Triphenylamine-modified difluoroboron dibenzoylmethane derivatives: Synthesis, photophysical and electrochemical properties

Article abstract of DOI:10.1016/j.dyepig.2016.09.036

Two new triphenylamine functionalized difluoroboron (BF₂) β-diketonate complexes, one asymmetrical 2 and the other symmetrical 3, were synthesized and fully characterized. The effects of the triphenylamine substituent on the photoluminescence,

Full text of DOI:10.1016/j.dyepig.2016.09.036

Accepted Manuscript
Triphenylamine-modified difluoroboron dibenzoylmethane derivatives: Synthesis,
photophysical and electrochemical properties
Hao Zhang, Chun Liu, Jinghai Xiu, Jieshan Qiu
PII:
S0143-7208(16)30733-1
10.1016/j.dyepig.2016.09.036
DYPI 5488
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 21 July 2016
Revised Date: 12 September 2016
Accepted Date: 13 September 2016
Please cite this article as: Zhang H, Liu C, Xiu J, Qiu J, Triphenylamine-modified difluoroboron
dibenzoylmethane derivatives: Synthesis, photophysical and electrochemical properties, Dyes and
Pigments (2016), doi: 10.1016/j.dyepig.2016.09.036.
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Graphical Abstract
Triphenylamine-modified
difluoroboron
dibenzoylmethane
derivatives: synthesis, photophysical and electrochemical properties
Hao Zhang, Chun Liu,* Jinghai Xiu and Jieshan Qiu
[*] Dr. C. Liu, Corresponding Author
State Key Laboratory of Fine Chemicals, Dalian University of Technology, Linggong
Road 2, Dalian 116024, China.
Tel.: +86-411-84986182.
E-mail: cliu@dlut.edu.cn

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Triphenylamine-modified difluoroboron
dibenzoylmethane
derivatives: synthesis, photophysical and electrochemical properties
Hao Zhang, Chun Liu,* Jinghai Xiu and Jieshan Qiu
[*] Dr. C. Liu, Corresponding-Author
State Key Laboratory of Fine Chemicals, Dalian University of Technology, Linggong
Road 2, Dalian 116024, China
Tel.: +86-411-84986182.
E-mail: cliu@dlut.edu.cn
Keywords: Difluoroboron complex, Triphenylamine, ICT emission, Photostability, Polarity probe.

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Abstract:
Two new triphenylamine functionalized difluoroboron (BF2) β-diketonate
complexes, one asymmetrical 2 and the other symmetrical 3, were synthesized and
fully characterized. The effects of the triphenylamine substituent on the
photoluminescence, redox properties and photostability of these BF2 complexes were
investigated. These new compounds exhibit long-wavelength absorption and strong
intramolecular charge transfer emission and reversible oxidation and reduction waves
in cyclic voltammetry experiments. It was found that they were highly fluorescent in
toluene with large Stokes shifts, and demonstrated excellent photostability
comparable to their parent BF2 nucleus. Meanwhile, these new complexes are highly
sensitive to the solvent polarity, which indicates that they could be used as highly
sensitive polar probes besides excellent emitting materials.

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1. Introduction
Difluoroboron complexes, classified as
N,N-bidentate, O,O-bidentate and
N,O-bidentate compounds [1, 2], have been extensively investigated in recent
years for their various applications in bioprobes, fluorescent indicators and
photosensitizers [3-9]. Among them, of particular importance are difluoroboron
β-diketonate complexes, which have outstanding properties such as large molar
extinction coefficients, intense emission, redox activities, high photostability
and sensitivity to the surrounding medium [10-13]. Owing to these properties,
they have been widely employed in near-IR probes [14, 15]. optical materials
[16, 17], solar cells [18], mechanochromic luminescent materials [19, 20] and
organic field-effect transistors [21]. For example, a class of difloroboron
β-diketonate complexes reported by Fraser’s group showed mechanochromic
luminescence [22-25]. In 2015, Lu et al. prepared some luminescent organogels
based on the difluoroboron complexes [26]. Untill now, many new
difluoroboron complexes have been synthesized and the study on their
structure-property relationship has become a research hotspot [20, 27-29].
Difluoroboron dibenzoylmethane derivatives (BF2dbms), as a special type of
difloroboron β-diketonate complexes, have also attracted considerable attention
as fluorophores due to their unique optical properties [17, 30, 31]. Recently,
Wang et al. reported the synthesis and spectrocopic properties of new
diaroylmethanatoboron difluoride derivatives [32]. In 2013, Gao et al.

