Light-controlled self-assembly of a dithienylethene bolaamphiphile in water

Article abstract of DOI:10.1039/d0dt02001j

The self-assembly of bolaamphiphiles comprised of a central photochromic dithienylethene (DTE) chromophore was investigated in aqueous media. Irradiation at 254 nm induced a conversion from the open to closed states of the DTE chromophores. Whereas, in water, irradiation produced a photostationary state of 20 : 80 (open/closed), in methanol the ratio improved to 10 : 90 (open/closed). The open → closed transition was accompanied by the formation of 1D nanofibers during incubation in darkness. This journal is

Full text of DOI:10.1039/d0dt02001j

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Douglas W. Stephen et al.
N-Heterocyclic carbene stabilized parent sulfenyl, selenenyl,
and tellurenyl cations (XH , X = S, Se, Te)
+
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DOI: 10.1039/D0DT02001J
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Light-controlled self-assembly of a dithienylethene
bolaamphiphile in water
Cassidy Creemer, ‡a Haydar Kilic, ‡b,c Kwang Soo Lee, Nurullah Saracoglu,* and Jon R. Parquette*
a
b
a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
The self-assembly of bolaamphiphiles comprised of a central
photochromic dithienylethene (DTE) chromophore was
investigated in aqueous media. Irradiation at 254 nm induced a
conversion from the open to closed states of the DTE
chromophores. Whereas, in water, irradiation produced
a
photostationary state of 20:80 (open/closed), in methanol the ratio
improved to 10:90 (open/closed). The open→closed transition was
accompanied by the formation of 1D nanofibers during incubation
in darkness.
Many biomolecular systems achieve and modulate their
1
functional characteristics via noncovalent self-assembly. Self-
assembly provides a convenient, albeit often empirical strategy,
2
to fabricate materials in the nanoscale regime. Strategies to
create nanostructured materials by controlled self-assembly
3
-5
offer a diverse array of applications in optoelectronics,
Fig. 1 Photoisomerization of DTE photochromic unit and structures of DTE
bolaamphiphile 1.
6
-9
10
11, 12
biomaterials, drug delivery, and molecular machines,
inter alia.1
3,
14
The potential for these materials to display
adaptivity, self-healing, and other forms of “intelligent” control of self-assembly in water with light has been less
1
5, 22-25
behavior would be enhanced by the capability to transition studied.
1
5
between multiple states.
Molecules that dynamically
A potential strategy to control the nanoscale organization of
modulate their structural features via external triggers offer a a self-assembled material would be incorporation of a
1
6, 17
strategy to create such smart materials.
“Molecular photoresponsive chromophore within the building blocks that
switches,”1
8-20
that switch between multiple configurational comprise the nanostructure. Dithienylethene (DTE)
states upon irradiation have potential to induce changes in photoswitches
2
1
26-28
have been employed to control the
2
9-31
32
33
local monomer packing as a mechanism to modulate long-range morphology of modified surfaces,
polymers, organogels,
nanostructural order. Although photoswitches have been nanofibers, and liquid crystals. Herein, we report the light-
exploited to modulate supramolecular structure and controlled self-assembly of photoresponsive DTE-
3
4
35
morphology in organic media, the
bolaamphiphiles in aqueous media. We have recently
demonstrated that lysine-1,4,5,8-naphthalenetetracarboxylic
acid diimide (Lys-NDI) amphiphiles and bolaamphiphiles
a. Department of Chemistry and Biochemistry, The Ohio State University, 100 W.
undergo efficient self-assembly into 1D-nanostructures in
1
8th Ave. Columbus, Ohio 43210, E-mail: parquett@chemistry.ohio-state.edu
4,36, 37
water.
In these systems, self-assembly was driven by the
b.
Department of Chemistry, Faculty of Sciences, Atatürk University, Erzurum, Turkey
intermolecular - stacking of the NDI group in addition to
amphiphilic phase separation. Therefore, based on these
systems, photoresponsive monomers 1 and 2 were designed by
replacing the NDI chromophores with a light-responsive, DTE
2
5240, E-mail: nsarac@atauni.edu.tr
c.
Oltu Vocational Training School, Atatürk University, Erzurum, Turkey 25400
Both authors contributed equally to this work.
‡
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
3
6
chromophore, which serves as the hydrophobic, aromatic
