Synthesis and characterization of two different azarubrenes

Article abstract of DOI:10.1039/C8CC01662C

The synthesis, properties and solid state X-ray single crystal structures of two new rubrene-derivatives, viz diazarubrene (4,4′-(6,12-diphenyltetracene-5,11-diyl)dipyridine) and tetraazarubrene (5,6,11,12-tetra(pyridin-4-yl)tetracene), are reported. Both the azarubrenes are more oxidatively stable than rubrene itself and show very similar optical properties but differ in their crystal packing from that of rubrene.

Full text of DOI:10.1039/C8CC01662C

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
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Synthesis and Characterization of Two Different Azarubrenes
Gaozhan Xie,a Sebastian Hahn,a Frank Rominger,a Jan Freudenberg,ab and Uwe H. F. Bunzac*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
N
OH
The synthesis, properties and solid state X-ray single crystal
structures of two new rubrene-derivatives, viz diazarubrene (4,4'-
(6,12-diphenyltetracene-5,11-diyl)dipyridine) and tetraazarubrene
(5,6,11,12-tetra(pyridin-4-yl)tetracene) are reported. Both
azarubrenes are more oxidatively stable than rubrene itself and
show very similar optical properties but differ in their crystal
packing from that of rubrene.
Si
N
i
iii
ii
O
N
Ph
Ph
N
2
1
DAR
14%
N
Rubrene (5,6,11,12-tetraphenyltetracene) is
a benchmark
iv
O
semiconductor and has been studied extensively in the field of
organic electronics.1 So far, the highest reported hole-
transport mobility for rubrene has reached 40 cm2V-1s-1. 2 Also,
rubrene promises powerful performance in next-generation
solar cells with its high singlet fission quantum efficiency of
nearly unity.3 Nevertheless, rubrene oxidizes easily in ambient
surroundings,4 hampering its practical applications. Thus,
various substituted rubrenes have been prepared and
investigated. Douglas et. al. and Sakamoto et. al. have
reported a variety of fluorinated rubrenes.5 The replacement
of hydrogen atoms by electron-withdrawing fluorine atoms
lowers the highest occupied molecular orbital (HOMO) and
lowest unoccupied molecular orbital (LUMO) energies, thus
enhancing stability of rubrene against oxidation. Mamada and
coworkers have developed and characterized a series of
rubrene analogues with thienyl or furyl substituents,6 which
showed increased photooxidation stability. However, to our
knowledge, azarubrenes are unknown at the present.
Introducing nitrogen atoms into π-system to form pyridine-like
species stabilizes the frontier molecular orbitals of molecules
and potentially convert parent hydrocarbon structures from
p-type to n-type heterocyclic semiconductors.7,8 Therefore, we
designed and synthesized two novel azarubrenes by
introducing nitrogen atoms into the phenyl peripheries of
rubrene.
O
N
DARO2
83%
Scheme 1. Synthetic route to Diazarubrene (DAR). i) K2CO3, THF/ CH3OH = 1:1, room
temperature (r.t.), 30 min; ii) benzophenone, n-BuLi, THF, 0 ℃ to r.t., 12 h; iii) me-
thanesulfonyl chloride, triethylamine, 1,1,2,2-tetrachloroethane, 146 ℃, 12 h; iv) O2,
dichloromethane, r.t., 5 d.
N
N
N
N
Cl
Cl
Cl
Cl
i
ii
O
O
3
N
N
TARO2 67%
N
N
48%
TAR
Scheme 2. Synthetic route to tetraazarubrene (TAR). i) 4-Pyridinylboronic acid;
Pd2dba3, Xphos, K3PO4, 1,4-dioxane, 100
5 d.
℃, 72 h; ii) O2, dichloromethane, r.t.,
The classic one-pot approach to rubrene using the allene
protocol has been applied extensively. Some new derivatives
of rubrene, such as 5,11-bis(3-thienyl)-6,12-diphenyltetracene
and 5,11-bis(4-cyanophenyl)-6,12-diphenyltetracene, have
been synthesized using this approach.9 To access DAR
) we prepared the carbinol . However, mesylation followed
(Scheme
1
2
by heating in o-xylene to synthesize DAR from 2, using the
allene method failed.
