(S2→S0) and (S1→S0) luminescence of dimethylaminostyryl-β-Diketonates of boron difluoride

Article abstract of DOI:10.1016/j.dyepig.2014.04.016

Spectral-luminescence properties of several dimethylaminostyryl-β- diketonates of boron difluoride with different α-substituents have been investigated through stationary and time-resolved spectroscopy. Quantum chemistry studies of the structure of excited states of 2,2-difluoro-4- (4′-dimethylaminostyryl)-6-phenyl-1,3,2-dioxaborine have been performed. Two luminescence centers have been revealed for the compound under study: an intensive band in the red-orange region of the spectrum caused by an S ₁-S₀ transition and a short-wave luminescence in the blue region of the spectrum caused by an S₂-S₀ transition.

Full text of DOI:10.1016/j.dyepig.2014.04.016

Accepted Manuscript
(S →S ) and (S →S ) Luminescence of Dimethylaminostyryl-β-diketonates of Boron
2
0
1
0
Difluoride
E.V. Fedorenko, A.G. Mirochnik, A. Yu. Beloliptsev, V.V. Isakov
PII:
S0143-7208(14)00144-2
10.1016/j.dyepig.2014.04.016
DYPI 4344
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 25 December 2013
Revised Date: 10 April 2014
Accepted Date: 12 April 2014
Please cite this article as: Fedorenko EV, Mirochnik AG, Beloliptsev AY, Isakov VV, (S →S ) and
2
0
(S →S ) Luminescence of Dimethylaminostyryl-β-diketonates of Boron Difluoride, Dyes and Pigments
1
0
(2014), doi: 10.1016/j.dyepig.2014.04.016.
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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
(S2→S0) and (S1→S0) Luminescence of Dimethylaminostyryl-β-diketonates of Boron
Difluoride
Fedorenko1 E.V., Mirochnik1 A.G., Beloliptsev1 A. Yu., Isakov2 V.V.
1 - Institute of Chemistry, Far-Eastern Branch of the Russian Academy of Sciences,
159, Prosp. 100-letiya Vladivostoka, Vladivostok 690022, Russian Federation.
Fax: 007(4232)311 889. E-mail: gev@ich.dvo.ru
2 - G.B. Elyakov Pacific Institute of Bioorganic Chemistry Far-Eastern Branch of the Russian
Academy of Sciences,
159, Prosp. 100-letiya Vladivostoka, Vladivostok 690022, Russian Federation.
Spectral-luminescence properties of several dimethylaminostyryl-β-diketonates of boron
difluoride with different α-substituents have been investigated through stationary and
time-resolved spectroscopy. Quantum chemistry studies of the structure of excited states
of 2,2-difluoro-4-(4′-dimethylaminostyryl)-6-phenyl-1,3,2-dioxaborine have been per-
formed. Two luminescence centers have been revealed for the compound under study: an
intensive band in the red-orange region of the spectrum caused by an S1-S0 transition and
a short-wave luminescence in the blue region of the spectrum caused an by S2-S0 transi-
tion.
Introduction
Traditionally, polymethine dyes are used as photosensitizers in photography, fluores-
cence probes, initiators for polymerization, active and passive components for tunable lasers and
non-linear optical materials [1-5]. The prospects of their further development requires more ex-

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tensive studies of electron structure of different types of conjugated molecules. Recently, a sig-
nificant number of papers devoted to the studies on polymethine dyes based on β-diketonates of
boron difluorides have been published [6-10]. Boron-containing dyes are of interest due to their
thermochromic [11], non-linear optical [12, 13] and two-photon absorption [14, 15] properties
and their possible use in organic light-emitting diodes [16, 17].
It should be mentioned that in the investigation of polymethine dyes based on β-
diketonates of boron difluoride the main focus was made on the effect of the structure and the
length of the polymethine chain on optical properties of the dyes [7-14]. Here, the role of the se-
cond α-substituent remains almost completely unstudied. In the present paper, a series of new β-
diketonates of boron difluoride with а dimethylaminostyryl substituent were synthesized, and
their structures and spectral-luminescence properties have been investigated by spectral methods
and quantum chemistry calculations.
Materials and Equipment
Materials
The initial β-diketonates of boron difluoride were prepared and purified according to the
following papers: benzoylacetonate and acetylacetonate of boron difluorde [18], anisoy-
lacetonate of boron difluoride [19], 2,2-difluoro-4-(9H-fluorene-2-yl)-6-methyl-1,3,2-
dioxaborine [20], 2,2-difluoro-4-(naphth-2-yl)-6-methyl-1,3,2-dioxaborine [21], 2,2-difluoro-4-
(9-anthracyl)-6-methyl-1,3,2-dioxaborine [22].
General procedure of synthesis of dimethylaminostyryl-β-diketonates of boron difluorides
The β-diketonate of boron difluoride(0.002 mol) and 4-dimethylaminobenzaldehyde [(0.002 mol
(0.004 mol for compound 6)] were heated in acetic anhydride (5 mL) in a water bath for 30 min.
Then, the solution was cooled, and the product was then filtered off and washed with ace-

