Magic surface clustering of borazines driven by repulsive intermolecular forces

Article abstract of DOI:10.1002/anie.201300948

It's a kind of magic: Hydroxy pentaaryl borazine molecules self-assemble into small clusters (see structure) on Cu(111) surfaces, whereas with symmetric hexaaryl borazine molecules large islands are obtained. Simulations indicate that the observed magic cluster sizes result from long-range repulsive Coulomb forces arising from the deprotonation of the B-OH groups of the hydroxy pentaaryl borazine. Copyright

Full text of DOI:10.1002/anie.201300948

Angewandte
Chemie
Communications
Self-Assembly on Surfaces
It’s a kind of magic: Hydroxy pentaaryl
borazine molecules self-assemble into
small clusters (see structure) on Cu(111)
surfaces, whereas with symmetric hex-
aaryl borazine molecules large islands are
obtained. Simulations indicate that the
observed “magic” cluster sizes result
from long-range repulsive Coulomb
forces arising from the deprotonation of
S. Kervyn, N. Kalashnyk, M. Riello,
B. Moreton, J. Tasseroul, J. Wouters,
T. S. Jones, A. De Vita,* G. Costantini,*
D. Bonifazi*
&&&& — &&&&
“Magic” Surface Clustering of Borazines
Driven by Repulsive Intermolecular
Forces
À
the B OH groups of the hydroxy pen-
taaryl borazine.
Angew. Chem. Int. Ed. 2013, 52, 1 – 5
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1
These are not the final page numbers!

.
Angewandte
Communications
DOI: 10.1002/anie.201300948
Self-Assembly on Surfaces
“Magic” Surface Clustering of Borazines Driven by Repulsive
Intermolecular Forces**
Simon Kervyn, Nataliya Kalashnyk, Massimo Riello, Ben Moreton, Jonathan Tasseroul,
Johan Wouters, Tim S. Jones, Alessandro De Vita,* Giovanni Costantini,* and Davide Bonifazi*
Dedicated to Professor Maurizio Prato on the occasion of his 60th birthday
The different types of covalent bonds that carbon atoms can
form yield a variety of fascinating structures with unique
structural, chemical, and physical properties.[1] For example,
discrete and extended polycyclic aromatic hydrocarbons
(PAHs), such as graphene[2] or graphene nanoribbons[3] are
emerging as novel, transformational molecular materials.[4]
Replacing carbon by isostructural atoms is also emerging as
a versatile functionalization strategy, for example, to tune
optoelectronic and mechanical properties.[5] In particular, the
tunneling microscopy (STM) imaging. Whereas borazine
1 forms virtually faultless monolayers of interleaved mono-
meric units, hydroxypentaaryl borazine 2 yields exclusive
formation of discrete “magic”[15] clusters.
Following the experimental routes developed for mole-
cule 1,[13b] 2 was obtained after reaction of aniline with BCl3
upon subsequent addition of two equivalents of 2-mesityl-
lithium (MesLi) and one equivalent of water (Scheme 1).[13c]
A small transparent crystal of borazine 2, suitable for X-ray
diffraction, was obtained by solvent evaporation (space
=
À
substitution of C C bonds by B N covalent couples leads to
isoelectronic molecular mimics bearing strong local dipole
moments.[6] This polarity significantly affects the electronic
properties[7] triggering the formation of self-assembled archi-
tectures.[8] In this respect, borazine (B3N3H3) and its deriva-
tives[9] have played a significant role as precursors for
preparing bulk[10] and thin-layer[11] BN-based ceramics and
ultrathin insulators,[12] and B-trimesityl-N-triphenylborazine
derivative 1 was recently proposed as a viable active layer for
optoelectronic devices.[13b] The study of borazine interactions
on solid surfaces has to date been largely unexplored,
although understanding and controlling the assembly of
borazine molecular layers[14] could provide the conceptual
basis to engineer functional supramolecular materials.
À
group: P21/c) and confirms the presence of the acidic B
OH function.
