Surface immobilized azomethine for multiple component exchange

Article abstract of DOI:10.1039/c7sm01456b

Diazonium chemistry concomitant with in situ electrochemical reduction was used to graft an aryl aldehyde to indium-tin oxide (ITO) coated glass substrates. This served as an anchor for preparing electroactive azomethines that were covalently bonded to th

Full text of DOI:10.1039/c7sm01456b

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Surface immobilized azomethine for multiple
component exchange†
Cite this:DOI: 10.1039/c7sm01456b
a
a
Michael Lerond,
Daniel Belanger *b and W. G. Skene
*
´
Diazonium chemistry concomitant with in situ electrochemical reduction was used to graft an aryl
aldehyde to indium-tin oxide (ITO) coated glass substrates. This served as an anchor for preparing
electroactive azomethines that were covalently bonded to the transparent electrode. The immobilized
azomethines could undergo multiple step-wise component exchanges with different arylamines. The
write–erase–write sequences were electrochemically confirmed. The azomethines could also be reversibly
hydrolyzed. This was exploited for multiple azomethine-hydrolysis cycles resulting in discrete electroactive
immobilized azomethines. The erase–rewrite sequences were also electrochemically confirmed.
Received 22nd July 2017,
Accepted 6th September 2017
DOI: 10.1039/c7sm01456b
rsc.li/soft-matter-journal
Introduction
Component exchange is the replacement of the constitutional
components of a compound by reversible bond formation.1
Such dynamic processes have been beneficial for catalysts2 and
drug discovery3,4 through combinatorial means. This approach
has also been extended to include other tangible applications
Fig. 1 Schematic representation of reversible azomethine formation and
such as selective membrane development,5 sensors,6 and component exchange.
bioactive compounds.7 Component exchange has also been
expanded to other advances, including polymer brushes for
both the reversible bonding of their side chains8 and tethering materials in organic electronic applications such as electro-
to silica particles.9 Adaptive and self-healing materials are chromic24–26 and photovoltaic devices,27 as well as light emitting
additionally possible by taking advantage of reversible bond diodes.28,29
formation.10–12
While single step-wise component exchanges are possible
Of the many functional groups13–16 that support component with azomethines,30 successive exchanges are challenging. This
exchange, azomethines are interesting for reversible bond is especially the case for obtaining unique and pure final
formation.17 This is in part owing to their component exchange products. Multiple substitutions must be done step-wise with
that can occur by transimination, transamination,18 and each exchanged product being isolated by purification followed
hydrolysis-imination19 (Fig. 1). These dynamic processes can by subsequent exchange. Multiple exchanges with straight-
be moderated by many stimuli including pH,20 temperature,21 forward rinsing would be ideal for regenerating the original
and Lewis22 and mineral acid23 catalysts. The stability of the azomethine. This would lead to multiple write-erase cycles,
azomethine bond, and hence its propensity for component and ultimately, to efficient continuous exchanges.
exchange, can also be tailored by electronic effects and degree
Recently, two consecutive component exchanges to pattern a
of conjugation. Additionally, the optical and electronic properties surface were demonstrated by immobilizing an azomethine on
of azomethines have made them interesting candidates as active a surface.31 In this case, the electroactive products formed by
component exchange were isolated by rinsing the surface on
which they were physisorbed. An electrochromic polyazomethine
a
´
´
´
Laboratoire de caracterisation photophysique des materiaux conjugues,
was further demonstrated to be covalently attached to a surface
by imination of its terminal aldehyde with a surface immobilized
triethoxypropylamine.32 We were therefore motivated to pursue a
surface mediated approach for multiple exchanges. This would
provide the means for perpetual property modification of
electroactive compounds by multiple component exchange.
The advantage of this approach would be the isolation of
´
´
´
Departement de Chimie, Pavillon JA Bombardier, Universite de Montreal,
´
´
CP 6128, succ. Centre-ville, Montreal, Quebec, H3C 3J7, Canada.
E-mail: w.skene@umontreal.ca
b
´
´
´
`
´
Departement de Chimie, Universite du Quebec a Montreal, Case Postale 8888,
´
´
succursale Centre-Ville, Montreal, Quebec H3C3P8, Canada.
