Mechanofluorochromism of NIR-emitting dyes based on difluoroboron β-carbonyl cyclic ketonate complexes

Article abstract of DOI:10.1016/j.dyepig.2018.03.069

New difluoroboron β-carbonyl cyclic ketonate complexes CnB (n = 2, 16), in which carbazole was linked to difluoroboron β-diketonate directly, and DCnB (n = 2, 16), where two terminal carbazole units were bridged by vinyl groups to link to difluoroboron β-

Full text of DOI:10.1016/j.dyepig.2018.03.069

Accepted Manuscript
Mechanofluorochromism of NIR-emitting dyes based on difluoroboron β-carbonyl
cyclic ketonate complexes
Fushuang Zhang, Lu Zhai, Siwei Feng, Wenhua Mi, Jingbo Sun, Meng Sun, Jinyu
Zhao, Ran Lu
PII:
S0143-7208(18)30315-2
10.1016/j.dyepig.2018.03.069
DYPI 6654
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 11 February 2018
Revised Date: 27 March 2018
Accepted Date: 28 March 2018
Please cite this article as: Zhang F, Zhai L, Feng S, Mi W, Sun J, Sun M, Zhao J, Lu R,
Mechanofluorochromism of NIR-emitting dyes based on difluoroboron β-carbonyl cyclic ketonate
complexes, Dyes and Pigments (2018), doi: 10.1016/j.dyepig.2018.03.069.
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ACCEPTED MANUSCRIPT
Mechanofluorochromism of NIR-emitting dyes based on
difluoroboron β-carbonyl cyclic ketonate complexes
1

ACCEPTED MANUSCRIPT
Mechanofluorochromism of NIR-emitting dyes based on
difluoroboron β-carbonyl cyclic ketonate complexes
a
a
b
a
a
a
Fushuang Zhang, Lu Zhai, Siwei Feng, Wenhua Mi, Jingbo Sun, Meng Sun,
a
*,a
Jinyu Zhao and Ran Lu
a
State Key Laboratory of Supramolecular Structure and Materials, College of
Chemistry, Jilin University, Changchun 130012, P. R. China
b
College of Life Sciences, Jilin University, Changchun 130021, P. R. China
Abstract
New difluoroboron β-carbonyl cyclic ketonate complexes CnB (n = 2, 16), in which
carbazole was linked to difluoroboron β-diketonate directly, and DCnB (n = 2, 16),
where two terminal carbazole units were bridged by vinyl groups to link to
difluoroboron β-diketonate core, were synthesized. They exhibited reversible
mechanofluorochromic (MFC) behavior under grinding/fuming or heating treatment.
Particularly, the emission of DC2B and DC16B emerged in the range of 650-850 nm
in the solid-state. They gave red and rose-red luminescence in the as-synthesized
crystals and in ground powders, respectively, during MFC processes. Such MFC
materials emitting NIR (near-infrared) light were seldom reported. In addition,
compared with C2B, C16B showed high-contrast mechanofluorochromism because
the long alkyl chain might decrease the strength of π-π interactions in the as-
synthesized crystals, leading to the emission appeared at the high-energy region (483
nm). The disassembling of parts of π-aggregates and the improvement of the
molecular planarity led to relative obvious red-shift of the emission (509 nm) upon
grinding.
2

ACCEPTED MANUSCRIPT
Keywords: difluoroboron β-diketonate, carbazole, mechanofluorochromism, NIR
emission
*
Corresponding author. State Key Laboratory of Supramolecular Structure and Materials, College of
Chemistry, Jilin University, Changchun 130012, P. R. China
E-mail: luran@mail.jlu.edu.cn
3

ACCEPTED MANUSCRIPT
1
. Introduction
As one kind of stimuli-responsive luminescent organic materials,
mechanofluorochromic (MFC) dyes have received much attention on account of their
1
-2
potential applications in sensors, memory chips, data storage and security inks.
Generally, the emitting colors of MFC compounds can be altered induced by external
3
-4
forces of stretching, grinding, pressing and shearing. It has been known that the
changes in the molecular packing or/and molecular conformation driven by
mechanical forces are responsible for the MFC behavior, and the
5
-7
mechanofluorochromism is often reversible.
However, the design of MFC dyes is
still challenging because the aggregation-caused quenching (ACQ) often results in the
5
decrease of the emission intensity in the solid-state. Solid-state NIR-emitting dyes
were seldom reported since the molecules with low-lying excited states often contain
extended conjugated systems or donor/acceptor moieties, which would lead to strong
8
π-π interactions as well as ACQ effect. It should be noted that the difluoroboron β-
9
diketonate (BF bdk) complexes are attractive fluorophores, and a number of MFC
2
materials based on BF bdk have been documented to date. Fraser and co-workers
2
found that the solid-state emission of difluoroboron β-diketonate complexes bearing
6
arenes changed upon the stimulation of mechanical forces. Šket et al. synthesized the
difluoroboron complex of 1-phenyl-3-(3,5-dimethoxyphenyl)-propane-1,3-dione, and
two kinds of polymorphs were obtained. By controllable alteration of the molecular
arrangements through external forces, varying solid-state emission was achieved from
1
0
the same dye. Liu et al. revealed that the force induced formation of emissive H-
aggregates and energy transfer to H-aggregates were responsible for the
1
1
mechanofluorochromism of difluoroboron dibenzoylmethane in the solid-state. Our
group previously reported that some D-π-A type difluoroboron β-diketonate
4

