Electronic coupling mediated by furan, thiophene, selenophene and tellurophene in a homologous series of organic mixed valence compounds

Article abstract of DOI:10.1039/c4cc03806a

Charge delocalization in the mixed-valent monocationic forms of phenothiazine-decorated chalcogenophenes is explored by cyclic voltammetry, optical absorption and EPR spectroscopy. Single units of furan, thiophene, selenophene and tellurophene are found to mediate electronic coupling between the phenothiazines attached to their 2- and 5-positions roughly equally well. Electronic communication seems to occur mostly through the butadiene-like backbone of the chalcogenophenes. the Partner Organisations 2014.

Full text of DOI:10.1039/c4cc03806a

ChemComm
View Article Online
View Journal | View Issue
COMMUNICATION
Electronic coupling mediated by furan, thiophene,
selenophene and tellurophene in a homologous
series of organic mixed valence compounds†
Cite this: Chem. Commun., 2014,
0, 10883
5
Received 19th May 2014,
Accepted 29th July 2014
Ann Christin Jahnke, Mariana Spulber, Markus Neuburger, Cornelia G. Palivan and
Oliver S. Wenger*
DOI: 10.1039/c4cc03806a
www.rsc.org/chemcomm
Charge delocalization in the mixed-valent monocationic forms of
phenothiazine-decorated chalcogenophenes is explored by cyclic
voltammetry, optical absorption and EPR spectroscopy. Single units
of furan, thiophene, selenophene and tellurophene are found to
mediate electronic coupling between the phenothiazines attached to Scheme 1 Molecular structure of the systems investigated in this study.
their 2- and 5-positions roughly equally well. Electronic communication
7
k,8
seems to occur mostly through the butadiene-like backbone of the
chalcogenophenes.
and tellurophene bridges have received little to no attention.
A very recent study reported that oligofurans can mediate stronger
electronic coupling between ferrocenyl redox units than oligothio-
phenes. We are unaware of prior comparative studies of charge
delocalization encompassing the entire chalcogenophene series
8a
The ability of p-conjugated oligomers and polymers to become
highly conducting upon oxidation is of key importance for various
1
(
opto)electronic applications. Numerous studies have investigated
(Scheme 1).
charge delocalization and charge transport phenomena in oxidized
oligo- and polythiophenes. Lately there has been increasing interest
Our study is based on four isostructural compounds with
phenothiazine (PTZ) groups attached at the 2- and 5-positions
of furan (1), thiophene (2), selenophene (3), and tellurophene (4).
An X-ray crystal structure of the selenophene compound is
shown in Fig. 1. The structure of the thiophene analogue (2)
2
in oligomers and polymers of the other chalcogenophenes, in
3
4
particular selenophene, but also furan and tellurophene. However,
despite important recent progress, the synthesis of oligofurans
5
7m
and oligotellurophenes remains nontrivial. From a fundamental
had been previously reported. Synthetic procedures and product
characterization data are given in the ESI.†
perspective and with the application potential of furan- or
tellurophene-based materials in mind, it seemed worthwhile to
explore to what extent an unpaired electron can delocalize over
individual chalcogenophene units with the heteroatoms varying
along the series O, S, Se, Te. Charge transport in oligomers and
charge delocalization in monomers are different issues, but in
order to tailor the properties of an oligomer or polymer it seems
desirable to understand the electronic structure of its monomeric
building blocks as detailed as possible.
Cyclic voltammetry reveals two essentially reversible oxidation
+
waves in the potential range between 0 and 0.6 V vs. Fc /Fc for all
four compounds (Fig. 2), corresponding to consecutive one-electron
oxidation of both PTZ moieties. Chalcogenophene-based oxidations
9
are expected at much higher potentials. In CH CN with 0.1 M
3
TBAPF the splitting between peak potentials (DE) increases along
6
the chalcogenophene series, ranging from 216 mV for furan to
Mixed valence compounds have been frequently employed for
exploration of charge delocalization phenomena, and both metal-
based as well as purely organic redox-active units have been used for
6
this purpose. A considerable number of thiophene-bridged mixed
7
valence compounds have been studied, but selenophene, furan,
Department of Chemistry, University of Basel, St. Johanns-Ring 19, CH-4056 Basel,
Switzerland. E-mail: oliver.wenger@unibas.ch
†
Electronic supplementary information (ESI) available: Synthesis protocols and
product characterization data, additional optical spectroscopic and EPR data.
