Synthesis and photophysical properties of halogenated derivatives of (dibenzoylmethanato)boron difluoride

Article abstract of DOI:10.1016/j.saa.2016.12.025

A series of (dibenzoylmethanato)boron difluoride (BF₂DBM) derivatives with a halogen atom in one of the phenyl rings at the para-position were synthesized and used to elucidate the effects of changing the attached halogen atom on the photophysi

Full text of DOI:10.1016/j.saa.2016.12.025

Accepted Manuscript
Synthesis and photophysical properties of halogenated derivatives
of (dibenzoylmethanato)boron difluoride
Yuriy N. Kononevich, Nikolay M. Surin, Viacheslav A.
Sazhnikov, Evgeniya A. Svidchenko, Vladimir M. Aristarkhov,
Andrei A. Safonov, Alexander A. Bagaturyants, Mikhail V.
Alfimov, Aziz M. Muzafarov
PII:
DOI:
Reference:
S1386-1425(16)30749-1
doi: 10.1016/j.saa.2016.12.025
SAA 14838
Spectrochimica Acta Part A: Molecular and Biomolecular
Spectroscopy
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Revised date:
Accepted date:
4 June 2016
16 December 2016
18 December 2016
Please cite this article as: Yuriy N. Kononevich, Nikolay M. Surin, Viacheslav A.
Sazhnikov, Evgeniya A. Svidchenko, Vladimir M. Aristarkhov, Andrei A. Safonov,
Alexander A. Bagaturyants, Mikhail V. Alfimov, Aziz M. Muzafarov , Synthesis and
photophysical properties of halogenated derivatives of (dibenzoylmethanato)boron
difluoride. The address for the corresponding author was captured as affiliation for all
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ACCEPTED MANUSCRIPT
Synthesis and Photophysical Properties of Halogenated Derivatives of
(Dibenzoylmethanato)boron Difluoride
Yuriy N. Kononevich,1 Nikolay M. Surin,2 Viacheslav A. Sazhnikov,3 Evgeniya A.
Svidchenko,2 Vladimir M. Aristarkhov,3 Andrei A. Safonov,3 Alexander A.
Bagaturyants,3,4 Mikhail V. Alfimov3,5 and Aziz M. Muzafarov1,2
1A.N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of
Sciences, 119991 Moscow, Russian Federation
2N.S. Enikolopov Institute of Synthetic Polymeric Materials, Russian Academy of
Sciences, 117393 Moscow, Russian Federation
3 Photochemistry Center, Russian Academy of Sciences, 119421 Moscow, Russian
Federation
4 National Research Nuclear University MEPhI, 115409 Moscow, Russian
Federation
5 Moscow Institute of Physics and Technology (State University), 141707
Dolgoprudny, Russian Federation
Abstract. A series of (dibenzoylmethanato)boron difluoride (BF2DBM)
derivatives with a halogen atom in one of the phenyl rings at the para-position
were synthesized and used to elucidate the effects of changing the attached halogen
atom on the photophysical properties of BF2DBM. The room-temperature
absorption and fluorescence maxima of fluoro-, chloro-, bromo- and iodo-
substituted derivatives of BF2DBM in THF are red-shifted by about 2-10 nm
relative to the corresponding peaks of the parent BF2DBM. The fluorescence
quantum yields of the halogenated BF2DBMs (except the iodinated derivative) are
larger than that of the unsubstituted BF2DBM. All the synthesized compounds are
able to form fluorescent exciplexes with benzene and toluene (emission maxima at
λem = 433 and 445 nm, respectively). The conformational structure and electronic
spectral properties of halogenated BF2DBMs have been modeled by DFT/TDDFT

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calculations at the PBE0/SVP level of theory. The structure and fluorescence
spectra of exciplexes were calculated using the CIS method with empirical
dispersion correction.
1. Introduction
(Dibenzoylmethanato)boron difluoride (BF2DBM) is one of the most interesting
molecules of the difluoroboron β-diketonate family due to its ability to form
emissive exciplexes with methylbenzenes (MB) in solutions [1–9]. Recently, we
stated that BF2DBM can also form fluorescent exciplexes with methylbenzenes on
the surface of SiO2 micro- and nanoparticles and in polymer matrices [10–14]. It
was shown that BF2DBM can be used as a promising molecular sensor for
detection of benzene, toluene and xylenes (BTX) in the gas phase [11, 13].
Since BTX are volatile organic compounds of great environmental significance
and can present serious health dangers, the development of optical chemical
sensors capable of monitoring BTX at trace concentrations attracts great interest
[15, 16]. Among BTX compounds, benzene has the highest toxicity and
consequently has the lowest permissible exposure limit. Therefore, it is important
to determine the concentration of each individual component in a BTX mixture.
It is known that the use of multisensor arrays might be an advantage for
performing quantitative detection of specific components in vapor mixtures [17-
20]. Upon exposure to an analyte mixture, such arrays consisting of colorimetric or
fluorescent sensing elements generate unique response patterns that can be
processed using corresponding pattern recognition software. The use of sensor
array technology requires multiple molecular sensors with various sensing
properties. For example, Khan et al. [16] used four different types of
solvatochromic dyes and two different types of polymers to make five different
types of sensing membranes.
It should be noted that complexation of singlet-excited BF2DBM molecule with
ground-state methylbenzene molecules results in the appearance of specific new
red-shifted emissions with the fluorescence quantum yield that is several times