ACCEPTED MANUSCRIPT
1
2
complexes with ICT emission exhibited MFC behavior. Up to data, limited studies
1
3
focused on MFC materials with NIR emission, which may have promising
1
4
15
applications in bioimaging and security inks. Moreover, the length of the alkyl
1
6
chains were found to be another factor to affect the MFC properties. With these in
mind, we synthesized new carbazole modified difluoroboron β-carbonyl cyclic
ketonate complexes CnB and DCnB (n = 2, 16, Scheme 1), which exhibited
reversible MFC behavior.
2
. Experimental Section.
2
.1. Materials and methods
1
13
H NMR and C NMR spectra were measured using a Mercury Plus instrument at
4
00 MHz and 101 MHz using CDCl and DMSO-d as solvent. FT-IR spectra were
3 6
measured using a Nicolet-360 FT-IR spectrometer by incorporation of samples in KBr
disks. The UV-vis absorption spectra were obtained on a Shimadzu UV-3100
spectrophotometer. Fluorescent emission spectra were obtained on a Cary Eclipse
fluorescence spectrophotometer. Mass spectra were obtained with Agilent 1100 MS
series and AXIMA CFR MALDI-TOF (Compact) mass spectrometers. C, H, and N
elemental analyses were taken on a Vario EL cube elemental analyzer. XRD patterns
were obtained on an Empyrean X-ray diffraction instrument. The single crystal of
C2B was obtained in 1,4-dioxane and n-hexane via solvent diffusion, and selected for
X-ray diffraction analysis on a Rigaku RA XIS-RA PID diffractometer using
graphite-monochromated MoK α radiation (λ = 0.71073 Å). The crystal was kept at
room temperature during data collection. Differential scanning calorimetry (DSC)
o
curves were obtained on a Netzsch DSC 204F1 at a heating rate of 10 C/min.
5

ACCEPTED MANUSCRIPT
THF and toluene were dried over sodium and benzophenone under nitrogen.
CH Cl was dried over calcium hydride. The other chemicals and reagents were used
2
2
as received without further purification. Compounds 1 and 2 were prepared according
1
7
to the reported procedures.
2
.2. Synthesis
-Ethyl-9H-carbazole-3-carbonyl chloride (3a)
1
2b
9
The solution of 9-ethyl-9H-carbazole-3-carboxylic acid (2a, 1.0 g, 4.2 mmol ) in
CH Cl (10.0 mL) was cooled at 0 C. Then, SOCl (0.5 mL, 4.3 mmol) was added,
°
2
2
2
followed by adding four drops of dry DMF. The mixture was warmed to room
temperature and stirred at room temperature for 3 h. It should be noted that the excess
SOCl was removed completely under vacuum when the reaction was finished. The
2
residuum of 9-ethyl-9H-carbazole-3-carbonyl chloride (3a) was not further purified
and used directly in the next step.
Scheme 1 Synthetic routes for C2B, C16B, DC2B and DC16B.
6

ACCEPTED MANUSCRIPT
9
-Hexadecyl-9H-carbazole-3-carbonyl chloride (3b)
Following the synthetic procedure of 3a, compound 3b was synthesized from 9-
hexadecyl-9H-carbazole-3-carboxylic acid (2b, 1.00 g, 2.3 mmol) and SOCl (0.25
2
mL, 3.5 mmol), and it was used directly in the next step without further purification.
4
3
2
,2-Difluoro-4-methyl-5,6,7,8-tetrahydro-2H-2λ ,3λ -benzo[d] [1,3,2] dioxaborinine
1
2b
(4)
LDA (2 M in THF, 12.70 mL, 25.40 mmol) was added to dry THF (10.0 mL) in 100
mL flask under N atmosphere, and the mixture was cooled to -78 °C. Cyclohexanone
2
(2.7 mL, 25.40 mmol) in dry THF (5.0 mL) was then added dropwise. After stirring at
-78 °C for 1 h, acetyl chloride (1.00 g, 12.70 mmol) in dry THF (5.0 mL) was added
dropwise. After that, the solution was allowed to warm to room temperature and
stirred at room temperature for another 18 h. The mixture was quenched by HCl (2 M),
and the crude product was extracted with ethyl acetate (3 × 50 mL). The organic
layers were combined and dried with anhydrous MgSO . After removal the solvent, a
4
yellow solid, 1-(2-hydroxycyclohex-1-ene-1-yl)ethan-1-one was complexed with
boron trifluoride diethyl etherate complex directly. In detail, the crude intermediate
was dissolved in dry CH Cl (25 mL), and boron trifluoride diethyl ether (3.3 mL,
2
2
2
5.40 mmol) was added. The mixture was refluxed under an atmosphere of nitrogen
for 2 h. Excess CH Cl was removed under vacuum to give the crude product, which
2
2
was purified by column chromatography (silica gel, CH Cl /petroleum ether, v/v =
2
2
1
0/1), followed by recrystallization from the mixed solvent of CH Cl /petroleum ether
2 2
1
to afford 4 (1.42 g) in 59 % yield. m.p. 73.9-74.1 °C. H NMR (400 MHz, CDCl ) δ
3
2
.58 (t, J = 5.9 Hz, 2H), 2.39 (t, J = 5.6 Hz, 2H), 2.30 (s, 3H), 1.82-1.75 (m, 4H) (Fig.
7

