Three component assemblies by orthogonal H-bonding and donor-acceptor charge-transfer interaction

Article abstract of DOI:10.1039/c3cc47376g

Three component supramolecular assemblies from a mixture of an aromatic donor (D), acceptor (A) and external structure directing agent (ESDA) are achieved by orthogonal noncovalent interactions involving two different types of H-bonding and alternate D-A

Full text of DOI:10.1039/c3cc47376g

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Three Component Assembly by Orthogonal H-Bonding
and Donor-Acceptor Charge-Transfer Interaction
Cite this: DOI: 10.1039/x0xx00000x
Haridas Kara and Suhrit Ghosh* a
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
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S2) for gelation suggesting necessity of both CTꢀinteraction and
orthogonal Hꢀbonding for gel formation.
Three component supramolecular assemblies from a mixture
of an aromatic donor (D), acceptor (A) and external structure
directing agent (ESDA) is achieved by orthogonal
noncovalent interactions involving two different types of H-
bonding and alternate D-A stacking. ESDA containing amide
or urea produces charge-transfer gel and sol, respectively,
owing to their contrasting morphology.
Elegant functions performed with ultimate perfection by biological
selfꢀassembled systems provide the inspiration to study
supramolecular assemblies of abiotic units 1 and explore their utility
2
3
in diverse applications including biology and material science.4
Their diversity in terms of structure and function increases
exponentially with growing number of different building blocks
integrated in a single system by nonꢀcovalent forces. While there
exists many examples of two component assemblies,5 three
component assemblies remain scarce.6 In the recent past we have
shown that an external structure directing agent (ESDA) can induce
Scheme 1: TopꢀStructure of donorꢀacceptor building blocks and
ESDA; Bottomꢀ Schematic showing threeꢀcomponent assembly
gelation of
a naphthaleneꢀdiimide (NDI) building block by
Gelation was also attempted in few other nonpolar organic solvents
with varying D: A ratio but no other combination except1:1 D/ A in
MCH/ CHCl3 (90/ 10) produced gel (Fig S3). This indirectly
suggests alternate DꢀA sequence rather than random distribution of
the two units as in that case 1:1 stoichiometry may not be
prerequisite for gelation. Trace amount of a protic solvent MeOH
destroys the gel with concomitant disappearance of the color (Fig 1a)
confirming Hꢀbonding is the cause while DꢀA stacking is the
consequence. This is also confirmed by UV/vis spectra (Fig 1a)
which shows drastically reduced CTꢀband in presence of MeOH (for
solvent dependent UV/vis spectra see Fig S4). To elucidate the
molecular arrangements in the gel state, powder XRD studies were
carried out with the xerogel (Fig 1b). Intense and sharp reflection is
seen in the low angle that corresponds to d = 46.6 Å along with
relatively less intense peaks for d/2 (23 Å), d/ 3 (11.6 Å) and d/ 4
(7.9 Å) indicating lamellar organization. The observed d spacing of
46.6 Å corroborates well with the summation (52.03 Å) of simulated
lengths of ESDAꢀA and NDIꢀ1 (for energy minimized structures see
Fig S5) supporting the proposed mode of coꢀassembly (Scheme 1).
orthogonal Hꢀbonding.7 To explore this supramolecular design
further, we have taken one step forward and examined coꢀassembly
of an aromatic donor (D) and acceptor (A) chromophore in presence
of a suitable ESDA (Scheme 1) by DꢀA chargeꢀtransfer (CT)
8ꢀ10
interaction
and two different types of Hꢀbonding operating
simultaneously in orthogonal directions.11
NDIꢀ1 (A) and Pyꢀ1 (D) contain a carboxylic acid group that is
complementary to the pyridine group of the ESDA. The ESDA also
contains an amide or urea for concurrent Hꢀbonding in orthogonal
direction. Three component assemblies were tested with NDIꢀ1 +
Pyꢀ1 + ESDAꢀA (1:1:2) in methyl cyclohexane (MCH) while
macroscopic precipitation was noticed. However, in presence of 10
% CHCl3 the mixture produces a homogeneous yellowish solution
upon heating which at rt transforms to a red transparent gel (Fig 1)
suggesting alternate DꢀA stacking. Tg was estimated to be 31°C (c =
10 mM) suggesting weak nature of the gel which was also reflected
in very low yield stress value (0.3 Pa at c = 12 mM) that was
obtained from the rheological measurements (Fig S1). As a control,
few other combinations such as (NDIꢀ1 + ESDAꢀA), (NDIꢀ1 + Py +
ESDAꢀA), (NDIꢀ2 + Py + ESDAꢀA), (NDIꢀ2 + Pyꢀ1 + ESDAꢀA)
and (Pyꢀ1 + ESDAꢀA) were tested while none found suitable (Fig
1
To gain more insight into the interꢀmolecular interactions, H NMR
of the gel was compared with those of the individual components
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(Fig 1c). Notably the amide proton (Hb) of the ESDAꢀA is downfield
shifted by ~ 0.5 ppm suggesting it is involved in Hꢀbonding.
