Use of solvent to regulate the degree of polymerisation in weakly associated supramolecular oligomers

Article abstract of DOI:10.1039/c4cc01514b

Using a tetrathiafulvalene functionalised calix[4]pyrrole (TTF-C[4]P; 1) and alkyl diester-linked bis-dinitrophenols (2-4), it was found that the solvent polarity and linker length have an effect on the molecular aggregation behaviour. 2D ¹H NO

Full text of DOI:10.1039/c4cc01514b

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Use of solvent to regulate the degree of
polymerisation in weakly associated
supramolecular oligomers†
Cite this: Chem. Commun., 2014,
50, 5497
Received 27th February 2014,
Accepted 2nd April 2014
Steffen Bahring,‡a Dong Sub Kim,‡b Troels Duedal,a Vincent M. Lynch,b
¨
Kent A. Nielsen,a Jan O. Jeppesen*a and Jonathan L. Sessler*b
DOI: 10.1039/c4cc01514b
www.rsc.org/chemcomm
Using a tetrathiafulvalene functionalised calix[4]pyrrole (TTF-C[4]P; 1)
and alkyl diester-linked bis-dinitrophenols (2–4), it was found that the
solvent polarity and linker length have an effect on the molecular
aggregation behaviour. 2D 1H NOESY, DOSY NMR and UV-vis-NIR
spectroscopic studies, as well as single crystal X-ray diffraction
analyses support these conclusions.
Considerable effort has been devoted to understanding and
establishing well-defined supramolecular oligomeric systems under
conditions where a high degree of polymerization is achieved via
strong binding.1–3 Typically hydrogen bonding or metal coordination
is used to create the requisite strong interactions.4–7 Increasingly, we
have become interested in the use of weaker intermolecular forces as
a basis for supporting self-assembly. In the context of this broad
effort, we recently reported the supramolecular polymerisation
of a pair of structurally rigid calix[4]pyrrole compounds, namely a
benzo-annulated tetrathiafulvalene functionalised calix[4]pyrrole
(TTF-C[4]P) and a dinitrophenyl (or methylpyridinium) functionalised
calix[4]pyrrole (DNP-C[4]P).8,9 With these components, well-defined
1D polymeric structures were seen, both in the solid state and in
trichloromethane solution. Assuming an isodesmic model, a binding
constant of 7.9 Â 104 MÀ1 (CDCl3) was calculated for the individual
Fig. 1 Graphical representation of the proposed assembly between TTF-
functionalised C[4]P 1 and the bis-dinitrophenyl end-capped flexible
linkers 2–4.
constituent–constituent interactions. While effective self-assembly understanding of self-assembly under conditions of weak monomer–
was seen, this first generation system cannot be considered as falling monomer interaction, we have designed a new system. This is based
within the truly weak binding regime. This is a regime that has on thiopropyl functionalised TTF-C[4]P 110 and three monomers of
not been extensively explored. Thus, the determinants that differing lengths, namely the linked bis-dinitrophenyl diesters 2–4
control self-assembly or which modulate the length and nature (Fig. 1). As detailed below, self-assembly occurs readily in the solid
of the resulting polymeric (or short oligomeric) supramolecular state; however, the extent of oligomerisation and the underlying
constructs remain poorly understood. In an effort to increase our binding constants in solution were found to be highly solvent
dependent and to vary with the choice of linker in a manner that
was not strictly correlated with spacer length (Fig. 1).
a Department of Physics, Chemistry and Pharmacy, University of Southern Denmark,
Campusvej 55, 5230 Odense M, Denmark. E-mail: joj@sdu.dk
Initial evidence of an interaction between 1 and the ditopic
electron acceptors 2À4 came from the fact that an immediate colour
change (yellow to green) occurs when 1 is mixed with any of the
other three putative monomers i.e. 2À4 in trichloromethane.
This colour change is ascribed to a donor–acceptor charge-transfer
(CT) interaction between the electron-rich TTF units in 1 and the
electron-poor 3,5-dinitrobenzoate moieties in 2À4. Further support
b Department of Chemistry, The University of Texas at Austin, 105 E. 24th Street,
Stop A5300, Austin, Texas 78712-1224, USA. E-mail: sessler@cm.utexas.edu
† Electronic supplementary information (ESI) available: Experimental procedures,
extra UV-Vis, NMR spectroscopic analyses, atomic force microscopic images, and
single crystal X-ray crystallographic data. CCDC 988770–988772. For ESI and
crystallographic data in CIF or other electronic format see DOI: 10.1039/c4cc01514b
‡ Co-first authors who contributed equally to this work.
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Fig. 2 (A) UV-vis-NIR absorption spectra of 1, 2 and 1 + 2 as recorded in
trichloromethane at 298 K. (B) Partial 1H NMR spectra (600 MHz, 298 K) of
(a) 2 (0.7 mM), (b) 1 (0.7 mM), (c) 1 + 2 (0.7 mM of 1 and 2, respectively) and
(d) 1 + 2 (7.0 mM of 1 and 2, respectively) all in trichloromethane-d1. (C) 2D
NOESY NMR spectroscopy of 1 (7 mM) and 2 (7 mM) in trichloromethane-d1.