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component (Fig. 1). Flanking Fmoc-protecting groups were Accordingly, imaging a sample of 1-open, preparedVibewyAdrtiilculetOinnglinae
added to provide additional π-π stacking capabilities. The colorless 10 mM solution to 1.0 mMD,OI:re10v.e10a3le9/dD0mDTi0n2im00a1lJ,
tendency of bolaamphiphiles to form 1D rods or tubes over nonspecific aggregation with a few liposomal aggregates (Fig.
spherical structures has been shown to increase for monomers 2A). Irradiation with 254 nm light for ~60 mins in water
having rigid internal segments, which results in a more ordered produced a violet solution due to the formation of the closed-
3
8,39
packing of the hydrophobic core.
Thus, we reasoned that form of the DTE. As the solution was kept in darkness, TEM and
the cyclization/cycloreversion reaction between the open- and AFM images of the solution at the PSS revealed a transition in
closed-ring isomers of DTE upon irradiation would alter both the morphology over the course of 24-48 h from the nonspecific
conformational flexibility and/or the -stacking capability of the aggregates to uniform 1D nanofibers with diameters of ~14 nm
monomer, thereby modulating the rigidity of the interior and heights of ~4.8 nm (Fig. 2B-D). Immediately following
segments, and the resulting structure, of the assemblies.
irradiation, a slow transition to amorphous ribbons was
Two DTE-lysine bolaamphiphiles displaying positively observed by TEM that converted to nanofibers after 24-48 h.
charged (1) or zwitterionic headgroups (2) were prepared by
amidation
of
4,4'-(cyclopent-1-ene-1,2-diyl)bis(5-
4
0
methylthiophene-2-carboxylic acid)(DTE-CO
2
H) as described
in the supporting information (Scheme S1). Bolaamphiphile 1,
displaying ammonium head groups was prepared by
bisamidation
of
DTE-CO
2
H
with
tert-butyl(2-
aminoethyl)carbamate followed by acidic removal of the t-Boc
groups, amide coupling with Fmoc-Lys(Boc)-OH and final
deprotection. Similarly, bolaamphiphile 2, containing
a
zwitterionic headgroup and flanking Fmoc groups, was obtained
by coupling two equivalents of Nα-Fmoc-L-Lys-OMe with DTE-
CO
2
H and subsequent deprotection.
Monomers 1 and 2 contain a central DTE photochromic unit
capable of undergoing a reversible cyclization/cycloreversion
reaction between open- and closed-ring isomers upon UV
irradiation.41 The photochromic behavior of 1 was studied by
UV-Vis spectroscopy in water, THF, MeOH, CDCl , and DMF (Fig.
3
S1). The open form of 1 exhibited an absorption band with
a max at ~260-270 nm. Irradiation of the colorless open form of
1
(10 mM) at 254 nm produced a photo-stationary state (PSS)
after 40-60 minutes, resulting in the formation of a deep violet-
colored solution with absorption bands at 352 and 526 nm, and
2
8
a concomitant decrease in the 266 nm band. The amount of
the closed-form of the DTE chromophore, as determined by
HPLC and H-NMR (Figs. S4,5), produced at the PSS was highly
1
Fig. 2: Tapping-mode AFM images of 1 (A) in the open state and, (B) 24 h
after irradiation in water with 254 nm light. (C) TEM images of 1 after
exposure to 254 nm light and subsequent aging for 0 h and (D) 24 h. Samples
dependent on solvent. For example, the amount of closed-form
of 1 present at the PSS was 90% in methanol; 80% in H
2
O, 63%
were prepared by dissolving 1 in water (10 mM), prior to diluting to 500
M
in THF, and 6% in DMF. The solvent dependence of the
(TEM) and evaporating onto a carbon-coated copper grid with uranyl acetate
isomerization yield at the PSS has been reported by other as a negative stain (TEM) or diluting to 100
M and evaporating onto freshly
groups.4
2, 43
cleaved mica (AFM).
The self-assembly characteristics of 1-2 were explored by
transmission electron microscopy (TEM) and atomic force
To induce maximal isomerization, 1 was irradiated in
microscopy (AFM) in both the open state and at the PSS methanol at 10 mM, resulting in a 90% conversion to the closed
following irradiation with 254 nm light. We initially explored the state, then the solvent was evaporated and replaced with
photoisomerization and assembly of 2. Although irradiation of water. Imaging of this sample after 24 h revealed only short,
2
-open with 254 nm light produced 48% of the closed form (2- non-uniform twisted fibers, in contrast to the initially formed
closed) at the PSS in water (Fig. S2), both the open and closed nanofibers produced by direct irradiation in water (Figs. 3A and
states of 2 remained monomeric under the self-assembly S6).
conditions (10 mM, 24 h). Reasoning that the central DTE
chromophore was not providing a sufficient driving force, via

