Self-assembled phthalocyanine derivatives on highly ordered pyrolytic graphite

Article abstract of DOI:10.1142/S1088424614500552

A novel unsymmetrically substituted hydroxy-functionalized Zn(II) phthalocyanine (Pc) 1, bearing long aliphatic chains, namely, dodecyloxy units, has been designed and synthesized to investigate the influence of the terminal hydroxyl group on the formation of self-assembled nanostructures. The symmetric derivative, octadodecyloxy-Zn(II)Pc (2) has been also synthesized and used as reference compound for comparison purposes. The supramolecular organization of the Pcs has been carried out by spin-coating on a highly ordered pyrolytic graphite (HOPG) surface and has been investigated by atomic force microscopy (AFM) and scanning electron microscopy (STM). AFM and STM studies showed that unsymmetrically substituted hydroxy-functionalized Zn(II)Pc1 gives rise to the formation of wire-like structures in different lengths from nanometer to micrometer scales, whereas in the case of the symmetrical Zn(II)Pc2 the formation of the wires on HOPG was less pronounced.

Full text of DOI:10.1142/S1088424614500552

Journal of Porphyrins and Phthalocyanines
J. Porphyrins Phthalocyanines 2014; 18: 771–777
DOI: 10.1142/S1088424614500552
Published at http://www.worldscinet.com/jpp/
Self-assembled phthalocyanine derivatives on highly
ordered pyrolytic graphite
S. Gokhan Colaka and Mine Ince*a,b◊
a Advanced Technology Research & Application Center, Mersin University, Ciftlikkoy Campus, TR-33343 Mersin,
Turkey
b Department of Energy Systems Engineering, Mersin University, Tarsus Faculty of Technology, TR-33480 Mersin,
Turkey
Dedicated to Professor Nagao Kobayashi on the occasion of his 65th birthday
Received 13 July 2014
Accepted 18 August 2014
ABSTRACT:A novel unsymmetrically substituted hydroxy-functionalized Zn(II) phthalocyanine (Pc) 1,
bearing long aliphatic chains, namely, dodecyloxy units, has been designed and synthesized to investigate
the influence of the terminal hydroxyl group on the formation of self-assembled nanostructures. The
symmetric derivative, octadodecyloxy-Zn(II)Pc (2) has been also synthesized and used as reference
compound for comparison purposes. The supramolecular organization of the Pcs has been carried out by
spin-coating on a highly ordered pyrolytic graphite (HOPG) surface and has been investigated by atomic
force microscopy (AFM) and scanning electron microscopy (STM). AFM and STM studies showed
that unsymmetrically substituted hydroxy-functionalized Zn(II)Pc 1 gives rise to the formation of wire-
like structures in different lengths from nanometer to micrometer scales, whereas in the case of the
symmetrical Zn(II)Pc 2 the formation of the wires on HOPG was less pronounced.
KEYWORDS: phthalocyanines, self-assembly nanostructures, molecular wires, supramolecular
electronics.
molecular photovoltaics, optoelectronic applications, and
INTRODUCTION
in the study of photoinduced electron transfer processes
The construction of self-assembled nanostructures by
due to their suitable photophysical properties, self-
meansofsupramolecularinteractionssuchaselectrostatic,
organization ability, and remarkable chemical and thermal
hydrogen bonding, p–p stacking, and coordinative
bonding (metal–ligand) has received much attention
in the field of optoelectronics since the spontaneous
stability[3]. Pcsaremacroheterocycliccompoundshaving
18-p electron system and are synthetic analogs of the
naturally occurring porphyrins. The electronic absorption
self-organization of the molecules into highly ordered
spectrum of Pcs shows two main bands, the Q-band and
supramolecularstructures, withcontrolleddimensionsand
size is a key point for the development of efficient devices
the Soret or B-band. The Q-band, associated to p–p*
HOMO–LUMO transition, is usually found in the region
[1]. Generally, the performance of such optoelectronic
of 620–700 nm and is responsible for the deep green
devices is dominated by molecule-to-molecule and
or blue color of these compounds. These bands can be
molecule-to-surface supramolecular interactions [2]. In
easily tuned by the careful choice of the metal atom in the
particular, Pcs, amongtheorganicdyes, areoneofthemost
macrocycles cavity and by placing adequate substituents
extensively studied functional molecular components for
at the macrocycles peripheral and axial positions [4]. The
unique physicochemical properties of Pcs have prompted
the incorporation of these macrocycles in widespread
◊SPP full member in good standing
applications in materials science. Up to date, a wide range
of supramolecular interactions have been used in the
*Correspondence to: Mine Ince, email: mine.ince@mersin.edu.
