Solid-state fluorescence properties and reversible piezochromic luminescence of aggregation-induced emission-active 9,10-bis[(9,9- dialkylfluorene-2-yl)vinyl]anthracenes

Article abstract of DOI:10.1039/c3tc00017f

In this work, we have synthesized 9,10-bis[(9,9-dialkylfluorene-2-yl)vinyl] anthracene derivatives (FLA-Cn) with propyl, pentyl, and dodecyl side chains to investigate their fluorescence properties. The results show that FLA-Cn exhibit not only an aggregation-induced emission effect but also reversible piezofluorochromic (PFC) behaviour. Interestingly, the fluorescence emission and grinding-induced spectral shifts (Δλ_PFC) of FLA-Cn solids are alkyl length-dependent: the longer alkyl-containing FLA-Cn solids show more blue-shifted emission and larger Δλ_PFC. Moreover, the fluorescence emission of ground FLA-C12 solid can recover spontaneously at room temperature, Powder wide-angle X-ray diffraction and differential scanning calorimetry experiments reveal that the transformation between crystalline and amorphous states under various external stimuli is responsible for the PFC behaviour, and the spontaneous recovering emission of amorphized FLA-C12 solid is ascribed to its low cold-crystallization temperature. This work demonstrates once again the accessibility of tuning the solid-state optical properties of organic fluorophores by combining the simple alternation of molecular chemical structure and the physical change of aggregate morphology under external stimuli.

Full text of DOI:10.1039/c3tc00017f

Journal of
Materials Chemistry C
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PAPER
Solid-state fluorescence properties and reversible
piezochromic luminescence of aggregation-induced
emission-active 9,10-bis[(9,9-dialkylfluorene-2-yl)vinyl]
anthracenes
Cite this: J. Mater. Chem. C, 2013, 1,
028
2
Lingyu Bu, Mingxiao Sun, Deteng Zhang, Wei Liu, Yalong Wang, Meng Zheng,
Shanfeng Xue and Wenjun Yang*
In this work, we have synthesized 9,10-bis[(9,9-dialkylfluorene-2-yl)vinyl]anthracene derivatives (FLA-Cn)
with propyl, pentyl, and dodecyl side chains to investigate their fluorescence properties. The results
show that FLA-Cn exhibit not only an aggregation-induced emission effect but also reversible
piezofluorochromic (PFC) behaviour. Interestingly, the fluorescence emission and grinding-induced
spectral shifts (DlPFC) of FLA-Cn solids are alkyl length-dependent: the longer alkyl-containing FLA-Cn
solids show more blue-shifted emission and larger DlPFC. Moreover, the fluorescence emission of ground
FLA-C12 solid can recover spontaneously at room temperature, Powder wide-angle X-ray diffraction and
differential scanning calorimetry experiments reveal that the transformation between crystalline and
amorphous states under various external stimuli is responsible for the PFC behaviour, and the
spontaneous recovering emission of amorphized FLA-C12 solid is ascribed to its low cold-crystallization
temperature. This work demonstrates once again the accessibility of tuning the solid-state optical
properties of organic fluorophores by combining the simple alternation of molecular chemical structure
and the physical change of aggregate morphology under external stimuli.
Received 3rd January 2013
Accepted 21st January 2013
DOI: 10.1039/c3tc00017f
www.rsc.org/MaterialsC
organic molecules exhibiting aggregation-induced/crystalliza-
tion-enhanced emission (AIE/CEE) have been the new mainstay
Introduction
9–13
Stimuli-responsive materials, especially piezo- or mechano-u-
orochromic (PFC) materials, have attracted much attention due
to their potential applications in optical recording and uo- tions,
of PFC materials.
The common structural characteristics of
AIE molecules are the strongly twisted skeleton conforma-
14,15
which could hinder solid-state intermolecular close
1–3
rescence sensors.
