Arene effects on difluoroboron β-diketonate mechanochromic luminescence

Article abstract of DOI:10.1039/c0jm04326e

Difluoroboron β-diketonate (BF₂bdk) dyes display reversible mechanochromic luminescence (ML) in the solid state. A series of BF ₂bdk dyes with methyl, phenyl, naphthyl and anthracyl groups (i.e. arene substituents and bdk ligands: Me-Ph = mbm, Ph-Ph = dbm, Np-Ph = nbm, An-Ph = abm) were prepared to test the effects of π conjugation length and arene size on ML properties. Solid-state emission spectra were recorded for powders, spin-cast films, and dye coated weighing paper. The materials emit at various wavelengths from blue to red, depending on the conjugation length. Additionally, emission spectra were recorded for smeared solids and their recovery was tracked at room temperature over time. All dyes except for BF₂mbm show emission changes upon mechanical perturbation, with increasing π conjugation correlating with more dramatic, redshifted fluorescence, however, their recovery is significantly affected by the aromatic substituents. The thermal and structural properties of the dyes in the solid state were also investigated by differential scanning calorimetry (DSC) and atomic force microscopy (AFM).

Full text of DOI:10.1039/c0jm04326e

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PAPER
Arene effects on difluoroboron b-diketonate mechanochromic luminescence†
a
a
b
Tiandong Liu, Alan D. Chien, Jiwei Lu, Guoqing Zhang and Cassandra L. Fraser
a
a
*
Received 11th December 2010, Accepted 2nd February 2011
DOI: 10.1039/c0jm04326e
Difluoroboron b-diketonate (BF2bdk) dyes display reversible mechanochromic luminescence (ML) in
the solid state. A series of BF2bdk dyes with methyl, phenyl, naphthyl and anthracyl groups (i.e. arene
substituents and bdk ligands: Me–Ph ¼ mbm, Ph–Ph ¼ dbm, Np–Ph ¼ nbm, An–Ph ¼ abm) were
prepared to test the effects of p conjugation length and arene size on ML properties. Solid-state
emission spectra were recorded for powders, spin-cast films, and dye coated weighing paper. The
materials emit at various wavelengths from blue to red, depending on the conjugation length.
Additionally, emission spectra were recorded for smeared solids and their recovery was tracked at room
temperature over time. All dyes except for BF2mbm show emission changes upon mechanical
perturbation, with increasing p conjugation correlating with more dramatic, redshifted fluorescence,
however, their recovery is significantly affected by the aromatic substituents. The thermal and
structural properties of the dyes in the solid state were also investigated by differential scanning
calorimetry (DSC) and atomic force microscopy (AFM).
in the solid state. Interestingly, BF2dbks are highly sensitive to
the surrounding environment. Changes in the temperature,13,14
solvent polarity,15 solid-state matrix,16,17 crystalline size,18 pro-
cessing conditions and solid-state morphology19,20 all cause
significant variation in dye emission, and recently, two new
solid-state luminescent properties were discovered. For example,
in poly(lactic acid) (PLA) matrices, difluoroboron dibenzoyl-
methane (BF2dbm) complexes display both molecular-
weight-dependent fluorescence and room temperature
phosphorescence,21,22 useful for ratiometric oxygen sensing and
tumor hypoxia imaging.23,24 BF2dbm dyes also show mechano-
chromic luminescence in the solid-state. For instance, the emis-
sion maxima for difluoroboron avobenzone (BF2AVB)20 are
460 nm before and 542 nm after smearing. This 82 nm shift is one
of the largest reported for ML molecules. A unique property of
BF2AVB is that its ML emission is reversible and rewritable after
smearing. BF2bdk dyes with alkyl chain substituents are also
under investigation for their ML properties,25,26 and the intro-
duction of a heavy atom allows for the production of negative
images (i.e. scratched regions are dark, due to triplet emission
quenching, or ML quenching, at room temperature).27 Struc-
ture–property studies are important to gain insight into the
mechanism of mechanochromic luminescence and to control
emission wavelengths and recovery times. BF2bdk dyes are ideal
for this purpose because a number of analogues are known and
synthetically accessible. Here, we explore the effects of simple
arene substitution on ML properties by comparing BF2
benzoylmethane systems with methyl, phenyl, naphthyl, and
anthracyl substituents, namely, BF2mbm (1), BF2dbm (2),
BF2nbm (3), and BF2abm (4) respectively. Both p conjugation
length and steric effects are probed by this series, and different
Introduction
Mechanochromism is of growing research interest because
molecules that change color in response to pressure can find
application in dynamic functional materials, sensors, and logic
gates with real-time response to mechanical stimuli. Mechano-
chromic luminescence (ML) refers to materials for which the
emission color changes upon physical perturbation such as
crushing, smearing or rubbing.1 Though ML is a rare pheno-
menon, systems based on organic compounds,2–4 inorganic
complexes,5 and polymers6 are known. Often, molecules that
respond to mechanical force are organized into flexible assem-
blies, such as colloids, gels, or liquid crystals.7,8 Packing profiles
reveal intermolecular interactions such as hydrogen bonding,8
p–p stacking,9 or aurophilic interactions10 but in certain cases,
covalent bonds are also involved.11,12 Applying forces to these
molecular solids can affect their packing morphology or induce
a breakage or formation of new intermolecular interactions or
chemical bonds. Though several mechanistic models have been
advanced, structure–property relationships are still not well
understood for most mechanochromic luminescent materials.
