Supramolecular polymeric nanowires: Preparation and orthogonal modification of their photophysical properties

Article abstract of DOI:10.1039/c1jm14138d

A series of three-dimensional shape-persistent molecules with three conjugated arms perpendicular to a planar core were developed to self-assemble into supramolecular polymeric nanowires through multiple hydrogen-bonding interactions. After introducing bulky functional groups, aggregation of the nanowires was inhibited, and single molecular nanowires were obtained in concentrated solutions. Therefore, these nanowires had large surface areas with functional groups appended on the surface. Moreover, the photophysical properties of the functional groups including emission peaks and fluorescent lifetime were not changed after self-assembly. Some nanowires emitted high fluorescence after incorporating various chromophores on the side chains of the three-dimensional skeleton through effective fluorescence resonance energy transfer. For example, 1-BTHex showed a quantum efficiency of about 7.9% in solution, similar to the model compound DHBT. However, in the solid state the fluorescence of DHBT was almost quenched with a quantum efficiency lower than 1% due to π-π interactions, but 1-BTHex also gave much higher quantum efficiency, about 6%, which was close to that in solution.

Full text of DOI:10.1039/c1jm14138d

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PAPER
Supramolecular polymeric nanowires: preparation and orthogonal
modification of their photophysical properties†
a
a
a
b
a
Ting Lei, Chu-Yang Cheng, Zi-Hao Guo, Cui Zheng, Ye Zhou, Dehai Liang and Jian Pei
b
a
*
*
Received 24th August 2011, Accepted 4th October 2011
DOI: 10.1039/c1jm14138d
A series of three-dimensional shape-persistent molecules with three conjugated arms perpendicular to
a planar core were developed to self-assemble into supramolecular polymeric nanowires through
multiple hydrogen-bonding interactions. After introducing bulky functional groups, aggregation of the
nanowires was inhibited, and single molecular nanowires were obtained in concentrated solutions.
Therefore, these nanowires had large surface areas with functional groups appended on the surface.
Moreover, the photophysical properties of the functional groups including emission peaks and
fluorescent lifetime were not changed after self-assembly. Some nanowires emitted high fluorescence
after incorporating various chromophores on the side chains of the three-dimensional skeleton through
effective fluorescence resonance energy transfer. For example, 1-BTHex showed a quantum efficiency
of about 7.9% in solution, similar to the model compound DHBT. However, in the solid state the
fluorescence of DHBT was almost quenched with a quantum efficiency lower than 1% due to p–p
interactions, but 1-BTHex also gave much higher quantum efficiency, about 6%, which was close to
that in solution.
a shape-persistent structure with three conjugated arms perpen-
dicular to a planar core, in which both R1 and R2 are modified
with functional groups and the direction of three arms is the self-
assembly direction. Herein, we present the synthesis of a series of
side chain functionalized monomers, which also form supramo-
lecular polymers through hydrogen bonding. Four different
functional moieties are introduced to modify the photophysical
properties of the desired nanowires. The fluorescence resonance
energy transfer (FRET) principle was applied to choose chro-
mophores on the side chains. We chose benzothiadiazole and
porphyrin units as the emitting groups, because their absorption
features match well with the emission of the skeleton. The
introduction of the functional groups on the side chains does not
change their one-dimensional growth tendency, and the
Introduction
Organic nanowires (ONWs) have attracted increasing attention
because they open up a new bottom-up approach to fabricate
nano-optoelectronic devices,1 including explosive detectors,2
organic field effect transistors,3 and others.4 Recently, p–p
interactions have provided an excellent approach to form
organic nanowires in many systems through various growing
conditions.1–4 Owing to the complexity, weakness, and low
directionality of p–p interactions,5–9 the growing conditions of
nano/microstructures always change after introducing functional
groups, and the expected morphologies are sometimes difficult to
realize. Therefore, controlled growth and preparation of nano-
wires or even single molecular wires with special functional
groups to realize certain properties are still of great challenges.
