Synthesis and luminescent properties of pyrenylvinylene-substituted tripylborane

Article abstract of DOI:10.1016/j.jorganchem.2009.01.015

A novel luminescent compound, bis((E)-2-pyren-1-yl-vinyl)-2,4,6-triisopropylphenylborane (3) was synthesized by hydroboration reaction and was fully characterized. The obtained compound was further investigated by single-crystal X-ray diffraction analysis

Full text of DOI:10.1016/j.jorganchem.2009.01.015

Journal of Organometallic Chemistry 694 (2009) 1723–1726
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Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Synthesis and luminescent properties of pyrenylvinylene-substituted tripylborane
*
Yuuya Nagata, Yoshiki Chujo
Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8515, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel luminescent compound, bis((E)-2-pyren-1-yl-vinyl)-2,4,6-triisopropylphenylborane (3) was syn-
thesized by hydroboration reaction and was fully characterized. The obtained compound was further
investigated by single-crystal X-ray diffraction analysis and DFT calculations. The extended structure tells
us their herringbone structures with closely faced pairs of the molecules. Comparing the photolumines-
cent spectra between solution-state and solid state, the spectrum of the solid state of the compound 3
Received 8 November 2008
Received in revised form 27 December 2008
Accepted 8 January 2009
Available online 14 January 2009
exhibited dramatically red-shifted fluorescent emission. This change also supports the efficient p-stack-
ing behavior of the compound 3.
Keywords:
Tripylborane
Luminescent compound
DFT calculation
Ó 2009 Elsevier B.V. All rights reserved.
Hydroboration reaction
1. Introduction
2-pyren-1-ylethenyl-substituted trivalent organoborane. The crys-
tal structure and the packing diagrams were determined by single-
crystal X-ray diffraction analysis.
Organic field-effect transistors (OFETs) have attracted a consid-
erable interest for electronic applications, and even light-emitting
OFETs have recently been developed. Air-stable n-type organic
semiconducting materials are quite important for the p–n junction
diodes. Fluorinated fused aromatics [1], heterocyclics [2–5] and
diimide derivatives [6–9] are promising n-type materials. The de-
sign strategy of these n-type semiconductive materials has been
2. Results and discussion
2.1. Synthesis
First, the reaction between 1-ethynylpyrene 2 and 2,4,6-triiso-
propylphenylborane (tripylborane, 1) [tripyl = 2,4,6-triisopropyl-
phenyl] was carried out as shown in Scheme 1. To a dehydrated
and degassed THF solution of 2 was added a half equivalent of 1
in THF dropwise at room temperature in dry argon atmosphere.
After 24 h of stirring, the solvent THF was removed under vacuum.
The orange residue was purified by recrystallization from hot hex-
ane to give orange crystals in 25% isolated yield. The structure of
the compound 3 was confirmed by 1H, 11B and 13C NMR, elemental
analysis and single-crystal X-ray diffraction analysis. The 1H NMR
spectrum of the compound 3 showed the peaks assignable to the
protons of two pyrenyl groups, a tripyl group and vinyl groups.
The high value of the coupling constant (J = 17.54 Hz) shows that
the vinyl protons are in the trans configuration. The integral ratios
of the peaks for the compound 3 were in good agreement with the
theoretical values. In the 11B NMR spectrum, a peak around 63 ppm
corresponding to the typical three-coordinated boron atom was
observed.
based on the modification of the
p-extended p-type semiconduc-
tive materials by electron-withdrawing groups. There have been
some attempts to incorporate pyrene into conjugated oligomers
for the generation of semiconducting layers in OFETs [10,11]. Fur-
thermore, pyrene derivatives are chemically and photochemically
stable comparing with linear polyacenes such as anthracene, tetra-
cene and pentacene.
On the other hand, we have synthesized a wide variety of orga-
noboron polymers including boron atoms in the polymer backbone
by means of hydroboration polymerization [12–14]. These poly-
mers exhibited strong fluorescence emission and n-type electronic
conductivity [15,16] due to the high electron affinity of boron
atoms. Especially, the extension of p-conjugation by hydroboration
reaction has some inherit advantages. For example, this method
does not require any transition metal catalyst which is usually
used to prepare
p-extended molecules. Therefore, metal catalysts
that sometimes cause adverse affect for electronic properties
cannot be in the obtained products in principle. We report here
on the synthesis and the properties of the first example of
2.2. Crystal structure
The crystal structure of the compound 3 was determined by
* Corresponding author.
single-crystal X-ray diffraction analysis. The boron center displays
E-mail address: chujo@chujo.synchem.kyoto-u.ac.jp (Y. Chujo).
0022-328X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2009.01.015

