A Sm(II)-mediated cascade approach to dibenzoindolo[3,2- b ]carbazoles: Synthesis and evaluation

Article abstract of DOI:10.1021/ol500841b

Previously unstudied dibenzoindolo[3,2-b]carbazoles have been prepared by two-directional, phase tag-assisted synthesis utilizing a connective-Pummerer cyclization and a SmI₂-mediated tag cleavage-cyclization cascade. The use of a phase tag allows us to exploit unstable intermediates that would otherwise need to be avoided. The novel materials were characterized by X-ray, cyclic voltammetry, UV-vis spectroscopy, TGA, and DSC. Preliminary studies on the performance of OFET devices are also described.

Full text of DOI:10.1021/ol500841b

Letter
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A Sm(II)-Mediated Cascade Approach to
Dibenzoindolo[3,2‑b]carbazoles: Synthesis and Evaluation
Matthew T. Levick,† Iain Grace,‡ Sheng-Yao Dai,† Nicholas Kasch,§ Christopher Muryn,† Colin Lambert,‡
Michael L. Turner,*,† and David J. Procter*,†
†School of Chemistry, University of Manchester, Oxford Road, Manchester, M13 9PL, U.K.
‡Department of Physics, Lancaster University, Lancaster, LA1 4YB, U.K.
§School of Physics and Astronomy, University of Manchester, Manchester, M13 9PL, U.K.
S
* Supporting Information
ABSTRACT: Previously unstudied dibenzoindolo[3,2-b]carbazoles
have been prepared by two-directional, phase tag-assisted synthesis
utilizing a connective-Pummerer cyclization and a SmI2-mediated tag
cleavage−cyclization cascade. The use of a phase tag allows us to exploit
unstable intermediates that would otherwise need to be avoided. The
novel materials were characterized by X-ray, cyclic voltammetry, UV−vis
spectroscopy, TGA, and DSC. Preliminary studies on the performance
of OFET devices are also described.
ndolo[3,2-b]carbazoles, nitrogen containing analogues of
increased.2 Various methods have been employed to enhance
the stability of the larger acenes to allow for a more detailed
study of their properties, mainly via either extensive
functionalization of the aromatic core or the inclusion of
heteroatoms such as sulfur or nitrogen into the structure.3
Herein we describe the synthesis, optical and electrochemical
properties, and preliminary transistor performances of dibenzo-
indolo[3,2-b]carbazoles [DBICs], a previously unknown class
of aza-heptacenes based on an extended indolo[3,2-b]carbazole
framework (Figure 1B). The first synthesis of DBICs involves
the utilization of a two-directional Pummerer4-type cyclization
and a two-directional radical cascade mediated by SmI2.5
Crucially, these two key steps incorporate the introduction and
removal of a phase tag that facilitates the purification of
unstable intermediates that would otherwise have to be
avoided. Attractively, the use of tag introduction/cyclization
and tag cleavage/cyclization6 delivers a route in which the use
of a phase tag adds no strategically redundant steps to the
synthesis.
I
pentacene, have been shown to be promising organic
semiconductors with mobilities as high as 1 cm2 V s−1, with the
advantage that the nitrogen atom allows for easy structural
modification either to alter electronic properties or enhance
solubility allowing for solution processing of devices (Figure
1A).1 Larger acenes beyond pentacene are less well studied
owing to the decreased stability as the conjugation length is
We have previously employed Pummerer-type cyclizations7
in the synthesis of a range of different targets including bis-
oxindole-based monomers for use in semiconducting poly-
mers,8 and benzo[b]carbazole end-capped oligothiophenes for
preliminary evaluation as semiconductors.9 Here we report an
extension of this methodology in the first synthesis of
dibenzoindolo[3,2-b]carbazoles.
Bis-hydroxyamides 1, synthesized according to previously
described procedures,6,7 underwent oxidation and selective
double Pummerer-type cyclization upon capture of a fluorous
Figure 1. (A) Indolo[3,2-b]carbazole motif in organic semiconductors.
(B) Unknown dibenzoindolo[3,2-b]carbazole (DBIC) motif. (C) This
study: 2-D, tag-assisted synthesis and study of DBICs.
Received: March 20, 2014
Published: April 8, 2014
© 2014 American Chemical Society
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Letter
thiol HS(CH2)2(CF2)7CF3 to give linear regioisomers (>5:1) 2
as a 1:1 mixture of diastereoisomers after rapid purification by
fluorous10 solid phase extraction (FSPE)11 (Scheme 1).
