Color-tunable solid-state emission of 2,2′-biindenyl-based fluorophores

Article abstract of DOI:10.1002/anie.201104533

A colorful bunch: Dyes based on 2,2′-biindenyl fluorophores exhibit tunable solid-state emission colors that cover the whole visible region from blue to red. The nonplanar conformation of the dyes in crystals result in bright deep-blue emission for 1-3, a

Full text of DOI:10.1002/anie.201104533

Communications
DOI: 10.1002/anie.201104533
Fluorescent Dyes
Color-Tunable Solid-State Emission of 2,2’-Biindenyl-Based
Fluorophores**
Zhiyun Zhang, Bo Xu, Jianhua Su, Leiping Shen, Yongshu Xie, and He Tian*
Dedicated to Professor Klaus Mꢀllen on the occasion of his 65th birthday
Solid-state emissive organic chromophores are very impor-
tant for practical thin-film-device applications, such as organic
luminescent displays and light sources.[1,2] However, most
organic luminescent materials face major challenges because
of low emission efficiency in the aggregated state. To reduce
aggregation-caused quenching,[3] several strategies, such as
the introduction of bulky substituents into the original
fluorophores, have been proposed.[4] Tang, Park, and their
respective co-workers observed the aggregation-induced
emission (AIE) phenomenon, which overcomes fluorescence
quenching in the aggregated state.[5] To date, a large number
of AIE-active dyes based on silole or tetraphenylethene as the
core structure have been developed.[6] Despite successful
examples of solid-state emissive materials based on the AIE
effect, it is still challenging to design novel core structures for
fluorophores with tunable solid-state emission that covers the
whole visible (especially red) region, and with large Stokes
shifts.
Linear aromatic systems that consist of carbon-bridged
phenylenevinylene are of great interest because of their
unique luminescent properties for organic displays.[7] We have
designed and synthesized a series of biindenyl derivatives
(1a–4a) by incorporating the dimethylmethylene bridge in
Figure 1. a) Chemical structures of 1a–4a and BDY-IN. b) Single-
crystal structures of 1a, 4a, and two forms of BDY-IN: BDY-O and
BDY-R.
=
order to lock the C C bond with one of the adjacent aryl
light, respectively. This is the first example of the same
bodipy-derived chemical structure with different solid-state
emission colors. The longest maximum emission wavelength is
up to 660 nm with a very large Stokes shift.
groups (Figure 1a). Considerable progress has been made in
the synthesis and properties of biindenes,[8] but, to the best of
our knowledge, their luminescent characteristics have not
been reported to date. The employment of 2,2’-biindene as
the core structure allows the achievement of solid-state
emission that covers a wide region from deep blue to red.
Unexpectedly, these four compounds are poorly emissive in
solution, but show an intense deep-blue or green emission in
the crystal state. When the adjacent aryl group is boradi-
azaindacene (bodipy),[9,10] BDY-IN (Figure 1) crystallizes in
two forms (BDY-O and BDY-R) from different solvents. It is
noteworthy that these two forms emit distinct orange and red
The crystal structures of all compounds are shown in
Figure 1b and Figure S1 in the Supporting Information, and
detailed data of the crystal analyses are given in Table S1.
Microcrystalline solids of 1a–4a exhibit moderate thermal
stabilities with decomposition temperatures (5% weight loss)
over 2408C (Figure S2). Solids of BDY-O and BDY-R lost
about 10% of their weight in the temperature ranges 95–
1628C and 116–2458C, respectively. BDY-IN molecules were
crystallized with lattice solvent molecules, that is, chloroform
and dichloromethane molecules are observed in BDY-O and
BDY-R, respectively (Figure S3). The 10% weight loss can be
attributed to the loss of solvent molecules in the crystal lattice.
The photophysical properties of all fluorophores were
measured in dilute solutions in various solvents (Table S2).