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Table 1. Photophysical, experimental and calculated electrochemical properties of rubrene, DAR, and TAR.
[a]
[a]
[b]
[b]
Absmax
Emsmax
Eox
Ered
HOMO (eV)
mes.[c]/cal.[d]
LUMO (eV)
mes.[c]/cal.[d]
gap (eV)
mes.[e]/cal.[d]
Comp.
(nm)
(nm)
(V)
(V)
rubrene
DAR
527
525
525
556
558
561
0.37[f]
0.54[f]
0.71[g]
– 2.11[f]
– 1.98[f]
– 1.73[f]
– 5.17/– 4.95
– 5.34/– 5.24
– 5.51/– 5.54
– 2.69/– 2.52
– 2.82/–2.82
– 3.07/–3.13
2.48/2.43
2.52/2.42
2.44/2.41
TAR
[a] Absorption peaks and maximum emissions in dichloromethane. [b] Oxidation and reduction potentials measured in CV using ferrocene/ferrocenium as the
reference redox system and internal standard (−4.8 eV). [c] Calculated from CV measurements (EHOMO = –4.80 eV – Eox (0/+); ELUMO = –4.80 eV – Ered (0/–)). [d]
Calculated by Gaussian 09 B3LYP/6-311++G**//BP86/def2-TZVP. [e] Estimated from EHOMO and ELUMO (Egap = ELUMO – EHOMO). [f] Calculated from half-wave potentials. [g]
Calculated from anodic peak potential.
Fortunately, when 1,1,2,2-tetrachloroethane was used as
As shown in Figure 1, DAR and TAR exhibit absorption and
solvent, DAR was obtained in fairly poor yield of 14%. The emission spectra similar to each other and to those of rubrene;
simplicity of the synthesis renders the yield acceptable, but this result is expected, as the chromophore, tetracene, is
unfortunately this scheme did not work for TAR, so an identical in all cases. Despite the presence of two or four
alternative route was developed, starting from literature nitrogen atoms in the periphery, DAR and TAR are still easily
known tetrachlorotetracene 3 10
synthesize thienyl and furyl substituted rubrene derivates by as expected by the breakup of the tetracene system (see
Stille coupling. From TAR was easily obtained by the Figure S1 in Electronic Supplementary Information, ESI).
coupling of to 4-pyridinylboronic acid, through a fairly
efficient Suzuki reaction, employing
.
3 had been employed to oxidized in air into DARO2 and TARO2; both emit blue-shifted,
3,
3
Figure 2 shows a comparison of fluorescence intensity for
a
Buchwald-type solutions of rubrene, DAR, and TAR as a function of time in
catalyst.11 The desired TAR forms in 48% after column ambient air. After 24 h, the fluorescence intensity of rubrene,
chromatography. Interestingly, in previous work, Douglas et. al. DAR and TAR have decreased by 52%, 24%, and 10%
found that the reaction of
3 and phenyl magnesium bromide respectively. Both DAR and TAR therefore are considerably
by Kumada-Tamao-Corriu cross-coupling undergoes an indene less sensitive towards oxidation in solution, but by no means
annulation reaction.12 However, similar types of air-stable for prolonged periods of time.
transformation do not occur under our conditions. Introducing
more nitrogen atoms by coupling of with substituted of the four products, i.e. the azarubrenes but also their
pyrimidines was not successful, using the above-developed oxidized products. Single crystals of DAR DARO2, and TARO2
We were able to grow single crystals for XRD analysis of all
3
,
conditions.