tr, tel: +90 324-361-0001-4980, fax: +90 324-361-0153
Copyright © 2014 World Scientific Publishing Company

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S.G. COLAK AND M. INCE
fabrication of Pc-based nanoscale functional systems
to improve their photophysical and (opto)electronic
properties on surfaces and in liquid crystalline phases
as a result of interactions between the dyes and/
or substrate [5]. For instance, Pcs bearing flexible
peripheral substituents represent typical examples of
discotic mesogens in which the molecules form efficient
columnar stacks and some of them produce high-
charge carrier mobility, due to intense p–p interactions
between the macrocycle cores [6]. Concerning the
“bottom-up” approach for the construction of Pcs based
systems on surfaces, the formation of two-dimensional
self-assembled monodispersed molecules on HOPG
has been achieved by using the single-molecule magnet
alkoxy-substituted bis(phthalocyaninato)terbium(III)
[7]. Similarly, the highly ordered self-assembled struc-
ture of octa-alkoxyl substituted Pc on the HOPG surface
has been also reported [8]. The enhanced interactions
of Pc molecules with the substrate as the result of the
presence of octa-alkoxy groups allows the formation
of quadratic lattices which could be used as templates
for trapping individual molecules. As an interesting
example, Torres and co-workers reported a covalently
linked Pc-fullerene conjugate which is able to form
highly organized supramolecular films and fibers on
HOPG [9].
EXPERIMENTAL
General
All chemicals were purchased from Aldrich and used
without further purification. Column chromatography
was carried out on silica gel Merck-60 (230–400 mesh,
60 Å), and TLC was carried out on aluminium sheets
percolated with silica gel 60 F254 (E. Merck). The IR
spectra were performed with Perkin–Elmer, FT-IR/
MIR-FIR (ATR, Attenuated total reflectance) spectro-
photometer. MALDI TOF-MS spectra were determined
on a BRUKER Microflex LT. NMR spectra were
recorded with a Bruker AC-400 instrument. UV-vis
spectra were recorded with a Analytic JENA S 600
UV-vis spectrophotometer. Atomic Force Microscopy
(AFM) and Scanning Tunneling Microscopy (STM)
studies were carried out using a Park System XE-100
SPM instrument under ambient conditions using a
commercial scanning probe microscope. The AFM
topographic images were analyzed using Park Systems
XEI software. The samples of Pcs in an organic solvent
were spin-coated onto HOPG surface and measured
under ambient conditions.
Synthesis
In this context, the present paper is focused on the
synthesis of a novel unsymmetrically substituted hydroxy-
functionalized Zn(II)Pc (1) (Chart 1) which bears long
aliphatic dodecyloxy chains for the construction of pro-
perly ordered Pc-based nanostructures on a HOPG surface.
Moreover, a symmetric derivative, octadodecyloxyZn(II)
Pc (2) was synthesized and used as reference compound.
Supramolecular organization of the Pcs on HOPG was
examined by AFM and STM in order to investigate
the influence of terminal hydroxyl groups on the self-
assembly features of the Pcs. The self-organizational
differences between the unsymmetrical Zn(II)Pc 1 and the
structurally related symmetric Zn(II)Pc 2 has been also
discussed.
Synthesis of Zn(II)Pc derivatives 1 and 2. A mix-
ture of 4,5-bis(dodecyloxy)phthalonitrile (3) (620 mg,
1.25mmol), 4-[(4-hydroxymethyl)phenyl-ethynyl]phth-
alonitrile (4) (80 mg, 0.31 mmol) and Zn(OAc)2 (90 mg,
0.49 mmol) in DMAE (6 mL) was heated at reflux
for 24 h with stirring under argon atmosphere. After
cooling to room temperature, the solvent was removed
and the residue was washed with a MeOH/H2O (5:1)
mixture. The crude product was purified by column
chromatography on silica gel (toluene/THF, 12:1).
The symmetric Zn(II)Pc 2 was eluted first, obtaining
210 mg (0.10 mmol, 32% yield), followed by Zn(II)Pc
1, 155 mg (0.08 mmol).
R1
R1
R1
R1
N
OH
N
N
N
N
1: R1 = OC12H25, R2 =H, R3 =
2: R1 = R2 = R3 = OC12H25
Zn
N
N
N
R2
R1
R3
Chart 1. Molecular structures of Zn(II)Pcs 1 and 2
R1
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773
Zinc(II) 9,10,16,17,23,24-hexadodecyloxy-2-[2´-(4´-
well-knownprocess.Theapproachrelyingonthestatistical
reaction of two different phthalonitrile precursors A and
B in a ratio of 3:1 to 9:1 appears to be the most commonly
applied. However, this method produces a mixture of all
the possible macrocycles (A4, A3B, A2B2, AB3, etc.), and
the isolation of desired Pcs from the mixture usually
requires extensive chromatographic purification. In this
regard, the steric effect of the substituents and ratio
and reactivity of the phthalonitrile precursors have to
be taken into account to facilitate the separation and to
increase the yield of the desired unsymmetric compound.
