Synthesis and optical reactivity of 6,13-α-diketoprecursors of 2,3,9,10-tetraalkylpentacenes in solution, films and crystals

Article abstract of DOI:10.1039/c3tc31824a

Tetraalkylpentacenes having alkyl chains at 2,3,9,10-positions (Et-PEN, Pr-PEN and Hex-PEN) were prepared from their precursors Et-PDK, Pr-PDK and Hex-PDK, respectively. Photoreactions proceeded both in solutions, thin-films, and crystals, thus the properties of Et-PDK in films can be studied despite the instability of the pentacenes in solution. Et-PEN showed significantly different aggregation-nature compared with the parent pentacene. The hole mobilities of Et-PEN and Pr-PEN in films were 3.4 × 10^-6 and 8.1 × 10^-7 cm² V^-1 s^-1, respectively, determined by space-charge-limited current measurement, comparable with the order 10^-6 cm² V^-1 s^-1 of the electron mobility of Alq₃.

Full text of DOI:10.1039/c3tc31824a

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Materials Chemistry C
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Synthesis and optical reactivity of 6,13-a-
diketoprecursors of 2,3,9,10-tetraalkylpentacenes
Cite this: DOI: 10.1039/c3tc31824a
in solution, films and crystals†
Shuhei Katsuta,a Hiroyuki Saeki,b Katsuki Tanaka,c Yuki Murai,b Daiki Kuzuhara,a
Masahiro Misaki,b Naoki Aratani,a Sadahiro Masuo,cd Yasukiyo Uedab
ad
*
and Hiroko Yamada
Tetraalkylpentacenes having alkyl chains at 2,3,9,10-positions (Et-PEN, Pr-PEN and Hex-PEN) were
prepared from their precursors Et-PDK, Pr-PDK and Hex-PDK, respectively. Photoreactions proceeded
both in solutions, thin-films, and crystals, thus the properties of Et-PDK in films can be studied despite
the instability of the pentacenes in solution. Et-PEN showed significantly different aggregation-nature
compared with the parent pentacene. The hole mobilities of Et-PEN and Pr-PEN in films were
Received 14th September 2013
Accepted 25th November 2013
DOI: 10.1039/c3tc31824a
3.4 ꢀ 10ꢁ6 and 8.1 ꢀ 10ꢁ7 cm2 Vꢁ1
s
ꢁ1, respectively, determined by space-charge-limited current
www.rsc.org/MaterialsC
measurement, comparable with the order 10ꢁ6 cm2 Vꢁ1 sꢁ1 of the electron mobility of Alq3.
method is also a powerful tool for the synthesis of novel acene
derivatives9 including substituted acenes and higher acenes.10
Introduction
In order to expand the capability of this photo-conversion
methodology, 2,3,9,10-tetraalkylpentacenes were selected as
synthesis targets. 2,3,9,10-Tetraalkylpentacenes are fascinating
materials for their applications to soluble organic
semiconducting materials, liquid-crystalline pentacenes,
supramolecular-pentacenes and so on. In that sense, 2,3,9,10-
tetraalkylpentacenes are good candidates for the purpose, since
the hole mobilities can be controlled by the chain length of the
substituents and crystallinities. However the lack of a general
synthesis method of 2,3,9,10-tetraalkylated pentacenes,11 due to
the instability at 6,13-positions, prohibits its wider applications.
Over the past 15 years, synthesis of organic semiconducting
materials is one of the most active areas of organic chemistry.1
Among various novel organic semiconducting materials repor-
ted so far,2 pentacene is still one of the most attractive materials
due to its inherent high carrier mobility in organic eld-effect
transistors (OFETs).3 From the point of view of material devel-
opment, various synthesis methods4 have been reported while
the study of the synthesis of pentacene derivatives is still limited
due to pentacene's instability and insolubility. We have previ-
ously reported 6,13-a-diketo pentacene (PDK), which can be
converted to the parent pentacene by photo-induced decar-
bonylation-reaction5 (Strating–Zwanenberg reaction) in solu-
tion or thin lm state (Fig. 1).6 Our group reported the
fabrication of pentacene-based OFETs using PDK with 0.86 cm2
Vꢁ1 sꢁ1 of hole mobility in a top-contact device and this value is
comparable with that of vapour-deposited pentacene.7 PDK was
also applied to the printable pn-type organic photovoltaics
(OPVs) of pentacene and fullerenes.8 This photo-conversion
aGraduate School of Materials Science, Nara Institute of Science and Technology,
8916-5, Ikoma, 630-0192, Japan. E-mail: hyamada@ms.naist.jp; Fax: +81-743-72-
6042; Tel: +81-743-72-6041
bGraduate School of Science and Technology, Kobe University, Kobe, 657-8501, Japan
cDepartment of Chemistry, School of Science and Technology, Kwansei Gakuin
University, Sanda, 669-1337, Japan
dCREST, JST, Chiyoda-ku, 102-0075, Japan
† Electronic supplementary information (ESI) available: 1H and 13C NMR spectra
of the compounds, UV-vis absorption spectra during the photoreaction, IR spectra
of the lm, and hole mobilities of acenes measured by the SCLC method. See DOI:
10.1039/c3tc31824a
Fig. 1 Photochemical synthesis of PEN and R-PENs from PDKs.