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greater than that of the uncomplexed BF2DBM [3, 5]. In principle, these unusual
properties of molecules of this type can be used to create perspective sensor arrays
suitable for quantitative discrimination of individual constituents within
complex methylbenzene mixtures. To our knowledge, however, there have been no
studies on exciplex interaction between methylbenzenes and any BF2DBM
derivatives.
The exciplex emission wavelength depends on the difference between the
ionization potential of the donor and the electron affinity of the acceptor, thereby
providing a simple approach to tuning the molecular orbital energy levels and
spectral position of the exciplex fluorescence band.
Halogenation is known to be an efficient technique for tuning the frontier orbital
energy levels [21]. The aim of this work was to synthesize halogen substituted
derivatives of BF2DBM and to study their spectral properties and their ability to
form fluorescent exciplexes with methylbenzenes. We have synthesized four
derivatives of BF2DBM containing F, Cl, Br, and I atom in the para-position of one
of the phenyl ring.
The room-temperature spectral properties of the synthesized compounds were
studied in THF solutions. Since the BF2DBM/MB exciplexes can be easily formed
both in dilute and concentrated solutions of methylbenzenes [8], the absorption and
fluorescence properties of exciplexes of halogenated BF2DBMs with benzene and
toluene were studied in neat benzene and toluene.
To better understand the structure and electronic properties of newly synthesized
compounds and their exciplexes with methylbenzenes, quantum-chemical
calculations were carried out. The time-dependent density functional theory
(TDDFT) is the most widely-used theoretical approach to simulate the optical
properties of organic molecules and dyes (see, for example, recent papers by
Adamo and co-workers [22-26]). In particular, Adamo et al. have stated that the
PBE0 hybrid exchange-correlation functional can be used as a multipurpose
approach since it yields appropriate simulations for many chemical and physical
properties, thus providing a theoretically consistent description of both ground- and

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excited-state properties. Recently, we have used the DFT and TDDFT calculations
at the PBE0/SVP level of theory to determine the gas-phase geometry and spectral
properties of unsubstituted and methyl-substituted BF2DBM-CH3 [27]. It was
stated that the PBE0 functional in combination with the SVP basis set provides
good agreement with several experimentally determined parameters of these
compounds. Therefore, in this study, the DFT-TDDFT/PBE0/SVP calculations
have been performed to reproduce UV-visible spectroscopic transitions of
halogenated derivatives of BF2DBM.
Calculation of the structure and spectra of exciplexes is a more complex
problem. Recently, we have shown that reasonable agreement with experimental
values for exciplexes of unsubstituted BF2DBM with methylbenzenes may be
obtained by using the CIS method with empirical dispersion correction for
geometry optimization and the CIS(D) method for single-point calculation of
transition energies [28]. The same approach was applied in this work for
exciplexes of halogenated BF2DBMs with benzene and toluene.
2. Experimental
2.1. Materials and methods
All solvents were purified before use. Tetrahydrofurane (THF) was distilled
from sodium/benzophenone, and dichloromethane was distilled over phosphorus
pentoxide. Ethyl benzoate, 4'-fluoroacetophenone, 4'-chloroacetophenone, 4'-
bromoacetophenone and 4'-iodoacetophenone were purchased from Acros
Organics and use without purification. The reactions were monitored by thin-layer
chromatography (TLC) using Fluka silica gel (60 F 254) plates (0.25 mm).
Visualization was made with UV light. Melting points are uncorrected. Infrared
1
spectra were recorded on a Bruker Equinox 55/S spectrometer (Germany). Н
NMR spectra were recorded on a Bruker WP 250 SY spectrometer (250.13 MHz)
13
19
(Germany). C and F NMR spectra were recorded on a Bruker Avance II
spectrometer (300 MHz; Germany). Chemical shifts are reported relative to
1
chloroform (δ=7.25 ppm) for Н NMR, chloroform (δ=77.00 ppm) and dimethyl

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13
sulfoxide (δ=39.51 ppm) for C NMR. High-resolution mass spectra (HRMS)
were measured using a Bruker micrOTOF II instrument with electrospray
ionization (ESI) (Germany). Spectroscopic grade solvents (Aldrich) were used in
UV-vis absorption and fluorescence measurements. The absorption spectra were
recorded on a Shimadzu UV-25001PC spectrophotometer (Japan). The
fluorescence spectra were measured on a scanning spectrofluorimeter developed
and constructed at ISPM RAS equipped with a 150 W arc xenon lamp as the
pumping source, two Seya-Namioka-type monochromators and two
photoelectronic multipliers [29]. Fluorescence quantum yields were measured
using POPOP as a reference (φF = 0.93). Experimental uncertainties are estimated
to be 10% for emission quantum yield determination, 5% for relative emission
intensities, 1 nm for absorption and emission peaks.
2.2. General procedure for the synthesis of 1,3-diaryl-propane-1,3-diones (2 a-
d)
The solution of 4-halogenacetophenone (0.05 mol) and ethyl benzoate (0.05
mol) in dry THF (30 mL) was added to suspension of sodium hydride (1.32 g,
0.055 mol) in dry THF (20 mL) under inert atmosphere. The resulting mixture was
heated at reflux for 2 h. Suspension was cooled to room temperature and poured
into water (100 mL), acidified with acetic acid (10 mL) and extracted by ethyl
acetate (150 mL). Organic layer was separated by diluted funnel and washed with
aqueous Na2CO3, then H2O. After drying over Na2SO4, the solvent was evaporated
in vacuo. The residual product was purified by recrystallization from isopropanol.
2.2.1. 1-Phenyl-3-(4-fluorophenyl)propane-1,3-dione (2 a).
Yield 58 %; Mp = 81-83 °C; IR (CsI, cm-1): 3076, 1601, 1552, 1533, 1502,
1
1483, 1302, 1225, 1159, 1138, 1107, 1066, 1009, 848, 769; H NMR (300 MHz,
CDCl3): δ 6.80 (s, 1H, CO-CH-CO), 7.16 (t, 2H, J = 8.5 Hz, Ar), 7.45-7.59 (m, 3H,
Ar), 7.95-8.03 (m, 4H, Ar), 11.46 (s, 1H, OH); 13C NMR (75 MHz, CDCl3): δ 92.8,
115.8 (d, J = 21.8 Hz), 127.1, 128.7, 129.6 (d, J = 9.1 Hz), 132.5, 135.2, 165.4 (d,
19
J = 253.8 Hz), 185.1 (d, J = 1.1 Hz); F NMR (282 MHz): δ -107.45; HRMS

