Thin layer cyclic voltammetry: An efficient tool to determine the redox characteristics of large dendrimers

Article abstract of DOI:10.1039/b208719g

Dendrimers with an electroactive bis(phenanthroline) copper(I) core have been prepared and thin layer cyclic voltammetry (TLCV) found to be an efficient tool to determine their redox characteristics in spite of the slow electron transfer kinetics observed

Full text of DOI:10.1039/b208719g

Thin layer cyclic voltammetry: an efficient tool to determine the redox
characteristics of large dendrimers
Yannick Rio,a Gianluca Accorsi,b Nicola Armaroli,*b Delphine Felder,a Eric Levillain*c and
Jean-François Nierengarten*a
a Institut de Physique et Chimie des Matériaux de Strasbourg, Groupe des Matériaux Organiques, Université
Louis Pasteur et CNRS, 23 rue du Loess, 67037, Strasbourg Cedex, France.
E-mail: niereng@ipcms.u-strasbg.fr
b Istituto per la Sintesi Organica e la Fotoreattività, Laboratorio di Fotochimica, Consiglio Nazionale della
Ricerche, via Gobetti 101, 40129, Bologna, Italy. E-mail: armaroli@frae.bo.cnr.it
c
Ingénierie Moléculaire et Matériaux Organiques, Université d’Angers et CNRS, 2, bd Lavoisier, 49045,
Angers Cedex, France. E-mail: eric.levillain@univ-angers.fr
Received (in Cambridge, UK) 6th September 2002, Accepted 10th October 2002
First published as an Advance Article on the web 28th October 2002
Dendrimers with an electroactive bis(phenanthroline) cop-
per( ) core have been prepared and thin layer cyclic
I
voltammetry (TLCV) found to be an efficient tool to
determine their redox characteristics in spite of the slow
electron transfer kinetics observed for the largest com-
pounds.
In light of their unique structures and properties, dendrimers
have been extensively studied in recent years.1 Of particular
current interest is the use of such architectures to mimic
globular proteins owing to the ability of a dendritic framework
to surround active core molecules, thus creating specific site-
isolated microenvironments capable of affecting the properties
of the core moiety.2 Such dendritic effects can be conveniently
analysed by cyclic voltammetry (CV) measurements. However,
increasingly slow electron transfer kinetics have been observed
for large electroactive dendrimers and the shielding of the
central core is so effective in some cases that charge transfer
could not be detected by classical CV measurements.2–4 This
kinetic effect appears to be a severe limitation for the
determination of the redox characteristics of large dendrimers.
In this paper, we show that thin layer cyclic voltammetry
(TLCV) is an efficient alternative tool for such a purpose.
Dendrimers with an electroactive bis(phenanthroline)cop-
per(
) core substituted with Fréchet-type5 dendrons have been
I
used for the present study (Fig. 1). The synthesis of the
corresponding ligands L0-4 is shown in Scheme 1. Compound
L0 was obtained from 2,9-dimethyl-1,10-phenanthroline (neo-
cuproine) by deprotonation of the methyl groups with LDA to
generate the corresponding dicarbanionic species, followed by
reaction with benzyl bromide.
Fig. 1 Dendrimers (Ln)2Cu+ (Bz = benzyl).
Compound 1 was prepared under similar conditions from
neocuproine and p-[(t-butyldimethylsilyl)oxy]benzyl bromide.
Treatment of 1 with tetra-n-butylammonium fluoride (TBAF)
and subsequent reaction of the resulting diphenol 2 with the
benzylic bromides GnBr in the presence of K2CO3 and
18-crown-6 (18-C-6) in refluxing acetone yielded the corre-
sponding dendritic ligands Ln. Finally, treatment of L0-4 with
Cu(CH3CN)4.BF4 in CH2Cl2 at room temperature afforded the
corresponding copper(
yields. The coordination of the ligands L0-4 to the copper(
I
) complexes (L0-4)2Cu+ in quantitative
I
)
cation was easily shown by 1H-NMR spectroscopy. Effectively,
the signal corresponding to the methylene groups directly
attached to the phenanthroline moiety observed at ca. 3.4 ppm
in the ligands L0-4 is shifted to ca. 2.9 ppm in the complexes.
This behaviour is characteristic of such compounds6 and
originates from the ring current effect of one phenanthroline
moiety on the 2,9-substituents of the second one in the complex.
Scheme 1 Reagents and conditions: (i) LDA, THF, 278 °C, 3 h then benzyl
bromide, 278 °C to rt, 12 h (L0: 55%); (ii) LDA, THF, 278 °C, 3 h, then
p-[(tert-butyldimethylsilyl)oxy]benzyl bromide, 278 °C to rt, 3 h (36%);
(iii) TBAF, THF, 0°C, 3 h (97%); (iv) GnBr, K2CO3, 18-C-6, D, 48 h (n =
1: 76%; n = 2: 95%, n = 3: 96%, n = 4: 99%).
The dendritic copper( ) complexes were also characterized by
I
mass spectrometry. In all the cases, the pseudo-molecular ion
peak corresponding to [M 2 BF4]+ is observed as the base peak
providing clear evidence for the monodispersity of the com-
2830
CHEM. COMMUN., 2002, 2830–2831
This journal is © The Royal Society of Chemistry 2002