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aggregation-induced emission
dibenzoylmethane derivatives [31].
investigated
the
of
difluroboron
Triphenylamine (TPA) has been used as a building block to construct
numerous organic electro-luminescent materials [33-36], because of its
noncoplanarity of the three phenyl substituents, strong electron donor,
prominent light-to-electrical energy conversion efficiencies, and good hole
transporting capability. A series of new bis- and tris-β-diketone-boron
difluorides using TPA as the terminal groups have been also reported by Lu et
al [36-38]. More recently, Ono and coworkers have synthesized difluoroboron
complexes containing n-butyl-substituted TPA moiety and investigated their
photoluminescence properties. These complexes could be used to fabricate
dye-sensitized solar cells, providing the conversion efficiencies in a range of
2.7-4.4% [35]. However, TPA-modified asymmetrical and symmetrical
difluoroboron complexes based on BF2dbm (
reported. In this work, we design and synthesize two new TPA-bearing
difluoroboron complexes (asymmetrical, Scheme 1) and (symmetrical,
1, Scheme 1) have not been
2
3
Scheme 1) to understand how the TPA moiety affect the properties of the
corresponding complexes and what the differences in photoelectric
performances between symmetrical and asymmetric TPA-modified
difluoroboron complexes are.
In this paper, an asymmetric compound
2 and a symmetric complex 3 have
been successfully synthesized (Scheme 1). Investigations on the

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structure-property relationships, such as photostability, optical and
electrochemical properties, have been carried out. Meanwhile, theoretical
calculations (DFT) were also performed to support and provide further insights
into our experimental findings. Interestingly, the results show that
3 is able to
monitor the ethanol contents in toluene with a limit of detection up to 0.0091%.
Here Scheme 1. Molecular structures of complexes 1, 2 and 3.
2. Experimental
2.1 Materials and measurements
CH2Cl2 was freshly distilled from CaH2. All the other chemicals and reagents were
1
purchased from commercial sources and used without further purification. H NMR,
13C NMR and 19F NMR spectra were recorded on a 400 MHz Varian Unity Inova
spectrophotometer. Mass spectra were recorded with a MALDI micro MX
spectrometer. UV-vis absorption spectra were recorded by Lambda 750S UV/vis/NIR
spectrophotometer (PerkinElmer). Fluorescence emission spectra were recorded by
HITACHI F-7000 fluorescence spectrophotometer. Fluorescence lifetimes were
determined on a steady and transient state fluorometer (FLS920, Edinburgh Analytical
Instruments Ltd.). IR spectra were recorded on a FTIR NEXUS Spectrometer using
KBr disks and wave numbers were given in cm-1. Melting points were determined
using X-4 digital melting point apparatus. Cyclic voltammograms of all the
difluoroboron β-diketonate complexes were recorded on an electrochemical

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workstation (BAS100B/W, USA) at room temperature. Crystallographic data were
collected using a Bruker SMART CCD diffractometer and graphite−monochromated
Mo−Kα radiation (λ = 0.71073 Å).
2.2 Synthesis
Compound 8 [39] and ligands 5-6 [40] were conveniently synthesized according to
the literature method and then used to synthesize difluoroboron β-diketonate
complexes via a distinctive step as previously reported (Scheme 2) [41].
2.2.1. Compound 8.
Sodium hydride (0.19 g, 8.0 mmol) was added quickly to a dry flask containing a
solution of methyl benzoate (1.25 mL, 10.0 mmol) in 20 mL of dry THF. The mixture
was heated to 60 °C. 1-(4-Bromophenyl)ethanone (0.99 g, 5.0 mmol) in dry THF (20
mL) was added dropwise to the mixture. The reaction mixture was refluxed under an
atmosphere of nitrogen for 24 h. Then the mixture was poured into water and
neutralized with hydrochloric acid until pH = 7. The mixture was extracted twice with
ethyl acetate. After the solvent was removed, the residue was purified by column
chromatography (silica gel, ethyl acetate/petroleumether, V/V = 300/1) to afford a
pale yellow solid (1.02 g, 3.37 mmol, 66%). 1H NMR (400 MHz, CDCl3, 25
°
C) δ
= 8.6 Hz, 2H, ortho-C6H4),
J = 7.4Hz, 1H, para-C6H5 ), 7.50
7.98 (d,
7.63 (d,
J
= 7.4 Hz, 2H, ortho-C6H5 ), 7.86 (d,
= 8.6 Hz, 2H, meta-C6H4), 7.57 (t,
J
J
(t,
J = 7.4 Hz, 2H, meta-C6H5), 6.82 (s, 1H, CHCO) (Fig. S3).
2.2.2. Ligand 5.