Thus, the condensation reaction of 4,5-bis(dodecyloxy)
phthalonitrile (3) (four equivalents) with the corres-
ponding hydroxy-functionalized phthalonitrile 4 (one
equivalent) in the presence of zinc(II) acetate generates
a statistical mixture of compounds in which the major
components are symmetrically substituted Pc 2, followed
by unsymmetrically substituted hydroxy-functionalized
Pc 1. Purification by silica gel column chromatography
gave rise to Zn(II)Pc 1 in 27% and Zn(II)Pc 2 in 32%
yield.
(hydroxymethyl)-phenyl)ethynyl]phthalocyaninato(2-)-
N29, N30, N31, N32 (1). Yield 155 mg (27%). 1H NMR (THF-
d8, 400 MHz): d, ppm 9.7 (s, 2H; Ar-H), 9.3 (s, 1H; Ar-H),
9.06 (d, 1H, J = 8 Hz, Ar-H), 8.6 (s, 1H; Ar-H), 8.5 (s, 1H;
Ar-H), 8.5–8.4 (m, 2H; Ar-H), 8.1 (d, J = 8 Hz, 1H, Ar-H),
7.82 (d, J = 8 Hz, 2H, Ar-H), 7.54 (d, J = 8 Hz, 2H, Ar-H),
4.73 (d, J = 4 Hz, 2H, Ar-H), 4.6–4.4 (m, 12H; Alkyl-H),
2.2–2.1 (m, 12H; Alkyl-H), 1.9–1.7 (m, 12H; Alkyl-H),
1.3–1.2 (m, 96 H, Alkyl-H) 0.9–0.8 (m, 18H; Alkyl-H). IR
(ATR): n, cm-1 2955, 2853, 1763, 1712, 1602, 1485, 1384,
1281, 1045, 881, 744. UV-vis (THF): lmax, nm (log e) 687
(4.96), 672 (4.91), 611 (4.3), 356 (4.76). MS (MALDI,
dithranol): m/z 1813.2 [M]+.
Zinc(II) 2,3,9,10,16,17,23,24-octakis(dodecyloxy)-
phthalocyaninato(2-)-N29,N30,N31,N32 (2).Yield 210mg
(32%). 1H NMR (THF-d8, 400 MHz): d, ppm 8.8 (s, 8H;
Ar-H), 4.6–4.5 (m, 16H; Alkyl-H), 2.1–2.0 (m, 16H;
Alkyl-H), 1.4–1.2 (m, 144H; Alkyl-H), 0.9–0.8 (m, 24H;
Alkyl-H). IR (ATR): n, cm-1 2956, 2918, 2849, 1730,
1606, 1495, 1456, 1381, 1277, 1201, 1094, 1072, 853,
798. UV-vis (THF): lmax, nm (log e) 672 (5.05), 646
(4.31), 607 (4.30), 355 (4.68). MS (MALDI, dithranol):
m/z 2052.4 [M]+.
Due to the presence of the long alkyl chains, both
Pc compounds are highly soluble in common organic
solvents such as CH2Cl2, CHCl3, toluene, and THF. The
structure and purity of both compounds was checked by
1H NMR, IR, UV-vis and MALDI-TOF spectroscopy.
1
Generally, the H NMR spectra of Zn(II)Pc derivatives
RESULTS AND DISCUSSION
are poorly resolved in non-coordinating solvents like
CHCl3 due to aggregation. However, Zn(II)Pcs 1 and 2
gave well-resolved and informative 1H NMR spectra in a
coordinating solvent such as THF-d8 where each proton
of the Pc macrocycle was easily assigned with signals
Synthesis
The hydroxy-functionalized Zn(II)Pc 1 was synthe-
sized following the routes depicted in Scheme 1. The
4,5-bis(dodecyloxy)phthalonitrile (3) [10], and 4-[(4-
hydroxymethyl)phenyl-ethynyl]phthalonitrile (4) [6c]
were successfully synthesized according to literature
procedures. It is known that the synthesis of unsymme-
trically substituted Pcs consisting of three identical
isoindole subunits and a fourth different one (A3B)
by statistical cross-condensation of two-differently
substituted phthalonitrile precursors is a relatively
1
between 7.8 and 9.6 ppm. Figure 1 shows the H NMR
spectrum of Zn(II)Pc 1 as an example.
