Crystal structures and emission properties of the BF2 complex 1-phenyl-3-(3,5-dimethoxyphenyl)-propane-1,3-dione: Multiple chromisms, aggregation- or crystallization-induced emission, and the self-assembly effect

Article abstract of DOI:10.1021/ja501977a

It is known that electron donating groups have quite a different effect on the π-delocalization of a conjugate system when bonded at ortho and para as compared to meta positions in the phenyl ring. In the present work, the BF ₂ complex of 1-phenyl-3-(3,5-dimethoxyphenyl)-propane-1,3-dione (1), a molecule with two methoxy groups in one of the phenyl rings at meta positions, was prepared. Compound 1 exists as two polymorphs having different mutual orientations of the two methoxy groups: in polymorph A away from each other (termed anti), while in polymorph B one methoxy group is oriented toward the other (syn-anti). In both crystals, the molecules which are antiparallel (the subPh rings as well as dioxaborine are on opposite sides) form stacks through face-to-face π-π interactions, while in polymorph A the crystal packing is further stabilized by intermolecular C(phenyl)-H···F and C(methoxy)-H···F hydrogen bonds. Solid A possesses numerous chromic effects, including mechano-, thermo-, and chronochromism, though the latter to a lesser extent, as well as the effect of rearrangement of the amorphous phase into a more stable crystalline phase A, associated with crystallization-induced emission enhancement (CIEE). The solid-state emission can be repeatedly switched regarding its color and efficiency with excellent reversibility by external stimuli. On the other hand, crystalline solid B undergoes thermal interconversion of syn-anti to the anti conformer. Compound 1 shows a solvatochromic effect (SE), is aggregation-induced emission (AIE) active, and through the sublimation process displays self-assembling crystalline platelike microstructures or microfibers that reveal an obvious optical waveguide effect.

Full text of DOI:10.1021/ja501977a

Article
pubs.acs.org/JACS
Crystal Structures and Emission Properties of the BF2 Complex
1‑Phenyl-3-(3,5-dimethoxyphenyl)-propane-1,3-dione: Multiple
Chromisms, Aggregation- or Crystallization-Induced Emission, and
the Self-Assembly Effect
†
̌
Petra Galer, Romana C. Korosec, Maja Vidmar, and Boris Sket*
̌
̌ ̌
Faculty of Chemistry and Chemical Technology, University of Ljubljana, Askerceva 5, SI-1000 Ljubljana, Slovenia
S
* Supporting Information
ABSTRACT: It is known that electron donating groups have
quite a different effect on the π-delocalization of a conjugate
system when bonded at ortho and para as compared to meta
positions in the phenyl ring. In the present work, the BF2
complex of 1-phenyl-3-(3,5-dimethoxyphenyl)-propane-1,3-
dione (1), a molecule with two methoxy groups in one of
the phenyl rings at meta positions, was prepared. Compound 1
exists as two polymorphs having different mutual orientations
of the two methoxy groups: in polymorph A away from each
other (termed anti), while in polymorph B one methoxy group is oriented toward the other (syn−anti). In both crystals, the
molecules which are antiparallel (the subPh rings as well as dioxaborine are on opposite sides) form stacks through face-to-face
π−π interactions, while in polymorph A the crystal packing is further stabilized by intermolecular C(phenyl)−H···F and
C(methoxy)−H···F hydrogen bonds. Solid A possesses numerous chromic effects, including mechano-, thermo-, and
chronochromism, though the latter to a lesser extent, as well as the effect of rearrangement of the amorphous phase into a more
stable crystalline phase A, associated with crystallization-induced emission enhancement (CIEE). The solid-state emission can be
repeatedly switched regarding its color and efficiency with excellent reversibility by external stimuli. On the other hand,
crystalline solid B undergoes thermal interconversion of syn−anti to the anti conformer. Compound 1 shows a solvatochromic
effect (SE), is aggregation-induced emission (AIE) active, and through the sublimation process displays self-assembling
crystalline platelike microstructures or microfibers that reveal an obvious optical waveguide effect.
sensory systems as sensitive and selective chemosensors and
bioprobes of the turn-on type.8 Conventional fluorescent
sensors commonly operate in a photoluminescence turn-off
mode: an initially highly emissive dye becomes nonfluorescent
when its molecules are induced to aggregate in the presence of
a chemical species or biological analyte. In contrast, the unusual
AIE effect has allowed the development of new type of sensing
system that operates in a photoluminescence turn-on mode9 by
taking advantage of luminogen aggregation.
It is also known that in some cases the same fluorophores are
present in the different polymorphs and consequently crystal-
to-crystal interconversion with change in color emission can
take place.10 Mechanochromism10b,11 is another phenomenon
of color change resulting from mechanical grinding or pressing
of a solid sample, and subsequent reversion to its original color
through treatment such as heating or recrystallization. Fraser et
al.1d published a communication describing the polymorphism
and reversible mechanochromic luminescence (ML) of solid-
state difluoroboron avobenzone (BF2AVB). The morphology-
sensitive solid-state emission of BF2AVB was evident on
1. INTRODUCTION
Organoboron complexes form one of the important classes of
fluorescent dyes.1 Most dyes exhibiting fluorescence in dilute
solution display quenched or reduced fluorescence intensity in
the solid state.2 However, some systems exhibit enhanced solid
state emission3 in which restriction of intramolecular rotation
has been identified as the main reason for this effect.4 Other
intra- or intermolecular phenomena are involved as well, such
as conformational changes, π−π stacking, hydrogen bonding,
and so forth, which cause rearrangements of the energy levels of
emissive excited states.5 In some cases, the crystalline phase
proved to be a more efficient emitter than the amorphous
phase,6 showing the influence of molecular packing on the solid
state emission. The effect is known as crystallization-induced
emission enhancement (CIEE). A series of molecules that are
nonemissive in the dissolved state are induced to emit
efficiently when aggregated in the solid state; the effect is
known as aggregation-induced emission (AIE).1b,4,6,7 Various
theories have been used to explain AIE phenomena, such as
planarity and rotability, intramolecular restrictions, intermo-
lecular interactions, aggregation caused quenching (ACQ) to
AIE transformation, and so forth. AIE luminogens are therefore
promising materials for utilization in optoelectronic and
Received: February 25, 2014
Published: April 25, 2014
© 2014 American Chemical Society
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mechanical perturbation. Single-crystal XRD revealed that
BF2AVB molecules can form multiple emissive aggregation
states with different intermolecular interactions. Investigation of
mechanical properties of green and cyan BF2AVB crystals12
using qualitative deformation tests and nanoindentation
techniques confirmed that the cyan form is much softer and
more compliant compared to the green one. ML experiments
on green- and cyan-emitting polymorphs of BF2AVB revealed
that, upon smearing, the plastically deformable cyan form
showed a prominent color change to yellow, while in the harder
green form red-shifted emission was detected. On the basis of
crystal structure analysis, the authors suggested that the
presence of slip planes in cyan form is responsible for its
higher plasticity compared to green form which has a nearly
isotropic structure.
2. RESULTS AND DISCUSSION
Preparation of Polymorphs and Crystallography.
