Synthesis and optical properties of a bis(diphenylphosphino)dithienosilole- digold(I) complex

Article abstract of DOI:10.1002/hc.20715

Treatment of 2,6-bis(diphenylphosphino)-4,4-bis(4-n-butylphenyl) dithienosilole with chloro(dimethylsulfide)gold(I) in acetone gave the corresponding digold complex as pale green solids. The complex was highly emissive and showed blue photoluminescence wi

Full text of DOI:10.1002/hc.20715

Heteroatom Chemistry
Volume 22, Number 3/4, 2011
Synthesis and Optical Properties of a
Bis(diphenylphosphino)dithienosilole-digold(I)
Complex
Joji Ohshita,1 Yuta Tominaga,1 Tomonobu Mizumo,1
Yusuke Kuramochi,2 and Hideyuki Higashimura2
1Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University,
Higashi-Hiroshima 739-8527, Japan
2Tsukuba Labs, Sumitomo Chemical Co., Ltd., Tsukuba 300-3294, Japan
Received 17 November 2010; revised 17 January 2010
properties of siloles are ascribed to the interaction
ABSTRACT: Treatment of 2,6-bis(diphenylphos-
between the silicon σ∗ and the butadiene π∗-orbital
phino)-4,4-bis(4-n-butylphenyl)dithienosilole
with
(σ∗–π∗ conjugation) [5], which lowers the LUMO en-
ergy level to enhance the electron affinity.
chloro(dimethylsulfide)gold(I) in acetone gave the
corresponding digold complex as pale green solids.
The complex was highly emissive and showed blue
photoluminescence with the quantum efficiencies
of ꢀ = 0.99 and 0.30 for its solution and powders,
respectively. The complex showed good film forming
properties and its spin coating from the toluene
solution afforded a thin solid film that also showed
We have studied the synthesis and properties
of dithienosiloles (DTSs) as a class of silole deriva-
tives [6–11]. DTS has a silole ring fused to a bithio-
phene unit, which exhibits extended conjugation at-
tributable 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,
organic transistors, and organic photovoltaic cells
[9]. Recently, we found that a DTS derivative with
diphenylphosphino-substituents (DTSP1) showed
good photoluminescence (PL) properties (Chart 1)
[10]. Interestingly, the PL quantum yields in solu-
tions increased about four times by oxidation of the
phosphorous atoms (DTSPO). It was also found that
a DTS-phosphine oxide compound showed electro-
luminescence properties, thus being applicable to
OLEDs. However, they could form the films only by
vapor deposition and attempted film formation by
spin coating and drop-cast failed.
ꢀ
C
emissive properties with ꢀ = 0.08.
2011 Wiley
Periodicals, Inc. Heteroatom Chem 22:514–517, 2011;
View this article online at wileyonlinelibrary.com. DOI
10.1002/hc.20715
INTRODUCTION
Silole derivatives have been well studied as func-
tional materials, such as organic semiconductors,
emissive materials, and photovoltaic materials [1–
3]. Applications of siloles in organic light-emitting
diodes (OLEDs) as electron-transporting materi-
als were first reported by Tamao, Yamaguchi, and
coworkers [4]. The excellent electron-transporting
On the other hand, gold complexes have re-
ceived much attention as the emissive materi-
als that may be usable for organic devices, such
as OLEDs, sensors, and so on [12–14]. In the
hope of obtaining solution processable DTS-based
materials with efficient PL properties, we prepared
Dedicated to Professor Kin-ya Akiba on the occasion of his 75th
birthday.
Correspondence to: Joji Ohshita; e-mail: jo@hiroshima-u.ac.jp.
Contract grant sponsor: Shorai Foundation for Science.
2011 Wiley Periodicals, Inc.
c
ꢀ
514