ACCEPTED MANUSCRIPT
†
13
S7, ESI ). C NMR (101 MHz, CDCl ) δ 191.88 (s), 190.01 (s), 108.99 (s), 32.44 (s),
3
†
18
2
3.13 (s), 22.24 (s), 21.91 (s), 20.89 (s) (Fig. S8, ESI ).
4
3
3
-(2,2-Difluoro-5,6,7,8-tetrahydro-2H-2
λ
,3λ
-benzo[d][1,3,2]dioxaborinin-4-yl)-9-
ethyl-9H-carbazole (C2B)
Following the synthetic procedure of 4, complex C2B was synthesized from the
ligand (9-ethyl-9H-carbazol-3-yl)(2-hydroxycyclohex-1-en-1-yl)methanone, which
was prepared from 3a (1.00 g, 4.20 mmol), cyclohexanone (0.87 mL, 8.40 mmol) and
LDA (4.20 mL, 8.40 mmol) in dry THF, and boron trifluoride diethyl etherate
complex (1.1 mL, 8.40 mmol) in dry CH Cl (25 mL). The crude product was purified
2
2
by column chromatography (silica gel, CH Cl /petroleum ether, v/v = 10/1), followed
2
2
by recrystallization from the mixed solvent of CH Cl /petroleum ether to afford C2B
2
2
1
(
0.85 g) in 55 % yield. m.p. 174.5-175.0 °C. H NMR (400 MHz, CDCl ) δ 8.68 (s,
3
1
H), 8.14 (d, J = 7.7 Hz, 1H), 8.01 (d, J = 8.7 Hz, 1H), 7.55 (t, J = 5,6 Hz, 1H), 7.46
(dd, J = 13.2, J = 8.5 Hz, 2H), 7.34 (t, J = 7.4 Hz, 1H), 4.43-4.38 (m, 2H), 2.81 (t, J =
5
.8 Hz, 2H), 2.72 (t, J = 6.5 Hz, 2H), 1.93-1.87 (m, 2H), 1.75-1.71 (m, 2H), 1.47 (t, J
†
13
=
7.2 Hz, 3H) (Fig. S9, ESI ). C NMR (101 MHz, CDCl ) δ 189.94 (s), 184.48 (s),
3
1
42.88 (s), 140.62 (s), 128.16 (s), 126.93 (s), 124.14 (s), 123.34 (s), 122.87 (d, J = 1.8
Hz), 120.81 (s), 120.53 (s), 109.24 (s), 108.25 (d, J = 13.5 Hz), 37.94 (s), 32.95 (s),
†
2
6.79 (s), 23.11 (s), 21.33 (s), 13.82 (s) (Fig. S10, ESI ). MALDI-TOF MS: m/z:
+
†
calculated for C H BF N O : 367.2; found: 365.3 [M-2] (Fig. S11, ESI ); FT-IR
2
1
20
2
2
2
-1
(
KBr, cm ): 2925, 2848, 1530, 1473, 1367, 1034, 747. Elem. Anal. Calcd. (%) for
C H BF NO : C, 68.69; H, 5.49; N, 3.81. Found: C, 68.38; H, 5.43; N, 3.93.
21
20
2
2
4
3
3
-(2,2-Difluoro-5,6,7,8-tetrahydro-2H-2
λ
,3
λ -benzo[d][1,3,2]dioxaborinin-4-yl)-9-
hexadecyl-9H-carbazole (C16B)
8

