The effect of substituent number on mechanochromic luminescence of β-diketones and the corresponding boron complexes

Article abstract of DOI:10.1016/j.dyepig.2019.03.022

In this work, we designed and synthesized a series of β-diketones and the corresponding boron complexes (including mono-substituted, meta-position di-substituted and tri-substituted compounds). The effect of substituent number on their photophysical properties and mechanochromic luminescence (ML) behavior in different states was systematically studied. As verified by experimental and calculated results, the mechanical responsive properties of the six molecules were found structurally dependent, which were mainly modulated by conjugation effect and steric effect derived from different substitution. The di-substituted m-bdk and m-BF2bdk displayed the most prominent ML behavior, while the mono-substituted molecules showed almost no response to mechanical force. Moreover, the tri-substituted compounds (tri-bdk and tri-BF2bdk) showed fast self-erasing properties, highlighting the promising applications in self-erasing or self-healing devices and optical force sensors with high temporal resolution. Our study not only provides an effective strategy for modulating the optical properties, ML performance and recovery time by tuning the substituent number at molecular scale for the first time, but also facilitates the in-depth understanding of structure–property relationships in β-diketonate based dyes.

Full text of DOI:10.1016/j.dyepig.2019.03.022

Accepted Manuscript
The effect of substituent number on mechanochromic luminescence of β-diketones
and the corresponding boron complexes
Mengwei Li, Yi Han, Zidan Zhang, Xuehao He, Yulan Chen
PII:
S0143-7208(18)32822-5
https://doi.org/10.1016/j.dyepig.2019.03.022
DYPI 7411
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 22 December 2018
Revised Date: 11 March 2019
Accepted Date: 11 March 2019
Please cite this article as: Li M, Han Y, Zhang Z, He X, Chen Y, The effect of substituent number on
mechanochromic luminescence of β-diketones and the corresponding boron complexes, Dyes and
Pigments (2019), doi: https://doi.org/10.1016/j.dyepig.2019.03.022.
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ACCEPTED MANUSCRIPT

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The Effect of Substituent Number on
Mechanochromic Luminescence of
β-Diketones and the Corresponding Boron
Complexes
Mengwei Li,a,b Yi Han,a,b Zidan Zhang,c Xuehao He,a and Yulan Chen*,a,b
a
Institute of Molecular Plus, Tianjin Key Laboratory of Molecular Optoelectronic
Science, Department of Chemistry, Tianjin University, Tianjin, 300072 (P. R. China)
b
Collaborative Innovation Center of Chemical Science and Engineering, Tianjin (P.
R. China)
c Division of Polymer Chemistry and Materials, Department of Chemistry, Katholieke
Universiteit Leuven, Leuven, 3001 (Belgium)
* E-mail: yulan.chen@tju.edu.cn. Phone/Fax: +86-22-27404118
ABSTRACT
In this work, we designed and synthesized a series of β-diketones and the
corresponding boron complexes (including mono-substituted, meta-position
di-substituted and tri-substituted compounds). The effect of substituent number on
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their photophysical properties and mechanochromic luminescence (ML) behavior in
different states were systematically studied. As verified by experimental and
calculated results, the mechanical responsive properties of the six molecules were
found structurally dependent, which were mainly modulated by conjugation effect and
steric effect derived from different substitution. The di-substituted m-bdk and
m-BF2bdk displayed the most prominent ML behavior, while the mono-substituted
molecules showed almost no response to mechanical force. Moreover, the
tri-substituted compounds (tri-bdk and tri-BF2bdk) showed fast self-erasing
properties, highlighting the promising applications in self-erasing or self-healing
devices and optical force sensors with high temporal resolution. Our study not only
provides an effective strategy for modulating the optical properties, ML performance
and recovery time by tuning the substituent number at molecular scale for the first
time, but also facilitates the in-depth understanding of structure–property relationships
in β-diketonate based dyes.
