Boron-nitrogen substituted dihydroindeno[1,2-: B] fluorene derivatives as acceptors in organic solar cells

Article abstract of DOI:10.1039/c9cc05103a

The electrophilic borylation of 2,5-diarylpyrazines results in the formation of boron-nitrogen doped dihydroindeno[1,2-b]fluorene which can be synthesized using standard Schlenk techniques and worked up and handled readily under atmospheric conditions. Th

Full text of DOI:10.1039/c9cc05103a

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Boron-Nitrogen Substituted Dihydroindeno[1,2-b]fluorene
Derivatives as Acceptors in Organic Solar Cells
Received 00th January 20xx,
Accepted 00th January 20xx
Matthew M. Morgan,a Maryam Nazari,a Thomas Pickl,a J. Mikko Rautiainen,b Heikki M. Tuononen,b
Warren E. Piers*,a Gregory C. Welch*,a and Benjamin S. Gelfand.a
DOI: 10.1039/x0xx00000x
The electrophilic borylation of 2,5-diarylpyrazines results in the conjugated molecules have seen the most promise. Most of
formation of boron-nitrogen doped dihydroindeno[1,2-b]fluorene the focus to date has been on well-studied BODIPY-based
which can be synthesized using standard Schlenk techniques and frameworks15 or boron containing subpthalocyanines,16-19 but
worked up and handled readily under atmospheric conditions. more recently boron-nitrogen bipyridine derivatives20, 21 have
Through transmetallation via diarylzinc reagents a series of demonstrated high device performances.
derivatives were sythesized which show broad visible to near-IR
To realize the potential of BN substituted frameworks as
light absorption profiles that highlight the versatility of this BN alternatives to fullerene acceptors there is a need for novel
substituted core for use in optoelectronic devices. The synthesis is cores that are easy to synthesize on reasonable scales via
efficient, scalable and allows for tuning through changes in methods that allow modular functionalization22 in order to
substituents on the planar heterocyclic core and at boron. tune frontier orbital energies to match available donor species.
Exploratory evaluation in organic solar cell devices as non-
B
N
fullerene acceptors gave power conversion efficiencies of 2%.
N
B
B
I
N
Heteroatom doped polycyclic aromatic hydrocarbons
(PAHs) are of interest in the field of organic materials because
electronic properties can be manipulated while maintaining
structural similarity. In this context, the substitution of C=C
units with the isoelectronic and isosteric B-N subunit.1-7 has
been a fruitful strategy for providing novel extended π-
materials,8-10 including those for which the analogous all
carbon frameworks are unavailable.11 Accordingly, researchers
have begun to use BN containing PAHs in organic light emitting
diodes (OLEDs),12 organic field effect transistors (OFETs)7,12 and
more recently in organic solar cells (OSCs).13, 14
OSCs are typically made using a blend of polymer or small
molecule donors and acceptors, and matching optical,
electronic, and packing properties are crucial to good device
performance. To date there are far fewer electron accepting
materials known, with fullerene-based acceptors being
generally the most effective. More economically feasible,
environmentally stable and easily tunable (in terms of
optoelectronic and physical properties) acceptor materials are
therefore desirable and this is where boron and BN doped
N
B
II
We have recently reported the 1,5-dibora-4a,8a-diaza
indacene core23 (I) and, based on the photophysical properties
of these tricyclic derivatives, envisioned that the analogous BN
doped version of the more π-extended pentacyclic
dihydroindeno[1,2-b]fluorene framework II would have
favourable acceptor properties for application in OSCs. The all
carbon compounds have been studied extensively by Haley
and co-workers in the context of conducting organic
materials.24 Here we report a convenient, scalable synthesis of
a family BN-doped dihydroindeno[1,2-b]fluorenes that are air
and water stable and exhibit tunable absorption properties
depending on the nature of the substituents on either the B
atom or the position marked in yellow on the flanking aryl
rings. Furthermore, we demonstrate good initial OSC
performance of one derivative when utilized as a non-fullerene
acceptor paired with polymer-based donors.