Compound 1 was prepared from 1-phenyl-3-(3,5-dimethox-
yphenyl)-propane-1,3-dione with BF3·OEt2 in benzene.18 The
resulting yellow solid was filtered off (termed solid A, Figure
1b, entry (i), up). When dissolved in CH3CN or CH2Cl2 and
Commercially available avobenzone reacted with boron
trichloride or tribromide to afford boron-bisavobenzone
salts.13 The complexes were found to exhibit unusual solvent-
dependent fluorescence in solution and hydrochromic fluo-
rescence in the solid state.
It was found later that various substituted difluoroboron
dibenzoylmethane derivatives1f,14 also exhibit ML in the solid
state. In an article published recently,1i a mechanistic
investigation of mechanochromic luminescent organoboron
materials was carried out. By correlating solution data with
solid-state results it was concluded that two coupled processes
are responsible for the BF2-dbm ML observed in the solid state:
force-induced emissive H-aggregate formation and energy
transfer to the emissive H-aggregates.
Development of luminescent materials is a hot topic of
current interest because of its implications for the development
of organic light-emitting diodes.15 Molecular structures and
conformations of dye molecules, as well as their morphological
packing arrangements, affect their photophysical processes in
the solid state. By preparing the BF2 derivative, the β-diketone
ring closure locked the conformation and blocked intra-
molecular rotations, making the crystals stronger emitters.
Many boron-containing materials possess impressive optical
properties.16
It should be mentioned that several substituted difluorobor-
on dibenzoylmethanes reported in the literature have a
substituent bonded at the para position in one or both phenyl
rings. However, it is known that electron donating groups have
quite a different effect on π-delocalization of conjugate systems
when bonded at ortho and para or at meta positions. It was
found that the absorption and fluorescence spectra of the
alkoxysilyl derivatives of dibenzoylmethanatoboron difluoride
(DBMBF2) depend on the position of the O-allyl or O-propyl
alkoxysilyl substituent.17 The highest fluorescence quantum
yield is obtained from substitution at the para position, whereas
that at the meta position gives the largest bathochromic shift in
the fluorescence spectrum.
Figure 1. (a) Normalized fluorescence spectra of 1 in the solid state
(λex = 370 nm); solid A (black), solid B (red), solid C (blue). Solid,
dotted, and dashed lines indicate pristine samples (as-synthesized),
monocrystals and partially ground pristine solid A, respectively. (b)
Legend together with photographs of the crystalline solids; (i), (ii),
(iii), (iv) under UV lamp irradiation (below) and under ambient light
(up) and monocrystals; (v) polymorph A, (vi) polymorph B under
365 nm UV lamp (right) and ambient light (left).
then the solvent evaporated, a solid with a different tint of
yellow was obtained (termed solid B, Figure 1b, entry (iii), up).
Under UV black light excitation (λex = 365 nm), solid A and
solid B exhibited changeable color from green to turquoise blue
and fluorescent yellow emissions, respectively; see Figure 1b,
entries (i, ii, iii), below. To clarify the reason for the alteration
of luminescence color and intensity of solids A (partially
ground pristine solid A is a 1.3-times more efficient emitter,
with a hypsochromic shift of 2 nm, than the pristine solid A),
XRD analyses were carried out (Figure S1, Supporting
Information (SI)). On the basis of the peak broadening in
the diffraction pattern, the average size of crystallites of the
pristine solid A and the corresponding partially ground crystals
were estimated as 142 and 35 nm, respectively. These results
confirmed our hypothesis about the dependence of the solid-
state emitting properties upon the size of the crystals according
to the observation of Mirochnik et al.19
On recrystallization from CH2Cl2/hexane, compound 1
formed two different types of crystals: greenish-yellow-emitting
prismlike crystals (termed polymorph A, Figure 1b, entry (v),
right) and yellow-emitting platelike crystals (termed polymorph
B, Figure 1b, entry (vi), right). Simulated X-ray diffraction
(XRD) curves showed distinctly different patterns for the two
polymorphs. In comparison, XRD spectra of pristine samples of
solids A and B totally coincided with the calculated diagrams of
polymorphs A and B, respectively, proving the presence of only
one phase (Figure 5, entry (i)). Also, the emission spectra of
monocrystals A and B were nearly identical to those of the
corresponding parent pristine solids, as shown in Figure 1a.
ORTEP drawings of polymorphs A and B (Figure 2) display
the main difference in mutual orientation of the two methoxy
groups; in polymorph A away from each other (termed anti)
and in polymorph B the methoxy group bonded at C3″
position is oriented toward the other (called syn−anti). The
polymorphs are triclinic and crystallize in the P1 space group
(additional crystal data, data collection, and refinement are
In the present article, we describe the BF2 complex of 1-
phenyl-3-(3,5-dimethoxyphenyl)-propane-1,3-dione,18 a mole-
cule with two methoxy groups in one of the phenyl rings at
meta position. Different mutual orientations of the two
methoxy groups in the crystal polymorphs result in two
rotamers anti and syn−anti. Due to the small energy barrier
between them, which is attributable to rotation of one of the
methoxy group around the single C−O bond, interconversion
of the rotamers by external stimuli can take place. Besides many
other interesting photophysical properties, dye 1 exhibits
especially rich chromic effects.
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overlapping for each pair. Contrary to polymorph A, there
are no remarkably strong intercontacts. The other differences in
the molecular arrangement of the two polymorphs are mostly
reflected in the π−π stacking geometry. The higher degree of
overlap results in stronger π−π interactions which helps to
explain the relatively large red-shift of Fmax and the reduction of
efficiency observed in the solid state emission of polymorph B
versus polymorph A.
To provide a more effective understanding of the emission
behavior of polymorphs A and B we employed DFT
calculations in the gas phase with complete geometry
optimization for the ground-state structures at the B3LYP/6-
31G(d,p) level using the Spartan’14 software package for all
four possible rotation isomers (Figure S3, SI): anti, syn−anti,
anti−syn, syn, based on the mutual orientation of the methoxy
groups in the substituted phenyl ring.
Figure 2. ORTEP drawings and atom labeling of the polymorphs.
summarized in Table S1, SI). In the crystals the molecules are
not planar, both arene planes being out of the central
diketonate plane (for the Ph and subPh rings by 6.03° and
15.53°, or 15.38° and 2.74°, for polymorphs A and B,
respectively; see Table S2, SI). Selected geometric parameters
of the polymorphs in comparison with the p-methoxy derivative
are given in Table S3, SI.
Figure 4 shows the highest occupied molecular orbital
(HOMO) and the lowest unoccupied molecular orbital
In polymorph A (Figure 3, left), the molecules which are
antiparallel (subPh rings as well as dioxaborine are on opposite
Figure 4. Molecular orbital diagrams for the HOMO and LUMO of
the anti isomer of 1.
(LUMO) of the anti isomer. The π-orbital of the entire
structure lies on the LUMO, which is predominantly π* in
character, while the HOMO is localized on the whole of the
substituted phenyl group. The oxygen atoms of both methoxy
groups are involved in the HOMO but not in the LUMO. The
relatively high energy level of the occupied orbitals of the subPh
group prevents the HOMO from locating the π-orbital on the
1,3-diketone moiety. There is not much difference in the
molecular orbital diagrams of the HOMO and LUMO for the
anti isomer and the other possible structures syn−anti, anti−
syn and syn. The energy levels of frontier molecular orbitals are
given in the Supporting Information, Table S4.
