Supramolecularly Engineered Amphiphilic Macromolecules: Molecular Interaction Overrules Packing Parameters

Article abstract of DOI:10.1002/anie.201611715

We report molecular interaction-driven self-assembly of supramolecularly engineered amphiphilic macromolecules (SEAM) containing a single supramolecular structure-directing unit (SSDU) consisting of an H-bonding group connected to a naphthalene diimide chromophore. Two such SEAMs, P1-50 and P2-50, having the identical chemical structure and hydrophobic/hydrophilic balance, exhibit distinct self-assembled structures (polymersome and cylindrical micelle, respectively) due to a difference in the H-bonding group (hydrazide or amide, respectively) of the single SSDU. When mixed together, P1-50 and P2-50 adopted self-sorted assembly. For either series of polymers, variation in the hydrophobic/hydrophilic balance does not alter the morphology reconfirming that self-assembly is primarily driven by directional molecular interaction which is capable of overruling the existing norms in packing parameter-dependent morphology control in an immiscibility-driven block copolymer assembly.

Full text of DOI:10.1002/anie.201611715

Angewandte
Communications
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International Edition: DOI: 10.1002/anie.201611715
German Edition: DOI: 10.1002/ange.201611715
Supramolecular Chemistry
Supramolecularly Engineered Amphiphilic Macromolecules:
Molecular Interaction Overrules Packing Parameters
Prithankar Pramanik, Debes Ray, Vinod K. Aswal, and Suhrit Ghosh*
Abstract: We report molecular interaction-driven self-assem-
bly of supramolecularly engineered amphiphilic macromole-
cules (SEAM) containing a single supramolecular structure-
directing unit (SSDU) consisting of an H-bonding group
connected to a naphthalene diimide chromophore. Two such
SEAMs, P1-50 and P2-50, having the identical chemical
structure and hydrophobic/hydrophilic balance, exhibit dis-
tinct self-assembled structures (polymersome and cylindrical
micelle, respectively) due to a difference in the H-bonding
group (hydrazide or amide, respectively) of the single SSDU.
When mixed together, P1-50 and P2-50 adopted self-sorted
assembly. For either series of polymers, variation in the
hydrophobic/hydrophilic balance does not alter the morphol-
ogy reconfirming that self-assembly is primarily driven by
directional molecular interaction which is capable of over-
ruling the existing norms in packing parameter-dependent
morphology control in an immiscibility-driven block copoly-
mer assembly.
Scheme 1. Structure of the SEAMs and a schematic showing direc-
tional molecular interaction-driven distinct self-assembly.
A
mphiphilic block copolymers[1] produce wide-ranging ele-
gant mesoscopic structures[2] with close relevance for nano-
technology and biomedicine.[3] In a majority of examples the
morphology of a block copolymer nanostructure relies on the
packing parameters[4] determined by the relative length and
volume of the hydrophobic and hydrophilic blocks which does
not endow precision and structural control at the molecular
scale. On the other hand for bio-macromolecules, an alter-
ation in the sequence of a single amino acid or mismatch in
one base pair could make vital difference to the overall
structure and function. To explore such possibilities in
synthetic polymers, we have studied the self-assembly of
supramolecularly engineered amphiphilic macromolecules
(SEAM; Scheme 1) which consist of a supramolecular struc-
ture-directing unit (SSDU) located at the junction of a hydro-
philic polymer and a hydrophobic wedge. We asked the
question whether in such systems, a specific directional
molecular interaction among the single SSDU (Scheme 1)[5]
would be able to dictate the formation of a mesoscopic
structure by overruling the packing parameters. Here we
show that the nanostructures generated from P1-50 and P2-50
(Scheme 1), which differ merely by a single H-bonding group
of the SSDU, are fully regulated by the self-assembly motif of
the SSDU, and the packing parameters are ignored. Remark-
ably even in their mixture the individual SSDUs maintain
their self-identity which results in self-sorting.
P1-50 and P2-50 were synthesized (see Scheme S1 in the
Supporting Information) by RAFT polymerization of the
oligo-oxyethylene-attached hydrophilic methacrylate deriva-
tive using functional chain-transfer agents containing the
corresponding SSDU and the hydrophobic wedge. P1-50 and
P2-50 were isolated in 72% and 80% yield, respectively. The
polymers were characterized unambigously by 1H NMR
spectroscopy (see Figure S1 in the Supporting Information).
