Carbazole-containing difluoroboron β-diketonate dyes: Two-photon excited fluorescence in solution and grinding-induced blue-shifted emission in the solid state

Article abstract of DOI:10.1039/c7tc04751g

Mechanochromic luminescent (MCL) materials have attracted considerable attention in recent years because of their potential applications in various fields and the fundamental importance of understanding the mechanism of mechanochromic luminescence. The most reported MCL materials show a bathochromic shift after external mechanical stimulus. Here we report new MCL materials with hypochromic shift of the emission after grinding. These materials are based on carbazole-containing difluoroboron β-diketonate complexes. These complexes show strong fluorescence in both the solid state and organic solutions. Our study reveals that the emission of structural symmetrical complexes is less influenced by the polarity of the solvent than those of unsymmetrical complexes and they retain strong fluorescence in highly polar solvents. In addition, the good planarity of the carbazole group and the ICT character of the complexes make them exhibit obvious two-photon excited fluorescence. In the solid state, their emission colours can be tuned reversibly by external stimuli, and they exhibit an unusual hypochromic shift after grinding. Single crystal X-ray analysis of the complexes reveals that the initial close molecular packing is responsible for the blue-shifted emission. Interestingly, both hypsochromic and bathochromic shifts of the emission are realized by different treatments of the samples. Our work provides information on the mechanism of the mechanochromic luminescent properties of organic luminescent materials and expands the potential applications of MCL materials.

Full text of DOI:10.1039/c7tc04751g

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PAPER
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Nguyên T. K. Thanh, Xiaodi Su et al.
Fine-tuning of gold nanorod dimensions and plasmonic properties using
the Hofmeister effects
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Carbazole-Containing Difluoroboron β-Diketonate dyes: two-
photon excited fluorescence in solution and grinding-induced
blue-shifted emission in solid state
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
a
b
a
DOI: 10.1039/x0xx00000x
Peng-Zhong Chen, Jian-Xin Wang, Li-Ya Niu,* Yu-Zhe Chen and Qing-Zheng Yang*
www.rsc.org/
Mechanochromic luminescent (MCL) materials have attracted considerable attention in recent years because of their
potential application in various fields and the fundamental importance to understand the mechanism of mechanochromic
luminescence. The most reported MCL materials show bathochromic shift after external mechanical stimulus. Here
we report new MCL materials with hypochromic shift of the emission after grinding. These materials are based on
the carbazole-containing difluoroboron β-diketonate complexes. These complexes show strong fluorescence in both the
solid state and organic solutions. Our study reveals that the emission of structural symmetrical complexes are less
influenced by the polarity of solvents than those of unsymmetrical complexes and they retain strong fluorescence in high
polar solvents. In addition, the good planarity of carbazole group and the ICT character of complexes make them exhibit
obvious two-photon excited fluorescence. In solid state, their emission colours can be tuned reversibly by the external
stimuli, and exhibit unusual hypochromic shift after grinding. Single crystal X-ray analysis of complexes reveals that the
initial close molecular packing is responsible for the blue-shifted emission. Interestingly, both hypsochromic and
bathochromic shifts of the emission are realized by different treatments of the samples. Our work provide the information
on the mechanism study of mechanochromic luminescent properties of organic luminescent materials and expand the
potential application of the MCL materials.
as pyrene derivatives, donor-acceptor π-conjugated dyes and
aggregation-induced-emission (AIE) type dyes, with MCL
Introduction
8
properties in solid state have been prepared. The most
Mechanochromic luminescent (MCL) material shows emission
1
colour change in response to mechanical stress in bulk solids.
reported MCL materials show red-shifted and weakened
emission after external mechanical grinding. The changes of
emission colour and intensity result from the phase transition
between the crystalline and amorphous state, and the
emission of the amorphous state is related to excimers and
MCL materials have attracted increasing attention in recent
years because of their potential application in various fields, as
well as the fundamental importance to understand the
2
mechanism of mechanochromic luminescence. The emission
9
excited aggregates. However, up to now, MCL materials that
properties of MCL materials strongly depend on the molecular
arrangements, which are determined by the conformation of
3
molecules and non-covalent intermolecular interactions. The
exhibit blue-shifted emission upon external stimuli are rarely
8
a, 10
reported.
