Molecular packing and morphological stability of dihydro-indeno[1,2-: B] fluorenes in the context of their substitution pattern

Article abstract of DOI:10.1039/c7ra09401a

The synthesis of a series of dihydroindeno[1,2-b]fluorene (IF) derivatives with various side chain substituents is reported, these have been investigated by single-crystal X-ray analysis in terms of their molecular packings and the thermal stability of these morphologies observed by DSC measurements. It is shown that symmetrically substituted IFs bearing longer linear aliphatic side chains tend to crystallize in thermolabile coplanar layers, in which the aliphatic and dihydroindeno[1,2-b]fluorene core segments are spatially segregated. In contrast to that, the attachment of aryl side chains to the dihydroindenofluorene core results in a stabilization of the cofacial morphology, which can be observed by the absence of thermal phase transitions. In addition to this, asymmetrically substituted derivatives, so called "mixed indenofluorenes" (MIFs), bearing pairwise linear aliphatic side chains of variable length, as well as aryl substituents have been synthesized. These derivatives tend to exhibit thermostable morphologies with an enhanced tendency to form edge-to-face contacts of the dihydroindenofluorene structures.

Full text of DOI:10.1039/c7ra09401a

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Molecular packing and morphological stability of
dihydro-indeno[1,2-b]fluorenes in the context of
their substitution pattern†
Cite this: RSC Adv., 2017, 7, 47183
*
M. Hempe and M. Reggelin
The synthesis of a series of dihydroindeno[1,2-b]fluorene (IF) derivatives with various side chain substituents
is reported, these have been investigated by single-crystal X-ray analysis in terms of their molecular
packings and the thermal stability of these morphologies observed by DSC measurements. It is shown
that symmetrically substituted IFs bearing longer linear aliphatic side chains tend to crystallize in
thermolabile coplanar layers, in which the aliphatic and dihydroindeno[1,2-b]fluorene core segments are
spatially segregated. In contrast to that, the attachment of aryl side chains to the dihydroindenofluorene
core results in a stabilization of the cofacial morphology, which can be observed by the absence of
thermal phase transitions. In addition to this, asymmetrically substituted derivatives, so called “mixed
indenofluorenes” (MIFs), bearing pairwise linear aliphatic side chains of variable length, as well as aryl
substituents have been synthesized. These derivatives tend to exhibit thermostable morphologies with an
enhanced tendency to form edge-to-face contacts of the dihydroindenofluorene structures.
Received 24th August 2017
Accepted 2nd October 2017
DOI: 10.1039/c7ra09401a
rsc.li/rsc-advances
The scaffold of dihydroindeno[1,2-b]uorenes was used for the
Introduction
design of both, small molecule and polymer light-emitting
materials.17–19 It has been shown by Mullen et al., that there is
¨
One of the most important correlations in the eld of opto-
electronic organic materials is the interplay between the optical
properties and the morphology of the molecules in the bulk of
a solid state lm.1 It is well known that the morphology of the
molecules has an impact on the charge carrier and exciton
dynamics2–5 and can lead to excimers, which may inuence the
pristine emission color and intensity upon excitation.6
A powerful strategy to utilize this behavior for the rational
design of high-performance materials has been termed side-
chain engineering and was successfully applied in the eld of
organic photovoltaic devices.7–10 Furthermore, the inuence of
side chain substituents on the emission properties of poly-
uorenes has been intensively investigated. Besides the stabi-
lization of the deep blue emission color by use of aryl
substituents,11 it has been shown that thin lms of alkylated
polyuorenes are prone to the formation of polymorphs with
distinct optical properties.12–16
a connection between the molecular structure, the microscopic
morphology and the optical properties of poly(dihydroindeno-
uorenes) (PIFs).20 In addition to this, theoretical studies
demonstrated that the energy transfer between the polymer
chains of PIFs is dependent on the interchain distance of the
polymers and can be very efficient.21,22 Besides the intermolec-
ular distance between chromophores the relative orientation of
the emitting molecules is of paramount importance. It has been
shown, that a cofacial arrangement of the emitting aromatic
motifs in combination with an intermolecular face-to-face
˚
distance of 3–4 A can lead to the formation of aggregates,
which may determine and alter the emission properties of their
thin lms.6,23 Therefore, efficient and color pure emitting
materials should provide a high morphological stability of their
amorphous solid state, which can be observed by the absence of
thermal phase transitions.24
To learn more about the molecular packing and the
morphological stability of dihydroindeno[1,2-b]uorene-based
materials with respect to their substitution pattern, a series of
monomeric derivatives with different side chain substituents
has been synthesized in this work. The set of molecules covers
symmetrically substituted dihydroindenouorenes (IFs) with
linear aliphatic side chains of variable length, as well as mole-
cules bearing sterically demanding tert-butylphenyl substitu-
ents. In addition, asymmetrically substituted derivatives, so
called “mixed indenouorenes” (MIFs), compromising pairwise
linear aliphatic side chains of variable length, as well as aryl
Shiing the blue emission color into a spectral region in
which the human eye is more sensitive, higher homologs of
bridged poly-para-phenylene derivatives have been developed.
