Three-dimensional shape-persistent fluorescent nanocages: Facile dynamic synthesis, photophysical properties and surface morphologies

Article abstract of DOI:10.1002/chem.200800154

A study was conducted to investigate facile dynamic synthesis, photophysical properties, and surface morphologies of three-dimensional (3D) shape-persistent fluorescent nanocages. The 3D functionalized body was utilized to couple with suitable planar caps that enabled achieving quantitative formation of C₃ symmetrical nanocages, with intensive visible luminescence through dynamic covalent chemistry (DCC). The study demonstrated that DCC has emerged as an efficient synthetic strategy, due to its error-checking and proof-reading features, generating thermodynamically controlled products through reversibility. The selective introduction of three conjugated arms generated fluorescent property and offered the interactions between elements involved in the synthesis process.

Full text of DOI:10.1002/chem.200800154

DOI: 10.1002/chem.200800154
Three-Dimensional Shape-Persistent Fluorescent Nanocages: Facile Dynamic
Synthesis, Photophysical Properties, and Surface Morphologies
Jia Luo, Ting Lei, Xiaoguang Xu, Fang-Mei Li, Yuguo Ma,* Kai Wu,* and Jian Pei*[a]
Functional nanostructures with intriguing topology and
special properties have played important roles in self-assem-
bly, host–guest chemistry, catalysis and nanotechnology.[1]
Among these nanosized units, shape-persistent artificial ar-
chitectures provide fixed conformations, which might offer
many advantages for predictable assembly, nanofabrication,
or guest inclusion.[1,2] Recently, many rigid cages with con-
fined cavities through multivalent assembly have attracted
considerable interest due to their applications in delivery,
extraction, detection and various microreactors.[1a,c,3] How-
ever, the synthesis of shape-persistent molecules is usually
laborious, and the structural diversification in different di-
mensions from a common parent is also difficult; on the
other hand, rational design of molecules with desired func-
tionalities still remains a great challenge. Therefore, it is im-
perative to develop a well-organized structure to meet the
requirement of readily available diversification and rational
design of derivatives with controllable solubility and proces-
sibility, which also provides the possibility to agilely mimic
multivalent assembly in nature.[4]
In our previous contribution, we reported a unique 3D
skeleton 4a as shown in Scheme 1 for pure blue emitters ap-
plied in organic light-emitting diodes.[5] The success of syn-
thesizing such skeleton provides us with convenience to real-
ize the structural diversity of 3D derivatives through orthog-
onally and distinguishably modifiable sites. As a result, such
a 3D structure would offer various derivatives with diverse
functionalities. Herein, we utilize this 3D functionalized
“body” (a covalent template) to couple with suitable planar
“caps” to achieve quantitative formation of C3 symmetrical
nanocages 1a–c with intensive visible luminescence through
dynamic covalent chemistry (DCC). DCC emerges as an ef-
ficient and versatile synthetic strategy due to its “error-
checking” and “proof-reading” features, generating thermo-
dynamically controlled products by virtue of its reversibili-
ty.[6] Additionally, the selective introduction of three conju-
gated arms not only guarantees the fluorescent property, but
also offers the interactions between host and guest.
Scheme 2 illustrates the synthetic route to nanocages 1a–c
and their reduced forms H12-1a–c. First, demethylation of
3D skeleton 4a by BBr3 followed by reacting with n-hexyl-
bromide or 2-(2-(2-methoxyethoxy)ethoxy)ethyl 4-methyl-
benzenesulfonate (TEG-OTs) under basic condition afford-
ed 4b or 4c in high yield. Subsequently, efficient Pd-cata-
lyzed Suzuki cross-coupling reaction between 4 and 4-for-
mylphenylboronic ester gave desired aldehyde 2 in good
yields (75–86%). Compared with weak noncovalent interac-
tions such as hydrogen-bonding and metal–ligand coordina-
tion, imine condensation between aldehydes and amines
seems to be the most valid choice to construct more stable
nanocages.[6e] First, the formation of the nanocages from al-
dehyde 2a and two equivalents of amine 3[7] was carried out
in refluxing CHCl3 or CH2Cl2 solution. However, amounts
1
of precipitates were formed and H NMR spectra of the in
situ mixture suggested that an undesirable mixture was
formed even after extended heating time, which might be
owing to the poor solubility of the imine intermediates and
starting materials in CHCl3 or CH2Cl2. 1,1,2,2-Tetrachloro-
ethane (TCE) was employed as a co-solvent to promote the
condensation reaction. To our delight, refluxing the mixture
[a] J. Luo, T. Lei, X. Xu, F.-M. Li, Prof. Y. Ma, Prof. K. Wu, Prof. J. Pei
Key Laboratories of Bioorganic Chemistry and
Molecular Engineering and of Polymer Chemistry and
Physics of Ministry of Education, College of Chemistry
Peking University, Beijing 100871 (PR China)
Fax: ( +86)10-62758145
E-mail: jianpei@pku.edu.cn
Supporting information for this article is available on the WWW
under http://www.chemistry.org or from the author.
