Doping of an organic molecular semiconductor by substitutional cocrystallization with a molecular n-dopant

Article abstract of DOI:10.1039/b610806g

Dopants for organic molecular semiconductors that yield immobile dopant ions are necessary for the creation of stable molecular semiconductor p-n junctions, the basis for almost all traditional inorganic semiconductor devices. We present evidence for the substitutional cocrystallization of tris(4-nitrophenyl)methyl radical (1) with small amounts of tris[4- (dimethylamino)phenyl]methyl radical (2), resulting in n-doped 1 with immobile 2⁺ counterions. Cyclic voltammetry indicates that electron transfer from 2 to 1 is favored by 0.51 eV. The powder X-ray diffraction patterns of pure 1 and 1 doped with 2 are very similar, indicating substitutional cocrystallization. The electrical conductivity of doped 1 increases with increasing concentration of 2, and the conductivity is constant over time. Variable-temperature conductivity measurements of 1 doped with 2% and 5% 2 indicate that the activation energy of conduction is 0.32 eV at both dopant concentrations. The Royal Society of Chemistry 2007.

Full text of DOI:10.1039/b610806g

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Doping of an organic molecular semiconductor by substitutional
cocrystallization with a molecular n-dopant{
William W. Porter III and Thomas P. Vaid*
Received 27th July 2006, Accepted 2nd November 2006
First published as an Advance Article on the web 14th November 2006
DOI: 10.1039/b610806g
Dopants for organic molecular semiconductors that yield immobile dopant ions are necessary for
the creation of stable molecular semiconductor p–n junctions, the basis for almost all traditional
inorganic semiconductor devices. We present evidence for the substitutional cocrystallization of
tris(4-nitrophenyl)methyl radical (1) with small amounts of tris[4-(dimethylamino)phenyl]methyl
radical (2), resulting in n-doped 1 with immobile 2+ counterions. Cyclic voltammetry indicates
that electron transfer from 2 to 1 is favored by 0.51 eV. The powder X-ray diffraction patterns of
pure 1 and 1 doped with 2 are very similar, indicating substitutional cocrystallization. The
electrical conductivity of doped 1 increases with increasing concentration of 2, and the
conductivity is constant over time. Variable-temperature conductivity measurements of 1 doped
with 2% and 5% 2 indicate that the activation energy of conduction is 0.32 eV at both dopant
concentrations.
polyacetylene p–n junction.15 However, the current–voltage
Introduction
curve of that junction exhibited hysteresis, and the junction
was presumably unstable over time because the dopant
counterions, Na+ and AsF62, are mobile and can migrate
to form NaAsF6 and undoped polyacetylene. The same
instability is expected in any organic molecular or polymeric
semiconductor doped with an element or molecule that results
in a small, mobile counterion. Most of the subsequent develop-
ment of organic semiconductor devices, therefore, has focused
on device architectures that utilize undoped organic semi-
conductors. Organic solar cells, for example, are based on
heterojunctions between two different, and undoped, organic
semiconductors. In contrast, inorganic solar cells are simply
silicon p–n homojunctions or, in thin-film solar cells, p–n
heterojunctions (of doped semiconductors). Doped inorganic
semiconductors are able to maintain the space–charge region
characteristic of a p–n junction because the dopant atoms
are covalently bound within the crystalline lattice and are
therefore immobile.
Organic molecular and polymeric semiconductors1 are being
widely investigated as the active components of electronic
devices.2,3 The performance of organic thin-film transistors4–6
and organic solar cells7,8 continues to improve, and displays
that utilize organic light-emitting diodes9 are now commer-
cially available. As impressive as these gains have been, signifi-
cant advances in the performance of organic semiconductor
devices remain desirable. For example, the light-to-electrical
energy conversion efficiency of organic photovoltaics,8 while
recently greatly improved,10 is still substantially lower than
that of photovoltaics made with crystalline inorganic semi-
conductors such as silicon and gallium arsenide.