¨
¨
¨
Clemens-Schopf-Institut fur Organische Chemie und Biochemie, Technische Universitat
Darmstadt, Alarich-Weiss-Straße 4, 64287 Darmstadt, Germany. E-mail: re@chemie.
tu-darmstadt.de
† Electronic supplementary information (ESI) available: Seven X-ray
crystallographic les of compounds 6a–d, 7a, 13a and 13c, experimental
procedures and characterization data. CCDC 1568960–1568966. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/c7ra09401a
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substituents have been synthesized. The molecular packing
behavior of these derivatives was investigated by single-crystal
X-ray analysis and was correlated to their thermal behavior
observed by DSC measurements.
Results & discussion
Synthesis
The synthesis of the symmetrically substituted dihydroindeno-
uorenes was achieved starting from the 2,5-diphenyltereph-
thalic ester 1, following two strategies based on reaction
sequences reported in the literature (Scheme 1).17,18,25 Along
route A, the terephthalic ester 1 was transformed to the bis- Scheme 2 Synthesis of the mixed substituted derivatives 13a–d.
carbinols 2 by the attack of metallated carbon nucleophiles.
Without further purication, the biscarbinols 2 were subse-
quently cyclized to the dihydroindeno[1,2-b]uorenes. It is
noteworthy that the cyclization reaction using hydrochloric
acetic acid (method 1) was only feasible in the case of the aryl
substituted IFs. The synthesis of alkylated IFs was achieved by
using methanesulfonic acid and poly phosphoric acid (method
2) under mild reaction conditions.
coupling reaction, leading to the benzoate 10 in good yield
over two steps. Using an excess amount of the metallated
species of 4-bromo-tert-butylbenzene 11, compound 10 was
converted to the unisolated carbinol intermediate, which was
subsequently cyclized to the dihydroindeno[1,2-b]uorene
scaffold 12. The derivatization of compound 12 was achieved in
analogy to the unsubstituted dihydroindenouorene 5, yielding
the homologous series of methyl-MIF 13a, butyl-MIF 13b, hexyl-
MIF 13c, and octyl-MIF 13d.
Alternatively, alkylated IFs have been synthesized using
a different strategy based on the work of Deuschel and Char-
donnens (Route B).26,27 Following this reaction sequence, the
terephthalic ester 1 was saponied and the resulting tereph-
thalic acid 3 subsequently cyclized to the diketone 4 using
sulfuric acid.28 The diketone 4 was reduced to the dihy-
Single-crystal X-ray analysis
The inuence of the geminal substituents on the molecular
geometry (Fig. 1) and packing behavior (Fig. 2) of the symmet-
rically substituted derivatives (6a–d and 7a), as well as of the
mixed substituted compounds methyl-MIF 13a and hexyl-MIF
13c were investigated using single-crystal X-ray analysis (for
details, see the ESI† and Table 1). Compounds butyl-MIF 13b
and octyl-MIF 13d were obtained as ne powders only.
droindenouorene 5 in a Wolff-Kishner reaction28 and aer-
29
´
wards alkylated using a reaction protocol by Lardıes et al.
