Synthesis and properties of stimuli-responsive ferrocene-and azobenzene-based copolymers P(FHEMA-co-MAAT)s

Article abstract of DOI:10.1016/j.jorganchem.2018.10.033

Three novel polymers (P(FHEMA-co-MAAT)s) having redox and photoresponsive moieties were prepared by free radical polymerization. The structures and molecular weights of the synthesized copolymers were characterized by nuclear magnetic resonance (¹H NMR) and gel permeation chromatography (GPC) respectively. Thermal properties, optical features and electroactive properties were studied with the help of thermogravimetric (TG), differential thermogravimetric (DTG) analysis, ultraviolet-visible spectroscopy (UV-vis) and cyclic voltammetry (CV). It was seen that synthesized copolymers had excellent redox and photoresponsive properties. These results exhibited that these polymers can be applied further for redox/photo responsive information storage devices.

Full text of DOI:10.1016/j.jorganchem.2018.10.033

Accepted Manuscript
Synthesis and properties of stimuli-responsive ferrocene-and azobenzene-based
copolymers P(FHEMA-co-MAAT)s
Amin Khan, Li Wang, Haojie Yu, Muhammad Haroon, Raja Summe Ullah, Ahsan
Nazir, Tarig Elshaarani, Muhammad Usman, Shah Fahad, Kaleem-ur-Rehman
Naveed
PII:
S0022-328X(18)30819-2
DOI:
https://doi.org/10.1016/j.jorganchem.2018.10.033
JOM 20618
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 16 September 2018
Revised Date: 24 October 2018
Accepted Date: 31 October 2018
Please cite this article as: A. Khan, L. Wang, H. Yu, M. Haroon, R.S. Ullah, A. Nazir, T. Elshaarani,
M. Usman, S. Fahad, K.-u.-R. Naveed, Synthesis and properties of stimuli-responsive ferrocene-and
azobenzene-based copolymers P(FHEMA-co-MAAT)s, Journal of Organometallic Chemistry (2018), doi:
https://doi.org/10.1016/j.jorganchem.2018.10.033.
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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
Synthesis and properties of stimuli-responsive ferrocene-and azobenzene-
based copolymers P(FHEMA-co-MAAT)s
Amin Khan, Li Wang , Haojie Yu*, Muhammad Haroon, Raja Summe Ullah, Ahsan Nazir, Tarig
Elshaarani, Muhammad Usman, Shah Fahad, Kaleem-ur-Rehman Naveed
State Key Laboratory of Chemical Engineering, College of Chemical and Biological
Engineering, Zhejiang University, Hangzhou 310027, P.R. China
Abstract
Three novel polymers (P(FHEMA-co-MAAT)s) having redox and photoresponsive moieties
were prepared by free radical polymerization. The structures and molecular weights of the
synthesized copolymers were characterized by nuclear magnetic resonance (1H NMR) and gel
permeation chromatography (GPC) respectively. Thermal properties, optical features and
electroactive properties were studied with the help of thermogravimetric (TG), differential
thermogravimetric (DTG) analysis, ultraviolet-visible spectroscopy (UV-vis) and cyclic
voltammetry (CV). It was seen that synthesized copolymers had excellent redox and
photoresponsive properties. These results exhibited that these polymers can be applied further for
redox/photo responsive information storage devices.
Keywords Redox responsive; photo responsive; organometallic
1. Introduction
Organometallic complexes containing photoresponsive group are of great interest for the world
as these complexes can provide dual stimuli effect1. Among available organometallic systems,
Correspondence to Prof. Li Wang, E-mail: opl_wl@dial.zju.edu.cn and Haojie Yu, E-mail: hjyu@zju.edu.cn
Tel: +86-571-8795-3200; Fax: +86-571-8795-1612.