Development of such materials is important to
in-depth understand the underlying mechanism of
mechanochromic luminescence in molecular level and expand
the potential application of the MCL materials.
external mechanical-force can alter the molecular
arrangements, subsequently changing in the emission colours
4
and intensity.
5
Herein, we report carbazole-containing difluoroboron β-
diketonate (BF bdk) complexes with significant MCL
Impressive MCL materials based on organic fluorophores
6
and luminescent metal complexes have been reported. Since
2
1-4
properties. They exhibited intense fluorescence in solid state
and their emission colours can be tuned by the external stimuli
such as grinding, heating and exposure to solvent vapour due
to the changes in molecular arrangements. It is noteworthy
that these materials show unusual blue-shifted emission after
grinding. The initial and mechanically perturbed states of the
materials were stable and readily characterized. Single crystal
the first example of MCL material was reported by Sagara et al.
7
in 2007, an increasing number of organic fluorophores, such
X-ray analysis of complexes 1-4 provided structural reason for
the blue-shifted emission. Our results indicated that the initial
molecular packing ways determined the direction of emission
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Photophysical properties in solutions
The photophysical properties of complexes
1-4 were
studied in solvents with different polarities. The data were
summarized in Table 1.
Each complex displayed a relative small absorption band
at approximately
* transitions delocalized along the
S1). While the intense absorption band at wavelength ranging
from = 380 nm to 500 nm showed high molar extinction
λ
< 400 nm, which was attributed to the
π–
π
π
-electronic system (Fig.
λ
4
-1
-1
coefficient (10 M cm ), and was assigned to the ICT
transition from carbazole unit to dioxaborine ring. The
absorption spectra were slightly affected by the polarity of
solvents. In general, the absorption maximum was closely
related with the sizes of conjugated systems. The absorption
Chart 1. The molecular structures of complexes 1-4
maximum from complex
hypsochromic shift in the same type solvent (e.g.
nm, at 440 nm, at 400 nm in THF) due to the reduced
conjugated length. While complex , which contained two
dioxaborine rings at the 3, 6-positions of central carbazole
units, showed the absorption maximum at = 414 nm in THF,
approximately 14 nm red-shift compared with that of
4,
1
and
2
displayed gradually
4
at = 460
λ
shifts for the MCL materials. These attributes made those
complexes excellent organic fluorescent materials.
1
2
3
λ
2.
