Covalent interlinking of an aldehyde and an amine on a Au(111) surface in ultrahigh vacuum

DOI: 10.1002/anie.200702859

Source and publish data:

Angewandte Chemie - International Edition p. 9227 - 9230 (2007)

Update date:2022-08-05

Topics:

Authors:

Weigelt, Sigrid

Busse, Carsten

Bombis, Christian

Knudsen, Martin M.

Gothelf, Kurt V.

Strunskus, Thomas

Woell, Christof

Dahlbom, Mats

Hammer, Bjork

Lgsgaard, Erik

Besenbacher, Flemming

Linderoth, Trolle R.

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Article abstract of DOI:10.1002/anie.200702859

Getting a reaction: A condensation reaction occurs between a dialdehyde and an amine coadsorbed on a Au(111) surface in an ultrahigh vacuum. The self-assembled structures formed by the diimine reaction product on the surface have been investigated by scanning tunneling microscopy (see image). A solvent-free reaction path is proposed from DFT calculations. (Figure Presented).

Full text of DOI:10.1002/anie.200702859

Angewandte

Chemie

DOI: 10.1002/anie.200702859

Scanning Tunneling Microscopy

Covalent Interlinking of an Aldehyde and an Amine on a Au(111)

Surface in Ultrahigh Vacuum**

Sigrid Weigelt, Carsten Busse, Christian Bombis, Martin M. Knudsen, Kurt V. Gothelf,*

Thomas Strunskus, Christof Wöll, Mats Dahlbom, Bjørk Hammer, Erik Lægsgaard,

Flemming Besenbacher, and Trolle R. Linderoth*

Organized molecular structures are central to the formation

of advanced functional surfaces in the field of nanotechnol-

ogy. The principles of supramolecular chemistry have recently

been applied extensively to molecules adsorbed at surfaces

under ultrahigh-vacuum (UHV) conditions,and complex

molecular structures formed by self-assembly based on weak,

reversible interactions,such as van der Waals forces, ^[1]dipole–

dipole interactions,^[2]hydrogen bonding,^[3,4]or metal com-

plexation,^[5]have been revealed by using local-probe scanning

tunneling microscopy (STM). Similar studies on the covalent

interlinking of molecules adsorbed directly on surfaces under

UHV conditions have been surprisingly scarce. Chemical

reaction has been induced locally with the STM tip,^[6]and a

few cases of monocomponent polymerization have been

reported,^[7]but in several instances compounds anticipated to

react under normal solution conditions have been shown to

instead form structures based on noncovalent interactions.^[4,8]

Herein,we perform a two-component condensation reaction

between aldehydes and amines coadsorbed on a Au(111)

surface,and investigate the conformation and lateral ordering

of the imine reaction products by submolecular-resolution

STM. Covalent interlinking is confirmed by comparison to the

STM signature of the reaction product formed ex situ,as well

as by near-edge X-ray absorption fine structure (NEXAFS)

spectroscopy. A solvent-free reaction path,based solely on

internal proton transfers,is proposed from ab initio density

functional theory (DFT) calculations.

Imines are of fundamental importance in synthetic

chemistry,and the basic reaction mechanism for their

formation from an aldehyde and an amine is shown in

Scheme 1a.^[9]Thin films of imines^[10]and imides^[11]have been

[*] M. M. Knudsen, Prof. K. V. Gothelf

Danish National Research Foundation: Centre for DNA Nano-

technology at iNANO and Department of Chemistry

University of Aarhus (Denmark)

Fax: (+45)8619-6199

E-mail: kvg@chem.au.dk

Dr. S. Weigelt, Dr. C. Busse, Dr. C. Bombis, Dr. M. Dahlbom,

Prof. B. Hammer, Prof. E. Lægsgaard, Prof. F. Besenbacher,

Prof. T. R. Linderoth

Interdisciplinary Nanoscience Center (iNANO), and Department of

Physics and Astronomy

University of Aarhus (Denmark)

Fax: (+45)8942-3690

E-mail: trolle@inano.dk

Scheme 1. a) Generalized solution-catalyzed formation of an imine

from an aldehyde and an amine. b) Thermally induced reaction

between a dialdehyde and octylamine coadsorbed on a Au(111)

surface in UHV.

Dr. C. Busse, Dr. T. Strunskus, Prof. C. Wöll

Lehrstuhl für Physikalische Chemie I

Ruhr-Universität Bochum, 44780 Bochum (Germany)

grown on surfaces by vapor deposition polymerization. In our

study of imine formation at the extreme low-coverage limit

[**] We acknowledge financial support fromthe EU programs FUN-

SMART and PICO-INSIDE, as well as fromthe Carlsberg Founda-

tion, the Danish Technical and Natural Science Research Councils,

and the Danish National Research Foundation. We thank A. H.

