Effect of topology structure on the electrochemical behavior of hydrogen-bonded self-assembled Poly(4-vinylpyridine) -ferrocenyl dendron complexes

Article abstract of DOI:10.1016/j.jorganchem.2017.06.021

In this report, two new redox-responsive complexes with different dendritic tail chain number (P4VP- nFc, n = 1, 2) were prepared via the hydrogen-bonded self-assembly of poly(4-vinylpyridine) and ferrocenyl-modified percec-type dendrons. These complexes exhibited a crystalline phase for P4VP-1Fc and liquid crystalline for P4VP-2Fc at room temperature. The cyclic voltammetry measurement shows that the electrode process of P4VP-nFc (n = 1, 2) films was diffusion controlled and the reversibility of the electrode process for the P4VP-2Fc films became better than that for the P4VP-1Fc. Moreover, the redox peak current |i_p| increased with increasing dendritic tail chain number n in the complex, because the complex film formed with 2Fc-COOH has the looser stacking of alkyl chains, which was favorable for the electrolyte diffusion and charge transfer. The present results demonstrate that the electrochemical activity of the redox-responsive poly(4-vinylpyridine) -ferrocenyl dendron complex can be easily tuned using the dendritic tail chain with different numbers. These complexes may be used as new kinds of modified electrodes.

Full text of DOI:10.1016/j.jorganchem.2017.06.021

Journal of Organometallic Chemistry 846 (2017) 223e229
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Effect of topology structure on the electrochemical behavior of
hydrogen-bonded self-assembled Poly(4-vinylpyridine) -ferrocenyl
dendron complexes
,
,
Zhiyu Cheng a, Chongqing Li b, Yongfu Qiu a *, Xueyi Chang a **, Guiping Tan a, Biye Ren b
a School of Environment and Civil Engineering, Guangdong Engineering and Technology Research Center for Advanced Nanomaterials, Dongguan University
of Technology, Guangdong 523808, PR China
b Research Institute of Materials Science, South China University of Technology, Guangzhou 510640, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
In this report, two new redox-responsive complexes with different dendritic tail chain number (P4VP-
nFc, n ¼ 1, 2) were prepared via the hydrogen-bonded self-assembly of poly(4-vinylpyridine) and
ferrocenyl-modified percec-type dendrons. These complexes exhibited a crystalline phase for P4VP-1Fc
and liquid crystalline for P4VP-2Fc at room temperature. The cyclic voltammetry measurement shows
that the electrode process of P4VP-nFc (n ¼ 1, 2) films was diffusion controlled and the reversibility of the
electrode process for the P4VP-2Fc films became better than that for the P4VP-1Fc. Moreover, the redox
peak current jipj increased with increasing dendritic tail chain number n in the complex, because the
complex film formed with 2Fc-COOH has the looser stacking of alkyl chains, which was favorable for the
electrolyte diffusion and charge transfer. The present results demonstrate that the electrochemical ac-
tivity of the redox-responsive poly(4-vinylpyridine) -ferrocenyl dendron complex can be easily tuned
using the dendritic tail chain with different numbers. These complexes may be used as new kinds of
modified electrodes.
Received 7 June 2017
Received in revised form
21 June 2017
Accepted 22 June 2017
Available online 24 June 2017
Keywords:
Ferrocene
Dendrons
Self-assembly
Liquid-crystalline
© 2017 Elsevier B.V. All rights reserved.
1. Introduction
electrode in electrochemical and biological sensors, the ferrocenyl-
modified functional percec-type dendrons would be as building
The technique of the hydrogen-bonded self-assembly is one of
powerful tools to create new nano-structural and functional ma-
terials in the past decade due to their facile preparation and
interesting properties [1,2]. The percec-type dendrons with as the
primary structure of a protein, prepared via a programmed
branching sequence, have been investigated intensively [3e5].
Which utilized as powerful building blocks for construction of
complex macromolecular and supramolecular systems have arouse
people's interests due to their abundant variation in the composi-
tion and structure during recent years [6e8].
block for construction of redox-controlled supramolecular systems
and applied as sensors and supercapacitors.