Fig 2: Variable temperature UV/vis spectra of the three component
assembly in presence of (a) ESDAꢀA and (b) ESDAꢀU; (c) Plot of
agg Vs Temperature; inset in (a)ꢀ image of the CTꢀsol of NDIꢀ1 (2.5
mM) + Pyꢀ1 (2.5 mM) + ESDAꢀU (5.0 mM) in MCH / CHCl3 (90:
10).
Notably Fig 2a indicates appearance of the CTꢀband prior to the
gelation (Tg = 31 °C) indicating DꢀA complexation actually induced
gelation. This was also confirmed by cursory observation (Fig S10)
where we noticed red coloration even before gelation, upon cooling
of a three component mixture. This raised the question on the
sequence of different interactions involved in the gelation. To gain
better insight we examined the FTꢀIR spectra of the ESDAꢀA alone
which showed (Fig S11) existence of Hꢀbonded amide groups even
at elevated temperature where the CTꢀband diminishes. This
indicates that the ESDAꢀA selfꢀassembles first to produce a
multivalent receptor which facilitates the D and A components to
assemble in alternate fashion and only beyond a certain threshold of
the DꢀA complexation (which is achieved at the Tg) the selfꢀ
assembly leads to gelation. However, this does not explain the lack
of gelation for ESDAꢀU containing three component system. Thus to
understand the reason for anomalous gelation property, we checked
their morphology. TEM images (Fig 3) of NDIꢀ1 + Pyꢀ1 + ESDAꢀA
shows fibrillar morphology that is typical for a gelator. In contrary
NDIꢀ1 + Pyꢀ1 + ESDAꢀU combination produces ill defined particles
with diameter less than 10 nm which was also supported by dynamic
light scattering (DLS) measurements revealing (Fig S12) presence of
aggregates with Dh < 10 nm. Further we examined the effect of D/ A
ratio [(D + A) = ESDAꢀU] and observed gel formation (Fig S13)
only at D: A = 1: 3 unlike the red solution at D: A = 1: 1. TEM
images of the diluted gel showed elongated fibres (Fig S14) and in
Fig 1: a) Selected region of UV/vis absorption spectra of the gel
produced by {NDIꢀ1 (2.5 mM) + Pyꢀ1 (2.5 mM) + ESDAꢀA (5.0
mM)} in MCH/ CHCl3 (90: 10) and its fate after MeOH addition.
Inset shows images of the corresponding samples. b) Powder XRD
data of the xerogel; c) H NMR spectra (selected region) of the gel
(in CDCl3/ MCH = 10: 90) and individual components (in CDCl3).
1
The Hd proton that is adjacent to the nitrogen atom of the pyridine
ring is also downfield shifted possibly due to the generation of δ+
charge on the N atom in the Hꢀbonded state. Furthermore the NDI
ring protons (Ha) experience most prominent upfield shift in gel state
compared to sol as a result of shielding effect in the alternate DꢀA
stacking. Same is also true for the pyrene ring protons although to a
lesser extent which is consistent with previous reports on alternate
DꢀA stacking. 12 A comparison of the FTꢀIR spectra of the gel with
those of the individual components (Fig S6) further supports the Hꢀ
bonded three component assembly (Scheme 1) although detail
analysis was not possible due coincidental overlap of several peaks.
