Fluorenone organic crystals: Two-color luminescence switching and reversible phase transformations between π-π Stacking-directed packing and hydrogen bond-directed packing

Article abstract of DOI:10.1021/cm500441r

Organic solid-state luminescence switching (SLS) materials with the ability to reversibly switch the luminescence by altering the mode of molecular packing without changing the chemical structures of their component molecules have attracted considerable interest in recent years. In this work, we design and synthesize a new class of 2,7-diphenylfluorenone derivatives (compounds 1-6) that exhibit prominent aggregation-induced emission (AIE) properties with high solid-state fluorescence quantum yields (29-65%). Among them, 2,7-bis(4-methoxyphenyl)-9H-fluoren-9-one (2) and 2,7-bis(4-ethylphenyl)-9H- fluoren-9-one (6) display reversible stimuli-responsive solid-state luminescence switching. Compound 2 transforms between red and yellow crystals (the emission wavelength switches between 601 and 551 nm) under the stimuli of temperature, pressure, or solvent vapor. Similarly, compound 6 exhibits SLS behavior, with luminescence switching between orange (571 nm) and yellow (557 nm). Eight X-ray single-crystal structures, characterization of the photophysical properties, powder X-ray diffraction, and differential scanning calorimetry provide insight into the structure-property relationships of the solid-state fluorescence behavior. The results indicate that the variable solid-state luminescence of the fluorenone derivatives is attributed to the formation of different excimers in different solid phases. Additionally, the stimuli-responsive reversible phase transformations of compounds 2 and 6 involve a structural transition between π-π stacking-directed packing and hydrogen bond-directed packing. The results also demonstrate the feasibility of our design strategy for new solid-state luminescence switching materials: introduction of both π-π stacking and hydrogen bonding into an AIE structure to obtain a metastable solid/crystalline state luminescence system.

Full text of DOI:10.1021/cm500441r

Article
pubs.acs.org/cm
Fluorenone Organic Crystals: Two-Color Luminescence Switching
and Reversible Phase Transformations between π−π Stacking-
Directed Packing and Hydrogen Bond-Directed Packing
Mao-Sen Yuan,† Dong-En Wang,† Pengchong Xue,‡ Wenji Wang,† Jian-Chun Wang,† Qin Tu,†
Zhiqiang Liu,§ Yang Liu,§ Yanrong Zhang,† and Jinyi Wang*,†
†College of Science, Northwest A&F University, Yangling, Shaanxi 712100, P. R. China
‡College of Chemistry, Jilin University, Changchun, Jilin 130012, P. R. China
§State Key Laboratory of Crystal Materials, Shandong University, Jinan, Shandong 250100, P. R. China
S
* Supporting Information
ABSTRACT: Organic solid-state luminescence switching
(SLS) materials with the ability to reversibly switch the
luminescence by altering the mode of molecular packing
without changing the chemical structures of their component
molecules have attracted considerable interest in recent years.
In this work, we design and synthesize a new class of 2,7-
diphenylfluorenone derivatives (compounds 1−6) that exhibit
prominent aggregation-induced emission (AIE) properties with high solid-state fluorescence quantum yields (29−65%). Among
them, 2,7-bis(4-methoxyphenyl)-9H-fluoren-9-one (2) and 2,7-bis(4-ethylphenyl)-9H-fluoren-9-one (6) display reversible
stimuli-responsive solid-state luminescence switching. Compound 2 transforms between red and yellow crystals (the emission
wavelength switches between 601 and 551 nm) under the stimuli of temperature, pressure, or solvent vapor. Similarly, compound
6 exhibits SLS behavior, with luminescence switching between orange (571 nm) and yellow (557 nm). Eight X-ray single-crystal
structures, characterization of the photophysical properties, powder X-ray diffraction, and differential scanning calorimetry
provide insight into the structure−property relationships of the solid-state fluorescence behavior. The results indicate that the
variable solid-state luminescence of the fluorenone derivatives is attributed to the formation of different excimers in different solid
phases. Additionally, the stimuli-responsive reversible phase transformations of compounds 2 and 6 involve a structural transition
between π−π stacking-directed packing and hydrogen bond-directed packing. The results also demonstrate the feasibility of our
design strategy for new solid-state luminescence switching materials: introduction of both π−π stacking and hydrogen bonding
into an AIE structure to obtain a metastable solid/crystalline state luminescence system.
dendrimers,22,23 and polymers24−28 have been studied.