were obtained by slow evaporation of saturated solutions of
dichloromethane under ambient surroundings; TAR single
crystals were grown successfully by slow evaporation of a THF
solution in the glovebox. Rubrene displays three different
polymorphs, orthorhombic (a = 26.78 Å, b = 7.17 Å, c = 14.26
Å), monoclinic (a = 8.74 Å, b = 10.13 Å, c = 15.64 Å,
and triclinic (a = 7.02 Å, b = 8.54 Å, c = 11.95 Å,
β
α
= 91°),
= 93°,
β = 106°,
8.27 Å, c = 11.86 Å,
γ
= 96°).13 As shown in Table 2, DAR (a = 7.07 Å, b =
α
= 90°, β = 106°, γ = 98°) crystallizes in the
triclinic crystal system and its molecular packing (Figure 3a and
3e) is close to isomorphous to that of triclinic rubrene crystals,
grown by the solvent diffusion method. Both of them exhibit a
planar tetracene backbone with slanted stacks. Noticeably,
Figure 1. Normalized absorption and emission spectra of rubrene, DAR and TAR
DAR exhibits a
than that of rubrene13)
the absence of short-axis displacement along crystal
direction (see Figure 3e and Figure S2 in ESI), indicating it
might have more efficient cofacial -stack interactions and
electronic couplings than rubrene.14 This is further supported
π
-π
stacking distance of 3.6 Å (0.2 Å shorter
measured in dichloromethane.
,
a long-axis displacement of 6.1 Å and
,
a
π
by Hirshfeld surface analysis15
(Figure 3g,h): Similar to triclinic
rubrene,16 the main feature is a large, light blue colored region
corresponding to π-stacking of the tetracene core. However,
red regions on dnorm result exclusively from C-H···π close
contacts – H···H close contacts are less prominent and N···H
Figure 2. Normalized time-dependent emission spectra of rubrene, DAR and TAR
close contacts contribute only 5.4% to the total surface area.
measured in dichloromethane.
2 | J. Name., 2012, 00, 1-3
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Table 2. Unit Cell Data of DAR, TAR and rubrene.
Comp.
crystal system
space group
a (Å)
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
V (Å3)
Rubrene[a]
DAR
triclinic
triclinic
triclinic
P1
P1
P1
7.02
7.07
7.76
8.54
8.27
11.95
11.86
21.77
93
90
98
106
106
90
96
98
684
660
TAR
18.66
101
3068
[a] The data of triclinic rubrene crystal is reported by Huang et al.12
single crystals from other solvents only led to very small
crystals.
We could also obtain specimen of the oxidized species,
which are, as expected, the oxygen adducts formed through a
formal 4+2-cycloaddition of DAR or TAR with O2. The
molecular packing of these species is complicated (see Figure
S3 in ESI).
Quantum chemical calculations show the influence of the
pyridine units on the energetic position of HOMO and LUMO.
(
Figure 4) Optimized geometries of rubrene, DAR and TAR are
twisted as reported for rubrene.17 The FMOs are stabilized by
0.3 eV (DAR) and 0.6 eV (TAR) each in comparison to rubrene,
pointing to an effect of around 0.15 eV per nitrogen atom,
fairly additive.
In this work, we have introduced two or four pyridine
substituents to rubrene and have modulated its frontier orbital
positions, without touching the tetracene unit, working on the
peripheral aromatic substituents. While this concept is not
new, pyridine-substituted rubrenes have to our knowledge
never been prepared and
3 is a versatile starting material for
the introduction of different substituents to rubrene. In future
we will exploit this concept further and introduce other
electron accepting substituents into the rubrene core and
investigate their structure, optical and electronic properties
with the final goal to develop
a high mobility n-
channel-material based on the rubrene framework.
G.X. thanks the CSC (Chinese Scholarship Council) for a
fellowship. We thank the DFG (SFB 1249) for generous
support.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
Figure 3. Different views of single crystal structures of DAR (a), TAR (b), DARO2 (c), and
TARO2 (d). Visualization of overlap and packing of DAR (e) and TAR (f). THF molecules
are omitted for clarity in the TAR crystal structures. g) dnorm Hirshfeld surface for DAR. h)
Fingerprint plot for DAR.
Notes and references
More interesting is the structure and the packing of triclinic
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