Keywords: Mechanochromic luminescence; Substituent number; β-Diketones;
Difluoroboron β-diketonate; Self-erasing
1. Introduction
Mechano-responsive luminescent materials are a class of smart materials, and
have attracted considerable attention nowadays, due to their tunable luminescence in
response to external mechanical stimuli.[1-5] Up to now, various types of ML
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materials have been created, including metal, organic, polymer and liquid crystalline
compounds.[6-8] Among them, organic dyes with their intrinsic merits of diversity,
tailorability, multifunctionality, and impressive optical properties are receiving
intense research interest in the development of mechanochromic luminescent (ML)
materials.[9-11] Mostly, the ML phenomena were caused by the switching between
ordered and amorphous states.[12-14] In other cases, the ML properties resulted from
the transformation from one packing model to another,[15,16] or related to the
variation of crystal sizes.[17] Though evidence has been provided for the proposed
mechanisms, comprehensive studies for better understanding the relationship between
molecular structure and mechanochromic properties and the development of new ML
organic dyes with high performance, such as high quantum luminescent yield,
sensitive ML transformation, are highly needed, which are very critical for promotion
of their promising applications in optical sensor, memory storage, security
technologies and in biological evaluation.[18-22]
Difluoroboron β-diketonate (BF2bdk) materials are noted for their outstanding
photoluminescent properties. They have several attributes such as intense emission in
both solution and solid states, aggregation induced emission and room-temperature
phosphorescence.[23,24] The highly emissive feature of BF2bdk derivatives could be
accounted for the planar and conjugated nature of the dioxaborine core.[25] Similar to
their boronated counterparts, unique optical properties have also been observed in
boron-free β-diketone derivatives (bdk). These advantages endow them with wide
applications in various areas such as imaging and optical devices.[26,27] Besides,
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their mechano-responsive properties have attracted much attention until recently. As
pioneered by Fraser’s group, many bdk and BF2bdk dyes exhibit obvious ML
behavior in either solid or film.[28-31] Prior reports describe the effects of alkyl chain
length, halide substitution and arene size on the ML of difluoroboron
β-diketonate.[32-35] However, the majority of the BF2bdk dyes are mono-substituted.
The influence of substituent number of BF2bdk moiety was not yet considered,
although it is conceived that it is also a common strategy for modulating solid-state
ML properties through tunable conjugation and steric effects, as well as packing
style.[36] Considering all the above, we envision that introducing several β-diketone
moieties into one molecule may be a novel and efficient approach to modulate the
overall molecular conjugation and steric stress, leading to tunable solid-state optical
properties and ML behavior. To this end, we herein developed a series of β-diketones
and the corresponding boron complexes with different substituent number, including
mono-substituted, meta-position di-substituted and tri-substituted compounds
(Scheme 1). Systematical investigations of their photophysical properties and ML
properties in different states, as well as their optical recovery properties were
performed, which not only allows for a more in-depth understanding of the
relationship between molecular structure, solid-state emission and recovery time, but
also opening up an efficient avenue to develop high performance mechano-responsive
β-diketone based materials delicately controlled at the molecular level.
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Scheme 1. Structures of β-diketones and their corresponding boron complexes.
2. Experiments
2.1 Materials and instruments
Unless noted otherwise, all chemicals were purchased from Aldrich, Acros or
Adamas and used without further purification. Solvents were dried by usual methods
before use. All reactions were performed under an atmosphere of nitrogen and
1
monitored by TLC. H NMR (400 MHz) and 13C NMR (100 MHz) spectra were
recorded on a 400 MHz Bruker AV 400 spectrometer. High resolution mass spectra
(HRMS) were obtained with a miorOTOF-QII mass spectrometer (Bruker Daltonics)
or a micro mass GCT-TOF mass spectrometer. UV-vis absorption spectra were
obtained on a PerkinElmer Lambda 750 UV/VIS/NIR spectrophotometer.
Fluorescence spectra and quantum yields were recorded on a Hitachi F-7000
fluorescence spectrophotometer. The transient fluorescence decay curves of
m-BF2bdk/poly(lactic acid) blends were measured on an Edinburgh FLS 920
fluorescence spectrometer. Melting points were measured using a SGW X-4A melting
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point apparatus (Shanghai Shenguang Instrument And Meter Co., Ltd.). Films were
fabricated by spin-coating ~ 10 drops of dye (1 mM, THF) onto a piece of cover glass
o
(1 cm × 1 cm) rotating at 3000 rpm. Films were dried in vacuum at 40 C for 10 min
before measuring. KW-4A spin-coater (SETCAS Electronics Co., Ltd) was used to
cast BF2bdks films on cover glasses for fluorescence. XRD patterns were collected by
using a Rigaku D/Max-2500 diffractometer. Geometric structures of molecules were
studied theoretically in the gas phase. Density functional theory (DFT) calculations
were performed by Gaussian 09 software at the B3LYP functional with the 6-31G*
basis set level.
2.2 Synthesis
2.2.1 Synthesis of 4-dodecyloxyacetophenone
Mixture of 4-hydroxybenzaldehyde (6.8 g, 0.05 mol), 1-bromodecane (13.75 g,
0.055 mol) and potassium carbonate (17.25 g, 0.25 mol) in acetone (50 mL) was
refluxed for overnight. Inorganic salts were filtered off, and acetone was evaporated
under reduced pressure. The residue was crystallized from ethanol to afford
4-dodecyloxyacetophenone as a colorless solid (12.5 g, 82%). 1H NMR (CDC13, 400
MHz): δ 7.92 (d, J = 8.8 Hz, 2H, ArH), 6.92 (d, J = 8.8 Hz, 2H, ArH), 4.02 (t, J = 6.6
Hz, 2H, CH2), 2.55 (s, 3H, CH3), 1.80 (m, 2H, CH2), 1.46 (m, 2H, CH2), 1.40–1.22
(m, 16H, CH2), 0.88 (t, J = 6.8 Hz, 3H, CH3). 13C NMR (CDCl3, 100 MHz): δ 196.8,
163.2, 130.6, 114.1, 68.3, 31.9, 29.7, 29.6, 29.6, 29.6, 29.4, 29.1, 26.3, 26.0, 22.7,
14.1. HRMS (ESI-TOF, m/z): [M + Na]+ C20H32NaO2, calculated: 327.2295, found,
327.2287.
