Mechanistic study on the solution-phase n-doping of 1,3-dimethyl-2-aryl-2, 3-dihydro-1H-benzoimidazole derivatives

Article abstract of DOI:10.1021/ja403906d

The discovery of air-stable n-dopants for organic semiconductor materials has been hindered by the necessity of high-energy HOMOs and the air sensitivity of compounds that satisfy this requirement. One strategy for circumventing this problem is to utilize stable precursor molecules that form the active doping complex in situ during the doping process or in a postdeposition thermal- or photo-activation step. Some of us have reported on the use of 1H-benzimidazole (DMBI) and benzimidazolium (DMBI-I) salts as solution- and vacuum-processable n-type dopant precursors, respectively. It was initially suggested that DMBI dopants function as single-electron radical donors wherein the active doping species, the imidazoline radical, is generated in a postdeposition thermal annealing step. Herein we report the results of extensive mechanistic studies on DMBI-doped fullerenes, the results of which suggest a more complicated doping mechanism is operative. Specifically, a reaction between the dopant and host that begins with either hydride or hydrogen atom transfer and which ultimately leads to the formation of host radical anions is responsible for the doping effect. The results of this research will be useful for identifying applications of current organic n-doping technology and will drive the design of next-generation n-type dopants that are air stable and capable of doping low-electron-affinity host materials in organic devices.

Full text of DOI:10.1021/ja403906d

Article
pubs.acs.org/JACS
Mechanistic Study on the Solution-Phase n‑Doping of 1,3-Dimethyl-
2-aryl-2,3-dihydro‑1H‑benzoimidazole Derivatives
Benjamin D. Naab,† Song Guo,‡ Selina Olthof,§ Eric G. B. Evans,∥ Peng Wei,† Glenn L. Millhauser,*,∥
Antoine Kahn,*,§ Stephen Barlow,*,⊥ Seth R. Marder,*,⊥ and Zhenan Bao*,†
†Departments of Chemical Engineering and Chemistry, Stanford University, 359 N-S Axis Stauffer III, Stanford, California 94303,
United States
‡Department of Chemistry and Biochemistry, The University of Southern Mississippi, Hattiesburg, Mississippi 39406, United States
§Department of Electrical Engineering, Princeton University, B420 Engineering Quadrangle, Princeton, New Jersey 08544, United
States
∥Department of Chemistry and Biochemistry, University of CaliforniaSanta Cruz, 1156 High Street, Santa Cruz, California 95064,
United States
⊥School of Chemistry and Biochemistry and Center for Organic Photonics and Electronics, Georgia Institute of Technology, 901
Atlantic Drive, Atlanta, Georgia 30332, United States
S
* Supporting Information
ABSTRACT: The discovery of air-stable n-dopants for organic
semiconductor materials has been hindered by the necessity of
high-energy HOMOs and the air sensitivity of compounds that
satisfy this requirement. One strategy for circumventing this
problem is to utilize stable precursor molecules that form the
active doping complex in situ during the doping process or in a
postdeposition thermal- or photo-activation step. Some of us
have reported on the use of 1H-benzimidazole (DMBI) and
benzimidazolium (DMBI-I) salts as solution- and vacuum-
processable n-type dopant precursors, respectively. It was initially suggested that DMBI dopants function as single-electron
radical donors wherein the active doping species, the imidazoline radical, is generated in a postdeposition thermal annealing step.
Herein we report the results of extensive mechanistic studies on DMBI-doped fullerenes, the results of which suggest a more
complicated doping mechanism is operative. Specifically, a reaction between the dopant and host that begins with either hydride
or hydrogen atom transfer and which ultimately leads to the formation of host radical anions is responsible for the doping effect.
The results of this research will be useful for identifying applications of current organic n-doping technology and will drive the
design of next-generation n-type dopants that are air stable and capable of doping low-electron-affinity host materials in organic
devices.
transfer from the dopant to the host material with a chemical
reaction. The Leo group reported several precursor molecular
n-dopants that were air-stable in a series of papers;1,7,8 these
include the use of organic salts, which are assumed to
decompose on heating to liberate the corresponding highly
reducing organic radicals, and the use of hydride-reduced forms
of organic cations, such as leuco crystal violet (LCV). Another
recent approach to air-stable dopants involves coupling of
electron transfer to the cleavage of dimeric organometallic
sandwich compounds; these react with organic electron
acceptors, including relatively weak acceptors such as 6,13-
bis(triisopropylsilylethynyl)pentacene, to form monomeric
cations and acceptor radical anions.9,10 Bao et al. has developed
n-dopants based on reduced 1,3-dimethyl-2-phenyl-2,3-dihy-
dro-1H-benzo[d]imidazoles (DMBI; Figure 1) and oxidized
INTRODUCTION
■
The controllable doping of inorganic semiconductors has been
critical to the development of modern electronics. Similarly, it
is envisioned that the development of efficient doping methods
for organic materials will aid in the improvement of existing
organic electronic devices and in the design of new organic-
based electronic devices.1 A particular challenge in developing
n-dopants is that typically to dope electron-transport materials
of interest by direct one-electron transfer, dopants must have
low ionization energies, which leads to instability in air. The
earlier reported n-type dopants for organic semiconductors
were reducing metals such as Li, Na, and Cs.2,3 However, the
applicability of these dopants is limited by their high reactivity
and diffusivity and by the requirement that they be vacuum-
deposited. Molecular dopants such as cobaltocenes4,5 and
6
Ru(terpy)2 are anticipated to be more stable with respect to
diffusion, but do not avoid the problem of air sensitivity. One
Received: April 19, 2013
approach to circumvent this problem is to couple the electron
Published: September 6, 2013
© 2013 American Chemical Society
15018
dx.doi.org/10.1021/ja403906d | J. Am. Chem. Soc. 2013, 135, 15018−15025

Journal of the American Chemical Society
Article
functions as a single-electron donor (Figure 1). However, the
mechanism was not studied. Furthermore, it was unclear
whether the doping reaction began in solution or after the film
was deposited and what effect a solution-phase reaction might
have on the doping efficacy. To better understand the doping
process of DMBI derivatives, the doping products with
PC61BM were characterized by electron spin resonance, ESR,
and UV−vis−near-IR spectroscopy. Here, we report that the
rate law, kinetic isotope effect, and linear free-energy relation-
ships together most strongly support the conclusion that the
dopant functions as a hydride atom donor to the host material;
this hydride transfer is then followed by an electron-transfer
step between host molecules. Additionally, the energy-level
alignment was probed by ultraviolet and inverse photoelectron
spectroscopy (UPS and IPES, respectively) to better under-
stand the energetics of the doping process and to confirm the
n-doping by the shift of the Fermi level, EF, toward the LUMO.
