Oxidative electrochemical aryl C-C coupling of spiropyrans

Article abstract of DOI:10.1039/c3cc42396d

The isolation and definitive assignment of the species formed upon electrochemical oxidation of nitro-spiropyran (SP) is reported. The oxidative aryl C-C coupling at the indoline moiety of the SP radical cation to form covalent dimers of the ring-closed SP form is demonstrated. The coupling is blocked with a methyl substituent para to the indoline nitrogen.

Full text of DOI:10.1039/c3cc42396d

ChemComm
View Article Online
View Journal | View Issue
COMMUNICATION
Oxidative electrochemical aryl C–C coupling of
spiropyrans†
Cite this: Chem. Commun., 2013,
4
9, 6737
a
b
a
ab
Oleksii Ivashenko,z Jochem T. van Herpt,z Petra Rudolf, Ben L. Feringa and
Received 2nd April 2013,
Accepted 11th June 2013
ab
Wesley R. Browne*
DOI: 10.1039/c3cc42396d
www.rsc.org/chemcomm
The isolation and definitive assignment of the species formed upon
electrochemical oxidation of nitro-spiropyran (SP) is reported. The
oxidative aryl C–C coupling at the indoline moiety of the SP radical
cation to form covalent dimers of the ring-closed SP form is demon-
strated. The coupling is blocked with a methyl substituent para to the
indoline nitrogen.
1
Since its discovery in 1952 by Fischer and coworkers, the
reversible photochemistry of spiropyrans (SP) has seen wide-
2
–4
spread application,
building on their solvato- and acido-
5
chromism and their sensitivity to certain metal ions, providing
systems with dual, triple or even quadruple stimuli responsiv-
6
ity. The ring-closed SP form is neutral and non-planar and is
converted by irradiation with UV light to its thermally unstable
merocyanine (MC) zwitterionic planar form (Fig. 1).
The redox chemistry of spiropyrans has received much less
attention than their photochemistry, despite several reports of
7–9
electrochromism both in solution and on surfaces. The oxidative
electrochemistry is characterized by an irreversible oxidation at
ca. 1.05 V (Fig. 2a) and mechanisms for the processes that follow
10–12
oxidation were first proposed over a decade ago.
It was estab-
lished that the oxidation is centred on the indoline moiety of the
Fig. 2 (a) Cyclic voltammetry of 1 at a glassy carbon (GC) electrode in 1,2-dichloro-
ethane (0.1 M TBAPF ). (b) The oxidation at 1.05 V is overall a four-electron ECCE
process, E: 1 e oxidation of (two equiv. of) 1, C: dimerization of 1 by aryl C–C coupling
6
ꢀ
2
+
ꢀ
2+
to form H
2
2 , C: double deprotonation to form 2, E: 2 e oxidation of 2 to form 2 .
2+
ꢀ
ꢁ
+
At point (ii) and (iii), 2 undergoes 1 e reduction to 2 and 2, respectively.
10,12
SP,
which was then proposed to undergo fast isomerization to a
Fig. 1 ‘Ring-closed’ spiropyran (SP) and ‘ring-open’ merocyanine (MC) forms of
1
2 4 7 15
. R is –C H –OC (Q O)–C H .
ring-open MC radical cation. The oxidized open form was proposed
12
to undergo rapid irreversible coupling to form a C–O–C or
C–O–O–C link between the phenol moieties of two molecules of
a
Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4,
747 AG Groningen, The Netherlands. E-mail: w.r.browne@rug.nl;
+
13
9
MC (Fig. S1a, b, ESI†). Recently it has been proposed that p-radical
cation dimers of the MC form are obtained upon electrochemical
oxidation and that neutral p-dimers are obtained upon subsequent
Tel: +31 50 363 4428
b
Stratingh Institute for Chemistry, University of Groningen, Nijenborgh 4,
9
747 AG Groningen, The Netherlands
14,15
reduction (Fig. S2c, ESI†).
An important observation, with regard
†
Electronic supplementary information (ESI) available: Synthesis and spectro-
to dimerization, is that encapsulation of SP in a porous matrix
(i.e., zeolites or mesoporous silicates) renders the voltammetry of
scopy of compounds 1–3. See DOI: 10.1039/c3cc42396d
These authors have contributed equally to this manuscript.
‡
This journal is c The Royal Society of Chemistry 2013
Chem. Commun., 2013, 49, 6737--6739 6737

View Article Online
Communication
ChemComm
13
spiropyrans partially or even fully reversible. Although this
supports a dimerization model, ring-opening to the MC form upon
13
oxidation was not observed when the SP was trapped. Other
2
mechanisms have been proposed also; however, definitive assign-
ments of the processes involved and species formed have, until now,
not been made, despite the effective utilisation of SP in redox
responsive systems, e.g., sensors, molecular logic, requiring a clear
understanding of the processes involved.