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2.2.2 Synthesis of s-bdk
In a two-necked flask, 4-dodecyloxyacetophenone (3.04 g, 10 mmol) and NaH (1.2
g, 30 mmol) were dissolved in THF (30 mL). After stirring for 20 min under N2, the
mixture solution was gently heated to refluxing and stirred, until the color of solution
turned deep red. A solution of methyl benzoate (1.36 g, 10 mmol) dissolved in THF
(10 mL) was added dropwise, and the reaction was monitored by TLC. After cooling
to room temperature, the reaction mixture was quenched carefully with NH4Cl (aq)
and extracted with EtOAc. The organic phase was combined, dried over anhydrous
MgSO4 and concentrated. The product was purified by column chromatography
eluting with PE/CH2Cl2 (5:1, v/v) and the residue was recrystallized in hexane to
1
afford the product as a colorless powder (2.45 g, 60%). m.p. 56.5–57.4 oC. H NMR
(CDC13, 400 MHz): δ 17.01 (s, 1H, OH), 7.97 (d, J = 8.2 Hz, 4H, ArH), 7.57–7.46
(m, 3H, ArH), 6.97 (d, J = 8.3 Hz, 2H, ArH), 6.81 (s, 1H, CH), 4.03 (t, J = 6.4 Hz,
2H, OCH2), 1.81 (m, 2H, CH2), 1.47 (m, 2H, CH3), 1.27 (m, 16H, CH2), 0.88 (t, J =
6.3 Hz, 3H, CH3). 13C NMR (CDCl3, 100 MHz): δ 186.3, 183.9, 162.9, 135.6, 132.2,
129.3, 128.7, 127.9, 127.0, 114.5, 92.4, 77.36, 77.0 , 76.7, 68.3, 32.0, 29.7, 29.7, 29.6,
29.6, 29.4, 29.4, 29.2, 26.0, 22.7, 14.2. HRMS (ESI-TOF, m/z): [M + Na]+
C27H36NaO3, calculated: 431.2557, found: 431.2563.
2.2.3 Synthesis of s-BF2bdk
s-bdk (408 mg, 1 mmol) was dissolved in CH2Cl2 (10 mL) under nitrogen, and
stirred at room temperature to give a pale yellow solution. Boron trifluoride diethyl
etherate (184 µL, 1.5 mmol) was added via syringe, and the solution turned bright
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yellow instantly. The mixture was stirred at room temperature overnight. Solvent was
removed under reduced pressure and the residue was recrystallized in CH2Cl2/hexane
to afford the product as a bright yellow solid (397 mg, 87%). m.p. 129.0–130.0 oC. 1H
NMR (CDC13, 400 MHz): δ 8.14 (t, J = 8.2 Hz, 4H, ArH), 7.68 (t, J = 7.3 Hz, 1H,
ArH), 7.55 (t, J = 7.6 Hz, 2H, ArH), 7.11 (s, 1H, CH), 7.02 (d, J = 8.3 Hz, 2H, ArH),
4.08 (t, J = 6.4 Hz, 2H, OCH2), 1.83 (m, 2H, CH2), 1.47 (m, 2H, CH2), 1.27 (m, 16H,
13
CH2), 0.88 (t, J = 6.3 Hz, 3H, CH3). C NMR (CDCl3, 100 MHz): δ 182.3, 181.3,
165.5, 134.67, 132.4, 131.7, 129.1, 128.6, 123.8, 115.1, 92.44, 68.8, 32.0, 29.7, 29.6,
29.6, 29.4, 29.4, 29.0, 26.0, 22.7, 14.2. HRMS (GCT-TOF, m/z): [M]• C27H35BF2O3,
calculated: 456.2647, found: 456.2652.