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tic acid. The products were purified by flash silica gel chromatography (eluent: chloroform) and
recrystallized.
2,2-Difluoro-4-(p-dimethylaminostyryl)-6-phenyl-1,3,2-dioxaborine (1). Black crystals
with metallic luster, yield: 0.40 g (60%), m.p. 216-217°С (acetonitrile). IR (KBr): 1582, 1558,
1494, 1373, 1172, 1132, 1101, 1055 cm-1. 1H NMR (500 MHz, CDCl3): 3.11 (s, 6H), 6.53
(s,1H), 6,57 (d, J= 15.5 Hz, 1H), 6.72 (d, J=9.1 Hz, 2H), 7.50 (m, 2H), 7.56 (d, J=9.1 Hz, 2H),
13
8.05 (m, 2H), 8.11 (d, J=15.5 Hz, 1H); C NMR (125 MHz, CDCl3): 40.2, 97.0, 112.1, 114.4,
122.2, 128.3, 128.8, 132.1, 132.7, 133.9, 149.4, 153.1, 179.0, 181.0. Anal. calc. for
C19H18BF2NO2: C, 66.89; H, 5.32; N, 4.11. Found: C, 66.54; H, 5.23; N, 4.20.
2,2-Difluoro-4-(p-dimethylaminostyryl-6-(p-methoxyphenyl)-1,3,2-dioxaborine (2). Black
solid, yield: 0.46 g (62%), m.p. 204-205°С (acetonitrile). IR (KBr): 1596, 1577, 1539, 1521,
1506, 1456, 1435, 1373, 1257, 1254, 1159, 1120, 1045, 1012 cm-1. 1H NMR (500 MHz, CDCl3):
3.11 (s, 6H), 3.92 (s, 3H), 6,46 (s, 1H), 6.57 (d, J=15.2 Hz, 1H), 6,79 (m, 2H), 7.00 (m, 2H), 7.56
(m, 2H), 7.56 (m, 2H), 8.05 (m, 3H); 13C NMR (125 MHz, CDCl3): 40.4, 55.6, 96.1, 112.6,
114.3, 125.0, 130.8, 131.6, 147.9, 164.8, 179.6. Anal. calc. for C20H20BF2NO2: C, 64.72; H,
5.43; N, 3.77. Found: C, 64.83; H, 5.23; N, 3.56.
2,2-Difluoro-4-(p-dimethylaminostyryl-6-(9H-fluoren-2-yl)-1,3,2-dioxaborine (3). Black
solid, yield: 0.57 g (67%), m.p. 238-239°С (acetonitrile). IR (KBr): 1600, 1517, 1479, 1365,
1324, 1286, 1163, 1126, 1045 cm-1.1H NMR (500 MHz, CDCl3): 3.11 (s, 6H), 6.59 (s, 1H), 6.58
(d, J=15.1 Hz, 1H), 6.71 (m, 2H), 7.43 (m, 3H), 7.55 (m, 2H), 7.87 (m, 2H), 8.10 (m, 2H), 8.27
(s, 1H); 13C NMR (125 MHz, CDCl3): 36.8, 40.1, 97.2, 112.1, 114.7, 120.0, 121.1, 125.0, 125.3,
127.2, 127.7, 128.5, 130.8, 131.9, 140.3, 143.7, 144.7, 147.8, 148.8, 153.1, 180.2. Anal. calc. for
C26H22BF2NO2 : C, 72.75; H, 5.17; N, 3.26. Found: C, 72.90; H, 5.11; N, 3.31.
2,2-Difluoro-4-(p-dimethylaminostyryl-6-(naphth-2-yl)-1,3,2-dioxaborine (4). Black sol-
id, yield: 0.51 g (65%), m.p. 201-202°С (acetonitrile). IR (KBr): 1595, 1558, 1516, 1490, 1375,
1184,1157, 1132, 1043 cm-1. 1H NMR (500 MHz, CDCl3): 3.11 (s, 6H), 6.60 (d, J=15.1 Hz, 1H),

