Synthesis of dithienobismoles as novel phosphorescence materials

Article abstract of DOI:10.1021/om100560n

Dithienobismoles having a bismole ring fused with a bithiophene system were prepared by the reactions of β,β′-dilithiobithiophenes with aryldihalobismuthanes, as novel phosphorescence materials.

Full text of DOI:10.1021/om100560n

Organometallics 2010, 29, 3239–3241 3239
DOI: 10.1021/om100560n
Synthesis of Dithienobismoles as Novel Phosphorescence Materials
Joji Ohshita,*,† Shigenori Matsui,† Roh Yamamoto,† Tomonobu Mizumo,†
Yousuke Ooyama,† Yutaka Harima,† Toshihiro Murafuji,*,‡ Keisuke Tao,‡
Yusuke Kuramochi,§ Takashi Kaikoh,§ and Hideyuki Higashimura*,§
†Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University,
Higashi-Hiroshima 739-8527, Japan, ‡Department of Chemistry, Graduate School of Medicine,
Yamaguchi University, Yamaguchi 753-8512, Japan, and Tsukuba Laboratories,
Sumitomo Chemical Co. Ltd., Tsukuba 300-3294, Japan
§
Received June 7, 2010
Summary: Dithienobismoles having a bismole ring fused with a
well studied, which provides useful building blocks for the
preparation of materials with even better conjugation than
the parent bithiophene, not only by fixing the tricyclic units
into a complete plane but also by electronic effects of the
bridging atoms. Examples include dithienothiophene,4
dithienopyrrole,5 dithienoborole,6 and dithienophosphole7
and their oligomers and polymers. The unique properties
and functionalities of these compounds have been demon-
strated.
In this regard, we prepared dithienosiloles having a Si-
bridged bithiophene system.8 In the course of our studies
concerning the functionalities of dithienosiloles, we found
that dithienosiloles are generally highly emissive. For exam-
ple, the solid-state fluorescence quantum yield of a dithie-
nosilole with diphenylphosphino substituents is as high as
0.8 in the solid state (DTS1 in Chart 1).9 To explore further
the scope of heteroatom-bridged bithiophene systems, we
prepared dithienobismoles and investigated their optical
properties. Bismuth is the heaviest abundant element of little
environmental and biological concern, and its heavy-atom
effects would lead to phosphorescence properties of the
compounds.10
bithiophene system were prepared by the reactions of β,β0-
dilithiobithiophenes with aryldihalobismuthanes, as novel phos-
phorescence materials.
Organic phosphorescence compounds are of current im-
portance, because of their utility as emissive materials for
high-performance organic light-emitting diodes (OLEDs),
and many efforts to develop organic phosphorescence sys-
tems have been made.1,2 However, most of the efficient
organic phosphorescence compounds reported so far are
complexes with a noble-metal or rare-earth-metal center
and there is urgent need for the development of new systems
without such rare-metal elements. On the other hand, oligo-
and polythiophenes are currently receiving considerable
attention as functional organic materials in the field of
organic electronics.3 The extended π-conjugation in these
systems arising from the fairly good coplanarity of adjacent
thiophene rings, and the sufficient thermal and chemical
stability, allow the use of oligo- and polythiophenes as active
components in organic electronic devices, such as OLEDs,
thin-film transistors, sensors, and photovoltaic cells. Re-
cently, bridging a bithiophene unit with a heteroatom at
the β,β0-position to form a fused tricyclic system has been
Dithienobismoles DTBi1-DTBi4 were obtained in 41, 57,
30, and 20% yields, respectively, by the reactions of dilithio-
bithiophenes and aryldihalobismuthanes, as shown in
Scheme 1. Their structures were verified by spectroscopic
analysis as well as by elemental analysis. The solid-state
structures of DTBi2-DTBi4 were determined by X-ray
diffraction studies, and the ORTEP drawing of DTBi3 is
*To whom correspondence should be addressed. J.O.: tel, þ81-82-
424-7743; fax, þ81-82-424-5494; e-mail, jo@hiroshima-u.ac.jp. T.M.:
e-mail, murafuji@yamaguchi-u.ac.jp. H.H.: e-mail, higashimura@
sc.sumitomo-chem.co.jp.
(1) Highly Efficient OLEDs with Phosphorescent Materials; Yersin,
H., Ed.; Wiley-VCH: Weinheim, Germany, 2008.
