Electrochromic polymeric films derived from (diphenylamino)phenyl- substituted metallophthalocyanines

Article abstract of DOI:10.1246/cl.2009.82

Copper and zinc phthalocyanines decorated with four triphenylamines were synthesized and a green film was formed by oxidative linkage between two peripheral triphenylamines. The film deposited onto an ITO electrode exhibited a reversible electrochromic sw

Full text of DOI:10.1246/cl.2009.82

82
Chemistry Letters Vol.38, No.1 (2009)
Electrochromic Polymeric Films Derived from
(Diphenylamino)phenyl-substituted Metallophthalocyanines
Mutsumi Kimura,ꢀ1;2 Kazumasa Yasuta,1 Naoya Adachi,1 Yoko Tatewaki,2 Tadashi Fukawa,1 and Hirofusa Shirai2
1Department of Functional Polymer Science, Faculty of Textile Science and Technology, Shinshu University, Ueda 386-8567
2Collaborative Innovation Center of Nanotech FIBER (nanoFIC), Shinshu University, Ueda 386-8567
(Received November 10, 2008; CL-081057; E-mail: mkimura@shinshu-u.ac.jp)
Copper and zinc phthalocyanines decorated with four tri-
phenylamines were synthesized and a green film was formed
by oxidative linkage between two peripheral triphenylamines.
The film deposited onto an ITO electrode exhibited a reversible
electrochromic switching among green, red-brown, and blue.
Metallophthalocyanines (MPcs) have been investigated mo-
lecular materials in many fields including non-linear optics, xe-
rography, optical data storage, molecular electronics, catalysis,
photodynamic cancer therapy, solar energy conversion, sensors,
and organic light emitting diodes.1 In order to fabricate electron-
ic and photonic devices using MPcs, it is necessary to construct
uniform thin films in which the molecular ordering can be con-
trolled. Vacuum sublimation, the Langmuir–Blodgett film for-
mation, spin coating, and electrodeposition have been applied
to prepare uniform MPc films onto the surface of various sub-
strates.2 Electrochemical polymerization has great advantages
for the control of resultant film thickness and the tuning of their
electronic conductivity.3 Electrochemical polymerization in-
volves the polymerization of monomers and the deposition of in-
soluble one- or two-dimensional structures onto the substrates.
Two-dimensional porphyrin–oligothiophene copolymers have
been constructed by electrochemical polymerization reported
by Segawa et al.4 However, two-dimensional network structures
of MPcs, in which MPcs were linked with conjugated oligomers,
have not been as extensively explored.5 In this study, we report
syntheses of novel MPcs having triphenylamine (TPA) substitu-
ents and their electrochromic behavior of two-dimensional poly-
meric MPcs prepared by the electrochemical polymerization.
We found the formation of two-dimensional network by the
electrochemical oxidation of peripheral TPAs. Shirota et al. re-
ported the formation of stable amorphous glasses of TPA-substi-
tuted MPcs, in which TPA units were linked with MPcs by ether
linkages.6 We also expect the formation of amorphous thin films
of MPcs by the electrochemical polymerization.
Phthalocyanine precursor 1 was synthesized by the Suzuki
coupling reaction between 3-iodophthalonitrile and 4-(di-
phenylamino)phenylboronic acid in the presence of Pd(PPh)3
(Scheme 1). Copper and zinc phthalocyanines 2 and 3 were pre-
pared from 1 by refluxing in 2-(dimethylamino)ethanol with
CuCl2 and ZnCl2, respectively. After purification by column
chromatography and preparative recycling HPLC, ca. 25% of
pure products were isolated. Precursor 1, final ZnPc 3 were fully
characterized by 1H and 13C NMR spectroscopy, matrix-assisted
laser desorption ionization time-of-flight (MALDI-TOF) mass
spectrometry.7 MPcs 2 and 3 are soluble in organic solvents such
as CH2Cl2, THF, and DMF. The introduction of TPAs improved
the solubility of MPcs in organic solvents.
Scheme 1.
Figure 1. UV–vis spectra of 1 and 2 in dichloromethane and the
inset shows the steady-state fluorescence spectra of 1 and 3 upon
the excitation at the absorption maxium of 1.
Copper and zinc phthalocyanines 2 and 3 exhibited a sharp
Q band at 706 and 707 nm, respectively (Figure 1). The positions
of Q band was shifted to the longer wavelength compared to cop-
per and zinc tetra(t-butyl)phthalocyanines lacking TPAs (ꢀmax at
Q band = 680 nm),8 suggesting the electronic interaction be-
tween MPc and TPAs. Broad absorptions are observed in the
range of 300–600 nm for 2 and 3, ascribed to the sum of the
absorption band of TPAs and the Soret bands of MPcs.8 While
the precursor 1 emitted a strong fluorescence at 516 nm at the
excitation at the absorption maximum of 1 (ꢀex ¼ 400 nm),
the emission from 3 was mostly from ZnPc (ꢀem ¼ 717 nm,
and quantum yield ðꢁfÞ ¼ 0:17) and the residual fluorescence
from TPA was very weak. This result suggests an efficient intra-
molecular energy transfer from TPA to ZnPc.
Copyright Ó 2009 The Chemical Society of Japan