Difluoroboron b-diketonate (BF2bdk) complexes are well-
known fluorophores with intense emission both in solution and
aDepartment of Chemistry, University of Virginia, McCormick Road,
Charlottesville, VA, 22904, USA. E-mail: fraser@virginia.edu; Fax: +1
434-924-3710; Tel: +1 434-924-7998
bDepartment of Materials Science, University of Virginia, McCormick
Road, Charlottesville, VA, 22904, USA. E-mail: jl5tk@virginia.edu
† Electronic supplementary information (ESI) available: Additional
emission spectra for powders, unannealed films and crystals, melting
points. See DOI: 10.1039/c0jm04326e
This journal is ª The Royal Society of Chemistry 2011
J. Mater. Chem., 2011, 21, 8401–8408 | 8401

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processing conditions were tested. Samples were explored in
different forms, as crystals or powders obtained from synthesis
on quartz substrates, as more amorphous films spin cast from
CH2Cl2 solutions onto glass, and as solvent cast films on
weighing paper. Effects of thermal annealing and mechanical
perturbation on emission and visual appearance were also
studied.
(20 mL) were added sequentially to a 50 mL round bottom flask.
After stirring the mixture for 10 min, a suspension containing
NaH (167 mg, 6.61 mmol) in THF (10 mL) was added dropwise
at room temperature under N2. The mixture was stirred for 20 h
before saturated aqueous NaHCO3 (1 mL) was added to quench
the reaction. THF was removed in vacuo before 1 M HCl (20 mL)
was added. The aqueous phase was extracted with CH2Cl2 (3 ꢃ
20 mL). The combined organic layers were washed with distilled
water (2 ꢃ 10 mL) and brine (10 mL), and dried over Na2SO4
before concentration in vacuo. The residue was purified by
column chromatography on silica gel eluting with hexanes/ethyl
acetate (6 : 1) to give 2-naphthoyl benzoylmethane, nbm, as
a yellow solid: 700 mg (62%). 1H NMR (300 MHz, CDCl3)
d016.98 (s, 1H, ArCOH), 8.62 (s, 1H, 10-ArH), 8.08–7.87 (m, 6H,
3 ,40,50,80-ArH. 200,600-ArH), 7.62–7.49 (m, 5H, 60,70-ArH.
300,400,500-ArH), 7.01 (s, 1H, COCHCO); MS (MALDI) m/z ¼
275.13 (M + H+), calcd m/z ¼ 275.11.
Experimental
Materials
Solvents CH2Cl2 and THF were dried and purified by passage
through alumina columns. Acetophenone, BF3$Et2O (purified,
redistilled), and all other chemicals were reagent grade from
Sigma-Aldrich and were used as received without further puri-
fication.
