Planar and distorted indigo as the core motif in novel chromophoric liquid crystals

Article abstract of DOI:10.1039/c5nj01594d

Symmetrically bis-substituted indigo derivatives with long peripheral alkyl chains were synthesised by the reductive condensation of corresponding isatin derivatives. Their thermotropic mesomorphism was investigated with respect to different substitution

Full text of DOI:10.1039/c5nj01594d

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DOI: 10.1039/C5NJ01594D
Journal Name
ARTICLE
Planar and Distorted Indigo as Core Motif in Novel Chromophoric
Liquid Crystals
Received 00th January 20xx,
Accepted 00th January 20xx
[a,b]
[a]
[a]
Jan H. Porada, Jörg Neudörfl and Dirk Blunk *
DOI: 10.1039/x0xx00000x
Symmetrically bis‐substituted indigo derivatives with long peripheral alkyl chains were synthesised by reductive
condensation of corresponding isatin derivatives. Their thermotropic mesomorphism was investigated with respect to
different substitution patterns, including position and lateral modifications of the substituents. A systematic investigation
of structure‐property relationships revealed that only substitution at the 6,6′ posiꢀons affords the calamiꢀc shape
necessary to form smectic or nematic liquid crystalline phases. This finding is rationalised on the basis of a structural
analysis of N,N′‐diacetyl indigo and the consequences for 5,5′‐ and 6,6′‐bis‐substituted derivatives. Some of the liquid
crystalline substances exhibit dichroism, which is especially pronounced in highly ordered phases. Additionally, the 6,6′
substitution leads to a significantly enhanced activity concerning the photochemical trans‐cis isomerization of the N,N′‐
diacetylated indigo derivatives.
www.rsc.org/
the side‐chains of
a
polymer backbone
(side‐chain
1
b, c, 8
Introduction
polymers).
to have liquid‐crystalline properties.
c) Polymerization of chromophoric monomers to a main‐
Such side‐chain polymers are often designed
1
a
The spatial anisotropic alignment of chromophores is an
essential requirement for many high‐tech applications in which
a macroscopic directed effect is generated by interaction of
light and matter. Examples can be found in the field of non‐
1
b, 9
chain polymer or copolymer.
d) the structural integration of the chromophore into a
1
2
3
molecular framework of a low molecular weight anisometric
linear optics, organic photovoltaics, laser technology or
1
0
4
molecule, capable of forming a liquid crystalline phase. This
method is by far the least common but nevertheless a very
promising approach.
supramolecular photo‐chemistry. Due to the rather small
HOMO‐LUMO gap, which causes the colour of the
chromophore, further applications in the field of molecular
5
electronics are possible. Common methods of approaching
the challenge of molecular alignment are shown in Figure 1
and described below. They are:
a) Embedding a chromophoric guest in an anisotropic host
matrix. Such a matrix usually consists of low molecular weight
or polymeric liquid crystals or polymers which have been
aligned by stretching or even oriented in a polar order by
1
b, c
various poling techniques.
chromophore orientation can be achieved by choosing a polar
Field induced switching of the
Figure 1. Possible methods to align chromophores.
6
7
matrix or an appropriate LC cell geometry.
The latter method (d) leads to materials with a significantly
higher chromophore density then host‐guest‐systems (a)
without the possibility of phase separation or physical
demixing. In contrast to polymers (b) or (c), the product can be
synthesized and purified in a precise manner. The presumably
higher solubility of such chromophoric mesogens compared to
polymeric systems would be advantageous for technical
applications.
b) Covalent integration of a chromophoric substructure into
a.
Dr. Jan H. Porada, Dr. Jörg Neudörfl, Dr. Dirk Blunk
Universität zu Köln, Institut für Organische Chemie
Greinstr. 4, 50939 Köln (Germany)
E-mail: d.blunk@uni-koeln.de – Fax: (+49) 221-470 3064
Homepage: www.oc.uni-koeln.de/blunk
Dr. Jan H. Porada,
b.
Universität Stuttgart, Institut für Physikalische Chemie
Pfaffenwaldring 55, 70569 Stuttgart (Germany),
Electronic Supplementary Information (ESI) available: [A description of the
experimental details, the analytical data, UV‐Vis spectra, the calculations of the
In this context indigo seemed to be a suitable chromophoric
molecular core as a starting point for the design of colored
order parameters,