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(ESI) m/z calcd. for C15H11FNaO2 [(M+Na)+]: 265.0635, found 265.0624;
Elemental analysis calcd. (%) for C15H11FO2: C, 74.37; H, 4.58; F, 7.84; Found: C,
74.23; H, 4.75; F, 7.84.
2.2.2. 1-Phenyl-3-(4-chlorophenyl)propane-1,3-dione (2 b).
Yield 74 %; Mp = 92-94 °C; IR (CsI, cm-1): 3060, 1597, 1525, 1483, 1306,
1
1227, 1095, 1012, 843, 762; H NMR (300 MHz, CDCl3): δ 6.87 (s, 1H, CO-CH-
13
CO), 7.44-7.59 (m, 5H, Ar), 7.89-7.99 (m, 4H, Ar), 11.50 (s, 1H, OH); C NMR
(75 MHz, CDCl3): δ 92.9, 127.1, 128.4, 128.7, 128.9, 132.7, 133.6, 134.9, 138.6,
184.4, 185.7; HRMS (ESI) m/z calcd. for C15H11ClNaO2 [(M+Na)+]: 281.0340,
found 281.0328; Elemental analysis calcd. (%) for C15H11ClO2: C, 69.64; H, 4.29;
Cl, 13.70; Found: C, 69.66; H, 4.36; Cl, 13.61.
2.2.3. 1-Phenyl-3-(4-bromophenyl)propane-1,3-dione (2 c).
Yield 61 %; Mp = 91-93 °C; IR (CsI, cm-1): 3467, 1589, 1549, 1477, 1304,
1
1227, 1072, 1009, 762; H NMR (300 MHz, CDCl3): δ 6.81 (s, 1H, CO-CH-CO),
7.45-7.65 (m, 5H, Ar), 7.82-7.87 (m, 2H, Ar), 7.96-8.00 (m, 2H, Ar), 11.52 (s, 1H,
13
OH); C NMR (75 MHz, DMSO-d6): δ 93.4, 127.0, 127.5, 128.9, 129.4, 131.9,
133.2, 133.7, 134.4, 184.1, 185.5; HRMS (ESI) m/z calcd. for C15H11BrNaO2
[(M+Na)+]: 324.9835, found 324.9824; Elemental analysis calcd. (%) for
C15H11BrO2: C, 59.43; H, 3.66; Br, 26.36; Found: C, 57.99; H, 3.89; Br, 25.64.
2.2.4. 1-Phenyl-3-(4-iodophenyl)propane-1,3-dione (2 d).
Yield 55%; Mp = 92-94 °C; IR (CsI, cm-1): 3467, 3057, 2960, 1676, 1589, 1525,
1
1485, 1392, 1304, 1275, 1227, 1066, 1007, 762; H NMR (300 MHz, CDCl3): δ
6.80 (s, 1H, CO-CH-CO), 7.46-7.59 (m, 3H, Ar), 7.69 (d, 2H, J = 8.5 Hz, Ar), 7.83
(d, 2H, J = 8.2 Hz, Ar), 7.97 (d, 2H, J = 7.0 Hz, Ar); 13C NMR (75 MHz, CDCl3):
δ 92.9, 99.8, 127.2, 128.6, 128.7, 132.6, 135.0, 135.3, 137.9, 184.6, 186.0; HRMS
(ESI) m/z calcd. for C15H11INaO2 [(M+Na)+]: 372.9696, found 372.9667;
Elemental analysis calcd. (%) for C15H11IO2: C, 51.45; H, 3.17; I, 36.24; Found: C,
49.58; H, 3.44; I, 40.32.