Measurements
2-Anthracenoyl benzoyl methane (abm). The anthracyl ligand
was prepared from 2-acetyl anthracence (301 mg, 1.34 mmol)
and methyl benzoate (252 mL, 2.01 mmol) as described for nbm
except that 3 equivalents of NaH were used and the reaction was
refluxed for 12 h. After purification by column chromatography
(3 : 1 hexanes/EtOAc to remove impurities, then CH2Cl2 to elute
the product), the diketone, abm, was obtained as a red solid: 362
mg (84%). 1H NMR (300 MHz, CDCl3) d 16.96 (s, 1H, ArCOH),
8.75 (s, 1H, 10-ArH), 8.60 (s, 1H, 100-ArH), 8.46 (s, 1H, 90-ArH),
8.10–7.95 (m, 6H, 30,40,50,80-ArH. 200,600-ArH), 7.61–7.50 (m, 5H,
60,70-ArH. 300,400,500-ArH), 7.01 (s, 1H, COCHCO); MS (MALDI)
m/z ¼ 324.07 (M + H+), calcd m/z ¼ 324.12.
1H NMR (300 MHz) spectra were recorded on a Varian Unity
Inova spectrometer in CDCl3 unless otherwise indicated. Reso-
nances were referenced to the signal for residual protio chloro-
1
form at 7.260 ppm. H NMR coupling constants are given in
hertz. Mass spectra were recorded using an Applied Biosystems
4800 spectrometer with a MALDI TOF/TOF analyzer. Melting
points were measured using a Laboratory Device Mel-Temp II
instrument. Differential scanning calorimetry (DSC) measure-
ments were performed using a TA Instruments DSC 2920
modulated DSC. Analyses were carried out in modulated mode
under a nitrogen atmosphere (amplitude ¼ ꢀ1 ꢁC; period ¼ 60 s;
heating rate ¼ 5 C minꢂ1; range ꢂ10 to 300 C). The reported
Tm and Tc values are from the first heating cycle and peak
maximum values are reported. Solid state fluorescence emission
spectra were recorded on a Horiba Fluorolog-3 Model FL3-22
spectrofluorometer (double-grating excitation and double-
grating emission monochromators). A Laurell Technologies WS-
650S spin-coater was used to cast BF2dbks films on cover glasses
for fluorescence, X-ray diffraction, and AFM measurements.
Approximately 10 drops of CH2Cl2 solutions (1 mg mLꢂ1) were
spun for 1 min at a rotator speed of 2000 rpm. Films on weighing
paper were prepared by applying several drops of CH2Cl2 sample
solution (1 mg mLꢂ1) onto the paper substrate and allowing the
surface to dry under air. The weighing paper was then taped to
the outside of a glass vial for ease of smearing and luminescence
analysis. The photograph in Fig. 3 was taken with an Apple
iPhone 4 digital camera with the default setting (no flash).
Tapping mode AFM (DI 3000, Digital Instrument, CA) was used
to characterize the morphology of spin cast films with a scan rate
of 0.8 Hz over an area of 20 ꢃ 20 mm.
ꢁ
ꢁ
Difluoroboron-diketonate synthesis
A representative synthesis is provided for difluoroboron 2-
naphthoyl benzoyl methane (BF2nbm) (3).
Difluoroboron 2-naphthoyl benzoyl methane, BF2nbm (3).
Boron trifluoride diethyl etherate (88 mL, 0.70 mmol) was added
to a solution of 2-naphthoyl benzoyl methane (191 mg, 0.70
mmol) in CH2Cl2 (20 mL) under N2. After stirring the mixture at
room temperature for 12 h, the solvent was removed in vacuo.
(Note: under reflux, boronation reactions are complete after ꢄ1
to 2 h.) The residue was recrystallized from acetone to give
BF2nbm as a yellow powdery solid: 147 mg (65%). Data are in
accord with literature values.10 1H NMR (300 MHz, CDCl3)
d 8.80 (s, 1H, 10-ArH), 8.22 (d, J ¼ 8.1, 2H, 200,600-ArH), 8.10–7.92
(m, 4H, 30,40,50,80-ArH), 7.73–7.59 (m, 5H, 60,70,300,400,500-ArH),
7.35 (s, 1H, COCHCO); MS (MALDI) m/z ¼ 345.00 (M + Na+),
calcd m/z ¼ 345.09.
Difluoroboron acetyl benzoyl methane, BF2mbm (1). Pale
yellow needle-like crystals precipitated from acetone/hexanes.29
1H NMR (300 MHz, CDCl3) d 8.08–8.04 (m, 2H, 20,60-ArH),
7.72–7.66 (m, 1H, 40-ArH), 7.56–7.50 (m, 2H, 30,50-ArH), 6.59 (s,
1H, COCHCO), 2.42 (s, 3H, CH3CO); MS (MALDI) m/z ¼
233.02 (M + Na+), calcd m/z ¼ 233.06.
