Dimension-controlled assemblies of modified bipyrroles stabilized by electron-withdrawing moieties

Article abstract of DOI:10.1039/c6cc03423c

Benzoyl-substituted bipyrroles possessing aliphatic chains were synthesized and formed a variety of dimension-controlled assembled structures as mesophases through various intermolecular interactions.

Full text of DOI:10.1039/c6cc03423c

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Dimension-controlled assemblies of modified
bipyrroles stabilized by electron-withdrawing
Cite this:DOI: 10.1039/c6cc03423c
moieties†
Kazuto Nakamura,a Nobuhiro Yasudab and Hiromitsu Maeda*a
Received 24th April 2016,
Accepted 3rd May 2016
DOI: 10.1039/c6cc03423c
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Benzoyl-substituted bipyrroles possessing aliphatic chains were
synthesized and formed a variety of dimension-controlled assembled
structures as mesophases through various intermolecular interactions.
Assemblies comprising p-electronic molecules are essential for
fabricating electronically functional materials.1 The introduction
of interacting moieties into p-electronic units can result in
dimension-controlled organized structures via p–p stacking and
other interactions. As an example, dipole–dipole interactions are
important for controlling molecular orientations and resulting
assembled structures.2 Interaction strengths vary, resulting in
unique thermal transition behaviours and related properties.
We have focused on pyrrole derivatives, which possess hydrogen-
bond donating NH sites, as p-electronic units that form assembled
structures.3 Pyrrole rings are well-known building blocks in biotic
functional dyes, such as chlorophyll and haem, in which they are
Fig. 1 (a) Synthesis and (b) structures of dibenzoylbipyrrole derivatives.
stabilized by aromatic macrocycles.4 Therefore, the design and induce van der Waals interactions, which cause the formation of
synthesis of stable linear oligopyrroles would be valuable for the dimension-controlled assemblies.
development of p-electronic molecules and their assemblies.
Following a literature procedure,6 benzoyl-substituted bipyrroles
Although directly linked oligopyrroles are less stable due to their 2a–d and 3a–d, possessing four and six alkoxy chains, respectively,
electron-rich states, their potential as assembly building units were synthesized from the corresponding benzamides7 in 16–43%
1
should be examined via the introduction of electron-withdrawing yields, and identified by H/13C NMR and MALDI-TOF-MS. Based
moieties. As an example, Geier et al. have reported a benzoyl- on theoretical studies at B3LYP/6-31G(d,p),8 the anti conformation
substituted pyrrole dimer (bipyrrole) 1, prepared from 2,20-bipyrrole5 of the two NH sites in 2,20-bipyrrole, as a core moiety of 2a–d and
and N,N-dimethylbenzamide (Fig. 1a), although properties such as 3a–d, was more stable at 2.78 kcal molꢀ1 than the corresponding
assembled behaviour were not investigated in detail.6 Based syn conformation, whereas the syn conformation of CQO and
on this background, we report the synthesis and assembled pyrrole NH sites in 2-benzoylpyrrole, as a partial moiety of 2a–d
structures of bipyrroles a-substituted with modified benzoyl and 3a–d, was more stable at 4.06 kcal molꢀ1 than the corres-
units (2a–d and 3a–d, Fig. 1b). Aliphatic chains, such as OC8H17, ponding anti conformation. These results were consistent with
OC12H25 and OC16H33, were introduced into 2b–d and 3b–d to the appropriate orientations of dipoles in the building units as
well as the preferred conformations of benzoyl-substituted
a Department of Applied Chemistry, College of Life Sciences, Ritsumeikan University,
Kusatsu 525–8577, Japan. E-mail: maedahir@ph.ritsumei.ac.jp
b Research and Utilization Division, Japan Synchrotron Radiation Research Institute,
Sayo 679–5198, Japan
bipyrroles. The bipyrrole derivatives, mainly existing as more
symmetric anti conformations, would be suitable building units
for the formation of aggregates. In fact, changes in the variable
temperature (VT) UV/vis absorption spectra of 3c in octane (1 mM)
showed two-step transitions upon cooling (Fig. 2a). Upon cooling
from 80 to 0 1C, the absorption at around 400 nm was red-shifted,
suggesting that the molecular conformation was fixed to provide
† Electronic supplementary information (ESI) available: Synthetic procedures, DSC
profiles, POM images, XRD patterns and CIF files. CCDC 1474906 and 1474907. For
ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c6cc03423c
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with anti orientations had dihedral angles of 01 (completely
parallel) and 11.581 for 2a and 3a, respectively,10 whereas benzoyl
units, whose CQO moieties were oriented in the same direction
as pyrrole NH, resulted in distortions in the core bipyrrole unit,
with dihedral angles of 33.01 and 58.01/51.41 for 2a and 3a,
respectively. On the other hand, columnar structures of 2a and
3a were constructed by aggregation, with repeat p-plane distances
of 3.38 Å and 3.37/7.33 Å, respectively. In the packing structures,
2a formed tilted columnar structures, whose components have a
dihedral angle of 85.91 with those in neighbouring columns
(Fig. 3a), whereas, in 3a, disordered solvents expanded the
distance between two molecules (Fig. 3b). Columns of 2a and
3a were arranged in 1D chain-like forms through hydrogen
bonding with N(–H)ꢁ ꢁ ꢁO(QC) distances of 2.88 Å and 2.99 Å,
respectively. Notably, pyrrole NH units in the columnar struc-
tures were oriented in the same direction for both 2a and 3a in
the stacking pairs.