CCDC 1002373. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c4cc03806a
Fig. 1 X-ray crystal structure of compound 3.
This journal is ©The Royal Society of Chemistry 2014
Chem. Commun., 2014, 50, 10883--10886 | 10883

View Article Online
Communication
ChemComm
3 6
Fig. 2 Cyclic voltammograms recorded in CH CN with 0.1 M TBAPF . The
ꢁ1
scan rate was 100 mV s
.
Fig. 3 Absorption spectra recorded in absence and in presence of various
4 2 3
amounts of Cu(ClO ) oxidant in CH CN. The asterisks mark the IVCT band.
Table 1 Differences in electrochemical potentials for one-electron oxi-
dation of the two PTZ units and comproportionation constants in CH
3
CN
By analogy to many previously investigated bis(triarylamines)
and related organic mixed valence compounds we associate the
with 0.1 M TBAPF
6
+
+
Compd
DE [mV]
K
c
low-energy absorptions of 1 –4 (marked by the red asterisks) with
6b,f,h,7g,j,k,p,s,11
3
intervalence charge transfer (IVCT) bands.
The optical
1
2
3
4
216
267
283
291
4.5ꢀ 10
3.4ꢀ 10
6.3ꢀ 10
8.6ꢀ 10
4
4
4
absorption spectra with properly determined extinction coefficients
were fitted to a sum of Gaussian functions in order to estimate the
dipole moment (m ) associated with the IVCT transition (Fig. S1
ge
in the ESI†). Eqn (1) requires nmax (the energetic position of the
ꢁ
1
2
91 mV for tellurophene (Table 1). Using these DE values one absorption band maximum) and e(n) input values in units of cm
DE/59mV
ꢁ1
ꢁ1
calculates comproportionation constants (K
c
=
10
CN (Table 1). In general, moment in units of Debyes (D).
are governed by several factors including electrostatic
and ion pairing effects, as well as the strength of electronic
)
and M
cm , respectively, and yields the transition dipole
3
4
12
between 4.5 ꢀ 10 and 8.6 ꢀ 10 in CH
3
DE and K
c
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
Ð
eðuÞ ꢂ du
max
1
0
mge ¼ 0:09584 ꢂ
(1)
interaction (HAB) between the two redox centers. Electrostatic
and ion pairing effects often dominate, and therefore conclusions
u
about H based on electrochemical data should be made with The mge values for the furan-, thiophene-, selenophene-, and
AB
8b
+
+
+
+
caution. In the present case the observed increase of DE and K
c
tellurophene-based compounds (1 , 2 , 3 , 4 ) are between 3.4
along the series of chalcogenophenes 1–4 is difficult to reconcile and 4.7 D (Table 2). Eqn (2) relates mge to the electronic coupling
with electrostatics because the distance between redox centers is matrix element HAB which quantifies the strength of the electronic
13
not decreasing along this series.
interaction between the two redox centers.