Synthesis of b-diketones
The methyl (mbm), naphthyl (nbm) and anthracyl (abm) b-
diketone ligands were prepared by Claisen condensation in the
presence of NaH as previously described.28 (Note: mbm and dbm
are commercially available from Aldrich.)
2-Naphthoyl benzoyl methane (nbm). Acetophenone (500 mg,
Difluoroboron dibenzoyl methane, BF2dbm (2). Pale yellow
4.12 mmol), methyl-2-naphthoate (1.03 g, 5.36 mmol) and THF
powder precipitated from acetone/hexanes. Data are in accord
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Table 1 Emission maxima and bandwidths for powders 1–4 on quartza
with literature values.15 1H NMR (300 MHz, CDCl3) d 8.19–8.15
(m, 4H, 20,60-ArH, 200,600-ArH), 7.74–7.68 (m, 2H, 40-ArH, 400-
ArH), 7.60–7.54 (m, 4H, 30,50-ArH, 300,500-ArH), 7.21 (s, 1H,
COCHCO); MS (MALDI) m/z ¼ 295.06 (M + Na+), calcd m/z ¼
295.07.
BF2bd
lP
lTA
lS
fwhmP
fwhmTA
fwhmS
BF2mbm (1)
BF2dbm (2)
BF2nbm (3)
BF2abm (4)
433
531
512
643
429
529
511
622
434
534
515
634
87
100
63
87
110
63
90
101
95
117
115
117
Difluoroboron 2-anthracenoyl benzoyl methane, BF2abm (4).
Boron trifluoride diethyl etherate (210 mL, 1.67 mmol) was added
dropwise to a stirred solution of 2-anthracenoyl benzoyl methane
(abm) (362 mg, 1.12 mmol) in CH2Cl2 (20 mL) under N2. After
stirring the mixture at room temperature for 12 h, saturated
aqueous NaHCO3 (1 mL) was added to quench the reaction.
(Note: under reflux, boronation reactions are complete after ꢄ1
to 2 h.) The aqueous phase was extracted with CH2Cl2 (3 ꢃ 20
mL) and the combined organic layers were washed with distilled
water (2 ꢃ 10 mL) and brine (10 mL), and dried over Na2SO4
before concentration in vacuo. The residue was recrystallized
from acetone to give BF2abm as a red powdery solid: 186 mg
(42%). 1H NMR (300 MHz, CDCl3) d 9.02 (s, 1H, 10-ArH), 8.65
(s, 1H, 90-ArH), 8.47 (s, 1H, 100-ArH), 8.23–7.97 (m, 6H,
30,40,50,80-ArH. 200,600-ArH), 7.74–7.52 (m, 5H, 60,70,300,400,500-
ArH), 7.36 (s, 1H, COCHCO); MS (MALDI) m/z ¼ 395.05 (M +
Na+), calcd m/z ¼ 395.10.
a
P ¼ powder; TA ¼ thermally annealed; S ¼ smeared; fwhm ¼ full width
at half maximum. Units are shown in nm.
the fluorescence colors for the films range from blue to red in this
series (1: blue; 2: yellow-green; 3: green; 4: red), further demon-
strating that solid-state emission color is readily modulated by
arene substitution. Emission from the anthracene derivative 4 is
very dim compared to the other analogues; hence, the films look
dark in Fig. 1. Emission maxima, lem, span a ꢄ200 nm range for
this film sample set: BF2mbm: 434 nm, BF2dbm: 534 nm,
BF2nbm: 502 nm, and BF2abm: 641 nm. Emission colors on
weighing paper are comparable to spin-cast films on glass, and
qualitative tests with filter paper as a substrate, commonly used
for vapochromism studies,30 are also similar.
