Switching the emission of tetrakis(4-methoxyphenyl)ethylene among three colors in the solid state

Article abstract of DOI:10.1039/c3nj00063j

Emission of a luminogen could be switched among three colors in the solid state by transformation among three different aggregation states. The partly amorphous solid of the luminogen exhibits excitation dependent emission due to the contribution of both amorphous and crystalline parts to the photoluminescence intensity.

Full text of DOI:10.1039/c3nj00063j

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LETTER
Switching the emission of
tetrakis(4-methoxyphenyl)ethylene among
three colors in the solid state†
Chenyu Li,a Xiaoliang Luo,a Weijun Zhao,a Cuihong Li,a Zhengping Liu,a
Zhishan Bo,a Yuping Dong,b Yong Qiang Dong*a and Ben Zhong Tang*cd
Cite this: NewJ.Chem., 2013,
37, 1696
Received (in Montpellier, France)
15th January 2013,
Accepted 12th March 2013
DOI: 10.1039/c3nj00063j
www.rsc.org/njc
mixture has been reported to be tuned reversibly among
reddish-orange, yellow, and green.7 Jia and co-workers have
also achieved multicolor emission switching of two luminogen-
functionalized peptides through modulation of molecular
packing patterns and mechanochemical reaction.8 Further-
more, there is still no clear design strategy for such materials.
Emissions of many panel-like organic luminescent materials
are weakened or even quenched at high concentrations or in
aggregate states, which is notoriously known as aggregation-
caused quenching (ACQ).9 We observed a phenomenon of aggre-
gation-induced emission (AIE) which is the exact opposite of the
ACQ effect: a group of propeller-shaped molecules are nonemissive
in solution, but are induced to emit intensively by aggregate
formation due to restriction of the rotation of their aryl rings.4c,10
Some AIE active compounds exhibit increased PL intensity and
blue-shifted emission when transforming from amorphous to
crystalline states, which has been coined crystallization induced
emission enhancement (CIEE).11 Thus emission of those CIEE
luminogens could be switched between two different colors
through modulating their molecular packing pattern.
Emission of a luminogen could be switched among three colors in the
solid state by transformation among three different aggregation
states. The partly amorphous solid of the luminogen exhibits excitation
dependent emission due to the contribution of both amorphous and
crystalline parts to the photoluminescence intensity.
Considerable effort has been devoted to the development of
stimuli-responsive luminescent materials due to their fundamental
importance and potential applications in sensors,1 memory2 and
security inks.3 Of particular interest are those luminogens whose
emission can be switched between different colors or between dark
and bright states in the solid state through modulating their
molecular packing pattern.4
The emission of some luminogens can be modulated reversibly
by repeatedly tuning luminogen aggregates between amorphous
and crystalline5 or between two different crystalline states6 through
solvent fuming, heating or mechanical stress. However, emissions
of many luminescent materials can be switched between only two
luminescent states as the materials can merely form two emissive
states. Though several luminogens do emit multicolor lights in
various states, reversible luminescence switching among multiple
colors has rarely been achieved. Emission of a liquid crystal
In this letter, we attached methoxyl groups to the p-positions
of phenyl rings in the TPE molecule. Through introducing weak
interactions (C–Hꢀ ꢀ ꢀO and C–Hꢀ ꢀ ꢀp) in the AIE molecule we
obtained a luminogen exhibiting morphology dependent emis-
sion. The luminescence of the compound could be switched
among three colors in the solid state through vapor, thermal
and mechanical stimuli.
a Beijing Key Laboratory of Energy Conversion and Storage Materials,
College of Chemistry, Beijing Normal University, Beijing, 100875, China.
