1,4-Bis(alkenyl)-2,5-dipiperidinobenzenes: Minimal fluorophores exhibiting highly efficient emission in the solid state

Full text of DOI:10.1002/anie.200900963

Angewandte
Chemie
DOI: 10.1002/anie.200900963
Fluorescent Materials
1,4-Bis(alkenyl)-2,5-dipiperidinobenzenes: Minimal Fluorophores
Exhibiting Highly Efficient Emission in the Solid State**
Masaki Shimizu,* Youhei Takeda, Masahiro Higashi, and Tamejiro Hiyama
Motivated by the progress in such optoelectronic devices as
organic light-emitting diodes (OLEDs),[1] semiconductor
lasers,[2] and fluorescent sensors,[3] scientists have paid much
attention to p-conjugated compounds that emit visible light in
the solid state.[4] However, the development of organic
molecules exhibiting highly efficient solid-state luminescence
is an extremely challenging task because the molecular
aggregation of chromophores, which is inherent in the solid
state, typically results in the concentration quenching of
luminescence. Therefore, examples of organic solids that
exhibit intense emission with high absolute quantum yield are
quite limited.[5] The common structural feature of emissive
organic solids is that they contain multiple aromatic rings and/
or polycyclic aromatic hydrocarbon skeletons, which often
cause problems in solubility, an undesirable property for the
purification and processing of organic materials. Meanwhile,
the emission colors of p-conjugated molecules are closely
related to their conjugation lengths. Organic materials with
good solubility have the potential of being applicable to
solution processing, which is a fascinating method for the easy
and low-cost fabrication of large-area optoelectronic devices.
Therefore, it is intriguing to determine the minimum size of
the aromatic component of highly emissive organic solids,
from both fundamental and practical viewpoints.[6] We report
herein that 1,4-bis(alkenyl)-2,5-dipiperidinobenzenes 1–5,
film. In addition, the emission colors can be tuned in the range
from blue to red simply by modifying the alkenyl moieties.
In the course of our research on the development of new
synthetic methods for organofluorine compounds, we found
that
2,5-dimethoxy-1,4-bis[(E)-3,3,3-trifluoroprop-1-enyl]-
benzene in powder form showed blue fluorescence (lem
=
441 nm) with a moderate quantum yield of 0.31.[7,8] This
observation prompted us to explore this molecular/electronic
structure as a minimal chromophore of emissive organic
solids. Then, we prepared the corresponding counterpart 3 as
a single stereoisomer from 2,5-dibromoterephthalaldehyde[9]
(Scheme 1).[10] Benzene 3 is soluble in organic solvents such as
Scheme 1. Preparation of 3: a) Ph2P(O)CH2CF3, Bu4NF, THF, RT, and
then I2, hn, RT, 28%; b) piperidine, Pd(OAc)2/(S)-binap (cat.), NaOtBu,
toluene, 1008C, 83%. binap=2,2’-bis(diphenylphosphanyl)-1,1’-
binaphthyl.
CH2Cl2, CH3CN, THF, and toluene, and it was easily purified
by column chromatography on Al2O3 followed by recrystal-
lization from CH2Cl2/hexane. Both crystals and a spin-coated
thin film of 3 were found to exhibit remarkably intense green
emission (lem = 523 and 526 nm) upon excitation with UV
light (l = 360 nm) with extremely high quantum yields (Ff) of
0.98 and 1.00, respectively (Figure 1 and Table 1).[11] More-
over, 3 dispersed in a thin film of polystyrene also emits a
brilliant green light, maintaining the excellent efficiency of
the solid-state emission (Ff = 1.00). The absorption spectrum
of 3 in cyclohexane and the fluorescence spectra (lex =
360 nm) in cyclohexane, CH2Cl2, or CH3CN are shown in
which contain only one benzene ring as the aromatic
component, serve as novel fluorophores and emit intense
visible light with high to excellent quantum yields in the solid
state such as in the crystal, neat film, and doped polystyrene
[*] Prof. Dr. M. Shimizu, Y. Takeda, M. Higashi, Prof. Dr. T. Hiyama
Department of Material Chemistry, Graduate School of Engineering
Kyoto University, Kyoto University Katsura
Nishikyo-ku, Kyoto 615-8510 (Japan)
Fax: (+81)75-383-2445
E-mail: m.shimizu@hs2.ecs.kyoto-u.ac.jp
Homepage: http://npc05.kuic.kyoto-u.ac.jp/e-index.htm
[**] This work was supported by a Grant-in-Aid for Creative Scientific
Research (No. 16GS0209) from the Ministry of Education, Culture,
Sports, Science and Technology (Japan).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200900963.