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2.3. General procedure for the synthesis of β-diketonatoboron difluorides (3 a-d)
Boron trifluoride-diethyl etherate (0.012 mol) was added to a solution of 1,3-
diaryl-propane-1,3-dione (0.01 mol) in dry dichloromethane (30 mL). The reaction
mixture was stirred for 1 h at room temperature. After removing the solvent by
evaporation in vacuo, the residue was purified by recrystallization from toluene.
2.3.1. 2,2-Difluoro-4-phenyl-6-(4-fluorophenyl)-1,3,2-(2H)-dioxaborine (3 a).
Yield 87 %; Mp = 223-225 °C; IR (CsI, cm-1): 1599, 1549, 1491, 1373, 1238,
1
1161, 1043; H NMR (300 MHz, CDCl3): δ 7.14 (s, 1H, CO-CH-CO), 7.20-7.27
(m, 2H, Ar), 7.55 (t, 2H, J = 7.3 Hz, Ar), 7.70 (t, 1H, J = 7.9 Hz, Ar), 8.12-8.21
13
(m, 4H, Ar); C NMR (75 MHz, CDCl3): δ 93.2, 116.6 (d, J = 22.3 Hz), 128.9,
19
129.2, 131.7 (d, J = 10.1 Hz), 135.3, 165.5 (d, J = 261.2 Hz), 182.0, 183.6; F
10
11
NMR (282 MHz): δ -101.56 (m, Ar-F), -141.02 (20%, B-F), -141.09 (80%, B-
F); HRMS (ESI) m/z calcd. for C15H10BF3NaO2 [(M+Na)+]: 313.0621, found
313.0609; Elemental analysis calcd. (%) for C15H10BF3O2: C, 62.11; H, 3.48; B,
3.73; F, 19.65; Found: C, 62.25; H, 3.60; B, 3.74; F, 19.38.
2.3.2. 2,2-Difluoro-4-phenyl-6-(4-chlorophenyl)-1,3,2-(2H)-dioxaborine (3 b).
Yield 76%; Mp = 244-246 °C; IR (CsI, cm-1): 1591, 1547, 1485, 1373, 1248,
1153, 1099, 1041; 1H NMR (300 MHz, CDCl3): δ 7.15 (s, 1H, CO-CH-CO), 7.50-
7.58 (m, 4H, Ar), 7.70 (t, 1H, J = 7.3 Hz, Ar), 8.05-8.15 (m, 4H, Ar); 13C NMR (75
MHz, CDCl3): δ 93.3, 129.0, 129.2, 129.6, 130.1, 130.3, 131.7, 135.5, 142.0,
181.7, 183.7; 19F NMR (282 MHz): δ -140.48 (20%, 10B-F), -140.54 (80%, 11B-F);
HRMS (ESI) m/z calcd. for C15H10BF2NaClO2 [(M+Na)+]: 329.0325, found
329.0307; Elemental analysis calcd. (%) for C15H10BClF2O2: C, 58.78; H, 3.29; B,
3.53; Cl, 11.57; F, 12.40; Found: C, 58.51; H, 3.34; B, 3.60; Cl, 11.38; F, 12.11.
2.3.3. 2,2-Difluoro-4-phenyl-6-(4-bromophenyl)-1,3,2-(2H)-dioxaborine (3 c).
Yield 92 %; Mp = 238-240 °C; IR (CsI, cm-1): 1589, 1547, 1481, 1371, 1039; 1H
NMR (300 MHz, CDCl3): δ 7.15 (s, 1H, CO-CH-CO), 7.56 (t, 2H, J = 7.9 Hz, Ar),
7.68-7.74 (m, 3H, Ar), 7.99 (d, 2H, J = 8.5 Hz, Ar), 8.14 (d, 2H, J = 7.3 Hz, Ar);
13C NMR (75 MHz, CDCl3): δ 93.3, 129.0, 129.2, 130.1, 130.7, 130.8, 131.7,

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132.6, 135.6, 181.8, 183.7; 19F NMR (282 MHz, CDCl3): δ -140.51 (22%, 10B-F), -
11
140.58 (78%, B-F); HRMS (ESI) m/z calcd. for C15H10BF2NaBrO2 [(M+Na)+]:
372.9820, found 372.9791; Elemental analysis calcd. (%) for C15H10BBrF2O2: C,
51.34; H, 2.87; B, 3.08; Br, 22.77; F, 10.83; Found: C, 51.29; H, 2.63; B, 2.87; Br,
23.26; F, 10.47.
2.3.4. 2,2-Difluoro-4-phenyl-6-(4-iodophenyl)-1,3,2-(2H)-dioxaborine (3 d).
Yield 71 %; Mp = 228-230 °C; IR (CsI, cm-1): 1585, 1539, 1479, 1400, 1369,
1
1248, 1155, 1095, 1034, 1001; H NMR (300 MHz, CDCl3): δ 7.15 (s, 1H, CO-
CH-CO), 7.55 (t, 2H, J = 7.6 Hz, Ar), 7.70 (t, 1H, J = 7.3 Hz, Ar), 7.81 (d, 2H, J =
13
8.6 Hz, Ar), 7.91 (d, 2H, J = 7.9 Hz, Ar), 8.13 (d, 2H, J = 7.3 Hz); C NMR (75
MHz, CDCl3): δ 93.3, 103.9, 129.0, 129.2, 129.9, 131.3, 131.7, 135.6, 138.6,
182.1, 183.7; 19F NMR (282 MHz): δ -140.43 (21%, 10B-F), -140.50 (79%, 11B-F);
HRMS (ESI) m/z calcd. for C15H10BF2NaIO2 [(M+Na)+]: 420.9682, found
420.9649; Elemental analysis calcd. (%) for C15H10BF2IO2: C, 45.27; H, 2.53; B,
2.72; F, 9.55; Found: C, 44.98; H, 2.38; B, 2.70; F, 9.41.
2.4. Computational details
Geometry optimization of structures of halogenated BF2DBMs was performed
using density functional theory (DFT) for the ground state and time-dependent
density functional theory (TDDFT) for the first excited singlet state. Excitation
energies along with corresponding oscillator strengths were calculated at optimized
geometries by the TDDFT method. Hybrid PBE0 functional and Ahlrichs' double-
zeta polarized SVP basis set were used in all calculations. Stuttgart–Dresden
effective core potential (ECP) and the corresponding def2-SVP basis set were used
for the iodine atom. Geometry optimization of exciplexes was performed by the
CIS method in combination with the dispersion correction for HF method with BJ
damping using the SVP basis set. Transition energies at optimized geometries were
calculated by the CIS(D) method with the triple-zeta TZVP basis set.
Further details are given
elsewhere [27, 28].