In our previous work, we developed a novel supramolecular
polymer constructed by multiple hydrogen-bonding interactions,
which formed uniform nanowires with high solid quantum effi-
ciency.10 As shown in Fig. 1, our three-dimensional skeleton has
aBeijing National Laboratory for Molecular Sciences (BNLMS) Key Labs
of Bioorganic Chemistry College of Chemistry and Molecular Engineering,
Peking University, Beijing, 100871, China. E-mail: jianpei@pku.edu.cn;
Fax: (+86) 10-62751845; Tel: (+86) 10-62751845
bKey Laboratory of Polymer Chemistry and Physics of the Ministry of
Education, College of Chemistry and Molecular Engineering, Peking
University, Beijing, 100871, China
† Electronic Supplementary Information (ESI) available: Data of the
experiment details, 1H NMR spectra, 13C NMR spectra and
MALDI-TOF. See DOI: 10.1039/c1jm14138d
Fig. 1 The design strategy of the functional supramolecular polymers.
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monomers can easily self-assemble into supramolecular poly-
meric nanowires. Interestingly, single molecular wires are also
obtained from concentrated solutions due to steric hindrance of
the chromophores. Moreover, the nanowires emit different
colors depending on the attached chromophores. The introduc-
tion of various chromophores can also test our assumption about
whether this strategy might have broad applications for different
types of functional groups.
and hexane. Furthermore, we also synthesized a model
compound DHBT to understand the photophysical properties
(Scheme 2).
Nanowire growth and characterization
We used a vapor-diffusion strategy to grow nanowires. These
compounds were firstly dissolved in THF to form homogenous
solutions, and then CH2Cl2 was introduced by gradual vapor
Results and discussion
Synthesis
Scheme 1 illustrates the synthesis routes for the functionalized
monomers. Compound 2 was readily obtained through four
steps from commercially available materials, as described in
our previous report.11 A demethylation reaction of 2 using
BBr3 was smoothly performed at room temperature and then
followed by an alkylation with 12-bromododecan-1-ol to
afford 3 in 79% yield. Firstly, PBr3 was employed to convert
hydroxyl groups to bromide terminal groups, which resulted in
very low yield. Then we used 4-toluenesulfonyl chloride to
convert hydroxyl groups to achieve tosylate ester 4. A Suzuki–
Miyaura coupling reaction was carried out between 4 and
4-methoxycarbonylphenylboronic pinacol ester to afford 5.
The usage of the more active palladium catalyst Pd2(dba)3 and
the ligand PCy3 led to 73% yield. Compound 5 reacted with
TPPOH, TPABTOH, HexBTOH, and MeBTOH using
K2CO3 in DMF following a hydrolysis reaction using LiOH
to afford the target monomers, 1-TPP, 1-BTTPA, 1-BTHex,
and 1-BTMe, in high yields. All the compounds are readily
soluble in THF, and show low solubility in CH2Cl2, CHCl3,
Scheme 2 Structures of the self-assembling monomers used in this study.
Scheme 1 Synthesis routes for the functionalized monomers.
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diffusion to decrease the solubility of the compound. Scanning
electron microscopy (SEM) was employed to investigate the
formed aggregates (Fig. 2). SEM images show that all the
monomers formed well-defined nanowires which are hundreds
of nanometres wide and micrometres long. This result is
rarely reported in previous literature,1–4 because even for a
small change in the molecular structure, the self-assembly
morphology dramatically changes.1a,b In our system, the 1D
growth tendency was retained well because the hydrogen-
bonding interaction was strong and the bonding sites were
isolated from the ‘‘external environment’’ by the shape-persis-
tent skeleton.
Fig. 3 TEM images of (a) 1-TPP and (b) 1-BTTPA nanowires directly
drop-cast onto a micro-grid.
Transmission electron microscopy (TEM) and atomic force
microscopy (AFM) were also employed to investigate the
formation of the nanowires. As shown in Fig. 3, nano/microwires
of 1-TPP and 1-BTTPA were formed with bundles of smaller
nanowires of several nanometres in width. Similar results were
also observed in AFM images (Fig. 4), at larger scale, 1-TPP and
1-BTTPA formed wires ranging from tens to hundreds of
nanometres in width. A detailed analysis by AFM reveals that
nanowires of 1-TPP had a smallest height of 3.5 nm and those of
1-BTTPA showed a smallest height of 3.8 nm (white arrows in
Fig. 4b and d), which agreed well with the molecular size given by
molecular modeling (Fig. 5). This is direct evidence of the exis-
tence of single molecular wires. We also observed the side
attached single molecular wires as shown by the red arrows.