The first step in the formation of the molecular layers is
the thermal deposition of the borazine derivatives onto
a room-temperature Cu(111) surface under ultrahigh vacuum
(UHV). At any coverage, high-resolution low-temperature
(77 K) STM images of borazine 1 acquired on Cu(111) reveal
highly ordered islands (Figure 1) of molecules lying “flat”
with the central borazine core parallel to the substrate. The
individual molecules are imaged as groups of three lobes with
a slightly distorted triangular shape (Figure 1c), where each
lobe is attributable to a Mes group.[16]
At variance with the standard surface-templated organ-
ization picture,[17] the absence of low-lying reactive molecular
groups capable of directional interactions with the substrate is
expected to induce low corrugation of the molecular adsorp-
tion potential. This situation suggests that the Cu(111)
substrate plays a negligible role in the assembly. The assembly
Herein, we report the first bottom-up preparation of
borazine-based supramolecular architectures on a metal sur-
face. We find that hexaaryl and hydroxypentaaryl borazine
derivatives assemble in very different architectures on Cu-
(111) surfaces, as revealed by low-temperature (LT) scanning
[*] S. Kervyn,[+] J. Tasseroul, Prof. Dr. J. Wouters, Prof. Dr. D. Bonifazi
Department of Chemistry and Namur Research College (NARC),
University of Namur (UNamur)
[**] D.B. gratefully acknowledges the EU through the ERC Starting Grant
“COLORLANDS” project, the FRS-FNRS (FRFC contracts no.
2.4.550.09and MIS no. F.4.505.10.F), the “Loterie Nationale”, the
Science Policy Office of the Belgian Federal Government (BELSPO-
IAP 7/05 project), the “TINTIN”ARC project (09/14-023), the MIUR
through the FIRB “Futuro in Ricerca” (“SUPRACARBON”, contract
no. RBFR10DAK6) and the University of Namur (internal funding).
S.K. thanks FRIA-FNRS and the University of Namur for his doctoral
fellowships. J.T. thanks FRIA-FNRS for his doctoral fellowship. G.C.,
T.S.J., and A.D.V. thank the EPSRC for support of this work through
the ULISSE grant (EP/G044864/1). G.C. also acknowledges finan-
cial support from The Royal Society through Grant no. RG100917
and from the Warwick-Santander Fund. Some of the equipment
used in this research was obtained through Birmingham Science
City: “Innovative Uses for Advanced Materials in the Modern
World” with support from Advantage West Midlands and part
funded by the European Regional Development Fund.
Rue de Bruxelles 61, Namur 5000 (Belgium)
E-mail: davide.bonifazi@fundp.ac.be
Prof. Dr. D. Bonifazi
Department of Pharmaceutical and Chemical Sciences and INSTM
UdR Trieste; University of Trieste
Piazzale Europa 1, Trieste 34127 (Italy)
M. Riello,[+] Prof. Dr. A. De Vita
Physics Department, King’s College London
London, WC2R 2LS (UK)
E-mail: alessandro.de_vita@kcl.ac.uk
Dr. N. Kalashnyk,[+] B. Moreton, Prof. Dr. T. S. Jones,
Prof. Dr. G. Costantini
Department of Chemistry, University of Warwick
Gibbet Hill Road, Coventry, CV4 7AL (UK)
E-mail: g.costantini@warwick.ac.uk
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201300948.
[+] These authors contributed equally.
2
www.angewandte.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2013, 52, 1 – 5
These are not the final page numbers!

Angewandte
Chemie
tions. Different from molecule 1, hydroxy borazine 2
self-assembled into small isolated clusters in which
each molecule appears as two aligned Mes lobes
(Figure 2d). Clusters of different sizes were
observed, composed of a minimum of 7 to a max-
imum of about 25 molecules (Figures 2a,b). The
cluster shapes are well-defined and recurrent,
although they do not generally correspond to regular
geometries and appear as different isomers. A
notable exception is the smallest observed cluster,
hexagonal in shape and composed of six outer and
one central borazine 2 (Figures 2c,d).
