Self-assembled naphthalenediimide derivative films for light-assisted electrochemical reduction of oxygen

Article abstract of DOI:10.1039/c1cc10139k

This naphthalene diimide derivative, DC18, forms highly conjugated semiconducting stacked assemblies over electrodes after electrochemical conditioning. These molecular materials are very efficient towards electrochemical photoreduction of oxygen under visible light.

Full text of DOI:10.1039/c1cc10139k

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COMMUNICATION
Self-assembled naphthalenediimide derivative films for light-assisted
electrochemical reduction of oxygenw
Evandro Castaldelli, Eduardo Rezende Triboni and Gregoire Jean-Franc¸
´
ois Demets*a
a
b
Received 7th January 2011, Accepted 18th March 2011
DOI: 10.1039/c1cc10139k
This naphthalene diimide derivative, DC18, forms highly
conjugated semiconducting stacked assemblies over electrodes
after electrochemical conditioning. These molecular materials
are very efficient towards electrochemical photoreduction of
oxygen under visible light.
Fig. 1 Molecular structure of DC18.
Naphthalimide and diimide derivatives are very promising
compounds for the design of molecular materials and devices
diimide bromide or DC18 (Fig. 1): a nitrogen-functionalized
derivative with long aliphatic chains, aiming highly self-
organized structures over electrodes held together by strong
solvophobic and electronic interactions. These are crucial to
preserve the whole structure of the molecular material even
during electrochemical swelling as well as to ensure highly
reproducible electrochemical responses. DC18 films have
shown high sensitivity to visible light, which was used in this
work to promote biased photoreduction of oxygen.
1
such as organic field-effect transistors, sensors, molecular
2
3
4
junctions and models for photosynthesis. Their photophysical
and electrochemical properties as well as their thermal and
chemical stabilities make them suitable for the assembly of
atmosphere-resistant solid state devices, which is not the case
for most of the so-called ‘‘synthetic metals’’ and organic
devices. As they form stable radical-anion species after reduction,
it is possible to use them as electron acceptors in complex
supramolecular structures such as photochemical diads and
DC18 was synthesized as a bromide and it is a cationic
species. All the details of its preparation and characterization
may be found in the ESI.w DC18 films were prepared over
fluorine-doped tin oxide glass electrodes (FTO) and glassy
carbon electrodes (GCEs) from n-heptane suspensions
5
–8
triads or as bulk n-type organic semiconductors. It is quite
easy to change and even tune their physical and chemical
properties using several functional groups. These can be
bonded to their nitrogen atoms or directly to the aromatic
system affecting their photophysical properties. These groups
also exert a strong influence on the way these molecules
ꢀ
2
ꢀ3
ꢀ3
(0.3 mL cm ; 1 x 10 mol dm ) leading to thin, white and
ꢀ
9
ꢀ2
mol cm ). Initial
homogeneous films (calc. 4 ꢁ 10
potential cycles of voltammetry for these films on GCE
2
9
aggregate, to form a variety of molecular stacks. In this
(+ = 2 mm) or FTO electrodes (0.25 cm ) revealed ill-defined
context, 1,4,5,8-naphthalene diimides (NDIs) may be regarded
as efficient and stable building blocks for photosensitive
molecular materials. Their photophysical properties have been
extensively investigated during the last decades, but there is
still a lot to be done to understand better their electro-
chemistry especially under the influence of visible light, since
voltammograms that change gradually during the cycling
process (Fig. 2). Successive potential cycles (100 cycles from
0.2 to ꢀ0.85 V vs. Ag/AgCl) lead to well-defined and intense
voltammograms showing that the films suffer a conforma-
tional rearrangement. After this electrochemical conditioning,
1
two reversible peaks become clearly visible (E1/2 = ꢀ0.19 V
1
0
2
few papers deal with this subject.
In this work we have studied the electrochemical and
and E1/2 = ꢀ0.61 V vs. Ag/AgCl) that can be assigned to the
ꢂ
ꢀ
ꢂ
ꢀ
2ꢀ
DC18 2 DC18 and DC18 2 DC18 pairs, in agreement
1
1
0
photoelectrochemical behavior of a NDI derivative, N,N -(ethyl-
00
with other diimides data from the literature.
00
00
N ,N ,N -dimethyloctadecane ammonium)-1,4,5,8-naphthalene
The color of the film changes from white to red after this
process which is expected from highly conjugated electron
systems such as p-stacked assemblies (Fig. 3). The formation
of molecular wire-type structures enhances the global bulk
electrical conductivity of these materials which behave as
a
Departamento de Quı´mica, Faculdade de Filosofia Cieˆncias e Letras
de Ribeira˜o Preto, Universidade de Sa˜o Paulo,
Av. Bandeirantes 3900 CEP 14040-901, Ribeira˜o Preto, SP,
Brazil. E-mail: greg@usp.br; Fax: +55 16 3602 4838;
Tel: +55 16 3602 4860
Instituto de Quı ´m ica, Universidade de Sa˜o Paulo,
Av. Lineu Prestes 748, CEP 05508-900, Sa˜o Paulo, SP, Brazil
1
organic semi-conductors. Surface AFM images reveal a
b
turbostratic structure that was not observed before the
electrochemical rearrangement (see ESIw). The rearrangement
is due to negative charge injection in the film, expelling
bromine ions and neutralizing the positive charge of the
sidechains, promoting a more compact film structure in which
w Electronic supplementary information (ESI) available: Experimental
0
00
00
00
data for the synthesis and characterization of N,N -(ethyl-N ,N ,N -
dimethyloctadecane ammonium)-1,4,5,8-naphthalene diimide bromide.
See DOI: 10.1039/c1cc10139k
This journal is c The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 5581–5583 5581