These result demonstrated that these nano/microwires were
formed through the side-by-side attachment of single molecular
wires. Previously, we reported the formation of single molecular
wires in highly diluted solution (10ꢀ6 M),10 and we assumed that
the nanowires were formed by the packing of lots of single
molecular wires. However, herein the single molecular wires were
found in highly concentrated solutions (10ꢀ4 M), and their
packing was also observed from AFM images, which proved our
previous assumption. Unlike flexible dodecyl chains in 1-C12, we
considered that the bulky functional groups of the molecules
hindered the fusion of single molecular wires into larger assem-
blies. Therefore, we obtained the single molecular wires in
concentrated solutions.
Photophysical properties and FRET process
The photophysical features of the monomers and nanowires
were investigated in detail and are summarized in Table 1. As
illustrated in Fig. 6, the absorption spectra of the monomers in
solution exhibited two main absorption peaks, one from the
main skeleton (around 322 nm) and another from the func-
tionalized side groups (around 400–450 nm). When excited at
the maximum absorption of the skeleton (around 322 nm) of
the monomers, the emission from the functionalized side
groups appeared, and the emission from the skeleton was
dramatically quenched. These results indicated that intra-
molecular energy transfer processes from the skeleton to the
monomers in solution clearly happened. The absorption and
emission spectra of the nanowires were also measured by drop-
casting their suspensions onto quartz plates. A shoulder in the
absorption band of the 3D skeleton was observed, which was
attributed to the relatively more rigid skeleton after hydrogen
bonding.10 Noticeably, the absorption peaks of all the side
groups remained almost unchanged, indicating the van der
Waals interactions, especially p–p interactions, of the side
Fig. 4 AFM height images (a, c) at large scale; (b, d) small scale and
small scale section analysis of (a, b) 1-TPP; (c, d) 1-BTTPA nanowires on
mica substrates.
Fig. 2 SEM images of the morphologies formed by (a) 1-TPP; (b) 1-
BTTPA; (c) 1-BTMe; (d) 1-BTHex in THF/CH2Cl2 at 0.5 mg mLꢀ1
.
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Fig. 5 Estimation of the size of 1-TPP by molecular modelling
(molecular models were minimized using MMFF94 force field).
groups were very weak. Interestingly, because of the narrow
absorption of the Soret band (420 nm)12 of the porphyrin
groups, only one vibrational emission was transferred in the
process, and the emission from the skeleton was split into two
peaks (Fig. 6a).
We also measured the time-resolved fluorescence decays of 1-
C12, 1-BTTPA, and 1-BTMe in dilute solution, as summarized in
Table 2. When excited at 339 nm, 1-C12 showed a fluorescent
lifetime of 1.24 ns (69%) and 8.2 ns (31%). In contrast, the
fluorescent lifetime of the skeleton (monitored at 411 nm)
dramatically decreased in 1-BTTPA and 1-BTMe (Fig. 7). The
measured fluorescent lifetimes were 1.0 ns (81%), 2.1 ns (19%) for
1-BTTPA and 1.1 ns (92%), 2.7 ns (8%) for 1-BTMe. The
dramatic decrease of the lifetime of the main skeleton suggested
that the energy transfer was not a trivial energy transfer (radia-
tive energy transfer). Meanwhile, because the skeleton and the
functional groups are well separated with non-conjugated
aliphatic chains, the energy transfer did not follow the Dexter
Fig. 6 (a) UV-vis, PL spectra and fluorescence microscopy images of (a,
b) 1-TPP, (c, d) 1-BTTPA, (e, f) 1-BTMe in THF solution (1.0 ꢁ 10ꢀ6 M)
or as nanowires. All the monomer emission spectra were excited at the
maximum absorption peak of the main skeleton. The emission of 1-TPP
from the solid state was very weak and is not shown in the spectra.
