Synthesis, photophysical and solvatochromic properties of diacetoxyboron complexes with curcumin derivatives

Article abstract of DOI:10.1016/j.saa.2019.05.031

Five novel diacetoxyboron complexes of β-diketones incorporated curcumin moiety were designed and synthesized. Their photophysical behaviors were investigated by UV–vis absorption and fluorescence spectroscopy in different solvents and solid state. It was observed that their fluorescence spectra yielded a blue to yellow-green emission in solution and emitted a yellow to red emission in solid state. Especially, the complex 3b displayed the strongest emission and the highest quantum efficiency (Φ_u = 0.98) in toluene among these complexes. The CIE coordinate of the complex 3b in solid state was positioned in an ideal red region of the chromaticity diagram. The maximal emission of these complexes exhibited a large wavelength shift and Stokes shift increased with the increase of the solvent polarity. Their dipole moment differences between the ground and excited states were also estimated by using the Lippert–Mataga equation. Meanwhile, their HOMO, LUMO energy levels and energy band gaps were calculated by cyclic voltammetry.

Full text of DOI:10.1016/j.saa.2019.05.031

Accepted Manuscript
Synthesis, photophysical and solvatochromic properties of
diacetoxyboron complexes with curcumin derivatives
Heng Lyu, Dan Wang, Lian Cai, Dun-Jia Wang, Xiao-Ming Li
PII:
S1386-1425(19)30507-4
https://doi.org/10.1016/j.saa.2019.05.031
SAA 17126
DOI:
Reference:
Spectrochimica Acta Part A: Molecular and Biomolecular
Spectroscopy
To appear in:
Received date: 12 February 2019
Revised date:
Accepted
date:
11 May 2019
11 May 2019
Please cite this article as: H. Lyu, D. Wang, L. Cai, et al., Synthesis, photophysical
and solvatochromic properties of diacetoxyboron complexes with curcumin derivatives,
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, https://doi.org/
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0.1016/j.saa.2019.05.031
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ACCEPTED MANUSCRIPT
Synthesis, photophysical and solvatochromic properties of
diacetoxyboron complexes with curcumin derivatives
a
a
a
a,
b
Heng Lyu , Dan Wang , Lian Cai , Dun-Jia Wang *, Xiao-Ming Li
a
Hubei Key Laboratory of Pollutant Analysis and Reuse Technology, College of Chemistry and
Chemical Engineering, Hubei Normal University, Huangshi 435002, China
Key Laboratory of Experimental Marine Biology, Institute of Oceanology, Chinese Academy of
b
Sciences, Qingdao, 266071, China
Abstract: Five novel diacetoxyboron complexes of β-diketones incorporated
curcumin moiety were designed and synthesized. Their photophysical behaviors were
investigated by UV–vis absorption and fluorescence spectroscopy in different
solvents and solid state. It was observed that their fluorescence spectra yielded a blue
to yellow-green emission in solution and emitted a yellow to red emission in solid
state. Especially, the complex 3b displayed the strongest emission and the highest
quantum efficiency (Φ = 0.98) in toluene among these complexes. The CIE
u
coordinate of the complex 3b in solid state was positioned in an ideal red region of
the chromaticity diagram. The maximal emission of these complexes exhibited a
large wavelength shift and Stokes shift increased with the increase of the solvent
polarity. Their dipole moment differences between the ground and excited states were
also estimated by using the Lippert–Mataga equation. Meanwhile, their HOMO,
LUMO energy levels and energy band gaps were calculated by cyclic voltammetry.
Keywords: Diacetoxyboron complex, curcumin derivative, photophysical properties,
solvatochromic effect, electrochemical properties
_
___
*Corresponding author. Tel: +86-714-6515602; Fax: +86-714-6573832
E-mail address: dunjiawang@163.com (D.-J. Wang)
1

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1
. Introduction
For more than two decades, the synthesis and design of tetracoordinate
organoboron compounds have caused tremendous attention because of their
impressive photophysical properties with application in semiconductors [1], organic
light-emitting diodes [2,3], chemical sensors [4], fluorescent probes [5,6], and
two-photon materials [7]. In recent years, many new functional difluoroboron
β-diketonates were synthesized and their photophysical and photochemical properties
were investigated in detail [8–12]. These difluoroboron complexes showed the large
Stokes shifts, intense fluorescence emission, high quantum yields, high molar
absorption coefficients and second-order nonlinear optical properties. Similarly,
Curcumin and its analogues possess two conjugated α,β-unsaturated β-diketone
moieties, which can also form a keto-enol tautomerism [13–15]. Therefore, the
researchers presented the curcumin boron complexes as a near-infrared imaging
fluorescent probe [16] and a chemical sensor [17]. Meanwhile, some novel
difluoroboron complexes with curcumin analogues were reported and their
photophysical properties were also studied [18–20].
However, there is no report on the diacetoxyboron complexes with curcumin
analogues so far. Recently, we reported the synthesis and photophysical behaviors of
some new diacetoxyboron derivatives with bis-β-diketone [21]. In this work, we
designed and synthesized several novel diacetoxyboron derivatives of β-diketones
1
13
incorporated curcumin moiety and their structures were confirmed by H NMR, C
NMR, IR, ESI–MS spectroscopy and elemental analysis. The photophysical
2

