Cruciform 9,10-distyryl-2,6-bis(p-dialkylamino-styryl)anthracene homologues exhibiting alkyl length-tunable piezochromic luminescence and heat-recovery temperature of ground states

Article abstract of DOI:10.1039/c3tc32035a

A series of 2,6-bis(p-dialkylaminostyryl)-9,10-distyrylanthracene (FCn) cruciforms with N-alkyl chains of different lengths have been synthesized, and their aggregation-enhanced fluorescence and piezochromic luminescence (PFC) behaviours are investigated. These 9,10-distyrylanthracene-containing cruciforms exhibit relatively low fluorescence quantum yields (Φ) in THF solution (Φ ≈ 10%) and moderate aggregation-enhanced emission in aqueous media (Φ ≈ 25%), but strong and chain length-dependent solid-state fluorescence emission. Grinding and pressing experiments indicate that they are all effective PFC materials in terms of mechanical stress-induced spectral shifts (Δλ_PFC = 23-54 nm), moreover, the longer alkyl-containing FCn shows a larger Δλ_PFC. Powder X-ray diffraction and differential scanning calorimetry measurements reveal that the transformation between the crystalline and amorphous states upon external stimuli is responsible for the reversible PFC behaviour. It is found that increasing the N-alkyl length could effectively decrease the cold-crystallization temperature of the ground states to render the PFC states with a tunable heat-recovering temperature, and ground FC10 and FC12 solids can recover spontaneously to their original states at room temperature.

Full text of DOI:10.1039/c3tc32035a

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Materials Chemistry C
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PAPER
Cruciform 9,10-distyryl-2,6-bis(p-dialkylamino-
styryl)anthracene homologues exhibiting alkyl
length-tunable piezochromic luminescence and
heat-recovery temperature of ground states
Cite this: J. Mater. Chem. C, 2014, 2,
1913
*
M. Zheng, D. T. Zhang, M. X. Sun, Y. P. Li, T. L. Liu, S. F. Xue and W. J. Yang
A series of 2,6-bis(p-dialkylaminostyryl)-9,10-distyrylanthracene (FCn) cruciforms with N-alkyl chains of
different lengths have been synthesized, and their aggregation-enhanced fluorescence and
piezochromic luminescence (PFC) behaviours are investigated. These 9,10-distyrylanthracene-containing
cruciforms exhibit relatively low fluorescence quantum yields (F) in THF solution (F z 10%) and
moderate aggregation-enhanced emission in aqueous media (F z 25%), but strong and chain length-
dependent solid-state fluorescence emission. Grinding and pressing experiments indicate that they are
all effective PFC materials in terms of mechanical stress-induced spectral shifts (DlPFC ¼ 23–54 nm),
moreover, the longer alkyl-containing FCn shows a larger DlPFC. Powder X-ray diffraction and differential
scanning calorimetry measurements reveal that the transformation between the crystalline and
amorphous states upon external stimuli is responsible for the reversible PFC behaviour. It is found that
increasing the N-alkyl length could effectively decrease the cold-crystallization temperature of the
ground states to render the PFC states with a tunable heat-recovering temperature, and ground FC10
and FC12 solids can recover spontaneously to their original states at room temperature.
Received 15th October 2013
Accepted 2nd December 2013
DOI: 10.1039/c3tc32035a
www.rsc.org/MaterialsC
compounds identied are still an isolated event. Therefore,
there is a great demand for the exploitation of new PFC mate-
Introduction
rials and the accumulation of relative structure–property
knowledge. In this context, the systematically comparable
molecular design and tunable recovery behaviour of PFC
materials are extremely desirable to both the investigation of
the structure–property relationship and various applications.
We have been interested in the optical and optoelectronic
properties of linear- and two-dimensional (2-D) cross-conju-
gated anthracene-based oligomers and polymers.11a,13–26 In
principle, an anthracene ring could be linked in two different
ways, 9,10- or 2,6-linkages, to form two conjugation pathways
with different backbone conformations and quinoid characters.