However, current studies of the SLS materials are primarily
INTRODUCTION
■
Organic solid-state luminescence switching (SLS) materials that
exhibit a reversible stimuli-responsive change of luminescence
color in the solid state without changing the chemical structure
of their component molecules have attracted special attention
limited to several classes of compounds, such as anthrace-
ne,9−11,22b,23 pyrene,7,8,22a,24 cyanostilbene,5,12,13,14b and di-
fluoroboron avobenzone16 derivatives. The design and syn-
in recent years.1−4 Their tuning and switching to the solid-state
thesis of new organic systems that display reversible switching
between two luminescent states in the solid state still remain
luminescence are achieved through altering the mode of their
molecular stacking upon physical perturbation, such as heating,
important. One of the primary difficulties in exploring new SLS
materials is that a theory to guide the design and preparation of
grinding, and exposure to chemical vapor. This physical
SLS compounds has not yet been developed.1,7b The
approach to switching the luminescence properties of organic
molecular-level understanding of the relationship between the
solids is considered to be more promising and easier than
organic molecular packing characteristics and the resulting
controlling molecular structures by chemical reactions, which is
optical properties is also insufficient. The luminescent proper-
frequently hampered by insufficient conversion, irreversible
ties of a given molecular system usually vary significantly
depending on its molecular aggregation state, and it is very
difficult to forecast whether a designed molecular model will
possess an SLS nature and even more difficult to judge its
reactions, or the need for strictly maintained operating
conditions.1
Since a stretching-induced fluorescence-switching polymer
was prepared using cyano stilbene derivative-doped low-density
polyethylene by Lowe and Weder in 2002,5 the exploration and
̈
synthesis of new SLS materials has become an active field of
research. In the past several years, a number of SLS systems
based on metal complexes,6 organic small molecules,7−21
Received: December 3, 2013
Revised: March 5, 2014
Published: March 7, 2014
© 2014 American Chemical Society
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spectroscopic characteristics in its aggregated state, because
even a tiny alteration of the chemical structure can greatly
influence the SLS behavior of an organic compound. Mean-
while, the fluorescent efficiency of organic luminescent
compounds generally becomes very weak in the solid state
because of the aggregation-caused quenching (ACQ) effect.
Having sufficiently high fluorescence efficiency is a
fundamental issue for luminescent materials in practical
applications. In 2001, Tang and co-workers first discovered
the anti-ACQ phenomenon of aggregation-induced emission
(AIE): a nonemissive compound in organic solution that
exhibits an obviously enhanced fluorescence in its aggregated
state.29 In recent studies, several AIE compounds have been
found to exhibit stimuli-responsive fluorescence switching
behavior.6c,13,14,23 Therefore, the design and preparation of
new solid-state luminescence switching materials based on AIE
systems may be an efficient strategy. It is well-known that for an
SLS material, the existence of a metastable solid/crystalline
state is a prerequisite.1,8,9 Therefore, modifying and controlling
the molecular arrangement in the solid state is very important
for the development of SLS materials. As Araki and Wang
mentioned,8,9a π−π stacking and hydrogen bonding are two
important factors that determine the mode of molecular
packing in the solid/crystalline state. Because of the
competition between π−π stacking and hydrogen bonding,
the introduction of the two factors to the same molecular
system may result in a metastable solid/crystalline structure,
e.g., a π−π stacking-directed structure or a hydrogen bond-
directed structure. Therefore, we speculate that the introduc-
tion of the two factors (π−π stacking and hydrogen bonding)
into an AIE system may be an effective design strategy for new
SLS materials.
more, eight single crystals of all six AIE compounds
(compounds 2 and 6 have two different crystal phases, namely
2R, 2Y and 6O, 6Y) have been obtained, and their X-ray crystal
structures have been determined. These results provide
structural evidence that reveals the structure−property relation-
ships of their solid-state fluorescence behavior and also yields
further insight into the mechanism of luminescence switching.
EXPERIMENTAL SECTION
■
Synthesis and Characterization. The six title compounds 1−6
were synthesized via a one-step Suzuki coupling reaction with 2,7-
dibromo-9H-fluoren-9-one and the corresponding phenylboronic acid.
Details of the synthesis, purification, and single-crystal preparation
processes, along with 1H and 13C NMR spectroscopy data, mass
spectrometry results, elemental analysis characterization, and X-ray
single crystal structures, are given in the Supporting Information.
Spectroscopic Characterization. 1H and 13C NMR spectra were
1
recorded using a Bruker Avance spectrometer (500 MHz for H and
125 MHz for 13C) in CDCl3. Element analyses (C, H) were performed
using a German Vario EL III elemental analyzer. Mass analyses were
performed using an Agilent 5973N MSD Spectrometer. UV−visible
absorption spectra for the solutions were recorded with a Shimadzu
UV-2550 spectrometer. Photoluminescence (PL) spectra were
recorded using a Shimadzu RF-5301PC fluorescence spectrometer.
The fluorescence quantum yield (Φ) in solution was determined using
rhodamine B in ethanol as a reference according to a previously
reported method.30 Quantum yields of the solid-state powder were
determined with a PTI C-701 calibrated integrating sphere system.31
Steady-state fluorescence spectra and decay curves were obtained using
an Edinburgh FLS920 fluorescence spectrometer equipped with a 450-
W Xe lamp and a time-correlated single photon counting (TCSPC)
card. Reconvolution fits of the decay profiles were performed with
F900 analysis software to obtain the lifetime values.