Results and discussion
Molecular design and synthesis
a
b
4
c
d
e
Solvents
λ
abs
ε*10
λ
em
Φ
f
f
τ /ns
Difluoroboron
2
β-diketonate (BF bdk) derivatives, an
-1
-1
(nm)
(cm M )
7.20
(nm)
474
495
498
514
521
530
intramolecular charge transfer (ICT) type dyes, exhibited rich
1
photophysical properties in both solution and solid state. The
variation of molecular conformation and intermolecular
Toluene
440
443
440
445
442
441
0.93
0.89
0.91
0.84
0.48
0.07
2.26
3.00
3.48
3.92
3.07
0.94
1
1
2
3
4
CHCl
3
7.53
THF
8.26
interactions between BF
2
bdk molecules usually lead to
DCM
6.93
significantly red-shifted emission in solid state than that in
Acetone
6.90
12
solutions. The multiple emissive properties made BF
2
bdks
CH
3
OH
5.30
suitable for investigation the relationship between the
emission
properties
and
corresponding
molecular
Toluene
402
406
400
406
402
401
4.40
5.19
4.78
4.60
4.62
4.04
444
475
475
495
506
534
0.87
0.75
0.79
0.70
0.57
0.05
3.43
4.83
5.36
6.00
5.80
0.64
CHCl
3
arrangements. We previously designed complex
4
by attaching
THF
carbazole groups to 4, 6-positions of dioxaborine ring due to
the good planarity and strong electron-donating capability of
DCM
1
carbazole. The structural features of complex
3
Acetone
4 are beneficial
CH
3
OH
for its strong emission in solution and solid state, as well as its
self-assembly into ordered structures via multiple
Toluene
412
417
414
418
417
414
6.08
6.74
6.18
5.44
5.90
5.28
436
443
450
451
460
476
0.88
0.92
0.95
0.93
0.81
0.39
2.00
2.11
2.52
2.50
3.30
3.48
intermolecular interactions such as
14
bonding and dipole-dipole interactions. Complex
π-π stacking, hydrogen
CHCl
3
4
formed
THF
yellow and red emissive crystals in dichloromethane/hexane
and chloroform/hexane, respectively. The emission colours of
DCM
Acetone
complex
4
in solid states were effectively tuned between
CH
3
OH
yellow and red by the external stimuli, such as grinding,
heating, solvent vapour, to modulate the molecular packing
Toluene
460
466
460
467
464
463
7.56
9.36
481
504
489
513
506
525
0.93
0.89
0.93
0.80
0.81
0.40
1.87
1.95
2.14
2.12
2.45
1.98
1
5
CHCl
3
ways. In addition, the red-emissive crystal exhibited blue-
shifted emission after grinding, which may be ascribed to
change of the molecular arrangement from a highly compact
way to relative loose packing modes. These properties inspired
THF
10.82
10.13
10.56
3.91
DCM
Acetone
3
CH OH
a
b
us further design and synthesize complexes 1-3, which differed
Absorption maximum. Extinction coefficients calculated at the
absorption maxima. Fluorescence emission maxima. The absolute
fluorescence quantum yields. Fluorescence lifetime were measured with
c
d
in their molecular sizes and structural symmetry, as a result
influencing the electronic property and the overall ICT
e
a 405 nm EPLEDs (picosecond pulsed LEDs) light source and monitored at
the emission maximum. All fluorescence lifetimes are fitted with single-
exponential decays unless indicated.
property. The detailed synthetic routes of complexes
shown in Scheme S1 in Supporting Information.
1-3 were
2
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in methanol, while the symmetric complexes
3
and
4
still
displayed strong fluorescence in methanol (Table 1).
The fluorescence decay profiles of complex
1-4
were
fitted by using a single-exponential function, with fluorescence
lifetime in the nanosecond region (Table 1), a result in the
accordance with the singlet-excited state. For all complexes,
the radiative rate constant, which is defined as K
much higher than the non-radiative rate constant, defined as
(1−Φ , in most solvents, indicating minimal interacting
r
=
Φ
F F
/τ , is
K
r
=
F F
)/τ
of the emitting state with the energetically low-lying localized
state reflecting in the relatively high fluorescence quantum
yields (Table S1). In addition, the emission intensities of
complexes
1
-
4
in DCM solutions almost kept constant under
= 400 nm for 300
continuous irradiation by a Xe lamp at λ
minutes, suggesting good photostability of these complexes in
Fig. 1 The normalized fluorescence spectra of complexes 1 (a), 2 (b), 3 (c) and 4
(
d) in different solvents.
solution (Fig. S3).