As a sequence, the electronic and photonic properties of
anthracene-based conjugated molecules are highly dependent
on both the nature of building blocks and the way in which they
are linked. For example, most 9,10-bis(arylvinyl)anthracene
derivatives exhibit unique aggregation-induced emission (AIE)
and PFC behaviour related to the strongly twisted backbone
conformations.8d,10c,11,25–30 Contrarily, planar 2,6-bis(arylvinyl)-
anthracene derivatives show aggregation-quenched emission
and non-PFC behaviour. Therefore, it is very interesting to know
whether the center-crossed chromophores integrated by 2,6-
and 9,10-bis(arylvinyl)anthracenes are AIE- and PFC-active since
2-D cross-conjugated cruciforms have many unique and inter-
esting optical and optoelectronic properties.31 To the best of our
Tuning and switching solid-state luminescence by an external
mechanical stimuli (such as grinding, compressing, shearing,
deformation, etc. named piezo- or mechano-uorochromism
(PFC)), instead of the chemical alteration of molecular struc-
tures not only has potential applications in chemo- and
mechano-sensors, data storage, security inks and optoelec-
tronic devices, but also in gaining understanding of solid-state
photophysical properties.1–6 Up to now, there has been an
increasing interest in the design, synthesis and characterization
of PFC materials, and some pyrene-, anthracene-, dibenzo-
fulvene-, tetraphenylethene-, carbazole-, and cyanostilbene-
based derivatives, etc. which have been shown to be promising
PFC materials, and their structure–property relationship has
been investigated.7–12 The results of these studies reveal that the
PFC behaviour is closely related to the changes of the molecular
aggregation morphology, such as the packing mode of the
crystal molecules or the phase transition from the crystalline to
amorphous state. However, PFC materials are still at the initial
state of investigation, and their numbers and the under-
standing of the phenomenon are limited. Moreover, most PFC
Key Laboratory of Rubber-plastics of Ministry of Education, School of Polymer Science
& Engineering, Qingdao University of Science & Technology, 53-Zhengzhou Road,
Qingdao, 266042, China. E-mail: ywjph2004@qust.edu.cn
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knowledge, there are no reports on the construction of cruci- spectrophotometer. The peak wavelength of the lowest energy
forms with AIE and/or PFC behaviour at present, and organic absorption band was used as the excitation wavelength for PL
materials exhibiting chain length-dependent PFC behaviour are measurement. The uorescence quantum yield (F) was
also scarce, although a number of organic uorophores with determined at room temperature by the dilution method
chain length-dependent solid-state uorescence have been using uorescein in water (pH ¼ 11) as the reference in which
reported in the past few years.7,11a,25,30,32,33 In the current work, the absolute values of maximal absorption are less than 0.1.34
we have designed and synthesized a series of 2,6-bis(p-di- Powder wide-angle X-ray diffraction (PWXD) experiments
alkylaminostyryl)-9,10-distyrylanthracenes (FCn, Scheme 1) to were performed on a powder X-ray diffractometer (INCA
examine emphatically the PFC behaviour and the effect of the N- Energy, Oxford Instruments) operating at 3 kW. Differential
alkyl length. We now report that these cruciform homologues scanning calorimetry (DSC) curves were determined on a
ꢁ1
ꢀ
are not only effective PFC materials, but also that their heat- Netzsch DSC 204F1 at a heating rate of 10 C min
.
recovering temperature of ground states can be effectively tuned
by changing the N-alkyl lengths.
Synthetic procedure for the various 4-dialkylaminobenzal-
dehydes
Experimental section
Synthesis
A mixture of aniline (0.26 g, 2.8 mmol), 1-alkylbromide
(7.5 mmol), and K2CO3 (1.38 g, 10 mmol) in DMF (20 mL) was
stirred for 24 h at 80 ꢀC and then cooled to room temperature
and poured into water and extracted with ether. Aer the
solvent was removed by a rotatory evaporator, the crude
product was dried in vacuo and then added to a stirring
solution of phosphoryl trichloride (5 mL) and DMF (20 mL) at
room temperature and stirred for 6 h. The resulting mixture
was poured into ice-water and extracted with ether. The
solvent was evaporated and the crude product was separated
by column chromatography on silica gel using hexane–ethyl
acetate (15/1) as the eluent. Colourless or a slight yellow oil
was obtained in high yields of 80–90%. These compounds
were characterized by 1H NMR and the relative data was as
follows.
