Pushing to the low limits: Tetraazaanthracenes with very low-lying LUMO levels and near-infrared absorption

Article abstract of DOI:10.1039/c7cc05224c

Here we propose the combination of the 4-alkoxythiazole donor motif with highly photostable tetraazaanthracenes as electron-acceptor units. The segregated frontier orbitals in these dyes afford optical band gaps of 1.4-1.1 eV. Cyclic voltammetry confirmed the very low-lying LUMO levels that are attributed to the highly electron-deficient tetraazaanthracene moiety.

Full text of DOI:10.1039/c7cc05224c

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Pushing to the Low Limits: Tetraazaanthracenes with Very Low-
Lying LUMO Levels and Near-Infrared Absorption
Received 00th January 20xx,
Accepted 00th January 20xx
Dominique Mario Gampe,a,† Stefan Schramm,a,b,† Florian Nöller,a Dieter Weiß,a Helmar Görls,c
Panče Naumovb and Rainer Beckerta,*
DOI: 10.1039/x0xx00000x
www.rsc.org/
Here we propose the combination of the 4-alkoxythiazole donor
motif with highly photostable tetraazaanthracenes as electron-
acceptor units. The segregated frontier orbitals in these dyes
afford optical band gaps of 1.4‒1.1 eV. Cyclic voltammetry
confirmed the very low-lying LUMO levels that are attributed to
the highly electron-deficient tetraazaanthracene moiety.
The recent advances in organic photovoltaics owe greatly to
the continual efforts to improve bulk heterojunction solar cells
(BHJ).1 A vast number of both polymers2 and small molecules3
have been designed and investigated with regard to their
electron-donating properties towards fullerene-based
acceptors,4 especially the phenyl-C61-butyric acid methyl ester
(PC61BM), which is reaching its performance limit.5 Small-
molecule acceptors are now generally thought to be
advantageous relative to the conventional fullerene
derivatives due to their processability from solution,6 cost-
effective synthesis,7 tunable frontier orbital energies8 and
improved efficiency of light harvesting.9 Since the recent
results have indicated superior performance of BHJs based on
small molecule acceptors relative to the traditional fullerene-
based devices,10 this line of pursuit calls for new acceptor
materials and manifold donor-acceptor pairs as an alternative
to the design of new potential donor species.
Chart 1 Structures of the target molecules.
building blocks by applying simple molecular design
principles.12 In that regard, 4-alkoxythiazoles are known to
excel with their photochemical properties13 and the propensity
to donate electrons to the benzo[1,2,5]thiadiazole (Bt)
acceptor moieties.14 The Bt-based acceptor structures are
easily functionalized15 and have been already applied to
construct D—A systems.16 As it has been demonstrated
recently tetraazaanthracenes can easily be obtained from Bt
substructures, whereby the LUMO energy is drastically
decreased to levels that are applicable to electron-accepting
materials.17 Here we present that combination of 4-alkoxy-
thiazole donors and tetraazaanthracene acceptor moieties
fulfill these requirements and are viable candidates for
advanced solar cell applications.
Long-wavelength absorbing small molecules are typically
designed by using the charge transfer (CT) in donor
‒acceptor
Chart 1 depicts the designed target structures, which were
prepared via the synthetic route shown in Scheme 1. Hantzsch
(D
—
A) type dyes.11 Since the highest occupied molecular
orbital (HOMO) and the lowest unoccupied molecular orbital
(LUMO) are determined by the respective electron-donating
and electron-accepting moieties, the energy levels of a D—A
type dye can be tuned by incorporating the appropriate
cyclization
α-bromoethylphenylacetate (20
17 18 19) afforded the 4-hydroxythiazoles 14
These Bt derivatives were transformed into the 4-nhexyloxy-
thiazoles 11 12 and 13 via Williamson-type etherifications
with iodohexane. Reductive desulfurization of the Bt-thiazoles
11 12 13) with sodium borohydride afforded the o-
between
the
21) and the carbothioamides
15 and 16 18
appropriate
,
(
,
,
,
.
