Electrochemical and spectroscopical characterization of ferrocenyl chalcones

Article abstract of DOI:10.1149/1.3439669

We report on the electrochemical and spectroscopical characterization of 10 ferrocenyl chalcones. These compounds were synthesized via the Claisen-Schmith condensation from acetylferrocene and the corresponding benzaldehyde or pyridine carboxaldehyde. The compounds were obtained in good yields (>60%) and purified by recrystallization. All compounds exhibit a quasi-reversible electrochemical behavior and chemical reversibility, as evidenced by Δ E_p of 72-90 mV and I_pa/I_pc values equal to 1.0-1.17. The redox formal potential (E^o) for these compounds range between 665 and 774 mV. The heterogeneous electron-transfer rate constants calculated from the Nicholson method are in the range of (2.0-4.4) × 10^-2 cm/s. Their diffusion coefficients are on the order of (1.65-3.7) × 10^-5 cm² /s, and the number of electrons transferred equals 1. 1-ferrocenyl-3-(p-N,N -dimethylaminophenyl)prop- 2-en-1-one shows a behavior similar to dimethylaniline and undergoes electropolymerization, producing cationic oligomers and dimeric species. The UV/visible spectra show the characteristic bands for aromatic systems in the UV region (200 and 280 nm) and bands in the visible (400 and 500 nm). The molar absorbance coefficient (ε) values are in the ranges (2.1-7.5) × 10⁴ L/ (mol cm) for λmax of 200-300 nm and (1.8-17.6) × 10² L/ (mol cm) for λ_max of 300-500 nm.

Full text of DOI:10.1149/1.3439669

F104
Journal of The Electrochemical Society, 157 ͑8͒ F104-F110 ͑2010͒
0013-4651/2010/157͑8͒/F104/7/$28.00 © The Electrochemical Society
Electrochemical and Spectroscopical Characterization
of Ferrocenyl Chalcones
*
Rocío A. Cardona, Kenneth Hernández, Laura E. Pedró, Myrna R. Otaño,
Ingrid Montes, and Ana R. Guadalupez
Department of Chemistry, University of Puerto Rico, Río Piedras Campus, San Juan 00931-3346,
Puerto Rico
We report on the electrochemical and spectroscopical characterization of 10 ferrocenyl chalcones. These compounds were syn-
thesized via the Claisen–Schmith condensation from acetylferrocene and the corresponding benzaldehyde or pyridine carboxal-
dehyde. The compounds were obtained in good yields ͑Ͼ60%͒ and purified by recrystallization. All compounds exhibit a
quasi-reversible electrochemical behavior and chemical reversibility, as evidenced by ⌬Ep of 72–90 mV and Ipa/Ipc values equal
to 1.0–1.17. The redox formal potential ͑Eo ͒ for these compounds range between 665 and 774 mV. The heterogeneous electron-
Ј
transfer rate constants calculated from the Nicholson method are in the range of ͑2.0–4.4͒ ϫ 10−2 cm/s. Their diffusion coeffi-
cients are on the order of ͑1.65–3.7͒ ϫ 10−5 cm2/s, and the number of electrons transferred equals 1.
1-ferrocenyl-3-͑p-N,N-dimethylaminophenyl͒prop-2-en-1-one shows a behavior similar to dimethylaniline and undergoes elec-
tropolymerization, producing cationic oligomers and dimeric species. The UV/visible spectra show the characteristic bands for
aromatic systems in the UV region ͑200 and 280 nm͒ and bands in the visible ͑400 and 500 nm͒. The molar absorbance coefficient
͑␧͒ values are in the ranges ͑2.1–7.5͒ ϫ 104 L/͑mol cm͒ for ␭max of 200–300 nm and ͑1.8–17.6͒ ϫ 102 L/͑mol cm͒ for ␭max of
300–500 nm.
© 2010 The Electrochemical Society. ͓DOI: 10.1149/1.3439669͔ All rights reserved.
Manuscript submitted December 15, 2009; revised manuscript received April 12, 2010. Published June 15, 2010.
Ferrocene derivatives have been extensively studied for many
reasons. First, they exhibit the well-defined one-electron redox pro-
cess that is characteristic of ferrocene. The redox process is electro-
chemically reversible or quasi-reversible, and the formal redox po-
from Buehler Co. Nanopure water ͑17 M⍀ cm, Barnstead͒ was
used in all experiments. Thin-layer chromatography ͑TLC͒ was done
with 250 ␮m Al2O3 plates ͑Whatman͒.