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Bao and co-workers have reported the synthesis and OFET found to be almost 35% by HPLC analysis. The high reactivity of
application of 2,3,9,10-tetramethylpentacenes12 while synthesis aryne 3a attributed to the low purity. Pure 6a was obtained by
methods applied by them were inadequate for the synthesis of multiple-recrystallization with hexane. Purication of 6b and 6c
other 2,3,9,10-tetraalkylpentacenes due to the inaccessibility of was difficult by recrystallization due to their high solubility.
2,3,9,10-tetraalkylpentacenequinones. In order to establish the Attempts to purify 6b and 6c using middle-pressure liquid
general synthesis route toward 2,3,9,10-tetraalkylpentacenes, chromatography, gel permeation chromatography and normal-
we have considered applying the photo-conversion method. phase HPLC also failed. Finally purication by recycle reversed-
Thus photoreaction of 2,3,9,10-tetraalkylpentacene-precursors phase HPLC (COSMOSIL Cholester Packed Column, 10 mml I.
(R-PDKs, Et-PDK: R ¼ C2H5, Pr-PDK: C3H7 or Hex-PDK: C6H13
)
D. ꢀ 250 mm) succeeded to afford pure 6b and 6c. OsO4
would afford 2,3,9,10-tetraalkylpentacenes (R-PEN: Et-PEN: oxidation of adducts 6 afforded diols 7 and the subsequent
R ¼ C2H5, Pr-PEN: C3H7 or Hex-PEN: C6H13) as shown in Fig. 1. Swern oxidation afforded desired Et-PDK, Pr-PDK and Hex-PDK.
Herein we will report the synthesis and optical reactivities of Solubility of Et-PDK is as high as >60 mg mlꢁ1 in CH2Cl2.
PDKs, and mobilities of the lms of R-PENs obtained from the Compared with PDK, solubility of Et-PDK was improved in all
corresponding R-PDKs by spin-coating and irradiation.
solvents, and especially in dichloromethane, toluene and THF
as shown in Table S1.† Solubility of Pr-PDK and Hex-PDK is also
high. High solubility is an important property for the fabrica-
tion of solution-processed devices.
The UV-vis absorption spectra of R-PDKs in toluene are
shown in Fig. 2. R-PDKs show characteristic weak and broad
forbidden n–p* absorption peaks at around 460 nm. No
signicant difference of absorption with the change of alkyl
chains was observed.
The photoreactions of R-PDKs to R-PENs were performed.
The 0.2 mM solutions of R-PDKs in toluene were degassed by Ar
bubbling for 10 min in the dark. Then the solutions were irra-
diated with a blue LED (StockerYale SpecBright™ Blue LED,
wavelength: 470 nm, intensity: 2.5 mW cmꢁ2). The change in
the absorption spectra was measured with an interval of 30 s
during photoreaction. The change of UV-vis spectra and time-
proles during the photoreaction under an Ar atmosphere is
shown in Fig. 3 and ESI, Fig. S1† for Et-PDK. Before irradiation,
only the broad n–p* peak at around 467 nm was observed. As
the intensity of this peak decreased gradually, the intensities of
the new peaks at 580, 537, 500, 468, 432 and 407 nm assigned to
Et-PEN increased. Appearance of isosbestic points at 486, 439,
379 and 368 nm indicates the quantitative photo-conversion
from Et-PDK to Et-PEN. According to the progress, the colour of
the solution changed from yellow to purple. The absorbance of
pentacene became constant aer 40 min irradiation. The solu-
bility of Et-PEN is improved compared to the parent pentacene,
thus precipitation of Et-PEN during photoreaction was not
Results and discussion
Synthesis
Synthesis of Et-PDK, Pr-PDK and Hex-PDK is shown in Scheme
1. Kumada-coupling reaction of alkyl magnesium bromides
with 1,2-dichlorobenzene under a Ni catalyst afforded 1,2-dia-
lkylbenzenes 1.13 Bromination of 1 afforded 1,2-dialkyl-4,5-
dibromobenzenes 2. Modern precursors for arynes such as
2-(trimethylsilyl)phenyl-triuoromethanesulfonate or 2-(iodo)
phenyl-triuoromethanesulfonate would allow milder and
selective cycloaddition, but introduction of two alkyl chains to
such precursors is very difficult, thus compounds 2 are the only
practical precursors of 1,2-dialkyl-4,5-benzynes. Diels–Alder
reaction of benzynes 3, which were prepared in situ from 2 and
n-BuLi, with exo-methylene 4 afforded Diels–Alder adducts 5.