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Fig. 4 Successive photochronoamperometric curves at E = ꢀ0.67 V
for the DC18 film (under white lamp illumination (on), and in the dark
ꢀ3
(
off)) recorded in oxygen saturated water, 0.1 mol dm KCl, naked
2
GC electrode area A = 0.031 cm .
observed, causing only an increase in the faradaic current,
which means a better electronic conduction of the film and
more effective electron transfer at the interface, since interfacial
oxygen concentrations are kept constant. Photochrono-
amperometric measurements at the reduction potential,
switching the light source on and off (Fig. 4), demonstrate
that the current intensity may be modulated by it with
ꢀ
1
ꢀ3
Fig. 2 Cyclic voltammograms (10 mV s ; 0.1 mol dm KCl)
ꢀ2
ꢀ9
recorded for a DC18 film (4 ꢁ 10 mol cm ) casted onto GCE
ion/ioff = 41%, and the maxima–minima behavior of such
during electrochemical conditioning in the absence of oxygen. Top:
successive curves (gray lines) are a clear indication that the
reduction yield is limited by oxygen interfacial concentration.
Electrochemical impedance spectroscopy (EIS) data
2
nd cycle (black line), 50th cycle (grey dots) and 100th cycle (black
dots). Bottom: after conditioning and in the presence of O (sat.), in
2
the dark (gray), under white light (dotted). The black solid line
voltammogram stands for the naked electrode in the same conditions.
(
DE = 80 mV; E = ꢀ0.23 V; 0.1 Hz–1 MHz) can be fitted
in a classic Randles circuit since the spectra are marked by
depressed circles in high frequency domains followed by
Warburg regions, which are diffusion controlled regions
(
Fig. 5). In all cases the constant phase element (CPE), which
impedance is given by ZCPE(o) = Z
(jo)ꢀa, ranged from 4.3
to 5.6 ꢁ 10 F and their exponents (a) were always around
0
ꢀ
7
1
2
0
.8, which indicates a capacitive behavior. In this case it is
the double layer capacitance (Table 1).
The apparent electron-transfer rates may be obtained from
1
eqn (1) below.
3
RT
2
ðnFÞ ARctC1
app
et
k
¼
ð1Þ
where n is the number of electrons involved, A is the area,
Fig. 3 Absorption spectra of (a): DC18 film at ꢀ0.579 V vs. Ag/AgCl,
C
N
is the oxygen concentration (we have considered
ꢂ
ꢀ
(
b): DC18 film at 0.200 V vs. Ag/AgCl, (c): DC18 and (d): DC18 in
ꢀ4
ꢀ3
1
.9 ꢁ 10 mol dm ) and the other symbols have their usual
ethanol.
aromatic cores get closer and the sidechains can maximize
9
their interaction. This aggregation is confirmed by reflectance
spectra, which reveal broad bands (Fig. 3a and b) and
low-energy absorption edges when compared to DC18 in
solution (Fig. 3c and d).
The presence of oxygen induces important changes in the
electrode’s behavior: the second DC18 reduction step is
completely suppressed and the remaining wave becomes
irreversible, indicating a quantitative electron transfer to the
solution interface below ꢀ0.3 V. White tungsten/halogen lamp
light intensifies this effect, increasing the reduction current by
about 40%. This fact can be explained essentially in terms
of photoconduction, once no significant potential shifts are
Fig. 5 EIS Nyquist plots for the films in different conditions
(DE = 80 mV, E = ꢀ0.23 V). Inset: the Randles circuit.
5
582 Chem. Commun., 2011, 47, 5581–5583
This journal is c The Royal Society of Chemistry 2011