Bis(diphenylphosphino)dithienosilole-digold(I) Complex 515
CHART 1 Structures of emissive DTS derivatives. UV ab-
sorption and emission maxima, and quantum yields in CHCl3
(solids).
bis(diphenylphosphino)dithienosilole-(AuCl)2 com-
plexes in which the intermolecular Au-Au, Au-Cl,
and/or Au-S interaction would exist to facilitate the
film formation.
FIGURE 1 Structure of model compound, and HOMO
and LUMO profiles derived from DFT calculations at
B3LYP/LanL2DZ.
radiative decay in DTSP2. In fact, fitting the decay
curve afforded the higher nonradiative kinetic con-
stant of knr = 6.0 × 108 s−1 than the radiative one
(kr = 1.7 × 108 s−1). The UV spectrum of DTSP2-Au
in chloroform showed an absorption maximum at
369 nm, which was blue-shifted from that of DTSP2
by 7 nm. This is probably due to the reduced contri-
bution of the phosphorous lone pairs to the conjuga-
tion. This lowers both the HOMO and LUMO energy
levels of the DTS system but affects the LUMO level
primarily decreasing the HOMO–LUMO energy gap
[10]. However, the degree of this blue-shift induced
by the coordination to Au was smaller than that from
DTSP1 to DTSPO (19 nm), suggesting the interac-
tion of the gold(I) units with the DTSP2 system to
an extent. Indeed, DFT calculations on a model com-
pound at B3LYP/LanL2DZ showed that the Au or-
bital contributes to the HOMO (Fig. 1), and the time-
dependent calculations at the same level indicated
that the major absorption is ascribed to the HOMO–
LUMO transition. The emission band of DTSP2-Au
appeared at 432 nm, again slightly blue-shifted from
that of DTSP2. The emission life time in chloroform
was determined to be τ = 3.17 ns, indicating that the
emission is ascribed to fluorescence. The PL quan-
tum yields were markedly increased by the coordi-
nation to gold to reach ꢀ = 0.99 and 0.30, for the
solution and powders of DTSP2-Au, respectively.
Blue-colored PL from DTSP2-Au is shown in Fig. 2.
The HOMO and LUMO energy levels of DTSP2-Au
RESULTS AND DISCUSSION
Bis(diphenylphosphino)DTS-digold(I)
complexes
were obtained as shown in Eq. (1). Treatment
of DTSP1 with (Me2S)AuCl in acetone readily
afforded DTSP1-Au as crude precipitates. However,
DTSP1-Au could not be purified by recrystallization
due to its low solubility. Attempted purification
of DTSP1-Au by repeated reprecipitation failed.
To increase the solubility, we introduced n-butyl
groups on the phenyl rings in DTSP2. The reaction
of DTSP2 with (Me2S)AuCl in acetone, followed
by recrystallization of the resulting precipitates
gave complex DTSP2-Au as pale green fine plate
solids, which were soluble in toluene, THF, and
chloroform, but insoluble in acetone, ethanol, and
hexane, and melted at 236.0◦C without decompo-
sition. The structure of DTSP2-Au was verified by
spectroscopic analysis and combustion elemental
analysis. The 1H and 13C NMR spectra showed
well-resolved sharp signals, indicating the absence
of any considerable intermolecular interaction in
the solution phase. The 31P NMR signal appeared
at 20.17 ppm, lowfield shifted from that of DTSP1
[10] (−18.55 ppm).
Optical properties of DTSP2 and DTSP2-Au
were examined as shown in Table 1. DTSP2 exhib-
ited lower PL quantum efficiencies than those of
DTSP1, both in chloroform and in the solid state.
The flexible butyl groups seemed to enhance the non-
Heteroatom Chemistry DOI 10.1002/hc

516 Ohshita et al.
TABLE 1 Optical Properties of DTSP2 and DTSP2-Au
UV abs (λmax/nm)
Compound
In CHCl3
Film
PL (λem/nm) (ꢀ)a In CHCl3
Powder
Film
DTSP2
DTSP2-Au
376
369
ndb
374
449 (0.30)
432 (0.99)
483 (0.19)
445 (0.33)
ndb
440, 466c (0.08)
aExcited at the absorption maximum.
bNot determined.
cShoulder peak.
were estimated by the work function measurement
and the optical band gap to be −5.93 and −2.83 eV,
respectively.
DTSP2-Au by solution processes seem to allow its
applications into organic electronic devices. The ap-
plications to OLEDs and sensors are being studied
and will be reported elsewhere.
As we expected, DTSP2-Au exhibited good film-
forming properties by solution processes and a
transparent thin solid film was obtained by spin coat-
ing of the toluene solution. The UV absorption and
emission spectral data of the film are also summa-
rized in Table 1. A new emission shoulder peak ap-
peared at 466 nm as shown in Fig. 3 and the PL
quantum yield of the film was determined to be
ꢀ = 0.08, much lower than that of the powders.
Some intermolecular interactions concerning gold
atoms would be involved in the film being responsi-
ble for the changes of the optical properties.
In conclusion, we prepared diphosphino-DTS-
digold(I) complex DTSP2-Au. The complex forma-
tion with gold(I) remarkably enhanced PL quantum
yields of DTSP2 in the solution phase as well as in
the solid state, but only slightly affected the emis-
sion λmax and color. Good film-forming properties of
EXPERIMENTAL
All reactions were carried out in dry nitrogen or ar-
gon. Ether was distilled from sodium-benzophenone
ketyl immediately before use. 2,6-Dibromo-4,4-
bis(4-n-butylphenyl)dithienosilole [11] and DTSP1
[10] were prepared as reported in the literature.
Mass spectra were measured on a JEOL SX-102A
spectrometer. NMR spectra were recorded on JEOL
LA-400 spectrometers. UV and emission spectra
were measured on Shimadzu UV-3150 and RF-5000
spectrophotometers, respectively. Emission quan-
tum yields were determined in an integration sphere
attached by a Hamamatsu Photonics C7473 Multi-
Channel Analyzer. PL life time of DTSP2-Au was
determined by monitoring the intensity at 450 nm,
excited at 380 nm. The work function measurement
was carried out on a Riken Keiki AC-2 photoelectron
spectrometer.
FIGURE 2 Emission from a chloroform solution of DTSP2-
Au excited at 364 nm.
FIGURE 3 PL spectra of DTSP2-Au.
Heteroatom Chemistry DOI 10.1002/hc