Powder X-ray Diffraction and Thermal Analysis. For phase
identification, powder X-ray diffraction (XRD) patterns were recorded
at room temperature with a Bruker AXS X-ray powder diffractometer
using Cu Kα radiation. The data were recorded in the 2θ mode with a
step size of 0.02626°. Differential scanning calorimetry (DSC)
measurements were performed to enable a better understanding of
the phase transitions, and the curves were obtained with a NETZSCH
thermal analyzer (DSC 204 F1) at heating and cooling rates of 10 K/
min under an N2 atmosphere.
Single-Crystal X-ray Diffraction. Eight single crystals were
obtained through slow diffusion of their respective organic solutions
for several days at room temperature. (For details, see the Supporting
Information). Because all the title crystals are stable under ambient
conditions, the data collection was performed without any inert gas
protection at room temperature on a Bruker SMART APEX-II CCD
area detector using graphite-monochromated Mo Kα radiation (λ =
0.71073 Å). The data reduction and integration and global unit cell
refinements were performed using the INTEGRATE program of the
APEX2 software package. Semiempirical absorption corrections were
applied using the SCALE program for the area detector. The structures
were solved by direct methods and refined using the full-matrix least-
squares methods on F2 using SHELX.32
Fluorenone, which has a planar aromatic configuration and
an exposed oxygen atom, can satisfy our proposed design
strategy of SLS molecules because planar aromatics are prone
to π−π stacking and exposed oxygen atoms are potential
hydrogen-bonding sites. In this study, we present a new class of
stimuli-responsive luminescence switching materials in which
fluorenone is used as the primary building block. Their
molecular structures are shown in Scheme 1; all of the six target
Scheme 1. Molecular Structures of Compounds 1−6
RESULTS AND DISCUSSION
■
AIE and Photophysical Properties. Contrary to fluorene
derivatives, which generally exhibit excellent luminescent
properties and have been widely studied as optoelectronic
materials, fluorenone compounds, which contain keto groups,
are almost nonemissive in solution. It is usually thought that
this class of molecules possesses a higher charge-transfer π−π*
state than the n−π* state.33 In this case, the radiative transition
is forbidden according to Kasha’s rule, and the single-molecular
emission is very weak. As shown in Figure 1a and 1b, our
synthesized fluorenone compounds 1−6 are no exception, and
compounds 1−6 exhibit typical AIE properties and possess
high solid-state fluorescence quantum yields (29−65%).
Among them, compounds 2 (2,7-bis(4-methoxyphenyl)-9H-
fluoren-9-one) and 6 (2,7-bis(4-ethylphenyl)-9H-fluoren-9-
one) exhibit SLS behavior, and their crystals undergo clear
luminescence switching accompanied by corresponding rever-
sible phase transformations under external stimuli. Further-
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Figure 1. (a) Photos of the THF solutions (20 μM) and as-prepared solid powders of compounds 1−6 under natural light and (b) 365-nm UV light.
(c) UV−visible absorption spectra of the THF solutions (20 μM) of compounds 1−6. (d) PL spectra of the THF solutions (20 μM) of compounds
1−6. (e) PL spectra of the THF/water solutions (20 μM, 80% water) of compounds 1−6. (f) normalized PL spectra of the as-prepared solid
powders of compounds 1−6. (g) UV−visible absorption spectra and (h) normalized PL spectra of the concentrated THF solutions (60 μM) of
compounds 1−6.
they exhibit almost no fluorescence (fluorescence quantum
yield Φ ∼ 1%; see Table 1) in dilute tetrahydrofuran (THF)
solution. However, their respective as-prepared powders exhibit
strong fluorescence under 365-nm UV light with Φ ranging
from 29% to 65% (Φ in the solid state is obtained in a
calibrated integrating sphere; see Figure 1a and 1b). Thus,
these compounds exhibit typical AIE behavior.
A solvent-poor-solvent photoluminescence (PL) test, which
is commonly used for studying the AIE phenomenon,29,34,35
was performed to explore the luminescent behavior of the
prepared fluorenone derivatives. Because water is a poor
solvent of compounds 1−6, the molecules will aggregate in
THF/water mixtures with high water contents. Thus, the
luminescence from such systems is primarily attributed to
molecule aggregation, not the single molecule in solution.
Figure 1d and 1e shows the PL spectra of compounds 1−6 in
THF (20 μM) and THF/water (80 vol% water, 20 μM). The
six title compounds all exhibit drastic changes in the fluorescent
intensity from the nonfluorescent THF solutions to the
strongly fluorescent THF/water mixtures, with the fluorescence
quantum yield increasing by several 10-fold (Table 1). This
finding confirms that all six of the fluorenone compounds are
highly AIE-active.
