Role of the Open-Shell Character on the Pressure-Induced Conductivity of an Organic Donor–Acceptor Radical Dyad

Article abstract of DOI:10.1002/chem.201800881

Single-component conductors based on neutral organic radicals have received a lot of attention due to the possibility that the unpaired electron can serve as a charge carrier without the need of a previous doping process. Although most of these systems ar

Full text of DOI:10.1002/chem.201800881

A Journal of
Accepted Article
Title: Role of the open-shell character in the pressure-induced
conductivity in an organic radical D-A dyad
Authors: Jaume Veciana, Manuel Souto, Maria Chiara Gullo, Hengbo
Cui, Nicola Casati, Fabio Montisci, Harald O. Jeschke, Roser
Valentí, Imma Ratera, and Concepció Rovira
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To be cited as: Chem. Eur. J. 10.1002/chem.201800881
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Role of the open-shell character in the pressure-induced
conductivity in an organic donor-acceptor radical dyad
Manuel Souto,[a] Maria Chiara Gullo,[a] HengBo Cui,[b] Nicola Casati,[c] Fabio Montisci,[d] Harald O.
Jeschke,[e] Roser Valentí,[e] Imma Ratera,[a] Concepció Rovira,[a] and Jaume Veciana*[a]
Abstract: Single-component conductors based on neutral organic
radicals have received a lot of attention due to the possibility that the
unpaired electron can serve as a charge carrier without the need of
a previous doping process. Although most of these systems are
based on delocalised planar radicals, we present here a nonplanar
and spin localised radical based on a tetrathiafulvalene (TTF)
moiety, linked to a perchlorotriphenylmethyl (PTM) radical by a
conjugated brige, which exhibits a semiconducting behavior upon
application of high pressure. The synthesis, electronic properties and
crystal structure of this neutral radical TTF-Ph-PTM derivative (1) are
reported and implications of its crystalline structure on its electrical
properties dicussed. On the other hand, the non-radical derivative 2,
which is isostructural with the radical 1, shows an insulating behavior
at all measured pressures. The different electronic structures of
these two isostructural systems have a direct influence on the
conducting properties as demonstrated by band structure DFT
calculations.
The first single-component metal was reported by Kobayashi
et al. in 2001 based on a Nickel complex in which conductive p-
electrons are delocalised over the tetrathiafulvalene(TTF)-type
large ligand skeleton decreasing the HOMO-LUMO gap.[3] There
are also many Au bis(dithiolene) complexes with single-
component conductor behaviour[4–6] and in some of them
additional properties of chirality have been introduced.[7] Mori et
al. have recently reported some purely-organic single-
component conductors based on TTF-based molecules
functionalised with a catechol unit which exhibit high electrical
conductivity due to the hydrogen bond-promoted delocalisation
of charge carriers upon partial oxidation of the molecules.[8,9]
More recently, a single-component and pure organic system
based on a zwitterionic TTF-extended dicarboxylate radical
exhibiting very high metallic conduction in a broad range of
temperatures has been reported.[10]
On the other hand, neutral organic radicals have also been
proposed as pure organic single-component conductors since
the unpaired electron can serve as a charge carrier without the
need of a previous doping process.[11] Most of the reported
examples are based on highly delocalised and planar systems,
such as phenalenyl- and thiazolyl-based radicals,[12–15] and some
of them enhance the electrical conductivity with the compression
of the crystal lattice under high pressure.[16–19]
Organic molecular materials exhibiting electrical conductivity,
normally based on two components such as charge transfer
salts, is a research field that has received a lot of attention
during the last decades.[1] More interesting is the development of
single-component conductors since they offer the possibility for
three-dimensional electronic structure because the absence of
any counterion.[2] However, realisation of purely-organic single-
component conductors is a major challenge since organic
materials are normally insulating due to the full-filled valence
band and the large energy band between the valence and
conduction bands. From a molecular point of view, these two
reasons are equivalent to the closed-shell electron configuration
of the HOMO and the large excitation energy from the HOMO to
the LUMO. Thus, in order to produce single-component purely-
organic molecular conductors it is necessary to remove
electrons from the HOMO (or add electrons to the LUMO) and
reduce the HOMO-LUMO energy gap.