E-mail: dongyq@bnu.edu.cn; Tel: +86-10-5880-6896
b College of Materials Science and Engineering, Beijing Institute of Technology,
Luminogen 1 is nearly nonemissive in solution. However,
when a large amount of water was added to the solution, 1
emitted strong green light with a peak at 491 nm due to the
formation of aggregates in acetonitrile–water mixture (Fig. S1,
ESI†). Thus, similar to other derivatives of TPE, luminogen 1 is
AIE active.10b
Beijing, 100081, China
c Department of Chemistry, The Hong Kong University of Science & Technology
(HKUST), Clear Water Bay, Kowloon, Hong Kong, China. E-mail: tangbenz@ust.hk
d Institute of Biomedical Macromolecules, MOE Key Laboratory of Macromolecular
Synthesis and Functionalization, Department of Polymer Science and Engineering,
Zhejiang University, Hangzhou, 310027, China
† Electronic supplementary information (ESI) available: Experimental procedures,
crystal structure of 1 (CIF), UV and PL spectra for 1, DSC thermograms and powder
XRD patterns of 1 in different aggregation states. CCDC 918901. For ESI and
Crystals of 1 were obtained by slow evaporation of its
acetone solution and its molecular packing pattern is shown
crystallographic data in CIF or other electronic format see DOI: 10.1039/c3nj00063j in Fig. S2 (ESI†). The crystal (1CA) emits deep-blue light with
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of luminogen may depend on the conformations of its molecules.
The varied equilibria of the weak interactions (such as C–Hꢀ ꢀꢀO
and C–Hꢀ ꢀ ꢀp, Fig. S2, ESI†) between molecules in the solid
state may afford different balanced conformations, hence the
morphology dependent emission. It is impractical to obtain
conformation information on molecules in amorphous or crystalline
powder form, thus details of the molecular conformations of 1CB
and 1Am are not accessible. Fortunately, the single crystal structures
of tetrakis(4-propoxyphenyl)ethylene (TPPE, CCDC 865201) and
tetrakis(4-butoxyphenyl)ethylene (TBPE, CCDC 865202) with PL
spectra peaking at 448 and 446 nm respectively,12 have been
reported. The methoxy, propoxy and butoxy groups display
similar inductive effects on the photophysical properties of the
TPE core. Thus different emissions from the three TPE cored
luminogens should be caused by their varied conformations. As
we expected, the average value of the torsion angles of the phenyl
rings in 1CA (551) is larger than those for TPPE (471) and TBPE
(461).12 The larger torsion angle induces lower conjugation,
which coincides with the bluer emission of 1CA than those of
TPPE and TBPE. Thus the morphology dependent emissions of
such propeller-shaped molecules may be due to the various
conformations in different aggregates.
Fig. 1 Photos of (a) 1CA, (b) 1Am, (c) 1CB, and (d) 1Am fumed with acetone.
(e) PL spectra (excitation wavelength: 338 nm), and (f) powder XRD patterns of
samples (a–d) in the images. Photos were taken under UV illumination. I, Heating
to melt and quickly cooling; II, 110 1C for 10 min; III, fuming with acetone vapour,
5 min.
The weak interaction between molecules in the loosely
a peak at 421 nm upon excitation. As many AIE luminogens packed aggregates of 1 could be readily broken and rebuilt,
have been reported to exhibit morphology dependent lumines- hence facilitating its transformation between different aggre-
cence, we prepared the pure amorphous solid of 1 (1Am, gate states and emission switching.
Fig. 1b) through quenching its melt. 1Am emits green light
Mechanochromic luminescent materials have received
with a peak at 491 nm, which is about 70 nm red-shifted considerable attention due to their potential applications
compared with that of 1CA. When we tried to fume 1Am with in mechanical sensors and optical storage.12,13 Deep blue emissive
acetone vapor, 1CA was obtained again. Thus the emission of 1CA transforms to green emissive powder (Fig. 2b under 365 nm
luminogen 1 can be switched reversibly between green light UV light) upon grinding in a mortar. However, when excited with
(1Am) and deep-blue light (1CA) through repeated heating and 254 nm light from a UV lamp, the ground powder emits blue light.
solvent fuming.
