Efficient solid-state emission and reversible mechanofluorochromism of a tetraphenylethene-pyrene-based β-diketonate boron complex

Article abstract of DOI:10.1039/c9ra03847g

A new twisted donor-acceptor (D-A) dye (BF₂-TP) that was composed of tetraphenylethene and pyrene connected with a β-diketonate boron moiety has been synthesized and characterized. Such a dye showed unique intramolecular charge transfer (ICT) features, which were evidenced by spectral analysis and theoretical calculations. More importantly, BF₂-TP solid samples exhibited an obvious mechanofluorochromic (MFC) behavior. Upon grinding with a spatula, the as-prepared powder sample illustrated a remarkable red shift of 62 nm, with considerable color contrast from yellow (562 nm) to orange red (624 nm). Its fluorescence color can be reversibly switched by repeating both the grinding-fuming and grinding-annealing processes. The mechanochromism is attributed to the phase transformation between amorphous and crystalline states. The results obtained would be helpful for designing novel MFC materials.

Full text of DOI:10.1039/c9ra03847g

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Efficient solid-state emission and reversible
mechanofluorochromism of a tetraphenylethene-
pyrene-based b-diketonate boron complex†
Cite this: RSC Adv., 2019, 9, 19641
ab
Ting Sun,ab Feng Zhao,
Gaolei Xi,c Jian Gong,b Mengyu Sun,b Chang Dongb
*
and Jingyi Qiub
A new twisted donor–acceptor (D–A) dye (BF2-TP) that was composed of tetraphenylethene and pyrene
connected with a b-diketonate boron moiety has been synthesized and characterized. Such a dye showed
unique intramolecular charge transfer (ICT) features, which were evidenced by spectral analysis and
theoretical calculations. More importantly, BF2-TP solid samples exhibited an obvious
mechanofluorochromic (MFC) behavior. Upon grinding with a spatula, the as-prepared powder sample
illustrated a remarkable red shift of 62 nm, with considerable color contrast from yellow (562 nm) to orange
red (624 nm). Its fluorescence color can be reversibly switched by repeating both the grinding–fuming and
grinding–annealing processes. The mechanochromism is attributed to the phase transformation between
amorphous and crystalline states. The results obtained would be helpful for designing novel MFC materials.
Received 22nd May 2019
Accepted 10th June 2019
DOI: 10.1039/c9ra03847g
rsc.li/rsc-advances
the application of mechanical forces.8a,9 Generally, the dyes
possess nonplanar p-conjugated structures and hence afford
Introduction
a looser molecular packing pattern in the crystal states, and can
be easily damaged under external stimuli. As is well known, the
derivatives of tetraphenylethene,6a,10 triphenylacrylonitrile,11
triphenylamine,12 and cyanoethylene,13 as well as some boron
complexes14 are preferentially considered to act as MFC mate-
rials. Although the mechanisms for mechanochromism are
somewhat clear, MFC dyes with extremely large MFC shis (>60
nm) and obvious color contrast under simple mechanical force
are still rare.15 Lots of MFC materials, showing the MFC shis
within tens of nanometers, have been synthesized.10a,14,16 MFC
materials with bright solid-state emission and considerable
color contrast are crucial for their usage in real world.17 It is very
important to rationally design and synthesize such new dyes
with excellent MFC performances.
As one class of typical uorescent dyes, b-diketonate boron
complexes display impressive optical properties including high
uorescence quantum yields in the solution and in the solid
states, large molar extinction coefficients, electron mobilities,
and high electron affinities.18,19 Therefore, they possess potential
applications in laser dyes, imaging cells uorophores, organic
eld effect transistors, and organic light emitting diodes
(OLEDs).20–23 Recent studies show that some b-diketonate boron
complexes exhibited excellent MFC behavior, and numerous
derivatives belonging to this family have been described. For
example, Nguyen and co-workers24 prepared a series of b-diket-
onate boron derivatives, and the photophysical properties of
these dyes were reported to be related to the alkyl chains length.
