Assembly of crystalline halogen-bonded materials by physical vapor deposition

Article abstract of DOI:10.1021/ja8029784

Polycrystalline halogen-bonded assemblies fabricated by physical vapor deposition (PVD) exhibit controllable morphologies and microstructures. Although the solid-state packing may vary going from a solution crystal growth process (used for chromophore single-crystal determination) to a vapor-phase deposition process (used for PVD film fabrication), the corresponding film microstructures are independent of the substrate surface chemistry. Copyright

Full text of DOI:10.1021/ja8029784

Published on Web 06/05/2008
Assembly of Crystalline Halogen-Bonded Materials by Physical Vapor
Deposition
Tanya Shirman,† Dalia Freeman,† Yael Diskin Posner,‡ Isai Feldman,‡ Antonio Facchetti,*,§ and
Milko E. van der Boom*,†
Departments of Organic Chemistry and Chemical Research Support, The Weizmann Institute of Science,
RehoVot 76100, Israel, and Department of Chemistry and the Materials Research Center,
Northwestern UniVersity, 2145 Sheridan Road, EVanston, Illinois 60208
Received April 22, 2008; E-mail: a-facchetti@northwestern.edu; milko.vanderboom@weizmann.ac.il
Organic supramolecular assemblies are often based on a combination
of noncovalent interactions, including van der Waals forces and π-π
stacking, as well as hydrogen bonding (HB).1 The design of proper
molecular synthons with well-defined HB functionalities requires
accurate control over the corresponding film microstructure. HB
interactions have been used to control the growth and microstructure
of films used in a variety of applications including nonlinear optics,
sensors, and transistors.2 Halogen bonding (XB) is an intermolecular
interaction involving a halogen atom (a Lewis acid) and a neutral or
anionic Lewis base.3 XB plays an important role in natural systems4
and has been used for crystal engineering, liquid crystals, and template
synthesis.5 Interestingly, XB exhibits characteristics similar to HB in
directionality and strength; however, actual studies on these issues are
Figure 1. (a) One-dimensional (1D) crystal packing of compound 2 showing
short intermolecular contacts as dashed lines: red, O· · ·I; purple, I· · ·I; blue,
O· · ·H-C; green, F· · ·H-C. (b) Three-dimensional packing of compound 2
showing the layered structure. (c) Helical structure of compound 1.8 Hydrogen
atoms are omitted for (b, c).
far more rare.6 Because of these similarities, an intriguing question is
whether molecules with XB functionalities (e.g. 1, 2) also enable the
fabrication of thin films exhibiting well-defined microstructural
characteristics.
Here we report the growth of highly crystalline halogen-bonded
thin films grown by physical vapor deposition (PVD) on silicon
substrates. Moreover, we demonstrate that the combination of substrate
which is aligned in the same plane. In addition, there are also relatively
short NO· · ·H-C contacts (d ) 2.37 Å, θ ) 169.8°). Introducing the
N-oxide group results in drastic changes in the molecular packing;
however, in both cases, a halogen-bonded network is formed.
surface chemical functionalization with
a
monolayer of
Cl3Si(CH2)11CN (3) and the XB compound molecular structure plays
a major role in determining the film’s morphology and microstructure.
Compound 3 was selected because of the strong interaction between
the CN group and Lewis base moieties of XB molecules.7 Stilbazoles
18 and 2 are ideal molecular candidates owing to the (i) presence of
both XB donor and acceptor functionalities, resulting in the formation
of unimolecular halogen-bonded networks by a one-step deposition
process, (ii) high thermal stability, and (iii) excellent volatility, a
prerequisite for a reliable PVD process.
Differential scanning calorimetry (DSC) of compounds 1 and 2
reveals no thermal transitions before melting, which occurs at 190 and
210 °C, respectively. Thermogravimetric analysis (TGA) reveals
efficient sublimation, demonstrating the volatility and thermal robust-
ness necessary for vacuum PVD. Thin films of compounds 1 and 2
(10 and 100 nm thick) were grown by PVD on bare silicon and silicon
substrates functionalized with a 3-based monolayer. The substrates were
maintained at room temperature during the growth process. 1H NMR
analysis of the PVD-deposited films did not show evidence of
decomposition of compounds 1 and 2. The XB films were characterized
by atomic force microscopy (AFM) and X-ray diffraction (XRD).
AFM images reveal that the film morphology of compound 1
consists of uniform grains regardless of the substrate surface (Figures
2a,b and S2). In contrast, the morphology of the assemblies grown
from the N-oxide derivative (2) depends on the substrate surface
chemical properties. Densely packed grains (height ) 30-60 nm) are
observed on bare silicon substrates (Figure 2c), whereas uniformly
distributed islands (height ) 20-35 nm) are formed on 3-function-
alized substrate monolayers (Figure 2d). The root-mean-square (rms)
roughness of the areas between the islands (∼0.2 nm) corresponds to
the roughness of the 3-based monolayers. Clearly, the N-oxide moiety
of compound 2 plays a dominant structural role in controlling both
the packing of the single crystals grown from solution and the
morphology of the thin films. The interfacial interactions of compound
2 with the substrate surface are apparently strong enough to govern
the growth mode of the entire assembly.
The syntheses of 2 is reported in the Supporting Information. Single-
crystal analysis of compound 1 reveals the presence of a helical XB
motif,8 whereas the molecular packing of the N-oxide derivative 2
consists of a strikingly different layered motif with an interlayer
distance of 3.36 Å (Figure 1). Interestingly, the network of compound
2 consists of two different XB interactions. The first involves one of
the two iodine atoms with the N-oxide moiety (d ) 2.82 Å, θ )
173.9°). This moiety is also in short contact with a second iodine atom
of an adjacent molecule (d ) 3.75 Å, θ1 ) 102.4°, θ2 ) 171.9°),
† Department of Organic Chemistry, The Weizmann Institute of Science.
‡ Department of Chemical Research Support, The Weizmann Institute of Science.
§ Northwestern University.
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10.1021/ja8029784 CCC: $40.75
2008 American Chemical Society