The synthesis of the mixed substituted dihydroindeno-
uorene derivatives was achieved starting from 2-bromo uo-
rene 8 (Scheme 2). In a sequenced one-pot Miyaura borylation/
Suzuki–Miyaura cross-coupling reaction cascade the corre-
sponding uorenyl pinacole boronate ester was generated in
situ, which aerwards was reacted with 2-bromo-
methylbenzoate 9 leading to 2-uorenyl-methylbenzoate 10.
Here, the use of the stronger base K3PO4 upon the addition
of the electrophile 9 promotes the Suzuki–Miyaura cross-
The total length (C2–C12 distance) of the indeno[1,2-b]uo-
˚
rene scaffold was determined to be 11.05(3) A and displays only
minor variations by deformations resulting from packing
Fig.
a function of the geminal substitution pattern. (a) Methyl-IF 6a, (b)
Scheme 1 Reaction sequence of the symmetrically substituted dihy- tBuPh-IF 7a, (c) methyl-MIF 13a (H atoms have been omitted for
1 Exemplary dihydroindeno[1,2-b]fluorene geometries as
droindeo[1,2-b]fluorenes.
clarity).
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Fig. 2 Molecular packings obtained by single-crystal X-ray analysis and illustrations with intermolecular distances (H atoms have been omitted
for clarity). (a) Methyl-IF 6a, (b) butyl-IF 6b, (c) hexyl-IF 6c, (d) octyl-IF 6d, (e) tBuPh-IF 7a, (f) methyl-MIF 13a, (g) hexyl-IF 13c.
effects. The bond angle of the sp3-bridging carbon atoms to the derivatives, the length of the terphenyl C–C-bond axis of the
˚
˚
carbon atoms of the aromatic rings shows a slightly distorted aromatic backbone was measured to be 1.469 A to 1.473 A, while
˚
tetrahedral geometry and has been determined in the range of this bond was shortened to 1.463 A in the case of the aryl
100.4ꢀ and 101.6ꢀ. In the case of the fourfold alkylated substituted compound tBuPh-IF 7a. Depending on the kind of
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substituents, the mixed substituted derivatives show bond the dihydroindeno[1,2-b]uorene moieties arrange in
length patterns which match those of the corresponding a coplanar orientation, in which the sterically demanding tert-
symmetrically substituted compounds.
butylphenyl side chains form a cavity that hosts a solvent
Because of packing effects, the compounds methyl-MIF 13a molecule within the crystals analyzed. The minimal intermo-
˚
and hexyl-MIF 13c (see ESI†) show a deformation of the dihy- lecular edge-to-edge distance was identied to be 4.629 A, while
˚
droindeno[1,2-b]uorene backbone, which is almost perfectly the interlayer distances were determined to 10.479 A and
˚
planar in the case of the symmetric substituted derivatives 10.513 A. Comparing the molecular arrangement of tBuPh-IF 7a
(6a–6d and 7a). The tetra-methylated dihydroindeno[1,2-b]u- with the dispirouorene-substituted dihydroindeno[1,2-b]uo-
orene 6a crystallizes orthorhombically in the space group Pbca. rene DSF-IF reported by Poriel et al.,19 it can be observed that the
The molecules within a unit cell arrange in a “herringbone”- uorene substituents of DSF-IF lead to a edge-to-face orienta-
˚
type structure, exhibiting a C–C-distance of 3.813 A as the tion of the dihydro-indenouorene core structures, while in the
shortest intermolecular contact. In this arrangement, the case of the tetraaryl-substituted derivative 7a a preservation of
molecular packing might be stabilized by C–H/p-interactions. the strictly coplanar orientation can be observed. This behav-
The derivative butyl-IF 6b crystallizes triclinically in the iour is in good agreement with the molecular packing of the
space group P1, as already reported by Zhang et al.30 Within the tetra-(4-tolyl)-substituted derivative 3Ph reported by Wong et al.,
ꢀ
31
˚
molecular packing, two coplanar arranged molecules are sur- adopting a intermolecular edge-to-edge distance of 3.58 A.
