH), 3.97-3.87 (m, 4 H, H-2, H-3, H-4,

3

2

OCH), 3.73 (m, 1 H, H-5), 3.69 (dd, J_5,6a= 4.3 Hz, J_6a,6b= 10.9

Hz, 1 H, H-6a), 3.61 (dd, ³J_5,6b= 1.8 Hz, ²J_6a,6b= 10.9 Hz, 1 H, H-

6b), 3.13, 3.06 (each s, each 3 H, CH₃) ppm; ¹³C-NMR (150 MHz,

CDCl₃): δ = 166.8 (SCO), 158.5 (C-7), 152.0 (C-13), 148.0 (C-10),

142.5 (C-22), 141.1 (C-16), 136.3 (C-19), 135.0, 134.9, 134.8,

134.7 (HC=CH₂), 127.9 (C-15, C-17), 127.6 (C-21, C-32), 124.6

(C-14, C-18), 123.3 (C-9, C-11), 117.8 (OCH₂CH=CH₂), 116.9

(OCH₂CH=CH₂), 116.8 (OCH₂CH=CH₂), 116.7 (C-8, C-12), 116.

6 (OCH₂CH=CH₂), 96.5 (C-1), 79.1 (C-2), 74.5 (C-3), 74.4 (C-4),

74.0 (OCH₂CH=CH₂), 72.4 (C-5), 72.4 (OCH₂CH=CH₂), 72.3

(OCH₂CH=CH₂), 71.4 (OCH₂CH=CH₂), 68.7 (C-6), 37.0 (2 CH₃)

ppm; IR (ATR): ñ = 2920, 2853, 1668, 1233, 1130,1088, 987 cm^-

¹; MS (ESI): m/z: calcd. for C₃₉H₄₅O₇N₃S + Na⁺: 722.2870 [M];

found 722.2857.

(E)-p-[p’-(N,N-Dimethyl-S-thiocarbamoyl)biphenylazo]phenyl

2,3,4,6 tetra-O-acetyl-α-D-mannopyranoside (18): The boronic

ester 17 (23.6 mg, 36.1 µmol), the thiocarbamte 15 (10.0 mg,

30.5 µmol), potassium carbonate (4.8 mg, 61.0 µmol) and TBABr

(5.0 mg, 15.5 µmol) were dissolved in a toluene/H₂O mixture (2:1,

6 ml) and degassed and after addition of a catalytic amount of

tetrakis(triphenylphospine)palladium(0), the reaction mixture was

stirred at 80 °C for 4 h and 16 h at room temperature. The phases

were separated, the organic phase was washed with water (10

mL) and dried over MgSO₄. It was filtered and the solvent

removed under reduced pressure and purified by column

chromatography (cyclohexane/ethyl acetate, 1:1). The product

was obtained as an orange solid (10.3 mg, 14.6 µmol, 48 %).

(E)-p-[p’-Mercaptobiphenylazo]phenyl α-D-

mannopyranoside (20): The carbamate 18 (37 mg, 55 µmol) and

potassium hydroxide (115 mg, 2.03 mmol) was dissolved in

methanol, degassed with ultrasound for 10 minutes and was

stirred for 2 h at 80 °C. The reaction mixture was neutralized with

2 N HCl and concentrated under reduced pressure. The residue

was dissolved in acetone and filtered to remove precipitated

potassium chloride. After removal of the solvent, the product was

R_f= 0.35 (cyclohexane/ethyl acetate, 1:1); m.p. 182 °C; [α]²⁰

D

1

+85.6 (c = 0.15, CH₂Cl₂); H-NMR (500 MHz, CDCl₃) δ = 7.99-

7.92 (m, 4 H, H-9, H-11, H-14, H-18), 7.75-7.51 (m, 6 H, H-15, H-

17, H-20, H-21, H-23, H-24), 7.26-7.20 (m, 2 H, H-8, H-12), 5.61

(m, 2 H, H-1, H-3), 5.49 (dd, ³J_1,2= 1.8 Hz, ³J_2,3= 3.4 Hz, 1 H, H-

2), 5.40 (dd~t, ³J_3,4= ³J_4,5= 9.8 Hz, 1 H, H-4), 4.33 (m, 1 H, H-6a),

4.15-4.05 (m, 2 H, H-5, H-6b), 3.09 (s, 6 H, 2 CH₃), 2.22, 2.06,

2.05, 2.04 (each s, each 3 H, 4 OAc) ppm; ¹³C-NMR (126 MHz,

CDCl₃): δ = 170.5, 170.0, 170.0, 196.7 (4 COCH₃), 166.8 (SCO),

157.6 (C-7), 151.9 (C-13), 148.5 (C-10), 142.7 (C-16), 141.1 (C-

19), 136.3 (C-22), 136.1 (C-21, 23), 127.9 (C-15, C-17), 127.7 (C-

20, C-24), 123.9 (C-14, C-18), 95.7 (C-1), 69.4 (C-5), 69.3 (C-2),

68.8 (C-3), 65.9 (C-4), 62.1 (C-6), 37.0 (2 CH₃), 20.9, 20.7, 20.7

(4 COCH₃) ppm; IR (ATR): ñ = 1746, 1365, 1211, 1030, 819 cm^-1;