‐orbital axis vectors and DFT calculations is given. CCDC mesogens. This is not only due to its rather small chromogenic
1033834 (12) and CCDC 811039 (23) contain the supplementary crystallographic
motif and its high thermal and photochemical stability, but
also to the C2h‐symmetry of the indigo molecule. Due to the
data for this paper. These data can be obtained free of charge from The Cambridge
Crystallographic Data Center via http://www.ccdc.cam.ac.uk/data_request/cif.].
See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
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ARTICLE
Journal Name
latter, it can easily be substituted in a symmetric way A modified one pot methodology was developed uViseiwnAgrttichleeOlnelisnes
generating rod‐like molecules. Furthermore many synthetic toxic toluene as solvent to transforDmOI:s1u0b.1s0t3i9tu/Ct5eNdJ0i1s5a9t4iDn
approaches towards indigo and its analogues – including derivatives of type
B into the corresponding indigo derivatives
donors like selenium, sulphur or oxygen – are known, which of type (Scheme 2). This procedure was also applied
C
will give broad access to a wide structural diversity of suitable successfully with respect to the synthesis of analogue
molecules. Once the basic structure‐property relations are structures in our previous publication about phasmidic indigo
1
5
studied, a desymmetrisation by corresponding synthetic derivatives. The pendant groups were introduced via Suzuki‐
strategies can broaden the structural diversity and give access Miyaura coupling of the bromoisatins of type (Scheme 2)
with the corresponding boronic acid of the pendant groups 4‐
A
to polar mesophases.
Despite the fact that indigo was intensely studied since the
16
7.
end of the 19th century, the reason for its remarkably low For further diversification and to suppress the formation of
frequency absorption (indicating its small HOMO‐LUMO gap) hydrogen bonds, which are known to cause the high melting
1
7
was inscrutable for decades. It was only in the 1960s that temperature of pure indigo, the vinylogue amide functions
quantum mechanical calculations revealed that the biggest were masked by acetylation, affording the indigo derivatives of
contribution to the chromophoric properties originates from type D (Scheme 2). The acetylation method published by
1
8
the double‐crossed donor‐acceptor substructure, which is Liebermann and Dickhut using acetic anhydride as solvent,
formed by the vinylogue amides. The aromatic rings contribute while reactive towards indigo did not result in any conversion.
to the chromophoric system mainly by stabilizing the coplanar Also, when using polar aprotic solvents, conversion could be
conformation of the molecule and only marginally by observed in NMP, but not in DMF or DMSO. The origin of this
1
1
mesomeric effects. The position of the absorption maximum effect is not completely understood, yet. The whole sequence,
12
can be fine‐tuned by the type of the peripheral substituents.
starting from the boronic acids 4‐7 and bromoisatines A is
Here we present our results concerning the synthesis and the shown in Scheme 2.
thermotropic properties of a variety of symmetrically bis‐
substituted indigo derivatives.
Results and Discussion
Synthesis
The structural requirements for mesogenic behaviour were
established by the introduction of pendant groups with long
aliphatic chains at the lateral positions of the phenyl rings of
the indigo unit. Though numerous synthetic approaches to
indigo are known, most of them are inadequate for the
synthesis of complex derivatives, due to their aqueous PCl
reaction conditions which are incompatible with the extended
Scheme 2. General synthetic route to the symmetrically bis‐substituted indigo
derivatives of type C and D. a) K PO (aq, 2 eq), Pd(PPh (2 mol%), DME, 90°C; b) 1.
(1.05 eq), 2. PhSH (2.2 eq), toluene, 100°C; c) AcCl/Ac O, NMP, 90°C.
3
4
3 4
)
5
2
aliphatic moieties of the pendant groups. An exception which In order to evaluate the influence of the pendant groups on
employs organic solvents, is the reaction of isatin (1) with mesomorphic behaviour, four boronic acids 4‐7, shown in
phosphorus pentachloride in benzene and the subsequent Figure 2, were employed. Those differ in the chain type being
reductive condensation of the resulting 2‐chloro‐3H‐indole‐3‐ an alkyl‐ (
one (2), finally using zinc dust or hydrogen iodide as reducing the substituent with a fluorine atom (
agent to yield indigo (3) with varying amounts of indirubin as While 4‐6 were synthesised according to published
4) or alkoxy‐chain (5‐7) or in a lateral substitution of
6
) or a methyl group ( ).
7
1
5, 19
by‐product. Actually, this procedure was one of the first indigo procedures
the 4‐dodecyloxy‐2‐methyl‐phenyl boronic
7) was obtained by selective bromination of the 4‐
13
syntheses and was described by Baeyer in 1879. The acid (
formation of indirubin can be avoided by applying refined position of m‐cresole in acetonitrile,
20
followed by
reductive conditions, i.e. using thiophenol as the reducing etherification with bromododecane and treatment with n‐
agent in benzene (Scheme 1) under inert atmosphere as butyl lithium and trimethylborate and subsequent acidic
1
4
published by Katritzky et al.
workup.
Scheme 1. Synthesis of indigo (3) with thiophenol as mild reducing agent performed in
14
an organic solvent, as published by Katritzky.
Figure 2. The boronic acids of the employed pendant groups 4-7.
2
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DOI: 10.1039/C5NJ01594D
Figure 3. The four symmetrically 4‐dodecyloxyphenyl‐bissubstituted indigo‐derivatives 8‐11.
Mesomorphic Properties
12 and 13 shown in Figure 4. The introduction of methyl
groups as possible alternative was not performed, because the
dimethyl indigo is known to exhibit a low thermal stability and
The symmetrically substituted compounds of type
C were
investigated, in order to observe how shape anisotropy
affected the mesogenic properties of bis‐substituted indigo
derivatives.
2
1
readily oxidizes to the methyl isatin
The four resulting regioisomeric 4‐dodecyloxyphenyl‐
bissubstituted indigo‐derivatives 8‐11 are shown in Figure 3.
Their thermotropic behaviour was investigated by polarization
microscopy (PM) and differential scanning calorimetry (DSC)
and is summarized in Table 1.
Table 1. Thermal data[a] of the four symmetrically 4-dodecyloxyphenyl-bissubstituted
indigo-derivatives 8-11 shown in Figure 3.
Cr
●
●
●
●
●
—
SmX
—
I
8
9
— / —
350 / 348
325 / 320
— / —
253 / 251.1 (37.6)
Decomposition
Decomposition
217 / 218.2 (30.1)
●
—
—
●
Figure 4. The N,N′‐diacetylated indigo derivatives with 4‐dodecyloxyphenyl pendant
groups in positions 5,5′ (12) or 6,6′(13).
10
11
—
[
a] Form of designation: PM / DSC [°C] (ΔH [kJ/mol]), SmX: smectic phase of
With compound 12 and 13 a dramatic reduction of the melting
temperatures, accompanied by an increase of the solubility
unknown type.
was achieved with respect to their parent compounds
, respectively.
9 and
As seen from Table 1, compounds
8 and 11 show a direct
1
0
transition from crystal to isotropic liquid, whereas the
compounds and 10 melt at much higher temperatures, while
undergoing decomposition. Shortly before decomposition of Figure 4.
compound , a fan‐like texture, indicating a smectic phase,
9
[a]
Table 2. Thermal data of the N,N′‐diacetylated indigo derivatives 12 and 13 shown in
9
Cr
●
●
●
I
was observed by PM. Further characterization of the phase
was not possible, due to decomposition. The different melting
behaviour of these four compounds can be rationalized with
regard to the role of the hydrogen bonds. It can be assumed
[
b]
1
1
2
3
127 / 124.8 (53.2)
181 / 180.6 (10.7)
SC
SmA
162 / 160.5 (26.7)
201 / 203.9 (7.0)
●
●
[a] Form of designation: PM / DSC [°C] (ΔH [kJ/mol]), [b] could only be observed
that
indigo which causes the high melting temperatures. In
compounds and 11 these intermolecular H‐bonds are
9
7and 10 form a hydrogen bond network, similar to in the first heating cycle, SC: soft crystalline phase, SmA: smectic A phase.
1
Upon heating, 12 undergoes a phase transition at 124.8°C into
a soft crystalline phase SC which could only be observed on
first heating. At 160.5°C this SC phase melts into the isotropic
liquid. Compound 13 shows a phase transition at 180.6°C into
the SmA phase, as determined by PM. The phase grows from
the isotropic melt as bâtonnets, which finally converge into a
fan like texture. Upon shearing homeotropic alignment could
be achieved. By conoscopy a uniaxial, optical positive
character was identified, which is in accordance with an
orientation of the molecular long axis perpendicular to the
layers and parallel to the viewing axis.
8
sterically hindered by the 4‐dodecyloxy substituents, which
leads to a significant reduction of the melting temperatures.
This interpretation is supported by the fact that
exhibit unusual high solubility in solvents as chloroform,
whereas and 10 only form colloidal suspensions in
chloroform.
Due to their rod like molecular shape the indigo derivatives
and 10 seemed to have the most promising structural motifs
for designing liquid crystalline compounds. To overcome the
high melting temperatures the vinylogue amide functions were
masked by acetylation leading to the diacetylated compounds
8 and 11
9
9
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In addition to our previously published phasmidic indigo methylated pendant groups of 16 reduce the staVibewiliAtryticloefOntlhinee
derivatives, 13 now represents the first example of a calamitic crystalline phase of the N,N′‐unsubstitutDedOI:c1o0m.10p3o9u/Cn5dNeJ0n1o5u94gDh
liquid crystal that possesses an indigo core. To gain insight into to melt, without decomposition, into an isotropic phase after
the effect of different pendant groups on the mesomorphic transitioning through a soft crystalline phase SC.
behaviour, the structures summarized in Figure 5 were The mesomorphic behaviour of N,N′‐diacetylated compounds
examined. The influence of the oxygen atom in the alkoxy 12 and 17 differ clearly with respect to each other. The soft
chain was evaluated by comparison to analogue alkyl crystalline phase SC, which was found in 12 cannot be
derivatives bearing no oxygen. Then the effect of lateral detected in compound 17. Further, the melting point is
substituents on the pendant groups was investigated. A reduced by 53°C with respect to 12 and solidifies as an
differentiation between electronic and steric effects was isotropic glass. For the corresponding laterally fluorinated
achieved by using fluorine or methyl groups as the lateral compound 18 a stabilization of the crystalline phase by 10°C
substituent. The thermal data of these compounds are could be observed compared to 12
.
presented in Table 3. The influence of the four varied The laterally methylated analogue 19, much like 17, shows
parameters, i.e. the substitution positions, the N‐substituent significant destabilization of the crystalline phase by
R1, the chain type R2 and the lateral substituent X (see Figure approximately 50°C with respect to 12. None of the 5,5′‐bis‐
5
) will be discussed in the following part. In this discussion the substituted, N,N′‐diacetylated compounds possess liquid
main subdivisions are given at first by the substitution crystalline phases.
positions and after that by the N‐substituent R1.
Table 3. Thermal data of the indigo derivatives shown in Figure 5.
Cr
●
●
I
1
1
1
1
1
1
2
2
2
2
2
2
4 ●
— / —
—
345/341
SmX
Decomposition
Decomposition
266/268.6 (56.6)
—
—
●
●
●
●
—
●
●
●
5 ● — /116.6 (38.3) Cr
2
320/322.9 (36.5) SmX
—
—
—
—
—
6 ●
7 ●
8 ●
9 ●
0 ●
— / —
— / —
— / —
— / —
— / —
— /241.4 (4.4)
— / —
SC
—
—
—
—
[b, c]
108/106.8 (56.2)
— / —
— / —
288/ —
169/170.9 (32.6)
110/110.6 (47.2)
Decomposition
1 ● — /165.8 (13.7) Cr
2 ● — / —
3 ● — /99.2 (18.8) SC
4 ● 141/140.7 (15.5) SmA 145/145.9 (1.3)
5 ● — / — — / —
2
— /239.2 (7.9) SmF/I 295/296.8 (56.1)
— /141.0 (20.5) SmF/I 261/263.8 (44.2)
172/168.1 (7.6) SmA 176/178.0 (6.9)
—
[d]
2
N
—
149/149.3 (1.9)
199/198.0 (50.3)
●
●
—
[a] Form of designation: PM / DSC [°C] (ΔH [kJ/mol]), [b] could only be observed
in the first heating cycle, [c] solidifies as amorphous glass, [d] values from cooling
cycle, Cr: crystalline phase, SmX: smectic phase of unknown type, SC: soft
crystalline phase, SmF/I: smectic F or smectic I phase, SmA: smectic A phase, N:
nematic phase.
The 6,6′‐bis‐substituted indigo derivatives already show clear
differences at the N,N′‐unsubstituted stage, in contrast to the
5
,5′‐bis‐substituted derivatives. Compared to the bis‐
alkoxyphenyl substituted compound 10, the bis‐alkylphenyl
substituted compound 20 shows a reduction of the melting
temperature by approx. 30°C, but decomposition still occurs.
For both, the N,N′‐unsubstituted laterally fluorinated
compound 21 and especially the lateral methylated compound
Figure 5. Compilation of the substitution patterns of the synthesised indigo derivatives
with pendant groups in the positions 5,5′ or 6,6′ and N,N′‐disubstitution, respectively.
22 a decrease in stability of the crystalline phase in comparison
to 10 and the formation of a high order liquid crystalline phase
were observed. While the decrease of the crystal phase
stability is approximately 80°C for the fluorinated compound
21, it is 180°C for the methylated analogue 22. Both
compounds clear into the isotropic phase without
decomposition. The liquid crystalline phase found for 21 and
22 was assigned either to be of SmI or SmF type, based upon
the following observations:
Shearing is possible, but the high resistance indicates a high
degree of molecular order. Furthermore, the high transition
For the 5,5′‐bis‐substituted indigo derivatives a direct
comparison between the N,N′‐unsubstituted compounds
9 and
1
4
differing only by an oxygen atom in the chain reveals a
minor stabilization of the crystalline phase, caused by the
alkoxy oxygen. Upon introduction of lateral fluorinated
pendant groups of compound 15 the formation of a second
crystalline phase Cr2 was observed at 116.8°C, which
decomposes at 320°C, forming a smectic phase while
decomposing, also observed in the compound 9. The laterally
4
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enthalpies of the liquid crystal to isotropic transition support derivatives was already reported for 6,6′‐dichloroindigo in
View Article Online
2
2
DOI: 10.1039/C5NJ01594D
this assumption.
The phase growth from the isotropic melt is dendritic, as is can easily be generated within the LC phase by preparation on
typically observed for highly ordered phases. treated substrates and reach dimensions not easily accessible
crystalline form. Large and homogeneous oriented domains
A smectic phase is more reasonable than a columnar one, due with crystalline materials. The orientation can be frozen by
to the particular molecular shape and the non‐curved rapid cooling, so that e.g. the appearance of dichroism in the
polar/apolar‐interface of these compounds.
LC phase might also be valuable for technical applications. The
13 are presented in the supporting
Conoscopy revealed an optical biaxial character in each UV‐Vis spectra of
domain. An orientation of the bisectrix parallel or orthogonal information.
to the viewing axis could be excluded. Such an orientation
cannot be explained without assuming an intrinsic tilt of the
molecules with respect to the layer normal. Therefore a SmB‐
phase can be excluded.
8‐
Evidently, the bis‐substitution in the positions 6 and 6′ has a
greater influence on the phase behaviour compared to bis‐
substitution in the positions 5 and 5′. Depending on the 6,6′‐
bis‐substituent the stability of the crystal phase can be
reduced tremendously. An explanation could be that sterically
Figure 7. The liquid crystalline phase of 22 at 260°C with only one polarizer horizontal
left) and vertical (right) exhibits strong dichroism.
demanding substituents in these positions cause a change of
(
the H‐bond network. The three dimensional network, known
1
7
from indigo, in which each molecule is connected with each
one H‐bond to four other molecules oriented perpendicular is
given up in favour to strands where each molecule is
connected by two H‐bonds to two others in parallel
orientation. As a result of such uniform alignment of light
absorbing molecules a pronounced dichroism should be
observed, which was found for the 6,6′‐bis‐substituted indigo
derivatives 21 and 22 in the LC phase. Figure 6 shows the UV‐
Vis‐absorption spectra of 22 with two maxima in the visible
range. The absorption at 604 nm causes a blue visual
impression, whereas absorption at 398 nm relates to a yellow
colour. Indeed, as consequence of the superposition, the
substance appears green.
The N,N′‐diacetylated, 6,6′‐bis‐alkylphenyl substituted 23 leads
to a different phase sequence as well as a lower clearing
temperature, compared to its alkoxy‐analogue 13. At 99.2°C a
phase transition into a soft crystalline phase SC2 was detected,
which manifests in a paramorphic “quasi fingerprint texture”
when cooling from the upper SmA phase. Such a texture is
known from the soft crystalline E phase, but at this stage of
investigation a definite assignment cannot be given. The SmA
phase of 23 was identified with PM and conoscopy observing
an identical appearance as the SmA phase of 13
.
The corresponding laterally fluorinated or methylated
compounds 24 and 25 exhibit complementary properties with
respect to their N,N′‐unsubstituted analogues, but also with
respect to each other. The crystalline phase of the lateral
methylated compound 25 shows a much higher stability
compared to the laterally fluorinated compound 24, which is
the opposite case as observed for the N,N′‐unsubstituted
analogues. The methylated indigo derivative 25 melts directly
into the isotropic phase at 198.0°C without the previous
formation of a liquid crystalline phase. The lateral methyl
group even stabilizes the crystalline phase by approximately
1.2
1.0
0.8
0.6
0.4
0.2
0.0
292
604
398
2
0°C compared to the compound 13 bearing no lateral
substituent.
The fluorinated compound 24 shows a phase transition at
1
40.7°C into the SmA phase followed by a transition at 145.9°C
300
400
500
600
700
800
into the nematic phase, which clears at 149.3°C. Both phase
types have been identified by characteristic textures observed
by PM. The SmA phase shows a fan like texture and has an
identical appearance as described for 13. The nematic phase
exhibits a schlieren texture including singularities with the
disclination strength of s = ±½ and s = ±1. The phase
assignments were confirmed by wide angle X‐ray scattering