In all cases, the unsubstituted phenyl ring deviates from the
central diketonate planarity by practically the same angle of
about 17°, except for the syn isomer, in which the angle is
13.2°, while the dihedral angles for the dimethoxy phenyl ring
depend on the orientation of the methoxy groups (Table S5,
SI). In addition, the anti orientation of the methoxy group in
anti and anti−syn isomers toward the difluoroborine group is
reflected in a lower energy (Table S4, SI). The higher stability
of the anti−syn isomer in comparison with the anti one (ΔE =
3.21 kJ/mol) can be explained by the smaller dihedral angle
between the plane through the subPh ring and the central
diketonate moiety (12.1° and 15.0°, respectively). Rotation of
the subPh ring around the C3−C1″ bond by 180° in the syn−
anti isomer results in the anti−syn orientation lowering the
energy by 5.51 kJ/mol.
Figure 3. Partial view of the crystal packing through π−π stacking
interactions of polymorphs A and B with the given ring (C1′−6′;
C1″−6″ and O1C1C2C3O3) centroid−centroid separations. The
types of overlapping of the molecules are shown. Hydrogen atoms are
omitted for the sake of clarity.
sides) form stacked trimers and dimers through face-to-face
π−π stacking interactions with two types of overlap arrange-
ment. The first, type A (Figure 3) consists of both an arene−
arene and a dioxaborine−arene pair stacked in such a way that
the first molecule overlaps with either a subPh ring, or the Ph
and dioxaborine rings of the two adjacent molecules. The total
degree of overlap is 6% between parallel subPh rings and 20%
of the surface area of the Ph ring at the ring centroid−centroid
(Cg−Cg) distances of 4.183, 4.144, and 4.031 Å, respectively.
The second, type B (Figure 3) includes the interactions
between Ph and dioxaborine rings in the stacked dimer with a
Cg−Cg separation of 3.863 Å; the degree of overlap of each
pair is 13% of the Ph ring’s surface area. The crystal packing is
further stabilized by intermolecular interactions classified as
C(phenyl)−H···F and C(methoxy)−H···F hydrogen bonds; see
Figure S2, SI.
In polymorph B (Figure 3, right), the antiparallel molecules
are packed in stacks with the arenes offset relative to each other
forming two types of dimer. The Ph ring overlaps with the
subPh ring and vice versa (type C overlapping) with 31% and
57% of overlap at the Cg−Cg distance of 3.827 Å, and type D
where both dioxaborine rings are paired with subPh rings at a
distance of 3.815 Å with 16% of the subPh ring area
Chromic Processes. Since the solid state emission
properties strongly depend on the molecular structure and its
packing in the crystal, alteration of molecular arrangements in
response to heat, pressure, dropwise treatment11e with a
solvent, and so forth are manifested as changes in emission
color and intensity.
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When pristine solid A with a fluorescence band centered at
490 nm (the spectrum is plotted in Figure 1a) was thoroughly
ground in an agate mortar for 10 min, dramatic changes were
observed in luminescence color and intensity with a drastic red-
shift to 526 nm and a reduction of emission efficiency by as
much as 5.3-fold. (J0/J = 5.3 where J and J0 are the emission
efficiency obtained as the integrated area under the
fluorescence peaks of thoroughly ground and pristine sample
A, respectively.) The observed weak and red-shifted emission
band can be attributed to the effect of a loose packing pattern in
the amorphous solid. Additionally, mechanochromic emission
changes in solid A were also investigated by powder X-ray
diffraction (XRD) measurements. The XRD pattern of the
unground sample showed intense and sharp reflection peaks, in
good agreement with the simulated pattern of polymorph A
(Figure 5, entry (i), solid A). After grinding, the XRD profile
entries (i, ii), solid B. Furthermore, upon dropwise treatment of
the well ground solid B with dichloromethane, the amorphous
phase rearranged into the more stable crystalline phase A
(Figure 5, entry (iii), solid B) via a partial dissolution and
recrystallization process, accompanied by an obvious blue-
shifted band at 490 nm with a pronounced enhancement in
fluorescence intensity. But when the resulting solid A (Figure 5,
entry (iii)) was completely dissolved in CH2Cl2 with
subsequent evaporation of the solvent under vacuum, a
reversible transformation from crystalline sample A to B
occurred, which is further supported by the XRD diagrams in
Figure 5, entry (iv). In contrast, the transition from one
crystalline phase to another was not achieved after dissolving
pristine solid B in various solvents such as CH2Cl2 or CH3CN
followed by evaporation.
Because of the more pronounced chromic effects of solid
sample A (owing to its better organizability or crystallinity) its
morphology-sensitive solid-state emission was studied by
fluorescence spectroscopy in greater detail after mechanical-
or heat-induced perturbation. (Note all spectra were measured
as soon as possible on solid samples freshly ground in an agate
mortar.) When the pastel-yellow-emitting well ground powder
of solid A (Figure 6, form 1) was briefly heated with a heat gun
Figure 5. XRD pattern changes for powder sample of 1, solid A (left)
and solid B (right). The measurements were carried out in the order
(i) pristine samples (black), with calculated XRD curves of polymorph
A and polymorph B also shown for comparison (red), (ii) well ground
samples after grinding for 10 min, (iii) after dropwise treatment with
CH2Cl2, and last (iv) after dissolution and exposure to a vacuum.
showed only a few weak and significantly broadened signals,
indicating a transition from a crystalline to an amorphous phase
containing microcrystallites which might have been produced
during the grinding process or by formation from a metastable
amorphous phase, which partially recovered spontaneously at
room temperature (Figure 5, entry (ii), solid A). However, this
speculation was further more closely examined by differential
scanning calorimetry (DSC), XRD, and fluorescence spectros-
copy (FS) measurements, and the results reported in the
subsection DSC Measurements. Next, after careful treatment of
the well ground powder of solid A with drops of dichloro-
methane, the yellow emission gradually reverted back to the
original blue-shifted color, as well the restoration of clear
reflection peaks, which well fit those of the pristine solid A
(Figure 5, entry (iii), solid A), demonstrating reversion from
the amorphous to the crystalline phase. Consequently, the data
indicates that the various emission colors observed upon
grinding may be ascribed to the mechanochromic effect,
whereas the changes in the fluorescence efficiency are
associated with the crystallization-induced emission enhance-
ment (CIEE) activity of solid A.
Figure 6. Reversible switching emission of solid A by repeated
grinding−heating cycles: cycle A (black), cycle B (red), excitation
wavelength = 370 nm. Photographs were taken in an agate mortar
under UV illumination.
(∼5−7 s), the resulting solid exhibited blue emission under UV
blacklight excitation (λex = 365 nm), (form 2). The
corresponding emission maximum shifted from 523 to 474
nm, which closely matches the starting, unground solid
emission, suggesting that the ground sample can revert back
to the original crystal state, and indicating the repeatability of
amorphization and crystallization by a grinding−heating cycle
with almost no loss of emittance color and intensity (Figure 6,
cycle A).