The synthesis and structure of FCn is depicted in Scheme 1.
The general route to FCn started from 2,6-bis(diethylphos-
phoryl-methyl)-9,10-dibromoanthracene (1). Intermediate 1
was rst subjected to a Heck coupling with styrene to afford
the intermediate 2. The Wittig–Horner reaction of 2 and each
4-dialkylaminobenzaldehyde with different lengths of alkyl
chain gave the desirable cruciforms FCn in good yields
and purity aer purication by column chromatography on
silica gel.
Regents and solvents
Tetrahydrofuran (THF) was distilled over metallic sodium
and N,N-dimethylformamide (DMF) over calcium hydride
before use. Styrene, 4-dimethylaminobenzaldehyde, aniline,
1-alkylbromides and others were all commercially available
analytical-grade chemicals and were used as received, unless
otherwise claimed. 2,6-Bis(diethylphosphorylmethyl)-9,10-
dibromo-anthracene was from a previous work.16
4-Dibutylaminobenzaldehyde. 1H NMR (500 MHz, CDCl3, d):
9.67 (s, 1H), 7.66 (d, J ¼ 8.5 Hz, 2H), 6.61 (d, J ¼ 9 Hz, 2H), 3.32
(t, 4H), 1.41 (m, 8H), 0.84 (t, 6H).
4-Diheptylaminobenzaldehyde. 1H NMR (500 MHz, CDCl3,
d): 9.67 (s, 1H), 7.67 (d, J ¼ 8.5 Hz, 2H), 6.61 (d, 2H), 3.32 (t, 4H),
1.60 (m, 4H), 1.31 (m, 16H), 0.86 (t, 6H).
4-Dioctylaminobenzaldehyde. 1H NMR (500 MHz, CDCl3, d):
9.71 (s, 1H), 7.71 (d, J ¼ 7.5 Hz, 2H), 6.67 (d, 2H), 3.31 (t, 4H),
Measurement
1H (500 MHz) and 13C NMR (125 MHz) spectra were recorded 1.61 (m, 4H), 1.32 (m, 20H), 0.91 (s, 6H).
on a Bruker-AC500 spectrometer with CDCl3 as the solvent
4-Didecylaminobenzaldehyde. 1H NMR (500 MHz, CDCl3, d):
and tetramethylsilane (TMS) as the internal standard, and are 9.68 (s, 1H), 7.68 (d, J ¼ 8.5 Hz, 2H), 6.62 (d, J ¼ 9 Hz, 2H), 3.32
expressed in ppm (d). Elemental analysis was performed on a (t, 4H), 1.55 (m, 4H), 1.26 (t, 28H), 0.87 (t, 6H).
Perkin-Elmer 2400. UV-vis absorption spectra were recorded
on
measurements were carried out with a Hitachi F-4600 (t, 4H), 1.57 (m, 4H), 1.28 (t, 36H), 0.85 (t, 6H).
4-Didodecylaminobenzaldehyde. 1H NMR (500 MHz, CDCl3,
Hitachi U-4100 spectrophotometer. Fluorescence d): 9.67 (s, 1H), 7.69 (d, J ¼ 9 Hz, 2H), 6.63 (d, J ¼ 9 Hz, 2H), 3.32
a
Scheme 1 Synthesis and structure of the cruciforms FCn.
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111.7, 109.9, 51.1, 31.9, 29.6, 28.4, 27.3, 22.7, 14.1. Anal. calcd
for C74H92N2: C, 88.04; H, 9.19; N, 2.77. Found: C, 88.11; H, 9.14;
N, 2.74.
