O-Doped Nanographenes: A Pyrano/Pyrylium Route Towards Semiconducting Cationic Mixed-Valence Complexes

Article abstract of DOI:10.1002/anie.201914025

Herein we report an efficient synthesis to prepare O-doped nanographenes derived from the π-extension of pyrene. The derivatives are highly fluorescent and feature low oxidation potentials. Using electrooxidation, crystals of cationic mixed-valence (MV) complexes were grown in which the organic salts organize into face-to-face π-stacks, a favorable solid-state arrangement for organic electronics. Variable-temperature electron paramagnetic resonance (EPR) measurements and relaxation studies suggest a strong electron delocalization along the longitudinal axis of the columnar π-stacking architectures. Electric measurements of single crystals of the MV salts show a semiconducting behavior with a remarkably high conductivity at room temperature. These findings support the notion that π-extension of heteroatom-doped polycyclic aromatic hydrocarbons is an attractive approach to fabricate nanographenes with a broad spectrum of semiconducting properties and high charge mobilities.

Full text of DOI:10.1002/anie.201914025

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
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Accepted Article
Title: O-doped nanographenes: the pyrano/pyrylium route towards
semiconducting cationic mixed valence complexes
Authors: Luka Ðorđević, Cataldo Valentini, Nicola Demitri, Andrea
Folli, Damien Murphy, Cecile Meziere, Magali Allain, Marc
Sallé, Samuel Mañas-Valero, Eugenio Coronado, and Davide
Bonifazi
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201914025
Angew. Chem. 10.1002/ange.201914025
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http://dx.doi.org/10.1002/ange.201914025

10.1002/anie.201914025
Angewandte Chemie International Edition
O-doped nanographenes: the pyrano/pyrylium
route towards semiconducting cationic mixed
valence complexes
Luka Ðorđević, Cataldo Valentini, Nicola Demitri, Cécile Mézière,
Magali Allain, Marc Sallé, Andrea Folli, Damien Murphy, Samuel
Mañas-Valero, Eugenio Coronado and Davide Bonifazi*
[*] Dr. L. Ðorđević, C. Valentini, Dr. A. Folli, Prof. Dr. D.
Murphy, Prof. Dr. D. Bonifazi; School of Chemistry, Cardiff
University, Main Building, Park Place, Cardiff CF10 3AT,
United Kingdom. Email: bonifazid@cardiff.ac.uk
Dr. N. Demitri; Elettra – Sincrotrone Trieste, S.S. 14 Km
163.5 in Area Science Park, 34149 Basovizza, Trieste, Italy
C. Mézière, M. Allain, Prof. Dr. M. Sallé; MOLTECH-Anjou - UMR
CNRS 6200, UNIV Angers, SFR Matrix, 2 Boulevard Lavoisier,
49045 Angers Cedex, France
S. Mañas-Valero, Prof. Dr. E. Coronado; Instituto de Ciencia
Molecular, Universitat de València, Catedrático José Beltrán
2, 46980 Paterna, Spain
[**]D.B. gratefully acknowledge the EU through the ERC Starting Grant
“COLORLANDS”, the MC-RISE “INFUSION” and MC-ITN “PHOTOTRAIN”
projects and the School of Chemistry at Cardiff University for
financial support. The work in Spain has been financed by the
MINECO (grants MAT2017-89993-R and Excellence Unit María de
Maeztu MDM-2015-0538) and the Generalidad Valenciana (Prometeo
Programme). The authors thank Prof. R. Herges (Kiel University)
for providing the AICD program, Prof. A. Stanger (Technion Israel
1
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10.1002/anie.201914025
Angewandte Chemie International Edition
institute of Technology) for sharing the Aroma package and Dr.
A. Fermi (Cardiff University) for the spectro-electrochemical
measurements.
Supporting information for this article is available on the WWW under
http://www.angewandte.org or from the author. Crystallographic data
for the structures reported in this paper have been deposited at the
Cambridge Crystallographic Data Centre as supplementary publication
no. 1557894 (1O), 1920493 (5), 1557895 (2O·1/3CH2Br2), 1557896
(3OCl2·1/5CH2Br2), 1912053 ((1O)10(PF6)6·(THF)16) and 1936956
((3O)3(ClO4)2·THF·1/2H2O).