Next, when the blue-emitting solid obtained was further
heated above the melting point, the color disappeared
immediately and after just a few seconds at room temperature
(rt), a virtually nonemissive, dark yellow solid emerged (form
3). To our surprise only a slight touch with the pestle changed
its emission color to bright yellow; see Figure 6, form 4. The
associated spectral band was significantly intensified with little
fluctuation in its emission maxima (Fmax ≈ 534 nm) in
comparison to the initial state, form 3. The solid form 4, after
intensive grinding followed by heating, could be converted back
to the blue-emitting form 2 as shown in Figure 6, cycle B.
Interestingly, besides being repeatedly switched in the solid
state, its emissions could be reversibly tuned between the two
In comparison, pristine solid B emits rather weak fluorescent
yellow light at 527 nm with an emission efficiency 8.4-fold
lower than that of the pristine solid A (for the spectrum see
Figure 1a). Although mechanical grinding of pristine solid B
caused little change in its physical appearance, emission color,
and intensity, the XRD pattern disappeared entirely when it was
thoroughly ground and so confirmed a morphological transition
from the crystalline to the amorphous phase; see Figure 5,
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cycles (A and B) by the grinding−heating process, without
exhibiting any degradation in luminescence.
to-phase transition, suggesting that crystalline phase B
converted to thermodynamically more stable crystalline phase
A. An intermediate phase at 162 °C was carefully prepared by
heating crystalline B to 162 °C directly in the DSC instrument.
The XRD pattern of this intermediate phase confirmed phase
transformation of crystalline solid B to crystalline A, since all
the diffraction peaks were consistent with those of crystalline
phase A; see Figure 8, entry (iv). On cooling, an exothermic
peak was observed at 164.6 °C (ΔHs = 67.7 J/g) due to
solidification. The XRD pattern confirmed that besides phase C
at least one additional crystalline phase was present (Figure 8,
entry (v)). For this reason, the second heating and cooling
cycles in both cases were slightly different. The melting point of
a new pure phase C, obtained after solidification from the melt
of crystalline A, was at 173.5 °C (ΔHm = −74.5 J/g) and
solidification at 161.2 °C (ΔHs = 71.1 J/g); see Figure 7a. The
mixture of phase C and additional phase(s) melted at 172.8 °C
(ΔHm = −70.3 J/g) and solidified at 164.6 °C (ΔHs = 66.8 J/
g); see Figure 7b.
DSC Measurements. To investigate the thermal properties
and the structural changes after heating and grinding in more
detail, DSC, XRD and fluorescence spectroscopy measurements
of crystalline solids A and B were carried out. DSC
measurements were performed in two heating and cooling
cycles under an argon atmosphere. In the first cycle, the heating
profile of solid A (Figure 7a) showed no peak up to the
To investigate further the stability of both polymorphs A and
B, single point DFT calculations at the B3LYP/6-31G(d,p)
level were carried out, using the geometries derived from the
crystal structures. The data indicate that molecule A is 13.29
kJ/mol more stable than molecule B.
The DSC curves of amorphous solid A (amA) and B (amB),
Figure 9a, which were obtained by intensive grinding in an
Figure 7. Two DSC heating and cooling cycles of (a) crystalline solid
A and (b) crystalline solid B.
temperature of melting at 174.5 °C (ΔHm = −100.1 J/g),
indicated by the sharp endothermic signal. On cooling, an
exothermic peak was observed at 161.7 °C (ΔHs = 73.1 J/g)
due to solidification of a new crystalline phase (termed solid
C). The presence of this phase was proved by XRD; see Figure
8, entry (ii), and FS analyses. Thus, its emission spectrum
Figure 9. (a) DSC curves of amorphous solid A and amorphous solid
B obtained after intense grinding of crystalline samples. (b)
Normalized fluorescence spectra of amorphous A (green) and
intermediate solids A, prepared after the first (blue) and the second
(red) crystallization peak. The spectrum of pristine solid A (black,
dotted line) is also shown for comparison.
agate mortar for 15 min, indicated that the yellow dye
molecules of amA or amB undergo a glass transition at 32
°C,14a followed by two exothermic peaks due to crystallization
in the temperature range from 35−60 °C and from 60−120 °C,
respectively. The XRD patterns of the intermediate solids of
amA or amB, prepared after the first (60 °C) and the second
(120 °C) crystallization peak, confirmed the presence of
crystalline solid A, even though their emission under UV light
illumination was yellow and bluish green, respectively. The
fluorescence spectra of amA and the intermediate solids are
shown in Figure 9b. The average size of the crystallites obtained
at 60 and 120 °C was calculated using the Scherrer equation as
31 and 54 nm, respectively. (The XRD patterns of the
intermediate solids of amA are depicted in Figure S4, SI.)
Closer inspection, however, reveals that the emission peak of
thoroughly ground sample A progressively reverted back to the
shorter wavelengths, quickly at 120 °C and slowly at room
temperature. The time-dependent spontaneous recovery of the
amorphous morphological structure back to the thermodynami-
Figure 8. XRD pattern changes during annealing of crystalline solids A
and B; (i, iii) pristine samples, (ii, v) after melting and cooling to rt,
and (iv) after heating crystalline B to 162 °C and cooling to rt.
showed a substantial red-shifted band at 549 nm, accompanied
by 14.5-fold weaker emission efficiency versus pristine solid A
(Figure 1, entry (iv)). In contrast, the first heating cycle of
crystalline solid B (Figure 7b) displayed a weak exothermic
peak at 161.3 °C (ΔH = 0.6 J/g) before melting at 174.2 °C
(ΔHm = −93.4 J/g). The peak at 161.3 °C belongs to a phase-
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cally stable crystalline state A, which is reflected in its
changeable, blue-shifted emission band, is known as a
chronochromic phenomenon.20 In our case, photolumines-
cence restoration was stabilized after ∼1.5 days under ambient
conditions with a blue-shift of 8 nm, although it never
recovered the identical spectral profile of pristine solid A, but
could be switched back to the initial (red-shifted) emission
wavelength by the grinding process. The data suggest that the
ground sample A is in a metastable amorphous state and can
crystallize promptly in the solid state on heating.
On further heating in the DSC instrument (after
crystallization), a small difference in the melting temperature
was observed for both investigated samples amA and amB of
173.6 °C (ΔHm = −99.1 J/g) and 171.5 °C (ΔHm = −90.7 J/
g), respectively (Figure 9a). This temperature corresponds to
the melting temperature of crystalline A (see Figure 7a).
Solid Films and the Self-Assembly Effect. While
exploring morphology changes during thermal treatment of
the crystalline solid B, we discovered another very interesting
physical property−its appreciable vapor pressure. For this
purpose, the DSC experiment was performed in such a way that
a larger hole was made in the lid. An endothermic peak with an
onset value at 160 °C was observed just before the phase
transformation and melting, and a difference in sample weight
occurred during the heating cycle (Δm = −17.2%) also
confirming a low degree of sublimation, see Figure 11a. From
this curve we established the sublimation temperature and
applied this property of the compound for preparation of thin
solid films. Sublimation is the key process responsible for
production of self-assembling microstructures.