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Y. Nagata, Y. Chujo / Journal of Organometallic Chemistry 694 (2009) 1723–1726
Table 1
Selected bond lengths (Å) and angles (°) for 3.
3 (X-ray)
B(1)–C(1)
B(1) –C(19)
B(1) –C(37)
1.531(4)
1.549(3)
1.579(4)
C(1) –B(1) –C(19)
C(1) –B(1) –C(37)
C(19) –B(1) –C(37)
115.6(2)
124.2(2)
120.1(2)
3 (DFT)
B(1) –C(1)
B(1) –C(19)
B(1) –C(37)
1.5511
1.5512
1.5905
C(1) –B(1) –C(19)
C(1) –B(1) –C(37)
C(19) –B(1) –C(37)
118.3
120.9
120.9
Scheme 1. Synthesis of compound 3.
a plane geometry, as shown in Fig. 1 and Table 1. Two pyrenyl
groups were found to be in the same plane and the tripyl group
was perpendicular to the pyrenyl groups. The extended structure
illustrates herringbone structures with closely faced pairs of the
molecules as shown in Fig. 2. The
p-stacking distance is a critical
parameter for efficient charge hopping in the solid state material.
The distance between a pair of the pyrenyl groups was approxi-
mately 2.8 Å. According to the previous study [17], the crystal
packing of the non-substituted pyrene was similar to that of the
Fig. 2. Plot illustrating the extended structure of compound 3. All hydrogen atoms
and tripyl groups were omitted for clarity.
compound 3 and the distance between a pair of pyrene molecules
was about 3.5 Å. Therefore, the compound 3 seems to be promising
as an organic electronic material.
2.3. DFT calculation
DFT calculation was carried out using GAUSSIAN 03 suit of program
[18] for further understanding of the behavior of the luminescence.
The structure of the compound 3 was optimized using the B3LYP/
6-31(d) method. The orbital diagrams were generated by using the
GAUSSVIEW program [19] as shown in Fig. 3. There was a good agree-
ment between the geometry of the compound 3 optimized by DFT
method and the structure determined by single-crystal X-ray dif-
fraction as shown in Table 1. The lowest unoccupied molecular
orbital (LUMO) is located on boron atom and the extension of the
Fig. 1. Structure of compound
3 with thermal ellipsoids drawn to the 50%
probability level.
p-conjugation length via the boron atom was observed as pointed
Fig. 3. HOMO and LUMO diagrams of compound 3. Scaling radius is 75% and molecular orbital surface isovalue is 0.02.