Gao et al.25,26 synthesized a series of b-diketonate boron
complexes, the excellent photoluminescence properties in the
Mechanouorochromic (MFC) dyes refer to a kind of “smart”
material that exhibits tunable and switchable solid state uo-
rescence in response to mechanical stimuli including pressing,
grinding, crushing, or rubbing, and can be reverted to the
original state by recrystallization, thermal treatment, or expo-
sure to solvent vapors.1,2 Currently, these dyes have sparked
tremendous interest owning to their unique properties and
promising applications in the elds of optoelectronic devices,3
mechanical sensors,4 security ink,5 rewritable media,6 and
uorescent switches,7 and so on. MFC dyes convert the uo-
rescence color via changing their chemical structure or aggre-
gate morphology.8 The former is a general method to alter the
emission of a dye, but obtain limited success because of the
irreversible and incomplete chemical reactions in the
condensed phase. Most of the reported MFC dyes, the uores-
cence color change are achieved through switch of their
morphology under mechanical stimuli. These dyes can convert
from crystal phase to amorphous state, or between two different
crystalline phases upon modication of the intermolecular
interactions, and produce a meta-stable state before and aer
aSchool of Chemistry and Materials Science, Guizhou Education University, Guizhou
550000, P. R. China
bCollege of Chemistry and Chemical Engineering, Anyang Normal University, Anyang
455000, P. R. China. E-mail: zhaofeng@aynu.edu.cn
cTechnology Ceter, China Tobacco Henan Industrial Co., Ltd., Zhengzhou 450000, P.
R. China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c9ra03847g
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solid state and sensitive to mechanical forces can generally be uorescence quantum yield for the solid sample was obtained
observed. Usually, the dyes with moderate intramolecular charge using an Edinburgh FLS920 steady state spectrometer using an
transfer (ICT) properties exhibit high MFC capability. Moreover, integrating sphere. Density functional theory (DFT) calculations
it is worth mentioning that the extended p-conjugated dyes were done employing the Gaussian 09W suit of programs at the
constructed by two p-conjugated structures intersecting at a b- B3LYP/6-31G(d) level. XRD patterns were measured on a Bruker
diketonate boron core have recently attracted signicant interest D8 Focus Powder X-ray diffraction instrument.
since their rigid p-conjugated skeletons bring a range of desired
attributes. This phenomenon compelled us to explore novel MFC
dyes based on b-diketonate boron complexes. In addition, pyrene
derivatives are considered to be excellent luminogens due to their
polarity sensitive vibronic emission, high charge carrier mobility,
long uorescence lifetime, chemical stability and also possessing
large planar surface through effective p–p stacking between the
molecules.27 Herein, we designed and synthesized a novel D–A
type MFC dye BF2-TP, which composed of two chromophores of
a pyrene unit and a tetraphenylethene moiety (Scheme 1). The
steric hindrance comes from the two bulky substituents attach-
ing on the central b-diketonate boron core endows the
compound with a high torsional molecular conformation,
resulting in effective inhibition of close p-packing and
enhancement of emission quantum yields in the solid states.
Results demonstrated that BF2-TP gave strong emission in solu-
tion and in the solid state. In addition, BF2-TP exhibit reversible
mechanouorochromic behavior upon the treatment of
grinding/fuming or thermal annealing, and the emission color
changed between yellow and orange red with large MFC shis
(Dlmax ¼ 62 nm), showing a remarkable mechanochromism.
These results are important for the development of novel MFC
dyes that respond to mechanical stimulation, and extending the
viability of MFC function into organic uorescent materials.
Synthesis
Tetrahydrofuran (THF) was dried over sodium and distilled
under nitrogen immediately prior to use. Dichloromethane
(DCM) was distilled under normal pressure over sodium hydride
under nitrogen before use. The other chemicals were used as
received without further purication. Column chromatography
was carried out on silica (200–300 mesh). Compounds 3 and 5
were synthesized according to the literatures.25,28
The synthetic routes for b-diketonate boron complex BF2-TP
were shown in Scheme 2. Firstly, the tetraphenylethene deriv-
ative 3 was synthesized by Suzuki–Miyaura coupling between 1
and boric acid 2. Then the key intermediate 6 was obtained
through the Friedel–Cras alkylation reaction and the next
Friedel–Cras acyl reaction by using pyrene 4 as the starting
material. Finally, the target molecules BF2-TP was prepared
between 3 and 6 in the presence of sodium hydride in anhy-
drous THF followed by complexation with boron triuoride
diethyl etherate, to give a yield of 62%. BF2-TP has good solu-
bility in common organic solvents such as THF, DCM, chloro-
form, and toluene, but show poor solubility in aliphatic
hydrocarbon solvents (such as cyclohexane and n-hexane) and
protic solvents (such as methanol and ethanol). The interme-
diates and the nal products have been carefully puried and
fully characterized by 1H and 13C NMR, MALDI-TOF mass
spectrometry, and C, H, N elemental analyses.