Adducts 5 were unstable under air, thus one-pot oxidation to
pentacene skeletons 6 by DDQ was applied. Purity of 6a was
Scheme 1 Synthesis of R-PDKs.
Fig. 2 UV-vis absorption spectra of R-PDKs in toluene.
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Fig. 5 Absorption (black line) and fluorescence (black dotted-line,
excited at 537 nm) spectra of Et-PEN in toluene.
Fig. 3 Photoreaction of Et-PDK in toluene under an Ar atmosphere.
The UV-vis absorption spectra were recorded at 0, 10, 20, 30, and
40 min during irradiation.
for Hex-PEN. These values are comparable with the reported
value 0.09 of pentacene in cyclohexane.14
observed. Photo-conversions of Pr-PDK to Pr-PEN or Hex-PDK to
Hex-PEN are shown in the ESI, Fig. S2 or S3.† In both cases, the
conversion proceeded smoothly similar to the case of Et-PDK.
And also precipitation of the formed pentacenes Pr-PEN or Hex-
PEN was not observed.
In order to conrm the complete conversion of R-PDKs to R-
PENs in thin lms, photoreactions were monitored by IR and
UV-vis measurement. Thin lms were prepared by drop casting
of Et-PDKs in CHCl3 (20 mg mlꢁ1) to washed glass substrates.
Photoreactions were conducted under an Ar atmosphere.
Conversion was monitored by IR measurement of lms, in
which the characteristic absorption of the carbonyl moiety at
1728 and 1745 cmꢁ1 had completely disappeared aer 90 min of
photo-conversion (Fig. 6). The change of UV-vis spectra of Et-
PDK in the thin lm under an Ar atmosphere is shown in Fig. 7.
Before irradiation, only a broad n–p* peak at around 400–525
nm was observed. Absorption of Et-PDK completely dis-
appeared and new absorption that is characteristic of Et-PEN at
around 400–625 nm appeared aer 90 min photo-irradiation.
Photoreactions of Pr-PDK or Hex-PDK to Pr-PEN or Hex-PEN in
lms are also shown in the ESI, Fig. S6–S9.† The absorption
spectrum of Et-PEN in the thin lm is signicantly different
from that of PEN (Fig. 7). In the case of Et-PEN, the absorption
spectrum of the thin lm is similar to that in the solution. In the
case of the PEN lm, absorption around 600–700 nm, which
originates from aggregation of PEN, was observed. This
The photoreactions were further monitored by 1H-NMR
spectroscopy. The changes in the NMR spectra during photo-
irradiation of Et-PDK under an Ar atmosphere is shown in
Fig. 4. Et-PDK was placed in degassed CDCl3 and was irradiated
with the blue LED under an Ar atmosphere. During the photo-
reaction, the singlet peak at 5.25 ppm (HC) due to bridgehead
protons of Et-PDK gradually decreased while new singlet peaks
due to Et-PEN emerged at 7.70 (HD), 8.55 (HE), and 8.88 (HF)
ppm. From the results of absorption and 1H-NMR spectral
changes, quantitative photo-conversion was conrmed. The
alkylated pentacenes are not stable enough for isolation. The
colour of Et-PEN solution was bleached in 3 min under air.
Absorption and uorescence spectra of the obtained Et-PEN,
Pr-PEN and Hex-PEN in toluene are shown in Fig. 5, ESI Fig. S4
and S5.† Compared to the parent pentacene, red-shi of
absorption was not observed. The absolute uorescence
quantum yields were 0.08 for Et-PEN, 0.10 for Pr-PEN and 0.09
Fig. 4 Change of 1H-NMR spectrum during photoreaction of Et-PDK Fig. 6 IR spectra before and after photoreaction of Et-PDK in the
in CDCl3 under an Ar atmosphere. * toluene.
thin film.
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nature is remarkably observed in the colour of crystals as shown
in Fig. 8. Et-PEN is a red crystal while PEN is a dark-blue crystal
due to strong interaction, depending on the p–p interaction
between molecules.
In order to further investigate the difference of aggregation
nature, crystal growth of PEN and Et-PEN was measured by AFM.