These mechanistic results are then discussed in the context of
the doping of organic semiconductors. Importantly, the
mechanism of DMBI dopants is distinct from that of electron
transfer dopants, and the consequences of this distinction
should be considered when employing DMBI or related
hydride donor dopants, especially when the goal is to compare
DMBI dopants to other n-dopants where electron transfer is
not coupled to C−H bond cleavage. Furthermore, because the
doping reaction for DMBI dopants is coupled to a hydride or
hydrogen atom transfer, the energetics of the doping process
cannot be solely determined by the alignment of the DMBI
imidazoline SOMO and the host LUMO. Rather, the feasibility
of doping depends on the free energy of electron transfer from
DMBI• to the host and on the difference in the strengths of the
DMBI−H bond and the C−H bond formed on hydride or
hydrogen-radical reduction of the acceptor. The results of this
study will clarify the ambiguity regarding the doping
mechanism of DMBI dopants and help define the scope and
limitations of current organic n-doping technology based upon
organic materials in which electron transfer is coupled to
cleavage of a C−H bond.
Figure 1. Structure of DMBI dopants and PC61BM. The naming
system for the dopants is such that the phenyl substituent position and
identity precede DMBI for all compounds where R3 = H (e.g., p-
MeO−DMBI where R1 = H and R2 = MeO). The names of
compounds for which R3 = D are prefixed by “d-“ (e.g., for R3 = D, R1
= H, R2 = NMe2 the name is d-NMe2-DMBI). In the particular cases
of p-NMe2-DMBI and d-p-NMe2-DMBI, their names have been
further abbreviated to N-DMBI and d-N-DMBI respectively.
benzimidazolium (DMBI-I) salts as solution- and vacuum-
processable n-type dopants, respectively.11,12
C60 films n-doped
using DMBI-I exhibited conductivities as high as 5 S/cm. More
recently, the tuning of the workfunction of graphene was
accomplished by the careful addition of DMBI dopants.13 In
order to design new and improved dopants in a rational
manner, a detailed understanding of the doping kinetics and
more generally the mechanism of the doping reaction would be
valuable; however, with the exception of the organometallic
dimers mentioned above,14 there has been very little study of
the mechanism of solution-phase doping reactions. Further-
more, the hypothesized n-doping mechanism of DMBI dopants
has been discussed in research reports following the initial
communication,15 and comparative studies between DMBI
dopants and electron transfer n-dopants have been reported.16
However, there is still a lack of understanding of the doping
mechanism. Although reduced heterocycles have been studied
extensively as hydride donors, the goal of the present study is to
shed light on the mechanism by which free carriers are
generated by organic hydride donors when doped into
semiconducting hosts such as PC61BM (Figure 1).
RESULTS
■
In the initial report on n-doping PC61BM with 4-(1,3-
dimethyl-2,3-dihydro-1H-benzoimidazol-2-yl)phenyl)-
dimethylamine (p-NMe2-DMBI, shortened to N-DMBI) it was
speculated that N-DMBI undergoes a C−H bond homolysis in
an annealing step and that the resulting imidazoline radical
Mechanisms under Consideration. Several mechanisms
have been reported in the literature for reductions by organic
hydrides; these are shown in Figure 2 for the case of DMBI and
PC61BM.17,18 We have performed experiments designed to help
elucidate which mechanism(s) is(are) operative in generating
Figure 2. Four reaction mechanisms for the n-doping reaction of PC61BM with DMBI derivatives. In each case the hydrogen reduced side product
PCBHx will be a radical, as indicated, if x is odd, but a closed-shell species, when x is even.
15019
dx.doi.org/10.1021/ja403906d | J. Am. Chem. Soc. 2013, 135, 15018−15025

Journal of the American Chemical Society
Article
anionic species when DMBI dopants react with organic
semiconductors. To emphasize the −H group of DMBI
dopants, we use DMBI-H here to represent DMBI in the
reduced state, DMBI+ to represent the benzimidazolium form,
and DMBI• to represent the neutral imidazoline radical.
PC61BM and other fullerenes are known to undergo exergonic
hydrogenation reactions with a wide variety of hydrogen-atom
donors, and C60 is known to accept multiple hydrogen
atoms.19,20 Thus, PC61BM hydrides, PC61BMH and higher
order open- or closed-shell hydrides PC61BMHx, are likely
products of the doping reaction; indeed, analogous derivatives
of C60 have been detected using IR spectroscopy after
photoinduced doping with LCV.7 For the doping reaction of
PC61BM, DMBI+ and PC61BM•− are also reasonable products,
as radical anions have been found in similarly n-doped thin
films.8 All four reaction mechanisms proposed may yield the
same final products, and evidence for their formation is
presented in the following section. Another possibility is that a
hydride is transferred to the carbonyl group of PC61BM rather
than to the fullerene cage; however, as noted below, we have no
evidence for the formation of the aldehyde expected to be
formed on subsequent loss of methoxide from the ester−
hydride adduct. Moreover, DMBI derivatives do not react with
simple esters, such as benzyl acetate and ethyl acetate, and are
capable of doping C60, where no ester group is present. Table 1
outlines the distinguishing characteristics of the four reaction
mechanisms for the solution reaction of DMBI and PC61BM,
based on the assumption that the first step in each case is rate-
determining. Some of the expected results if the second steps of
these mechanisms were rate determining are noted in the Table
1 footnotes. The assumption of a rate-determining first step is
supported by the mechanistic data presented below and is
consistent with general observations reported in the literature.