Bipyrroles 2b–d and 3b–d, possessing aliphatic chains,
afforded yellow or yellowish-green solids from CHCl3/MeOH.
The thermal transition behaviours of 2b–d and 3b–d, examined
by differential scanning calorimetry (DSC), were dependent
on the number and length of aliphatic chains (Table 1). DSC of
2b–d and 3b–d showed thermal transitions at temperatures (1C)
of 168/4 and 8/173 (2b), 153/134/27 and 31/153 (2c), 147/40 and
39/107/118/145 (2d), 83 and 58/93 (3b), 68/56 and 55/65/74 (3c),
and 60/33/20 and 48/61 (3d), upon 1st cooling and 2nd heating,
respectively, suggesting the formation of mesophases. Polarized
optical microscopy (POM) of 2b showed fibre-like textures upon
cooling from the isotropic liquid (Iso), whereas POM of 2c and
2d as mesophases showed coffee bean-like textures (Fig. 4a). On
the other hand, 3b–d mesophases exhibited mosaic-like textures
via POM.
Fig. 2 (a) Changes in variable temperature (VT) UV/vis absorption spectra
of 3c in octane (1 mM) upon cooling from (i) 80 to 0 1C and (ii) 0 to ꢀ60 1C
and (b) changes in VT 1H NMR spectra of 3c in octane-d18 (1 mM) upon
cooling from (i) 40 to (v) ꢀ40 1C via (ii) 20, (iii) 0 and (iv) ꢀ20 1C.
planar geometries. Further cooling from 0 to ꢀ60 1C caused a new
band to appear at 440 nm, ascribed to the formation of aggre-
1
gates. VT H NMR spectral changes of 3c in octane-d18 (1 mM)
showed broad less clear signals at lower temperatures, suggesting
the formation of aggregates (Fig. 2b).
Single-crystal X-ray analysis of 2a and 3a showed their solid-
state molecular conformations and assemblies (Fig. 3), especially
using synchrotron radiation X-rays at BL40XU SPring-8 for 3a.‡9
Molecules in the solid state showed similar conformations, as seen
in theoretical studies. The two pyrrole units in bipyrrole moieties
Synchrotron X-ray diffraction (XRD) analysis (BL40B2, SPring-8)
of 2c showed three mesophases, including two lamellar phases
and a rectangular columnar (Colr) phase at 10, 80 and 140 1C,
respectively, upon cooling (Fig. 4b). The two lamellar phases
exhibited layer distances of 2.93 and 3.32 nm at 10 and 80 1C,
respectively, suggesting that the shorter distance at low tempera-
ture was due to effective interactions. At higher temperatures, the
changes in molecular orientations and interdigitation between
aliphatic chains resulted in another lamellar phase, supported by
p–p stacking and van der Waals interactions for core units and
aliphatic chains, respectively, with a layer distance that was
approximately equal to the tilted orientation (ca. 451) for the
pffiffi
ꢀ
molecular length (4.85 nm)8 providing 4:85 2 ¼ 3:43 nm.