Change of AFM images during photo-conversion from PDK to PEN
is shown in Fig. 9. Before photo-irradiation, a at and uniform
lm of PDK was observed. Aer 5 min irradiation, small penta-
cene crystals arose and they grew with further irradiation. The
small crystals gradually grew in height and width and then fused
to neighboring microcrystals. Thus, the total number of grains
decreased with increasing grain size (Fig. 9). In the case of Et-PDK,
a at and uniform lm of Et-PDK was also observed before photo-
irradiation with 0.563 nm of roughness. However the crystal
growth of Et-PEN was not observed during irradiation and the lm
was still at aer 60 min (Fig. 10), although the photo-conversion
occurred similarly as conrmed by UV-vis and IR measurement.
The mobilities of Et-PEN and Pr-PEN prepared from the
precursors by irradiation using a metal halide lamp were
measured by space-charge-limited current (SCLC) measurements
(ESI, Fig. S10†).15 The lm thickness of Et-PEN and Pr-PEN was
about 45 and 60 nm, respectively. The lm surface was at and
smooth, which enables the lm to be measured by the SCLC
method. Under the same irradiation conditions, the PEN lm
Fig. 7 Absorption spectra of Et-PEN in solution or thin film and
absorption of PEN in the thin film.
difference indicates the weak intermolecular interaction of Et-
PEN in the lm compared with PEN.
The difference of aggregation nature was also measured by
uorescence from solid Et-PEN or PEN (Fig. 8). Changes of
uorescence during photo-conversion of Et-PDK or PDK to Et-
PEN or PEN in the solid state were monitored. The uorescence
spectrum from solid Et-PEN is similar to that in the solution
and no aggregation was observed aer 30 min irradiation. In
sharp contrast to Et-PEN, PEN shows additional uorescence
around 650–800 nm, which originates from the aggregated
pentacene, during photo-conversion. This indicates that non-
substituted pentacene molecules have strong p–p stacking in
crystals, but ethyl groups disrupt the aggregation due to a p–p
interaction between molecules. A difference of p–p stacking
Fig. 8 Change of fluorescence in crystals of Et-PDK (top) and PDK
(bottom) during the photoconversion to Et-PEN and PEN, respectively. Fig. 9 Changes of AFM images (10 mm ꢀ 10 mm) during photoreaction
Inset: Pictures of crystals of Et-PDK and PDK before and after the of PDK to PEN (top) and the change of grain numbers (circles) and size
photoirradiation.
(triangles) during photoreaction of PDK to PEN (bottom).
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Experimental section
General methods
1H-NMR and 13C-NMR spectra were recorded on a JEOL JNM-
AL300 or a JNM-ECX400 spectrometer using tetramethylsilane
as an internal standard. ESI mass spectra were recorded on a
JEOL JMS-MS T100LC spectrometer. Melting points were
measured with a J-Science Group RFS-10. IR spectra were
recorded on a JASCO FT/IR-4200. UV-vis absorption spectra
were recorded on a JASCO UV/VIS/NIR Spectrophotometer
V-670. Fluorescence spectra and uorescence quantum yields
were obtained on a Hamamatsu Absolute PL Quantum Yield
Measurement System C9920-02. AFM images were taken on a
JEOL JSPM-5200. SCLC measurement was performed on the
device of [ITO (200 nm)/PEDOT:PSS (20 nm)/R-PEN/MoO3
(5 nm)/Au (50 nm)].
Fig. 10 Changes of AFM images (10 mm ꢀ 10 mm) during photoreac-
tion of Et-PDK to Et-PEN.
Photoreaction
Photoreactions in solutions were performed with a blue LED
(StockerYale SpecBright™ Blue LED; wavelength: 470 nm,
intensity: 2.5 mW cmꢁ2) and were monitored on an Ocean
Optics DH-2000-BAL and HR4000. Photoreactions in thin lms
were performed with a metal halide lamp (NPI, PCS-MH375RC,
intensity: 5750 lm).
prepared from PDK was not smooth enough and the mobility
could not be measured. The mobilities of Et-PEN and Pr-PEN
were obtained as 3.4 ꢀ 10ꢁ6 and 8.1 ꢀ 10ꢁ7 cm2 Vꢁ1 sꢁ1
,
respectively, which are comparable with the order 10ꢁ6 cm2 Vꢁ1
sꢁ1 of the electron mobility of Alq3, a typical electron transporting
material.16 The lower hole mobilities of Et-PEN and Pr-PEN
compared with general hole transporting materials are probably
due to the amorphous properties of the lms. In general the
carrier mobility of hole-transporting materials is higher than that
of electron transporting materials,17 but the carrier balance
between hole and electron transporting abilities is important for
the organic electroluminescence (EL) materials. Thus Et-PEN and
Pr-PEN can be good candidates for p-type materials in combi-
nation with general electron transporting materials like Alq3.