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ferrocene-based polymers can provide many exciting applications due to the unusual properties
of ferrocene moiety2. Ferrocene has many unique features such as good thermal stability, 3D
structure, high redox activity and easy solution processability3, 4. A redox process of ferrocene
can alter the properties of the polymeric material. Thus, depending on their redox state (oxidized
or reduced) the polymer usually presents different chemical, electronic, optical or mechanical
properties. These attributes have initiated a new era of polymers containing ferrocene
component5, 6. On the other hand, photoresponsive chromophores have also got importance in the
past few decades7, 8. Among photoresponsive moieties, azobenzene-based moieties have received
considerable attention due to their several properties such as rapidly reversible
photoisomerization, low flammability, bright color and good stability9-11. Azobenzene shows two
significant isomerization trends depending on the type of external exposure. The first trend
originates due to the ultraviolet irradiation which causes trans to cis isomerization. This
isomerization brings a decrease in absorption intensity at 330-380 nm (π-π* transition) and an
increase in absorption intensity at 420-450 nm (n-π* transition). The second substantial trend can
be observed due to the visible light or heat which causes cis to trans isomerization12. These
photo conformational changes in azobenzene can change its dipole moment, absorption capacity
and geometry13. The communication of electronic states can be manipulated by interlinking
ferrocene and azobenzene group with a π-conjugated system or an organic linker with suitable
length14. Nishihara and co-workers were among the pioneers who reported the unique properties
of ferrocene-based azobenzenes in which the one-electron oxidation of the compound decreased
the thermodynamic stability of the cis-azobenzene group bonded to ferrocene15. This oxidation
provided a corridor towards a trans rich mixture in the Fe3+ state using monochromatic green
light and a cis-rich mixture due to the electrochemically reduced state of ferrocene (Fe2+). This
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electro-photo probe-based system proved as a good on-off switching model. Alternatively,
different kinds of functional groups can alter the cis-trans isomerization of organic azobenzene
as well16, 17. Thus, the photoisomerization behavior of azobenzene can be changed by the
introduction of the appropriate functional group. It is expected that introducing a methyl group
on the electron-rich benzene rings of azobenzene can decrease the photo-oxidation, photoinduced
side reactions and π−π stacking of chromophores18, 19. What is noteworthy is that, on a
combination of ferrocene and azobenzene in one system, the ferrocenyl center affects the
isomerization of azobenzene by intra-molecular interactions20. By taking this all idea into the
account, different ferrocene-and azobenzene-based copolymers (P(FHEMA-co-MAAT)s) have
been synthesized and their reversible electro/photo responsive properties have been analyzed in
this work.
2. Experimental section
2.1 Materials
Tetrahydrofuran (THF, analytical reagent (AR)), dichloromethane (DCM, AR), methanol (AR),
pyridine (AR) and triethylamine (Et3N, AR) were purchased from Sinopharm Chemical Reagent
Co., Ltd. Ferrocene carboxylic acid (AR) was purchased from Aladdin. Hydroxyethyl
methacrylate (HEMA, 96%) and methacrolyl chloride (95%) were provided by Acros Organics.
O-aminoazotoluene (97%) was purchased from Tokyo Chemical Industry (TCI) Co., Ltd.
Tetrabutylammonium tetrafluoroborate (Bu4NBF4) and azobisisobutyronitrile (AIBN) were
supplied by J& K Scientific Co. Ltd. Activated 4A-type molecular sieves were used to dry the
pyridine. DCM, THF and Et3N were firstly dried by activated 4A-type molecular sieves. After
that, DCM and Et3N were distilled over calcium hydride while THF was distilled over
potassium. Other chemicals were used without further purification.
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2.2 Synthesis of ferrocene carbonyl chloride
Ferrocenecarbonyl chloride was prepared in a schlenk line system to ensure moisture free
environment as reported before21. In a typical reaction, ferrocene carboxylic acid (10.3601 g,
45.0 mmol) was firstly dried under vacuum at 50 °C for 30 min and then dissolved in 75.0 mL of
freshly distilled DCM. After that, pyridine (7.20 mL, 90.36 mmol) was added to the previous
solution followed by the dropwise addition of oxalyl chloride (7.75 mL, 90.36 mmol) at 25 °C.
The reaction mixture was stirred for 30 min first at 25 °C and then refluxed for 5 h. The contents
of the reaction flask were evaporated under vacuum and petroleum ether (80.0 mL) was added.