No obvious difference is observed in the absorption and
emission spectra of 1a–4a in solvents in which the fluores-
cence quantum yield (FF) of the compounds is low. BDY-IN
shows an absorption band in a wavelength region similar to
1a–4a; the position of this band changes slightly as the
solvent is varied (Figure S5). The absorption bands (lab)
appear at 496–504 nm and can be assigned to the p–p*
[*] Z. Zhang, B. Xu, Prof. Dr. J. Su, L. Shen, Prof. Dr. Y. Xie,
Prof. Dr. H. Tian
Key Laboratory for Advanced Materials and Institute of Fine
Chemicals, East China University of Science and Technology
Shanghai 200237 (P.R. China)
E-mail: tianhe@ecust.edu.cn
[**] This work was supported by NSFC/China (20972053) and the
National Basic Research 973 Program (2011CB808400). Z.Z. thanks
Dr. Z.-Y. Xia, Dr. J.-H. Huang, and Dr. X. Li for the helpful discussion.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201114533.
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Angew. Chem. Int. Ed. 2011, 50, 11654 –11657

transition of the bodipy unit. In sharp contrast, the lumines-
cence of a solution of BDY-IN changes dramatically as the
solvent is changed. In a nonpolar solvent (toluene), BDY-IN
shows a single sharp emission maximum at 529 nm, in polar
solvents, in addition to the sharp signal around 525 nm, a new
emission band emerges in the red region with a maximum
intensity at 665 nm. Unlike most bodipy derivatives with a
high FF value in solution, BDY-IN shows emission with a low
FF value but with a large Stokes shift. The FF value decreases
with increasing solvent polarity: toluene 9.4%, chloroform
3.9%, THF 2.8%, dichloromethane 0.7%, and acetonitrile
0.3%.
Solutions of 1a emitted a small amount of observable light
when illuminated with a UV lamp, whereas microcrystalline
1a emitted a large amount of light (Figure S6), thus indicating
that the crystal formation resulted in the light emission. The
AIE feature can be characterized by the enhancement of the
emission efficiency from the isolated molecules to the
aggregated particles. The photoluminescence (PL) spectrum
of 1a in CH3CN/H2O mixtures is shown in Figure 2a and that
of BDY-IN in THF/H2O mixtures in Figure 2b. As water is a
amount of water is added to the THF solution, and the
emission band at 543 nm reaches a maximum in 70% aqueous
THF. When the fw value is increased from 70% to 90%, the
PL spectra of 1a and BDY-IN show a decrease in the intensity
and a red-shift of the emission band. The nanoparticles from
the aqueous mixtures were obtained by solvent evaporation
(Figure 2c,d). The nanoparticles formed in the 70% aqueous
mixture are larger than those obtained from the 90% aqueous
mixture. This result is probably due to the change in
conformation and packing mode of the dye molecules in the
aggregates.[11] In the mixture with the lower water fraction,
the dye molecules may steadily assemble in an ordered
fashion to form more emissive crystalline aggregates. How-
ever, in the mixture with the higher water fraction, the dye
molecules quickly agglomerate in a random way and form
particles that emit at longer wavelengths but with lower
quantum yields.
The interesting phenomena observed from the aggregates
suspended in aqueous media demonstrate that the emission
behavior of 1a and BDY-IN can be tuned by changing the
aggregation state. Various solvents, such as dichloromethane/
ethanol, chloroform/ethanol, acetone, and THF, were selected
to prepare single crystals. When excited at 365 nm (Figure 3),
the crystals of 1a show an intense blue luminescence with
Figure 2. Photoluminescence (PL) spectra of a) 1a in CH3CN and
CH3CN/water mixtures, and b) BDY-IN in THF and THF/water mix-
tures (insets: plots of relative photoluminescence signal intensities
against water content fw of the solvent mixtures). Scanning electron
microscopy (SEM) images of c) 1a nanoaggregates obtained from
CH3CN/H2O, and d) BDY-IN nanoaggregates obtained from THF/H2O
at a concentration of 10 mm (volume of water: 1 70%, 2 90%).