A thorough discussion of the mechanisms and data justifying
the assumption of a rate-determining first step will be presented
below.
Reaction Products. The doping products were charac-
terized by ESR and UV−vis−near-IR spectroscopy and by mass
spectrometry. Thin films of PC61BM and C60 were deposited
on the walls of 3 mm quartz ESR tubes by evaporation of
deoxygenated toluene solutions under vacuum at 70 °C;
solutions were prepared under a nitrogen atmosphere (O2 <0.3
ppm). Undoped C60 and PC61BM films and solutions exhibited
no appreciable radical signal by ESR spectroscopy and no UV−
vis−near-IR absorptions attributable to radical anions. How-
ever, codeposition of C60 with o-MeO-DMBI revealed a strong
1
signal consistent with previous reports for the S = /2 C60
radical anion. The observed signal for doped C60 consisted of
two superimposed signals one at 298 K at a g value of 1.999
with a peak-to-peak line width of 32.3 G and the other at a g
value of 2.001 with a peak-to-peak line width of 4.1 G; various
explanations for the double signals have been discussed in the
literature.21,22 In the present case, the ratio of the two signals
could be controlled by varying the mole percent of the dopant,
which strongly suggests that the signals originate from distinct
chemical environments that arise at low and high doping
concentrations, respectively (Supporting Information, Figure
S1). Similarly, the doping of PC61BM revealed an isotropic
radical with a g value of 1.998 (Figure 3). Double integration of
Table 1. Comparison of Proposed Mechanisms Shown in
Figure 2 for the n-Doping Reaction of PC61BM with N-
DMBI, Assuming in Each Case That the First Step Is Rate
Determining
influence of
solvent
rate-
determining
mech step (RDS)
isotope
effect (MH
vs MD)
dielectric
a
b
obsd rate law
(ε)
LFER
I
electron
transfer
d[DMBI-H]/dt =
−k[DMBI-H]
[PC61BM]
weak
potentially
strong
σ, σ+
σ, σ+
σ•
II
H− transfer
d[DMBI-H]/dt =
−k[DMBI-H]
[PC61BM]
primary
primary
primary
potentially
strong
III
IV
H• transfer
d[DMBI-H]/dt =
−k[DMBI-H]
[PC61BM]
unknown
weak
single
molecule
dissociation
d[DMBIH]/dt =
−k[DMBI-H]
σ•
Figure 3. X-band ESR spectra of undoped and o-MeO-DMBI doped
PC61BM.
a
b
ε is the solvent dielectric constant. Linear free-energy relationship
parameters: σ, widely used Hammett parameter; σ+, resonance
parameter gauging stabilization of positive charge; σ•, a parameter
gauging effects on radical stability. In all cases, a rate-determining
second step will lead to a more complex rate law and different isotope
effects. Use of the steady-state approximation leads to the following
rate laws and isotope effects. Mechanism I: rate = k1k2[DMBI-
H][PCBM]2/(k−1[PCBM•−]); primary isotope effect (where k1 and k2
are rate constants for the first and second steps and k−1 is the rate
constant for the reverse of the first step). Mechanism II: rate =
k1k2[DMBI-H][PCBM]2/(k−1[DMBI+]); isotope effect reduced to a
minor equilibrium effect on k1/k−1. Mechanism III: rate = k1k2[DMBI-
H][PCBM]2/(k−1[PCBMH•]); isotope effect as for II. In mechanism
IV, the steady-state approximation cannot be readily applied, since
both DMBI• and the H• radical should be treated as highly reactive
intermediates.
constant C60 mass samples with varying dopant concentrations
were used to estimate the spin concentration by comparison
with solutions of the stable nitroxyl radical (2,2,6,6-
tetramethylpiperidin-1-yl)oxyl (TEMPO) in toluene. At
moderate concentrations a linear relationship between the
spin density and doping concentration was revealed (Support-
ing Information, Figures S2 and S3). It is important to be aware
that the measured spin concentration is not necessarily
equivalent to the radical product yield because of possible
spin-pairing, broadening, or quenching effects.23 However, the
observation of a linear relationship between the dopant and
spin concentrations indicates the formation of the latter may be
attributed to the former, and the substantial amount of spin
present implies that the fullerene radicals are not minor
15020
dx.doi.org/10.1021/ja403906d | J. Am. Chem. Soc. 2013, 135, 15018−15025

Journal of the American Chemical Society
Article
byproducts of the doping reaction. This conclusion was further
substantiated by the UV−vis−near-IR spectra of doped
solutions of PC61BM.
observed C:N ratio differs from a perfect 1:1 salt possibly due
to the presence of excess PC61BM or chlorobenzene trapped in
the precipitate.
Negative ion MALDI-TOF spectra of the concentrated
filtrate left from the precipitation reaction between N-DMBI
and PC61BM show a strong peak at m/z 910.1053, again
attributed to PC61BM. However, minor deviations in the
isotopic envelope for this peak from that calculated for PC61BM
and, more significantly, differences in the isotopic envelopes
between samples formed using N-DMBI and its deuterated
analogue d-N-DMBI may be due to the presence of PCBMHx/
PCBMDx species (Supporting Information, Figures S6−S11).
Overlap with the PC61BM isotopic envelope and the possibility
of hydrogen-transfer reactions in the ionization process,
however, preclude specific identification of the value(s) of x
in this case. In addition, the mass spectra do not show alcohol
or aldehyde products resulting from hydride reduction of the
ester.
To summarize the product analysis data, radical anions of
fullerenes were identified in the doping products in solution by
UV−vis−near-IR and ESR spectroscopy, and in thin films by
ESR spectroscopy. Quantitative ESR showed that the radicals
are a major product and that the density of radicals is directly
proportional to the dopant density at moderate to low doping
concentrations. Mass spectra of the insoluble precipitate of the
doping reaction contained both PC61BM and N-DMBI ions,
and the C:N ratio as determined by C,H,N elemental analysis
provides evidence for a near 1:1 stoichiometry for these two
components. Evidence for PC61BM hydrides may be found by
comparing the observed isotope ratio for the concentrated
filtrate remaining after the doping reaction to the expected
isotope distribution for PC61BM. Similarly, the Leo group has
positively identified fullerene C−H bond stretches in the IR
spectra of LCV-doped thin films of C60.7 The above product
characterizations served as the starting point for identifying the
reaction mechanism(s) by which DMBI dopes fullerenes to
produce charge carriers.
