Supramolecular stacks of asymmetric zinc phthalocyanines functionalized with one tetrathiafulvalene unit

Article abstract of DOI:10.1246/cl.2010.812

An asymmetric hybrid compound, in which one tetrathiafulvalene (TTF) unit is attached to a zinc phthalocyanine (ZnPc) ring, has been studied by spectroscopy and electrochemistry. Its Langmuir-Blodgett film-forming properties have also been examined.

Full text of DOI:10.1246/cl.2010.812

doi:10.1246/cl.2010.812
812
Published on the web June 26, 2010
Supramolecular Stacks of Asymmetric Zinc Phthalocyanines
Functionalized with One Tetrathiafulvalene Unit
Mutsumi Kimura,*1,2 Satoshi Otsuji,1 Junko Takizawa,2 Yoko Tatewaki,2 Tadashi Fukawa,1 and Hirofusa Shirai2
1Department of Functional Polymer Science, Faculty of Textile Science and Technology,
Shinshu University, Ueda, Nagano 386-8567
2Collaborative Innovation Center of Nanotech FIBER (nanoFIC), Shinshu University, Ueda, Nagano 386-8567
(Received May 14, 2010; CL-100470; E-mail: mkimura@shinshu-u.ac.jp)
An asymmetric hybrid compound, in which one tetrathia-
fulvalene (TTF) unit is attached to a zinc phthalocyanine (ZnPc)
ring, has been studied by spectroscopy and electrochemistry.
Its Langmuir-Blodgett film-forming properties have also been
examined.
Since the discovery of semiconductive properties in
phthalocyanines and condensed aromatic carbons, an extremely
large number of one-dimensional assemblies of organic and
metal-organic compounds have been investigated as molecular
metals.1 To obtain highly electrical conducting assemblies, the
conjugated molecular components should be arranged in close
spatial proximity like in crystals. The use of molecular self-
assembly through noncovalent intermolecular interactions pro-
vides one possible direction in the exploration of readily
accessible soft conducting materials. In this context, there have
been many attempts to construct well-defined supramolecular
structures involving one-dimensional stacks of conjugated
molecular components.2 Metallophthalocyanines (MPcs) and
tetrathiafulvalenes (TTFs) have been used as molecular compo-
nents for molecular metals.3 Due to their excellent conductive
properties, hybrid compounds composed of MPcs and TTFs
have been synthesized in the search for novel optoelectronic
materials.4 Bryce et al. reported the synthesis of Pcs decorated
with eight peripheral TTF units.4a-4d Cook et al. demonstrated
liquid crystalline properties of asymmetric Pc possessing two
TTFs.4e Nolte et al. reported the formation of self-assembled
nanofibers made of tetra(TTF-crown ether)phthalocyanine.4f
These compounds had spacer groups between the Pc core and
TTFs. Here, we present the synthesis of a novel hybrid TTF-Pc
compound in which one redox-active TTF is attached directly to
the MPc ring to obtain novel molecular systems with unusual
electrochemical and structural properties.4g
The synthesis of our target molecule 3 is shown in
Scheme 1. The TTF derivative 1 was obtained in 70% yield
by the standard phosphite-induced cross-coupling of 1,3-
dithiole-2-one and -2-thione fragments.5 Novel phthalocyanine
precursor 2 was synthesized in 65% yield by the coupling of 4,5-
dichlorophthalonitirile with 1 bearing two cyanoethyl units in
the presence of K2CO3. The target asymmetric ZnPc 2 was
prepared from 1 and 4-tert-butylphthalonitrile with a molar ratio
of 1:3 with ZnCl2 in dimethylaminoethanol.6 The required ZnPc
3 was separated by column chromatography as a second fraction.
Compounds 2 and 3 were characterized by 1H NMR and
MALDI-TOF mass spectra.
Scheme 1.
Figure 1. UV-vis spectra of 3 (solid line) and ZnPc(t-Bu)4
(dashed line) in CH2Cl2.
-
max = 678 nm, the TTF-substituted 3 shows a split Q-band
absorption at 674 and 697 nm. The splitting of the Q-band
appears to be similar to that of low-symmetric Pc derivatives,
and the red-shifting is attributed to the enlargement of the ³-
conjugated system.7 In the 300-450 nm region, the spectrum of 3
shows superimposed bands of TTF and the Soret band of ZnPc.
Compound 3 exhibits a fluorescence peak at 713 nm, which is
longer than that of ZnPc(t-Bu)4 (686 nm), which is also
indicative of the enlargement of the ³-conjugated system.
Fluorescence quantum yield of 3 in CH2Cl2 is 0.04. This value is
very much lower than that obtained for ZnPc(t-Bu)4 lacking the
TTF moiety: cf. ZnPc(t-Bu)4, Φ = 0.21. This fluorescence
quenching is likely due to an intramolecular electron-transfer
reaction between the excited singlet state of the ZnPc and TTF.
An electrochemical study of 3 was performed through cyclic
¹3
Figure 1 shows the absorption spectra of 3 and symmetric
tetra(tert-butyl)phthalocyaninatozinc complex (ZnPc(t-Bu)4) in
CH2Cl2. While ZnPc(t-Bu)4 exhibits a single sharp Q-band at
voltammetry measurement in CH2Cl2 containing 0.1 mol dm
TBAP as the supporting electrolyte. Compound 2 exhibits two
reversible oxidation waves at E1/2 = +0.61 and +0.83 V vs.
Chem. Lett. 2010, 39, 812-813
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