ꢁ
5
While 4 ꢀ 10 M solutions of the charge-neutral forms of 1–4
in CH CN are essentially colorless (dotted black traces in Fig. 3),
oxidation with Cu(ClO ) produces new absorptions in the near-
mge ꢂ umax
e ꢂ R
HAB ¼
(2)
3
4
2
infrared and visible spectral ranges. In all four cases the lowest-
In eqn (2), e is the elemental charge and R is the effective
charge transfer distance. In compounds 1–4 the N–N distance is
ꢁ
1
energy absorption band is centered around 14 500–15 000 cm
,
reaching its maximum intensity after addition of 1 equivalent of expected to vary between 4.58 and 5.32 Å based on molecular
Cu(II) oxidant (red traces, bands marked with asterisks) from which modeling and X-ray crystal structure data (Table 1). Prior
+
+
we conclude that these bands are due to the mixed-valent 1 –4
species. As frequently observed, addition of a second equivalent of troaromatic anions have reached the conclusion that R is equal
Cu(ClO oxidant is unable to quantitatively convert the mono- to to roughly 2/3 of the geometrical N–N distance (dNN) in these
the dications. However, the abovementioned low-energy bands systems. Therefore it seems plausible to assume that R E 2/3ꢂ
studies of mixed-valent bis(triarylamine) cations and dini-
4 2
)
6f
14
+
+
clearly lose intensity when more than 1 equivalent of oxidant is dNN for 1 –4 , and this leads to estimates for the electronic
added, and for 2 –4 a new absorption band around 19 500 cm
becomes detectable (dash-dotted black traces). In the case of (Table 2). For comparison, the structurally related N,N,N ,N -
molecule 1 addition of a second equivalent of oxidant rapidly leads tetraanisyl-p-phenylenediamine cation has HAB = 3240 cm
2+
2+
ꢁ1
ꢁ1
coupling matrix elements in the range from B3300 to B4100 cm
0
0
ꢁ
1 11a
.
to sample decomposition.
The solvent dependence of the IVCT band is relatively weak,
1
0884 | Chem. Commun., 2014, 50, 10883--10886
This journal is ©The Royal Society of Chemistry 2014

View Article Online
ChemComm
Communication
Table 2 Geometrical N–N distances, energetic positions of IVCT absorp-
tion band maxima, dipole moments associated with the IVCT transitions,
and electronic coupling matrix elements characterizing the interaction
between PTZ redox units
Table 3 EPR gyromagnetic factors and hyperfine coupling constants for
compounds 1 –4
+
+
Compd
g
a
N
[G]
H
a [G]
+
ꢁ1
ꢁ1
1
2.0032
2.0032
2.0024
2.0030
4.2
4.8
4.9
5.2
4.8
3.9
3.6
4.0
Compd
dNN [Å]
n
max [cm
]
m
ge [D]
H
AB [cm
]
+
2
+
+
a
3
1
4.58b
5.16
14 970
14 560
14 660
15 020
3.4
4.7
4.3
3.8
3580
4125
3815
3325
+
+
4
2
+
b
3
5.16a
+
4
5.32
rather suggests that furan and tellurophene mediate electronic
coupling somewhat less well than thiophene and selenophene,
but the differences are relatively small when considering the
uncertainty associated with the procedure used for determina-
tion of mge and HAB. We are therefore lead to the conclusion that
single units of furan, thiophene, selenophene, and tellurophene
mediate electronic coupling between two amine redox centers
similarly well. This is consistent with a picture in which the
electronic communication between the 2- and 5-positions of a
chalcogenophene is mostly mediated by the butadiene backbone
of the heterocycle, and the EPR data is compatible with this view.
A recent study of ferrocene-based mixed valence compounds has
a
b
Determined by molecular modeling. Determined from X-ray crystal
7
m
structures.
ꢁ1
n
max red-shifts between B500 and B1500 cm when changing
from CH CN to CH Cl (Fig. S2, ESI†).
The EPR spectra of 1 –4 in CH
3
2
2
+
+
3
CN indicate that the unpaired
14
1
electron spin interacts with the nuclear spins of N and H (solid
traces in Fig. 4). Simulation of the experimental spectra (dotted
traces in Fig. 4) yields the gyromagnetic factors g, and the hyperfine
coupling constants reported in Table 3. All EPR spectra are centered
at values of the gyromagnetic factors g ranging from 2.0024 to
2
.0032, typical for organic radicals. The simulations indicate an
1
5
reached the same conclusion for phosphole bridges.
