Tunable Low-LUMO Boron-Doped Polycyclic Aromatic Hydrocarbons by General One-Pot C-H Borylations

Article abstract of DOI:10.1021/jacs.9b04675

Boron-doping has long been recognized as a promising LUMO energy-lowering modification of graphene and related polycyclic aromatic hydrocarbons (PAHs). Unfortunately, synthetic difficulties have been a significant bottleneck for the understanding, optimiz

Full text of DOI:10.1021/jacs.9b04675

Article
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Cite This: J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Tunable Low-LUMO Boron-Doped Polycyclic Aromatic
Hydrocarbons by General One-Pot C−H Borylations
†,§
Jeffrey M. Farrell,†,§ Carina Mutzel, ‡ David Bialas,‡ Maximilian Rudolf,† Kaan Menekse,‡
̈
‡
,†,‡
̈
Ana-Maria Krause, Matthias Stolte, and Frank Wurthner*
†Institut fu
̈
r Organische Chemie, Universitat Wurzburg, Am Hubland, 97074 Wurzburg, Germany
̈
̈
̈
‡
̈
̈
̈
Center for Nanosystems Chemistry, Universitat Wurzburg, Theodor-Boveri-Weg, 97074 Wurzburg, Germany
S
* Supporting Information
ABSTRACT: Boron-doping has long been recognized as a
promising LUMO energy-lowering modification of graphene and
related polycyclic aromatic hydrocarbons (PAHs). Unfortu-
nately, synthetic difficulties have been a significant bottleneck for
the understanding, optimization, and application of precisely
boron-doped PAHs for optoelectronic purposes. Herein, a facile
one-pot hydroboration electrophilic borylation cascade/dehy-
drogenation approach from simple alkene precursors is coupled
with postsynthetic B-substitution to give access to ten ambient-
stable core- and periphery-tuned boron-doped PAHs. These include large hitherto unknown doubly boron-doped analogues of
anthanthrene and triangulene. Crystallographic, optical, electrochemical, and computational studies were performed to clarify
the effect of boron-doped PAH shape, size, and structure on optoelectronic properties. Our molecular tuning allowed the
synthesis of molecules exhibiting visible-range absorption, near-unity fluorescence quantum yields, and, to our knowledge, the
most facile electrochemical reductions of any reported ambient-stable boron-doped PAHs (corresponding to LUMO energy
levels as low as fullerenes). Finally, our study describes the first implementation of a precise three-coordinate boron-substituted
PAH as an acceptor material in organic solar cells with power conversion efficiencies (PCEs) of up to 3%.
extensively used acceptors in organic electronics and photo-
voltaics (e.g., fullerenes or rylene diimides).9
INTRODUCTION
■
Graphene1 and related “nanographene”2 substructures have
captivated chemists and physicists with their unique structural,
(opto-)electronic, and magnetic properties. Concurrently,
intense research has surrounded the incorporation of boron
into these and related polyaromatic hydrocarbon (PAH)
scaffolds.3 The substitution of sp2-hybridized carbon for
neutral, three-coordinate boron renders these scaffolds
electron-deficient owing to the empty p orbital of boron.