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properties of these diacetoxyboron complexes in solution and solid state were
described in detail. Meanwhile, the spectroscopic behaviors of these complexes were
investigated in different solvents at room temperature and their dipole moment
differences between the ground and excited states were estimated by using the
Lippert–Mataga equation.
2
. Experimental
2
.1. Chemicalsand methods
Acetylacetone, benzaldehyde, 4-methoxybenzaldehyde, 4-methylbenzaldehyde,
1-naphthaldehyde, cinnamaldehyde, boron trioxide, tributyl borate, acetic anhydride,
n-butylamine, tetrabutylammonium hexafluorophosphate, and boric acid were
purchased from Shanghai Sun Chemical Technology Co., Ltd.. Other reagents
(quinine sulfate, dichloromethane, chloroform, ethyl acetate, toluene, tetrahydrofuran,
acetone and acetonitrile, petroleum ether, dimethyl sulfoxide, ethanol, benzene and
hydrochloric acid) were analytically pure.
Elemental analyses were taken with PerkinElmer 2400CHN elemental analyzer.
Melting points were measured on Beijing Tech X-4 digital melting-point apparatus.
1
13
TM
H and C NMR spectra were recorded with Bruker Avance III 300 MHz NMR
spectrometer in CDCl solution. FT-IR spectra were performed with Nicolet FTIR
3
5700 spectrophotometer in KBr disks. Mass spectra were determined by Finnigan
LCQ Advantage Max spectrometer. UV-vis absorption spectra were carried out on
Hitachi U-3010 spectrophotometer. The emission spectra were recorded with Varian
Cary Eclipse fluorescence spectrometer and fluorescence lifetimes were performed
3

ACCEPTED MANUSCRIPT
on an Edinburgh FS5 spectrofluorometer by the time-correlated single-proton
counting (TCSPC) (picoseconds pulsed diode laser EPL-450, λ = 450 nm).
ex
Quantum yields (Φ) were measured relative to quinine sulfate in 0.1M sulfuric acid
(
Φ = 0.55, λ = 366 nm) at room temperature by the standard method [22]. Cyclic
s
ex
voltammetry (CV) measurements were recorded on a CHI 660E electrochemistry
workstation with a three-electrode cell (a platinum disk electrode as the working
electrode, a platinum wire as the counter electrode and an AgCl/Ag electrode as the

1
reference electrode) in a 0.1 mol L n-Bu NPF solution in CH Cl at a scanning rate
4
6
2
2

1
of 50 mV s .
2
.2. Synthesis of curcumin derivatives (1a5a)
In a round bottom flask, boron trioxide (0.49 g, 7.0 mmol) and acetylacetone (1.0
mL, 1.0 mmol) was dissolved in ethyl acetate (10 mL) and refluxed for 1h under
stirring. To this solution, the aromatic aldehyde (20 mmol) and tributyl borate (4.8
mL, 20 mmol) were added and kept for stirring to a yellow-green solution. Then
n-butylamine (2.0 mL, 20 mmol) were added and stirred for 4h at room temperature.
Then 5% hydrochloric acid (15 mL) was added dropwise to the reaction mixture and
adjusted to pH = 2. The organic layer was separated and aqueous layer was extracted
with ethyl acetate. The organic phase was dried over anhydrous MgSO and distilled
4
by rotary evaporator. The residues were cooled to give crystal products and
recrystallized from chloroform to obtain the curcumin derivatives (1a–5a) in 26–51%
yields.
1
,7-Diphenylhepta-1,6-diene-3,5-dione (1a): Yellow green powder, yield 35%, m.p.
4

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1
1
44145 °C; H NMR (300 MHz, CDCl ): δ 15.91 (s, 1H, enol OH), 7.67 (d, 2H, J =
3
15.9 Hz, C=CH), 7.587.55 (m, 4H, ArH), 7.417.39 (m, 6H, ArH), 6.64 (d, 2H,
J = 15.8 Hz, C=CH), 5.86 (s, 1H, enol C=CH) ppm; IR (KBr): ν 3441 (m), 3067
(m), 2955 (w), 2869 (w), 1631 (s), 1588 (s), 1400 (s), 1351 (m), 1138 (m), 976 (m),

1
+
7
55 (s), 682 (m) cm ; ESIMS m/z: 277.16 [M+H] ; Anal. calcd. for C H O : C
1
9
16
2
82.58, H 5.84; found C 82.85, H 5.88.
1
,7-Di-p-tolylhepta-1,6-diene-3,5-dione (2a): Yellow green powder, yield 43%,
1
m.p. 213214 °C; H NMR (300 MHz, CDCl ): δ 16.00 (s, 1H, enol OH), 7.64 (d, 2H,
3
J = 15.9 Hz, C=CH), 7.46 (d, 4H, J = 8.0 Hz, ArH), 7.20 (d, 4H, J = 7.9 Hz, ArH),
6.59 (d, 2H, J = 15.8 Hz, C=CH), 5.82 (s, 1H, enol C=CH), 2.38 (s, 6H, ArCH3)
ppm; IR (KBr): ν 3441 (m), 3065 (m), 2949 (w), 2856 (w), 1631 (s), 1592 (s), 1400