Synthesis
All solvents and chemicals were of reagent grade quality,
obtained commercially and used without further purication
except as noted. For spectral measurements and photoreaction,
spectral-grade toluene was used. Thin-layer chromatography
(TLC) and column chromatography were performed on an Art.
5554 (Merck KGaA) and Silica Gel 60N (Kanto Chemical Co.),
respectively.
Conclusions
General procedure for the synthesis of 1,2-dialkylbenzenes
In this paper, the photochemical synthesis of 2,3,9,10-tetraal-
1,2-Diethylbenzene 1a. Under an argon atmosphere, alkyl
kylpentacenes and their properties were discussed. By using magnesium bromide (0.88 mol in 240 ml Et2O) was added
these precursors, the rst synthesis of 2,3,9,10-tetraalkylpenta- dropwise to the stirred mixture of 1,2-dichlorobenzene (36.0 ml,
cenes Et-PEN, Pr-PEN, and Hex-PEN was achieved. The synthe- 0.318 mol) and Ni(dppp)Cl2 (1.0 g) in dry-Et2O (160 ml) at 0 ꢂC.
sized pentacene derivatives were characterized by UV-vis, The reaction was carefully allowed to warm to room tempera-
uorescence, and 1H-NMR spectroscopy. Since the photo- ture and stirred for 30 min. Then, the reaction was heated to
conversion proceeded in lms the properties of Et-PDK in lms reux and stirred overnight. Aer cooling the reaction mixture
ꢂ
can be studied despite the instability of the pentacenes in to 0 C, 6 M-HCl was added dropwise. The organic phase was
solution. Irradiation-induced conversions in thin lms were diluted with Et2O, washed with H2O, water and brine, dried over
conrmed by IR and UV-vis spectra of the lm, but the AFM Na2SO4, and then concentrated under reduced pressure to give
measurement of the Et-PEN lm showed a clear difference of the crude product. The crude product was puried by distilla-
1
crystallinity from the non-substituted pentacene lm. In the tion to afford the product. Colourless oil; H-NMR (300 MHz,
PEN lm, the growth of pentacene-crystal pillars was clearly CDCl3): d ¼ 7.19–7.12 (m, 4H), 2.66 (q, J ¼ 7.6 Hz, 4H), 1.22 ppm
observed, however no crystals grew in the Et-PEN lm. Such a (t, J ¼ 7.6 Hz, 6H).
1
low aggregation nature of Et-PEN was also conrmed by uo-
1,2-Dipropylbenzene 1b. Colourless oil; H-NMR (300 MHz,
rescence measurement of the solid. Due to the low crystallinity, CDCl3): d ¼ 7.23–7.10 (m, 4H), 2.59 (t, J ¼ 7.9 Hz, 4H), 1.75–1.55
Et-PEN and Pr-PEN showed hole mobilities of 3.4 ꢀ 10ꢁ6 and (m, 4H), 0.99 ppm (t, J ¼ 7.3 Hz, 6H).
8.1 ꢀ 10ꢁ7 cm2 Vꢁ1 sꢁ1, respectively, by SCLC measurement.
1,2-Dihexylbenzene 1c. Colourless oil; 1H-NMR (300 MHz,
Such moderate mobilities might be suitable for the hole CDCl3): d ¼ 7.16–7.09 (m, 4H), 1.62–1.52 (m, 4H), 1.41–1.28 (m,
transporting materials of organic EL.
12H), 0.89 ppm (t, J ¼ 6.6 Hz, 6H).
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2H), 2.68 (t, J ¼ 7.8 Hz, 8H), 1.61–1.54 (m, 8H), 1.39–1.27 (m,
24H), 0.88 ppm (t, J ¼ 6.9 Hz, 13H); 13C-NMR (75 MHz, CDCl3): d
¼ 141.4, 139.0, 138.2, 130.3, 126.7, 120.3, 50.2, 32.8, 31.8, 31.2,
29.4, 22.6, 14.1 ppm; IR (ATR): n ¼ 2962, 2924, 2854, 1219, 904,
770 cmꢁ1; HRMS (ESI+): m/z calculated for C48H64Na (M+ + Na):
663.49057, found: 663.49028.
General procedure for the synthesis of 1,2-dibormo-4,5-
dialkylbenzenes 2
Under an argon atmosphere, Br2 (72 g, 0.45 mol) was added
dropwise to the stirred mixture of starting material (0.203 mol)
and I2 (2.6 g) in CH2Cl2 (240 ml) at 0 ꢂC. The reaction was
allowed to warm to room temperature and stirred overnight.