E-mail: belanger.daniel@uqam.ca
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c7sm01456b
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Fig. 2 Model azomethine compounds and those immobilized on an ITO coated glass electrode prepared and investigated for component exchange
and reversible imination/hydrolysis.
exchanged products by simple rinsing of the substrate post were sonicated in ethanol for 10 min They were then rinsed
exchange. Within this context, we exploited well-known diazonium with solvent and air-dried. The cleaned substrates were sub-
surface chemistry33–37 to bond an aryl aldehyde38,39 (4s; Fig. 2) to sequently stored in a covered Petri dish until used. An aceto-
indium-tin oxide (ITO) coated glass electrodes that are commonly nitrile solution of tetrabutylammonium tetrafluoroborate
used in electrochromic applications. The modified electrodes (TBABF4; 0.1 M) and 1,3-dioxolane-4-benzenamine (1 mM) were
served as platforms for multiple component exchanges of bubbled with nitrogen for at least 10 min, followed by the
covalently bonded azomethines. The covalent attachment of addition of an equivalent of tert-butyl nitrite. The reaction
the anchor for component exchange and its capacity to sustain mixture to form the diazonium salt was reacted for 1 min
both successive component exchanges by arylamine substitution before immersing a previously cleaned ITO substrate. The latter
of electroactive azomethines and their reversible hydrolysis are served as the working electrode. Electrografting was done by
herein presented.
cyclic voltammetry by applying 10 successive sweeps between
0.3 and ꢀ1.2 V vs. silver wire at 100 mV sꢀ1. A platinum mesh
and Ag wire electrode were used as counter and pseudo
reference electrodes, respectively. Afterwards, the functiona-
lized ITO substrate was rinsed with ethanol. It was then
sonicated in ethanol for 10 min to remove traces of the
electrolyte and any physisorbed reagents and products. Electro-
grafting was confirmed by cyclic voltammetry, time-of-flight
secondary ion mass spectrometry (ToF-SIMS). Electrochemical
blocking properties of the electrode after grafting were assessed
with a 5 mM solution of K3[Fe(CN)6] and 5 mM K4[Fe(CN)6] in
0.1 M KCl with the pH adjusted to 5.
Experimental section
1H and 13C NMR spectra were recorded at room temperature in
acetone-d6 and DMSO-d6 at 400 and 100 MHz, respectively,
using the solvent residual peak as an internal standard. The
prepared compounds were purified with a commercial automated
flash chromatography system.
Cyclic voltammetry experiments were performed with a Solar-
tron model 187 potentiostat. A Bio-Logic VSP potentiostat was
used for square wave voltammetry measurements. All measure-
ments were carried out at room temperature with a conventional
three-electrode configuration consisting of a platinum or ITO
working electrode, an auxiliary platinum electrode and a silver
wire pseudo reference electrode. The solvent in all experiments
was acetonitrile and the supporting electrolyte was TBAPF6
(0.1 M), unless otherwise stated. Ferrocene (0.1 mM) was added
at the end of the cyclic voltammetry measurements of 1–3 as an
Component exchange
After electrochemical grafting, the acetal protected derivative
was hydrolyzed to generate 4s. This was done by immersing
the surface in distilled water with H2SO4 (0.19 mM) for 2 h.
The substrate was subsequently rinsed with ethanol and dried.
It was then submerged in a dichloromethane solution of the
corresponding arylamine (0.05 mM for 3s and 5 mM for both
1 and 2s) for 2 to 24 h to form 1s–3s. Afterwards, the ITO
substrate was rinsed with ethanol, dried and then it was either
characterized immediately or stored in a covered Petri dish for
future characterization. Similar conditions were used to hydrolyze
the arylazomethines to afford 4s. Meanwhile, dynamic component
exchange was done by immersing the functionalized ITO sub-
internal reference. This was to directly compare their oxidation
0
potentials. An apparent redox potential (E0 ) of 0.5 V was deter-
mined by taking the average between the anodic and cathodic peak
potentials for the reversible ferrocene/ferricenium redox couple.