Chemistry Letters Vol.38, No.1 (2009)
83
appeared. The appearance of new peak at 480 nm ascribed to the
generation of the one-oxidation product of TBA.9 The
decreasing of Q band intensity at 0.7 V suggests an oxidation
of phthalocyanine ring.12 At oxidation potential higher than
1.0 V, the board absorption band around 800 nm corresponding
to TBA2þ grew up. The spectral changes as a function of the ap-
plied potential are reversible and the film color remains at each
potential.
In conclusion, novel MPcs 2 and 3 directly connected with
TPAs were synthesized and electrochemically polymerized to
give electroactive and electrochromic films through the oxida-
tive dimerization of TPA segments. Thin films made of TPA-
substituted MPcs are expected to find applications in organic
devices due to their unique optical and electronic properties.
Figure 2. (a) Repeated potential scanning electro-polymeriza-
tion of 2 ([2] = 1 mM); (b) Absorbance changes of polymerized
2 film on an ITO electrode at different potentials (vs. Ag/AgCl)
in an electrolyte solution.
This work was supported by projects for ‘‘Creation of Inno-
vation Center for Advanced Interdisciplinary Research Areas’’
in Special Coordination Funds for Promoting Science and Tech-
nology from the Ministry of Education, Culture, Sports, Science
and Technology, Japan.
Electrochemical characteristics of TPAs, in which electro-
active nitrogen is linked to three electron-rich phenyl groups
in a propeller-like geometry, have well investigated.9 The anodic
oxidation of TPA leads to TPA cation radical and the generated
cation radical TPA dimerized to form tetraphenylbenzidine
(TBA).10 Electrochemical studies of 1 and 2 were performed
in CH2Cl2 solution containing 0.1 M Bu4NClO4. The cyclic
voltammogram of 1 indicates an irreversible oxidation at
Ep,a ¼ 1:07 V in the first scan and a new pair of redox peaks
corresponding to the formation of TBA was observed at
E1=2 ¼ 0:68 V in the second scan.10 Repeated potential cycling
between 0 and 1.4 V vs. Ag/AgCl of 2 showed both monomer
oxidation and electrochemical polymerization to form an elec-
troactive film. Figure 2a shows multiple voltamograms of 2
using an ITO glass as a working electrode. CuPc 2 also shows
the appearance of new redox couple at E1=2 ¼ 0:65 V, indicating
the electrochemical dimerization of TPA segments of 2.10 Upon
the repeated scans between 0 and 1.4 V, the peak currents of
two redox couples increased and a green film was deposited onto
the surface of ITO glass.11 MALDI-TOF mass spectrum of the
green material showed a dominant set of peaks corresponding
to m=z of 4638 (trimer), 6184 (tetramer), 7730 (pentamer), and
9275 (hexamer).
Electropolymerized film of 2 grown on the ITO glass was
discovered to be quite durable. Upon washing and equilibrating,
the opto-electrochemical properties of polymer 2 in monomer-
free electrolyte were probed, and the film’s redox activity was
found to be rather stable, exhibiting less than a 5% loss of elec-
troactivity after 50 repeated scans between 0 and 1.4 V. The
polymer exhibited at least two oxidation waves at 0.65 V and
