3-Boryl-2,2′-bithiophene as a versatile core skeleton for full-color highly emissive organic solids

Article abstract of DOI:10.1002/anie.200604935

Seeing the light: Tuning the electron-donating ability of the π-conjugated framework of bithiophene has resulted in intense solid-state emissions with maxima ranging over a wide visible region (see picture). Even a deep-red fluorescence with a large Stoke

Full text of DOI:10.1002/anie.200604935

Angewandte
Chemie
DOI: 10.1002/anie.200604935
Emissive Organic Solids
3-Boryl-2,2’-bithiophene as a Versatile Core Skeleton for Full-Color
Highly Emissive OrganicSolids**
Atsushi Wakamiya, Kenji Mori, and Shigehiro Yamaguchi*
The design and synthesis of highly emissive organic materials
which can fluoresce even in the solid state is a fundamental
and important requirement for various optoelectronic and
photonic applications, such as organic light-emitting diodes,
organic lasers, and sensors.[1] Whereas a number of molecules
are known to be highly fluorescent in dilute solution, most of
them tend to show a decreased fluorescence in the solid state
because of certain intermolecular interactions that result in
significant quenching of the emission. Aproblem that has to
be overcome to achieve an intense solid-state emission is how
to suppress the quenching processes. We recently reported the
Figure 1. Schematic presentation of the molecular design of emissive
synthesis of boryl-substituted oligo(p-phenyleneethynylene)s
and related compounds for the rational design of highly
emissive organic solids.[2] While p systems bearing boryl
groups generally have strong fluorescence in solution,[3–12]
we showed that the introduction of bulky dimesitylboryl
groups at the side positions of the electron-donating p frame-
work is particularly effective for obtaining fluorescence
quantum yields close to unity, even in the solid state. This
property is attributable to two factors: First, the steric bulk of
the boryl groups prevents intermolecular interaction. Second,
a large Stokes shift resulting from the intramolecular charge-
transfer (CT) transition from the electron-donating p system
to the electron-accepting boron moiety diminishes the self-
quenching in the condensed phase. On the basis of this design
principle, we now disclose 3-boryl-2,2’-bithiophene-based p-
conjugated materials as a new highly emissive system
(Figure 1). The employment of the oligothiophene as an
electron-donating p skeleton allows us to achieve not only an
intense solid-state emission, but also full-color emissions
covering a wide range—from blue (477 nm) to deep red
(660 nm).
organic solids and the structures of 3-boryl-2,2’-bithiophene derivatives
1–7. Mes=mesityl.
Scheme 1. Reagents and conditions: a) 1. nBuLi, diethyl ether, À788C,
2. Mes2BF; b) NBS, CH2Cl2; c) for 3 and 4: ArB(OH)2, [Pd2-
(dba)3]·CHCl3, 2-(2’,6’-dimethoxybiphenyl)dicyclohexylphosphine (S-
Phos), K3PO4, toluene, 508C; for 5–7: ArSnBu3, [Pd2(dba)3]·CHCl3,
P(2-furyl)3, THF, reflux. dba=trans,trans-dibenzylideneacetone.
The parent compound 3-dimesitylboryl-2,2’-bithiophene
(1) was synthesized by the lithiation of 3-bromo-2,2’-bithio-
phene followed by reaction with Mes2BF (Scheme 1). Dibro-
mination of 1 with two equivalents of N-bromosuccinimide
(NBS) produced the dibromide 2. Aseries of p-extended
derivatives 3–7 were prepared by palladium-catalyzed cou-
pling reactions of 2 with appropriate aryl stannanes or aryl
boronic acids.[13] All of the boryl-substituted bithiophene
derivatives produced are stable in air and water, and can be
purified by column chromatography on silica gel. These
compounds have a high thermal stability; for example, the
decomposition temperatures for a 5% weight loss (Td5) of 6
and 7 are 416 and 3858C, respectively.