By comparing the UV/Vis (Figure S2) and 1H NMR (Fig-
ure S3) spectra of the polymers with those obtained for the
SSDU-functionalized chain-transfer agents, it is obvious that
the dithioester group remains intact at the other chain
terminal of the polymer. Gel permeable chromatography
analysis showed (Figure S4) monomodal peaks corresponding
to molecular weights corroborating with those values
obtained theoretically or estimated from the end group
analysis using NMR spectroscopy (Table S1). The SSDU, by
design, consists of a naphthalene diimide (NDI) chromophore
and a H-bonding functional group (hydrazide and amide,
respectively, for P1-50 and P2-50) to facilitate synergistic
operation of the H-bonding and aromatic interaction
[*] P. Pramanik, Prof. Dr. S. Ghosh
Polymer Science Unit, Indian Association for the Cultivation of
Science
2A and 2B Raja S. C. Mullick Road, Kolkata, 700032 (India)
E-mail: psusg2@iacs.res.in
Dr. D. Ray, Dr. V. K. Aswal
Solid State Physics Division, Bhabha Atomic Research Centre
Trombay, Mumbai, 400085 (India)
Supporting information, including synthesis and characterization of
the polymers, sample preparation, detail information on the physical
studies and additional physical data, and the ORCID identification
number(s) for the author(s) of this article can be found under:
http://dx.doi.org/10.1002/anie.201611715.
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(Scheme 1).[5,6] UV/Vis spectra of both the polymers in THF
show (Figure 1a) sharp absorption bands with vibronic
features indicating non-interacting NDI. In H2O, the spectra
exhibit a bathochromic shift of 3.0–4.0 nm together with
(Figures 2a, S7) show hollow spherical objects for P1-50
with an average diameter of about 300 nm indicating poly-
mersome formation.[8] A similar spherical morphology was
also observed in the AFM image (Figure S9). The size
obtained from microscopy images corroborates with the
dynamic light scattering (DLS) data (Figure 2d) revealing
a single peak with a particle size of 300–400 nm. In contrast,
TEM images of P2-50 (Figures 2b, S8) reveal a spherical
micellar morphology with a relatively small diameter (80–
120 nm) corroborating with the DLS (Figure 2e). Intriguingly
the DLS data of the P2-50 solution after 100 h show (Fig-
ure 2e) complete disappearance of the initial peak at 80–
120 nm and the emergence of a new relatively broad peak
corresponding to larger particle sizes indicating a morphology
transition. Time-dependent DLS studies of over 100 h indi-
cate (Figure S10) a gradual increase in the particle size and
a saturation after 72 h. Nonetheless, no such variation was
observed for P1-50 (Figures 2d, S10) suggesting stability of its
polymersome structure. The intrinsic viscosity of the freshly
prepared aqueous solutions of P1-50 and P2-50 were com-
parable as expected because of the similar spherical morphol-
ogy. The value remained almost unchanged after 72 h for P1-
50 while that for P2-50 showed a five-fold increase (Figure 2 f)
further confirming a morphology transition in this case
selectively leading to larger entangled structures. This could
be clearly visualized in the TEM images of the aged sample of
P2-50 revealing cylindrical micelles (Figure 2c, S8)[9] with
a length and diameter in the range of 1.5 mm–4.5 mm and 50–
130 nm, respectively. Likewise the AFM images (Figure S9)
also show a transformation of the initially formed spherical
structure to elongated fibrils after aging. In these three
distinct assembled structures (polymersomes, spherical
micelles, cylindrical micelles), organization of the alkyl
chains of the hydrophobic wedge is expected to be different.
Figure 1. a) Absorbance b) emission (lex =340 nm) and c) FTIR spec-
tra of P1-50 (black) and P2-50 (gray) in solvent (dotted line) (THF-UV
and PL, CHCl3-IR) and water (solid line); C=1.0 mgmLꢀ1 for absorb-
ance and emission spectra and 10 mgmLꢀ1 for FTIR spectra.
a reversal of the intensity for the peaks at 381 and 360 nm and
an overall reduction in the intensity, which is indicative of an
aromatic interaction among the NDI chromophores[5] and is
further supported by fluorescence quenching (Figure 1b). A
significant upfield shift of the NDI ring protons in the
1H NMR spectra of the polymers in D2O compared to the
spectra in CDCl3 (Figure S5) reconfirmed the p-stacking.
Variable-temperature UV/Vis studies (Figure S6) show no
change in the spectral features (except the increase in the
baseline intensity) even above 808C indicating a very high
thermal stability. The increase in baseline intensity at elevated
temperatures is attributed to scattering beyond the lower
critical solution temperature (LCST)[7] of the hydrophilic
block which was estimated to be 44 and 428C for P1-50 and
P2-50, respectively (Figure S6).