A fundamentally different approach to organic semicon-
ductor devices, and one that may offer significant performance
advances and even completely new devices, is through doped
organic semiconductors. Almost all of the research on the
devices cited above has involved undoped organic semi-
conductors. That is in stark contrast to traditional inorganic
semiconductor devices, which, in the vast majority of cases,
rely upon junctions between n- and p-doped semiconductor
regions for their operation.11 The different path in the develop-
ment of organic semiconductor devices can be traced to the
origins of modern organic semiconductor research. The classic
discovery by Heeger, MacDiarmid, Shirakawa, and coworkers
involved a doped organic semiconductor: p-doped12–14 and
n-doped14 polyacetylene. Soon after the discovery of doped
polyacetylene, the same group reported the construction of a
A method for doping organic semiconductors that yields
immobile dopant counterions would allow stable organic p–n
junctions to be made, and, in principle, allow any traditional
inorganic semiconductor device architecture to be realized with
organic semiconductors. Several research groups are exploring
a variety of methods for creating immobile dopants for organic
semiconductors. Gregg and coworkers have thoroughly
studied a singly reduced perylene diimide with a covalently
attached cation that acts as an n-dopant when cocrystallized
with a perylene diimide host.16–21 Utilizing a similar strategy,
Lonergan and coworkers have covalently attached ions to
polyacetylene22,23 and formed polyacetylene p–n junctions.24,25
Leo and coworkers have co-sublimed a very strong reducing
agent, bis(terpyridine)rutheniuum0, with the semiconductor
zinc phthalocyanine (ZnPc) to create n-doped ZnPc, while
co-sublimation of ZnPc and the strong oxidizing agent
tetrafluorotetracyanoquinodimethane (F4-TCNQ) resulted in
Center for Materials Innovation and Department of Chemistry,
Washington University, St. Louis, Missouri 63130, USA.
E-mail: vaid@wustl.edu
{ Electronic supplementary information (ESI) available: 1H NMR
spectra of 2 and 2-H quenched with I2, cyclic voltammograms of 1 and
2, and a graph of conductivity of doped 1 as a function of time. See
http://dx.doi.org/10.1039/b610806g/
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J. Mater. Chem., 2007, 17, 469–475 | 469

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p-doped ZnPc. A ZnPc p–i–n (i = intrinsic, or undoped)
homojunction was created by sequential sublimation of the
appropriate materials.26 The p–i–n junction acted as an
electrical diode and was stable for weeks under vacuum.
That stability is somewhat surprising in light of other
experiments that showed that F4-TCNQ quickly diffused
through ZnPc at room temperature.27
synthesis, isolation, and characterization of 1 and 2. The
electrical properties of 1 doped with 2 will be discussed.
Experimental
General procedures and materials
All manipulations were carried out using Schlenk line or
glovebox techniques unless otherwise noted. All manipulations
involving radicals were carried out with light excluded.
Reagents were purchased from commercial suppliers and
used as received unless their purification is noted as follows.
Tetrabutylammonium hexafluorophosphate, [NBu4][PF6], was
recrystallized twice from ethanol. Acetonitrile was distilled
Our own efforts have focused on a strategy that we have
dubbed ‘‘isostructural doping’’.28–33 It is closely analogous to
the standard method for doping inorganic semiconductors.
Silicon, for example, is n-doped by the formal replacement of a
small fraction of the silicon atoms with an atom with one more
valence electron, such as phosphorus. An isostructural dopant
for an organic semiconductor is created by replacing, for
example, a carbon atom in the molecular semiconductor with a
nitrogen atom, while maintaining charge neutrality of the
molecule. A small amount of the isostructural n-dopant is then
cocrystallized with the parent molecular semiconductor to
create an n-doped material. Because the dopant is isostructural
with the semiconductor, it should cocrystallize substitutionally
with the semiconductor and will therefore be immobile.
The dopant has an excess electron (compared to the parent
semiconductor), and therefore should be easily ionized to dope
the semiconductor and form an immobile cation. We have
found that cocrystals of molecular semiconductors and
relatively high proportions (at least 10%) of isostructural
dopants can be formed.28,30–32 However, in no case has an
isostructural dopant been a strong enough electron donor or
acceptor to electrically dope its parent semiconductor.