 nm
Figure 6. UV‐Vis spectra of 22 (20 mg/L in NMP).
Under the polarization microscope, when applying only one
polarizer domains ranging from blue to yellow can be
observed. Upon rotating the polarizer by 90° the colour
appearance of the blue and yellow domains are inverted
(
WAXS). Additionally the order parameter S for each phase
type could be obtained from the intensity profile of the wide
angle area according to the method of Davidson et al. (see
(
Figure 7), indicating that the absorption maxima belong to
electronic excitation with the vectors of the transition dipole
moments being oriented orthogonal. Dichroism in indigo
2
3
supporting information).
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DOI: 10.1039/C5NJ01594D
Figure 8: Rotational distortions of the ring systems with respect to the central double bond induced by N,N′‐disubstitution of the indigo motive. When the molecule is positioned in
the centre of the xy‐plane with the double along the x‐axis a rotational distortion around the x‐axis causes a twist (left), around the y‐axis a buckle and around the z‐axis a skew
2 2
(right). Whereas for the twist and the buckle the symmetry point group is reduced from C2h to C , it is retained C2h for the skew as this distortion is along the C ‐axis of the
molecule.
2
6
The nematic phase of 24 possesses a value S=0.47 at 148°C, al. In the POAV1 theory the p‐orbital axis vector is defined to
which is within the typical range of the nematic phase. The have equal angles  ≥ 90° to all three ‐bonds. ‐90°
determined order parameter for the SmA‐phase is S=0.55 at precisely quantifies the pyramidalisation from which the s p