To quantify the energy transfer (ET) efficiency, we measured
the fluorescence quantum yields of the donor in the presence and
absence of acceptor using eqn (1),13
Qdonor ðacceptorÞ
Qdonor
JET ¼ 1 ꢀ
(1)
in which Qdonor and Qacceptor correspond to the fluorescence
quantum efficiency of the donor in the absence and the presence
of the acceptor, respectively. Herein, the Qdonor, the quantum
efficiency of 1-C12, was 24%. Clearly, 1-BTMe and 1-BTHex
showed the highest ET efficiencies up to 70% (Table 3), which
may be attributed to the perfect match between their absorption
and the emission of the main skeleton. The FRET process
became much more efficient in the solid state. For 1-BTTPA, the
€
mechanism but rather the Forster mechanism. On the other
hand, when excited at the absorption of the functional groups of
1-BTMe (390 nm) and monitored at the emission of the func-
tional groups (526 nm), the fluorescent lifetime was 8 ns, similar
to the lifetime of the reference compound DHBT (10 ns).
Accordingly, the photophysical properties of functional groups,
including emission peaks and fluorescent lifetime, are not
changed.
Table 1 Summary of the optical properties of the monomers in dilute solution and in nanowires
In solution (10ꢀ6 in THF)
Nanowires
Compound
labs (nm)b
lemi (nm)
labs (nm)
lemi (nm)
1-TPP
323 (5.54), 420 (6.22)
322 (5.07), 449 (4.43)
322 (5.31), 416(4.54)
322 (5.47), 420 (4.77)
324 (5.18)
392, 440, 655
421, 583
408, 526
410, 526
411
324, 429
317, 426
324, 426
325, 356, 425
327
N/Ac
577
528
529
424
519
1-BTTPA
1-BTMe
1-BTHex
1-C12a
DHBT
414 (4.10)
529
420, 520
a
Data come from our previous report. b Extinction coefficients, log 3 (Lꢀ1 molꢀ1 cmꢀ1), are shown in parentheses. c Not measured, because the peak was
very weak.
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Table 2 Excitation wavelength (lex), monitoring wavelength (lem) and
fluorescence lifetime (s) obtained from the time-resolved fluorescence
measurement in THF solution (10ꢀ6 M)
‘‘orthogonal modification’’. In typical p–p interacting systems,
molecules are required to be closely packed (0.34 nm). The strong
interactions of the functional groups always change their pho-
tophysical or electronic properties. Furthermore, we also
measured the quantum efficiency in the solid state. Except
1-TPP, all the nanowires exhibited much higher quantum effi-
ciencies than their side-chain functional groups. For example,
1-BTHex showed a quantum efficiency of about 7.9% in solution,
similar to model compound DHBT in dilute solution. However,
in the solid state the fluorescence of DHBT was almost quenched
with a quantum efficiency lower than 1%, but 1-BTHex also gave
a much higher quantum efficiency of about 6%. This result was
both the contribution from the reduced side-chain interactions
and the energy transfer from the skeleton.
lex
[nm]
lem
[nm]a
Compound
sa
1-C12
1-BTTPA
1-BTMe
339
339
339
390
390
411
411
411
526
526
1.24 ns (69%), 8.2 ns (31%)
1.0 ns (81%), 2.1 ns (19%)
1.1 ns (92%), 2.7 ns (8%)
8 ns
DHBT
10 ns
a
Percentages of different decay lifetimes were calculated and are shown
in parentheses.
Conclusions
In summary, we have developed four functionalized monomers
which readily self-assemble into supramolecular polymeric
nanowires in solution. The three-dimensional monomers have
a shape-persistent structure with three conjugated arms perpen-
dicular to a planar core. The conjugated arms are designed to
form multiple hydrogen bonds, and the planar core is function-
alized to modify their photophysical properties. After the func-
tionalization, all the monomers kept their one dimensional
self-assembly tendency very well and formed nano/microwires in
solution. Noticeably, single molecular wires were also obtained
in a concentrated solution due to the strong hydrogen-bonding
interactions and weak intermolecular interactions of the func-
tional groups.