The observed cluster population shows distinct
preferred “magic” peak values for clusters containing
7, 10, 11, 12, and 13 molecules (Figure 3). While the
7-mers and 13-mers occur with high frequency (28%
and 20%, respectively), clusters bigger than the 13-
mer are much less abundant, with the 14-mer being
exceptionally rare. The interdigitation of aromatic
substituents observed for borazine 1 is still present in
all these clusters (Figure 2d). However, purely
attractive short-range vdW forces cannot explain
Scheme 1. Top: synthetic pathway for hydroxy- and methyl-bearing borazines 2
and 3. Below: chemical formula of trimesityl borazine 1 and ORTEP representa-
tion of 2 as determined by X-ray diffraction analysis (blue N, red O, yellow B,
gray C; atomic displacement parameters, obtained at 293 K, are set at the 50%
probability level).
Figure 1. STM images for borazine 1 on Cu(111), deposited at 300 K,
imaged at 77 K. a) Large area of the Cu(111) surface, top right:
molecular island; bottom: bare Cu(111) surface. b)–c) Expanded views
of the molecular islands; the three circles in the triangle in (c)
correspond to the three Mes moieties; the shaded rectangle corre-
sponds to the experimental unit cell. d) Calculated structure super-
imposed on the imaged monolayer.
Figure 2. STM images of borazine 2 on Cu(111). a) Isolated molecular
clusters, highlighting the regular 7-mers. b) Expanded view of a Cu(111)
step edge, revealing two enantiomers of an irregular cluster.
c) Expanded view of two 908-rotated regular 7-mers. d) Calculated
molecular model superimposed onto the 7-mer (purple circles corre-
spond to the Mes moieties).
is instead driven by van der Waals (vdW) attractive forces
through intermolecular interdigitation of the Mes and Ph
rings of each molecule with those of its six nearest neighbors
the observed formation of small clusters. A simple kinetic
Monte Carlo (KMC) simulation using a discrete honeycomb
lattice reveals that for any realistic vdW bonding strength,
large molecular islands are the only possible assembly
outcome in the relevant temperature/coverage range. This
result suggests that repulsive interactions must also be present
and determine the observed cluster distribution.
We analyzed our findings by means of a simple model
where each molecule is allowed to bind with up to six
neighbors, consistent with the general hexagonal shape of the
(Figure 1d).
A Molecular Dynamics simulation of the
system[18] for 2 ns at 250 K followed by cooling and equilibra-
tion at 77 K for 8 ns produced a stable interdigitated structure
whose detailed features are in excellent agreement with the
observed topography (Figure 1d).
In a second series of experiments, borazine 2 was sublimed
on Cu(111) under the same submonolayer coverage condi-
Angew. Chem. Int. Ed. 2013, 52, 1 – 5
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
3
These are not the final page numbers!

.
Angewandte
Communications
mer. However, multiple deprotonations in large clusters
would entail repulsions between many pairs of deprotonated
molecules. This situation is expected to destabilize the large
clusters for any fixed fraction of deprotonated molecules as
the repelling pair number scales quadratically with cluster
size. Consistently, we very rarely observe clusters larger than
approximately 20 molecules.
These considerations were incorporated into a simple
model Hamiltonian based on a discrete hexagonal lattice and
including an attractive nearest-neighbor energy term and
a repulsive 1/r3-scaling term (see Supporting Information).
The rates associated with the cluster energies produced by
this Hamiltonian were then used to predict the equilibrium
cluster population, by means of a KMC simulation where
cluster pairs exchange individual molecules, while 14-mers are
allowed to occasionally split into two 7-mers. The distribution
predicted by the model is reported in Figure 3 (blue histo-
gram). The good agreement found confirms that the dipole-
induced repulsion and the vdW attraction are the key
parameters controlling the assembly in this system. This
suggests that tuning their ratio in modified borazines of
similar size could selectively promote 7-mers over larger
clusters containing more than one deprotonated monomer.