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All calculations were performed in the gas phase. The solvent effects were not
taken into account for the following reasons. The first is the absence of regular
solvatochromic shifts in the case of unsubstituted BF2DBM. Experimentally
observed shifts in solvents of different polarity are small and irregular. Another
reason is that there is a lack of a developed and accepted technique for the solvent
effects in the case of exciplexes. The main reason is the fact that the exciplexes
are designed to operate at interfaces between solid surfaces and the gas phase.
3. Results and Discussion
3.1. Synthesis of halogen substituted diketonatoboron difluorides
The halogen-substituted diketonatoboron difluorides 3 a-d were synthesized in
two steps according to the route outlined in Scheme 1. β-Diketones 2 a-d were
obtained by the reaction of ethyl benzoate with corresponding acetophenones 1 in
the presence of sodium hydride in THF. Products were purified by recrystallization
from isopropanol. The yield of products was 55-74 %.
Boron difluoride complexes 3 a-d were obtained in good yields (71-92 %) by
the reaction of the corresponding β-diketones 2 a-d with boron trifluoride diethyl
etherate in dichloromethane. The reaction proceeded to completion in 1 h at room
temperature. After completion of the reaction, the solvent was removed in vacuo
and the residue was recrystallized from toluene. Crystalline substances with an
intense yellow color were formed.
Hal: a – F, b – Cl, c – Br, d – I.
Scheme 1. Synthesis of halogen-substituted diketonatoboron difluorides.

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3.2. Steady-state absorption and fluorescence spectra
The obtained results are shown in Figs. 1 and 2 and are summarized in Table 1.
The spectra of fluorinated, chlorinated, brominated, iodinated and unsubstituted
BF2DBM are indicated as F, Cl, Br, I, and H respectively. Fig. 1 presents the room-
temperature absorption spectra of the halogen substituted compounds in THF
solution. For comparison, the figure shows the spectrum of the unsubstituted
parent BF2DBM.
4,5x104
4,0x104
3,5x104
H
F
Cl
Br
3,0x104
2,5x104
I
2,0x104
1,5x104
1,0x104
5,0x103
0,0
250
300
350
400
450
Wavelength (nm)
Fig. 1. Electronic absorption spectra of halogenated BF2DBMs and unsubstituted
DBMBF2 in THF at 293 K. The dye concentration is 2  10-5 M for all compounds.
The absorption spectrum of parent BF2DBM and that of a halogenated BF2DBM
consists of four bands. A structured strong band with the molar extinction
coefficient ε in the range of 30000–42000 M–1 cm–1 is centered in the region of
320–410 nm. Two resolved peaks at 380 and 360 nm and a shoulder at about 350
nm of the parent BF2DBM are vibronic bands [31]. Obviously, similar peaks for
halogenated BF2DBMs in the region of 365-390 nm can be attributed to vibronic
transitions. In the 220–320 nm spectral region, the spectrum exhibits three low-
intensity bands at about 230 nm, 270 nm and 290 nm, respectively, with ε in the
range of 5500-7500 M–1 cm–1. The spectra of chlorinated and brominated

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compounds are very similar. On going from fluoro- to chloro-, bromo-, and iodo-
substituted BF2DBM, both vibronic peaks of the long-wavelength absorption band
shift progressively by about 5 nm (from 365 and 381nm for BF2DBM-F to 370 and
386 nm for BF2DBM-Cl and BF2DBM-Br, and to 374 and 392 nm for BF2DBM-I,
respectively). The low-intensity maxima in the short-wavelength region also
exhibit noticeable red shifts on passing from F to I (for example, a red shift from
300 nm to 310 nm is clearly observed for bromo- and iodo-derivatives).
1,0
H
F
Cl
0,8
Br
I
0,6
0,4
0,2
0,0
400
450
500
550
Wavelength (nm)
Fig. 2. Normalized fluorescence spectra (λex = 365 nm) of halogenated and
unsubstituted BF2DBM in THF (c = 2  10-5 M) at room temperature.
Figure 2 shows the normalized fluorescence spectra of the parent BF2DBM and
synthesized BF2DBMs and figure 3 shows photograph of the fluorescence of
halogenated BF2DBMs in THF. When excited at 365 nm, both unsubstituted and
halogen-substituted BF2DBMs exhibits partially structured fluorescence band with
vibronic peaks at about 400 and 420 nm and a shoulder at about 480 nm. Both
peaks of the fluorescence band shift batochromically on going from F to Cl, Br and
I (Table 1). The band shape and fluorescence maximum are independent of the
excitation wavelength. The fluorescence excitation spectra of all compounds nearly
coincide with the low-energy absorption bands. The short-wavelength fluorescence

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emission maxima are red shifted by about 15 nm from the long-wavelength
absorption maxima.
Fig. 3. Photograph of solutions of halogenated BF2DBMs in THF excited with
black light at 365 nm (from left to right: F, Cl, Br, and I, respectively).
Note that similar slight red shifts in absorption and fluorescence maxima on
going from F to I were recently observed for lipid derivatives of halide substituted
difluoroboron dibenzoylmethanes in CH2Cl2 solution [32].
The obtained values of fluorescence quantum yields are listed in Table 1. For
halogen-substituted BF2DBMs, methyl-substituted BF2DBM with F = 0.30 [27]
can be taken as a model compound owing to similar structural symmetry. The
halogenation of BF2DBM is expected to decrease the fluorescence quantum yield
due to the heavy-atom effect on the intersystem crossing rate. As can be seen in
Table 1, the presence of a halogen atom on the phenyl group has a pronounced
effect on the magnitude of the quantum yield. However, halogenation quenches the
fluorescence emission of BF2DBM in an irregular, non-monotonic manner as the
atomic number of the attached halogen increases. This may indicate that the
rotation and vibration of the phenyl moieties may be the main paths of deactivation
of the first excited singlet state of the parent BF2DBM (F = 0.12 in THF) and its
para-substituted derivatives. The role of torsional motions of the two relatively