Synthesis of 2,6-bis(diethylphosphorylmethyl)-9,10-distyryl-
anthracene
A pressure tube containing styrene (1.11 g, 10.1 mmol), 2,6-
bis(diethylphosphorylmethyl)-9,10-dibromoanthracene (1.35
g, 2.13 mmol), Pd(OAc)2 (52 mg, 0.21 mmol), tri(o-tolyl)
phosphine (0.38 g, 1.21 mmol), N(C2H5)3 (7 mL) and THF (7
mL) was sealed with a helix teon cap under nitrogen and
then reuxed at 80 ꢀC for 24 h. Aer cooling, the solvents were
evaporated and the residue was separated by column chro-
matography on silica gel using ethyl acetate–hexane (3/1) as
the eluent. Yield: 1.16 g (80.6%). 1H NMR (500 MHz, CDCl3, d):
8.35 (d, J ¼ 9 Hz, 2H), 8.21 (s, 2H), 7.89 (d, J ¼ 11.5 Hz, 2H),
7.69 (d, J ¼ 7.5 Hz, 4H), 7.48 (m, 6H), 7.39 (m, 2H), 6.92 (d, J ¼
17 Hz, 2H), 4.03 (m, 8H), 3.33 (d, 4H), 1.23 (m, 12H). Anal.
calcd for C40H44O6P2: C, 70.37; H, 6.50; O, 14.06; P, 9.07.
Found: C, 70.46; H, 6.44.
2,6-Bis(4-dioctylaminostyryl)-9,10-distyrylanthracenes (F8).
1H NMR (500 MHz, CDCl3, d): 8.30 (d, J ¼ 9.5 Hz, 2H), 8.15
(s, 2H), 7.90 (d, J ¼ 16.5 Hz, 2H), 7.75 (m, 6H), 7.48 (m, 4H),
7.39 (m, 6H), 7.11 (m, 4H), 6.96 (d, J ¼ 16.5 Hz, 2H), 6.59 (d, J ¼
8.5 Hz, 4H), 3.25 (t, 8H), 1.55 (m, 16H), 1.26 (m, 32H), 0.87 (m,
12H). 13C NMR (500 MHz, CDCl3, d): 147.9, 137.4, 134.9, 134.5,
132.1, 130.0, 129.2, 128.9, 128.4, 128.1, 127.7, 127.0, 126.7,
125.4, 124.5, 124.1, 122.6, 111.7, 109.9, 51.1, 31.9, 29.7, 29.4,
28.4, 27.3, 22.7, 14.1. Anal. calcd for C78H100N2: C, 87.91; H,
9.46; N, 2.63. Found: C, 88.01; H, 9.44; N, 2.60.
2,6-Bis(4-didecylaminostyryl)-9,10-distyrylanthracenes (F10).
1H NMR (500 MHz, CDCl3, d): 8.30 (d, J ¼ 9 Hz, 2H), 8.15 (s,
2H), 7.90 (d, J ¼ 16.5 Hz, 2H), 7.74 (m, 6H), 7.46 (m, 4H), 7.40
(m, 6H), 7.11 (m, 4H), 6.96 (d, J ¼ 16 Hz, 2H), 6.60 (d, J ¼ 8.5
Hz, 4H), 3.26 (d, 8H), 1.56 (m, 32H), 1.30 (m, 32H), 0.86 (m,
12H). 13C NMR (500 MHz, CDCl3, d): 147.9, 137.4, 134.9, 134.5,
132.1, 130.0, 129.2, 128.9, 128.2, 128.0, 127.7, 127.0, 126.7,
General synthetic procedure for various 2,6-bis(p-dialkyl-
aminostyryl)-9,10-distyrylanthracenes (FCn)
Potassium tert-butoxide (0.16 g, 1.43 mmol) was added to 125.4, 124.4, 124.1, 122.6, 111.7, 110.2, 51.2, 31.9, 30.2, 29.7,
the mixture of 2,6-bis(diethylphosphorylmethyl)-9,10-distyryl- 29.4, 28.0, 27.4, 27.1, 22.8, 14.2. Anal. calcd for C86H116N2: C,
anthracene (0.13 g, 0.19 mmol) and 4-dialkylamino 87.70; H, 9.93; N, 2.38. Found: C, 87.82; H, 9.88; N, 2.35.