Then, the reaction was quenched with aq-NaHSO3. The organic
phase was washed with aq-NaHCO3, water and brine, dried over
Na2SO4, and then concentrated under reduced pressure to give
the crude product. The crude product was puried by distilla-
tion to afford the product.
General procedure for the synthesis of pentacene-diols
Tetraethylpentacene-diol 7a. Under an argon atmosphere,
OsO4 (1.0 ml, 0.02 M in t-BuOH) was added to the stirred
mixture of 6a (50 mg, 0.12 mmol) and N-methylmorpholine
N-oxide (NMO) (50 mg, 0.42 mmol) in acetone (2.0 ml) at room
temperature. The reaction solution was stirred for 2 days. Then,
the reaction was quenched with aq-Na2S2O4. The reaction
mixture was extracted with EtOAc. Combined organic extracts
were washed with H2O, water and brine, dried over Na2SO4, and
concentrated under reduced pressure to give the crude product.
The crude product was puried by column chromatography
(CHCl3) and GPC to afford the pure product (31.9 mg,
0.071 mmol, 59%). White powder; mp 224.2–225.7 ꢂC; 1H-NMR
(300 MHz, CDCl3): d ¼ 7.76 (s, 2H), 7.70 (s, 2H), 7.58 (s, 2H), 7.56
(s, 2H), 4.59–4.57 (t, 2H), 4.18–4.16 (m, 2H), 2.82–2.74 (m, 8H),
1.33–1.24 ppm (m, 12H); 13C-NMR (75 MHz, CDCl3): d ¼ 140.65,
140.59, 136.6, 135.0, 131.43, 131.41, 125.7, 124.35, 124.33,
122.35, 122.32, 68.5, 51.08, 51.05, 25.5, 15.0 ppm; IR (ATR):
n ¼ 2964, 2940, 2873, 1217, 1057, 1013, 906, 766; HRMS (ESI+):
m/z calculated for C32H34O32 (M+): 450.2558, found: 450.2559.
Tetrapropylpentacene-diol 7b. Prepared as reported for 7a,
starting from 6b (1.98 mmol) and obtaining pure 7b (1.03 mmol,
52%). White powder; mp 79.3–80.5 ꢂC; 1H-NMR (300 MHz,
CDCl3): d ¼ 7.73 (s, 2H), 7.67 (s, 2H), 7.55 (s, 2H), 7.53 (s, 2H),
4.58–4.56 (m, 2H), 4.23–4.12 (m, 2H), 2.75–2.66 (m, 8H), 1.72–
1.59 (m, 8H), 1.05–0.96 ppm (m, 12H); 13C-NMR (75 MHz, CDCl3):
d ¼ 139.32, 139.27, 136.6, 134.9, 131.38, 131.33, 126.8, 124.4,
122.4, 68.5, 51.1, 34.9, 24.18, 24.15, 14.2 ppm; IR (ATR): n ¼ 2956,
2930, 2863, 1216, 1057, 1071, 1012, 909, 773; HRMS (ESI+): m/z
calculated for C32H34O32Na (M+ + Na): 529.3082, found: 529.3082.
Tetrahexylpentacene-diol 7c. Prepared as reported for 7a,
starting from 6c (1.93 mmol) and obtaining pure 7c (0.78 mmol,
57%). Colourless viscous oil; 1H-NMR (300 MHz, CDCl3):
d ¼ 7.72 (s, 2H), 7.66 (s, 2H), 7.54 (s, 2H), 7.52 (s, 2H), 4.57–4.54
(m, 2H), 4.18–4.12 (m, 2H), 2.72 (t, J ¼ 7.7 Hz, 8H), 1.67–1.57 (m,
8H), 1.42–1.31 (m, 24H), 0.89 ppm (t, J ¼ 6.7 Hz, 12H); 13C-NMR
(75 MHz, CDCl3): d ¼ 139.57, 139.53, 136.5, 134.9, 131.38,
131.32, 126.7, 124.4, 122.3, 68.5, 51.1, 32.8, 31.8, 31.16, 31.12,
29.4, 22.6, 14.1 ppm; IR (ATR): n ¼ 2960, 2926, 2857, 1219, 1077,
1013, 906, 776; HRMS (ESI+): m/z calculated for C48H66O2Na (M+
+ Na): 697.4960, found: 697.4961.
1
1,2-Dibormo-4,5-diethylbenzene 2a. Colourless oil; H-NMR
(300 MHz, CDCl3): d ¼ 7.38 (s, 2H), 2.57 (q, J ¼ 7.5 Hz, 4H),
1.20 ppm (t, J ¼ 7.5 Hz, 6H).