The mixture was stirred for 2 h at 90 °C at this stage. At last, the solvent was evaporated to get
the dried ferrocene monocarbonyl chloride.
2.3 Synthesis of 2-(methacryloyloxy)ethyl ferrocene carboxylate (FHEMA)
FHEMA was prepared as reported before22. To obtain FHEMA, ferrocene monocarbonyl
chloride (6.1400 g, 24.6 mmol) was firstly dissolved in 60.0 mL of THF. Afterward, pyridine
(1.93 mL, 24.6 mmol) and HEMA (3.01 mL, 24.7 mmol) were added to the previous solution
under Ar atmosphere. This reaction mixture was refluxed for 5 h. After completing the reflux,
the precipitates were removed from the reaction mixture by filtration. After that, the obtained
filtrate was evaporated on a rotary evaporator. This crude product was washed successively by a
saturated solution of Na2CO3 (250 mL × 3) and deionized water (250 mL × 3) to obtain the final
pure product. This product was placed in a vacuum oven at 40 oC to get FHEMA.
2.4 Synthesis of methacrylo-2-aminoazotoluene (MAAT)
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MAAT was prepared according to the reported procedure23. In a typical reaction, o-
aminoazotoluene (AAT) (4.6011 g, 20.4 mmol) was firstly dried under vacuum for 30 min at
room temperature and then dissolved in 80.0 mL of DCM. After that, triethylamine (3.00 mL,
21.5 mmol) was added in the same reaction flask at the room temperature followed by the
dropwise addition of methacrolyl chloride (2.50 mL, 25.5 mmol) at 0 ºC. The reaction was
carried out for 2 h at this stage and then for 8 h at room temperature. Afterward, the precipitates
were removed from the reaction mixture by filtration. This crude product was washed
successively by a saturated solution of Na2CO3 (250 mL × 3) and distilled water (250 mL × 3) to
obtain the final pure product. This product was placed in a vacuum oven at 40 oC to get reddish
yellow MAAT.
2.5 Synthesis of poly 2-(methacryloyloxy) ethyl ferrocene carboxylate (PFHEMA)
For obtaining poly 2-(methacryloyloxy) ethyl ferrocene carboxylate, free radical polymerization
was carried out24. In a typical synthesis, FHEMA (500.3 mg, 1.75 mmol) and AIBN (2.9 mg,
0.0176 mmol) were dissolved in DMF (2.0 mL). The flask was purged with Ar gas first for 15
min and then the reaction was left for 12 h at 80 ºC. The reaction mixture was precipitated in 250
mL of methanol to remove the unreacted monomers. After that, the solvent with precipitates was
centrifuged. The obtained residual solid was dissolved in THF and precipitated again in 250 mL
of methanol. The final polymer was obtained with centrifugation and dried in a vacuum oven for
the complete removal of residual solvents.
2.6 Synthesis of poly (methacrylo-2-aminoazotoluene) (PMAAT)
For obtaining poly (methacrylo-2-aminoazotoluene), free radical polymerization was used24. In a
typical synthesis, MAAT (306.1 mg, 1.04 mmol) and AIBN (1.4 mg, 0.0085 mmol) were
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dissolved in DMF (2.0 mL). The flask was purged with Ar gas first for 15 min and then the
reaction was left for 12 h at 80 ºC. The reaction mixture was precipitated in 250 mL of methanol
to remove the unreacted monomers. After that, the solvent with precipitates was centrifuged. The
obtained residual solid was dissolved in THF and precipitated again in 250 mL of methanol. The
final polymer was obtained with centrifugation and dried in a vacuum oven for the complete
removal of residual solvents.