This “doping” allows manipulation of the electronic, photo-
physical, and magnetic properties of polyaromatic systems
without compromising their planar geometries. Such boron
doping has been championed as a way forward for the band
gap manipulation of graphene3d and for the development of n-
type organic semiconductors.3a,b,4
We have recently reported the synthesis of an ambient-stable
doubly boron-doped perylene 4a (Table 1)10 using the
chemistry of cationic three-coordinate boron compounds
(borenium ions).11 Our newly devised one-pot reaction
sequence entailed N-heterocyclic carbene (NHC) borenium
hydroboration, ring-closing dehydrogenative electrophilic
borylation, TEMPO-mediated dehydrogenation and hydrol-
ysis.12 This facile synthesis from a simple, readily accessible
alkene precursor contrasted starkly with traditionally difficult
boron-doped PAH syntheses. We anticipated that, if applicable
to other alkenes, this approach would allow the synthesis and
study of boron-doped polybenzenoid systems of varying sizes,
boron substitution patterns and periphery substitutions
(Figure 1). Herein, we have used this method to synthesize
a series of boron-containing PAHs including large and
unprecedented doubly boron-doped anthanthrenes and
triangulenes. Furthermore, we have succeeded in subsequent
postsynthetic substitutions of boron-bound hydroxyl groups
for mesityl groups. We have studied the effects of structural
changes on photophysical properties, electrochemical proper-
ties, and solid-state packing through experimental and
computational means. Through structural tuning, we report
Despite great promise, significant obstacles exist for boron-
doped PAHs as a class of materials. Namely, difficult syntheses
have impeded their study. This is especially true of boron-
doped polybenzenoid π-scaffolds bearing only boron and
carbon5 for which very few general syntheses are known.5j−l,6
These represent substructures and potential building blocks of
boron-doped graphene.7 The synthetic challenges of boron-
doped polybenzenoid PAHs have been detrimental to the
diversity of available structures and have hampered the focused
tuning of properties. Although some examples have been used
in organic electronics,5h,n,o,8 this class of molecules has yet to
match the appealing properties and low LUMO levels of more
Received: May 1, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/jacs.9b04675
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c
Table 1. One-Pot Syntheses of Polyaromatic Borinic Acids
Figure 1. Approach herein for the synthesis of boron-doped PAHs by
(A) one-pot hydroboration/electrophilic borylation cascade/dehy-
drogenation, and (B) mesityl substitution at the boron centers.
RESULTS AND DISCUSSION
■
Synthesis of Compounds 1−10. All alkene precursors
used in our study were synthesized by straightforward
Mizoroki−Heck reactions (see Supporting Information).
Conveniently, our one-pot reaction conditions could be used
largely unchanged10 for the synthesis of polyaromatic borinic
acids 1−3, 5 and 6 from their respective alkene precursors
(Table 1). Each of these precursors was added to a solution of
an NHC-borenium salt, which was generated in situ by the
combination of HNTf2 and an air- and moisture-stable NHC-
borane adduct13 at room temperature in chlorobenzene. Each
reaction was then stirred at 110 °C for 4−5 h to effect
hydroboration and electrophilic borylation reactions. For
electron-deficient fluorinated substrate 4b this step required
a longer reaction time at higher temperature in o-dichloro-
benzene. For each synthesis, dehydrogenation was effected by
reaction with a TEMPO radical solution at 80 °C for 24−36 h.
Hydrolytic workup allowed the isolation of the desired
polycyclic borinic acid products by silica gel chromatography
(1−3, 5−6) or acidic aqueous extraction (4b). 1-Boraphena-
lene 1 (49%),14 1,6-diborapyrene 2 (39%), 1,8-diborapyrene 3
(31%), 3,9-diboraperylene 4b (12%), 1,7-diboraanthanthrene
5 (22%), and 2,8-diboratriangulene 6 (15%) (Table 1) were
characterized by multinuclear NMR spectroscopy and HRMS
(see Supporting Information). Compounds 1−6 show no signs
of degradation when stored as solids under ambient conditions
for months. The larger compounds 5 and 6, however, show
1
signs of degradation by H NMR spectroscopy in aerobic
solutions within days to weeks (especially upon exposure to
ambient light). Although all-carbon triangulene is a notoriously
unstable ground state diradical,15 the core of compound 6 is
isoelectronic with a triangulene dication and is therefore
reasonably stable.
a
We envisioned that postsynthetic substitution of the B-
hydroxyl moiety would offer even further flexibility to our
synthetic approach. We therefore probed borinic acids 2, 5, 6,
and previously reported 4a for substitution of B-hydroxyl
groups with commonly encountered B-mesityl groups. These
borinic acids were each reacted with BBr3 at room temperature
for 26 h, concentrated in vacuo, and subsequently reacted with
mesityl magnesium bromide for 16−20 h in toluene to give
compounds 7−10 (21−29%) after workup and column
chromatography (Scheme 1). Compounds 7−10 were
characterized by multinuclear NMR spectroscopy and HRMS
(see Supporting Information). Compounds 7−10 are all
ambient stable and compounds 9 and 10 show improved
stability compared to their precursors 5 and 6. Compounds 7−
10 also show markedly increased solubility in halogenated
solvents compared to their respective borinic acid precursors.