 =
m
2
4
n
1
44°C, which is unusually low. This is most likely a result of hybridization and the average sp hybridization may be
the bent shape of the distorted N,N′‐diacetyl indigo core which calculated. The degree of pyramidalisation of the C2‐atom C2
is discussed in more detail below.
= (C2)‐90° is a direct measure for the buckling of the
molecule.
Conformational Aspects
However, in the POAV1 theory an equal ‐bond hybridization
For the examined N,N′‐diacetylated compounds liquid in C3v symmetry is assumed, which may lead to significant
crystalline behaviour was exclusively found with a substitution deviations of the actual hybridization, when the bond angles
pattern in the 6 and 6′ posiꢀons. In order to gain insight into ij to the surrounding atoms substantially differ. This case is
the origins of this observation, the inherent molecular given for the here examined indigoid structures and a more
m
distortions of N,N’‐diacetyl indigo in general were analysed precise description of the axis vector and the s p‐hydridisation
and in a later step compared to those of the peripherally bis‐ is then obtained from the POAV2 theory which treats the ‐
2
7
substituted N,N’‐diacetyl indigos 12 and 23
.
bond hybridizations individually. The dihedral angle 
‐orbital
When the planar indigo molecule is N,N′‐disubstituted, the ring between two POAVs accurately designates the

systems, which are connected by the central double bond, alignment and the actual twist within the molecule and is
experience rotational distortions in all three spatial dimensions separated from the buckling distortion. The torsion angle C2‐
with respect to the double bond as illustrated in Figure 8.
It is known that N,N′‐disubstitution of indigo leads to a the performed calculations of the POAVs are given in the
buckling and twisting of the central double bond. This supporting information. The angle is defined as the angle
phenomena is caused by the steric demand of the substituent between two lines, one passing through the centre of C5/C6
precisely quantifies the twist of the indigo motif. Details on
C2′