In principle, changes in the solid-state stacking and intense p–p interaction and might result in these

uorescence of conjugated organic materials could be achieved materials possessing mechanical stress-dependent stable and
by chemical or physical methods. The chemical method metastable solid morphologies that show different optical and
involves solid-state chemical reactions that provoke chemical optoelectronic properties. However, PFC materials are still at
4
structural changes of molecules and are usually incomplete
and irreversible. The physical method, which involves the
change of molecular packing mode instead of chemical alter-
the initial stages of investigation, and the kinds of PFC mate-
rials and in-depth understanding of PFC phenomena are
limited. Therefore, there is still a great demand on exploitation
ation of the molecular structure, has been attracting consider- of new PFC materials and accumulation of knowledge of their
able interest because switching and tuning the solid-state color structure–property relationships. In this context, the design and
and/or luminescence properties by controlling the molecular synthesis of new PFC materials with comparable molecular
packing mode could not only provide practical applications in structures and the investigation of PFC behaviour are very
optical recording and sensing but also prot the fundamental interesting but scarce.
understanding of photophysical properties. While there have
Very recently, it was reported that 9,10-bis(p-alkoxystyryl)
anthracences only with sufficient long alkyl chains (decyl,
undecyl and dodecyl) are effective PFC materials; in contrast,
been a variety of reports on organic and organometallic chro-
1–3,5–8
10i
mophores with PFC behaviour,
recently, some conjugated
the shorter alkyl-containing 9,10-bis[(N-alkylcarbazol-3-yl)vinyl]
13
anthracenes show more remarkable PFC behaviour. These
two forms of chain length-dependent PFC behaviour underline
not only the complexity of the structure–property relationship
of PFC materials but also the signicant effect of the nature of
Key Laboratory of Rubber-Plastics of Ministry of Education/Shandong Provincial Key
Laboratory of Rubber-Plastics, School of Polymer Science and Engineering, Qingdao
University of Science & Technology, 53 Zhengzhou Road, Qingdao 266042, China.
E-mail: ywjph2004@qust.edu.cn
2
028 | J. Mater. Chem. C, 2013, 1, 2028–2035
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Scheme 1 Synthesis and structure of FLA-Cn.
end-arylenes and substituents at the aryl rings on the solid- Piezouorochromic and stimulus-recovering experiments
state uorescence and piezochromic luminescence of 9,10-
Pressing experiment: a quantity of FLA-Cn and KBr powder was
bis(arylvinyl)anthracene derivatives. To further understand this
simply mixed in a mortar and then pressed with an IR pellet
phenomenon and develop new PFC materials, in the current
press under a pressure of 1500 psi for 1 min at room
work, we have designed and synthesized some AIE-active 9,10-
temperature. Annealing experiment: the pressed sample was
bis[(9,9-dialkyluorene-2-yl)vinyl]anthracenes with propyl,
pentyl and dodecyl as side alkyl chains (FLA-Cn, Scheme 1) to
investigate their uorescence properties. We report here that
FLA-Cn not only exhibit chain length-dependent PFC behav-
iour, in that longer alkyl-containing FLA-Cn show larger spec-
tral shis, but also the uorescence color and emission of
ground FLA-C12 solid can recover spontaneously at room
temperature.
ꢀ
put into an oven whose temperature was 20 C over T (T is the
c
c
cold-crystallization temperature of each compound) for 3 min.
Solvent-fuming experiment: the pressed sample was put above
the dichloromethane level and exposed to the vapor for 1 min
at room temperature. Grinding experiment: at room tempera-
ture, pristine FLA-C12 solid was put on a glass plate and
ground with a metal spatula and then allowed to stand for a
given time.
Experimental section
Regents and solvents
Synthesis
9
,10-Bis(chloromethyl)anthracene. To a stirred mixture of
anthracene (1.78 g, 10 mmol), anhydrous ZnCl (1.64 g, 12
mmol), and paraformaldehyde (1.50 g, 50 mmol) in dioxane
20 mL) was slowly added concentrated aqueous hydrochloric
All the chemicals were purchased from Aldrich Chemical Co.
