Preparation and optical properties of dithienosilole-arylphosphine alternate oligomers

Article abstract of DOI:10.1080/10426507.2010.525763

Dithienosilole-arylphosphine alternate oligomers were prepared by the Stille coupling reactions of bis(tributylstannyl)dithienosilole with bis(4-bromophenyl)phenylphosphine and-phosphine oxide. The resulting oligomers are dark orange solids, and they can

Full text of DOI:10.1080/10426507.2010.525763

Phosphorus, Sulfur, and Silicon, 186:1303–1307, 2011
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Taylor & Francis Group, LLC
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ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426507.2010.525763
PREPARATION AND OPTICAL PROPERTIES
OF DITHIENOSILOLE-ARYLPHOSPHINE
ALTERNATE OLIGOMERS
Joji Ohshita, Yasunori Kurushima, Masakazu Arita, and
Tomonobu Mizumo
Department of Chemistry and Chemical Engineering, Faculty of Engineering,
Hiroshima University, Higashi-Hiroshima, Japan
Abstract Dithienosilole-arylphosphine alternate oligomers were prepared by the Stille cou-
pling reactions of bis(tributylstannyl)dithienosilole with bis(4-bromophenyl)phenylphosphine
and -phosphine oxide. The resulting oligomers are dark orange solids, and they can be spin-
coated to thin films on substrates. They are emissive in solutions as well as in films.
Keywords Emission; oligomer; phosphine; phosphine oxide; silole; thiophene
INTRODUCTION
Silole-containing π-conjugated systems have been extensively studied as active com-
ponents for organic electronic devices, such as organic light emitting diodes (OLEDs), or-
1
ganic thin film transistors (OTFT), organic photovoltaic cells (OPV), and organic sensors,
since applications of silole derivatives to OLEDs as electron-transporting materials were
2
demonstrated by Tamao et al. The excellent electron-transporting properties of siloles are
∗
∗
∗
∗
ascribed to the interaction between the silicon σ - and the butadiene π -orbital (σ – π
1
–3
conjugation), which lowers the LUMO energy level to enhance the electron affinity.
Previously, we reported the synthesis of dithienosiloles (DTSs) with a silole ring
4
fused to a bithiophene unit. This system exhibits extended conjugation compared with
that of silole-free bithiophene, attributable not only to the coplanarity of the two thiophene
∗
∗
rings fixed by the tricyclic system, but also to the σ – π conjugation, and it has been
demonstrated by us and other research groups that DTS-based compounds are applicable to
OLEDs, OTFTs, and OPVs.1 Recently, we found that DTS derivatives with arylphosphine
a,5
6
substituents show excellent photoluminescence (PL) properties (Chart 1).
Interestingly, these compounds exhibit efficient blue-green PL, even in the solid
states with the maximum quantum yield of ϕ = 0.79 for compound 1a. This is presumably
due to the packing in the solid states, which suppresses the rotation with respect to the
Received 30 June 2010; accepted 15 September 2010.
Financial support from Shorai Foundation for Science and Technology and Electric Technology Research
Foundation of Chugoku is gratefully acknowledged.
Address correspondence to Joji Ohshita, Department of Chemistry and Chemical Engineering, Faculty of
Engineering, Hiroshima University, Higashi-Hiroshima 739-8527, Japan. E-mail: jo@hiroshima-u.ac.jp
1
303

1
304
J. OHSHITA ET AL.
Chart 1 Structures of emissive DTSs. UV absorption and emission maxima, and quantum yields in CHCl3 (data
in solid).
C P and C Si bonds to decrease the probabilities of nonradiative decay from the excited
states of these compounds. It was also demonstrated that vapor-deposited films of these
DTS-phosphine compounds are usable as emissive layers in multilayered OLEDs. In the
hope of obtaining solution processable DTS-based materials with efficient PL properties,
we prepared oligomers composed of an alternate arrangement of DTS and arylphosphine
units.
RESULTS AND DISCUSSION
First, we examined the reactions of dilithiodithienosilole and dichlorophenylphos-
phine as shown in Equation (1). However, all attempts to prepare the expected product with
a DTS-P alternate structure were unsuccessful, and a viscous oil with the molecular weight
less than Mw = 1000 was always obtained under various reaction conditions.
This is probably due to the steric hindrance for the P C bond formation,
and we introduced phenylene spacers in this system, as oligomers 3a and 3b in

