Water-soluble J-type rosette nanotubes with giant molar ellipticity

Article abstract of DOI:10.1021/ja105028w

A new self-assembling tricyclic module (× K1) featuring the Watson-Crick H-bonding arrays of guanine and cytosine fused to an internal pyridine ring was synthesized. When dissolved in water at room temperature, this module rapidly self-assembles into hexa

Full text of DOI:10.1021/ja105028w

Published on Web 10/11/2010
Water-Soluble J-Type Rosette Nanotubes with Giant Molar Ellipticity
Gabor Borzsonyi,†,‡ Rachel L. Beingessner,† Takeshi Yamazaki,† Jae-Young Cho,†
Andrew J. Myles,† Marek Malac,†,§ Ray Egerton,†,§ Masahiro Kawasaki,⊥ Kazuo Ishizuka,#
Andriy Kovalenko,†,| and Hicham Fenniri*,†,|
National Institute for Nanotechnology, Departments of Chemistry, Physics, and Mechanical Engineering, UniVersity
of Alberta, 11421 Saskatchewan DriVe, Edmonton, Alberta, T6G 2M9, Canada., JEOL USA, Inc., 11 Dearborn Road,
Peabody, Massachusetts 01960, and HREM Research, 14-48 Matsukazedai, Higashimastuyama, 355-0055, Japan
Received June 9, 2010; E-mail: hicham.fenniri@ualberta.ca
Abstract: A new self-assembling tricyclic module (×K1) featuring
the Watson-Crick H-bonding arrays of guanine and cytosine
fused to an internal pyridine ring was synthesized. When dis-
solved in water at room temperature, this module rapidly self-
assembles into hexameric rosettes, which then stack to form
J-type rosette nanotubes (RNTs) with increased inner/outer
diameters and the largest molar ellipticity ever reported (4 × 106
deg·M-1 ·m-1). Using a combination of imaging and spectroscopic
techniques we established the structure of ×K1-RNT and have
shown that the extended π system of the self-assembling module
resulted in a new family of J-type RNTs with enhanced inter-
modular electronic communication.
Since their discovery in 1936 by Scheibe1 and Jelley,2 J-
aggregates3 have seen a broad range of applications because they
display coherent, cooperative phenomena such as superradiance and
giant oscillator strength as a result of the long-range delocalization
of their electronic excitation.3a,4 J-aggregates have for example been
used as organic photoconductors,5 optical switches,6 NLO devices,7
critically coupled resonators,8 LEDs,9 sensitizers,10 photovoltaics,11
nanowires,12 and sensors.13
J-aggregates are usually dye crystallites in which the transition
dipoles of the constituent molecules strongly couple to form a
collective narrow line width optical transition possessing oscillator
strength derived from the aggregated monomers.14 In this report
we use self-assembly and self-organization strategies15 to build
water-soluble J-type nanotubular architecture with unprecedented
control over dimensions, supramolecular organization, and physical
properties relative to earlier reports on J-aggregates. This approach
offers opportunities for further tunability of electronic properties
through subtle changes in the core structure and functionalization
of the self-assembling molecular modules.
Figure 1. ×K1 (1) and K1 (2) modules, corresponding hexameric rosettes
(middle), and RNTs (lower) (X ) CF3CO2-).
established in the case of its bicyclic congener, ×K1 was specifically
designed to (a) self-assemble in water to form RNTs with increased
inner/outer diameters relative to K1 (2) and (b) engender an
extended π system that would enhance electronic transport along
the RNT’s main axis.
Following is the synthesis, self-assembly, and characterization
of ×K1 using circular dichroism (CD), UV-vis and fluorescence
spectroscopy, scanning electron microscopy (SEM), transmission
electron microscopy (TEM), tapping mode atomic force microscopy
(TM-AFM), energy filtering TEM (EFTEM), off-axis electron
holography (EH), and quantitative phase technology (QPt). We also
report remarkable optical properties suggesting enhanced electronic
communication for this new class of RNTs.
The synthesis of ×K1 (Scheme 1) began by transforming
commercially available barbituric acid in three steps to bicycle 3.
Three consecutive regioselective SNAr reactions at positions 4, 2,
and 7 with benzyl alcohol, methylamine, and allylamine, respec-
tively, provided the functionalized compound 6. However, for the
third SNAr reaction to proceed, it was necessary to electronically
The G∧C motif16 is a self-complementary hybrid molecule of
the DNA bases guanine and cytosine. This motif and its deriva-
tives17 (2, Figure 1) were studied extensively in water and organic
solvents and were shown to undergo hierarchical self-assembly into
hexameric rosettes which further organize into a linear stack called
a rosette nanotube (RNT).16c,18 Here we present a water-soluble
tricyclic variant 1, termed ×K1. This molecule features the same
H-bonding arrays, but separated by an internally fused pyridine
ring. Building upon the design criteria formulated and successfully
† National Institute for Nanotechnology, University of Alberta.
‡ Department of Chemistry, University of Alberta.
§ Department of Physics, University of Alberta.
| Department of Mechanical Engineering, University of Alberta.
⊥ JEOL USA, Inc.
# HREM Research.
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10.1021/ja105028w  2010 American Chemical Society