During the course of the synthesis we found that the
parent compound 1 shows an intense sky-blue fluorescence
with a maximum wavelength of 477 nm (FF 0.66) in THF. This
long emission wavelength is remarkable considering the short
p-conjugation length of the bithiophene unit. To demonstrate
its uniqueness we first compared it to its regioisomer,
5-dimesitylboryl-2,2’-bithiophene (8).[6b]
[*] Dr. A. Wakamiya, K. Mori, Prof. Dr. S. Yamaguchi
Department of Chemistry
Graduate School of Science, Nagoya University, and
SORST, Japan Science and Technology Agency
Chikusa, Nagoya 464-8602 (Japan)
Fax: (+81)52-789-5947
E-mail: yamaguchi@chem.nagoya-u.ac.jp
Figure 2 shows the crystal structure of 1.[14] While 8 has a
coplanar bithiophene framework, the bithiophene skeleton of
the 3-boryl-substituted 1 is significantly twisted with a
dihedral angle of 56.08, which apparently arises from the
[**] This work was partially supported by SORST, JST, and the TOYOTA
Motor Co.
Supportinginformation for this article is available on the WWW
under http://www.angewandte.org or from the author.
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absorption spectra. More noteworthy is that the knr value of
1 (2.8 ” 107 sÀ1) is also less than that of 8 (2.8 ” 108 sÀ1), thereby
demonstrating the suppressed radiationless decay process in
1, despite the larger Stokes shift in 1 than 8. In addition to
these differences, it is also worth noting that the fluorescence
spectra of 1 show only a subtle solvent effect (lem = 457
(cyclohexane), 477 (THF), 478 nm (MeOH)), which is
indicative of a less-polarized excited state produced by the
intramolecular charge-transfer transition,[15] unlike ordinary
donor–acceptor-type p-electron systems.
In the present p-electron systems the emission comes
from the singlet excited state generated by the intramolecular
charge-transfer transition from the bithiophene moiety to the
boron moiety, and thus the emission wavelength is highly
dependent on the electron-donating ability of the p-conju-
gated framework of the bithiophene unit (Figure 4 and
Table 1). Thus, the extension of the p conjugation with
mesityl (3) or phenyl (4) groups shifts the emission maximum
to 510 nm (green) and 543 nm (greenish yellow), respectively.
Figure 2. ORTEP drawingof 1 (50% probability for thermal ellipsoids).
steric congestion of the 3-boryl group. Despite this difference,
the longest wavelength absorption maximum of both com-
pounds are comparable (around 370 nm) in the UV/Vis
spectra (Figure 3), although the molar extinction coefficient
of 1 is much smaller (loge = 3.65) than that of 8 (loge = 4.51).
According to TD-DFT calculations (B3LYP/6-31G(d)), the
absorption bands of both 1 and 8 can be essentially assigned to
the intramolecular charge-transfer transition from the highest
occupied molecular orbital (HOMO) delocalized over the
bithiophene moiety to the lowest unoccupied molecular
orbital (LUMO) mainly localized on the boron center. In
contrast to this similarity in the absorption spectra, significant
differences are observed in the fluorescence properties. First,
the wavelength of the emission maximum (lem) of 1 is more
than 40 nm longer than that of 8. The Stokes shift of 1 exceeds
100 nm (5990 cmÀ1), whereas that of 8 is only 56 nm
(3500 cmÀ1). This large Stokes shift is one notable character-
istic of the present 3-boryl-bithiophene skeleton; presumably,
it may be attributed to the change from the twisted structure
to the planar structure in the excited state. Second, the
lifetime of the singlet excited state (ts) of 1 (12 ns) is much
longer than that of 8 (1.7 ns). According to calculations of the
radiative (kr) and nonradiative (knr) rate constants from the
FF and ts values, the kr value (5.5 ” 107 sÀ1) of 1 is much less
than that of 8 (3.1 ” 108 sÀ1). This finding is consistent with the
transition of 1 being less-allowed, as observed in the
Figure 4. Fluorescence of 3-borylbithiophene derivatives 1 and 3–7:
a) Emission spectra measured in THF and b) photographs under
irradiation at 365 nm.