813
stacking of aromatic components and the packing of alkyl
chains. The electrical conductivity of 20-layer LB films of 3 was
measured by using evaporated gold electrodes with a gap
distance of 500 ¯m. The LB films of 3 exhibit a low conductivity
¹1
in the range 10¹10-10¹9 S cm at room temperature.3a After
exposure of the LB films to I2 vapor, the conductivity increased
¹1
to 10¹5-10¹4 S cm due to the partial oxidations of TTFs or
ZnPcs in the LB film.
In summary, novel asymmetric hybrid ZnPc 3 directly
connected with one TTF unit was synthesized and organized
through the ³-³ interaction between aromatic components and
the close packing of alkyl chains in the LB film. The integration
of molecular components will provide new molecular materials
with interesting solution and solid-state properties.
Figure 2. (a) Surface pressure vs. surface area isotherms for 2
(dashed line) and 3 (solid line). (b) UV-vis spectrum of 20-layer
LB film of 3 deposited on hydrophobic quartz substrate. The inset
shows angular dependence of the Q band intensity for LB film.
SCE. These potentials are almost in accord with the reported
potentials for tetrakis(alkylsulfanyl)TTF derivatives.4b,4c Since
Zn(II) does not undergo redox processes within the potential
window in CH2Cl2, all oxidation processes of 3 can be ascribed
as phthalocyanine ring-centered and TTF oxidations. Compound
3 displays two TTF redox waves at +0.60 and +0.81 V vs. SCE
similar to those of the building block 2, and the phthalocyanine-
ring oxidation [ZnPc(¹1)/ZnPc(¹2)] is observed at around
+0.4 V. The oxidation potential shifts to negative by 100 mV
compared to ZnPc(t-Bu)4,8 suggesting that the phthalocyanine
ring in 3 is easy to oxidize through the electron-donating
property of the direct-substituted TTF moiety. These results of
UV-vis, fluorescence, and electrochemical analyses suggest the
intramolecular electronic interactions between ZnPc and TTF.
The Langmuir-Blodgett (LB) technique is a method for
fabricating ordered molecular structures from organic functional
molecules. Well-organized thin films have been created from
MPcs and TTFs through the LB technique.9 The pressure-
surface area (³-A) isotherms for 2 and 3 on pure water at 18 °C
are shown in Figure 2a. Compound 3 shows a steep increase of
pressure for the monolayer formation at the air-water interface.
The occupied area of 3 is larger than that of 2 (0.28 nm2/
molecule). Assuming the molecular dimension of 3 from the
computer-generated molecular model (Scheme 1), the observed
occupied area is in agreement with the estimated area for the
edge of ZnPc moiety (0.525 nm2/molecule). This indicates that
the phthalocyanine ring planes are oriented perpendicular to the
water surface with the side alkyl chains extended into air.
Monolayers of 3 were transferred onto a hydrophobic quartz
substrate by vertically dipping the substrate through the
monolayer at a surface pressure of 20 mN m¹1. Y-type deposi-
tions occurred with a transfer ratio of about 1.0. Absorption
spectrum of 20-layer LB film of 3 is shown in Figure 2b.
Compared to the spectrum of 3 in CH2Cl2, the Q band of the LB
film is broadened compared to that in solution. This spectral
change is attributed to the formation of stacks among ZnPcs.
The orientation of the molecules in the built-up LB film was
analyzed by using the angular dependence of the absorption
intensity using polarized light. When the electric vector of the
incident light is coupled most strongly with the transition
moment of the chromophore, the absorption intensity is
maximum.10 The relative absorption intensity minimizes at
ª = 0° (ª is the angle between the incident light and substrate
normal) suggesting the transition moment is tilted by less than
54.7° from the axis perpendicular to the substrate. These results
indicate that 3 orient within the built-up LB films through the
This work was supported by a project for “Innovation
Creative Center for Advanced Interdisciplinary Research Areas”
in Special Coordination Funds from the Ministry of Education,
Culture, Sports, Science and Technology, Japan.
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Chem. Lett. 2010, 39, 812-813
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/