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A mixture of 4-(diphenylamino)phenylboronic acid (0.29 g, 2.0 mmol), compound
8 (0.30 g, 1.0 mmol), and K2CO3 (0.21 g, 1.5 mmol) in distilled water (9 mL) and
THF (15 mL) was charged and bubbled by nitrogen for 20 min, and then Pd(PPh3)4
(20.0 mg, 0.017 mmol) was added. The mixture was refluxed for 24 h under nitrogen.
After cooling to room temperature, the mixture was extracted with ethyl acetate three
times. Then the organic phase was combined and dried over anhydrous Na2SO4. After
removing the solvent under reduced pressure, the residue was purified by column
chromatography (silica gel, petroleum ether/ethyl acetate, V/V = 100/1) to give a light
1
1
yellow solid (0.28 g, 0.60 mmol, 60%). H NMR (400 MHz, CDCl3, 25
NMR (400 MHz, CDCl3, 25 C) 8.05 – 7.86 (m, 4H, ortho-C6H5CO,
C6H4CO), 7.62 (d, = 8.6 Hz, 2H, meta-C6H4CO), 7.55 – 7.37 (m, 5H, meta-,
para-C6H5CO, C6H4), 7.31 – 7.19 (m, 4H, meta-C6H5), 7.14 – 7.00 (m, 8H,
ortho- para-C6H5, C6H4 ), 6.90 (s, 1H, CHCO) (Fig. S4).
°C) H
°
δ
J
,
2.2.3. Compound 10.
A mixture of 4-(diphenylamino)phenylboronic acid (0.29 g, 2.0 mmol), compound
7 (0.19 g, 1.0 mmol), and K2CO3 (0.21 g, 1.5 mmol) in distilled water (4 mL) and
ethanol (12 mL) was charged and then Pd(OAc)2 (23.0 mg, 0.10 mmol) was added.
The mixture was refluxed for 3 h, monitored by TLC (petroleum ether/ethyl acetate,
V/V = 10/1). After cooling to room temperature, the saturated salt water was added
and the mixture was extracted with ethyl acetate for three times. Then the organic
phase was combined and dried over anhydrous Na2SO4. After removing the solvent
under reduced pressure, the residue was purified by column chromatography (silica

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gel, petroleum ether/ethyl acetate, V/V = 50/1) to give a yellow green solid (0.39 g,
1.07 mmol, 98%). 1H NMR (400 MHz, CDCl3, 25
C8H7O), 7.66 (d, = 8.4 Hz, 2H, C8H7O), 7.51 (d,
°
C) δ 8.01 (d,
= 8.8 Hz, 2H, C6H4), 7.31 –
para-C6H5, C6H4), 2.63
J = 8.4 Hz, 2H,
J
J
7.26 (m, 4H, meta-C6H5), 7.16 – 7.01 (m, 8H, ortho-
,
(s, 3H, CH3) (Fig. S5).
2.2.4. Compound 11
A mixture of 4-(diphenylamino)phenylboronic acid (0.29 g, 2.0 mmol),
compound (0.21 g, 1.0 mmol), and K2CO3 (0.21 g, 1.5 mmol) in distilled
.
9
water (4 mL) and ethanol (12 mL) was charged and then Pd(OAc)2 (23.0 mg,
0.10 mmol) was added. The mixture was refluxed for 3 h, monitored by TLC
(petroleum ether:ethyl acetate, V/V = 10:1). After cooling to room temperature,
the saturated salt water was added and the mixture was extracted with ethyl acetate
for three times. Then the organic phase was combined and dried over anhydrous
Na2SO4. After removing the solvent under reduced press, the residue was
purified by column chromatography (silica gel, petroleum ether/ethyl acetate,
1
V/V = 50/1) to give a yellow green solid (0.36 g, 0.95 mmol, 96%). H NMR
(400 MHz, CDCl3, 25
°
C)
δ
8.08 (d,
J = 8.4 Hz, 2H, C8H7O2), 7.63 (d, J = 8.4
Hz, 2H, C8H7O2), 7.50 (d,
J
= 8.7 Hz, 2H, C6H4), 7.29 – 7.26 (m, 4H,
meta-C6H5), 7.18 – 7.01 (m, 8H, ortho-, para-C6H5, C6H4), 3.93 (s, 3H, OCH3)
(Fig. S6).
2.2.5. Ligand 6.