Electrografting
Commercially available (Delta Technologies) indium-tin oxide strate in a previously prepared dichloromethane solution of the
(ITO) coated glass substrates (17 O squareꢀ1; 2.5 cm ꢁ 0.5 cm) given arylamine (0.05 or 5 mM). A catalytic amount of p-toluene
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sulfonic acid was used to convert 2s to 1s. The electrodes were and washed with methanol to afford the title compound
1
then rinsed with ethanol and the exchange surface was immersed (27 mg, 88%). H NMR (DMSO-d6) d = 8.72 (s, 1H), 7.74–7.72
in a tetrabutylammonium hexafluorophosphate (TBAPF6; 0.1 M) (d, J = 7.5 Hz, 2H), 7.29–7.27 (d, J = 7.8 Hz, 2H), 4.65 (s, 2H), 4.27
solution in acetonitrile and characterized by square wave voltam- (s, 1H), 4.16 (s, 1H), 2.34 (s, 3H) ppm. 13C NMR (DMSO-d6)
metry. This was done by connecting the coated substrate to a d = 157.9, 140.4, 134.2, 129.4, 127.8, 105.1, 69.2, 66.9, 62.8,
potentiostat with a platinum mesh and a silver wire electrode 21.1 ppm. MS (m/z): calculated for (C18H18FeN)H+: 304.0786,
as the counter and pseudo reference electrodes, respectively. found: 304.0783.
The dynamic component exchange - rinse - square wave
1,3-Dioxolane-4-nitrobenzaldehyde. In a two-neck-round-
measurement cycle was done for each azomethine. Finally, bottom-flask equipped with a Dean–Stark trap, were added
ToF-SIMS (4s) and X-ray photoelectron spectroscopy (XPS) nitrobenzaldehyde (1 g, 6.6 mmol), ethylene glycol (0.37 mL,
(1s, 2s, 3s) measurements were done to confirm the compounds 6.6 mmol), and p-toluene sulfonic acid (12 mg, 1 mol%) in
formed on the ITO substrate.
anhydrous toluene (6 mL). The mixture was heated at 120 1C
N,N-Bis(4-nitrophenyl)-1,4-phenylenediamine. Synthesized overnight under an inert atmosphere with the azeotrope dis-
according to a known procedure.40 1,4-Phenylenediamine tillate isolated in the Dean–Stark trap. Upon cooling, the
(0.54 g, 5.00 mmol), 4-fluoronitrobenzene (1.41 g, 10.0 mmol), mixture was dried over MgSO4, filtered, and the solvent evapo-
and K2CO3 (2.76 g, 20 mmol) were stirred in anhydrous DMSO rated in vacuo. The product was purified by flash chromatogra-
at 90 1C for 3 days. The mixture was then poured into cold phy (ethyl acetate/hexanes 1 : 3 vol%). The title compound was
1
deionized water (200 mL). The resulting precipitate was filtered. isolated as a white powder (1.20 g, 93%). H NMR (acetone-d6)
Afterwards it was dissolved in ethyl acetate (150 mL), washed d = 8.29–8.27 (d, J = 8.8 Hz, 2H), 7.77–7.75 (d, J = 8.5 Hz, 2H), 5.90
with brine, dried with anhydrous Na2SO4, and then filtered. The (s, 1H), 4.13 (m, J = 8.2 Hz, 2H), 4.06 (m, J = 5.4 Hz, 2H) ppm.
title compound was isolated as a red powder after concentrat- 13C NMR (acetone-d6) d = 148.0, 136.8, 127.2, 123.3, 103.1,
1
ing the filtrate under reduced pressure, (1.58 g, 90%). H NMR 65.5 ppm. MS (m/z): calculated for (C9H9NO4)H+: 196.0613,
(acetone-d6) d = 8.19–8.17 (d, J = 9.3 Hz, 4H), 8.19–8.17 (d, J = found: 196.0604.
9.3 Hz, 4H) 7.28–7.26 (d, J = 9.3 Hz, 4H), 7.02–7.00 (d, J = 8.8 Hz,
1,3-Dioxolane-4-benzenamine. 1,3-Dioxolane-4-nitrobenz-
2H), 6.82–6.80 (d, J = 8.8 Hz, 2H), 4.99 (s, 2H) ppm. 13C NMR aldehyde (500 mg, 2.56 mmol), PtO2 (5 mg, 0.02 mmol), and
(acetone-d6) d = 130.1, 129.5, 126.4, 123.2, 122.6, 122.5, 116.7 MgSO4 (1g, 8.31 mmol) were added to a mixture of 3:1 vol%
ppm. MS (m/z): calculated for (C18H14N4O4)H+: 351.1098, found: THF/ethanol (8 mL). The reaction mixture was allowed to react
351.1088.