benzaldehyde (0.56 mmol) in anhydrous THF (15 mL) at room
2,6-Bis(4-didodecylaminostyryl)-9,10-distyrylanthracenes
temperature under N2 protection. The mixture was stirred (F12). 1H NMR (500 MHz, CDCl3, d): 8.31 (d, J ¼ 8 Hz, 2H), 8.15 (s,
overnight and then 100 mL of methanol was added. The 2H), 7.92 (d, J ¼ 16.5 Hz, 2H), 7.73 (m, 6H), 7.49 (m, 4H), 7.39 (m,
precipitate was collected and puried by column chromatog- 6H), 7.11 (m, 4H), 6.97 (d, J ¼ 16.5 Hz, 2H), 6.60 (d, J ¼ 8.5 Hz, 4H),
raphy on silica gel using hexane–CH2Cl2 (3/1) as the eluent. The 3.27, 3.26, 3.24 (t, 8H), 1.54 (m, 48H), 1.30 (m, 32H), 0.87 (m,
green to orange solids were obtained in 57–73% yields, and 12H). 13C NMR (500 MHz, CDCl3, d): 148.0, 137.5, 134.9, 134.5,
their 1H and 13C NMR data and element analysis were as 132.1, 130.0, 129.2, 128.9, 128.2, 128.0, 127.7, 126.9, 126.7, 125.5,
follows.
124.4, 124.1, 122.6, 111.7, 110.2, 51.3, 32.1, 31.9, 30.5, 30.2, 29.7,
2,6-Bis(4-dimethylaminostyryl)-9,10-distyrylanthracenes (F1). 29.4, 28.0, 27.4, 27.1, 22.8, 14.2. Anal. calcd for C94H132N2: C,
1H NMR (500 MHz, CDCl3, d): 8.32 (d, J ¼ 9 Hz, 2H), 8.18 (s, 2H), 87.52; H, 10.31; N, 2.17. Found: C, 87.61; H, 10.25; N, 2.13.
7.92 (d, J ¼ 16.5 Hz, 2H), 7.73 (m, 6H), 7.50 (m, 6H), 7.45 (m, 4H),
7.13 (m, 4H), 6.96 (d, J ¼ 16.5 Hz, 2H), 6.70 (d, J ¼ 8 Hz, 4H), 2.98
Piezochromic and stimulus-recovering experiments
(m, 12H). 13C NMR (500 MHz, CDCl3, d): 147.9, 137.3, 134.8,
Grinding experiment. Pristine FCn solid was put on a glass
plate and ground with a metal spatula at room temperature.
Pressing experiment: 5–7 mg of FCn solid and ca. 250 mg of KBr
powder were simply mixed in a mortar (for the sake of mini-
mizing the amount of uorophore), and then pressed with an IR
pellet press under the pressure of 1500 psi for 30 seconds at room
temperature. Annealing experiment: the ground or pressed FC1–
FC8 samples were placed into an oven with a temperature 20 ꢀC
134.5, 132.2, 130.6, 129.3, 128.9, 128.2, 128.0, 127.9, 127.0, 126.7,
125.4, 124.4, 124.1, 122.6, 118.5, 111.7, 50.2. Anal. calcd for
C50H44N2: C, 89.25; H, 6.59; N, 4.16. Found: C, 89.36; H, 6.53; N,
4.13.
2,6-Bis(4-dibutylaminostyryl)-9,10-distyrylanthracenes (F4).
1H NMR (500 MHz, CDCl3, d): 8.33 (d, J ¼ 9 Hz, 2H), 8.23 (s, 2H),
7.90 (d, J ¼ 16.5 Hz, 2H), 7.75 (m, 6H), 7.51 (m, 4H), 7.42 (m,
6H), 7.11 (m, 4H), 6.98 (d, J ¼ 16.5 Hz, 2H), 6.63 (d, J ¼ 8 Hz, 4H),
3.30 (t, 8H), 1.59 (m, 8H), 1.37 (m, 8H), 0.97 (m, 12H). 13C NMR
above the respective cold-crystallization temperature and stood
for 3 minutes, and the ground or pressed F10 and FC12 samples
(500 MHz, CDCl3, d): 147.9, 137.3, 134.9, 134.5, 132.2, 130.8,
were stood at room temperature. Solvent-fuming experiment: in a
sealed CH2Cl2-containing beaker, the sample was suspended
above the solvent level and exposed for 30 seconds at room
temperature. Aer external stimuli, the uorescence images and
129.3, 128.9, 128.2, 128.0, 127.9, 127.0, 126.7, 125.4, 124.4,
124.1, 122.6, 118.6, 111.7, 50.8, 29.5, 20.4, 14.1. Anal. calcd for
C
62H68N2: C, 88.52; H, 8.15; N, 3.33. Found: C, 88.41; H, 8.19; N,
3.37.
