Strained and unstrained macrocycles composed of carbazole and butadiyne units: Electronic state and optical properties

Article abstract of DOI:10.1021/jo300441k

Cu(OAc)₂ catalyzes dehydrogenative condensation of 3,6-bis(2-ethynylphenyl)carbazole in the presence of O₂ to afford the cyclization product 1 and cyclodimer 2. Compound 1 contains bent carbazole and butadiyne groups, while 2 has a l

Full text of DOI:10.1021/jo300441k

Note
pubs.acs.org/joc
Strained and Unstrained Macrocycles Composed of Carbazole and
Butadiyne Units: Electronic State and Optical Properties
Tomohito Ide,† Sho Sakamoto,‡ Daisuke Takeuchi,† Kohtaro Osakada,*,† and Shigeru Machida†,‡
†Chemical Resources Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503 (Mail Box R1-03),
Japan
‡Department of Chemical Science and Engineering, Tokyo National College of Technology, 1220-2, Kunugida-machi, Hachioji,
Tokyo 193-0997, Japan
S
* Supporting Information
ABSTRACT: Cu(OAc)2 catalyzes dehydrogenative condensation
of 3,6-bis(2-ethynylphenyl)carbazole in the presence of O2 to
afford the cyclization product 1 and cyclodimer 2. Compound 1
contains bent carbazole and butadiyne groups, while 2 has a less
strained structure with Z shape around the two parallel butadiyne
groups. Optical properties of the compounds are discussed based
on the electronic states estimated from electrochemical measure-
ment and density functional theory calculation.
trained macrocyclic molecules with bent π-conjugated
systems, such as cyclophanes, show characteristic linear
were isolated in 38 and 10% yields, respectively, after
purification using silica gel column chromatography.
S
and nonlinear optical properties as well as redox behaviors.1
Properties of [n]cycloparaphenylenes vary depending on their
ring size;2 smaller cycloparaphenylenes, for example, exhibit
longer fluorescence wavelength and lower oxidation potential
than the larger ones.2a,d Octadehydrodibenzo[12]annulene,
which has strained butadiyne units, shows fluorescence
maximum at 532 nm in spite of its small π-conjugated system.3
Although many strained hydrocarbon macrocycles containing a
bent π-conjugated system have been reported,4 there are a
limited number of those having heteroaromatic units.5 Herein
we report the synthesis, structure, and electronic state of
macrocyclic molecules composed of carbazole and diphenylbu-
tadiyne units. They are expected to exhibit characteristic
properties derived from the strained ring system and the
presence of a photo- and electrochemically active carbazole
group.
X-ray analysis of 1 revealed its cyclic structure with ring strain
(Figure 1a,b). The inner C−CC bond angles (169.8 and
169.9°) are deviated from those of strain-free linear diphenyl-
1,3-butadiyne. The distortion is less significant than that of
butadiyne units in octadehydrodibenzo[12]annulene (C−C
C angle = 12.6−15.0°).6 The carbazole unit is also distorted,
and the angles between the five-membered ring and the six-
membered rings are 13.2 and 14.4°. The 13C NMR peaks of
alkyne carbons of 1 are shifted to downfield (see Supporting
Information). It can be ascribed to decrease of sp character and
increase of sp2 character due to the bent structure. DFT
computation of a homodesmic reaction in Scheme 27 by
accurate B2GP-PLYP-D38/def2-TZVPP level of theory using
BLYP-D3/def2-SVP level geometry computed by ORCA9
indicated that the molecular strain energy (64.7 kJ mol−1)
was in the same range of octadehydrodibenzo[12]annulene
(69.1 kJ mol−1) calculated similarly.
Scheme 1 outlines the synthesis of carbazole−diphenylbuta-
diyne hybrid macrocycles 1 and 2. 2-(Triisopropylsilylethynyl)-
benzeneboronic acid 3 was obtained by Sonogashira coupling
of 1-bromo-2-iodobenzene with (triisopropylsilyl)acetylene
(99% yield) and boration (89% yield). Suzuki coupling reaction
of 3 with 3,6-dibromocarbazole 4 formed 5 (91% yield), which
was converted into 6 by removal of the TIPS groups with
TBAF (84% yield). Addition of Cu(OAc)2 catalyst to a dilute
solution of 6 (pyridine/ether = 3:1, 10 mM) and stirring the
solution for 40 h under air caused intra- and intermolecular
coupling of the alkynyl groups to afford a mixture of cyclization
product 1 and cyclic codimer 2. These macrocyclic compounds
Preliminary X-ray diffraction study of 2 revealed its molecular
structure having two carbazole units whose NH groups are
orientated to the opposite direction, as shown in Scheme 1.10
The structure with two “Z”-shaped groups linked with two
carbazole groups is confirmed by theoretical calculation (vide
1
infra). Compound 2 showed sharp H NMR signals at 60 °C
(see Supporting Information), whereas some of the aromatic
hydrogen signals are broadened at room temperature. The
Received: March 3, 2012
Published: April 23, 2012
© 2012 American Chemical Society
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Note
temperature-dependent change of the NMR spectrum can be
assigned to fluxional behavior of the molecule such as
synchronized rotation of the carbazole−phenyl single bonds.