in a sealed hydrogen reactor at 70 psi overnight in a contained
(E)-4-((4-Methylbenzylidene)amino)-N,N-bis(4-nitrophenyl)aniline fumehood. Caution: The reaction conditions are pyrophoric
(1). N,N-Bis(4-nitrophenyl)-1,4-phenylenediamine (20 mg, 0.14 mmol) and explosive, requiring special handling conditions. The
was added to a solution of p-tolualdehyde (17.5 mL, 0.14 mmol) that pressure was returned to room and the mixture was filtered
was dissolved in anhydrous dichloromethane (3 mL). p-Toluene over a celite pad. The filtrate was then evaporated to afford the
sulfonic acid was then added in a catalytic amount and the mixture title compound as a yellow oil (405 mg, 96%), which was not
was stirred at room temperature for 4 h. The product (50 mg, 80%) purified. 1H NMR (acetone-d6) d = 7.14–7.12 (d, J = 8.5 Hz, 2H),
was isolated as a bright orange powder after recrystallization from 6.63–6.61 (d, J = 8.5 Hz, 2H), 5.54 (s, 1H), 4.69 (s, 2H) 4.03
1
dichloromethane/hexanes. H NMR (acetone-d6) d = 8.68 (s, 1H), (m, 2H), 3.90 (m, 2H, CH2) ppm. 13C NMR (acetone-d6) d = 149.1,
8.09 (d, J = 3 Hz, 8H), 7.43–7.41 (d, J = 9.2 Hz, 2H), 7.40–7.36 136.8, 127.8, 114.1, 103.1, 65.5 ppm. MS (m/z): calculated for
(d, J = 11.2 Hz, 2H), 7.10, 7.08 (d, J = 9.2 Hz, 2H), 6.98–6.94 (C9H11NO2)H+: 166.1926, found: 166.1913.
(d, J = 14 Hz, 2H), 2.23 (d, 3H) ppm. 13C NMR (acetone-d6)
d = 156.6, 145.9, 140.5, 139.8, 136.2, 128.9, 127.7, 122.6, 117.7,
117.3, 21.3 ppm. MS (m/z): calculated for (C26H20N4O4)H+:
453.1576, found: 453.1557.
Results and discussion
(E)-N-Phenyl-1-( p-tolyl)methanimine (2). Aniline (1 mL, The toluyl derivatives (1–3) were first prepared. These were
10 mmol) and p-tolualdehyde (1.19 mL, 10 mmol) were stirred examined as both their identity and properties could be accurately
without solvent at 60 1C for 2 h. The title compound was measured using conventional means. For example, their electro-
isolated (1.75 g, 90%) as off-white crystals by recrystallizing chemical redox processes could be measured by cyclic voltam-
1
from dichloromethane/hexanes. H NMR (acetone-d6) d = 8.54 metry. Simplified 1H-NMR spectra in the aromatic region
(s, 1H), 7.87–7.85 (d, J = 8.1 Hz, 2H), 7.40–7.39 (d, J = 7.0 Hz, were also expected with the toluyl derivatives. This feature is
2H), 7.35–7.33 (d, J = 7.8 Hz, 2H), 7.25–7.23 (d, J = 7.6 Hz, 4H), beneficial for easily following the component exchange by
2.41 (d, 3H) ppm. 13C NMR (acetone-d6) d = 153.4, 142.8, 135.2, tracking the characteristic imine proton. The varying electronic
130.5, 130.2, 129.9, 126.7, 121.9, 21.7 ppm. MS (m/z): calculated groups of the derivatives were further expected to lead to
for (C14H13N)H+: 196.1118, found: 196.1121.
unique imine proton that would be characteristic to each
N-Ferrocene-1-( p-tolyl)methanimine (3). Amino ferrocene compound, further making component exchange easy to track.
(22 mg, 0.1 mmol) and p-tolualdehyde (13.1 mL, 1 eq.) were The toluyl derivatives were further expected to mimic the
dissolved in anhydrous methanol (1 mL). The reaction was then phenyl substitution of the grafted derivatives 1s–4s on the
stirred at room temperature for 5 h. The precipitate was filtered electrode surface. It was also thought that the methyl group
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would additionally prevent unwanted radical cation coupling41
and mitigate an irreversible anodic behavior. Owing to the weak
donating effect of the methyl group, the oxidation potentials
of the toluyl derivatives were expected to be minimally shifted
(ca. o50 mV) to less positive potentials than their counterparts
immobilized on the electrode.