2.7 Synthesis of poly (2-(methacryloyloxy) ethyl ferrocene carboxylate-co-(methacrylo
aminoazo toluene) P(FHEMA-co-MAAT)
For the typical copolymerization of FHEMA and MAAT, free radical polymerization technique
was adapted19. The synthetic procedure for all the obtained polymers is same with the difference
in the mole ratios of monomers (FHEMA, MAAT). Therefore, the synthesis of P(FHEMA-co-
MAAT)-1 was taken as an example. In the typical synthesis, FHEMA (0.3401 g, 0.99 mmol),
MAAT (0.2801 g, 0.97 mmol) and AIBN (0.0038 g, 0.02 mmol) were dissolved in DMF (2.0
mL). The flask was then purged with Ar gas for 0.5 h to remove any dissolved oxygen. The
reaction was carried out for 12 h at 80 ºC. The reaction mixture was precipitated in 250 mL of
methanol to remove any unreacted monomers. After that, the solvent with precipitates was
centrifuged. The obtained residual solid was dissolved in THF and precipitated again in 250 mL
of methanol. Finally, the precipitates were centrifuged, filtered and dried in a vacuum oven to
obtain a yellowish solid.
2.8 Characterization
The 1H NMR spectra of the synthesized copolymers were measured using a Bruker Advanx-600
MHz NMR spectrometer. Gel permeation chromatography (GPC) analysis was carried out with
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waters 1524/2414 as GPC instrument and THF as the mobile phase. All the copolymers were
calibrated against polymethylmethacrylate (PMMA). Thermogravimetric analysis (TGA) was
performed with a heating rate of 10 ºC/min using TA-Q500 (Metler-Toledo). Differential
scanning calorimetry (DSC) was executed with a heating rate of 10 ºC/min using Perking Elmer
(PE) DSC 7. UV-vis spectra of the samples were measured using the Unico spectrophotometer.
The concentration of the solution was 0.05 mM. The CV analysis was measured with a CHI-
630A electrochemical analyzer (CH Instruments, Inc., Austin, Texas). For all the samples, the
concentration of polymers and electrolyte (Bu4NBF4) was 0.5 mM and 0.1 M respectively. A
platinum wire electrode was used as the counter electrode and silver (Ag) electrode as a
reference electrode. The glassy carbon working electrode was given a mirror finish using 0.05
µm Al2O3 paste followed by washing under ultrasonication with alcohol/deionized water for 30 s
and finally dried at room temperature prior to use.
3. Results and discussions
3.1 Synthesis and characterization of FHEMA
Esterification reaction between ferrocene carbonyl chloride and HEMA was carried out to
1
synthesize FHEMA (Scheme 1a). The structure of FHEMA was confirmed by H NMR (Fig.
1a). The data of chemical shifts for FHEMA was as follows: δ (ppm) = 6.07 (1H, Ha), 5.71 (1H,
Hb), 4.71 (2H, Hc), 4.46 (2H, Hd), 4.38 (4H, He, Hf), 4.18 (5H, Hg), 1.88 (3H, Hh).
3.2 Synthesis and characterization of MAAT
The synthesis of MAAT was successfully performed by amidation reaction of o-
aminoazotoluene and methacrolyl chloride (Scheme 1b). The structure of MAAT was elucidated
by 1H NMR (Fig. 1b). Following chemical shifts were obtained in a result: δ (ppm) = 9.44 (1H,
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Ha), 7.29-7.77 (7H, Hb-Hh) 5.89 (1H, Hi), 5.54 (1H, Hj), 2.66 (3H, Hk), 2.31 (3H, Hl), 1.98 (3H,
Hm).
3.3 Synthesis and characterization of PFHEMA, PMAAT and P(FHEMA-co-MAAT)s
Free radical polymerization was executed to synthesize PFHEMA, PMAAT and P(FHEMA-co-
MAAT)s. Homo-polymerization and copolymerization of FHEMA and MAAT were performed
using AIBN as a free radical initiator. The synthetic routes of homopolymers and copolymers
have been given in Scheme 1(c-e). Different mole ratios of monomers were taken to synthesize
different copolymers. The characteristic peaks of the synthesized homopolymers and copolymers
were confirmed by 1H NMR spectra as shown in Fig. 1(c and d) and Fig. 2 respectively.
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Scheme 1 Synthetic reactions for (a) FHEMA, (b) MAAT, (c) PMAAT, (d) PFHEMA and (e)
P(FHEMA-co-MAAT).