˚
from P2O5 onto 3 A molecular sieves and vacuum transferred
immediately prior to use. Pyridine was distilled from KOH
˚
onto 3 A molecular sieves and vacuum transferred immediately
prior to use. Dimethylformamide (DMF) was vacuum-distilled
˚
from P2O5 and stored over activated 3 A molecular sieves in a
nitrogen-filled glove box. Ethereal solvents were distilled from
a purple sodium benzophenone solution and hydrocarbon
solvents were distilled from purple sodium benzophenone
solutions with added tetraglyme. Pyridine-d5 was vacuum
˚
transferred onto 3 A molecular sieves from KOH and stored
in a nitrogen-filled glove box. Acetonitrile-d3 was vacuum
˚
transferred onto 3A molecular sieves from P2O5 and stored
in a nitrogen-filled glove box. Tris(4-dimethylaminophenyl)
methane was prepared by the reaction of 2+Cl2 with NaBH4.36
Addition of an aqueous solution of KI to commercial 2+Cl2
yielded a precipitate of 2+I2,37 which was purified by
recrystallization from CH2Cl2–hexanes. Published procedures
In the current study we have relaxed the requirement that a
dopant be exactly isostructural with its parent molecular
semiconductor, and instead sought a pair of molecules that are
sufficiently structurally similar to be substitutionally cocrys-
tallized, and which we also expect to undergo a spontaneous
electron transfer. Previous solution phase electrochemical
studies of tris(4-nitrophenyl)methyl radical (1, Fig. 1) and
tris[4-(dimethylamino)phenyl]methyl radical (2, Fig. 1) indi-
cated that 2 would spontaneously transfer an electron to 1.34
An additional indication that electron transfer from 2 to 1
would occur was provided by another study: a one-to-one
mixture of 12 and the bis(4-dimethylamino)trityl cation (a
molecule very similar to 2) has been shown to exist in solution
as an equilibrium mixture of a cation–anion pair and a pair
of neutral radicals, with the position of the equilibrium
dependent upon the polarity of the solvent.35 The similarity
of the structures of 1 and 2 led us to believe that they could
be substitutionally cocrystallized. Herein we describe the
were employed for the syntheses of ferrocenium hexafluoro-
2 38
phosphate (Fc+PF6
)
and tris(4-nitrophenyl)methane,39 and
a modified literature procedure was used for its deprotonation
to Na+12.40
1H NMR and 13C NMR spectra were recorded on a Varian
Mercury 300 MHz spectrometer with the solvent signal as the
standard. Infrared spectra were obtained on a Perkin-Elmer
Spectrum BX FT-IR system as Nujol mulls on NaCl plates.
UV-vis spectra were obtained using a Varian Cary 100 Bio
UV-vis spectrometer. ESR spectra were obtained on a Bruker
X-band spectrometer with 0.1 G modulation. ESR spectral
simulations were created with WINEPR SimFonia.41 X-Ray
powder diffraction was performed on a Rigaku Geigerflex
D/max diffractometer interfaced to a PC. Microanalysis
was performed by the University of Illinois Microanalysis
Laboratory, Urbana, IL.
Solution phase magnetic susceptibility of 2 was measured as
a solution in pyridine-d5 by the Evans method as described
previously.28,42 Solid state susceptibilities were measured at
room temperature on a Johnson Matthey Mark I magnetic
susceptibility balance.
Cyclic voltammetry and room temperature solid state
conductivity measurements were performed with an EG&G/
PAR 263A potentiostat with electrical connection to the inside
of a nitrogen-filled drybox, where all materials and solutions
were maintained during the measurements. Cyclic voltam-
metry was performed in anhydrous THF with 0.10 M
[Bu4N][PF6] supporting electrolyte and a ferrocene internal
standard. The working electrode and pseudoreference
Fig. 1 Structures of tris(4-nitrophenyl)methyl radical (1) and tris[4-
(dimethylamino)phenyl]methyl radical (2).
470 | J. Mater. Chem., 2007, 17, 469–475
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electrode were 0.50 mm diameter platinum disks and the
counter electrode was a 2.5 mm diameter platinum disk. Scans
were run at 50 mV s21. Solid state conductivity of pressed
powders was measured in a two-electrode configuration on a
lab-built apparatus that consists of a Delrin block with a
6.35 mm cylindrical hole and two 6.35 mm copper cylindrical
contacts. The material to be studied was inserted between the
two contacts within the Delrin block and a mass of 3.85 kg was
placed on the top contact, thus applying 12 bar of pressure. A
5.3 Hz low-pass filter was utilized in making cyclic voltam-
metry and conductivity measurements.
