Synthesis, structural, and photophysical properties of the first member of the class of pyrene-based [4]helicenes

Article abstract of DOI:10.1002/ejoc.201300487

A convenient route to a new class of pyrene-based [4]helicenes is presented. Wittig reaction of 7-tert-butyl-1,3-dimethyl-5-formylpyrene with benzyltriphenylphosphonium salts in the presence of nBuLi afforded 7-tert-butyl-1,3-dimethyl-5-(phenylethenyl)pyrenes, from which 4,5-naphthalene annulated aromatic [4]helicenes, namely 7-tert-butyl-1,3-dimethyl-13- methoxydibenzo[ij,no]tetraphene and 7-tert-butyl-1,3,12,14- tetramethyldibenzo[ij,no]tetraphene, were obtained by photoinduced intramolecular cyclization. The chemical structures of these [4]helicenes were determined on the basis of their elemental analyses and spectroscopic data. The helicity in the synthesized [4]helicene induced by the presence of the second methyl group in the fjord region is discussed in detail. The photophysical and electrochemical properties of these newly developed [4]helicenes were fully investigated by UV/Vis absorption and photoluminescence spectrphotometry, and cyclic voltammetry (CV), and the crystal structures were determined for two examples. A convenient route to a new class of pyrene-based [4]helicenes is presented along with their optoelectronic properties. The introduction of two methyl groups in the fjord region of the [4]helicene gives a more distorted structure and leads to a remarkable redshift of the absorption band. Copyright

Full text of DOI:10.1002/ejoc.201300487

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FULL PAPER
DOI: 10.1002/ejoc.201300487
Synthesis, Structural, and Photophysical Properties of the First Member of the
Class of Pyrene-Based [4]Helicenes
Jian-Yong Hu,[a,b] Xing Feng,[a] Arjun Paudel,[a] Hirotsugu Tomiyasu,[a] Ummey Rayhan,[a]
Pierre Thuéry,[c] Mark R. J. Elsegood,[d] Carl Redshaw,[e] and Takehiko Yamato*[a]
Keywords: Fused-ring systems / Wittig reactions / Cyclization / Helical structures / Optoelectronic properties
A convenient route to a new class of pyrene-based [4]heli-
cenes is presented. Wittig reaction of 7-tert-butyl-1,3-di-
methyl-5-formylpyrene with benzyltriphenylphosphonium
salts in the presence of nBuLi afforded 7-tert-butyl-1,3-
dimethyl-5-(phenylethenyl)pyrenes, from which 4,5-naphth-
alene annulated aromatic [4]helicenes, namely 7-tert-
butyl-1,3-dimethyl-13-methoxydibenzo[ij,no]tetraphene and
7-tert-butyl-1,3,12,14-tetramethyldibenzo[ij,no]tetraphene,
were obtained by photoinduced intramolecular cyclization.
The chemical structures of these [4]helicenes were deter-
mined on the basis of their elemental analyses and spectro-
scopic data. The helicity in the synthesized [4]helicene in-
duced by the presence of the second methyl group in the
fjord region is discussed in detail. The photophysical and
electrochemical properties of these newly developed [4]heli-
cenes were fully investigated by UV/Vis absorption and pho-
toluminescence spectrphotometry, and cyclic voltammetry
(CV), and the crystal structures were determined for two ex-
amples.
fluoro derivative, which was less active.[7] In addition, ni-
tration, acetylation, and bromination of the parent I were
investigated by Newman in the 1940s; his studies showed
preferential substitution at C-5.[8]
Introduction
Although described in the early 20th century literature
by Weitzenbock and co-workers, as well as by Mayer and
Oppenheimer,[1] the isolation of the first tetrahelicene (I;
Figure 1) was not confirmed until the early 1930s when
Cook and Heweit reinvestigated their work.[2,3] Parent
benzo[c]phenanthrene (B[C]Ph, I) is the smallest polycyclic
aromatic hydrocarbon (PAH) with a fjord region. As the
first member of the [n]helicene family,[4] it has a twisted
framework with an angle of 27° between the A and D rings,
as shown by the X-ray structure.[5] Focusing on structure–
activity relationships, the 3-, 4-, 5-, and 6-methylated deriv-
atives of such compounds were found to be tumorigenic in
mouse skin, but the 1- and 2-methyl derivatives were re-
ported to be less active than the parent (B[C]Ph).[6] Fluorine
substitution on the benzo ring was found to exhibit en-
hanced tumorigenicity relative to parent I, except for the 2-
Figure 1. Structures of tetrahelicenes I and II.
The influence of planarity on the metabolic activation
and DNA-binding properties of PAHs were studied by
Lakshman et al. in 2000,[9] who reported that the increased
nonplanarity in this type of PAH lowered their ability to
be metabolically activated to form DNA-damaging adducts.
Interestingly, this report also described a convenient syn-
thetic route to 1,4-dimethylB[C]Ph (II), its (Ϯ)-trans-9,10-
dihyrodiol, as well as the (Ϯ)-9β,10α,1α-epoxide. Compara-
tive metabolic activation and DNA-binding of B[C]Ph, 1,4-
DMB[C]Ph, and their dihydrodiols have been investigated
by using mammary carcinoma MCF-7 cells. This study also
showed that a methyl group in the fjord region can induce
helicity in these small hydrocarbons, resulting in atropisom-
erism (P and M helicity). Methyl substitution in the highly
congested fjord region increased skeletal distortion from
27° for the A/D ring angle in B[C]Ph (I) to 30° for that in
1,4-DMB[C]Ph (II).[10]
[a] Department of Applied Chemistry, Faculty of Science and
Engineering, Saga University,
Honjo-machi 1, Saga-shi, Saga 840-8502, Japan
E-mail: yamatot@cc.saga-u.ac.jp
Homepage: http://www.fusion.saga-u.ac.jp
[b] Emergent Molecular Function Research Group, RIKEN
Center for Emergent Matter Science (CEMS),
Wako, Saitama 351-0198, Japan
[c] Service de Chimie Moléculaire, DSM, DRECAM, CNRS URA
331, CEA Saclay,
91191 Gif-sur-Yvette, France
[d] Chemistry Department, Loughborough University,
Loughborough, LE11 3TU, UK
[e] Department of Chemistry, The University of Hull,
Hull, HU6 7RX, UK
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201300487.
