Synthesis and properties of cycloparaphenylene-2,7-pyrenylene: A pyrene-containing carbon nanoring

Article abstract of DOI:10.1039/c3cc48198k

The first synthesis of a pyrene-containing carbon nanoring, cyclo[12]paraphenylene[2]-2,7-pyrenylene ([12,2]CPPyr), is described. Through a sequence of stepwise macrocyclization using Pd and Ni catalysts followed by aromatization of L-shaped cyclohexane units afforded a [12,2]CPPyr in five steps from commercially available compounds. The UV-vis absorption and fluorescence spectra as well as TD-DFT calculations clarified the unique photophysical and electronic properties of the [12,2]CPPyr. This journal is

Full text of DOI:10.1039/c3cc48198k

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Synthesis and properties of cycloparaphenylene-
2,7-pyrenylene: a pyrene-containing carbon
nanoring†
Cite this: Chem. Commun., 2014,
50, 957
Received 26th October 2013,
Accepted 19th November 2013
Akiko Yagi,a Gandikota Venkataramana,a Yasutomo Segawaa and Kenichiro Itami*abc
DOI: 10.1039/c3cc48198k
www.rsc.org/chemcomm
The first synthesis of a pyrene-containing carbon nanoring, cyclo[12]- aryl-substituted CPPs such as tetraaryl[12]CPP,7a phenylene- or
paraphenylene[2]-2,7-pyrenylene ([12,2]CPPyr), is described. Through naphthylene-bridged [8]CPP dimers,7b and dodecaaryl[9]CPP,8
a sequence of stepwise macrocyclization using Pd and Ni catalysts further benzannulation of these CPP derivatives has not been
followed by aromatization of L-shaped cyclohexane units afforded a successful so far. In 2012, we reported the synthesis of [9]cyclo-
[12,2]CPPyr in five steps from commercially available compounds. 1,4-naphthylene ([9]CN), which can be considered as a benzan-
The UV-vis absorption and fluorescence spectra as well as TD-DFT nulated [9]CPP at its ‘‘K region’’.9b A number of interesting
calculations clarified the unique photophysical and electronic properties derived from benzannulation (high ring strain, chiral
properties of the [12,2]CPPyr.
conformation, and small Stokes shift compared to [9]CPP) have been
observed in [9]CN. As shown in Fig. 1, the other position on CPP
In recent years, considerable attention has been devoted to the for benzannulation is its ‘‘bay region’’ to convert a biphenyl
chemistry of carbon nanorings that are macrocyclic aromatic unit into a pyrene unit. Herein we report the synthesis of
hydrocarbons representing segments of carbon nanotubes cycloparaphenylene-2,7-pyrenylene (CPPyr), the first pyrene-
(CNTs).1 A carbon nanoring could not only serve as a template containing carbon nanoring. Structural and physical properties
for a structurally well-defined CNT,2 but is also an interesting of CPPyr would also be of interest since it is well known that
p-conjugated molecule in its own right. Among various possible pyrene and its derivatives show characteristic photophysical
carbon nanorings, cycloparaphenylene (CPP) is the simplest properties such as long fluorescence lifetime12 and excimer
carbon nanoring consisting solely of benzene rings. After many emission.13
years of efforts by several groups including ours, the chemical
We applied our general and modular synthetic strategy using a
synthesis of CPP has become possible in the last five years.3 benzene-convertible L-shaped cyclohexane unit 1 to the synthesis
Since then, several sizes of CPPs,4–6 CPP derivatives,7,8 and of CPPyr.3b In our previous syntheses of CPP derivatives, coupling
related carbon nanorings9,10 have been synthesized and their of the L-shaped unit and 1,4-diborylbenzene, 4,40-diborylbiphenyl,
interesting properties have come to light.11
2,6-diborylnaphthalene, or 5,50-dibromo-2,20-bipyridyl furnished
We next focused on the synthesis of benzannulated, p-extended macrocyclic intermediates for CPPs,4a,d cycloparaphenylene-2,6-
CPPs that are similar to the structure of armchair CNTs. Con- naphthylene,9a or cycloparaphenylene-2,5-pyridylidene,9c respec-
sidering the unique structural and photophysical properties tively. We envisioned that a [12,2]CPPyr 5, which consists of twelve
of CPPs, modification of electronic properties of CPPs by benzenes and two pyrenes, can be synthesized by a stepwise
benzannulation would provide novel macrocyclic aromatic
systems. Although there have been three examples of simple
a Department of Chemistry, Graduate School of Science, Nagoya University,
Chikusa, Nagoya 464-8602, Japan. E-mail: itami.kenichiro@a.mbox.nagoya-u.jp;
Fax: +81 52-788-6098; Tel: +81 52-788-6098
b JST, ERATO, Itami Molecular Nanocarbon Project, Nagoya University, Chikusa,
Nagoya 464-8602, Japan
c Institute of Transformative Bio-Molecules (WPI-ITbM), Nagoya University, Chikusa,
Nagoya 464-8602, Japan
† Electronic supplementary information (ESI) available: Experimental proce-
dures, spectra of new compounds, and details of the computational study. See
