Facile three-step synthesis and photophysical properties of [8]-, [9]-, and [12]cyclo-1,4-naphthalene nanorings: Via platinum-mediated reductive elimination

Article abstract of DOI:10.1039/c7cc07370d

Herein we report a facile three-step synthesis of [8]-, [9]-, and [12]cyclo-1,4-naphthalene nanorings as the conjugated segments of carbon nanotubes. The nanorings were created via a platinum-mediated assembly of 1,4-naphthalene-based units and subsequent reductive elimination in the presence of triphenylphosphine. This present platinum-mediated approach is attractive because of its simple three-step process to produce the targeted nanorings in a high overall yield. In addition, their photophysical properties were studied using UV-vis spectroscopy and photoluminescence (PL) spectroscopy, which further revealed their unique size-dependent properties.

Full text of DOI:10.1039/c7cc07370d

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DOI: 10.1039/C7CC07370D
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Facile Three-Step Synthesis and Photophysical Properties of [8]-,
[9]-, and [12]Cyclo-1,4-naphthalene Nanorings via Platinum-
Mediated Reductive Elimination
Received 00th January 20xx,
Accepted 00th January 20xx
Hongxing Jia, Yuyue Gao, Qiang Huang, Shengsheng Cui, Pingwu Du*
DOI: 10.1039/x0xx00000x
www.rsc.org/
CPPs have been synthesized using polycyclic aromatic hydrocarbons
(PAHs) as the building blocks, such as chrysene,19 anthracene,1, 10 and
pyrene.20 Our group reported the use of a larger PAH, hexa-peri-
hexabenzocoronene (HBC), as the building unit to synthesize highly
conjugated giant nanorings.6, 21 The first synthesis of a π-extended
naphthalene-based nanoring, [9]cyclo-1,4-naphthalene ([9]CN), has
been achieved from a L-shaped building unit by Itami and coworkers
in 2012.22 More recently, they have also synthesized a few even-
numbered carbon nanorings, [n]CNs (n = 8, 10, 12, and 16),
using a similar strategy.23 Unfortunately, despite the successful
synthesis, these studies displayed very poor yields for the final
products (0.1% for [8]CN, 1.2% for [9]CN, and 1.5% for [12]CN), which
has seriously hampered the further physical studies of naphthalene-
based CPPs and their utilization in bottom-up synthesis of uniform
CNTs. Therefore, the development of a new facile synthesis route is
highly desired.
Herein we report the facile three-step synthesis of [8]-, [9]-, and
[12]cyclo-1,4-naphthalenes nanorings as the conjugated segments
of carbon nanotubes. The nanorings were created by platinum-
mediated assembly of 1,4-naphthalene-based units and
subsequent reductive elimination in the presence of
triphenylphosphine. This present platinum-mediated approach is
attractive because of its simple three-step process to produce the
targeted nanorings in a high overall yield. In addition, their
photophysical properties were studied using UV-Vis spectroscopy
and photoluminescence (PL) spectroscopy, which further revealed
their unique size-dependent properties.
Cyclic π-conjugated organic materials have attracted much
attention in recent years. Cycloparaphenylenes (CPPs, Figure 1a) are
hoop-shaped aromatic hydrocarbons consisting of para-linked
phenylene rings, which have been received significant interest in the
last few years due to their highly symmetric topological structures as
the shortest structural segment of armchair carbon nanotubes (CNTs,
Figure 1b).1, 2 Moreover, CPPs have demonstrated many potential
applications in material science and supramolecular chemistry.3-6 It
has been proposed the CPPs can be serve as seed compounds for
8
diameter-controlled growth of CNTs.7, The bottom-up chemical
synthesis of CPPs was proposed a few decades ago1, 9, 10 but despite
the simple structure, this synthesis was only recently achieved, with
Bertozzi and Jasti reporting the first synthesis of [9]-, [12]-, and
[18]CPPs
in
2008.11
After long-
term exploration and unremitting efforts, CPPs with different
diameters ([5]-[16], [18]CPPs) have been achieved by different
research groups.11-18
Based on these fundamental studies, further π-extended CPP
structures can be considered as closer precursors of CNTs and have
attracted great attention. In the literature, different derivatives of
Figure 1. (a) Structure of [n + 4]CPP. (b) Structure of (8,8)armchair
carbon nanotubes (CNTs). The constituent [8]CN unit is shown in blue.
(c) Molecular structure of [8]CN.
In 2010, Yamago and coworkers developed a platinum-
mediated assembly strategy to synthesize [8]CPP.17 After that, more
CPPs with difference sizes were also successfully synthesized by the
same group.12-14 Similar to the synthesis of CPPs, the major
bottleneck for achieving CNs also lies in the increased strain energy
resulting from the curved structure. Inspired by the platinum-
mediated assembly strategy for the synthesis of CPPs,6, 17 we envision
that CNs could also be synthesized by the platinum-mediated
Hefei National Laboratory of Physical Science at the Microscale, CAS Key Laboratory
of Materials for Energy Conversion, Department of Materials Science and
Engineering, iChEM(Collaborative Innovation Center of Chemistry for Energy
Materials), University of Science and Technology of China (USTC), 96 Jinzhai Road,
Hefei, Anhui Province, 230026, P. R. China. Email: dupingwu@ustc.edu.cn; Fax: 86-
551-63606207.
