Highly emissive organic solids containing 2,5-diboryl-1,4-phenylene unit

Article abstract of DOI:10.1021/ja0637550

Wedisclosed a series of π-conjugated systems containing 2,5-bis(dimesitylboryl)-1,4-phenylene as the core unit and electron-donating amino groups at the terminal positions. The extension of the p_π-π* conjugation in the diborylphenylene moiety along the short axis of the π-conjugated framework as well as the incorporation of two bulky dimesitylboryl groups at the para-positions makes this moiety act as a unique bulky π-electron-accepting unit. As a consequence, these systems behave like donor-acceptor-donor quadrupolar π-electron systems and show a large solvatochromism in the fluorescence spectra. Moreover, these organoboron π systems exhibit intense fluorescence even in the solid state with the quantum yields of 0.73-0.90. Copyright

Full text of DOI:10.1021/ja0637550

Published on Web 11/22/2006
Highly Emissive Organic Solids Containing 2,5-Diboryl-1,4-phenylene Unit
Cui-Hua Zhao,† Atsushi Wakamiya,† Yuko Inukai,† and Shigehiro Yamaguchi*,†,‡
Department of Chemistry, Graduate School of Science, Nagoya UniVersity, and SORST, Japan Science and
Technology Agency, Chikusa, Nagoya 464-8602, Japan
Received May 29, 2006; E-mail: yamaguchi@chem.nagoya-u.ac.jp
Boron-containing π-electron materials have attracted increased
attention because of their unique properties stemming from the pπ-
π* conjugation between the vacant p orbital on the boron atom
with the π* orbital of the π-conjugated framework.1 The extensive
studies on the organoboron compounds have demonstrated their
potential uses for various photonic and optoelectronic applications,
such as nonlinear optics,2,3 two-photon excited emitters,4 organic
light-emitting diodes,5,6 and fluorescent sensors.7-9 Most of the
reported systems either contain the boron atoms in the main chain
or attach the boryl groups at the terminal positions of the
π-conjugated framework (Figure 1a and b). We now disclose a new
type of boron-based π-electron compounds, in which the boryl
groups are introduced at the side positions (Figure 1c). As a
π-electron-accepting core unit, we chose 2,5-bis(dimesitylboryl)-
1,4-phenylene and designed novel donor-acceptor-donor (D-A-
Figure 1. Schematic representation of organoboron π systems containing
boron atoms or boryl groups (a) in the main chain, (b) at the terminal
D) type quadrupolar π systems 1-4 (Figure 1d), in which the
electron-donating amino groups were introduced at the terminal
positions of the oligo(phenyleneethynylene) (OPE) or oligo-
(phenylenevinylene) (OPV) skeletons. We envisioned that the
extension of the pπ-π* conjugation along the short axis of the
π-conjugated framework as well as the incorporation of the bulky
boryl groups at the side positions would lead to unique properties.10
In fact, we found that these compounds showed intriguing intense
fluorescence properties in the solid state.
positions, and (c) at the side positions and (d) structures of 1-7.
The boryl-substituted OPEs 1-3 were prepared based on the
Sonogashira reaction of appropriate diboryldiethynylbenzenes with
aryl halides. The synthesis of the OPV derivative 4 was ac-
complished via a dilithiation of a dibromo-substituted distyrylben-
zene derivative followed by treatment with dimesitylfluoroborane.
All the compounds are stable to air and water and have high thermal
stability. The decomposition temperature for a 5% weight loss (Td5)
of 1 is 373 °C. Among the compounds, the structure of 1 was
determined by X-ray crystallography. This molecule has a signifi-
cantly twisted main chain, in which the dihedral angle between
the central and terminal benzene rings is 47.5°. This nonplanar
structure arises from the steric hindrance of the bulky dimesitylboryl
groups at the side positions.
Figure 2. Absorption and fluorescence spectra of 1.
Figure 3. Pictorial presentation of (a) HOMO and (b) LUMO of 1
calculated at the B3LYP/6-31G(d) level using its crystal structure.
The absorption and fluorescence spectra of the boryl-substituted
OPE 1 are shown in Figure 2. In cyclohexane, 1 shows a moderately
intense absorption band at 432 nm (log ꢀ ) 4.30). The TDDFT
calculation (B3LYP/6-31G(d)) indicates that this band is assignable
to the intramolecular charge transfer (CT) transition from the
HOMO delocalized over the OPE framework to the LUMO
localized on the diborylphenylene unit. Apparently, the vacant p
orbitals on the boron atoms substantially contribute to this LUMO
(Figure 3). In the fluorescence spectrum, despite the highly twisted
structure, 1 exhibits a very intense emission at 536 nm with a
quantum yield (ΦF) of 0.99. It is worth noting that this emission
band is strongly dependent on the solvent. A significant red shift
is observed from 536 nm in cyclohexane to 601 nm in THF and
627 nm in methanol, whereas only a subtle solvent dependence is