2
5
and was first observed for N,N′‐dimethyl indigo, but appears and the centre of C5′/C6′, the other one passing through C2
in more pronounced form in N,N′‐diacetyl indigo. The buckling and C2′. This angle specifies the lateral skew of the ring
is a direct result of the twist of the central ‐bond, since the systems with respect the central double bond. It is defined to
sp2 orbitals of the bonded atoms rehybridise with a higher p‐ be > 0 when the ring systems are shifted in a direction in



n
character (sp , with 2<n<3), which leads to a pyramidalisation which the atoms C5 and C5′ are more in line with the central
of the bond geometry. Origins of the skew are different bond double bonds than C6 and C6′.
lengths and pyramidalisations for different atoms within each Additionally, four further characteristic angles were
ring system.
determined. The effective ring twist  describes the torsion
Because buckling and twisting are interrelated to each other it between the ring systems and is defined as the dihedral angle
is nontrivial to separately describe these distortions as of C5 ‐ C6 ‐ C6′ ‐ C5′. This twist originates from the torsion C2‐
required for a quantitative comparison between different
of the central double bond but may be altered in magnitude
C2′
N,N′‐diacetylated indigos. A powerful tool was found in the p‐ by compensation or amplification due to further internal twists
orbital axis vector (POAV) analysis, as introduced by Haddon et within the ring systems. βC5 is defined as the angles between
This journal is © The Royal Society of Chemistry 20xx
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Page 7 of 11
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New Journal of Chemistry
Journal Name
two lines, one passing through the C5 carbon atom and the system, hence an effective ring twist
ARTICLE

and an overall bent  of
View Article Online
DOI: 10.1039/C5NJ01594D
opposing C7a atom in the same phenyl ring, the other one zero. Consequently, β and β are zero, indicating a parallel
5
6
passing through the C5′ and the C7a′ of the phenyl ring on the alignment for each set of lines. The seemingly high skew of
 =
other side of the central double bond. βC6 is correspondingly 11.63° is an artefact caused by its definition, since for a
defined for lines passing through C6 and C3a on the one side stepwise alignment it determines the magnitude of the step
and C6′ and C3a′ on the other side. These angles can have and not the lateral skew. Other possible definitions as for
values from 0° for parallel alignment to 90° for orthogonal instance the dihedral angle between the centre of C5/C6, the
alignment and serve as a measure of the bent of the indigoid centre of C5′/C6′, C2′ and C2 indicate a skew of 0°.The
structure with respect to the corresponding substitution situation is very different for the C ‐symmetric conformer,
2
position 5 and 6. The overall bent
 is defined as the sum of resulting from the combination of the distortions twist, buckle
two angles each between two lines. The central line is passing and skew in all three spatial dimensions. In this conformer the
through C2 and C2′. The plane lines are connecꢀng the centre ring systems of the indigo structure are distorted in a
of C5/C6 and centre of C3a/C7a on the one ring system and disrotatory way by the twist and buckle deformation, but in a
the centre of C5′/C6′ and centre of C3a′/C7a′ on the other ring conrotatory way for the skew deformation.
system. The sum of the angle between the central line and
each plane line describes the position unspecific bent of the
molecule. The plane lines are on the same plane as the lines
used to define βC5 and βC6 which allows comparing these
different measures for the bent directly.
Computational Results
Figure 9. Calculated geometries of N,N′‐diacetyl indigo in the actual C
2
‐symmetry (left)
and the hypothetical C‐symmetry (right).
i
The molecular geometry of N,N′‐diacetyl indigo obtained from
X‐ray diffraction was discussed and compared to quantum
mechanical calculations on the AM1‐SCF‐level by Grimme et
For the sake of clarity it is helpful to first describe the effect of
each distortion separately. The torsion of the central double
2
8
al. However, in this discussion different angle definitions
bond by
 just by itself twists the ring planes, such that  = ,
were applied. The C ‐symmetric molecular shape of this
2
since the defined peripheral bond vectors of the dihedral angle
are perpendicular to the rotational axis. In this case the 5 and
calculated structure is in accordance with the overall
molecular geometry found from the X‐ray diffraction, but
shows strong deviations with respect to the magnitude of the
angles. Therefore, we decided to recalculate the molecular
5
′ posiꢀons migrate to the opposite face of the central double
bond than the 6 and 6′ posiꢀons.
The magnitude of β and β is identical, but smaller than
2
9

, as
the defining lines are not perpendicular to the axis of rotation.
The overall bent would stay zero for only a twist deformation
geometry using the more recent B3LYP density functional
and the 6‐311G(d)‐basis set, as implemented in the Gausian
5
6
3
0

0
3
package (see supporting information). Though in the
since all lines are along the rotational axis. The buckling of the
C2‐symmetric conformer is a result of the pyramidalisation of
crystal structure solely the C ‐symmetric species was found,
2
we also considered an alternative Ci‐symmetric molecular
geometry in our calculations, which also minimizes the steric
strain and in principle may exist in solution or fluid phase. In
C‐2 and C‐2′ by
 and ′, respectively, with identical algebraic
signs. Therefore, the buckling without twist or skew would
lead to a bending of the central double bond, leaving the 5, 5′,
such a C ‐symmetric conformation the pyramidalisation
 is of
i
6
and 6′ posiꢀons in the same plane, parallel to the double
opposing algebraic sign for C2 and C2′, thus the angle of
torsion (C2‐C2‘) of the ring systems is 180°, which leads to a
step‐like arrangement. Both possible geometries were found
as local energetic minima and are shown in Figure 9. However,
the difference of the sum of electronic and zero point energies
of both the C ‐ and C ‐symmetric conformations was computed
bond, resulting in = 0 and βC5 = β . The overall bent would