UV−vis absorption and fluorescence spectra of 1 and 2 in
CHCl3 are shown in Figure 2. Weak absorption peak of 1 at
Scheme 1. Synthesis of Carbazole−Diphenylbutadiyne
Hybrid Macrocycles
Figure 2. UV−vis absorption spectra (solid line) and fluorescence
spectra (hashed line) of 1 and 2 in CHCl3 (c = 0.01 mM for UV−vis
and c = 0.001 mM for fluorescence spectra).
366 nm is assigned to charge-transfer (CT)-type absorption.
Bathochromism of 1 was observed in THF due to CT nature of
excitation (see Supporting Information). TD-DFT (BHLYP/
def2-TZVP//BLYP-D3/def2-SVP) calculation suggested that
the first transition of 1 (calculated to be 360 nm with oscillator
strength f = 0.26) mainly consists of transition from HOMO
delocalized on the carbazole unit to LUMO delocalized on the
diphenylbutadiyne unit (Figure 3). The onset of absorption in
Figure 1. Thermal ellipsoid drawings of 1 with selected bond lengths
(Å) on bottom and bond angles (°) on top.
Scheme 2. Homodesmic Reaction for Estimation of Strain
Energy of 1
Figure 3. Frontier orbitals (B3LYP/def2-SVP//BLYP-D3/def2-SVP)
of 1 and 2. Plots (isosurface = 0.03) of LUMO (top) and HOMO
(bottom).
the UV−vis spectra of 1 and 2 is observed at 390 and 382 nm,
respectively. Since the wavelength corresponds to the energy
gap of π−π* transition, the energy gap of 1 is comparable to
that of 2 regardless of the bent structure of 1.
Table 1 summarizes HOMO, LUMO, and energy gaps of 1,
2, and 6. Results of related compounds 1F and 6F having a
fluorene group instead of the carbazole group of 1 and 6 are
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Note
The higher HOMO level of 1F compared to that of 6F is usual,
although the macrocycle obtained in this study, 1 with the N-
hetero-π-conjugated system, has a lower HOMO level than that
of 6. It is speculated that localization of the HOMO orbital on
carbazole and electron-donating property of N atom are
important factors for reversed order of HOMO level shifting.
The fluorescence maxima of 1 and 2 are observed at 436 nm
(ΦF = 0.038) and at 426 nm (ΦF = 0.035), respectively. The
red shift of 1 compared with 2 can be attributed to the ring
strain. Photoexcitation of the carbazole ring is followed by
electron transfer from the carbazole to the butadiyne group,
consistent with the low ΦF values because the fluorescence
corresponds to the relaxation from the LUMO of 1.
Combination of the electron-poor butadiyne group and the
electron-rich carbazole group resulted in the fluorescence
properties via electron transfer from the photoexcitation state.
In summary, the macrocycle with bent carbazole 1 and its
unstrained dimer 2 were newly synthesized, and their structures
were revealed by X-ray crystallography. Electrochemical
oxidation potential of the carbazole did not change in spite
of the strained structure. DFT calculation suggested that the
HOMO level of 1 is slightly lower than that of 2. The strained
N-hetero-π-conjugated system of 1 shows a different energy
level shift on the frontier orbital from the hydrocarbon
derivative 1F. The levels and delocalization of the orbitals
affected the optical properties of the strained molecule 1.
Table 1. HOMO, LUMO, and Energy Gaps of Macrocycles
1, 2, Precursor 6, and Their Analogues 1F and 6F Calculated
by B3LYP/def2-SVP//BLYP-D3/def2-SVP Level of theory
compound
HOMO/eV
LUMO/eV
energy gap/eV
1
2
6
−5.53
−5.34
−5.44
−5.59
−5.83
−1.92
−1.71
−1.15
−2.05
−1.27
3.61
3.63
4.29
3.54
4.56
a
1F
a
6F
a
Chemical structures of 1F and 6F are shown below.