ACCEPTED MANUSCRIPT
Following the synthetic procedure of 4, complex C16B was synthesized from the
ligand
(9-hexadecyl-9H-carbazol-3-yl)(2-hydroxycyclohex-1-en-1-yl)methanone,
which was prepared from 3b (1.00 g, 2.30 mmol), cyclohexanone (0.48 mL, 4.60
mmol) and LDA (2.30 mL, 4.60 mmol) in dry THF, and boron trifluoride diethyl
etherate complex (0.58 mL, 4.60 mmol) in dry CH Cl (25 mL). The crude product
2
2
was purified by column chromatography (silica gel, CH Cl /petroleum ether, v/v =
2
2
1
0/1), followed by recrystallization from the mixed solvent of CH Cl /petroleum ether
2 2
1
to afford C16B (0.80 g) in 62 % yield. m.p. 91.0-91.5 °C. H NMR (400 MHz, CDCl )
3
δ 8.69 (s, 1H), 8.15 (d, J = 7.7 Hz, 1H), 8.01 (d, J = 3.8 Hz, 1H), 7.55 (t, J = 7.3 Hz,
1
H), 7.45 (t, J = 9.6 Hz, 2H), 7.34 (t, J = 7.4 Hz, 1H), 4.33 (t, J = 7.2 Hz, 2H), 2.82 (t,
J = 5.9 Hz, 2H), 2.72 (t, J = 6.6 Hz, 2H), 1.92-1.85 (m, 4H), 1.77-1.71 (m, 2H), 1.24
†
13
(
d, J = 4.8 Hz, 26H), 0.88 (t, J = 6.8 Hz, 3H) (Fig. S12, ESI ). C NMR (101 MHz,
CDCl ) δ 189.91 (s), 184.50 (s), 143.41 (s), 141.11 (s), 128.17 (s), 126.91 (s), 124.14
3
(s), 123.32 (s), 122.79 (d, J = 3.7 Hz), 120.77 (s), 120.52 (s), 109.49 (s), 108.45 (s),
1
08.31 (s), 43.45 (s), 32.96 (s), 31.94 (s), 29.59 (ddd, J = 23.8, 11.2, 6.4 Hz), 28.93
†
(
s), 27.25 (s), 26.83 (s), 23.14 (s), 22.71 (s), 21.36 (s), 14.14 (s) (Fig. S13, ESI ).
+
MALDI-TOF MS: m/z: calculated for C H BF NO : 563.6; found: 561.6 [M-2]
3
5
48
2
2
†
-1
(
Fig. S14, ESI ); FT-IR (KBr, cm ): 2916, 2849, 1543, 1490, 1474, 1373, 1038, 741.
Elem. Anal. Calcd. (%) for C H BF NO : C, 74.59; H, 8.59; N, 2.49. Found: C,
3
5
48
2
2
7
4.35; H, 9.04; N, 2.39.
9
-Ethyl-3-((E)-2-((E)-8-((9-ethyl-9H-carbazol-3-yl)methylene)-2,2-difluoro-5,6,7,8-
4
3
tetrahydro-2H-2
λ
,3λ
-benzo[d][1,3,2]dioxaborinin-4-yl)vinyl)-9H-carbazole (DC2B)
1
9
9

ACCEPTED MANUSCRIPT
Compounds 4 (0.63 g, 3.35 mmol) and 1a (0.75 g, 3.35 mmol) were dissolved in dry
toluene (20 mL), and four drops of piperidine were added. The mixture was refluxed
for 2 h. Water (80 mL) was added to the cooled solution and the mixture was
extracted with CH Cl (3 × 50 mL). The organic layers were combined and dried with
2
2
anhydrous MgSO . After removal the solvent, the crude product was purified by
4
column chromatography (silica gel, CH Cl /petroleum ether, v/v = 10/1), followed by
2
2
recrystallization from the mixed solvent of CH Cl /petroleum ether to afford DC2B
2
2
1
(
0.98 g) in 49 % yield. m.p. 278.0-279.0 °C. H NMR (400 MHz, CDCl ) δ 8.27 (d, J
3
=
17.8 Hz, 3H), 8.16-8.10 (m, 3H), 7.71 (d, J = 8.6 Hz, 1H), 7.56 (d, J = 8.6 Hz, 1H),
7
.50 (t, J = 7.4 Hz, 2H), 7.39-7.27 (m, 6H), 7.00 (d, J = 15.1 Hz, 1H), 4.32-4.25 (m,
H), 2.90 (t, J = 5.5 Hz, 2H), 2.71 (t, J = 6.0 Hz, 2H), 1.90-1.84 (m, 2H), 1.44-1.39
4
†
13
(
m, 6H) (Fig. S15, ESI ). C NMR (101 MHz, DMSO-d ) δ 176.67 (s), 175.33 (s),
6
1
1
1
1
2
49.13 (s), 142.09 (s), 140.24 (t, J = 15.5 Hz), 129.52 (s), 128.46 (s), 128.18 (s),
26.56 (d, J = 20.6 Hz), 126.01 (s), 125.60 (s), 124.47 (s), 123.62 (s), 123.03 (s),
22.65 (s), 122.28 (d, J = 7.8 Hz), 120.86 (s), 120.06 (s), 119.68 (s), 114.24 (s),
09.93 (s), 109.67-109.26 (m), 54.89 (s), 43.72 (s), 37.33 (s), 26.74 (s), 22.78 (s),
†
2.18 (s), 21.80 (s), 13.77 (s) (Fig. S16, ESI ). MALDI-TOF MS: m/z: calculated for
+
†
-1
C H BF N O : 598.5; found: 596.8 [M-2] (Fig. S17, ESI ); FT-IR (KBr, cm ):
38
33
2
2
2
2
928, 2847, 1740, 1581, 1473, 1231, 1189, 1108, 1025, 963, 731. Elem. Anal. Calcd.
(
%) for C H BF N O : C, 76.26; H, 5.56; N, 4.68. Found: C, 75.87; H, 5.73; N, 5.05.
3
8
33
2
2
2
3
2
-((E)-(2,2-Difluoro-4-((E)-2-(9-hexadecyl-9H-carbazol-3-yl)vinyl)-6,7-dihydro-2H-
4
3
λ
,3λ -benzo[d][1,3,2]dioxaborinin-8(5H)-ylidene)methyl)-9-hexadecyl-9H-
carbazole (DC16B)
1
0