Meanwhile, obvious spectral changes in the position of
emission peaks of the six compounds in THF (20 μM) and
THF/water (20 μM) were observed. The PL maxima (λem) of
compounds 1−6 in the THF/water mixtures were red-shifted
150 nm relative to those in the pure THF solutions (20 μM);
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Table 1. Spectroscopic Data for Compounds 1−6
a
a
solution in THF
solution in THF/water (80% water)
crystalline powder
b
c
d
λabs (nm)
λem (nm)
Φ
τ (ns)
λabs (nm)
λem (nm)
Φ
λem (nm)
Φ
τ (ns)
1
288, 323, 337
290, 340
360, 375
368, 385
0.011
0.012
−
1.0
286, 341, 471
294, 334, 476
534
548
0.48
0.43
534
601
551
573
529
561
571
557
0.61
0.32
0.57
0.29
0.65
0.40
0.39
0.52
18.0
4.47
14.5
4.22
15.4
9.12
7.17
14.8
e
2R
2Y
3
286, 324, 336
279, 322, 335
291, 341
363, 378
364, 378
373, 388
362, 379
0.013
0.015
0.012
0.015
−
−
−
1.0
285, 334, 479
283, 327, 470
288, 338, 480
270, 340, 458
560
528
552
545
0.31
0.52
0.40
0.41
4
5
f
6O
292, 325, 339
6Y
a
b
c
With c = 2.0 × 10−5 mol/L. Fluorescence quantum yield was determined using rhodamine B in ethanol as a standard. Fluorescence lifetime.
d
e
Fluorescence quantum yield in the solid state was obtained using a calibrated integrating sphere.30 The red crystal and yellow crystal of compound
f
2 are named 2R and 2Y, respectively. The orange crystal and yellow crystal of compound 6 are named 6O and 6Y, respectively.
Figure 2. (a) Photos of both the red crystal 2R and yellow crystal 2Y of compound 2 under natural light (NL) and 365-nm UV light (UV). (b)
Photos of the crystalline phase transformation of 2Y from yellow to red to yellow; i: heating at 115 °C for approximately 1 min, and ii: heating to
melt and then cooling quickly. (c) UV−visible absorption spectra of 2R and 2Y crystalline powders. (d) Normalized PL spectra of 2R and 2Y
crystalline powders, quenched 2R (after melting 2R and then quenching), and heated 2Y (heating 2Y at 115 °C for 1 min) crystalline powders.
this effect was accompanied by the disappearance of the
vibronic structure in the spectra (Figure 1e). Furthermore,
drops of the dilute solutions of these compounds (<30 μM) on
a thin-layer chromatography plate did not luminesce, even after
solvent evaporation, because the spot can be regarded as a
dilute solid-state solution. These phenomena are similar to
those reported in Tao’s study.34 However, the spectral change
is different from the spectral characteristics of the majority of
previously reported AIE dyes, whose enhanced luminescence is
caused by restricted intramolecular rotation of peripheral
aromatic rings or the effects of intramolecular planarization in
their aggregated states.29,35 In those AIE systems, the
luminescence, whether from their organic solutions or
aggregated states, originates from the single molecule, and
their peak positions of PL maxima are not significantly different
in the two different states. Here, the shapes of the greatly red-
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shifted, unstructured luminescence spectra of the six fluorenone
compounds in THF/water and the compounds’ long
fluorescence lifetime in the solid state (see Table 1) are typical
features of excimer emission. In fact, the excimers of the six title
compounds not only exist in aggregated states but also form
quite easily in concentrated organic solutions. Figure 1g shows
the absorption spectra of the THF solutions of compounds 1−
6 with concentration of 60 μM. In comparison with Figure 1c,
the emergence of a new, wide absorption band that peaks at
approximately 450 nm in Figure 1g indicates the formation of
the excimers. These kinds of excimers formed from such
prearranged ground-state aggregates or dimers are generally
termed “static excimers”.36 In their corresponding normalized
PL spectra in 60-μM THF solutions (Figure 1h), two primary
emission bands peaked at approximately 380 and 540 nm are
observed. On the basis of the above results, we can deduce that
the peak at approximately 380 nm with a vibronic structure
originates from single-molecule emission and that the peak at
approximately 540 nm originates from excimer emission.
Notably, the six title compounds display similar absorption
(Figure 1c) and emission (Figure 1d) properties in dilute THF
solutions (20 μM), suggesting that their ground- and excited-
state electronic structures are similar. However, their absorption
(Figure S1b in the Supporting Information) and emission
(Figure 1f) spectra in the solid state, especially the emission
peaks ranging from 534 nm (green) to 601 nm (red), are
significantly different, which may imply that there are
differences in their respective excimers.
Figure 3. (a) Powder XRD profiles of crystals 2R and 2Y, quenched
2R, and heated 2Y. (b) DSC profiles of the red crystal 2R and yellow
crystal 2Y.
Luminescence Switching Properties of Compound 2.