observed in the absorption maxima (λmax ) 432 nm in cyclohexane
and 437 nm in THF). This result confirms that the diborylphenylene
in the D-A-D system can work as the π-electron-accepting unit
only in the excited state.
Notably, even in the solid state, 1 shows an intense fluorescence.
In the spin-coated film, 1 shows an emission at 562 nm and retains
a high quantum yield of 0.90, as determined by the calibrated
integrating sphere system. It is also interesting to note that the
emission spectrum is almost identical to that in benzene in terms
of not only the maximum wavelength (559 nm in benzene), but
also the fwhm, indicative of a comparable environment arisen in
† Nagoya University.
‡ SORST, Japan Science and Technology Agency.
9
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J. AM. CHEM. SOC. 2006, 128, 15934-15935
10.1021/ja0637550 CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Photophysical Properties of 1, 5, and 6
Table 2. Photophysical Properties of Boryl-Substituted
Compounds in the Spin-Coated Films
c
compd
λ
abs/nma
λ
em/nmb
ΦF
τ/ns
c
compd
λ
abs/nma
λ
em/nmb
ΦF
1
benzene
filmd
442
447
465
489
389
395
559
562
520
575
430
434
0.98
0.90
0.98
0.29
0.92
0.39
5.4
1.3/6.9 (37/63)e
2.2
1
2
3
4
7
447
442
400 (sh)
471
400 (sh)
562
555
504
596
476
0.90
0.86
0.85
0.73
0.43
5
6
benzene
filmd
0.4/3.0 (85/15)e
1.0
benzene
filmd
0.2/0.7 (93/7)e
a Only the longest absorption maxima are shown. b Excited at the longest
absorption maxima. c Absolute quantum yield determined by a calibrated
integrating sphere system within ( 3% errors. d Spin-coated from THF
solution. e Amplitudes of two lifetimes are given in the parentheses.
a Only the longest absorption maxima are shown. b Excited at the longest
absorption maxima. c Absolute quantum yield determined by a calibrated
integrating sphere system within ( 3% errors.
state.11 Development of such materials is a very fundamental subject
for realizing many new optoelectronic applications, such as organic
lasers. The present results not only disclose a new class of emissive
organoboron materials, but also provide a firm direction in the
design of emissive organic solids. The diborylphenylene skeleton
is a unique skeleton for the emissive D-A-D systems, in terms
of the high electron-accepting ability as well as the steric bulkiness.
We believe that the present simple idea, incorporating the boryl
groups as the side groups, will be the basis of further rational
designs for functional emissive materials.
Acknowledgment. This work was partially supported by
SORST, JST. C.-H. Zhao is grateful to the Japan Society for the
Promotion of Science (JSPS) for a Postdoctoral Fellowship.
Supporting Information Available: Experimental procedures,
X-ray crystal data for 1, theoretical calculation of 1, photophysical data
including the absorption and emission spectra of 1, 5, and 6, and
complete list of ref 2e. This material is available free of charge via the
Internet at http://pubs.acs.org.
the solid state and in the dilute benzene solution. A similar
fluorescence property was also observed for the powder of 1.
To elucidate the effect of the diborylphenylene skeleton, we
compared the photophysical properties of 1 with those of 5 and 6,
which have electron-accepting cyano groups or neutral bulky
triisopropylsilyl (TIPS) groups, respectively, in place of the
dimesitylboryl groups (Table 1).
The dicyanophenylene skeleton also behaves as the π-electron-
accepting unit mainly in the excited state, similar to the diborylphen-
ylene unit. Thus, 5 exhibits large solvatochromism in the fluores-
cence spectra (λmax ) 482 nm in cyclohexane, 602 nm in THF),
while in the absorption spectra only a slight blue shift is observed
(λmax ) 464 nm in cyclohexane, 454 nm in THF). The most notable
fact is that although 5 shows an intense fluorescence in benzene
(ΦF ) 0.98), in the spin-coated film, the ΦF of 5 significantly
decreases to 0.29, in sharp contrast to the retained high ΦF of 1.
This demonstrates the importance of a sterically bulky skeleton for
attaining the intense solid-state emission, which can prevent the
intermolecular interaction and hence the nonradiative energy
migration through the Dexter mechanism. On the other hand,
although compound 6 contains the bulky TIPS groups, the emission
of it still significantly decreases from a ΦF of 0.92 in benzene to
a ΦF of 0.39 in the solid-state. It is worth noting that the Stokes
shift of 6 in the film is 2275 cm-1, which is much smaller than
those for 1 (4578 cm-1). This result suggests that the electron-
accepting character of diborylphenylene skeleton also plays an
important role for the intense solid-state emission, possibly because
it gives rise to the intramolecular CT transition with a large Stokes
shift, which suppresses the self-quenching through the Fo¨rster
mechanism.
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