Experimental
Materials and measurements
1-(2,7-Di-tert-butylpyren-4-yl)ethanone (6)
1H and 13C NMR spectra were recorded on a Bruker-Avance 400
MHz spectrometer by using CDCl3 as the solvents. Elemental
analyses were performed with a PerkinElmer 240C elemental
analyzer by investigation of C, H, and N. MS spectra were
measured on Agilent Technologies 6224 spectrometer and
MALDI-TOF MS Performance (Shimadzu, Japan). UV-visible
spectra were carried out on a Shimadzu UV-2550 spectropho-
tometer. Fluorescence measurements were measured on a Cary
Eclipse Fluorescence Spectrophotometer. The absolute
AlCl3 (12.2 g, 91.8 mmol) is added in 1,2-dichloroethane (100 mL)
followed by the addition of acetyl chloride (7.2 g, 91.8 mmol)
slowly over 30 min at 5 C. Compound 5 (9.42 g, 30.0 mmol) in
1,2-dichloroethane (40 mL) was then added slowly, and the
reaction mixture was stirred for 2 h at room temperature and kept
for 2 h at 35 ꢀC. Then the mixture was cooled to room
ꢀ
Scheme 1 The molecular structure of BF2-TP.
19642 | RSC Adv., 2019, 9, 19641–19647
Scheme 2 Synthetic routes of BF2-TP.
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temperature and the solvent was removed under reduced pres- photophysical data are summarized in Table S1.† From the UV-
sure. An orange solid appeared aer dilute HCl (200 mL) was vis absorption spectra (Fig. 1a), a relative strong absorption
added to the ask. The solid is ltered and washed with water, band at l ¼ 280–360 nm was attributed to p–p* local electron
followed by recrystallization from ethanol to give a white solid transitions of the conjugate system, which did not shi with
(9.63 g). Yield 90% mp 133.0–135.0 ꢀC. 1H NMR (400 MHz, CDCl3) increasing polarity of the deduced from the solvent-dependent
d 9.25 (s, 1H), 8.61 (s, 1H), 8.30–8.25 (m, 3H), 8.08–8.01 (m, 2H), absorption spectra. For example, in hexane, the CT band of
2.96 (s, 3H), 1.61 (s, 18H) (Fig. S3†); 13C NMR (100 MHz, CDCl3) BF2-TP centered at 411 nm and red-shied gradually with
d 201.93, 149.50, 149.06, 134.73, 132.37, 130.78, 128.89, 128.33, increasing solvent polarity, its emission band reached 429 nm
127.12, 126.69, 124.53, 123.90, 123.66, 123.52, 122.93, 121.88, in DMSO. Form the uorescence spectra (Fig. 1b), along with
35.50, 35.22, 31.94, 31.88, 29.92 (Fig. S4†); TOF LC/MS: m/z:
calculated for C26H28O: 356.2140; found: 356.2132 (Fig. S5†).
Anal. calcd. for C28H22O: C 87.60, H 7.92; found: C 87.71, H 7.83.