The SLS properties of the six title fluorenone derivatives were
investigated. Two of them (compounds 2 and 6) exhibit
intriguing solid-state two-color luminescence switching by
thermal, mechanical, or organic solvent vapor stimuli.
almost identical to that of the red crystal 2R with decreased
peak intensities. Likewise, the powder XRD of quenched 2R
(which was obtained by melting 2R and then quenching)
exhibited weak but clear reflection peaks that were in good
agreement with the peaks observed in 2Y, which suggested the
same molecular arrangement in quenched 2R as in the crystal
2Y. Differential scanning calorimetry (DSC) measurements
were also performed for 2R and 2Y (Figure 3b) to understand
the phase transformation. The two different crystals have
almost the same melting points (217.4 °C for 2R and 218.0 °C
for 2Y), whereas the yellow crystal 2Y displays an exothermic
peak at approximately 109 °C, which indicates that yellow
crystal 2Y could undergo a phase transformation to the
thermodynamically more-stable red crystal. This result is
identical to that of the heating experiment with 2Y.
Apart from thermally induced reversible phase trans-
formation, mechanical and organic solvent vapor stimuli can
also produce similar results. For example, grinding the 2Y
yellow crystalline powder yielded a red powder that exhibited
luminescence the same as that of 2R, and exposure of the red
crystalline powder 2R to dichloromethane fumes yielded a
yellow powder (Figure S4 in the Supporting Information). All
of the original crystals and treated crystals were very stable over
the course of the experiments at room temperature in air, which
suggests that these compounds have potential for application as
an optical recording material.
As shown in Figure 2a, the two crystals of compound 2,
which were obtained from THF and dichloromethane, exhibit
entirely different color and luminescent properties. Upon
crystallization from THF, compound 2 packed to form red
rhombic crystals (2R) that exhibit red emission at 601 nm,
which is the same as that of the as-prepared crystalline powder
precipitated directly from the reaction solution. In contrast,
upon crystallization from dry dichloromethane, yellow
rectangle crystals (2Y) were obtained, and they exhibit yellow
emission at 551 nm. Interestingly, when heating the yellow
crystal 2Y to 115 °C using a microscopic melting-point
apparatus with a heating rate of ∼2 K/min under ambient
condition (or directly heating the yellow crystal 2Y at 115 °C
for approximately 1 min), we observed the color change of
crystal 2Y from yellow to red, with an emission peak at 601 nm
(Figure 2b, 2c, and 2d). Thus, thermal treatment changed not
only its emission spectrum but also its absorption spectrum.
This result implies that external stimuli cause the static excimer
to change from one form in the yellow crystal to another in the
red crystal. Upon heating the red crystal to its melted state at
approximately 220 °C and then quickly cooling (quenching) it,
a yellow solid sample (crystalline powder), which exhibited
almost the same color and luminescence (552 nm) as the
original yellow crystal 2Y (551 nm), was formed (Figure 2b, 2c,
and 2d).
To explore the reversible phase transformation and
luminescence mechanism in depth, significant efforts were
exerted to obtain single crystals of the title compounds. Eight
single crystals, including two each from compounds 2 (2R and
2Y) and 6 (6O and 6Y), were obtained. Their X-ray single-
crystal structures were determined, and selected crystallo-
graphic data are given in Table 2.
To gain more insight into the SLS properties of compound 2,
we studied the phase characteristics of the related samples using
powder XRD analysis. As shown in Figure 3a, the powder
pattern of the heated 2Y (after thermal treatment at 115 °C for
1 min) is different from that of the yellow crystal 2Y but is
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Table 2. Selected Crystallographic Data for Compounds 1−6
1
2R
2Y
3
4
5
6O
6Y
formula
fw [g·mol−1
C25H16O
C27H20O3
392.43
C27H20O3
392.43
C27H20O3
C27H20O3
392.43
C29H24O3
C29H24O
388.48
C29H24O
]
332.38
392.43
420.48
388.48
crystal color
pale yellow
orthorhombic
red
yellow
reddish-orange
orthorhombic
yellow
reddish-orange
orthorhombic
orange
yellow
crystal
system
monoclinic
orthorhombic
orthorhombic
monoclinic
monoclinic
space group
a [Å]
Cmc21
P21/c
Cmc21
Pbcn
C2221
Pnma
P21/n
P21/c
34.844(3)
6.8460(5)
7.2121(6)
90.00
11.0929(11)
8.7240(6)
21.0781(18)
104.607(2)
1973.9(3)
4
41.590(3)
6.9057(4)
7.0345(5)
90.00
21.194(2)
11.4105(11)
8.0612(6)
90.00
10.0490(8)
14.9454(14)
13.6607(11)
90.00
11.5953(17)
7.1473(18)
26.346(3)
90.00
13.7657(12)
9.4828(8)
16.2158(15)
94.677(1)
2109.7(3)
4
20.7401(18)
5.8060(4)
18.9202(15)
111.537(2)
2119.2(3)
4
b [Å]
c [Å]
β [deg]
V [Å3]
Z
1720.4(2)
4
2020.3(2)
4
1949.4(3)
4
2051.7(3)
4
2183.4(7)
4
ρcalcd
1.283
1.321
1.290
1.337
1.270
1.279
1.223
1.218
[g/cm3]
μ [mm−1
]
0.077
0.085
0.083
0.086
0.082
0.082
0.072
0.072
T [K]
298(2)
298(2)
298(2)
298(2)
298(2)
293(2)
298(2)
298(2)
θmin−θmax
3.03−25.01
2.38−25.02
2.94−25.01
3.24−25.01
2.73−25.02
2.91−25.02
2.61−25.01
2.88−25.02
[deg]
R/wR
[I > 2σ(1)
0.0346/0.0781
0.0450/0.0969
0.0489/0.0959
0.0436/0.0850
0.0335/0.0759
0.1405/0.3184
0.0588/0.1549
0.0894/0.2094
]
Figure 4. X-ray crystallographic packing of red crystal 2R (a−c) and yellow crystal 2Y (d−f). (a) Side view of red crystal 2R packing. (b) Top view
and illustration of the π−π stacking in 2R. (c) Side view and illustration of the hydrogen bonding in 2R. (d) Side view of red crystal 2Y packing. (e)
Side view of 2Y yellow crystal packing: the gray molecules are parallel to each other, and the orange molecules are also parallel to each other.