also included for comparison (B3LYP/def2-SVP//BLYP-D3/
def2-SVP level). The HOMO level of 1 is slightly lower than
that of 6 in spite of the strained structure. The relative HOMO
levels are confirmed by electrochemical oxidation potential of
the compounds, shown in Figure 4. The first oxidations of 1
EXPERIMENTAL SECTION
■
Synthesis. 1-Bromo-2-(triisopropylsilylethynyl)benzene.11
PdCl2(PPh3)2 (35.1 mg, 0.050 mmol), CuI (14.0 mg, 0.074 mmol),
Et3N (80 mL), 1-bromo-2-iodobenzene (0.63 mL, 5.00 mmol), and
triisopropylsilylacetylene (1.3 mL, 5.84 mmol) were placed in a
Schlenk tube under argon. The mixture was stirred at room
temperature for 12 h. The reaction mixture was evaporated to
dryness. The residual oil was dissolved in hexane and through a short
plug of silica gel. The solvent was removed, and the resulting oil was
dried with heating under vacuum to give 1-bromo-2-
1
(triisopropylsilylethynyl)benzene as colorless oil (1.68 g, 99%): H
NMR (500 MHz, CDCl3) δ 7.59 (d, J = 8 Hz, 1H), 7.53 (dd, J = 8, 2
Hz, 1H), 7.26 (ddd, J = 8, 8, 1 Hz, 1H), 7.17 (ddd, J = 8, 8, 1 Hz, 1H),
1.18 (m, 21 H); 13C NMR (125 MHz, CDCl3) δ 133.8, 132.3, 129.3,
126.8, 125.7, 125.6, 104.7, 96.2, 18.7, 11.3.
Figure 4. Cyclic voltammograms of 1, 2, and 6 (0.20 mM) in CH2Cl2
containing 0.10 M nBu4N·PF6.
2-(Triisopropylsilylethynyl)benzeneboronic acid (3). To a solution
of 1-bromo-2-(triisopropylsilylethynyl)benzene (0.846 g, 2.5 mmol) in
THF (25 mL) at −84 °C was added 2.6 M nBuLi in hexane (1.2 mL,
3.0 mmol). After 30 min, trimethyl borate (0.56 mL, 5.0 mmol) was
added to the cold solution at the temperature. The reaction mixture
was allowed to warm slowly to room temperature and was stirred for
additional 12 h. The reaction mixture was quenched with 2 N HCl (20
mL). After the mixture was stirred for 1 h at room temperature, the
reaction mixture was washed with water (×2) and brine and then
extracted with hexane. The organic phases were dried over Na2SO4,
and the solvent was removed under vacuum. Silica gel column
chromatography (hexane/ethyl acetate 10:1) of the residue gave 2-
(triisopropylsilylethynyl)benzeneboronic acid as a white solid (0.677 g,
89%): 1H NMR (500 MHz, CDCl3) δ 7.99 (d, J = 7 Hz, 1H), 7.54 (d,
J = 7 Hz, 1H), 7.42 (tt, J = 7, 1 Hz, 1H), 7.39 (tt, J = 7, 1 Hz, 1H),
5.94 (s, 2H), 1.16 (m, 21 H); 13C NMR (125 MHz, CDCl3) δ 135.5,
133.1, 130.7, 128.4, 126.9, 108.4, 95.8, 18.6, 11.3; HRMS (ESI-TOF-
MS) calcd for C34H52B2O3Si2 + Na 609.3546, found m/z = 609.3549
[2M − H2O + Na+], dehydrated dimer was observed.
(Epa = 1.05 vs Ag/Ag+) and 6 (Epa = 1.04 V) are observed at the
same potential in spite of the strained structure of 1.
Compound 2 shows double oxidation of two carbazole units
(Epa = 0.98 and 1.15 V), which is observed due to Coulomb
repulsion by a carbazole cation of the mono-oxidized species.
The above Epa value indicated that the HOMO level of 2 is
higher than that of 1. It is contrasted with strained π-conjugated
macrocycles such as [n]cycloparaphenylene,2d [n]-
cyclothiophene,4b and six-porphyrin nanoring4d that undergo
the first electrochemical oxidation at lower potentials than the
corresponding linear molecules due to their higher HOMO
level from ring strain.
The LUMO levels of 6, 2, and 1 are lowered in this order as
shown in Table 1. Thus, the energy gap of 1 is comparable to
that of 2 as expected from UV−vis spectra. Orbital energy
levels of 1 and 6 are compared with 1F and 6F (Table 1) that
have a fluorene group instead of the carbazole group of 1 and 6.
Although 1F (and 6F) is not prepared in this study, the
optimized structure of 1F is similar to that of 1, and the angle
between the five-membered ring and six-membered rings is
14.0°. The LUMO of 1F is delocalized on the diphenylbuta-
diyne skeleton, and its level is roughly the same as that of 1.