Generally, emission maxima show a bathochromic shift with
extended p conjugation of the aromatic moieties; however, the
phenyl-naphthyl derivative 3 emits at a shorter wavelength than
the diphenyl dye 2, which is an exception to this trend. There is
growing evidence in solution and computational studies to
suggest that the entire ligand structure influences emission for
symmetrical structures like dbm; whereas, in unsymmetrical
structures, it is the larger arene donor that dominates in intra-
molecular charge transfer (ICT) processes.24 These results
suggest that this may extend to solid state emission as well;
however here, differential intermolecular interactions and dye–
dye stacking between dbm and nbm systems could also be
playing an important role. In this case, BF2dbm may have
stronger intermolecular interactions (ground- and excited-state
dimers) than BF2nbm.
Results and discussion
Dye synthesis
Boron dyes 1–4 were synthesized via a previously reported
method.23 Namely, the diketone ligand precursors were obtained
by Claisen condensation between appropriate ketones and esters,
using NaH as the base to deprotonate the ketones in all cases.
The anthracene ligand, abm, requires three equivalents NaH and
refluxing conditions for 12 hours, whereas the other ligands can
be produced at room temperature after 12 hours or less with
lower base loadings (i.e. 1.5 equivalents vs. the ketone). Ligands
show good solubility in common organic solvents such as
CH2Cl2 and EtOAc, with the exception of abm which shows poor
solubility in EtOAc. The reaction between boron trifluoride
etherate (BF3$Et2O) and the corresponding b-diketones in
refluxing dichloromethane generates the difluoroboron-diketo-
nate products 1–4 within two hours, in moderate yield after
recrystallization (e.g. 3: 65% yield). Sample 1 was obtained as
needle-shaped crystals and 2–4 as powders.
Thermal annealing
The melting point and melting and crystallization temperatures
(ꢁC) of the dyes were measured by DSC (Table 3). Temperatures
increase with molecular weight as expected. No additional phase
transitions were noted.
Spin-cast films of 1–4 were annealed at 110 ꢁC for 10 minutes
and associated normalized emission spectra are provided in
Fig. 1B. The spectrum for the methyl derivative BF2mbm, 1,
Solid-state emission color
For an initial comparison of fluorescence colors, dyes were
investigated as powders, spin-cast films on glass, and solvent cast
films on weighing paper. As isolated, solid-state boron dyes 1–4
(lem, nm) show cyan (433; crystals), green-yellow (531, with high
energy peak at 446; microcrystalline powder), green (512;
Table 2 Emission maxima and bandwidths for spin-cast films 1–4 on
glassa
powder), and red (643; powder) emission respectively (lex
¼
BF2bdk
lA
lTA
ls
fwhmA
fwhmTA
fwhmS
365 nm) (Table 1, Fig. S1, ESI†). Amorphous films of dyes 1–4
were generated by spin casting methylene chloride solutions (10
drops, 1 mg mLꢂ1) onto glass (1 ꢃ 1 in). After air drying for at
least two hours, spectra were recorded under ambient atmo-
sphere (Table 2, Fig. 1). More sample (20 drops, 1 mg mLꢂ1) was
used to generate films for the images to better visualize the colors
and their changes upon thermal annealing. Fig. 1 illustrates that
BF2mbm (1)
BF2dbm (2)
BF2nbm (3)
BF2abm (4)
434
534
502
641
435
531
487
584
433
541
516
636
83
100
96
79
109
76
82
102
109
117
123
162
a
A ¼ amorphous; TA ¼ thermally annealed, S ¼ smeared; fwhm ¼ full
width at half maximum. Units are shown in nm
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connecting the centers of the adjacent dye molecules and the long
axis of a molecule.31 If the angle is small (e.g., less than 32ꢁ for
cyanine dyes), aggregation will induce a bathochromic shift in
absorption and emission, which is called J-aggregates. Aggre-
gation causes a hypsochromic shift when the angle is larger than
32ꢁ for cyanine dyes and is attributed to H-aggregates. The
previously reported crystallography data of BF2dbm8 revealed
a slippage angle of ꢄ25ꢁ. If this small angle is associated with
J-aggregates for this system too, it could maybe explain the
emergence of the small peak at 440 nm that arises upon thermally
annealing and is bathochromically shifted relative to monomer
emission. The hypsochromic shift of the main emission peak for
BF2dbm after heating may also be ascribed to aggregation in the
crystalline structures that form. A dramatic blue shift was
observed previously for crystalline BF2AVB20 and
BF2dbmOC12H25 states.25
Previously, it was reported that BF2dbm tends to form
different stacking profiles. Two types of arrangements are
common.8,32 In the first case, the phenyl rings of adjacent
BF2dbm molecules overlap. In the second case, the diketonate
and a phenyl ring of two adjacent molecules stack. In both cases,
adjacent boron diketonates moieties face in opposite directions.