The emission spectra of complexes 1-4 showed obvious
Two-photon absorption measurements
positive solvatochromicity properties (Fig. 1 and Table 1). The
emission bands became broad and red-shifted significantly
Complexes 1-4 showed the fluorescence upon excitation
with increasing the polarity of solvents, consistent with the at 780 nm laser pulse and their fluorescence exhibited profile
strong ICT character of the solvent relaxed emissive state. It and peak position identical with their respect one-photon
was notable that the ICT character, compared to the excited fluorescence (Fig. S4), suggesting the emission was
conjugation length, was likely the dominant factor that generated from the same excited state regardless of the
affected the maximal emission of these complexes, especially excitation modes. There is no one-photon absorption in the
in the solvents of high polarity such as methanol. For example, wavelength after 700 nm for all complexes in different
the conjugation length of
followed order . However, the emission maxima of
in methanol were located at = 525 nm, 530 nm and
32 nm, respectively. The red-shifted wavelengths from plotting logarithmic forms of fluorescence integrates against
1, 2 and 4 was sequenced as the solvents, indicating the emission is the two-photon excited
1
7
4
>
1
>
2
4,
fluorescence (TPEF). The fluorescence spectra of complexes
1-4 at different laser intensities were shown in Figure S5. By
1
and
2
λ
5
toluene to methanol are 44 nm, 56 nm and 90 nm for the input laser powers, the slopes were determined to be 2.09,
complexes and , respectively. This might be related with 1.97, 1.81 and 1.83, respectively, further confirming the two-
the intrinsic electronic asymmetry in complexes
compared to . DFT calculation revealed that the HOMO
orbital was mainly located on the carbazole unit, whereas the in GM = 1 × 10 cm .S.photon .molecule ) of complexes
LUMO orbital was distributed on the dioxaborine rings, as well were firstly measured in toluene, THF and dichloromethane
as the anisole group for and methyl group for and (Fig. (DCM) solution, respectively, at 10 nm intervals from 720 nm
4,
1
2
1
8
1
and
2
photon excitation mechanism.
4
The two-photon-absorption cross sections (δ, expressed
-50
4
-1
-1
1-4
1
2
3
S2). This result suggested that ICT from the carbazole unit to
the dioxaborine ring was significant regardless of whether
1
6
anisole or methyl was present as a competing donor group.
This asymmetry promoted symmetry breaking in the excited
state leading to more polar excited states, which are stabilized
in high polarity environments, leading to a red-shifted
emission. Although complex
system than that of complex
3
displayed larger conjugation
, its maximal emission bands
2
were centred at higher energy region in all solvents used in
this work. Since carbazole was a stronger electron-donating
group than methyl, ICT process was aligned in one direction in
complex
ring in complex
A-D-A type), which weakened the whole ICT process and was
the possible reason for the blue-shifted emission of complex
compared to that of complex . In addition, the structural
2
(D-A type). The presence of another dioxaborine
3
resulted in the ICT process in two directions
(
3
2
symmetry of complexes also significantly influenced their
fluorescence quantum yield in solvents of high polarity. The
Fig. 2 Solvent dependence of the measured TPA spectra of complex 1(a), 2(b),
emission of the asymmetric complexes
1
and
2
were very weak
3(c) and 4(d).
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to 900 nm, by using a femtosecond pulsed laser source. During
measurements, no obvious degradation was observed for all
samples. It was noteworthy that for all of the complexes
except 2, the maximal δ were located at significantly shorter
wavelength than twice those of their respective one-photon
absorption maxima, indicating the fact that the lowest excited
singlet excited state was strong one-photon allowed but two-
1
9
photon forbidden (Fig. S6-S8).
The TPA spectra of in the three solvents are presented
in Figure S9. Depending on the effective conjugation and ICT
over the entire molecules, complex displayed the largest TPA
(285 GM at 750 nm in toluene, 432 GM at 770 nm in THF,
62 GM at 700 nm in DCM), while complexes showed the
significantly less values (< 200 GM) compared to . For
complexes and , the TPA spectra in different solvents
displayed the similar band shape and variations in maximum
values following the order: toluene < THF < DCM (Fig. 2a,b and
1-4
4
δ
7
1-3
4
1,
2
4
δ
d). While the highest
that in DCM, and the
δ
δ
for complex
3 in THF was larger than
in toluene still had the smallest values
(
Fig. 2c). We speculated that the charge separation assisted by
solvents of intermediate polarity was favourable for the
enhancement of TPA values in THF and DCM, compared with
that in the less polar toluene.