In the simplest case, component exchange can occur by
displacement with an amine added to the reaction mixture. The
process is typically acid catalyzed with exchange rates being
accelerated upon heating the reaction mixture. The conditions
required for component exchange were investigated by NMR.
First, model compounds (1–3) were prepared by condensing
p-toluyl aldehyde with the corresponding aryl amine. These
were prepared as authentic compounds to assign the imine
proton by 1H-NMR. Each azomethine had a unique singlet
imine proton signal at ca. 8.6 ppm. The imine resonances
of each compound were also non-overlapping, making each
compound readily identifiable in a mixture of azomethines.
Component exchange was first examined by converting 2 to 3.
This was done by adding an equimolar amount of amino
ferrocene to a DMSO-d6 solution of 2. The resulting NMR spectra
were recorded at given time intervals. As seen in Fig. 3, the
region between 8.5 and 9 ppm in the 1H-NMR spectrum of 2
consists uniquely of its imine proton. Upon adding equimolar of
amino ferrocene in DMSO-d6, a new singlet formed at 8.77 ppm
after stirring for 1.5 h at room temperature, concomitant with
the decrease of the original imine proton. Based on the NMR
spectra measured for 1–3, the peak formed corresponded to 3,
resulting from component exchange. The conversion of 2 to 3
was found to be 72% after 1.5 h, based on the integrations of the
imine peaks. The conversion of 2 into 3 was 80% after 24 h at
room temperature. Trace amounts of water (5 vol% determined
by NMR) in the hygroscopic DMSO were assumed to catalyze the
reaction. Meanwhile, the increased degree of conjugation of 3
Fig. 3 Truncated 1H-NMR spectra of 2 in DMSO-d6 (A), 2 with equimolar
amino ferrocene (B), after 1.5 h stirring at room temperature (C), and after
24 h stirring at room temperature (D). The imine proton of 2 ( ) and 3 (
)
are highlighted to illustrate the dynamic component exchange.
relative to 2 and the azomethine weakened by the electron contingent on the stability of the product formed and the aryl
withdrawing nitro groups of 1 were assumed to be the driving amine to be incorporated.
forces for the room temperature mediated component exchange.
While the collective NMR data confirm that dynamic
The capacity of 2 to be converted into 1 by dynamic compo- component exchange is possible, 1H-NMR is not conducive
nent exchange was also investigated by 1H-NMR. Unlike the for tracking component exchange on immobilized surfaces.
conversion of 2 into 3, the addition of equimolar amounts Moreover, electrochemical analyses, are by far, more sensitive
of 4-aminodinitrotriphenylamine to 2 in DMSO-d6 did not than NMR spectroscopy. As such, the reduced amounts of
spontaneously form 1 by component exchange. No conversion azomethine grafted onto the electrodes and the subsequent
was observed even upon heating at 40 1C for 6 h. The conver- mixture of products relative to those under conventional NMR
sion of 2 into 1 began only with the addition of a catalytic concentrations can be tracked contingent on the redox potentials.
amount of scandium triflate. This Lewis acid was selected The redox potentials of the small library of azomethines were
because it is known to catalyze component exchange.42 More therefore electrochemically evaluated. This was also to verify that
importantly, it is invisible in the spectral window used for the each azomethine and their corresponding constitutional amines
NMR studies, especially in the imine region. The component and tolualdehyde each had a unique redox potential among the
exchange was sluggish with 35% conversion only after 24 h at library of compounds. This is desired to readily identify each
40 1C in the presence of a Lewis acid. The 1H spectra remained component even within a mixture of compounds.
relatively clean during the exchange, confirming the direct
The azomethines and their corresponding reagents were
conversion of 2 into 1 with minimal secondary products and evaluated by square wave voltammetry (Fig. 4) rather than by
no degradation. The conversion could be increased by adding conventional cyclic voltammetry. This was in part owing to the
10 equivalents of 4-aminodinitrotriphenylamine. The NMR increased sensitivity of the pulsed method. More importantly,
studies confirmed that the azomethines can sustain compo- any electrochemically formed products by irreversible electron
nent exchange with the extent and rate of conversion transfer that would otherwise contaminate the voltammogram
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more positive potential with each anodic voltammetry cycle.