ACCEPTED MANUSCRIPT
The synthetic method for DC16B was similar to that for DC2B except using
compounds 4 (1.00 g, 2.4 mmol) and 1b (0.22 g, 2.4 mmol) as reactants. The crude
product was purified by column chromatography (silica gel, CH Cl /petroleum ether,
2
2
v/v = 10/1), followed by recrystallization from the mixed solvent of
CH Cl /petroleum ether to afford DC16B (1.00 g) in 42 % yield. m.p. 152.3-153.9 °C.
2
2
1
H NMR (400 MHz, CDCl ) δ 8.31 (d, J = 17.9 Hz, 3H), 8.21 (s, 1H), 8.13 (dd J =
3
1
1.6, 7.7 Hz, 2H), 7.73 (d, J = 7.5 Hz, 1H), 7.59 (d, J = 8.1 Hz, 1H), 7.50 (t, J = 7.4
Hz, 2H), 7.38 (t, J = 8.2Hz, 4H), 7.30 (dd, J = 15.7, 7.9 Hz, 2H), 7.04 (d, J = 15.1 Hz,
1
1
H), 4.23 (dd, J = 13.5, 6.8 Hz, 4H), 2.95 (t, J = 5.2 Hz, 2H), 2.76 (t, J = 6.0 Hz, 2H),
†
.92-1.84 (m, 6H), 1.24 (d, J = 5.1 Hz, 52H), 0.88 (t, J = 6.8 Hz, 6H) (Fig. S18, ESI ).
1
3
C NMR (101 MHz, CDCl ) δ 176.66 (s), 176.30 (s), 148.86 (s), 142.60 (s), 141.33
3
(s), 140.91 (t, J = 26 Hz), 129.67 (s), 127.90 (s), 127.29 (s), 126.44 (s), 126.22 (s),
1
25.70 (s), 123.90 (s), 123.55 (s), 123.04 (s), 122.80 (d, J = 4.6 Hz), 120.70 (d, J = 52
Hz), 120.09 (s), 119.72 (s), 113.70 (s), 109.25 (d, J = 8 Hz), 109.05 (s), 108.63 (d, J =
6 Hz), 99.98 (s), 43.15 (s), 31.95 (s), 29.82–29.46 (m), 29.36 (d, J = 16 Hz), 28.92 (d,
1
J = 20 Hz), 27.25 (d, J = 16 Hz), 27.04 (s), 23.45 (s), 22.71 (s), 22.20 (s), 14.15 (s)
†
(
Fig. S19, ESI ). MALDI-TOF MS: m/z: calculated for C H BF NO : 991.2; found:
66
89
2
2
+
†
-1
9
89.2 [M-2] (Fig. S20, ESI ); FT-IR (KBr, cm ): 2920, 2850, 1248, 1740, 1581,
1
749, 1255, 1193, 1112, 1029, 967, 728. Elem. Anal. Calcd. (%) for C H BF N O :
6
6
89
2
2
2
C, 79.97; H, 9.05; N, 2.83. Found: C, 79.59; H, 9.31; N, 3.17.
3
. Results and Discussion
3
.1. Synthesis
The synthetic routes to N-alkylcarbazole modified difluoroboron β-diketonate
1
7
complexes CnB and DCnB (n = 2, 16) are shown in Scheme 1. Compounds 1-2 and
1
1

ACCEPTED MANUSCRIPT
1
2b
3
-4 were prepared according to the reported procedures. The chlorination of 2a and
2
b with SOCl afforded 3a and 3b, respectively, without further purification. It should
2
be noted that the excess SOCl must be removed completely before use in the next
2
step. After the enol anion was generated from cyclohexanone in presence of LDA, the
α-acylation reaction underwent with 3a to yield the ligand (9-ethyl-9H-carbazol-3-
yl)(2-hydroxycyclohex-1-en-1-yl)methanone, which was complexed with boron
trifluoride diethyl etherate directly without further purification to yield C2B in 55 %
1
2b
yield.
Similarly, C16B was synthesized from cyclohexanone, LDA and 3b,
followed by complexation with boron trifluoride diethyl etherate complex, in 62 %
yield. Compound 4 was synthesized from cyclohexanone, LDA and acetyl chloride,
followed by complexed with boron trifluoride diethyl etherate complex. Then, the
Knoevenagel condensation reaction between compounds 4 and 1 in the presence of
piperidine in anhydrous toluene gave DC2B and DC16B in a yield of 49 % and 42 %,
1
9
1
13
respectively. The target molecules were characterized by H NMR, C NMR, FT-IR,
MALDI-TOF mass spectrometry and C, H, and N elemental analyses. The single
crystal structure of C2B (CCDC 1810906) further confirmed the molecular structure.
The bond length of (O)C1-C6 in aliphatic six-member ring was 1.383 Å, which was
shorter than the bond length of C6-C7(O) (1.417 Å, see Fig. 4a), so we deduced that
cyclohexenol was involved in C2B. In the FT-IR spectrum of DC2B, a vibration
-1
absorption peak at 965 cm was detected, meaning the trans-form of the vinyl groups.
Similarly, the vinyl groups in DC16B also existed in the trans-form. The synthesized
complexes are soluble in common solvents of THF, CH Cl and toluene.
2
2
3
.2. Photophysical properties in solutions
1
2