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Photochemical ring closure of appropriate stilbenes to constant (J = 1.8 Hz) at δ = 8.25 (8-H) and 9.73 (6-H) ppm,
phenanthrenes has proven to be a useful method for the as well as two singlets at δ = 7.69 (2-H) and 8.61 (4-H) ppm.
synthesis of several angular-fused PAHs,[11] and as an exten- Similarly, a set of doublets with an ortho coupling constant
sion of this method, the synthesis of fjord-region dihydrodi- (J = 9.2 Hz) at δ = 8.02 and 8.12 ppm were assigned to
ols has been documented.[12] However, for this study, the the protons at the 9- and 10-positions on the pyrene ring,
suitability of this method was not known because the intro- respectively. The singlet peak at δ = 10.49 ppm was assigned
duction in the fjord region of at least one methyl group in to the formyl proton. These data strongly support a struc-
the course of pyrene-based photocyclization had not then ture of 7-tert-butyl-1,3-dimethylpyrene-5-carbaldehyde (2),
been documented.
and strongly suggest that the tert-butyl group on the pyrene
Electrophilic substitution of pyrene only occurs at the ring protects the pyrene from electrophilic attack at the 6,8-
1-, 3-, 6-, and 8-positions, and not at the other positions (2, positions, as well as the methyl groups at the 1,3-positions
4, 5, 7, 9, and 10).[13,14] Therefore pyrenes substituted at the inhibiting electrophilic attack at the 4,10-positions.
latter positions must be prepared in ways other than by di-
The reaction of 2 and (4-methoxybenzyl)triphenylphos-
rect electrophilic substitution of pyrene itself. Regioselective phonium chloride (3) with n-butyllithium in THF gave
electrophilic substitution at the 5- or 5,9-positions is still the desired 7-tert-butyl-1,3-dimethyl-5-(4-methoxyphenyl-
challenging. We previously reported the convenient prepar- ethenyl)pyrene [(E)-4; Scheme 2]. Only the E isomer was
ative synthesis of 7-tert-butyl-1,3-dimethylpyrene from isolated in 72% yield by silica gel column chromatography
pyrene in five steps, which involved the formylation of and recrystallization from hexane and dichloromethane.
7-tert-butyl-1,3-dimethylpyrene and Wolff–Kishner re- The structure of (E)-4 was determined by elemental analysis
duction.[15,16] This compound is a convenient starting mate- and spectroscopic data. In 1H NMR spectroscopy, a singlet
rial for the preparation of 5- and 5,9-disubstituted 1,3-di- from the olefinic protons of the E isomer should be ob-
methylpyrenes by electrophilic substitution because the served at a lower field (δϾ 7.4 ppm) than those of the Z
active 6- and 8-positions of the pyrene ring are protected olefinic protons (δϽ 6.9 ppm).[17] As expected, the 1H
by the tert-butyl group.
NMR spectrum (300 MHz, CDCl3) of 4 shows a pair of
In this work, the electrophilic aromatic substitution of 7- doublets (J = 15.9 Hz) at δ = 7.33 and 7.92 ppm for the E-
tert-butyl-1,3-dimethylpyrene selectively afforded 5-mono- olefinic protons and a singlet at δ = 3.88 ppm for the meth-
formyl substitution product depending on the Lewis acid oxy protons. Similarly, two methyl protons on the pyrene
catalysts that were used for the preparation of the first ring are observed as singlets at δ = 2.94 and 2.98 ppm. The
members of a new class of pyrene-based [4]helicenes. These structure of compound 4 was also established by the molec-
contain at least one methyl group in the fjord region. Their ular ion at m/z = 418 in its mass spectrum.
photophysical and electrochemical properties will be pre-
sented in full.
Results and Discussion
Synthesis
7-tert-Butyl-1,3-dimethylpyrene (1) was prepared accord-
ing to the previously reported procedure.[15,16] Pyrene 1 was
then formylated with dichloromethyl methyl ether under
various conditions. Under the optimum conditions, the re-
action took place at room temperature for 3 h in the pres-
ence of titanium tetrachloride selectively at the 5-position
to give the corresponding 5-monoformylated product 2 in
69% yield (Scheme 1).
Scheme 2. Synthetic route to 7-tert-butly-1,3-dimethyl-13-methoxy-
dibenzo[ij,no]tetraphene (5).
When a solution of (E)-4 (50 mg, 0.12 mmol) and a stoi-
chiometric amount of iodine (31 mg, 0.12 mmol) in benzene
(260 mL) was irradiated with a high-pressure mercury lamp
Scheme 1. Formylation of 7-tert-butyl-1,3-dimethylpyrene (1).
(400 W) at room temperature for 6 h, the photocyclized
The structure of 2 was assigned by spectroscopic data product, 7-tert-butyl-1,3-dimethyl-13-methoxydibenzo[ij,no]-
and elemental analysis. The 1H NMR spectrum (300 MHz, tetraphene (5), was obtained in only 10% yield along with
CDCl3) of 2 shows a set of doublets with a meta coupling recovered starting compound. Prolonging the reaction time
2
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Pyrene-Based [4]Helicenes
to 12 h led to an increase in the yield to 25%. Irradiation = 2.94 and 2.98 ppm for the two pyrene methyl groups. This
with a stoichiometric amount of iodine plus a large amount NMR spectrum also shows a pair of doublets (J = 15.3 Hz)
of propylene oxide[18,19] (1.53 mL, 21.1 mmol) in the ab- at δ = 7.30 and 8.01 ppm for the E-olefinic protons, which
sence of air led to an increase in yield to 70% (Scheme 2).
shows the compound to be in the E configuration.