Figure 11. (a) DSC heating curve of crystalline sample B. (b)
Normalized fluorescence emission spectra of the gently smeared
crystalline solid B (black, dotted line) and the spin-cast film of 1
(black, solid line) in response to thermal stimuli monitored by CFM.
solution at 4500 rpm. The resulting solid film was naturally
dried on standing at room temperature under an air
atmosphere in the dark for a few days, before further
investigation was done. Our success in obtaining a pure
amorphous solid was supported by XRD, SEM, and CFM. The
solid film (for the CFM microphotograph and corresponding
XRD curve see Figure S5a, SI) exhibited a broad remarkably
red-shifted emission band at 553 nm comparable to those of
the pristine solid samples (Figure 11b, black solid curve). To
our surprise, a small mechanical stimulus to the amorphous
film, such as a slight touch with a spatula, formed some
crystalline regions even before heating, as revealed by SEM
analysis, suggesting a strong tendency of the amorphous form
toward ordering; see Figure 12, entries (i, ii). A little thicker
layer of the solid film obtained by spin-coating showed lower
stability, converting spontaneously at room temperature to a
more orderly state, namely a dendrite-like textured solid, within
Next, the pristine solid B was gently smeared on a glass
substrate and slowly heated to 162 °C in a hot stage
microscope. The solid film microstructures before and after
thermal treatment were examined by scanning electron
microscopy (SEM) and confocal fluorescence microscopy
(CFM).
From SEM, we clearly observed that the starting platelike
crystals transformed to prismlike ones (Figure 10, entries (i, ii))
Figure 10. SEM images of morphology changes during thermal
treatment of crystalline solid B.
and notably, from CFM a change in fluorescence spectra
occurred from 528 to 481 nm as shown in Figure 11b, black-
and red-dotted lines. Both mentioned methods were used to
redetermine the heat-induced interconversion between the two
crystalline phases. When heating was continued above the
melting point to 195 °C, after cooling crystalline particles like
scales were formed with a significantly red-shifted emission
peak at 554 nm. The particle morphology is displayed in Figure
10, entry (iii), and the fluorescence spectrum in Figure 11b,
blue dotted curve.
Figure 12. SEM images of (i) the amorphous phase obtained by spin-
coating, (ii) mechanical pressure on the amorphous film with a spatula,
(iii) dendrites obtained by heating the amorphous film to 120 °C, and
(iv) crystallization of the amorphous form by heating for a minute at
162 °C.
Next, we attempted to create a more stable amorphous film
of 1 on a glass slide by spin-coating from dilute CH2Cl2
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1 h (Figure S5b, SI). Its emission band was nearly identical to
that of the pure amorphous solid being red-shifted to 555 nm
(Figure 11b, red solid line). Furthermore, when the pure thin
amorphous film (Figure 12, entry (i)) was heated to 120 °C, a
dendritic-like solid was also formed (Figure 12, entry (iii)).
After careful thermal annealing at 162 °C for a relatively short
time (because of its tendency to sublimation), the amorphous
film crystallized in the more stable crystal state A, as depicted in
Figure 12, entry (iv). Besides particle morphology, the figure
also clearly displays the particle size, which is comparable with
that of the amorphous form; see entry (i).
In order to evaluate the morphological structures and optical
properties of the sublimate, an experiment was performed in
which a second glass plate was placed a few millimeters above
the amorphous film. Sublimation was carried out in two
different ways. In the first case, the spin-cast amorphous film
was heated at 160 °C until the sample had mostly sublimed,
followed by cooling the cover glass bearing the sublimate to
room temperature. The CFM image (Figure 13, entry (i))
conditions for a week. The resulting sublimate was analyzed by
CFM and SEM. Evidently, a quite different sort of organized
structure was formed. As seen from the SEM (Figure S6b, SI)
and CFM microphotographs (Figure 13, entry (ii)), extremely
thin (up to 1 μm) fluorescent crystalline microfibers with
lengths of approximately several hundreds of micrometers,
divided into segments, were formed. Upon excitation, these
crystalline fibers emit green light of 555 nm (the fluorescence
spectrum is shown in Figure 11b, blue solid curve).
To sum up, compound 1 is capable of self-assembly whereby
its molecules pack one-dimensionally to give crystalline
microstructures. Careful study by SEM and CFM revealed
that the dazzling light emission which is seen along the
microstructures (highlighted by the red circles in Figure 13,
entries (i, ii)), indicates an obvious optical waveguide effect,
operating in the light transmission process of the micro-
particles, as previously noticed and described by Tang and co-
workers.4e,20 On the basis of the SEM and CFM images (see
Figure S7, SI), we assume that even the microstructures in
phase A (Fmax = 475 nm), crystallized from the spin-cast
amorphous film, possess the waveguide effect.
Many larger, thicker microfibers, which can be observed even
with the naked eye, up to 1 mm in length, were formed when
an ethanolic solution of solid B was slowly evaporated on
standing at room temperature (an optical image of the fibers is
shown in Figure S6c, SI).
Solvatochromism. The optical properties of dye 1 were
investigated in solution where free rotation around single bonds
exists. Such motion around the C−O bond of the methoxy
group causes equalization of the two polymorphs. The
absorption and emission spectra of solid A and solid B in
CH3CN solution were identical, and both samples also gave
1
identical H and 13C NMR spectra. However, the molecules of
1 in solution are characterized by the existence of a set of
rotational states, which decreases the probability of the
occurrence of a planar conformation and hinders overlapping
of the molecules typical of the crystal state.
The effects of several solvents of various polarities on the
electronic absorption and fluorescence spectra were examined.
It is known that the surrounding medium (solvent) can effect a
change in position, intensity and shape of the absorption or
emission bands, an effect known as solvatochromism.1a,b,21 In
general, dye molecules with a large change in their permanent
dipole moment on excitation exhibit strong solvatochromism.
We found that the absorption spectrum of 1 was only slightly
altered (in the range of 361−371 nm) with increasing solvent
polarity (Figure 14). This observation implies that the ground-
state electron distribution is not much affected, possibly due to
lower polarity in the ground state than in the excited state.
In the emission spectra on irradiation at the charge transfer
(CT) band (370 nm), compound 1 emitted from 431 nm in
hexane (nonpolar solvent) to 534 nm in CH3CN (polar
solvent). The fluorescence emission peak underwent a red-shift
with increasing polarity of the solvent, confirming a π−π*
transition (Figure 14). According to the theory of solvato-
chromism greater differences between λmax and Fmax in more
polar solvents indicate that the CT character in the first
electronically excited singlet state is more polarized than that in
the ground state. The difference between the ground- and
excited-state dipole moments (Δμ) of the solute molecule 1
was then determined from the Lippert−Mataga, Bakhshiev,
Kawski−Chamma−Viallet, and Reichardt equations22 based on
a linear correlation between the wavenumbers of the ultra-
Figure 13. CFM images of fluorescent (i) platelike or needlelike and
(ii) microfiber structures of the sublimate. Fluorescent images were
taken under a fluorescence microscope with 405 nm excitation. SEM
microphotographs of the self-organized structures show different levels
of connection: (iii) fully filled with material connecting two crystals at
the interface, (iv) partially filled, (v) separated segments containing
residual material connecting two crystals at the interface, (vi) divided
into invidual units, and last (vii) whole pointed-platelike structures.
shows that fluorescent structures with Fmax = 564 nm were
formed (the spectrum is plotted in Figure 11b, green solid
line). A scanning electron microscope was also used to examine
the morphology of the resulting solid (Figure S6a, SI). The
results suggest that the platelike structures are built from
segments (see Figure 13), which can be classified into three
different levels of connection: the first, fully filled with material
connecting two crystals at the interface (iii), the second,
partially filled (iv), and the third, empty (vi) (see Figure 13).