MS (ESI): m/z: calcd. for C₃₅H₃₈O₁₁N₃S: 708.2222 [M]; found

708.2223.

obtained as an orange amorphous solid (16.5 mg, 35 µmol). [α]²⁰

D

+66.7 (c = 0.09, DMSO); ¹H-NMR (500 MHz, DMSO-d₆) δ = 7.91-

7.87 (m, 6 H, H-9, H-11, H-14, H-18, H-15, H-17), 7.85-7.81 (m, 2

H, H-20, H-24), 7.71-7.69 (m, 2 H, H-21, H-23), 7.29-7.27 (m, 2

H, H-8, H-12), 5.55 (d, ³J_1,2= 1.4 Hz, 1 H, H-1), 5.10 (m, 1 H, OH),

4.86 (m, 1 H, OH), 4.80 (m, 1 H, OH), 4.46 (m, 1 H, OH), 3.87 (m,

1 H, H-3), 3.70 (m, H-2), 3.60 (m, 1 H, H-6a), 3.52-3.50 (m, 2 H,

H-4, H-6b) ppm; ¹³C-NMR (500 MHz, DMSO-d₆) δ =158.9 (C-13),

151.2 (C-10), 146.8 (C-7), 141.2 (C-16), 138.2 (C-22), 135.5 (C-

19), 127.8 (C-20, C-24), 127.7 (C-21, C-23), 127.4 (C-15, C-17),

124.3 (C-9, C-11), 122.9 (C-14, C-18), 117.02 (C-8, C-12), 98.5

(C-1), 75.2 (C-5), 70.4 (C-3), 69.7 (C-2), 66.6 (C-4), 60.9 (C-6)

ppm; IR (ATR): ñ = 3249, 2923, 1589, 1131,1006, 2848, 557 cm^-

¹; MS (ESI): m/z: calcd. for C₂₄H₂₄O₆N₂S: 468.1355 [M]; found

468.1353; ε (DMSO) = 17631.11 ± 302.48 L × mol^-1cm^-1

.

(E)-p-[p’-(N,N-Dimethyl-S-thiocarbamoyl)biphenylazo]phenyl

2,3,4,6 tetra-O-allyl-α-D-mannopyranoside (19): The boronic

ester 17 (241 mg, 373 µmol), the thiocarbamate 15 (247 mg, 804

µmol), cesium carbonate (237 mg, 727 µmol) and TBABr (25 mg,

78 µmol) were dissolved in toluene/H₂O (10:1, 6 mL). The solution

was degassed and tetrakis(triphenylphosphine)palladium was

added (57 mg, 49 µmol). The reaction mixture was stirred at 90

°C for 16 h, diluted with ethyl acetate (30 mL) and washed with 2

M aq. HCl (20 mL). The organic phase was dried over MgSO₄,

filtered, and the filtrate concentrated under reduced pressure.

Column chromatography (cyclohexane / ethyl acetate, 4:1) gave

the product as an orange oil (70 mg, 100 µmol, 27 %). R_f= 0.05

(cyclohexane/ethyl acetate, 2:1); [α]²⁰D +95.0 (c = 0.10, CH₂Cl₂);