However, the gray and the orange molecules are not parallel, and the dihedral angle between the two fluorenone units is 24.2°. (f) Top view of 2Y
yellow crystal packing and illustration of the hydrogen bonding and C−H···π interactions.
The different molecular packings of red crystal 2R and yellow
crystal 2Y are shown in Figure 4. For red crystal 2R, the
obvious characteristic molecular pairs can be observed in the
packing diagram. π−π stacking interactions with a contact
distance of 3.47 Å join every two up−down adjacent molecules
(Figure 4a and 4b). Intermolecular π−π interactions play a
major role in the molecular packing, and we can conclude that
the crystal of 2R is a π−π stacking-directed structure. Weak C−
H···O hydrogen bonds also exist in the 2R crystal (Figure 4c),
and they hold the molecule pairs together to result in the
formation of a 3D structure. However, for yellow crystal 2Y, the
gray molecules at the top are not parallel to the orange
molecules at the bottom, and there are no π−π interactions in
the crystal (Figure 4d and 4e). The molecules are linked by
hydrogen bonds with bond lengths of 2.50 Å and arranged into
2D sheets (Figure 4f), which are further stacked into 3D
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structures based on weak C−H···π interactions (Figure S5 in
the Supporting Information). Therefore, we can regard the
hydrogen bonds as the guiding force in the crystal packing of
2Y. In addition, the emission maxima of compound 2 in its
THF/water (80% water) solution (20 μM) and concentrated
THF solution (60 μM) (Figure 1e and 1h) are located at 547
and 550 nm, respectively, which are similar to the maxima of
yellow crystal 2Y (551 nm). This similarity indicates that the
structures of their aggregated states in concentrated THF and
THF/water solutions are the same as the hydrogen bond-
directed structure of yellow crystal 2Y, and this type of
structure is kinetically controlled. Thus, we can understand the
phase transformation from yellow crystal 2Y to red crystal 2R
as being a structural transition from the kinetically stable
hydrogen bond-directed packing to the thermodynamically
stable π−π stacking-directed packing.
Notably, the crystal packing of compound 1 (Figure S8 in the
Supporting Information) is very similar to that of yellow crystal
2Y. However, there is no phase transformation or luminescence
switching observed in the crystal of 1, which is different from
2Y. In the crystal structure of red crystal 2R, the hydrogen atom
of the methoxy group forms a weak hydrogen bond with the
closest oxygen atom of the carbonyl group (CO) (Figure
4c), which aids in the stabilization of the crystal structure.
However, there is no formation of such a hydrogen bond in
crystal 1. This is the reason why compound 1 does not form
such a packing.
Regarding the different luminescence mechanisms of 2R and
2Y, at the single molecule level, the most obvious difference
between 2R and 2Y lies in the torsional configuration, i.e., the
dihedral angles between the fluorenone core and the 2,7-linked
phenyl rings are different (Figure S6 in the Supporting
Information). For 2Y, the dihedral angles between the
fluorenone and the 2,7-linked phenyl rings are both 32.2°.
For 2R, the two dihedral angles are not identical: one is 17.3°,
and the other is 32.2°. Theoretical calculations based on the
single molecule suggest that both the highest occupied
molecular orbitals (HOMOs) and the lowest unoccupied
molecular orbitals (LUMOs) of 2R and 2Y are very similar
(Figure S13 in the Supporting Information), which indicates
that the small differences in their molecular structure does not
cause significant changes in their absorption and emission
spectra. This result further illustrates that their luminescence
does not come from the single molecule species but rather the
excimers.