Electronic Supplementary Information (ESI) available: See
DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
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assembly of naphthalene-based units and subsequent reductive ternaphthalene (5), which is another key precursor for the synthesis
elimination of platinum to form our target molecules, although the of [n]CNs. Compound 5 was well characterizDeOdIb: 1y01.1H03(VF9iie/gCwu7ArCretCicS0le47O)3na7l0ninDde
para position in the naphthalene unit is more bulky which probably 13C NMR (Figure S5), as well as ESI mass spectrometry (Figure S6).
causes more hindrance than that in the phenylene unit. Herein we The synthesis details can be found in the SI. Following a similar
report the synthesis of [8]-, [9]-, and [12]CNs with higher yields based procedure to that of [8]CN, compound 5 was reacted with Pt(COD)Cl2
on this platinum-mediated strategy. In addition, the CNs’ to form the intermediate 6. Finally, two types of carbon nanorings
photophysical properties using both steady-state and time-resolved [9]CN (3.0%) and [12]CN (3.5%) were obtained after reductive
spectroscopies in THF are experimentally investigated.
elimination of the intermediate 6. This result suggests that the
assembly of compound 5 with Pt(COD)Cl2 mainly afforded a mixture
of tri- and tetra-nuclear platinum macrocycle precursors. The
synthesis procedure for [9]CN and [12]CN is shown in Figure 3.
Figure 2. Synthesis procedures for [8]CN and [10]CN. Reagents and
conditions: (i) 1 (1.0 equiv.), Bpin-bpin (3.0 equiv.), Pd(dppf)Cl2 (5
mol%), KOAc (5.0 equiv.), 1,4-dioxane, 100 °C, 36 h; (ii) 2 (1.0 equiv.),
Pt(COD)Cl2 (1.0 equiv.), CsF (4.0 equiv.), THF, 40 °C, 24 h; (iii) PPh3
(10.0 equiv.), toluene, 110 °C, 36 h.
Figure 3. Synthesis procedures for [9]CN and [12]CN. Reagents and
conditions: (i) 4 (1.0 equiv.), Bpin-bpin (3.0 equiv.), Pd(dppf)Cl2 (5
mol%), KOAc (5.0 equiv.), DMF, 120 °C, 36 h; (ii) 5 (1.0 equiv.),
Pt(COD)Cl2 (1.0 equiv.), CsF (4.0 equiv.), THF, 40 °C, 24 h; (iii) PPh3
(10.0 equiv.), toluene, 110 °C, 36 h.
To accomplish the above-mentioned goal, the naphthalene-
based units should be functionalized with boronate esters or
trialkyltin groups, which can be readily reacted with the platinum
17, 24-26
complex to form macrocycle precursors.6,
With these
macrocycle precursors in hand, the final [n]CNs products can be
obtained after reductive elimination of platinum. Initially, 4,4′-
bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane-2-yl)-1,1′-
The subsequent increase of the naphthalene unit to make
larger nanorings was hampered by the poor solubility. As a result,
using the platinum-mediated assembly and reductive elimination
approach, [8]-, [9]-, [10]-, and [12]CNs were synthesized and pure [8]-,
[9]-, and [12]CNs were obtained by careful separation. The structures
of these three nanorings were confirmed by MALDI-TOF-MS
binaphthalene (2) was synthesized from 4,4′-dibromo-1,1′-
binaphthyl (1) via the Miyaura borylation reaction in the presence of
Pd(dppf)Cl2, bis(pinacolato)diboron, and KOAc. The synthesis details
are provided in the supporting information (SI). The molecular
1
structure of compound 3 was characterized by H NMR (Figure S1),
13C NMR (Figure S2) and ESI mass spectrometry (Figure S3). Next,
compound 2 was treated with one equivalent of Pt(COD)Cl2 (COD =
1,5-cyclooctadiene) in the presence of four equivalents of cesium
fluoride in THF for 24 hours under argon. The resulting platinum-
mediated complex 2 was obtained in a 40% yield as a white
precipitate, which was subsequently subjected to reductive
elimination by treating with ten equivalents of triphenylphosphine as
a coordination ligand under refluxing anhydrous toluene for 36 hours.
After purification through preparative thin-layer chromatography,
the target product, [8]CN (2.6%) was obtained. These results indicate
that the assembly of compound 2 with Pt(COD)Cl2 can successfully
provide a tetra-nuclear platinum macrocycle precursor. Interestingly,
[10]CN was also detected by mass spectroscopy (Figure S10), but
pure [10]CN was very difficult to isolate because of its much lower
yield. The synthesis procedure of [8]CN is summarized and shown in
Figure 2.