Other solvents and reagents were analytical grade and used as
received, unless otherwise stated. Tetrahydrofuran (THF) was
distilled over metallic sodium and dimethyl formamide (DMF)
over calcium hydride before use.
2
(
acid (40 mL) at room temperature. The mixture was gentle
reuxed for 3 h and allowed to stand for 16 h at room temper-
ature. The resulting ne granular solid was separated by ltra-
Measurements
2
tion and washed with H O and dioxane to afford the crude
1
13
H and C NMR spectra were recorded on a Bruker-AC500 (500 product. The crude product was recrystallized from toluene to
1
MHz) spectrometer with CDCl
TMS) as the internal standard. Elemental analysis was per- CDCl
formed on a Perkin-Elmer 2400. UV-vis absorption and diffuse 9,10-Bis(diethoxylphosphorylmethyl)anthracene. A mixture
reectance absorption spectra were recorded on a Hitachi U- of 9,10-bis(chloromethyl)anthracene (7.80 g, 28.3 mmol) and
100 spectrophotometer. Fluorescence measurements were triethyl phosphite (50 mL) was stirred and gently reuxed
3
as solvent and tetramethylsilane give a yellowish solid (1.81 g, 64.1%). H NMR (500 MHz,
(
3
): d 8.40 (m, 4H), 7.62 (m, 4H), 5.61 ppm (s, 4H).
4
carried out with a Hitachi F-4600 spectrophotometer. The peak overnight. The excess triethyl phosphite was removed by
wavelength of the lowest energy absorption band was used as distillation under reduced pressure. The crude product was
the excitation wavelength for the PL measurement. The uo- separated by silica gel column chromatography using ethyl
rescence quantum yield (F) was determined at room tempera- acetate/petroleum ether (1/1, v/v) as the eluent. A yellow solid
1
ture by the dilution method using uorescein in water (pH ¼ 11) was obtained (10.4 g, 76.8%). H NMR (500 MHz, CDCl
3
): d 8.32
as the reference. Powder wide-angle X-ray diffraction (PXD) (d, 4H), 7.53 (d, 4H), 4.21 (q, 8H), 3.78 (d, 4H), 1.11 ppm (t, 12H).
experiments were performed on a powder X-ray diffractometer 2-Bromo-9,9-didodecyluorene. To mixture of 50%
INCA Energy, Oxford Instruments) operating at 3 kW. Differ- aqueous NaOH (12 mL), DMSO (40 mL) and 2-bromouorene
a
(
ential scanning calorimetry (DSC) curves were determined on a (2.1 g, 8.2 mmol) was slowly added 1-bromododecane (4.9 mL,
ꢀ
ꢁ1
Netzsch DSC 204F1 at a heating rate of 10 C min
.
19.6 mmol) at room temperature. The mixture was stirred at
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ꢀ
1
9
0 C overnight and then cooled to room temperature. Aer
FLA-C5: H NMR (500 MHz, CDCl
3
): d 8.49 (q, 4H), 8.01–7.98
that, the mixture was added to brine and extracted with (d, 2H), 7.79–7.26 (m, 18H), 7.06–7.03 (d, 2H), 2.03 (t, 8H), 1.65
1
3
dichloromethane. The organic phase was dried over MgSO and (qui, 8H), 1.15–1.09 (m, 16H), 0.75 ppm (t, 12H). C NMR (125
4
the solvent was removed. The residue was puried with column MHz, CDCl ): d 151.53, 151.13, 141.32, 140.81, 138.14, 136.30,
3
chromatography (silica gel, petroleum ether) to give 3.55 g of 132.86, 129.77, 127.12, 126.88, 126.61, 125.54, 125.24, 124.42,
1
yellow liquid (75% yield). H NMR (500 MHz, CDCl
1
4
3
): d 7.65 (d, 122.95, 121.02, 119.96, 119.79, 55.14, 53.39, 40.31, 23.49, 22.26,
70: C, 91.59; H, 8.41. Found: C,
H), 7.54 (d, 1H), 7.45 (d, 2H), 7.31 (s, 3H), 1.98 (t, 4H), 1.66 (qui, 13.98 ppm. Anal. calcd for C64
H), 1.53–1.15 (m, 36H), 0.88 ppm (t, 6H). 91.77; H, 8.52%.