C O M M U N I C A T I O N S
Scheme 1. Synthesis of ×K1a
subsequent reductive amination of aldehyde 8 with protected
L-lysine provided compound 9. Finally, deprotection of 9 with TFA/
thioanisole furnished the target ×K1 motif (1) as a TFA salt.
Characterization of ×K1 by SEM revealed that this tricyclic
compound rapidly self-assembles into RNTs when dissolved in
water (pHfinal ) 2.8) (Figure 2C) at room temperature. This is an
advantage over the bicyclic G∧C base 2 which requires heat to
drive the entropically driven self-assembly process as a result of
its reduced hydrophobic character.18b TM-AFM (Figure 2A) and
TEM (Figure 2B) of uranyl acetate-stained samples established the
RNTs’ outer diameter of 4.2 ( 0.2 and 4.4 ( 0.2 nm, respectively,
in excellent agreement with the theoretical value 4.3 nm. These
measurements were further verified using three direct TEM imaging
methods (without staining agent), from which the diameter of a
single RNT was measured to be 4.5 ( 1 nm (EFTEM), 4.4 ( 0.75
nm (EH), and 4.9 ( 0.8 nm (QPt).19 In contrast with EFTEM, EH
and QPt use elastic scattering of the fast electrons in TEM instead
of the inelastically scattered electrons to obtain quantitative
measurements. Based on these results, the inner and outer diameters
of ×K1-RNTs relative to K1-RNTs increased by 0.4 and 0.9 nm,
respectively.
CD and UV-vis spectroscopic techniques were used to monitor
the self-assembly process in solution. The CD spectra of ×K1 (2.1
× 10-5 M) feature two couplets centered at 257 and 380 nm with
positive maxima at 264 and 388 nm (Figures 3B). The CD signal
intensity increased ca. 40× over 7 days reaching a giant molar
ellipticity, with an unprecedented value of 4 × 106 deg ·M-1 ·m-1
for a chiral helical stack.20 Briefly heating a dilute solution of ×K1
(1.9 × 10-5 M) to 100 °C (10 s) or acidification with TFA (0.1%
v/v) leads to the complete disappearance of the CD profile, thus
suggesting that the induced CD is the result of the supramolecular
chirality of the RNTs. In agreement with these studies, variable
temperature CD (VT-CD) established the reversibility of the CD
profile.19 The intensity of the CD couplet is proportional to the
fourth power of the dipole strength and the relative orientation of
the chromophores and is inversely proportional to the square of
the interchromophoric distance.21 Since the latter is probably the
same for RNTs assembled from K1 and ×K1, the very large molar
ellipticity recorded is therefore an indication of the much larger
transition dipole strength and an optimal relative orientation of the
corresponding dipole moments.
a (a) POCl3, DMF, reflux, 15 h, 90%; (b) malonitrile, ꢀ-alanine, CH3CN,
30 h, 50%; (c) 180 °C, 48 h, 75%; (d) BnOH, Et3N, CH2Cl2, -40 °C, 2 h,
70%; (e) CH3NH2 (2 M in THF), DIPEA, CH2Cl2, 0 °C, 2 h, 60%; (f)
Boc2O, DIPEA, DMAP, THF, rt (room temperature), 12 h then allylamine,
DIPEA, CH2Cl2, rt, 2 h, 50%; (g) Cl3CONCO, CH2Cl2, rt, 5 h then 7 N
NH3 in MeOH, rt, 12h, 74%; (h) Boc2O, DIPEA, DMAP, THF, rt, 12 h,
96%; (i) OsO4 in t-BuOH, 50% NMO in H2O, THF, rt, 6 h, then NaIO4 in
H2O, rt, 48 h, 85%; (j) Protected L-Lysine, DIPEA, 1,2-DCE, rt, 10 min,
then Na(OAc)3BH, rt, 48 h, 82%; (k) 94/6 (v/v) TFA/thioanisole, rt, 96 h,
87%.
Figure 2. Imaging of ×K1 RNTs (0.1 mg/mL in water) by (A) TM-AFM
(5 µm scan, height scale ) 0-10 nm), (B) TEM, and (C) SEM. White
arrows point to individual RNTs; scale bars in nm.
UV-vis, CD, and fluorescence spectroscopy unveiled the
remarkable optical properties of the RNTs assembled from ×K1
(×K1-RNT). First, the UV-vis spectra of K1 versus ×K1 show a
significant shift of all the maxima toward the red in the latter case.