Y. Nagata, Y. Chujo / Journal of Organometallic Chemistry 694 (2009) 1723–1726
1725
before. We carried out TD–DFT calculations to obtain further in-
sight into the origin of the electronic transitions for the compound
3. The calculation reveals that the lowest energy transition of the
compound 3 corresponds to promotion of electrons from the
HOMO to the LUMO levels. The calculated excitation wavelength
of the compound 3 was 492 nm with 1.4642 of the oscillator
strength f. This wavelength was in approximately consistent with
the result of UV–Vis absorption spectroscopy.
2.4. Optical properties
The optical properties of the compound 3 were investigated by
UV–Vis absorption and fluorescence emission spectroscopy. The
absorption of the compound 3 is shown in Fig. 4. The compound
3 showed the strong absorption peak at 437 nm (e = 49 800). This
absorption peak should be caused by the HOMO–LUMO transition
on the plate of the pyrenylvinylene–borane–pyrenylvinylene from
the DFT calculation. Fig. 5 shows the fluorescence spectra of the
compound 3 in dichloromethane and in the solid state. The sample
of the solid state of the compound 3 was prepared by using chloro-
form as a solvent (10 mg mlÀ1) and by spinning the solution onto
quartz substrates at 2000 rpm for 30 s. The excitation wavelength
of 437 nm was set to the absorption maximum from the UV–Vis
Fig. 6. Photographs of fluorescence of the compound 3 under UV light at 365 nm.
Left: dichloromethane solution. Right: solid state.
absorption spectrum. The fluorescence emission peak from the
dichloromethane solution was 490 nm. On the other hand, the
solid state of the compound 3 showed dramatically red-shifted
emission peak at 530 nm. This large red-shift should be due to an
efficient
p-stacking of the plane molecules of the compound 3.
Photographs of photoluminescence of the solution and the solid
state are shown in Fig. 6. The dichloromethane solution exhibited
a green emission and the spin-coated solid exhibited a yellow-
green emission.
2.5. Conclusion
In conclusion, a pyrenylvinylene-substituted tripylborane was
successfully prepared by hydroboration of ethynylpyrene with
tripylborane. The crystal structure was revealed by single-crystal
X-ray diffraction analysis. The packing diagram displayed herring-
bone structures with closely faced pairs of the molecules. DFT cal-
culation and UV–Vis absorption spectroscopy indicated the
delocalization of
emission spectra reflected the strong
p
-electron via the boron center. The fluorescence
-stacking of the compound
p
3. Efforts are underway to fabricate and evaluate OFETs utilizing
the compound 3.
Fig. 4. UV–Vis spectra of compound 3 in CH2Cl2 (5.0 Â 10À5 M).
3. Experimental
3.1. General
1H, 13C and 11B NMR spectra were recorded on a JEOL JNM-
EX400 instrument. The chemical shift values were expressed
relative to Me4Si (1H and 13C NMR) as an internal standard and
BF3 Á OEt2 (11B NMR) as an external standard. UV–Vis spectra were
obtained on a SHIMADZU UV-3600 spectrophotometer, and sam-
ples were analyzed in CH2Cl2 at room temperature. Fluorescence
spectra were recorded on a Perkin Elmer LS50B luminescence spec-
trometer, and samples were analyzed in CH2Cl2 at room tempera-
ture. The melting point (m.p.) of the compound 3 was measured
with a Yanaco Micro Melting Point apparatus (Model MP-S3). Ele-
mental analysis was performed at the Microanalytical Center of
Kyoto University. All procedures were performed under argon
atmosphere. X-ray diffractions were collected on a Rigaku R-AXIS
RAPID-F graphite-monochromated Mo Ka radiation diffractometer
with an imaging plate. A symmetry related absorption correction
was carried out by using the program abscor [20]. The analysis
was carried out with direct methods (SHELX-97 [21] or SIR92 [22])
using YADOKARI-XG [23]. The program ORTEP3 [24] was used to generate
the X-ray structural diagrams.
Fig. 5. Fluorescence spectra of compound 3 in CH2Cl2 (1.0 Â 10À7 M) and in the
solid state excited at 437 nm.

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Y. Nagata, Y. Chujo / Journal of Organometallic Chemistry 694 (2009) 1723–1726
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state purification system (Glasscontour System, Joerg Meyer,
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3.2.1. Bis((E)-2-pyren-1-yl-vinyl)-2,4,6-triisopropylphenylborane (3)
Tripylborane 1 (108 mg, 0.50 mmol) in THF (1 ml) was slowly
added to 1-ethynylpyrene (226 mg, 1.00 mmol) in THF (4 ml). After
stirring the mixture for 24 h at room temperature, the solvent was
evaporated and the crude product was purified by recrystallization
from hexane. After the emerged crystals were dried under reduced
pressure, the model compound 4 (83 mg, yield 25%) was obtained
as an orange solid. m.p. 245 °C. 1H NMR (CDCl3, d, ppm): 8.61 (2H,
d, J = 17.54 Hz, –CH@CH–B–), 8.55 (2H, d, J = 8.04 Hz), 8.36 (2H, d,
J = 9.50 Hz), 8.22–8.16 (6H, m), 8.12–8.04 (6H, m), 8.00 (2H, t,
J = 7.55 Hz, pyrene-7H), 7.80 (2H, d, J = 17.54 Hz, –CH@CH–B–),
7.17 (2H, s, tripyl-H), 3.12–3.01 (1H, m, –CH–(CH3)2 on 4-position
of tripyl), 2.87–2.77 (2H, m, –CH–(CH3)2 on 2 and 6-position of tri-
pyl), 1.43 (6H, d, J = 7.07 Hz, –CH–(CH3)2 on 4-position of tripyl),
1.28 (12H, d, J = 6.58 Hz, –CH–(CH3)2 on 2 and 6-position of tripyl).
13C NMR (CDCl3, d ppm): 150.73, 149.75, 148.18, 139.90, 138.07,
132.21, 132.07, 131.40, 130.77, 129.41, 128.14, 127.45, 126.09,
125.70, 125.46, 125.11, 124.98, 124.82, 124.54, 122.69, 120.05,
35.31, 34.40, 24.60, 24.28. 11B NMR (CDCl3, d, ppm): 63.67. Anal.
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6.88%.
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