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independent phenyl groups of BF2DBM molecule was discussed by Harju et al.
[4]. The halogenation (for example, bromination) significantly increases the mass
of the halogen-substituted phenyl moiety so that its vibration and rotation rate
along the bond linked to the central core may be remarkably slowed. It should be
noted that a similar increase in the quantum yield (from 0.68 to 0.76) was also
observed on passing from F to Cl for BF2DBM haloderivatives containing methyl
group at the para-position of the second phenyl ring [27].
Table 1. Optical properties of halogenated BF2DBMs and unsubstituted BF2DBM
in THF (c = 2  10–5 M). The wavelengths of the second absorption and
fluorescence peaks are given in parentheses.
 (M-1 cm-1)a
F
Compound
3 a
λabs (nm)
381 (365)
386 (369)
387 (370)
392 (374)
382 (365)
λF (nm)
29900
396 (412)
399 (419)
401 (421)
407 (426)
396 (414)
14.0
17.9
22.0
8.5
41900
3 b
41900
3 c
32400
3 d
34700
12.0
DBMBF2
a Coefficients are given for the short-wavelength peaks.
3.3. Fluorescence emission spectra of exciplexes of halogenated BF2DBMs with
benzene and toluene
Previous studies [1–8] indicated that the formation of highly fluorescent
exciplexes with benzene and methylbenzenes is a specific feature of unsubstituted
BF2DBM. The supersedence of the original BF2DBM fluorescence peaks in
acetonitrile by a new red-shifted emission on addition of benzene was first
described by Chow et al. [1]. The formation of highly fluorescent exciplexes of
singlet-excited BF2DBM with benzene, toluene and p-xylene in the pure solvents
was then studied in detail in a subsequent research [2-7]. To examine whether there
exist fluorescent complexes of halogenated BF2DBMs with methylbenzenes, we
have studied the emission spectra of halogenated BF2DBMs in pure benzene and
toluene. It was found that the structured fluorescence bands of halogenated
BF2DBMs, which are characteristic of their solutions in THF (Fig. 2), have almost

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totally disappeared in neat benzene and toluene. As in the case of the parent
BF2DBM, the fluorescence band of a halogenated BF2DBM is replaced by a red-
shifted, broad, and structureless emission band. The steady-state fluorescence
spectra of halogenated BF2DBMs in neat benzene are presented in Fig. 5. The
observed coincidence of the fluorescence excitation spectra and the absorption
spectra shown in Fig. 4 as well as the appearance of the red-shifted structureless
emission bands indicate that excitation of halogenated BF2DBMs in the presence
of benzene results in formation of corresponding BF2DBM-Hal/benzene emissive
exciplexes. This suggests that even in the case of iodine-substituted BF2DBM its
fluorescence lifetime is long enough for the exciplex formation with benzene
molecule. Figure 6 shows photograph of the fluorescence of halogenated
BF2DBMs in benzene.
1,0
0,8
F
Cl
Br
I
0,6
0,4
0,2
0,0
300
350
400
450
Wavelength (nm)
Fig. 4. Electronic absorption spectra of halogenated BF2DBMs and unsubstituted
DBMBF2 in benzene at 293 K. The dye concentration is 2  10-5 M for all
compounds.
Note that when the fluorescence maxima of Cl-, Br-, and I-derivatives of
BF2DBM monomers in THF solutions exhibit a progressive red shift on going
from Cl to I, the emission maxima of corresponding exciplexes are located very

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close to each other (λmax ≈ 433 nm for Cl-, Br-, and I-derivatives.). They are red-
shifted by about 5-6 nm relative to the emission maximum of BF2DBM-F/benzene
exciplexes (λmax = 427 nm). The latter value is very close to the maximum of
unsubstituted BF2DBM (λmax = 426 nm) in benzene [2].
0,03
F
Cl
Br
I
0,02
0,01
0,00
400
450
500
550
600
Wavelength (nm)
Fig. 5. Room-temperature fluorescence spectra (λex = 375 nm) of brominated,
fluorinated, chlorinated and iodinated BF2DBMs (indicated as Br, F, Cl and I,
respectively) in neat benzene (c = 1 × 10–5 M).
The absorption and emission spectra of halogenated BF2DBMs dissolved in pure
toluene exhibit similar behavior (Fig. 7 and Fig. 8, respectively). Figure 9 shows
photograph of the fluorescence of halogenated BF2DBMs in toluene. As shown in
Fig. 8, the exciplex emission maximum of BF2DBM-F in toluene is located at
about 440 nm. According to the literature, the unsubstituted BF2DBM in pure
toluene exhibits an emission maximum at 438 nm [2]. The maxima of chloro-,
bromo-, and iodo-derivatives are centered in the region of 445 nm. Thus, their
emission maxima in toluene are red shifted as compared to those in benzene by 12
nm.

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Fig. 6. Photograph of solutions of halogenated BF2DBMs in benzene excited with
black light at 365 nm (from left to right: F, Cl, Br, and I, respectively).
1.0
0.8
F
Cl
0.6
0.4
0.2
0.0
Br
I
300
350
400
450
Wavelength (nm)
Fig. 7. Electronic absorption spectra of halogenated BF2DBMs and unsubstituted
DBMBF2 in toluene at 293 K. The dye concentration is 2  10-5 M for all
compounds.