K1 showed maxima at 236 and 285 nm, whereas ×K1 showed
maxima at 246 and 363 nm in addition to a unique, sharp, and
lower energy band at 388 nm (Figure 3A). This band is unique to
deactivate the ring by protecting the methylamine in compound 5
with an electron-withdrawing group. Ring closure of 6 using
trichlorocarbonyl isocyanate/NH3, followed by protection of the
amine, generated tricycle 7. Oxidative cleavage of alkene 7 and
Figure 3. (A) UV-vis, (B) circular dichroism, and (C) fluorescence (λexc ) 388 nm) spectra of ×K1-RNTs (ca. 2 × 10-5 M in water).
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C O M M U N I C A T I O N S
×K1-RNT as it has never been observed for bicyclic G∧C
derivatives such as K116c,18 or nonassembled ×K1. Because this
band is narrow and red-shifted and grew over time and because it
decreased with increasing temperature (Figures S3, S4), dilution
(Figures S5, S6),19 or titration with TFA (data not shown), we
propose that ×K1 modules assumed a J-type arrangement1-3 within
the RNTs.
In summary, here we describe the design, synthesis, and
characterization of a new class of water-soluble RNTs from a
tricyclic self-assembling module (×K1). UV-vis and CD experi-
ments revealed interesting optical properties. In particular, the
formation of highly ordered J-type RNTs suggests long-range
intermodular electronic communication relative to the parent RNTs
(derived from 2). Detailed photophysical studies to determine the
size of the coherent domain of the J-aggregates within the ×K1-
RNTs are underway. Our results suggest also that by further
extending the ring system of the G∧C motif, we should be able to
realize an electronically conducting RNT with tremendous practical
and fundamental potential.4-13
Second, the growth of the J-band is associated with the growth
of a couplet centered on the same λmax (388 nm) (Figures 3B,
S2). Variable temperature UV-vis and CD showed that this
couplet is associated with the supramolecular organization of
×K1-RNT, as it disappears with heating and grows upon cooling
(Figure S4).
Acknowledgment. We thank NSERC, NRC, and the University
Third, a subtle hypochromic effect (ca. 8%) was observed
over 7 days (Figure 3A) whereas a pronounced hyperchromic
effect (up to ca. 50%) was recorded upon thermally induced
disassembly (Figure S5).19 While this result suggests that RNT
formation has already substantially progressed prior to the initial
measurements (i.e., within minutes), the UV-vis and CD
profiles, notably the appearance and growth of the red-shifted
band along with the CD couplet, suggest that the formation of
J-type RNTs proceeds in at least two stages: the first step (within
minutes) leading to rapid formation of RNTs and the second
(within days) during which ×K1 modules adopt a particularly
favorable supramolecular arrangement for exciton coupling
within the RNT construct. We refer to these states as confor-
mational states I and II (CS-I, CS-II). As evidenced by time-
dependent SEM,19 the shape, dimension, and hierarchy/
aggregation states of ×K1-RNTs were the same in CS-I and
CS-II. However, the transition from CS-I to CS-II clearly has a
dramatic effect on the relative orientation of the transition dipoles
of the self-assembling modules and their exciton delocalization.3
of Alberta for supporting this program.
Supporting Information Available: Synthetic procedures for ×K1;
UV-vis, CD and fluorescence studies; SEM, TEM, AFM, EH, QPt
and EFTEM procedures and studies; and computational methods. This
material is available free of charge via the Internet at http://pubs.acs.org.
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