The introduction of electron-donating carbazolyl (5) or
diphenylamino (6) groups to the terminal phenyl rings
causes further red-shifts to 557 nm (yellow) and 600 nm
(reddish orange), respectively, with the retention of very high
quantum yields (exceeding 0.90). Moreover, the introduction
of an electron-donating 5-(diphenylamino)-2-thienyl group
further shifts the emission wavelength. Thus, compound 7 has
a deep-red emission at 660 nm with a Stokes shift of 195 nm in
THF. This example is of particular interest from the viewpoint
of converting a blue excitation light into a deep-red emission.
Such emitting material with a large Stokes shift of about
200 nm is rather rare among the number of red fluorescence
dyes reported to date.[16–18] Notably, the Stokes shift of 7
(6350 cmÀ1) is comparable to that of 1 (5990 cmÀ1). This fact
demonstrates that the unique property of the 3-boryl-
bithiophene skeleton directly affects the properties of its p-
extended derivatives.
Figure 3. UV/Vis absorption and fluorescence spectra of borylbithio-
phenes 1 and 8 in THF.
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Table 1: Photophysical data of 3-borylbithiophene derivatives 1 and 3–8.
Keywords: boron · conjugation · fluorescence · Stokes shift ·
thiophene
.
Compd
Absorption
labs (loge)[a]
[nm]
Fluorescence
Stokes shift
Lifetime
lem (FF)[c]
[nm] ([cmÀ1])
t [ns]
[b]
[nm]
1
3
4
5
6
7
8
THF
371 (3.65)
386
384 (3.80)
385
415 (4.02)
424
422 (4.26)
430
449 (4.31)
456
465 (4.30)
479
477 (0.66)
486 (0.55)
510 (0.92)
515 (0.87)
543 (0.90)
552 (0.85)
557 (0.93)
564 (0.87)
600 (0.90)
601 (0.60)
660 (0.38)
657 (0.30)
429 (0.53)
106 (5990)
100 (5330)
126 (6430)
130 (6560)
128 (5680)
128 (5470)
135 (5740)
134 (5530)
151 (5610)
145 (5290)
195 (6350)
178 (5660)
56 (3500)
12.0
8.9
7.5
5.2
5.0
3.0
1.7
film[d]
THF
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film[d]
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film[d]
THF
film[d]
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The 3-borylbithiophene derivatives 1 and 3–7 show
intense fluorescence not only in solution, but also in the
solid state (Figure 4b). The decrease in the FF value of the
spin-coated film relative to that of the THF solution is within
20% for 1 and 3–5, and about 33% and 21% even for the
orange-emissive 6 and red-emissive 7, respectively. It is also
worth noting that no significant difference in the fluorescence
spectra of the THF solution and the spin-coated film, in terms
of both the emission maximum wavelength and the full-width
at half-maximum (fwhm), is observed for all the compounds.
The intense solid-state emission is a rather general property
for the present 3-boryl-bithiophene-based p systems.
We also investigated the electrochemical properties of the
present systems to evaluate their potential applicability in
electronic devices. Compounds 6 and 7 not only show
reversible oxidation waves by cyclic voltammetry
(6: +0.38 V; 7: + 0.14 V, versus ferrocene/ferrocenium
(Fc/Fc+)), they also show reversible reduction waves (6:
À2.24, À2.61 V; 7: À2.18 V; see the Supporting Information).
The high reversibility in their redox process demonstrates the
substantial stability of the produced charged species, and is
indicative of their potential use as emissive ambipolar-trans-
porting materials.
In summary, a series of emissive 3-boryl-2,2’-bithiophene-
based p-electron materials have been designed and synthe-
sized. The results show the versatility of the 3-borylbithio-
phene skeleton for attaining an intense solid-state emission,
with the maxima ranging over the whole visible region.
Notably, even a red-emitting material with an extremely large
Stokes shift of 195 nm could be obtained. The present results
demonstrate the generality and effectiveness of our molecular
design for creating highly emissive organic solids. Afurther
study in which these materials are utilized for optoelectronic
applications is now in progress.
Received: December 6, 2006
Published online: March 22, 2007
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Communications
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