Scheme 1. Structures and structural schematic diagram of three-dimen-
sional skeletons.
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for 3 h resulted in a homoge-
nous solution, in which nano-
cage 1a was quantitatively con-
structed, as evidenced by
1H NMR spectroscopy. After
removal of the solvents under
reduced pressure, nanocage 1a
was obtained as
a
slightly
yellow solid without further pu-
rification.
Figure 1 compares the partial
1H NMR spectra (300 MHz,
CDCl3, 298 K) of 2a, 3 and
nanocage 1a. Compared with
that of 2a in Figure 1a, the
signal assigned to CHO (d=
1
9.91 ppm) in the H NMR spec-
trum of 1a (Figure 1c) disap-
peared, and a new signal (d=
7.69 ppm) assigned to imine
protons emerged, which shifted
upfield (about 0.44 ppm) rela-
tive to that of the correspond-
ing proton in normal imine
groups.[8] Furthermore, after the
transformation from amine 3 to
imine 1a, the signals of CH2
showed downfield shift from
d=3.88 ppm (singlet in Fig-
ure 1b) to 4.94 ppm (dd in Fig-
ure 1c), which was due to gemi-
nal
coupling
(J=15.6 Hz)
caused by the unsymmetrical
chemical environment in our
rigid nanocage. Moreover, its
chemical shift moved downfield
(about 0.26 ppm) in comparison
with those of the corresponding
imines within an open scaf-
fold.[8] MALDI-TOF MS analy-
sis of 1a, as shown in Figure 2,
further confirmed the proposed
cage structure and the quan-
titative formation: only an
ion peak at m/z=1729.8 for
[M+H]+ (calcd for C126H84N6O3:
Scheme 2. Synthetic route to fluorescent cages 1a, 1b, and 1c, and their reductive cages H12-1a, H12-1b, and
H12-1c.
1728.7) was observed, and the isotopic distribution fit well
with the theoretically predicted pattern. All the characteri-
zation data verified the formation and the purity of cage 1a.
Figure 3 illustrates the 1H NMR spectra of the mixture
(2a/3 1:2) in CDCl3/C2D2Cl4 1:1 at 333 K as time elapsed. A
signal of formyl protons at chemical shift 9.98 ppm appeared
and finally all signals of formyl protons disappeared as time
went by. These results indicated that the system was gradu-
ally transformed to imine, and eventually became the sym-
metrical species after formation of many unsymmetrical in-
termediates. After 40 min, the signal at about d=5.00 ppm
emerged, concomitant with decrease of the singlet signal at
d=3.88 ppm. The transformation became complete in about
1
130 min. Obviously, the clean H NMR spectra indicate that
there are no oligomers or polymers in the final products. As
the 1H NMR spectra show, there are only two signals
emerged at about 10 ppm, which means only two kinds of
1
formyl protons could be observed in H NMR spectra. Thus,
we proposed that the formation of the imine bonds con-
tained a self-accelerating process. After one arm of a cap
was connected to the “main body”, the reaction rates for
the formation of the other two imine bonds became much
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J. Pei, K. Wu, Y. Ma et al.
faster, so the only intermediate
1
observed through H NMR was
one-end capped structure, al-
though its structure need to be
further elucidated. Apparently,
such a phenomenon can be at-
tributed to the multivalent
effect directed by our rigid
structure, which accelerated the
reaction rates in contrast with
previous reports.[4b]
Nanocages 1b and 1c from
2b or 2c and
3 were also
formed quantitatively in reflux-
ing CHCl3 solution. Due to the
introduction of n-hexyl and
TEG chains, the solubility of
such skeletons in common or-
ganic solvents was significantly
improved. We also observed
that the solubility of the inter-
mediates and products turned
out to be crucial in the thermo-
dynamically controlled process.