Fig. 4 (a) The molecular conformation of complex 1 in front view and side view,
respectively. (b) The molecular structures of complex 1 in one unit cell. (c)
Molecular stacking structures along the short and long molecule axis,
respectively. (d) The intermolecular interactions between adjacent molecules.
δ
2
0
Photophysical properties in solid state
f
543 nm (Φ = 10.1%) compared with crystals of 1 and 2. The
emission of all three complexes in solid state was significantly
red shifted compared to those in DCM solution, indicating
strong intermolecular interactions between chromophores in
solid state.
In addition to their outstanding photophysical properties
in solution, complexes 1-4 also showed intense fluorescence in
solid state. The crystalline powders of complexes 1-3 suitable
for X-ray structural analysis were obtained by slow
evaporation of their DCM/hexane or DCM/iPrOH mixed
To gain further insight into the relationship between the
molecular packing arrangements and their emission behaviour,
solutions, respectively (Fig. 3). Crystal of
fluorescence at approximately λem = 627 nm with fluorescence
quantum yield (Φf) of 12.6%. Crystal of showed yellow
1 displayed red
single crystal structures of complexes
The crystal data were summarized in Table S2.
In all crystals of antiparallel alignment of neighbouring
1-3 were determined.
2
1-3
emission colour with the emission band centred at λem = 565
nm (Φf = 22.4%), approximately 62 nm blue-shift compared to
that of crystal of 1. While the crystal of 3 which contains bis-
dioxaborine rings, its emission further blue-shifted to λem
=
Fig. 5 (a) The molecular conformation of complex 2 in front view and side view,
respectively. (b) The molecular structures of complex 2 in one unit cell. (c)
Molecular stacking structures along the c-axis direction. (d) The intermolecular
interactions between adjacent molecules.
Fig. 3 The normalized emission spectra of crystals of 1-3. The inset is the
fluorescence microscope images of crystals of 1-3.
4
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molecules were observed, indicating that the ground-state degree of overlap is 42% between the carbazole rings of the
dipole moment has a major impact on the packing of the adjacent molecules at the ring centroid-centroid (Cg-Cg)
2
1
dyes. Crystal of
1
belonged to a crystal system of monoclinic distances of 4.21 Å (Fig. S11). However, negligible overlap
ioxaborine ring and carbazole plane was
space group P2 /n and the unit cell contained four individual between the
1
d
……
nearly planar molecules (Fig. 4a and b). In the crystal, the observed. Strong CH F hydrogen bonds were also observed
central dioxaborine adopted boat conformation, with the between the nearest mutually perpendicular molecules, linking
meso-C and B atoms deviating from the diketonate plane by the packing motifs together (Fig. S12).
approximately 4.1° and 19.1°, respectively (Fig. S10). The
molecules packed along one direction in a brickwork type of planes being out of the central carbazole plane by 21.4° and
arrangement, forming infinite two-dimensional packing arrays 13.7°, respectively (Fig. 6a). The ioxaborine rings at both ends
Fig. 4c). Within the array, one molecule overlapped with four were in different configuration, one being almost coplanar,
Complex 3 in the crystal was not planar, both diketonate
d
(
adjacent molecules between the carbazole and anisole rings while the boron atom in other dioxaborine ring being out of
with the distance of approximately 3.4 Å, indicating strong
π
interactions between the neighbours (Fig. 4d). The degree of complex
-
π
the diketonate plane by 22.8°. The crystal structures of
belonged to a crystal system of tetragonal space
3
overlap was almost 100% of the anisole rings surface area. group P4 /n with eight molecules of anisotropic directions in
2
Whereas, it was noteworthy that there was no significant one unit cell (Fig. 6b). In the crystals the packing arrays were
overlap between dioxaborine ring and the adjacent molecules. oriented towards diverse directions and linked together by the
…
However, the BF in the dioxaborine ring acted as a bifurcated intermolecular CH F hydrogen bonding, forming a wavy-like
…
2
o
acceptor with an angle of 110.49 . Within each packing array, motif (Fig. S13). In each packing array complex
3
experienced
…
fluorine atoms formed strong CH F hydrogen bonds towards head-to-tail antiparallel molecular packing with multiple
…
…
…
the adjacent molecules with the distances of 2.57 and 2.88 Å, intermolecular interactions such as
π
-
π
stacking, C-H
π and C-
…
…
further stabilized the molecular packing.