Fig. 1 CPP and two benzannulated, p-extended CPPs: CN and CPPyr.
Chem. Commun., 2014, 50, 957--959 | 957
DOI: 10.1039/c3cc48198k
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Fig. 2 (a) UV-vis absorption (solid lines) and fluorescence (broken lines)
spectra of dichloromethane solution of 5 (blue line), chloroform solution
of [16]CPP (red line) and chloroform solution of pyrene (orange line).
(b) Fluorescence of 5.
singlet signal belonging to the paraphenylene moieties was
observed in a similar chemical shift (7.68 ppm) to that of
[16]CPP (7.69 ppm).4a
The UV-vis absorption and fluorescence spectra of the
[12,2]CPPyr 5 were investigated to elucidate the effect of pyrene
on the electronic properties of [12,2]CPPyr structures. The
spectra of 5 taken as a dichloromethane solution are depicted
in Fig. 2. Absorption peaks were observed at 328 nm and 343 nm
with a shoulder on the long-wavelength side. The most intense
absorption maximum (labs) was observed at 343 nm and the
Scheme 1 Synthesis of 5. Reaction conditions: (a) Pd(PPh3)4, Ag2O, THF,
reflux. (b) Ni(cod)2, 2,20-bipyridyl, 1,4-dioxane, 90 1C. (c) NaHSO4ꢃH2O, 1,2,4-
trichlorobenzene, 150 1C. MOM = CH2OCH3, B(pin) = 4,4,5,5-tetramethyl-
1,3,2-dioxaborolane-2-yl, THF = tetrahydrofuran.
molecular absorption coefficient (e) was 6.1 ꢁ 105 cmꢀ1 Mꢀ1
.
Similar to [12]–[18]CPPs, 5 showed intense blue photolumines-
cence. Emission maxima (lem) were observed at 430 nm under
excitation at 343 nm. Excimer emission was not observed even
at the concentration of 10ꢀ3 M. The absolute fluorescence
coupling of the L-shaped unit 1 and 2,7-diborylpyrene 2, followed quantum yield (FF) of 5 determined by a calibrated integrating
by the aromatization of cyclohexane moieties. The ring size of sphere system was 0.21 ꢂ 0.03. This value is lower than those of
the optimized structure of 5 (ca. 20 Å) is almost identical to that CPPs ([9]–[16]CPP, FF = 0.73–0.90).11a On the other hand, the
of [16]CPP.4a From a hypothetical homodesmotic reaction, the fluorescence lifetime of 5 was long (ts = 25.6 ns) in comparison
strain energy of 5 was estimated to be 37.9 kcal molꢀ1, which is with CPPs ([9]CPP and [12]CPP, ts = 10.6, 2.2 ns, respectively).
also similar to that of [16]CPP (35.6 kcal molꢀ1).11a
Taking the long fluorescence lifetime of pyrenes12 into considera-
Our synthesis of 5 is shown in Scheme 1. The L-shaped unit tion, inserted pyrenes would contribute to the long fluorescence
1 and 2,7-diborylpyrene 2 were synthesized from commercially lifetime of 5. The radiative and nonradiative decay rate constants
available reagents according to reported methods.3b,14 The (kr = 8.4 ꢁ 106 sꢀ1, knr = 3.1 ꢁ 107 sꢀ1) were determined from the
Suzuki–Miyaura cross-coupling of 2 (1.0 equiv.) with an excess observed quantum yield and fluorescence lifetime as well as the
amount of 1 (5 equiv.) produced a pyrene-containing U-shaped following equations: FF = kr ꢁ ts and kr + knr = tsꢀ1. The kr values are
unit 3 in 69% yield. The Ni(cod)2/2,20-bipyridyl-promoted one- or two orders of magnitude lower than those of CPPs (kr = 6.9 ꢁ
homocoupling of 3 afforded the corresponding box-shaped 107 sꢀ1 and kr = 4.0 ꢁ 108 sꢀ1 for [9]CPP and [12]CPP, respectively),
macrocycle 4 in 17% yield. In order to inhibit oligomerization whereas the knr values are comparable to each other (B107 sꢀ1). The
of 3, the reaction was performed under dilute conditions (1 mM). spectra of pyrene and [16]CPP are also shown in Fig. 2. The UV-vis
Finally, 4 was subjected to aromatization with NaHSO4 (20 equiv.) spectrum of 5 seems to be a combination of two independent
in 1,2,4-trichlorobenzene at 150 1C and a [12,2]CPPyr 5 was spectra of pyrene and [16]CPP. The fluorescence spectrum of 5 is
obtained in 18% yield as a pale yellow solid. The high- red-shifted from those of pyrene and [16]CPP.
temperature reaction in 1,2,4-trichlorobenzene was needed to
To understand the electronic nature of the [12,2]CPPyr 5, a
accomplish the aromatization step. The solid of 5 was soluble TD-DFT (time-dependent density functional theory) study was
in chlorinated solvents such as dichloromethane and chloroform, performed at the B3LYP/6-31G(d) level. Fig. 3 shows the energy
whereas it had poor solubility in other solvents such as ethyl diagrams and pictorial representations of six frontier molecular
acetate, methanol, and hexane. In the 1H NMR spectrum of 5, the orbitals (MOs) of 5. The orbitals of HOMO ꢀ 1 and HOMO ꢀ 2 as
two singlets that would be expected for a 2,7-disubstituted pyrene well as those of LUMO + 1 and LUMO + 2 are degenerate. In
were observed: d = 8.06 and 8.37 ppm in CDCl3.15 A broadened comparison with [16]CPP, the shapes and energy levels of HOMO
958 | Chem. Commun., 2014, 50, 957--959
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Notes and references
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Chem. Commun., 2014, 50, 957--959 | 959