Thomsen and M. Nielsen for their help with the synthesis of the

molecules. We acknowledge fruitful discussions with C. Schalley

and G. Witte.

from reactants coadsorbed directly on an inert herringbone-

pﬃﬃﬃ

reconstructed Au(111)-(22 3) surface under UHV condi-

tions,we used the dialdehyde and the aliphatic amine shown

in Scheme 1b. Similar salicylaldehyde-derived compounds

are known to react in solution with amines to form imines.^[12]

The dialdehyde molecules adsorb with their backbones

parallel to the substrate and form well-ordered structures of

monomolecular height (Figure 1a).^[13]The molecular top-

Supporting information for this article is available on the WWW

under http://www.angewandte.org or fromthe author.

Angew. Chem. Int. Ed. 2007, 46, 9227 –9230

ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

9227

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ography is typically dominated by bright protrusions at the

while the sample was held at room temperature (Tꢀ 300 K).

This procedure resulted in a highly ordered structure

(Figure 1c),different from the structures formed by exposing

the surface to the individual reactants. The protrusions in the

STM images associated with the tert-butyl groups form

parallel double rows with the aromatic backbone visible

between two bright protrusions belonging to neighboring

rows. Features with a length matching the length of the

octylamine molecule extend from the aromatic end group

from the side opposite to the tert-butyl group,that is,from the

position originally occupied by the aldehyde group. Alkyl

chains from adjacent molecules align pairwise,similarly to the

lamellar motif observed for the octylamine structure. All

these STM features for the codeposited molecular structure

are consistent with it being formed by the diimine product,as

overlaid in Figure 1c. To confirm this hypothesis,the diimine

was synthesized ex situ by conventional solution-phase tech-

niques and was subsequently evaporated onto a gold substrate

held at a temperature of approximately 120 K to prevent

thermally activated on-surface reactions. Adsorption of the

ex situ reaction product indeed resulted in small structural

domains (Figure 1e) identical to those observed after surface

reaction (Figure 1c). Figures 1c,e depict mirror domains

constructed from opposite surface enantiomers of the di-

imine,as clarified by the overlaid structures. The dimensions

of the unit cells for the in situ and ex situ structures are

identical within experimental error (a = (16.9 Æ 0.9), b =

(26.8 Æ 1.3) , f= (107 Æ 3)8),with the aromatic backbones

tilted at an angle ab,h110i = (25 Æ 4)8 with respect to the

nearest close-packed direction of the Au(111) surface. We

can,thus,conclude that the structures resulting from diimines

formed in situ and ex situ are identical. In Figure 1 f,the

structure is shown in an STM imaging mode that suppresses

the bright tert-butyl,^[13]thereby highlighting the positions of

the aromatic backbone and alkyl chains. The positions of the

alkyl chains with respect to the aromatic backbone match the

expected conformation of the diimine product,which is

stabilized by an intramolecular hydrogen bond between the

imine nitrogen atom and the hydroxy group.

Annealing the molecular surface structures at 400–450 K

led to larger ordered domains of diimine molecules,as

observed for products formed both in situ (Figure 1d) and

ex situ,signaling completion of the reaction,increased surface

mobility,and possibly also switching of the molecular

conformation^[13]at this elevated temperature. Upon deposi-

tion of the diimine formed ex situ,we also observed free

amines on the substrate,indicating either some fragmentation

upon evaporation or the presence of surplus amines in the

molecular powder.

To obtain spectroscopic confirmation that an in situ 2D

surface reaction had occurred,we performed NEXAFS

spectroscopy studies. Carbon 1s spectra of the reactants and

of the ex situ and in situ products on the Au(111) surface are

shown in Figure 2. The peak at E_p1*= 284.9 eV is attributed to

a superposition of peaks arising from carbon atoms on the

aromatic backbone^[14]and is shared by the dialdehyde and the

ex situ and in situ reaction products. As expected,this peak is

absent for the octylamine,which only shows a broad

resonance arising from s orbitals of the alkyl chains. The

ends of the molecules arising from the tert-butyl groups.

Octylamine forms a densely packed lamellar structure as

shown in Figure 1b.