2
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Abstract: Herein we report an efficient synthesis to prepare O-doped
nanographenes, which derive from the longitudinally and
latitudinally p-extension of pyrene. The derivatives are highly
fluorescent and feature low-oxidation potentials. Exploiting
electrooxidation, crystals of cationic mixed valence (MV) complexes
were grown in which the organic salts organize into face-to-face p-
p stacks, a favorable solid-state arrangement for organic
electronics. Variable-temperature EPR measurements and relaxation
studies suggest a strong electron delocalization along the
longitudinal axis of the columnar p-stacking architectures. Electric
measurements of single crystals of the MV salts exhibited a
semiconducting behavior with a remarkable high conductivity at room
temperature. These findings further support the idea for which the
p-extension of polycyclic aromatic hydrocarbons featuring precise
heteroatom doping topologies, is an attractive approach to enable
nanographenes with a broad spectrum of semiconducting properties and
high charge mobilities.
Keywords: polycyclic aromatic hydrocarbons – heteroatom doping –
oxonium – pyrylium – pyrano rings – molecular graphenes – EPR – mixed
valence complexes – electrocrystallization.
Figure to the table of content
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Introduction
The engineering of polycyclic aromatic hydrocarbons (PAHs)[1] has
recently enticed great attention for materials exploration as
organic semiconductors.[2] Replacing carbon atoms with other isolobal
atoms, i.e. doping,[3] is emerging as a powerful approach to control
the chemical, supramolecular and optoelectronic properties of PAHs.
Recent examples include period 2 elements like B-,[4] N-,[5] and O-[6]
PAHs. Furthermore, the doping of PAHs allows the synthesis of
structures that are difficult to be prepared as full-carbon.[7]
Fig. 1. Structural frameworks of all-carbon (top) and O-doped (center) PAHs
discussed in this work, along with the synthetic approach (bottom).
Inspired by the chemical structures of natural anthocyanins[8] and
artificial p-expanded xanthenium cations,[9] we inferred that pyranyl
and pyrylium rings[10] could be used to prepare O-doped congeners of
PAHs 1C-3C and used to engineer p-type organic semiconductors. Out
of the three 1C-3C PAHs (Fig. 1, top), attention has been placed only
on dibenzanthanthrene (1C)[11] and its derivatives,[12] finding
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applications in single-molecule transistors,[13] optical interaction
with alkali atoms for quantum memory,[14] single emitter for super-
resolution microscopy[15] and semiconductors with high mobility in
solution-processed OFETs.[12b]
Building on this hypothesis, herein we report a robust synthetic
approach towards the preparation unprecedented oxygen-containing O-
doped PAHs 1O-3O (Fig. 1, middle), in which two carbon atoms have
been replaced by two oxygen atoms. At the structural level, this
entails the replacement of two fused benzene rings by two pyranyl
cycles. The resulting structures reflect disubstituted pyrenes (with
a latitudinal or longitudinal substitution pattern, depending on the
targeted isomer) with two phenols as key core and peripheral
moieties, respectively (Fig. 1, bottom). To favor solubility, all
molecules were decorated with t-butyl groups (Scheme 1).
Capitalizing on the Pummerer oxidative cyclisation[16] of peri-
xanthenoxanthene (PXX), we expected that this protocol could also
lead to isolated pyranyl rings,[17] that in turn could be oxidized
into pyrylium cations (Fig. 1, bottom). Interestingly, mixed valence
(MV) complexes could be formed at the solid state between the
electron-donor pyranyl molecules and its cationic pyrylium
derivatives, generating p-type organic semiconductors.
Results and Discussion
We first prepared 1,6-dibromopyrene (4) and 1,8-diiodopyrene (10)
precursors through regioselective bromination (Br2 in CHCl3) and
iodination (I2 and KIO3 in CH3COOH/H2O/H2SO4) of pyrene, respectively
(Scheme 1).[18] Regarding the 1,3-dibromo isomer (7), the pyrenyl core
was first mono tert-butylated and then dibrominated with NBS. Suzuki-
Miyaura cross-coupling reaction between the relevant dihalide
pyrenyl precursor and 4-tert-butyl-2-methoxyphenylboronic acid,
using [Pd(PPh3)4] catalyst, gave access to phenol-bearing pyrenes 6,
9 and 12 in excellent yields, after demethylation reaction with BBr3.