UV-visible spectroscopy
It is well-established that the electronic absorption
spectra of Pcs strongly depends on the solvent polarity
[4]. For example, in non-coordinating solvents such as
toluene or CHCl3, often, the absorption corresponding to
C12H25
O
O
CN
CN
OC12H25
OC12H25
OC12H25
OC12H25
C12H25
O
C12H25
O
C12H25
C12H25O
C12H25O
N
N
3
N
N
N
N
N
N
N
N
Zn(OAc)2
Zn
Zn
+
N
+
N
N
N
DMAE, reflux
NC
NC
N
N
OC12H25
OC12H25
C12H25O
C12H25O
OH
C12H25O
C12H25O
OH
4
2
1
Scheme 1. Synthetic route to symmetrical and hydroxy-functionalized Zn(II)Pcs 1 and 2
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S.G. COLAK AND M. INCE
Fig. 1. 1H NMR spectrum of Zn(II)Pc 1 in THF-d8
the Q-band observed as broad band in the range of 600–
700 nm due to intermolecular p–p interaction between
two Pc macrocycles. However, this interaction can be
broken up in coordinating solvents such as THF which
provides axial coordination to the zinc metal center that
avoidsmacrocycleaggregation.UV-visabsorptionspectra
of Zn(II)Pcs 1 and 2 in toluene are depicted in Figs 2 and 3.
For comparison purposes, UV-vis experiments were also
carried out in a polar and coordinating solvent such as
THF. ZnPc(II) 1 exhibits a split Q-band with maximum at
690nmintoluene.TheabsorptionspectrumofZn(II)Pcs1
Fig. 3. UV-vis absorption spectra of Zn(II)Pc 2 in THF (solid
line) and toluene (dashed line) (~1 × 10-5 M)
in THF is very similar to the one in toluene with
absorption maximum at 687 nm. This result suggests that
Zn(II)Pc 1 does not display any important aggregation
features in toluene.
The UV-vis absorption spectrum of symmetric Zn(II)
Pc 2 in THF exhibits sharp Q-band with an absorption
maximum located at 672 nm corresponding to the
monomeric species (Fig. 3). When the solvent changes
from THF to toluene, similar absorption feature is
observed. Nevertheless, a small red shift of the Q-band
(ca. 5 nm) was observed in toluene. On the other hand,
in comparison with symmetric Zn(II)Pc derivative 2,
Fig. 2. UV-vis absorption spectra of Zn(II)Pcs 1 in THF (solid
line) and toluene (dashed line) (~1 × 10-5 M)
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Fig. 4. (a) Tapping-mode AFM images of Zn(II)Pc 1 on HOPG. (b) AFM image of a magnified area of (a)
Fig. 5. (a) High-resolution STM analysis of a Zn(II)Pc 1 molecular wire on HOPG. (b) The length and height profile along the
red line in (a) (line scan). (c) Schematic representation of the possible organization of Zn(II)Pc 1 on HOPG surface. (d) Estimated
molecular dimensions of Zn(II)Pc 1: macrocyclic core is to be 1.1 nm, Pc core with terminal hydroxyl group is to be 1.9 nm and the
length of dodecyloxy chain is 1.6 nm
the Q-bands of Zn(II)Pc 1 undergo a red shifting, i.e.
15 nm, with concomitant splitting of the Q-band which
symmetrically substituted Zn(II)Pc 2 does not exhibit.
investigated by AFM and STM. A freshly prepared
solution of Zn(II)Pcs in toluene (c~10-5 M) was deposited
by a spin-coating technique on HOPG and allowed to
evaporate. AFM studies of Zn(II)Pc 1 on HOPG surfaces
revealed the formation of wire-like structures in different
lengths from nanometer to micrometer scales (Fig. 4a).
Detailed AFM investigations confirmed that Zn(II)Pc 1
self-assembles into well-ordered p-stacks on a HOPG
Organization of Zn(II)Pcs on a HOPG surface
Supramolecular organization of unsymmetrical Zn(II)
Pc 1 and symmetric Zn(II)Pc 2 on HOPG surface was
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S.G. COLAK AND M. INCE
Fig. 6. (a) High-resolution STM analysis of Zn(II)Pc 2 on HOPG. (b) Magnified STM image of the selected area
surface (Fig. 4b). The lengths of the wires on the surface
are in the range of 3–10 mm, and the diameter of the wires
are between 40–60 nm.
STM (Fig. 6). This could be explained by the fact that the
strong p–p interactions between Pc molecules are more
pronounced in the case of symmetric Pc compounds.