In the present study, redox-active P4VP- nFc (n ¼ 1, 2) complexes
would be as examples. They were prepared via the hydrogen-
bonded self-assembly of poly(4-vinylpyridine) with ferrocenyl-
modified percec-type dendrons, 4-(11-ferrocenylundecyloxy) ben-
zoic acid (1Fc-COOH) or 3,5-bis(11-ferrocenylundecyloxy) benzoic
acid (2Fc-COOH) in DMF, respectively. The chemical structures of
P4VP-1Fc and P4VP-2Fc were shown in Fig. 1. The mesomorphous
structure and thermal properties were monitored with XRD, TG,
DSC and POM. The electrochemical behavior of the P4VP- nFc (n ¼ 1,
2) was investigated with CV measurements.
The design of new functional percec-type dendrons for special
applications is desired [3]. Since ferrocene and its derivatives, as
one of the most important organometallic electron mediators
[9e11], play the role of electron transfer between redox matrix and
2. Experimental
2.1. Materials
* Corresponding author.
** Corresponding author.
E-mail addresses: qiuyf1979@foxmail.com (Y. Qiu), 2016027@dgut.edu.cn
(X. Chang).
Poly(4-vinylpyridine) (P4VP, Aldrich, MW ¼ 60,000 g/mol),
Methyl 4-hydroxybenzoate (Aldrich, purity 99%), and methyl 3, 5-
http://dx.doi.org/10.1016/j.jorganchem.2017.06.021
0022-328X/© 2017 Elsevier B.V. All rights reserved.

224
Z. Cheng et al. / Journal of Organometallic Chemistry 846 (2017) 223e229
dihydroxybenzoate (Aldrich, purity 97%) were used without further
treatment as received. The synthesis of 11-bromoundecyl ferrocene
(compound 3 in Fig. 2.) was reported in our previous study in detail
[12]. Other reagents were all commercial chemicals of analysis
grade and purified following the standard procedures.
2.2. Synthesis of ferrocenyl-modified percec-type dendrons
(1Fc-COOH and 2Fc-COOH). Ferrocenyl-modified percec-type
dendrons, 4-(11-ferrocenylundecyloxy) benzoic acid (1Fc-COOH) or
3,5-bis(11-ferrocenylundecyloxy) benzoic acid (2Fc-COOH) were
synthesized in our previous report following the method reported
by Percec et al. [3,13] 1Fc-COOH and 2Fc-COOH were synthesized
by mucleophilic substitution reaction between 11-bromoundecyl
ferrocene with methyl 4-hydroxybenzoate, methyl 3, 5-
dihydroxybenzoate, respectively, followed by a hydrolysis process.
Synthetic routes for 1Fc-COOH and 2Fc-COOH are shown in Fig. 2.
The 1H NMR spectrums of 1Fc-COOH and 2Fc-COOH are shown in
Fig. 3. Their NMR results are listed as follows, For 1Fc-COOH: 1H
NMR (CDCl3, TMS) d 8.05 (d, 2H, Ar-H), 6.93 (d, 2H, Ar-H), 4.12e4.03
(m, 9H, H(Cp), 4.02 (t, 2H, -CH2-O-Ar), 2.28 (t, 2H, Cp-CH2-), 1.80 (m,
2H, -CH2-CH2-O-Ar), 1.49e1.29 (m, 16H, -CH2-); For 2Fc-COOH: 1H
NMR (CDCl3, TMS) d 7.19 (s, 2H, Ar-H), 6.65 (s, 1H, Ar-H), 4.09e4.04
(m, 18H, H(Cp), 3.94 (t, 4H, -CH2-O-Ar), 2.23 (t, 4H, Cp-CH2-), 1.76
(m, 4H, -CH2-CH2-O-Ar), 1.48e1.28 (m, 32H, -CH2-).
Fig. 1. Chemical structures of P4VP-1Fc (left) and P4VP-2Fc (right).
Fig. 2. Synthetic routes of 11-bromoundecyl ferrocene (3), 1Fc-COOH, and 2Fc-COOH.