6-(2,7-Di-tert-butylpyren-4-yl)-2,2-diuoro-4-(4-(1,2,2-triphenyl-
vinyl)phenyl)-2H-1,3,2-dioxaborinin-1-ium-2-uide (BF2-TP)
A mixture of 3 (2.00 g, 5.13 mmol) and 6 (1.66 g, 4.66 mmol) was
dissolved in dry THF (80 mL), and then NaH (60%, 1.03 g, 25.75
mmol) was added. Under an atmosphere of nitrogen, the
mixture was reuxed with stirring for 24 h. Aer the reaction
was over, the mixture was cooled to room temperature. And
then the mixture was acidied with dilute HCl and extracted
with DCM. Aer solvent removal, the solid residue was dried
under vacuum. The obtained solid was dissolved in dry DCM (80
mL), and boron triuoride diethyl etherate (2.95 mL, 3.31 g,
23.30 mmol) was added to the solution. The mixture was stirred
for 24 h under an atmosphere of nitrogen at room temperature.
Then the mixture was poured into the water, and extracted with
DCM. Aer solvent removal, the crude product was puried by
column chromatography (silica gel, DCM/petroleum ether, v/v
¼ 2/3), affording a yellowish green solid (2.31 g). Yield 65%.
1H NMR (400 MHz, CDCl3) d 8.94 (s, 1H), 8.65 (s, 1H), 8.34 (d, J ¼
5.2 Hz, 2H), 8.29 (s, 1H), 8.10–8.04 (m, 2H), 7.98 (d, J ¼ 8.4 Hz,
2H), 7.25 (d, J ¼ 8.4 Hz, 3H), 7.18–7.15 (m, 9H), 7.06 (s, 6H), 1.60
(s, 9H), 1.59 (s, 9H) (Fig. S6†); 13C NMR (100 MHz, CDCl3)
d 186.69, 182.30, 151.96, 149.65, 149.50, 143.91, 142.88, 142.64,
139.50, 133.17, 132.20, 131.33, 131.23, 131.09, 129.65, 128.73,
128.54, 128.27, 128.05, 127.77, 127.33, 127.05, 126.62, 125.45,
124.34, 123.43, 121.09, 98.10, 35.54, 35.27, 31.84 (Fig. S8†).
HRMS (MALDI-TOF) m/z: [M + H]+ calcd. for C53H46BF2O2:
763.3559; found: 763.7036 (Fig. S9†). Anal. calcd. for
C
53H45BF2O2: C 83.46, H 5.95; found: C 83.54, H 5.81.
Preparation of the samples for mechanouorochromism study
The grinding powders were obtained by grinding the as-
prepared powder with a pestle in the mortar. The fumed
samples were prepared by fuming the grinding powder with
DCM for 2 min. The annealing samples were obtained by
ꢀ
heating the ground powders at 200 C for 5 min.
Result and discussion
Absorption and uorescence studies
Optical properties of the compound BF2-TP in solvents with
different polarities was investigated by UV-vis and uorescence
spectroscopy. The UV-vis absorption and uorescence emission
Fig. 1 Normalized UV-vis absorption (a) and PL (b, lex ¼ 420 nm)
spectra of BF2-TP in different solvents (1.0 ꢁ 10ꢂ5 mol Lꢂ1). (c) Lip-
pert–Mataga plot: fluorescence emission maximum energy of BF2-TP
spectra are demonstrated in Fig. 1, and the corresponding as a function of solvent polarity.
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the increase of the polarity of the solvent, the emission bands of
BF2-TP red-shied rapidly and became broad. In the same time,
the large Stokes shi is also observed. For example, BF2-TP
emitted blue light appeared at 507 nm in hexane and green light
located at 645 nm in DMSO with the band broadening, and was
accompanying with large Stokes shi from 4548 cmꢂ1 in hexane
to 7806 cmꢂ1 in DMSO. Combining the obvious transition to the
low-energy direction, the signicant Stokes shis, and the
extension of the emission bands indicated that ICT transitions
of BF2-TP take place in polar solvents.29 It should be mentioned
that the uorescence spectra of BF2-TP exhibited vibrational
structures in non-polar solvents, such as hexane and cyclo-
hexane, which recommends two separated close-lying excited
states. This phenomenon suggests that the locally excited (LE)
state is responsible for the emission of BF2-TP in nonpolar
solvents.30 The effects of solvents on the emission features can
be further evaluated by the relationship between the emission
maximum energy and a function of the Lippert solvent polarity
(Lippert–Mataga). It illustrated the conformational change of
the excited state surface prior to the emission.31 As shown in
Fig. 1c, the slope can be used to assess the change in dipole
moment upon excitation, and the break in the linear relation-
ship suggests the existence of two different excited states. The
deviation of the emission maximum energy in cyclohexane and
hexane from the linear relationship followed by those in other
solvents are in support of the fact that the emission of BF2-TP
origins from the LE state in non-polar solvents, and from the
ICT state in polar solvents.32 Moreover, the uorescence
quantum yield (Ff) of BF2-TP (Table S1†) showed a notable
decrease with the increasing solvent polarity. The Ff value was
from 0.369 to 0.083 when the solvent was changed from apolar
(e.g., hexane) to highly polar (e.g., DMSO), further conrming
unique ICT nature of compound BF2-TP.