The synthetic pathway to assemble the BN-
dihydroindenylfluorenes,
which
engages
established
electrophilic borylation methodologies,25-29 is summarized in
Scheme 1 (See ESI for details). 2,5-Diarylpyrazines 1-Pz and 2-
Pz (incorporating tBu and N(4-tBuPh)2 groups in the R position,
respectively) were prepared from 2,5-dibromopyrazine via
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standard palladium catalysed cross coupling procedures30 in
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DOI: 10.1039/C9CC05103A
good yields. Utilizing the electrophilic borylation conditions
R
R
R
Cl
Cl
Ar
Ar
B
B
BCl3
2
N
N
N
ZnAr2
TBP, AlCl3
N
N
N
CH2Cl2
Cl
Cl
CH2Cl2
Ar
B
B
RT, 16hrs
RT, 10 min Ar
R
R
R
Pz
Cl
Ar = Tol,
CH3
F
54-55%
F
F
R = tBu, series 1
R = N(4-tBuPh)2, series 2
87%
68%
Ar = FAr,
74-75%
Scheme 1 Synthesis of compounds 1-Cl, 2-Cl, 1-Ar and 2-Ar.
reported by Ingelson et al. to prepare BN-fluorenes,31 1-Pz/2-
Pz were treated with BCl3, AlCl3 and the bulky base 2,4,6-tri-
tertbutylpyridine (TBP) in dichloromethane to yield 1-Cl and 2-
Cl in 87% and 68% yields respectively. These compounds were
isolated in atmospheric conditions by washing the solids with
Figure 2 a) (top) Normalized absorption spectra in dichloromethane solutions of 1-Cl
and 2-Cl. b) (bottom) Normalized absorption spectra in dichloromethane of arylated
water and hexanes and showed no decomposition due to derivatives. For analogous plots of molar absorptivity vs wavelength, see ESI.
hydrolysis of the B-Cl bonds. Substitution of the Cl ligands at
All six of the BN-doped dihydroindeno[1,2-b]fluorenes are
all highly coloured and have broad absorption profiles with
two major bands (Figure 2 and Table 1). TD-DFT (PBE0/Def-
TZVP) analysis shows that these absorptions are associated
with π- π* transitions involving the planar pentacyclic core of
the molecule; Figure 3 shows the frontier orbitals of 1-FAr as a
representative set, with details on the other compounds given
in the ESI. The shorter wavelength absorption is primarily due
to the HOMO-LUMO+1 transition and for the four compounds
with electron withdrawing groups on B (i.e. Cl and FAr), the
longer wavelength band (which exhibits some vibrational
structure in the 1 series) is due to the HOMO-LUMO transition.
For 1-Tol and 2-Tol (with more electron donating aryl groups
on B), the HOMO is associated more with the π system of the
tolyl groups and the π- π* absorption observed involves
HOMO-1 and HOMO-4 orbitals (Figure S2).
boron with aryl groups was smoothly accomplished via
treatment with 2 equivalents of a base-free diaryl zinc reagent,
ZnAr2 (Ar = tolyl,32 2,4,6-F3C6H233) to give the tetraarylated
compounds 1-Tol, 1-FAr, 2-Tol and 2-FAr in moderate to good
yields. The use of completely ether or THF-free ZnAr2 reagents
The position of these absorptions is strongly affected by
the nature of the R group, with a significant red shift in both
bands observed for the NAr2 derivatives of the 2 series. The
nature of the group on B does not dramatically affect the
position of the high energy band, but does shift the lower
energy absorption, particularly when substituting Cl for Ar.