1,2-Dirbormo-4,5-dipropylbenzene 2b. Colourless oil;
1H-NMR (300 MHz, CDCl3): d ¼ 7.37 (s, 2H), 2.50 (t, J ¼ 7.8 Hz,
4H), 1.65–1.50 (m, 4H), 0.97 ppm (t, J ¼ 7.3 Hz, 6H).
1,2-Dirbormo-4,5-dihexylbenzene 2c. Colourless oil; 1H-NMR
(300 MHz, CDCl3): d ¼ 7.36 (s, 2H), 2.51 (t, J ¼ 7.9 Hz, 4H), 1.57–
1.48 (m, 6H), 1.38–1.26 (m, 20H), 0.93–0.85 ppm (m, 10H).
General procedure for the synthesis of Diels–Alder adducts
Diels–Alder adduct 6a. Under an argon atmosphere, n-BuLi
(1.0 ml, 1.65 M in hexane) was added dropwise to the stirred
mixture of starting material 2a (0.476 g, 1.62 mmol) and exo-
mꢂethylene 4 (60.0 mg, 0.38 mmol) in dry-toluene (2.5 ml) at
0 C. The reaction was allowed to warm to room temperature
ꢂ
and stirred for 3 h. Then, the reaction was cooled to 0 C and
DDQ (0.20 g) was added at one-portion. Aer 15 min stirring,
the reaction was quenched by addition of silica gel. The reaction
mixture was ltered off and washed with CHCl3. The ltrates
were concentrated under reduced pressure to give the crude
product. The crude product was puried by column chroma-
tography (CHCl3 : hexane ¼ 1 : 6) and recrystallization with
hexane to afford the pure product (23.7 mg, 0.057 mmol, 15%).
White powder; mp 185.5–186.2 ꢂC; 1H-NMR (300 MHz, CDCl3): d
¼ 7.60 (s, 4H), 7.46 (s, 4H), 6.99 (m, 2H), 5.23 (m, 2H), 2.74 (q, J
¼ 7.5 Hz, 8H), 1.25 ppm (t, J ¼ 7.5 Hz, 12H); 13C-NMR (75 MHz,
CDCl3): d ¼ 141.5, 140.1, 138.2, 130.3, 125.6, 120.3, 50.1, 25.4,
15.1 ppm; IR (ATR): n ¼ 3000, 2933, 2871, 899, 778 cmꢁ1; HRMS
(EI): m/z calculated for C32H32 [M+]: 416.2504, found: 416.2503.
Diels–Alder adduct 6b. Prepared as reported for 6a, starting
from 2b and obtaining pure 6b (20%) by recycle reverse-phase
HPLC (COSMOSIL Cholester Packed Column, acetonitrile, 4 ml
minꢁ1). White powder; mp 36.5 ꢂC; 1H-NMR (300 MHz, CDCl3):
d ¼ 7.58 (s, 4H), 7.43 (s, 4H), 6.99 (m, 2H), 5.23 (m, 2H), 2.67 (t, J
¼ 7.7 Hz, 8H), 1.63 (dq, J ¼ 15.1, 7.5 Hz, 8H), 0.97 ppm (t, J ¼
7.3 Hz, 12H); 13C-NMR (75 MHz, CDCl3): d ¼ 141.5, 138.7, 138.2,
130.3, 126.7, 120.3, 50.2, 34.9, 24.2, 14.1 ppm; IR (ATR): n ¼
2963, 2928, 2868, 1218, 1091, 910, 776 cmꢁ1; HRMS (ESI+): m/z
calculated for C36H40Na (M+ + Na): 495.3027, found: 495.3026.