Furthermore, the minimum layer distance formed by vertically
oriented molecules was ca. 3.3 nm in a completely interdigitated
form, suggesting that the layers should form in a tilted orienta-
tion. At 140 1C, a Colr phase was constructed through van der
Waals interaction with parameters a = 5.95 nm, b = 3.93 nm and
c = 0.38 nm based on a single molecule in the unit disc (Z = 4 for
r = 0.8). The space for one molecule became larger with increasing
mobility of the constituent molecules, resulting in low densities
at high temperatures. On the other hand, upon cooling, XRD
of 2d, possessing four long alkyl chains, showed three meso-
phases, comprising two lamellar phases and a Colr phase at 40,
Fig. 3 Single-crystal X-ray analysis of (a) 2a and (b) 3a as assembled
structures as (i) top views showing intermolecular N–Hꢁ ꢁ ꢁOQC hydrogen
bonding and (ii) side views of columnar structures from the directions of
arrows in (i). Atom colour code: brown, pink, blue and red refer to carbon,
hydrogen, nitrogen and oxygen, respectively.
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Table 1 Summary of transition behaviours for 2b–d and 3b–d
Coolinga
Heatinga
2b
2c
Crb 4 Crb 168 Iso
lamellar 27 lamellar
134 Colr 153 Iso
Crb 8 Crb 174 Iso
lamellar 31 lamellar 154c Iso
2d
lamellar 40 lamellar
lamellar 39 lamellar 107
lamellar 118 Colr 145 Iso
110d Colr 147 Iso
3b
3c
3d
Cole 84 Iso
Cole 58 Coltet 93 Iso
Crf 55 Crf 74 Iso
Coltet 48 Colhex 61 Iso
Crf 56 Crf 68 Iso
Coltet 10 Coltet 33 Colhex
61 Iso
a
Transition temperatures (1C, peak onset) from DSC 1st cooling and
2nd heating scans (5 1C minꢀ1). Cr with lamellar-based unidentified
structure. Colr phase was exhibited before melting as identified using
XRD patterns. Transition temperature of the broad peak was sup-
ported by the change in XRD patterns. Coltet and Colhex mixed phases,
b
c
d
e
f
depending on the cooling processes. Cr with unidentified structures.
Fig. 5 (a) XRD pattern of 3b at 70 1C upon cooling and possible
assembled models, wherein Colhex and Coltet phases consisted of trimeric
and dimeric unit discs, respectively, and the peaks labeled as h and t were
derived from the respective phases. (b) XRD patterns of 3d at (i) 28 1C upon
cooling and (ii) 45 1C upon cooling and possible assembled models. The
sharper halo pattern in the Coltet phase suggested that monomeric disc
units induced strong interactions among aliphatic chains.
Fig. 4 (a) POM textures of (i) 2c at 153 1C and (ii) 2d at 147 1C upon
cooling and (b) possible assembled models of 2c.
110 and 147 1C, respectively. The two lamellar phases were
fabricated by the layers with distances of 3.81 and 3.97 nm at 25
and 100 1C, respectively. At 120 1C, a Colr phase was formed
by van der Waals interaction with the parameters a = 6.50 nm,
b = 2.98 nm and c = 0.39 nm based on a single molecule in the at 80 1C upon cooling, the peak pattern of the Coltet phase was
unit disc (Z = 2 for r = 0.6). The formation of tilted columns of initially less intense than that of the Colhex phase, but the peak
2c and 2d in the Colr phases correlated with the single-crystal intensity became greater than that of the Colhex phase after
X-ray structure of 2a. In contrast to 2c and 2d, 2b exhibited annealing at 5 min. Therefore, the Coltet and Colhex phases were
complicated XRD patterns because shorter aliphatic chains in found to be in more stable and metastable states, respectively,
2b allowed more effective intermolecular interactions.
at this temperature. In contrast, at 40 1C upon cooling, the peak
The number of aliphatic chains in benzoyl-substituted ratio of Colhex to Coltet did not change by annealing. Further-
bipyrroles was important for influencing dimension-controlled more, due to rate-dependent changes in the XRD patterns, at
assemblies. Synchrotron XRD of 3b suggested the formation of 40 1C upon cooling, the peak pattern of the Colhex phase with a
two mesophases at 70 1C upon cooling, namely, hexagonal cooling rate of 10 1C minꢀ1 was initially more intense than that
columnar (Colhex) and tetragonal columnar (Coltet) phases, with with a cooling rate of 5 1C minꢀ1. This observation suggested that
the unit parameters a = 2.82 and 2.06 nm, respectively, and 3b might form soft crystals at low temperatures. The balance
c = 0.77 nm in both cases. The Colhex and Coltet phases were between the Colhex and Coltet phases can be ascribed to building
fabricated from trimeric (Z = 3 for r = 1) and dimeric (Z = 2 for unit stabilities of the columnar structures; the trimeric assembly
r = 1.2) unit discs, respectively (Fig. 5a). Upon cooling from the in Colhex is kinetically more stable than the dimeric assembly in
Iso, a phase containing both Coltet and Colhex structures was Coltet upon cooling from the Iso. However, the trimeric assembly
observed. Through time-dependent changes in the XRD patterns, in Colhex was thermodynamically less stable than the dimeric
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assembly in Coltet, resulting in a phase transition from Colhex to Prof. Tomonori Hanasaki, Ritsumeikan University, for POM
Coltet. Furthermore, the Coltet phase formed above 58 1C upon measurements and Prof. Hitoshi Tamiaki, Ritsumeikan University,
heating from mixed states, also suggesting that Coltet was more for various measurements.