Synthesis and characterization of the compounds.— FCs were
synthesized in Dr. I. Montes’ laboratory following the Claisen–
Schmidt reaction, as previously published.11,14,15 1H and 13C NMR
spectra were taken in a 500 MHz Bruker-AV spectrometer, while the
UV/vis spectra were done in a Perkin-Elmer ͑Lambda 35͒ spectro-
photometer.
Synthesis of FCs.— 1 mmol of acetylferrocene was added to a 50
mL erlenmeyer flask and dissolved in a 1:1 ethanol/water solution
with 10 mmol of NaOH at room temperature. In another flask, 1
mmol of the desired aromatic derivative was mixed with 3–5 mL of
ethanol. The acetylferrocene solution was added slowly to the solu-
tion of the substituted benzaldehyde or pyridine carboxaldehyde un-
der continuous stirring. The reaction progress was followed by TLC
until completion. The resultant precipitate was collected by vacuum
filtration and recrystallized. Compounds 2a and 2e were recrystal-
lized from acetone, 2d from ethanol, and 2j from 1:1 ethanol/water.
Mixtures of 1:1 acetone/water were used to recrystallize compounds
2b, 2f, 2g, 2h, 2i, and 2k, while a 2:1 mixture was used for com-
pound 2c.
tential ͑Eo ͒ depends on the nature of the substituents attached to the
Ј
cyclopentadienyl rings.1,2 Through a variety of synthetic routes, de-
rivatives can be made with tunable redox and spectroscopical
properties.2,3 They have found applications in different fields such as
biosensors,3-5 drug delivery systems,6 electrocatalyts,7,8 and
optoelectronics,9 among others.
A family of ferrocene derivatives that have drawn considerable
attention from scientists in recent years is that of the ferrocenyl
chalcones ͑FCs͒.10-12 These compounds not only possess the redox
activity of ferrocene, but they also show intense colors.12 An FC is a
compound where ferrocene is linked to an aromatic group through
an enone moiety. Several derivatives of FCs have been synthesized
and characterized structurally.10-13 More recently, studies on FCs
have focused on their biological properties, in particular their poten-
tial antiplasmodial and anticancer activity.11
In our case, we are interested in exploiting these compounds for
applications in biosensing and electrocatalysis. For that purpose, we
conducted a study of the electrochemical and UV/visible ͑UV/vis͒
behavior of several FCs ͑Fig. 1͒. To the best of our knowledge, few
studies have been published using FCs for these applications.
Methylation of DMAPP.— In
a
round flask,
1
mmol of
1-ferrocenyl-3-͑p-N,N-dimethylaminophenyl͒
prop-2-en-1-one
͑DMAPP͒ and 8 mmol of sodium bicarbonate were dissolved in
2-methyl tetrahydrofuran. The mixture was stirred under N2, while
16 mmol of dimethylsulfate was added dropwise. After 1 h stirring
at room temperature, the temperature was raised to 60°C for 2 h.
The resulting precipitate was collected and dried. The purification
was done by recrystallization from 1:1 acetone/water mixture.
Experimental
Chemicals.— Ferrocene
3-nitrobenzaldehyde ͑99%͒,
͑98%͒,
acetylferrocene
͑95%͒,
͑98%͒,
4-nitrobenzaldehyde
4-methoxybenzaldehyde ͑98%͒, 4-chlorobenzaldehyde ͑98%͒,
4-͑dimethylamino͒ benzaldehyde ͑Ն99%͒, 2-fluorobenzaldehyde
͑97%͒, 3-fluorobenzaldehyde ͑97%͒, 2-pyridinecarboxaldehyde
͑99%͒, 3-pyridinecarboxaldehyde ͑98%͒, 4-pyridinecarboxaldehyde
͑97%͒, tetrabutylammonium perchlorate ͑TBAP͒, acetonitrile
͑ACN͒-d3 ͑99.6% D͒, chloroform-d ͑99.8% D͒, acetone, and ethanol
͑American Chemical Society grade͒ were obtained from Aldrich.
ACN ͑Burdick and Jackson͒ was dried over 4 Å molecular sieves.
Diamond paste ͑no. 40-6244͒ and metadi fluid ͑no. 40-6032͒ were
Electrochemistry.— All electrochemical experiments were done
in a BAS 100 B/W. A three-electrode electrochemical cell was used
with Pt ͑area = 0.0197 cm2͒ or glassy carbon ͑area = 0.080 cm2͒,
Ag/AgCl ͑NaCl 3 M͒, and nichrome as the working, reference, and
counter electrodes, respectively. The Pt electrode surface was pol-
ished with diamond paste/metadi fluid in a microcloth and rinsed
with water. Subsequently, the electrode was cycled in 1 M H2SO4
solution from Ϫ200 to 1200 mV at 100 mV/s for 50 cycles, rinsed
with water and air dried.