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6.66 (s, 1H), 6.71 (m, 2H), 7.65-7.51 (m, 5H), 7.90 (m, 2H), 7.99 (m, 2H), 8.12 (d, J=15.1 Hz,
13
2H); C NMR (125 MHz, CDCl3): 40.1, 97.5, 112.1, 114.4, 122.1, 123.4, 127.1, 127.9, 128.6,
129.0, 129.7, 130.4, 132.1, 136.0, 149.4, 153.2, 178.6, 180.8. Anal. calc. for C23H20BF2NO2: C,
70.61; H, 5.15; N, 3.58. Found: C, 70.43; H, 5.14; N, 3.58.
2,2-Difluoro-4-(p-dimethylaminostyryl-6-(anthr-9-yl)-1,3,2-dioxaborine (5). Dark green
solid, yield: 0.52 g (59%), m.p. 114-115°С (acetic acid). IR (KBr): 1558, 1538, 1519, 1483,
1
1437, 1371, 1282, 1221, 1184, 1142, 1043, 1012 cm-1. H NMR (500 MHz, CDCl3): 3.13 (s,
6H), 6.35 (s, 1H), 6.55 (d, J=15.1 Hz, 1H), 6.73 (m, 2H), 7.60-7.46 (m, 8H), 8.04 (m, 2H), 8.24
(m, 3H), 8.89 (s,1H), 13C NMR (125 MHz, CDCl3): 40.1, 105.6, 112.2, 113.7, 122.3, 125.2,
125.3, 125.4, 125.6, 126.6, 127.3,128.6, 129.2, 130.9, 132.5, 151.1, 153.5, 181.1, 182.0. Anal.
calc. for C27H22BF2NO2 : C, 73.49; H, 5.03; N, 3.17. Found: C, 73.46; H, 5.04; N, 3.15.
2,2-Difluoro-4,6-di(p-dimethylaminostyryl-1,3,2-dioxaborine (6). Black solid, yield: 0.49
g (60%), m.p. 220-221°С (acetonitrile). IR (KBr): 1603, 1558, 1519, 1487, 1471, 1373,1228,
1192, 1115, 1036 cm-1. 1H NMR (500 MHz, CDCl3): 3.09 (s, 12H), 5.90 (s, 1H), 6.47 (d, J=15.2
13
Hz, 2H), 6.74 (d, J=8.8 Hz, 4H), 7.52 (m, 4H), 7.97 (d, J= 15.1 Hz, 2H); C NMR (125 MHz,
CDCl3): 40.1, 100.9, 112.4, 115.4, 131.3, 140.7, 178.0. Anal. calc. for C23H25BF2N2O2: C, 67.33;
H, 6.14; N, 6.83. Found: C, 67.43; H, 6.24; N, 6.76.
Instrumentations and methods
The luminescence and excitation spectra were registered on a Shimadzu-RF5301 spec-
trometer. A solution of rhodamine 6G was used as a standard for measuring the fluorescence
quantum yield (ϕF = 0.96) [23]. The measurements of fluorescence lifetime by time-correlated
single-photon counting (TCSPC) were performed on a FluoTime 200 device (PicoQuant) with a
PDL 800-B (370 nm) excitation source and a TimeHarp device as the SPC controller. The life-
time data were analyzed using the FluorFit 4.0 software from PicoQuant. The absorption spectra

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were registered on a Shimadzu-UV2550 spectrometer in a cell of 10x10 mm size. The IR spectra
were registered on an IRAffinity-1 Shimadzu spectrometer.
Computational details
The quantum chemistry simulations of electron absorption spectra of the compounds in
different conformations were performed in a cluster approximation with the use of GAMESS-US
program complexes [24]. The structural parameters, energy characteristics, and electron struc-
tures of the compounds were determined at full geometry optimization in the 6-311G(d, p) basis
by the non empirical (ab initio) method and the density functional method with the exchange-
correlation potential B3LYP. The electron absorption spectra of the compound 1 with the ac-
count of excited singlet and triplet states were calculated by the TDDFT method in the 6-
311G(d, p) basis with the potential B3LYP.
Results and Discussion
The dyes 1-6 were synthesized from β-diketonates of boron difluoride by aldol-crotonic
condensation with para-N,N-dimethylaminobenzaldehyde, similarly to the method described in
[25] for 2,2-difluoro-4-mehtylnaphtho-[2,1-е]-1,3,2-dioxaborine (Fig. 1).
N(CH3)2
R
CH3
R
O
H
(CH3CO)2O
1000C
O
O
+
(CH3)2N
B
O
O
F
F
B
F
F
R:
1 - C6H5;
2 - p-CH3OC6H4;
4
3
N(CH3)2
5
6