1
spectrometry (Figures S7-S9). Moreover, the H NMR data (Figures
S11-S13) are consistent with the data reported by Itami and
coworkers.23 Compared with the yields of the reported method using
a curved naphthalene-based precursor (0.1% for [8]CN, 1.2% for
[9]CN, and 1.5% for [12]CN),23 our present approach has much higher
yields (2.6% for [8]CN, 3.0% for [9]CN, and 3.5% for [12]CN).
Therefore, platinum-mediated assembly strategy is another
interesting pathway for synthesis of [n]CNs from easily
prepared starting materials 1 and 4.
Encouraged by these results, we turned our attention to
synthesizing larger [n]CNs using this approach. The starting material
was functionalized with one more naphthalene unit to get 4,4′′-
dibromo-1,1′:4′,1′′-ternaphthalene (4). After
a similar Miyaura
borylation reaction, compound 4 was readily transformed into 4,4′′-
bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane-2-yl)-1,1′:4′,1′′-
2 | J. Name., 2012, 00, 1-3
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Figure 4. (a) UV-vis absorption (solid line) and fluorescence (dash line)
spectra of [8]CN (black), [9]CN (red), and [12]CN (blue). (b)
Luminescent photographs of [8]CN (yellow), [9]CN (green), and
[12]CN (blue) in THF under UV irradiation at λ = 365 nm.
Fluorescence decay tests were conducted usingVaienwaAnrtoicsleeOconlninde
pulsed laser system in degassed THF solutioDnOaIt: 1r0o.o10m39t/eCm7CpCe0ra7t3u7r0eD.
The fluorescence decay of [n]CNs followed first-order kinetics with a
lifetime (τ) = 5.3 ns for [8]CN, 1.7 ns for [9]CN, and 1.0 ns for [12]CN,
as measured by the single-photon counting method (Figure 5).
According to the equation kr = ΦF/τ,27 their radiation decay rate
constant (kr) could also be determined to be 4.2 × 107 s-1 for [8]CN,
2.0 × 108 s-1 for [9]CN, and 4.2 × 108 s-1 ns for [12]CN.
Table 1. Absorption and emission data for [n]CNsa
Conclusions
In conclusion, we developed a novel and efficient three-step
synthesis approach to achieve [8]-, [9]-, and [12]CNs nanorings based
on the platinum-mediated assembly of naphthalene units and
subsequent reductive elimination. This synthesis route provides a
good strategy to overcome the strain energy resulting from the
curved structure and successfully achieved the target molecules with
a relatively high yield. This method may open a new way for the
synthesis of [n]CNs from easily prepared starting materials 2 and 5
and facilitate their further utilization in bottom-up synthesis of
uniform CNTs. In addition, their photophysical properties in THF
were experimentally investigated, revealing their unique size-
dependent properties.
a Data obtained at room temperature in THF. b Maximum absorption.
Maximum emission upon excitation at 350 nm. Lifetime.
Fluorescence quantum yields.
c
d
e
The photophysical properties of the [8]-, [9]-, and [12]CNs in
THF were further investigated using UV-vis absorption spectroscopy,
steady-state fluorescence spectroscopy, and time-resolved
fluorescence decay. The absorption and emission spectra are shown
in Figure 4 and the data are summarized in Table 1. Interestingly,
unlike CPPs, which absorb UV-vis light at similar wavelength
regardless of their different sizes,12 the UV-vis absorption spectra of
the [8]-, [9]-, and [12]CNs clearly exhibit size-dependent properties:
The maximum absorption (λabs) were blue-shifted with an increase in
the size of the [n]CNs nanorings. This behavior is consistent with
reports by Itami and coworkers.23 The fluorescence emission
measurements were performed under an excitation at 350 nm, and
all [n]CNs emitted strong fluorescence in a variety of colors (yellow
for [8]CN, green for [9]CN, and blue for [12]CN) in THF solutions). The
fluorescence spectra also exhibit size-dependent properties. With
increasing size of [n]CNs, the maximum emission (λem) were
significantly blue-shifted. The λem peaks are 556 nm, 486 nm, and 463
nm for [8]-, [9]-, and [12]CNs, respectively. The fluorescence
quantum yields (ΦF) of [n]CNs were determined by using anthracene
in ethanol as the reference. The ΦF values are provided in Table 1
and the highest ΦF is 0.42 for [12]CN. Similarly, the ΦF values of
[n]CNs increased with increasing size.
This work was financially supported by the National Key Research
and Development Program of China (2017YFA0402800), the National
Natural Science Foundation of China (51772285, 21473170), and the
Fundamental Research Funds for the Central Universities.
Notes and references
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TOC
Facile three-step synthesis of [8]-, [9]-, and [12]cyclo-1,4-
naphthalenes nanorings were firstly accomplished by platinum-
mediated assembly of 1,4-naphthalene-based units and
subsequent reductive elimination reactions.
4 | J. Name., 2012, 00, 1-3
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