2,6-Bis(4-diheptylaminostyryl)-9,10-distyrylanthracenes (F7).
emission spectra were recorded at room temperature.
1H NMR (500 MHz, CDCl3, d): 8.31 (s, 2H), 8.16 (s, 2H), 7.90
(d, J ¼ 16.5 Hz, 2H), 7.74 (m, 6H), 7.50 (m, 4H), 7.41 (m, 6H),
7.10 (m, 4H), 6.96 (d, J ¼ 16.5 Hz, 2H), 6.59 (d, 4H), 3.26 (d, 8H),
1.57 (m, 16H), 1.30 (m, 24H), 0.90 (m, 12H). 13C NMR (500 MHz,
Results and discussion
General photophysical properties
CDCl3, d): 147.9, 137.4, 134.9, 134.5, 132.1, 130.0, 129.2, 128.9, As expected, these cruciforms show the same absorption and
128.2, 128.0, 127.9, 127.0, 126.7, 125.4, 124.5, 124.1, 122.6, emission spectra (Fig. 1a) and uorescence quantum yields
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(F ¼ 10–12%) in THF solution because of their identical Table 1). Alkyl chain length-dependent solid-state uorescence
conjugated skeletons, indicating the N-alkyl chain lengths emission is an interesting phenomenon and has been reported
do not substantially affect their solution photophysical in other organic uorophores. However, there are few reports on
properties.
the cruciform uorophores exhibiting chain length-dependent
However, it is noted that the F values of FCn in THF solid-state uorescence, which will be further investigated
solution are obviously higher than those of the linear 9,10- elsewhere.
distyrylanthracene derivatives whose solutions are weak or
non-uorescent. This implies that the non-radiative decay
caused by the free intramolecular torsion of the two styryl
Most linear and branched 9,10-diarylvinylanthracene deriv-
moieties at the 9,10-positions of the anthracene core has been
depressed in the cruciform conguration. As a result, FCn
may show a weak AIE effect in aqueous media. This is evi-
denced by the emission spectra and uorescence images of
FC4 in THF solution and a THF–water (1/9) mixture (Fig. 1b).
The emission spectrum in aqueous media is slightly blue-
shied relative to that in THF solution, and the F value in the
aqueous dispersion is only about twice that in THF solution
Piezouorochromic and recovering behaviour
atives have been found to show PFC behaviour. However, two
branches of FCn, 2,6-bis(p-dialkylaminostyryl)anthracenes
and 9,10-distyrylanthracene, do not show PFC behaviour. To
investigate whether cruciforms FCn exhibit mechanochromic
luminescence, we rst simply ground the pure FCn solids on
the glass plate using a metal spatula. It is observed that all the
FCn solids showed changed uorescence colors upon
grinding (Fig. 2). Interestingly, the uorescence colors of the
(inset in Fig. 1b).
ground FC1–FC8 solids are stable and remain unchanged
Nevertheless, FCn are still AIE-active molecules and exhibit
over 24 hours at room temperature, but ground FC10 and
FC12 can spontaneously change back to original colors
within seconds at room temperature (bottom, Fig. 2).
strong solid-state uorescence. This is probably due to the
highly distorted 9,10-distyryl moieties caused by the strongly
steric hindrance of the 1,8-positions of the anthracene core,
Considering that the difference among these cruciforms is
even aer stacking, disturb the intermolecular close packing, to
diminish the intermolecular quenching effects. In addition, it is
noted that the uorescence emission of the pristine FCn solids
depends on the N-alkyl length (le column in Fig. 2 and
only the N-alkyl length, it is concluded that the alkyl chains
play a crucial role in determining the solid-state optical
properties and the heat-recovering behaviour of the ground
states. To further understand the recovering properties of the
ground FC1–FC8 solids, the ground solids are thermally
annealed or exposed to solvent vapor (fuming on dichloro-
methane at room temperature). The results show that the
uorescence colors of the ground states are almost converted
to the original colors, and regrinding the annealed or fumed
samples affords the same uorescence color changes as the
rst grinding. This process is reproducible, indicating a
reversible PFC phenomenon.