This is consistent with its electropolymerization on the electrode.
Only in the case of 4-aminodinitrotriphenylamine was a cathodic
process observed. In fact, three distinct reversible processes were
observed (ꢀ0.87, ꢀ1.02, and ꢀ1.23 V), in part owing to the nitro
group reduction. The collective electrochemical data nonetheless
confirm that component exchange can be readily tracked among
the constitutional components and hydrolyzed by-products.
Of the many electrodes that support diazonium electro-
chemical coupling, ITO was chosen. This was owing to its
relevance in plastic electronics, especially as the commonly
used transparent electrode in electrochromic applications.
Although 4s38 was identified as the best anchor for following
dynamic component exchange electrochemically, the grafting
method was optimized using 1,3-dioxolane-4-benzenamine
on glassy carbon electrodes. Its surface immobilization was
confirmed by the electrochemical blocking analyses with the
ferricyanide/ferrocyanide redox system. The diazonium pre-
cursor required for preparing 4s was synthesized from com-
mercially available 4-nitrotolualdehyde. Its aldehyde was first
protected as the acetal, followed by reducing the nitro group to
afford the required amine. The latter was converted to the
diazoninum salt in situ by tert-butylnitrite (1 mM) in a solution
of acetonitrile. The acetal was selected owing to straightforward
preparation and its capacity to support the electrografting.
Fig. 4 Square wave voltammograms of 1 ( ), 1s ( ), 2 ( ), 2s ( ), 3 ( ),
3s ( ) and 4s (m) in degassed acetonitrile with TBAPF6 as the electrolyte
measured at 100 mV sꢀ1, [C] = 0.1 M.
and make the exchange/hydrolysis process complicated to Also, it can readily be hydrolyzed using soft conditions that
follow electrochemically do not contribute to the square wave would not cleave the anchor from the surface. Diazonium
signal. Therefore, the redox processes assigned uniquely to the grafting to form 4s on clean ITO coated glass substrates was
component exchange processes and their constitutional done by cyclic voltammetry from 0.3 to ꢀ1.2 V for 10 cycles.
components should be readily resolved by square wave voltam- Afterwards, the substrates were rinsed with ethanol and soni-
metry. Unlike with 1–3 measured by cyclic voltammetry, the cated in ethanol for 10 min and then dried.
internal reference ferrocene could not be added to each sample
Electrografting to afford 4s on the ITO electrode was
for directly comparing the oxidation potentials. This was confirmed by ToF-SIMS, AFM, and electrochemical blocking
because, in most cases, its signal masked the square wave of measurements (Table S1 and Fig. S6, ESI†). It should be noted
the given azomethine. This aside, it is evident that 1–3, and the that formation of 4s could not be confirmed by XPS. This was
tolualdehyde used in their preparation each have a unique owing to substantial carbon and oxygen rich contamination
oxidation potential in the range of 0.50 to 1.9 V vs. Ag0 (Table 1). that was observed even with cleaned native ITO substrates
Similarly, the arylamines used to prepare 1–3 had unique (Table S2, ESI†) and the absence of a unique signature to
oxidation potentials that did not overlap with the azomethines differentiate 4s from the back ground signal. In contrast, 1s
(see Fig. S2, ESI†). Meanwhile, the oxidation potential of aniline and 3s could be confirmed by XPS, according to their respective
occurred between 1 and 1.1 V. Its oxidation potential shifted to characteristic N1s (399.5 and 406.3 eV) and Fe2p3/2 (708.0 eV)
binding energies. Meanwhile, their attachment to the surface
via the imine bone was confirmed by the characteristic NQCH
related N1s and C1s bonding energies at 399.5 and 285.6 eV,
Table 1 Electrochemical data of the compounds prepareda
b
respectively (Fig. S7 and S8, ESI†). ToF-SIM measurements were
also done on a 4s coated electrode. The surfaces were first
thoroughly rinsed and sonicated to remove both any physi-
sorbed reagents and electrolyte that were used for preparing 4s.
The ToF-SIM surface analysis method desorbs ca. 10 nm of the
surface and the resulting mass of the fragments are monitored.