Compounds 1-Cl and 1-FAr are strongly emissive, exhibiting
quantum yields of 0.63 and 0.85, respectively, and Stokes
shifts of ≈ 2500 cm-1. The localization of the LUMO on the
pyrazine core gives this absorption some charge-transfer
character that may enhance the emission Stokes shift. While
we suspect the NAr2 analogs 2-Cl and 2-FAr may also be
emissive, we cannot detect this due to the low energies of the
absorption bands. Interestingly, the tolyl-substituted
derivative 1-Tol is not emissive; this is likely due to the
fundamentally different nature of the HOMO in this compound
(Figure S2) as compared to 1-Cl and 1-FAr.
Figure 1. Molecular structure of 1-FAr. Hydrogen atoms have been omitted for clarity.
Thermal ellipsoids drawn at 50% probability level. Selected bond lengths (Å) B(1)-N(1)
1.642(6), B(1)-C(1) 1.618(6), C(1)-C(2) 1.404(6), C(2)-C(3) 1.453(6), C(3)-C(4) 1.398(6),
N(1)-C(3) 1.348(6), N(1)-C(4) 1.327(5).
was critical to the success of these reactions. All six of these
new BN-dihydroindenylfluorenes compounds were fully
characterized and proved remarkably bench-stable with
samples remaining spectroscopically pure even after several
weeks under ambient conditions. The structures of 1-Cl (Figure
S1) and 1-FAr (Figure 1) were confirmed through single crystal
X-ray diffraction analysis. The structures indicate the
pentacyclic core is perfectly planar, with B-N bond lengths of
1.615(6)Å (1-Cl) and 1.642(6) (1-FAr).
2 | J. Name., 2012, 00, 1-3
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Table 1. Summary of photophysical, electrochemical, and OSC device data.
opt
elec
λabs,DCM (ε)
λFl (φf)[a]
Eg
HOMO
(eV)[d]
LUMO
(eV)[d]
Eg
Voc
FF
PCE
Comp
Jsc (mA/cm2)[e]
/nm (104M-1cm-1)
/nm(%)[b]
(eV)[c]
(eV)[d]
(V)[e]
(%)[e]
(%)[e]
332 (5.55)
482 (1.59)
451 (10.51)
670 (3.68)
338 (4.63)
462 (1.59)
451 (6.93)
640 (2.54)
336 (4.01)
512 (1.01)
455 (7.49)
803 (2.26)
547
(0.85)
1-FAr
2-FAr
1-Tol
2-Tol
1-Cl
2.27
1.48
2.38
1.64
2.12
1.24
-6.24
-5.18
-6.01
-5.13
-6.41
-5.22
-3.88
-3.66
-3.71
n/a
2.36
0.87
0.79
0.96
0.89
n/a
5.4
1.2
1.4
0.8
n/a
n/a
40
31
1.9
0.3
0.4
0.2
n/a
n/a
--
1.52
2.30
n/a
not
emissive
not
emissive
598
31
27
-4.31
-4.18
2.10
1.04
n/a
n/a
(0.63)
2-Cl
--
n/a
[a] Excited at 482 nm (1-FAr), 512 nm (1-Cl). [b] absolute quantum yield determined in CH2Cl2 solution using an integrating sphere. [c] calculated using the onset of
absorption maxima. [d] ELUMO = –(4.8+Ered), EHOMO = –(4.8+Eox).[e] All values based on OSC devices made from 1:1(Comp:PTB7-Th) weight ratio C6H5Cl solutions with
a total solid concentration of 10 mg/mL (See ESI for further details).
devices. Proof-of-concept OSC devices were first fabricated
using the compound 1-FAr and three common high-
performance donor polymers, namely PTB7-Th, PBDB-T, and
PPDT2FBT (see Figure S11 for chemical structures). Although
all compounds were subjected to preliminary testing, 1-FAr
was selected for detailed studies since it has the deepest
electronic energy levels (Figure S12) and strong visible
absorption, both complementary to the low-band gap donor
polymers. OSC devices were constructed using the inverted
architecture with active layers processed from C6H5Cl in air
(Full details are found in the supporting information). The
three active layers PTB7-Th:1-FAr, PBDB-T:1-FAr, and
PPDT2FBT:1-FAr resulted in working devices with photodiode
behaviour (Table 1 and Figure S11). In all cases panchromatic
Figure 3 HOMO, LUMO and LUMO+1 orbitals of 1-FAr based on calculations at the
PBE0/Def-TZVP level.