1
+
(
s), 1351 (m), 1141 (m), 977 (m), 819 (m) cm ; ESIMS m/z: 305.18 [M+H] ; Anal.
calcd. for C H O : C 82.86, H 6.62; found C 82.57, H 6.56.
2
1
20
2
1
,7-Bis(4-methoxyphenyl)hepta-1,6-diene-3,5-dione (3a): Orange powder, yield
o
1
5
1%; m.p. 167168 C; H NMR (300 MHz, CDCl ): δ 16.10 (s, 1H, enol OH), 7.62
3
(d, 2H, J = 15.8 Hz, C=CH), 7.51 (d, 4H, J = 8.6 Hz, ArH), 6.91 (d, 4H, J = 8.6 Hz,
ArH), 6.50 (d, 2H, J = 15.8 Hz, C=CH), 5.79 (s, 1H, enol C=CH), 3.84 (s, 6H,
OCH ) ppm; IR (KBr): ν 3440 (m), 3046 (m), 2963 (w), 2867 (w), 1631 (s), 1600 (s),
3
1512 (m), 1400 (s), 1350 (m), 1250 (m), 1177 (m), 1143 (m), 977 (m), 826 (s), 733 (s)

1
+
cm ; ESIMS m/z: 337.07 [M+H] ; Anal. calcd. for C H O : C 74.98, H 5.99;
2
1
20
4
found C 75.66, H 6.03.
1
,7-Di(naphthalen-1-yl)hepta-1,6-diene-3,5-dione (4a): Orange powder, yield:
5

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o
1
4
5%; m.p. 190191 C; H NMR (300 MHz, CDCl ): δ 16.08 (s, 1H, enol OH), 8.55
3
(
d, 2H, J = 15.6 Hz, C=CH), 8.28 (d, 2H, J = 8.2 Hz, ArH), 7.927.82 (m, 6H,
ArH), 7.637.49 (m, 6H, ArH), 6.77 (d, 2H, J = 15.5 Hz, C=CH), 5.96 (s, 1H,
enol C=CH) ppm; IR (KBr): ν 3431(m), 3057 (m), 2951 (w), 2864 (w), 1630 (s),

1
1
589 (s), 1399 (s), 1347 (m), 1138 (m), 966 (m), 798 (m), 765 (s) cm ; ESIMS m/z:
+
3
77.23 [M+H] ; Anal. calcd. for C H O : C 86.15, H 5.35; found C 86.43, H 5.31.
27
20
2
1
,11-Diphenyl-undeca-1,3,8,10-tetraene-5,7-dione (5a): Orange powder, yield
o
1
2
6%; m.p. 214215 C; H NMR (300 MHz, CDCl ): δ 15.86 (s, 1H, enol OH),
3
7
.497.31 (m, 12H, ArH or C=CH), 6.92 (d, 4H, J = 5.4 Hz, ArH), 6.18 (d, 2H, J
15.1 Hz, C=CH), 5.70 (s, 1H, enol C=CH) ppm; IR (KBr): ν 3440 (m), 3086 (m),
045 (m), 2949 (w), 2853 (w), 1631 (s), 1591 (s), 1400 (s), 1351 (m), 1119 (m), 995
=
3

1
+
(
m), 748 (m), 687 (m) cm ; ESIMS m/z: 658.13 [2M+H] ; Anal. calcd. for
C H O : C 84.12, H 6.14; found C 84.41, H 6.18.
2
3
20
2
2
.3. Synthesis of diacetoxyboron complexes (1b–5b)
In a round bottom flask, acetic anhydride (8 mL) and boric acid (0.06 g, 1 mmol)
was added and refluxed for 30 min to give a light yellow solution. The curcumin
derivatives 1a–5a (0.5 mmol) were dissolved in benzene (20 mL) and heated to
reflux in a three-necked round bottom flask. Then the above solution of boron acetate
(1 mmol) in acetic anhydride was added dropwise slowly into the mixture over 30
min. The reaction mixture was kept refluxing for 5 h and complexes were precipitated
out. The precipitates were filtered off and recrystallized from chloroform to obtain
diacetoxyboron complexes (1b–5b) in 57–71% yields.
6

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Diacetoxyboron 1,7-diphenylhepta-1,6-diene-3,5-dionate (1b): Yellow powder,
o
1
yield 57 %, m.p. 217218 C; H NMR (300 MHz, CDCl ): δ 7.93 (d, 2H, J = 15.7
3
Hz, C=CH), 7.597.57 (m, 4H, ArH), 7.437.41 (m, 6H, ArH), 6.74 (d, 2H, J =
1
3
1
5.7 Hz, C=CH), 6.25 (s, 1H, enol C=CH), 2.11 (s, 6H, COCH ) ppm; C NMR
3
(
75 MHz, CDCl ): δ 179.4, 172.4, 146.6, 134.2, 131.5, 129.1, 128.1, 120.9, 103.2,
3
23.2 ppm; IR (KBr): ν 3064 (m), 2956 (w), 2866 (w), 1717 (s), 1602 (s), 1564 (s),

1
1
404 (s), 1371 (s), 1270 (s), 1181 (m), 1045 (s), 979 (m), 768 (m), 688 (m) cm ;
+
ESIMS m/z: 426.80 [MNa] ; Anal. calcd. for C H BO : C 68.34, H 5.24; found C
2
3
21
6
68.62, H 5.29.
Diacetoxyboron 1,7-di-p-tolylhepta-1,6-diene-3,5-dionate (2b): Orange powder,
o
1
yield 61 %, m.p. 240241 C; H NMR (300 MHz, CDCl ): δ 7.87 (d, 2H, J = 15.5
3
Hz, C=CH), 7.44 (d, 4H, J = 7.4 Hz, ArH), 7.19 (d, 4H, J = 7.4 Hz, ArH), 6.67 (d,
2
H, J = 15.6 Hz, C=CH), 6.22 (s, 1H, enol C=CH), 2.38 (s, 6H, ArCH ), 2.11 (s,
3
1
3
6
H, COCH ) ppm; C NMR (75 MHz, CDCl ): δ 179.2, 172.3, 146.4, 142.3, 131.7,
3
3
1
29.8, 129.1, 120.0, 102.9, 23.2, 21.6 ppm; IR (KBr): ν 3061 (m), 2953 (w), 2849 (w),
726 (s), 1603 (s), 1555 (s), 1401 (s), 1371 (s), 1258 (s), 1169 (m), 1049 (s), 950 (m),
1