Recently, we have reported a new type of neutral nonplanar
and spin-localized radical conductor which is based on a
monopyrrolo(MP)-TTF unit linked to a perchlorotriphenylmethyl
(PTM) radical through
a phenyl-vinylene bridge (MPTTF-
PTM).[20] This system is insulating at ambient pressure and
becomes semiconducting at high pressure due to the increased
electronic bandwidth
W and charge reorganization which
decreases the Coulomb energy repulsion U. This charge
reorganization is occurring in the solid state through an
intramolecular charge transfer (ICT) between the electron-donor
TTF unit and the electron-acceptor PTM radical through the
conjugated bridge; as it has been observed in solution for many
other similar systems.[21–24] However, until now it has not been
possible to study in detail the effect of the electronic structure
since the open- and closed-shell derivatives crystallized in a
different manner.[25]
[a]
Dr. M. Souto, M.C. Gullo, Dr. I. Ratera, Dr. C. Rovira, Dr. J. Veciana
Institut de Ciència de Materials de Barcelona (ICMAB-
CSIC)/CIBER-BBN
Campus Universitari de Bellaterra, 08193, Bellaterra, Spain
E-mail: vecianaj@icmab.es
In order to study the effect of both the open-shell structure
and the bridge length, we have designed and synthesized a
shorter dyad in which both donor and acceptor units are linked
through a phenyl-vinylene bridge (TTF-Ph-PTM) (1, Scheme 1).
In addition, we have also prepared the non-radical analogue
(TTF-Ph-PTM-H; 2), which shows the same crystal structure as
1, in order to study, for the first time, the influence of the
electronic structure (open- versus closed-shell) on the
conducting properties. Interestingly, radical dyad 1 becomes
semiconductor from 11 GPa whereas the isostructural non-
radical dyad 2 remains insulating at all measured pressures
demonstrating unambiguously the key role played by the radical
character of dyad 1. Band structure calculations based on
[b]
[c]
[d]
[e]
Dr. H. Cui
Condensed Molecular Materials Laboratory, RIKEN,
Wako-shi, Saitama, 351-0198, Japan
Dr. N. Casati
Paul Scherrer Institut
WLGA/229, CH-5232 Villigen PSI, Switzerland
F. Montisci
Department of Chemistry and Biochemistry, University of Bern
Freiestrasse 3, 3012 Bern, Switzerland
Dr. H. O. Jeschke, Dr. R. Valentí
Institut für Theoretische Physik, Goethe-Universität Frankfurt
Max-von-Laue-Straße 1, 60438 Frankfurt am Main, Germany
Supporting information for this article is given via a link at the end of
the document.
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density functional theory (DFT) confirm that
significant smaller bandgap than 2 which explains the different
conducting behavior.
1
shows
a
an indirect measurement of the Coulomb repulsion energy U,
that gauge the energy needed to transfer intramolecularly one
electron from the donor to the acceptor site. The estimated
disproportionation potential of radical dyad 1 is 0.60 V which is
only slightly lower than that of the related MPTTF-PTM dyad
(0.64 V)[20] suggesting a similar Coulomb energy repulsion for
both systems.
Scheme 1. Molecular structure of radical dyad 1 and non-radical dyad 2.
Initially, we describe the synthesis of the TTF aldehyde
precursor which is based on a palladium-catalyzed direct C-H
arylation of TTF with the corresponding aryl bromide.[26]
Treatment of TTF (3) with 4-bromobenzaldehyde (4) in presence
of cesium carbonate and catalytic amounts of palladium acetate
and tri-tert-butylphosphonium tetrafluorobrate in refluxing THF
provided the TTF-Ph-CHO (5) (Scheme 2) in 22% yield after
purification by column chromatography.