This is consistent with our modelꢀs predictions (see Support-
ing Information).
To check our conclusions and investigate possible molec-
ular symmetry effects on the “magic” clustering process
(assuming a conformation of the aryl rings perpendicular to
the plane of the borazine ring, molecules 1 and 2 belong to the
D3h and C2v point groups, respectively), we synthesized
a reference, non-acidic methylpentaaryl borazine derivative
3 (point group: C2v, see Scheme 1 and Supporting Informa-
tion). Upon deposition on Cu(111), borazine 3 assembles into
large islands similar to 1 (Figure S7), strongly supporting the
conclusion that the “magic” cluster building of 2 is driven by
long-range Coulomb repulsions arising from deprotonation
Figure 3. Experimental (red) and theoretical (blue) cluster size distribu-
tion of borazine 2.
B3N3 core (Supporting Information, Figure S8). This readily
highlights the 7-mer as a first stable “magic” cluster in which
all molecules have at least three molecular neighbors (Fig-
ure 2d). From here, the formation of an 8-mer and a 9-mer
involves the iterated addition of a molecule bound to just two
neighbors, which makes these clusters progressively less
stable, consistent with the observations.
The 10-mer is the next expected “magic” number in which
all molecules have at least three molecular neighbors (see also
Figure S8 and text in the Supporting Information). We note
À
that the acidic B OH moiety in molecule 2 (pKa ꢀ 9) can
undergo deprotonation, leading to negatively charged ad-
sorbed molecules, each effectively carrying a standing neg-
ative electrostatic dipole as result of the metal substrate
(image–charge) screening. Deprotonation is expected to be
promoted by polarization of neighboring molecules, screening
the dipole formation.[19] Thus, borazine 2 molecules nested
“inside” a cluster will be the most likely ones to undergo
deprotonation. Any pair of electric dipoles associated with
a couple of deprotonated molecules will however involve
a repulsive long-range 1/r3 energy term, where r = distance
between dipoles.
À
À
reactions, as these reactions can occur in B OH but not in B
CH3.
In conclusion, we have demonstrated the first bottom-up
preparation of borazine-based supramolecular architectures
on a metal surface. Detailed STM studies show that molecule
1 self-organizes into large islands, whereas OH-bearing
borazine 2 undergoes exclusive “magic” cluster building.
These findings are rationalized as a delicate interplay of short-
range vdW attractions between neighboring molecules and
long-range Coulomb repulsions between deprotonated
charged molecules. This picture is supported by theoretical
modeling and further experiments on reference molecule 3
where deprotonation cannot occur and clustering is never
observed. More generally, our theoretical model further
suggests that the dipole strength and the vdW attraction
between monomers could be independently varied in experi-
ments designed to control the degree of monodispersion in
the cluster size population.
Indirect evidence of multiple deprotonation already
occurring inside 10-mers (the smallest cluster with two non-
perimeter molecules, Figure S8) is provided by STM images
showing many different geometric isomers (Figure S5). This
effect, consistent with the presence of repulsion forces within
the cluster, is even more pronounced in the observed 11-mer
and 12-mer population (Figure S6). The next in the series, the
13-mer cluster, is once more predicted to be distinctively
stable, being the first “magic” cluster hosting two non-
peripheral non-nearest-neighbor molecules able to undergo
deprotonation while still only moderately repelling each
other. This is consistent with the high observed occurrence of
13-mers (Figure 3).
The following 14-mer is the smallest cluster capable of
decaying into two magic clusters. Such a process would be
promoted by the repulsion between two charged molecules
hosted in the 14-mer, which could explain the relatively rare
occurrence of 14-mers and the high occurrence of 7-mers
(Figure 3). Non-neighboring deprotonated molecule pairs
could be accommodated in any cluster larger than the 14-
Received: February 2, 2013
Revised: April 5, 2013
Published online: && &&, &&&&
4
www.angewandte.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2013, 52, 1 – 5
These are not the final page numbers!

