Intense solid-state blue emission with a small Stokes' shift: π-stacking protection of the diphenylanthracene skeleton

Article abstract of DOI:10.1039/b901794a

Intense blue emissions with a small Stokes' shift both in solution and in the solid state were attained by introducing (pentafluorophenyl)dimethylsilyl groups to a 9,10-diphenylanthracene skeleton as the π-stacking tethers. The Royal Society of Chemistry

Full text of DOI:10.1039/b901794a

Showcasing research from Shigehiro Yamaguchi’s
laboratory, Nagoya University, Japan.
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Chemical Communications
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Number 21 | 7 June 2009 | Pages 2953–3124
Title: Intense Solid-State Blue Emission with a Small Stokes’ Shift:
pi-Stacking Protection of Diphenylanthracene Skeleton
A highly blue-fluorescent compound with a small Stokes’shift
was synthesized by introducing (pentafluorophenyl)dimethylsilyl
groups into a diphenylanthracene skeleton as the π-stacking
tethers. This compound showed high fluorescence quantum
yields even in the solid state (ΦF = 0.93 in a crystal), despite
substantial self-absorption.
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Intense solid-state blue emission with a small Stokes’ shift: p-stacking
protection of the diphenylanthracene skeletonw
Azusa Iida and Shigehiro Yamaguchi*
Received (in Cambridge, UK) 28th January 2009, Accepted 23rd February 2009
First published as an Advance Article on the web 19th March 2009
DOI: 10.1039/b901794a
Intense blue emissions with a small Stokes’ shift both in solution
and in the solid state were attained by introducing (pentafluoro-
phenyl)dimethylsilyl groups to a 9,10-diphenylanthracene skeleton
as the p-stacking tethers.
the skeleton by forming this kind of interaction so as to
suppress the non-radiative decay process from the excited
state and make the radiative process more dominant, besides
serving as a bulky group to prevent any undesired inter-
molecular interaction in the solid state.
Compound
1 was readily synthesized from 9,10-bis-
Because of their potential use in the field of optoelectronics,
such as high performance organic electroluminescent devices,1
light-emitting field-effect transistors,2 and lasers,3 continuous
efforts have been devoted to exploring efficient, stable, and
pure color-emitting organic solids. Several approaches to gain
an intense solid-state emission have been reported, such as
dendritic substituent protection,4 cross dipole stacking,5
aggregation-induced emission,6 enhanced intramolecular
charge transfer transition,7 and J-aggregate formation.8 None-
theless, it still remains a challenging issue to attain an intense
solid-state blue emission with a small Stokes’ shift, because the
small Stokes’ shift gives rise to a large overlap between the
absorption and fluorescence bands and thus serious self-
absorption in the solid state, leading to fluorescence quenching.
Consequently, the hitherto reported molecules showing
intense blue emissions in the solid state generally have relatively
large Stokes’ shifts.9 We now report a new design concept to
attain this property, which is to rigidify an inherently emissive
skeleton with p-stacking protection (Fig. 1). We envisioned
that, if the molecule has thoroughly quantitative fluorescence
efficiency, the self-absorption in the solid state would no
longer result in fluorescence quenching, even in the case of a
small Stokes’ shift.
(2-bromophenyl)anthracene. This key precursor, synthesized
by Suzuki–Miyaura coupling of 9,10-diiodoanthracene with
2-bromophenylboronic acid, was converted to 1 in 18% yield
through the halogen–lithium exchange reaction with t-BuLi in
THF followed by treatment with (C6F5)Me2SiCl.
The absorption and fluorescence spectra of 1 in solution and
in the solid state are shown in Fig. 2. In THF, 1 shows
absorption and emission spectra similar to those of the parent
DPA. The Stokes’ shift of 1 is very small (Dl = 9 nm,
550 cmꢀ1) and its fluorescence quantum yield is almost
quantitative (FF = 0.99). In the crystal, 1 shows a red-shifted
fluorescence having a maximum at 445 nm with a small
shoulder around 420 nm. Apparently, this shoulder corres-
ponds to the shortest l0–0 band observed in the fluorescence
spectrum in solution. The significant decrease in the intensity
of this band is a result of self-absorption. Despite this, the
quantum yield of the crystal is as high as 0.93.10 To confirm
the generality of this property, we also prepared its cast film,
which showed a high fluorescence efficiency of FF = 0.81.
To demonstrate the effect of the (pentafluorophenyl)silyl
groups in 1, we also synthesized other silyl-substituted
derivatives, ortho-Me3Si-substituted 2, meta-Me3Si-substituted 3,
and ortho-Me2PhSi-substituted 4 (Fig. 1), whose photo-
physical properties are summarized in Table S1 (ESIw).
Whereas all these compounds show fluorescence spectra similar
to that of 1 with high quantum yields (0.96–0.99) in THF,
these values decrease in the solid state (Fig. 3). For instance,
the quantum yields of the phenyl analog 4 in the crystal and in
a thin film are 0.75 and 0.43, respectively. The more simple
Me3Si-substituted 2 also shows high fluorescence yield in the
crystal (FF = 0.94), suggesting that if a sufficiently rigid
9,10-Diphenylanthracene (DPA) is well known as a blue
fluorescent compound. In THF, it shows a high fluorescence
quantum yield of 0.9710 with a small Stokes’ shift (Dl = 14 nm,
870 cmꢀ1). This intense emission, however, decreases in the
solid state due to self-absorption. To overcome this problem,
we sought to modify its skeleton. Thus, we designed a DPA
derivative 1 in which (pentafluorophenyl)dimethylsilyl groups
were introduced at the ortho positions of the 9,10-phenyl
groups as the p-stacking tethers (Fig. 1).z A pentafluorophenyl
group is known to form a strong face-to-face p-stacking with a
phenyl group, because of its electron-accepting nature.11 We
expected that the pentafluorophenyl groups in 1 would rigidify
Department of Chemistry, Graduate School of Science,
Nagoya University, Furo, Chikusa, Nagoya, 464-8602, Japan.
E-mail: yamaguchi@chem.nagoya-u.ac.jp; Fax: +81 52 789 5947;
Tel: +81 52 789 2291
w Electronic supplementary information (ESI) available: Experimental
procedures, spectral data for all new compounds, photophysical data,
and crystallographic data of 1 and 4. CCDC 717643 and 717644. For
ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/b901794a
Fig. 1 p-Stacking protection of the anthracene fluorophore: (a)
schematic representation and (b) structures of DPA derivatives 1–4.
ꢁc
This journal is The Royal Society of Chemistry 2009
3002 | Chem. Commun., 2009, 3002–3004