Figure 3. a) Photographs of crystals of 1a–4a and two forms of BDY-
IN (BDY-O and BDY-R) taken under UV irradiation at 365 nm. PL
spectra of b) crystals and c) amorphous films of 1a–4a and BDY-IN.
emission around 417 nm, which is similar to the emission of 1a
in CH3CN/H2O mixtures (3:7 v/v, 10 mm). Needle-like crystals
of the two forms of BDY-IN exhibit distinct orange (BDY-O,
582 nm) and red (BDY-R, 661 nm) emissions. It is noteworthy
that the nanoparticles of BDY-IN formed in THF/water (3:7
v/v, 10 mm) show a yellow emission at 543 nm (Figure 2b).
Unfortunately, we were unable to determine the conforma-
tion of the nanocrystal formed in THF/water (3:7 v/v, 10 mm)
by X-ray diffraction crystallography because of its small size,
poor solvent for these dyes, the dye molecules aggregated in
solvent mixtures with high water fractions (fw). For 1a, weak
PL signals are recorded at fw ꢀ 40%, and the PL intensity
starts to rise at fw > 40% with an increase in intensity of up to
about 255 times in 70% aqueous mixture. A solution of BDY-
IN in THF shows two emission bands (524 and 665 nm); the
band in the red region (665 nm) is quenched when a small
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Communications
and many efforts to obtain a yellow-emitting solid of BDY-IN
Figures S11 and S12, the main supramolecular interactions
À
failed. Amorphous films of 1a and BDY-IN were prepared by
spin-coating solutions of the dyes in CH2Cl2 onto quartz
plates; the PL spectra of the films are shown in Figure 3c.
Compared with the crystals, the emission of the amorphous
film of 1a is red-shifted by about 70 nm with a much lower FF
value (Table 1). Similar emission behaviors to 1a were
observed for 2a and 3a in their aggregated states, however,
are hydrogen bonds of C H···p and H···F. The weak face-to-
face intermolecular p–p stacking interaction between two
bodipy units with a contact distance of 3.83 ꢀ was found in
BDY-R, but not in BDY-O. The emission of BDY-R shows an
obvious red-shift of about 79 nm compared with BDY-O
because of the more planar conformation and face-to-face p–
p stacking interaction, which increase the degree of p conju-
gation.
In conclusion, we designed a series of novel 2,2’-biindenyl-
based fluorophores based on the AIE strategy, and success-
fully obtained two crystal forms of the novel bodipy-derived
dye BDY-IN with tunable solid-state emission colors and
large Stokes shifts. The solid-state emission ranges from deep-
blue to red and can be readily tuned by the variation of the
substituents on the 2,2’-biindenyl fluorophore or by adjust-
ment of the aggregation state. These results are highly
valuable for the further design of relevant compounds with
tunable solid-state emissions.
Table 1: Emission characteristics of 1a–4a and BDY-IN in amorphous
and crystalline states.[a]
lem [nm][b]
FF [%]
Amorphous
Amorphous
Crystalline
Crystalline
1a
2a
3a
4a
476
479
483
512
656
417
412
415
5.3
4.2
3.8
12.8
<2
76.0
76.3
77.6
33.2
<2
513
BDY-IN
582[c]/661[d]
[a] lem =emission maximum, FF =fluorescence quantum yield (mea-
sured by using an integrating sphere with an error of Æ3%). [b]Æ1 nm.
[c] BDY-O. [d] BDY-R.