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Sodium hydride (0.048 g, 2.0 mmol) was added quickly to a dry flask containing a
solution of compound 11 (0.19 g, 0.5 mmol) in 10 mL of dry THF. The mixture was
o
heated to 60 C. Compound 10 (0.18 g, 0.5 mmol) in dry THF (10 mL) was added
dropwise to the mixture. The reaction mixture was refluxed under an atmosphere of
nitrogen for 24 h. Then the mixture was poured into water and neutralized with
hydrochloric acid until pH = 7. And the mixture was extracted twice with ethyl acetate.
After the solvent was removed, the residue was purified by column chromatography
(silica gel, ethyl acetate/petroleum ether, V/V = 100/1) to afford a pale yellow solid
1
(0.17 g, 0.24 mmol, 46%). Mp: 113 – 115 °C. H NMR (400 MHz, CDCl3, 25 °C) δ
8.07 (d, J = 8.4 Hz, 4H, C6H4CO), 7.71 (d, J = 8.4 Hz, 4H, C6H4CO), 7.55 (d, J = 8.7
Hz, 4H, C6H4), 7.33 – 7.27 (m, 8H, meta-C6H5), 7.17 - 7.02 (m, 16H, ortho-,
13
para-C6H5, C6H4), 6.93 (s, 1H, CHCO) (Fig. S7). C NMR (100MHz, CDCl3, 25
°C) δ = 184.8, 183.7, 147.8, 147.1, 144.3, 133.4, 132.9, 129.0, 127.7, 127.4, 126.3,
124.4, 123.0, 92.6 (Fig. S8). IR (KBr): ν = 3433, 2960, 2921, 1734, 1589, 1488, 1379,
1327, 1278, 1197, 825, 788, 753, 696, 617, 511 cm-1. MALDI-TOF-MS (m/z): calcd
for C51H38N2O2 [M]+ 710.2933; found 710.2956 (Fig. S9)
.
2.2.6. Complex 1.
Boron trifluoride–diethyl etherate (0.13 mL, 1.0 mmol) was added to a solution of
compound 4 (0.11 g, 0.5 mmol) in dry dichloromethane (15 mL) under nitrogen. The
reaction mixture was stirred for 2 h. After removal of the solvent, the residue was
purified by chromatography on silica gel (petroleum ether/dichloromethane, V/V =
5/1) to afford a light yellow solid (0.088 g, 0.32 mmol, 65%). 1H NMR (400 MHz,

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8.13 (d, = 7.5 Hz, 4H, ortho-C6H5) 7.68 (t,
para-C6H5), 7.54 (t, = 7.5 Hz, 4H, meta-C6H5), 7.20 (s, 1H, CHCO)
CDCl3, 25
°
C)
δ
J
J
= 7.5 Hz, 2H,
Fig.
J
(
S10)
2.2.
.
7
. Complex 2.
Boron trifluoride–diethyl etherate (0.13 mL, 1.0 mmol) was added to a
solution of compound (0.095 g, 0.2 mmol) in dry dichloromethane (15 mL)
5
under nitrogen. The reaction mixture was stirred for 2 h. After removal of the
solvent, the residue was purified by chromatography on silica gel (petroleum
ether/dichloromethane, V/V = 1/1) to afford a bright red solid (0.061 g, 0.12
1
mmol, 60%). Mp: 254 – 256
°
C. H NMR (400 MHz, CDCl3, 25 °C)
δ
8.21-8.17 (m, 4H, ortho-C6H5CO, C6H4CO), 7.76 (d,
J = 8.4 Hz, 2H,
meta-C6H4CO), 7.63 – 7.48 (m, 5H, meta-, para-C6H5CO, C6H4), 7.35 – 7.27
(m, 4H, meta- C6H5), 7.22 (s, 1H, CHCO), 7.19 – 7.05 (m, 8H, ortho-,
13
o
para-C6H5, C6H4) (Fig. S11). C NMR (100 MHz, CDCl3, 25 C)
δ= 182.7,
148.9, 147.6, 147.2, 135.0, 130.9, 129.7, 129.5, 129.3, 128.9, 128.1, 126.8,
19
o
125.1, 123.7, 122.8, 93.2 (Fig. S12). F NMR (376 MHz, CDCl3, 25 C)
δ =
-139.95, -140.01 (Fig. S13). IR (KBr): ν = 3063, 2960, 1727, 1589, 1549, 1487,
1373, 1280, 1162, 1043, 828, 807, 772, 755, 697, 613, 511 cm-1.
MALDI-TOF-MS (m/z): calcd for C33H24NO2F2B [M]+ 515.1868; found
515.1843 (Fig. S14).
2.2.8. Complex 3.

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Boron trifluoride-diethyl etherate (0.13 mL, 1.0 mmol) was added to a solution
of compound
6 (0.14 g, 0.2 mmol) in dry dichloromethane (15 mL) under
nitrogen. The reaction mixture was stirred for 2 h. After removal of the solvent,
the residue was purified by chromatography on silica gel (petroleum
ether/dichloromethane, V/V = 1/1) to afford a dark red solid (0.045 g, 0.06
mmol, 30%). Mp: 269 – 271 °C. 1H NMR (400 MHz, CDCl3, 25 °C) δ 8.21 (d, J = 8.6
Hz, 4H, C6H4CO), 7.76 (d, J = 8.6 Hz, 4H, C6H4CO), 7.56 (d, J = 8.7 Hz, 4H, C6H4),
7.32 – 7.27 (m, 8H, meta-C6H5), 7.23 (s, 1H, CHCO), 7.17- 7.05 (m, 16H, ortho-,
13
para-C6H5, C6H4) (Fig. S15). C NMR (100 MHz, CDCl3, 25 °C) δ = 181.5,
148.5, 147.0, 146.9, 135.1, 131.7, 130.9, 130.5, 130.1, 129.7, 129.2, 129.1,
127.7, 126.5, 124.7, 123.3, 122.5, 92.7 (Fig. S16). 19F NMR (376 MHz, CDCl3,
25 °C
) δ = -140.05, -140.12 (Fig. S17). IR (KBr): ν = 3036, 2925, 1721, 1588, 1541,
1487, 1372, 1331, 1287, 1203, 1043, 827, 801, 756, 697, 614, 516 cm-1
.
MALDI-TOF-MS (m/z): calcd for C51H37N2O2BF2 [M]+ 758.2916; found 758.2902
(Fig. S18).
4.3. X-ray structural analysis
Crystals suitable for X-ray analysis were grown by slow diffusion of n-hexane into
the respective solution of 2 or 3 in chloroform at room temperature. Reflection data
were collected at 296 (2) K using a graphite monochromator with Mo-Kα radiation (λ
= 0.71073 Å) on a Bruker SMART APEX(II) CCD diffractometer. The collected
frames were processed with the software SAINT and an absorption correction
(SADABS) was used to the collected reflections. The resulting structure was solved by