Figure 1. STM images of reactants and products in the reaction of

Scheme 1b on a Au(111) surface. The molecular structures are over-

laid and outlined on certain images. a) Self-assembled island formed

by the dialdehyde (V_t=À1.0 V, I_t=À0.77 nA, bar size 2 nm). b) Lamel-

lar structure formed by octylamine (V_t=+2.0 V, I_t=0.41 nA, bar size

2 nm). c) Self-assembled structure formed by the diimine prepared

in situ at roomtemperature ( V_t=+1.9 V, I_t=0.27 nA, bar size 2 nm).

The unit cell of the structure is indicated. d) Large ordered domain

formed by the diimine prepared in situ and annealed at 450 K

(V_t=À2.2 V, I_t=À0.42 nA, bar size 4 nm). e) Self-assembled structure

formed by the diimine prepared ex situ and dosed onto a sample at

120 K (V_t=À2.0 V, I_t=À0.29 nA, bar size 2 nm). The unit cell of the

structure is indicated. f) Self-assembled structure formed by the

diimine prepared ex situ and observed in the backbone-imaging mode

(V_t=+1.9 V, I_t=0.38 nA, bar size 2 nm).^[13]

To perform the 2D surface reaction,the dialdehyde was

first deposited on the Au(111) substrate to create a submo-

nolayer coverage of molecular islands. Subsequently,the

surface was brought close to saturation of the first monolayer

by exposing it to a vapor of octylamine (p ꢀ 1–5 10^À8mbar),

9228 www.angewandte.org

ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Angew. Chem. Int. Ed. 2007, 46, 9227 –9230

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Angewandte

Chemie

Figure 2. Carbon 1 s NEXAFS spectra of octylamine (green), the dialde-

hyde (blue), the diimine prepared ex situ (black), and the diimine

prepared in situ (red). The spectra were obtained at an incidence angle

of 308.

peak at E_p2*= 286.3 eV stems from the carbon atom in the

aldehyde group,^[14]and is present for the dialdehyde and

absent for the ex situ product,which does not contain an

aldehyde group. For the in situ product (red curve),the

aldehyde peak is significantly reduced,which is the expected

spectroscopic signature for the formation of the imine. A peak

for the imine carbon atom is not observed in the spectrum,as

it is presumably superimposed on the peaks arising from the

aromatic carbon atoms.^[15]

Figure 3. Optimum reaction pathway identified by theoretical model-

ing. The numbers on the graph correspond to the molecular conforma-

tions shown below the graph. See text for details.

It is surprising that the imine formation proceeds under

the extreme UHV conditions,because in the conventional

reaction scheme (Scheme 1a),the last step,in which the

tetrahedral intermediate is converted into the imine product,

is normally catalyzed by the solvent,which acts as a combined

proton donor/acceptor. To establish a plausible reaction path

in the absence of a solvent,state-of-the-art,ab initio DFT

calculations were performed (Figure 3). To reduce the

computational load,the amine and the dialdehyde were cut

off beyond the reacting groups and saturated with hydrogen

atoms,as shown in the starting configuration 1. In the first

step of the reaction path,the amino group undergoes a

nucleophilic addition to the carbonyl group with formation of

the tetrahedral hemiaminal intermediate 2. Next,the phenol

hydroxy group donates a proton to the hydroxy group of the

hemiaminal,and water is eliminated,configuration 3. Finally,

a rotation around the bond between the phenyl ring and the

iminium carbon atom takes place (see Figure S1 in the

Supporting Information),and the iminium group donates a

proton to the phenoxy group (configurations 4–6). In this

reaction pathway,all energy barriers are less than 1.17 eV,

enabling the reaction to proceed at the annealing temperature

of 400 K (an Arrhenius expression gives 0.02 reaction events

per second with a prefactor of 10¹³s^À1). Experimentally,we

observed that formation of the imine also proceeds at room

temperature. This observation indicates the existence of a

more favorable energy pathway or reduced energy barriers

caused by catalytic effects of the underlying metal substrate.

An important feature of the proposed reaction mechanism is

that the phenol hydroxy group acts as an internal proton

donor/acceptor and,thereby,substitutes for the solvent

present under normal solution-phase conditions. Direct con-

version of the tetrahedral intermediate into the imine product

in the absence of a proton-donor/acceptor group has pre-

viously been shown to result in an energy barrier of

approximately 2.4 eV,^[16]which would make the reaction

unlikely at moderate temperatures.

In summary,we have covalently interlinked an aldehyde

and an amine coadsorbed on a Au(111) surface under UHV

conditions. The imine reaction product was observed by STM

to form laterally ordered structures similar to those obtained

from the product synthesized ex situ. The results offer new

prospects for the formation of laterally ordered surface

nanostructures or polymers of high thermal and chemical

stability through interlinking by strong covalent bonds,as

opposed to the weaker,reversible interactions underlying

molecular self-assembly on surfaces.