Z. Cheng et al. / Journal of Organometallic Chemistry 846 (2017) 223e229
225
2.3. Preparation of P4VP- nFc (n ¼ 1, 2)
2.5. Measurements
The complex P4VP-1Fc was prepared according to the following
procedure: 0.40 g (0.84 mmol) of 1Fc-COOH was dissolved in 10 mL
of DMF and slowly added dropwise into 10 mL of DMF containing
0.05 g (0.84 mmol of repeat unit) of P4VP under stirring at room
temperature. The mixture was stirring for 72 h in an Ar atmosphere.
The solvent was slowly vaporized until yellow precipitate
appeared. The resulting solid complex was separated by filtration
and washed with DMF, and then dried under vacuum for 72 h at
room temperature. The yield of the P4VP-1Fc was 82%. P4VP-2Fc
was prepared following the similar procedure. The yield of the
P4VP-2Fc was 74%. The synthetic routes of P4VP- nFc (n ¼ 1, 2) were
illustrated in Fig. 4.
1H NMR spectrum was operated in CDCl3 solution on a Varian
INOVA 500NB spectrometer. X-ray diffraction (XRD) of the P4VP-
nFc (n ¼ 1, 2) powder was run in transmission geometry with an
X'pert PRO diffractometer (40 KV and 40 mA) using Cu Ka radiation
(wavelength
l
¼ 0.154 nm) with scan rate 0.01ꢂ/sec at room tem-
perature. The differential scanning calorimetry (DSC) was carried
out with 4e6 mg of sample in a 6 mm aluminum pan on a Netzsch
DSC 204 under nitrogen atmosphere at heating or cooling rate of
10 ꢂC minꢀ1 following the temperature sequence as -60 ꢂC/ 120 ꢂC
/ -60 ꢂC / 120 ꢂC. Thermogravimetry (TG) was performed with a
Netzsch TG 209 under nitrogen atmosphere. Polarized optical mi-
croscope of Zeiss Axiophot was used with a Linkam hot stage. CV
measurements were performed on a CHI 660C electrochemical
workstation (CH Instruments, Shanghai) at 25 ꢂC in 0.1 M NaCl
solution using a three-electrode system. The modified GC electrode
with P4VP- nFc (n ¼ 1, 2) was used as the working electrode, a
platinum wire as the counter electrode, and saturated calomel as
the reference electrode (SCE).
2.4. Preparation of modified electrodes with P4VP- nFc (n ¼ 1, 2)
5.0
m
L solution of the P4VP- nFc (n ¼ 1, 2) in chloroform with
0.03 mol Lꢀ1 was spread evenly onto the freshly burnished glassy
carbon electrode (effective surface area 0.07 cm2) with a micro-
syringe. The coverage
G of the complex on the electrode was esti-
mated as 2 ꢁ 10ꢀ6 mol cmꢀ2. The GC modified electrode with the
P4VP- nFc (n ¼ 1, 2) film was then dried in Ar atmosphere.
Fig. 3. 1H-NMR spectra of 1Fc-COOH (A) and 2Fc-COOH (B) in CDCl3 solvent.
Fig. 4. Synthetic routes of P4VP-nFc (n ¼ 1, 2).

226
Z. Cheng et al. / Journal of Organometallic Chemistry 846 (2017) 223e229
3. Results and discussion
3.1. Formation of P4VP- nFc (n ¼ 1, 2)
As well known, both building block P4VP and nFc-COOH (n ¼ 1,
2) have good solubility in the DMF. However, the formation of
hydrogen-bonded self-assembly complexes P4VP-nFc (n ¼ 1, 2)
were observed as yellow precipitates with poor solubility in the
DMF.
The complexation between P4VP and nFc-COOH (n ¼ 1, 2) were
further verified with FT-IR spectra. In Fig. 5A, it can be observed that
pure P4VP has a symmetric ring stretching mode at 993 cmꢀ1. Once
complexed with 1Fc-COOH, a new band arises at 1012 cmꢀ1, which
can be attributed to hydrogen bonding between the pyridine and
hydroxyl moieties of the polymer and the 1Fc-COOH, respectively.