form immediately when the addition began. The suspension
was stirred for 12 h. The precipitate was collected by filtration
and washed with CH3CN. The product was recrystallized by
the slow concentration of a solution in 70 mL of pyridine
to 15 mL. The deep green microcrystals were collected by
filtration and held under vacuum at 80 uC for 4 h. Yield:
145 mg, 61%. UV-vis in CH3CN, saturated: lmax = 619 nm. IR
(Nujol, cm21): 3093 (m), 2428 (vw), 1579 (m), 1567 (s), 1501
(vs), 1333 (vs, br), 1314 (s), 1270 (m), 1180 (m), 1125 (w), 1103
(s), 866 (s), 857 (s), 847 (s), 833 (m), 762 (m), 749 (m), 701 (s),
674 (w). Anal. calcd for C19H12N3O6: C, 60.32; H, 3.20; N,
11.11. Found: C, 59.85; H, 3.44; N, 10.72%.
For variable-temperature conductivity measurements,
samples were prepared as pressed pellets of 6.4 mm diameter
in a nitrogen-filled glovebox. The thickness of the pellets
was computed using their mass and an estimated density of
1.50 g cm23, calculated from the crystal structure of 1.43 The
two surfaces of the pressed pellet were coated in silver paint
and an electrical connection to each was made with 25 mm
diameter platinum wire. The entire sample was then coated in
poly(vinyltoluene) by evaporation of a solution of the polymer
in toluene. The sample was protected from exposure to air by
sealing in two Ziploc1 bags and then quickly transferring the
sample from the bags to the cryostat of a Quantum Design
Physical Properties Measurement System (PPMS). The system
was interfaced to an EG&G/PAR 263A potentiostat for
current–voltage measurements. A cooling and heating rate of
1 K min21 was used, and at 5 K intervals a current–voltage
curve was recorded over a potential range of +4 to 24 V at a
sweep rate of 100 mV s21. For the sample with 2% doping,
conductivity was calculated from the data from +4.0 to +3.5 V
and 24.0 to 23.5 V because there was a small amount of
non-linearity in the current–voltage curves.
Tris[(4-dimethylamino)phenyl]methyl iodide (2+I2)37. 1H
NMR (DMSO-d6): d 7.28 (d, 6H, J = 9.0 Hz), 7.00 (d, 6H,
J = 9.0), 3.22 (s, 18H). 13C NMR (DMSO-d6, partial
spectrum): d 176.3, 155.3, 139.2, 125.8, 112.6. UV-vis in
CH3CN, 7.8 6 1027 M: lmax = 593 nm, e = 9.9 6 104; lmax
=
551 nm, e = 9.5 6 104 L mol21 cm21 (overlapping peaks fit
with two Gaussians peaks). IR (Nujol, cm21): 1679 (w), 1656
(w), 1582 (s), 1524 (w), 1360 (s, br), 1298 (m), 1229 (m), 1173
(s, br), 1135 (s), 1066 (m), 959 (w), 940 (s), 912 (m), 838 (m),
825 (m), 798 (w), 758 (w), 743(w), 666 (w).
Tris[(4-dimethylamino)phenyl]methyl radical (2). A mixture
of 2+I2 (845 mg, 1.69 mmol), sodium amalgam (38.9 mg Na,
1.69 mmol; 4.01 g Hg), and 10 mL of DMF was stirred at 22 uC
for 12 h. The red solution was filtered to remove the mercury.
The DMF was removed under reduced pressure and 60 mL
toluene was added to the residue. The solution was filtered
to remove NaI. The toluene was removed under reduced
pressure, and the product was recrystallized by dissolving in
40 mL THF, adding 10 mL heptane, and concentrating the
solution to 20 mL. The product was collected by filtration and
dried under vacuum for 4 h. Yield: 390 mg, 62% [the product
typically contains 10% tris(4-dimethylaminophenyl)methane].
UV-vis in THF, 3.3 6 1024 M: lmax = 405 nm. IR (Nujol,
cm21): 2795 (m), 1884 (w), 1678 (w), 1658 (w), 1610 (s, br),
1563 (w), 1517 (s), 1444 (m), 1349 (s, br), 1225 (m), 1201 (m),
1185 (m), 1164 (m), 1129 (m), 1060 (m), 947 (s), 836 (w), 812
(s), 795 (w), 752 (w). Anal. calcd for C25H30N3: C, 80.60; H,
8.12; N, 11.28. Found: C, 80.05; H, 8.39; N, 10.98%.
1
Tris(4-nitrophenyl)methane39. H NMR (CDCl3): d 8.24 (d,
6H, J = 8.8 Hz), 7.29 (d, 6H, J = 9.0 Hz), 5.86 (s, 1H). 13C
NMR (CDCl3): d 148.2, 147.5, 130.3, 124.5, 56.1.