Final Pummerer oxidative ring-closure with CuO in PhNO2 under reflux
conditions yielded O-doped PAHs 1O-3O in very good yields.
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Pyrene
b,c
a
d
Br
Br
I
I
Br
Br
4 (32%)
7 (43%
over 2 steps)
tBu
10 (13%)
e
e
e
tBu
tBu
tBu
tBu
O
O
O
R
O
O
R
R
R
R
R
O
tBu
tBu
tBu
R = CH3 5 (93%)
R = H 6 (95%)
R = CH3 8 (97%)
R = H 9 (93%)
R = CH3 11 (91%)
R = H 12 (92%)
f
f
f
g
g
g
tBu
tBu
tBu
tBu
O
O
O
O
O
O
1O (76%)
tBu
2O (83%)
3O (92%)
tBu
tBu
Scheme 1. Synthetic routes to 1O, 2O and 3O. Reagents and conditions: (a) Br2,
t
CHCl3, r.t.; (b) BuCl, AlCl3, CH2Cl2, r.t.; (c) NBS, THF, 30 °C; (d) I2, KIO3,
CH3COOH/H2O/H2SO4, 40 °C; (e) 4-tert-butyl-2-methoxyphenylboronic acid, [Pd(PPh3)4],
NaOH, dioxane/H2O, reflux; (f) BBr3, CH2Cl2, 0 °C → r.t.; (g) CuO, PhNO2, reflux.
Suitable crystals for X-ray diffraction (Fig. 2) were prepared by
slow evaporation (for 1O) and vapor exchange (for 2O). Attempts to
obtain crystals for 3O failed, and only the X-ray structure of a
dichlorinated neutral derivative (3OCl2) was obtained after chemical
oxidation experiments (Fig. S1d, S2d and S7). At the solid state,
molecule 1O establishes C-H· · · p interactions in a herringbone
arrangement with a distance of 3.45(11) Å and an angle of ~50° (Fig.
2c). Molecule 2O forms pillars through π-π stacks (Fig. 2h-g) with
an interplanar distance of 3.34(15) Å and a relative orientation of
~80° (Fig. 2f).
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Fig. 2. X-Ray crystal structures of 1o and 2o. a) Top-view, b) side-view and c)
packing mode of 1O. d) Top-view, e) side-view and h) packing mode of 2O. f,g)
Details related to the stacking mode of 2O, top- and side-view. Space groups: P21/I
(1O) and C2/c (2O). Atom colors: red O, gray C.
Molecules 1O, 2O and 3O present the lowest electronic transitions at
466, 487 and 506 nm, respectively (Table 1, Fig. 3 and S8), with the
strongest absorptivity found for 1O and 3O. As displayed by TD-DFT
calculations (Tables S20-S22, Fig. S47 and S49), these bands are
attributed to π-π* transitions. Comparison between absorption
profiles (in THF) of O-doped PAH 1O and the (only) previously reported
full-carbon compound 1C, revealed a blue-shifted lowest energy
absorption maximum (from 586 to 466 nm for 1C and 1O, respectively,
Fig. S10). The increase in the optical band gap is caused by
replacement of two benzenoid rings with (slightly) anti-aromatic
pyrano rings, which causes an increase in the LUMO level (from DFT
calculations: ELUMO = −2.80 and −1.80 eV for 1C and 1O, respectively),
while the HOMO level is barely affected (EHOMO = −4.85 and −4.72 eV
for 1C and 1O, respectively). Emission profiles for 1O, 2O and 3O
displayed mirroring luminescence profiles with narrow Stokes’ shifts
(4, 9 and 18 nm, respectively), high quantum yields (Ffl = 93, 88
and 68%, respectively) and short lifetimes (t = 2.2, 6.2 and 6.2 ns,
respectively).