The structure of product 5 was determined by elemental
When a solution of (E)-7 (50 mg, 0.12 mmol) and a stoi-
analyses and spectroscopic data. Its mass spectrum shows chiometric amount of iodine (31 mg, 0.12 mmol) in the
a molecular ion at m/z = 416. The 1H NMR spectrum presence of propylene oxide (1.53 mL, 21.1 mmol) in benz-
(300 MHz in CDCl3) of 5 shows a singlet at δ = 1.62 ppm ene (260 mL) was irradiated with a high-pressure mercury
for the tert-butyl group, two singlets at δ = 2.50 (1-Me) and lamp (400 W) under the same conditions as described
2.97 (3-Me) ppm for two methyl groups, and a singlet at δ above, the photocyclization product, 7-tert-butyl-1,3,12,14-
= 3.90 ppm for a methoxy group. Compared with the tetramethyldibenzo[ij,no]tetraphene (8), was obtained in
methyl peaks of (E)-4, the methyl peak at δ = 2.50 ppm for 40% yield as a white crystalline solid. The low yield of the
5 is more shielded by 0.44 ppm. Similarly, the 1H NMR compound is a result of the substitution of two methyl
spectrum of compound 5 shows a pair of doublets (J = groups in the fjord region, which leads to pronounced steric
9.3 Hz) at δ = 8.70 and 7.69 ppm for 9-H and 10-H, and a hindrance during the cyclization reaction.
pair of doublets (J = 9.0 Hz) at δ = 8.02 and 8.20 ppm for
The structure of compound 8 was determined by elemen-
4-H and 5-H, respectively. A set of characteristic double tal analysis, spectroscopic data, and the presence of the mo-
doublets at δ = 7.22 ppm (dd, J = 2.4, 2.4 Hz) was assigned lecular ion at m/z = 414 in its mass spectrum. The 1H NMR
to 12-H. Similarly, 14-H can clearly be seen to be deshielded spectrum (300 MHz, CDCl3) of 8 shows four distinct sing-
at δ = 8.03 ppm by the adjacent rings, a common conse- lets at δ = 1.82 (14-Me), 2.06 (12-Me), 2.59 (1-Me), and
quence of extended benzene rings. Furthermore, the helical 2.96 (3-Me) ppm for the protons of four methyl groups, two
structure of compound 5 was confirmed by its single-crystal sets of doublets (J = 8.7 Hz) at δ = 7.98 and 8.74 ppm for
X-ray analysis (see Figure 7).
10-H and 9-H, and two sets of doublets (J = 9.1 Hz) at δ =
To investigate this helicity in more detail, two methyl 8.04 and 8.21 ppm for 5-H and 4-H, respectively (Figure 2).
groups were introduced into the fjord regions of the synthe- As expected, the protons of the two methyl groups in the
sized [4]helicene. An additional methyl group at the 14-posi- fjord region of the [4]helicene structure are significantly
tion could lead to increased skeletal distortion, even more shielded at δ = 1.82 (14-Me) and 2.59 (1-Me) ppm by the
than that reported in benzo[c]phenanthrene.[20] Thus, the adjacent rings, and there is an absence of meta coupling in
reaction of 2 and (3,5-dimethylbenzyl)triphenylphosphon- the aromatic protons, a common consequence of the distor-
ium bromide (6) with n-butyllithium in THF gave the de- tion from planarity of peripheral rings.[20]
sired 7-tert-butyl-1,3-dimethyl-5-(3,5-dimethylphenylethyn-
yl)pyrene [(E)-7] in 75% yield (Scheme 3).
1
Figure 2. H NMR spectra (300 MHz, CDCl3) of 8.
Scheme 3. Synthetic route to 7-tert-butly-1,3,12,14-tetramethydi-
benzo[ij,no]tetraphene (8).
Photophysical and Electrochemical Properties
The new [4]helicenes 5 and 8 are very soluble in common
The structure of compound (E)-7 was established by ele- organic solvents, such as hexane, CH2Cl2, CHCl3, and tolu-
mental analysis, spectroscopic data, and the molecular ion ene, owing to the presence of the solubilizing tert-butyl
1
at m/z = 416 in its mass spectrum. The H NMR spectrum group. The UV/Vis absorption and fluorescence emission
(300 MHz, CDCl3) of (E)-7 shows a singlet at δ = 2.41 ppm spectra of the [4]helicenes 5 and 8, and the pre-cyclization
for six protons of the methyl groups and two singlets at δ products,
7-tert-butyl-1,3-dimethyl-5-(phenylethenyl)pyr-
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enes 4 and 7, were recorded in dilute dichloromethane the differences in the π-conjugation systems caused by the
solution at room temperature (Figures 3, 4, and 5). The cyclization reaction. The pronounced hypochromic absorp-
spectroscopic data are presented in Table 1, together with tion of the absorption bands at 350–400 nm can be ascribed
those of 7-tert-butyl-1,3-dimethylpyrene (1). The absorption to the absence of the phenylethenyl unit, the decrease in
spectrum of 7-tert-butyl-1,3-dimethylpyrene (1) is almost aromaticity, the increase in distortions from planarity, and,
identical to that of the parent pyrene with three well-re- consequently, the decrease in π conjugation after cycliza-
solved, sharp absorption bands observed in the region 300– tion. The UV spectra of 5 and 8 in CH2Cl2 show a large
350 nm. The slight bathochromic shift observed is ascribed number of transition bands (Figure 3), typical of PAHs.[23]
to the increased electron density on the pyrene ring arising
from the electron-donating nature of the tert-butyl and
methyl groups at the 7- and 1,3-positions, respectively. The
substituted pyrene 1 is more bathochromically redshifted
than 2,7-di-tert-butylpyrene (9)[21] because the alkyl groups
are in different positions on the pyrene core and the nodal
planes in the HOMO and LUMO of pyrene pass through
the 2,7-positions, and thus substitution at the 2,7-positions
has a limited effect on the optical properties.[22]
Figure 3. Normalized UV/Vis absorption spectra of 4 and 7 (before
cyclization), and 5 and 8 (after cyclization), and, for comparison,
1, recorded at a concentration of ca. 10–5 m at room temperature.