Figure 1. Fluorescence spectra of 3 in the solid states (lex =360 nm).
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Table 1: Photophysical properties of 1–5.
Cmpd labs [nm][a]
(e)[b]
lmax (Ff)[c,d]
solution[a]
439 (0.31)
crystals[e] thin film[f] PS film[g]
1
2
3
331
(4600)
351
(4900)
380
441 (0.73) 439 (0.65) 439 (0.68)
486 (0.94) 480 (0.89) 487 (1.00)
523 (0.98) 526 (1.00) 543 (1.00)
469 (0.53)
523 (0.49)
563 (0.57)[h]
567 (0.57)[i]
566 (0.66)
(3500)
4
5
410
(6300)
425
596 (0.89) 591 (0.80) 595 (1.00)
638 (0.43) 641 (0.34) 659 (0.15)
Figure 3. Molecular and crystal structures of 3. Hydrogens are omitted
656 (0.09)
for clarity.
(4400)
[a] 1ꢀ10À5 m in cyclohexane. [b] e: mÀ1 cmÀ1. [c] Emission maxima upon
irradiation with UV light (lem =310 nm for 1, 320 nm for 2, 360 nm for 3
and 4, 365 nm for 5). [d] Absolute quantum yield determined with a
calibrated integrating sphere system. [e] Prepared by recrystallization
from mixed solvents: 1 from toluene/hexane; 2 from Et2O/hexane; 3–5
from CH2Cl2/hexane. [f] Prepared by spin-coating from a solution in THF
for 1 and 2, and in toluene for 3–5. [g] Dispersed in a polystyrene (PS)
film. [h] In CH2Cl2. [i] In CH3CN.
actions, such as p–p stacking, which lead to the quenching of
fluorescence.
DFT calculations (B3LYP/6-31G(d) level) of 3 disclose
that the HOMO is delocalized primarily over the benzene
ring and the nitrogens, and the LUMO is evident in the
CF3C C C6H4 C CCF3 framework (Figure 4).[13] Moreover,
= À
À =
Figure 2. A large Stokes shift (142 nm) and remarkable
solvatochromism of the fluorescence (lem = 523 nm in cyclo-
hexane; 563 nm in CH2Cl2; 567 nm in CH3CN) were
Figure 4. HOMO (a) and LUMO (b) diagrams of 3.
the time-dependent DFT calculations indicate that the
absorption band at 380 nm could be the intramolecular
charge-transfer transition from the HOMO to the LUMO.
This is consistent with the observed large Stokes shift and the
solvatochromism.
Figure 2. Absorption (left) and fluorescence spectra (lex =360 nm;
right) of 3 in solution.