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Scheme 1.
In organic solvents, the obtained compounds form solutions of either red or violet color
with an intensive luminescence in the red-orange region of the spectrum. The quantum yield in
non-polar solvents was in the range 0.2 - 0.8 and it decreases along the solvent polarity increases.
The dye solutions are characterized with sovatochromism and luminescence solvatochromism
(Table 1).
In the ground or excited states, β-diketonates of boron difluoride are known to interact
with some solvents, thus forming exciplexes with aromatic compounds [26, 27] and hydrates
with water [28, 29]. Upon heating in acetonitrile or dimethylformamide the decomposition of
boron difluoride complexes was observed with the formation of β-diketones [30]. Therefore, to
avoid the effect of specific solvatation, chlorinated hydrocarbons with different polarities were
selected for the study of the solvatochromism phenomenon: dichloromethane, chloroform, and
carbon tetrachloride (ε(CH2Cl2) = 9.08, ε(CHCl3) = 4.81, ε(CCl4) = 2.24 [31]).
As Table 1 shows, in β-diketonates of boron difluoride with p-N,N-dimethylaminostytryl
groups, one observes a bathochromic shift of absorption and luminescence spectra as the solvent
polarity is increased. Here, the increase of Stokes shift also takes place as well (Table 1). For
compounds 1-4 in dichloromethane, the Stokes shift is about 2300-2700 cm-1. In comparison to
other compounds, compound 5 is characterized by an abnormally high Stokes shift of 2894 cm-1
(Table 1). Among primary photo-processes which cause the emergence of fluorescence with
the increased Stokes shift, a prominent place is held by structural relaxation - changes in the
molecule geometry resulting in the energy decrease in the electron-excited state, in comparison
with its initial excited Franck-Condon state (FCS), which the molecule acquires after the light
quantum absorption. The difference in the energies of excited states would constitute the in-
crease of the Stokes shift, and the structurally relaxed state (SRS) is often characterized by an
intramolecular charge transfer. For most of the polymethine dyes, the Stokes shift increase is re-
lated to ‘flattening’ of the dye molecule in the excited state [32]. In dyes 1-6, structural changes

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promoting the formation of a planar structure are observed. In dyes 1-4, 6 free rotation is of the
aromatic substituent possible, and unproblematic is the location of α-substituent and β-
diketonate cycle in the same plane contributing to efficient π-π conjugation. As distinct from
other compounds, in 5 the anthracene group is rotated relative to the chelate cycle due to steric
hindrance. The geometry of the molecule of β-diketonate of boron difluoride with an anthracene
fragment is examined in detail in [22] by the example of 2,2-difluoro-4-(9-anthracyl)-6-methyl-
1,3,2-dioxaborine. According to X-ray structural analysis data, the angle of the anthracene sub-
stituent planes with the chelate cycle is equal to 69°. Probably, even a slight reduction of inter-
plane angle of the anthracene fragment with β-diketonate cycle from photoexcitation of 5 should
involve a marked decrease of the excited state energy and larger Stokes shift than in 1-4. By con-
trast, in symmetrical dye 6 observed is the lowest value of the Stokes shift in the series of com-
pounds under study: 1907 cm-1 in dichloromethane. When the solvent polarity is reduced, the
decrease of the Stokes shift for 6 is lower than in 1-5.
Quantum chemistry calculations of absorption spectra of dyes for FCS and SRS geome-
tries were performed for example 1. Quantum chemistry calculations have been performed for a
series of β-diketonates of boron difluorides [33-37]. The authors of these foregoing papers note
good agreement between experimental (in nonpolar solvents and in the gas phase) and calculated
absorption spectra for the Franck-Condon geometry of the excited state. For 1, unlike the com-
plexes described in the above papers, no correlation between the calculated and experimental
spectra within the scope of the Franck-Condon geometry was observed (Table 2). In the calculat-
ed absorption spectrum for 1 in FCS first band was observed at 418.73 nm with an oscillator
strength of 1.017, although the maximum of the experimental absorption spectrum of 1 in nonpo-
lar solvents (hexane and carbon tetrachloride) equals to 509 nm. The earlier studied β-
diketonates had hydrocarbon substituents with similar donor-acceptor properties, whereas the S0-
S1 transition was accompanied by an insignificant redistribution of the electron density delocal-
ized over the whole molecule [35, 37]. As was mentioned in [38], in the presence of α-

