Highly organized phthalocyanine assembly onto gold surface through spontaneous polymerization

Article abstract of DOI:10.1246/cl.2007.304

Soluble μ-dihydroxysilicon-tetra-t-butylphthalocyanine was immobilized onto OH-coated gold surface. The phthalocyanine was immobilized in a sigmoidal manner with two threshold times, and the process was successfully analyzed with QCM and AFM studies. UV-vis studies indicated the highly organized polymer structure deposited onto the surface. Copyright

Full text of DOI:10.1246/cl.2007.304

304
Chemistry Letters Vol.36, No.2 (2007)
Highly Organized Phthalocyanine Assembly onto Gold Surface
through Spontaneous Polymerization
Masaaki Ishikawa,1;2 Michiya Fujiki,ꢀ1;2 and Masanobu Naito1;2
1Graduate School of Materials Science, Nara Institute of Science and Technology, 8916-5 Takayama, Ikoma 630-0101
2CREST-JST (Japan Science and Technology Agency), 4-1-8 Hon-cho, Kawaguchi 332-0012
(Received November 17, 2006; CL-061354; E-mail: fujikim@ms.naist.jp)
450
Soluble
ꢀ-dihydroxysilicon-tetra-t-butylphthalocyanine
(a)
(b)
ttbPcSi(OH)2/HOC6SAu
ttbPcH2/HOC6SAu
2.5
2.0
1.5
1.0
0.5
0.0
was immobilized onto OH-coated gold surface. The phthalocya-
nine was immobilized in a sigmoidal manner with two threshold
times, and the process was successfully analyzed with QCM and
AFM studies. UV–vis studies indicated the highly organized
polymer structure deposited onto the surface.
400
350
300
250
200
150
100
50
t Bu
t Bu
ttbPcSi(OH)2/C6SAu
N
M
N
N
N
N
N
N
N
region III
t Bu
t Bu
region II
ttbPcSi(OH)2: M=Si(OH)2
ttbPcH2: M=H2
Organizing molecules into supramolecular, polymeric, and
crystalline architecture onto a solid surface at the nano- and up-
ward scales is one of the key challenges of supramolecular, poly-
mer, material, and surface science. For electronics, optoelectron-
ics, and nonlinear optics device applications, various quasi-one-
dimensional (Q1D) semiconductors, including molecular solids
made of ꢁ-building blocks and ꢂ=ꢁ-conjugating polymers, are
of particular importance as candidates due to their effective
quantum confinement.1 Exploring such an ideal self-organiza-
tion, however, remains an issue due to the lack of proper model
supramolecular system and detecting apparatus with ultrahigh
sensitivity. This is presumably due to limited knowledge and un-
derstanding on the deposition process of the building block at the
liquid/solid interface and on the nature of attracting forces be-
tween the block and interface.
region I
0
200
1
10
100
Immersion time/min
(c)
(d)
(e)
Wavelength (λ )/nm
250
300
400
500
600 700 800
0.03
0.02
0.01
0.00
2.5
2.0
1.5
1.0
0.5
0.0
To elucidate spatio-temporal evolutional organization of a
building block into a ꢁ-conjugating architecture at the liquid/
solid interface in a controlled manner, we chose ꢀ-dihydroxy-
silicon-tetra-t-butylphthalocyanine (ttbPcSi(OH)2, Figure 1a).2
This served as a model of soluble ꢁ-type square planer building
blocks with reactive bi-functional OH groups at the center of
Pc ring and the OH-coated gold surface treated with 6-hydroxy-
hexane-1-thiol (HOC6SAu),2 one of the most useful solid
surfaces with a functional OH group. For comparison, free-base
tetra-t-butylphthalocyanine (ttbPcH2) was used as a model of
soluble Pc without reactive functional groups.2
x8
(f)
ε
The present work demonstrated a combined study of the
9-MHz quartz crystal microbalance (QCM) measurement with
atomic force microscopy (AFM) observation and elucidated
a rapid self-propagation of ttbPcSi-based nanoarrays onto
the OH-coated gold substrate, leading to self-deposition of
ttbPcSi-based ultrathin film with high degree of polymerization
by UV–vis studies.
The amount of the adsorbed ttbPcSi(OH)2 on the electrode
increased in a sigmoidal manner with two threshold immersion
times at 10 and 90 min, as shown in Figure 1b. On the other hand,
ttbPcSi(OH)2 on n-alkane-coated QCM gold electrode treated
with hexane-1-thiol (C6SAu)2 showed a very weak adsorption
even with a prolonged immersion time. Interestingly, even
ttbPcH2 was adsorbed on the OH-coated gold surface by one
third compared to the corresponding ttbPcSi(OH)2 at a pro-
longed immersion time. These results led to the idea that
40,000
35,000
30,000
25,000
20,000
)/cm-1
15,000
Wavenumber (
ν
Figure 1. (a) Chemical structure of ttbPc derivatives in this
work. (b) The amount of immobilized ttbPcSi(OH)2 and ttbPcH2
on the OH-coated gold surface, and ttbPcSi(OH)2 on the alkane-
coated gold surface by QCM method under 1:5 ꢁ 10ꢂ5 M
chloroform solution. Typical AFM images of the deposited thin
film. [(c) 1 min, (d) 10 min, (e) 120 min; Area: 2 ꢁ 2 mm, Height:
10 nm] (f) UV–vis spectra of the deposited thin film (thick solid
line), ttbPcSi(OH)2 (thin solid line), and oligo-(ttbPcSiO)n (thin
dotted line) in CHCl3.
ttbPcSi(OH)2 spontaneously adsorbed on the OH-coated gold,
probably due to attractive cooperative tandem interactions:3 (i)
OH/ꢁ interaction, (ii) OH/HO interaction, (iii) OH/N inter-
Copyright ꢀ 2007 The Chemical Society of Japan