pounds.† The absorption spectra of the complexes (Ln)2Cu+
recorded in CH2Cl2 exhibit the intense p–p* ligand-centered
bands in the UV and the much weaker and broad metal-to-
ligand charge-transfer (MLCT) bands with a maximum at 456
Surprisingly, the copper( ) bis-phenanthroline core of the
I
largest compound (L4)2Cu+ presents a nice and well-resolved
signal characteristic of a reversible one-electron process (Fig.
3). Indeed, the voltammograms are similar for all the com-
pounds under these experimental conditions and reveal identical
nm, typical of bis(2,9-dialkylphenanthroline)copper(
phores.7 In room temperature CH2Cl2 solutions, all the
copper( ) complexes exhibit luminescence from the thermally
equilibrated lowest singlet and triplet MLCT excited states.7,8
The luminescence characteristics (lmax = 714 nm; Fem
I
) chromo-
redox potentials (0.62 V vs. Fc+/Fc) for all the copper(
I)
I
complexes (Ln)2Cu+.
In conclusion, we have shown that the very weak electro-
activity observed for the copper( ) bis-phenanthroline core of
=
I
0.0011 ± 0.0002; t = 220 ± 20 ns) have been found to be
substantially independent of the generation number under these
conditions (CH2Cl2, air-free solutions).
dendrimer (L4)2Cu+ in classical CV can be enhanced by TLCV
(note that experimentally, in TLCV with scan rates close to 1
mV s21, it can be expected to observe a reversible process with
an electron transfer kinetic up to 1026 cm s21). Therefore,
TLCV is an efficient tool to determine the redox characteristics
of large electroactive dendrimers.
The CV‡ of (L0)2Cu+, performed at 0.1 V s21 in semi-
infinite diffusion conditions in CH2Cl2/0.5 M n-Bu4PF6,
exhibits a classical signal for a reversible one-electron process
corresponding to the oxidation of the copper cation9 at 0.62 V
vs. Fc+/Fc. By increasing the size of the dendritic shell, the
electron transfer kinetic is attenuated as judged by a decrease in
the peak current and an increase in the peak potential
differences (Fig. 2). The electron transfer kinetics of the highest
generation compound (L4)2Cu+ is estimated to be close to 5 3
1024 cm s21, compared to 5 3 1023 cm s21 for (L0)2Cu+. The
latter kinetic effect has been observed for several examples of
dendrimers with an electroactive core and illustrates the
dendritic shell effect that leads to a more hindered approach of
the core to the electrode.3,4
Notes and references
† Selected data for (L3)2Cu+: 1H-NMR (CDCl3, 200 MHz): d 2.54 (m, 8H),
2.93 (m, 8H), 4.86 (s, 8H), 4.94 (s, 16H), 5.00 (s, 32H), 6.04 (d, J = 8 Hz,
8H), 6.40 (d, J = 8 Hz, 8H), 6.56 (m, 12H), 6.61 (d, J = 2 Hz, 8H), 6.68
(d, J = 2 Hz, 16H), 7.29 (m, 80H), 7.68 (d, J = 8 Hz, 4H), 7.97 (s, 4H), 8.46
(d, J
=
8 Hz, 4H). FAB-MS: m/z 3809 ([M 2 BF4]+, calcd. for
C252H216N4O28Cu: 3808.5). Anal. Calcd. for C252H216N4O28CuBF4.H2O
(3898.85): C 77.27, H 5.61, N 1.43%. Found: C 77.09, H 5.46, N 1.25%. For
(L4)2Cu+: 1H-NMR (CDCl3, 200 MHz): d 2.49 (m, 8H), 2.89 (m, 8H), 4.77
(s, 8H), 4.93 (m, 16H), 5.99 (d, J = 8 Hz, 8H), 6.37 (d, J = 8 Hz, 8H), 6.53
(m, 28H), 6.61 (d, J = 2 Hz, 8H), 6.64 (d, J = 2 Hz, 48H), 7.29 (m, 160H),
7.59 (d, J = 8 Hz, 4H), 7.90 (s, 4H), 8.37 (d, J = 8 Hz, 4H). MALDI-TOF-