Bis(diphenylphosphino)dithienosilole-digold(I) Complex 517
31P NMR (in CDCl3 from H3PO4) δ20.17; Anal calcd
for C52H48Au2Cl2P2S2Si: C, 48.34; H, 3.74, found: C,
48.04; H, 3.44.
Preparation of DTSP2. To
a
solution of
2.0
g
(3.2 mmol) of 2,6-dibromo-4,4-bis(4-n-
butylphenyl)dithienosilole in 30 mL of ether, 6.5 mL
of a 1.57 M n-BuLi solution in hexane was added
dropwise at −80◦C and the mixture was allowed
to warm to room temperature and stirred for 1 h.
To this, 1.2 mL (6.5 mmol) of chlorodiphenylphos-
phine was added and the resulting mixture was
stirred for 3 h. After hydrolysis with water, the or-
ganic layer was separated and the aqueous layer
was extracted with chloroform. The organic layer
and the extract were combined and dried over an-
hydrous magnesium sulfate. The solvent was evap-
orated, and the residue was subjected to silica
gel column chromatography eluting with chloro-
form/hexane to give 0.63 g (24% yield) of DTSP2:
mp 51.0–55.0◦C; FAB-MS m/z 826 (M+); 1H NMR
(in CDCl3) δ0.91 (t, 6H, J = 7.24 Hz, CH3), 1.26–
1.42 (m, 4H, CH), 1.49–1.63 (m, 4H, CH2), 2.60
(t, 4H, J = 7.91 Hz, CH2Ph), 7.17 (d, 4H, J =
7.59 Hz, m-phenylene), 7,31–7.43 (m, 20H, Ph),
7.47–7.54 (m, 6H, phenylene and DTS ring); 13C
NMR (in CDCl3) δ13.93, 22.37, 33.42, 35.74 (n-Bu),
127.99 (phenylene), 128.40 (p-Ph), 128.43 (d, JP-C
= 6.7 Hz, m-phenyl), 128.82 (phenylene), 132.97 (d,
JP-C = 19.3 Hz, o-Ph), 135.42 (phenylene), 137.91
(d, JP-C = 8.1 Hz, i-Ph), 139.62 (d, JP-C = 32 Hz,
DTS), 140.20 (phenylene), 141.69 (d, JP-C = 8.2 Hz,
DTS), 145.48 (DTS), 156.42 (DTS); Anal calcd for
C52H48P2S2Si: C, 75.51; H, 5.85, found: C, 75.39; H,
5.65.
DTSP1-Au was prepared in a similar fashion
to that mentioned above. However, attempted re-
crystallization was unsuccessful and repeated re-
precipitation from chloroform/ethanol and chloro-
form/hexane did not affect the purity. Data for
DTSP1-Au after reprecipitation: UV (in CHCl3) λmax
369 nm; PL λem 433 nm, ꢀ 0.92 (in CHCl3); λem 433
nm, ꢀ 0.28 (solid); 1H NMR (in C6D6) δ6.83–7.11 (m,
20H), 7.19–7.25 (m, 6H), 7.44 (dd, J = 8.0 and 1.2
Hz, 4H, o-Ph), 7.48 (br d, J = 8.0 Hz, 2H, DTS).
ACKNOWLEDGMENT
We thank Dr. Yousuke Ooyama of Hiroshima Uni-
versity for the fruitful discussion on the optical prop-
erties of DTSP2-Au.
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Preparation of DTSP1-Au and DTSP2-Au. A
mixture of 0.10 g (0.12 mmol) of DTSP2 and 71
mg (0.24 mmol) of chloro(dimethylsulfide)gold(I)
in 2 mL of acetone was stirred at room tempera-
ture for 5 h. The resulting precipitate was collected
and recrystallized from ethanol/chloroform = 1/1 to
give 20 mg (20% yield) of DTSP2-Au as pale green
solids: mp 236.0◦C; FAB-MS m/z 1255 (M+-Cl for
35Cl); 1H NMR (in CDCl3) δ0.94 (t, 6H, J = 6.77
Hz, CH3), 1.37 (m, 4H, CH2), 1.60 (m, 4H, CH2),
2.64 (t, 4H, J = 7.72 Hz,CH2-Ph), 7.22 (d, 4H, J
= 7.72 Hz, phenylene), 7.47–7.63 (m, 24H), 7.76
(d, 2H, JH-P = 8.71 Hz, DTS); 13C NMR (in CDCl3)
δ13.95, 22.47, 33.39, 35.79 (Bu), 125.68 (phenylene),
128.82 (phenylene), 129.34 (d, JP-C = 11.9 Hz, m-
pPh), 129.46 (d, JP-C = 65.5 Hz, i-Ph), 131.51 (d,
JP-C = 60.2 Hz, DTS), 132.28 (d, JP-C = 2.2 Hz, p-
Ph), 133.50 (d, JP-C = 14.1 Hz, o-Ph), 135.46 (pheny-
lene), 142.42 (d, J = 14.8 Hz, DTS), 144.47 (d, J =
11.1 Hz, phenylene), 146.40, 157.42, 157.47 (DTS);
Heteroatom Chemistry DOI 10.1002/hc