Although the molecular pattern of symmetric Pc 2 seems
to be similar to that of unsymmetrical derivative, it
should be noted that, in the case of Zn(II)Pc 1 molecular
patterns were obtained from the nanowire which is
shown in Fig. 4.
WithintheunsymmetricalPcmolecules,thelongdodecyl
chains are hydrophobic and the remaining part, the –OH
moiety is relatively hydrophilic. On the hydrophobic
HOPG surface, wire self-aggregates were only observed
with unsymmetric Pc 1. However, organization of 1 was
not detected on hydrophilic surfaces such as mica and
quartz. Furthermore, amorphous and irregular structures
were observed during studies carried on hydrophilic
surfaces with both unsymmetric and symmetric Pcs. This
result shows that hydrophobic surfaces play an important
role in wire formations. Thus, molecules surrounded with
long –OR moieties are stabilized on the hydrophobic
surface by the assistance of appropriate interaction of the
long chains in the main core [11].
To gain further insight into the self-organization
of Zn(II)Pc 1 on HOPG surface, STM studies were
performed. High-resolution STM analysis of a Zn(II)Pc 1
molecular wire on HOPG is shown in Fig. 5. The STM
image is given in Fig. 5 was obtained by focusing on the
molecular wire structure determined by AFM measure-
ment (Fig. 4). STM studies reveal the formation of
single stacks of parallel-oriented ZnPc 1, in which the Pc
molecules are tilted with respect to the surface (Fig. 5b).
This nanowire arrangement on the surface could result
from the intermolecular p–p interactions of macrocyclic
core as well as Zn—OH interactions between the central
metal of Zn(II)Pc 1 and the terminal oxygen which
hindering the formation of a closed-packing array on
the HOPG.
A closer look at the STM images indicates that
single stacks of Pcs molecules are organized in ordered
parallel lines, in which the alkyl chains of one stack are
all arranged in the same direction. The periodicity of
organization of single molecule into nanowire is about
2 nm which shows a good agreement with the dimension
of the molecule which is shown in Fig. 5c. In this context,
the larger molecular wire structures (diameter 40–60 nm)
observed by AFM studies (Fig. 4) are assumed to be
composed of many single wires with an about 2 nm
diameter aligned parallel to each other.
SUMMARY AND CONCLUSIONS
A
novel unsymmetrically substituted hydroxy-
functionalized Pc 1 bearing long aliphatic dodecyloxy
chains has been synthesized as the building block for
the construction of multipurpose materials for potential
optoelectronic applications. UV-vis absorption studies
showed that both unsymmetrically and symmetrically
On the other hand, under similar conditions, the
absorption of symmetric Zn(II)Pc 2 on the HOPG surface
leads the formation of closed-packed array revealed by
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substituted Zn(II)Pcs derivatives 1 and 2 respectively
1491–1546. (d) Elemans JAAW, van Hameren R,
Nolte RJM and Rowan AE. Adv. Mater. 2006; 18:
1251–1266.
exhibited monomeric forms in toluene. AFM studies
of unsymmetrical substituted Pcs 1 on the graphitic
surface revealed the formation of wire-like structures in
different lengths from nanometer to micrometer scale.
On the other hand, symmetrically substituted Zn(II)Pc
2 exhibited the formation of the closed-packed array
rather than wires on HOPG surface. Interestingly, the
supramolecular organization of both Pcs was not observed
in solution which was confirmed by optical absorption
measurements. This result showed that the morphology
of self-organization on the surface strongly depends on
the features of the substrate, the chemical structure of the
molecule, and molecule-to-molecule as well as molecule-
to-surface interactions. Indeed, two different types of
the self-organization, namely closed-packed arrays and
properly ordered molecular wires were observed with
symmetrical and unsymmetrical Zn(II)Pcs, respectively,
on the HOPG surface. According to these results we can
assume that the both intermolecular p–p and metal-ion
coordination between two unsymmetrically substituted
Pc units (1) are the driving force for the formation of
nano-wire structures, whereas strong p–p interactions
in the case of symmetric Zn(II)Pc 2 deposited on the
HOPG surface facilitate the formation of closed-packed
array. Although the results are still preliminary, further
detailed investigations are necessary to better understand
the supramolecular organization of these Pcs in different
solvents and substrates. The research directed on these
lines are undergoing in our group.
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Acknowledgements
This work has been supported by Mersin University
(BAP-FBE ESMB (SGC) 2014-2 YL). SGC thanks The
Scientific and Technological Research Council of Turkey
(TUBITAK) (grant no. 112T565) for financial support.
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