Experimental Section

The STM experiments were performed in a UHV system equipped

with a noncommercial variable-temperature Aarhus STM (see also

http://www.specs.de).^[17]The Au(111) surface was prepared by argon-

ion sputtering at 1.5 kV followed by annealing at 850 K. Details on

the synthesis of the dialdehyde (1,4-bis[(5-tert-butyl-3-formyl-4-

hydroxyphenyl)ethynyl]benzene)^[18]and the ex situ synthesis of the

diimine

(1,4-bis[(5-tert-butyl-3-octylimino-4-hydroxyphenyl)ethy-

nyl]benzene) can be found in the Supporting Information. The

compounds were evaporated from a heated glass crucible. Octyl-

amine (99%,Aldrich) was held in a glass vial and dosed through a

leak valve.

For the in situ reaction (for STM imaging),a subsaturation

coverage of the dialdehyde (typically less than 1/3 monolayer,as

estimated by STM) was formed by deposition on a sample held at

room temperature. Subsequently,the sample was nearly saturated

with octylamine by dosing for 1–2 min at a pressure of 1–5

10^À8mbar,while the sample was held at room temperature. To

complete the reaction and to enhance the order on the surface,the

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Communications

sample was annealed at temperatures in the range of 400–450 K. The

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order of the deposition of the reactants did not influence the outcome.

For imaging,the STM was cooled to temperatures in the range of 120–

160 K.

NEXAFS spectra were obtained at the beamline HE-SGM at

BESSY II. The reference spectra shown in Figure 2 were obtained for

a saturated monolayer of octylamine (deposition of multilayer at

108 K and subsequent heating to 260 K),a saturated monolayer of the

dialdehyde (deposition of multilayer at room temperature and

[3] J. A. Theobald,N. S. Oxtoby,M. A. Phillips,N. R. Champness,

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the in situ reaction, = of the saturation coverage of the dialdehyde

was evaporated (coverage determined using X-ray photoelectron

spectroscopy (XPS)),and the sample was subsequently cooled to

239 K and exposed to octylamine (27 Langmuir),resulting in the

adsorption of a multilayer of octylamine. The sample was then

annealed at 410 K to desorb multilayers and complete the reaction.

The peak assignment was made based on the peak positions from the

literature of 284.5 eV for aromatic carbon atoms,286.6 eV for

carbonyl carbon atoms,^[14]and 285.9 eV for imine carbon atoms.^[15]

Small shifts of the measured peak positions as compared with those in

the literature as well as between the dialdehyde and the diimine are

due to the difference between a chemisorbed monolayer and a

physisorbed multilayer.

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Org.

All theoretical calculations were made using the DFT code

DACAPO. The exchange–correlation functional PW91 was

employed. The molecules were described in a large (17 15 10 )

supercell,and the wave functions were expanded on a basis of plane

waves with energies up to 25 Ry. Ultrasoft pseudopotentials were

used to describe the atomic cores. The populations of the one-electron

states were stabilized by a broadening of the states according to Fermi

statistics at k_BT= 0.1 eV. The reaction energy pathways and transition

states were determined using the climbing nudged elastic band

(CNEB) method. The positions of the N = 48 atoms in the setup were

optimized until the 3N norms of the 3N forces perpendicular to the

Electron. 2001 , 2 ,75; b) X. D. Wang,K. Ogino,K. Tanaka,H.

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À1

individual reaction paths were smaller than 0.2 eV or until the

energy barrier was insignificant compared to the barriers along other

parts of the reaction paths.

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Received: June 27,2007

Revised: August 17,2007

Published online: October 29,2007

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Microsc. 1988, 152,663.

[18] K. V. Gothelf,A. H. Thomsen,M. Nielsen,R. S. Brown,

WO2004050231, 2004.

Keywords: adsorption · imines · scanning probe microscopy ·

self-assembly · surface chemistry

[1] M. C. Blüm,E. Cavar,M. Pivetta,F. Patthey,W. D. Schneider,

Angew. Chem. 2005 , 117 ,5468; Angew. Chem. Int. Ed. 2005 , 44 ,

5334.

9230 www.angewandte.org

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Angew. Chem. Int. Ed. 2007, 46, 9227 –9230

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Article Doi

Covalent interlinking of an aldehyde and an amine on a Au(111) surface in ultrahigh vacuum

DOI: 10.1002/anie.200702859

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Article abstract of DOI:10.1002/anie.200702859

Full text of DOI:10.1002/anie.200702859

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