For each compound, a 10 mL of ϳ1 ϫ 10−3 M solution was
prepared in 0.1 M TBAP/ACN. Cyclic voltammetry ͑CV͒ was run
for each compound at scan rates of 20, 50, 100, 200, 500, 1000, and
*
Electrochemical Society Student Member.
z E-mail: anlupe@uprrp.edu
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Journal of The Electrochemical Society, 157 ͑8͒ F104-F110 ͑2010͒
F105
O
O
Fe
Fe
N(CH3)2
N
2c
Figure 1. Synthesis of FCs.
2000 mV/s. 1 and 10 cycles were done for each scan rate, and the
electrode was cleaned between scan rates. Each experiment was run
three times, every time with a fresh solution. The background was
recorded for each compound under the same experimental condi-
tions of the sample. The cell was degasified with N2 for 10–15 min
before each measurement. To determine the number of electrons
transferred, a 10 mL TBAP 0.1 M/ACN solution containing ϳ1
ϫ 10−3 M of both ferrocene and the FC was prepared. A CV of the
solution was recorded at 100 mV/s between 0 and 1200 mV. Chro-
nocoulometry ͑CC͒ was done in a similar solution but without the
ferrocene to determine the diffusion coefficient of the compounds.
The applied potential pulse was from 200 to 1200 mV for 250 ms.
UV/vis spectroscopy.— The molar extinction coefficients of the
most prominent UV/vis bands of the compounds were calculated
from calibration curves following Beer’s law. A 1 ϫ 10−3 M stock
solution of each compound was prepared in ACN followed by dilu-
tions with concentrations in the ͑2–1000͒ ϫ 10−6 M range. Absorp-
tion spectra of all compounds were taken from 190–820 nm. The
cell path was 1.00 cm.
Figure 2. 1H and 13C NMR of DMAPP ͑2c͒ in CD3CN.
caniova and co-workers.16 See tables in the supporting information
section17 for a summary of the spectral analysis of the studied com-
pounds.
Results and Discussion
Synthesis and chemical characterization.— The FCs differ in
the aromatic derivative attached to the double bond. The o- and
m-fluorophenyl derivatives ͑2f, 2g͒ have not been reported until
now. All the compounds were obtained in good yields ͑Ͼ60%͒ and
Electrochemical characterization.— The redox activity of the
FCs was studied using CV and CC. Figure 3A-C shows the CVs, Ipa
1
were characterized by H and 13C NMR after purification. Figure 2
vs the square root of the sweep rate curve ͑insert log Ipa vs log ͒,
v
shows the H and 13C NMR spectra of DMAPP ͑2c͒. The H NMR
shows three characteristic regions: the ferrocene ͓␦ ͑ppm͒
= 4.894 ͑s, 2H͒, 4.555 ͑s, 2H͒, 4.184 ͑s, 5H͔͒, the
p-N,N-dimethylaminephenyl ͓␦ ͑ppm͒ = 7.612 ͑d, J = 9 Hz, 2H͒,
6.758 ͑d, J = 8.5 Hz, 2H͒, 3.003 ͑s, 6H͔͒, and the double bond ͓␦
͑ppm͒ = 7.600 ͑d, J = 13.5 Hz, 1H͒, 7.082 ͑d, J = 15.5 Hz, 1H͔.
1
1
and the Anson plot of DMAPP ͑2j͒ ͑insert: Q vs time, ms͒. The same
electrochemical measurements were done for all FCs. Table I sum-
marizes the electrochemical parameters of all the compounds.
The number of electrons transferred was determined by compar-
ing the cathodic peak currents for ferrocene and each individual FC
from a cyclic voltammmogram of a solution containing both com-
pounds at the same concentration and correcting by differences in
their diffusion coefficients. Using this approach, from the Randles
and Sevcik equation,18 assuming a comparable rate of heteroge-
neous electron transfer ͑ks͒, the n values ranged from 0.87 to 1.47.
Based on these data, we conclude that all FC compounds exhibit the
expected 1e− redox process centered on the ferrocene moiety.