The absorption band at 404 nm for 5 due to the methoxy
group also indicates a departure of the aromatic rings from
The UV/Vis absorption spectra of compounds 4 and 7 planarity. On the other hand, the different shape of the UV
(before cyclization) are broad, less well resolved, and the spectrum of 8 confirms the increase in the nonplanarity of
longest-wavelength hyperchromic absorption maxima of 4 the aromatic rings to avoid steric crowding by overlapping
and 7 occur at 368 and 373 nm, respectively (Table 1). These methyl groups at C-1 and C-14.
are redshifted by 15–19 nm relative to 1 due to the introduc-
Upon excitation, dilute solutions (ca. 10–6 m) of com-
tion of the phenylethenyl unit at the 5-position leading to pounds 4, 7, and 1 in dichloromethane at room temperature
an extension of the π conjugation. In addition, of these show a broad blue emission band (Figures 4 and 5). Com-
compounds, the 3,5-dialkyl-substituted phenylethenyl pared with the emission band of 7-tert-butyl-1,3-dimeth-
pyrene 7 displays a slightly greater bathochromic shift (ca. ylpyrene (1) at 403 nm, the emission bands of 4 and 7 are
5 nm) compared to the (4-methoxyphenylethenyl)pyrene 4, redshifted to 433 and 455 nm, respectively. Both com-
which can be attributed to the stronger electron-donating pounds were observed in the visible pure-blue region. All
nature of the two alkyl groups than that of the methoxy the fluorescence emission bands are broad but not identical;
group.
the emission band of 4 shows a small shoulder at 452 nm.
The UV spectra of the dibenzo[ij,no]tetraphene deriva- The emission band of compound 7 is broad but only
tives 5 and 8 are almost identical, with the absorption slightly redshifted (ca. 22 nm) compared with 4, which has
bands observed in the range of 300–410 nm (Figure 3). been ascribed to the high alkyl substitution in the com-
However, the quite different shapes of the UV spectra of 4 pound, and therefore 7 exhibits a higher Stokes shift than
and 7, and the UV spectra of 5 and 8 can be ascribed to 4.
Table 1. Photophysical and electrochemical data for 1, 4, 7, 5, and 8.
[e]
λmax abs [nm]
λmax PL [nm][c]
Stokes shift [nm]
Soln.[a] Film[b] Soln./thin film/doped film
Φf[d]
Eg
HOMO[f]
[eV]
LUMO[g]
[eV]
Soln.[a] Film[b]
Soln.[a]
Film[b]
[eV]
1
4
7
5
8
353
368
373
371
382
n.d.
n.d.
n.d.
370
381
403 (222)
433 (222)
455 (221)
419 (220)
422 (219)
n.d.
n.d.
n.d.
414 (320)
450 (315)
50
65
83
48
40
n.d.
n.d.
n.d.
44
0.09/n.d./n.d.
0.77/n.d./n.d.
0.88/n.d./n.d.
0.11/0.08/0.10
0.13/0.07/0.12
n.d.
n.d.
n.d.
3.12
3.04
n.d.
n.d.
n.d.
–5.76
–5.72
n.d.
n.d.
n.d.
–2.64
–2.68
69
[a] Measured in dichloromethane at room temperature. [b] Measured in thin neat films. n.d.: not detected. [c] PL: photoluminescence.
The values in parentheses are the excitation wavelengths. [d] Measured in dichloromethane, thin neat films, and 1 wt.-% doped poly(methyl
methacrylate) (PMMA) films. [e] Optical band gap estimated from the onset point (onset) of the absorption band of the thin film: Eg =
1240/λonset. [f] Calculated from the oxidation potentials. [g] Calculated from the HOMO energy levels and Eg.
4
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emission spectra. Upon excitation, the emission band of
2,7-di-tert-butyldibenzo[ij,no]tetraphenes (10)[21] are red-
shifted in comparison with cyclized 5 and 8; the consider-
able hypsochromic blueshift of 5 and 8 is ascribed to their
more twisted structure.
Figure 4. Normalized fluorescence spectra of 4 (before cyclization)
and 5 (after cyclization), and, for comparison, 1, recorded at a
concentration of ca. 10–6 m at room temperature.
The normalized UV/Vis absorption spectra and emission
spectra of 5 and 8 in thin neat films are shown in Figure S3–
1 in the Supporting Information, and the spectroscopic data
are summarized in Table 1. In the UV spectra of both 5 and
8, a small hypsochromic shift (ca. 1 nm) is observed relative
to the corresponding spectra in solution, which indicates
that these two compounds have similar conformations in
both states.[24] Interestingly, compared with their corre-
sponding spectra in solution, the photoluminescence spec-
tra of 5 and 8 show a main emission peak at 414 nm with
a small hypsochromic shift (ca. 5 nm) for 5 and a main
emission peak at 450 nm with a large bathochromic shift
(ca. 28 nm) for 8, respectively. However, no excimer emis-
sion is observed in the emission spectra of 5 and 8 owing
to their twisted molecular structures after cyclization (see
Figure S3–1). We also examined the fluorescence emission
spectra of thin films of poly(methyl methacrylate) (PMMA)
doped with 1 wt.-% 5 and 8.[25] The spectra each show deep-
blue emission with a maximum peak at 414 and 419 nm
for 5 and 8, respectively (Figure 6), almost identical to the
corresponding emissions in solution, which indicates that
the π stacking of the pyrene units may be inhibited in doped
films. These results suggest that these newly developed pyr-
Figure 5. Normalized fluorescence spectra of 7 (before cyclization)
and 8 (after cyclization), and, for comparison, 1, recorded at ca.
10–6 m concentration room temperature, compared with that of 1.
The emission spectra of compounds 5 and 8 (after cycli-
zation) in CH2Cl2 are also shown in Figure 4 and Figure 5,
and the data are presented in Table 1. Compared with the
band of 1, the bands (major) of 5 and 8 at 419 and 422 nm,
respectively, are redshifted. The fluorescence emission
bands are sharper than those of the starting compounds 4
and 7 with a small shoulder at longer wavelength.
Remarkably high quantum yields are observed for com-
pounds 4 and 7 because of greater π conjugation along the
phenylethenyl unit at the 5-position in the pyrene ring
(Table 1). The high fluorescence quantum yields of 4 and
7 are due to the electron-donating groups attached to the
phenylethenyl unit. The fluorescence quantum yields of 5
and 8 are low compared with those of 4 and 7 because
of low aromaticity after cyclization and consequently lower
electron delocalization around the aromatic core. Although
the shift is not pronounced, the maximum emission bands
of 5 and 8 are less redshifted compared with those of 4 and
7. The less redsifted values of 5 and 8 are due to the substi-
tuted groups causing a loss of co-planarity after cyclization.