Fig. 2 Absorption and fluorescence spectra of 1 in THF (black), the
crystal (blue), and as a thin film (light blue).
Fig. 3 Histograms of the quantum yield of 1–4, and DPA in THF
(black), the crystal (blue) and as a thin film (light blue).
environment is realized in the crystalline state, such high
quantum yields can be achieved. However, the FF of 2
in a film decreases to 0.73. The higher FF of 1 in a film
compared to that of 2 demonstrates the important role of the
(pentafluorophenyl)silyl group for attaining high fluorescence
efficiency, irrespective of the states of the compound.
Fig. 4 ORTEP drawings of (a) 1A, (b) 1B, and (c) 4 (50% thermal
probability for ellipsoids). Hydrogen atoms are omitted for clarity.
is 4.0 ꢂ 106
s
ꢀ1, which is much smaller than that of 4
To elucidate the origin of the intense solid-state emission of
1, its crystal structure was determined and compared to that of
the phenyl analog 4. Compound 1 contains two crystallo-
graphically independent molecules 1A and 1B (Fig. 4). In 1A,
the pentafluorophenyl groups are arranged right above the
terminal benzene moiety of the anthracene skeleton with an
interfacial distance of B3.36 A, indicative of tight p-stacking.
In 1B, the pentafluorophenyl groups slightly protrude from
the anthracene skeleton, but there is still a definite interaction
between them (the interfacial distance, B3.37 A). These
structures are in contrast to the structure of 4, in which the
phenyl groups are located relatively far from the anthracene
skeleton, probably due to steric repulsion. In both compounds
1 and 4, no specific intermolecular interaction is observed. The
intramolecular p-stacking interaction observed in 1A and 1B
likely increases the structural rigidity in the solid state,
leading to the high fluorescence efficiency. This argument was
supported by an excited state dynamics study. The time-
resolved fluorescence spectroscopy showed the single-
exponential decays from the singlet excited state both in 1
and 4 in the crystalline state. The fluorescence lifetimes (ts) of 1
and 4 are 17.4 and 13.0 ns, respectively. The non-radiative
decay rate constant for 1, calculated from the FF and ts,
(1.9 ꢂ 107 sꢀ1), indicating that the non-radiative decay process
is suppressed due to the enhanced rigidity of the skeleton.
It is interesting to compare the present result with the fact
that p-stacking often leads to fluorescence quenching through
the formation of a non-emissive aggregate or excimer/exciplex.
In compound 1, its slightly red-shifted l0–1 emission band at
445 nm in the crystal compared to that in solution (432 nm)
suggests that a certain electronic interaction indeed occurs
between the anthracene and pentafluorophenyl groups.
Nevertheless, this interaction does not result in fluorescence
quenching in the present system. The reason for this pheno-
menon is not clear at this moment.
The present property, i.e., intense blue emission with a small
Stokes’ shift, is crucial to gain an intense white light emission
from a neat film without any binder. We prepared a white
light-emitting film using 1 as a blue-emitting host material,
together with green-emissive 3-(dimesitylboryl)bithiophene 5
(lem = 510 nm, FF = 0.92 in THF)7b and red-emissive
perphenylated BODIPY 6 (lem = 609 nm, FF = 0.73 in
poly(methyl methacrylate) matrix) (Fig. 5).12 A spin-coated
film was prepared from a mixture of these compounds in
CH2Cl2 in the ratio 1 : 5 : 6 = 2000 : 20 : 1. This film indeed
showed a white emission with CIE coordinates of (0.32, 0.32)
ꢁc
This journal is The Royal Society of Chemistry 2009
Chem. Commun., 2009, 3002–3004 | 3003