Although the hydrophobic
content in these polymers is only
6 wt% they still show self-assem-
bly at very low concentrations
(CAC < 0.5 mgmLꢀ1) and high
temperatures indicating the strong
impact of the directional interac-
tion between the SSDU involving
synergistic H-bonding and aro-
matic interactions. In fact H-bond-
ing was directly probed by FTIR
spectroscopy (Figure 1c). For P1-
50 in CDCl3, two peaks at 3524 and
3423 cmꢀ1 are assigned to the OH
stretching (from residual water)
and non-bonded NH stretching,
respectively. In D2O, the appear-
ance of a distinct peak at 3334 cmꢀ1
indicates strong H-bonding among
Figure 2. HRTEM images (negative staining with uranyl acetate for a and c) of a) P1-50, b) P2-50
(freshly prepared), and c) P2-50 (aged) in aqueous solution. Dynamic light scattering (DLS) plot of
d) P1-50 and e) P2-50 in initial state (black line) and aged state (gray line). f) Intrinsic viscosity in
initial state and aged state of P1-50 (black) and P2-50 (gray). g) Variation of GP for the Laurdan dye
with time after it is encapsulated in the aqueous solution of P1-50 or P2-50 (Concentration of
polymers=1.0 mgmLꢀ1 and laurdan dye=0.01 mm). For HRTEM and DLS, concentration of the
the hydrazide groups. Likewise for
P2-50, similar observations were
made by comparing the spectra in
CDCl3 and D2O confirming H-
bonding interactions among the
amides in P2-50. TEM images
polymers were 1.0 mgmLꢀ1
.
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To test that, we probed the fluidity of the hydrophobic
domain by a well-known microviscosity-sensitive hydropho-
bic dye Laurdan (Figures 2g, S11).[10] Fluidity at its vicinity
can be measured by the generalized polarization (GP)
function defined as shown in Equation (1),
I440 ꢀ I490
ð1Þ
GP ¼
I440 þ I490
where I440 and I490 are the steady-state emission intensities
(from the excitation spectra) at 440 and 490 nm, respectively.
Laurdan was encapsulated in the aggregates of P1-50 and P2-
50 and from time-dependent fluorescence excitation spectra
(Figure S11) GP was estimated according to the above-
mentioned formula. At the beginning, GP for P1-50 was 0.5
(Figure 2g) indicating a highly ordered microenvironment as
reported for membranes. The value did not change (Fig-
ure 2g) during the tested time span (21 h) supporting stable
polymersome assembly. In sharp contrast, for P2-50, values of
GP close to zero for freshly prepared solution clearly indicate
lack of ordering among the alkyl chains which is expected for
micellar structures. Interestingly, in this case the value
gradually increases and saturates at about 0.3 after 4 h
(Figure 2g) which can be related to the morphology transition
from spherical to cylindrical micelles.[11] From the increase in
GP, it is evident that in cylindrical structure the alkyl chains
are organized more effectively than the initially formed
spherical aggregates and that could be the driving force for
the transformation. Nonetheless, even after prolonged aging
the value of GP of encapsulated Laurdan in P2-50 did not
reach that of P1-50 indicating the fluidity of the hydrophobic
domain is gel-like for P1-50 while liquid-crystal-like in P2-
50.[10]
To get more structural insight, small-angle neutron
scattering (SANS) experiments were performed in D2O.
Distinctly different scattering patterns for P1-50 and P2-50
(Figure 3a) clearly support formation of different mesoscopic
structures. Fresh P2-50 data could be well fitted using
a spherical core–shell micellar model which considers hydro-
philic chains (characterized by the radius of gyration, Rg)
attached to the hydrophobic core of the micelle (character-
ized by the core radius, Rc). The analyzed value of Rc is found
(Table 1) to be larger than the roughly estimated length of the
hydrophobic block (ca. 35 ꢀ). This could be arising from the
propagation of hydrophobic environment up to some part of
the hydrophilic block.[12] Upon aging, the shape of the
micelles has been found to transform from spherical to long
cylindrical ones as evidenced in the DLS/TEM measure-
ments. In this case, the length of these long cylindrical
micelles will be reflected in the further lower Q region which
is beyond the accessible Qmin of this SANS instrument (i.e. 2p/
Qmin ꢁ 350 ꢀ). Nonetheless the inset of Figure 3a highlights
the formation of elongated structures through the simulated
scattering data for aged P2-50 (considering long cylindrical
micelles) extended up to the lower-Q region. On the other
hand, P1-50 has been found to form large unilamellar vesicles
(LUV). These vesicles have been characterized by the
thickness of the hydrophobic component (t; Table 1) as the
measurement of the radius of vesicles is limited by the Qmin of
Figure 3. a) SANS data of P1-50 and P2-50 (fresh and aged). Inset:
Extended fitted scattering data up to lower-Q re-gion for cylindrical
micellar structure of aged P2-50. b,c) SANS profiles for aqueous
solutions of P1 and P2 series of polymers. The solid curves are the
theoretical fits to the experimental data.