rounded by four molecules in an almost orthogonal but among
The mixed substituted derivative methyl-MIF 13a crystallizes
themselves cofacial alignment. These molecules are slightly monoclinically in the space group Cc. The four dihydroindeno
shied towards each other with an intermolecular C–C-distance [1,2-b]uorenes of one unit cell arrange in pairs of lamellas,
˚
of 3.522 A (p–p-stacking).
where the two layers are rotated towards each other. The
The derivative hexyl-IF 6c crystallizes triclinically in the space intermolecular distances between the molecules of the two
˚
ꢀ
groupe P1. In an unit cell, two coplanar arranged molecules kinds of lamellas have been measured to be 10.267 A and
˚
˚
form layers in which the molecules are slightly shied towards 10.592 A, while the minimal edge-to-edge distance is 4.329 A.
each other. The aromatic systems within the molecular packing
exhibit a minimal intermolecular edge-to-edge-distance of trigonally in the space group R3. The molecules within a unit
The molecules of the derivative hexyl-MIF 13c crystallize
ꢀ
˚
˚
4.420 A and a minimal interlayer distance of 3.416 A.
cell arrange in a layered and stacked radial orientation of three
Similarly to the hexyl-substituted compound hexyl-IF 6c, the dihydroindeno[1,2-b]uorene molecules per layer. The minimal
˚
ꢀ
derivative octyl-IF 6d crystallizes in the space group P1 (triclinic) intermolecular distance within one layer is 4.716 A, while the
˚
as well. The molecules within the elementary cell arrange in the intermolecular distance between two layers expands to 8.385 A.
same orientation as in the case of the hexyl-IF 6c, whereas the
Along the investigated series of molecules it can be observed
˚
˚
interlayer distance expands from 11.153 A to 11.228 A. Within that by the extension of the alkyl side chains, a widening of the
the molecular packing, the closest intermolecular edge-to-edge molecular packing takes place. In the case of the symmetrically
˚
contacts are identied to be 3.797 A, while the minimal inter- substituted derivatives, the close “herringbone”-style packing
˚
layer distance is 3.390 A.
mode of the methyl-IF 6a changes to a strictly coplanar orien-
As is the case for the aliphatic substituted derivatives, the tation of the dihydroindenouorene segments for compounds
tetraaryl substituted compound tBuPh-IF 7a crystallizes tri- 6b–6d. This orientation can also be observed in the case of the
ꢀ
clinically in the space group P1. Within the molecular packing, tetraaryl substituted derivative tBuPh-IF 7a, while the mixed
Table 1 Crystallographic data for compounds 6a–6d, 7a, 13a, 13c
Methyl-IF 6a
24H22
310.41
Orthorhombic
Pbca
6.7189(8)
13.926(1)
19.061(2)
90
90
90
1783.49
4
1.156
Butyl-IF 6b
Hexyl-IF 6c
Octyl-IF 6d
tBuPh-IF 7a
Methyl-MIF 13a
Hexyl-MIF 13c
Formula
C
C36H46
478.73
Triclinic
ꢀ
P1
9.350(3)
13.930(4)
19.084(4)
109.98(3)
91.28(2)
98.95(2)
2299.92
3
C44H26
590.93
Triclinic
ꢀ
P1
8.453(1)
10.868(1)
11.903(2)
92.85(1)
108.07(1)
104.44(1)
997.087
1
C52H78
703.14
Triclinic
ꢀ
P1
8.502(1)
10.722(2)
14.069(2)
82.38(2)
73.88(2)
75.30(2)
1189.9
C60H62
903.19
Triclinic
ꢀ
P1
11.660(4)
11.794(3)
11.944(3)
102.39(2)
115.41(3)
107.53(3)
1297.12
1
C42H42
546.75
Monoclinic
Cc
15.416(2)
20.078(2)
12.162(2)
90
118.63(2)
90
3304.15
4
C52H62
687.05
Trigonal
ꢀ
R3
MW [g molÀ1
]
Crystal system
Space group
˚
a [A]
43.677(2)
43.677(2)
12.140(1)
90
90
120
20 056.5
3
1.028
0.058
293(2)
˚
b [A]
˚
c [A]
a [deg]
b [deg]
g [deg]
3
˚
V [A ]
Z
1
rcalc [g cmÀ3
]
1.037
0.058
293(2)
0.984
0.055
293(2)
0.982
0.054
293(2)
1.156
0.065
293(2)
1.099
0.062
293(2)
m [mmÀ1
T [K]
]
0.065
293(2)
Qmin–Qmax [deg}
R/wR [I > 2s(I)]
2.925–25.184
0.0691/0.1233