When the dye solids were heated, the distance between the
overlapping molecules increased,8 which implies that the
observed spectral changes may be associated with a change in
interdimeric interactions. Reasons for these arrangements are
not known, but the strong electron withdrawing difluoroboron
group may function as a directing unit and arranging large
dipoles opposite to each other may increase the distance, and
lower the repulsion and overall energy of the system. Compared
to BF2dbm, the emission spectrum of BF2nbm is more narrow;
the lower energy side of the peak is absent. An amorphous spin-
cast BF2nbm has a major peak at 502 nm accompanied by
a shoulder at 487 nm, which may arise from the amorphous and
ordered aggregate emission respectively. The monomeric emis-
sion of BF2nbm in methylene chloride solution is at 483 nm,
which could also be a contributing factor in the solid state. Here
too, the relative intensity of the blueshifted peak was enhanced
after thermal annealing and the full width at half maximum
(fwhm) narrowed from 96 nm to 76 nm for the spin-cast film,
similar to previous observations with BF2AVB.20
Fig. 1 Solid-state emission spectra and images of 1–4 as films spin-cast
from CH2Cl2 solutions (1 mg mLꢂ1) onto glass. (A) Before (amorphous)
and (B) after thermal annealing. (1–3: lex ¼ 369 nm; 4: lex ¼ 397 nm.)
Table 3 Melting and crystallization temperatures for
boron dyes 1–4
a
a
Tc
BF2bdk
Tm
In contrast to the nbm complex, BF2abm exhibited a dramatic
hypsochromic shift after annealing, so differences do not arise
from size alone. For example, for the amorphous film of 4, the
emission maxima shifted from 641 nm to 584 nm (57 nm), the
peak broadened considerably, with an increased intensity blue
shoulder for BF2abm. Here too, the dramatic hypsochromic shift
may originate from aggregation. (Compare to BF2abm in dilute
solution: lem ¼ 595 nm.)
BF2mbm (1)
BF2dbm (2)
BF2nbm (3)
BF2abm (4)
157
193
228
254
130
181
210
214
a
DSC; peak values.
showed little change upon annealing, but the intensity decreased.
The emission spectrum of BF2dbm, 2, slightly blue shifted (ꢄ2 to
3 nm), broadened (fwhm increased ꢄ9 to 10 nm), and a new peak
emerged at ꢄ440 nm (Tables 1 and 2). In previous studies, the
short wavelength band has been attributed to monomeric emis-
sion, which authors say is more prominent in larger crystals.8
(Compare to BF2dbm lem ¼ 417 nm in CH2Cl2.) However,
aggregate emission states may be a more likely explanation. The
aggregation behavior of dyes has been extensively studied and
has been classified into H-aggregates and J-aggregates based on
the slippage angle, which is defined as the angle between the line
To gain further insight into the solid-state morphology and
ways that samples change upon heating, atomic force micros-
copy (AFM) was employed (Fig. 2). As-spun films had similar
ring-like morphologies upon evaporation of solvent by spin-
casting. Data confirm the aggregation of compounds 1–3 after
thermal treatment. BF2mbm may sublime or decompose during
thermally annealing, consistent with decreased emission intensity
in the image in Fig. 1B, however, it still formed larger bead-
shaped species. For BF2dbm and BF2nbm, square- and triangle-
shaped crystalline species were observed respectively. BF2abm
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were smeared (Fig. S2, ESI†). For example, the crystalline
BF2dbm has two emission peaks at 446 nm and 522 nm respec-
tively. However, only a broader longer wavelength emission still
existed when the crystals were crushed. Mirochnik attributed this
to size-dependent fluorescence in solid state BF2dbks. The short
wavelength peak was assigned to the monomeric emission, which
only appeared in large-sized crystals and the long wavelength
peak was assigned to interdimeric associations.12 An alternative
explanation is that the original sample has both ordered and
amorphous components. When the sample is crushed, the short
wavelength peak associated with crystalline aggregate emission
disappears and a more disordered, amorphous state is generated,
which gives rise to a broad, redshifted emission.