Reaction Kinetics. Having already established the products
of the doping reaction that lead ultimately to the generation of
charge carriers in DMBI doped fullerene thin films, a thorough
mechanistic understanding of this process was sought. The
growth of the PC61BM•− absorbance band around 1030 nm in
the solution reaction of DMBI derivatives and PC61BM was
used to monitor the rate of the reaction, d[PC61BM•−]/dt. The
initial reaction rates, (d[PC61BM•−]/dt)t=0, of mixtures of N-
DMBI and PC61BM at various reactant concentrations were
determined, and the results are presented in Table 2. It is clear
that when doubling the concentration of either reactant, the
Figure 4. UV−vis−near-IR spectra of N-DMBI, PC61BM, and N-
DMBI/PC61BM at 298 K. Conditions: [N-DMBI] = 4.8 × 10−4 M,
[PCBM] = 2.2 × 10−4 M, the mixture [N-DMBI] = 2.4 × 10−4 M and
[PCBM] = 5.5 × 10−4 M.
The black and red lines in Figure 4 are the UV−vis−near-IR
spectra of N-DMBI and PC61BM in chlorobenzene, respec-
tively. Both reactants have no significant absorption between
700 and 1200 nm. After N-DMBI and PC61BM were mixed in
chlorobenzene, a new band with a maximum absorption at
1030 nm began to form. The spectrum of the mixed solution
after reaction overnight at room temperature is shown as the
dotted blue spectrum of Figure 4. The band shape of the blue
spectrum is consistent with the previously reported absorption
spectra of C60 and PC61BM•− in solution.24 The identity of
•−
the 1030 nm band was further confirmed by its similarity to
that seen when PC61BM was treated with [Ru(Cp*)(TEB)]2
(Cp* = pentamethylcyclopentadienyl; TEB = 1,3,5-triethyl-
benzene) in chlorobenzene; [Ru(Cp*)(TEB)]2 is a strong
electron donor that cleanly reduces acceptors such as fullerenes
without the formation of side products (Supporting Informa-
tion, Figure S5).9,14
At higher concentrations of DMBI dopants and PC61BM or
C60 in toluene black insoluble precipitates form; these were
shown to contain fullerene radical anions by ESR spectroscopy
(Supporting Information, Figure S4). The composition of the
precipitates that formed between N-DMBI and PC61BM or C60
was investigated by DART-TOF-MS and C,H,N elemental
analysis. DART-TOF-MS identified a peak consistent with
PC61BM in the negative-ion mode, and DMBI cations were
identified in positive-ion ESI spectra. However, it should be
stated that an m/z ratio corresponding to a particular species is
not unambiguous evidence for the presence of that species in
the doped film, since the species may arise by the ionization or
decomposition of another species. For instance, the observed
DMBI+ ion may in principle arise from DMBI-H, although this
would suggest that DMBI coprecipitated with PC61BM without
undergoing a reaction, which seems unlikely, given the UV−
vis−near-IR and ESR results. Elemental analysis of the
precipitates gave a C,N ratio consistent with a near 1:1 mixture
of dopant cation to PC61BM, further supporting the assignment
of these ions. (Anal. Calcd: C, 90.72; H, 2.99; N, 3.57; C:N
ratio, 25.4. Found: C, 86.97; H, 2.73; N, 2.84; C:N, 30.62.) The
Table 2. Initial Reaction Rates for the Mixture Solution of N-
DMBI and PC61BM in Chlorobenzene
a
b
[N-DMBI] (M) [PC61BM] (M) initial rate (au)
normalized rate
2.40 × 10‑3
1.20 × 10‑3
2.40 × 10‑3
1.20 × 10‑3
1.20 × 10‑3
5.50 × 10−4
5.50 × 10−4
2.75 × 10−4
2.75 × 10−4
1.38 × 10−4
8.01 × 10−4
3.72 × 10−4
3.86 × 10−4
1.79 × 10−4
9.64 × 10−5
8.31
3.86
4.00
1.86
1.00
a
Initial rate obtained by plotting the evolution of the intensity of the
fullerene radical anion absorption with time in the first few percent of
b
the reaction. The normalized rate is the rate relative to that of the
slowest reaction.
15021
dx.doi.org/10.1021/ja403906d | J. Am. Chem. Soc. 2013, 135, 15018−15025

Journal of the American Chemical Society
Article
initial reaction rate doubles. This is consistent with the
following rate expression:
−d[PC61BM]/dt = k[DMBI‐H][PC61BM]
This expression is further supported by fitting of the
temporal evolution of the PC61BM•− absorption, which is
good when PC61BM is in excess. This rate expression suggests a
reaction mechanism in which the first step of the reaction
between N-DMBI and PC61BM is bimolecular, an observation
that is consistent with mechanisms I−III, where the first step in
each case is rate determining. This result is inconsistent with
mechanism IV or with the more complicated rate laws that are
predicted when the second step is rate determining (Table 1).
In mechanisms I−III, the first step involves transfer of an
electron, hydride anion, and hydrogen radical, respectively. As
discussed earlier, a primary kinetic isotope effect (KIE) would
be expected for the first step of mechanisms II and III but not
for that of mechanism I. The synthesis of deuterated N-DMBI
(d-N-DMBI) is discussed in detail in the Supporting
Information. The reaction between PC61BM and d-N-DMBI
indeed exhibited a strong kinetic isotope effect (kH/kD) of 8.6
at room temperature, implying that C−H bond cleavage and/or
formation takes place during the rate-determining step. This
result is inconsistent with mechanism I and suggests that DMBI
dopants react first via donation of either a hydride or hydrogen
atom. Furthermore, the observed primary KIE justifies the
assumption of a rate-determining first step for mechanisms II−
IV.