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the Direct or Patterson methods (SHELXTL 97) in conjunction with standard
difference Fourier techniques and then refined by full-matrix least-square technique on
F2. All hydrogen atoms were positioned geometrically and non-hydrogen atoms were
refined anisotropically. Relevant crystal data for the structures are collected in the
Supporting Information.
3. Results and Discussion
3.1 Synthesis and characterization
The synthetic routes to the BF2 complexes 1, 2 and 3 were illustrated in Scheme 2.
First, the Claisen condensation reaction of compound 7 with methylbenzoate in the
presence of sodium hydride in anhydrous THF afforded compound 5 in a yield of
66%. Then, the Suzuki-Miyaura coupling reaction between compound 8 and
4-(diphenylamino)phenylboronic acid gave ligand 5 in a yield of 63%. Next, ligand 5
reacted with boron trifluoride-diethyl etherate directly in anhydrous CH2Cl2, affording
2 in a yield of 60%. Similarly, compounds 10 and 11 were synthesized from
compound 7 and 9 through the Suzuki-Miyaura coupling reaction, respectively.
Ligand 6 was prepared via the Claisen condensation reaction between compounds 10
and 11 in a yield of 46%. Finally, 6 reacted with boron trifluoride-diethyl etherate
directly to give 3 in a yield of 30%. Complex 1, the dibenzoylmethane skeleton was
used as a reference, which was characterized by 1H NMR spectroscopy. Complexes 2
1
13
19
and 3 were characterized by H NMR, C NMR, F NMR and MALDI-TOF mass
spectrometry.

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Here Scheme 2. Synthetic routes to complexes 1, 2 and 3.
3.2 X-ray crystallographic analysis of BF2 complexes
The molecular and crystal structures of 2 and 3 were investigated by X-ray
crystallographic analysis. Single crystals of 2 and 3 suitable for X-ray diffraction
analysis were obtained by slow diffusion of hexane into a solution of 2 or 3 in
chloroform. ORTEP drawings of 2 and 3 are shown in Fig. 1 and selected
crystallographic data are provided in Table S1 (see Supporting Information). Key
bonding parameters are given in Table 1. Overall, both compounds 2 and 3 have
slightly distorted tetrahedral structures with geometrical parameters in line with
expectations. In the two molecules, it was found that two fluorine atoms were
distributed over the different side of the diketonate six-member ring. The sp3 orbits
hydridized at boron centers of 2 appeared as a distorted tetrahedron with bond angle
O1-B-O2 of 111.6° (3), slightly bigger than what is observed in 3 (109.4° (3)) owing
to its asymmetric structure. And the angles of F1-B-F2 in 2 and 3 were 111.7° (4) and
111.6° (3), respectively. These bond angles are in accordance with the common
difluroboron diketonate analogues [10]. In unsymmetric 2, the B-O bonds in the
BF2-chelating moiety are not equal and these bond lengths are 1.465(5) Å and 1.496(5)
Å. In the symmetric 3, the B-O bonds are almost equivalent and these bond lengths
are 1.483(5) Å and 1.488(5) Å. The average B-O bond lengths of 2 and 3 were 1.480
and 1.484 Å [35] and their average B-F bond lengths were 1.373 (5) and 1.371 (5) Å.
Thus it seems that all bond lengths around the boron atom are typical for