1
+
8
18 (m) cm ; ESIMS m/z: 454.89 [MNa] ; Anal. calcd. for C H BO : C 69.46,
2
5
25
6
H 5.83; found C 69.75, H 4.77.
Diacetoxyboron
1,7-Bis(4-methoxyphenyl)hepta-1,6-diene-3,5-dionate (3b):
o
1
Orange-red powder, yield 63 %, m.p. 229230 C; H NMR (300 MHz, CDCl ): δ
3
7.85 (d, 2H, J = 15.5 Hz, C=CH), 7.51 (d, 4H, J = 8.5 Hz, ArH), 6.89 (d, 4H, J =
8.5 Hz, ArH), 6.57 (d, 2H, J = 15.5 Hz, C=CH), 6.17 (s, 1H, enol C=CH), 3.85 (s,
7

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1
3
6
H, OCH ), 2.11 (s, 6H, COCH ) ppm; C NMR (75 MHz, CDCl ): δ 178.7, 172.3,
3
3
3
1
62.4, 145.9, 131.0, 127.2, 118.5, 114.5, 102.7, 55.4, 23.2 ppm; IR (KBr): ν 3045 (m),
959 (w), 2865 (w), 1717 (s), 1601 (s), 1543 (s), 1398 (s), 1368 (s), 1263 (s), 1173
2

1
+
(
m), 1039 (s), 975 (m), 828 (s) cm ; ESIMS m/z: 486.80 [MNa] ; Anal. calcd. for
C H BO : C 64.68, H 5.43; found C 64.37, H 5.48.
2
5
25
8
Diacetoxyboron 1,7-Di(naphthalen-1-yl)hepta-1,6-diene-3,5-dionate (4b): Red
o
1
powder, yield 71 %, m.p. 241242 C; H NMR (300 MHz, CDCl ): δ 8.79 (d, 2H, J
3
=
15.4 Hz, C=CH), 8.24 (d, 2H, J = 8.3 Hz, ArH), 7.957.87 (m, 6H, ArH),
7.657.48 (m, 6H, ArH), 6.90 (d, 2H, J = 15.4 Hz, C=CH), 6.39 (s, 1H, enol
1
3
C=CH), 2.18 (s, 6H, COCH ) ppm; C NMR (75 MHz, CDCl ): δ 179.3, 172.4,
3
3
142.8, 133.8, 132.1, 131.7, 131.3, 128.9, 127.4, 126.5, 126.1, 125.4, 123.3, 123.0,
103.6, 23.3 ppm; IR (KBr): ν 3052 (m), 2954 (w), 2858 (w), 1722 (s), 1606 (s), 1555

1
(
s), 1398 (s), 1372 (s), 1273 (s), 1055 (s), 953 (m), 857 (m), 773 (s) cm ; ESIMS
+
m/z: 526.88 [M+Na] ; Anal. calcd. for C H BO : C 73.83, H 5.00; found C 73.56, H
3
1
25
6
5
.05.
Diacetoxyboron
Purple-red powder, yield 68 %, m.p. 211212 C; H NMR (300 MHz, CDCl ): δ
1,11-Diphenyl-undeca-1,3,8,10-tetraene-5,7-dionate
(5b):
o
1
3
7.68 (dd, 2H, J = 14.8, 10.7Hz, C=CH), 7.48 (d, 4H, J = 5.9 Hz, ArH), 7.387.35
(m, 6H, ArH or C=CH), 7.076.89 (m, 4H, ArH), 6.26 (d, 2H, J = 14.9 Hz,
1
3
C=CH), 6.07 (s, 1H, enol C=CH), 2.10 (s, 6H, COCH ) ppm; C NMR (75 MHz,
3
CDCl ): δ 178.6, 172.3, 146.6, 144.2, 135.8, 129.9, 128.9, 127.7, 126.7, 124.5, 103.1,
3
23.2 ppm; IR (KBr): ν 3081 (m), 3039 (m), 2952 (w), 2848 (w), 1712 (s), 1603 (s),
8

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1543 (s), 1399 (s), 1370 (s), 1274 (s), 1143 (m), 1048 (s), 995 (m), 757 (m), 688 (m)

1
+
cm ; EIMS m/z: 478.82 [MNa] ; Anal. calcd. for C H BO : C 71.07, H 5.52;
2
7
25
6
found C 71.38, H 5.47.
3
. Results and discussion
3
.1. Chemistry
The synthetic route of diacetoxyboron complexes with curcumin derivatives was
illustrated in Scheme 1. The curcumin derivatives (1a5a) were synthesized by
condensation reaction of the corresponding aromatic aldehydes with acetylacetone in
the presence of boron trioxide, tributyl borate and n-butylamine in ethyl acetate
according to literatures [13]. Then, the curcumin derivatives were converted to
diacetoxyboron complexes by the chelation reaction with boron acetate in benzene.
The products were recrystallized from chloroform to obtain the pure diacetoxyboron
complexes (1b–5b).
<
Scheme 1>
The structure elucidation of these diacetoxyboron complexes was based on the
1
13
IR, H NMR, C NMR, ESI–MS spectroscopy and elemental analysis. The IR
spectra of these diacetoxyboron complexes (1b5b) showed the CO and enolic CC

1
1
stretching vibrations at 16061601 cm and 15641543 cm were red-shifted 24–30

1
1
cm and 24–57 cm compared to the curcumin derivatives, respectively [23,24], and

1
the disappearance of corresponding O–H vibration bands at 3441–3431 cm which
9

ACCEPTED MANUSCRIPT
existed in the curcumin derivative ligands due to their keto–enol tautomerism.