On the other hand, the different structural characteristics
Figure 6. Emission spectra of thin films of PMMA doped with
of the compounds after cyclization are clearly observed in 1 wt.-% 5 and 8.
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ene-based [4]helicenes 5 and 8 might be promising candi- of the helicene 5 (Figure 7) contains long and short C–C
dates in deep-blue organic light-emitting diodes bonds. The longer bonds are those mostly affected by intra-
(OLEDs).[26]
molecular torsion (namely C3–C4, C4–C11, C11–C12,
The fluorescence quantum yields of 4, 7, 5, and 8 re- C12–C27), with an average of 1.43 Å. Note that in the pe-
corded in dilute dichloromethane solutions and thin films ripheral rings, the C8–C9 bond length [1.363(4) Å] is signifi-
are also listed in Table 1, together with those of 1. The Φf cantly shorter than its counterpart C4–C11 [1.461(4) Å],
values of the cyclized [4]helicenes 5 and 8 are moderate, which experiences more twisting to reduce steric hindrance
ranging from 0.07 to 0.13, whereas the Φf values of the pre- with the more rigid pyrene core. The torsion angles along
cyclization products 4 and 7 are quite high, ranging from the inner helical rim of the fjord region C27–C12–C11–C4
0.77 to 0.88. The low quantum yields obtained in solution and C12–C11–C4–C3 are –18.5(5) and –21.5(5)°, respec-
and in thin films for the [4]helicenes 5 and 8 indicate that tively. Nonbonding distances [Å] were calculated to be
excitons are not confined to the backbone of these mo- 5.951 Å for C30–O1, 5.397 Å for C27–C2, and 2.991 Å for
lecules due to their non-planar twisted molecular struc- C12–C6. These values are also a convenient measure of the
tures; some energy loss may occur during the exciton mi- helicity.[9]
grations,[27] thereby resulting in low quantum yields. How-
ever, the high fluorescence quantum yields for the pre-cycli-
zation products 4 and 7 in solution indicate that the exci-
tons are completely confined to the backbone of 4 and 7,
arising from the extended π-delocalization between the rigid
phenylethenyl groups and the central pyrene core, thereby
giving higher quantum yields.
To investigate the redox behavior of these new [4]heli-
cenes and estimate their frontier molecular orbital levels we
carried out cyclic voltammetry (CV) experiments on 5 and
8 by using a conventional three-electrode cell with 0.1 m
tetrabutylammonium hexafluorophosphate (Bu4NPF6) as
supporting electrolyte in dichloromethane and ferrocene as
the internal standard. The cyclic voltammograms are shown
in Figure S4–1 in the Supporting Information. Both 5 and
8 exhibit one reversible oxidation process in the anodic
sweep, but no reduction signal was detected for either of
these [4]helicenes during the cathodic scan. As shown in
Figure 7. X-ray structure of compound 5.
A similar molecular geometry is observed for the helical
skeleton of 8 with the corresponding torsion angles along
the inner helical rim of the fjord region C13–C12–C11–C33
and C12–C11–C33–C32 being 21.8(3) and 33.2(3)°, respec-
tively (Figure 8). These values indicate a more twisted con-
formation due to the bulkier Me groups at C13 and C32 in
8 (Figure 8, b, 47.83°) compared with those of the H atoms
at C3 and C27 in 5 (Figure 7, b, 41.42°).
Figure S4–1, the oxidation wave of 8 is shifted to a less posi-
tive potential compared with that of 5. This implies that the
removal of the first electron from 8 is easier than from 5.
The half-potentials (E1/2) of the first oxidation waves are
1.36 and 1.32 V, respectively, relative to ferrocene. The
HOMO energy levels were estimated according to the equa-
tion of HOMO = –(E1/2 + 4.4 eV) to be –5.76 and –5.72 eV
for 5 and 8, respectively.[28] These values further indicate
that these [4]helicenes are potential deep-blue emitters for
optoelectronic device applications such as OLEDs.[29] The
energies of the band gaps were determined by the absorp-
tion edge technique,[28] and the LUMO levels calculated by
subtracting the gap from the energy of the HOMO. The
electrochemical and electronic data for 5 and 8 are pre-
sented in Table 1.
X-ray Molecular Structures
To investigate the molecular structures of the newly de-
veloped pyrene-based [4]helicenes in the solid state, we ob-
tained suitable crystals of 5 and 8 by vapor diffusion of
hexane into their dichloromethane solutions. The crystallo-
graphic data for compounds 5 and 8 are summarized in the
Exp. Sect. and in Table S1 in the Supporting Infor-
mation.[30] The molecular geometry of the helical skeleton Figure 8. X-ray structure of compound 8.
6
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Pyrene-Based [4]Helicenes
CHO) ppm. HRMS (FAB, EI): calcd. for C31H30O [M]+ 314.1671;
found 314.1670. C23H22O (314.42): calcd. C 87.86, H 7.05; found
Conclusions
The formylation of 7-tert-butyl-1,3-dimethylpyrene (1) C 87.85, H 7.06.
with dichloromethyl methyl ether in the presence of TiCl4
successfully afforded 7-tert-butyl-1,3-dimethylpyrene-5-carb-
aldehyde (2). Wittig reaction of the 5-formylated pyrene 2
7-tert-Butyl-1,3-dimethyl-5-(4-methoxyphenylethenyl)pyrene [(E)-4]:
The Wittig reagent was prepared from triphenylphosphane and 4-
methoxybenzyl chloride in dry benzene. n-Butyllithium in hexane
with (4-methoxybenzyl)triphenylphosphonium chloride and (1.20 mL, 1.89 mmol) was slowly added to a solution of this Wittig
reagent (794 mg, 1.89 mmol) in dry THF (15 mL) at 0 °C under
argon. The mixture was stirred for 10 min and a solution of 2
(200 mg, 0.632 mmol) in dry THF (15 mL) was injected under the
same conditions. After the addition, the mixture was warmed to
room temperature, stirring for 6 h under argon. The mixture was
quenched by a large amount of ice/water and extracted with ethyl
acetate (2ϫ 100 mL). The combined extracts were washed with
water, dried with MgSO4, and concentrated. The residue was puri-
fied by column chromatography over silica gel (Wako C-300, 200 g)
with hexane/dichloromethane (5:1) as eluent to give (E)-4 (NMR
(3,5-dimethylbenzyl)triphenylphosphonium bromide gave
the corresponding arylethenylpyrenes 4 and 7. Photoin-
duced intramolecular cyclization of these arylethenylpyr-
enes in the presence of iodine and propylene oxide led to
the corresponding dibenzo[ij,no]tetraphene derivatives 5
and 8 containing methyl groups in the fjord region. As ex-
pected, the presence of two methyl groups in the fjord re-
gion of the [4]helicene 8 gives a more distorted structure.