Angewandte
Chemie
[11] L. Ci, L. Song, C. Jin, D. Jariwala, D. Wu, Y. Li, A. Srivastava,
Keywords: borazine · scanning tunneling microscopy ·
self-assembly · supramolecular chemistry
.
Z. F. Wang, K. Storr, L. Balicas et al., Nat. Mater. 2010, 9, 430 –
435.
[12] a) M. Corso, W. Auwꢅrter, M. Muntwiler, A. Tamai, T. Greber, J.
Osterwalder, Science 2004, 303, 217 – 220; b) J. Xue, J. Sanchez-
Yamagishi, D. Bulmash, P. Jacquod, A. Deshpande, K. Wata-
nabe, T. Taniguchi, P. Jarillo-Herrero, B. J. LeRoy, Nat. Mater.
2011, 10, 282 – 285.
[1] a) J. L. Delgado, M. ꢁ. Herranz, N. Martin, J. Mater. Chem.
2008, 18, 1417 – 1426; b) L. Chen, Y. Hernandez, X. Feng, K.
Mꢂllen, Angew. Chem. 2012, 124, 7758 – 7773; Angew. Chem. Int.
Ed. 2012, 51, 7640 – 7654.
[13] a) S. Kervyn, T. Nakanishi, J. Aimi, A. Saeki, S. Seki, B.
Champagne, D. Bonifazi, Chem. Lett. 2012, 41, 1210 – 1212; b) S.
Kervyn, O. Fenwick, F. Di Stasio, Y. Sig Shin, J. Wouters, G.
Accorsi, S. Osella, D. Beljonne, F. Cacialli, D. Bonifazi, Chem.
Eur. J. 2013, 19, 7771 – 7779; for the first report describing OH-
functionalized borazines by-products see: c) S. Allaoud, B.
Frange Inorg. Chem. 1985, 24, 2520 – 2523.
[2] A. K. Geim, K. S. Novoselov, Nat. Mater. 2007, 6, 183 – 191.
[3] L. Dçssel, L. Gherghel, X. Feng, K. Mꢂllen, Angew. Chem. 2011,
123, 2588 – 2591; Angew. Chem. Int. Ed. 2011, 50, 2540 – 2543.
[4] a) F. Chen, N. J. Tao, Acc. Chem. Res. 2009, 42, 429 – 438; b) T. N.
Hoheisel, S. Schrettl, R. Szilluweit, H. Frauenrath, Angew.
Chem. 2010, 122, 6644 – 6664; Angew. Chem. Int. Ed. 2010, 49,
6496 – 6515.
[14] a) J. V. Barth, G. Costantini, K. Kern, Nature 2005, 437, 671 –
679; b) J. A. A. W. Elemans, S. Lei, S. De Feyter, Angew. Chem.
2009, 121, 7434 – 7469; Angew. Chem. Int. Ed. 2009, 48, 7298 –
7333; c) D. Bonifazi, S. Mohnani, A. Llanes-Pallas, Chem. Eur. J.
2009, 15, 7004 – 7025; d) R. Otero, J. M. Gallego, A. L. Vꢆzquez
de Parga, N. Martꢇn, R. Miranda, Adv. Mater. 2011, 23, 5148 –
5176.
[15] a) M. Bçhringer, K. Morgenstern, W.-D. Schneider, R. Berndt, J.
Phys. Condens. Matter 1999, 11, 9871 – 9878; b) A. D. Jewell,
S. M. Simpson, A. Enders, E. Zurek, E. C. H. Sykes, J. Phys.
Chem. Lett. 2012, 3, 2069 – 2075; c) C. Iacovita, P. Fesser, S.
Vijayaraghavan, M. Enache, M. Stçhr, F. Diederich, T. Jung,
Chem. Eur. J. 2012, 18, 14610 – 14613.
[16] a) F. Rosei, M. Schunack, P. Jiang, A. Gourdon, E. Lægsgaard, I.