stable than Colhex
.
XRD of 3d exhibited the formation of Coltet and Colhex
mesophases at 28 and 45 1C, respectively, upon cooling. The
Coltet and Colhex phases, with a = 2.35 and 3.10 nm and c = 0.75
and 0.76 nm, respectively, consisted of monomeric (Z = 1 for
r = 0.8) and dimeric (Z = 2 for r = 1) unit discs, respectively
(Fig. 5b). The c values, which were twice the general p–p
stacking, revealed the stacking of two molecules with opposite
orientations to prevent the overlap of aliphatic chains in
stacking molecules in the unit disc of Colhex. The unit disc con-
sisting of a single molecule in Coltet induced effective inter-
actions in the aliphatic chains rather than in the core units,
providing a small calculated density of o1. In contrast to 3b
and 3d, 3c exhibited complicated XRD patterns due to its more
effective intermolecular interactions than those in 3b and 3d.
The larger number of flexible points (alkoxy oxygens) in 3b–d
may require longer aliphatic chains for effective intermolecular
interactions, those for p-units, polarized NH and CO units and
aliphatic chains, compared to 2b–d.
In summary, benzoyl-substituted bipyrrole derivatives were
found to form dimension-controlled assemblies through inter-
molecular interactions. Tetraalkoxy-substituted 2c and 2d
formed lamellar and tilted Colr structures, whereas hexaalkoxy-
substituted 3b and 3d formed mainly vertically oriented Coltet
and Colhex structures. Multiple interactions were present in the
pyrrole-based molecules, enabling a variety of assembly modes
depending on the numbers and lengths of aliphatic chains.
Interactions between dipoles of constituent p units, which are
not easy to examine in order to discuss in detail, are essential for
the formation of assemblies. Further modifications and mole-
cular design will provide fascinating dimension-controlled
assembled structures.
Notes and references
‡ Crystal data for 2a (from EtOAc/n-heptane): C26H24N2O6, Mw = 460.47,
monoclinic, P21/c (no. 14), a = 15.361(4) Å, b = 4.9550(13) Å, c = 14.843(4) Å,
b = 98.245(7)1, V = 1118.1(5) Å3, T = 93(2) K, Z = 2, DC = 1.368 g cmꢀ3
,
m(Cu-Ka) = 0.809 mmꢀ1, R1 = 0.0329, wR2 = 0.0938, GOF = 1.074
(I 4 2s(I)). CCDC 1474906. Crystal data for 3a (from EtOAc/iPr2O):
%
C28H28N2O8ꢁC4H8O2, Mw = 520.54, triclinic, P1 (no. 2), a = 7.9706(19) Å,
b = 10.519(3) Å, c = 17.440(4) Å, a = 87.803(4), b = 86.847(4), g = 78.375(4)1,
V = 1429.4(6) Å3, T = 90(2) K, Z = 4, DC = 1.312 g cmꢀ3, m(synchrotron
radiation) = 0.120 mmꢀ1, R1 = 0.0551, wR2 = 0.1213, GOF = 1.132
(I 4 2s(I)). CCDC 1474907.
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This work was supported by Grants-in-Aid for Scientific
Research (B) (No. 26288042) and on Innovation Areas (‘‘Photo-
synergetics’’ Area 2606, No. 26107007) from the MEXT. We
thank Prof. Atsuhiro Osuka and Mr Takanori Soya, Kyoto
University, for single-crystal X-ray analysis, Prof. Hikaru Takaya,
Kyoto University, and Dr Ryohei Yamakado, Ritsumeikan University,
for synchrotron radiation XRD measurements (BL40XU at
SPring-8: 2015A1388), Dr Noboru Ohta, JASRI/SPring-8, for
synchrotron radiation XRD measurements (BL40B2 at SPring-8),
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