Scheme 1. Two Directional, Tag-Assisted Synthesis of
a
DBICs
Figure 2. Crystal structures of DBIC-2 (top) and DBIC-3 (bottom).
2 show twisting of the alkyl chains, which implies that the
packing structure is being dominated by the aromatic cores,
with the alkyl chains twisting to accommodate the packing of
the aromatic core.
DSC traces for compounds DBIC-2 and DBIC-3 both
showed single melting point transitions at 194 and 229 °C,
respectively. DBIC-1, with the longer branched alkyl chain,
forms two mesophases, identified by polarized optical
microscopy, that were observed at 100 and 104 °C before
the material melted into an isotropic liquid at 106 °C.14
The HOMO energy levels of DBIC-1 and DBIC-2 suggest
that devices should operate under ambient conditions (Table
1).15 The introduction of methyl groups onto the backbone
(DBIC-3) raises the HOMO energy level by 0.2 eV. All
HOMO energy levels are raised relative to those reported for
indolo[3,2-b]carbazoles in the literature and better match the
work function of gold which should allow for effective charge
injection (see Supporting Information (SI)). UV−vis spectros-
copy showed an onset of absorption at around 515 nm for all
three compounds, significantly red-shifted relative to indolo-
[3,2-b]carbazoles (λonset ≈ 450 nm)1a as a consequence of the
extended conjugation length (see SI).
Preliminary OFET devices were fabricated by vacuum
sublimation of DBIC-2 and DBIC-3 onto OTS treated SiO2/
Si++ wafers, while fabrication of devices based on DBIC-1 was
performed by spin coating from a 0.5% w/w solution in THF.
DBIC-1 and DBIC-2 were found to show good transfer
characteristics and similar mobilities of 2.6 × 10−3 and 4.0 ×
10−3 ( 1 × 10−3) cm2 V s−1, respectively, whereas DBIC-3
exhibited a mobility 2 orders of magnitude lower (3.1 × 10−5).
This is possibly a consequence of the tighter crystal packing of
DBIC-2 relative to DBIC-3. Fabrication of devices based on
DBIC-2 using substrates maintained at 25 °C did not exhibit
normal transfer characteristics making it difficult to obtain
accurate values for the devices’ performance. It was found that
when a heated substrate (100 °C) was used during the
evaporation, this problem was alleviated. Transfer and output
curves for DBIC-2 are shown in Figure 3.
a
RF = −CH2CH2C8F17. FSPE = fluorous solid phase extraction. HFIP
= 1,1,1,3,3,3-hexafluoro-2-propanol.
Deprotonation and alkylation of 2 using LDA and a
dibromo-alkylating agent gave key intermediates 3 that proved
highly unstable toward conventional chromatography. Fortu-
nately, purification by FSPE (rapid filtration through fluorous
silica gel, eluting with a fluorophilic solvent) gave 3 with little
decomposition. Finally, treatment of 3 with SmI2 and a proton
source at 60 °C triggered cleavage12 of both fluorous tags and a
double cyclization to give the desired DBICs after oxidation of
the product mixture with p-benzoquinone (p-BQ). The novel
aza-heptacenes were obtained in overall yields of 10−17% in 5
steps (average yield/step, 63−70%) from known and readily
accessible hydroxyamides 1 (Scheme 1).
Slow evaporation from THF delivered crystals of DBIC-2
and DBIC-3 suitable for X-ray crystallographic analysis (Figure
2).13 DBIC-2 was found to pack in a lamellar-type structure
(with the aromatic cores arranged in layers separated by regions
of aggregated alkyl chains with an interlamellar spacing of 15 Å)
in which the aromatic cores pack in a herringbone-like manner
similar to pentacene but offset along the long axis of the
molecule such that only three rings from each molecule are
overlapping in the edge-to-face orientation. Each molecule is
surrounded by six others via a shortest carbon−carbon distance
of 3.5 Å, two in a completely offset parallel face-to-face
arrangement and four in an edge-to-face manner.