In Fig. 5B, the carbonyl stretching band appearing at 1678 cmꢀ1 for
pure 1Fc-COOH indicated that their carboxyl group was not in a free
but in a cyclic dimeric form. The carbonyl stretching vibration shifts
to higher wavenumbers at 1717 cmꢀ1 in the P4VP-1Fc complex,
indicating that the hydrogen bond is formed between the pyridine
and hydroxyl moieties of bound 1Fc-COOH.
3.2. Structural analysis and thermal properties of P4VP- nFc (n ¼ 1,
2)
XRD measurements were performed to reveal the meso-
morphous structure of P4VP-nFc (n ¼ 1, 2). The XRD patterns in the
wide angle range for the P4VP-nFc (n ¼ 1, 2) were obtained and
shown in Fig. 6A. For P4VP-2Fc only one broad peak appears at
Fig. 5. Infrared spectra of P4VP, 1Fc-COOH and P4VP-1Fc at wavenumber regions (A)
1100-900 cmꢀ1 and (B) 1800-1500 cmꢀ1
.
2q
z 20ꢂ, which corresponding to the distance between the con-
formationally disordered alkyl chains, indicating a non-crystalline
structure in the solid complexes at room temperature. However,
in the wide angle region, the P4VP- 1Fc display more than one
sharp diffraction peaks at 2q
of 15e30ꢂ, which indicates the pres-
ence of crystalline phases in the complexes.
Fig. 6B is DSC curves of the complexs P4VP-nFc (n ¼ 1, 2)
recorded during the second heating run. There exists a weak
endothermic peak Ti at 82 ꢂC in second heating curve for P4VP- 2Fc.
This weak endothermic transition Ti was considered corresponding
to the transition from liquid crystal to isotropic phase. While for
P4VP-1Fc, there are two clear endothermic transitions in the DSC
curve between -60 ꢂC and 120 ꢂC. The first stronger endothermic
transition belongs to the melting of the side chain crystals of bound
1Fc-COOH in P4VP-1Fc. The corresponding melting point Tm (peak
value) is 62 ꢂC. The preceding XRD diffraction profiles also support
the existence of these crystal phases in the complexes at room
temperature.
It is therefore important to directly observe the liquid crystal
phases in the complexes with Polarized Optical Micrographs
(POM). Fig. 7 illustrates POM photographs of P4VP- 2Fc taken at 25
and 90 ꢂC. As expected, using birefringent, a random polygon-like
SmA or SmC texture was observed under POM above room tem-
perature for P4VP- 2Fc. This birefringence only appears at tem-
peratures shown in Fig. 7a below Ti and disappears when the
temperature is above Ti (shown in Fig. 7b). POM photos further
illustrate the existence of a crystal phase in the complexes of P4VP-
1Fc at room temperature in Fig. 7c. In Fig. 7d, the POM photos at
80 ꢂC with the birefringent random polygon-like SmA or SmC
texture, indicates that there is a transition phase from ionic liquid
crystal to isotropic phase between two endothermic transition
temperatures. Furthermore, an isotropic phase without birefrin-
gent texture was observed from P4VP-1Fc at 105 ꢂC (Fig. 7e) above
Ti (92 ꢂC), suggesting the liquid crystal SmA or SmC phase changed
to isotropic.
Fig. 6. (A) XRD patterns of the P4VP-nFc (n ¼ 1, 2) complexes at room temperature; (B)
DSC curves of the complexs P4VP-nFc (n ¼ 1, 2) recorded during the second heating
run at the rate of 10 ꢂC minꢀ1
.