Sodium tris(4-nitrophenyl)methyl (Na+12)40. Tris(4-nitrophe-
nyl)methane (761 mg, 2.01 mmol), sodium methoxide (119 mg,
2.21 mmol), and 60 mL of pyridine were heated to reflux
for 12 h to give a deep blue solution. The solution was
concentrated to 15 mL to yield a purple precipitate, which
was collected by filtration. Recrystallization by concentrating
a CH3CN–toluene solution gave a purple, microcrystalline
product that was held under vacuum at 80 uC for 4 h. Yield:
Cocrystallization of 2 and 1. In a typical cocrystallization, 1
(93.2 mg, 0.246 mmol) and 2 (4.8 mg, 0.013 mmol) were
dissolved in 70 mL of pyridine. The solution was slowly
concentrated to 2–3 mL to precipitate a green solid. The
product was collected by filtration and held under vacuum for
4 h. Isolated yield: 75.6 mg, 77%.
1
664 mg, 83%. H NMR (CD3CN): d 7.91 (d, 6H, J = 9.3 Hz),
7.33 (d, 6H, J = 9.3). 13C NMR (CD3CN, partial spectrum): d
150.8, 140.5, 128.3, 125.6. UV-vis in CH3CN, 7.5 6 1025 M:
l
max = 792 nm, e = 2.9 6 103 L mol21 cm21. IR (Nujol, cm21):
1590 (m), 1573 (s), 1505 (s), 1397 (s), 1331 (m), 1231 (w), 1215
(w), 1174 (m), 1106 (s), 966 (w), 861 (m), 848 (s), 828 (m), 780
(s), 758 (m), 701 (s), 682 (w), 636 (w), 614 (w). Anal. calcd for
C19H12N3NaO6: C, 56.87; H, 3.01; N, 10.47. Found: C, 56.39;
H, 3.14; N, 10.35%.
Results and discussion
Synthesis
44–48
49
Both 1
and 2
have been studied spectroscopically,
generally as solutions that were generated in situ. Single
crystals of 1 large enough for X-ray structure determination
have been grown by the reduction of tris(4-nitrophenyl)bro-
momethane (1-Br) with a zinc rod.43 However, we found that
none of the reported procedures for the generation of either
2
Tris(4-nitrophenyl)methyl radical (1). A solution of Fc+PF6
(250 mg, 0.756 mmol) in 15 mL of CH3CN was added
dropwise over 1 h to a stirred solution of Na+12 (252 mg,
0.628 mmol) in 20 mL of CH3CN. A green precipitate began to
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radical 1 or 2 was suitable for their synthesis and isolation
in bulk.
reported hyperfine couplings for other 4-substituted tri-
phenylmethyl radicals as a starting point, a simulated spectrum
(which also has lines spaced by 0.151 G) was created with
Because of the low solubility of 1, its synthesis by the
reduction of 1-Br with zinc or other metals was complicated by
a difficult separation from the salt byproduct. We instead took
advantage of the low solubility of 1 for its isolation after the
oxidation of 12. Tris(4-nitrophenyl)methane (1-H) was pre-
pared by nitration of triphenylmethane with a mixture of
sulfuric acid and nitric acid.39 Deprotonation of 1-H by
sodium methoxide gave Na+12.40 Oxidation of a solution of
aH,ortho = 2.550 G, aH,meta = 1.115 G, aN = 0.500 G, aH,methyl
0.337 G. The g-value of 2 is 2.0040.
=
Magnetic susceptibility
(i) Solution phase. The solubility of 1 was too low for a
solution phase determination of its magnetic susceptibility.
The Evans method42 was used to determine the magnetic
susceptibility of 2 as a solution in pyridine-d5. Concentrations
of 0.0451, 0.0349, and 0.0171 M (the presence of 2-H was
accounted for) gave magnetic moments of 0.488, 0.509, and
0.514 mB, respectively. Those values are all significantly lower
than the free-electron value of 1.73 mB, indicating that some
reversible p- or s-dimerization of 2 is occurring in solution.
The increasing magnetic moment with decreasing concentra-
tion is consistent with a reversible dimerization.
Na+12 in CH3CN with a solution of Fc+PF6 in CH3CN
2
resulted in the immediate precipitation of 1, which was purified
by recrystallization from pyridine.