2.2.4 Synthesis of m-bdk
In a two-necked flask, 4-dodecyloxyacetophenone (5.0 g, 16.5 mmol) and NaH
(1 g, 24.8 mmol) were dissolved in THF (25 mL). After stirring for 20 min under N2,
the mixture solution was gently heated to refluxing and stirred, until the color of
solution turned deep red. A solution of dimethyl isophthalate (1.45 g, 7.5 mmol)
dissolved in THF (5 mL) was added dropwise, and the reaction was monitored by
TLC. After cooling to room temperature, the reaction mixture was quenched carefully
with HCl (1 N) until pH < 7. The solid was filtered off, and washed with EA. After
recrystallization from THF/acetone, the product was obtained as a needle crystal (1.45
g, 25%). m.p. 108–110 oC. 1H NMR (CDC13, 400 MHz): δ 17.04 (s, 2H, OH), 8.54 (s,
1H, ArH), 8.13 (d, J = 7.5 Hz, 2H, ArH), 8.00 (d, J = 7.9 Hz, 4H, ArH), 7.61 (t, J =
7.4 Hz, 1H, ArH), 6.98 (d, J = 8.1 Hz, 4H, ArH), 6.87 (s, 2H, CH), 4.05 (m, J = 5.5
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Hz, 4H, OCH2), 1.82 (m, 4H, CH2), 1.47 (m, 4H, CH2), 1.27 (m, 32H, CH2), 0.88 (t, J
13
= 6.3 Hz, 6H, CH3). C NMR (CDCl3, 100 MHz): δ 186.5, 183.1, 163.1, 136.3,
130.4, 129.5, 129.0, 127.6, 125.5, 114.5, 92.6, 68.4, 31.9, 29.7, 29.7, 29.6, 29.6, 29.4,
29.4, 29.1, 26.0, 22.7, 14.1. HRMS (GCT-TOF, m/z): [M + H]+ C48H67O6, calculated:
739.4932, found: 739.4938.
2.2.5 Synthesis of m-BF2bdk
m-bdk (739 mg, 1 mmol) and CH2Cl2 (60 mL) was added to a 2-neck round bottom
flask under nitrogen to give a pale yellow suspension solution. Boron trifluoride
diethyl etherate (368 µL, 3 mmol) was added via syringe, and the solution was heated
under reflux overnight. The solid was filtered and washed with small portion of
CH2Cl2, and the product was obtained as a yellow powder (709 mg, 85%). m.p.
o
1
254.0–255.0 C. H NMR (CDC13, 400 MHz): δ 8.77 (s, 1H, ArH), 8.38 (d, J = 7.7
Hz, 2H, ArH), 8.23 (d, J = 8.0 Hz, 4H, ArH), 7.74 (t, J = 7.8 Hz, 1H, ArH), 7.20 (s,
2H, CH), 7.06 (d, J = 8.0 Hz, 4H, ArH), 4.11 (t, J = 6.3 Hz, 4H, OCH2), 1.85 (m, 4H,
CH2), 1.49 (m, 4H, CH2), 1.27 (m, 32H, CH2), 0.88 (t, J = 6.1 Hz, 6H, CH3). HRMS
(GCT-TOF, m/z): [M + H]+ C48H65B2F4O6, calculated: 835.4898, found: 835.4919.
2.2.6 Synthesis of tri-bdk
In a two-necked flask, 4-dodecyloxyacetophenone (1.003 g, 3.3 mmol) and NaH
(198 mg, 4.95 mmol) were dissolved in THF (20 mL). After stirring for 20 min under
N2, the mixture solution was gently heated to refluxing and stirred, until the color of
solution turned deep red. A solution of trimethyl benzene-1,3,5-tricarboxylate (252
mg, 1 mmol) dissolved in THF (10 mL) was added dropwise, and the reaction was
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monitored by TLC. After cooling to room temperature, the reaction mixture was
quenched carefully with HCl (1 N) until pH < 7. The solid was filtered off, and
washed with EA. After recrystallization from THF/acetone, the product was obtained
o
1
as a pale yellow powder (42 mg, 4%). m.p. 135.5–136.5 C. H NMR (CDC13, 400
MHz): δ 17.11 (s, 3H, OH), 8.67 (s, 3H, ArH), 8.03 (d, J = 8.2 Hz, 6H, ArH), 7.00 (d,
J = 8.2 Hz, 6H, ArH), 6.94 (s, 3H, CH), 4.05 (t, J = 6.2 Hz, 6H, CH2), 1.83 (m, 6H,
13
CH2), 1.47 (m, 6H, CH2), 1.27 (m, 48H, CH2), 0.88 (t, J = 6.2 Hz, 9H, CH3). C
NMR (CDCl3, 100 MHz): δ 186.4, 182.4, 163.3, 136.8, 129.6, 128.4, 127.3, 114.54,
92.8, 68.4, 32.0, 29.7, 29.7, 29.6, 29.6, 29.4, 29.4, 29.2, 26.0, 22.7, 14.2. HRMS
(GCT -TOF, m/z): [M + H]+ C69H97O9, calculated: 1069.7127, found: 1069.7133.