13C NMR spectra show the expected number of signals according to
the number of carbon atoms with chemical shifts as follows: fer-
rocene
͓␦
͑ppm͒ = 82.59,
73.19,
70.78,
70.36͔,
p-N,N-dimethylaminephenyl ͓␦ ͑ppm͒ = 153.07, 131.02, 112.90,
40.38͔, the carbonyl ͓␦ ͑ppm͒ = 193.13͔, and double bond ͓␦
͑ppm͒ = 141.72, 123.62͔. All FC compounds showed signals corre-
The formal redox potential ͑Eo ͒ of the FCs varies from
Ј
sponding to these three distinct regions. The 1H chemical shifts
corresponding to the ferrocene and double-bond regions were basi-
cally unaltered by the aromatic derivatives. As expected, the 1H
chemical shifts of the aromatic derivative changed depending on the
substituents on the phenyl or the position of the nitrogen atom in the
pyridine. Similar results were observed for the 13C NMR where the
ferrocene, carbonyl, and double-bond signals did not change signifi-
cantly. The 13C NMR for the aromatic derivative followed the ex-
pected pattern depending on the compound molecular structure. Our
results compare favorably with previously published values by Sol-
+665 mV for DMAPP ͑2c͒ to +773 mV for the 2-pyridinyl deriva-
tive ͑2h͒. Compared to ferrocene ͑Eo ͒ , the Eo values are higher by
Ј
Ј
at least +206 mV ͑DMAPP͒ to +314 mV ͑2-pyr͒. These Eo
Ј
changes can be explained by electronic effects. As reported by Wu
and collaborators,11 the carbonyl group on FCs is highly polarized,
and as such it withdraws electron density from the ferrocene moiety,
making its oxidation more difficult, a fact that is reflected on higher
Eo values. Simultaneously, but to a lesser extent, the substituents
Ј
also exert an electronic influence by modulating the electron with-
drawing capacity of the carbonyl group; this explains the small but
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F106
Journal of The Electrochemical Society, 157 ͑8͒ F104-F110 ͑2010͒
330
still significant Eo differences among the various derivatives. This
Ј
A
is consistent with our observation that the nitro derivatives ͑Hammet
220
values of ␴ = 0.81 and ␴ = 0.71͒ exhibit higher Eo ͑as expected
Ј
p
m
for electron withdrawing groups͒, while an electron donating sub-
110
stituent such as N,N-dimethylamino ͑Hammet value of ␴ = −0.63͒
p
shows a lower Eo . In general, the log Eo correlates satisfactorily
0
Ј
Ј
͑r = 0.935͒ with the corresponding Hammet values12,19,20 or the
various substituents ͑see Fig. 4͒.
--1
220 mV/s
1100 mV/s
The Ipc/Ipa ratio, with the exception of the DMAPP ͑vide infra͒,
is close to 1, showing that the redox process of the FCs is chemi-
cally reversible. The ⌬Ep values range between 72 and 90 mV,
which is higher than the expected Nernstian value of 59 mV per
electron, indicating that the redox process is electrochemically
quasi-reversible.18 The plots of Ipc ͑or Ipa͒ vs the square root of the
sweep rate shows a linear dependence, and the slopes of the plots of
log Ipc ͑or Ipa͒ vs log sweep rate are close to 0.5, evidencing that the
redox process is diffusion controlled. The diffusion coefficients
͑Dox͒ are between 1.65 and 3.7 ϫ 10−5 cm2/s, which closely agrees
with the Dox of ferrocene, 2.4 ϫ 10−5 cm2/s.21
--2
5500 mV/s
22000 mV/s
--3
--4
2
0
0
0
1
5
0
0
1
0
0
0
5
0
0
0
--500
PPotentiaal, mV vs AAg/AgCl ((NaCl 3 M)
330
--4.4
yy = -6.15523 + 0.4466739x R= 00.99881
--4.6
225
--4.8
--5
--5.2
220
--5.4
--5.6
1
Electropolymerization of DMAPP.— Figure 5A shows the CV
of 1 mM DMAPP derivative in 0.1 M ACN/TBAP. We observe three
well-defined oxidations with ͑a͒ Epa = +694 mV, ͑b͒ +974 mV,
1
.5
2
2.5
3
33.5
115
LLog v
110
and ͑c͒ +1521 mV and a cathodic process with ͑e͒ Epc
=
+632 mV. On top of the cathodic wave, there is a barely discernible
shoulder around ͑d͒ +760 mV. Figure 5B shows that on reversing
the potential at +850 mV, there is only one reversible wave with
5
yy = 0.138898 + 0.5566797x R= 00.99898
0
Eo = +665 mV. We assigned this wave to the oxidation and reduc-
0
1
0
2
0
3
0
4
0
550
Ј
//2
v1
tion of the ferrocene nucleus in the DMAPP in analogy with other
FCs. The ͑b͒ second and ͑c͒ third oxidations, which are chemically
irreversible are assigned to the successive loss of the nitrogen elec-
trons in the p-N,N-dimethylamino group. The shoulder at ͑d͒
+760 mV corresponds to the reduction of the cationic radical that is
produced after the first oxidation of this group.