Commercially available 2+Cl2 (crystal violet) was converted
to 2+I2 by precipitation from aqueous solution by the addition
of aqueous KI. Reduction of 2+I2 with sodium amalgam
in DMF yielded 2, which was inevitably contaminated with
10–20% of tris(4-dimethylaminophenyl)methane (2-H, leuco
crystal violet). The 2-H could not be separated from 2, and
reductions of 2+I2 with different metals (magnesium or zinc)
and in different solvents (THF or pyridine) also led to some
2-H as a side product. The amount of 2-H relative to 2 could
be quantified by 1H NMR before and after oxidation of a
sample with I2, which converts both 2-H and 2 to 2+I2 (see
ESI{). The electrochemical reduction of 2+ has also been shown
to produce 2 and a small amount of 2-H.50 The contamination
of 2 with 2-H was not a major concern for our doping studies,
as the small amount of 2-H would either be electrically inactive
in the doped crystals of 1, or would itself act as an n-dopant
for 1, as 2-H has previously been shown to n-dope C60.51
(ii) Solid state. The magnetic susceptibilities of two samples
of microcrystalline 1 were measured at room temperature and,
after a correction for diamagnetism, indicated magnetic
moments of 1.74 mB and 1.62 mB for the two samples measured.
Those susceptibilities are close to the free-electron value and
indicate that there is little intermolecular spin–spin interaction
for 1 in the solid state. Measurements of the magnetic
susceptibility of solid 2, after a correction for its diamagnetism,
indicated a magnetic moment of 0 mB. The total lack of
paramagnetism in solid state 2 indicates complete spin pairing,
most likely through the formation of p-dimers, and is con-
sistent with the reversible dimerization observed in solution.
Spectroscopy
The 1H NMR spectra of Na+12 and 2+I2 are normal, while the
radicals 1 and 2 have no detectable 1H NMR signal (except for
that of the 2-H impurity in 2). The UV-vis spectra of Na+12,47
1,44 2+I2,52 and 2 49 all match previously reported spectra.
The ESR spectrum of 1 as a dilute solution in pyridine,
shown in Fig. 2, agrees with a spectrum simulated using
Electrochemistry
34
34,54,55
The electrochemistry of 1
and 2
have been investi-
gated previously, but we examined both again by cyclic voltam-
metry. The cyclic voltammogram of a saturated solution of
Na+12 in THF showed its fully reversible 10/2 couple at
20.65 V versus ferrocene+/0 (see ESI{). The cyclic voltammo-
gram of 2+I2 in THF indicates that its reversible 2+/0 couple
occurs at 21.16 V versus ferrocene+/0, followed by an irrever-
sible reduction at approximately 22.35 V (see ESI{). The
published hyperfine couplings.48 The g-value of
1 was
calculated to be 2.0085 (measured relative to external
TCNE2 at g = 2.0027853), larger than the reported value of
2.0037 for 1 in the solid state.45
An ESR spectrum of a dilute solution of 2 in benzene (Fig. 3,
top) consists of a series of lines spaced by 0.151 G (as
determined by a Fourier transform of the spectrum). Using
Fig. 3 ESR spectrum of a dilute solution of 2 in benzene (top) and a
Fig. 2 ESR spectrum of a dilute solution of 1 in pyridine.
472 | J. Mater. Chem., 2007, 17, 469–475
simulated spectrum (bottom).
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relative potentials of 10/2 and 2+/0 mean that the electron
transfer reaction of eqn 1 is favored by 0.51 eV (12 kcal mol21),
1 + 2 = 12 + 2+
(1)
so the equilibrium constant of the reaction is 5 6 108 (at 22 uC
in THF with 0.10 M [NBu4][PF6]). The previously reported
electrochemistry of 1 and 2 in benzonitrile34 revealed a similar
potential difference of 0.52 V between 10/2 and 2+/0, indicating
an equilibrium in benzonitrile that favors electron transfer to a
similar extent. While the energetics of electron transfer are
somewhat different in the solid state, the solution phase
electrochemical results are a strong indication that 2 as a solid
state dopant for 1 will be almost completely ionized to 2+.
The cyclic voltammogram of a one-to-one mixture of 1 and
2 (and a ferrocene standard) in THF shows the 10/2 and 2+/0
waves at their normal potential and an additional, larger,
reversible wave at 21.75 V (see Fig. 4). It has not been possible
to definitively identify the species responsible for the new
redox wave, but it may be the tightly associated ion pair 2+12.