interaction of the unpaired electron with two equivalent nitrogen
nuclei (a
nuclei (a
H-atoms. Nitrogen hyperfine coupling constants (a ) between 4.2
and 5.2 G for 1 –4 are compatible with complete delocalization of
the unpaired electron over two N nuclei on the EPR timescale. In
the case of 4 the presence of an impurity somewhat complicates
analysis of the EPR data (see ESI† for details) but reliable para-
meters can nevertheless be determined. For 1 –3 there is no
We have conducted the first comparative study of charge
delocalization across single units of the entire chalcogenophene
series, and our findings are relevant in the greater context of mole-
N
ranging from 4.2 to 5.2 G), and two equivalent hydrogen
varying from 3.6 to 4.8 G) for the two chalcogenophene
H
N
16
+
+
cular electronics, for instance for the design of new chalcogeno-
phene-based charge-conducting oligomers and polymers.
+
Funding from the Swiss NSF through grant number
200021_146231/1 is gratefully acknowledged. CGP acknowledges
+
+
the Swiss NSF for a R’Equip grant for the EPR spectrometer.
indication for hyperfine interaction with the chalcogenophene
+
heteroatom. For 4 interaction of the unpaired electron with the Notes and references
tellurium atom cannot be rigorously excluded (see ESI†).
1
(a) X. Guo, M. Baumgarten and K. M u¨ llen, Prog. Polym. Sci., 2013, 38,
The combined data sets from cyclic voltammetry, optical
absorption, and EPR spectroscopy are in line with strong charge
delocalization in 1 –4 . DE and K increase systematically along
c
the chalcogenophene series (Table 1), but for mge and HAB this
trend is not followed (Table 2). The optical absorption data
1832–1908; (b) J. L. Br ´e das, J. P. Calbert, D. A. da Silva and J. Cornil,
Proc. Natl. Acad. Sci. U. S. A., 2002, 99, 5804–5809.
2 (a) J. Roncali, Chem. Rev., 1992, 92, 711–738; (b) T. Otsubo, Y. Aso
and K. Takimiya, J. Mater. Chem., 2002, 12, 2565–2575.
3 (a) E. Poverenov, Y. Sheynin, N. Zamoshchik, A. Patra, G. Leitus,
I. F. Perepichka and M. Bendikov, J. Mater. Chem., 2012, 22,
+
+
14645–14655; (b) A. Patra and M. Bendikov, J. Mater. Chem., 2010,
2
0, 422–433; (c) Y. H. Wijsboom, A. Patra, S. S. Zade, Y. Sheynin, M. Li,
L. L. W. Shimon and M. Bendikov, Angew. Chem., Int. Ed., 2009, 48,
443–5447; (d) M. Heeney, W. Zhang, D. J. Crouch, M. L. Chabinyc,
5
S. Gordeyev, R. Hamilton, S. J. Higgins, I. McCulloch, P. J. Skabara,
D. Sparrowe and S. Tierney, Chem. Commun., 2007, 5061–5063.
(a) O. Gidron, A. Dadvand, Y. Sheynin, M. Bendikov and D. F. Perepichka,
Chem. Commun., 2011, 47, 1976–1978; (b) O. Gidron and M. Bendikov,
Angew. Chem., Int. Ed., 2014, 53, 2546–2555; (c) A. A. Jahnke and D. S.
Seferos, Macromol. Rapid Commun., 2011, 32, 943–951; (d) A. A. Jahnke,
G. W. Howe and D. S. Seferos, Angew. Chem., Int. Ed., 2010, 49,
4
1
0140–10144.
(a) O. Gidron, Y. Diskin-Posner and M. Bendikov, J. Am. Chem. Soc.,
010, 132, 2148–2149; (b) A. A. Jahnke, B. Djukic, T. M. McCormick,
5
6
2
E. B. Domingo, C. Hellmann, Y. Lee and D. S. Seferos, J. Am. Chem.
Soc., 2013, 135, 951–954; (c) C. R. B. Rhoden and G. Zeni, Org.
Biomol. Chem., 2011, 9, 1301–1313.