To gain a better insight into optical properties of BF2-TP,
density functional theory (DFT) calculations was performed at the
B3LYP/6-31G(d) level basis in the Gaussian 09W program
package. As shown in Fig. 2, the HOMO is delocalized on the tert-
butyl substituted pyrene fragment, while the LUMO is mainly
distributed over the dioxaborine ring, as well as the tetraphe-
nylethylene group. Thus, it can be inferred from the orbital
distribution pattern that ICT occurs largely from the tert-butyl
substituted pyrene unit to the dioxaborine ring regardless of
whether tetraphenylethylene was present as a competing donor
unit. In addition, the geometry optimized structures of BF2-TP
demonstrate highly twisted spatial conformation. The dihedral
angles between tert-butyl substituted pyrene (B) and the dioxa-
borine ring (C) is 43.7ꢀ, and the dihedral angles between the side
benzene rings (A) in tetraphenylethene and the dioxaborine ring
(C) is 16.9ꢀ. Such torsional angles in BF2-TP will prevent the
molecules from packing in a close p–p stacking mode in the
solid state and thus may endow it with MFC feature.
Fig. 2 The frontier orbital contributions of BF2-TP calculated by DFT
in Gaussian 09 at the B3LYP/6-31G(d) level.
quantum efficiencies of the as-prepared solid states was 0.46.
Interestingly, aer grinding with a mortar and pestle, the
powder changed its emission color into orange red, demon-
strating a property of mechanochromic uorescence. Aer
exposure to the vapor of DCM for 2 min, the emission color of
the ground powder changed back to initial yellow and the as-
prepared powder was obtained again. The mechano-
uorochromic process is reproducible, which meant that the
uorescence transformation between yellow and orange red is
reversible switched. As shown in Fig. S1,† the dye displayed
excellent reversibility with almost no fatigue in response
throughout the six cycles. The PL spectra measurement was
applied to monitor such a reversible color switching under
external stimuli. As depicted in Fig. 3a, the emission spectrum
of BF2-TP is obvious with the initial emission centered at lmax
562 nm shiing to 624 nm aer the mechanical grinding; that
is, a spectral red-shi of 62 nm occurred simply by grinding.
Aer fuming treatment, the emission peak of the ground solid
blue-shied to the initial wavelength. It is noted that the vapor
fuming progress can be replaced by thermal annealing. When
the ground sample of BF2-TP is annealed at 200 ꢀC for 5 min, it
could recover to the emission color of the original state corre-
sponding as-prepared sample. At the same time, the recovery of
the emission spectrum of BF2-TP could also be observed by
heating its ground sample.
Mechanouorochromic properties
The crude product was puried on a silica-gel column to afford
an orange powder. The BF2-TP powder demonstrated strong
yellow emissions under 365 nm UV-light illumination, the
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Fig. 4 Photos of the luminescence writing/erasing process of BF2-TP
on filter papers under 365 nm UV light illumination: (a) fluorescence
emission of as-prepared powder; (b) the letter of “A” was written with
a spatula; (c) the paper was erased by vapor fuming or thermal
annealing; (d) rewritable the letter of “B” generated with a spatula.
of the untreated sample, indicating that the as-prepared BF2-TP
is well-ordered arrangement crystalline structure. In sharp
contrast, all of the diffraction peaks displays diffuse and
depressed reections aer grinding, verifying that the ground
Fig. 3 (a) Normalized fluorescent spectra of BF2-TP in different solid-
states: as-synthesized, grinding and fuming, lex ¼ 370 nm. (b) Photos
of BF2-TP color changes under grinding and fuming stimuli.