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substituents with significantly different in electron-donor properties, one observes a predominant
realization of intramolecular charge transfer (ICT) state during the excitation of a molecule of
boron difluoride β-diketonate. The presence of solvatochromism in compounds 1-6 indicates that
the S0-S1 transition is the transition with a charge transfer. The compounds under study differ
from other β-diketonates of boron difluoride by the presence of a strong electron donor (dime-
thylamino-group) and a strong acceptor (BF2) in the molecule. The presence of both the strong
donor and the strong acceptor in the same molecule results in the fact that the highest occupied
molecular orbital (HOMO) is localized mainly on the dimethylaminostyryl group, whereas the
lowest unoccupied molecular orbital (LUMO) is localized mainly on the chelate cycle (see Fig.
1), i.e. the S0-S1 transition is the transition with a charge transfer. The optimization of the excited
state geometry revealed that the LUMO SRS is substantially different from the LUMO FCS. The
electron density decreases on the dimethylaminostyryl group and is homogeneously distributed
between phenyl and chelate rings (see Fig. 1), which causes the increase of the charge transfer
upon the molecule excitation. When the SRS geometry is used in calculations of the absorption
spectrum, one observes better agreement with the experiment than for the FCS geometry (see
Fig. 2, Table 2).
Figure 3 shows geometrical changes occurring in molecule 1 in the transition to the SRS
state. The lengths of the phenyl ring bonds linked to the N,N-dimethylamino group (Ph1) under-
goes virtually no changes. Whereas the lengths of bonds between the Ph1 ring and the double
bond and between the double bond and the chelate cycle increase substantially, they decrease
between the chelate cycle and Ph2. At the same time, there occur changes in the lengths of С-С
bonds of the chelate cycle. As seen from Figure 3, the above changes cause the formation of a
chain of alternating one-and-half and double bonds, similarly to alternation of bonds in polyme-
thine dyes molecules [39-41].
The low-intensity bands (see Fig. 4) are present in the short-wave range of excitation and
luminescence spectra for the dyes under study, and the spectra of short-wave excitation and lu-

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minescence are symmetric reflection (see Fig. 4, insert). The presence of the second group of
symmetric reflection bands of excitation and luminescence indicates the existence of the second
luminescence center in the dye molecule. As was demonstrated in [34,42], low-intensity bands
attributed to Sn-S0 transitions can be observed in the short-wave range of the luminescence spec-
trum. The excitation spectrum of short-wave luminescence really corresponds to the bands of S0-
S2 transitions in the absorption spectrum. Here, one observes the dependence of luminescence
spectra on the excitation wavelength and vice versa (Fig. 5). When the luminescence of 1 is ex-
cited by a 350 nm wavelength, which belongs to the group of short-wave bands of the excitation
spectrum, a short-wave band with a maximum at 406 nm emerges in the luminescence spectrum.
For the SRS state of 1, the intensity of S0-S2 transition in the calculated absorption spectrum be-
comes, as opposed to the FCS, almost equal to that of S0-S1 transition (Fig. 2, Table 2). Here, in
the case of the SRS, the contribution of the transfer from HUMO to LUMO+1 related to the
charge transfer from dimethylaminostyryl group to the whole molecule increases from 15 up to
30%, as compared to the FCS (Fig. 1, Table in Supplementary materials). In the experimental
absorption spectra of 1 in all three solvents, observed are short-wave bands of S0-S2 transition
(342 and 359 nm) corresponding to the SRS state.
Figure 6 presents the time-resolved luminescence spectrum of 1 upon excitation at 370
nm corresponding to the excitation band of S0-S2 transition. In the moment of excitation, bands
with maxima at 463 (S2-S0) and 528 (S1-S0) nm in the luminescence spectrum are observed.
Within one nanosecond, the short-wave band disappears, and the maximum of the long-wave
band bathochromically shifts to 550 nm. It indicates the initial transition to the state S2, whose
deactivation includes luminescence directly from the state S2 and S2-S1 transition. In the station-
ary spectra, unlike the time-resolved spectra, the intensity of S2-S0 luminescence is much lower
than that for S1-S0. When the solvent polarity is increased, the rate of S2-S1 transition decreases,
and the intensity of S2-S0 luminescence increases. In the stationary spectrum of 1 in carbon tetra-
chloride, the short-wave luminescence is not manifest, it can only be registered by the time-