Chemistry Letters Vol.36, No.2 (2007)
305
action, (iv) ꢁ=ꢁ interaction, and (v) CH/ꢁ interaction. It is
therefore assumed that ttbPcH2 spontaneously deposited on the
OH-coated gold by OH/ꢁ, OH/N, ꢁ=ꢁ, and CH/ꢁ interactions
between the OH-coated gold and ttbPc.
To further clarify the adsorption mechanism and deposition
kinetics of ttbPcSi(OH)2 on the OH-coated gold, the change
in mass were classified into three regions I (0–10 min), II (10–
90 min), and III (90–120 min), as shown in Figure 1b. Although
ttbPcSi(OH)2 in the region I gradually deposited on the gold,
ttbPcSi(OH)2 in the region II steeply deposited exceeding the
10 min-threshold, and the region III was a plateau region with
a constant value.
that the Q-band of the thin film was further blue-shifted by
2991 cmꢂ1 (ꢀꢄmax ꢄ 97 nm), when compared to that of the
oligo-(ttbPcSiO)n, suggesting a greater DP of the deposited
ttbPcSi with the H-type assembly. In addition, formation of
Si–O–Si bonding in the backbone was confirmed by IR signal
band at 1013 cmꢂ1, assigned to ꢅas (Si–O–Si). This idea was
supported from an estimated molecular image height of
ca. 50 nm (ca. 150 Pc) for the deposited ttbPcSi nanoarrays
measured by QCM analysis, as shown in Figure 1b.
In conclusion, this letter first characterized organization of
soluble ttbPcSi(OH)2 bearing two reacting Si–OH groups at
the chloroform/OH-coated gold interface by means of QCM,
AFM, and UV–vis experiments. Self-immobilization (Avrami
index, n ꢄ 0:7) and self-polymerization (Avrami index, n ꢄ
1:5) of ttbPcSi(OH)2 were successfully analyzed by the
Avrami equation from time-course QCM experiment. AFM
study proved the spatio-temporal organization of ttbPcSi-based
nanoarrays at the interface, leading to an almost uniform
ultrathin film. The present built-up approach may be regarded
as a programmed switching from a supramolecular approach to
hierarchical polymer organizations.
The Avrami equation,4 ꢃðtÞ ¼ 1 ꢂ expðꢂKꢃtnÞ, was success-
fully applicable to analyze the apparent sigmoidal behavior,2
where ꢃ is the weight fraction of ttbPcSi(OH)2 deposited onto
the substrate with immersion time t at room temperature relative
to a maximum frequency shift, ꢀFmax, at the prolonged immer-
sion of ca. 120 min, K a temperature-dependent constant, n
Avrami index depending on nucleation and propagation manners
of ttbPcSi(OH)2, although this equation was originally formulat-
ed to characterize isothermal nucleation and crystallization
kinetic of polymers. The value of ꢀFmax in this study was chosen
as 350 Hz, which is responsible for a maximum deposit value
ttbPcSi thin solid film. Although the evaluated n value of ca.
0:7 in the region I infers an inhomogeneous nucleation, the n
value ca. 1:5 in the region II is an intermediate between Q1D
and Q2D self-organizations of ttbPcSi(OH)2 stacks. The total