The success of this synthetic method implies that our borinic
acids can be used as synthetic equivalents of boron-doped, all-
Performed in o-C6H4Cl2 with step 2 carried out at 160 °C for 24 h
b
and hydrolytic workup by acidic aqueous extraction. Step 2 carried
out for 4 h and step 3 carried out for 24 h. Isolated yields. Unless
otherwise indicated, reactions performed in C6H5Cl with step 2
carried out at 110 °C for 5 h, step 3 carried out at 80 °C for 36 h, and
hydrolytic workup by silica gel chromatography.
c
ambient-stable boron-doped PAHs with fluorescence quantum
yields as high as 95% and facile electrochemical reductions
representative of LUMO energies comparable to fullerenes.
Most importantly, our lowest-LUMO energy boron-doped
PAH could be exploited in organic thin-film transistors
(OTFTs) with n-type charge carrier mobility and in inverted
bulk-heterojunction (BHJ) solar cells with power conversion
efficiencies of up to 3%.
B
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Scheme 1. Synthesis of B-Mesityl Boron-Doped PAHs
Figure 3. Solid-state structure (a) and packing (b) of 8. C: black, B:
yellow-green. H atoms omitted for clarity.
bond angles about boron of 360° (Figures 2 and 3). Hydrogen
bonding is observed between the borinic acid protons of 2, 4b,
and 5 and cocrystallized coordinating solvents rather than
solvent coordination to boron. This characteristic is shared by
borinic acid 4a.10
Accessible π-surfaces are observed in the solid-state
structures of 2, 4b, and 5. These compounds form continuous
π-stacks (Figure 2b,e,h). The smallest of the series, 2, shows
interplanar distances of 3.2 Å but with severely slipped stacking
and little overlap of π-systems (Figure 2b,c). On the other
hand, the solid-state π-stacks of 4 exhibit columnar structures
with considerable overlap of π-systems and interplanar
distances of 3.4 Å (Figure 2e,f). The C−H for C−F
substitution of 4b has relatively little impact of the solid-
sp2-hybridized polybenzenoid frameworks (i.e., potential
precursors for the bottom-up construction of boron-doped
graphitic structures).
Crystallographic Studies. Crystals suitable for X-ray
crystallography could be obtained from DMSO solutions of
2 and 4b, a dioxane solution of 5, and a solution of 8 in 1:1
dichloromethane/hexanes. For all compounds investigated by
X-ray crystallography, structures confirm expected connectivity
and agree well with optimized structures obtained by DFT
calculations at the B3LYP/6-31++G** level of theory (see
Supporting Information). All of the examined π-systems are
planar and boron retains trigonal planar geometry with sums of
Figure 2. Solid-state structures of 2 (a), 4b (d), 5 (g), solid-state packing of 2 (b), 4b (e), 5 (h), and a top-down view of two stacked molecules of
2 (c), 4b (f), 5 (i). C: black, B: yellow-green, O: red, F: green, H: gray. Solvent molecules and selected H atoms omitted for clarity.