In addition to the visual observations, we studied the excitation
Amorphous solids will crystallize upon heating before melting, dependent emission of 1 using a PL spectrofluorometer. The PL
thus we annealed 1Am at 110 1C; however, 1Am did not reform spectra also blue-shifted from 490 nm to 446 nm when the
1CA upon heating but afforded another crystal (1CB) of luminogen excitation light changed from 399 to 300 nm. Thus the emission
1, which emits blue light with a peak at 442 nm. The powder X-ray of the ground solid can be switched between blue and green just
diffraction (XRD) pattern of 1CB also reveals that 1CB is a new through changing the excitation light between 254 nm and
crystalline phase which is different from 1CA. The exothermic 365 nm. What is the cause of the mechanochromic and excitation-
peak at 82 1C in the differential scanning calorimetry (DSC) dependent fluorescence of luminogen 1?
thermogram of 1Am (Fig. S3, ESI†) indicated this cold crystal-
lization process (from 1Am to 1CB). The DSC thermograms
suggest that 1CA and 1CB melt at similar temperatures. 1CB
could be transformed to green emissive 1Am again by quenching
its melt. Thus the luminescence of 1 can also be tuned reversibly
between green (1Am) and blue (1CB) through pure thermal
methods. It is a pity that direct transformation between 1CA
and 1CB cannot be achieved, though we have tried heating and
solvent fuming, while both crystals can be transformed to 1Am
first and then to 1CA through solvent fuming or to 1CB through
heating. Thus, it is clear that emission of 1 can be tuned
reversibly among three colors.
Why does luminogen 1 exhibit morphology dependent
emission? The propeller-like conformation of molecule of 1 rules
Fig. 2 (a, b) Photos of ground solid obtained on grinding 1CA in a mortar taken
out any specific strong intermolecular interactions (such as p–p
stacking or H/J-aggregates) between molecules, thus, the emission
under a (a) 254 nm and (b) 365 nm UV lamp. (c) Excitation dependent PL spectra
of the ground solid.
c
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Organic crystals can be transformed into an amorphous
Letter
powder upon grinding. Powder XRD and DSC of the ground
and annealed powder from 1CA were measured to disclose the
mechanism of mechanochromic fluorescence. The diffraction
pattern of the ground solid exhibits some reflections that agree
with those of 1CA and the annealed crystal but does not have as
many or as sharp peaks, hinting at a not absolutely amorphous
phase. The diffraction curve of the annealed sample displays
many sharp and intense reflection peaks that coincide with
those of 1CA, indicating their same crystalline order. The
exothermic peak at 59 1C in the DSC thermogram of the ground
solid (Fig. S4b, ESI†) indicates the phase transformation from the
amorphous to the crystalline phase. Thus the mechanochromic
fluorescence of 1 is caused by the amorphization of crystals upon
grinding and the emission reverts back to deep-blue upon heating.
None of the pristine crystal, amorphous solid and annealed
ground solid of 1 exhibit excitation dependent emission
(Fig. S5, ESI†). Why does the ground solid behave differently?
As grinding a large amount of sample in a mortar affords a not
absolutely amorphous solid, thus the emission of the ground
solid may originate from both amorphous solid and pristine
crystals. The excitation spectra (Fig. S6, ESI†) of pristine crystals
and amorphous solid revealed that when excited with light
below 344 nm, both crystals and amorphous solid contribute to
the PL spectra. When the excitation wavelength is larger than
344 nm the contribution of pristine crystals to the PL intensity
drops quickly, and no PL signal is observed when the excitation
wavelength is larger than 390 nm. On the other hand, as the
excitation increases from 270 to 400 nm, the PL intensity
(490 nm) of the amorphous solid increases, thus contributing more
to the PL spectrum. It is clear that the decreased contribution from
the crystals and the increased contribution from the amorphous
solid of luminogen 1 with excitation increase induced the excitation
dependent PL spectra of ground solid of 1.
As it is difficult to obtain absolutely amorphous solid
through grinding a large amount of sample in a mortar, thus
we sheared a tip of a crystal of 1 with a spatula on the inner wall
of a quartz cell. The PL spectrum of the ground powder
was independent of excitation and fit well with that of the
amorphous solid of 1, (Fig. S7, ESI†) indicating its amorphous
essence. The emission of the ground powder reverted back to
deep-blue when heated, which was different from that of the
amorphous solid of 1. Though the PL spectrum of the ground
solid fit well with that of 1Am, trace amounts of tiny crystals of
1 in the ground solid may serve as nuclei to afford 1CA on
heating. Fuming with solvent vapor also transformed the
ground solid to the original deep blue crystals, which coincided
with 1Am. Thus the emission of 1 could be switched reversibly
between green and deep-blue through repeated grinding and
heating or repeated grinding and fuming processes (Fig. 3).