Fig. 3 (A–C) X-ray single crystal structures of (1Á2)n, (1Á3)n and (1Á4)n. (D and
F) Dihedral angles for the linkers present in monomers 2À4 as deduced
from the solid state structural parameters. (E) Binding constants calculated
from UV-vis-NIR spectroscopic titrations carried out in trichloromethane.
for this conclusion came from UV-vis-NIR and NMR spectroscopic
studies involving 2. Specifically, a new CT band at 566 nm is seen in
the UV-vis-NIR spectrum of 1 + 2 recorded in trichloromethane that
is not present in the spectrum of the individual components for (1Á2)n, (1Á3)n and (1Á4)n, respectively.8 The observed variations in
(Fig. 2A). In addition, upfield shifts in the signals ascribed to the binding affinities are ascribed to differences in steric hindrance
nitrophenyl aryl protons and downfield shifts are seen for the pyrrole between the monomeric constituents, which are relatively reduced
NH signals in the 1H NMR spectrum recorded of a mixture of 1 + 2 in (1Á4)n and (1Á2)n in comparison to (1Á3)n, as inferred from the
(CDCl3) as compared to the individual components. These effects individual structures in question and from inspection of the
become more pronounced at higher concentrations (Fig. 2B). They dihedral angles for the spacers linking the electron deficient sub-
are also seen when 2 is replaced by either 3 or 4. In addition, 2D 1H units in 2À4 (cf. Fig. 3D and E).
NOESY NMR studies reveal clear cross peaks between protons on 1
and 2 (Fig. 2C).
Support for the notion that the individual subunit–subunit
interactions are highest in the case of (1Á4)n, as compared to (1Á2)n
Diffraction grade single crystals from all three combinations of and (1Á3)n, came from 2D diffusion ordered (DOSY) NMR spectro-
1 and 2À4, corresponding to (1Á2)n, (1Á3)n and (1Á4)n, were scopic studies (see ESI,† Fig. S14). For instance, in the case of similar
obtained (Fig. 3A–C; see ESI† for crystallographic tables). The equimolar mixtures in CDCl3, the lowest diffusion coefficient was
corresponding X-ray diffraction structures revealed that extended seen in the case of 1 and 4, as would be expected for systems that
supramolecular aggregates are stabilised in the solid state. Based undergo a higher relative degree of self-association.
on the structural parameters, these polymeric structures are
Of the three self-assembled systems considered in this study,
thought to be stabilised through a combination of donor acceptor namely (1Á2)n, (1Á3)n and (1Á4)n, the best solubility was observed
and hydrogen bond interactions involving the pyrrolic NHs of 1 in the case of (1Á2)n. This ensemble was thus used to examine
and the nitro groups present in 2À4. However, the specific the effect of solvent. Towards this end, two solvents, namely
structures differ from one another. For instance, while the 1,2-dichloroethane (DCE) and methylcyclohexane (MCH), with
structures of (1Á2)n and (1Á3)n both consist of monomeric subunits greater and lower polarity, respectively, than trichloromethane,
arranged in a head-to-tail fashion, their monomeric subunits were used. Based on variable temperature UV-vis-NIR spectro-
arranged in a linear fashion, gauche interactions are present in scopic studies, an inverse relationship between the calculated
the linking subunit of (1Á3)n that are not present in (1Á2)n. Even binding constants (Table 1) and the solvent polarity was
more dramatic differences are seen in the case of (1Á4)n; here, the observed. The effect was notable, with the use of MCH providing
individual subunits are arranged in a T-like manner (Fig. 3).
An effort was made to correlate the X-ray single crystal structures relative to what is seen in trichloromethane (Table 1).
with the extent of association observed in solution. Towards this Continuous-variation (Job) plots for 1 and 2 provided further
an increase in the Ka value by more than two orders of magnitude
end, absorption changes produced at 490 nm were monitored as a support for the contention that the Ka value is higher in MCH
function of concentration when 1 is mixed with 2, 3 or 4 in than in trichloromethane (Fig. 4A). For instance, in MCH, the
trichloromethane. The observed changes could be fit well to a 1 : 1 mole fraction at which the concentration of the complex was
binding profile in all cases. This allowed Ka values of 1.5 (Æ 0.3) Â maximal was 0.5, as would be expected for an association that
103, 1.3 (Æ 0.3) Â 103, and 3.8 (Æ 0.6) Â 103 L molÀ1 to be calculated was 1 : 1 (or 2 : 2, or 3 : 3, etc.). However, a slight deviation was
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Table 1 Thermodynamic properties of the supramolecular aggregates, (1Á2)n, formed from the ditopic donors and acceptors 1 and 2 as deduced from
temperature-dependent absorption spectroscopic analyses carried out in 1,2-dichloroethane (DCE) and methylcyclohexane (MCH). The values listed
assume an isodesmic model
Solvent
Concentration [mM]
DH [kJ molÀ1
]
Tm [K]
Ke [L molÀ1
a
]
DS [J molÀ1 KÀ1
]
DPNa
CHCl3
1,2-Dichloroethane
Methylcyclohexane
1.6
0.4
0.1
NA
NA
265.0
330.8
1.5 (Æ0.3) Â 103
3.9 (Æ0.3) Â 102
4.8 (Æ0.1) Â 105
NA
1.7
1.0
16
À28.1
À72
À106
À249
a
Values determined at 293 K.