C6
be more acute than βC5 and βC6 as line between the centres of
C5/C6 and C3a/C7a runs perfectly perpendicular to the axis of
distortion, while the defining lines β and β of are not. The
skew deformation by itself leaves both ring systems on the
same plane, thus the effective ring twist stays zero. As the
5
6
2
i

to be ∆H=32.1 kJ/mol, favouring the C ‐symmetric conformer.
2
ring systems are rotated conrotatory the defining lines for βC5
Thus the existence of the C ‐symmetric conformer in solution
i
and βC6 each stay parallel, hence βC5 = βC6 = 0. The defining
or a fluid phase can be neglected.
The calculated and experimental results are summarized in
Table 4. The stepwise arrangement of the two ring systems in
angles for the overall bent
opposing algebraic signs and will cancel out, resulting in
As indicated in Figure 8 the deformations twist and buckle, by
itself, result in a chiral C ‐symmetric conformer. By inverting

are of the same magnitude, but of

= 0.
the C ‐symmetric conformer only allows a relief of the steric
i
strain of the N‐acetyl bond of 38.4° by an extremely high
torsion (N‐C8). As a result here the most dominant
pyramidalisations are found for N and C8, whereas the
pyramidalisations for C2 and C3 are significantly lower than in
2
the algebraic sign of either
 and ′ or  the enantiomer of
the respective conformation is formed. Since enantiomers are
energetically degenerated, there is no directional preference
for the bending or the torsion of the double bond.
the C ‐symmetric conformation. The inversion symmetry of
2
the C ‐conformer implies opposing algebraic signs for each ring
i
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PNl ee aws eJ do ou rnn oa tl ao df j uC sht emm ai rs gt ir ny s
Page 8 of 11
View Article Online
Table 4. Comparison of characteristic angles and distances of N,N′‐diacetyl indigo obtained by X‐ray diffraction and DFT‐computation applied to the C ‐ and C‐symmetric
2
i
DOI: 10.1039/C5NJ01594D
conformers.
2
8
C
(
i
‐symmetry
DFT calc.)
C
2
‐symmetry
N,N′‐diacetyl indigo (exp.)
(DFT calc.)
[
a]
[b]








C2


[°] / m
[°] / m
±1.71 / 0.002
±7.07 / 0.029
±0.23 / 0.000
±2.77 / 0.005
180.00 / 1.362
±11.64 / 1.514
±159.12 / 1.413
±38.40 / 1.438
11.63
5.44 / 0.017
3.91 / 0.009
1.35 / 0.001
1.85 / 0.002
‐24.15 / 1.364
12.44 / 1.509
162.06 / 1.407
‐21.02 / 1.424
4.28
5.90 / 0.020
5.99 / 0.021
1.72 / 0.002
[a]
[b]
[b]
N
[a]
[c]
C3


[°] / m
[°] / m
[a]
[b]
C8
[
1.36 / 0.001
b]




(C2‐C2′) [°] / dC2‐C2’ [Å]
‐19.37 / 1.349
[
b]
[c]



(C2‐C3)



[°] / dC2‐C3 [Å]
[°] / dC2‐N [Å]
[°] / dN‐C8 [Å]
13.87 / 1.501
[
b]
[c]
(C2‐N)
157.67 / 1.414
[
b]
[c]
(N‐C8)
‐20.49 / 1.411

Ln (5/6‐5′/6′) to Ln (2‐2′) [°]
(C5‐C6‐C6’‐C5’)[°]
C5 = Ln (5‐7a) to Ln (5′‐7a′) [°]
C6 = Ln (3a‐6) to Ln (3a′‐6′) [°]
= Ln (5/6‐3a/7a) to Ln (2‐2′) +
Ln (5′/6′‐3a′/7a′) to Ln ( 2‐2′) [°]
7.37
‐19.17
52.90
32.44

0.00
0.00
0.00
‐24.67
49.14
23.45
β
β

[c]
0.00
43.65
51.44
m
m

X
denotes the angle of pyramidalisation of the atom X, m the degree of s‐character of the s p hybrid orbital, (X‐Y) the dihedral angle between the s p‐orbitals of the
two neighbouring atoms X and Y and dX‐Y the distance between X and Y. Pointy brackets indicate the average value from both sides of the central double bond. “Ln”
denotes a line which was defined to pass through the specified atoms. The angle between the lines was determined using the Diamond 3 software from Crystal Impact.
The notation X/Y indicates that a dummy atom at the centre between atom X and Y was used. [a] obtained from POAV1 analysis, [b] obtained from POAV2 analysis, [c]
significant asymmetry. A detailed list of all the calculated parameters is given in the supporting information.
The conformers, that result when twisting and buckling occur Though the DFT‐calculation of the C
together, still retain C ‐symmetry, because the C ‐axis for gives much better results than obtained by the AM1‐SCF‐
2
‐symmetric conformer
2
2
both, singly distorted conformers are identical, but now are method, it can be seen that the pyramidalisation  and the
m
diastereomeric. The buckling in one direction energetically s‐character of the s p hybrid orbital of the atoms C2, C3 and
discriminates the direction of the twist and vice versa. In the especially N are generally underestimated, when compared to
preferred conformation the acetyl groups are on the convex the crystal structure, whereas C8 is slightly overestimated. On
face of the molecule, due to the reduction of steric strain. Thus the other hand the torsion (C2‐C2′) of the central ‐bond was
the 5 and 5′ posiꢀons, which are para relaꢀve to the amide predicted to be 4.8° higher than actually found, which is also
groups, are pushed towards the concave molecular face. While reflected in the difference of the bond lengths, whereas the
the degrees of the effective ring twist

and the overall bent

average torsion (C2‐N) was computed to be smaller by 4.4°. It
stay nearly unaffected by the combination of these distortions, has to be mentioned that due to crystal packing effects the
there is now a substantial difference between βC5 and βC6, these average torsions angles show significant asymmetric
since the angle between the lines defining βC5 are now more deviations of 7‐12°, however the more consistent bond length
acute than the overall bent
less acute. Just considering the twist and the buckle The most significant deviations were found in connection to
deformation the difference between and each β angle should the nitrogen atom. It seems that the steric strain, which is
 and the lines defining βC6 are now substantiate this trend.