,9-Didodecyluorene-2-carbaldehyde. To a solution of 2-
H
9
bromo-9,9-didodecyluorene (3.5 g, 6.03 mmol) in anhydrous
THF (15 mL) was added dropwise n-BuLi (2.5 mL, 7.24 mmol) at
Results and discussion
Synthesis and characterization
ꢀ
ꢁ
78 C. Aer stirring for 1 h, anhydrous DMF (2.87 g, 35.0
ꢀ
mmol) was added and stirred for an additional 12 h at 0 C. The FLA-Cn could be facilely synthesized by the Wittig–Horner
resulting mixture was added to water and extracted with reaction of 9,9-dialkyluorene-2-carbaldehydes (FL-CHO) and
dichloromethane. The organic layer was separated and washed 9,10-bis(diethoxyphosphorylmethyl)anthracene in high yields
with diluted HCl and then saturated aqueous NaHCO
solvent was removed by evaporation and the residue was puri- bromo-9,9-dialkyluorene with n-butyllithum in anhydrous
ed by column chromatography using dichloromethane/petro- THF followed by the addition of anhydrous DMF. The structure
leum ether (1/10, v/v) as the eluent to give a colorless oil (1.43 g, and composition of the target compounds (FLA-Cn) were
3
. The of 75–85% (Scheme 1). FL-CHO was from the treatment of 2-

1
1
13
4
5% yield). H NMR (500 MHz, CDCl ): d 10.05 (s, 1H), 7.85–7.76 unambiguously characterized by H and
C NMR and
3
(
3
m, 4H), 7.30 (s, 3H), 2.00 (t, 4H), 1.67 (qui, 4H), 1.25–1.12 (m, elemental analysis.
6H), 0.87 ppm (t, 6H).
Other 9,9-dialkyluorene-carbaldehyde intermediates were Aggregation-induced emission (AIE) in THF–water mixtures
synthesized as described for 9,9-didodecyluorene-2-carbalde-
hyde except that different 1-alkylbromides were used. Aer
purication, they were used without characterization.
To understand whether a water-nonsoluble conjugated organic
molecule is AIE active, a simple and practical method is usually
to add a large amount of water into the organic solution of the
9,10-Bis[(9,9-didodecyluorene-2-yl)vinyl]anthracene (FLA-
14,15
compound and record the changes of uorescence intensity.
C12). 9,9-Didodecyluorene-2-carbaldehyde (0.167 g, 0.46
mmol) and 9,10-bis(diethylphosphorylmethyl)anthracene (0.10
g, 0.21 mmol) were dissolved in 20 mL of anhydrous THF.
Potassium tert-butoxide (0.19 g, 1.67 mmol) was added and the
resulting suspension was stirred for 5 h at room temperature.
Methanol was added into the mixture and the resulting solid
was collected by ltration. The crude product was puried by
silica gel column chromatography (petroleum ether/dichloro-
methane ¼ 4/1, v/v). Aer recrystallization from chloroform–
FLA-Cn are highly soluble in common organic solvents, such as
THF, dichloromethane, chloroform and toluene. Since water is
the non-solvent for FLA-Cn, the molecules of FLA-Cn must have
aggregated in THF–water mixtures with high water content like
10,13,15
other 9,10-diarylvinylanthracene derivatives.