Further we examined ESDAꢀU (Scheme 1) which differs with
ESDAꢀA only in the urea functionality instead of amide. NDIꢀ1 +
Pyꢀ1 + ESDAꢀU (1:1:2) in MCH/ CHCl3 (90: 10) also shows intense
red color (insetꢀFig 2a) indicating CTꢀcomplexation. But to our
surprise, no gelation was observed even for samples with tenfold
higher concentration. To examine the interꢀcomponent interactions
DLS
a peak appeared (Fig S14) corresponding to average
hydrodynamic diameter of ~ 900 nm in sharp contrast to <10 nm for
1: 1 mixture. In absence of any donor the mixture resulted in
macroscopic precipitation. Based on these results a possible model is
shown in Fig 3.
1
we studied selfꢀassembly in this case also by H NMR as in case of
ESDAꢀA (Fig 1c) and noticed (Fig S7) similar behavior indicating
existence of Hꢀbonding and DꢀA alternate stacking. To understand
possible reasons for lack of gelation ability for ESDAꢀU, we
compared the association constants (K) for the CTꢀcomplex in
presence of this two different structure directing agents by
concentration dependent UV/ vis experiments (Fig S8, S9).13 The K
values were estimated to be 42 (extinction coefficient of the CTꢀ
band =3.77 x 103 Mꢀ1cmꢀ1) and 98 Mꢀ1 (extinction coefficient of the
CTꢀband = 1.96 x 103 Mꢀ1cmꢀ1) for ESDAꢀA and ESDAꢀU,
respectively,14 suggesting actually better assembly in case of the urea
containing ESDA. Further their thermal stabilities were compared by
variable temperature UV/vis (Fig 2a, 2b). With increasing
temperature, the CTꢀband gradually decreases in linear fashion in
both cases suggesting disassembly and lack of any cooperative
nature. To have a comparison, the respective αagg (T) (mole fraction
of aggregate at temperature T) values were estimated (see supporting
information for detail). A plot of αagg vs. T (Fig 2c) indicates stiffer
slope for ESDAꢀA suggesting lower stability.
Fig 3: Possible mode of assembly that leads to contrasting TEM
morphology for three component organization in presence of ESDAꢀ
A (left) and ESDAꢀU (right).
In case of ESDAꢀU, the supramolecular polymerization grows with a
curvature and thus produces ill defined tiny particles which can be
compared to cyclic in conventional step growth polymerization.
Noteworthy that in a previous report we showed ureaꢀurea Hꢀ
bonding with a longitudinal displacement which caused slipped
assembly for the attached dialkoxynaphthalene chromophores.15 But
for alternate DꢀA assembly in the present system, faceꢀtoꢀface
arrangement is possibly more favorable and thus to accommodate
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both these structural demands, the ESDAꢀU stacks with longitudinal
as well as rotational displacement that leads to the formation of
small particles. However, this curvature (arising out of rotational
displacement) may not be essential in presence of excess acceptor as
in that case there will be several segments containing AꢀA sequence
which is anyway known to adopt a slipped stacking that is
compatible with the Hꢀbonding motif of urea. Thus elongated
fibrillar morphology and gelation is noticed in case of 1: 3 D/ A.
This also confirms absence of any other type of possible Hꢀbonding
(such as carboxylic acidꢀurea) in the mixture containing ESDAꢀU,
because in that case varying stoichiometry of D/A would not have so
drastic effect on morphology and gelation.
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functionality in the ESDA, either a 1D fibre or particles are
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16
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ESDA for tuning the coꢀassembly properties.
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Notes and references
a Indian Association for the Cultivation of Science, Polymer Science Unit,
2A & 2B Raja S. C. Mullick Road, Kolkata, Indiaꢀ700032. Email:
psusg2@iacs.res.in
† We thank Mr. M. R. Molla for providing NDIꢀ1. HK thanks CSIR for a
research fellowship. SG thanks SERB for funding (SR/S1/OCꢀ18/2012).
Electronic Supplementary Information (ESI) available: [Synthesis and
characterization, materials, methodology of all experiments and
additional spectral data]. See DOI: 10.1039/c000000x/
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K values are for the DꢀA complexation and does not reflect the
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Three Component Assembly by Orthogonal H-
Bonding and Donor-Acceptor Charge-Transfer
Interaction
Haridas Kar and Suhrit Ghosh*
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