Figure 5. Hammett plots for the reaction between PC61BM and para-
substituted DMBI derivatives in chlorobenzene.
electron/proton net H• transfer in which the electron is largely
transferred in the transition state, but the proton is not, would
not lead to development of a large positive charge on C-2 in the
transition state and only a weak dependence of the rate on the
identity of the para substituent (i.e., a small negative value of ρ)
would be expected. In the reverse case, where proton transfer is
followed by electron transfer, a negative charge would be
developed on C-2 and a positive value of ρ would be expected.
Only a H• transfer approximated as asynchronous hydride
transfer/reverse electron transfer could potentially give the
observed LFER, and this seems unlikely, given the relatively
poor electron-donating and -accepting abilities expected for
PCBMH− and DMBI+, respectively.
A series of para-substituted DMBI compounds were
synthesized and reacted with PC61BM in chlorobenzene at 85
°C (Table 3). The rate constants of the reaction between the
If the rate-determining step involves formation of ionic
intermediates, a strong dependence of the reaction rate on the
solvent polarity would be expected, although presumably polar
radical-transfer reactions of the type mentioned above could
also exhibit polarity dependence. Unfortunately, few solvents
are capable of solvating both starting materials and the DMBI+/
PC61BM•− product, thereby limiting our ability to study solvent
effects for this reaction. The rate constants in chlorobenzene
and toluene were found to be similar; however, both of these
solvents have rather low polarities (ε = 5.62 and 2.38,
respectively). Consequently, our results do not distinguish
between hydride and hydrogen-atom transfer. However, the
implications of our results are the same regardless; namely, that
acceptor hydrogenation is important for the action of DMBI
and related dopants, and thus the thermodynamics and kinetics
of the doping process cannot be understood by considering
only the dopant ionization energy and host electron affinity, as
is the case for dopants that operate through simple one-electron
transfer.
Arrhenius and Eyring plots based on variable-temperature
reaction constants are displayed in Figure 6. The parameters
obtained from the Arrhenius and Eyring plots are given in
Table S1 (Supporting Information). Significantly, the negative
value of ΔS⧧ (−92 J mol−1 K−1 for N-DMBI/PC61BM in
chlorobenzene) is consistent with an associative bimolecular
reaction. The free energy for direct electron transfer from N-
DMBI to PC61BM, as in mechanism I, was estimated to be 1.18
eV at 300 K from electrochemical data (Supporting
Information, Table S2 and Figures S12−S16), meaning that
ΔG⧧ for such a reaction should be somewhat larger. However,
the value of ΔG⧧ extracted from the variable-temperature rate
data is 0.63 eV, significantly lower than the free energy for
Table 3. LFER Values Obtained from the Literature25,28 and
Plotted against the Reaction Constant Ratios for Various p-
X-N-DMBI Compounds for the Doping Reaction with
PC61BM in Chlorobenzene (ClBz) at 85 °C
log(kX/kH)
X group
σ
excess PC61BM in ClBz
excess DMBI in ClBz
H
0.00
0.78
0.00
−1.53
0.30
0.00
−2.10
0.43
NO2
MeO
F
−0.27
0.06
−0.30
−0.40
0.81
−0.25
−0.32
0.87
Cl
0.23
NMe2
CN
CH3
CF3
−0.83
0.66
−1.00
0.21
−0.99
0.37
−0.17
0.54
−0.89
−0.78
various substituted DMBI derivatives and PC61BM correlate
strongly with both Hammett σ and σ+ parameters25 but do not
strongly correlate with any radical σ scales (Figure 5).26 The ρ
value for the σ scale was −1.41, which indicates that in the
transition state of the reaction significant positive charge
develops on the C-2 of the imidazoline ring. The magnitude of
the observed ρ value is comparable to those reported for other
hydride-transfer processes.27 The lack of a correlation with any
radical σ scale does not completely rule out mechanism II, as
these scales only correlate with nonpolar homolysis reactions.
Some radical hydrogen abstractions correlate well with polar
LFER scales as a consequence of the polar nature of the
transition state when proton and electron transfers are
asynchronous.26 In the present case an asynchronous
15022
dx.doi.org/10.1021/ja403906d | J. Am. Chem. Soc. 2013, 135, 15018−15025

Journal of the American Chemical Society
Article
of EF upon doping and its ultimate position in the gap of the
semiconductor. The ionization energy of o-MeO-DMBI was
also measured on Au and was found to be 4.26 eV, consistent
with its air stability (Supporting Information, Figure S18).
DISCUSSION
■
For a chemical transformation with competing reaction
mechanisms, different mechanisms might dominate under
specific reaction conditions. For example, different mechanisms
are observed for the reaction between TIPS-pentacene and
1,2,3,4,5-pentamethylrhodocene dimer when the reactant
concentration ratio is changed from <0.1 to >10.14 The initial
reaction rates for various p-X-DMBI and PC61BM were
measured at two extreme concentration ratios, with either
reactant in large excess (Table 2). No evidence for such a
change in the mechanism was found for the reaction between p-
X-DMBI and PC61BM. This result also shows that the
hydrogenated fullerene products are not interfering with the
interpretation of the initial reaction rates.
Fullerenes are capable of accepting as many as six electrons,
which may complicate the interpretation of kinetic data. To
control the possibility of multiple reductions, PC61BM was
employed in large excess such that the anion product can only
be PC61BM•− (any PC61BM2− would comproportionate with
remaining PC61BM to yield PC61BM•−). Additionally, the same
absorbance features are present when PC61BM is in large excess
and when the dopant is in large excess, further confirming that
the same radical anion product is formed, although different
hydrogen-reduced fullerenes may be formed. Furthermore, the
absorption spectra in this study compared well with known
spectra for fullerene radical anions but not with that of fullerene
dianions.24
It is possible that the DMBI dopants operate via a different
mechanism depending on the host material used. For instance,
Cheng and co-workers oxidized DMBI-H with Fe3+ and
confirmed the formation of DMBI-H•+ by an ESR and
deuterium labeling study.17 Thus, electron transfer may precede
C−H bond homolysis when strongly electron-accepting hosts
(oxidants) that have low affinities for hydride or hydrogen
atoms are employed. Electron transfer was found to precede
C−H bond homolysis for the reaction between the related LCV
dopant and the strong electron acceptor 2,3-dichloro-5,6-
dicyano-1,4-benzoquinone (DDQ).30 These results demon-
strate that the doping mechanism may depend on the dopant/
host combination, and it is unlikely that a single mechanism will
account for all dopant/host combinations.
