Bulky 4-tritylphenylethynyl substituted boradiazaindacene: Pure red emission, relatively large Stokes shift and inhibition of self-quenching

Article abstract of DOI:10.1039/b808681h

Bulky 4-tritylphenylethynyl substituted boradiazaindacene with pure red emission, relatively large Stokes shift, high fluorescence quantum yield, and low self-quenching was efficiently synthesized and qualified as a potential EL dopant. The Royal Society of Chemistry.

Full text of DOI:10.1039/b808681h

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Bulky 4-tritylphenylethynyl substituted boradiazaindacene: pure red
emission, relatively large Stokes shift and inhibition of self-quenchingw
Dakui Zhang,a Yugeng Wen,b Yi Xiao,*a Gui Yu,*b Yunqi Liub and Xuhong Qian*c
Received (in Cambridge, UK) 22nd May 2008, Accepted 30th June 2008
First published as an Advance Article on the web 6th August 2008
DOI: 10.1039/b808681h
Bulky 4-tritylphenylethynyl substituted boradiazaindacene with
pure red emission, relatively large Stokes shift, high fluorescence
quantum yield, and low self-quenching was efficiently synthe-
sized and qualified as a potential EL dopant.
most known BODIPY dyes are green emitters. Some red-
emissive BODIPY dyes previously reported are ‘‘excessively
red’’ (considerable emission spectra in the near infrared re-
gion), and might not be suitable candidates for display appli-
cations. Secondly, self-quenching should be avoided by both
increasing the Stokes shift and inhibiting molecular aggrega-
tion. It is a challenge to develop fluorophores with relatively
large Stokes shifts in combination with high fluorescence
quantum yields and narrow emission spectra.6 Although it is
a good method to introduce bulky and rigid side chains to
strengthen steric hindrance for inhibition of molecular aggre-
gation, this strategy is rarely attempted on the BODIPY
platform.7
Fluorescent dyes based on boradiazaindacene (BODIPY or
BDP),1 show excellent optical properties and photostability,
and play increasingly important roles in many fields such as
molecular probes,2 laser dyes,3 and solar energy harvesters.4 In
addtion, because of their typically narrow emission spectra
which may result in attractively high color purity, they are
expected to demonstrate further application as potential
emitters for organic light-emitting diodes (OLEDs).
With these considerations, we introduced two 4-tritylphenyl-
ethyl groups into the 2,6-positions of the BODIPY core
(compound 3). It is reasonable to expect that the extended
conjugation will lead to a red shift of the emission maxima,
and the bulkiness of the substituents will efficiently inhibit
concentration quenching due to the reduction of p–p stacking.
Additionally, with substituents on all the positions of BODIPY,
our compounds were expected to have larger Stokes shifts than
the reported 3,5-bisphenylethyl substituted BODIPY8 which has
no substituents on positions 1,2,6,7. According to previous
experiences on BODIPY derivatives, steric hindrance effect
and nonplanarity induced by higher substitution degree will
lead to an increase in Stokes shift, due to the change in
geometrical structure upon excitation.5b
However, there are few reports on the applications of
BODIPY dyes as EL dopants.5 It is a pity that most BODIPY
dyes have rather weak fluorescence in the solid state due to
their small Stokes shifts (5–20 nm, in most cases) which will
lead to serious self-quenching at high concentration. Moreover,
due to their high planarity in molecular structures, they have
an inherent tendency to aggregate compactly in solid state,
which further aggravates self-quenching. In the reported
devices, the doping amounts of the BODIPY dye in the host
materials were controlled to be very low (o1%) in order to
avoid self-quenching. It should be noted that lower doping
concentrations may lead to not only the lower EL intensities,
but also poor color purity because of the incompletely-
quenched shorter wavelength emission from the host materials.
On the other hand, in OLED applications, red-emissive
materials with good color purity, high efficiency and good
stability have been in great demand while efficient blue- and
green-emissive materials have been developed to meet the
commercial applications. Red-emissive materials remain as
the stumbling block in realizing full color OLED. We expect
that BODIPY dyes can solve such a problem. First, we need to
develop novel BODIPY derivatives with pure red emission, as
The target compound 3 and the reference compound 4 were
efficiently synthesized as shown in Scheme 1. Iodination of the
precursor BODIPY 1 with excess N-iodosuccinimide (NIS)
gives the 2,6-bisiodo-BODIPY 2 in high yield (70%) while
4-tritylphenylethyne was synthesized via a four-step route9
from chlorotriphenylmethane. Sonogashira coupling reaction
of the above two intermediates produced compound 3 with
satisfactory yield (71%). Noticeably, this facile synthetic work,
together with the previous reports,8,10 confirmed that
Pd-catalyzed Sonogashira coupling reactions were very
efficient and versatile methods for developing new longer
wavelength BODIPY derivatives.
a State Key Laboratory of Fine Chemicals, Dalian University of
Technology, Zhongshan Road 158, Dalian, China.