Cocrystallization of 1 and 2
The single-crystal X-ray structure of 1 has been published.43
The powder X-ray diffraction pattern of microcrystalline 1,
obtained by crystallization from pyridine, matches well with a
pattern simulated using the single-crystal diffraction data (see
Fig. 5). The small variations in peak intensities between the
two patterns are probably due to the preferred orientation of
the crystallites in the powder diffraction sample. Cocrystals of
a 9 : 1 mixture of 1 and 2 were obtained from pyridine, and
their powder diffraction pattern matches reasonably well with
that of pure 1. No other crystalline phases are evident. Our aim
in the cocrystallization of 1 and 2 was to obtain substitutional
cocrystals that are isomorphous with crystals of pure 1, and
the powder diffraction patterns are consistent with that result.
We were unable to obtain highly crystalline samples of a 1 : 1
mixture of 1 and 2, which we would expect to be the salt 122+.
The difficulty in obtaining crystalline 122+ may be due to the
large solubility difference between 1 and 2 (2 is much more
soluble than 1 in every solvent), combined with the reversibility
of the charge transfer between 1 and 2. However, the low
conductivity of the 1 : 1 precipitate of 1 and 2 (see below) is
Fig. 5 Simulated powder X-ray diffraction pattern of
1 (top),
experimental powder X-ray diffraction of 1 (middle), and powder
X-ray diffraction of 1 cocrystallized with 10% 2 (bottom).
most consistent with it being mainly 122+, since phases
containing 1 and a small amount of 2 and phases containing
2 and a small amount of 1 would both be more conductive, as
in the doped materials described below.
Solid state electrical conductivity of 1 doped with 2
(i) Room-temperature conductivity. Cocrystals of 1 and 2
Fig. 4 Cyclic voltammogram of a 1 : 1 : 1 molar mixture of 1 and 2
and ferrocene in THF.
were obtained by cocrystallization from pyridine. Solid state
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At molar concentrations of 2% and 5% Na+, the conductivity
Table 1 Room-temperature electrical conductivity of 1, 2, and
cocrystals of 1 and 2.
of doped 1 is only 5 6 1028 and 6 6 1028 V21cm21
,
respectively (see Fig. 6). The Na+12 apparently forms tightly
bound ions pairs in the solid state that do not easily transfer an
electron to 1.
% 1
% 2
Conductivity/V21cm21
100
99
98
97
95
90
50
0
0
1
2
3
5
10
50
100
1.5 6 1029
9.5 6 1028
6.2 6 1027
7.7 6 1027
1.2 6 1026
9.1 6 1027
4 6 10211
5 6 10212
The mobility (or lack of mobility) of 2+ in 1 was examined
by measuring the conductivity of doped samples as a function
of time. If the 2+ was mobile there would be an ionic
component to the conductivity that would decay over time,
and the movement of 2+ might also affect the electrical
conductivity of doped 1. The conductivity of pressed pellets of
1 doped with 2%, 5%, and 10% 2 was measured in a two point
configuration, with the applied potential switched from +4 V
to 24 V at intervals as short as 1 s, and no change in the
current was evident at any time scale. Additionally, a constant
bias of 4 V was applied for several hours, and no decay in the
current occurred (see ESI{).
conductivity measurements of pressed pellets of microcrystal-
line samples were performed in a nitrogen-filled glovebox.
The conductivity data for 1, 2, and cocrystals with a range of
compositions are given in Table 1. For the cocrystals of 1
doped with 1%, 2%, 3%, 5%, and 10% 2, each entry in Table 1
represents the average of four measurements: two samples
from each of two separate cocrystallizations at a given dopant
concentration. Those results are shown graphically in Fig. 6.
Although there is a fairly large uncertainty in the measured
(ii) Variable-temperature conductivity. The conductivity of
pressed pellets of 1 doped with 2% 2 and 1 doped with 5% 2
were measured as a function of temperature from 300 K to the
lowest temperature at which the resistance was still measurable
with our instrument, which was 200 K for the 2% doped
sample and 170 K for the 5% doped sample. For the 5%
doped sample, the measurements were repeated as the
temperature was raised from 170 K to 300 K, and no hysteresis
was observed between the cooling and warming cycles. The
data are shown in Fig. 7. For both samples, electrical
conduction is an activated process with an activation energy
of 0.32 eV, which is comparable to that observed in other
neutral radical conductors.56 The fact that the activation
energy is the same in both samples indicates that the activated
process is the same in both materials. The activation energy
probably represents the energy required to overcome the
Coulombic attraction to move an electron from a 12 anion
neighboring a 2+ cation to a 1 radical without a cationic
neighbor. Alternatively, the activated process could be the
simple hopping of an electron from 12 to a neighboring 1, or
the measured activation energy may be due to a combination
of these two processes.21
conductivities, there is clearly
a monotonic increase in
conductivity from pure 1 through 1 doped with 1%, 2%,
3%, and 5% 2, as expected if 2 is an n-dopant in 1. The
measured conductivity for the 1% doping level appears to be
anomalously low, possibly because some of the 2 was oxidized
to 2+ by an impurity present in 1 at less than 1% concentration.