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β-diketonatate of BF2 [42]. After the generation of the chelating BF2 ring and
introduction of the triphenylamine unit, the structural rigidity is improved and the
conjugated region becomes extensive. These facts discussed above can make a
contribution to the intense orange fluorescence of the novel BF2 complexes (Table 2).
Here Fig. 1. ORTEP drawing of 2 (a) and 3 (b) (30% probability ellipsoids, hydrogen atom labels
have been omitted for clarity).
Here Table 1. X-ray selected bond lengths (Å) and angles (°) around the boron atom in the BF2
complexes.
3.3 UV-vis absorption spectra of BF2 complexes
The UV-vis absorption and the fluorescence emission spectra of 1, 2 and 3 in
toluene are shown in Fig. 2, and the corresponding photophysical data are
summarized in Table 2. As a whole, the electron-donating moiety (TPA) at the
p
-position of the phenyl ring induced a remarkable red shift of the absorption
maximum. As shown in Fig. 2, exhibited absorption at 300-400 nm with a
large extinction coefficient, which is typical for difloroboron β-diketonate
conpounds. In sharp contrast, complexes and displayed a broad and
1
2
3
bathochromic absorption with λmax at 462 and 493 nm, respectively. Owing to
the electron donating ability of triphenylamine and the electron accepting
ability of difluoroboron β-diketone moiety, that is, the strong push-pull system
[43], the maximum absorption band of
2 and 3 can be ascribed to the ICT

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transition, which could be supported by the solvent polarity-dependent
fluorescence emission spectral changes (Fig. 7-8, Fig. S1–S2) and theoretical
calculations (Fig. 5). Meanwhile, the molar absorption coefficient of the
maximum absorption peak for
3
was up to 4.1×104 M-1cm-1
，which was higher
than
1
(2.9×104 M-1cm-1) and
2
(2.4×104 M-1cm-1), showing strong light
harvesting abilities.
Here Fig. 2. Normalized UV-vis absorption (left) and fluorescence emission (right) spectra of BF2
complexes; c=1.0 × 10-5 mol·dm-3 in toluene, 25 °C.
Here Table 2. Photophysical data of BF2 complexes in toluene.
3.4 Fluorescent emission spectra of BF2 complexes
The triphenylamine substituent on the BF2dbm core 1 had significant influence on
emission properties of the complexes (Fig. 2 and Table 2). When a triphenylamine
moiety was introduced into the para position of one phenyl ring, the BF2dbm
derivative 2 exhibited the emission at 587 nm with a large red shift of 145 nm
compared with 1 (442 nm). The BF2dbm derivative 3, a symmetric molecule, had a
similar emission behavior and the emission maximum was red-shifted to 590 nm.
Both 2 and 3 showed orange emission in toluene with a fluorescence quantum yield
(Фf) of 0.92 and 0.95, respectively, while the BF2dbm core 1 showed a blue
fluorescence with a quantum yield of 0.66. According to the fluorescence properties
of 2 and 3, such as longer wavelength emissions, larger Stokes shifts, broader

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emission bands and solvent polarity-dependent emission (Fig. 7-8, Fig. S1–S2 ), it is
clear that ICT is the major mechanism responsible for these phenomenon [44]. The
fluorescence lifetimes of the BF2 complexes were also measured and the results were
collected in Table 2. Complexes 1-3 exhibited a similar single-exponential decay in
toluene. It was found that the introduction of a triphenylamine moiety did not make an
obvious difference in terms of the fluorescence lifetime (1, τf = 3.92 ns; 2, τf = 3.89 ns).
However, the fluorescence lifetime shortened in ca. 25%, when two triphenylamine
moieties were introduced into the complex (3, τf = 2.95 ns). Thus, the introduction of
the triphenylamine moiety can significantly affect the photoluminescence properties
of difluoroboron derivatives.
The fluorescence behaviors in the solid state of complexes 1-3 were further
investigated. Similar to their solution behaviors, the emission was red-shifted when
the electron-donating triphenylamine moiety was present (2 and 3) compared to the
unsubstituted complex (1). Meanwhile, the emission band of 2 red-shifted to 686 nm
in the solid state from 587 nm in toluene (∆λ = 99 nm), and that of 3 red-shifted to
688 nm in the solid state from 590 nm in toluene (∆λ = 98 nm). The corresponding
shift was 93 nm for 1. In the solid state, complexes 2 and 3 show strong fluorescence
with sharp peaks and their solid-state emission bands are narrower than that of 1. The
reason for this might be that the introduction of triphenylamine moiety increases the
steric hindrance, prevents the molecules from packing compactly and thus avoids the
spectral broadening.