8
8
7
C
6
7
5
4
6
3
2
1
5
1
0
0
2
0
0
3
0
0
4
0
0
5
0
0
6600
TTime, ms
Figure 5C shows that on continuous cycling ͑50 cycles͒, the
original waves ͓͑a͒-͑e͔͒ started to decrease with the peak at ͑c͒
+1521 mV, decreasing faster than the others. Concurrently, we ob-
serve a small anodic wave growing between ͑f͒ 1070 and 1400 mV
with a cathodic counterpart between ͑g͒ 900 and 1300 mV. The
background current close to ͑h͒ 0 mV increases toward the negative
potentials. We attribute these changes to the electropolymerization
of DMAPP similar to what happens in the electrochemically induced
polymerization of N,N-dimethylaminoaniline ͑DMA͒. Oyama and
co-workers22 reported the electrochemical oxidation of DMA, a
compound that is structurally similar to the R group in DMAPP.
DMA oxidizes electrochemically at +800 and +1000 mV vs satu-
rated sodium chloride calomel electrode in 0.5 M Na2SO4. The first
oxidation generates a radical cation ͑DMA+•͒ that oxidizes further
into a dication ͑DMA2+͒. On continuous cycling, DMA+• electropo-
lymerizes forming an electroinactive cationic polymeric film at the
electrode surface.
4
3
yy = 1.72203 + 0.355876x R== 00.99933
2
1
0
2
4
6
8
1
0
1
2
1
4
116
//2
t , ms
t1
Figure 3. Cyclic voltammograms of a solution of 2j 1.00 mM in 0.101 M
TBAP/ACN at a platinum working electrode: ͑A͒ voltammogram at various
scan rates, ͑B͒ Ipa vs
Q vs time͒.
1/2
͑insert: log Ipa vs log ͒, and ͑C͒ Anson plot ͑insert:
v
v
Table I. Electrochemical parameters of the FCs.
Dox ͑ϫ10−5
͑cm2/s͒
͒
Eo
͑mV/s͒
⌬Ep
͑mV͒
Ј
Compound
n
Ipc/Ipa
Slope log Ipa vs log
v
2a
2b
2c
2d
2e
2f
2g
2h
2i
723.667 Ϯ 0.001
722.69 Ϯ 0.02
664.50 Ϯ 0.02
680.833 Ϯ 0.002
691.50 Ϯ 0.01
699.00 Ϯ 0.01
693.00 Ϯ 0.01
773.17 Ϯ 0.02
733.667 Ϯ 0.004
760.000 Ϯ 0.004
692.66 Ϯ 0.02
0.93 Ϯ 0.01
0.88 Ϯ 0.01
—
2.17 Ϯ 0.01
3.33 Ϯ 0.01
—
1.01 Ϯ 0.01
1.1 Ϯ 0.4
2.0 Ϯ 0.5
1.1 Ϯ 0.1
1.17 Ϯ 0.04
1.1 Ϯ 0.2
1.1 Ϯ 0.1
1.0 Ϯ 0.2
1.11 Ϯ 0.04
1.0 Ϯ 0.2
1.0 Ϯ 0.7
72.000 Ϯ 0.0001
90.00 Ϯ 0.02
47.67 Ϯ 0.03
86.333 Ϯ 0.002
75.67 Ϯ 0.01
79.67 Ϯ 0.02
79.333 Ϯ 0.004
72.000 Ϯ 0.004
83.33 Ϯ 0.01
82.67 Ϯ 0.01
74.00 Ϯ 0.01
0.453
0.487
0.433
0.478
0.487
0.471
0.467
0.400
0.473
0.467
0.455
0.87 Ϯ 0.02
0.90 Ϯ 0.02
1.44 Ϯ 0.13
1.47 Ϯ 0.10
0.90 Ϯ 0.01
1.25 Ϯ 0.01
0.91 Ϯ 0.01
—
3.7 Ϯ 0.8
3.4 Ϯ 0.1
1.65 Ϯ 0.2
1.73 Ϯ 0.08
3.17 Ϯ 0.01
2.80 Ϯ 0.01
3.12 Ϯ 0.01
—
2j
2k
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Journal of The Electrochemical Society, 157 ͑8͒ F104-F110 ͑2010͒
F107
2.87
2.86
2.86
2.85
2.85
2.84
2.84
2.83
2.83
2.82
2.82
p-NO2
m-NO2
m-F
p-Cl
Figure 4. Hammett plot for ferrocene de-
rivatives ͓log of Eo as a function of Ham-
Ј
p-OCH3
mett values ͑␴͔͒.