ACCEPTED MANUSCRIPT
The UV-vis absorption and fluorescence emission spectra of CnB and DCnB in
†
different solvents are shown in Fig. 1 and Fig. S1 (ESI ), and the corresponding
†
photophysical data are summarized in Table S1 (ESI ). It was clear that C2B gave
two absorption bands located at 329 nm and 400 nm in toluene, which were attributed
to π-π* transitions. With increasing the solvent polarity the absorption band at ca. 400
nm was red-shifted to a certain extent. Particularly, the fluorescence emission band of
C2B gave significant red-shift with increasing the polartity of the solvents. For
example, the emission was located at 451 nm in toluene and shifted to 539 nm in
DMSO. Thus, the Stokes shifts increased with increasing the solvent polarities, and
Figure 1. Normalized UV-vis absorption spectra of C2B (a) and DC2B (c), and
fluorescent emission spectra of C2B (b, λ = 400 nm) and DC2B (d, λ = 530 nm) in
ex
ex
-6
different solvents (5.0 × 10 mol/L).
1
3

ACCEPTED MANUSCRIPT
-1
-1
they were 2827 cm and 5897 cm in toluene and DMSO, respectively. Besides, C2B
was highly emissive in THF with a fluorescence quantum yield (Ф ) of 0.81 using
F
9
,10-diphenylanthracene in benzene (Ф = 0.85) as the standard, but Ф was only 0.07
F F
in DMSO. The broadening and the red-shift of the emission band, the decreasing of
-1
Ф and the increasing Stokes shift (2827-5897 cm ) with increasing thesolvent
F
2
0
polarities illustrated the occurrence of ICT for C2B in polar solvents. As depicted in
†
Fig. S1a-b (ESI ), complex C16B gave similar UV-vis absorption and fluorescence
emission spectra to C2B due to the same conjugated π-skeletons. When two N-
alkylcarbazolyl-vinyl units were linked to the difluoroboron β-diketonate core, the
absorption and emission bands were red-shifted compared with CnB on account of
the enhanced degree of conjugation. As shown in Fig. 1c-d, DC2B gave a weak
absorption at 339 nm and a strong absorption at 544 nm with a shoulder at 515 nm
due to π-π* transitions in toluene. With increasing the solvent polarities, the
structured absorption bands were combined into one, which was red-shifted to 552 nm,
5
61 nm and 567 nm in DCM, DMF and DMSO, respectively. Moreover, the emission
of DC2B appeared at 577 nm in toluene, and was red-shifted with increasing solvent
2
1
polarity due to the strong interaction between excited state and solvent molecules.
For example, the ICT emission of DC2B was red-shifted to 647 nm in DMSO.
DC16B exhibited similar electronic spectra to those of DC2B in solutions.
3
.3. Density function theoretical calculations
The Quantum chemical calculations were performed on C2B and DC2B by density
functional theory (DFT) calculations at the B3LYP/6-31G level so as to reveal their
electronic structures. As shown in Fig. 2, the HOMO and LUMO of C2B were mainly
distributed in the carbazole unit (electron donor) and difluoroboron β-diketone moiety
1
4

ACCEPTED MANUSCRIPT
Figure 2. Energy levels and molecular orbital surfaces in the optimized ground-state
structures.
(electron acceptor), respectively, suggesting a D-π-A type skeleton. DC2B is a typical
D-π-A-π-D molecule, in which difluoroboron β-diketone acts as an electron
withdrawing group, two carbazole groups are electron donors, and the vinyl moieties
are involved as π-bridges. It was found that the HOMO of DC2B was located at the
vinyl carbazole linked to cyclohexane and the LUMO was distributed in the acceptor
and vinyl groups. Therefore, it was understandable that ICT emission was detected for
CnB and DCnB. Meanwhile, the calculated LUMO energy level of DC2B was lower
than C2B, but HOMO energy level of DC2B was higher than C2B because of the
extended conjugation of DC2B. The low band gap for DC2B was in accordance with
that its absorption and emission emerged in low-energy region compared with C2B.
On the other hand, the absorption wavelengths (λ ), oscillator strengths (f) and
abs
dominant excitation characters of C2B and DC2B were calculated with the TD-
DFT/B3LYP/6-31G(d) functional. It was clear that their maximum absorption peaks
1
5