optical absorption and polymer emission quenching was
observed (Figure S13). Device metrics are summarized in Table
S1. The best OSCs were those based on PTB7-Th:1-FAr active
The modification of the electronic structure across the
layer with an open-circuit voltage of 0.89V, a short-circuit
series is further supported by the electrochemical data
current of 5.6 mA cm2 and a fill factor of 40% giving a
(Figures S3-10). The HOMO and LUMO energy levels of these
respectable initial power conversion efficiency (PCE) of 2%
molecules are estimated by cyclic voltammetry using the onset
(average PCE of 1.5% over 10 measurements). OSCs based on
of both the reduction and oxidation peaks and are summarized
the PBDB-T:1-FAr and PPDT2FBT:1-FAr active layers had PCEs
in Table 1. Using 1-FAr as an example, the estimated HOMO
of 1%, owing to lower photocurrent generation. An
and LUMO energy levels are -6.41 eV and -4.31 eV,
examination of the active layer surface using atomic force
respectively, giving an electrochemical energy gap of 2.10 eV
microscopy revealed highly uniform and very smooth surfaces
which matches closely with the photophysical gap of 2.12 eV.
(Figure S14). This, coupled with a low shunt resistance implies
Across most of the series the optical and electrochemical band
minimal phase separation, thus we targeted the higher
gaps are in good agreement, except 2-Cl where the values
performing PTB7-Th:1-FAr system for further optimization.
differ by 0.20 eV, and 2-Tol where the electrochemical
Unfortunately changing the donor:acceptor ratio or using
reduction peak could not be observed. This lack of a reduction
volatile solvent processing additives did not improve the OSC
wave could be due to the low solubility of 2-Tol as even for 2-
device PCE (Table S2). Screening of the other materials 1-Tol,
FAr which shows higher solubility, the oxidative couple is
2-Tol, and 2-FAr using PTB7-Th gave OSCs with inferior
significantly smaller in magnitude than the reduction (Figures
perforce (ca. PCE 0.2-0.4%) when compared to the OSCs using
S9-10).
the 1-FAr based blends (Table S3), thus identifying the 1-FAr as
The broad absorption profiles and the favorable frontier
the target compound to structurally evolve into a new family
orbital energies led us to explore the use of this family of BN-
of non-fullerene acceptors.
dihydroindenylfluorenes as non-fullerene acceptors34, 35 in OSC
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J. Name., 2013, 00, 1-3 | 3
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K. Cnops, B. P. Rand, D. Cheyns, B. Verreet, M. A. Empl
16.
17.
In conclusion, electrophilic borylation was utilized to
synthesize a new family of air and moisture stable B-N doped
π−conjugated organic molecules with promising properties for
application as electron acceptors. Organic solar cells using 1-
FAr as the acceptor molecule led to devices with power
conversion efficiencies of 2%. While device performance is
low, the good open-circuit voltages (ca. > 0.8V) and uniform
film formation make this system attractive for targeted
acceptor synthesis. Indeed, the R groups employed (tBu and
NAr2) are far from ideal and use of planar electronic deficient
capping units20, 36-38 offer a clear direction for further research
aimed at performance optimization of this class of acceptors in
OSC devices.
Funding for this work was provided by NSERC of Canada in
the form of a Discovery Grant to W.E.P. and the U of Jyväskylä
and Emil Aaltonen Foundation to H. M. T. W.E.P. and G. C. W.
acknowledge the Canada Research Chair secretariat for a Tier I
CRC (2013–2020). and a Tier II Chair, respectively. M. M. M.
thanks NSERC of Canada for PGSD Scholarship support. M. N.
thanks the U Calgary for an Eyes High Scholarship.
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