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resolved spectroscopy method. However, in the stationary spectra of the dyes in dichloro-
methane, the short-wave luminescence is observed, and its quantum yield is ≈ 0.002 at the quan-
tum yield of the long-wave luminescence S1-S0 being 0.05-0.2. In alcohol solutions, the quantum
yield of luminescence from S2 in the stationary spectrum reaches 0.1-0.2 and becomes compara-
ble to that from the level S1 (Fig. 5).
The effect of the anthracene α-substituent (5) on the dye spectral properties should be
specially considered. As was shown earlier by the X-ray structural analysis method, in the mole-
cule 2,2-difluoro-4-(9-anthracyl)-6-methyl-1,3,2-dioxaborine [22], the dihedral angle between
the planes of the anthracene fragment and the chelate ring equals to 69°. This large value of the
dihedral angle between the chelate cycle and the α-substituent is indicative of weakening of the
π-π interaction between the rings and allows to consideration the anthracene fragment and the
chelate ring conjugated with the dimethylaminostyryl group as two practically independent
chromophores. In fact, when the dependence of the luminescence spectrum on the excitation
wavelength was studied, an additional luminescence center in the stationary luminescence of 5 in
ethanol and dichloromethane was observed. The third group of reflection symmetrical bands of
excitation and luminescence (λex = 244, 283 nm and λlum = 314, 368 nm) was only noticed in
spectra of 5 (Fig. 7). Here, the excitation band at 244 nm corresponds to the absorption band of
the anthracene fragment (Fig. 7).
Table 1 presents the data on the effect of the structure of the second α-substituent in di-
methylaminostyryl β-diketonates of boron difluoride on complex spectral-luminescence proper-
ties. The introduction of an electron-donor substituent into the para-position of the phenyl ring,
for instance, of the methoxy-group (2), causes an insignificant hypsochromic shift of the maxima
of luminescence and absorption spectra relative to 1 (see Table 1), as opposed to a strong batho-
chromic shift in the spectra in the transition from dibenzoylmethanate of boron difluoride to its
methoxy-derivatives [43]. The reason of this, as was demonstrated by quantum chemistry calcu-
lations, consists in the localization of HOMO on the chelate ring and the dimethylaminostyryl

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group (Fig. 1), and the second α-substituent being not included in the molecule π-system. This is
why the effect of a donor group in the para-position of the phenyl ring on the position of the lu-
minescence spectrum maximum is not manifest. During the excitation, the electron density is
transferred from the aminostytryl group to the whole molecule. Therefore, the increase of the
charge transfer efficiency must be aided by the presence in the para-position of the phenyl ring
of an acceptor substituent, rather than a donor substituent. Really, as was shown in [44] for a
similar boron chelate, the introduction of chlorine into the para-position of the phenyl ring re-
sulted in the bathochromic shift of the absorption spectrum maximum by 10 nm, in the case of a
nitro-group – by 30 nm. Along the π-system of an aromatic substituent is increased, one observes
a bathochromic shift of absorption and luminescence spectra. Thus, upon the substitution of phe-
nyl radical 2-fluoryl (3) by 1-naphthyl (4), there occurs a bathochromic shift of absorption and
luminescence spectra by approximately 10 nm (Table 1). At further increase of the π-system,
when, for instance, the second dimethylaminostyryl substituent is introduced (6), the shift of ab-
sorption and luminescence spectra maxima attains as much as 50-60 nm relative to 1.
When hydrochloric acid is added to solutions 1-6, the solution decolorized and a change
in the luminescence color from red to either blue or green was observed (see Table 3, Fig. 8, 9).
Thus, β-diketonates of boron difluoride containing the N,N-dimethylamino-group participate in
the reactions characteristic for amines: they form a hydrochloride in the reactions with hydro-
chloric acid (Scheme 2). The reaction with hydrochloric acid occurs in water miscible solvent,
such as ethanol, acetone, dioxane and hydrochloric acid vapors.
CH3
H
N
+
CH3
N
CH3
CH3
R
R
HCl
O
O
O
O
B
B
NaOH
F
F
F
F
Scheme 2.

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The donor amino-group is transformed into an acceptor ammonium cation, and the redis-
tribution of electron density of the dye molecule takes place. The optimization of geometry of
the cation obtained through protonation of dimethylamino-group 1 (cation 1) demonstrates the
increase of the length of C-N bond between the dimethylamino-group and the phenyl ring from
1.374 up to 1.477 Å and the disruption of the molecule coplanarity takes place: the torsion angle
between the plane of the aromatic ring by the amino-group and the plane encompassing the mol-
ecule main part is equal to 25°. The disruption of coplanarity of molecule 1 upon protonation re-
sults in the reduction of π-conjugation in cation 1. The structures of the HOMO and LUMO un-
dergo significant changes in the transformation of the dye from the neutral form to its cation
(Fig. 10). As distinct from molecule 1, in cation 1 HOMO is localized on the second aromatic α-
substituent, β-diketonate ring, and the double bond rather than on the aminostyryl group. Due to
this fact, in the hydrochloride series 1-5 the positions of luminescence spectra maxima are batho-
chromically shifted in accordance with the increase of π-donor capacity of the second α-
substituent in the same manner as in the series of benzoylacetonates and dibenzoylmethanates
[30]. The lowest donor capacity is characteristic for the anthracene group rotated relative to the
chelate cycle. In accordance with the increase of donor capacity, substituents can be grouped as
follows: 9-anthracyl <methoxyphenyl<fluoryl<naphthyl (Table 3).
The cation HOMO is located on the β-diketonate ring, double ring, and aromatic ring by
the amino-group (Fig. 10). The localization of the major part of HOMO and LUMO electron
density on the chelate ring and the double bond promotes the reduction of СТ character of S0-S1
transition. The latter results in the hypsochromic shift of the cation absorption and luminescence
spectra, as compared to those of the dye neutral form. The calculated absorption spectrum for
cation 1 is shown in Fig. 11, the assignment of absorption bands is presented in Supplementary
Materials.
Conclusions