Meanwhile, we have also carried out the pressing experiment
of the FCn samples (5–7 mg of FCn solid was mixed with KBr to
minimize the amount of uorophores) with an IR pellet press
(30 seconds at 1500 psi) at room temperature. Since FCn are
organic and non-polar and KBr is an inorganic salt, FCn should
Fig. 1 Absorption and emission spectra at 1.0 ꢂ 10ꢁ5 M: (a) FCn in THF Fig. 2 Fluorescence images of pure FCn solids under as-prepared
solutions; (b) FC4 in THF and THF–H2O (1/9) mixture (the inset is the (pristine) and various external stimuli states illuminated by a UV lamp of
corresponding fluorescence images and F).
365 nm. Piezofluorochromic and recovering behaviour.
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Table 1 Peak emission wavelengths (l, in nm) and thermal transition data of FCn solids under various external stimuli
a
b
c
d
Cpds
lpristine
lpressed
lannealed
lre-pressed
lfumed
DlPFC
Tm /ꢀC
Tcc /ꢀC
FC1
FC4
FC7
FC8
FC10
FC12
591
597
563
541
538
551
616
613
595
598
597
596
593
588
565
544
552
547
615
613
593
597
597
597
593
588
563
547
554
555
23
25
30
54
45
49
>300
245
143
136
nde
nde
138
95
72
50
#RT
<RT
a
b
c
Peak emission wavelengths of as-prepared solids. Pressing-induced spectral shi, DlPFC ¼ lpressed ꢁ lannealed
.
Isomeric melt transition
temperature. d Cold-crystallization temperature of ground solids. e Not determined.
disperse in KBr non-molecularly and show solid-state proper- spectra whose peak wavelengths are in a relatively narrow
ties. Overall, the pressing gives the same effectiveness as the range of 616–595 nm, although their peak emission wave-
grinding, and the uorescence colours can be changed revers- lengths in pristine solids are very different (597–538 nm)
ibly by thermal annealing, solvent-fuming, and repressing (Table 1). Moreover, most pressed samples can revert back to
(Fig. 3). Interestingly, the uorescence colours of the pressed the uorescence emission similar to the pristine solids upon
FC10 and FC12 samples can remain almost unchanged over annealing or solvent-fuming. In terms of pressing-induced
5
min at room temperature, which allows the spectral spectral shi (DlPFC ¼ lpressed ꢁ lannealed), FCn with longer
measurement. The phenomena that piezochromic lumines- N-alkyl chains show a more remarkable PFC behaviour
cence can retain a relatively longer time using the KBr-diluted (DlPFC ¼ 45–54 nm), and those with shorter N-alkyl chains
sample than the pure solid was observed by us in the previous have DlPFC of 23–30 nm (Table 1). These DlPFC values are
long alkyl-containing PFC materials.25 This implies that the KBr qualitatively consistent with the corresponding uorescence
matrix could decrease the molecular motion of the dispersed images observed (Fig. 3). Overall, FCn solids are a class of
uorophores and delay the recovery process. These observa- effective and chain length-dependent PFC materials, and
tions suggest that we have not only obtained a new class of PFC their alkyl length-dependency is similar to those of
materials with cruciform conguration, but also realized the 9,10-bis(p-alkoxystyryl)anthracenes,27,30 b-cyano-substituted
tunable thermal-recovering behaviour by changing the N-alkyl bis(p-alkoxystyryl)benzenes,32b and 9,10-bis[(9,9-dialkyl-
lengths.
uorene-2-yl)vinyl]anthracenes,25 but thoroughly different from
In addition to the above visual uorescence images, the that of 9,10-bis[(N-alkylcarbazol-3-yl)vinyl]anthracenes.11a These
uorescence emission spectra in the pressing experiment dichotomous alkyl length-dependent PFC behaviours underline
were recorded on a luminescence spectrophotometer (Fig. 4), the complexity of the structure–property relationship and also
and the corresponding spectroscopic data is summarized in provide new material systems for further comparable
Table 1. All the ground FCn samples show similar emission investigation.