Complex was almost coplanar in the crystal, the The C-H F hydrogen bond was even observed between
carbazole plane being out of the central diketonate plane by alternative molecules and occurred at the methyl group and
H F hydrogen bonds between adjacent molecules (Fig. 6c).
…
…
2
o
8
.0 (Figure 5a). The unit cell of crystal of
space group C2/c, consisting of eight molecules (Fig. 5b). distortion (Fig. S14). Two types of overlap arrangement were
Complex displayed two differently directed stacking motifs observed in this crystal (Fig. 6d). In type I, the molecules hardly
with almost mutually perpendicular orientations (Fig. 5c). In overlapped with each other, indicating negligible
each packing motif, the molecules were antiparallel packed by interactions. In type II, the molecules showed significantly
multiple intermolecular interactions such as stacking, overlap in the carbazole units. The total degree of overlap was
and CH F hydrogen bonding between adjacent 65% of the carbazole ring’s surface area at the ring centroid-
molecules and arranged in slip-stacks along the long molecular centroid (Cg-Cg) distances of 3.44 Å. However, the ioxaborine
2 was monoclinic, fluorine atom in the dioxaborine rings with large configuration
2
π
-
π
π-π
…
…
……
CH
π
d
o
axis with a slip angle of approximately 50.1 (Fig. 5d). The total rings showed negligible interactions with the other molecule.
The multiple interactions between chromophores were
Fig. 6 (a) The molecular conformation of complex 3 in the side views. (b) The
molecular structures of complex 3 in one unit cell. (c) The intermolecular
interactions between adjacent molecules. (d) The molecular overlaps between
adjacent molecules.
Fig. 7 The normalized emission spectra and fluorescence microscopy images of
complexes 1 (a, b), 2 (c, d) and 3 (e, f) before and after grinding. The scale bar is
0.5 mm.
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responsible for the red-shifted emission spectra in the solid 1.82 ns compared to that of emission at
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λ
em = 612 nm and the
states compared to that in solution. On the other hand, these pristine crystal samples, but comparable with that in solutions.
interactions might be interrupted by external stimuli, resulting After strong grinding with enough time (fully ground), the
in the changes in the emission properties. We investigated the emission band further blue-shifted to λem = 597 nm with the
mechanochromic luminescence properties of complexes disappearance of peak at em = 557 nm, exhibiting broad and
1-
3
λ
upon grinding of different extents, which induced gradually structure-less features. The fluorescence lifetime of the fully
changes in their emission spectra and fluorescence lifetimes ground samples increased to 14.13 ns compared with the
(
Fig. S15). The pristine crystal of
maximum at em = 627 nm. After gently crushing (partly
ground) crystal of on glass slides with a pestle, we were also observed for complexes
surprised to find that the emission spectrum blue-shifted to the emission shifted from em = 566 nm of the pristine crystal
em = 612 nm with a shoulder peak at em = 557 nm (Fig. 7a to em = 553 nm with a shoulder peak at em = 535 nm. While
exhibited a more obvious blue-shift compared with
and , from λem = 542 nm of the initial crystal to λem
1
showed its emission pristine crystals.