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0,03
F
Cl
Br
I
0,02
0,01
0,00
400
450
500
550
600
Wavelength (nm)
Fig. 8. Room-temperature fluorescence spectra (λex = 375 nm) of fluorinated,
chlorinated, brominated and iodinated BF2DBMs (indicated as F, Cl, Br and I,
respectively) in neat toluene (c = 1 × 10–5 M).
Fig. 9. Photograph of solutions of halogenated BF2DBMs in toluene excited with
black light at 365 nm (from left to right: F, Cl, Br, and I, respectively).
In general, the fluoro-substituted BF2DBM derivative shows the exciplex
emission maxima in benzene and toluene very close to those of the parent

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BF2DBM. The emission maxima of chloro-, bromo-, and iodo-derivatives of
BF2DBM are batochromically shifted by about 7 nm relative to the exciplex
fluorescence maxima of unsubstituted BF2DBM in benzene and toluene. The
fluorescence quantum yields of halogenated BF2DBMs except iodo-substituted
derivative are somewhat higher compared to the parent molecule.
The results obtained indicate that the halogenated BF2DBM derivatives can also
be used, along with the parent BF2DBM, as molecular sensors for fluorescence-
based detection of BTX.
Table 2. Optical properties of halogenated BF2DBMs in benzene and toluene (c =
2  10–5 M). The wavelengths of the second absorption and fluorescence peaks are
given in parentheses.
Benzene
Toluene
 (M-1 cm-1)a
32000
Compound
λabs (nm)  (M-1 cm-1)a
λF (nm)
427
λabs (nm)
366 (380)
371 (386)
372 (387)
376 (391)
λF (nm)
442
366 (381)
371 (386)
372 (387)
376 (391)
34500
35500
46000
46500
3 a
3 b
3 c
3 d
433
33600
444
433
46000
445
434
46500
444
a Coefficients are given for the short-wavelength peaks.
3.4. Computational analysis
The UV-vis absorption and the emission spectra of a halogenated derivative
were calculated at the geometries optimized for the ground state and the first
singlet excited state, respectively. Files with optimized structures of halogenated
derivatives are included in Supporting Materials. As an example, Table S1 in
Supporting Information (SI) compares selected equilibrium bond lengths and
dihedral angles of brominated BF2DBM in the S0 and S1 states. In addition, the
corresponding values are also given for the parent BF2DBM. In general,
halogenation of BF2DBM leads to only small changes in bond lengths both in the
ground and the first excited states in comparison with those of the parent
BF2DBM. The main changes are observed for dihedral angles.

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The four atoms (C1, O1, C3, and O2) of the central boron-containing ring
exhibit a nearly planar configuration in all of the compounds. For BF2DBM in the
ground state, the BF2 and C2H1 groups are tilted from the plane by about 30º and
5º, respectively. For halogenated BF2DBMs in the ground state, these dihedral
angles decrease by ~13º and ~2.5º, respectively (Table S1, SI). On the contrary, the
dihedral angles between the electron-donating phenyl groups and central boron-
containing accepting ring increases from about 1º for the parent BF2DBM to ~6.5-
8º (unsubstituted phenyl group) and ~10.5-11.5º (halogen-containing phenyl
group) for its haloderivatives, indicating that the overall molecular coplanarity is
partially destroyed.
Fig. 10. HOMO (top) and LUMO (bottom) molecular orbitals for BF2DBM-Br at
the PBE0/SVP level of theory.

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On electronic excitation, the dihedral angles do not change markedly, but bond
lengths between phenyl groups and the central ring are shortened by about 0.03 Å.
The inner C-C bonds of all the rings are also altered (Table S1).
At the DFT/PBE0/SVP level, calculations predicted that the frontier molecular
orbitals (MOs) involved in the excitation and emission of BF2DBMs exhibit a
significant displacement of electron density from one part of a molecule to the
other in the HOMO to LUMO transitions.
Calculations show that the electron-withdrawing halogen substituents diminish
the electron density on the unsubstituted phenyl ring. As an example, Fig. 10
shows the frontier MOs of BF2DBM-Br. In the ground state, the HOMO electron
density is mainly shifted to bromine atom. However, the LUMO electron density is
distributed almost homogeniously. The most remarkable difference in the ground-
state and excited-state electron density distributions is illustrated in Fig. S2 (SI) for
the iodo-derivative of BF2DBM.
Table 3. Comparison of experimental and calculated optical properties of
BF2DBM-Br in THF and in vacuo.
Electronic λmax,a nm λcalc ,b nm
f c
Configurationd (coefficiente)
transition
(eV)
S0 → S1
387
293
274
338 (3.67) 0.905
293 (4.23) 0.027
285 (4.35) 0.032
HOMO → LUMO
(0.997)
(HOMO–2) → LUMO
(0.970)
(HOMO–1) → LUMO
(0.919)
S0 → S2
S0 → S3
(HOMO–3) → LUMO
(0.339)
S0 → S4
238
278 (4.46) 0.048
(HOMO–3) → LUMO
(0.859)
(HOMO–1) → LUMO
(0.340)
a
b
Experimental long-wavelength absorption peak in THF. Computed transition
c
d
wavelength (in vacuo). Computed oscillator strength (in vacuo). Only the main
configurations are presented. e Coefficient of the excitation in the wavefunction.

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The excitation energies were calculated for PBE0/SVP optimized structures by
the TDDFT method at the same level of theory. Results obtained for BF2DBM-Br
are listed in Table 3. It is seen that TDDFT calculations reproduce most of the
features found in the experimental spectra. Though the energy of the first excited
singlet state is somewhat overestimated, calculations accurately predict the
overall positions of four absorption bands and their relative intensities. Some
overestimation of energy of the longest-wavelength transition is known to be one
of the disadvantages of the TDDFT method.
The HOMO to LUMO transition is predicted to be that with substantial
oscillator strength for BF2DBM-Br as shown in table 3. The computed absorption
spectra of BF2DBM and BF2DBM-Br obtained by the same computational method
are compared in Fig. 2S in SI.
Table 4. Comparison of experimental and calculated optical properties of
BF2DBM -Hals in THF and in vacuo, respectively.
Compound
λa, nm (eV);
f calc
λa, nm (eV)
experim
λf, nm (eV);
f calc
λf, nm (eV)
experim
DBMBF2
331.0 (3.75);
0.779
380 (3.26)
355.5 (3.49);
0.772
395 (3.14)
DBMBF2-F
DBMBF2-Cl
DBMBF2-Br
DBMBF2-I
DBMBF2-CH3
329.7 (3.76);
0.799
381 (3.25)
386 (3.21)
387 (3.20)
392 (3.16)
386 (3.21)
352.8 (3.51);
0.792
396 (3.13)
399 (3.11)
401 (3.09)
407 (3.05)
400 (3.10)
335.6 (3.69);
0.886
357.7 (3.47);
0.880
337.7 (3.67);
0.905
359.2 (3.45);
0.917
346.5 (3.58);
0.890
365.8 (3.39);
0.915
339.3 (3.65);
0.880
358.8 (3.46);
0.861