MS: m/z 7203 ([M 2 BF4]+, calcd. for C476H408N4O60Cu: 7201.8). Anal.
Calcd. for C476H408N4O60CuBF4 H2O (7294.82): C 77.79, H 5.68, N
0.76%. Found C 77.72, H 5.64, N 0.57%.
‡
CV has been performed in a three-electrode cell equipped with a
platinum millielectrode of 0.126 cm2 area and a platinum wire counter
electrode. A silver wire served as quasi-reference electrode and its potential
has been checked against the ferrocene/ferricinium couple (Fc+/Fc) before
and after each experiment. The electrolytic media involved CH2Cl2 and 0.5
mol L21 of n-Bu4PF6. All experiments have been performed in a glove box
containing dry, oxygen-free ( < 1 vpm) argon, at room temperature.
Electrochemical experiments have been carried out with an EGG PAR
273A potentiostat with positive feedback compensation. Based on repetitive
measurements, absolute errors on potentials have been found to be around
± 5 mV.
The setup for the TLCV experiments is described in ref. 10.
1 G. R. Newkome, C. N. Moorefield and F. Vögtle, Dendrimers and
Dendrons: Concepts, Syntheses, Applications, VCH, Weinheim, 2001;
J. M. J. Fréchet and D. A. Tomalia, Dendrimers and other Dendritic
Polymers, Wiley, Chichester, 2001.
2 S. Hecht and J. M. J. Fréchet, Angew. Chem., Int. Ed., 2001, 40, 74 and
references therein; C. B. Gorman and J. C. Smith, Acc. Chem. Res.,
2001, 34, 60.
3 C. S. Cameron and C. B. Gorman, Adv. Funct. Mater., 2002, 12, 17.
4 P. J. Dandliker, F. Diederich, M. Gross, C. B. Knobler, A. Louati and E.
M. Sanford, Angew. Chem., Int. Ed., 1994, 33, 1739; P. J. Dandliker, F.
Diederich, J.-P. Gisselbrecht, A. Louati and M. Gross, Angew. Chem.,
Int. Ed., 1995, 34, 2725; K. W. Pollak, J. W. Leon, J. M. J. Fréchet, M.
Maskus and H. D. Abruna, Chem. Mater., 1998, 10, 30; N. Armaroli, C.
Boudon, D. Felder, J.-P. Gisselbrecht, M. Gross, G. Marconi, J.-F.
Nicoud, J.-F. Nierengarten and V. Vicinelli, Angew. Chem., Int. Ed.,
1999, 38, 3730; D. K. Smith, J. Chem. Soc., Perkin Trans. 2, 1999,
1563; C. M. Cardona, T. D. McCarley and A. E. Kaifer, J. Org. Chem.,
2000, 65, 1857; N. Solladié, M. Gross, J.-P. Gisselbrecht and C.
Sooambar, Chem. Commun., 2001, 2206.
Fig. 2 Voltammograms of the (L0)2Cu+ (A) and (L4)2Cu+ (B) in CH2Cl2/n-
Bu4PF6 (0.5 M) at 100 mV s21
.
An investigation has been performed by CV in finite
diffusion conditions‡ (TLCV)10 in order to observe the
electrochemical behaviour of these dendrimers at low scan
rates. Fig. 3 displays the results obtained in TLCV at 2 mV s21
for (L4)2Cu+.
5 C. J. Hawker and J. M. J. Fréchet, J. Am. Chem. Soc., 1990, 112,
7638.
6 C. O. Dietrich-Buchecker, P. A. Marnot, J.-P. Sauvage, J.-P. Kintzinger
and P. Maltese, Nouv. J. Chimie, 1984, 8, 573.
7 N. Armaroli, Chem. Soc. Rev., 2001, 30, 113.
8 D. Felder, J. F. Nierengarten, F. Barigelletti, B. Ventura and N.
Armaroli, J. Am. Chem. Soc., 2001, 123, 6291; R. M. Everly and D. R.
McMillin, J. Phys. Chem., 1991, 95, 9071.
9 P. Federlin, J.-M. Kern, A. Rastegar, C. O. Dietrich-Buchecker, P. A.
Marnot and J.-P. Sauvage, New J. Chem., 1990, 14, 9.
10 F. Gaillard and E. Levillain, J. Electroanal. Chem., 1995, 398, 77.
Fig. 3 Voltammogram of the dendrimer (L4)2Cu+ in CH2Cl2/n-Bu4PF6 (0.5
M) at 2 mV s21 in TLCV.
CHEM. COMMUN., 2002, 2830–2831
2831