ACCEPTED MANUSCRIPT
A series of N,N-dimethylaminostyryl β-diketonates of boron difluoride with different α-
substituents have been synthesized. Spectral-luminescent properties of the obtained dyes have
been investigated through stationary and time-resolved spectroscopy methods. It has been
demonstrated that the introduction of an electron-donor substituent into the para-position of the
phenyl α-substituent results in an insignificant hypsochromic shift of the luminescence and ab-
sorption spectra maxima. The bathochromic shift of the luminescence and absorption spectra
maxima is observed upon introduction of the electron-acceptor substituent into the para-position
of the phenyl ring and upon the increase of π-system of the aromatic α-substituent. For the com-
pounds under study, two luminescence processes are observed: an intensive band in the red-
orange range of the spectrum caused by S1-S0 transition and the short-wave luminescence in the
blue range of the spectrum caused by S2-S0 transition. As is demonstrated by quantum chemistry
simulations, the molecule geometry in the S1 state corresponds to the geometry of the structurally
relaxed state.
Supplemental Materials
Supplemental Materials include excitation and luminescence spectra for compounds 1-6
and the results of quantum chemistry calculations for compound 1 and its cation (energies, types,
and compositions of some molecular orbitals; energies and oscillator strengths of singlet-singlet
transitions in the electron absorption spectrum).
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Table 1.
Spectral-luminescence properties of dimethylaminostyryl β-diketonates boron difluoride.
СH2Cl2
λfl,
СHCl3
λfl,
СCl4
λfl, nm
Com-
pound
∆υST,
∆υST,
∆υST,
λabs, nm (lg ε)
λabs, nm (lg ε)
λabs, nm (lg ε)
ϕ
ϕ
ϕ
cm-1
cm-1
cm-1
nm
nm
406,
427, 0.44
598
394,
416,
566
370,
388,
560
399,
418,
576
348 (4.19), 362
(4.19), 537 (4.87)
430,
627
348 (4.15), 362
(4.16), 528 (4.83)
342 (4.13) , 359
(4.10), 509 (4.81)
2673
2386
2540
2516
2216
2104
2213
2076
0.53 1978
0.44 1905
0.56 1869
0.64 848
1
2
3
4
0.026
358 (4.43), 376
(4.32), 534 (4.92)
396,
612
358 (4.43), 376
(4.32), 528 (4.93)
386,
0.38
594
354 (4.43),
370(4.27), 506 (4.84)
0.15
0.20
0.05
482 (3.42), 549
(4.81)
450,
638
482 (3.42), 549
(4.92)
442,
0.54
612
353 (4.46), 520
(4.92)
276 (4.23), 374
(4.23), 393 (4.37),
549 (4.92)
274 (4.38), 374
(4.23), 393 (4.37),
543 (4.83)
276 (4.23), 366
(4.29), 385 (4.46),
520 (5.10)
435,
637
406,
0.37
612
435,
544
254 (5.09),
254 (5.09), 283
(4.24), 330 (4.01),
346 (3.98), 380
308,
366,
407,
570
283(4.24), 330
(4.01), 346(3.98),
380 (3.60), 533
(4.80)
278 (3.75), 340
(3.23), 460 (4.17),
590 (4.96)
254 (5.09),326
(4.25), 342(4.22),
503 (4.93)
415,
641
436,
0.06
627
2894
1907
2812
1705
0.19 2336
0.74 1672
5
6
0.008
0.54
(3.60), 540 (4.82)
309 (4.02), 337
(4.02), 460 (4.33),
598 (5.13)
273(3.92),334 (3.67), 398,
430,
675
440,
0.79
656
448 (3.89),
481(4.14), 556 (4.46)
415,
613