The blue-shift emission induced by partly grinding were
and (Fig. 7c-f). For complex
λ
1
2
3
2,
λ
λ
λ
λ
λ
and b). Time resolved fluorescence spectra revealed that the complex
3
1
two emission peaks had quite different decay profiles (Table complex
2
S3, Fig. S16). The peak at λem = 612 nm showed the lifetime of = 499 nm (shoulder peak at λem = 528 nm) after partly ground.
.94 ns, similar with the pristine crystal samples of 5.80 ns. Fully grinding further induced the emission changes of
However, the band at λem = 557 nm showed a short lifetime complexes and but with a reverse tendency. The band at
λem = 535 nm was enhanced with respect to the band at λem
5
2
3
=
5
53 nm for complex
2
after fully grinding. On the contrary,
complex
3 experienced a gradually red-shifted emission with
increasing grinding time. The emission spectra of fully ground
samples finally reached λem 556 nm, even more
bathochromic-shift than that of pristine crystal of . The
fluorescence lifetime for complex changed indistinctly in the
=
3
2
values from 3.29 ns (pristine, λem = 566 nm) to 3.23 ns (partly
ground, λem = 553 nm) and 2.99 ns (fully ground, λem = 535 nm)
(
Fig. S17). However, grinding induced obvious reduction in the
fluorescence lifetime for complex from 5.68 ns (pristine, λem
542 nm) to 2.37 ns (partly ground, λem = 499 nm) and 2.38 ns
fully ground, λem = 556 nm) (Fig. S18). It should be
3
=
(
emphasized that the fluorescence quantum yields enhanced
after grinding, from 12.6%, 22.4% and 10.0% of the pristine
samples, to 38.2%, 45.2% and 11% of the fully ground samples,
for complexes 1, 2 and 3, respectively. The variation in the
photophysical properties of the three complexes upon grinding
were ascribed to the changes of their molecular packing ways,
which were reflected in the different absorption spectra of the
pristine, partly ground and fully ground samples (Fig. S19).
X-ray diffraction (XRD) provided further essential
information on the phase transition during the grinding
process (Fig. 8). The pristine crystals of the three complexes
showed sharp and intense peaks. Partly grinding induced some
variations in the XRD patterns compared with those of pristine
crystals, indicating the changes of molecular packing, which
was the possible reason for the blue-shifted emission after
partly grinding. For fully ground samples, the XRD patterns
only revealed broad and featureless reflections, suggesting the
formation of amorphous state.
The emission of amorphous ground materials was usually
assigned to two different species, the excimers or excited
9
aggregates (i. e. H-dimers). The assignment of the emission of
9
a, 9c
amorphous materials to excimers is widely accepted.
In
such case, the amorphous states should exhibit the similar
emission properties with that of high concentrated solutions,
in which the fluorescence of excimers can be observable. In
Fig. 8 The XRD patterns of crystals of 1 (a), 2 (b) and 3 (c) before and after
grinding.
high concentration solution (20 mM) of complex
2 in
chloroform, the emission band was centered at λem = 575 nm,
6
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Fig. 9 The normalized emission spectra of complexes 1 (a), 2(b) and 3 (c) in different conditions. Reversible switching emission of complexes 1 (d), 2(e) and 3 (f)
by repeated grinding−heaꢀng cycles.