(2) For recent examples, see: (a) Chopra, N.; Lee, J.; Zheng, Y.; Eom,
S. H.; Xue, J.; So, F. Appl. Phys. Lett. 2008, 93, 143307. (b) Su, S. J.;
Gonmori, E.; Sasabe, H.; Kido, J. Adv. Mater. 2008, 20, 4189. (c) Lyu, Y. Y.;
Kwak, J.; Jeon, W. S.; Byun, Y.; Lee, H. S.; Kim, D.; Char, K. Adv. Funct.
Mater. 2009, 19, 420. (d) Su, S. J.; Takahashi, Y.; Chiba, T.; Takeda, T.; Kido,
J. Adv. Funct. Mater. 2009, 20, 4189.
(5) (a) Ogawa, K.; Rasmussen, S. C. Macromolecules 2006, 39, 1771.
(b) Zhou, E.; Nakamura, M.; Nishizawa, T.; Zhang, Y.; Wei, Q.; Tajima, K.;
Yang, C.; Hashimoto, K. Macromolecules 2008, 41, 8302. (c) Steckler, T. T.;
Zhang, X.; Hwang, J.; Honeyager, R.; Ohira, S.; Zhang, X.-H.; Grant, A.;
Ellinger, S.; Odom, S. A.; Sweat, D.; Tanner, D. B.; Rinzler, A. G.; Barlow, S.;
€
ꢀ
(3) For reviews, see: (a) Mishra, A.; Ma, C.-Q.; Bauerle, P. Chem.
Bredas, J.-L.; Kippelen, B.; Marder, S. R.; Reynolds, J. R. J. Am. Chem. Soc.
Rev. 2009, 109, 1141. (b) Perepichka, I. F.; Perepichka, D. F.; Meng, H.;
Wudl, F. Adv. Mater. 2005, 17, 2281. (c) Osaka, I.; McCullough, R. D. Acc.
Chem. Res. 2008, 41, 1202. (d) Gunes, S.; Neugebauer, H.; Sariciftci, N. S.
Chem. Rev. 2007, 107, 1324.
2009, 131, 2824.
(6) Kim, S.; Song, K.; Kang, S. O.; Ko, J. Chem. Commun. 2004, 68.
ꢀ
ꢀ
(7) (a) Baumgartner, T.; Bergmans, W.; Karpati, T.; Neumann, T.;
ꢀ
Nieger, M.; Nyulaszi, L. Chem.—Eur. J. 2005, 11, 4687. (b) Dienes, Y.;
ꢀ ꢀ ꢀ
Durben, S.; Karpati, T.; Neumann, T.; Englert, U.; Nyulaszi, L.; Baumgartner,
T. Chem.—Eur. J. 2007, 13, 7487. (c) Dienes, Y.; Eggenstein, M.; Karpati,
T.; Sutherland, T. C.; Nyulaszi, L.; Baumgartner, T. Chem.—Eur. J. 2008,
14, 9878. (d) Ren, Y.; Dienes, Y.; Hettel, S.; Parvez, M.; Hoge, B.;
Baumgartner, T. Organometallics 2009, 28, 734.
(8) (a) Chen, J.; Cao, Y. Macromol. Rapid Commun. 2007, 28, 1714.
(b) Ohshita, J. Macrmol. Chem. Phys. 2009, 210, 1360.
(9) Ohshita, J.; Kurushima, Y.; Lee, K. H.; Ooyama, Y.; Harima, Y.
Organometallics 2007, 26, 6591.
(4) (a) Barbarella, G.; Favaretto, L.; Sotgiu, G.; Antolini, L.; Gigli,
G.; Cingolati, R. Chem. Mater. 2001, 13, 4112. (b) Fong, H. H.; Pozdin,
V. A.; Amassian, A.; Malliaras, G. G.; Smilgies, D.-M.; He, M. Q.; Gasper, S.;
Zang, F.; Sorensen, M. J. Am. Chem. Soc. 2008, 130, 13202. (c) Lin, J.; Qin,
F.; Li, C. M.; Bao, Q.; Chan-Park, M. B.; Zhang, W.; Qin, J.; Ong, B. S.