In attempts to distinguish mechanochromism from size
dependent crystalline emission, 1–4 powders were applied to the
surface of a quartz cuvette and the emission properties were
investigated after smearing (Fig. 3). Thermally annealed spin-
cast films were also monitored after smearing for comparison
(Fig. 4). These experiments show that aromatic groups influence
BF2bdk ML properties—the presence or absence of mechano-
responsiveness, the emission wavelength shift, and the recovery
rate—in measurable ways. Processing methods can play a role
too. Mechanochromic effects of each dye, 1–4, will be discussed
in turn below.
The emission spectrum of solid BF2mbm, 1, has a major peak
at 433 nm accompanied by a shoulder peak at 459 nm (Fig. 3A),
the shoulder emission of the powder species only slightly
increasing after thermally annealing at 110 ꢁC for 10 min.
Mechanical perturbation removed the enhancement of the
shoulder peak, and resulted in an emission spectrum that
matches the less ordered sample before annealing. Amorphous
films of BF2mbm are even less responsive to mechanical force
(Fig. 4A). Emission spectra are nearly identical for spin-cast,
annealed and smeared films. Correspondingly, for films too, no
visible color change is noted. Thus, under the conditions tested,
BF2mbm cannot be considered mechanochromic.
The ML behaviors of BF2dbm are shown in Fig. 3B and 4B.
The emission spectra of smeared BF2dbm powders and films
were nearly identical to respective amorphous powder and film
samples before annealing. The short wavelength emission that
emerged after annealing disappeared, which may be interpreted
as the loss of the aggregated species. In tracking the emission of
the smeared samples over time, they kept blueshifting gradually
until the main emission overlapped with that of the ordered,
thermally annealed sample. Notably, recovery dynamics of
BF2dbm powders and films have significant differences. The films
on glass recovered in less than 2 hours, whereas the powders on
the quartz cuvette took up to a day to return to the initial ther-
mally annealed state. Clearly, BF2dbm aggregated and formed
a thermally preferred structure, which induced the hypsochromic
shift. Mechanical force disrupted the organization in the ther-
mally stable structure. This process is represented by the red-shift
in emission after the smearing.
Fig. 2 AFM images of BF2mbm (1), BF2dbm (2), BF2nbm (3), and
BF2abm (4) spin-cast films on glass substrates. A ¼ as-spun films and B ¼
after thermal annealing.
samples formed larger rings after annealing, but it is not as
obvious visually that crystalline species emerged. Generally
speaking, compounds 2–4 show increased crystallinity, greater or
lesser hyposochromic shifts and a new emission peak at short
wavelength after thermally annealing. H aggregates may be
involved.
For BF2nbm, mechanochromic luminescent effects are man-
ifested differently. The emission maximum (lem ¼ 513 nm) of the
powder on a quartz substrate did not change before and after
annealing (Fig. 3C); however, the emission peak was drastically
broadened after smearing. The fwhm increased from 62 nm to 96
nm. In the first ten minutes after smearing, the bandwidth
Mechanochromic luminescence
Mechanical responsiveness of the dyes was also tested for crys-
talline and powdery solids obtained from synthesis as well as for
thermally annealed spin-cast films. BF2mbm and BF2dbm crys-
talline solids showed significant changes in emission when they
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Fig. 3 Normalized emission spectra of 1–4 powders on quartz. (A)
Fig. 4 Normalized emission spectra of amorphous films of 1–4 on cover
glass. (A) BF2mbm, lex ¼ 369 nm. (B) BF2dbm, lex ¼ 369 nm. (C)
BF2nbm, lex ¼ 369 nm. (D) BF2abm, lex ¼ 397 nm. (AS ¼ as-spun film;
TA ¼ thermally annealed film; times indicate after smearing; m ¼ minute,
h ¼ hour, d ¼ day).
BF2mbm, lex ¼ 369 nm. (B) BF2dbm, lex ¼ 369 nm. (C) BF2nbm, lex
¼
369 nm. (D) BF2abm, lex ¼ 397 nm. (AI ¼ as isolated powder; TA ¼
thermally annealed powder; times indicate after smearing; m ¼ minute, h
¼ hour, d ¼ day).
narrowed slightly, but after that, the spectra remained
unchanged after monitoring for two days. Of the difluoroboron
dyes that we have investigated in this way, BF2nbm is the first
one that is largely stable after smearing. Clearly, the aromatic
substituent is not just influencing the emission maxima, but the
recovery dynamics too.