C
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g
Table 2. Optoelectronic Properties of Boron-Containing Polyaromatic Compounds 1−10 and PC61BM
λem
Stokes shift
lifetime τ1
first E1/2 red
second E1/2 red
compound
λabs [nm] (ε [M−1 cm−1])
374 (8800), 348 (9600)
425 (17 500), 383 (12 700)
466 (11 600), 358 (6800)
560 (23 700), 524 (18 600)
540 (26 800), 504 (19 300), 473 (8400)
525 (32 600), 491 (26 400), 433 (12 400)
547 (5900), 509 (8800), 479 (9300), 450 (7800),
377 (24 100)
467 (14 100), 411 (14 600)
[nm]
[cm−1
]
Φ
[ns]
[V]
[V]
a
d
1
489
561
586
601
563
548, 586
573, 619
6290
5700
4390
1220
760
0.27
0.44
0.51
0.63
0.95
0.35
0.42
6.4
7.1
5.9
5.4
6.2
3.2
6.0
−1.98
−
−1.84
−1.82
−1.64
−1.47
−1.70
a
a
d
d
2
3
−1.47
d
d
−1.46
a
10
,
d
d
4a
4b
−1.30
a
d
d
−1.13
b
d
d
5
b
800
830
−1.39
d
6
−1.53
−
b
e
e
7
635
668
622
633, 684
−
5660
1400
1310
1120
−
<0.01
0.74
0.28
0.34
−
4.2
5.6
2.6
2.9
−1.15
−1.62
−1.41
−1.51
b
e
e
8
611 (31 900), 417 (10 100)
575 (34 700), 541 (28 800), 455 (19 600)
591 (5100), 550 (7200), 434 (22 600), 404 (37 200)
−1.07
b
e
e
9
−1.17
b
e
10
−1.34
−
−1.55
c
9b
,
f
f
PC61BM
695, 492, 430, 328
−
−1.17
a
b
c
Optical measurements carried out at 298 K in CHCl3. Optical measurements carried out at 298 K in CH2Cl2. Optical measurements carried out
at 298 K in cyclohexane. Electrochemical measurements carried out at 298 K in 0.1 M n-Bu4NPF6 in DMSO. Electrochemical measurements
carried out at 298 K in 0.1 M n-Bu4NPF6 in CH2Cl2. Electrochemical measurements carried out at 298 K in n-Bu4NPF6 0.1 M o-dichlorobenzene.
See Supporting Information for all measurement details. Electrochemical reduction potentials were calibrated with ferrocene as an internal
d
e
f
g
standard and are referenced vs Fc+/0
.
state structure, which is remarkably similar to previously
reported compound 4a.10 For compound 5, which bears the
largest π-system, π-stacking is observed with larger interplanar
distances of 3.7 Å (Figure 2h,i). In this case, periphery phenyl
groups are positioned on top of neighboring diboraanthan-
threne cores and impede closer packing. Nevertheless,
accessible π-surfaces and solid-state π-stacking are not
commonly encountered for ambient-stable boron-doped
PAHs.5a,8a,b This molecular design possibility may be attractive
for organic electronic applications where charge carrier
transport is desired along closely stacked π-scaffolds.
Conversely, π-stacking is not observed for 8, which bears
sterically demanding boron-bound mesityl groups oriented
orthogonal to its π-system (Figure 3b). Clearly the steric
demands of this mesityl substitution prevents the close
approach of π-systems of neighboring molecules of 8.
Redox Properties. Suitably low LUMO energy levels are
crucial for application of small molecules as acceptor materials
in organic electronics. Electrochemical reduction potentials of
boron-doped polyaromatic structures, although more positive
than their undoped parent hydrocarbon structures,3a have not
yet been reported at levels corresponding to common acceptor
compounds such as PC61BM (E1/2 = −1.17 V vs Fc+/0, 0.1 M
n-Bu4NPF6 o-dichlorobenzene,9b ELUMO = −3.99 eV).16
Moreover, strategies to tune this property are not well
established given relatively few examples. In order to address
these issues, the electrochemical properties of compounds 1−
10 were studied by cyclic voltammetry in 0.1 M n-Bu4NPF6
DMSO solution (1−6) or in 0.1 M n-Bu4NPF6 dichloro-
methane solution (7−10) based on solubility. For singly
boron-doped compound 1, a single, reversible reduction could
be observed at −1.98 V (Table 2). This first reduction occurs
at the most negative potential of all compounds studied. This is
somewhat unsurprising given both its onefold (vs twofold)
boron doping and its small π-scaffold size for electron
delocalization.
readily reduced ambient-stable core boron-doped PAH yet
reported. Doubly boron-doped triangulenes 6 and 10 each
exhibited only one reversible reduction at −1.53 V and −1.34
V, respectively. Second reductions for 6 and 10 were
irreversible and occurred at −2.17 V and −2.09 V, respectively.
The C6F5-substituted borinic acid 4b exhibits a first reduction
at −1.13 V, which is 0.17 V more positive than its C6H5
analogue 4a. Mesityl compounds 7−10 also exhibit more
positive reduction potentials than their respective borinic acid
precursors 2, 4a, 5, and 6 by between 0.19 and 0.32 V.