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Table 1. Optoelectronic properties of 1O-3O in air-equilibrated THF. Half-
wave potentials (ꢀꢁ/ꢂ) were determined by CV (THF, 0.1 M n-Bu4NPF6) and
referenced versus Fc+/Fc couple.
ꢌꢍꢊ
ꢊ/ꢋ
ꢌꢍꢋ
ꢊ/ꢋ
ꢇꢈꢉ
[b]
[e]
[f]
[c]
[a]
ꢃfl
ꢀ
ꢀ
λabs
λem
ꢀꢄꢄ
ꢀꢎꢏꢐꢏ
ꢀꢑꢒꢓꢔ
ꢀ
ꢅꢆ
Molecule
[eV]
[eV]
[eV]
[nm]
[nm]
[%]
[V]
[V]
[V]
1O
2O
3O
466 470 93 2.64 −2.54 0.10 0.54
−4.90
−2.26
487 496 88 2.50 −2.42 0.29 0.72[d] −5.09
−2.59
−2.59
506 524 68 2.36 −2.02 0.15 0.54
−4.95
[a]
ꢃfl measured by optical dilute method and Coumarin 153 as reference. [b]Optical
bandgap. [c]From the cathodic peak of an irreversible process. [d]From the anodic
peak of an irreversible process. [e]Formal potential of Fc+/Fc couple is assumed to
be −4.8 eV vs vacuum. [f]Calculated using ꢀꢕꢕ.
Fig. 3. a-c) UV-Vis absorption (black) and emission (red) spectra for 1O-
3O in THF at room temperature. d) FMO energies obtained by CV (full) and
E00 (dashed) along with orbital densities calculated at the B3LYP/6-31(d,p)
level of theory (isoval: 0.02 a.u.); the optical bandgap shrinks following
the order 1O>2O>3O. e) Absorption and emission colours for 1O-3O in THF at
r.t.
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Cyclic voltammetry (CV) was used to assess the redox properties in
THF (Table 1, Fig. S11-S13). The CV profiles of 1O and 3O feature two
reversible oxidation couples (0.10 and 0.54 V for 1O and 0.15 and
0.54 V for 3O), whereas only one reversible oxidation event was
detected for 2O at 0.29 V, together with an irreversible process at
0.72 V. Only irreversible reduction waves were observed in the redox
window of THF. Comparing the diffusion coefficients measured for 1O,
2O and 3O by DOSY-NMR (1.10 × 10-5, 9.89 × 10-6 and 1.11 × 10-5 cm2 s-1,
respectively) with those obtained from the first cathodic events
(1.24 × 10-5, 9.55 × 10-6 and 1.13 × 10-5 cm2 s-1 measured at 0.18, 0.34
and 0.20 V, respectively), we could assert that the first oxidative
waves correspond to the formation of the radical cation species (1O•+,
2O•+ and 3O•+, see Section 5 in the SI). These findings suggest that
all oxidation states are likely accessible by titration with tris(4-
bromophenyl)ammoniumyl hexafluoroantimonate (BAHAF), a one-e- oxidant
(Ered = 0.70 V vs Fc/Fc+).[19]
Fig. 4. X-band CW-EPR spectra (-) and simulations (-) of radical cations 1O•+, 2O•+
and 3O•+ in THF solutions as generated with BAHAF (left). Iso-spin density
distribution maps for the spin density value of 0.002 electron/bohr by DFT at the
UB3LYP/EPR-II level of theory for the radical cations (right).
The effects of oxidation are particularly apparent in EPR spectra
if unpaired electrons are formed. Titration of one eq. of BAHAF into
solutions of 1O, 2O and 3O gave radical cations 1O•+, 2O•+ and 3O•+, the
latter exhibiting X-band CW-EPR signals characteristic of S = 1/2
H
electron spin systems (Fig. 3). The isotropic giso and aiso values
show agreement with those estimated from theory (Table S5). Upon
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further additions of BAHAF (>1 eq.), the intensity of the EPR signals
decreased until complete disappearance (>2 eq.), and this coincided
with the emergence of the EPR signal of BAHAF. For instance, closed-
shell, EPR-silent, cations 1O2+ and 3O2+ are obtained in the case of
1O and 3O (Fig. S35). Given the irreversibility of the second
oxidation process for 2O, we did not perform any EPR characterization
for this molecule in the presence of an excess of BAHAF. To shed
further light on the properties of the oxidized species, we also
performed UV-Vis-NIR spectroelectrochemical (Fig. S17-S20) and
spectrophotometric titration experiments with both BAHA (magic blue)
and BAHAF (Fig. 5 and S21-S24). In all cases, the absorption spectra
readily underwent dramatic changes, evidencing the progressive
disappearance of the electronic transitions at 410-510 nm and the
simultaneous appearance of new red-shifted bands in the NIR spectral
region, typical of radical cations species. We further attempted to
characterize the dications in solution by 1H-NMR spectroscopy (Fig.