Z. Cheng et al. / Journal of Organometallic Chemistry 846 (2017) 223e229
227
3.3. Electrochemical behavior of P4VP- nFc (n ¼ 1, 2)
the potential range of -0.2 to þ1.0 V in 0.1 M NaCl solution. A couple
of distinct redox peaks are observed from each curve. The anodic
peak potential Epa of the P4VP- 1Fc and P4VP- 2Fc are 0.55 and
0.50 V, respectively. The cathodic peak potential Epc of the com-
plexes are the same 0.42 V for n ¼ 1 and 2. Thus, the potential
The present question is whether the structure difference of
P4VP- nFc (n ¼ 1, 2) would significantly affect the electrochemical
activity of redox-active ferrocenyl moieties in complex. In order to
answer this question, we further studied electrochemical behavior
of these complexes with CV technique. The electron transfer reac-
tion of P4VP-nFc (n ¼ 1, 2) are shown in Fig. 8.
difference
D
E (¼ Epa - Epc) is 0.13 and 0.08 V for P4VP- 1Fc and P4VP-
2Fc. Generally, the kinetic reversibility of the redox couple can be
assessed according to the separation of the anodic and cathodic
peak potentials in CV [14]. The complexes with two tails demon-
Fig. 9 depicts CV curves of the Glass Carbon (GC) electrode
modified with P4VP- nFc (n ¼ 1, 2) at a scan rate of 0.1 V sꢀ1 within
strates a better reversible redox process due to its lower DE.
Fig. 7. Polarized optical micrographs of the complexes P4VP-2Fc at (a) 25 ꢂC, (b) 90 ꢂC and the complexes P4VP-1Fc at (c) 25 ꢂC,(d) 80 ꢂC,(e) 105 ꢂC.
Fig. 8. The electron transfer reaction of (A) P4VP-1Fc and (B) P4VP-2Fc.

228
Z. Cheng et al. / Journal of Organometallic Chemistry 846 (2017) 223e229
0.02e1 V sꢀ1 for P4VP- nFc (n ¼ 1, 2). When v ¼ 0.02e0.1 V sꢀ1
, DE
6
3
slightly increases for the P4VP- nFc (n ¼ 1, 2) with increasing v.
Once v > 0.2 V sꢀ1
, DE significantly grows sharply with lnv and then
P4VP-1Fc
linearly for the two complexes. This suggests that electrode pro-
cesses of the P4VP- nFc (n ¼ 1, 2) modified GC electrode is almost
reversible at lower scan rates but become quasi-reversible at higher
scan rates, especially at v > 0.2 V sꢀ1. Fig. 10D illustrates the
dependence of anodic peak current ipa on the square root of scan
rate v1/2 for the complexs P4VP-nFc (n ¼ 1, 2). The data for one
modified electrodes can be fitted with two straight lines of different
slopes at v ¼ 0.01e0.1 V sꢀ1 and 0.2e1 V sꢀ1, respectively. It is well
known that the peak current jipj linearly increases with the square
root of the scan rate v1/2 for a diffusion controlled electrode process
[15]. Hence, the complexs P4VP-nFc (n ¼ 1, 2) is with almost
reversible process at lower scan rates and with quasi-reversible
process at higher scan rates, which are features of diffused control.
P4VP-2Fc
μ
0
-3
-0.2 0.0 0.2 0.4 0.6 0.8 1.0
E / V vs. SCE
Fig. 9. CVs of the P4VP-nFc (n ¼ 1, 2) with scan rate of 0.1 V sꢀ1 in 0.1 M NaCl solution
at room temperature.
4. Conclusions
Fig. 10A and B are the CV behaviors of the P4VP- 1Fc and P4VP-
2Fc at different scan rates. Which show that the reduction peak Epc
of the complexes P4VP- nFc (n ¼ 1, 2) shifts leftwards and the
oxidation peaks Epa shifts rightwards with increasing the scan rate
Two new redox-responsive complexes with different dendritic
tail chain number (P4VP- nFc, n ¼ 1, 2) were prepared via the
hydrogen-bonded self-assembly of poly(4-vinylpyridine) and
ferrocenyl-modified percec-type dendrons. These complexes
exhibited an ordered mesomorphous and crystalline phase for
v, thus the peak separation
D
E gradually increases with the scan
rate. Fig. 10C depicts plots of
DE against ln v with the scan rate over
0
Fig. 10. (A, B) CVs of the P4VP-1Fc (A) and P4VP-2Fc (B) in 0.1 M NaCl solution at room temperature with different scan rates v; (C) Peak potentials (Ep-E0 ) of P4VP-1Fc (a, d) and
P4VP-2Fc (b, c) films as a function of scan rate v; (D) Plots of ipa vs. v1/2 for the complexs P4VP-nFc (n ¼ 1, 2) at the scan rate ranging from 0.02 to 1.00 V sꢀ1 in 0.1 M NaCl solution at
25 ꢂC.