Presumably, lone-pair donation from borinic acid oxygen
atoms to the empty p orbital of boron impedes reduction.
These examples show that both our pre- and postsynthetic
periphery substitutions are viable methods to predictably tune
the electron-accepting ability of boron-doped PAHs. The
changes to reduction potentials occur as anticipated, with the
substitution of more donating groups for more electron
withdrawing groups leading to more positive reduction
potentials.
It is worth noting that the periphery substitution of C6H5 for
C6F5 in 4b has a comparable effect on reduction potential
(ΔE1/2 = +0.17 V vs 4a) to the B−OH for B-mesityl
substitution of 8 (ΔE1/2 = +0.23 V vs 4a). Indeed, both 4b and
8 exhibit comparable or more facile reductions than PC61BM
(Figure 4) and both compounds are ambient-stable. However,
4b possesses accessible π-surfaces evidenced by solid state π-
stacking whereas 8 does not (Figure 2e, 3). These results
For doubly boron-doped borinic acids 2−5 and mesityl
derivatives 7−9, two reversible one-electron reductions could
be observed with first reductions occurring at much more
positive potentials between −1.07 V (8) and −1.47 V (2)
(Table 2). Diboraperylene 8 is, to our knowledge, the most
Figure 4. Cyclic voltammograms of (a) 4b (7 × 10−4 M in 0.1 M n-
Bu4NPF6 DMSO, 298 K) and (b) 8 (3 × 10−4 M in 0.1 M n-Bu4NPF6
CH2Cl2, 298 K). Dotted blue lines indicate the potential of the first
reduction of PC61BM in 0.1 M n-Bu4NPF6 o-dichlorobenzene.9b
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suggest that, even when the diboraperylene scaffold is rendered
electron-deficient, steric protection of boron is not a
prerequisite for ambient stability. Furthermore, π-stacking,
which is often useful for charge-carrier transport in n-type
semiconductor materials, is attainable while retaining low
LUMO energy levels and ambient stability. In addition, larger
variations in reduction potentials are observed within the series
of similarly substituted borinic acids of different shapes and
sizes (1−3, 5−6, 4a) than for B−OH vs B-mesityl substitution
(Table 2). This suggests that careful choice of the core boron-
doped π-structure is more important for reduction potentials
than the choice of hydroxyl vs mesityl substituents at boron.
With this in mind, investigation of the impact of the core
structure on electrochemical behavior is a critical point. Thus,
a comparison can be made between 2, 4a, and 5, which are
each similarly substituted. The core of each of the three
molecules differs in size from the smallest diborapyrene 2 to
diboraperylene 4a to the largest diboraanthanthrene 5.
Notably, the most readily reduced species is the intermediate
case, diboraperylene 4a. Clearly, the electrochemical reduction
potentials of boron-doped PAHs are not solely dominated by
either π-scaffold size (for charge delocalization) or the ratio of
boron to carbon within the π-scaffold (for a greater proportion
of formally empty p orbitals). Importantly, this warns against
the arbitrary pursuit of large or excessively boron-doped
structures as targets for electron acceptor small molecules.
Notwithstanding, our calculations of the adiabatic electron
affinities17 of 2, 4a, and 5 at the B3LYP/6-31++G** level of
theory (1.84, 2.09, and 2.01 eV, respectively) correctly
correspond to the order of experimentally obtained first
reduction potentials for the series. Such calculations may
therefore provide insight into the most promising future targets
for boron-doped polyaromatic core structures in terms of
electron-accepting capability.