X-ray diffraction and DSC analysis
To understand the PFC mechanism and the effect of N-alkyl
chains on the thermal-recovering behaviour of cruciform FCn
solids, powder wide-angle X-ray diffraction and differential
scanning calorimetry (DSC) experiments on pristine and
ground FC1–FC8 solids were conducted. The X-ray diffraction
patterns of the pristine FC1–FC8 solids show sharp and
intense reections indicative of a well-ordered microcrystal-
line structure (Fig. 5a). In contrast, the diffractograms of the
ground samples display broad and featureless reections
along with a series of overlapped peaks, reecting notable
amorphous features. The formation of amorphous states upon
grinding was further conrmed by DSC experiment. As shown
in Fig. 5b, there are no thermal transitions for the pristine
solids before isotropic melt transition (Tm); however, each
ground solid shows the respective cold-crystallization
transition.
Furthermore, FC4 and FC8 solids are molten and then
rapidly put into liquid N2, which should afford real amor-
Fig. 3 Fluorescence images of the FCn samples mixed with KBr under
various external stimuli illuminated by a UV lamp of 365 nm.
phous states. It is shown that the quenched solids have the
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Journal of Materials Chemistry C
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Fig. 4 Fluorescence emission spectra of the FCn samples mixed with KBr after various external stimuli. The spectra were recorded at room
temperature.
same emission spectra as the ground or pressed samples (Tcc ꢁ 20 ꢀC), but recover rapidly upon annealing above
(Fig. 6). These ndings indicate that the transformation respective Tcc (Tcc + 10 ꢀC). Thus Tcc can be regarded as
between the amorphous and crystalline states under the critical thermal recovering temperature of ground FCn.
various external stimuli is responsible for the reversible PFC The spontaneous recovery behaviour of ground FC10
behaviour. Further inspection of the DSC curves of the and FC12 at room temperature could be ascribed to the
ground solids reveals that the cold-crystallization tempera- low Tcc. It could be thought that long alkyl chain should not
tures (Tcc) of the ground FCn solids decreases gradually with be in favor of molecular close packing and endow the mate-
the length of N-alkyl chains (Fig. 5b and Table 1). Based on rials with a low thermal transition temperature and easily-
the decreased rate of Tcc from FC1 to FC8, the Tcc values of the destroyed packing structure. Unfortunately, the single
ground FC10 and FC12 should be at or below room temper- crystals of FCn are still unobtainable at present, and
ature. This implies that changing the length of the N-alkyl this impedes the discovery of the real reason. Overall, we
chains can signicantly affect the Tcc of amorphous can endow these PFC cruciforms with a tunable heat-recov-
FCn solids. We have observed that the uorescence ering temperature, which could better satisfy various
colors of the ground samples are stable below respective Tcc applications.
Fig. 5 Powder X-ray diffraction patterns at room temperature (a) and DSC curves upon the heating procedure (b) of FCn solids before and after
grinding.
1918 | J. Mater. Chem. C, 2014, 2, 1913–1920
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Journal of Materials Chemistry C
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Conclusions
We have demonstrated that anthracene-centered cruciforms
(FCn) integrated by the aggregation-induced emission (AIE)
9,10-distyrylanthtracene and the aggregation-quenched emis-
sion 2,6-bis(p-dialkylaminostyryl)anthracenes are still AIE-
active dyes and exhibit remarkable piezochromic luminescence
with mechanical stress-induced spectral shis (DlPFC) of 23–
54 nm. Increasing the N-alkyl length can not only enlarge the
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low cold-crystallization temperature. Cruciform dyes with
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Acknowledgements
We thank the NSF of China (no. 51173092, 51073083,
51303091), NSF of Shandong Province and Qingdao City (no.
ZR2010EM023, ZR2012EMQ003, 13-1-4-207-jch), Open Project
of State Key Laboratory of Supramolecular Structure and
Materials of Jilin University (no. SKLSSM201207), and the
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