Table 1: SANS data of P1-50 and P2-50 in D2O (C=10.0 mgmLꢀ1).
SEAM
Structural parameters
Structure
P2-50:
fresh
radius of hydrophobic core Rc =56.8ꢃ1.9 ꢀ, spherical
polydispersity s=0.32,
micelle
radius of gyration of hydrophilic chains
Rg =16.6ꢃ0.6 ꢀ
P2-50:
aged
cross-sectional core radius
Rcs =58.4ꢃ2.0 ꢀ,
cylindrical
micelle
polydispersity s=0.37,
radius of gyration of hydrophilic chains
Rg =16.6ꢃ0.6 ꢀ
P1-50
membrane thickness t=44.6ꢃ1.6 ꢀ
unilamellar
vesicle
the SANS instrument. Notably the estimated hydrophobic
thickness of the vesicle (ca. 45 ꢀ) formed by P1-50 is
significantly shorter than twice the length of the hydrophobic
segment of P2-50 indicating efficient inter-digitation of the
peripheral alkyl chains.
Generally the morphology of block copolymer aggregates
depends on the packing parameter (p). Spherical micelles,
cylindrical micelles or polymersomes are formed when p ꢂ 1/
3, 1/3 < p ꢂ 1/2, and 1/2 < p ꢂ 1, respectively.[4] Contrary to
such existing norms, the current examples of self-assembly of
SEAMs are unique because P1-50 and P2-50 form distinctly
different morphologies although having identical p parame-
ters as in both cases the hydrophobic wedge and the hydro-
philic polymer are the same. Therefore it is evident that the
assembly is not driven by immiscibility, rather it is overruled
by specific self-assembly motifs of the particular SSDU. Now
the SSDU consists of hydrazide and amide in P1-50 and P2-
50, respectively, which significantly differs in terms of
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flexibility. The rigid hydrazide groups produce a stiff chain by
extended hydrogen bonding which allows the hydrocarbon
chains to crystallize and form a bilayer that eventually
generates a polymersome structure. On the other hand the
flexibility of the amide group due to the presence of the two
methylene units between the amide group and the NDI
chromophore appears to allow the H-bonded chains adequate
flexibility and therefore the supramolecularly assembled P2-
50 collapses to a spherical micellar structure (kinetic product)
which eventually reorganizes to generate the more stable
cylindrical micelles. To what extent does molecular assembly
overrule the packing parameter? To answer this question we
have studied P1-25 and P1-100 (Scheme 1) where the SSDU
remains the same like in P1-50, but the degree of polymer-
ization of the hydrophilic block changes significantly
(Table S1). They showed exactly identical SANS profiles
(Figure 3b) suggesting polymersome structures in all cases.
Likewise, in P2 series (Scheme 1), for all three polymers
micellar structures were evident from the very similar SANS
pattern (Figure 3c). Microscopy images of P1-25, P1-100, P2-
25, and P2-100 (Figures S13–16) corroborate with the results
obtained from SANS measurements and therefore are con-
sistent with the hypothesis. For P1-100 or P2-100, the hydro-
phobic wedge is merely 3 wt% with respect to the entire
polymer and still they exhibit facile assembly which reveals
the supremacy of directional interaction between the SSDU
which ignores the hydrophobicity factor and overtakes the
classical norms of block copolymer self-assembly depending
on packing parameters.
When mixed together, will P1-50 and P2-50 co-assemble
or form self-sorted structures? To probe that by FRET, we
have synthesized P1-50-R, P1-50-G, and P2-50-R (Figure 4a)
which are structurally similar to P1-50 or P2-50 except for the
presence of attached red or green dyes (10–15%). Absorption
and emission spectra of these polymers (Figure S17) confirm
the successful attachment of the rhodamine dye (R) to P1-50-
R and P2-50-R while in P2-50-G the naphthalene-monoimide
dye (G) is attached.[13] A mixture of P1-50-G + P1-50-R in
THF shows negligible emission at the acceptor (R) site when
excited at the absorption wavelength of the donor (440 nm)
indicating no FRET (Figure 4b) as expected because there is
no self-assembly in THF. But in aqueous medium, a strong
emission is noticed at 594 nm which is attributed to FRET.