 interactions, for assembly in water, bolamphiphile 2 was
designed with two appended, terminal Fmoc groups to enhance
the potential for intermolecular  interactions.
2
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nanofibers with diameters of ~13 nm and AFM heights of 4.0-
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DOI: 10.1039/D0DT02001J
.5 nm (Figs. 4B,D, and S7). It is noteworthy that sonication of
4
the open state of 1 did not induce self-assembly after 24 h (Fig.
S6).
The photoisomerization of the DTE ring system leads to
conversion from a non-planar, conformationally flexible
structure to a rigid, planar ring. However, a comparison of the
CD spectra of the open and closed forms of 1 exhibited
transitions only in the region 200-350 nm (Fig. S3). In the
spectral region of 520-530 nm, corresponding to the

 absorption of the closed DTE chromophore, no Cotton
Fig. 3 TEM images of 1-closed after 254 nm irradiation in methanol, evaporation, effects were observed. The lack of CD signals in this region
and redissolution in water (10 mM): (A) without sonication after 24 h and (B) 10
min after sonication (20% amp., 5 min, pulsed 10 s on, 5 s off). TEM images were
recorded after diluting in water (1 mM) and evaporating onto a carbon-coated
lack of 2closed to assemble. This likely emerged from the steric
copper grid with 2% aq. uranyl acetate as a negative stain.
suggested that  aggregation of the DTE chromophore was
not sufficiently promoting self-assembly, consistent with the
requirements of the axially-oriented, trans methyl substituents
in the DTE chromophore, which also rendered the closed DTE
form chiral. Changes in the flexibility of the bolaamphiphile
induced by photoisomerization of the DTE unit, likely mediated
the observed differences in self-assembly characteristics of the
open and closed forms of 1. A model for the self-assembly
process could be envisaged based on the extended length of 1-
closed of ~3.5 nm, which was similar to the heights of the helical
fibers measured by AFM (Fig. 4B). Thus, the fully formed
nanofibers likely emerge from the helical twisting of smaller
fibrils, with diameters of ~ 4 nm, created by the amphiphilic
aggregation of 1closed (Fig. 5).
Fig. 5 Notional depiction of amphiphilic assembly of 1-closed into 1D nanofibers.
Fig. 4 Tapping-mode AFM (top) and TEM (bottom) images of 1: (A/C) 1-open (10
mM) and (B/D) 1-closed. Samples of 1-closed were prepared by irradiation with
In conclusion, the self-assembly of bolaamphiphiles
containing a central DTE chromophore was controlled by
2
54 nm light in methanol for ~40 min (for 1-closed), evaporation and redissolution
in water (10 mM), sonication (20% amp., 5 min., pulsed) and aging for 24 h in
darkness. AFM and TEM images were recorded after diluting in water (1 mM) and photochemical interconversion between open and closed
evaporating onto a carbon-coated copper grid with 2% aq. uranyl acetate as a states. The open→closed transition induced a change from
negative stain (TEM) or onto freshly cleaved mica (AFM).
small, nonspecific aggregates to 1D nanofibers in aqueous
media.
We hypothesized that the self-assembly of 1-closed could
be accelerated by ultrasonication. Ultrasonication accelerates
Acknowledgements
the self-assembly process by producing mechanical forces that
facilitate the reorganization of initial aggregates into elongated
This material is based upon work supported by the National
4
4-46
structures with optimal intermolecular interactions.
Science Foundation (CHE-1708390) and by the U.S. Army
Research Laboratory and the U.S. Army Research Office under
grant number W911NF-14-1-0305. H. Kilic is grateful for the
doctoral fellowship (2214/A) to the Scientific and Technological
Research Council of Turkey (TÜBİTAK). N. Saracoglu is thankful
for the supports to the Council of Higher Education (CoHE) of
Accordingly, sonication of an aqueous solution of 1-closed (10
mM, 20% amplitude, 1 min, pulsed 10s on, 5s off), formed by
irradiation in methanol, induced a rapid transition from the
initial aggregates to short nanofibers within 10 min (Figs. 3A,B;
4
A,C) and, after 24 h, into uniformly elongated, helical
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TOC Graphic
Light-controlled self-assembly of a dithienylethene bolaamphiphile in water
Cassidy Creemer, Haydar Kilic, Kwang Soo Lee, Nurullah Saracoglu,* and Jon R. Parquette*
In this work, we report the light-driven self-assembly of a photochromic dithienylethene bolaamphiphiles
in aqueous media. The open→closed transition that occurred upon irradiation was accompanied by the
formation of 1D nanofibers.
UV
H
H
H
H
N
N
N
N
Vis
*R
S
S
R*
*R
S
S
R*
O
O
O
O