Scheme 3. Synthesis of dyads 5 and 2.
Scheme 2. Synthesis of TTF-Ph-CHO precursor 5.
The syntheses of dyads 1 and 2 is summarised in Scheme 3.
First, non-radical dyad 2 was obtained as a red solid through a
Horner-Wadsworth-Emmons reaction of compound 5 and the
phosphonated PTM derivative 6[27] in 30% yield. Then, the
subsequent deprotonation of 2 with TBAOH and oxidation of
formed carbanion with AgNO3 yields radical dyad 1 (66 %) as a
black solid and stable under ambient conditions (Scheme 3).
Electrochemical properties of dyads 1 and 2 were studied
by cyclic voltammetry (CV) that was performed in CH2Cl2 at
room temperature (Figure 1). Non-radical dyad 2 exhibits two
redox waves that are assigned to the step-wise oxidation of the
TTF moiety to TTF·+ and TTF2+ with redox potentials at 0.39 and
0.92 V, respectively. On the other hand, radical dyad 1 exhibits
three redox waves assigned to the redox processes PTM-/PTM·,
TTF/TTF·+ and TTF·+/TTF2+ at -0.16, 0.44 and 0.97 V,
respectively. The oxidation potentials related to the oxidation of
the TTF moiety shift 0.05 V towards more positive potentials
from 2 to 1 indicating a stronger interaction between the donor
and acceptor groups in 1.[23,24] The estimated disproportionation
(Edisp) potential in solution can be calculated from the difference
between the first oxidation and reduction potentials and provide
Figure 1. Cyclic voltammogram of radical dyad 1 (red line) and 2 (black line) in
CH2Cl2 versus Ag/AgCl using TBAPF6 as electrolyte, under Argon and scan
rate of 0.1 V/s.
The UV-vis-NIR spectra of dyads 1 and 2 were recorded in
CH2Cl2 at 300 K (Figure 2). The spectrum of non-radical dyad 2
shows two principal bands at 318 and 430 nm. On the other
hand, absorption spectrum of radical dyad 1 shows an additional
intense band at 385 nm, characteristic of PTM radical
chromophores, and two-less resolved peaks at 440 and 550 nm
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attributed to the delocalization of the unpaired electron into the
conjugated framework. In addition, a broad absorbance is
observed in the 700-1000 nm range which is assigned to an
intramolecular charge transfer (ICT) process from the TTF to the
PTM radical as it has been observed in other similar TTF-PTM
systems.[21,23,24] Thus, the optical gaps calculated from the
lowest-energy bands for dyads 1 and 2 were found to be 1.2 and
2.0 eV, respectively, in agreement with the presence of the
single unoccupied molecular orbital (SUMO) in radical dyad 1
which appears at lower energies than the LUMO of 2.[24]
Absorption spectra of radical dyad 1 were measured in
different solvents in order to compare the solvatochromic
behavior observed in related TTF-PTM radical dyads.[21,23,28] This
solvatochromism is assigned to the bistability phenomenon
between neutral and zwitterionic species since polar solvents
can induce an intramolecular electron transfer process from the
electron-donor TTF unit to the electron-acceptor PTM radical. In
the case of dyad 1, this phenomenon of bistability was only
observed in strong polar solvents such as DMF or DMSO with
the appearance of a new band at 512 nm assigned to the
anionic form of the PTM (Figure S2) and indicating the
coexistence of neutral and zwitterionic species as it has been
observed for the MPTTF-PTM dyad.[23] Moreover, the absorption
spectrum of 1 was compared with those of other related dyads
(TTF-PTM[21] and MPTTF-PTM[23]) observing that the ICT band
was always in the same range of energies (Figure S3) and that
the estimated HOMO-SUMO gap of 1 (1.2 eV) is smaller than
the one of MPTTF-PTM dyad (1.3 eV).