Fig. 7 Time-resolved fluorescence decays of 1-C12, 1-BTTPA and 1-
BTMe excited at 339 nm (absorption of the main skeleton), and moni-
tored at 411 nm (emission of the main skeleton). The fluorescence lifetime
of the main skeleton were largely shortened because of FRET process.
The investigation of their photophysical properties shows that
the photophysical properties of the side groups were not affected
by the self-assembly in the solid state, which is rarely achieved in
p–p interacting systems. The monomers showed clear intra-
molecular FRET processes both in dilute solution and in the
solid state, and the energy transfer efficiencies were also calcu-
lated. After they self-assemble into supramolecular nanowires,
the intermolecular energy transfers become dominant in the
supramolecular nanowires. Except for 1-TPP, all the nanowires
exhibited much higher quantum efficiencies than their side-chain
functional groups. For example, the fluorescence of DHBT was
almost quenched in the solid state with a quantum efficiency
lower than 1% due to p–p interactions, but 1-BTHex gave
a much higher quantum efficiency of about 6%, which is close to
the value in solution. These organic nanowires have very large
surface areas with functional groups appended on the surface.
Therefore, they may show potential applications in explosives
detection, hazardous gas sensing, and biological applications.
Table 3 Energy transfer efficiencies measured in solution and in the
solid state
a
a
b
b
Compound
FFL
JET
FFL
JET
1-TPP
1-BTTPA
1-BTMe
1-BTHex
1-C12 (donor)
9.7%
18%
7.9%
7.3%
24%
60%
25%
67%
70%
N/A
very low
7.1%
very low
very low
22%
> 99%
70%
> 99%
> 99%
N/A
a
Measured at the peak of 410 nm, and using 9,10-diphenylanthracene as
the reference in dilute solution. Measured at the solid state using
b
a integrating sphere system. FFL is the quenched quantum efficiency of
the donor; JET is the energy transfer efficiency from the skeleton to
the side chain chromophores.
Experimental section
energy transfer efficiency increased from 25 to 70%, and for
1-BTMe and 1-BTHex, the efficiencies increased from 70 to 99%.
These results are attributed to the efficient intermolecular energy
transfer in the solid state. Interestingly, the emission from the
skeleton showed some fine vibrational structures, indicating that
three conjugated arms were solidified by the hydrogen-bonding
in self-assembly state (Fig. 6c). Similar to the absorption spectra,
the emission from the nanowires exhibited no obvious changes
compared with those in the solution. In this system, the prop-
erties of the functional groups are well retained, we call it
General methods
Chemicals were purchased and used as received. All air and water
sensitive reactions were performed under nitrogen atmosphere.
Toluene and tetrahydrofuran (THF) were distilled from sodium
and benzophenone ketyl. H and 13C NMR spectra were recor-
1
ded on a Varian Mercury plus 300 MHz and Bruker ARX-400
(400MHz). All chemical shifts were reported in parts per million
(ppm), 1H NMR chemical shifts were referenced to TMS (0 ppm)
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or CHCl3 (7.26 ppm), and 13C NMR chemical shifts were refer-
enced to CDCl3 (77.00 ppm). Absorption spectra were recorded
on a PerkinElmer Lambda 35 UV-vis Spectrometer. PL spectra
were recorded on a PerkinElmer LS55 Luminescence Spec-
trometer. MALDI-TOF mass spectra were recorded on a Bruker
BIFLEX III time-of-flight (TOF) mass spectrometer (Bruker
Daltonics, Billerica, MA, USA) using a 337 nm nitrogen laser
with dithranol as the matrix.
2009CB623601) from the Ministry of Science and Technology,
and National Natural Science Foundation of China.
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Scanning electron microscopy (SEM) images were obtained
using a cold field emission scanning electron microscope
(FESEM, Hitachi S-4800) operated at an accelerating voltage of
1.0 kV. All the samples were prepared by directly drop-casting
the nanowire suspensions on a silicon wafer. Molecular modeling
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Acknowledgements
This work was supported by the Major State Basic
Research Development Program (Nos. 2006CB921602 and
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