2.2.7 Synthesis of tri-BF2bdk
tri-bdk (1.069 g, 1 mmol) and CH2Cl2 (100 mL) was added to a 2-neck round
bottom flask under nitrogen to give a pale yellow suspension solution. Boron
trifluoride diethyl etherate (618 µL, 5 mmol) was added via syringe, and the solution
was heated under reflux overnight. After the solvent was evaporated under reduced
pressure, the residue was washed with small portion of EA and dried in vacuum. The
o
1
product was obtained as a yellow powder (950 mg, 78.4%). m.p. 245–246.0 C. H
NMR (CDC13, 400 MHz): δ 8.90 (s, 3H, ArH), 8.27 (d, J = 9.0 Hz, 6H, ArH), 7.34 (s,
3H, CH), 7.07 (d, J = 9.0 Hz, 6H, ArH), 4.12 (t, J = 6.5 Hz, 6H, CH2), 1.84 (m, 6H,
13
CH2), 1.47 (m, 6H, CH2), 1.28 (m, 48H, CH2), 0.89 (t, J = 6.8 Hz, 9H, CH3). C
NMR (CDCl3, 100 MHz): δ 184.2, 176.7, 166.8, 134.4, 132.9, 131.9, 123.1, 115.5,
93.5, 69.1, 31.9, 29.7, 29.7, 29.6, 29.6, 29.4, 29.4, 29.0, 25.9, 22.7, 14.1.
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MALDI-TOF, m/z: calculated for C69H93B3F6O9, 1212.7003; found, 1193.4013 ([M −
F]+). HRMS (GCT-TOF, m/z): [M + BH]+ C69H94B4F6O9, calculated: 1224.7169,
found: 1224.7212.
3. Results and discussion
3.1 Synthesis and Characterization.
To investigate the influence of substituent number on the optical and ML
properties of β-diketones and their boron complexes, the mono-substituted,
meta-position di-substituted and tri-substituted β-diketones (bdks) and their
complexes (BF2bdks) were designed and synthesized (Scheme 2). All the bdks were
prepared via Claisen condensation of 4-dodecyloxyacetophenone and appropriate
esters under basic conditions, to afford s-bdk, m-bdk and tri-bdk. Subsequently, the
corresponding BF2bdks were obtained by borylation of their bdks precursors with
1
BF3·Et2O in CH2Cl2. All the compounds were unambiguously characterized with H
13
and C NMR spectra, and high resolution mass spectroscopy (HRMS). The detailed
procedures and characterization data are provided in supporting information.
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Scheme 2. Synthetic routes of bdks and BF2bdks.
3.2 Optical Properties in Solution
UV-vis absorption spectra and fluorescence emission spectra of the bdks and
BF2bdks were measured in CH2Cl2. As shown in Figure 1a, with the substituent
number increased, the absorbance peak red-shifted from 356 nm for s-bdk to 370 nm
for tri-bdk, and the corresponding emission peak shifted bathochromically from 414
nm to 425 nm. More remarkable red-shift phenomena were observed for BF2bdks
(Figure 1b). The results indicated the enhanced conjugation from the
mono-substituted β-diketone based dyes (s-bdk and s-BF2bdk) to tri-substituted
analogues (tri-bdk and tri-BF2bdk). Besides, BF2bdks exhibited a bathochromic
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shift in UV-vis absorption and fluorescence emission, and higher fluorescence
quantum yields (ФF) than bdks, which was typical for the bdks/BF2bdks systems and
in accord with the increased planarity arising from boron complexation. (Table 1).[30]
Figure 1. Normalized UV-vis absorption and fluorescence spectra of (a) bdks and (b)
BF2bdks in CH2Cl2.
Table 1. Photophysical properties of bdks and BF2bdks in CH2Cl2
3.3 Theoretical Calculations
The optimized structures and electronic distribution of frontier molecular orbitals
of bdks and BF2bdks were studied theoretically, using density functional theory
(DFT) method at the level of B3LYP/6-31G*[37-39]. As shown in Figure 2, Scheme
S1 and Table S1，the β-diketone or difluoroboron β-diketone moieties were almost
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coplanar with phenyl rings which are adjacent to dodecyloxy groups, with torsion
angles (θ2, θ2’, θ2’’) ranging from ca. 6o to 9o. The torsion angle θ1 between benzene
core and β-diketone or difluoroboron β-diketone moieties became smaller with the
increase of substituent number, from ca. 17o to 11o for bdks and 15o to 13o for
BF2bdks. Both the torsion angles (θ1’, θ1’’) of tri-bdk (tri-BF2bdk) were smaller
than those of m-bdk (m-BF2bdk), owing to the enhanced conjugation of these
C3-symetric molecules. Meanwhile, for tri-bdk and tri-BF2bdk, they have additional
substituents and torsion angles (θ1’, θ1’’) which could contribute to the distorted
molecular structures. The result indicated that tri-bdk and tri-BF2bdk may possess a
relatively loose stacking style, and we would expect such distortion and steric
hindrance would disfavor the formation of well stacked H-aggregates.[25] We also
analyzed the electronic distribution of bdks and BF2bdks. For s-bdk and s-BF2bdk,
the π electrons of the highest occupied molecular orbitals (HOMO) and the lowest
unoccupied molecular orbitals (LUMO) were mostly localized on the β-diketone
moieties and the attached phenyls. From s-bdk to tri-bdk, the area of electronic
delocalization increased, implying the increased conjugation of these molecules with
the substituent number increase, which was in good agreement with the results from
the optical measurements. The calculated band gaps of bdks decreased (s-bdk >
m-bdk > tri-bdk), which was in accord with the red-shifted threshold of their UV-vis
absorption in CH2Cl2. Similar phenomena were found for BF2bdks, particularly with
a more planar core after boron complexation. Moreover, the non-covalent interaction
(NCI) method[40] was utilized for analyzing and visualizing the weak interactions in
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stacking structures. The optimized configurations and scatter maps of reduced density
gradient of the dimers, which can be served as the simplest stacking structures, are
displayed in Figure S1 and S2, which illustrated the existence of π-π stacking
interactions and weak hydrogen bonds. The DFT calculations and the NCI analysis
were performed by Guassian 09[41] and Multiwfn[42] software, respectively.