In the 2R crystal structure, π−π stacked molecular pairs
(dimers) form the excimers (Figure 4b). The two molecules
overlap with the electronegative five-membered ring (which
involves the carbonyl group of the five-membered ring) that
faces the electropositive benzene ring (Figure 4b), which
facilitates the two molecules forming an excimer in the ground
state, i.e., static excimer. When the excimers are excited, they do
not undergo any energy-consuming configurational rearrange-
ment. Consequently, the red crystal 2R has a shorter
fluorescence lifetime (τ = 4.47 ns, Figure 5) and a higher
fluorescence quantum yield (Φ = 0.32) at the emission
maximum of 601 nm than a typical π−π aggregate. For the
yellow crystal 2Y, its single-crystal structure reveals that the
excimers are formed by two adjacent and parallel 2Y molecules
through hydrogen bonds (Figure 4f). As shown in Figure S13
in the Supporting Information, upon one molecule being
excited, the electrons of the LUMO are confined mostly in the
fluorenone core, and greater electron density is present on the
Figure 5. Fluorescence lifetime profiles of (a) the red crystal 2R,
yellow crystal 2Y, and 2 in THF (20 μM) and (b) the orange crystal
6O, yellow crystal 6Y, and 6 in THF (20 μM).
oxygen atom of the carboxyl group. At this time, the oxygen
atom with high electronegativity is inclined to bind the
electropositive hydrogen atoms of an adjacent ground-state
molecule (the forbidden relaxation of the singlet excited state
allows sufficient time for the binding to occur). Opportunely,
the prearranged ground-state aggregate in such crystal packing
is very convenient for excimer formation. Because of the long
charge-transfer distance and the high stability of the photo-
excited state, the 2Y crystal exhibits a long fluorescence lifetime
(τ = 14.5 ns, Figure 5) at the yellow fluorescence peak of 551
nm.
Luminescence Switching Properties of Compound 6.
We observed the SLS behavior of compound 6 during its
preparation and purification. After the reaction, the solution of
compound 6 was naturally cooled from the reaction temper-
ature (80 °C), and a yellow floccule precipitated from the
reaction system. When filtered and rinsed with dichloro-
methane, the yellow floccule immediately turned into reddish-
orange crystals. Subsequently, the orange block single crystal
(6O) and the yellow lamellar single crystal (6Y) were obtained
by slow diffusion of their dichloromethane and THF solutions,
respectively, for several days at room temperature. Under 365-
nm UV light, they exhibited charming orange (571 nm) and
yellow (557 nm) emissions (Figure 6a and 6d), respectively.
Similar to compound 2, compound 6 can accomplish a
reversible phase transformation accompanied by two-color
emission switching. As shown in Figure 6b, upon heating at
approximately 90 °C for 1 min, the orange crystal quickly
changed its color to bright yellow, with an emission peak at 559
nm (Figure 6c and 6d). When the heated crystal was then
exposed to dichloromethane vapor for 1 min at room
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Figure 6. (a) Photos of the orange crystal 6O and yellow crystal 6Y of compound 6 under natural light (NL) and 365-nm UV light (UV). (b)
Photos of the crystalline phase transformation of 6O from orange to yellow to orange; i: heating at 90 °C for approximately 1 min, and ii: exposure
to CH2Cl2 vapor for approximately 1 min. (c) UV−visible absorption spectra of 6O and 6Y crystalline powders. (d) Normalized PL spectra of 6O
and 6Y crystalline powders, CH2Cl2-fumed 2Y (2Y exposed to CH2Cl2 vapor for ∼1 min), and heated 6O (produced by heating 6O at 90 °C for ∼1
min) crystalline powders.
temperature, it returned to an orange crystal with a 573-nm
luminescence (Figure 6d). The reversible phase transformation
was also verified by powder XRD analysis and DSC
measurements (Figure S12 in the Supporting Information).
The XRD reflection peaks of the heated 6O sample, which was
obtained by heating 6O at 90 °C for approximately 1 min,
matched well with those of the yellow crystal powder 6Y, and
those of the CH2Cl2-fumed 6Y sample matched well with those
of the orange crystal 6O.
Figure 7 shows the crystal packing structures of both the
orange crystal 6O and yellow crystal 6Y. In 6O (Figure 7a),
there are molecular pairs bound by π−π stacking interactions in
which the adjacent molecular planes overlap by about 40%
(Figure 7b); the distance between them is approximately 3.61
Å. It is this π−π stacking-bound excimer (dimer) that generates
the orange emission of the crystal upon excitation. Meanwhile,
weak hydrogen bonds and C−H···π interactions also play a role
in the stability of the crystal structure (Figure 7c). Surprisingly,
there seems to be a “Z-shaped channel” in this crystal structure
(Figure 7a). We tried to find some small solvent molecules
(e.g., CH2Cl2) in the “channel” to check whether solvent
molecules that were involved in the crystal packing affected the
transformation of the two different crystal phases. However, the
result was counterintuitive. In the case of 6Y, the aromatic
frameworks of the two adjacent molecules barely overlapped
(Figure 7d and 7e); thus, the π−π stacking interactions in the
structure are very weak. Hydrogen bonding, as the leading
intermolecular force in this packing, stabilizes the crystal
structure. The dimer, which is held together in a side-by-side
arrangement by hydrogen bonds (Figure 7f), can be easily
found in the packing.