Figure 6. Arrhenius and Eyring plots of reaction constants at variable
temperatures for the reaction of N-DMBI and PC61BM in
chlorobenzene.
electron transfer, which is inconsistent with the reaction
proceeding by mechanism I.
Doping Energetics. To confirm that charge carriers are
generated in thin films by the reaction between PC61BM and
DMBI derivatives, UPS and IPES spectra (Supporting
Information, Figure S17) were recorded for PC61BM spin-
coated on ITO, undoped or doped with 1 wt % o-MeO-DMBI.
The ionization energy (IE), electron affinity (EA), and single-
particle gap (IE−EA) for PC61BM were found to be 6.08, 3.7,
and 2.4 eV, respectively, in good agreement with previously
reported values.29 Upon doping PC61BM with 1 wt % of o-
MeO-DMBI, the line shapes of the spectra did not substantially
change except for a shift toward higher binding energy. This is
equivalent to an upward movement of EF in the gap, indicative
of n-doping, and the generation of free electrons. This
conclusion is further substantiated by the observed conductivity
increase that arises when PC61BM is doped by DMBI
derivatives. The results are summarized in Figure 7. Most
The dependence of the reaction rate on both the host and
dopant concentrations has important implications for the utility
of DMBI dopants, because this implies that the imidazoline
radical does not independently form. The relatively high
ionization energy of o-MeO-DMBI makes direct electron
transfer unfavorable to many common organic semiconductors,
and a drastic variation in the ionization energy of the DMBI
dopants reported herein is unlikely. For some acceptor
materials, such as fullerenes, a reaction is coupled to the
electron transfer event, thereby enabling the n-doping to
proceed. In such cases the feasibility of doping depends on the
free energy of electron transfer from DMBI• to the host and on
the difference in the strengths of the DMBI-H and the
hydrogenated acceptor C−H bonds. However, with other
materials, such as TIPS-pentacene, it is evident from solution
and thin film absorption spectra that the dopant is not actually
transferring an electron to the host material, possibly because
Figure 7. Energy level alignment of undoped and 1 wt % o-MeO-
DMBI doped PC61BM.
relevant to the current study, the energy difference between the
bottom edge of the LUMO and EF, measured with the
combination of UPS and IPES, decreases from 0.72 to 0.21 eV,
which corresponds to a roughly 500 meV shift of EF. It should
be noted that the initial EF position in the undoped film is not
particularly significant, as it depends on the nature and work
function of the substrate; the significance lies in the movement
15023
dx.doi.org/10.1021/ja403906d | J. Am. Chem. Soc. 2013, 135, 15018−15025

Journal of the American Chemical Society
Article
tetramethylsilane (TMS, 0.00 ppm) internal standard. Electrospray
ionization mass spectrometry (ESI-MS) and direct analysis in real time
mass spectrometry (DART-TOF-MS) were performed at the
University of Florida mass spectrometry facilities. Elemental analysis
was performed by Robertson Microlit.
the barrier to break the C−H bond is too high to initiate the n-
doping reaction and/or TIPS-pentacene is not a sufficiently
strong hydride acceptor; instead, it is proposed that only trap
sites with higher electron affinities are filled. Notably, trap filling
may be advantageous for OTFT applications where a
conductive film is not desired and where trap filling by a
dopant has been shown to enhance mobilities and permit
control over the threshold voltage.31−34 This attribute of DMBI
dopants was exploited in another work, where it was found that
TIPS-pentacene can be doped with o-MeO-DMBI to obtain
high-mobility n-channel transistors without increasing the bulk
conductivity, as is commonly observed when doping organic
transistors.35
Electron Spin Resonance (ESR) Detection Experiments. The
ESR measurements were performed on an X-band Bruker EMX
operating at 9.44 GHz with a Bruker SHQ cavity with variable
temperature control. The spectra were recorded under nonsaturating
microwave power conditions. The magnitude of modulation was
chosen to optimize the resolution and the signal-to-noise ratio of the
observed spectra. The g value was calibrated against an external 2,2-
diphenyl-1-picrylhydrazyl (DPPH) standard. Spin quantification
experiments were performed on thin films of doped C60 prepared by
evaporation of toluene solutions on the walls of the ESR tubes. Double
integrals of the spectra were compared to those of external (2,2,6,6-
tetramethylpiperidin-1-yl)oxidanyl (TEMPO) standards in toluene.
Ultraviolet−Visible−/Near-Infrared (UV−Vis−Near-IR) Spec-
troscopy Measurements. The samples for UV−vis−near-IR
measurements were prepared in a glovebox and then transferred
into polytetrafluoroethylene (PTFE) stopcock sealed quartz cuvettes
(175−2700 nm) with path lengths of 1 mm. The cuvettes were then
taken to a Varian Cary 5E ultraviolet−visible−near-infrared
spectrometer to monitor the reaction progress. A Quantum Northwest
TC 125 temperature controller was used to control the temperature of
the sample cuvette holder.
Ultraviolet and Inverse Photoelectron Spectroscopy (UPS
and IPES). For the photoelectron spectroscopy measurements the
spin-cast samples were transferred into an ultrahigh-vacuum system.
Samples containing the n-dopant were transferred via a transport
system without air exposure, while the more stable films were exposed
to air for a few seconds during the loading process. For the UPS
measurements, either the He I line (21.22 eV) or the He II line (40.8
eV) from a discharge lamp were used with an experimental resolution
of 150 meV. The IPES measurements were performed in the same
setup in the isochromat mode at an energy resolution of 400 meV
using a setup described elsewhere.37
Preparation of α-Deuteriodimethylaminobenzaldehyde. A
small dry argon-flushed vial was charged with deuterated dimethylfor-
mamide (260 μL, 3.3 mmol) and then cooled to 0 °C. To this was
slowly added phosphoryl chloride (150 μL, 1.65 mmol) dropwise, after
which the solution was warmed to room temperature. Dimethylaniline
(210 μL, 1.65 mmol) was then added, and the solution was heated to
90 °C for 2 h with stirring under argon. The reaction mixture was then
diluted with ice and neutralized to pH of 6−8 with 10 wt % aqueous
sodium acetate solution. The solids that formed were collected by
vacuum filtration and washed with water until the color cleared. The
crude product was stored under argon at −20 °C and used without
further purification.