Figure 2. Frontier molecular orbitals and energy levels of bdks and BF2bdks in the
gas phase, as calculated at the level of B3LYP/6-31G*.
3.4 Mechanochromic Luminescence
The mechanochromic luminescence (ML) behavior of these β-diketone and
difluoroboron β-diketone dyes were studied in the solid states. As the ML of bdks
showed a rapid room temperature recovery, the emission spectra were measured
immediately after the powders were ground. For s-bdk, the ground sample showed
slightly broadening emission band compared to the pristine powder (Figure S3). The
changes of its fluorescence emission were not easy to be detected by the naked eye
due to its low fluorescence quantum yield (ФF < 0.01), illustrating its lack of ML
functionality. The emission spectrum of the initially synthesized m-bdk powder
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showed two emission bands located at 472 nm and 543 nm, respectively. According
to the emission spectra of the recrystallized sample (Figure S4), the two peaks are
consistent with the emission from amorphous state and ordered crystal state,
respectively, which suggested that the amorphous and ordered m-bdk were both
involved in the as-synthesized sample. After grinding, the relative intensities of the
two peaks changed (Figure 3a). As a result, the PL intensity at 472 nm enhanced,
along with the distinct emission color changing from yellow to cyan. After fuming
with DCM, the reversible variation of the two bands occurred, indicating the partial
reversibility of its ML performance. The distinctly enhanced fluorescence quantum
yield of m-bdk (ФF = 0.117) compared to s-bdk may be attributed to the steric effect
and intermolecular O-H…O hydrogen bonds, which could restrict the intramolecular
bond rotational and vibrational freedom[43].
As for tri-bdk, the ML phenomenon was remarkably different from that of s-bdk
and m-bdk. The emission band of tri-bdk was narrow with a maximum located at 437
nm (ФF = 0.005). After the tri-bdk powder was ground, a new peak at longer
wavelength of 474 nm appeared, accompanied by the emission color changing from
blue to blue-green. The changes above could be spontaneously recovered back
quickly at room temperature. As shown in Figure 3b, d and S5, the yellow-green
emission of tri-bdk converted back to blue emission within 10 min, and the spectrum
of the recovered sample was almost the same as the original spectrum. Similar to
those of s-bdk and m-bdk, the tri-bdk film showed almost the same ML behavior with
that in the solid state, expect that its fluorescence spectrum recovered to the original
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one within 2 min, which was faster than that in the solid state. The fast recovery
properties for tri-bdk can be rationalized by invoking steric considerations. The
metastable amorphous emissive state would be under steric stress as a result of
additional substitution relative to s-bdk and m-bdk, and this could serve as the
driving force to return to the initially much less sterically hindered and more stable
state. The powder XRD patterns of the three compounds in different states were
studied to reveal the evolution of conformation and packing modes in different states
(Figure S6). Upon grinding, the XRD spectra of s-bdk and m-bdk were almost same
as that of the pristine states. Whereas for tri-bdk, the reflection peaks at 2θ < 15o were
weakened or disappeared, which indicated the molecular packing was partially
destroyed, leading to a more amorphous disordering phase.
Figure 3. Fluorescence spectra of (a) m-bdk and (b) tri-bdk in pristine powders,
ground states and after recovery by fuming the ground samples with solvent or at
ambient condition. Images of (c) m-bdk and (d) tri-bdk before and after ground under
illumination with 365 nm UV light.