Nanocages 1b and 1c also ex-
hibited the similar 1H NMR
and mass spectroscopic behav-
iors (See the Supporting Infor-
mation). In addition, the cage
formation reactions were car-
ried out quantitatively without
any added acid catalysis. All
the above data show that the
desired nanocages are thermo-
dynamically stable products ir-
respective of substituted “tails”
in planar truxene moiety. Al-
though the imine bonds were
reversible, our desired nanocag-
es were stable both in solution
and solid state.
Figure 1. Partial 1H NMR (CDCl3, 300 MHz, 298 K) spectra of a) 2a, b) 3, c) 1a produced in situ, and
1H NMR (C2D2Cl4, 300 MHz, 298 K) spectra of d) H12-1a. and N denote CDCl3 and C2D2Cl4, respectively.
&
To completely “fix” our
cages, reduction of 2 by NaBH-
(OAc)3 was carried out and the
1H NMR and MALDI-TOF MS
spectra indicated that all the six
imine bonds were reduced to
saturated amines. As shown in
Figure 1d, the resonance signals
of CH2 (Hk) in the “cap” were
shifted
upfield
by
about
1.05 ppm, resulting from the
imine transformation. In con-
sideration of the decrease of ri-
gidity after the reduction, the
Figure 2. MALDI-TOF MS spectrum of nanocage 1a. Insets a) and b) are corresponding to experimental and
theoretical isotopic distributions, respectively.
split resonance signal of Hk tended to merge. In addition, a
new singlet peak emerged at d=3.59 ppm which was as-
signed to the signal of CH2 converted from CH=N. MALDI-
TOF MS analysis of H12-1a further confirmed the formation
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Shape-Persistent Fluorescent Nanocages
COMMUNICATION
cage molecules were distributed
on the mica surface, and they
stood. However, when CHCl3
solution (3.0”10À6 m) of nano-
cage 1b was drop-cast onto the
mica surface, the TM-AFM ex-
periments and sectional analysis
showed the presence of many
clusters with larger size on the
substrate, and the surface mor-
phologies were very different
from those of nanocage 1a
shown in Figure 5b. The height
of these clusters were measured
to be in the range of 0.9–1.6 nm
(average of 30 measurements)
by sectional analysis. This im-
1
Figure 3. Partial H NMR spectra (CDCl3/C2D2Cl4 1:1) of the 2a and 3 (6.6 mm/13.2 mm) mixture at 333 K as a
function of time.
plied that the van der Waals in-
of the proposed reductive cage: only one ion peak at m/z=
1741.6 for [M+H]+ (calcd for C126H96N6O3: 1740.8) was ob-
served.
The photophysical properties of nanocages 1a–c and their
corresponding reduced forms H12-1a–c were first investigat-
ed in dilute solution. Figure 4 shows the absorption and pho-
toluminescent (PL) spectra of 2a, 1a, and H12-1a in dilute
C2H2Cl4 solution. Compounds 2a and 1a showed an identi-
cal absorption peak at 323 nm, being assigned to the
2,2’,7,7’-tetraphenyl-9,9’-spirobifluorene fragment.[9] More-
over, in their absorption spectra, both 2a and 1a exhibited a
shoulder at about 348 nm. This registers the similarity in the
effective conjugation length between aldehyde 2a and nano-
cage 1a. However, the onset of the absorption spectrum of
2a red-shifted from 410 nm to about 470 nm, and the ab-
sorption intensity of the shoulder feature decreased. After
reduction, H12-1a showed an absorption peak at 322 nm
with two shoulders at 310 and 300 nm. In dilute C2H2Cl4 so-
lution, the PL spectrum of 2a showed a peak at 450 nm. The
PL maximum lmax of 1a slightly red-shifted about 8 nm in
comparison with that of 2a. The reductive cage H12-1a emit-
ted blue light with a wavelength of 396 nm in C2H2Cl4 solu-
tion. Other cages 1b and 1c also showed similar absorption
and PL spectra. Both 2 and 1 emitted sharp greenish-blue
color under the UV light irradiation.