In the case of the protected acetal derivative of 4s, the Tof-SIM
measurements confirmed that it was formed based on the
observed 303 g molꢀ1 molecular ion. The ion peak corres-
ponding to 4s was 7-fold more abundant than its acetal
precursor, whereas the intensity of 4s increased 3-fold upon
briefly hydrolyzing the acetal. 4s fragmented into two major
Compound
Eox (V)
1
2
3
1s
2s
3s
4s
Aniline
1.30/1.59
1.91
0.50
1.25/1.53
1.44
0.42
—
—
4-Aminodinitrotriphenylamine
Amino ferrocene
0.70/0.91/1.33
0.20
a
Square wave voltammetry measured in degassed acetonitrile with
TBAPF6 as the electrolyte measured at 100 mV sꢀ1, [C] = 0.1 M.
b
Oxidation potential for the forward scan. Potential vs. silver wire.
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fragments whose masses were consistent with the assigned successive solutions of the arylamines. This was also to validate
structure. These masses were the principle fragments measured that each azomethine formed by reacting 4s with the various
apart from the major In2O3 and SnO2 masses, corresponding to arylamines could be tracked electrochemically. Also, that the
the ITO coating.
products could be identified among the various reagents
Both electrochemical blocking analyses and AFM measure- (Fig. S3, ESI†). For this, 4s was immersed in dichloromethane
ments confirmed that the electrode was not completely covered solution of the corresponding arylamine for 2 to 24 h. After-
by a uniform layer of 4s by diazonium electrografting. This is wards, the substrate was rinsed with ethanol and the square
advantageous as the electrode remains conductive. Therefore wave voltammogram was measured in acetonitrile containing
any change to the structure of 4s such as azomethine formation only the supporting electrolyte. The azomethine was then
and component exchange can readily be tracked by changes in hydrolyzed with sulfuric acid (0.19 mM) for 2 h. The substrate
the redox properties. The covalent attachment of 1s on the was then rinsed and the square wave voltammogram was once
electrode was confirmed by AFM. This was done by comparing again measured in neat acetonitrile with the supporting elec-
the native ITO substrate to the 1s formed by immersing trolyte. This was to ensure that the azomethine hydrolysis to
the 4s coated substrate in a dichloromethane solution of form 4s occurred and that it remained attached to the surface.
4-aminodinitrotriphenylamine. The surface was thoroughly A series of successive azomethine formation/hydrolysis cycles
rinsed with solvent, sonicated, rinsed, and then dried. This
procedure was to ensure that only the surface grafted 1s
remained and any physisorbed 4-aminodinitrotriphenylamine
was removed. As seen in Fig. 5A, the native ITO electrode
consists of large grains over the 2 mm ꢁ 2 mm scanned region
that are assigned to the metal oxide coating. After thoroughly
cleaning the electrografted substrate, the same scan region
(in Fig. 5B) showed additional small dispersed islands, corres-
ponding to 1s. The average particle size measured by AFM was
42.7 nm with particles ranging between and 31.2 and 55.8 nm
(Fig. S5, ESI†).
To test whether 4s could sustain component exchange
without degrafting from the electrode and assess the reaction
conditions necessary for azomethine formation and dynamic
component exchange, the surface anchored 4s was exposed to
Fig. 6 (A) Schematic representation of 4s - 1s - 4s - 2s - 4s - 3s -
4s - 1s successive azomethine formation and hydrolysis cycles done. (B)
Square wave voltammograms of original 1s ( ), 4s (m) formed by hydro-
lyzing 1s, 3s ( ) formed by adding amino ferrocene to the hydrolyzed
4s, 2s ( ) formed by hydrolyzing 3s to form 4s followed by adding aniline,
Fig. 5 AFM height (left) and phase (right) micrographs of native ITO
coated glass substrate (A) and 1s (B) on ITO coated glass substrate.
voltammograms measured in degassed acetonitrile and TBAPF6 used as
electrolyte, [C] = 0.1 M measured at 100 mV sꢀ1
.