2.570–25.183
0.0769/0.1227
2.639–25.186
0.0608/0.1356
2.560–25.184
0.1538/0.2016
2.941–25.349
0.1147/0.2738
2.785–25.185
0.0478/0.0957
2.468–25.187
0.0910/0.2535
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substituted derivatives methyl-MIF 13a and hexyl-MIF 13c tend been assigned to the melting points of the compounds at 143 ꢀC
ꢀ
ꢀ
to exhibit intercalated structures.
(butyl-IF), 95 C (hexyl-IF) and 87 C (octyl-IF).
In comparison to the tetraalkylated derivatives, a morpho-
logical stabilization can be observed in the case of the tert-
butylphenyl substituted compounds. The thermogram of the
rst heating cycle of the derivative tBuPh-IF 7a only exhibits
a weak endothermic signal, which was assigned to the glass
transition of the material. The stabilization effect of the aryl
side chains can be observed in the case of the mixed dihy-
droindenouorenes as well. The derivatives methyl-MIF 13a,
butyl-MIF 13b, hexyl-MIF 13c and octyl-MIFs 13d did not show
any signicant thermal transitions in the temperature range
between À10 ꢀC and 150 ꢀC. Note that only the butyl-substituted
compound butyl-MIF 13b shows a weak endothermic signal at
Thermal behavior (DSC)
To investigate the inuence of the geminal substituents on the
thermal behavior of the dihydroindeno[1,2-b]uorenes synthe-
sized, differential scanning calorimetry (DSC) measurements
have been performed (Fig. 3).
The thermogram of the symmetrically substituted derivative
methyl-IF 6a shows no thermal phase transitions in the range
between À10 ꢀC and 200 ꢀC. In sharp contrast to this, the
thermograms of the symmetrically substituted derivatives with
longer linear aliphatic side chains (butyl-IF, hexyl-IF, octyl-IF)
exhibit several signals in their rst heating cycle. The
measurements reveal weak endothermic signals at À18 ꢀC and
À36 ꢀC, which can be correlated to glass transitions of the hexyl-
IF 6c and octyl-IF 6d derivatives. In addition to this, the ther-
ꢀ
89 C, which could not be assigned.
Discussion
mograms of the three derivatives 6b–d show further exothermic We have observed that the tetramethyl-substituted derivative
signals below their melting temperature, which can be assigned methyl-IF 6a crystallizes in a “herringbone”-type structure,
to recrystallization processes and the formation of mesophases while longer linear aliphatic substituents lead to a widening of
that already have been reported by Elmahdy et al. in the case of this close packing mode and result in a coplanar arrangement
octyl-substituted dihydroindenouorene mono- and oligo- of the terphenylic systems. In the course of that, a spatial
mers.32 Furthermore, the DSC curve of the hexyl-IF 6c exhibits segregation of aliphatic and aromatic segments takes place,
ꢀ
one additional endothermic signal at a temperature of 73 C, which was already reported by Elmahdy et al. in the case of
which merges into the second exothermic recrystallization oligoindenouorenes.32 The interlayer distance of this align-
signal. The thermograms of the second heating cycle of the ment increases along the homologous series of the derivatives
tetraalkylated dihydroindenouorenes butyl-IF 6b, hexyl-IF 6c butyl-IF 6b, hexyl-IF 6c and octyl-IF 6d. The substitution of the
and octyl-IF 6d only exhibit endothermic signals, which have dihydroindeno[1,2-b]uorene scaffold with four tert-
Fig. 3 Differential scanning calorimetry (DSC) of the derivatives methyl-IF 6a, butyl-IF 6b, hexyl-IF 6c, octyl-IF 6d, tBuPh-IF 7a, methyl-MIF 13a,
butyl-MIF 13b, hexyl-MIF 13c and octyl-MIF 13d (N2-atmosphere, 10 K minÀ1, black line: first heating cycle, blue line: second heating cycle). The
traces have been shifted vertically for clarity.