The terminal segment is typically pointed (vii). We believe that
initially the segments were joined together by material
connecting two crystals at the interface (probably of
amorphous material), and that after some period of time they
separated (v) and the material connecting two crystals at the
interface disappeared (vi). In the second experiment, the
amorphous film was rapidly heated to 210 °C so that
sublimation occurred immediately and was then left at ambient
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approximately 300-fold higher that than in CH3CN (Figure 14
b).
AIE Phenomena. The light emission of 1 induced by
aggregate formation was also studied; in other words, the AIE
activity of 1 was investigated.
A stock solution of 1 in CH3CN (5 × 10−4 M) was prepared
and an aliquot (1 mL) of this solution transferred to a 25 mL
volumetric flask. After addition of a calculated volume of pure
solvent, water (a nonsolvent of 1) was added in two different
ways: (a) in the first case, dropwise under vigorous stirring to
give final 5 × 10−5 M CH3CN−H2O mixtures with different
final water fractions (fw = 0−90 vol %); (b) in the second case,
a calculated quantity of water was added at once to prepare
mixtures with water contents in the range of 0−99 vol % and
the same final concentration. (Note that the suspensions with
95 and 99 vol % of water were derived from the recalculated
initial acetonitrile concentration).
Figure 14. (a) Absorption (solid lines) and emission (excited at 370
nm, dotted lines) spectra of dye 1 in various solvents. (b) Optical
properties of solutions of dye 1: absorption (λmax) and fluorescence
(Fmax) maxima, extinction coefficients (ε), and relative fluorescence
quantum yield (Φf), estimated using quinine sulfate in 0.05 M H2SO4
as a standard. The photos of dye 1 in hexane, toluene, tetrahydrofuran
(THF), and acetonitrile were taken under illumination with a UV lamp
(365 nm).
Dye 1 was virtually nonluminescent when dissolved in
acetonitrille, as indicated by the negligibly small value of Φf and
the photograph in Figure 14b. In fact, CH3CN−H2O mixtures
with water contents up to 70 vol % were also practically
nonemissive. A closer check of their fluorescence spectra
revealed significantly broad and weak signals at Fmax ≈ 556 nm
with small blue-shifts versus the spectrum of 1 measured in
pure acetonitrile solution, as can be seen from the spectra
shown in the inset of Figure S8a in the SI. When fw = 10 was
further increased up to 70 vol %, the emission intensity
decreased to reach a straight line in the 70 vol % aqueous
mixture. Moreover, both mixtures (prepared in different ways)
up to 70 vol % of added water were observed to give identical
spectral profiles. However, when a larger amount of water (fw >
70 vol %) was added to the solution, the emission of dye 1 was
turned on, as shown by the plot in Figure 15 on the right,
violet−visible absorption and fluorescence maxima, and the
bulk solvent polarity function, which involves both the
dielectric constant and refractive index of the medium, or the
molecular-microscopic solvent polarity parameter in the case of
the Reichardt equation (see page S10 of the SI). All parameters
necessary for determination of Δμ in the ground and excited
states are collected in Tables S6−8, SI. The aforementioned
methods suggest that the dipole moment in the excited singlet
state (μe) is greater than the corresponding ground-state value
(μg). This demonstrates a substantial redistribution of the π-
electron densities between the two electronic states, leading to
more polar species in the photoexcited state which is
considerably affected by stabilization from the polar solvent
as a consequence of stronger solvent−solute interactions in the
excited state. It is worth noting from Table 1 that the
discrepancies in the estimated values of Δμ observed may be
due to approximations made in the different theories.
Table 1. Ground- and Excited-State Dipole Moments of 1 as
Calculated by Various Methods
a
ground-state dipole moment, μg
vacuum
6.5 D
toluene
7.6 D
CH3CN
8.9 D
difference between the ground- and excited-state dipole moment, Δμ
b
c
d
e
16.2 D
9.7 D
12.0 D
7.8 D
a
Theoretical values of the anti isomer with complete geometry
optimization obtained by employing the Spartan’08 suite of quantum
Figure 15. (Left) Emission spectrum of blue suspension of 1 (dotted
line) and time-resolved emission spectra of green suspension (solid
lines) in 5 × 10−5 M CH3CN/water mixture at 85% and 95% fraction
of water (fw), respectively. Inset: photographs of suspension taken
under UV lamp irradiation. (Right) Plot of (I/I0 − 1) values versus
composition of the aqueous mixtures. I0 = emission intensity in pure 5
× 10−5 M CH3CN solution. Exciting wavelength: 370 nm.
b
chemical programs. Value calculated from the Lippert−Mataga
c
d
model. Value calculated from the Bakhshiev model. Value calculated
e
from the Kawski−Chamma−Viallet model. Value calculated from the
Reichardt model.
Theoretical μg values in vacuum and in two solvents (being
lower in less polar toluene than in acetonitrile) were evaluated
by quantum chemical calculations using the DFT method
adopting B3LYP/6-31G(d) level of theory; see Table 1. The
Onsager cavity radius of solute molecule 1 (a = 6.2 Å) was
determined by Kirillova’s equation23 using the static alpha
polarizability parameter (α = 66.55 Å3) from the computed
wavefunction.
caused by the formation of luminogen aggregates in aqueous
mixtures with a high water content due to the insolubility of 1
in polar media such as water. The AIE activity of 1 is probably
associated with its molecular structure, particularly with freely
rotatable moieties in the molecule, which in dilute solution can
undergo an active intramolecular rotation process, while in the
aggregates this process is restricted, so the nonradiative
relaxation pathway is blocked, thus making compound 1 a
strong emitter.
Interestingly, the fluorescence quantum yield Φf is also
greatly dependent on the solvent polarity; thus, in hexane, it is
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The 75% aqueous mixture, prepared in the first way, emits
significantly blue-shifted light at 469 nm with a drastic increase
of its emission intensity (I/I0 = 196) which is reflected in the
rather narrow emission band (Figure S8a, SI). With increase in
fw, its emission was greatly enhanced and later (fw = 85 vol %)
reached its maximum efficiency value (I/I0 = 553); see Figure
15. When the crystals or assemblies grow to critical sizes,
precipitation occurs. This explains why the fluorescence
emission did not increase further despite a further increase in
the water fraction up to 90 vol %.