P21/c (no. 14), Z = 2, 10 480 reflections measured, 2874 unique
reflections (Rint = 0.0270) which were used in all calculations.
R1 = 0.0360, wR2 = 0.1364. CCDC 717644.
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¨
In conclusion, we have demonstrated that p-stacking
protection is effective to rigidify the skeleton, so that the
DPA derivative 1 can maintain an intense blue emission in
the solid state, despite its small Stokes’ shift. This result not
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strategy to other p-conjugated systems is currently in progress.
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Notes and references
z Crystal data for 1 (from ethyl acetate): C42H28F10Si2, M = 778.82,
triclinic, a = 9.5475(13), b = 12.9922(14), c = 15.7118(16) A,
a = 66.101(5)1, b = 80.559(6)1, g = 78.204(6)1, U = 1737.0(3) A3,
10 The absolute fluorescence quantum yields were determined with a
Hamamatsu C-9920-02 integrating sphere system. The dependence
of the FF on the crystal size has been carefully examined: see ESIw.
11 (a) C. R. Patrick and G. S. Prosser, Nature, 1960, 187, 1021;
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ꢀ
T = 100 K, space group P1 (no. 2), Z = 2, 11 475 reflections
measured, 5945 unique reflections (Rint = 0.0237) which were used
in all calculations. R1 = 0.0369, wR2 = 0.0946. CCDC 717643.
Crystal data for 4 (from ethyl acetate): C42H38Si2, M = 598.90,
monoclinic, a = 11.1085(18), b = 9.2128(14), c = 16.355(3) A,
b = 101.6544(7)1, U = 1639.3(5) A3, T = 100 K, space group
12 A. Wakamiya, N. Sugita and S. Yamaguchi, Chem. Lett., 2008, 37,
1094.
ꢁc
This journal is The Royal Society of Chemistry 2009
3004 | Chem. Commun., 2009, 3002–3004