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butylphenyl groups also results in the orientation of the mole- mesophases, which have been detected by DSC measurements.
cules in parallel layers, while the intermolecular edge-to-edge- In contrast to aliphatic substituents, the attachment of aryl side
distance within a layer further increases in comparison to the chains results in a stabilization of the morphology, which can
aliphatic-substituted derivatives.
be observed by the absence of thermal phase transitions in the
In contrast to the symmetric substitution pattern, the mixed temperature range investigated. This kind of behavior has
substituted dihydroindenouorenes methyl-MIF 13a and hexyl- previously been identied as one important material property
MIF 13c tend to crystallize in lamellar, intercalating structures for the improvement of OLED lifetimes.24,33–35 Therefore, the
with an increased number of edge-to-face contacts of the dihy- results obtained will be valuable for the development of future
droindenouorene core structures.
dihydroindeno[1,2-b]uorene-based materials and may be
The widening of the molecular packing with increasing transferable to related structures and applications.
linear aliphatic side chains directly corresponds to the thermal
behavior of the derivatives, which has been examined by DSC
measurements. For the compound methyl-IF 6a, which crys-
Conflicts of interest
tallizes in a “herringbone”-type structure, no thermal phase
transition could be observed up to a temperature of 150 C. In
There are no conicts to declare.
ꢀ
contrast to this, the coplanar arrangement of the symmetrically
substituted molecules of longer aliphatic side chain lengths Acknowledgements
results in a thermolabile morphological behavior, which leads
The author would like to thank Sabine Foro (Technical
to the existence of polymorphs and rather low melting points.
University of Darmstadt) for performing the single-crystal X-ray
In accordance with the observations of the symmetrically
¨
analysis and Christian Ruttiger (Rehahn group, Technical
aliphatic substituted derivatives, the attachment of four tert-
butylphenyl groups to the dihydroindenouorene core leads to
coplanar molecular packing. Unlike the thermolabile character
of the aliphatic substituents, the rigid nature of the aryl side
chains results in a stabilization of the morphology of the
material.
University of Darmstadt) for conducting the DSC measure-
ments. Generous support by Merck KGaA (Darmstadt) is grate-
fully acknowledged.
Notes and references
The stabilizing effect of the aryl side chains can be observed
in the case of the mixed substituted derivatives, as well. The
thermograms of the compounds methyl-MIF 13a, butyl-MIF 13b,
hexyl-MIF 13c and octyl-MIF 13d lack any signicant features up
to a temperature of 150 ꢀC. A possible explanation of this
behavior can be found in the enhanced tendency of the mole-
cules to form edge-to-face contacts of the dihydroindenouorene
core structures, which could be observed in the solid-state
structures of the derivatives methyl-MIF 13a and hexyl-MIF
13c. In that case, the aryl side chains disturb the spatial segre-
gation between the aliphatic and aromatic segments and result
in an interlocked arrangement of the molecules within the
molecular packing. The rigidity of the aryl side chains as well as
the enlargement of the intermolecular contact surface of the
aromatic systems could be the reason for the higher morpho-
logical stability and the absence of thermolabile mesophases.
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Conclusions
This work offers insights into the molecular packing and the
morphological stability of dihydroindeno[1,2-b]uorenes in
dependence of their substitution pattern.
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