3,6-Bis(2-(triisopropylsilylethynyl)phenyl)carbazole (5). A mixture
of 2-(triisopropylsilylethynyl)benzeneboronic acid 4 (321 mg, 1.1
mmol), Pd(PPh3)4 (60.8 mg, 0.053 mmol), K2CO3 (368 mg, 2.7
mmol), 3,6-dibromocarbazole 3 (156 mg, 0.48 mmol), degassed water
(50 mL), and dimethoxyethane (50 mL) was stirred at 80 °C in argon
for 23 h. The reaction mixture was washed with water and brine and
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Note
then extracted with ethyl acetate. The solvent was evaporated under
vacuum. The residue was purified by silica gel column chromatography
(hexane/ethyl acetate 5:1) to give 3,6-bis(2-(triisopropylsilylethynyl)-
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: kosakada@res.titech.ac.jp.
1
phenyl)carbazole as a pale yellow oil (298 mg, 91%): H NMR (500
MHz, CDCl3) δ 8.42 (d, J = 1 Hz, 2H), 8.11 (s, 1H), 7.72 (dd, J = 8, 1
Hz, 2H), 7.64 (dd, J = 7, 1 Hz, 2H), 7.48 (dd, J = 8, 1 Hz, 2H), 7.43
(d, J = 8 Hz, 2H), 7.39 (ddd, J = 8, 1 Hz, 2H), 7.28 (ddd, J = 7, 1 Hz,
2H), 0.94 (m, 21 H); 13C NMR (125 MHz, CDCl3) δ 145.3, 139.5,
134.1, 132.4, 130.0, 128.7, 127.9, 126.5, 123.7, 122.4, 121.5, 110.0,
107.0, 93.7, 18.7, 17.9, 12.5, 11.5; HRMS (ESI-TOF-MS) calcd for
C46H57NSi2 + Na 702.3922, found m/z = 702.3916 [M + Na+].
3,6-Bis(2-ethynylphenyl)carbazole (6). To a solution of 3,6-bis(2-
(triisopropylsilylethynyl)phenyl)carbazole 5 (260 mg, 0.38 mmol) in
THF (8 mL) was added 1 M TBAF in THF (0.36 mL, 0.36 mmol) at
room temperature. After stirred for 21 h at room temperature, the
reaction mixture was washed with water (×2) and brine and then
extracted with ethyl acetate. The organic phase was dried over Na2SO4,
and the solvent was evaporated to dryness. Silica gel column
chromatography (hexane/ethyl acetate 1:4) of the residue gave 3,6-
bis(2-ethynylphenyl)carbazole as a pale yellow solid (118 mg, 84%):
1H NMR (500 MHz, CDCl3) δ 8.32 (d, J = 1 Hz, 2H), 8.15 (s, 1H),
7.68 (dd, J = 9, 2 Hz, 2H), 7.66 (d, J = 7 Hz, 2H), 7.51−7.43 (m, 4H),
7.44 (ddd, J = 7, 1 Hz, 2H), 7.31 (ddd, J = 7, 1 Hz, 2H); 13C NMR
(125 MHz, CDCl3) δ 145.2, 139.4, 133.9, 131.9, 130.0, 129.0, 127.5,
126.5, 123.4, 121.2, 120.6, 110.1, 83.6, 79.9; HRMS (ESI-TOF-MS)
calcd for C28H17N + Na 390.1253, found m/z = 390.1251 [M + Na+].
Macrocycles 1 and 2. 3,6-Bis(2-ethynylphenyl)carbazole 6 (97.0
mg, 0.27 mmol) and Cu(OAc)2·2H2O (299 mg, 1.6 mmol) were
dissolved in pyridine (20 mL) and diethyl ether (7 mL). The mixture
was stirred at room temperature under air for 44 h. The reaction
mixture was washed with saturated aqueous NH4Cl (×2) and then
extracted with ethyl acetate. The organic phase was dried over Na2SO4,
and the solvent was removed under vacuum. The residue was
subjected to silica gel chromatography (hexane/ethyl acetate 3:1 then
2:1) to afford 1 and 2 from different fractions. Each product was
washed with a small amount of acetonitrile to give 1 as a yellow solid
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by a Grant-in-Aid for Scientific
Research on Innovation Areas, “Coordination Programming”
(22108509) from the Ministry of Education, Culture, Sports,
Science, and Technology, Japan. T.I. acknowledges a Research
Fellowship for Young Scientists from the Japan Society for the
Promotion of Science.
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(35.4 mg, 38%) and 2 as an ivory solid (10.1 mg, 10%). 1: H NMR
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* Supporting Information
NMR spectra, crystallographic parameters, and computational
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