On the basis of the packing structures of 6O and 6Y, we can
speculate about the process of molecular rearrangement during
the phase transformation. Upon heating the orange crystal 6O
at approximately 90 °C, the molecule facing the “Z-shaped
channel” is provided with enough energy to overcome the C−
H···π interactions and move toward the empty space along the
direction of the yellow arrows (Figure 7a) until a distance at
which the molecule combines with another by hydrogen
bonding to form a dimer (Figure 7f). Simultaneously, the
single-molecule configuration changes from a twisted to a
coplanar conformation because of the hydrogen bonding
(Figure S7 in the Supporting Information). The dimer bound
by π−π stacking interactions also struggles to separate to form a
new hydrogen-bonded dimer. Thus, the phase transformation
from a π−π stacking-directed structure (6O) to a hydrogen
bond-directed structure (6Y) is accomplished.
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Figure 7. X-ray crystallographic packing of 6O (a, b, c) and 6Y (d, e, f) crystals. (a) Side view of 6O orange crystal packing. (b) Top view and
illustration of π−π stacking in 6O. (c) Side view and illustration of the hydrogen bonding and C−H···π interactions in 6O. (d) Side view of 6Y
yellow crystal packing. (e) Top view and illustration of the π−π stacking in 6Y. (f) Side view and illustration of the hydrogen bonding in 6O.
Figure 8. Procedures of writing and erasing the luminescent images of (a) compound 2 and (b) compound 6. Scale bar: 1 cm.
We think that compounds 1, 3, 4, and 5 do not exhibit SLS
properties because they form strong and very stable crystal
packing structures (see Figures S8−S11 in the Supporting
Information), which are immune to external stimuli. To verify
our hypothesis, the concentration-dependent absorption
spectra of the THF/hexane (v:v = 1:1) solutions of compounds
1−6 were measured, which can monitor the formation of dimer
aggregates and acquire the dimerization constants KD. As
shown in Figures S14 and S15 in the Supporting Information,
upon increasing concentration, the new, wide absorption bands
between 400 and 500 nm appeared and arose, which indicates
the formation of dimer aggregates (static excimers). Though
the single-molecule (monomer) absorption at 285 nm also
arose, the increase became more and more slow with increasing
concentration, which indicates the consumption of monomers.
Then the dimerization constants KD values were calculated by
fitting of the UV/vis spectral data according to the isodesmic
model with nonlinear least-squares regression analysis (see
Supporting Information for details).37 Compounds 1, 3, 4, and
5 exhibit KD values higher than those of compounds 2 and 6.
The results show that the dimer aggregates of compounds 1, 3,
4, and 5 are stronger than those of compounds 2 and 6, which
may testify to the stable crystal packing structures of
compounds 1, 3, 4, and 5.
Demonstration of Writing and Erasing. As a demon-
stration of potential applications in optical recording and
security ink materials, stimuli-responsive writing and erasing
examples for compounds 2 and 6 are shown in Figure 8. The
red film (Figure 8a) was prepared by coating an ethanol
suspension of 2R red crystalline powder on a silicon wafer.
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When exposed to dichloromethane vapors using an “OK” sign
stamp for about 2 min, the luminescence of the fumed region of
the film changed from red to yellow. Thus, a yellow “OK” sign
was stamped on the red “paper”. The red “nails” of the “OK”
sign were obtained by subsequent pressing of their correspond-
ing positions. Upon heating the whole “paper” at 120 °C for 1
min, the yellow “OK” sign could be erased easily. A similar
process can be performed to draw and erase an orange
“benzene ring” on yellow “paper” using the yellow crystalline
powder of 6O, as shown in Figure 8b. The results suggest that
the materials have potential for application as optical recording
materials.
Chinese Scholars, the State Education Ministry, and the
Northwest A&F University.
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CONCLUSIONS
■
In this study, we discovered a new class of AIE solid-state
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ASSOCIATED CONTENT
* Supporting Information
■
S
Eight X-ray crystallographic files of compounds 1−6 in CIF
format; synthesis, characterization, relevant spectral and crystal
structural graphs of compounds 1−6; powder XRD and DSC
profiles of compound 2; solid-state luminescence switching
properties of compound 2 under mechanical and solvent vapor
stimuli; theoretical calculations and results and determination
of the dimerization constants. This material is available free of
charge via the Internet at http://pubs.acs.org.
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2012, 28, 4534−4542.
AUTHOR INFORMATION
Corresponding Author
*Phone: + 86-29-870 825 20. Fax: + 86-29-870 825 20. E-mail:
jywang@nwsuaf.edu.cn.
■
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This study was supported by the National Natural Science
Foundation of China (21202132, 21175107, and 21375106),
the Natural Science Foundation of Shaanxi (2012JQ2017), the
Ministry of Education of the People’s Republic of China
(NCET-08-602 0464), the Fundamental Research Funds for
the Central Universities (Z109021106 and Z109021303), the
Scientific Research Foundation for the Returned Overseas
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