S25-S28). While we observed the disappearance of signals from the
neutral species upon addition of BAHA, accompanied by presence of
new proton resonances, the interpretation was complicated by the
evident loss of intensity, presumably due to the strong tendency of
the oxidized species to aggregate (Fig. S28). Moreover, all attempts
to obtain single crystals suitable for X-ray diffraction of the
chemically-oxidized species were unsuccessful.
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Fig. 5. UV-Vis-NIR spectra obtained by titration of a) 1O, b) 2O and c) 2O with
BAHA (magic blue) in dry CH2Cl2 at 298 K.
The formation of the cationic species was further corroborated by
DFT calculations and their aromaticity was probed by nucleus-
independent chemical shift (NICS) and anisotropy of the induced
current density (AICD) analyses (SI, Section 11).[20] NICS(1)zz values
(Fig. S54-S57) for the pyrano rings show antiaromatic character
(+12.76, +10.93 and +14.66 ppm for 1O, 2O and 2O, respectively), while
oxidation by one-electron results in that ring being essentially
non-aromatic (−1.18, +0.73 and +0.30 ppm, respectively), and double
oxidation gives pyrylium rings with aromatic character (−18.06,
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−6.54 and −17.11 ppm, respectively) similar to the all-carbon neutral
PAH counterparts 1C-3C (Fig. S58-S59). In contrast to typical C-O
single bond (1.43 Å), the new formed C-O bonds (Fig. S44-S46) are
substantially shorter (1.37 Å) in the neutral 1O-3O species,
suggesting that the O-PAHs lone-pair electrons contribute in the π-
electron interactions. The C-O bonds were observed to shorten even
further in the calculated oxidized O-PAHs (1.35 Å for the radical
cations and 1.33-1.34 Å for the dications). TD-DFT simulations
(Tables S25-S30, Fig. S50) of the UV-vis absorption envelop resembled
that observed experimentally for the oxidized species.
We turned our attention to the electrocrystallisation technique[21]
to grow crystals of the cations suitable for X-ray diffraction,
performing electrooxidation experiments (SI, Section 8). Triclinic
shiny black crystals containing salts of mixed-valence (MV)
complexes for O-doped PAHs 1O and 3O were obtained in THF with n-
Bu4NPF6 and n-Bu4NClO4 as electrolytes, respectively (Fig. 6). While
the former electro-crystallizes in an asymmetric unit containing ten
crystallographically-independent O-doped molecules, six PF6− anions
and 16 solvent molecules, (1O)10(PF6)6·(THF)16, the latter depicts
three independent O-PAHs, two ClO4− anions, one THF and half H2O
molecules, (3O)3(ClO4)2·THF·1/2H2O. All attempts to grow suitable
crystals containing oxidized species of 2O for X-ray analyses failed.
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Fig. 6. X-ray crystals structures for MV complexes (1O)10(PF6)6·(THF)16 (top, P-1)
and (3O)3(ClO4)2·THF·1/2H2O (down, P-1), solvent molecules omitted for clarity. a)
Morphology of the crystals and X-ray structure of the relevant isolated O-doped
molecule, b) π-π stacking mode, c-d) solid-state columnar π-π stacks. *For the MV
complexes of 3O, each stacking mode between two 3o molecules gives rise to two
different off-set arrangements. Atom colors: red O, gray C., green Cl, yellow F,
orange P.
