T.M. Pappenfus et al. / Journal of Molecular Structure 1095 (2015) 96–99
97
Table 1
Crystal data and structure refinement for 3b.
Crystal data
3b
Empirical formula
Temperature (K)
Crystal system
Space group
a (Å)
C
26
H
24Br
173 (2)
Orthorhombic
Pca2
2 2 6
N O
1
29.497(3) Å
6.8609(5) Å
26.001(2)
5262.0(7)
8
b (Å)
c (Å)
3
V Å
Z
3
D (g/cm )
l
1.566
ꢂ1
(cm
)
31.330
44,571
10,770
0.0465
0.1009
Fig. 1. Molecular structures of indigo and isoindigo dyes.
No. of reflections measured
Unique reflections
R1 (I > 2.00r(I))
such as tert-butoxycarbonyl (Boc) can be removed as needed after
processing from solution simply by thermal treatment. In the case
of Boc-protected materials, thermal treatment yields the depro-
wR2 (All reflections)
Goodness of fit
1.026
tected material and gaseous products of CO
reported previously for diketopyrrolopyrrole-based compounds
6,7]. Very recently Boc-protected indigo dyes have been commu-
nicated and these materials allow for solution-processing of indigo
films via simple heat treatment [8]. We now report the application
of this chemistry to isoindigo dyes and outline the synthesis,
crystallographic characterization and physical properties of a
Boc-substituted isoindigo and compare its properties to that of
an alkyl-substituted isoindigo.
2
and isobutene as
suspended solid. To this suspension was added 10 mL of methanol.
The solid was filtered and washed sequentially with water
(5 ꢁ 2 mL), methanol (3 ꢁ 2 mL), and hexanes (3 ꢁ 2 mL) to afford
[
1
0.436 g of 3b (70%) as a bright red–orange solid. H NMR (300 MHz,
CDCl
3
) d 8.84 (d, 2H, J = 8.7 Hz), 8.07 (d, 2H, J = 1.8 Hz), 7.32 (dd,
: C,
2H, J = 8.7, 1.8 Hz), 1.68 (s, 18H). Calcd for C26
H
2 2 6
24Br N O
50.34; H, 3.90; N, 4.52. Found: C, 50.36; H, 3.97; N, 4.52.
X-ray crystallography
Experimental
Table 1 contains crystal data, collection parameters, and refine-
ment criteria for the crystal structure of 3b. Crystals of 3b were
grown by slow evaporation of chloroform/ethanol solutions. A
crystal was placed on the tip of a Mitigen micromount and X-ray
intensity data were measured at low temperature (173(2) K) with
Materials and methods
All syntheses were performed under an inert nitrogen atmo-
sphere. The following were purchased from Sigma–Aldrich and
used as received: anhydrous THF; di-tert-butyl dicarbonate solu-
tion (1.0 M in THF); and 4-(dimethylamino)pyridine (DMAP). The
anhydrous THF was dried further with 3 Å molecular sieves
graphite monochromated Mo K
a radiation (k = 0.71073 Å) on a
Rigaku XtaLAB mini diffractometer. The data were collected and
processed using CrystalClear [16] and then solved and refined
using SHELXL-97 [17]. Distances related to p-stacking are compli-
cated by the bowed nature of the isoindigo core. The two unique
molecules of the asymmetric unit alternate within a given stack
parallel with the b axis. The angle between least-squares planes
formed by the indole atom positions of one molecule of the asym-
metric unit and the neighboring molecule indole atom positions is
0
prior to use using standard procedures [9]. Compound 3c, 6,6 -di-
0
bromo-N,N -(2-ethylhexyl)-isoindigo,
was
purchased
from
0
SunaTech and used as received. 6,6 -dibromo-isoindigo (compound
a), was prepared as previously reported [10]. 1H NMR spectra
3
were recorded on a JEOL Eclipse 300 MHz spectrometer. The chem-
ical shifts are reported in ppm and referenced to the residual chlo-
roform peak (7.26). Elemental analyses were performed by Atlantic
Microlab, Inc., Norcross, GA. Thermogravimetric analyses were per-
formed on a TA Instruments Q50 analyzer under a nitrogen atmo-
sphere. Electrochemical measurements were performed with a
Pine Research Instrumentation WaveNow USB potentiostat using
methods previously described [11]. UV–vis spectra were recorded
on an Ocean Optics USB HR4000 fiber optic spectrometer.
Solution spectra were obtained in a 10 mm quartz cell. Thin film
measurements were obtained by drop-casting onto glass from
chloroform. DFT calculations were performed with the Gaussian
1
.2°. Intermolecular parallel distances between indole least-
squares planes formed by every other molecule within a stack
are 6.70 Å and 6.78 Å, between molecules containing Br1 and
Br3, respectively. Halving these distances provides an approximate
‘
parallel’ nearest neighbor indole-indole intermolecular p-stacking
distances of 3.35 Å and 3.38 Å.
Results and discussion
Synthesis
0
3 program [12]. Geometries and orbital energies were calculated
by means of the hybrid density functional B3LYP [13,14] with the
-31G(d,p) basis set. The input files were generated with
GaussView. Alkyl groups for 3c were replaced by n-propyl groups
to save computational time similar to the approach for other con-
jugated systems [15].
Our synthetic strategy was to focus on a dihalide of isoindigo so
that the resulting Boc-substituted isoindigo can be widely utilized
as a building block in palladium-catalyzed cross coupling reactions
to produce soluble small molecules and polymers [4]. One such
6
0
dihalide is 6,6 -dibromoisoindigo 3a which can be synthesized in
multi-gram quantities with little purification from commercially
available indoles [10]. Here we report the reaction of 3a with
di-tert-butyl dicarbonate and 4-(dimethylamino)pyridine (DMAP)
in THF to afford the Boc-protected isoindigo 3b using procedures
similar to those reported previously [18]. Unlike previous methods,
however, a pure compound is obtained in good yield without the
need for column chromatography.
Synthesis of compound 3b
To a suspension of 3a (0.424 g, 1.01 mmol) in THF (10 mL) was
added DMAP (0.308 g, 2.52 mmol) in one portion. Di-tert-butyl
dicarbonate (5.0 mL, 5.0 mmol) was added to the mixture and
the reaction was stirred for 24 h resulting in a bright red–orange