The photophysical and electrochemical properties of these
compounds were fully examined in solution and in thin analysis) as a light-yellow solid. Recrystallization from hexane/
dichloromethane (5:1, v/v) afforded the anti compound (E)-4
(190 mg, 72%); m.p. 150–152 °C. IR (KBr): ν = 2961, 1601,
films. Further studies on the chemical and physical proper-
ties of the new class of pyrene-based [4]helicenes are now
in progress.
˜
max
1504, 1455, 1247, 1171, 1025, 956, 880, 804, 565 cm–1. 1H NMR
(300 MHz, CDCl3): δ = 1.59 (s, 9 H, tBu), 2.94 (s, 3 H, Me), 2.98
(s, 3 H, Me), 3.88 (s, 3 H, OMe), 7.00 (d, J = 8.7 Hz, 2 H, Ar-H),
7.33 (d, J = 15.9 Hz, 1 H, -CH=CHa-), 7.65 (d, J = 8.7 Hz, 2 H,
Ar-H), 7.70 (s, 1 H, 2-HPy), 7.92 (d, J = 15.9 Hz, 1 H, -CHb=CH-
), 8.02 (d, J = 9.0 Hz, 1 H, 9-HPy), 8.18 (d, J = 9.0 Hz, 1 H, 10-
HPy), 8.21 (d, J = 1.8 Hz, 1 H, 8-HPy), 8.36 (s, 1 H, 4-HPy), 8.48
(d, 1 H, J = 1.8 Hz, 6-HPy) ppm. 13C NMR (75 MHz, CDCl3): δ =
19.7, 32.0, 35.3, 55.4, 114.2, 118.4, 121.1, 122.2, 123.5, 125.1, 126.5,
127.4, 127.5, 128.0, 130.0, 130.7, 131.2, 131.5, 131.8, 133.6, 148.5,
159.4 ppm. HRMS (FAB, EI): calcd. for C31H30O [M]+ 418.2297;
found 418.2299. C31H30O (418.57): calcd. C 88.95, H 7.22; found
C 88.96, H 7.20.
Experimental Section
General: 1H NMR spectra were recorded with a Nippon Denshi
JEOL FT-300 spectrometer. Chemical shifts are reported as δ val-
ues (ppm) relative to internal Me4Si. IR spectra were recorded as
KBr pellets with a Nippon Denshi JIR-AQ2OM spectrometer. UV/
Vis spectra were recorded with a Perkin–Elmer Lambda 19 UV/Vis/
NIR spectrophotometer in various organic solvents. Fluorescence
spectroscopic studies were performed in various organic solvents
in a semimicro fluorescence cell (Hellma®, 104F-QS, 10ϫ 4 mm,
1400 μL) with a Varian Cary Eclipse spectrophotometer. Fluores-
cence quantum yields were determined by using absolute methods.
Mass spectra were obtained with a Nippon Denshi JMS-HX110A
Ultrahigh Performance mass spectrometer at 75 eV by using a di-
rect-inlet system. Elemental analyses were performed with a Yan-
aco MT-5 analyzer. GLC analyses were performed by using a Shi-
madzu GC-14A gas chromatograph (silicone OV-1, 2 m; pro-
grammed temperature rise, 12 °Cmin–1; nitrogen carrier gas,
25 mLmin–1). Electrochemical properties and the HOMO and
LUMO energy levels were determined by using an Electrochemical
Analyzer.
7-tert-Butyl-1,3-dimethyl-5-(3,5-dimethylphenylethenyl)pyrene [(E)-
7]: The Wittig reagent was prepared from triphenylphosphane and
3,5-dimethylbenzyl bromide in dry benzene. n-Butyllithium in hex-
ane (0.933 mL, 1.50 mmol) was slowly added to a solution of this
Wittig reagent (692 mg, 1.50 mmol) in dry THF (15 mL) at 0 °C
under argon. The mixture was stirred for 10 min and the solution
of 2 (316 mg, 1 mmol) in dry THF (20 mL) was injected under the
same conditions. After the addition, the mixture was warmed to
room temperature, stirring for 6 h under argon. The mixture was
quenched by a large amount of ice/water and extracted with ethyl
acetate (2ϫ 100 mL). The combined extracts were washed with
water, dried with MgSO4, and concentrated. The residue was puri-
fied by column chromatography over silica gel (Wako C-300, 200 g)
with hexane/ethyl acetate (5:1) as eluent to give (E)-7 (1H NMR
analysis) as a light-yellow solid. Recrystallization from hexane/tolu-
ene (2:1) and washing with hexane afforded the anti compound (E)-
The preparation of 7-tert-butyl-1,3-dimethylpyrene (1) has been de-
scribed previously.[15,16]
7-tert-Butyl-1,3-dimethylpyrene-5-carbaldehyde (2): A solution of ti-
tanium tetrachloride (0.10 mL, 0.91 mmol) in CH2Cl2 (1 mL) was
added to a stirred solution of 1 (106 mg, 0.37 mmol) and dichlo- 7 (215 mg, 75%); m.p. 106 °C. IR (KBr): ν
romethyl methyl ether (74 mg, 0.64 mmol) in CH2Cl2 (5 mL) at
0 °C. This mixture was stirred for 2 h at room temperature. The (300 MHz, CDCl3): δ = 1.59 (s, 9 H, tBu), 2.41 (s, 6 H, Ar-Me),
= 2961, 1603, 1478,
˜
max
1460, 1360, 1206, 1106, 1016, 873, 805, 472 cm–1. 1H NMR
mixture was poured into a large amount of ice/water and extracted
with CH2Cl2 (2ϫ 25 mL). The combined organic layers were
washed with water (2ϫ 25 mL), dried with MgSO4, and concen-
trated under reduced pressure. The residue was recrystallized from
hexane to afford 2 as pale-yellow prisms (80 mg, 69%); m.p. 268–
2.94 (s, 3 H, Py-Me), 2.98 (s, 3 H, Py-Me), 7.00 (s, 2 H, Ar-H),
7.30 (d, J = 15.3 Hz, 1 H, -CH=CHa-), 7.33 (s, 1 H, Ar-H), 7.70
(s, 1 H, 2-HPy), 8.01 (d, J = 15.3 Hz, 1 H, -CHb=CH-), 8.05 (d, J
= 9.0 Hz, 1 H, 9-HPy), 8.18 (d, J = 9.0 Hz, 1 H, 10-HPy), 8.21 (d,