Here, we demonstrate that oxidation of nitro-BIPS spiropyran
(
1, Fig. 1), contrary to previous proposals (Fig. S1, ESI†), in fact Fig. 4 (a) CV of 2 at a GC electrode in CH Cl (0.1 M TBAPF ). (b) UV/vis absorption
2
2
6
results in the formation of covalent SP–SP dimers as a product of spectra of 2 in CH
3
CN before (grey line) and after irradiation at 365 nm (red dotted
line) and following thermal recovery of the original 2 spectrum (black dashed line).
oxidative carbon–carbon bond formation at the ‘para’ position of
16
the indoline moieties, to form a spiropyran dimer 2 (an ECCE
1
mechanism, Fig. 2b). Isolation and spectroscopic characterisation chromatography and characterised by H NMR, ATR-FTIR,
of 2, provides definitive assignment of the mechanisms associated UV/vis absorption and Raman spectroscopy, ESI-MS and cyclic
with the voltammetry of 1 (Fig. 2a). Furthermore, we show that voltammetry (see ESI† for details). The cyclic voltammetry of 2
oxidative dimerization can be blocked by introducing a methyl is shown in Fig. 4a. Two reversible oxidations are observed at
substituent in the indoline unit (3) and confirm that oxidation of 0.8 V and 1.0 V that are coincident with the redox waves those
1–3 does not lead to ring-opening to the MC form.
observed after the initial oxidative sweep for 1 (Fig. 2a).
In the first positive sweep in the CV of 1, irreversible oxidation
of the indoline nitrogen occurs at 1.05 V (Fig. 2a), followed by a dimer formation (i.e. [1
The mass of 2, 955.6 m/z, is in agreement with loss of H and
2
1
2
–H
2
]). The H NMR spectrum of 2 is similar to
reversible two step reduction at 0.83 V and 0.65 V on the reverse that of 1 indicating that it is in the ring-closed spiropyran form (see
sweep. The second cycle features two oxidation waves of the ESI†). Importantly 2 shows one signal less in the aromatic region and
1
7
product 2 and a decrease in the oxidation at 1.05 V.
a change in the coupling pattern for the indoline moiety indicating
UV/vis absorption spectroelectrochemistry shows that upon symmetric dimerization through formation of a phenyl–phenyl bond
oxidation of 1 (Fig. 3a(i)) an absorption band appears at 516 nm between the indoline units. These observations are in agreement with
5
(
with a shoulder at ca. 488 nm, Fig. 3a). The spectrum is earlier reports of Cu(II) mediated dimerization (vide infra).
1
consistent with the spectrum reported for the dication of
As for the H NMR spectra, the ATR FTIR spectra of 1 and 2
N,N,N ,N -tetramethylbenzidine (TMB , vide infra). Subsequent (Fig. S4, ESI†) are essentially the same with only minor differences in
reduction at 0.83 V (Fig. 2a(ii)) results in a decrease in absorbance the absorption band assigned to an aromatic CQC stretching mode,
0
0
2+
18
ꢀ1
ꢀ1
at 516 nm and the appearance of new absorption bands at which shifts from 1608 cm for 1 to 1613 cm for 2. The band
19
ꢀ1
4
43 nm, 478 nm, 770 nm, 867 nm and 985 nm.
assigned to the nitro stretch at 1338 cm and the spiro carbon
The resonance Raman spectrum of 2 (Fig. 3b) shows an mode at 956 cm are unaffected by dimerization as expected.
intense band at 1611 cm , assigned, tentatively, to an aromatic Importantly the characteristic absorption of the ring-closed form
CQC stretching mode and N-ring stretching mode of the indoline (i.e. the spiropyran form) at 956 cm indicates that 2 is formed in
+
ꢁ
ꢀ1
22,23
ꢀ1
24
ꢀ1
ꢀ1
unit at 1351 cm by comparison with the Raman spectrum the ring-closed SP state, and not the ring-open MC form, consistent
+
ꢁ
20,21
1
reported for TMB .
loss in NIR absorbance and an increase in absorption at 308 nm spectrum of 2 and the changes observed upon irradiation at 365 nm
with a weak absorbance tailing into the visible region. confirm that the spiropyran form of 2 is obtained upon oxidation of
Reduction to form 2 results in a complete with H NMR spectroscopic data (vide supra). The UV/vis absorption
Preparative oxidative electrolysis of 1 at 1.2 V, followed by 1. Irradiation of 2 in acetonitrile at 365 nm results in the appearance
isolation and purification of the products obtained upon subsequent of an absorption band in the visible region at 589 nm, with recovery
reduction at 0.2 V yielded 2 as the major product (Fig. 2b). of the original spectrum thermally (Fig. 4).