9

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(a)
(b)
Whereas n=17
(c)
Whereas n=16
(d)
Fig. 1 1HNMR spectra of (a) FHEMA, (b) MAAT, (c) PMAAT and (d) PFHEMA.
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The characteristic peak shifts for PFHEMA were as follows: δ (ppm) = 4.76 (2H, Ha), 4.45 (2H,
Hb), 4.33 (4H, Hc, Hd), 4.19 (5H, He), 1.66-2.01 (2H, Hf), 0.84-1.15 (3H, Hg).
The characteristic peak shifts for PMAAT were as follows: δ (ppm) = 8.86 (1H, Ha), 7.13-7.72
(7H, Hb-Hh) 2.50-2.61 (3H, Hi), 2.24-2.38 (3H, Hj), 1.71-1.98 (2H, Hk), 1.22-1.51 (3H, Hl).
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(a)
(b)
(c)
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1
Fig. 2 HNMR spectra of (a) P(FHEMA-co-MAAT)-1, (b) P(FHEMA-co-MAAT)-2 and (c)
P(FHEMA-co-MAAT)-3.
The chemical shifts for P(FHEMA-co-MAAT)-1 were as follows:
δ (ppm) = 8.86 (1H, Ha), 7.16-
7.77 (7H, Hb-Hh) 4.72 (2H, Hi), 4.35 (6H, Hj, Hk, Hl), 4.15 (5H, Hm), 2.60 (3H, Hn), 2.37 (3H,
Ho), 1.77-2.08 (4H, Hp, Hq), 0.91-1.28 (6H, Hr, Hs).
The chemical shifts for P(FHEMA-co-MAAT)-2 were as follows:
δ (ppm) = 8.84 (1H, Ha), 7.21-
7.78 (7H, Hb-Hh) 4.74 (4H, Hi), 4.39 (12H, Hj, Hk, Hl), 4.16 (10H, Hm), 2.62 (3H, Hn), 2.37 (3H,
Ho), 1.72-2.08 (6H, Hp, Hq), 0.86-1.23 (9H, Hr, Hs).
The chemical shifts for P(FHEMA-co-MAAT)-3 were as follows:
δ (ppm) = 8.83 (1H, Ha), 7.24-
7.77 (7H, Hb-Hh) 4.75 (6H, Hi), 4.42 (18H, Hj, Hk, Hl), 4.17 (15H, Hm), 2.66 (3H, Hn), 2.37 (3H,
Ho), 1.64-2.10 (8H, Hp, Hq), 0.81-1.28 (12H, Hr, Hs).
The synthetic details of the copolymers (P(FHEMA-co-MAAT)s) are mentioned in Table 1
whereas Table 2 shows the details of molecular weights and mole ratios of copolymers.
Step 1
Step 2
DMF
Step 3
Step 4
FHEMA (A)
MAAT (B)
AIBN
Ar purging
T
t
Mole ratio
Sample
g
mmol
g
mmol
g
mmol
0.02
0.03
0.04
A:B
mL
2.0
2.0
2.5
h
oC
h
P(FHEMA-co-MAAT)-1
P(FHEMA-co-MAAT)-2
P(FHEMA-co-MAAT)-3
0.3401
0.6801
1.0211
0.99
1.98
2.98
0.2801
0.2915
0.2811
0.97
1.01
0.98
0.0038
0.0049
0.0065
1:1
2:1
3:1
0.5
0.5
0.5
80
80
80
12
12
12
Table 1 Synthetic details of P(FHEMA-co-MAAT)s
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P(FHEMA-co-MAAT)sa
Mw
P(FHEMA-co-MAAT)sc
Sample
Mn
PDI
1.60
1.86
1.89
(x:y)b
1.19:1
2.41:1
3.20:1
m(FHEMA)
n(MAAT)
P(FHEMA-co-MAAT)-1
P(FHEMA-co-MAAT)-2
P(FHEMA-co-MAAT)-3
4970
8277
7887
7983
15472
14946
4
8
9
3
3
2
Table 2 GPC results and ratios of each unit
a: Determined by GPC data, b: determined by 1H NMR results whereas x and y are the number
of mole ratios attached with FHEMA and MAAT respectively and c: calculated with the help of
1
GPC and H NMR results whereas m and n are the number of repeating units for FHEMA and
MAAT respectively.