Chem. Mater. 2008, 20, 2057. (d) Zhan, X.; Tan, Z.; Domercq, B.; An, Z.;
Zhang, X.; Barlow, S.; Li, Y.; Zhu, D.; Kippelen, B.; Marder, S. R. J. Am.
Chem. Soc. 2007, 129, 7246. (e) Yang, H.-Y.; Yen, Y.-S.; Hsu, Y.-C.; Chou,
H.-H.; Lin, J. T. Org. Lett. 2010, 12, 16. (f) He, M.; Li, J.; Sorensen, M. L.;
Zhang, F.; Hancock, R. R.; Fong, H. H.; Pozdin, V. A.; Smilgies, D.-M.;
Malliaras, B. G. J. Am. Chem. Soc. 2009, 131, 11930.
ꢀ
ꢀ
ꢀ
(10) Only a few bismuth-containing compounds are known to exhibit
phosphorescence.11,12
r
2010 American Chemical Society
Published on Web 07/14/2010
pubs.acs.org/Organometallics

3240 Organometallics, Vol. 29, No. 15, 2010
Ohshita et al.
depicted in Figure 1a (for others, see Figures S-1 and S-2).
Their structures possess essentially the same geometry for the
dithienobismole tricyclic unit with almost complete planar-
ity, independent of the substituents. All the bond lengths and
angles are in the normal range, except for the small C-
(thiophene)-Bi-C (thiophene) angles of 78.3(2), 78.8(2),
and 77.9(3)° for DTBi2-DTBi4, respectively, presumably
due to the long Bi-C bonds. The structures were well
reproduced by DFT calculations at the level of B3LYP/
6-31G(d)/LANL2DZ, as presented in Figure 1b,c. These
dithienobismoles are solids and are soluble in common
organic solvents. DTBi2 was stable in the solid state and
could be stored for 10 months without detectable decom-
position in air at room temperature, under room light, but
underwent decomposition readily in a dilute solution. In
fact, allowing the chloroform solution (10-5 g/L) of DTBi2
to stand overnight led to the conversion of DTBi2 to
unidentified compounds (Figure S-4). Compound DTBi1
was less stable and underwent decomposition slowly even
in the solid state. In contrast, DTBi3 and DTBi4 were stable
even in solution and no spectral changes were observed when
the solutions stood for several weeks. It is likely that the
methyl substituents and the benzo annulated units kineti-
cally stabilize the bismole system.
dithienocyclopentadiene DTC1 (λmax 323 nm) (Chart 1).
Presumably, in the dithienobismole system, a σ*-π* type
interaction operates to lower the LUMO energy level as
shown in Figure 1c, similar to the case for dithienosilole. No
obvious participation of the Bi σ-orbital in the HOMO is
observed by the DFT calculations. The UV spectrum of
DTBi4 showed four maxima, two of which were at longer
wavelength than those of DTBi1-DTBi3, indicating the
extended conjugation in DTBi4, as expected.
The present dithienobismoles exhibited clear red emission
in solution under an argon atmosphere, and the spectra
revealed broad bands centered at 600-640 nm (Figure 2b),
together with those around 400 nm. The former disappeared
in air, indicating that this emission originated from phos-
phorescence (Figure S-5). The phosphorescence lifetimes in
CHCl3 were determined to be approximately τ = 2-6 μs for
DTBi2-DTBi4 (Table 1). The phosphorescence quantum
yields were approximately 0.2%, as given in Table 1. Com-
pounds DTBi2 and DTBi3 were also phosphorescent in the
solid state (Figure 3), while only fluorescence bands were
observed from the solids of DTBi1 and DTBi4. It is likely
that the trimethylsilyl groups in DTBi2 and DTBi3 prevent
the molecules from being stacked to suppress the concentra-
tion quenching even in the solid state. Usually, the smaller
the singlet-triplet energy splitting ΔE(S1-T1) of the mole-
cule, the higher the phosphorescence quantum yield. How-
ever, the splitting of the present dithienobismoles estimated
by theoretical calculations is ΔE(S1-T1) = 1.1-1.3 eV,
which is much larger than that for well-known phosphor-
escent metal complexes (ΔE(S1-T1) = 0.4-0.7 eV).1 Since
phosphorescence is no longer observed for organic com-
pounds with ΔE(S1-T1) greater than 1 eV usually, this is
indicative of the large bismuth heavy-atom effects.