y = 0.0251x + 2.8376
R² = 0.9352
p-N(CH3)2
-0.80
-0.60
-0.40
-0.20
0.00
0.20
0.40
0.60
0.80
1.00
Hammett Values (σ)
40
20
10
A
B
e
d
5
0
0
-20
-40
-60
-80
Background
20 mV/s
-5
-10
-15
200 mV/s
100 mV/s
500 mV/s
1000 mV/s
c
a
b
900
800
700
600
500
400
300
200
1600
1400
1200
1000
800
600
400
200
0
Potentiaal, mVV vss AAg/AgCll ((NNaaCCll 33 MM))
Potential, mV vs Ag/AgCl (NaCl 3M)
10
5
c
D
5
e
C
d
g
Figure 5. CV at a platinum electrode of
1.00 ϫ 10−3 M compounds ͑2c͒ and ͑2k͒
in 0.101 M TBAP/ACN: For ͑2c͒, ͑A͒ 1
cycle at 20, 100, 500, 2000 mV/s, ͑B͒ 200
mV/s in the window potential of ferrocene
oxidation, ͑C͒ 50 cycles at 200 mV/s
0–1600 mV/s scan window, ͑D͒ 50 cycles
at 200 mV/s 0–1200 mV/s scan window,
͑E͒ 50 cycles at 200 mV/s in supporting
electrolyte only after cycling in ͑D͒, and
͑F͒ voltammogram of ͑2k͒ at 200 mV/s.
0
-5
0
-5
a
-10
-10
-15
200 mV/s
200 mV/s
f
c
b
b
a
1600
1200
800
400
0
1500
1000
500
0
-500
Potential, mV vs Ag/AgCl (NaCl 3 M)
Potential, mV vs Ag/AgCl (NaCl 3 M)
2
5
E
F
1
0
0
-1
-2
-3
-5
200 mV/s
200 mV/s
1000
800
600
400
200
0
1500
1000
500
0
-500
Potential, mV vs Ag/AgCl (NaCl 3M)
Potential, mV vs Ag/AgCl (NaCl 3M)
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F108
Journal of The Electrochemical Society, 157 ͑8͒ F104-F110 ͑2010͒
Figure 6. Proposed mechanism for elec-
trochemical polymerization of DMAPP:
͑A͒ DMAPP molecular structure, ͑B͒
DMAPP resonance structures, and ͑C͒ for-
mation of the DMAPP dimer: Head to
ortho position attack ͑likewise happens for
the meta position͒.
In analogy to Oyama’s results, we propose that the small wave
formation of a high MW polymer ͑Fig. 6A-C͒. This is consonant
with what has been observed for DMA and aniline.22,23 In both
cases, the polymerization preferably occurs through a head to tail
attack initiated by the cation radical. Although the polymerization
also occurs at the ortho positions relative to the amino and dimethy-
lamino groups, the polymer grows mainly at the para position. In
our case, the para position is sterically hindered because of the pres-
ence of the ferrocenyl moiety.
that grows between +900 and +1400 mV ͑g, h͒ corresponds to the
oxidation and the reduction of a dimeric species produced by the
chemical reaction of two radical monocations generated after the
first oxidation of the N,N-dimethylamino group. The increment in
background signal at 0 mV could result from the reduction of pro-
tons produced during the electropolymerization. The production of
cationic oligomers, which are soluble in ACN, or the formation of
an electroactive polymeric film might be the reason for the voltam-
metric waves fading away eventually.
When the DMAPP is quaternized with dimethylsulfate ͑Fig. 5F͒,
only one voltammetric wave is observed with a Eo = +693 mV.
Ј
Likewise, the quaternization of the DMAPP prevents its electro-
chemical polymerization.