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Table 2.
Experimental and calculated absorption spectra compound 1 .
Exp. spectra
Franck-Condon exited state
Structurally relaxed state
λmax, nm
(lg ε)
f
Transition
f
Transition
λ, nm
λ, nm
89-90
(98.7)
89-90
(99.9)
509 (4.81)
418.73
1.017
499.74
0.613
88-90
(83.8)
89-91
(15.5)
88-90
(27.7)
89-91
(65.3)
342 (4.13) ,
359 (4.10)
325.76
0.212
347.49
0.604
Table 3.
Absorption and luminescence maxima of ethanol solution N,N-dimethylamino-styryl β-
diketonates boron difluoride and their hydrochlorides
Neutral form
λabs, нм
Hydrochloride
Compound
λlum, нм
635
λabs, нм
λlum, нм
452
1
2
3
4
5
6
530
530
387
412
631
483
545
650
443
522
542
625
425
531
251, 538
591
640
251, 368
413
401
691
461

ACCEPTED MANUSCRIPT
a
b
(91) НСМО+1 -0.0449
-0,0507
(90) НСМО -0.0908
-0,1016
(89) ВЗМО -0.2081
-0,2015
(88) ВЗМО-1 -0.2454
-0,2480
Fig. 1.

ACCEPTED MANUSCRIPT
1,0
0,8
0,6
0,4
0,2
0,0
FÑS
SRS
200
250
300
350
400
450
500
550
λ, nm
Fig. 2.
Fig. 3.

ACCEPTED MANUSCRIPT
50
25
0
600
300
0
1
2
350
400
450
λ, nm
400
600
800
λ
, nm
Fig. 4.
4
50
2
I, a.u.
3
1
λ
25
λ
λ
0
300
400
500
600
700
λ, nm
Fig. 5.

ACCEPTED MANUSCRIPT
I, a.u.
20
1
2
10
0
3
4
400
450
500
550
600
650
λ
, nm
Fig. 6.
300
0,6
I, a.u.
D
2
3
200
100
0
0,4
0,2
0,0
1
300
400
500
600
700
800
λ
, nm
Fig. 7.

ACCEPTED MANUSCRIPT
0,3
Dye + HCl
600
D
I, a.u.
2
0,2
0,1
0,0
400
200
0
1
Dye
3
4
5
200
300
400
500
600
700
800
λ, nm
Fig. 8.
a
b
Fig. 9.

ACCEPTED MANUSCRIPT
LUMO
HOMO
Fig. 10. HOMO and LUMO for cation 1.
1,00
0,75
0,50
0,25
0,00
1
f
1-H+
200
250
300
350
400
450
λ
, nm
Fig. 11. Theoretical absorption spectra compound 1 and it’s cation (1-H+) for Franck-Condon
state.

ACCEPTED MANUSCRIPT
Figure captions
Fig. 1. Orbitals involved in the absorption spectra for 1 (a) - Franck-Condon state, (b) -
structurally relaxed state.
Fig. 2. Theoretical absorption spectra of 1 Franck-Condon state (FCS) and structurally relaxed
state (SRS).
Fig. 3. Bond length in ground (bottom) and excited structurally relaxed (top) states.
Fig. 4. Spectra of solution 1 in chloroform С = 5ꢀ10-6 mol/l: 1 – excitation; 2 – luminescence; in
insert shown expanded short-wave part of spectra.
Fig. 5. Spectra of solution 4 in ethanol C = 5ꢀ10-6 mol/l: excitation: 1 - λreg = 443 nm, 2 - λreg
650 nm; luminescence: 3 - λex = 390 nm, 4 - λex = 562 nm.
=
Fig. 6. Time-resolved emission spectra of solution 1 in carbon tetrachloride C = 5ꢀ10-5 mol/l: 1 -
75.7 ns, 2 -75.8 ns, 3 – 76 ns, 4 – 76.4 ns. Time is the time after the laser pulse. λex = 370
nm.
Fig. 7. Spectra of solution 5 in dichloromethane C = 5ꢀ10-6 mol/l: 1 – absorption; 2 - excitation,
λreg = 313 nm; 3 - luminescence, λex = 256 nm.
Fig. 8. Spectra of solution in ethanol C = 5ꢀ10-6 mol/l: 1 - absorption of 2; 2 – absorption of
hydrochloride 2; 3 – luminescence of 2, λex = 550 nm; 4 – luminescence of hydrochloride
2, λex = 420 nm; 5 – luminescence of hydrochloride 2, λex = 365 nm.
Fig. 9. The pictures of 2 (left) and 2–H+ (right) on a solution in ethanol under day
light (a) and UV light (b).
Fig. 10. HOMO and LUMO for cation 1.
Fig. 11. Theoretical absorption spectra compound 1 and it’s cation (1-H+) for Franck-Condon
state.

ACCEPTED MANUSCRIPT
N,N-dimethylaminostyryl-β-diketonates of boron difluoride were synthesized.
Two luminescent processes were revealed for the compounds.
The quantum chemistry simulation of absorption spectra was calculated.
These compounds form hydrochloride with a bright luminescence.