At the 10% doping level, the conductivity is lower than at 5%
doping. It is likely that at the high doping level of 10% the
dopants are interacting with each other. At an even higher
concentration of 2, 50%, where the material is presumably
mainly 2+12, the conductivity is significantly lower than in
pure, undoped 1. Pure 2 has the lowest conductivity of all the
materials. The increasing conductivity with increasing con-
centration of 2 in 1, in addition to showing that 2 is a dopant
for 1, is a strong indication that cocrystallization of 1 and 2 has
occurred, since neither 2 nor 2+12 as a physical mixture with 1
would increase the conductivity of 1.
For comparison, two samples of sodium-doped 1 were
prepared by cocrystallization of 1 and Na+12 from pyridine.
Fig. 6 Room-temperature electrical conductivity of 1 doped with
varying concentrations of 2. At each concentration, identical symbols
indicate samples from the same cocrystallization.
Fig. 7 Pressed pellet conductivity of 1 doped with 2% (#) and 5%
(%) 2 as a function of temperature.
474 | J. Mater. Chem., 2007, 17, 469–475
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17 S.-G. Chen, H. M. Branz, S. S. Eaton, P. C. Taylor, R. A. Cormier
and B. A. Gregg, J. Phys. Chem. B, 2004, 108, 17329.
18 B. A. Gregg, J. Phys. Chem. B, 2004, 108, 17285.
19 B. A. Gregg, S.-G. Chen and R. A. Cormier, Chem. Mater., 2004,
16, 4586.
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84, 1707.
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28 T. P. Vaid, A. K. Lytton-Jean and B. C. Barnes, Chem. Mater.,
2003, 15, 4292.
29 J. A. Cissell, T. P. Vaid and A. L. Rheingold, J. Am. Chem. Soc.,
2005, 127, 12212.
30 X. Deng, W. W. Porter, III and T. P. Vaid, Polyhedron, 2005, 24,
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Conclusions
Our goal in this study was to create a dopant for a molecular
semiconductor that fulfilled two criteria: (1) the dopant would
substitutionally cocrystallize with the semiconductor to yield
immobile dopant ions in the doped semiconductor, and (2) the
dopant would spontaneously transfer an electron to the mole-
cular semiconductor in the solid state. The electrochemical
redox potentials of 1 and 2, along with their similar overall
molecular size and shape, led us to believe that they would
fulfil the roles as an appropriate molecular semiconductor and
dopant, respectively. Evidence for substitutional cocrystalliza-
tion was found in the similar powder X-ray diffraction
patterns of pure 1 and 1 cocrystallized with 2. Additional
evidence for substitutional cocrystallization comes from the
increased electrical conductivity of 1 doped with 2 compared to
pure 1, as both 2 and 2+12 have significantly lower conduc-
tivity than pure 1. And, of course, the increased conductivity in
doped 1 compared to pure 1 is evidence that criterion (2) has
been fulfilled. There was no indication of ionic conductivity in
doped 1, confirming that the 2+ dopants are immobile.
Systems such as the one presented herein will lead to new
organic semiconductor devices that utilize true p–n junctions.
31 W. W. Porter, III and T. P. Vaid, J. Org. Chem., 2005, 70, 5028.
32 W. W. Porter, III, T. P. Vaid and A. L. Rheingold, J. Am. Chem.
Soc., 2005, 127, 16559.
33 J. A. Cissell, T. P. Vaid and A. L. Rheingold, Inorg. Chem., 2006,
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Acknowledgements
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We thank David Sloop for assistance in acquiring ESR spectra
and Anup Gangopadhyay for assistance with the PPMS.
Funding was provided by the National Science Foundation
grant CHE 0133068.
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