This observation confirms co-assembly of the two polymers
(as expected in this case because both belong to the P1 series)
and puts the appended red and green emitting dye molecules
within their Forster distance (52.28 ꢀ).[13] Now under identi-
cal conditions, P1-50-G + P2-50-R shows a contrasting behav-
ior as FRET is negligible (blue line, Figure 4b) indicating
a self-sorted assembly.[14,15] Furthermore, the merged fluores-
cence microscopy image of freshly prepared P1-50-G + P2-50-
R (Figure 4c–e) of shows red and green emission from
different particles (Figure 4e) confirming self-sorting.[15] The
control experiment of P1-50-G + P1-50-R, in contrast, shows
a yellow emission (Figure 4h) for the merged image indicat-
ing that the red and green emissions arise from the same
particle. TEM images (Figures S18–19) also show co-exis-
tence of polymersome + spherical micelles (fresh sample) or
polymersome + cylindrical micelles (aged sample) and thus
Figure 4. a) Structure of fluorescently labeled SEAMs. b) Solvent-de-
pendent emission spectra of (P1-50-R+P1-50-G) and (P2-50-R+P1-50-
G). The intensity was normalized at the donor (P1-50-G) emission.
lex =440 nm. c–h) Fluorescence microscopy images of freshly prepared
P2-50-R+P1-50-G (left) and P1-50-R+P1-50-G (right). c,f) Green chan-
nel emission. d,g) Red channel emission. e) Merged image of (c) and
(d). h) Merged image of (f) and (g).
further support the self-sorting between P1-50 and P2-50. This
is believed to be a consequence of the fact that due to the
mismatch in the spacer length between the H-bonding group
and the NDI ring of the two different SSDU, a synergistic
effect of H-bonding and aromatic interaction was not
achieved.
In summary we have established that a directional
molecular interaction can overturn the classical norms of
block-copolymer self-assembly driven by packing parameters.
Consequently, two such block copolymers with identical
chemical structures and the same hydrophobic/hydrophilic
balance organized in a distinct manner form either cylindrical
micelles or polymersomes depending on whether the SSDU
contains an amide or a hydrazide functional group, respec-
tively. On the other hand for same series of polymers
containing either an amide or hydrazide group at the SSDU,
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the difference in the packing parameters by varying the
degree of polymerization of the hydrophilic block did not
alter the self-assembled structure. In a mixture of amide and
hydrazide-containing polymers self-sorted assembly was
noticed and all existing rules of polymer mixing/segregation
were ignored. In the recent past there has been great interest
in studying living crystallization driven self-assembly[9,16] of
block copolymers. Compared to those systems, the present
design is a significant step forward because in this case the
self-assembly is driven by specific directional molecular
interaction among a single SSDU present in the entire
polymer chain which brings new opportunities for control-
lable mesoscopic structure formation of the macromolecules
in solution and solid state with precision at the molecular
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Laurdan dye depends on the fluidity of its microenvironment
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Acknowledgements
P.P. thanks IACS for a research fellowship. S.G. thanks the
DST, Government of India, for funding through the Swarna-
Jayanti Fellowship (grant number DST/SJF/CSA-01/2-14-15).
We thank Mr. Supriya Chakraborty, IACS, for assisting us
with the TEM experiments. We also thank Prof. S. Sinha and
Ms. H. Khatra, Department of Organic Chemistry, IACS, for
helping us to carry out fluorescence microscopy imaging
experiments.
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Conflict of interest
The authors declare no conflict of interest.
Keywords: amphiphilic polymer · cylindrical micelle ·
fluorescence · polymersome · supramolecular assembly
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negligible FRET could arise due to interparticle donor–acceptor
communication. Likewise in Figure 4e, a few yellow emitting
objects can be noticed (in addition to red and green emitting
particles) which is clearly different to those observed in Fig-
ure 4h and is attributed to co-incidental overlapping of self-
sorted green and red emitting structures.
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Manuscript received: December 1, 2016
Revised: January 25, 2017
Final Article published: && &&, &&&&
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Angew. Chem. Int. Ed. 2017, 56, 1 – 6
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Supramolecular Chemistry
P. Pramanik, D. Ray, V. K. Aswal,
Amphiphilic block-copolymers: A direc-
tional molecular interaction overrules
classical packing parameters and enacts
new rules for the self-assembly of supra-
molecularly engineered amphiphilic
polymer assemblies. The self-assembly is
governed by a distinct H-bonding motif of
a single H-bonding moiety present in the
entire polymer chain.
S. Ghosh*
&&&& — &&&&
Supramolecularly Engineered
Amphiphilic Macromolecules: Molecular
Interaction Overrules Packing Parameters
6
www.angewandte.org
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2017, 56, 1 – 6
These are not the final page numbers!