General Procedure for Preparation of Benzimidazoles. A 20
mL vial was charged with N,N′-dimethyl-o-phenylenediamine (136
mg, 1.00 mmol) in methanol (2 mL); to this was added the
appropriate aromatic aldehyde (1.00 mmol). One drop of glacial acetic
acid was then added, and the solution was sonicated at room
temperature until precipitation was observed. The crude product was
then collected by vacuum filtration and recrystallized from methanol/
H2O to give the corresponding pure 1,3-dimethyl-2-arylbenzimida-
zoles.17
CONCLUSION
■
DMBI dopants have been shown to effectively n-dope
fullerenes, carbon nanotubes, and graphene to give highly
conductive electron-transport materials. They have also been
used to improve the air stability of n-channel OTFTs, to switch
the transport characteristics of p-channel materials in OTFT
configurations, and have been employed in electron-transport
layers for inverted solar cells.36 The utility that has already been
demonstrated for DMBI dopants provides the motivation to
study the fundamental reaction mechanism by which they
generate charge carriers; the goal of this research was to
illuminate this mechanism and to spur further developments in
the field of air-stable molecular n-dopants. It was found that
PC61BM and C60 spontaneously react with DMBI dopants to
form fullerene radical anions, stable organic cations, and
hydrogen-reduced fullerenes. The doping of the PC61BM films
was confirmed by UPS and IPES spectroscopy. Initial rates of
the reaction of substituted and deuterium-labeled DMBI
dopants were compared at various concentrations of the
dopant and host to determine the reaction order and nature of
the rate-determining step. The most probable reaction
mechanism involves a bimolecular hydride transfer from the
dopant to PC61BM in the rate-determining step, although a
hydrogen-atom transfer with polar character cannot be
definitively ruled out. Electron transfer to form the detected
PC61BM radical anions must succeed the hydrogen/hydride
atom transfer event, and most simply would occur via electron
transfer between the hydride-reduced PC61BM and unreduced
PC61BM. It is important to recognize that the mechanism of
DMBI dopants is distinct from that of electron-transfer
dopants, and this distinction should be considered when
employing DMBI or related hydride-donor dopants. Most
importantly, the efficacy of doping is inadequately described by
the offset between the ionization energy of the DMBI-H donor
or DMBI• radical and the electron affinity of the host material,
because DMBI-H dopants act first as hydride/hydrogen-atom
donors. However, our results do not exclude the possibility of
different mechanisms in other host materials.
ASSOCIATED CONTENT
■
EXPERIMENTAL SECTION
■
S
* Supporting Information
General Methods. Spectrophotometric grade (99.9%) chloroben-
zene was purchased from Alfa Aesar, dried over calcium hydride,
distilled, deoxygenated by three cycles of freeze−pump−thaw, and
stored in a nitrogen-filled glovebox. Toluene was passed through an
alumina column, refluxed over phosphorus pentoxide, distilled, and
deoxygenated by the freeze−pump−thaw method prior to use. 1H and
13C NMR spectra were recorded in deuterated chloroform or dimethyl
sulfoxide on Varian 500, 400, or 300 MHz NMR spectrometers. The
chemical shifts (δ) are reported in parts per million (ppm) relative to a
Text, figures, and tables giving a complete characterization
summary for all synthesized materials (13C and H NMR,
1
C,H,N elemental analysis, high-resolution mass spectra),
quantitative electron spin resonance ESR spectra, mass spectra
for doping products, UV−vis−near-IR comparison to [Ru-
(Cp*)(TEB)]2, electrochemical data, Arrenhius and Eyring
parameters, UPS and IPES data for PC61BM, and o-MeO-
1
DMBI, and H and 13C NMR spectra for the synthesized
15024
dx.doi.org/10.1021/ja403906d | J. Am. Chem. Soc. 2013, 135, 15018−15025

Journal of the American Chemical Society
Article
(17) Zhu, X.-Q.; Zhang, M.-T.; Yu, A.; Wang, C.-H.; Cheng, J.-P. J.
Am. Chem. Soc. 2008, 130, 2501−16.
compounds. This material is available free of charge via the
Internet at http://pubs.acs.org.
(18) Fukuzumi, S.; Nishizawa, N.; Tanaka, T. J. Org. Chem. 1984, 49,
3571−3578.
AUTHOR INFORMATION
■
(19) Henderson, C. C.; Cahill, P. A. Science 1993, 2790, 1−3.
(20) Jin, C.; Hettich, R.; Compton, R.; Joyce, D.; Blencoe, J.; Burch,
T. J. Phys. Chem. 1994, 98, 4215−4217.
(21) Watanabeb, T. Synth. Met. 1999, 103, 2374−2375.
(22) Hwang, Y. L.; Yang, C. C.; Hwang, K. C. J. Phys. Chem. A 1997,
101, 7971−7976.
(23) Chen, S.-G.; Branz, H. M.; Eaton, S. S.; Taylor, P. C.; Cormier,
R. A.; Gregg, B. A. J. Phys. Chem. B 2004, 108, 17329−17336.
(24) Konarev, D. V.; Drichko, N. V.; Graja, A. J. Chem. Phys. 1998,
95, 2143−2156.
(25) C., H.; Leo, A.; R.W., T. Chem. Rev. 1991, 91, 165−195.
(26) Heberger, K. J. Phys. Org. Chem. 1994, 7, 244−250.
(27) Zhu, X.-Q.; Li, X.-T.; Han, S.-H.; Mei, L.-R. J. Org. Chem. 2012,
77, 4774−83.
(28) Dust, J. M.; Arnold, D. R. J. Am. Chem. Soc. 1983, 105, 1221−
1227.