Looking at organization at the solid state of (1O)10(PF6)6·(THF)16
(Fig. 6a1-d1), one can clearly evidence the presence of a columnar
arrangement in which the molecules organize antiparallely (Fig. 6b1),
with an average interplanar spacing of 3.33(7) Å (shortest distance
between ring centroids of 3.367(12) Å with a lateral slippage of
0.83 Å). The PF6− counterions are evenly intercalated in the channels
(Fig. 4c1) together with solvent THF molecules. Similarly, in the
crystal of (3O)3(ClO4)2·THF·1/2H2O (Fig. 6a2-d2), molecule 3O segregates
in π-π stacks (π-π distances from 3.21 to 3.42 Å, Fig. 6c2), with
ClO4− anions and solvent molecules intercalating between different
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stacks. In this crystal, two stacking modes are present: a pincer-
like stack (Fig. 6b2, top), in which two independent molecules are
facing to each other in an antiparallel fashion with a relative angle
of about 90°, and a head-to-tail arrangement (Fig. 6b2, bottom),
where two molecules antiparallely stack. In both MV complexes, all
O-doped PAHs show very similar bond lengths (SI, Table S4) and short
interplanar π-π distances. These structures are consistent with a
material in which the charge is delocalized over the stacks.[22] A MV
salt would form a partially filled band structure, providing the
conduction carrier.
As crystals for (1O)10(PF6)6·(THF)16 were extremely fragile, we used
(3O)3(ClO4)2·THF·1/2H2O to probe the electronic properties of the
organic salts. We first performed variable temperature (VT) X-band
CW-EPR investigations (Fig. 7a). The EPR spectra are constituted by
what appears to be a (medium polarized) conduction electron spin
resonance line, most likely associated with delocalization of spin
density along the longitudinal axis of the π-stacking architecture.
This behavior has been observed in systems showing metal to
insulator/semiconductor transition.[23] As the intensity of the EPR
signal decreases when the temperature is lowered, an Arrhenius plot
has been used to interpret the VT-dependent EPR behavior (Fig. 7a-
b). The linear portion of the trend at low temperature was fitted
to a line of equation f(1/T) = –4761/T + 8.77 and an activation energy
(Ea) of 4.1 × 10–2 eV was obtained in good agreement with the value
derived from single-crystal conductivity measurements.
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Fig. 7. a) X-band CW-EPR VT-spectra of crystals (3O)3(ClO4)2·THF·1/2H2O between 298
K and 119 K at 10 kHz field modulation frequency; b) natural logarithm (•••) of
the EPR intensities versus 1/T Arrhenius plot, the fitting (-) give R2 = 0.9933;
c) 2-pulse Hahn echo decay measured at 15 K (-), 10 K (-) and 5 K (-). The pulse
sequence adopted to generate a primary electron spin echo is shown as an inset.
Microwave pulse lengths of πꢀ=ꢀ120ꢀns were used to suppress any potential proton-
electron spin modulation. (d) Echo-detected saturation recovery experiment at 15
K (green trace), 10 K (red trace) and 5 K (blue trace). The pulse sequence adopted
is shown as an inset, where πꢀ=ꢀ48ꢀns, τꢀ=ꢀ248 ns and the delay time T is variable.
Relaxation studies were also performed at of 5, 10 and 15 K to
calculate the phase memory time Tm (i.e., transversal relaxation or
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spin-spin relaxation time T2, Fig. 7c). Already at 20 K no Hahn echo
was detected when using a primary π /2−π −π pulse sequence, even when
shortening the inter-pulse delay, t, to the minimum. A fast Tm value
of 0.43 µs was measured at 5 K, which rapidly decreases to 0.34 µs
and 0.29 µs at 10 K and 15 K, respectively. Notably, solid-state Tm
values are ten times lower than those measured in frozen THF for
radical cation 3O•+, i.e. 4 µs at 5 K (Fig. S37). Overall, these
observations suggest that fast spin-spin relaxation routes occur in
the crystal due to the stacking of the paramagnetic radical cations.
Similarly, short relaxation time T1 (or longitudinal spin-lattice
relaxation time) was measured (Fig. S37). A decrease of the T1 value
was also observed as a function of the temperature (0.21 ms at 5 K,
0.07 ms at 10 K and 0.05 ms at 15 K), which is 120 times lower than
the T1 value (25 ms) measured for 3O•+ in solution. The dramatic
lowering of T1 further corroborates the presence of the ordered
crystal lattice in the electrochemically-growth solid material.