Since the emission of many reported difluoroboron β-diketonate dyes in solid-state
are dependent on substrate, thermal history and thickness of the film,[28] ML
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properties of BF2bdks were studied under the consideration of these effects. Changes
in different solid-state emission can be readily reproduced mechanically. Thin films
were firstly fabricated by spin coating dilute dye/THF solution on quartz substrates,
which were treated and measured in three states: “as spun” (AS), thermally annealed
(TA), and smeared (SM) states. Annealing conditions were screened by heating the
o
AS films at the range of 100~200 C for 10 min, followed by fluorescence emission
measurement. The optimized annealing temperature for s-BF2bdk, m-BF2bdk and
o
o
o
tri-BF2bdk were determined as 110 C,[33] 200 C and 180 C, respectively (Figure
S7). After the films were annealed at the abovementioned temperature for 10 min,
blue-shift emission was achieved for s-BF2bdk and m-BF2bdk, whereas there was a
red-shift phenomenon for tri-BF2bdk, comparing with the corresponding emission
from their AS films. The different shift of emission bands indicates different
thermodynamic stable forms exisiting in their aggregates which will be discussed in
detail below. As shown in Figure 4a, after the stimulation with a cotton swab, the TA
film of s-BF2bdk showed bathochromic shift of emission maximum only about 10
nm, accompanied with broadening fluorescence emission band (Table S2). In
contrast, the fluorescence emission of m-BF2bdk red-shifted more considerably by
about 80 nm, from 494 nm to 566 nm, with emission color turned from blue-greenish
to yellow (Figure 4c, 5). As for tri-BF2bdk, the film took on unique
mechano-hypsochromic emission character (Figure 4e). In detail, the fluorescence of
tri-BF2bdk annealing film could convert from orange-red (λem = 610 nm) back to the
yellow state (λem = 570 nm) upon mechanical treatment with a cotton swab (Figure 5).
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Figure 4. Fluorescence spectra of (a) (b) s-BF2bdk, (c) (d) m-BF2bdk, and (e) (f)
tri-BF2bdk in different states. Fluorescence spectra for (a) (c) (e) in film state, and
for (b) (d) (f) in solid state.
Figure 5. UV light excited images of s-BF2bdk, m-BF2bdk, and tri-BF2bdk in
different states.
As for powder samples, variation of photophysical properties of these BF2bdks
dyes before and after grinding can be distinguished by direct visualization more
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readily. The emission maximum of s-BF2bdk in powder and ground powder states
showed little difference, only with the broadening of emission band. As for
m-BF2bdk, the as-synthesized solid exhibited two emission peaks, located at 485 nm
and 525 nm. Upon recrystallization from THF/acetone, the relative PL intensity at 485
nm increased, which implied that the high energy emission band could be attributed to
the molecular ordered stacking state and the other one corresponded to the amorphous
one (Figure S8). Upon grinding with a metal spatula, the fluorescence of m-BF2bdk
recrystallized sample red shifted to 567 nm (Figure 4d) with high color contrast ML
from cyan to yellow, indicating the appearance of amorphous state under mechanical
stimuli. The yellow state then could be converted to the yellow-green state at room
temperature, and completely back to the cyan state by fuming with DCM (Figure S9).
In fact, when m-BF2bdk was mixed in poly(lactic acid), a solid-state solvent, with
different dye loadings, the polymer films with higher dye loading display red-shifted
emission maxima (Figure S10), likely due to the exciton coupling or
“fluorophore−fluorophore interaction” as previously reported.[44] Furthermore, the
double-exponential decay in lifetime for high dye loadings (Figure S11) has also been
detected in terms of dye−dye interactions, where excimeric species have probably
formed. The excimers are more susceptible to external perturbation to endow a
sensitive chromic response, as a result, force induced changes in packing allowing for
higher amorphization, which were in accord with the larger red-shift of fluorescence
concomitant with increased ФF (from 0.19 to 0.23). The results could be further
demonstrated by XRD pattern (Figure S12). After the recrystallizd m-BF2bdk sample
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was ground, the intense and sharp reflection peaks became weakened, which implied
the ordered packing was destroyed.
On the other hand, after stimulation with a spatula, the fluorescence emission of
tri-BF2bdk powder exhibited a red-shifted emission about 10 nm. Therefore, its
packing style was not influence seriously by mechanical force, which in accord with
the almost unchanged XRD pattern (Figure S12c). Besides, not like those of
s-BF2bdk and m-BF2bdk, the XRD profiles of tri-BF2bdk are less resolved, owing
to its poor crystallization capability. Notably, the photophysical property of
tri-BF2bdk powder could be effectively affected by the annealing. Red-shifted
emission by about 40 nm was detected after the powder was annealed at 180 oC for 20
min and cooled to room temperature (Figure 6). Then the emission color changed
significantly from orange-red to orange upon grinding, and further converted back to
yellow by fuming with DCM. The different ML behavior of m-BF2bdk and
tri-BF2bdk were possible regarding their steric effect: as the absorbance maxima of
m-BF2bdk blue shifted from 445 nm (DCM) to 372 nm (AS) (Figure S13),
H-aggregates could be formed in the film state; in contrast, as for tri-BF2bdk, the
increased flexible alky chains would decrease the strength of π-π interactions in the
as-synthesized sample, which could be transformed to a thermodynamically ordered
form under annealing. Indeed, UV-vis absorbance spectra of tri-BF2bdk in AS film
state showed the maximum absorbance peak at 431 nm (Figure S14), red-shifted by
about 10 nm than that in DCM solution, indicating the formation of J-aggregates.