(29) Akaike, K.; Kanai, K.; Yoshida, H.; Tsutsumi, J.; Nishi, T.; Sato,
N.; Ouchi, Y.; Seki, K. J. Appl. Phys. 2008, 104, 023710.
(30) Yamamoto, S.; Sakurai, T.; Yingjin, L.; Sueishi, Y. Phys. Chem.
Chem. Phys. 1999, 1, 833−837.
(31) Kim, J. H.; Yun, S. W.; An, B.-K.; Han, Y. D.; Yoon, S.-J.; Joo, J.;
Park, S. Y. Adv. Mater. 2013, 25, 719−24.
(32) Abe, Y.; Hasegawa, T.; Takahashi, Y.; Yamada, T.; Tokura, Y.
Appl. Phys. Lett. 2005, 87, 153506.
(33) Soeda, J.; Hirose, Y.; Yamagishi, M.; Nakao, A.; Uemura, T.;
Nakayama, K.; Uno, M.; Nakazawa, Y.; Takimiya, K.; Takeya, J. Adv.
Mater. 2011, 23, 3309−3314.
(34) Olthof, S.; Singh, S.; Mohapatra, S. K.; Barlow, S.; Marder, S. R.;
Kippelen, B.; Kahn, A. Appl. Phys. Lett. 2012, 101, 253303.
(35) Naab, B. D.; Himmelberger, S.; Diao, T.; Vandewal, K; Wei, P.;
Lussem, B.; Salleo, A.; Bao, Z. Adv. Mater. 2013, 25, 4663−4667.
(36) Cho, N.; Yip, H.-L.; Davies, J. A.; Kazarinoff, P. D.; Zeigler, D.
F.; Durban, M. M.; Segawa, Y.; O’Malley, K. M.; Luscombe, C. K.; Jen,
A. K.-Y. Adv. Energy Mater. 2011, 1, 1148−1153.
(37) Wu, C. I.; Hirose, Y.; Sirringhaus, H.; Kahn, A. Chem. Phys. Lett.
1997, 272, 43−47.
Corresponding Authors
glennm@ucsc.edu.
kahn@princeton.edu.
seth.marder@chemistry.gatech.edu.
zbao@stanford.edu.
stephen.barlow@chemistry.gatech.edu.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
B.D.N. gratefully acknowledges the National Defense Science
and Engineering Graduate Research Fellowship and the
National Science Foundation for a Graduate Research Fellow-
ship. This research was also funded by the National Science
Foundation Materials Network Program (NSF-DMR-1209468)
and Air Force Office of Scientific Research (FA9550-12-1-
0190). The work at Georgia Tech was supported by the
National Science Foundation through the Science and
Technology Center Program (DMR-0120967) and by the
Office of Naval Research (N00014-11-1-0313). The work at
Princeton University was supported by the National Science
Foundation (DMR-1005892). The work at the University of
California−Santa Cruz was supported by the National Institutes
of Health (NIH grant GM065790). We thank Dr. Swagat
Mohapatra for synthesis of the [Ru(Cp*)(TEB)]2 used.
REFERENCES
■
43.
(1) Lussem, B.; Riede, M.; Leo, K. Phys. Status Solidi A 2013, 210, 9−
̈
(2) Schneider, W. G.; Helfrich, W. Phys. Rev. Lett. 1965, 14, 229−
232.
(3) Kido, J.; Matsumoto, T. Appl. Phys. Lett. 1998, 73, 18−21.
(4) Chan, C. K.; Kahn, A.; Zhang, Q.; Barlow, S.; Marder, S. R. J.
Appl. Phys. 2007, 102, 014906.
(5) Chan, C. K.; Zhao, W.; Barlow, S.; Marder, S.; Kahn, A. Org.
Electron. 2008, 9, 575−581.
(6) Walzer, K.; Maennig, B.; Pfeiffer, M.; Leo, K. Chem. Rev. 2007,
107, 1233−71.
(7) Li, F.; Werner, A.; Pfeiffer, M.; Leo, K.; Liu, X. J. Phys. Chem. B
2004, 108, 17076−17082.
(8) Li, F.; Pfeiffer, M.; Werner, A.; Harada, K.; Leo, K.; Hayashi, N.;
Seki, K.; Liu, X.; Dang, X.-D. J. Appl. Phys. 2006, 100, 023716.
(9) Qi, Y.; Mohapatra, S. K.; Bok Kim, S.; Barlow, S.; Marder, S. R.;
Kahn, A. Appl. Phys. Lett. 2012, 100, 083305.
(10) Guo, S.; Kim, S. B.; Mohapatra, S. K.; Qi, Y.; Sajoto, T.; Kahn,
A.; Marder, S. R.; Barlow, S. Adv. Mater. 2012, 24, 699−703.
(11) Wei, P.; Oh, J. H.; Dong, G.; Bao, Z. J. Am. Chem. Soc. 2010,
132, 8852−3.
(12) Wei, P.; Menke, T.; Naab, B. D.; Leo, K.; Riede, M.; Bao, Z. J.
Am. Chem. Soc. 2012, 134, 3999−4002.
(13) Peng Wei; Liu, N.; Lee, H. R.; Adijanto, E.; Ci, L.; Naab, B. D.;
Zhong, J. Q.; Park, J.; Chen, W.; Cui, Y.; Bao, Z. Nano Lett. 2013, 13,
1890−1897.
(14) Guo, S.; Mohapatra, S. K.; Romanov, A.; Timofeeva, T. V;
́
Hardcastle, K. I.; Yesudas, K.; Risko, C.; Bredas, J.-L.; Marder, S. R.;
Barlow, S. Chem. Eur. J. 2012, 18, 14760−72.
(15) Lu, M.; Nicolai, H. T.; Wetzelaer, G.-J. a. H.; Blom, P. W. M.
Appl. Phys. Lett. 2011, 99, 173302.
(16) Menke, T.; Wei, P.; Ray, D.; Kleemann, H.; Naab, B. D.; Bao,
Z.; Leo, K.; Riede, M. Org. Electron. 2012, 13, 3319−3325.
15025
dx.doi.org/10.1021/ja403906d | J. Am. Chem. Soc. 2013, 135, 15018−15025