An experiment where the potential was cycled 50 times between
+0 and +1200 mV showed that the wave corresponding to the fer-
rocene nucleus decrease slightly while the one due to the first
DMAPP redox process decreased steadily ͑Fig. 5D͒. When this elec-
trode was removed from the solution, dried in air, and put in an
ACN solution with supporting electrolyte only, a voltammetric wave
Electron-transfer heterogeneous rate constant ͑k ͒
.— To deter-
s
mine the heterogeneous electron-transfer rate constant ͑ks͒, we
treated the CV data according to Nicholson.24 We constructed a
working curve of ⌬Epءn ͑n = 1e−͒ as a function of log ⌿ using the
values for ⌬Eءpn ͑V͒ and ⌿ published by Nicholson. The equation
for this curve is ⌬Eءpn = −0.02⌿3 + 0.0536⌿2 − 0.0545⌿ + 0.083.
By taking the product of the n and ⌬Ep values of each compound
and substituting in the cubic equation, we obtained the ⌿ values.
The ks was calculated from the following equation
with Eo = +696 mV was observed ͑Fig. 5E͒. As before ͑Fig. 5C͒,
Ј
this wave decreases continuously until it disappears completely. In
this experiment, the observation of the ferrocene wave confirms that
the reason for losing the ferrocene signal is the formation of low
molecular weight cationic species.
As has been reported for DMA,22 we propose that DMAPP un-
dergoes electrochemical polymerization after its first oxidation. The
electropolymerization results in the formation of a film at the glassy
carbon electrode. Contrary to the DMA case, DMAPP electropoly-
merization does not produce a high molecular weight polymer be-
cause the p position is not available. The formation of the oligomers
occurs through the chains growth at the meta positions These posi-
tions are sterically hindered thus decreasing the probability of the
␣/2
ks = ⌿͓Do␲͑F/RT͒␯͔1/2/͓͑Do/DR͒
͔
where ⌿ is the value obtained from the cubic equation, Do is the
diffusion coefficient ͑cm2/s͒, F is the Faraday constant ͑96,500
C/mol͒, R is 8.314472 J/mol K, T is 298 K, and ␣ = 0.5. Do/Dr is
assumed to be equal to 1. Table II summarizes the values of ks for all
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Journal of The Electrochemical Society, 157 ͑8͒ F104-F110 ͑2010͒
F109
all FCs showed three bands in the UV region and one in the visible
Table II. Heterogeneous electron-transfer rate constants „ks… for
region. The absorption coefficients ranges from 1.8 ϫ 102 to 7.5
the FCs.
ϫ 104 M−1 cm−1
.
It can also be observed in the spectra that the region between
275–328 nm is variable for all FCs. This region corresponds to the
n − ␲ electronic transition attributed to the carbonyl. As pointed
ks
͑cm/s͒
Compound
ء
2a
2b
2c
2d
2e
2f
2g
2h
2i
͑0.027 Ϯ 0.003͒
͑0.042 Ϯ 0.004͒
—
͑0.024 Ϯ 0.003͒
͑0.037 Ϯ 0.002͒
͑0.023 Ϯ 0.005͒
͑0.024 Ϯ 0.002͒
͑0.020 Ϯ 0.009͒
͑0.027 Ϯ 0.004͒
͑0.044 Ϯ 0.007͒
out in the electrochemical section, these differences demonstrate the
effect that the electrodonating and electrowithdrawing substituents
exert on the carbonyl polarization.12
Different from ferrocene which is yellow-orange solid, all FC
solids exhibit a strong purple reddish color. The color of the ACN
solution was the same as the solid. Some of the FCs exhibit solva-
tochromism. The solutions were stable for 24 h without protecting
them from the room light.
2j
Conclusion
In this study, we demonstrated the synthesis and characterization
of 10 FCs. Our results show that all compounds are chemically
reversible Ipa/Ipc = 1.0–1.17 for all compounds except DMAP and
are electrochemically quasi-reversible, ⌬Ep = 72–90 mV. The n
and Dox values reported agree with the values published for fer-
rocene ͑1e−, Dox = 2.4 ϫ 10−5 cm2/s͒.21 The ks values ranges
0.020–0.044 cm/s compared to 0.035–0.076 cm/s values for fer-
rocene.
the compounds. The ks are between 0.020 to 0.044 cm/s. When
compared to the ks reported for ferrocene of 0.035–0.076 cm/s,24 the
values for the FCs are comparable and in accordance with previous
papers.
UV/vis spectra and absorption coefficients.— Figure 7A-D shows
the UV/vis spectra of the compounds grouped by the nature of the
aromatic derivative. Table III summarizes the absorption coefficients
for the most prominent bands in the spectra. With few exceptions,
The DMAPP electrochemical parameters were difficult to obtain
because the proximity of the ferrocene and first DMAPP redox pro-
cess and the complication introduced by its electropolymerization.