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Table 4 Lifetimes of spin-cast films of 1–4 on cover glass
samples may not be fully amorphous at room temperature. This
was verified by XRD, that indicated some degree of crystallinity
for as-spun films of 1, but not for 2–4. Generally, emission
maxima redshift with increasing arene conjugation and after
smearing, the shift to lower energy becomes more distinct. The
dye containing the naphthyl unit showed unrecoverable emission
after mechanical perturbation, whereas the emissions of smeared
BF2dbm and BF2abm move towards short wavelength and
resemble the initial thermally annealed state over time. The
amorphous film species recover faster than the powders, which
may be a sample thickness effect. For these samples, thermal
annealing is the key step to observing mechanochromic lumi-
nescence, in a pronounced way or at all. When unannealed
samples are smeared, little change is observed in the spectra.
AFM studies revealed that 2–4 aggregated during annealing and
formed crystalline species, which are associated with the emer-
gence or increase of a higher energy emission band, perhaps due
to aggregate emission. The lifetimes of the smeared samples and
original amorphous film samples are in accord, which implies
that smearing also generates an amorphous state. Overall, the
formation and disappearance of the aggregated species appears
to be the reason for the luminescence variation upon thermally
annealing and mechanical perturbation, and strong intermole-
cular interactions may be the driving force for thermally induced
aggregation and spontaneous recovery at room temperature after
smearing.20
a
a
a
BF2dbk
sA
sTA
sS
BF2mbm (1)
BF2dbm (2)
BF2nbm (3)
BF2abm (4)
8.9
51.5
15.0
5.0
—
—
48.6
11.9
7.3
51.3
15.5
3.0
a
A ¼ amorphous, TA ¼ thermally annealed, S ¼ smeared. Units are
shown in ns
The dye with the even larger anthracene ring, BF2abm, showed
the most dramatic emission change upon smearing. The emission
maximum shift from 622 nm to 634 nm for the powder sample
(Fig. 3D) and from 584 nm to 636 nm for the thermally annealed
spin-cast film (Fig. 4D). Whether starting from the powder or the
film, smeared samples revert to the starting thermally annealed
emission, but the recovery rates are distinctly different. It takes
up to a day for the powder sample to recover, but only an hour
for the smeared spin-cast BF2abm film to return to the initial. In
both cases, the shoulder emission does not reappear, also sug-
gesting that it arises from thermally induced aggregation.
Thermal annealing is a key factor in observing ML for this
arene series. When mechanical perturbation was applied to
unannealed spin-cast 1–4 films on cover glass, their emission
spectra showed much less change compared to the annealed
samples (Fig. S3, ESI†). As has been observed previously for
BF2dbmOC12H25, spin-cast films and mechanically smeared
samples show spectral similarity, suggesting that both are
amorphous states. Multiple interactions, such as strong dipolar
nature of the molecules, arene stacking and Ar–H/F hydrogen
bonds, may be the driving force for the aggregation of BF2mbm,
BF2dbm, and BF2nbm, upon thermal treatment or recovery after
smearing.
In this study, solid-state dyes with different arene substituents
were explored. Numerous questions remain regarding steric and
electronic effects on mechanochromic luminescence properties.
Furthermore, substrate effects and quantifying the sensitivity of
the dyes to force also merit investigation. These topics, ways of
modulating the luminescence response, and uses for mechano-
chromic luminescent materials will serve as the subjects of future
reports.
The lifetimes of the spin-cast samples were also monitored
(Table 4). For BF2dbm and BF2nbm, the lifetimes slightly
decreased after annealing. But the smeared samples have nearly
identical lifetimes as the original samples, as could be expected,
given both are more amorphous states. BF2abm undergoes
a different process. Compared to the initial amorphous spin-cast
state, the lifetime increased after annealing but then decreased
significantly after smearing. Taken together, the AFM and
emission data point to different behavior for BF2abm after
thermal treatment and smearing. The reasons for this are not
clear.
Acknowledgements
We thank the National Science Foundation for support for this
research (CHE 0718879) and we thank the Beckman Foundation
for a Beckman Scholarship to Alan D. Chien and generous
support of undergraduate research at UVA.
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