1
Fig. 2 represents the comparison of H NMR spectra of P(FHEMA-co-MAAT) copolymer. The
results indicated that the peak area between 6.75-8.0 ppm belong to azobenzene, while the
signals between 4.0-5.0 ppm belong to ferrocene. The disappearance of a double bond having a
chemical shift between 5.5-6.5 ppm indicated that the unreacted monomers were completely
removed from the polymers. Fig. 3A shows the GPC curves of the different synthesized
polymers and it can be seen that the sample P(FHEMA-co-MAAT)-2 and P(FHEMA-MAAT)-3
did not show a mono distinct peak in comparison with P(FHEMA-co-MAAT)-1. This might be
due to the uncontrolled polymerization route. Fig. 3B and 3C show the thermal analysis (TGA
and DTG) and Fig. 3D explains the differential scanning calorimetry (DSC) of the synthesized
polymers. Fig. 3B explains the multistage degradation process of the polymers. The initial
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decomposition temperature of all the three copolymers (P(FHEMA-co-MAAT)s) was around
280 ºC which could be due to the putrefaction of the azo group. Afterward, the second stage of
weight loss appeared at around 365 ºC which is ascribed to the decomposition of ester bonds
(C=O and C–O) attached with the ferrocene. A large amount of carbon dioxide was released as a
result which might be responsible for this weight loss. Lastly, Fe-C bonds were decomposed,
forming a lot of Fe atoms which possibly catalyzed the degradation of polymers after 465 ºC.
Thermal stability of the copolymers might be increased due to the addition of azobenzene-based
monomer which had the thermal stability up to 270 ºC due to the high electron density and
conjugate structure. The similar results could be observed in the case of DTG curves (Fig. 3C).
b
100
a
(B)
(A)
80
c
a
b
c
60
d
d
e
e
40
20
0
100
200
300
T (oC)
400
500
16
18
20
22
24
26
Elution time (min)
22.0
21.5
21.0
20.5
20.0
19.5
19.0
18.5
18.0
17.5
40
35
30
25
20
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
(D)
c
(C)
a
b
d
30 40 50 60 70 80 90 100 110
c
e
e
d
b
a
50
100
150
200
100
200
300
400
500
T(oC)
T (oC)
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Fig. 3 (A) GPC curves of (a) PFHEMA, (b) PMAAT, (c) P(FHEMA-co-MAAT)-1, (d)
P(FHEMA-co-MAAT)-2 and (e) P(FHEMA-co-MAAT)-3; (B) TGA curves of (a) PFHEMA, (b)
PMAAT, (c) P(FHEMA-co-MAAT)-1, (d) P(FHEMA-co-MAAT)-2 and (e) P(FHEMA-co
-
MAAT)-3; (C) DTG curves of (a) PFHEMA, (b) PMAAT, (c) P(FHEMA-co-MAAT)-1, (d)
P(FHEMA-co-MAAT)-2 and (e) P(FHEMA-co-MAAT)-3 and (D) DSC curves of (a) PFHEMA,
(b) PMAAT, (c) P(FHEMA-co-MAAT)-1, (d) P(FHEMA-co-MAAT)-2 and (e) P(FHEMA-co
-
MAAT)-3.
Fig. 3D shows the DSC curves of synthesized polymers whereas inset shows the melting
temperature (Tm) of PFHEMA and PMAAT. The Tg of P(FHEMA-co-MAAT)-3 was found to be
lower than those of the other two polymers. This might be due to the least percentage of an azo
moiety in the polymer so its Tg became lower as compared to the other two polymers. The liquid
crystalline phases can be observed from the DSC curves of P(FHEMA-co-MAAT)-2 and
P(FHEMA-co-MAAT)-3. One possible explanation is that in these polymers the amount of
mesogenic ferrocene unit is higher. This mesogenic ferrocene group imparted liquid crystalline
properties to the polymer as explained by Amer and co-workers25. The azo group in the polymer
enhances the size, rigidity and the polarity of the polymer chains. This enhancement usually
results in the transition from the glass-like state to the rubber-like state at high temperatures.