be identical. However, the positive skew rotates the ring induced by the acetyl groups, is stronger compensated by the
systems in a way that the positions 5 and 5′ are more in line N pyramidalisation N and the torsion (C2‐N) than predicted.
with the central double bond, which shifts up the ring systems This allows a less twisted central
‐bond and a higher degree
on their planes and decreases both βC5 and β , while leaving of conjugation between the two ring systems. On the other
C6
the overall bent
molecular bending βC5 is now only slightly higher than
whereas the position specific molecular bending βC6 is to the computed gas phase structure. For both cases, the
significantly reduced compared to . As a result 5,5′‐bis‐ computed and the measured structure, the effective ring twist
substituted N,N′‐diacetyl indigo derivatives show high very accurately reflects torsion (C2‐C2‘) of the central ‐bond.
molecular bents and are not suitable for the formation of The overall bent – and thus βC5 and βC6 – was found to be

nearly unaffected. Thus the position specific hand, due to the higher distortion of the nitrogen bonds the
,
skew angle is significantly increased by 3.1°, when compared





mesophases, whereas the 6,6′‐bis‐substituted N,N′‐diacetyl substantially higher for the crystal structure than predicted for
indigo derivatives are much less bent. The molecular shape of the gas phase. However, in the crystal structure the two angles
the latter derivative is still not ideal rod‐like, but within the contributing to , one for each ring system, largely differ with
window of calamitic mesogens. Deviations in magnitude from 34.5° and 16.9°, indicating strong crystal forces. The higher
this general trend are caused by further internal distortions value was found for the ring system with the stronger
within each ring system.
pyramidalisation of atoms.
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Page 9 of 11
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New Journal of Chemistry
Journal Name
ARTICLE
Crystal Structures
pattern or from crystal packing effects, the geometries of a set
of correspondingly substituted N,N′‐diacetyl indigo structures
was computed using the same DFT‐method as described above
View Article Online
DOI: 10.1039/C5NJ01594D
For the peripherally bis‐substituted compounds 12 and 23 it
was possible to obtain crystals suitable for single crystal X‐ray
diffraction. Figure 10 and Figure 11 show two projections of
the molecular structures of 12 and 23, respectively.
(
see supporting information). With regard to the
computational effort the chain length of the substituent was
reduced to 4‐ethylphenyl (PhEt) and 4‐methoxyphenyl
(
PhOMe).
In Table
5
the experimentally determined molecular
geometries of 12 and 23 are compared to the computed
structures. The C2 pyramidalisation C2 is in good accordance
for all structures, slightly increased for 12. The torsion (C2‐C2‘)
of the central double bond shows a high conformity in all
experimentally determined geometries, but seems to be
systematically overestimated by 4‐5° in the computed
structures. Substantial differences were found for the
parameters, which are associated with the nitrogen atom and
the acetyl group. Compared to the unsubstituted N,N′‐diacetyl
indigo structure the pyramidalisation N of the nitrogen atom
is strongly decreased for the 5,5′‐bis‐substituted 12, whereas it
Figure 10. Structure of the 5,5′‐bis‐substituted compound 12 in the solid state, shown is significantly increased for the 6,6′‐bis‐substituted 23, also
in two projections. The anisotropic displacement parameters are drawn at the 50%
probability level. Oxygen atoms are drawn in red; nitrogen atoms in blue. Only one
crystallographic type of molecule was observed.
indicated by the higher torsion (C2‐N) between the nitrogen
and the neighbouring carbon of the central
‐bond. This
finding could be explained by the conjugation between the
electron pushing substituent in position 5 with the N‐acetyl
group in the same ring system as well as the N‐acetyl and the
carbonyl group of the opposing one – all involving the nitrogen
atoms. A conjugation for substituent in position 6 only occurs
with the carbonyl group of the same ring system, leaving the
nitrogen atom unaffected. The computed geometries of
correspondingly substituted confirm this trend but to a
diminished extent indicating strong crystal forces. In 12 the
steric strain induced by the acetyl groups is mainly
compensated by a high torsion (N‐C8) between nitrogen atom
and the carbon of the acetyl group, whereas 23 shows much
lower values for these torsions. The position specific bent
angles βC5 and βC6 of 12 are in good accordance with those of
the computed structures. In 23, on the other hand, the stain
seems to be compensated to a higher extent by internal
distortions of the ring systems causing them to be less twisted
Figure 11. Molecular structure of the 6,6′‐bis‐substituted compound 23 as found in the
crystalline state, shown in two projections. The anisotropic displacement parameters
are drawn at the 50% probability level. Oxygen atoms are drawn in red; nitrogen atoms against each other but generally more bent than 12, as
in blue. Only one crystallographic type of molecule was observed.
indicated by values for
 and . Both βC5 and especially βC6 of
compound 23 are higher than predicted computationally. All
Like in the unit cell of the parent structure N,N’‐diacetyl indigo computed structures show independently of their substitution
only one crystallographic type of molecule was found as a pattern only little deviation among each other for all
racemate of conformers in both crystal structures. However, determined parameters. Thus, it is very likely that the strong
2
3
was crystallized from NMP and crystal structure still distortions found in 23 rather originate from crystal effects
contains two molecules of NMP per molecule 23 with one than from its substitution pattern. The value for βC6 of 23 in
them being disordered.
the fluid phase may therefore be expected lower than found in
It is known for anilines that electron withdrawing substituents the crystal structure.
decrease the pyramidalisation of the nitrogen due to an The observed bent angle of approximately 130° for the 5,5′‐
increased donor‐acceptor conjugation between the nitrogen bis‐substituted indigo derivatives is within the suitable range
32
lone pair and the substituent. This effect is strongest for para‐ for bent‐core mesogens. However, these phases were not
substitution, but prevails to a lesser extent for meta‐ observed for the compounds presented in this article. The 6,6′‐
31
substituents. Electron pushing substituents as 4‐alkylphenyl bis‐substituted indigo derivatives have a more obtuse bent
or 4‐alkoxyphenyl show the opposite effect. In order to angle of approximately 150°. Bent angles around 150° are
estimate if the different distortions of 12 and 23 found in their considered to be on the border between calamitic and bent
crystal structures originate from the altered substitution core mesogens.
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PNl ee aws eJ do ou rnn oa tl ao df j uC sht emm ai rs gt ir ny s
Page 10 of 11
View Article Online
Table 5. Comparison of the experimentally determined characteristic angles and distances of compounds 12 and 23 as well as the computed values for 5,5’‐ and 6,6’‐bis‐
DOI: 10.1039/C5NJ01594D
substituted N,N’‐diacetyl indigo compounds with 4‐ethylphenyl (PhEt) and 4‐methoxyphenyl (PhOMe) substituents.
12 (5,5’)
23 (6,6’)
5,5’‐DiPhEt
5,5’‐DiOMe
6,6’‐DiPhEt
6,6’‐DiOMe
Exp.
Exp.
Calc.
Calc.
Calc.
Calc.
Space group
P2
1
/n
1
P2 /c
[a]
[b]