red-shifted and broadened compared with that of diluted samples, probably due to the thermodynamic instability of the
chloroform solution (λem = 495 nm) (Fig. S20a). We tentatively amorphous state. After heating at 200 °C for 15 min, to our
assigned this band to the emission of excimers. The time surprise, the ground complex
1
further blue shifted the
resolved spectroscopy provided the further evidence to emission to ca. λem = 535 nm (
Φ = 17.5%) with significant
support such assignment. The high concentration solution of reduced fluorescence lifetime 1.67 ns, which were similar with
complex exhibited much longer fluorescence lifetime (15.66 those of complex in solutions (Fig. 9a). Heating also induced
ns) than that of dilute solutions (4.83 ns) (Fig. S20b). These the emission blue-shifting from em = 535 nm to 518 nm (
results suggested the emission of em = 535 nm after fully 28.1%, = 2.60 ns) for ground complex and from em = 556
grinding of complex could not come from this excimer in the nm to 526 nm ( = 8.2 %, = 2.92 ns) for ground complex
2
1
λ
Φ =
λ
τ
2
λ
2
Φ
τ
3
amorphous state. The assignment of the emission of (Fig. 9b and c). Moreover, such blue-shifted emission was also
amorphous materials to the excited aggregates (i. e. H-dimers) realized when the samples were fumed with DCM vapours.
9
b
was proposed in literature. It was proposed that the The emergence of reflection peaks in the XRD patterns for the
aggregates (i. e. H-dimers) possess the lowest excited-states in heated samples indicated the regeneration of crystalline
amorphous state, and the energy transfer might happen from states, which were clearly different from the pristine
the species of high energy excited states to the H-dimers and crystalline states (Fig. S23). We speculated that the molecules
9
b
induce the emission of the H-dimers. This mechanism may in the new crystalline phase arranged more loosely than that
not be applicable to our system, given the blue-shifted of pristine crystalline states, resulting in the weak
emission after grinding. We propose that the grinding intermolecular interactions and blue-shifted emission. Further
interrupted the highly ordered crystalline structure into small grinding the new crystalline samples induced emission spectra
pieces of aggregates with different dye-dye interactions in the red shifting to
amorphous state, leading to a reduced excitonic delocalization complexes , respectively, probably due to the phase
region compared with that of initial crystalline state. As a conversion between crystalline states and amorphous states. It
result, the emission colours blue-shifted after grinding. was important to note that repeated grinding-heating cycles of
λem = 585 nm, 540 nm and 556 nm for
1-3
Differential scanning calorimetry (DSC) of the pristine such materials induced the reversible changes of the emission
crystals exhibited strong endothermic peaks at 277 °C, 187 °C spectra. (Fig. 9d-9f).
and 322 °C, which corresponded to the melting points of
complexes
temperatures below their respect melting points (200 °C for
and , 140 °C for ) were chosen to investigate the
thermochromic properties of complexes . The pristine
crystals of showed identical emission spectra before and
1, 2 and 3, respectively (Fig. S21). So the
Conclusions
1
3
2
In summary, we have prepared a series of difluoroboron
complexes based mechanochromic luminescent materials.
1-3
1-4
1-3
These complexes exhibited excellent photophysical properties
such as strong fluorescence in both solution and solid state,
two-photon excited fluorescence, as well as good photo and
thermal stability. Their emission colours in solid state were
effectively tuned by the external stimuli, such as grinding,
after heating, indicating that the initial crystalline phase was
quite stable (Fig. S22). Besides, the pristine crystals were also
inert to solvent vapour. However, heating or solvent fuming
further induced the emission changes of the fully ground
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heating and solvent fuming to modulate the molecular packing
ways. Interestingly, the shift directions of emission bands upon
grinding were tightly related with the Initial molecular packing
ways. A highly compact packing in the initial molecular
arrangements resulted in blue-shift in the emission bands
upon grinding, which was rarely reported in literature. While
red-shifted emission was observed when the initial molecular
arrangements were in loose packing ways. We propose that
the emission of amorphous state in our system may derived
from the reduced ground-state aggregates compared with the
initial crystalline structures, which was responsible for the
blue-shifted emission after grinding. Our specific and
interesting findings may inspire chemists to design and
prepare new MCL materials.
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There are no conflicts to declare.
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This work was financially supported by National Natural
Science Foundation of China (21525206, 21561130149 and
21472202) and the 973 program (2013CB933800).
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Graphical abstracts
Mechanochromic luminescent materials that exhibit blue-shifted emission upon grinding are reported.
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