3.4 Photo isomerization properties of P(FHEMA-co-MAAT)s
Photoinduced isomerization is considered as a backbone of azobenzene-containing polymers as
this behavior can bring a lot of exciting applications such as optical data storage26, molecular
switches27, display devices28, nonlinear optical devices29 and photochemical systems30.
According to the conformational and isomerization behavior, azobenzenes have been classified
into aminoazobenzene type, pseudo-stilbene type and azobenzene type molecules. Low (n-
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π*)

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and high (
π
-π*) absorption intensities can be observed depending on the type of source (UV/vis)
for the azobenzene type of components. The UV-vis spectroscopy can be used easily for the
trans back relaxation31. To
monitoring of these type of molecules due to relatively slow cis
-
characterize the photoisomerization behavior of the polymers, the sample solutions were
prepared in DMF.
The samples were irradiated using UV/vis light and their absorption spectra were recorded to
capture the photoisomerization process of copolymers at different time intervals. The irradiation
of UV light (364nm) caused trans to cis isomerization which triggered a decrease in absorption
intensity at around 390 nm from
π-π* transition and an increase in absorption intensity at around
430 nm from n- * transition. When the polymer solutions were exposed to visible light at
π
different times, the reverse transition of azobenzene moiety can be observed as shown in Fig. 4
(a-c). This spectral variation evidences that the synthesized polymers have photoisomerization
behavior, showing an appealing application as light stimuli-responsive materials.
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1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
-0.2
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
-0.1
-0.2
(a) Before UV irradiation
b) 5 min UV irradiation
c) 2 min vis irradiation
d) 5 min vis irradiation
e) 15 min vis irradiation
(a) Before UV irradiation
(c)
a
a
(b)
(a)
(
(
(
(
(
(
(
(
b) 5 min UV irradiation
c) 2 min vis irradiation
d) 5 min vis irradiation
e) 15 min vis irradiation
e
d
e
d
c
c
b
b
300
400
Wavelength (nm)
500
300
400
Wavelength (nm)
500
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
-0.1
-0.2
(a) BeforeUVirradition
b) 5minUVirradition
c) 2minvis irradition
d) 5minvisirradition
e) 15minvis irradition
(
(
(
(
a
e
d
c
b
300
400
Wavelength(nm)
500
Fig. 4 Photo induced studies of (a) P(FHEMA-co-MAAT)-1, (b)
P(FHEMA-co-MAAT)-2 and (c) P(FHEMA-co-MAAT)-3 at different time intervals.
3.5 Electrochemical properties of P(FHEMA-co-MAAT)s
The electroactive polymers can give a lot of potential applications. Cyclic voltammetry is used
extensively to explain and confirm the electro-responsive behavior of materials. The
electrochemical behavior of the synthesized polymer can be affected by several factors such as
scan rate, polarity/type of solvent and concentration of supporting electrolyte 32, 33. Therefore, it
is expected that changing these influential parameters wisely can provide the optimum conditions
that are important to find enhanced electrochemical properties.
Different solvents and scan rates affected the electrochemical properties of the synthesized
polymers. The synthesized polymers showed different behavior by changing the parameters
(scan rate and solvents). Organic solvents affected the shape of the CV curves due to the change
in the polarity of the solvent. From Fig. 5 to 7, the shape of the curve was found to be more
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compact in case of DMSO and DMF due to their high polarity while the shape was more
expanded in DCM and a little bit in THF because of their less polarity. Similarly, one redox peak
was observed in the case of DCM whereas the solutions of THF, DMF and DMSO showed two
or more than two redox peaks.
Fig. 5 CV curves of P(FHEMA-co-MAAT)-1 in (a) DCM, (b) DMF, (c) DMSO and (d) THF at
different potential scan rates (direction of arrows means the increase of peaks in terms of current
(oxidation and reduction) by increasing the scan rate) and (e) different organic solvents: (1)
DCM, (2) DMF, (3) DMSO and (4) THF at 0.1 V/s.