¹H-NMR (600 MHz, CDCl₃) δ = 7.97-7.90 (m, 4 H, H-9, H-11, H-

14, H-18), 7.74-7.72 (m, 2 H, H-16, H-17), 7.68-7.66 (m, 2 H, H-

20, H-24), 7.60-7.58 (m, 2 H, H-21, H-23), 7.21-7.18 (m, 2 H, H-

8, H-12), 6.04-5.85 (m, 4 H, HC=CH₂), 5.65 (d, ³J_1,2= 1.5 Hz, 1 H,

H-1), 5.40-5.22 (m, 6 H, 3 HC=CH₂), 5.16-5.12 (m, 2 H, HC=CH₂),

4.38 (ddt, ⁴J_OCH2,CH=CH2= 1.3 Hz, ³J_OCH2,CH=CH2= 5.7 Hz, ²J_{OCHa, OCHb}

= 12.3 Hz, 1 H, OCH), 4.29-4.21 (m, 4 H, 2 OCH₂), 4.13 (m, 1 H,

(E)-p-[p’-Mercaptobiphenylazo]phenyl 2,3,4,6 tetra-O-allyl-α-

D-mannopyranoside (21): The carbamate 19 (26.2 mg, 37.4

µmol) and potassium hydroxide (106 mg, 188 mmol) were

dissolved in methanol (2 mL) and the reaction mixture degassed

with ultrasound for 10 minutes. The reaction mixture was stirred

at 65 °C for one h and then 16 h at room temperature. It was

neutralized with ion exchange resin Amberlite IR120, the resin

was filtered off, and the filtrate was concentrated under reduced

pressure. The product was obtained as a dark orange syrup (11.6

1

mg, 18.4 µmol, 48 %). [α]²⁰D +97.1 (c = 0.07, CH₂Cl₂); H-NMR

(600 MHz, CDCl₃) δ = 7.97-7.91 (m, 4 H, H-9, H-11, H-14, H-18),

7.74-7.72 (m, 2 H, H-16, H-17), 7.68-7.66 (m, 2 H, H-20, H-24),

7.60-7.58 (m, 2 H, H-21, H-23), 7.21-7.19 (m, 2 H, H-8, H-12),

3

6.04-5.85 (m, 4 H, HC=CH₂), 5.65 (d, J_1,2= 1.5 Hz, 1 H, H-1),

5.40-5.22 (m, 6 H, 3 HC=CH₂), 5.16-5.12 (m, 2 H, HC=CH₂), 4.38

4

3

2

(ddt, J_OCH2,CH=CH2= 1.3 Hz, J_OCH2,CH=CH2= 5.7 Hz, J_{OCHa, OCHb}

=

12.3 Hz, 1 H, OCH), 4.29-4.21 (m, 4 H, 2 OCH₂), 4.13 (m, 1 H,

OCH), 4.08 (m, 1 H, OCH), 3.97-3.87 (m, 4 H, H-2, H-3, H-4,

3

2

OCH), 3.73 (m, 1 H, H-5), 3.69 (dd, J_5,6a= 4.3 Hz, J_6a,6b= 10.9

Hz, 1 H, H-6a), 3.61 (dd, ³J_5,6b= 1.8 Hz, ²J_6a,6b= 10.9 Hz, 1 H, H-

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6b), 3.52 (s, 1 H, SH) ppm; ¹³C-NMR (150 MHz, CDCl₃): δ = 158.6

(C-7), 152.0 (C-13), 148.0 (C-10), 135.0, 135.0, 134.9, 134.7

(HC=CH₂), 131.0 (C-23), 129.8 (C-21), 128.2 (C-24), 127.8 (C-

20), 127.8 (C-16), 127.7 (C-19), 127.6 (C-15, C-17), 127.4 (C-22),

124.7 (C-14, C-18), 123.3 (C-9, C-11), 117.8 (OCH₂CH=CH₂),

116.9 (OCH₂CH=CH₂), 116.8 (OCH₂CH=CH₂), 116.7 (C-8, C-12),

116. 6 (OCH₂CH=CH₂), 96.5 (C-1), 79.2 (C-2), 74.5 (C-3), 74.4

(C-4), 74.0 (OCH₂CH=CH₂), 72.4 (C-5), 72.4 (OCH₂CH=CH₂),

72.3 (OCH₂CH=CH₂), 71.4 (OCH₂CH=CH₂), 68.7 (C-6) ppm; IR

(ATR): ñ = 2921, 2856, 1597, 1497, 1233, 1130, 1102, 986 cm^-1;

MS (ESI): m/z: calcd. for C₃₆H₄₀O₆N₂S + Na⁺: 722.2870 [M]; found

722.2857; ε (CHCl₃) = 26549.98 ± 298.19 L × mol^-1cm^-1.

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Keywords: Glyco-SAMs • Azobenzenes • Photochemistry •

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Entry for the Table of Contents

FULL PAPER

Ellen Fast^[a]#, Alexander Schlimm^[b]#

Irene Lautenschläger^[b], Kai Uwe

,

Clausen^[b], Thomas Strunskus^[c], Carina

Spormann^[a], Thisbe K. Lindhorst^[a]*,

Felix Tuczek^[b]*

Page No. – Page No.

Glyco-SAMs were fabricated on Au(111) and their photoswitching properties were

extensively investigated with a powerful set of methods (IRRAS, XPS and

NEXAFS). In order to maximize the photoswitching capacity different concepts for

the assembly of the underlying SAM were evaluated.

Improving the Switching Capacity of

Glyco-SAMs on Au(111)

Article Doi

Improving the Switching Capacity of Glyco-Self-Assembled Monolayers on Au(111)

DOI: 10.1002/chem.201903644

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Authors:

Article abstract of DOI:10.1002/chem.201903644

Full text of DOI:10.1002/chem.201903644

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