I-V curves of single crystals of (3O)3(ClO4)2·THF·1/2H2O exhibit a
typical semiconductor behavior with a sigmoidal shape rather than a
linear behavior, consistent with the observed temperature dependence
of the conductivity (Fig. 8a-b).[24] The conductivity (s) was
calculated in the ohmic region (sohmic), i.e., around zero bias and,
as an extension, in the non-ohmic (snon ohmic) activated regime at higher
-
bias values. Considering that organic materials are typically poorly
conductive semiconductors (s <10-6 S cm-1 at 300 K) or just
insulators,[25] our mixed-valence crystalline complexes exhibit high
conductivity at room temperature (sohmic = 8.8 × 10-5 and 3.7 × 10-3 S
cm-1; snon ohmic = 2.4 × 10-4 and 5.1 × 10-3 S cm-1 for crystals a and b,
-
respectively). Exceptions include crystalline organic conductors and
superconductors based on electron donors (such as TTF) and acceptors
(such as TCNQ),[26] some organic and inorganic radicals,[27] and some
metal-organic frameworks (MOFs).[28]
16
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10.1002/anie.201914025
Angewandte Chemie International Edition
a
b
Fig. 8. Device and electrical response of two crystals ( and ) of
(3O)3(ClO4)2·THF·1/2H2O: a) I-V curve (•••) at 300K with the ohmic linear fit (-) and
2
a
non-ohmic fit (-). For crystal : The ohmic fit (R = 0.99752) gives a slope of
1.98±0.02 × 10-7 Ω-1 and a y-intercept value of 9±2 × 10-10 A; non-ohmic fit (R2 =
0.99752) gives a slope of 5.43±0.01 × 10-7 Ω-1 and a y-intercept value of -1.48±0.003
× 10-7 A. For crystal : The ohmic fit (R = 0.99998) gives a slope of 3.943±0.005
2
b
× 10-5 Ω-1 and a y-intercept value of 2±5 × 10-9 A; non-ohmic fit (R2 = 0.99957)
gives a slope of 5.46±0.02 × 10-5 Ω-1 and a y-intercept value of -7.26±0.0015 × 10-
6
A. b) Arrhenius plot of the conductivity vs. temperature (•••) together with the
linear fits (-, -) in the low- and high-temperature regimes. Conductivity values
calculated at high (s0,HT) and low (s0,LT) temperatures and associated activation
-1
a
energies (Ea,HT and Ea,LT). For crystal : s0,HT = 5.50±0.05 S cm and Ea,HT = 72.0±0.6
meV (R2 = 0.99359); s0,LT = 4.92±0.12 × 10-6 S cm-1 and Ea,LT = 37.15±0.14 meV (R2 =
0.99915); for crystal : s0,HT = 70.0±0.5 S cm and Ea,HT = 80.21±0.09 meV (R =
0.99817); s0,LT = 2.81±0.08 × 10-6 S cm-1 and Ea,LT = 63.11±0.07 meV (R2 = 0.99967).
Insets: top view (a) and lateral view (b) of the devices.
-1
2
b
Conclusions
In summary, we have developed an efficient synthesis for the
construction of O-embedded PAHs. This approach consists in
longitudinal or latitudinal extension of dihalogenated pyrenes, that
coupled with phenol moieties can be planarized through Pummerer
oxidative ring-closure (CuO in PhNO2). Electrochemical and chemical
17
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10.1002/anie.201914025
Angewandte Chemie International Edition
oxidation investigations showed that the cations of O-PAHs can be
easily accessed, prompting us to perform electrocrystallisation
experiments to form organic MV crystals. The organic salts organize
in columns at the solid state, depicting a strong electron
delocalization along the longitudinal axis of the p-stacks. Electric
measurements of single crystals of the MV salts exhibited a
semiconducting behavior with relatively high conductivity at room
temperature, further supporting the idea for which the heteroatom
approach is a viable route for empowering PAHs and nanographenes
with a broad spectrum of semiconducting properties and high charge
mobilities.
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