The absorp-
tion and emission spectra of FLA-Cn in THF–water mixtures are
shown in Fig. 1. FLA-Cn are AIE-active with weak uorescence in
molecularly dissolved solution with uorescence quantum
yields (F) of 3.8–4.9% but increased uorescence intensities in
aqueous suspensions with F of 21–29% at high water content
methanol, a yellowish-green solid was obtained (0.19 g, 75%
1
yield). H NMR (500 MHz, CDCl
3
): d 8.51 (q, 4H), 8.02–7.98 (d,
(
THF/water ¼ 1/9) (Fig. 1a–c). Because of the large internal steric
2
8
H), 7.79–7.23 (m, 18H), 7.06–7.03 (d, 2H), 2.03 (t, 8H), 1.85 (qui,
hindrance between the 1,8-positions of the anthracene ring and
the b-positions of the vinylene moieties, the conjugated back-
bone of 9,10-diaryl-vinylanthracene has been forced to twist
1
3
H), 1.10–1.25 (m, 72H), 0.85 ppm (t, 12H). C NMR (125 MHz,
): d 151.51, 151.12, 141.29, 140.77, 138.13, 136.27, 132.83,
29.72, 127.36, 127.09, 126.79, 125.54, 125.18, 124.31, 122.96,
CDCl
3
1
1
2
10i,15a,b
strongly.
In molecularly dissolved solution, the intra-
21.04, 119.91, 119.74, 55.11, 53.81, 40.33, 31.86, 30.04, 29.26,
3.83, 22.62, 13.99 ppm (shortage of some alkyl peaks was
molecular torsion motion is relatively free, which facilitates
nonradiative relaxation pathways and affords weak uorescence
emission. However, in aqueous media, the intramolecular
vibration and rotation are seriously restricted by molecular self-
aggregation, causing the aqueous suspensions to exhibit
strongly aggregation-induced emission. On the other hand,
there are no signicant differences for the absorption spectra of
FLA-Cn in solution and aqueous suspension (Fig. 1d). This
implies that the molecular stacking in aqueous media is prob-
ably loose.
probably due to peak overlap). Anal. calcd for C92
H, 10.31. Found: C, 90.01; H, 10.15%.
H126: C, 89.69;
Other FLA-Cn derivatives were synthesized and puried as
described for FLA-C12 except that different 9,9-dialkyl-uorene-
1
13
2
-carbaldehydes were used. Their H NMR, C NMR data and
elemental analysis were as follows:
1
FLA-C3: H NMR (500 MHz, CDCl ): d 8.48 (q, 4H), 8.01–7.98
3
(
(
1
1
1
d, 2H), 7.79–7.26 (m, 18H), 7.06–7.03 (d, 2H), 2.03 (t, 8H), 1.65
1
3
m, 8H), 0.85 ppm (t, 12H). C NMR (125 MHz, CDCl
3
): d
51.53, 151.17, 141.32, 140.81, 138.14, 136.37, 132.88, 129.77,
27.52, 127.15, 126.88, 125.54, 125.24, 124.42, 122.55, 121.14,
Absorption and emission spectra of pristine FLA-Cn solids
19.91, 119.74, 55.37, 53.37, 17.31, 14.52 ppm. Anal. calcd for The uorescence colors of pristine FLA-Cn solids are gradually
C
56
H54: C, 92.51; H, 7.49. Found: C, 92.12; H, 7.73%.
blue-shied with the increase of alkyl length, and FLA-C3 and
2
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Fig. 2 Fluorescence emission spectra of pristine FLA-Cn solids (a) and diffuse
reflectance absorption spectra of pristine and ground FLA-Cn solids (b).
aggregation behavior of FLA-Cn. It should be noticed that there
have been a variety of reports on the effect of length of alkyl
chains on the solid-state aggregation and uorescence proper-
10i,13,15b,16
ties of organic uorophores,
however, the reports on
the effect of length of alkyl chains on the PFC behaviour are
scarce. The chain length-dependent uorescence properties are
believed to be relative to both crystal packing modes (J- or H-
aggregation) and the distortion degree of the conjugated
backbone.