Optical properties of the present dithienobismoles
DTBi1-DTBi4 are summarized in Table 1. The absorption
maxima of DTBi2 and DTBi3 are at 356 and 359 nm (Figure 2a),
which are comparable to that of similarly substituted dithie-
nosilole DTS2 (λmax 354 nm) but red-shifted from that of
Chart 1
Table 1. Optical Properties of DTBi Derivatives
phosphorescence λem/nm (Φ/%)b
Scheme 1. Synthesis of Dithienobismoles
compd UV-vis abs/nma
in CHCl3
solid
d
τ/μse
f
DTBi1 346, 357
DTBi2 356, 371c
622 (0.2)
625 (0.2)
635 (0.2)
601 (0.2)
620
617
5.2
6.3
2.1
DTBi3 359, 376c
DTBi4 322, 336, 375, 392
d
a In CHCl3. b λex 380 nm; quantum efficiency (Φ) was determined by
using a 5,10,15,20-tetraphenylporphyrinatozinc solution (Φ = 3.3%) as
the reference, which shows red fluorescence at 600 nm.13 c Shoulder.
d Not observed. e In CHCl3; λex 380 nm and λobs 620 nm. f Not deter-
mined.
Figure 1. (a) ORTEP drawing of DTBi3 with ellipsoids at the 50% probability level. Hydrogen atoms are omitted for clarity. Selected
˚
˚
˚
bond lengths (A) and angles (deg): Bi-C(thiophene) = 2.241(3), 2.265(3) A; Bi-C(Ph) = 2.256(3) A; C(thiophene)-Bi-C(thiophene) =
78.2(1)°; C(thiophene)-Bi-C(Ph) = 93.5(1), 96.9(1)°. (b) HOMO and (c) LUMO profiles of DTBi3, derived from DFT calculations at
the level of B3LYP/6-31G(d)/LANL2DZ with ECP applied to Bi only.

Communication
Organometallics, Vol. 29, No. 15, 2010 3241
Figure 2. (a) UV-vis absorption and (b) emission spectra of dithienobismoles (λex 380 nm) in CHCl3. Emission spectra were recorded
in argon.
organic phosphorescence materials, and studies on the pre-
paration of bismole derivatives with higher phosphorescence
quantum yields are in progress.
Acknowledgment. Financial support from the Shorai
Foundation for Science and Technology and Electric
Technology Research Foundation of Chugoku is grate-
fully acknowledged. Combustion elemental analyses of
Figure 3. Photos of DTBi2 crystals in air at room temperature:
(a) under room light; (b) under UV laser irradiation at 375 nm.
DTBi1 were carried out at the Center of Instrumental
Analysis, Yamaguchi University, and the Elemental Ana-
lytical Center affiliated with the Faculty of Science,
Kyushu University.
In conclusion, we prepared dithienobismoles and demon-
strated their potential utilities as phosphorescence materials.
It is noteworthy that DTBi2 and DTBi3 showed stable
phosphorescence even in the solid state in air at room
temperature. The large heavy-atom effects of bismuth in
the system seem applicable to the development of novel
Supporting Information Available: CIF files giving crystal-
lographic data for compounds DTBi2-DTBi4, and text, fig-
ures, and a table giving experimental details, ORTEP drawings
of compounds DTBi2 and DTBi4, HOMO and LUMO profiles
of DTBi1, DTBi2, and DTBi4 derived from theoretical calcula-
tions, emission spectra of DTBi2 in chloroform as prepared and
after standing for 2 weeks at room temperature in air, and
emission spectra of DTBi2 in air and under nitrogen. This
material is available free of charge via the Internet at http://
pubs.acs.org.
(11) (a) Ballardini, R.; Varani, G.; Indelli, M. T.; Scandola, F. Inorg.
Chem. 1986, 25, 3858. (b) Strasser, A.; Volger, A. Inorg. Chem. Commun.
2004, 7, 528. (c) Smith, R. C.; Earl, W. J.; Protasiewicz, J. D. Inorg. Chim.
Acta 2004, 357, 4139.
(12) Kang, Y.; Song, D.; Schmider, H.; Wang, S. Organometallics
2002, 21, 2413.
(13) Quimby, D. J.; Longo, F. R. J. Am. Chem. Soc. 1975, 97, 5111.