ACCEPTED MANUSCRIPT
were derived from HOMO-LUMO transitions (Table S2). Because the frontier orbital
plots of HOMOs and LUMOs for C2B and DC2B were mainly located at the donor
and acceptor units, respectively, they possessed polar π-conjugated skeletons. It
suggested that the ICT emission might be observed.
3
.4. Mechanofluorochromic properties
As shown in Fig. 3a, C2B emitted strong green light in the as-synthesized sample, and
the grinding led to the emitting color becoming yellowish green. The emission band
was red-shifted to 519 nm in the ground powders compared with that at 504 nm in the
as-synthesized sample. When the ground powders were fumed with CH Cl vapor for
2
2
Figure 3. Normalized fluorescent emission spectra of C2B (a) and C16B (c) excited
at 400 nm, and XRD patterns of C2B (b) and C16B (d) in different solid-state.
1
6

ACCEPTED MANUSCRIPT
o
4
s or heated at 160 C for 5 s, the emitting color could be recovered to green. Such
changes could be repeated several times under grinding and solvent fuming/thermal
†
treatment (Fig. S3a, ESI ), suggesting a fully reversible MFC processes. So as to
reveal the MFC mechanism, the single crystal of C2B was obtained by slow
evaporation from the solution in dioxane/cyclohexane. As shown in Fig. 4, we could
find that the dihedral angle between carbazole and difluoroboron β-diketone moiety
o
was 25.3 in single crystal of C2B. It was similar to the DFT calculations, which
disclosed the dihedral angle of 28.1° between carbazole and difluoroboron β-diketone
†
plane in C2B (Fig. S2, ESI ). It was clear that the molecules C2B were packed in a
face-to-face mode, and the distance between two adjacent carbazole rings was 3.532
Å in the single crystal.Therefore, J-aggregates were involved in the single crystal of
C2B, in which the hydrogen bonds of C−H⋅⋅⋅F and C−H⋅⋅⋅π were also formed. In
detail, one of the fluorine atoms in the central molecule formed a hydrogen bond with
hydrogen in cyclohexanone ring (2.571 Å), and the distance for C−H⋅⋅⋅π was 2.820 Å.
The single crystal structure suggested that multiple intermolecular interactions would
suppress the intramolecular rotation in the crystalline state, leading to strong
luminescence of C2B in the solid-state. XRD patterns of C2B in different solid-state
Figure 4. Molecular configuration (a) and single crystal structure of C2B (b).
1
7

ACCEPTED MANUSCRIPT
were depicted in Fig. 3b. The as-synthesized sample of C2B gave several sharp and
strong diffraction peaks, illustrating well-ordered crystalline structure. However, most
of the diffraction peaks disappeared upon grinding, so we deemed that the ordered
crystalline structures were transformed into an amorphous state under external
mechanical force. After fuming with CH Cl vapor, the diffraction peaks similar to
2
2
those in the as-synthesized crystals emerged again. It meant that the reversible MFC
process of C2B was originated from the transformation between crystalline and
amorphous states. Furthermore, the UV-vis spectra of C2B in different solid-state
were obtained in order to confirm the MFC mechanism. An absorption band at 454
nm was detected for C2B in the as-synthesized crystals and was blue-shifted to 424
†
nm in the ground powders (Fig. S5a, ESI ). Compared with the absorption at 400 nm
of C2B in toluene, the red-shift of 54 nm was detected for the as-synthesized crystals,
suggests the presence of J-aggregates in the crystals; a feature which is in accord with
the result based on the single crystal structure. We deemed that parts of J-aggregates
were destroyed in the ground powders, so the absorption of the ground powders was
blue-shifted upon grinding. However, the emission of the ground powders of C2B
emerged in low-energy region compared with the as-synthesized crystals, which
might be due to greater planarity in ground powders might be better than that in as-
synthesized crystals.
Although the conjugated skeletons of C16B and C2B were the same, C16B with
long carbon chain in carbazole unit showed high-contrast MFC behavior compared
with C2B. The as-synthesized crystals of C16B emitted blue fluorescence centered at
4
83 nm, and the emitting color of the ground powders was changed into bluish green
with a maximal emission peak at 509 nm (Fig. 3c). Moreover, the reversible MFC
processes could be also realized upon the treatment of grinding and fuming with
1
8

ACCEPTED MANUSCRIPT
o
†
CH Cl vapor for 3 s or heating at 80 C for 2 s (Fig. S3b, ESI ). In addition, exposure
2
2
of the ground powders of C16B under the vapor of THF or CHCl for several seconds
3
†
also induced the recovery of the emission to 483 nm (Fig. S4, ESI ). Although the
emission of the ground powders was blue-shifted upon fumed with the vapor of
C H OH or CH OH, it could not be fully recovered, which might be due to the poor
2
5
3
solubility of C16B in C H OH or CH OH. Unfortunately, we could not gain the
2
5
3
single crystal structure of C16B. The XRD patterns of C16B in different solid-state
also illustrated that the transformation between crystalline and amorphous states was
reasonable for the MFC behavior of C16B (Fig. 3d). Additionally, the absorption at ca.
†
4
21 nm of C16B in the as-synthesized crystals was observed in Fig. S5b (ESI ), and
was ascribed to the electronic transition from the aggregated molecules (the
absorption at 401 nm for C16B in toluene). When the as-synthesized crystals of C16B
were ground, the absorption was blue-shifted to 416 nm. The changes of the
electronic spectra of C16B during MFC processes were similar to C2B, e.g., the
grinding lead to a red-shift of the emission and a blue-shift of the absorption.
However, why do C16B and C2B exhibit different MFC properties? Firstly, the long
alkyl chain in C16B might decrease the strength of π-π interactions in the as-
synthesized crystals, so that its emission bands appeared at the high-energy region
compared with C2B, for instance, the emission of C16B in crystals was located at 483
nm. Upon grinding, the disassembling of parts of π-aggregates and the improvement
of the molecular planarity led to the red-shift of the emission of C16B to 509 nm. The
high-contrast mechanofluorochromism of C16B was mainly originated from the high-
lying excited state in the as-synthesized crystals.
When two terminal carbazole units were bridged by vinyl groups to link to
difluoroboron β-diketonate cores, the large extended conjugation in DCnB induced
1
9