The reason behind obtaining two redox peaks might be related with the higher viscosity, less ion
mobility and higher resistance of the solvents in comparison with DCM. The appearance of one
redox peak showed that all the ferrocene units behaved equally whereas in high polar solvents
the oxidized form of ferrocene was more stable and it was difficult to reduce. The similar
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behavior can be observed from the previously reported results by Wang et. al 33. The synthesized
polymers also showed sensitivity toward potential scan rate. It was found that the peak current
values were increased by increasing the scan rate.
Fig. 6 CV curves of P(FHEMA-co-MAAT)-2 in (a) DCM, (b) DMF, (c) DMSO and (d) THF at
different potential scan rates (direction of arrows means the increase of peaks in terms of current
(oxidation and reduction) by increasing the scan rate) and (e) different organic solvents: (1)
DCM, (2) DMF, (3) DMSO and (4) THF at 0.1 V/s.
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Fig. 7 CV curves of P(FHEMA-co-MAAT)-3 in (a) DCM, (b) DMF, (c) DMSO and (d) THF at
different potential scan rates (direction of arrows means the increase of peaks in terms of current
(oxidation and reduction) by increasing the scan rate) and (e) different organic solvents: (1)
DCM, (2) DMF, (3) DMSO and (4) THF at 0.1 V/s.
This increasing trend explained that the charge transport of the electrode evidenced Fick’s law at
room temperature. The rate of electrode reaction was faster in DCM in comparison with the other
used solvents due to the smaller potential separation (peak to peak). The offered resistance by
DCM was smaller and charge diffusion rate was faster than other solvents which explained the
reason behind smooth occurring of redox reactions.
Similarly, the less steric hindrance between the synthesized copolymers eased the electron
transport characteristics which showed distinct oxidation and reduction peaks in CV studies.
These results indicated that reversible electrochemical processes can be achieved by using these
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synthesized polymers in this work. It was found that the synthesized copolymers in this work can
be used for the storage of optical and electrical information. For the better understanding,
schematic and pictorial views have been shown in Fig. 8.
Fig. 8 (a) Possible schematic view representing the information storage application and (b)
pictorial view of the information storage application based on polymers containing ferrocene and
azobenzene.
In the first step, UV light incidents and trans azobenzene changes to cis azobenzene which can
be regarded as state 2 (optical information storage stage). In the third state, electrical information
is stored, when ferrocene changes to ferrocenium ion (Fc to Fc+). The fourth state corresponds
with cis to trans isomerization of azobenzene after consuming visible light and at the end
ferrocenium ion turns into its initial state after retrieving the electrical information
(electrochemical stimulus).
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4. Conclusion
In this study, we focused on the stimuli-responsive behavior of ferrocene and azobenzene. For
this purpose, three different copolymers named as P(FHEMA-co-MAAT)-1, P(FHEMA-co-
MAAT)-2 and P(FHEMA-co-MAAT)-3 were synthesized using free radical polymerization and
1
characterized by H NMR spectra. Thermal stability of the polymers was confirmed using
TG/DTG analysis. These resulting copolymers showed excellent redox and photoresponsive
properties which were confirmed with the help of cyclic voltammetry and UV/vis spectroscopy.
It was concluded that when these moieties were combined together, they showed very useful
electro-photo responsive behavior which can be further used for information storage application.
Acknowledgment
Financial support from the National Natural Science Foundation of China (51673170, 21472168,
21372200 and 21611530689), the Science and Technology Innovation Team of Ningbo
(2011B82002), the Fundamental Research Funds for the Central Universities (2017FZA4023)
are gratefully acknowledged.
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•
Ferrocene and azobenzene-based polymers were synthesized by free radical
polymerization
•
•
•
Photo induced studies were performed by UV/vis spectroscopy
Electro responsive behavior was confirmed by Cyclic Voltammetry
The synthesized polymers can be a good choice for information storage devices