To understand the different uorescence emissions of
homologous and isomeric uorophores in crystalline states, an
effective and accurate way is to prepare and analyze the single
crystal structures. Unfortunately, good-quality single crystals of
FLA-Cn could not be obtained probably due to the existence of
four side alkyl chains in the twisted backbone, which is unfa-
vorable for the highly ordered alignment of molecules and the
formation of high-quality single crystals. Recently, Chi et al.
have reported that, in the crystalline state, 9,10-bis(p-alkox-
ystyryl)anthracenes (9,10-DSA) with long alkyl chains (decyl,
Fig. 1 Emission (PL) spectra (a–c) and absorption spectra (d) of FLA-Cn at 1.0 ꢂ undecyl and dodecyl) have more twisted conjugated backbones
ꢁ
5
1
0
M in water–THF mixtures; the insets are the fluorescence images with water
and weaker intermolecular interactions than those with short
alkyl chains (heptyl, octyl and nonyl), which is believed to be
contents of 0%, 50% and 90% under illumination with a 365 nm UV lamp.
10i
the reason why long alkyl-containing 9,10-DSA derivatives emit
shorter PL wavelengths and exhibit larger PFC spectral shis
FLA-C12 exhibit yellow (558 nm) and green (515 nm) emission, (DlPFC). We have recently found that shorter alkyl-containing
respectively (Fig. 2a, Table 1). On considering that the only 9,10-bis[(N-alkylcarbazol-3-yl)vinyl]anthracenes emit more blue-
13
difference between these molecules is the alkyl length, it is shied uorescence and show larger DlPFC
. Thus, it could not
concluded that the alkyl chain can affect the solid-state be deduced directly that the more blue-shied uorescence
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Table 1 Peak emission wavelengths (l, in nm) and thermal transition temperature data of FLA-Cn solids under various external stimuli
DlPFCa
T
b
m
/ C
ꢀ
T / C
c
ꢀ
Cpd
l
pristine
l
pressed
l
annealed
l
repressed
l
fumed
c
FLA-C3
FLA-C5
FLA-C12
558
530
514
574
554
554
556
528
514
575
555
552
556
532
515
18
26
40
281
180
65
145
132
#r.t.
a
b
c
DlPFC ¼ lpressed ꢁ lannealed
.
Isotropic melt transition temperature. Cold-crystallization temperature of ground FLA-Cn solids.
emission of longer alkyl-containing FLA-Cn is due to the more column, Fig. 3). Moreover, when the pressed samples are
twisted conjugated backbone arising from the long alkyl chains annealed before the isotropic melt transition or exposed to
because of the unavailability of crystal structures. Alternatively, solvent vapor (fuming above dichloromethane), the uores-
we have measured the diffuse reectance absorption spectra of cence colors are recovered to the original ones. Furthermore,
pristine FLA-Cn solids (black lines, Fig. 2b). It is shown that the when the fumed or annealed samples are re-pressed, the uo-
absorption spectrum of pristine FLA-C12 solid is obviously blue- rescence colors are again changed as the rst pressing. This
shied compared with those of pristine FLA-C3 and FLA-C5 process is very reproducible and indicates reversible PFC
solids, suggesting a more twisted conjugated backbone for FLA- behaviour. It is noted that the longer the alkyl chain of FLA-Cn,
C12 than FLA-C3 and FLA-C5 in pristine crystals. On the other the more remarkable the uorescence color change upon
hand, in comparison with the solution absorption spectra pressing (Fig. 3).