In the second way, the emission spectra of 1 remained
practically unchanged from 10 up to 70 vol % of added water,
as described previously in the first case. When the water
fraction was increased to 75% the yellow emission was switched
on (see Figure 15 on the right-hand side). Furthermore, very
prompt reversion (occurring within a few minutes) from the
initial yellow- to a green-emitting suspension was observed for
different proportions of added water varying from 75 to 99 vol
% (Figure 15, left). The intensity of the green emission
increased with increasing amount of water, reaching a
maximum at a 95% aqueous mixture (Figure S8b, SI).
Aggregation as well as diverse color emission was followed by
time-dependent fluorescence spectroscopy. For instance, after
reaching a 95% aqueous mixture, a broad overlapping signal
between 470 and 650 nm, centered at 532 nm appeared. In the
stationary state, with time the emission band slightly intensified,
becoming narrower and blue-shifted to 518 nm; see spectra in
Figure 15. (Note: since the photoluminescence spectrum
changed with time, all spectra were taken immediately after the
samples were prepared.)
in Figure 15, indicated by the circle. Thus, the three distinct
emission colors of 1 in suspensions with the same water
content (fw = 85 vol %) may be attributed to the different
conformations and packing modes of the dye molecules in the
aggregates.
To the best of our knowledge, this is the first time that the
way of adding water and not its content in the mixture was
utilized to fine-tune molecular packing during the aggregation
process, allowing control of the selective production one of the
desired solids. Evidently, a simple manipulation of the
composition of the mixture or a small variation in the assembly
environment can lead to a large change in the emission color
and fluorescence efficiency of 1. If water was added carefully
(drop by drop), molecules of 1 at fw > 70 vol % may cluster
together slowly in an ordered fashion to form more emissive
blue crystalline aggregates. When the water fraction becomes
high in one step (as is described in the second case), the
molecules may agglomerate quickly in a random way to form
less emissive yellow amorphous particles, which are then
converted over a short period of time into aggregates of the
crystalline phase B.
3. EXPERIMENTAL SECTION
Structural and other crystallographic data (listed in Table S1, SI) have
been deposited with the Cambridge Crystallographic Data Centre as
supplementary publication number CCDC 988545 (Polymorph A)
and 988546 (Polymorph B). A copy of the data can be obtained, free
of charge, on application to CCDC, 12 Union Road, Cambridge, CB2
1EZ, UK fax: +44(0)-1223−336033 or by e-mail: deposit@ccdc.cam.
ac.uk.
To further test our speculation on the origin of the three
different emission colors of varying efficiency, we prepared
experiments on a larger scale at 85% of water in order to isolate
both aggregates by filtration of the resulting steady-state
suspension. XRD analysis of the particles obtained from the
blue-emitting suspension showed a totally identical pattern to
that of solid A, while the pattern of the green suspension’s
aggregates was in accordance with that of solid B (Figure 16b).
The π−π stacking interaction was described by the degree of
overlap in terms of the area shared by the rings, which was calculated
in the following procedures: (a) project the area covered by subPh or
dioxaborine rings on the arene ring in stacked pair; (b) calculate the
intersection of the projected area of the ring and the area of arene ring;
(c) take the percentage of the intersection (area) with respect to the
area of arene (Ph or subPh) ring.
X-ray powder diffraction data of the solids was collected using a
PANalytical X’Pert PRO (HTK) diffractometer with Cu Kα radiation,
acquired from 2Θ angles of 5° to 35° in steps of 0.034° at rt.
The crystallite size was determined using a crystallite size and strain
analysis package included in Topas v2.1.
DSC analysis was performed in a Mettler Toledo DSC 1 instrument
under an inert argon atmosphere. Samples (mass from 2 to 3 mg) were
weighed in a light 20 μL Al pan. To prevent any evaporation of the
sample, a pierced lid was pressed onto the sample using a special tool.
For crystalline A and B samples, two heating and cooling cycles were
performed under an inert flow of argon (flow 50 mL/min). The
sample was heated from room temperature to 200 °C with a heating
rate of 10 K/min, hold at final temperature for 5 min and then cooled
again to room temperature at 10 K/min. This cycle was repeated once
again. For studying crystallization behavior from amorphous materials,
samples were heated under the same conditions. Prior to all
measurements, the furnace was purged together with an inserted
sample for 20 min to provide inert conditions. To estimate the
sublimation temperature of crystalline solid B and additionally confirm
that this sample sublimes before phase transformation and melting, a
dynamic DSC measurement was performed in slightly different
conditions, namely a larger hole with a diameter ∼1 mm was made in
the lid to enable sample evaporation. The experiment was not
performed in the open pan to avoid deposition of the sample in the
DSC cell. The DSC analyzer was calibrated with high purity gallium
and indium. Gallium was supplied by Physikalisch-Technische
Bundesanstalt Braunschweig, whereas indium came from Mettler
Toledo. DSC standards and all samples for DSC analysis were carefully
weighed on a Mettler Toledo MX5 balance.
Figure 16. (a) SEM images of aggregate A (top) and B (bottom). (b)
XRD diagrams: measured patterns of the aggregates are in black and
the simulated one is in red.
This was supported by the SEM images that revealed well-
defined prismlike and rectangular platelike aggregates A and B,
respectively (Figure 16a). However, it is noteworthy that the
fluorescence spectrum of the pure amorphous film well
overlapped with that of the yellow suspension, suggesting
that the crystallization or organization process favored the
formation of crystals of solid B occurring through the first step,
the so-called metastable amorphous state, as suggested by the
photographs and time-dependent emission plot vs wavelength
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The solvents used in this work purchased from Fluka and Sigma-
Aldrich were of spectrophotometric grade (≥ 99.7%) and used without
further purification.
switching emission on repeated grinding−heating processes,
without displaying fatigue.
In contrast, after intensive grinding of the fluorescent yellow
solid B in an agate mortar, followed by careful treatment with
drops of CH2Cl2 or direct heating in a DSC instrument, the
amorphous phase rearranged into the more stable crystalline
phase A.
Further, heat-induced phase-to-phase interconversion be-
tween the two crystalline samples was found; thus, crystalline
phase B converted to the thermodynamically more stable phase
A at 162 °C, before melting, accompanied by a hypsochromic
shift of 37 nm and a pronounced emission efficiency
enhancement of 8.4-fold. Notably, the reinverse process is
also accessible by complete dissolution of the resulting solid A
in various solvents and subsequent evaporation.
Using additional methods such as SEM and CFM, we
obtained supplementary evidence on the morphological
changes during thermal treatment of the pristine solid B and
a pure amorphous spin-cast film of 1. Our findings can be
summarized as follows. First the amorphous phase that
exhibited a broad emission band remarkably red-shifted to
553 nm has a strong tendency for ordering. A small mechanical
perturbation of the amorphous film formed some crystalline
regions even before heating, while it crystallized promptly into
crystalline phase A on thermal annealing. Next, making use of
its appreciable vapor pressure the morphological structures and
optical properties of the sublimate were revealed. Sublimation
of the spin-cast amorphous film was performed in two different
manners: slowly at 160 °C and rapidly at higher temperature to
generate two types of microstructures, fluorescent platelike
structures (emitting at 564 nm) or extremely thin crystalline
microfibers (Fmax = 555 nm) with lengths up to several
hundreds of micrometers, respectively. In both cases the
microstructures are constructed from segments in which the
molecules self-organize by one-dimensional ordering, and show
an obvious optical waveguide effect.