amount of ttbPcSi(OH)2 at the equilibrium region III led to the
idea that 2.2 mgꢃcmꢂ2 of ttbPcSi(OH)2 molecules, corresponding
to ca. 50 nm or ca. 150 Pc repeating units in length,5 self-organ-
ized into the corresponding ttbPcSi assembly.
To assess the validity of direct immobilization and sponta-
neous deposition of soluble ttbPcSi(OH)2, AFM images at three
regions I, II, and III were obtained. Although 1-min immersion
in the region I process provided ttbPcSi(OH)2 domains (Height:
0.3–3 nm) aligned with side-by-side organization (Figure 1c),
the domains immediately disappeared, and a 10-min immersion
procedure then induced an abrupt appearance of small nucleus
domains spread throughout the observing area (Figure 1d).
The domain areas then vertically grew as needle-like nanoarch-
itecture after further immersion time. These snapshots could be
consistent with the inhomogeneous nucleation of Q1D- and
Q2D-propagation kinetics from the surface based on the Avrami
plot. A prolonged immersion for 120 min eventually led to an
almost uniform ultrathin film based on ttbPcSi(OH)2 with
minimal defects (Figure 1e).
This research was partly supported by Grant-in-Aid for
Scientific Researches in a priority area ‘‘Super-Hierarchical
Structures’’ (No. 446) from MEXT. MF is acknowledged in part
for grant from MEXT, for Grant-in-Aid for Scientific Research
(No. 16205017).’’ MN is acknowledged in part for grant
from MEXT, for Grant-in-Aid for Scientific Research (No.
17750110).’’ The authors are grateful to Profs. Tsuyoshi Kawai
and Kotohiro Nomura for their valuable discussion and
comments, and to Shohei Katao and Fumio Asanoma for
ESI-MS measurement and elemental analysis.
References and Notes
1
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All UV–vis absorption spectra of the thin film onto the
OH-coated gold-on-sapphire3 and chloroform solution (5:0 ꢁ
10ꢂ6 M) of ttbPcSi(OH)2 and chemically synthesized oligo-
2
3
Supporting Information is available electronically on the
CSJ-Journal Web site, http://www.csj.jp/journals/chem-lett/
index.html.
a) T. Steiner, Angew. Chem., Int. Ed. 2002, 41, 48. b) G. R.
Desiraju, Acc. Chem. Res. 2002, 35, 565. c) M. Nishio,
CrystEngComm 2004, 6, 130.
2
(ttbPcSiO)n are displayed in Figure 1f. The thin film which
had a very broad Q-band at 19120 cmꢂ1 (523 nm), was greatly
blue-shifted by 4436 cmꢂ1 with fwhm of ca. 5000 cmꢂ1, com-
pared to that of the corresponding monomeric ttbPcSi(OH)2.
Compared to the Q-band of ttbPcSi(OH)2 (14684 cmꢂ1
,
681 nm), the Q-band of oligo-(ttbPcSiO)n with ca. 18mer
(16129 cmꢂ1 with fwhm of ca. 1350 cmꢂ1, ꢄmax ¼ 620 nm)
was blue-shifted by 1445 cmꢂ1 (ꢀꢄmax ꢄ 61 nm). This means
4
5
M. Avrami, J. Chem. Phys. 1939, 7, 1103.
T. Serizawa, K. Yamamoto, M. Akashi, Langmuir 1999, 15,
4682.
Published on the web (Advance View) January 20, 2007; doi:10.1246/cl.2007.304