Stensgaard, C. Joachim, F. Besenbacher, Science 2002, 296, 328 –
331; b) L. Gross, K.-H. Rieder, F. Moresco, S. M. Stojkovic, A.
Gourdon, C. Joachim, Nat. Mater. 2005, 4, 892 – 895.
[17] Selected examples: a) T. Classen, G. Fratesi, G. Costantini, S.
Fabris, F. Louis Stadler, C. Kim, S. de Gironcoli, S. Baroni, K.
Kern, Angew. Chem. 2005, 117, 6298 – 6301; Angew. Chem. Int.
Ed. 2005, 44, 6142 – 6145; b) L.-A. Fendt, M. Stçhr, N. Wintjes,
M. Enache, T. Jung, F. Diederich, Chem. Eur. J. 2009, 15, 11139 –
11150.
[5] S. Kervyn, C. Aurisicchio, D. Bonifazi in Functional Supra-
molecular Architectures for Organic Electronics and Nanotech-
nology (Eds.: P. Samori, F. Cacialli), Wiley, Weinheim, 2010;
b) T. A. Schaub, M. Kivala Arkivoc 2013, i, 175 – 184.
[6] a) Z. Q. Liu, T. B. Marder, Angew. Chem. 2008, 120, 248 – 250;
Angew. Chem. Int. Ed. 2008, 47, 242 – 244; b) D. Golberg, Y.
Bando, Y. Huang, T. Terao, M. Mitome, C. Tang, C. Zhi, ACS
Nano 2010, 4, 2979 – 2993; c) K. Watanabe, T. Taniguchi, T.
Niiyama, K. Miya, M. Taniguchi, Nat. Photonics 2009, 3, 591 –
594; d) K. Watanabe, T. Taniguchi, H. Kanda, Nat. Mater. 2004,
3, 404 – 409.
[7] a) A. J. V. Marwitz, A. N. Lamm, L. N. Zakharov, M. Vasiliu,
D. A. Dixon, S.-Y. Liu, Chem. Sci. 2012, 3, 825 – 829; b) A. J. V.
Marwitz, M. H. Matus, L. N. Zakharov, D. A. Dixon, S.-Y. Liu,
Angew. Chem. 2009, 121, 991 – 995; Angew. Chem. Int. Ed. 2009,
48, 973 – 937; c) C. Zhi, Y. Bando, T. Terao, C. Tang, H.
Kuwahara, D. Golberg, Adv. Funct. Mater. 2009, 19, 1857 –
1862; d) M. Cꢃtꢄ, P. Haynes, C. Molteni, Phys. Rev. B 2001, 63,
125207.
[8] M. J. D. Bosdet, C. A. Jaska, W. E. Piers, T. S. Sorensen, M.
Parvez, Org. Lett. 2007, 9, 1395 – 1398.
[9] a) A. Stock, E. Pohland, Ber. Dtsch. Chem. Ges. B 1926, 59,
2215 – 2223; b) S. J. Groszos, S. F. Stafiej, J. Am. Chem. Soc. 1958,
80, 1357 – 1360; c) A. Wakamiya, T. Ide, S. Yamaguchi, J. Am.
Chem. Soc. 2005, 127, 14859 – 14866; d) J. Haberecht, R. Nesper,
H. Grꢂtzmacher, Chem. Mater. 2005, 17, 2340 – 2347.
[10] a) J.-S. Li, C.-R. Zhang, B. Li, Appl. Surf. Sci. 2011, 257, 7752 –
7757; b) J. Li, S. Bernard, V. Salles, C. Gervais, P. Miele, Chem.
Mater. 2010, 22, 2010 – 2019.
[18] An island composed of 36 molecules of borazine 1 placed on top
of a four-layered metallic substrate slab yielding a (28.8 ꢈ
28.8 nm) surface unit cell, periodically repeated.
[19] J. B. Neaton, M. S. Hybertsen, S. G. Louie, Phys. Rev. Lett. 2006,
97, 216405.
Angew. Chem. Int. Ed. 2013, 52, 1 – 5
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!