C2


[°] / m
6.18 / 0.022
5.57 / 0.018
8.83 / 0.047
1.58 / 0.001
1.33 / 0.001
‐19.71 / 1.351
15.21 / 1.500
153.82 / 1.418
‐17.18 / 1.412
5.86
5.52 / 0.017
3.84 / 0.008
1.33 / 0.001
1.83 / 0.002
‐24.60 / 1.364
12.44 / 1.510
162.10 / 1.407
‐20.48 / 1.423
4.20
5.44 / 0.017
3.89 / 0.009
1.37 / 0.001
1.75 / 0.002
‐24.53 / 1.364
12.11 / 1.510
162.33 / 1.407
‐20.75 / 1.422
4.06
5.43 / 0.017
4.22 / 0.010
1.38 / 0.001
1.73 / 0.002
‐24.30 / 1.363
12.66 / 1.510
161.55 / 1.409
‐20.06 / 1.422
4.69
5.37 / 0.016
4.26 / 0.010
1.37 / 0.001
1.78 / 0.002
‐23.69 / 1.363
12.47 / 1.510
161.57 / 1.409
‐20.55 / 1.422
4.59
[
a]
[b]
[c]
N
[°] / m
[°] / m
[°] / m
2.72 / 0.004
1.62 / 0.001
2.34 / 0.003
‐19.46 / 1.362
12.44 / 1.499
163.80 / 1.413
‐31.52 / 1.419
3.91
[
a]
[b]
C3


[
a]
[b]
[c]
C8
b]
[


(C2‐C2‘) [°] / dC2‐C2’ [Å]
[
b]

(C2‐C3)

[°] / dC2‐C3 [Å]
[°] / dC2‐N [Å]
[°] / dN‐C8 [Å]
[
b]




(C2‐N)

[
b]
(N‐C8)


= Ln (5/6‐5′/6′) to Ln (2‐2′) [°]

(C5‐C6‐C6’‐C5’)[°]
‐26.05
‐18.68
‐26.08
‐25.80
‐23.95
‐22.59
β
β
C5 = Ln (5‐7a) to Ln (5′‐7a′) [°]
C6 = Ln (3a‐6) to Ln (3a′‐6′) [°]
50.60
26.11
54.29
32.41
49.97
23.83
48.54
22.74
50.07
24.04
49.01
24.13

= Ln (5/6‐3a/7a) to Ln (2‐2′) +
Ln (5′/6′‐3a′/7a′) to Ln ( 2‐2′) [°]
45.71
52.90
44.29
42.75
44.79
44.20
m
m

X
denotes the angle of pyramidalisation of the atom X, m the degree of s‐character of the s p hybrid orbital, (X‐Y) the dihedral angle between the s p‐orbitals of the
two neighbouring atoms X and Y and dX‐Y the distance between X and Y. Pointy brackets indicate the average value from both sides of the central double bond. “Ln”
denotes a line which was defined to pass through the specified atoms. The angle between the lines was determined using the Diamond 3 software from Crystal Impact.
The notation X/Y indicates that a dummy atom at the centre between atom X and Y was used. [a] obtained from POAV1 analysis, [b] obtained from POAV2 analysis, [c]
significant asymmetry. A detailed list of all the calculated parameters is given in the supporting information.
In this context the unusual low order parameter found for the The N,N′‐unsubstituted compounds, bearing pendant groups in
SmA phase of 24 can be understood as the higher degree of the 5,5′ and 6,6′ posiꢀons possess an approximately rod like
disorder caused by this bending. Such low bent angle molecular shape. Low solubility and high melting temperature,
mesogens have gained much attention recently, due to high caused by H‐bonding, dominate their thermotropic behaviour,
tendency to form uncommon phases types, such as the as they are characteristic for indigo. The 5,5′‐bis‐substituted
3
3
34
SmAPA or the SmAPR. By introduction of flexible linking derivatives exhibit liquid crystalline phases only shortly before
groups such as esters or imines, bent core liquid crystalline or during decomposition. Stable, highly ordered smectic
phases may be induced. Such a sterically deformed
could serve as an unusual and unique bent unit and will be laterally fluorinated or methylated pendant groups at 6 and 6′
subject of further investigation. positions. For these phases a pronounced dichroism was
‐system phases were found for the derivatives substituted with
The peripheral substitution pattern also has a tremendous observed.
influence on the kinetics of the trans‐cis‐photoisomerization of Upon acetylation of the vinylogue amide groups the solubility
the N,N′‐diacetylated indigo derivatives. Substitution in the 5 of all indigo derivatives was increased and the melting
and 5′ posiꢀons leads to a decrease in the rate of the temperatures were reduced. LC phases, however, were
isomerization compared to N,N′‐diacetyl indigo. The cis‐ exclusively found for the compounds substituted at the 6 and
product could not be observed even under long and intense 6′ posiꢀons. Based on a structural analysis of N,N′‐diacetyl
exposure to light. In contrast the substitution in the 6 and 6′ indigo it is supposed that the combination of buckling and
positions leads to a considerable increase of the isomerization twisting of the central double bond leads to molecular bent
rate. The cis‐product was observed after short time exposure which is specific for the substitution position. For 6,6′‐bis‐
to sunlight indicating an increase of the quantum yield for the substituted N,N′‐diacetyl indigos, the molecular bent is
isomerization process compared to N,N′‐diacetyl indigo. This reduced compared to the bent of the core system, whereas it
phenomenon could be exploited to generated new is amplified for the substitution positions 5 and 5′.
photoresponsive materials.
Acknowledgements
We gratefully acknowledge Prof. F. Gießelmann for allowing us
the access to his X‐ray facilities for diffraction on LCs, Prof.
Conclusion
We presented an efficient synthesis for a wide spectrum of
Schwarz for respective devices for single crystal X‐ray
peripherally substituted indigo derivatives. Substitution at the
measurements, and M. Moran and R. Callahan for fruitful
4
,4′ and 7,7′ posiꢀons lead to a considerable increase in
discussions.
solubility, which presumably originates from steric hindrance
of intermolecular H‐bonding.
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