Figure 3. ESR spectrum of radical dyad 1 in CH2Cl2/toluene (1:1) at 220 K.
Dyads 1 and 2 were crystallized by a slow evaporation from a
solution of hexane/CH2Cl2. Both dyads are isostructural showing
that they crystallized in the triclinic system with a P-1 space
group and the unit cell parameters are a = 9.2 Å, b = 11.4 Å and
c = 19.1 Å. The fact that both dyads 1 and 2 have the same
crystal structure is quite significant for the purpose of this study
since the previously reported non-radical and radical MPTTF-
PTM dyads crystallized in
a
different manner.[20,23,25] The
molecular structure of 1 and 2 shows dynamic disorder in the
phenyl ring and the TTF unit is distorted with respect to the
phenyl and vinylene bridge which has a trans-configuration
(Figure 4). The S2-C1-S1-C2 torsion angle in the TTF moiety is
7.7º and the average of TTF C=C bond length is 1.34 Å
indicating that the TTF moiety remains neutral in both cases.
The main difference between the molecular structure of 1 and 2
is the hybridization of the central C of the PTM unit (sp2 and sp3,
respectively) with C18-C21, C28-C21 and C22-C21 distances of
ca. 1.47 and 1.54 Å, for the radical and non-radical cases,
respectively.
Figure 4. ORTEP views of 1 at 300 K. The grey, green and yellow represents
the carbon, chlorine and sulfur atoms, respectively. Hydrogen atoms have
been omitted for clarity.
Figure 2. UV-vis-NIR spectrum of radical dyad 1 (red line) and 2 (black line) in
CH2Cl2 (0.05 M) at 300 K. The inset shows the low-energy range of the
absorption spectrum of dyad 1.
Molecules of dyads 1 and 2 are arranged in a head-to-tail
manner, as shown in Figures 5 and 6, with short Cl···Cl and
S···Cl contacts of 3.3 and 3.4 Å, respectively. Additionally, there
are short C···C interactions of 2.4 Å between adjacent phenyl
units of the bridge. It is important to mention that the TTF
moieties are not disposed in a herringbone manner with short
S···S interactions as it was observed for MPTTF-PTM radical
dyad.[20]
Magnetic properties of radical dyad 1 were studied in solution by
means of Electron Spin Resonance (ESR) (Figure 3). The ESR
spectrum of dyad 1 was recorded in CH2Cl2/toluene (1:1) at 220
K and shows at g-value of 2.004 two main lines with which
corresponds to the hyperfine coupling of the unpaired electron
with one 1H of the vinylene bridge. The isotropic coupling
constants showed the typical values for vinylene-substituted
PTM radicals (aH = 1.9 Hz, DHpp = 1.1 Hz).
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temperature dependence measurements of the resistivity of
radical dyad 1 were performed with three independent crystals
showing an insulating behavior at ambient pressure. In MPTTF-
PTM dyad,[20,30] increasing the pressure up to 11.5 GPa the
room-temperature resistivity of such crystals rapidly decreased
exhibiting a semiconducting behavior with an activation energy
(Ea) of 0.2 eV (Figure 8). On the contrary, three independent
crystals of non-radical dyad 2 were measured up to 21 GPa and
they were found always to be insulating under these conditions.
The room-temperature conductivity (srt) of radical dyad 1
continuously increased with a reversible behavior exhibiting a
value of 9·10-3 S cm-1 at 14.6 GPa with a Ea of 0.068 eV.
Interestingly, to compare with the results that were obtained
from high pressure measurements on MPTTF-PTM (at 15.2 GPa,
-1
srt and Ea were 0.76 S cm
and 0.067 eV, respectively),[20]
radical dyad 1 showed almost the same value of Ea, but srt was
almost two orders of magnitude smaller. As discussed for
MPTTF-PTM crystals, due to the incorporation of TTF units
where the MPTTF units are stacked along
b axis, the
conductivity increases with the enhancement of the
intermolecular overlap between the MPTTF-PTM molecules.