The experiment of time-resolved uorescence was carried
out (Fig. S2a†) and the corresponding data are illustrated in
Table S2.† The uorescence decay curves were tted by
a double-exponential decay for BF2-TP in solid states, which
revealed the mixture of two distinguished emission states in the
amorphous phase. The uorescent lifetime of ground sample
and as-prepared sample were 7.57 ns and 3.61 ns, respectively.
The longer lifetime of 7.57 ns indicted the formation of exci-
mers aer pressing.
Switchable mechanochromic dye of BF2-TP with a large shi
of 62 nm prompts us to evaluate it as a kind of smart material
with numerous potential applications. An example of such
applications is demonstrated in Fig. 4, aer being simply
pressed by streaking a metal spatula on a piece lter paper with
sprayed as-synthesized powder, an orange red letter appeared
on the yellow background due to the amorphization of BF2-TP
in the written “A” area under UV light illumination (Fig. 4b).
Interestingly, aer thermal annealing or vapor fuming the letter
“A” can be merged in the background because of the crystalli-
zation of BF2-TP in area of “A” (Fig. 4c), and the clear orange red
letter “B” can be written again (Fig. 4d). Such writing and
erasing process can be repeated many times through repeating
writing and fuming processes. The letters are invisible under
room lighting but become visible with UV illumination. On the
basis of its excellent MFC properties, BF2-TP may be utilized as
optical recording materials.
With the aim of getting insight into the mechano-induced
emission color changes, powder X-ray diffraction (XRD) was
used to study the crystalline states (Fig. 5a). Many sharp and
intense reection peaks were observed in the diffraction pattern
Fig. 5 (a) XRD patterns of BF2-TP in different solid states: as-prepared,
grinding, annealing and fuming. (b) DSC curves of BF2-TP in the
ground and the crystalline states.
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sample is amorphous. Furthermore, when fumed with DCM or
annealed, sharp reection peaks resemble to those of the as
prepared powder emerge out, suggesting the ground sample
can be readily converted back into an ordered crystalline lattice.
Accordingly to these results, the mechanochromism of BF2-TP
should be attributed to the crystalline–amorphous phase
transformations, which greatly inuences photophysical prop-
erties. Analysis by differential scanning calorimetry (DSC) also
substantiates the above claim, and the results are presented in
Fig. 5b. The as-prepared sample of BF2-TP demonstrated
a strong endothermic peak at 362 ꢀC, which corresponded to its
melt point. Aer grinding, there are two cold-crystallization
transition peaks at 167.0 C and 195.0 C before melting, indi-
cating that a metastable aggregation structure in the ground
sample is produced, and it would transfer to the more-stable
state. Aer treatment by annealing at 200 ꢀC for 5 min or
solvent fuming by DCM for 2 min, the cold-crystallization
transitions vanish and the shapes of the DSC curve is very
similar to that obtain from the as-prepared sample. These
results further conrm that the grinding treatment causes the
transition in morphology of BF2-TP, meanwhile, the
morphology transition can easily be recovered through fuming
or annealing treatment.
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Conclusion
In this work, a novel D-A structured compound BF2-TP showing
typical ICT characteristics has been successfully obtained and
potential application of BF2-TP has also been investigated. The
emission color of BF2-TP powders changed from initial yellow to
nal orange red under simple mechanical force, accompanied
with the remarkable spectral shi of 62 nm. Moreover, the
spectroscopic properties and morphological structures of the
solid BF2-TP can be smartly switched in the grinding-heating or
grinding–fuming uorochromic process. The result indicates
that the MFC behavior of BF2-TP is attributed to the reversible
phase transformation between crystal and amorphous states.
Thus, these intriguing properties enable BF2-TP for various
potential applications in security inks, rewritable sensors, and
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Conflicts of interest
There are no conicts to declare.
Acknowledgements
We are grateful to the Program for Innovative Research Team of
Science and Technology in the University of Henan Province
(18IRTSTHN004), and Anyang Normal University Program
(AYNU-2017-B16, AYNUKP-2018-B20) for nancial support of
this research.
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