General procedure for the synthesis of diketone-precursors
Et-PDK. Under an argon atmosphere, TFAA (3.6 ml) was
Diels–Alder adduct 6c. Prepared as reported for 6a, starting added to the stirred solution of dry-DMSO (3.6 ml) in dry-
ꢂ
from 2c and obtaining pure 6c (25%) by recycle reverse-phase CH2Cl2 (25.5 ml) at ꢁ78 C over 10 min. Aer being stirred at
HPLC (COSMOSIL Cholester Packed Column, acetonitrile : THF the same temperature for 15 min, 7a (0.619 g, 1.374 mmol) in
¼ 4 : 1, 4 ml minꢁ1). Colourless viscous oil; 1H-NMR (300 MHz, dry-CH2Cl2 (12.8 ml) was added to the reaction mixture over
CDCl3): d ¼ 7.58 (s, 4H), 7.43 (s, 4H), 6.99 (m, 2H), 5.25–5.20 (m, 20 min. Aer 2.5 h stirring at ꢁ60 ꢂC, the reaction mixture was
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ꢂ
cooled to ꢁ78 C and N,N-diisopropylethylamine (DIPEA) (12.8
ml) was added dropwise to the reaction mixture. The reaction
mixture was stirred at ꢁ78 ꢂC for 1.5 h and then allowed to
warm to room temperature over 30 min. 3 M HCl was added to
the reaction mixture and the organic phase was extracted with
CH2Cl2. The combined organic extracts were washed with water
and brine, dried over Na2SO4, and concentrated under reduced
pressure to give the crude product. The crude product was
puried by column chromatography on silica gel
(CH2Cl2 : hexane ¼ 3 : 2 to 3 : 1) to afford the product as yellow
powder (0.400 g, 0.895 mmol, 65%). Orange powder; mp 270.3–
M. Zagorska, D. Djurado and R. Demadrille, Chem. Soc.
Rev., 2010, 39, 2577.
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M. J. Kang, I. Doi, H. Mori, E. Miyazaki, K. Takimiya,
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1
ꢂ
271.4 C; H-NMR (300 MHz, CDCl3): d ¼ 7.85 (s, 4H), 7.61 (s,
4H), 5.24 (s, 2H), 2.80 (q, J ¼ 7.5 Hz, 8H), 1.30 ppm (t, J ¼ 7.5 Hz,
12H); 13C-NMR (100 MHz, CDCl3); d ¼ 185.6, 142.2, 132.6, 131.2,
126.0, 124.5, 60.6, 25.6, 14.9 ppm; IR (ATR): n ¼ 2973, 2933,
2873, 1750, 1731, 1223, 1112, 920, 773; HRMS (ESI+): m/z
calculated for C32H30O2 (M+): 446.2245, found: 446.2245.
Pr-PDK. Prepared as reported for Et-PDK, starting from 7b
(0.790 mmol) and obtaining pure Pr-PDK (0.398 mmol, 50%).
Orange powder; mp 101.1–102.0 ꢂC; 1H-NMR (300 MHz, CDCl3):
d ¼ 7.82 (s, 4H), 7.59 (s, 4H), 5.23 (s, 2H), 2.73 (t, J ¼ 7.8 Hz, 8H),
1.68 (dq, J ¼ 15.1, 7.5 Hz, 8H), 1.01 ppm (t, J ¼ 7.3 Hz, 12H); 13C-
NMR (75 MHz, CDCl3): d ¼ 185.7, 140.8, 132.5, 131.2, 127.1,
124.5, 60.6, 35.0, 24.1, 14.3 ppm; IR (ATR): n ¼ 2963, 2933, 2876,
1750, 1731, 1219, 1106, 913, 773; HRMS (ESI+): m/z calculated
for C36H38O2Na (M+ + Na): 525.2769, found: 525.2769.
Hex-PDK. Prepared as reported for Et-PDK, starting from 7c
(1.01 mmol) and obtaining pure Hex-PDK (0.506 mmol, 50%).
Orange viscous oil; 1H-NMR (400 MHz, CDCl3): d ¼ 7.82 (s, 4H),
7.58 (s, 4H), 5.23 (s, 2H), 2.74 (t, J ¼ 7.8 Hz, 8H), 1.64 (dt,
J ¼ 15.2, 7.6 Hz, 8H), 1.42–1.31 (m, 24H), 0.89 ppm (t, J ¼ 6.9 Hz,
12H); 13C-NMR (100 MHz, CDCl3): d ¼ 185.7, 141.1, 132.5, 131.1,
127.0, 124.5, 60.6, 32.9, 31.8, 31.0, 29.5, 22.7, 14.2 ppm; IR
(ATR): n ¼ 2949, 2927, 2857, 1750, 1732, 1216, 911, 776; HRMS
(ESI+): m/z calculated for C48H62O2Na (M+ + Na): 693.4647,
found: 693.4647.
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Acknowledgements
This work was supported by a Grant-in-Aid (no. 22350083 to
H.Y.), Research Fellow of the Japan Society for the Promotion of
Science (to S.K.) and the Green Photonics Project in NAIST
sponsored by the Ministry of Education, Culture, Sports,
Science and Technology, Japan. The authors also thank Prof.
Ken-ichi Nakayama, Dr Takao Motoyama and Ms Chika Ohashi
in Yamagata University for OFET-fabrications and discussions,
and Mr Takuya Ohtsubo in Kobe University for AFM measure-
ment. The authors also thank Ms Mika Yamamura and Ms
Yuriko Nishiyama in NAIST for ESI-MS measurements.
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