C O M M U N I C A T I O N S
are sufficient to vary the solid-state structure. Surprinsingly, this result
is independent of the Si surface chemistry.
In conclusion, the crystalline assemblies of 1 and 2, fabricated by
PVD, exhibit controllable film morphologies. In particular, the surface
morphology of 2-based assemblies can be varied from elongated
features, cone-shaped grains, and interconnected islands to relatively
large grains. Crystal growth morphology and molecular-based thin film
formation guided by monolayer-based interfaces is of major interest.9
On the other hand, the corresponding film microstructures are
independent of the substrate surface chemistry. This is very surprising
for an organic thin film.2 The molecular packing of 1-based assemblies
is identical to solution grown crystals, whereas the thin films of
compound 2 consist of a different polycrystalline structure. Apparently,
introduction of an N-oxide moiety results in a looser crystal packing,
which varies upon interaction with the substrate surface. However, in
both cases, film XRD experiments demonstrate that the solid-state
structure is independent of the substrate surface chemical functional-
ization. Since PVD is an established method for forming high-quality
molecular-based electronic materials,10 we believe that halogen-bonded
thin films may evolve into high-quality functional materials necessary
for the development of new organic optoelectronic devices.2,11
Figure 2. AFM images of 1- and 2-based films. (a, b) Compound 1 deposited
on bare silicon (10 and 100 nm, respectively); (c, d) compound 2 deposited on
bare silicon (10 nm) and on silicon substrates functionalized with a 3-based
monolayer (10 nm), respectively.
Acknowledgment. This research was supported by Minerva and
the U.S.-Israel Binational Science Foundation. M.E.vd.B. is the
incumbent of the Dewey David Stone and Harry Levine Career
Development Chair.
Supporting Information Available: Experimental details for the
formation of compound 2, PVD experiments with compounds 1 and 2.
This material is available free of charge via the Internet at http://
pubs.acs.org.
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