ACCEPTED MANUSCRIPT
their solid emission to red-shift to the NIR region. As shown in Fig. 5a, DC2B
emitted red light centered at 714 nm in the as-synthesized sample, which was blue-
shifted to 703 nm (rose red) in the ground powders. The emitting color could be
recovered to red when the ground powders were fumed with CH Cl vapor for 8 s or
2
2
o
heated at 240 C for 5 s. The MFC behavior was reversible under the treatment of
†
grinding/fuming or heating (Fig. S3c, ESI ). The XRD patterns of DC2B in different
solid-state also illustrated that the transformation between the crystalline and
amorphous states led to the reversible MFC properties (Fig. 5b). Meanwhile, DC2B
gave two strong absorption bands located at ca. 546 nm and ca. 590 nm in the as-
synthesized crystals. No obvious shift was detected for the two absorption bands after
Figure 5. Normalized fluorescent emission spectra of DC2B (a) and DC16B (c)
excited at 530 nm, and XRD pattern of DC2B (b) and DC16B (d) in different solid-
state.
2
0

ACCEPTED MANUSCRIPT
grinding the as-synthesized crystals of DC2B, except for the slight change of their
†
relative intensities (Fig. S5c, ESI ). It is suggested that the isolated and aggregated
molecules of DC2B were involved in both the as-synthesized crystals and the ground
powders. The grinding might change the ratio of the contents of the isolated and
aggregated molecules, leading to the change of the emitting colors. DC16B also
†
exhibited reversible MFC properties (Fig. 5c-d and Fig. S3d, ESI ), and the emitting
colors changed from red centered at 734 nm in as-synthesized crystals to rose red
centred at 720 nm in ground powders. It should be noted that the emission peaks in
as-synthesized and fumed samples of DC16B were fully overlapped, while the
emission band for DC2B in the fumed sample was broader than in the as-synthesized
crystals. We deemed that the long alkyl chain in DC16B might lead to looser
molecular packing in crystals compared with DC2B, which would be beneficial for
the recovery of luminescence. XRD patterns of DC16B in different solid-state further
illustrated that the transformation between crystalline and amorphous states are
responsible for MFC properties (Fig. 5d), which could be further proved by DSC
†
curves of DC16B in different solid-state. As shown in Fig. S6 (ESI ), the ground
powders of DC16B gave exothermic peaks at ca. 73.1 °C, except for the peaks
corresponding to the melting points, meaning that the amorphous state was
2
3
metastable. It should be noted that the emission bands for DC2B and DC16B in the
solid-state emerged in the range of 650-850 nm, such MFC materials with NIR
emission was seldom reported.
4
. Conclusions
In summary, new difluoroboron β-carbonyl cyclic ketonate complexes were
synthesized, and they gave ICT emission in the solid-state. Interestingly, reversible
MFC processes were detected for the four synthesized complexes. In detail, the
2
1

ACCEPTED MANUSCRIPT
emission of C2B emerged at 504 nm in crystals, and was red-shifted to 519 nm upon
grinding. The emitting color could be recovered to green from yellowish green upon
fuming the ground powders with CH Cl vapor. XRD patterns of C2B in different
2
2
solid-state illustrated that the transformation between the crystalline and amorphous
states were responsible for MFC behavior. However, C16B showed a red-shift of 26
nm upon the grinding as-synthesized crystals into ground powders, exhibiting relative
high-contrast mechanofluorochromism compared with C2B. The reason might
include the following points. The long alkyl chain in C16B would decrease the
strength of π-π interactions in the as-synthesized crystals, so that its emission band
appeared at high-energy region (483 nm). The disassembling of parts of π-aggregates
as well as the improvement of the molecular planarity led to the red-shift of the
emission of C16B to 509 nm upon grinding. Interestingly, in the case of DC2B and
DC16B, in which two carbozole units were bridged by vinyl moieties to link to
difluoroboron β-diketonate core, their emission appeared in the range of 650-850 nm
in the solid-state, and gave red and rose-red luminescence in the as-synthesized
crystals and in ground powders, respectively.
Acknowledgements
This work was financially supported by the National Natural Science Foundation of
China (51773076 and 21503090) and the Open Project of State Key Laboratory of
Supramolecular Structure and Materials (SKLSSM201813).
Supplementary data
Supplementary data related to this article can be found in the online version, at doi:
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