Z. Cheng et al. / Journal of Organometallic Chemistry 846 (2017) 223e229
229
P4VP-1Fc and liquid crystalline for P4VP-2Fc. The electrode process
References
of P4VP-nFc (n ¼ 1, 2) films was diffusion controlled and the
reversibility of the electrode process for the P4VP-2Fc films became
better than that for the P4VP-1Fc. The redox peak current jipj
increased with increasing dendritic tail chain number n in the
complex, because the more loose stacking of alkyl chains in the
complex film formed with 2Fc-COOH was more favorable for the
electrolyte diffusion and charge transfer. The present results
demonstrate that the electrochemical activity of the redox-
responsive poly(4-vinylpyridine) -ferrocenyl dendron complex
can be easily tuned using the dendritic tail chain with different
numbers. These complexes may be used as new kinds of modified
electrodes.
[1] Y. Habibi, L.A. Lucia, O.J. Rojas, Chem. Rev. 110 (2010) 3479e3500.
[2] P. Cordier, F. Tournilhac, C. Soulie-Ziakovic, L. Leibler, Nature 451 (2008)
977e980.
[3] B.M. Rosen, C.J. Wilson, D.A. Wilson, M. Peterca, M.R. Imam, V. Percec, Chem.
Rev. 109 (2009) 6275e6540.
[4] Z.K. Du, B.Y. Ren, X.Y. Chang, R.F. Dong, Z. Tong, Macromolecules 50 (2017)
1688e1699.
[5] Z.Y. Cheng, P.R. Xu, B. Guo, Y.F. Qiu, H.B. Fan, X.Y. Chang, B.Y. Ren, Ionics 21
(2015) 1489e1494.
[6] H.H. Cai, G.L. Jiang, C.Y. Chen, Z.B. Li, Z.H. Shen, X.H. Fan, Macromolecules 47
(2014) 146e151.
[7] H.H. Cai, G.L. Jiang, Z.H. Shen, X.H. Fan, Macromolecules 45 (2012) 6176e6184.
[8] J.W. Choi, B.K. Cho, Soft Matter 7 (2011) 4045e4049.
[9] J. Xiang, R. Burges, B. Haupler, A. Wild, U.S. Schubert, C.L. Ho, W.Y. Wong,
Polymer 68 (2015) 328e334.
[10] J. Xiang, T.K. Wang, Q. Zhao, W. Huang, C.L. Ho, W.Y. Wong, J. Mater. Chem. C 4
(2016) 921e928.
[11] J. Xiang, K. Sato, H. Tokue, K. Oyaizu, C.L. Ho, H. Nishide, W.Y. Wong, M.D. Wei,
Eur. J. Inorg. Chem. 7 (2016) 1030e1035.
Acknowledgment
The work described in this paper was supported by the Tech-
nology Planning Project of Guangdong Province (No.
2016B090918006), the Guangdong Innovation Team Project for
Colleges and Universities (No. 2016KCXTD023), the Foundation of
the Department of Education of Guangdong Province (No.
2016KQNCX167), the National Natural Science Foundation of China
(No. 21674039). We thank Qiu Yan (1941308558@qq.com) for her
linguistic assistance during the preparation of this manuscript.
[12] C.Q. Li, B.Y. Ren, Y. Zhang, Z.Y. Cheng, X.X. Liu, Z. Tong, Langmuir 24 (2008)
12911e12918.
[13] Z.Y. Cheng, B.Y. Ren, L. Rui, X.Y. Chang, T. Zhen, Chin. Chem. Lett. 23 (2012)
438e441.
[14] E. Laviron, J. Electroanal. Chem. 101 (1979) 19e28.
[15] E. Laviron, L. Roulier, J. Electroanal. Chem. 115 (1980) 65e74.