Fig. 3 Normalized (a) PL spectra of ground and annealed solid of 1 in the three
repeat cycles. Annealing conditions: 110 1C, 1 min. (b) Switching the fluorescence
of luminogen 1 by repeated shearing on the inner wall of a quartz cell and
heating.
Fig. 4 Procedure of repeated writing and erasing process using luminogen 1 as
an emissive material. Photoluminescent images were taken on
weighing paper under UV irradiation at 365 nm.
a piece of
obtained a piece of green emissive paper (Fig. 4). Then the
emission of the paper turned from green to deep blue, in
accordance with the 1Am to 1CA phase transition. Next we
wrote the Chinese word for teacher on the paper, and then a
green Chinese word appeared on the deep blue background due
to the amorphization of 1CA in the written area. The green
word was erased upon heating due to the transformation of 1 in
the word area from amorphous to crystal. Then we could repeat
the writing and erasing process many times through repeated
shearing and heating processes. Thus luminogen 1 may find
application in optical recording due to the high sensitivity and
low background noise of luminescence-based materials.
In conclusion, a luminogen exhibiting multicolor emission
switching was obtained through introduction of weak inter-
actions to an AIE active molecule. The partly amorphous
ground solid from 1CA exhibits excitation dependent emission.
The varied equilibria of the weak interactions afford morphology
dependent emission of the luminogen, and the loose packing
pattern endued by the propeller-like conformation of the AIE
active molecules facilitates transformation among different
aggregate states. Hence we provide a possible design strategy
for emission switching materials.
Experimental
If we start from 1CB, the emission of ground solid could be 1 was synthesized by a typical self-McMurry coupling reaction.
turned to deep-blue through fuming with solvent vapor, or to A 250 mL two-necked flask equipped with a magnetic stirrer
blue through heating (Fig. S8, ESI†).
was charged with zinc dust (3.3 g, 50 mmol) and 60 mL THF
The mechanochromic fluorescence of 1 prompted us to under a nitrogen atmosphere. The mixture was cooled to 0 1C,
study its potential application as an optical recording material. and TiCl4 (2.75 mL, 25 mmol) was added slowly by syringe. The
1CA was ground on one piece of weighing paper, and we mixture was refluxed for 2.5 h and cooled to 0 1C. The raw
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material 4,40-dimethoxybenzophenone (4.8 g, 20 mmol) in THF
(60 mL) was added to the mixture. Then the mixture was
refluxed until TLC showed complete conversion. The reaction
was quenched with saturated aqueous NH4Cl solution, and
extracted with diethyl ether. The organic layer was desiccated
with anhydrous magnesium sulfate for 2 h, and then filtered.
The solvents were removed by evaporation. Finally the resulting
residue was purified by recrystallization from acetone to give a
white powder 1 (3.5 g, 75%). M.p. 181–182 1C. 1H NMR (DMSO-
d6, 400 MHz) d = 3.67 (s, 12H, OCH3), 6.67–6.69 (d, 8H, Ar-H),
6.84–6.86 (d, 8H, Ar-H). 13C NMR (100 MHz, DMSO-d6, d):
157.38, 137.94, 136.18, 131.88, 113.11, 54.81. IR: 2832, 1604,
1509, 1455, 1296, 1239, 1167, 1032, 832. HRMS (ESI) calculated
for C30H28O4 [M+H]+: 453.2060; found, 453.2073.
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This work was partially supported by the National Science
Foundation of China (51173018), National Basic Research
Program of China (973 Programs 2013CB834704 and
2011CB935702), and the Fundamental Research Funds for the
Central Universities. The work was also partially supported by
RPC Grants of HKUST (RPC11SC09), the Research Grants
Council of Hong Kong (HKUST2/CRF/10).
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