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Table 1 EIS equivalent circuit parameters
These excitons are driven to the interfacial region where
electrons are readily captured by O
2
molecules. Charge
ꢀ
5
ꢀ7
CPE/10
Condition
Dark/N
Light/N
R
s
/O
R
ct/kO
o/10
Z
a
compensation forces electrons to flow from the electrode to the
bulk material. In spite of the strongly favored electron-transfer
process, DC18 films are still somehow resistive and this, allied to
bulky bromine anions, limits the whole process. Monolayers
and small counter-anions should enhance even more the
photo-induced electron transfer reaction for this system.
2
282
272
283
275
4.81
4.42
5.40
3.83
3.19
3.87
8.55
19.9
5.60
5.60
5.07
4.36
0.85
0.85
0.78
0.80
2
Dark/O
Light/O
2
2
meanings. Using this expression for the oxygen reduction
ꢀ
3
ꢀ1
reaction one finds 8.5 ꢁ 10
cm s in the dark and
The authors warmly thank Prof. Mario Jose Politi, for his
´
ꢀ3
ꢀ1
1
.2 ꢁ 10 cm s under illumination. These rates are very
precious help and CAPES for financial support.
reasonable and they increase by 41% upon illumination. The
driving force of this kind of photo-induced electron transfer
can be roughly modelled in terms of the Rehm–Weller expression
Notes and references
1
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(
eqn (2)), in which E00 is the energy of the singlet (0,0)
transition observed (Fig. 3b) and the last term is the solvent
effect, and this influence is negligible in polar solvents like
2 G.-F. Demets, E. Triboni, E. Alvarez, G. Arantes, P. Berci Fo and
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1
4–16
water.
2
Ze2
4 S. Langford, M. Latter and C. Woodward, Photochem. Photobiol.,
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0
et
0
ꢂꢀ
0
ꢀ
2
DG ¼ E ðDC18 =DC18Þ ꢀ E ðO2=O Þ ꢀ E00 ꢀ
es
ð2Þ
6
7
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0
DG = ꢀ2.8 eV, a very strongly favored reaction.
et
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In summary, we were able to chemically modify FTO and
glassy carbon electrodes with a 1,4,5,8-naphthalene diimide
derivative producing air-stable, conductive and photosensitive
electrodes which can electrocatalyse the oxygen reduction
reaction upon irradiation of visible light. AFM and SEM
images indicate a homogeneous lamellar structure and spectral
data indicate the formation of a p extended system. The
creation of DC18 radicals, after the first reduction potential,
lowers the optical absorption edge of the solid, due to the
creation of SOMO levels in the band gap, corresponding
to the formation of a n-type semiconductor. This energy
reduction makes exciton formation upon illumination easier.
9
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This journal is c The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 5581–5583 5583