1.38
1.12
1.3
A
B
1.2
1.1
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.00
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.00
2
2
aa
2c
b
190.0
250
300
350
400
450
500
550
600
650
700
750
820.0
190.0
250
300
350
400
450
500
550
600
650
700
750
820.0
Figure 7. UV/vis spectra of 2.0
ϫ 10−5 M compounds ͑2a-2j͒ in ACN
for: ͑A͒ 2a and 2b, ͑B͒ 2c, ͑C͒ 2d-2g, and
͑D͒ 2h-2j.
Wavelength, nm
Wavelength, nm
1.02
1.19
1.1
C
D
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
2d
2h
2e
2i
2f
2j
2g
-0.01
-0.01
190.0
250
300
350
400
450
500
550
600
650
700
750
820.0
190.0
250
300
350
400
450
500
550
600
650
700
750
800
850
900.0
Wavelength, nm
Wavelength, nm
Table III. Absorption coefficients of FCs.
␧͑ϫ104͒
͑L/mol
cm͒
␧͑ϫ104͒
͑L/mol cm͒
␧͑ϫ102͒
͑L/mol cm͒
␧͑ϫ102͒
͑L/mol cm͒
␭
͑nm͒
␭
͑nm͒
␭
͑nm͒
␭
max
͑nm͒
max
max
max
Compound
2a
2b
2c
2d
2e
2f
2g
2h
2i
198.82
204.24
200.17
199.17
199.17
199.94
201.46
201.53
201.53
199.49
5.2 Ϯ 0.3
5.7 Ϯ 0.5
7.5 Ϯ 0.2
5.7 Ϯ 0.3
3.8 Ϯ 0.2
5.6 Ϯ 0.1
3.4 Ϯ 0.2
6.2 Ϯ 0.9
6.5 Ϯ 0.3
4.7 Ϯ 0.6
—
—
3.7 Ϯ 0.3
2.44 Ϯ 0.05
—
2.1 Ϯ 0.2
2.2 Ϯ 0.3
2.0 Ϯ 0.1
2.9 Ϯ 0.1
3.29 Ϯ 0.04
3.1 Ϯ 0.2
315.46
390.05
393.44
328.33
384.89
386.42
387.94
390.73
391.41
394.80
290 Ϯ 1
15.6 Ϯ 0.8
421 Ϯ 9
517.55
503.30
—
488.06
497.23
498.00
497.23
500.59
493.81
506.70
20.3 Ϯ 0.5
16.8 Ϯ 0.8
—
16.1 Ϯ 0.3
15.8 Ϯ 0.5
15.4 Ϯ 0.1
15.5 Ϯ 0.6
16.1 Ϯ 0.1
1.8 Ϯ 0.3
16.5 Ϯ 0.3
280.87
252.39
—
303.11
295.47
297.00
301.22
296.47
275.45
280 Ϯ 2
17.6 Ϯ 0.6
16.5 Ϯ 0.1
16.1 Ϯ 0.7
15.6 Ϯ 0.2
1.8 Ϯ 0.3
14.4 Ϯ 0.3
2j
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F110
Journal of The Electrochemical Society, 157 ͑8͒ F104-F110 ͑2010͒
691, 3909 ͑2006͒.
The electropolymerization of DMAPP results in cationic oligomers
that are soluble and not suitable for further polymerization and the
formation of polymeric films at electrode surface.
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Eo of the FCs by modulating to a certain extent the polarization of
Ј
the carbonyl group. This result is consistent with the UV/vis varia-
ء
tions in the n − ␲ region that corresponds to the carbonyl absorp-
tion.
In summary, FCs with tunable redox and spectroscopical proper-
ties can be produced by the reaction of acetylferrocene and appro-
priate derivatives of aromatic aldehydes. These compounds could be
used as electrochemical and optical labels in sensor arrays. We are
currently exploring ways to immobilize these compounds at elec-
trode surfaces and studying their applications in biosensing and
electrocatalysis.
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Acknowledgments
We thank Linda Vargas, Yesenia Enríquez, and Tanya L. Díaz for
their assistance with preliminary studies. Rocío del A. Cardona
thanks NSF-PREM ͑NSF-DMR-0934195͒ and NIH-RISE ͑2R 25
GM 061151͒ for their kind support. K.H. acknowledges the support
from NIH-MARC ͑5T 34 GM 07821͒. M.O. acknowledges the sup-
port from Pfizer Pharmaceuticals.
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University of Puerto Rico assisted in meeting the publication costs of this
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