The surface morphologies of these nanocages 1a–c were
also investigated.[3h] According to molecular modeling, the
height of cage 1a is about 1.5 nm. The highly dilute C2H2Cl4
solution of nanocage 1a (3.0”10À6 m) was drop-cast onto the
mica substrate and the surface pattern was analyzed by tap-
ping-mode atomic force microscopy (TM-AFM). The TM-
AFM experiments and section analysis showed the presence
of monodisperse entities protruding from the surface of the
mica substrate, as shown in Figure 5a. Sectional analysis re-
vealed that these entities had a height of 1.37Æ0.14 nm
(average of 30 measurements), in consistence with the
height of nanocage 1a. This result suggested that the nano-
Figure 4. UV/Vis absorption and PL spectra of 2a, nanocages 1a, H12-1a
in dilute C2H2Cl4 solution (c=1.0”10À6 m). Emission spectra were record-
ed upon excitation at absorption maximum.
teraction between n-hexyloxy chains led to larger aggrega-
tion and non-uniform height fluctuation upon evaporation
of the solvent. Interestingly, once the R2 substituents were
swapped to the polar TEG groups, the surface pattern
changed: after the drop-casting of 1c (3.0”10À6 m in CHCl3)
onto the mica surface, the nanocages formed uniformed
morphologies, both horizontally and vertically, in Figure 5c.
Sectional analysis revealed that these uniform entities had a
height of 0.91Æ0.07 nm (average of 30 measurements). Be-
cause of the incapability of imaging in detail the flexible
TEG chains with AFM, we speculate that such a height
might be associated with the nanocages with their TEG
chains stretched over the polar mica surface, possibly due to
the strong interaction between polar TEG chains and hydro-
philic mica substrate. Figure 5d shows the structural sche-
matic diagram of nanocages 1a-c. Figure 5a–c also illustrate
the schematic representations of the surface morphologies
of nanocages 1a-c on the mica substrate.
In summary, we have developed a facile approach to syn-
thesizing a series of 3D shape-persistent fluorescent nano-
cages by applying the concept of dynamic covalent chemis-
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J. Pei, K. Wu, Y. Ma et al.
try. Time-dependent 1H NMR
analyses indicate that multiva-
lent bonds of these nanocages
between aldehydes and amines
can be facilely and effectively
constructed in situ. This process
is quantitative, and shape-per-
sistent artificial nanocage struc-
tures are selectively formed as
the most thermodynamically
stable species in the reaction
mixture. More interestingly, our
3D skeleton provides double
cavities in one molecule
through double capping, show-
ing the possibility to integrate
two different guests into one
molecule. These dynamic cages
are trapped to become fixed
ones by NaBH(OAc)3 in mod-
A
erate yields. Moreover, varia-
tions of the flexible substitution
groups in the planar moiety
hardly affect the conformations
of the nanocages; these substi-
tuting groups not only adjust
the solubility of the nanocages,
but also dramatically change
the aggregation behaviors and
the surface morphologies. Sys-
tematic investigation of their
photophysical properties shows
Figure 5. Tapping-Mode AFM height images (1 mm”1 mm), sectional analyses and schematic representations
of the surface assembling morphologies of a) 1a, b) 1b and c) 1c on mica substrate, and d) structural schemat-
3
ic diagram of 1 with its flexible chains R2 omitted. Calculated volume of each cavity is about 120 Š (opti-
mized by MMFF94).
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lab. All in all, these results demonstrate that the utilization
of such skeletons for 3D nanostructure assembly opens a
new pathway to exploit the mechanical bond at the molecu-
lar level in chemistry.
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Acknowledgements
This work was financially supported by the Major State Basic Research
Development Program (Nos. 2006CB921602 and 2007CB808000) and by
the National Natural Science Foundation of China (NSFC, 20632020,
50673002, 20425207). Mr. Jia Luo thanks the Peking University CDY
scholarship for generous support.
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Received: January 25, 2008
Published online: April 2, 2008
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