,
(
,
,
phenylenediamine substructures, which due to their instability
against oxygen were used as obtained after a short work-up
procedure (for further details see the Supporting
Information).19
dichlorodicyanopyrazine (
The
following
) provided the
cyclizations
with
7
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Scheme 1 Synthetic route to the target compounds 1, 2 and 3. i) NaAc, EtOH, 80 °C, 14: 63%, 15: 76%, 16: 48%; ii) I-C6H13, K2CO3, acetone, 70 °C, 11: 85%, 12: 74%; 13: 28%; iii)
NaBH4, CoCl2 (cat.), EtOH/THF, N2, short work-up, 8, 9, 10: quant.; iv) 5,6-dichloro-2,3-dicyanopyrazine (7), dioxane, N2, 110 °C, 4: 32%, 5: 58%, 6: 33%; v) DDQ, THF, N2, 1: 91%, 2:
57%, 3: 90%.
dihydrotetraazaanthracenes
dihydrotetraazaanthracene
and the dihydrospecies were oxidized with pyridyl-derivative
dichlorodicyanobenzoquinone. Utilizing this synthetic protocol, green and the dithiazole
Bt derivatives (11 12
tetraazaanthracenes in two steps and 30% yield. The target electronic absorption spectra of all compounds (Figure 1A).
4
,
6
and the thiazol-bridged bis- azaacenes, but the mechanism remains unexplored and
5
. To obtain the D
—
A derivatives obscure.20 All derivatives form deep-colored solutions—the
1
,
2
3
,
1
display blue-green, the dianthracene
dark yellow solutions. Strong π
2
π*
3
—
,
, 13) are easily transformed into transition bands in the UV and blue regions dominate the
materials were fully characterized and showed good Molar extinction coefficients (ε) of 2.14 (
1
), 3.82 (
) were determined at the respective
), 353 ( ) and 446 nm ( ). Broad CT bands
2) and
1
photostability under laboratory conditions in solution (such as 3.63·104 M-1cm‒
(3
THF, toluene, benzene) and in the solid state. When heated in maxima of 349 (
1
2
3
protic solvents, such as methanol, they are reduced back to extend over the whole visible and the near-infrared (NIR)
their dihydro precursors ( ). This behavior is known for region. Both A A type dyes and show long-
4
,
5,
6
—
D
—
1
2
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CT band of
3
extends between 550 and 1100 nm, with a
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maximum at 839 nm (ε = 6350 M‒ cm ) and a narrow band
gap of 1.13 eV. These long-wavelength absorptions seem
remarkable, since in contrast to the fullerene-based materials
they provide significant overlap with the solar spectrum of up
to about 1100 nm.21
1
‒1
DOI: 10.1039/C7CC05224C
The redox activity of the molecules was examined using cyclic
voltammetry (Figure 1B). All derivatives (
irreversible oxidation step at 0.8 V (against Fc/Fc+), which is
typical for 4-alkoxythiazoles (Figure S3
S5, SI).14 As expected,
the bis-thiazole can be oxidized twice, hence the irreversible
1, 2, 3) possess an
‒
3
oxidation at 1 V. Two reversible one-electron reduction steps
were detected that are attributed to the tetraazaanthracene
substructure, which forms a stable radical anion and dianion.
and are reduced at half-wave potentials of E1/2 = -0.52
and -1.3 V (against Fc/Fc+). The D
D molecule has an
1
2
—
A
—
3
extraordinary high electron affinity, as reflected in the
reductions at -0.37 and -1.00 V. These high reduction
potentials (
electrochemical stability against water and oxygen.22 The
electrochemical LUMO levels were calculated to -4.6 eV (
and -4.7 ( ) /-4.8 eV ( ) by means of DFT/TD-DFT, respectively
1, 2: ~0 V / 3: 0.16 V against SCE) secure
1, 2)
1
2
(Table 1, Figure 1C and SI). These low levels seem remarkable
by comparison with conventional acceptor materials, such as
PC61BM (ELUMO = -3.75 eV),23 PC71BM (-4.30 eV)24 or
Figure 1 A) UV/Vis absorption spectra (THF) of anthracene derivatives 1, 2 and 3; B)
Reduction cycles of the target molecules, measured in 1 M solution of TBAPF6 in THF,
calibrated externally against Fc/Fc+; C) Graphical depiction of the comparison between
the experimental (black lines) and theoretical (blue lines) obtained frontier orbital
energies.