J = 1.8 Hz, 1 H, 8-HPy), 8.36 (s, 1 H, 4-HPy), 8.48 (d, 1 H, J =
269 °C. IR (KBr): ν
= 2964, 2946, 1671 (C=O), 1589, 1503, 1.8 Hz, 6-HPy) ppm. 13C NMR (75 MHz, CDCl3): δ = 21.4, 30.9,
˜
max
1471, 1460, 1363, 1231, 1163, 898, 747, 472, 422 cm–1. 1H NMR
(300 MHz, CDCl3): δ = 1.62 (s, 9 H, tBu), 2.93 (s, 3 H, Me), 2.95 129.6, 129.9, 130.0, 131.4, 131.6, 131.9, 132.0, 133.7, 137.8, 138.3,
(s, 3 H, Me), 7.69 (s, 1 H, 2-HPy), 8.02 (d, J = 9.2 Hz, 1 H, 9-HPy),
148.5 ppm. HRMS (FAB, EI): calcd. for C32H32 [M]+ 416.2504;
8.12 (d, J = 9.2 Hz, 1 H, 10-HPy), 8.25 (d, J = 2.0 Hz, 1 H, 8-HPy), found 416.2496. C32H32 (416.60): calcd. C 92.26, H 7.14; found C
8.61 (s, 1 H, 4-HPy), 9.73 (d, J = 2.0 Hz, 1 H, 6-HPy), 10.49 (s, 1 H, 92.27, H 7.13.
32.0, 35.4, 118.5, 121.5, 122.2, 123.5, 124.7, 126.6, 126.9, 127.4,
Eur. J. Org. Chem. 0000, 0–0
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T. Yamato et al.
FULL PAPER
General Procedure for Photocyclization: The photoreactor was a cy-
lindrical glass vessel with an immersion well and two tapered joints.
A vertical joint was attached to a condenser to which an argon
source was fitted. The other joint was angled for withdrawal and
addition of samples. The vessel was flat-bottomed to allow a mag-
netic stirring bar to rotate. The immersion well was a double-walled
Pyrex tube cooled by water and containing a high-pressure quartz
Hg vapor lamp. Argon gas was bubbled through benzene for 20–
30 min, which was used to dissolve the sample and iodine. The
solution of pyrene, iodine, and propylene oxide was added to the
reaction vessel through the angled joint and the lamp was switched
on. The reaction was carried out under argon. The photoreactions
134.4, 135.2, 136.4, 146.9, 148.7 ppm. HRMS (FAB, EI): calcd. for
C32H30 [M]+ 414.2347; found 414.2356. C32H32 (414.58): calcd. C
92.71, H 7.29; found C 92.70, H 7.30. The structure of 8 was sup-
ported by single-crystal X-ray structure analysis.
Crystallographic Data of 5 and 8[30]
Crystal Data for 5: C31H28O, Mr = 416.53, monoclinic, P21/c, a =
30.160(2), b = 5.8496(3), c = 25.6873(14) Å, ß = 101.737(3)°, V =
4437.1(4) Å3, Z = 8, Dc = 1.247 gcm–3, μ(Mo-Kα) = 0.073 mm–1,
T = 150(2) K, colorless lath; 125835 reflections measured with a
Nonius–Kappa CCD diffractometer, of which 8336 are indepen-
dent; data corrected for absorption on the basis of symmetry equiv-
alent and repeated data (min. and max. transmission factors: 0.988
and 0.997) and Lorentzian and polarization effects, Rint = 0.0776,
structure solved by direct methods (Sir2002),[31] F2 refinement, R1
= 0.0633 for 4000 data with F2 Ͼ 2σ(F2), wR2 = 0.1692 for all data,
614 parameters. There are two molecules in the asymmetric unit,
one of which was modeled with two-fold disorder of the methyl
groups in the iBu group containing C54 (in the asymmetric unit of
the second molecule, not shown in Figure 7).
1
were monitored by H NMR spectroscopy and the change in color
of the iodine. After complete irradiation, work-up was performed
which included washing with 15% Na2S2O3·H2O and saturated
brine, drying with anhydrous MgSO4, filtering, and concentration
to dryness on a rotary evaporator. The residue obtained was either
washed through a short column of silica gel or different solvent
systems were used to obtain the pure compounds.
7-tert-Butyl-1,3-dimethyl-13-methoxydibenzo[ij,no]tetraphene (5): 7-
tert-Butyl-1,3-dimethyl-5-(4-methoxyphenylethenyl)pyrene [(E)-4,
50 mg, 0.12 mmol] in benzene (260 mL) was irradiated in the pres-
ence of I2 (31 mg, 0.12 mmol) and propylene oxide (1.53 mL,
21.1 mmol) for 6 h. Following work-up, the mixture was adsorbed
on silica gel by using dichloromethane and purified by column
chromatography with hexane as eluent. The viscous material ob-
tained from the column was dissolved in diethyl ether and left over-
night to leave a white solid. The compound was further recrys-
tallized from diethyl ether and washed with hot hexane to afford 5
¯
Crystal Data for 8: C32H30, Mr = 414.56, triclinic, P1, a =
8.2687(5), b = 8.4703(5), c = 16.9751(11) Å, α = 98.2723(9), β =
91.1145(9), γ = 110.5735(9)°, V = 1098.35(12) Å3, Z = 2, Dc
=
1.253 gcm–3, μ(Mo-Kα) = 0.073 mm–1, T = 150(2) K, yellow tablet;
17374 reflections measured with a Bruker APEX II CCD dif-
fractometer, of which 6783 are independent; data corrected for ab-
sorption on the basis of symmetry equivalent and repeated data
(min. and max. transmission factors: 0.937, 0.993) and Lorentzian
and polarization effects, Rint = 0.022, structure solved by direct
methods (SHELXS-97),[32] F2 refinement, R1 = 0.049 for 5638 data
with F2 Ͼ 2σ(F2), wR2 = 0.146 for all data, 409 parameters.