E-mail: xiaoyi@chem.dlut.edu.cn; Fax: +86-411-83673488;
Tel: +86-411-88993870
The absorption and emission spectra of compounds 3 and 4
in dichloromethane were recorded in Fig. 1, and the data were
b Beijing National Laboratory for Molecular Sciences, Institute of
Chemistry, Chinese Academy of Sciences, Beijing, China.
E-mail: yugui@mail.iccas.ac.cn
listed in Table 1. Compared with the parent BODIPY 1 (labs
=
498 nm, lflu = 508 nm),1a compounds 3 and 4 exhibit
significant red-shift in absorption and emission maxima, which
indicates that the phenylethynyl groups extend conjugation
efficiently. For compound 3, the emission band peak at 610 nm
has entered the red light region while compound 4 emits
c Shanghai Key Laboratory of Chemical Biology, East China
University of Science and Technology, Meilong Road 130, Shanghai,
China. E-mail: xhqian@ecust.edu.cn
w Electronic supplementary information (ESI) available: Experimental
section. CCDC 650117. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/b808681h
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Table 2 Electronic states studies of compounds 3 and 4
HOMO/
HOMO/
ox
onset
b
Ega/eV
E
Epox
Epredc
LUMOd/eV LUMOe/eV
2.01
2.11
a
0.73
0.81
1.07
1.00
À1.28
À1.30
À5.13/À3.12 À5.23/À2.69
À5.21/À3.10 À5.34/À2.67
Energy band gap, determined from UV-Vis absorption spectra.
b
ox
onset
E
= onset oxidation potential; Epox = oxidation peak potential;
potentials reported vs. ferrocene as internal standard, glassy carbon
working electrode, Ag/AgNO3 reference electrode, platinum counter
electrode, 0.1 M Bu4NPF6–CH2Cl2, scan rate 100 mV sÀ1 at 20 1C.
c
d
ox
Epred = reduction peak potential. HOMO = E
+ 4.4 eV;
onset
e
LUMO = HOMO À Eg. Energy levels estimated from theoretical
calculation.
energy level of the HOMO is a little higher than that of the
latter. These data indicate that the trityl groups play a role to
elevate the HOMO energy level without influencing the LUMO,
and thus reduce the energy gap and lead to spectra red-shifts of
compound 3 relative to compound 4. Another important in-
formation from the above studies is that both compounds may
be suitable for electroluminescence applications, because their
LUMO levels match well with those of normal electron transfer
layer materials (e.g. Alq3, À3.0 eV) and HOMO levels match
well with those of common hole transfer layer materials (e.g.
NPB, À5.2 eV).12
Scheme 1 Routes to 4-tritylphenylethyne and BODIPY derivatives 3
and 4. Reagents and conditions: (a) aniline, 220 1C and reflux with 2 M
HCl/MeOH, 95%; (b) NaNO2, HCl, 0 1C and KI, acetone 60 1C, 70%;
(c) ethynyltrimethylsilane, Pd(PPh3)2Cl2, CuI, DMF–NEt3 = 9 : 1,
85%; (d) 1 M NaOH, THF, 20 1C, 93%; (e) NIS, CH2Cl2, 70%;
(f) CuI, Pd (PPh3)4, DMF–NEt3 = 9 : 1, 65 1C, 71%, Sonogashira
coupling with compound 5; (g) Sonogashira coupling with ethynyl-
benzene, 76%.
X-Ray single crystal analysis revealed the nonplanar struc-
ture of compound 4. The dihedral angle between the pyrrole
plane and the phenyl moiety of phenylethynyl is 13.71 (shown
in Fig. 2, center). It may be that fast changes in geometrical
structures of the nonplanar fluorophore molecules upon ex-
citation result in a large Stokes shift. One previous example is
3,5-dinaphthyl-BODIPY (labs = 542 nm, lflu = 607 nm), in
which the twist angle between the naphthyl moiety and the
BODIPY core is up to 501.13 For this compound, a large
Stokes shift (65 nm) is observed though its fluorescence
quantum yield is relatively low (0.38 in CHCl3). Obviously,
larger Stokes shifts are accompanied with more non-emissive
decay of excitation energy. For our BODIPY derivative 4, the
dihedral angle is apparent but not very large, so that it has an
relatively large Stokes shift while maintaining a high fluores-
cence quantum yield. This explanation is also suitable for
compound 3, since it has the similar conjugation to compound
4, although single crystals were not obtained, due to its
amorphous nature induced by the tetraphenylmethane units.14
Although compounds 3 and 4 have similar Stokes shifts and
fluorescence quantum yields in solution, their solid-state spectral
Fig. 1 Normalized UV-Vis absorption and photoluminescence spec-
tra of compounds 3 (left) and 4 (right) in CH2Cl2.
orange–red light with an emission maximum at 594 nm. Both
compounds inherit the characteristics of high molar extinction
coefficients (e) and high fluorescence quantum yields (FF) from
the BODIPY family. However, they exhibit larger Stokes shifts
(35, 33 nm for compound 3 and 4, respectively) than most
BODIPY derivatives. Due to their relatively large Stokes shifts
in combination with the narrow emission bands, there are
smaller overlap areas between the absorption and emission
spectra (Fig. 1), which is a distinct aspect compared with the
very large spectra overlap areas observed in many other
BODIPY dyes. This is beneficial to inhibit self-quenching.