(Fig. 1d), the absorption maximum of FLA-C12 solid is hyp-
These naked-eye-visible uorescence color changes were
sochromic relative to its solution absorption band, and FLA-C3 recorded on a luminescence spectrophotometer. The spectral
and FLA-C5 solids show bathochromic absorption maxima data are summarized in Table 1, and the emission spectra of
relative to their solutions. In view of these absorption spectra, pressed and annealed FLA-Cn samples are depicted in Fig. 4. It
FLA-C12 may be H-aggregates though it has strong uo- can be seen that the emission spectra are very consistent with
15b
rescence, and FLA-C3 and FLA-C5 are possibly J-aggregates. the corresponding uorescence colors observed (Fig. 3). The
This deduction is also supported partly by the more blue-shied pressing-induced spectral shis (DlPFC ¼ lpressed ꢁ lannealed) of
emission for FLA-C12 than FLA-C3 and FLA-C5 solids (Fig. 2a). FLA-C12, FLA-C5 and FLA-C3 solids are 40, 26, and 18 nm,
respectively. This implies that FLA-Cn solids are also chain
length-dependent PFC materials, and alkyl chains have played a
functional role in tuning the PFC behavior of FLA-Cn although
such alkyl elements are normally inactive and undesirable for
optoelectronic properties. It is noted that the alkyl length-
Solid-state piezochromic luminescence
We have examined the PFC behaviour of FLA-Cn solids (mixed
with KBr to minimize the amount of uorophores) by pressing
with an IR pellet press (30 seconds at 1500 psi). Pristine FLA-
C12, FLA-C5 and FLA-C3 solids emit green, yellowish-green and
yellow uorescence, respectively. However, when they are
pressed, FLA-C12 and FLA-C5 are changed into yellow-emitting
solids and FLA-C3 becomes yellowish-orange emitting (le
dependent PFC behaviour of FLA-Cn is similar to those of 9,10-
1
0i
bis(p-dialkoxylstyryl)anthracenes
and b-cyano-substituted
5a
bis(p-alkoxystyryl)benzenes but thoroughly different from that
13
of 9,10-bis[(N-alkylcarbazol-3-yl)vinyl]anthracenes. The origin
for this dichotomous alkyl chain length-dependent PFC
behaviour is not clear at present, underlining the complexity of
the structure–property relationship of PFC materials.
Fig. 3 Fluorescence images of FLA-Cn solids mixed with KBr upon pressing and
Fig. 4 Emission spectra of FLA-Cn solids mixed with KBr upon pressing and
annealing under a 365 nm UV lamp.
annealing.
2
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Another interesting phenomenon is the PFC behaviour of ground and pristine solids. The PXD patterns of pristine FLA-C3
pure FLA-C12 solid. Just ground FLA-C12 solid emits yellow and FLA-C5 solids show sharp and intense reections, indica-

uorescence, however, when it is le standing at room tive of their well-ordered microcrystalline structures (Fig. 6a). In
temperature, the uorescence colour changes with the increase contrast, the ground samples display broad and featureless
of standing time and gradually recovers to the original green diffractograms reecting notable amorphous features. This
emission (Fig. 5, top). The lower part of Fig. 5 is the change indicates that grinding has induced the phase transition of FLA-
curve of the peak emission wavelength of ground FLA-C12 solid Cn solids between crystalline and amorphous states, which
versus the standing time. It is shown that the rapid recovering in should be responsible for the PFC behaviour of FLA-Cn solids.

uorescence colour and emission happens within the initial 10 The formation of amorphous state upon grinding was also
min (form 553 to 522 nm), and standing for 60 min, the uo- conrmed by DSC experiments. As shown in Fig. 6b, no addi-
rescence emission (515 nm) is almost identical with the pristine tional thermal transitions could be detected for all pristine
solid (514 nm) and almost unchanged further. In view of the solids before the isotropic melt transition (T ); however, there
m
changes in uorescence colour and emission spectra, grinding is clearly one exothermic transition peak at the lower-temper-
has the same effect as pressing. On the other hand, unlike FLA- ature region for ground FLA-C3 and FLA-C5 solids. This broad
C12, the uorescence colours and emission spectra of ground exothermic peak (T
FLA-C3 and FLA-C5 solids remain orange and yellow over 24 h at amorphized FLA-Cn solids upon heating. Amorphized FLA-C3
room temperature. These ndings suggest that we can endow and FLA-C5 solids have high T values, which result in their
,10-diarylvinyl-anthracene dyes with stable or self-recovering stable PFC behaviour at room temperature. It is known that the
c
) is ascribed to the cold-crystallization of
c
9
PFC behaviour by altering the end-arylenes and tuning the alkyl Tc is obviously lower than the Tm of a compound, and the
length. PFC materials with tunable recovering behaviour could ground FLA-C12 solid could cold-crystallize at or below room
ꢀ
be useful for various applications.