(a) W. Kaim, A. Klein and M. Gl o¨ ckle, Acc. Chem. Res., 2000, 33,
7
(
2
55–763; (b) S. F. Nelsen, Chem. – Eur. J., 2000, 6, 581–588;
c) K. D. Demadis, C. M. Hartshorn and T. J. Meyer, Chem. Rev.,
001, 101, 2655–2685; (d) D. M. D’Alessandro and F. R. Keene, Chem.
Rev., 2006, 106, 2270–2298; (e) F. Pevny, E. Di Piazza, L. Norel,
M. Drescher, R. F. Winter and S. Rigaut, Organometallics, 2010, 29,
5912–5918; ( f ) J. Hankache and O. S. Wenger, Chem. Rev., 2011, 111,
5138–5178; (g) D. Siebler, M. Linseis, T. Gasi, L. M. Carrella,
Fig. 4 Solid lines: experimental X-band EPR spectra recorded for the
3
monocationic forms in CH CN. Dotted lines: simulated spectra. See ESI.†
This journal is ©The Royal Society of Chemistry 2014
Chem. Commun., 2014, 50, 10883--10886 | 10885

View Article Online
Communication
ChemComm
R. F. Winter, C. Forster and K. Heinze, Chem. – Eur. J., 2011, 17,
8, 2023–2032; (s) L. G. Reuter, A. G. Bonn, A. C. St u¨ ckl, B. C. He,
P. B. Pati, S. S. Zade and O. S. Wenger, J. Phys. Chem. A, 2012, 116,
7345–7352; (t) M. Jenart, C. Niebel, J. Y. Balandier, J. Leroy,
A. Mignolet, S. Stas, A. Van Vooren, J. Cornil and Y. H. Geerts,
Tetrahedron, 2012, 68, 349–355.
8 (a) O. Gidron, Y. Diskin-Posner and M. Bendikov, Chem. – Eur. J.,
2013, 19, 13140–13150; (b) D. M. D’Alessandro and F. R. Keene,
Dalton Trans., 2004, 3950–3954.
4
540–4551; (h) A. Heckmann and C. Lambert, Angew. Chem., Int. Ed.,
2012, 51, 326–392; (i) P. J. Low, Coord. Chem. Rev., 2013, 257, 1507–1532.
7
(a) J. Guay, P. Kasai, A. Diaz, R. L. Wu, J. M. Tour and L. H. Dao,
Chem. Mater., 1992, 4, 1097–1105; (b) P. B ¨a uerle, U. Segelbacher,
A. Maier and M. Mehring, J. Am. Chem. Soc., 1993, 115, 10217–10223;
(
c) P. B ¨a uerle, U. Segelbacher, K. U. Gaudl, D. Huttenlocher and
M. Mehring, Angew. Chem., Int. Ed., 1993, 32, 76–78; (d) Y. B. Zhu
and M. O. Wolf, J. Am. Chem. Soc., 2000, 122, 10121–10125; (e) S. Le
Stang, F. Paul and C. Lapinte, Organometallics, 2000, 19, 1035–1043;
9 (a) H. Nakanishi, S. Inoue and T. Otsubo, Mol. Cryst. Liq. Cryst.,
1997, 296, 335–348; (b) S. Inoue, H. Nakanishi, K. Takimiya, Y. Aso
and T. Otsubo, Synth. Met., 1997, 84, 341–342.
(
f ) S. Fraysse, C. Coudret and J.-P. Launay, J. Am. Chem. Soc., 2003,
1
25, 5880–5888; (g) D. Rohde, L. Dunsch, A. Tabet, H. Hartmann 10 A. Hildebrandt and H. Lang, Organometallics, 2013, 32, 5640–5653.