tetracyano-p-quinodimethane
(TCNQ:-4.23 eV).25
Furthermore, they are in the range of Wang’s 4CN-4Cl-
perylenediimide that has an “ultralow LUMO level”
of -4.64 eV.26 Notably, a record-breaking low LUMO for non-
fluorinated materials was determined by CV and DFT to -4.8 eV
wavelength absorptions to 900 nm, which is related to optical
for bis-thiazole
3. To our knowledge, only three- and four-fold
band gaps of 1.45 (1) and 1.42 eV (2). Comparison with their
fluorinated TCNQ derivatives deploy such low LUMOs (-5.24 eV
corresponding Bt precursors 11 and 12 points to the acceptor
strength of the anthracenes, as the optical gaps are decreased
by 1 and 0.9 eV, respectively (Table 1). In the case of the
in F4TCNQ).25,27 Electrochemical band gaps of ΔECV = 1.3 (
and 1 eV ( ) were estimated via the HOMO levels of -5.9 (
and -5.8 eV (
with the optical gaps. The slightly narrower ΔECV (see Table 1)
1
1,
,
2
2
)
)
3
3
), respectively, and are in very good agreement
absorption spectra of D
—
A—
D molecules 13 to
3, a strong
bathochromic shift of ~300 nm (
≙
0.8 eV) was observed. The
Table 1 Summary of the photochemical data and the estimated frontier orbital energies.
11
433 [4.3];
242 [4.4]
2.52
-5.83
-3.28
1
664 [3.4];
349 [4.3]
1.45
-5.94
-4.63
1214
470 [4.3];
315 [4.5]
2.28
2
685 [3.6];
353 [4.6]
1.42
-5.91
-4.65
1314
545 [4.5];
365 [4.4]
1.97
3
839 [3.8];
446 [4.6]
1.13
-5.77
-4.80
λmax / nm [log ε]a
∆Eopt / eV b
EHOMO / eV c
ELUMO/ eVc
∆ECV/ eV d
-5.62 f
-3.18 f
2.44
-5.48 f
-3.42 f
2.06
2.55
1.31
1.26
0.97
-5.66
(0.17)
-3.34
(0.06)
2.32
(0.23)
-5.91
(0.03)
-4.67
(0.04)
1.24
(0.07)
-5.58
(0.04)
-2.92
(0.26)
2.66
(0.22)
-5.92
(0.01)
-4.82
(0.17)
1.10
(0.16)
-5.43
(0.05)
-3.18
(0.24)
2.25
(0.19)
-5.52
(0.25)
-4.77
(0.03)
0.77
(0.22)
EHOMO / eV (error) e
ELUMO/ eV (error) f
∆EDFT/ eV (error) d
a Measured in THF. b Calculated via the onset (10%) of the absorption spectra. c Recorded by cyclic voltammetry in 0.1 M solution of TBAPF6 in THF, calculated via
EHOMO = -[Eonset(ox vs. Fc+/Fc)+5.1 eV] and ELUMO = -[Eonset(red vs. Fc+/Fc)+5.1 eV]. d ∆E = -(EHOMO-ELUMO). e Calculated energies at B3LYP/6-31+G(d,p) level of theory
based on a B3LYP/6-31+G(d,p) PCM THF optimized ground state geometry, HOMO energies resulted from B3LYP/6-31+G(d,p) calculations, LUMO energies resulted
from PBE/6-31+G(d,p) calculations,29 ‘error’ stands for the difference between the experimental and theoretical estimated values. f Frontier orbital energies were
calculated via the outdated equations: EHOMO = -[Eonset(ox)-Eonset(Fc/Fc+)]-4.8 eV and ELumo = -[Eonset(red)-Eonset(Fc/Fc+)]-4.8 eV.
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Comparison between the energy levels of the anthracene
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derivatives and the Bt precursors (11
electron deficiency of the tetraazaanthracenes. While the
HOMO layers of 11 and 12 and as well as 13 and remain
constant (see Table 1), as they are located at the thiazole
donor part, the LUMOs decrease drastically. The reduction
, 12, 13) confirm the high
1
,
2
3
waves of
1
,
2
and
3
are shifted by 1.3 V to higher potentials
12 or 13
relative to the appropriate Bt precursor 11
,
;
consequently the LUMO levels and band gaps decrease by
~1.2 eV. Bt substituents are applied frequently as strong
electron-accepting unit in D—
A materials.28 However the
synthetic effort to prepare the tetraazaanthracenes seems
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In summary, this work describes successful combination of the
4-alkoxythiazole donor and tetraazaanthracene acceptor
building block. The derivatives were synthesized without
expensive transition metal catalysis and are fully
spectroscopically characterized. Broad absorption bands were
observed over the whole visible to the NIR region. Utilization
of tetraazaanthracene as electron-accepting moiety results in
narrow band gaps and record-breaking low LUMOs which
becomes evident by comparison with their Bt-acceptor bearing
precursors. Due to good solubility in common organic solvents,
such as chloroform or dichlorobenzene, and easy accessibility,
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the bis-thiazole
3 is particularly promising for application in
organic electronics.
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