Compound 2 was separated from unreacted 1 by column
The mechanism by which 2 forms upon oxidation of 1
(
Fig. 2b) can be understood by considering the electrochemistry
of the structurally analogous N,N-dimethylaniline (DMA) – which
represents the indoline unit of the spiropyran. DMA has already
2
5–27
been shown to undergo oxidative dimerization (Scheme 1).
1
6
Briefly, oxidation of DMA at 0.76 V vs. SCE yields initially
the radical cation DMA , which undergoes dimerization via
ꢁ
+
aryl–aryl C–C homocoupling. This is immediately followed by
0
0
loss of two protons to yield the neutral species N,N,N ,N -
tetramethylbenzidine (TMB). TMB undergoes two one-electron
ꢁ
+
2+
oxidation steps (TMB 2 TMB 2 TMB ). Hence an analo-
gous mechanism can be proposed for the formation of 2 from 1
(Fig. 2b). Indeed very recently, Natali et al. have reported
Fig. 3 (a) UV/vis absorption spectra of the product formed upon oxidation of 1
that a related nitro-BIPS compound reacts with Cu(II) salts to
form radical cations, which subsequently dimerise via an aryl
in CH
3
CN (0.1 M NaClO
4
) at (i) 1.2 V, and subsequent reduction at (ii) 0.8 V and
+
ꢁ
5
(
iii) 0.2 V. (b) Resonance Raman spectrum (l 785 nm) of the radical cation 2
.
6
738 Chem. Commun., 2013, 49, 6737--6739
This journal is c The Royal Society of Chemistry 2013

View Article Online
ChemComm
Communication
dimer with a ring-closed, spiropyran structure. Importantly, we
demonstrate that the species observed spectroscopically upon
oxidation are the oxidised states of the already dimerized
species (2) and not of oxidised or ring-opened merocyanine
monomers. Furthermore, we have shown that the dimer
formed exhibits reversible photochemical ring-opening.
In a broader context, the irreversible formation of 2 by oxida-
tion of 1 can be considered as permanent writing of a state for
Scheme 1 Mechanism for electrochemical oxidative dimerization of N,N- which the reversible oxidation waves of 2 at less positive potential
0
0
dimethylaniline (DMA) to N,N,N ,N -tetramethylbenzidine (TMB). The dimer could serve as a convenient readout modality, electrochemically
obtained can be oxidized to monocationic and dicationic states.
and spectroscopically, given the strong NIR absorption of the
monocation 2 . In addition the photochromic response of 1 is
+
retained in 2 enabling temporary information storage regardless
of whether it is in the monomer or dimer form.
The ‘‘Top Research School’’ (Bonus Incentive Scheme, the
Netherlands’ Ministry of Education, Science, and Culture) and
the ERC (Adv.G. 227897, JTH, BLF) are thanked for funding.
Notes and references
1
2
E. Fischer and Y. Hirshberg, J. Chem. Soc., 1952, 4522.
B. L. Feringa and W. R. Browne, Molecular Switches, John Wiley &
Sons, 2011.
3
4
I. Willner, M. Lion-Dagan and E. Katz, Chem. Commun., 1996, 623.
A. Koçer, M. Walko, W. Meijberg and B. L. Feringa, Science, 2005,
309, 755.
5
M. Natali and S. Giordani, Org. Biomol. Chem., 2012, 10, 1162.
6
Fig. 5 CV of 3 in acetonitrile (0.1 M TBAPF ) at a GC electrode at a scan rate of
ꢀ1
6 S. Guragain, B. P. Bastakoti, M. Ito, S. Yusa and K. Nakashima, Soft
Matter, 2012, 8, 9628.
0
.1 V s . Ipa = Ipc and Epa ꢀ Epc = 60 mV.
7
8
A. Doron, E. Katz, G. Tao and I. Willner, Langmuir, 1997, 13, 1783.
R. Baron, A. Onopriyenko, E. Katz, O. Lioubashevski, I. Willner,
S. Wang and H. Tian, Chem. Commun., 2006, 2147.
P. Mialane, G. Zhang, I. M. Mbomekalle, P. Yu, J.-D. Compain,
A. Dolbecq, J. Marrot, F. S ´e cheresse, B. Keita and L. Nadjo,
Chem.–Eur. J., 2010, 5572.
carbon–carbon coupling, to form the symmetrical dimer of the
ring-closed spiropyran (SP).