Absorption spectra were measured on a PerkinElmer Lambda 750
UV−vis spectrophotometer, and emission spectra on a PerkinElmer
LS 55 spectrofluorometer with an excitation wavelength at 370 nm.
Fluorescence spectra of solids were recorded with a front surface
accessory from PerkinElmer and of single-crystals using a Well Plate
Reader Accessory. The relative fluorescence quantum yields ΦF in
optically dilute solutions were estimated by standard methods24 with
quinine sulfate in 1 N H2SO4 as a reference (λex = 347 nm; ΦF =
0.508;24c n20 H2O = 1.333) at rt.
The solid-state emission efficiencies relative to the pristine solid A
were measured on prepared samples as follows: ∼1.5 mg of solid was
added to a 200-fold excess of nonabsorbing KBr powder and gently
homogenized in an agate mortar with a pestle to obtain an optically
thick sample (3 mm). Then the emission spectrum of the sample was
recorded over an integration range of 427−645 nm with an excitation
wavelength at 370 nm.
The sample treated dropwise with dichloromethane was prepared in
the following way: pristine solid sample (15 mg) was freshly ground in
an agate mortar for at least 10 min to obtain the well ground solid.
Then on adding a single drop of CH2Cl2 to the ground powder the
yellow solid locally changes to the blue luminescent solid after about 1
min following evaporation of the solvent. The process of adding
solvent drop by drop with intermediate evaporation repeated many
times to achieve conversion of the entire ground powder to the blue-
emitting crystalline solid A. The natural evaporation step in the
process occurring between the each added drop of CH2Cl2 is very
important to prevent the ground solid film becoming completely
dissolved. The resulting crystalline solid was scraped from the mortar
and examined by XRD measurement.
The morphological characteristics of the compounds obtained were
investigated by SEM microscopy (field-emission FE-SEM Zeiss
ULTRA PLUS) using an accelerating voltage of 2 kV. The samples
were placed on graphite tape and were not gold sputtered prior to
microscopy. Images were taken using a standard Evernhart-Thornley
secondary electron (SE) detector.
Optical microscopy was conducted in the reflective light mode using
a Zeiss Axio Imager microscope.
A Leica TCS SP5 laser scanning microscope mounted on a Leica
DMI 6000 CS inverted microscope (Leica Microsystems, Germany)
The absorption spectra of 1 are only slightly affected by the
solvent polarity, whereas the center of its emission band alters
from 431 nm in hexane (a nonpolar solvent) to 534 nm in
polar CH3CN with a huge decrease in fluorescence quantum
yield. The alterations of the Stokes shift caused by various
solvents elucidate the nature of the first electronically excited
singlet state, which is more polarized than that in the ground
state.
Dye 1 is also AIE active. Adding a large amount of water (fw
> 70 vol %) to the practically nonluminescent acetonitrile
solution turns on its emission, being blue (crystalline phase A)
or yellow (metastable amorphous solid, but which almost
within a few minutes converts to a green suspension
corresponding to phase B). The three distinct emission colors
of 1 in suspensions with the same water content may be
induced by diverse processes of aggregation. Fascinatingly, this
is the first time that the way of addition of water, and not its
content in the mixture, was utilized to fine-tune molecular
packing during the aggregation process.
equipped with an 20× objective and excited with a blue laser (λex
=
405 nm) was used for CFM imaging. Fluorescence emission was
detected in the range of 440−600 nm. For sequential excitation, a 50
mW 405 nm diode laser and the 476 nm line of a 25 mW argon laser
were used.
4. CONCLUSIONS
The BF2 complex of 1-phenyl-3-(3,5-dimethoxyphenyl)-pro-
pane-1,3-dione was shown to form two polymorphs, which are
different in the mutual orientation of the two methoxy groups,
resulting in changes in the modes of molecular packing. By
controllable alteration of its molecular arrangements through
external stimuli, varying solid-state emission was achieved from
the same fluorophore. We observed that solid A but not solid B
exhibited mechanochromic fluorescence and a striking CIEE
effect; for instance, solid A emited strongly in the crystalline
phase but only faintly in the amorphous phase (Fmax = 490 and
526 nm, respectively), with a decrease in emission efficiency of
as much as 5.3-fold. The yellow-emitting well ground powder of
solid A on dropwise treatment with solvent (CH2Cl2) or on
heating (thermochromism) reverted back to the initial blue-
emissive crystal state, as well as partially returning to the
original emission color spontaneously at room temperature
(chronochromism). Additionally, solid A exhibited reversible
Finally, it is particularly important to understand the role of
molecular packing in controlling the solid-state optical
properties of dye 1 for developing stimuli-sensitive materials.
We believe that this present work is worthy of attention not
only in the field of fundamental research, but also in practical
applications such as potential photoswitchable molecular
devices, luminescent and sensory materials in optical displays,
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AUTHOR INFORMATION
Corresponding Author
boris.sket@fkkt.uni-lj.si
■
(7) (a) Tong, H.; Dong, Y.; Haussler, M.; Lam, J. W. Y.; Sung, H. H.-
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Notes
†A part of a Ph.D. dissertation.
The authors declare no competing financial interest.
(8) (a) Hong, Y.; Lam, J. W. Y.; Tang, B. Z. Chem. Commun. 2009,
4332−4353. (b) Wang, M.; Zhang, G.; Zhang, D.; Zhu, D.; Tang, B. Z.
J. Mater. Chem. 2010, 20, 1858−1867.
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ACKNOWLEDGMENTS
■
108, 1517−1549. (b) Gonca̧ lves, M. S. T. Chem. Rev. 2009, 109, 190−
212. (c) Klymchenko, A. S.; Mely, Y. Prog. Mol. Biol. Transl. 2013, 113,
35−58.
This work was supported by the Ministry of Higher Education,
Science and Technology of the Republic of Slovenia and the
Slovenian Research Agency (PR-4020). We are deeply grateful
(10) (a) Zhao, B. Y.; Gao, H.; Fan, Y.; Zhou, T.; Su, Z.; Liu, Y.;
Wang, Y. Adv. Mater. 2009, 21, 3165−3169. (b) Luo, X.; Li, J.; Li, C.;
Heng, L.; Dong, Y. Q.; Liu, Z.; Bo, Z.; Tang, B. Z. Adv. Mater. 2011,
23, 3261−3265. (c) Abe, Y.; Karasawa, S.; Koga, N. Chem.Eur. J.
2012, 18, 15038−15048. (d) Harada, N.; Abe, Y.; Karasawa, S.; Koga,
N. Org. Lett. 2012, 14, 6282−6285.
̌
to Prof. Marjan Marinsek (Faculty of Chemistry and Chemical
Technology, University of Ljubljana) for all photographs taken
using SEM and optical microscopes, as well as for helpful
̌
discussion. We also thank Mojca Bencina, Ph.D. (National
Institute of Chemistry, Ljubljana, Slovenia) for assistance in
CFM measurements and the EN-FIST Centre of Excellence,
Ljubljana, Slovenia for use of the SuperNova diffractometer.
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