value being observed when comparing 1Á3 and 1Á4 (Fig. 3) in
trichloromethane. The effect of solvent is even greater, as
inferred from studies of 1Á2 in trichloromethane, DCE and
MCH. In the case of the latter solvent, a 1 : 1 binding stoichio-
metry was inferred from curve fits of the binding profile based
on UV-vis-NIR spectroscopic titrations and from Job plots. The
derived equilibrium constant for the individual interactions,
Ka = 4.8 Â 105 L molÀ1, translates to an effective DPN, of 16 at
0.1 mM in MCH assuming an isodesmic model. In contrast,
very little polymerisation is observed in trichloromethane, even
at 1.6 mM. The present results, which highlight the effect of
external parameters on the self-assembly of weakly associated
systems, may serve to increase our understanding of the factors
that influence aggregation in a way that might not be possible
using systems that self-associate with high binding affinity.
This work was supported by the National Science Foundation
(grant CHE-1057904 to J.L.S.), the Robert A. Welch Foundation
(grant F-1018 to J.L.S.), the Villum Foundation (to J.O.J.), and the
Danish Natural Science Research Council (FNU, Project 11-106744
to J.O.J.).
Fig. 4 (A) Continuous variation plots for the solution phase interaction
between 1 and 2 as observed in methylcyclohexane and trichloromethane,
respectively. (B) Plots of DPN as a function of monomer concentration in
different solvents, based on the binding constants in Table 1.
seen in trichloromethane, notwithstanding the reasonable fit to
a 1 : 1 binding profile that was observed under conditions of the
UV-vis binding titrations (vide supra).
This deviation is ascribed to the presence of a small quantity
of (TTF-C4P : bis-dinitrophenyl) complex with net 2 : 1 stoichio-
metry (Fig. 4A). Such deviations from pure 1 : 1 stoichiometry
are expected in the case of weaker net Ka values.
To analyze the effect of solvent polarity on the degree of
polymerisation (DPN), the DPN was plotted as a function of
concentration in MCH, DCE, and trichloromethane. In the case
of trichloromethane and DCE, the degree of polymerisation is
extremely low and does not show a meaningful increase as the
total concentration of monomers goes up. On the other hand,
in MCH strong concentration-dependent behaviour was seen
with the DPN increasing with concentration. This is consistent
with what is expected for a system characterised by a relatively
higher binding constant.
Notes and references
1 C. Schmuck and W. Wienand, Angew. Chem., Int. Ed., 2001, 40,
4363–4369.
2 M.-O. M. Piepenbrock, G. O. Lloyd, N. Clarke and J. W. Steed, Chem.
Rev., 2010, 110, 1960–2004.
3 B. Zheng, F. Wang, S. Dong and F. Huang, Chem. Soc. Rev., 2012,
41, 1621.
4 R. P. Sijbesma, F. H. Beijer, L. Brunsveld, B. J. Folmer,
J. H. Hirschberg, R. F. Lange, J. K. Lowe and E. W. Meijer, Science,
1997, 278, 1601–1604.
In summary, mixing the TTF functionalised ditopic C[4]P
receptor 1 with any of the three congeneric bis-dinitrophenyl
functionalised acceptors 2À4 leads to formation of supramolecular
self-assembled ensembles. Fully polymeric structures are seen in
the solid state in the case of all three acceptors.
In trichloromethane solution, the interactions are weak and
are seen to vary with the choice of linker. The highest intra-
component interactions are seen in the case of 4, followed by 2,
an observation ascribed to steric effects. The effect of structure
is rather large, with an order of magnitude difference in the Ka
5 Y. Li, T. Park, J. K. Quansah and S. C. Zimmerman, J. Am. Chem. Soc.,
2011, 133, 17118–17121.
6 X. Yan, B. Jiang, T. R. Cook, Y. Zhang, J. Li, Y. Yu, F. Huang,
H.-B. Yang and P. J. Stang, J. Am. Chem. Soc., 2013, 135,
16813–16816.
7 N. N. Adarsh and P. Dastidar, Chem. Soc. Rev., 2012, 41, 3039–3060.
8 J. S. Park, K. Y. Yoon, D. S. Kim, V. M. Lynch, C. W. Bielawski,
K. P. Johnston and J. L. Sessler, Proc. Natl. Acad. Sci. U. S. A., 2011,
108, 20913–20917.
9 D. S. Kim, V. M. Lynch, J. S. Park and J. L. Sessler, J. Am. Chem. Soc.,
2013, 135, 14889–14894.
10 K. A. Nielsen, W. S. Cho, J. O. Jeppesen, V. M. Lynch, J. Becher and
J. L. Sessler, J. Am. Chem. Soc., 2004, 126, 16296–16297.
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