1
Meanwhile, the new strong CO stretching vibrations at 17261712 cm were
assigned to the acetoxy moieties of diacetoxyboron complexes. The strong absorption

1
1
bands at 13721368 cm and 10551039 cm were attributed to the BO stretching

1
vibrations [23, 25] and the strong CO stretching vibrations at 12741258 cm were
due to the acetoxy moieties. Obviously, the curcumin derivatives have no strong
1
absorption bands in these ranges. In H NMR spectra of the diacetoxyboron
complexes (1b5b), the enolic proton signals at δ = 16.1015.86 ppm disappeared
completely and the new methyl proton signals of the acetoxy moieties at  =
2.182.10 ppm were observed in comparison with the curcumin derivatives. The
methylene proton signals at  = 6.396.07 ppm of the keto-enol tautomerism were
shifted 0.370.43 ppm to lower field with respect to the corresponding curcumin
derivatives [23,24], which was due to the strong coordination of the B(OAc)2
fragments. However, other chemical proton signals have no significant changes.
3
.2. UV-vis absorption properties
These diacetoxyboron complexes were the DAD electronic structure dyes,
which possessed electron donor groups at both ends of the conjugated system with
an electron acceptor group in the middle of the molecules. They exhibited high
absorption coefficients at 400–550 nm (Fig. 1 and Table 1) in dichloromethane

5
1
solution (1.0 ×10 mol L ), which were very suitable for photosensitizer and
fluorescent dyes. From Fig. 1, it was observed that these diacetoxyboron complexes
displayed the weaker absorption peaks at 240–280 nm, which were corresponding to
1
0

ACCEPTED MANUSCRIPT
the terminal aromatic moieties, and the stronger absorption peaks at 424–474 nm with

a shoulder at 450–502 nm except complex 4b, which were due to  transition of
the conjugated B(OAc) -chelating system. As can also be seen, the electron donating
2
abilities of the terminal aromatic moieties have a great influence on the spectroscopic
behavior of these diacetoxyboron complexes. For instance, the maximum absorption
wavelength of complexes 3b, 4b and 5b revealed a considerable bathochromic shift (>
5
0 nm) in comparison with that of the complex 1b, which was assignable to the
introduction of the strong electron-donating methoxyl moiety or the large
-conjugated aromatic moieties. Meanwhile, these diacetoxyboron complexes (1b–5b)
showed the much higher extinction coefficients (logεmax = 4.51‒4.70).

<
Figure 1>
<
Table 1>
3
.3. Fluorescence properties
The fluorescence behaviors of diacetoxyboron complexes (1b5b) were determined

5
1
in dichloromethane solution (1.0 ×10 mol L ). Their fluorescence spectra are
presented in Fig. 2 and the fluorescence data are listed in Table 1. From Fig. 2A, their
excitation spectra showed a broad band at 350–500 nm, which were similar to their
UV-vis absorption spectra, but excitation band of complexes 3b was much broader
and more intensive in these diacetoxyboron complexes. From Fig. 2B, their emission
1
1

ACCEPTED MANUSCRIPT
spectra displayed the strong fluorescence ranging from blue to yellow-green at
473–554 nm in dichloromethane solution. The terminal aromatic moieties of these
complexes exhibited a significant impact on their fluorescence behaviors. Compared
to complex 1b, the emission bands of complexes 3b, 4b and 5b showed the much
larger bathochromic shift (62–81 nm), which were due to the stronger
electron-donating or the larger -conjugated terminal aromatic moieties in the
B(OAc) -chelating system. Especially, the diacetoxyboron complex 3b exhibited the
2
strongest emission intensity in these complexes, which was in agreement with its
excitation spectra. Furthermore, the quantum yields (Φ ) of these diacetoxyboron
u
complexes were also measured relative to quinine sulfate (Φ = 0.55). From Table 2,
s
it can be pointed out that these complexes had high quantum yields (Φ = 0.45‒0.92)
u
in dichloromethane solution, especially the complex 3b exhibited the highest
quantum efficiency (Φ = 0.92) in these complexes, which was probably due to the
u
introduction of the strong electron donor methoxyl in the terminal aromatic moieties.
Meanwhile, the fluorescence lifetimes (τ) of complexes 1b5b in dichloromethane
solution were studied by time-resolved fluorescence spectroscopy. Their fluorescence
decays were fitted to a monoexponential function (Fig. 3) and the data were presented
in Table 1. It was found that the complex 4b had much longer lifetime values (τ =
5.99 ns) in dichloromethane among these complexes, which was probably due to the
introduction of the rigid naphthalene ring in the conjugated system. However, the
complex 2b exhibited much shorter lifetime values (τ = 1.26 ns) in these complexes,
mainly assigned to the methyl moieties containing high-energy oscillator C‒H groups
1
2