To examine the validity of the bis(alkenyl)-
dipiperidinobenzene framework as a minimal core of highly
emissive organic solids, we designed and prepared 1, 2, 4, and
5, in which the terminal carbons of the alkenyl moieties were
substituted with Me, H, EtO2C, and Ac groups, respectively,
in place of CF3 groups.[14] Their photophysical properties and
the fluorescence spectra of the crystals are shown in Table 1
and Figure 5c, respectively.[15] Bis(2-methylpropenyl) and
divinyl derivatives 1 and 2 in both the crystal and the spin-
coated film emit blue and greenish blue light, respectively
(Figure 5a), with high to excellent quantum yields, ranging
from 0.65 to 0.94. Crystals and a thin film of 4 show intense
orange fluorescence (lem = 596 and 591 nm) with excellent Ff
of 0.89 and 0.80, respectively. Moreover, diacetyl derivative 5
exhibit red fluorescence in crystals and a thin film with
emission maxima at 638 and 641 nm and quantum yields of
0.43 and 0.34, respectively. Since red fluorophores typically
have a tendency to aggregate in the solid state, which results
in concentration quenching of fluorescence, the quantum
observed, suggesting significant structural differences
between the ground and excited states, and solvent stabiliza-
tion of the charge-transfer excited state.
Single crystals of 3 suitable for X-ray analysis were
obtained by recrystallization from THF/EtOH. Compound 3
¯
crystallizes in the triclinic space group P1 with an inversion
center at the centroid of the benzene ring (Figure 3).[12] The
dihedral angle between the alkenyl moiety and the benzene
ring (]C1-C2-C3-C4) is 32.78, while the piperidyl groups are
more twisted out of the benzene plane (]C2-C5-N1-C6 was
72.68). Presumably as a result of the twisted conformations of
the piperidyl and the trifluoropropenyl groups, the benzene
planes of the adjacent two molecules of 3 in the crystal are far
away from each other (5.33 ꢀ), and thus molecules of 3 in the
crystal are prevented from intermolecular electronic inter-
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polystyrene films doped with 1–5 taken under UV light (l=365 nm).
c) Fluorescence spectra of microcrystals 1–5.
yields of solid 5, a red emitter, are rather high.[16] Further-
more, solids 1–4 dispersed in a polystyrene (PS) film also
show strong fluorescence in colors similar to those of crystals
and thin films with excellent quantum yields of 0.68 and 1.00
(Figure 5b). This behavior is quite attractive in view that the
benzenes could be potential dopant emitters in host materials.
It should also be noted that the Ff values of 1–5 in
cyclohexane are much lower than those in the solid state.
Thus, aggregation-induced enhanced emission is operative in
all cases probably because of the restriction of intramolecular
rotation.[17] These results demonstrate that 1,4-bis(alkenyl)-
2,5-dipiperidinobenzenes are unique and promising solid
fluorophores in multiple colors.
In summary, we have demonstrated that 1,4-bis(alkenyl)-
2,5-dipiperidinobenzenes are highly emissive fluorophores in
solid states, such as crystals and thin films, and even in
polystyrene film. The solid-state photoluminescent quantum
yields are high to excellent, and the fluorescent color is easily
tuned by modifying functional groups at the alkenyl moieties.
It is of great interest from fundamental and practical view-
points that the remarkably high efficiency of solid-state
emission and the brilliant full-color emission are achieved
with the compact p-conjugated system that consists of only
one benzene ring as an aromatic element. These character-
istics render the compact fluorophores unique and make them
potential candidates for organic emitters and for applications
to solid-state lighting devices. Further studies on electro-
chemical, thermal, and morphological properties of 1,4-
bis(alkenyl)-2,5-dipiperidinobenzenes, and their applications
in optoelectronic devices are in progress.
Received: February 19, 2009
Published online: April 7, 2009
Keywords: arenes · chromophores · fluorescence ·
.
luminescence · materials science
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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.
[13] The molecular structure was optimized by DFT methods at the
B3LYP/6-31G(d) level with the crystal structure as the initial
geometry.
[14] For the preparation of 1, 2, 4, and 5, see the Supporting
Information. These benzenes are also soluble in organic solvents
such as CH2Cl2, CHCl3, THF, and toluene.
[15] Absorption and fluorescence spectra of 1–5 in cyclohexane and
fluorescence spectra in thin neat films and in polystyrene films
are shown in the Supporting Information.
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