temperature since its isotropic melt point is only about 65 C.
Thus, the amorphized FLA-C12 solid induced by grinding is in a
thermodynamically and dynamically unstable morphology.
This is reason why ground FLA-C12 solid exhibits spontane-
ously recovering uorescence properties at room temperature.
X-ray diffraction and DSC analysis
To gain an insight into the PFC behaviour of FLA-Cn solids,
powder wide-angle X-ray diffraction (PXD) and differential
scanning calorimetry (DSC) experiments were conducted on the
Fig. 5 Fluorescence images (upper) and the peak emission wavelengths (lower)
of ground pure FLA-C12 solid at different standing times at room temperature.
Fig. 6 Powder X-ray diffraction patterns at room temperature (a) and DSC
curves (b) of ground and pristine FLA-Cn solids.
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To understand the chain length-dependent PFC behaviour of
FLA-Cn, the diffuse reectance absorption spectra of ground
samples were recorded (red lines, Fig. 2b). It is observed that
grinding does not signicantly affect the absorption maxima of
FLA-C3 and FLA-C5; however, the absorption maximum of FLA-
C12 is obviously red-shied by grinding. Thus, it could be
considered that the conjugated backbone of FLA-C12 becomes
less twisted upon grinding. Since the conjugated backbone of
FLA-C12 in pristine crystal is probably more twisted (vide supra)
and the intermolecular interactions are weaker (low Tc),
grinding could not only destroy easily the crystal structure of
FLA-C12 but also induce the backbone conformation to change
largely, resulting in a large DlPFC. The real origin for this unique
chain length-dependent PFC behaviour is still under investiga-
tion in our laboratory. Finally, it is noticed that the uorescence
color of the pressed FLA-C12 sample diluted with KBr is more
stable and can remain unchanged over 5 min at room temper-
ature. This implies that KBr could reduce the molecular motion
and delay the cold-crystallization rate of FLA-Cn. This is also an
interesting phenomenon that requires investigation.
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Conclusions
We have demonstrated new alkyl length-dependent solid-state

uorescence and piezochromic luminescence properties of AIE-
active 9,10-bis[(9,9-dialkyluorene-2-yl)vinyl]anthracenes (FLA-
Cn) with propyl, pentyl and dodecyl side chains. The uores-
cence emissions of pristine FLA-Cn solids are blue-shied with
the increase of the length of alkyl chains and the spectral shis
induced by pressing are increased with the length of alkyl
chains. This is thoroughly different from the behaviour
observed previously in 9,10-bis[(N-alkyl-carbazol-3-yl)vinyl]
anthracenes, implying that both the nature of end-arylenes
capped at divinylanthracene and the substituents on the aryl
rings have important effects on the solid-sate uorescence and
piezochromic luminescence of 9,10-diarylvinylanthracene
derivatives. PXD and DSC reveal that the transformation
between crystalline and amorphous states upon various
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room temperature, which is ascribed to the low cold-crystalli-
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Acknowledgements
We thank the NSFC of China (no. 51173092, 51073083), NSF of
Shandong Province (no. ZR2010EM023, ZR2012-EMQ003), and
Open Project of State Key Laboratory of Supramolecular Struc-
ture and Materials of Jilin University (no. SKLSSM201310).
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