ACCEPTED MANUSCRIPT
in the conjugated system, which led to an increase in the non-radiative decay.
<
<
Figure 2>
Figure 3>
The fluorescence behaviors of diacetoxyboron complexes (1b5b) were also
investigated in solid state at room temperature (Fig. 4 and Table 2). The
excitation spectra of these complexes were much broader than in solution and
red-shifted remarkably, which was attributed to the aggregation effect in solid state.
Their emission spectra showed that these complexes emitted a yellow to red emission
at 535–643 nm, which were red-shifted 62–94 nm in comparison with those of the
corresponding complexes in dichloromethane solution. Meanwhile, the Stokes shifts
of complexes (1b5b) in solid state were much larger than in solution. For example,
the complex 3b bore a Stokes shift of 115 nm. These fluorescence properties could
probably be assigned to their molecular conformation and intermolecular interactions
in solid state [26,27]. From Fig. 3B, the complex 3b revealed the strongest emission
intensity and largest Stokes shift in solid state. However, the complex 5b exhibited
the negligible emission intensities in solid state, which was much weaker than that
recorded in solution. It was probably because of the consecutive and strong
intermolecular  interactions between neighbouring dye molecules resulting in
fluorescence quenching in solid state [28]. In addition, the CIE color coordinates of
1
3

ACCEPTED MANUSCRIPT
complexes 1b–5b in solid state was determined from their emission spectra (Fig. 5
and Table 2). The results showed that the chromaticity coordinate for the complex 3b
was CIE (x = 0.641, y = 0.358), which was very close to the ideal red CIE (x = 0.64,
y = 0.34) [29]. It indicated that the diacetoxyboron complex 3b was promising for
applications in optoelectronic materials.
<
<
Figure 4>
Figure 5>
<
Table 2>
3
.4. Solvatochromic effect
The effect of the solvent on UV-vis absorption and fluorescence behaviors of
diacetoxyboron complexes (1b5b) was investigated at room temperature. The
UV-vis absorption and fluorescence spectra of these complexes in seven different
solvents (such as toluene, tetrahydrofuran, chloroform, dichloromethane, acetone,
acetonitrile and dimethyl sulfoxide) were shown in Figs. 6 and S1–S4. The changes
in their photophysical properties depending on solvent polarity were listed in Table 3.
From Table 3, the type of these solvents had a negligible effect on the UV-vis
absorption behaviors of these diacetoxyboron complexes, which indicated a small
difference in the dipole moments between the Franck–Condon excited state and the
1
4

ACCEPTED MANUSCRIPT
ground state [30]. In contrast to UV-vis absorption spectra, these complexes showed a
pronounced positive emission solvatochromism. The normalized fluorescence spectra
of diacetoxyboron complexes in various solvents were shown in Figs. 7A and S5–S8.
It was found that their fluorescence maxima caused a remarkable bathochromic shift

1
(
10–45 nm) and the Stokes shift values (562–1329 cm ) were significantly enlarged
with the increase in solvent polarity. For instance, the complex 3b exhibited a large
red-shift of 38 nm in emission peaks from 510 nm in low-polar toluene to 548 nm in
high-polar DMSO. A notable change from green to yellow could obviously be
observed when the solutions of the complex 3b were placed under 365 nm UV
illumination (Fig. 7B). These data suggested that the energy of the excited state was
more influenced by increasing the solvent polarity in comparison with the energy of
the ground state [31]. It also revealed that there was a significant difference between
the excited and ground state charge distribution in these diacetoxyboron molecules,
which was due to the stronger intermolecular interaction with polar solvents in the
excited state [32]. The fluorescence quantum yields of diacetoxyboron complexes
(
1b5b) in seven different solvents were also determined using quinine sulfate (Φ =
s
0
.55) as standard and presented in Table 3. It was found that these complexes
exhibited the much higher quantum yields in toluene, which meant that the
diacetoxyboron complexes were strong emissive in non-polar solvents and their
quantum yields were decreased with the increase of the solvent polarity. This was
because that the radiative decay process was decreased with increasing the solvent
polarity [9]. In addition, the fluorescence decay behaviors of complexes 1b5b were
1
5

ACCEPTED MANUSCRIPT
measured using time-resolved fluorescence spectroscopy in seven different solvents
(Figs. 8 and S9–S12) and lifetime values (τ) were summarized in Table 3. The results
showed that all the fluorescence decays of diacetoxyboron complexes in solutions
were well-fitted with a monoexponential function at their emission peak wavelength
and their lifetime values (τ) also changed depending on solvent polarity. From Table 3,
the fluorescence lifetimes (τ) of these complexes displayed a downward trend as a
whole with the increase of the solvent polarity, which could be explained by the
increase of the non-radiative decay in polar solvent resulted from these processes
such as quenching, energy transfer, and solvent-solute interaction.
<
<
<
Figure 6>
Figure 7>
Figure 8>
<
Table 3>
In order to gain further insight on solvatochromic behaviors of these
diacetoxyboron complexes, the correlation of the Stokes shift value (Δν = ν –ν )
abs
flu
with the solvent polarity parameter (E (30)) was investigated (the solvent parameters
T
are in Table S1). We plotted the fluorescence Stokes shift to the solvent E (30)
T
1
6