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Here Fig. 3. Emission spectra of complexes 1-3 in the solid state (λex = 350 nm for 1; λex = 400
nm for 2 and 3).
3.5 Electrochemical properties of BF2 complexes
The electrochemical properties of 1-3 were examined using cyclic voltammetry.
The redox curves are shown in Fig. 4 and the corresponding frontier orbital energy
levels based on the redox data are summarized in Table 3. Using Bu4NPF6 (0.1 M) as
the supporting electrolyte, values were obtained at a scan rate of 100 mV·s-1 with a
saturated calomel electrode (SCE) as the standard [45]. As for 1, no oxidation waves
were observed up to +2.0 V versus SCE and it gave one well-defined reversible peak
with half-wave potential (Eꢀꢃꢁ/ꢂꢄ) of -0.92 V. Presumably, the oxidation for 1 occured
outside the potential window, due to the electron-poor nature of the BF2 chelate. This
also indicated that the HOMO energy of 1 was lower than those of 2 and 3 because of
the presence of the electron-donating triphenylamine moiety. Compared with 1, the
voltammograms of 2 and 3 displayed reversible oxidation and reduction waves (Fig.
4), indicating that compounds 2 and 3 had amphoteric redox properties. The halfwave
ꢃ/ꢄ
ꢅꢆ
oxidation potentials (E ) for 2 and 3 were observed at +0.96 V and +0.97 V versus
the SCE, respectively. The halfwave reduction potentials (Eꢀꢃꢁ/ꢂꢄ) were -0.96 (2), and
-0.94 (3), which were close to the halfwave reduction potential of 1 (-0.92 V).
ꢃ/ꢄ
ꢀꢁꢂ
Compared with 1, the E
of 2 and 3 showed a slightly negative shift due to the
electron-donating effect of triphenylamine moiety. From the results in Fig. 4 and the
redox potential data in Table 3, we can calculate the energy gaps of 1, 2, and 3 in the

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order of 3.12 eV (1) > 2.37 eV (2) > 2.28 eV (3).
Here Fig. 4. Cyclic voltammograms of complexes 1, 2 and 3 in CH2Cl2.
Here Table 3. Frontier orbital energy levels of all BF2 complexes.
3.6 Theoretical calculations (DFT)
To gain an insight into the electronic structures and understand the nature of
excited states of BF2 complexes, DFT calculations were performed at the
B3LYP/6-31G(d) level with the Gaussian 09W program package [46]. The HOMO
and LUMO energy diagrams of
1
,
2
and
3 were shown in Fig. 5. It is clear that
the HOMO and LUMO densities of
1
are localized over the whole molecule.
However, for 2, the HOMO and LUMO densities are mainly located on the
triphenylamine moiety and the BF2dbm unit, respectively. The HOMO and LUMO
densities of 3 were also clearly separated from each other. These results also
further demonstrate that the ICT process predictably occurs from the donor to the
acceptor moiety in 2 and 3. The HOMO-LUMO gaps for 1, 2 and
3 are 3.84 eV,
2.23 eV and 2.21 eV, respectively, which is nearly in agreement with the results
based on the emission spectra and electrochemical data (Table 3). Both the HOMO
and LUMO energy increase when triphenylamine group was introduced into the
para position of the phenyl rings. Clearly, introducing the triphenylamine group into
the BF2dbm compounds affects the HOMO level significantly, resulting in a
marked decrease in energy gap (Eg).

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Here Fig. 5. HOMO and LUMO energies (eV) of complexes 1-3 calculated by using DFT at the
B3LYP/6-31G(d) level.
3.7 Photostability of BF2 complexes
The photostability of BF2 complexes under continuous irradiation was studied
according to the method described by Hartmann and co-workers [47], to test their
potential practical applications as new emitting materials. In this work, the BF2
complex was immobilized in an ethylcellulose (EC) film and its weight content is 0.5
wt%. The thickness of the film is about 5.88 ꢀm. Photostability of EC films
immobilized with BF2 complexes were estimated upon continuous irradiation with a
254 nm 16 W UV lamp under air atmosphere. The power density on the films is 21.5
W/m2. The photostability of 2 and 3 were studied by continuous irradiation and were
compared with that of the known 1. Both complexes 2 and 3 showed better
photostability than that of 1 (Fig. 6). In fact, 11% and 6% decreases in the
fluorescence intensity of 2 and 3 were observed respectively (60 min), compared to 52%
for 1. The results demonstrate that the introduction of a triphenylamine moiety into
the molecules of the BF2dbm core could enhance the photostability of the
corresponding complexes efficiently.
Here Fig. 6. Photo-degradation histogram of 1, 2 and 3 immobilized in ethyl cellulose. Irradiation
is performed with a WFH-204B 254 nm UV lamp. The power density on the films is 21.5 W/m2.

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3.8 The changes of the fluorescent intensity of BF2 complexes in
Ethanol/Toluene system
Donor-acceptor (D-A) systems [48] have drawn much attention for their optical
properties and applications in chemical sensors [49], photodetectors [50], and
non-linear optics [51]. Often their emission is solvent polarity sensitive, given charge
separation in the excited state [52]. As shown in Fig. 7, remarkable spectral changes
of 3 were observed in three common solvents with different polarity. The emission
maximum had a red-shift and the fluorescence intensity declined sharply with the
increase of the solvent polarity. To further study the effect of the solvent polarity on
the emission, 3 was selected as a probe to test its emission in different contents of
ethanol in toluene. The results in Figure 8 showed that 3 emitted intense orange
fluorescence in toluene, which was quenched when a small amount of ethanol was
added into the solution. In pace with the increase of ethanol fraction (vol%), the
emission weakened gradually with the differing ethanol content from 0 to 30.0%.
When the ethanol content was up to 20.0%, the emission peak almost vanished and
the emission intensity decreased by 90.4%. Moreover, it was found that the
fluorescence emission bands of 3 red-shifted significantly with the increase of the
ethanol content (or the solvent polarity), accompanied by the broadening of the
emission. These emission bands with the increase of the polarity of the solvents
demonstrate the ICT emission feature of 3 [37, 38, 53].