the second at 0.80 V. These potentials are almost agree with
those of TBA in the solution study.10 Upon both redox cycling
and potential stepping of films grown on the ITO glass, the poly-
mer films showed a color switching among green, red-brown,
and dark blue. The absorption spectra of the film were recorded
from 400 to 850 nm while incrementally stepping the potential to
different redox states of the polymer film of 2 (Figure 2b). For
the polymeric film of 2 at 0 V, the maximum of the Q band
was found at 710 nm and the position of Q band was almost same
as that of 2 in solution. This indicates that the propeller-like
geometry of TBA linkages in polymerized 2 prevents the aggre-
gation of CuPcs within the highly concentrated thin film. Upon
the oxidation at 0.7 V, the absorption intensity of the Q band
decreased and new peak at 480 nm having a shoulder at 530 nm
References and Notes
1
Phthalocyanines, Properties and Applications, ed. by C. C.
Leznoff, A. B. P. Lever, VCH, New York, 1989–1993, Vols. 1–3;
Phthalocyanines-Chemistry and Functions, ed. by H. Shirai, N.
Kobayashi, IPC, Tokyo, 1997; Phthalocyanine Materials, Synthe-
ses, Structure and Function ed. by N. B. McKeown, Cambridge
University Press, New York, 1998; The Porphyrin Handbook,
ed. by K. M. Kadish, K. M. Smith, R. Guilard, Academic Press,
San Diego, 2003, Vols. 16–20.
2
3
M. J. Cook, I. Chambrier, in The Porphyrin Handbook ed. by
K. M. Kadish, K. M. Smith, R. Guilard, Academic Press, San
Diego, 2003, Vol. 17, Chap. 108, pp. 37–127.
A. Audebert, F. Miomandre, in Conjugated Polymers Theory, Syn-
thesis, Properties, and Characterization ed. by T. A. Skotheim,
J. R. Reynolds, CRC Press, New York, 2007, Chap. 18, pp. 18-
1–18-40.
4
5
6
7
H. Segawa, F.-P. Wu, N. Nakayama, H. Maruyama, S. Sagisaka,
N. Higuchi, M. Fujitsuka, T. Shimizu, Synth. Met. 1995, 71, 2151.
T. Muto, T. Temma, M. Kimura, K. Hanabusa, H. Shirai, J. Org.
Chem. 2001, 66, 6109.
M. Ottmar, T. Ichisaka, L. R. Subramanian, M. Hanack, Y.
Shirota, Chem. Lett. 2001, 788.
CuPc 2 was characterized by MALDI-TOFMS. 1H NMR of 3
indicated the presence of isomers. Scheme 1 shows only the sym-
metrical C4v isomer.
8
9
M. J. Stillman, T. Nyokong, in Phthalocyanines, Properties and
Applications ed. by C. C. Leznoff, A. B. P. Lever, VCH, New
York, 1986, Vol. 1, pp. 133–289.
C. Lambert, W. Gaschler, E. Schmalzlin, K. Meerholz, C.
Brauchle, J. Chem. Soc., Perkin Trans. 2 1999, 577.
10 K. Y. Chiu, T. X. Su, J. H. Li, T.-H. Lin, G.-S. Liou, S.-H. Cheng,
J. Electroanal. Chem. 2005, 575, 95.
11 Upon the repeated scans of ZnPc 3 between 0 and 1.4 V, a green
film was also deposited. The polymer exhibited at least two oxida-
tion waves at 0.66 V and the second at 0.81 V.
12 J. Mack, M. J. Stillman, in The Porphyrin Handbook ed. by K. M.
Kadish, K. M. Smith, R. Guilard, Academic Press, San Diego,
2003, Vol. 16, Chap. 103, pp. 43–116; M. L’Her, A. Pondaven,
M. J. Stillman, in The Porphyrin Handbook ed. by K. M. Kadish,
K. M. Smith, R. Guilard, Academic Press, San Diego, 2003,
Vol. 16, Chap. 104, pp. 117–170.
Published on the web (Advance View) December 20, 2008; doi:10.1246/cl.2009.82