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(4s - 1s - 4s - 2s - 4s - 3s - 4s - 1s) was done with a The sample was then immersed in a dichloromethane solution
given substrate. The square wave voltammetry was measured at of amino ferrocene (0.05 mM) for h. to displace
4
each step. As seen in Fig. 6B, the voltammograms are consistent the aminodinitrotriphenylamine and form 3s. The latter was
with the formation of the given azomethine, with 4s being then immersed in a solution of aniline (5 mM) to form 2s. 1s
completely converted into the given azomethine. To further could be regenerated to complete the 1s - 3s - 2s - 1s
confirm the imination of the surface, substrates of 3s and 2s cycle by immersing 3s in a dichloromethane solution of
were examined by Tof-SIMS. In the case of 3s, analyses were 4-aminodinitrotriphenylamine with
a catalytic amount of
done with the originally formed 3s. It was then measured again p-toluene sulfonic acid. The square wave voltammogram after
after having undergone the 3s - 4s - 3s hydrolysis/imination each exchange was measured (Fig. 7). The electrochemical
cycle. Only the Tof-SIMS ion peaks (186, 201, and 91 g molꢀ1
)
measurements confirmed that only the azomethine product
were observed for both samples. These were consistent with the was formed by component exchange. The original azomethine
3s structure. Similarly, the originally formed 2s showed only and hydrolyzed products 4s and arylamines were not observed.
characteristic molecular ions that confirmed its structure. The exchange was also confirmed by XPS by monitoring the
Other background ions accounted for less than 9%, providing characteristic nitro and iron related peaks of 1s and 3s,
sound evidence for the covalent attachment of only 2s and 3s to respectively (Fig. S9 and S10, ESI†). For example, no NO2 related
the ITO substrate. The collective measurements provide sound peaks were observed when 1s was converted to 2s by immersing
evidence that the modified electrode can sustain multiple the 1s coated substrate in an aniline with a catalytic amount of
imination/hydrolysis cycles.
pTSOH (Table S2, ESI†). The absence of the characteristic NO2
Component exchange was initially done with 1s, with this signal at 406.3 eV confirms the complete disappearance of 1s.
starting azomethine being confirmed by square wave voltammetry. Similarly, the formation 3s by component exchange of 1s
with aminoferrocence was confirmed by the unique iron
related Fe2p XPS component. In this case, the exchange was
not catalyzed and it proceeded to only 33% conversion. This
was based on the ratio of the atomic percentages of the
characteristic peaks at binding energy of 708.0 and 406.3/533.8 eV.
The collective data further demonstrate that successive and
multiple component exchange is possible with surface
immobilized aldehyde anchors.
Conclusions
It was demonstrated that an aryl aldehyde could be electro-
grafted to ITO coated glass substrates. The modified electrode
could sustain azomethine formation, azomethine exchange
with arylamines, and azomethine hydrolysis, all of which could
be tracked electrochemically. Successive arylamine exchanges
were possible by immersing the substrate in the arylamine
solution followed by rinsing. Multiple azomethine hydrolysis-
formation cycles were also possible with the electrografted
electrode. The multiple write-erase and erase–rewrite cycles
served as a proof-of-concept for surface-mediated for continuous
component exchange. With arylamines of extended degrees of
conjugation, electrochromic substrates capable of changing
the color of their neutral and electrochemically generated states
in addition to modifying their redox properties are expected by
component exchange. This opens the possibility of perpetual
property modification by the judicious choice of the constituents
for component exchange.
Fig. 7 (A) Schematic representation of component exchange cycles
starting from 1s monitored electrochemically. (B) Square wave voltammo-
grams of original 1s
(
), 3s
(
) formed by displacing 4-aminodi-
nitrotriphenylamine with amino ferrocene, 2s ( ) formed by displacing
amino ferrocene with aniline, and 1s ( ) reformed by displacing aniline
with 4-aminodinitrotriphenylamine. Voltammograms measured in degassed
acetonitrile and TBAPF6 used as electrolyte, [C] = 0.1 M measured at
Conflicts of interest
100 mV sꢀ1
.
There are no conflicts to declare.
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20 R. Nguyen, N. Jouault, S. Zanirati, M. Rawiso, L. Allouche,
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Acknowledgements
The authors acknowledge both the Fonds de recherche du
´
Quebec – Nature et technologies and Canada Foundation for
Innovation for a Team and equipment/infrastructure grants,
respectively. The Centre for Self-Assembled Chemical Structures is
also acknowledged. Galyna Shul is also thanked for preliminary
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