Compounds 6 and 10 present a striking example of π-
scaffold shape as a critical factor for the electrochemical
behavior of boron-doped PAHs. Diboratriangulenes 6 and 10
offer a π-core isoelectronic with the dication of triangulene,
their isostructural all-carbon parent compound. Therefore, the
two-electron reduction products of 6 and 10 should possess
dianionic cores that are both isostructural and isoelectronic
with triangulene (for which a closed-shell Lewis structure
cannot be formulated). Notably, 6 and 10 were the only
compounds studied where the second measured electro-
chemical reductions were irreversible. Considering the
notorious instability of the parent triangulene hydrocarbon
(which eluded isolation until 2017),15 our observations of
irreversible second reductions suggest the formation of
Figure 5. UV−vis absorption (solid lines: 1−4 10−5 M in CHCl3, 298
K; 5−10 10−6 to 10−5 M in CH2Cl2, 298 K) and fluorescence (dashed
lines: 1−4 10−5 M in CHCl3, 298 K; 5−10 10−7 to 10−4 M in CH2Cl2,
298 K) spectra of boron-containing PAHs.
coefficients for the absorption maxima of 1−10 range from
9600 M−1 cm−1 (1) to 37 200 M−1 cm−1 (10) (Table 2).
Compounds 1−10 exhibit fluorescence in solution with
quantum yields ranging from very poor to excellent; however,
most are moderate to good (Table 2). Emission maxima for
the series also span a broad range of wavelengths from a blue
emission for the smallest compound 1 (λem = 489 nm) to a
deep red emission for compound 8 (λem = 668 nm) that
extends into the NIR. Fluorescence lifetimes of 1−10 range
from 2.6 ns (9) to 7.1 ns (2). The longest wavelength
absorption and emission maxima of compounds 2−5 and 7−9
are all significantly red-shifted compared to their parent
hydrocarbons pyrene (λabs = 335 nm, λem = 385 nm),18
perylene (λabs = 436 nm, λem = 468 nm)19 and anthanthrene
(λabs = 433 nm, λem = 437 nm).20 Compounds 1, 2, 3, and 7
also emit in the solid state with λem = 530 nm (Φ = 0.10), λem
= 566 nm (Φ = 0.05), λem = 643 nm (Φ = 0.03), and λem = 670
nm (Φ = 0.03), respectively. These emissions, ranging from
green to red, are bathochromically shifted with respect to
solution-phase emissions of the same compounds. No
́
unstable non-Kekule diboratriangulene dianions at moderate
potentials (−2.17 V and −2.09 V vs Fc+/0).
Optical Properties. For many prospective applications of
boron-containing PAHs, such as in OLEDs or as molecular
sensors, absorption and emission characteristics are important.
In organic photovoltaic applications, acceptor materials with
visible range absorption are advantageous over fullerene-based
competition whose weak visible-range absorption is suboptimal
for solar-light harvesting.9a Compounds 1−10 were therefore
studied by UV−vis absorption and fluorescence spectroscopy
in dichloromethane or chloroform solution (Figure 5). The
lowest energy absorption maxima for the series of boron-doped
polyaromatics span from the UV for the smallest compound 1
(λabs = 374 nm, Figure 5a) to the long wavelength absorption
of compound 8 (λabs = 611 nm, Figure 5b). Extinction
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Figure 6. (a) Device configuration of BHJ solar cells investigated herein (ITO/ZnO (30 nm)/8:polymer (1:1)/MoO3 (10 nm)/Al (100 nm)). (b)
J−V characteristics of the inverted BHJ solar cells based on 8 in 1:1 mixing ratio with donor polymers PBDB-T (blue) or PCE10 (violet) under
1000 W/m2AM 1.5G illumination measured in inert conditions. (c) UV−vis absorption (solid line) and EQE (symbols) spectra of inverted BHJ
solar cells.
measurable solid-state luminescence was observed for 4−6 or
8−10.
properties are independently modifiable. The substituent
effects on the hypsochromic or bathochromic shifts of UV−
vis absorption spectra agree well with those predicted by TD-
DFT calculations (Figures S112−S114 and Table S15).
Organic Thin-Film Transistors and Solar Cells. The
pursuit of nonfullerene acceptors for organic photovoltaics is
an ongoing area of research that hopes to improve upon the
fullerenes’ low open-circuit voltages, poor solubility, challeng-
ing structural modifications and suboptimal light-harvesting
abilities.9a,d Although the inclusion of three-coordinate boron
into an otherwise all-carbon π-scaffold is a known LUMO
energy-lowering modification, well-defined boron-doped PAHs
demonstrating this principle have not yet been reported as
acceptor materials in organic photovoltaics.21,22 Nevertheless,
we were inspired to assess 8 in this role due to its good
solubility in organic solvents, strong visible-range absorption
into the NIR, and its low LUMO energy level comparable to
commonly used PC61BM.