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Table 4 compares the experimental and computed characteristics of studied
compounds. The predicted trend for red shifts of the absorption and fluorescence
maxima in halogenated derivatives of BF2DBM is in good agreement with
experimentally obtained values.
Thus, calculations at the PBE0/SVP level of theory provide predictions which
are in good agreement with experimental data for spectral characteristics of new
halogenated derivatives of the BF2DBM monomer.
Fig. 11 shows the computed structure of BF2DBM-Br/benzene exciplex (top
view is given in Fig. S4, SI).
The calculations predict that two molecules form a stacked BF2DBM-
Br/benzene complex (Fig. 11) with an intermolecular face-to-face distance of about
2.86 Å between the nearest carbon atoms of the benzene ring and the dye moiety
(Fig. S4, SI). Predicted fluorescence emission maximum for such a structure is
located at 433 nm.
Fig. 11. Calculated structure of BF2DBM-Br/benzene exciplex. The distance
between two components is about 2.86 Å.
The energy of BF2DBM-Br/benzene exciplex formation was calculated to be ΔH
= –15.05 kcal/mol. For comparison, the calculated energy of BF2DBM/benzene
exciplex formation is ΔH = –15.44 kcal/mol. Thus, halogenation of BF2DBM does
not significantly affect the efficiency of the exciplex formation with benzene. The
calculated fluorescence maximum for BF2DBM-Br/benzene exciplex is located at
433 nm, which is in excellent agreement with the experimental value (Fig. 5). It
should be stressed that the same values of the exciplex fluorescence maxima were

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also obtained for halogenated and iodinated derivatives, in full accordance with the
experimental findings. Note that calculated emission maxima for corresponding
monomers exhibit progressive red shifting when going from chlorinated to
iodinated derivatives of BF2DBM (Table 4). This difference may be a consequence
of somewhat different nature of the HOMO-LUMO transitions in monomers and
exciplexes of BF2DBM-Hals.
Similar results were obtained for BF2DBM-Br/toluene exciplex. For
complexes with toluene, however, there are several possible equilibrium structures
with slightly different energies. Fig. 12 shows an equilibrium configuration
corresponding to ΔH = –17.58 kcal/mol and exhibiting a fluorescence maximum at
442 nm. The predicted value is very close to the experimental maximum (445
nm).
Fig. 12. Calculated equilibrium structure of BF2DBM-Br/toluene exciplex.
Two other equilibrium structures are shown in Figs. S5 (SI). The
corresponding computed transition energies are 430 and 448 nm, respectively.
Thus, calculations may predict the most probable structure and configuration for
exciplexes of this type. Files with optimized structures of exciplexes are included
in Supporting Materials.

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The obtained results indicates that computational approach based on the
CIS-D/CID(D) approximation may be a highly efficient method for modeling
spectral features of various exciplexes.
4. Conclusions
We have synthesized and investigated for the first time the photophysical
properties of four halogenated BF2DBMs. The absorption and fluorescence
spectral characteristics of the synthesized compounds were studied in THF
solutions at room temperature. Compared to the parent BF2DBM molecule in THF
solution, the absorption maximum peaks of halogen substituted compounds (in the
range from 311 to 345 nm) and the fluorescence emission peaks (from 394 to 412
nm) are red shifted by about 5-10 nm. Modeling at the PBE0/SVP level of DFT-
TDDFT theory predicts such a trend in the series of halogen atoms F, Cl, Br, I due
to lowering the HOMO-LUMO energy gap. It was stated that all the synthesized
halogenated compounds form exciplexes with benzene and toluene in neat solvents
with emission maxima at 433 nm and 445 nm, respectively. The equilibrium
structures of these exciplexes and their fluorescence maxima were calculated using
the CIS-D/CIS(D) method. The calculations predict the stacked arrangement of
two components at an equilibrium distance of about 2.9 Å. Predicted positions of
the emission maxima for such configurations are located in the regions of 433 and
442 nm for exciplexes with benzene and toluene, respectively. It is obvious that
predictions are very close to the experimental values.
Thus, halogenation of BF2DBM molecule can be used to tune the electronic
properties of its ground and excited states, as well as the spectral properties of the
corresponding exciplexes with methylbenzenes. This means that the synthesized
compounds have the potential to be used as molecular sensors in multisensor cross-
reactive arrays employing pattern recognition analysis.
Acknowledgments. This work was supported by the Russian Science
Foundation (project no. 15-13-00163).

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Graphical abstract

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Highlights
A series of halogenated in the para-position DBMBF2 derivatives were synthesized.
The absorption and fluorescence of the Hal-DBMBF2 derivatives are measured.
The fluorescence QY of the Hal-DBMBF2 (except I-DBMBF2) are larger than of DBMBF2.
All Hal-DBMBF2 are able to form fluorescent exciplexes with benzene and toluene.