as a white solid (34 mg, 70%); m.p. 140–141 °C. IR (KBr): ν
=
˜
max
2953, 2919, 1603, 1458, 1452, 1373, 1217, 1102, 1035, 875, 755,
675, 472 cm–1. 1H NMR (300 MHz, CDCl3): δ = 1.62 (s, 9 H, tBu),
2.50 (s, 3 H, 1-Me), 2.97 (s, 3 H, 3-Me), 3.90 (s, 3 H, OMe), 7.22
(dd, J = 2.4, 2.4 Hz, 1 H, 12-HAr), 7.50 (s, 1 H, 2-HAr), 7.69 (d, J
= 9.3 Hz, 1 H, 10-HAr), 7.90 (d, J = 9.0 Hz, 1 H, 11-HAr), 8.02 (d,
J = 9.0 Hz, 1 H, 5-HAr), 8.03 (s, 1 H, 14-HAr), 8.17 (d, J = 1.8 Hz,
1 H, 6-HAr), 8.20 (d, J = 9.0 Hz, 1 H, 4-HAr), 8.70 (d, J = 9.3 Hz,
1 H, 9-HAr), 8.89 (d, J = 1.8 Hz, 1 H, 8-HAr) ppm. 13C NMR
(75 MHz, CDCl3): δ = 19.4, 25.1, 31.9, 35.4, 55.2, 107.8, 109.2,
113.7, 115.4, 117.1, 117.9, 118.5, 122.8, 123.0, 126.1, 126.8, 127.1,
127.6, 128.9, 130.3, 130.9, 131.9, 132.3, 133.3, 148.7, 157.6 ppm.
HRMS (FAB, EI): calcd. for C31H28O [M]+ 416.2140; found
416.2148. C31H28O (416.55): calcd. C 89.38, H 6.78; found C 89.39,
H 6.76. The structure of 5 was supported by single-crystal X-ray
structure analysis.
Supporting Information (see footnote on the first page of this arti-
1
cle): H and 13C NMR spectra of 4, 5, 7, and 8, packing diagrams
of 5 and 8, normalized UV/Vis absorption and emission spectra of
5 and 8 recorded in thin neat films, cyclic voltammograms of 5 and
8, and summary of the crystal data of 5 and 8.
Acknowledgments
This work was performed under the Cooperative Research Program
of “Network Joint Research Center for Materials and Devices”,
Institute of Materials Chemistry and Engineering, Kyushu Univer-
sity. The authors would like to thank the Ocean Thermal Energy
Conversion (OTEC) at Saga University for financial support.
7-tert-Butyl-1,3,12,14-tetramethyldibenzo[ij,no]tetraphene (8): 7-
tert-Butyl-1,3-dimethyl-5-(3,5-dimethylphenylethynyl)pyrene [(E)-
7; 50 mg, 0.12 mmol] in benzene (260 mL) was irradiated in the
presence of I2 (31 mg, 0.12 mmol) and propylene oxide (1.53 mL,
21.1 mmol) for 10 h. Following work-up, the mixture was adsorbed
on silica gel by using dichloromethane and purified by column
chromatography with hexane as eluent to give pure 8 as a white
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2953, 2919, 1600, 1457, 1442, 1363, 1217, 1102, 1034, 873, 751,
665, 472 cm–1. 1H NMR (300 MHz, CDCl3): δ = 1.62 (s, 9 H, tBu),
1.82 (s, 3 H, 14-Me), 2.06 (s, 3 H, 12-Me), 2.59 (s, 3 H, 1-Me), 2.96
(s, 3 H, 3-Me), 7.21(s, 1 H, 13-HAr), 7.59 (s, 1 H, 11-HAr) 7.68 (s,
1 H, 2-HAr), 7.98 (d, J = 8.7 Hz, 1 H, 10-HAr), 8.04 (d, J = 9.1 Hz,
1 H, 5-HAr), 8.19 (d, J = 1.5 Hz, 1 H, 6-HAr), 8.21 (d, J = 9.1 Hz,
1 H, 4-HAr), 8.74 (d, J = 8.4 Hz, 1 H, 9-HAr), 8.86 (d, J = 1.5 Hz,
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23.3, 31.9, 35.4, 117.6, 119.7, 122.3, 123.2, 124.7, 126.0, 126.7,
127.2, 127.3, 128.9, 130.3, 130.6, 130.7, 130.8, 131.1, 131.9, 133.0,
8
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Received: April 4, 2013
Published Online: ᭿
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9

Job/Unit: O30487
/KAP1
Date: 31-07-13 17:07:50
Pages: 10
T. Yamato et al.
FULL PAPER
Pyrene-Based Helicenes
A convenient route to a new class of pyr-
ene-based [4]helicenes is presented along
with their optoelectronic properties. The
introduction of two methyl groups in the
fjord region of the [4]helicene gives a more
distorted structure and leads to a remark-
able redshift of the absorption band.
J.-Y. Hu, X. Feng, A. Paudel,
H. Tomiyasu, U. Rayhan, P. Thuéry,
M. R. J. Elsegood, C. Redshaw,
T. Yamato* ..................................... 1–10
Synthesis, Structural, and Photophysical
Properties of the First Member of the Class
of Pyrene-Based [4]Helicenes
Keywords: Fused-ring systems / Wittig reac-
tions / Cyclization / Helical structures / Op-
toelectronic properties
10
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Eur. J. Org. Chem. 0000, 0–0