To explore this possibility we first probed the charge carrier
transport properties of 8 by fabricating organic thin-film
transistors (OTFTs, see Supporting Information). Vacuum-
processed devices exhibit n-type charge carrier transport with
electron mobilities of up to 3 × 10−3 cm2 V−1 s−1 on Si/SiO2/
AlOx/TPA substrates measured under inert conditions
(Figures S115, S116, and Table S16). Although these values
are modest, they rank favorably among the few strictly three-
coordinate boron-doped PAHs reported as n-type semi-
conductors in OTFTs,5n,8a,c particularly for an amorphous
material (see AFM image: Figure S116).
After verifying the ability of 8 to mediate electron transport,
we tested 8 as a small-molecule acceptor in solution-processed
bulk-heterojunction (BHJ) solar cells with a simple inverted
device architecture (ITO/ZnO/8:polymer/MoO3/Al, Figure
6a). The active layer was processed by spin-casting 1:1 blends
of 8 and donor polymers PBDB-T or PCE10 (Figure S117) in
3 vol % 1,8-diiodooctane/chlorobenzene. To our delight, these
devices provided power conversion efficiencies (PCEs) of up
to 3% with appreciably high open circuit voltages (VOC) up to
0.94 V (Figure 6b,c and Table 3). Furthermore, the
wavelength-dependent external quantum efficiencies demon-
strate that both the donor polymer and the boron acceptor
contribute to the harvesting of solar light (Figure 6c, S119).
This proof of concept introduces precise three-coordinate
boron-doped PAHs as a new class of nonfullerene acceptors for
prospective use in organic solar cells.
The arrangement of boron atoms within the core impacts
photophysical properties. This is illustrated by the absorption
and emission maxima of 3, which are bathochromically shifted
with respect to those of 2 (Figure 5a). These isomers differ
only by the positioning of the B−OH moieties of the
diborapyrene core. In compound 3 the electron-rich alkene-
like residues are segregated to one side of the π-core opposite
to the electron-accepting boron centers. This modification
increases the HOMO energy level (see Supporting Informa-
tion for calculations) effectively reducing the optical bandgap
with respect to 2. TD-DFT calculations reproduce the
observed bathochromic shift of the UV−vis absorption
spectrum of 3 compared to that of 2 (Figure S112 and
Table S15). Notably, these two molecules possess nearly
identical first and second reduction potentials (and corre-
spondingly identical calculated LUMO energies). This suggests
that arrangement of boron within the π-core may be used to
tune the absorption and emission properties of a boron-
containing PAH without necessarily changing reduction
potentials. Periphery substitutions also significantly affect
photophysical properties. All examined mesityl-substituted
compounds (7−10) exhibit bathochromically shifted absorp-
tion and emission spectra compared to their parent borinic
acids 2, 4a, 5, and 6 (Figure 5b,c). This mesityl substitution
has relatively little impact on the fluorescence quantum yields
for these compounds except for 7 where it is severely
diminished (Φ < 0.01, Table 2) with respect to 2. On the
other hand, the C6F5 for C6H5 substitution of compound 4b
versus 4a leads to hypsochromically shifted absorption and
emission spectra (Figure 5b). Compound 4b exhibits a small
Stokes shift (760 cm−1) and a well-resolved vibronic
progression compared to most other compounds studied
(Figure 5b). This simple modification also leads to a marked
increase of the fluorescence quantum yield (Φ = 0.95)
concomitant with a slightly longer fluorescence lifetime (τ1 =
6.2 ns) than parent compound 4a (Φ = 0.63, τ1 = 5.4) (Table
2). These data suggest inhibition of nonradiative deactivation
by this C6F5 substitution. Comparing compounds 4b and 7−
10, the B-mesityl substitutions produced bathochromically
shifted absorption and emission spectra while the C6F5
substitution gave hypsochromically shifted spectra. Since
both of these periphery modifications facilitate electrochemical
reduction, it appears that photophysical and electrochemical
F
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Journal of the American Chemical Society
Article
results encourage the exploration of small molecules bearing
boron-doped polyaromatic carbon π-cores as new candidates
for nonfullerene acceptors in organic solar cells.