However, in radical dyad 1 the packing of the TTF units does not
have the same impact on the intermolecular overlap giving rise
to a smaller conductivity than in the MPTTF-PTM. In addition, a
pressure induced semiconducting state appears at higher
pressure. Reversibility and reproducibility of the experiments are
shown in the Supporting Information (Figures S7, S8, and S9).
Figure 5. Molecular packing of 1 (and the isomorphous 2) in the bc plane.
Figure 6. Molecular packing of 1 (and the isomorphous 2) in the ac plane.
Figure 7. Evolution of experimental crystal cell parameters of radical dyad 1 at
Single-crystal synchrotron-radiation diffraction experiments for
dyad 1 at 4 different pressures up to 11.5 GPa were performed
at the MS beamline of the Swiss Light Source.[29] Data at 2.0
GPa could also be used for a structural refinement, while a
continuous decay in the signal strength, possibly due to radiation
damage, did not allow for full refinement at higher pressure
values. The relative compression data, shown in Figure 7,
shows no major discontinuity and is consistent with the initial
cell. Reconstructed precession images at 2 and 11.5 GPa (see
Figure S6) also do not present any marked difference, at the
side of the volume contraction, with respect to each other,
showing no sign of a different metric. The main difference seems
to be the quenching of the disordered position in the phenyl
group, which becomes disordered in two positions in its own
plane, rather than off plane, as evident in the refined structure at
2.0 GPa.
different pressures.
105
104
P (GPa)
11.5
12.3
103
R
13.3
14.6
17.2
102
19.3
101
0
50
100 150 200 250 300
T (K)
Figure 8. Temperature dependence of the resistivity of radical dyad 1 at
different pressures.
Resistivity measurements on crystals of 1 and 2 were
performed under high-pressure conditions. Pressure and
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We used density functional theory to investigate the electronic
structure of the two isostructural materials TTF-Ph-PTM radical
dyad 1 and the non-radical dyad 2. The phenylene vinylene
bridge (Ph) is slightly disordered; two configurations of almost
equal probability are tilted by about 30 degree against each
other. In the radicals, there is also a slight disorder in the TTF
part of the molecule so we perform the calculations for the
majority configuration. We relaxed the hydrogen positions, most
of which are calculated rather than measured in the
experimental structure. We calculate the electronic structure with
the all electron full potential local orbital (FPLO) basis[31] with the
generalized gradient approximation (GGA)[32] exchange and
correlation functional (see Figure 9).
retaining the P 1 space group which makes the two radicals in
the unit cell symmetry equivalent and allows only for
a
ferromagnetic solution. This solution is 82 meV per radical lower
in energy than the nonmagnetic solution. Each of the molecules
carries exactly S = 1/2. The electronic structure is shown in the
SI (Figure S11). The more realistic case of an antiferromagnetic
solution with zero total magnetization requires breaking
inversion symmetry; the corresponding calculations converge
with a moment of 0.51 μB per radical. The total energy is 0.5
meV per radical below the ferromagnetic solution, indicating that
a state with zero net magnetic moment is the ground state, and
that the effective exchange interaction between radicals is
antiferromagnetic.
The
electronic
structure
of
the
antiferromagnetic solution is shown in Figure
9
(b). The
antiferromagnetic solution is insulating with a small gap of Eg =
22 meV. While we cannot describe, using simple DFT
calculations, the magnetically disordered state that is most likely
realized for temperatures exceeding a few Kelvin, we can clearly
establish that the TTF-Ph-PTM radical 1 has a semiconducting
ground state with a very small activation energy.
In summary, we have reported the synthesis and electronic
properties of a new organic non-planar donor-acceptor dyad
based on the electron-donor TTF linked to the electron-acceptor
PTM moiety in its radical (1) and non-radical (2) forms.
Interestingly, the radical dyad
1 exhibited semiconducting
behaviour under high pressure whereas the isostructural non-
radical analogue 2 was insulating at all measured pressure.