After the AS film was annealed, the relative intensity of the maximum absorbance
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enhanced with the ФF increased from 0.07 (AS film) to 0.30 (TA film), implying the
increased ratio of J-aggregates. Excitation spectra of tri-BF2bdk in AS and TA film
states at 600 nm showed similar results.[45] It is reasonable to speculate that the
torsion angles between aromatic rings decreased after tri-BF2bdk was annealed,
leading to the increased conjugation and red-shifted emission from yellow to
orange-red. Upon grinding, the ordered stacking was disturbed and the conjugation
decreased, which helped to explain the relatively large, nevertheless, unusual
blue-shift of fluorescence.
Figure 6. (a) Fluorescence spectra of tri-BF2bdk in powder state, TA state (T = 180 oC), ground state and
then fumed with CH2Cl2. (b) The corresponding images under illumination with 365 nm UV light.
Additionally, the force induced amorphization was further confirmed by DSC
analysis. The DSC curve of the ground m-BF2bdk displayed a crystallization
transition peak at 108 oC, suggesting that the ground powder presented in a metastable
amorphous phase and converted to a stable ordering phase via an endothermal
recrystallization process. As for tri-BF2bdk, the annealing process was helpful for
achieving a more stable state, as evidenced by the disappearance of endothermal
o
recrystallization process at 155 C, compared to the pristine powder (Figure S15).
Based on these findings, a rational assumption about the MCL mechanism of
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tri-BF2bdk was thereafter put forward as Figure 7. Contrary to tri-BF2bdk, no
considerable changes of its emission color were examined for s-BF2bdk, most
probably due the limited aromatic size presented in the molecule, leading to
insufficient change of torsion angles upon grinding.
Figure 7. Proposed assumption about ML mechanism of tri-BF2bdk
.
The current work shows that among the mono-substituted, di-substituted and
tri-substituted β-diketones and their boron complexes, substituent number plays an
important role on their optical and ML properties. The mono-substituted molecules
are almost ML inactive, while the tri-substituted ones, although with largest aromatic
size, showed limited ML performance yet fast self-erasing properties. Most prominent
ML behavior was surprisingly realized in di-substituted samples (m-bdk and
m-BF2bdk), no matter the thermal treatment process and the aggregation states. The
experiments indicate that as for the current β-diketione system, their molecular
packing modulated by aromatic effect and steric effect derived from different
substitution jointly influence the ML and recovery performance. Such understanding
of structure−property relationships in β-diketonate based dyes enables more rational
design of mechanochromic luminescent materials.
4. Conclusions
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To explore the effect of substituent number on the optical and ML properties of
β-diketones based dyes, a series of β-diketones and the corresponding boron
complexes were synthesized, including mono-substituted, di-substituted and
tri-substituted compounds. Their photophysical properties and ML behavior were
systematically studied. All the samples are emissive in solution and in the solid states,
and reversible ML processes were detected for the di- and tri-substituted samples.
Compared to the three β-diketones, their boronated counterparts presented more
efficient fluorescence emission and higher contrast ML based dyes. The mechanical
responsive properties of the six molecules were found structurally dependent, by
which the di-substituted samples (m-bdk and m-BF2bdk), regardless of the thermal
treatment process and their aggregation states, displayed the most prominent ML
behavior, while the mono-substituted molecules were non-responsive to force.
Additional substitution of the central phenyl group with a β-diketone moiety provided
highly symmetric tri-substituted compounds (tri-bdk and tri-BF2bdk), which
imparted the dyes with increased conjugation, as well as broad, red-shifted emission,
and high fluorescence quantum yields. However, they showed limited ML
performance in contrary to m-bdk and m-BF2bdk, respectively. Interestingly,
compared to mono-substituted and di-substituted compounds, their fast self-erasing
properties mainly stemming from the steric effect highlighted the promising
applications in self-erasing or self-healing devices or as an optical force sensor with
high temporal resolution.[26] Our study hence provides an effective strategy for
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modulating the optical properties, ML performance and recovery time in β-diketones
and their boronated counterparts by tuning the substituent number at molecular scale.
Acknowledgements
Financial support by the National Key Research and Development Program of
China (Grants 2017YFA0204503 and 2017YFA0207800), the National Natural
Science Foundation of China (Grants 21734006), the Thousand Youth Talents Plan
and the Natural Science Foundation of Tianjin (Grants 17JCJQJC44200) is gratefully
acknowledged.
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.org/xxx.
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