Table 3. Organic BHJ Solar Cell Performance of Boron-
Containing Polyaromatic Compound 8 in Combination
with Donor Polymers PBDB-T and PCE10 (1:1)
a
In conclusion, our facile, general C−H borylation synthesis
has allowed significant manipulation of boron-containing PAH
properties through structural tuning. This has provided both
insight on structure−property relationships and allowed
optimization of desired properties toward organic electronic
applications.
donor
b
polymer
VOC [V]
JSC [mA cm−2
]
FF [%]
PCE [%]
PBDB-T
PCE10
0.94 0.01
0.87 0.01
−3.26 0.07
−8.44 0.07
36
40
1
1
1.1 0.1 (1.2)
2.9 0.1 (3.1)
a
Statistical data taken from 6 independent devices under 1000 W m−2
AM1.5G illumination in the device architecture ITO/ZnO/BHJ/
MoO3/Al. Maximum PCEs in parentheses.
b
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.9b04675.
■
S
CONCLUSION
■
We have synthesized core- and periphery-tuned boron-
containing polybenzenoid PAHs via a facile and general C−
H borylation strategy. Moreover, we have successfully
converted so-formed borinic acids to B-mesityl substituted
compounds. This approach has allowed the synthesis of ten
boron-doped PAHs, including large and hitherto unknown
doubly boron-doped anthanthrenes and triangulenes, that are
stable under ambient conditions. X-ray crystallographic studies
have revealed π-scaffold accessibility of compounds 2, 4b, and
5 as evidenced by intimate π−π stacking.
Experimental details on synthetic procedures, NMR
spectroscopy, UV−vis and fluorescence spectroscopy,
cyclic voltammetry, X-ray crystallography and computa-
tions (PDF)
Crystal data for 9,10-bis(2,3,4,5,6-pentafluorostyryl)-
anthracene (CIF)
Crystal data for compound 2 (CIF)
Crystal data for compound 4b (CIF)
Crystal data for compound 5 (CIF)
Although often advertised as electron acceptors for
optoelectronic applications, the LUMO levels of boron-
doped PAHs have lagged somewhat behind more established
molecules such as PC61BM (E1/2= −1.17 V,9b ELUMO = −3.99
eV).16 We have used structural tuning in conjunction with
electrochemical studies to obtain four boron-doped PAHs (4b,
7, 8, and 9) with LUMO energy levels comparable to or lower
than that of PC61BM. Diboraperylene 8 is to our knowledge
the most readily reduced ambient stable boron-doped PAH yet
reported (E1/2 = −1.07 V, ELUMO = −4.08 eV).16 We have also
found that the core structures corresponding to the lowest
LUMO compounds did not correspond to the largest π-
surfaces or to core structures bearing the largest proportion
boron per carbon. These results suggest that ambitiously large
or excessively boron-doped targets may not necessarily lead to
desirable low LUMO levels. Rather, screening of core
structures experimentally or computationally and further
tuning properties with the more predictable electronic effects
of periphery substituents presents a more tactful approach. For
this reason, the facile access to different core structures
provided by our synthetic methodology is beneficial.
Compounds 1−10 are typified by visible range absorptions
which may suggest potential light-harvesting advantages over
weakly absorbing compounds (like PC61BM) in photovoltaic
applications.9a Furthermore, the ability to easily and
independently tune both visible-light absorption and electro-
chemical reduction potentials contrasts with the more fixed
properties of the fullerenes. Compounds 1−10 also show
emissions in the visible to NIR regions with quantum yields as
high as 95%. Both of these properties are appealing for
prospective boron-doped PAH OLED3a or sensing applica-
tions.23
Crystal data for compound 8 (CIF)
AUTHOR INFORMATION
Corresponding Author
*wuerthner@uni-wuerzburg.de
ORCID
■
Frank Wurthner: 0000-0001-7245-0471
̈
Author Contributions
§JMF and CM contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We thank the Alexander von Humboldt foundation for a
postdoctoral stipend for J.M.F. We thank Dr. Hagen Klauk and
̈
̈
Dr. Ute Zschieschang (MPI fur Festkorperforschung, Stutt-
gart) for providing us with TPA modified substrates.
■
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