ACCEPTED MANUSCRIPT
parameter in Figs. 9 and S13–S17. Their linear variations were obtained and the plots
were linear with correlation coefficients r = 0.94–0.97. The linear relationship of their
plots implied that the dipole–dipole interaction between the solute and solvent was
responsible for the larger solvent-dependent fluorescence shift in diacetoxyboron
complexes.
<
Figure 9>
According to the Lippert–Mataga equation (Eq.1) [33], the difference in dipole
moments between the excited and ground states (Δμ) can be calculated from the slope
of a plot of the Stokes shift (Δν) versus Δf:
2
2


   
f  constant
abs
flu
(1)
3
hca

1
where Δν is the Stokes shift in wavenumber (cm ), ν and ν are the wavenumbers
abs
flu
of the absorption and fluorescence maxima, respectively, h is the Planck’s constant, c
is the velocity of light, a is the cavity radius, and Δf is the orientation polarizability of
the solvent, defined as follows (Eq. 2):
2

2
1
n 1
f 

(2)
2
 1 2n 1
Where  is the solvent dielectric constant and n is the solvent refractive index. The
Lippert–Mataga plots of diacetoxyboron complexes (1b5b) in different solvents
were shown in Figs. 10 and S18–S22. It was observed that these plots revealed the
better linearity with correlation coefficients r = 0.94–0.96. The positive slope
1
7

ACCEPTED MANUSCRIPT
indicated the dipole moments of these complexes in the excited state were larger than
the corresponding dipole moments in the ground state [34]. Additionally, the dipole
moment difference between the ground and excited states (Δμ) of complexes (1b5b)
can be estimated by the slope of these lines from the Lippert–Mataga equation. The
cavity radii (a) of complexes 1b5b were calculated as 5.43, 5.56, 5.69, 5.85 and
5.66 Å, respectively, by using Eq. (3) [35].
3
M
1
/ 3
a  (
)
(3)
4
Nd
where M is the molecular weight of the solute, N is the Avogadro constant, and d is
3
an assumed molecular density of 1g/cm . The estimated dipole moment differences
(Δμ) were 17.15, 18.12, 23.21, 29.30 and 23.41 D (debye) for complexes 1b5b,
respectively. These large dipole moment differences suggested the occurrence of
intramolecular charge-transfer induced by the solvent after excitation [36]. Especially,
complexes 3b5b exhibited the higher Δμ values, indicating a stronger increase in the
excited state dipole moment and a more noticeable fluorescence solvatochromic
effects.
<
Figure 10>
3
.5. Electrochemical properties
Electrochemical behaviors of diacetoxyboron complexes 1b–5b were

4
1
determined by cyclic voltammetry (CV) in CH Cl solutions (5.0  10 molL )
2
2
+
using the ferrocenium/ferrocene (Fc /Fc) couple as the reference potential standard
1
8

ACCEPTED MANUSCRIPT
(Fig. 11), and electrochemical data were summarized in Table 4. These complexes all
onset
exhibited the irreversible oxidation (Eox
= +1.11, +1.01, +0.81, +1.13, +1.16 V vs.
+
onset
Fc /Fc for complexes 1b–5b, respectively) and reduction process (Ered
= 0.93,
+

0.96, 1.14, 1.10, 0.95 V vs. Fc /Fc for complexes 1b–5b, respectively) in the
cyclic voltammograms. It was found that complex 1b displayed the smaller oxidation
and reduction potentials in these complexes, which was due to the electron-donating
methoxy substituents in the conjugated B(OAc) -chelating unit. From the onset
2
potentials of the oxidation and reduction potentials, the HOMO and LUMO energy
onset
levels can be estimated from the empirical formula [37] : EHOMO (eV) = (Eox
+
onset
4.8) and ELUMO (eV) = (Ered
+ 4.8). The obtained HOMO energy levels were
5.91, 5.81, 5.61, 5.93, 5.96 eV for complexes 1b–5b, respectively, and the
LUMO energy levels were 3.87, 3.84, 3.66, 3.70, 3.85 eV for complexes
1
b–5b, respectively (Table 4). The electrochemical HOMO-LUMO gaps can be
electrochem
obtained from the formula: Egap = ELUMO  EHOMO, and the Eg
of complexes
1
b–5b were 2.04, 1.97, 1.95, 2.23, 2.11 eV, respectively (Table 4). It was observed
that the HOMO-LUMO gaps of complexes 2b and 3b were smaller in these
complexes, which was probably due to the introduction of the electron donor moieties
in the conjugated system [38]. Meanwhile, the optical band gaps of these complexes
can also be calculated using Tauc relation [39] from the UV-vis absorption spectra
opt
(
Table 4). Their values of Eg were 2.67, 2.58, 2.43, 2.38, 2.36 eV for complexes
1
b–5b, respectively, which was in agreement with their electrochemical band gaps.
1
9

ACCEPTED MANUSCRIPT
<
Figure 11>
<
Table 4>
4
Conclusions
In conclusion, several novel diacetoxyboron complexes of β-diketones
1
incorporated curcumin moiety were synthesized and characterized by FTIR, H NMR,
MS and elemental analysis. Their photophysical properties were studied by UV–vis
absorption and fluorescence spectroscopy in detail. It was found that their
fluorescence spectra yielded a blue to yellow-green emission at 473–554 nm in
dichloromethane solution and emitted a yellow to red emission at 535–643 nm in
solid state. The terminal aromatic moieties of these complexes exhibited a significant
impact on their fluorescence behaviors. Especially, the complex 3b displayed the
strongest emission intensity and the highest quantum efficiency (Φ = 0.98) in
u
toluene among these complexes. The CIE coordinate of the complex 3b [CIExy
(0.641, 0.358)] in solid state was positioned in an ideal red region of the chromaticity
diagram. Meanwhile, the effect of different solvents on absorption and fluorescence
properties of these complexes was also studied and the solvent dependence of their
spectral behaviors was discussed by using the solvent polarity parameter approach
and Lippert–Mataga plot. The results showed that these complexes exhibited a
pronounced positive fluorescence solvatochromism and the large dipole moment
2
0