DBIC-3 was found to pack in a herringbone motif, and
segregation of the alkyl chains is again observed with an
interlayer spacing of 16 Å (Figure 2). Each molecule is
surrounded by six others in the same manner observed for
DBIC-2 (except with only two rings on each adjacent molecule
overlapping in the edge-to-face orientation) via a shortest
carbon−carbon distance of 3.7 Å, four of these in an edge-to-
face arrangement and two in a completely offset parallel face-to-
face arrangement. The crystal structures of DBIC-1 and DBIC-
Analysis by powder-XRD indicates that the orientation of
DBIC-2 on the two different temperature substrates is almost
identical. A primary diffraction peak is observed at 2θ = 5.70°
(100 °C) (Figure 4) and 5.72° (25 °C), with second-, third-,
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Table 1. Summary of Optical and Electronic Properties
HOMO/
LUMO/
HOMO
c
LUMO
c
λonset
nm
/
Eg/
eV
λ(hole)
/
a
b
c
c
eV
eV
calcd
calcd
μ/cm2 V s−1
Ion/Ioff
Vth
t/eV
eV
d
e
DBIC-1
DBIC-2
DBIC-3
−5.13
−5.19
−4.99
−2.72
−2.79
−2.57
−
−5.70
−5.52
−
−0.77
−0.67
515
516
513
2.41 2.6 × 10−3 ( 5 × 10−4
2.40 4.0 × 10−3 ( 1 × 10−3
)
)
1 × 103
4 × 104
4 × 103
−1
−19
−13
−
0.11
−
0.076
f
2.42 3.1 × 10−5 ( 2.8 × 10−5
)
0.05
0.074
a
b
Estimated from cyclic voltammetry using the equation: EHOMO (eV) = −(5.1 − Eonset + E1/2[Fc]). Estimated from cyclic voltammetry (HOMO) and
c
d
optical band gap. See SI for computational method. Fabricated by solution processing from THF (0.5% w/w solution) onto OTS−SiO2.
e
f
Fabricated by vacuum evaporation onto OTS−SiO2 at 100 °C. Fabricated by vacuum evaporation onto OTS−SiO2 at 25 °C.
crystal structures, we estimated both transfer integrals (t) and
reorganization energies (λ) for transport through DBIC-2 and
DBIC-3. Using semiclassical Marcus hopping theory, the rates
of charge transfer (hole) were then calculated to be 0.22 and
0.02 s−1.16 The calculated reorganization energies are low and
close in value (0.076 and 0.074 eV), with the source of the
differing rates being derived from the large difference in the
respective transfer integrals.
In summary, we have synthesized the previously unstudied
dibenzoindolo[3,2-b]carbazoles by a novel, tag-assisted ap-
proach involving a two-directional Pummerer-type reaction and
a two-directional SmI2-mediated radical cascade. The use of a
phase tag allows us to exploit intermediates that are unstable to
purification by conventional chromatography and would
otherwise need to be avoided. Dibenzoindolo[3,2-b]carbazoles
show good ambient stability, with HOMO levels between −5.0
and −5.1 eV. The compounds were found to pack in a lamellar
“herring bone-type” structure. Preliminary OFET devices were
fabricated and found to provide modest mobilities of up to 4.0
× 10−3. The optimization of device performance is currently
underway in our laboratories.
Figure 3. Transfer (top) and output characteristics (bottom) for
DBIC-2.
ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental details for the preparation of compounds and
1
devices, H and 13C NMR spectra for new compounds, DSC
and CV traces. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: david.j.procter@manchester.ac.uk.
Notes
The authors declare no competing financial interest.
Figure 4. Powder-XRD for DBIC-2 and DBIC-3.
ACKNOWLEDGMENTS
■
and fourth-order diffraction peaks visible in both cases. The
primary diffraction peaks correspond to a d-spacing of 15.5 Å;
this matches closely with the predicted (1,0,0) reflection from
the single crystal data obtained, which gives a d-spacing of 15.7
Å. This hints that the compound may be adopting an
orientation in which the alkyl chains are pointing both
downward toward the substrate and upward away from the
substrate, with the aromatic core laid with its longest axis off
parallel by an angle of ∼35° relative to the substrate. DBIC-3
displays a primary diffraction peak of 6.39°, corresponding to a
d-spacing of 13.8 Å. This matches with the predicted primary
diffraction peak at 6.56° (0,1,−1), estimated from the single
crystal data, suggesting the aromatic core of the compound may
be adopting a conformation with its long axis at an angle of
∼50° relative to the substrate. Using the coordinates from the
We thank the EPSRC and SAFC Hitech (CASE award to
M.T.L.) for financial support.
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