and J. Fabian, J. Phys. Chem. B, 2006, 110, 8223–8231; 11 (a) C. Lambert and G. N o¨ ll, J. Am. Chem. Soc., 1999, 121, 8434–8442;
(
h) J. C. Lacroix, K. I. Chane-Ching, F. Maqu `e re and F. Maurel,
J. Am. Chem. Soc., 2006, 128, 7264–7276; (i) P. Rapta, O. Zeika,
D. Rohde, H. Hartmann and L. Dunsch, ChemPhysChem, 2006, 7,
(b) C. Lambert, G. N o¨ ll and J. Schelter, Nat. Mater., 2002, 1, 69–73;
(c) S. Barlow, C. Risko, S. J. Chung, N. M. Tucker, V. Coropceanu,
S. C. Jones, Z. Levi, J. L. Br ´e das and S. R. Marder, J. Am. Chem. Soc.,
2005, 127, 16900–16911; (d) J. Bonvoisin, J. P. Launay, M. Van der
Auweraer and F. C. De Schryver, J. Phys. Chem., 1994, 98, 5052–5057;
(e) D. Sun, S. V. Rosokha and J. K. Kochi, Angew. Chem., Int. Ed.,
2005, 44, 5133–5136.
8
63–870; ( j) S. A. Odom, K. Lancaster, L. Beverina, K. M. Lefler,
N. J. Thompson, V. Coropceanu, J. L. Br ´e das, S. R. Marder and
S. Barlow, Chem. – Eur. J., 2007, 13, 9637–9646; (k) G. N o¨ ll, M. Avola,
M. Lynch and J. Daub, J. Phys. Chem. C, 2007, 111, 3197–3204;
(
l) L. B. Gao, J. Kan, Y. Fan, L. Y. Zhang, S. H. Liu and Z. N. Chen, 12 (a) C. Creutz, M. D. Newton and N. Sutin, J. Photochem. Photobiol., A,
Inorg. Chem., 2007, 46, 5651–5664; (m) A. W. Franz, L. N. Popa,
F. Rominger and T. J. J. M u¨ ller, Org. Biomol. Chem., 2009, 7, 469–475;
1994, 82, 47–59; (b) R. J. Cave and M. D. Newton, Chem. Phys. Lett.,
1996, 249, 15–19.
(
n) J. Casado, S. R. Gonzalez, M. C. R. Delgado, M. M. Oliva, 13 N. S. Hush, Prog. Inorg. Chem., 1967, 8, 391.
J. T. L. Navarrete, R. Caballero, P. de la Cruz and F. Langa, 14 (a) S. F. Nelsen, A. E. Konradsson, M. N. Weaver and J. P. Telo, J. Am.
Chem. – Eur. J., 2009, 15, 2548–2559; (o) F. Zhang, G. Gotz,
E. Mena-Osteritz, M. Weil, B. Sarkar, W. Kaim and P. B ¨a uerle, Chem.
Sci., 2011, 2, 781–784; ( p) S. Barlow, C. Risko, S. A. Odom, S. J.
Zheng, V. Coropceanu, L. Beverina, J. L. Br ´e das and S. R. Marder,
J. Am. Chem. Soc., 2012, 134, 10146–10155; (q) J. M. Speck, R. Claus,
Chem. Soc., 2003, 125, 12493–12501; (b) S. F. Nelsen, M. N. Weaver,
J. I. Zink and J. P. Telo, J. Am. Chem. Soc., 2005, 127, 10611–10622;
(c) K. Lancaster, S. A. Odom, S. C. Jones, S. Thayumanavan,
S. R. Marder, J. L. Br ´e das, V. Coropceanu and S. Barlow, J. Am.
Chem. Soc., 2009, 131, 1717–1723.
A. Hildebrandt, T. R u¨ ffer, E. Erasmus, L. van As, J. C. Swarts and 15 D. Miesel, A. Hildebrandt, M. Korb, P. J. Low and H. Lang, Organo-
H. Lang, Organometallics, 2012, 31, 6373–6380; (r) Y. P. Ou, J. L. Xia, metallics, 2013, 32, 2993–3002.
J. Zhang, M. Xu, J. Yin, G. A. Yu and S. H. Liu, Chem. – Asian J., 2013, 16 C. Herrmann and J. Elmisz, Chem. Commun., 2013, 49, 10456–10458.
1
0886 | Chem. Commun., 2014, 50, 10883--10886
This journal is ©The Royal Society of Chemistry 2014