ACCEPTED MANUSCRIPT
differences between the ground and excited states. It implied a strong increase in the
excited state dipole moment and the charge-transfer character of the excited state. In
addition, the HOMO, LUMO energy levels and energy band gaps of these complexes
were calculated according to the onset oxidation and reduction potentials of the cyclic
voltammograms. These results indicated that these DAD electronic structure boron
complexes might be used as excellent photoelectric functional materials and
fluorescent molecular probes.
Acknowledgment
The authors gratefully acknowledge financial support from National Natural
Science Foundation of China (No. 31870328) and Hubei Key Laboratory of Pollutant
Analysis and Reuse Technology (No. PA160203).
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2
6

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Tables:
Table 1
Photophysicaldata of diacetoxyboron complexes 1b–5bin dichloromethane solution.
‒
1
Complexes
max (nm)
log εmax λex (nm)
λem (nm) τ (ns)
Φu
νstokes (cm )
1
2
3
4
5
b
b
b
b
b
424, 450(sh) 4.51
440, 466(sh) 4.64
464, 490(sh) 4.66
428
442
468
470
478
473
492
535
554
545
3.63 0.67
1.26 0.56
2.63 0.92
5.99 0.64
1.84 0.45
2443
2402
2860
472
4.65
3136
474, 502(sh) 4.70
2748
Table 2
Photophysical data for diacetoxyboron complexes 1b–5b in solid state.
‒
1
Complexes
λex (nm)
473
λem (nm)
535
stokes (cm )
CIEx
0.366
0.492
0.641
0.661
0.599
CIEy
0.602
0.502
0.358
0.338
0.397
1
2
3
4
5
b
b
b
b
b
2450
482
566
3079
513
628
3570
537
643
3070
550
639
2532
2
7

ACCEPTED MANUSCRIPT
Table 3
Spectroscopic and photophysical properties of complexes 1b–5bin various solvents.
‒
1
Complexes
Solvents
Toluene
THF
λmax (nm) log εmax λem (nm)
νstokes (cm )
2126
τ (ns)
3.58
3.49
3.60
3.63
3.43
3.24
3.13
Φu
1
2
3
4
5
b
b
b
b
b
424
422
426
424
422
422
428
4.61
4.56
4.54
4.51
4.61
4.55
4.68
466
468
472
473
473
476
480
0.72
0.73
0.66
0.67
0.57
0.64
0.52
2329
2288
2443
2555
2688
2531
CHCl3
CH Cl
2
2
Acetone
CH CN
3
DMSO
Toluene
THF
436
434
440
440
434
434
442
4.58
4.64
4.59
4.64
4.61
4.66
4.69
481
484
491
492
489
493
499
2146
2380
2360
2402
2592
2757
2584
1.34
1.32
1.25
1.26
1.19
1.23
1.27
0.88
0.75
0.63
0.56
0.58
0.55
0.51
CHCl3
CH Cl
2
2
Acetone
CH CN
3
DMSO
Toluene
THF
456
456
464
464
458
462
468
4.72
4.69
4.64
4.66
4.70
4.69
4.74
510
518
529
535
531
541
548
2322
2625
2648
2860
3002
3161
3120
2.45
2.38
2.52
2.63
2.29
2.09
2.26
0.98
0.95
0.91
0.92
0.88
0.85
0.83
CHCl3
CH Cl
2
2
Acetone
CH CN
3
DMSO
Toluene
THF
468
464
474
472
464
468
478
4.56
4.54
4.60
4.65
4.59
4.58
4.62
529
539
551
554
553
569
574
2464
2999
2948
3136
3469
3793
3499
5.13
5.75
5.73
5.99
4.67
4.50
4.24
0.84
0.75
0.73
0.64
0.58
0.43
0.38
CHCl3
CH Cl
2
2
Acetone
CH CN
3
DMSO
Toluene
THF
470
468
474
474
468
472
478
4.70
4.67
4.70
4.70
4.76
4.77
4.81
523
531
543
545
544
550
558
2156
2535
2681
2748
2985
3005
3000
2.57
2.38
1.93
1.84
1.77
1.81
2.03
0.74
0.61
0.47
0.45
0.41
0.33
0.38
CHCl3
CH Cl
2
2
Acetone
CH CN
3
DMSO
2
8

ACCEPTED MANUSCRIPT
Table 4 Electrochemical properties of diacetoxyboron complexes 1b–5b.
onset
onset
electrochem
opt
Complexes Eox
(V) Ered
(V) HOMO(eV) LUMO(eV) Eg
(eV)
Eg (eV)
1
2
3
4
5
b
b
b
b
b
+1.11
+1.01
+0.81
+1.13
+1.16
0.93
0.96
1.14
1.10
0.95
5.91
5.81
5.61
5.93
5.96
3.87
3.84
3.66
3.70
3.85
2.04
2.67
2.58
2.43
2.38
2.36
1.97
1.95
2.23
2.11
Schemes:
AcO OAc
B
O
O
O
O
B O , B(OC H )
2
3
4
9 3
B(OAc)3
O
O
+
R CHO
R
R
C H NH
Benzene
4
9
2
R
R
1a - 5a
1b - 5b
,
CH3,
R =
OCH3,
,
1
2
3
4
5
Scheme 1 Synthesis of diacetoxyboron complexes 1b–5b
2
9