The electronic states were investigated through cyclic voltam-
metry and theoretical calculation (B3LYP, 3-21G),11 and the
results were summarized in Table 2 (details in ESIw). Both
studies show consistent results: the energy level of the LUMO
for compound 3 is very close to that of compound 4, while the
Table 1 Optical properties of compounds 3 and 4
Compound
labs/nm
l
flu/nm DSa/nm (cmÀ1
)
e/MÀ1 cmÀ1
b
FF
Fig. 2 Crystal structure of 4 with ellipsoids shown at the 30%
probability level and all hydrogen atoms removed for the sake of
clarity. Left: View perpendicular to the BODIPY main axis. Center:
View from phenylethynyl to BODIPY core. Right: Crystal packing
diagram.
3
4
575
561
610
594
35 (998)
33 (990)
0.79 76 600
0.82 73 000
a
Stokes shift calculated from the absorption and emission energies.
The reference system used was rhodamine 6G in ethanol (FF = 0.95).
b
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of the standard red CRT phosphors of the Society of Motion
Picture and Television Engineers (SMPTE-C) and European
Broadcasting Union (EBU), respectively.15 Taking into
account the pure red EL spectrum of compound 3, as well as
its other favorable characteristics introduced in this paper, we
believe it is worthwhile to further optimize the device fabrica-
tion for higher performance.
Fig. 3 UV-Vis absorption spectra of compounds 3 and 4 in the solid-
state (left), intensity–doped concentration curves (centre) and emission
intensity of compounds 3 and 4 on films under their maximum doped
concentrations as dopants for SBS (right).
In conclusion, Sonogashira coupling reactions on the
2,6-positions of the BODIPY core were utilized as an
efficient method to develop novel derivatives with red-shifted
spectra. With the bulky substituents, 2,6-di-(4-tritylphenyl-
ethynyl) BODIPY exhibited relative large Stokes shift, inhibi-
tion of self-quenching effect and a pure red EL spectrum,
which qualified it as a potential candidate for red EL dopant.
This work was supported by National Natural Science
Foundation of China (No. 20406004, 20572012, 20536010)
and Program for Changjiang Scholars and Innovative
Research Team in University (IRT0711).
properties are quite different. As shown in Fig. 3, the solid
absorption spectrum of compound 4 become very broad and
considerably red-shifted compared with that in dichloro-
methane. Absorption of compound 3 however broadens only
slightly, with a negligible shift. Further, the fluorescence concen-
tration-quenching effect for compound 4 is very serious. The
fluorescence intensity for compound 4 doped in SBS (styrene-
butadiene block copolymer, M.W. = 200 000 g molÀ1, an inert
polymer matrix) film reaches a maximum at a doping concentra-
tion of only 1.4% above which the fluorescence intensity
decreases, a similar result to the reported doping concentration
limit (1%) of another green emissive BODIPY dopant.5a
However, for compound 3, the doping concentration limit is
increased to 2.7%. The maximum fluorescence intensity of
compound 3 is about 2.4-fold higher than that of compound 4
(Fig. 3, right), which indicates that compound 3 is superior to
compound 4 as a solid-state emitter. Molecules of compound 4
aggregate via forming compact p–p stacking, as is proved by the
X-ray single crystal analysis (Fig. 2, right, the shortest distance
between indacene and a benzene ring in another molecule is
3.389 A), which explains its broadened absorption spectra and
strong self-quenching effect. The tendency to form p–p stacking
for molecules of compound 3 is counteracted by the steric
hindrance from bulky trityl groups, so that the intermolecular
interactions are weakened and self-quenching is inhibited to some
extent.
Notes and references
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As a preliminary work for OLED investigations, electro-
luminescence spectra (Fig. 4) of compounds 3 and 4 were
measured in devices with a simple configuration without any
optimization (details in ESIw). The configuration was ITO/
PEDOT (30 nm)|BDP-PPV: 2% 3–4 (60 nm)|Alq3 (10 nm)|Al
(100 nm), where ITO was the anode, PEDOT was used as a
hole-injection layer, compounds 3 and 4 were doped into
BDP-PPV as an emitting layer, Alq3 was used as an electron-
transporting as well as a hole-blocking layer, and Al was used
as the cathode. The devices show EL peaks at 620 nm with
Commission Internationale de l’Eclairage (CIE) coordinates
(0.65, 0.34) for 3 and 608 nm (0.65, 0.35) for 4. The coordinates
are accordant or comparable with (0.64, 0.34) and (0.64, 0.33)
13 A. Burghart, H. Kim, M. B. Welch, L. H. Thoresen, J.
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National Television System Committee (NTSC) is stricter with
(0.67, 0.33) CIE color chromaticity coordinates.
Fig. 4 EL spectra for compounds 3 and 4.
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