Experimental Section
The 2,2’-biindenyl dyes were prepared according to the synthetic
routes shown in Scheme S1. Treatment of indanone 1 with sodium
hydride in dry tetrahydrofuran (THF) at room temperature resulted
in quantitative conversion to the respective enolates. Subsequent
oxidation of the enolates with copper(II) chloride in THF according
to the method of Saegusa[13a] afforded dimer 2. The subsequent
Grignard reaction between 2 and the respective arylmagnesium
bromide formed compounds 1a–4a. Dibromination of 2a gave the
corresponding benzyl bromide, which was heated in dimethylsulf-
oxide (DMSO) to obtain 3. Subsequent treatment of 3 with classical
methods for the preparation of bodipy dyes (addition of trifluoro-
acetic acid to a mixture of aldehyde and pyrrole gives the corre-
sponding dipyrromethane in situ, subsequent oxidation with 2,3-
dichloro-5,6-dicyanobenzoquinone affords the corresponding dipyr-
romethene, and treatment of the latter with an excess of trifluoro
boronetherate in the presence of triethylamine gives the correspond-
ing boron complex)[13b] led to the formation of BDY-IN. All reaction
intermediates and final products were characterized, and single
crystals of 1a–4a and BDY-IN were grown and analyzed by X-ray
diffraction crystallography. Two crystal forms of BDY-IN were
observed: a BDY-O phase with a monoclinic space group symmetry
P21/c was obtained when crystals were grown from a chloroform/
ethanol mixture, and a BDY-R phase with a triclinic space group
symmetry P-1 was obtained when crystals were grown from a CH2Cl2/
EtOH mixture. Synthesis and characterization of 1a–4a and BDY-IN,
and detailed experimental procedures for optical, X-ray diffraction,
thermogravimetric analysis, and DSC experiments are reported in the
Supporting Information. CCDC 762612 (1a), 762613 (2a), 762614
(3a), 762615 (4a), 804270 (BDY-O phase of BDY-IN), and 804269
(BDY-R phase of BDY-IN) contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via www.ccdc.
cam.ac.uk/data_request/cif.
4a is a special case (Figure 3 and Table 1). Because of the
more rigid conformation in the molecule, 4a shows green
emission around 510 nm in every aggregate state. Addition-
ally, the crystals of 1a–4a are phosphorescent at room
temperature (Figure S7), which is a phenomenon that is
rarely observed in pure organic chromophore systems.[12] As
expected, the Commision Internationale de L’Eclairage
(CIE) chromaticity diagram (Figure S8) of typical dye
samples exhibited emission colors tuned in the range from
deep-blue CIE (0.15, 0.04) to red CIE (0.62, 0.37) by
modifying the adjacent aryl moieties or the aggregation state.
To gain further insight into the mechanism of the AIE
system and the distinct emissive behavior of the crystal states
of these dyes, we checked the details of the molecular
conformation and packing in the single crystals. As shown in
Tables S3 and S4, and in Figure S9, the interplane angles
between the two indene aromatic rings have large values of
around 708 in all the crystals, thus indicating that core
structure 2,2’-biindene is far from planar. Furthermore, the
indene planes and the aryl substituents are not coplanar. In
1a–4a, the interplane angles between the indene moieties and
the 3-aryl rings vary from 378 to 708. These large interplane
angles make the conformations of 1a–4a strongly deviate
from a planar conformation and prevent the rings from
undergoing p–p stacking interactions, thus inducing intense
emissions in the crystal state. In BDY-O and BDY-R, the
corresponding interplane angles of the indene moieties and
the 3-phenyl rings are in the range of 378 to 588, the 3-phenyl
rings and the bodipy units possess interplane angles from 508
to 608. The most pronounced difference between the crystals
of BDY-O and BDY-R are the torsion angles between the
indene moieties and the bodipy units, which are 76.28 and
81.08 for BDY-O, and 22.68 and 9.08 for BDY-R. These angles
indicate that the conformation of BDY-O is more planar than
that of BDY-R. The two distinct crystal packing diagrams of
BDY-O and BDY-R are shown in Figure S10. As shown in
Received: June 30, 2011
Revised: September 21, 2011
Published online: October 13, 2011
Keywords: aggregation-induced emission · bodipy · dyes/
.
pigments · fluorescence
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