9
Since oxidation of nitro-spiropyran 1 produces a reactive
radical cation, which undergoes rapid C–C coupling at the para- 10 M. Campredon, G. Giusti, R. Guglielmetti, A. Samat, G. Gronchi,
A. Alberti and M. Benaglia, J. Chem. Soc., Perkin Trans. 2, 1993, 2089.
position of the aromatic indoline moiety, we anticipated and
1
1
1 J. F. Zhi, R. Baba and A. Fujishima, Ber. Bunsen Ges., 1996, 100, 1802.
2 M. J. Preigh, M. T. Stauffer, F. T. Lin and S. G. Weber, Faraday
Trans., 1996, 92, 3991.
indeed found that when the para position bears a methyl group
methyl-nitro-spiropyran 3), a stable cationic species is generated
(
1
1
3 A. Dom ´e nech, H. Garc ´ı a, I. Casades and M. Espl ´a , J. Phys. Chem. B,
upon oxidation – the methyl groups block dimerization – and
reversible electrochemistry is observed (Fig. 5).
2004, 108, 20064.
4 K. Wagner, R. Byrne, M. Zanoni, S. Gambhir, L. Dennany,
R. Breukers, M. Higgins, P. Wagner, D. Diamond, G. G. Wallace
and D. L. Officer, J. Am. Chem. Soc., 2011, 133, 5453.
5 O. Oms, K. Hakouk, R. Dessapt, P. Deniard, S. Jobic, A. Dolbecq,
T. Palacin, L. Nadjo, B. Keita, J. Marrot and P. Mialane, Chem.
Commun., 2012, 48, 12103.
The solid state Raman spectra of 1 and 3 are similar, as
expected (Fig. S5, ESI†). For both 1 and 3, the strongest features
in the spectrum are from the symmetric and asymmetric nitro-
1
ꢀ
1
ꢀ1
aromatic stretching modes at 1335 cm and 1574 cm , respec-
ꢀ1
16 ‘E’ = electron transfer, ‘C’ = chemical.
7 The CV of 1 under inert atmosphere (O
to that in air, excluding involvement of water or oxygen.
The UV/vis absorption spectrum of 3 shows a maximum absor- 18 H. Hasegawa, J. Phys. Chem., 1961, 65, 292.
tively. Other bands typical of ring-closed spiropyran 1 at 1230 cm
1
2
, H
2
O r 1 ppm) is identical
ꢀ1
and 1650 cm are present also.
1
2
2
2
9 Y. Hirata, M. Takimoto and N. Mataga, Chem. Phys. Lett., 1983,
7, 569.
bance at 341 nm and no absorption in the visible region, as for 1
Fig. S6, ESI†). Oxidation at 1.2 V results in a decrease in absorbance
9
(
0 V. Guichard, A. Bourkba, O. Poizat and G. Buntinx, J. Phys. Chem.,
1989, 93, 4429.
1 L. Boilet, G. Buntinx, C. Lapouge, C. Lefumeux and O. Poizat, Phys.
Chem. Chem. Phys., 2003, 5, 834.
at 341 nm and a concomitant increase in absorption at 306 nm and
ca. 457 nm. Subsequent reduction at 0.4 V resulted in reformation of
the initial spectrum. Hence, the additional methyl group renders the
2 M. De Backer and F. X. Sauvage, J. Electroanal. Chem., 2007, 602, 131.
oxidation of SP reversible and shows that oxidation does not lead to 23 G. Socrates, Infrared and Raman Characteristic Group Frequencies:
Tables and Charts, John Wiley & Sons, Ltd, Chichester, UK, 3rd edn,
ring-opening to the MC form.
2001.
We have demonstrated that one-electron electrochemical
oxidation of SP 1 to form an indoline centred radical cation is
followed by rapid and irreversible dimerization. In contrast to
previous proposals we have shown that dimerization proceeds
2
4 R. Delgado-Macuil, M. Rojas-L ´o pez, V. L. Gayou, A. Ordu n˜ a-D ´ı az and
J. D ´ı az-Reyes, Mater. Charact., 2007, 58, 771.
5 T. Mizoguchi and R. N. Adams, J. Am. Chem. Soc., 1962, 84, 2058.
6 Z. Galus, R. M. White, F. S. Rowland and R. N. Adams, J. Am. Chem.
Soc., 1962, 84, 2065.
2
2
via aryl C–C coupling of the indoline units to form a symmetric 27 H. Yang, D. O. Wipf and A. J. Bard, J. Electroanal. Chem., 1992, 331, 913.
This journal is c The Royal Society of Chemistry 2013
Chem. Commun., 2013, 49, 6737--6739 6739