Spin-polarized DFT band calculations confirmed that non-radical
dyad 2 is a band insulator with a large gap of 1 eV whereas
radical dyad 1 has a semiconducting ground state with a small
gap. This study shows that tuning the electronic structure
(reducing the HOMO-SUMO gap) of these family of radical
dyads by decreasing the bridge length between the acceptor
and donor units is a good strategy to enhance the transport
properties and has to be taken into account in the rational
design of neutral radical conductors.
Figure 9. Electronic bands and density of states of (a) TTF-Ph-PTM-H (2) and
(b) TTF-Ph-PTM radical (1) calculated using density functional theory.
Occupied states are marked in the DOS. TTF-Ph-PTM-H is a band insulator
with a large gap of Eg = 1.06 eV. For the TTF-Ph-PTM radical, spin-polarized
calculations are used, and the electronic structure corresponds to a magnetic
solution with zero total magnetic moment.
Acknowledgements
This work was supported the Spanish MINECO/FEDER grants
(“Mother”; MAT 2016-80826-R and “Fancy”; CTQ 2016-80030-
R), the Networking Research Center on Bioengineering,
Biomaterials, and Nanomedicine (CIBER-BBN), and the AGAUR,
Generalitat de Catalunya (grant 2014-SGR-17), by the EU Marie
Curie ITN iSwitch (grant 642196) and by Swiss National Science
Foundation, project 200020_162861. ICMAB is supported by the
Spanish Ministry of Economy and Competitiveness, through the
“Severo Ochoa” Programme for Centres of Excellence in R&D
(SEV- 2015-0496). M.S. is grateful to Spanish Ministerio de
Educación, Cultura y Deporte for a FPU. HO Jeschke, and R.
Valenti thank the Deutsche Forschungsgemeinschaft (DFG) for
funding through grant SFB/TRR49 and Steve Winter for useful
discussions. We thank Xavier Fontrodona (Univ. Girona) for
single-crystal X-ray diffraction measurements. We also thank
Amable Bernabé and Vega Lloveras (ICMAB-CSIC) for MALDI
and ESR spectroscopy.
In the case of TTF-Ph-PTM-H (2), we find a band insulator with
a gap of Eg = 1.06 eV. However, at the non-spin-polarized GGA
level, TTF-Ph-PTM radical dyad (1) has two extra bands at the
Fermi level which are half-filled (see Figure S10). We can use
band weights to determine that the central C atoms of the two
PTM radicals in the unit cell (Z = 2) provide the main contribution
to the half-filled bands. It is important to note that the TTF-Ph-
PTM radical dyad 1 is a Mott insulator at ambient pressure (it is
half-filled band and the ratio between electronic bandwidth W
and Coulomb repulsion is of the order of 1) with a similar
behaviour than the previously reported MPTTF-PTM radical
dyad.[20] As the open shells of the radicals likely give rise to
magnetism, we also performed spin-polarized calculations for
the TTF-Ph-PTM radical 1. The simplest way to do this is by
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10.1002/chem.201800881
Chemistry - A European Journal
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COMMUNICATION
Manuel Souto, Maria Chiara Gullo,
HengBo Cui, Nicola Casati, Fabio
Montisci, Harald O. Jeschke, Roser
Valentí, Imma Ratera, Concepció
Rovira and Jaume Veciana*
Page No. – Page No.
Role of the open-shell character in
the pressure-induced conductivity in
an organic donor-acceptor dyad
We present here a nonplanar and spin localised radical (1) based on a
tetrathiafulvalene (TTF) moiety, linked to a perchlorotriphenylmethyl (PTM) radical
by
a conjugated bridge, which exhibits a semiconducting behaviour upon
application of high pressure. On the contrary, the isostructural non-radical
derivative 2 shows an insulating behaviour at all measured pressures. The different
electronic structures of these two isostructural systems have a direct influence on
the conducting properties as demonstrated by band structure DFT calculations.
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