DITHIENOSILOLE-ARYLPHOSPHINE OLIGOMERS
1305
Table 1 Properties of oligomers 3a and 3ba
UV abs λmax/nm
PL λem/nm (ϕ/%)b
Oligomer
Yield/%
Mw (Mw/Mn)
in CHCl3
film
in CHCl3
film
3
3
a
b
46
31
2800 (1.2)
2500 (1.2)
416
418
420
433
513 (16)
491 (36)
560 (6)
609 (8)
aAfter reprecipitation from chloroform-hexane.
b
Excited at the absorption maximum.
Equation (2). Stille coupling reactions of bis(tributylstannyl)dithienosilole 4 with bis-
4-bromophenyl)phenylphosphine and -phosphine oxide (5a and 5b) in toluene at the
(
reflux temperature afforded the oligomers, after reprecipitation of the organic products
from chloroform-hexane.
The yields, molecular weights, and optical properties of 3a and 3b are summarized in
1
Table 1. The oligomer structures were verified by spectroscopic analysis. H NMR spectra
of these oligomers reveal only broad signals, but the integration ratios are almost consistent
◦
with the expected structures. They are dark orange solids and do not melt up to 300 C,
and are soluble in common organic solvents, such as THF and chloroform, but insoluble
in hexane and ethanol. As expected, the oligomers can be processed to thin solid films by
spin-coating and drop-cast on substrates, such as glass and quartz plates.
Absorption and emission spectra of oligomer 3a in chloroform reveal the maxima,
7
slightly red-shifted from those of diphenyldithienosilole 6 by 7 and 18 nm, respectively
(
Chart 2), indicating the effects of the phosphine linkage on the conjugation in 3a. Similar
6
red-shifts were reported for the introduction of phosphine substituents on a DTS ring.
However, the red-shifts of 3a from 6 are much smaller than those of 1a from 7 (Chart 2),
which are 24 and 45 nm for absorption and emission, respectively. It is likely that the degree
of n-π interaction between the phosphine and π-conjugated unit is reduced by expansion
of the π-conjugation.
Chart 2 DTS derivatives with their absorption and emission maxima in CHCl3.
Phosphine oxide 3b shows absorption and emission spectra similar to 3a in chloro-
form, as depicted in Figures 1a and 1b, again indicating reduced n-π interaction in these
oligomers. In their films, however, oligomer 3b exhibits a remarkably red-shifted band
compared to 3a by about 50 nm, as can be seen in Figure 1c. The reason for the red-shift
is unclear, but the existence of polar phosphine oxide groups in 3b may facilitate the inter-
molecular interaction of 3b. The emission quantum yields of the present oligomers were