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Here Fig. 7. Fluorescence spectral variation of 3 (10 µmol/L) in different solvents.
Here Fig. 8. Fluorescence spectral variation of 3 (10 µmol/L) in toluene with different ethanol
contents (volume fraction 0-30.0%, λex = 400 nm).
Similarly, the fluorescence properties of 1 and 2 were then investigated in the
EtOH/Toluene mixture at various ratios to compare the emission changes to the
solvent polarity (Fig. 9, and Fig. S1–S2). In Fig. 9, the quenching rate is the Y-axis
and the ethanol fraction (δ(EtOH)/%) is the X-axis. It was clear that the quenching
rates of 1, 2 and 3 were observed at 45.9%, 98.1% and 90.4% respectively, when the
content of ethanol was 20.0%. Therefore, the sensitivity of the emission of BF2
complexes to the solvent polarity is in the sequence of 2 > 3 > 1. The results
demonstrate that complexes 2 and 3 are potential sensor molecules for communicating
local environmental properties.
Here Fig. 9. Quenching rates of fluorescence intensity of 1, 2, and 3 (10 µmol/L) to different
ethanol contents in toluene at λem = 450 nm for 1; λem = 600 nm for 2 and 3 (λex = 350 nm for 1;
λex = 400 nm for 2 and 3).
Meanwhile, by fitting the fluorescence spectra data of 2 and 3 at λem = 600 nm in
the EtOH/Toluene mixture at various ratios, we found that the fluorescence intensity
of 3 showed an excellent linear relationship with the ethanol fraction when the content
of ethanol was in the range of 0 to 3.0% (Figure 10). The linear equation could be

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described as follows: I =1380.2-184.3×100δ(EtOH). The correlation coefficient is
0.995. Then the limit of detection (LOD) of this method was evaluated by the linear
regression equation. The algorithm can be described as follows:
LOD = 3S/K
Eq. 1
In this equation, S is the relative standard deviation whose numerical value is
obtained after enough blank tests. K is the slope of the linear regression equation. The
results of eleven blank tests showed that the relative standard deviation (S) was 0.56%.
The LOD of this method was obtained according to Eq. 1. Complex 3 can be used as a
polar sensitive probe to monitor the ethanol content in toluene directly, with a
detection limit of 0.0091%.
Here Fig. 10. Response of fluorescence intensity at λem = 600 nm of 3 (10µmol/L) to different
ethanol contents in toluene (λex = 400 nm).
4. Conclusions
In summary, novel triphenylamine-substituted BF2 complexes 2 and 3 have been
synthesized and characterized by means of spectroscopic, electrochemical methods
and X-ray crystal structure. These compounds exhibit long wavelength absorptions in
their UV-vis spectra and a red shift of the emission, and demonstrate enhanced
photostability compared to unmodified 1. Both 2 and 3 emit intense orange light in
toluene with the Фf up to 0.92 and 0.95, respectively. The electrochemical

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voltammograms of these complexes illustrate reversible oxidation and reduction
waves. Meanwhile, the TPA-modified complexes are highly sensitive to the solvent
polarity of the mixture of ethanol/toluene
.
Acknowledgment
The authors thank the financial support from the National Natural Science Foundation
of China (21276043 and 21421005), and Open Fund of Key Laboratory of
Biotechnology and Bioresources Utilization (Dalian Minzu University), State Ethnic
Affairs Commission & Ministry of Education.
Appendix A. Supplementary data
Characterization data, NMR and MS spectra of complexes, crystal structure data, and
fluorescence spectral variation curves of 1 and 2 in ethanol / toluene system.
Supplementary data related to this article can be found at
Notes and references
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List of Tables:
Table 1. X-ray selected bond lengths (Å) and angles (°) around the boron atom in the
BF2 complexes.
Table 2. Photophysical data of BF2 complexes in toluene.
Table 3. Frontier orbital energy levels of all BF2 complexes.

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Table 1. X-ray selected bond lengths (Å) and angles (°) around the boron atom in the BF2
complexes.
CCDC
Complexes
B-O1
B-O2
B-F1
B-F2
O1-B-O2 F1-B-F2
number
2
3
1445632
1.465(5) 1.496(5) 1.373(5) 1.373(5) 111.6 (3) 111.7(4)
1.483(5) 1.488(5) 1.363(5) 1.380(5) 109.4 (3) 111.6(3)
1445633