1
306
J. OHSHITA ET AL.
Figure 1 (a) UV absorption and (b) emission spectra of 3a and 3b in CHCl3. (c) Emission spectra of spin-coated
films of 3a and 3b on quartz plates.
determined in solutions as well as in spin-coated films, as listed in Table 1. It is noteworthy
that these oligomers show efficient orange emission even in films.
In conclusion, we prepared DTS-arylphosphine alternate oligomers by the Stille
coupling reactions, which are emissive in solutions as well as in films. Although the PL
quantum yields of the films of 3a and 3b are lower than those of 1a and 1b, the good film
forming properties clearly suggest their potential applications to OLEDs. Studies on the
applications of the present oligomers to OLED and the preparation of other oligomeric or
polymeric DTS-arylphosphine compounds are in progress.
EXPERIMENTAL
General Procedure and Materials
Toluene was distilled from sodium and stored over activated molecular sieves before
8
9
use. Monomers 4 and 5a were prepared as reported in the literature. Mass spectra were
measured on a Shimadzu QP5050A spectrometer. NMR spectra were recorded on a JEOL
LA-400 spectrometer. UV and emission spectra were measured on Shimadzu UV-3150 and
RF-5000 spectrophotometers, respectively. Emission quantum yields were determined in
an integration sphere attached by a Hamamatsu Photonics C7473 Multi-Channel Analyzer.
Preparation of Monomer 5b10
To an aqueous solution of hydrogen peroxide (10.5 mL, 30%), a solution of 5a in
chloroform (50 mL) was added slowly at room temperature, and the resulting mixture was
stirred for 2 h. To this, water (20 mL) was added, and the organic layer was separated.
The aqueous layer was extracted with chloroform, and the organic layer and the extracts
were combined then dried over anhydrous magnesium sulfate. After evaporation of the
solvent, the residue was chromatographed on a silica gel column eluting with hexane/ethyl
+
79
acetate = 1/2 to give the title compound as a colorless solid. MS m/z 434 (M for Br).
1
13
H NMR (400 MHz in CDCl3): δ = 7.48–7.64 (m). C NMR (100 MHz in CDCl3): δ =
1
1
27.44 (d, J = 3.3 Hz), 128.7 (d, J = 12.3 Hz), 131.10 (d, J = 104.3 Hz), 131.91 (d, J =
0.0 Hz), 131.92 (d, J = 12.8 Hz), 132.40 (d, J = 2.8 Hz), 133.49 (d, J = 10.6 Hz); one
carbon is missing due to overlapping with other signals. HR-mass calcd. for C18H14Br2OP:
434.91435; found: 434.91434.

DITHIENOSILOLE-ARYLPHOSPHINE OLIGOMERS
1307
Preparation of Oligomers 3a and 3b
A mixture of 4 (208 mg, 0.20 mmol), 5a (84 mg, 0.20 mmol), PdCl2(PPh3)2 (8 mg
0.01 mmol), CuI (2 mg, 0.01 mmol), and toluene (10 mL) was heated to reflux for 48 h
under dry nitrogen. After filtration of the resulting precipitates, the solvent was evaporated.
Reprecipitation of the residue from chloroform-hexane gave oligomer 3a as a dark red
◦
1
solid, mp >300 C. GPC Mw = 2800, Mw/Mn = 1.2. H NMR (400 MHz in CDCl3): δ
0.76–0.95 (m, 6H, CH2CH2CH2CH3), 1.22–1.40 (m, 4H, CH2CH2CH2CH3), 1.49–1.67
br m, 4H, CH2CH2CH2CH3), 2.53–2.64 (m, 4H, CH2CH2CH2CH3), 7.11–7.69 (m, 23H,
=
(
1
3
ring protons). C NMR (100 MHz in CDCl3): δ = 13.91, 22.35, 33.42, 35.82, 128.46,
1
28.55, 128.62, 128.74, 131.94, 132.06, 133.53, 133.65, 135.42, 135.44, no other sig-
nals were observed probably due to overlapping with major peaks. HR-MS calcd for
C18H14Br2OP: 434.91435; found: 434.91434.
◦
Polymer 3b was obtained in a manner similar to that above, 31% yield, mp >300 C.
1
GPC: Mw = 2500, Mw/Mn = 1.2. H NMR (400 MHz in CDCl3): δ = 0.76–1.00
(
m, 6H, CH2CH2CH2CH3), 1.26–1.41 (m, 4H, CH2CH2CH2CH3), 1.49–1.67 (br m, 4H,
CH2CH2CH2CH3), 2.59 (t, 4H, J = 7.9 Hz, CH2CH2CH2CH3), 7.14–7.33 (m, 6H, thio-
13
phene and benzene ring protons), 7.43–7.74 (m, 17H, benzene ring protons). C NMR
(
100 MHz in CDCl3): δ = 13.91, 22.34, 33.41, 35.75, 128.54, 133.49, 133.65, 135.42, no
other signals were observed probably due to overlapping with major peaks.
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