Synthesis, photophysical and electrochemical properties of novel and highly fluorescent difluoroboron flavanone β-diketonate complexes

Article abstract of DOI:10.1039/d0nj03525d

Difluoroboron β-diketonates complexes are highly luminescent with extensive properties such as their fluorescence both in solution and in solid state and their high molar extinction coefficients. Due to their rich optical properties, these compounds have been studied for their applications in organic electronics such as in self-assembly and applications in biosensors, bio-imaging and optoelectronic devices. The easy and fast synthesis of difluoroboron β-diketonate (BF2dbm) complexes makes their applications even more attractive. Although many different types of difluoroboron β-diketonates complexes have been studied, the cyclic flavanone analogues of these compounds have never been reported in the literature. Therefore, the present work aims to synthesize difluouroboron flavanone β-diketonate complexes, study their photophysical and electrochemical properties and assess their suitability for applications in optoelectronic devices. The synthesis was based on a Baker-Venkataraman reaction which initially provided substituted diketones, which were subsequently reacted with aldehydes to afford the proposed flavanones. The complexation was achieved by reacting flavanones and BF3·Et2O and in total 9 novel compounds were obtained. A representative difluoroboron flavanone complex was subjected to single crystal X-ray diffraction to unequivocally confirm the chemical structure. A stability study indicated only partial degradation of these compounds over a few days in a protic solvent at elevated temperatures. Photophysical studies revealed that the substituent groups and the solvent media significantly influence the electrochemical and photophysical properties of the final compounds, especially the molar absorption coefficient, fluorescence quantum yields, and the band gap. Moreover, the compounds exhibited a single excited-state lifetime in all studied solvents. Computational studies were employed to evaluate ground and excited state properties and carry out DFT and TDDFT level analysis. These studies clarify the role of each state in the experimental absorption spectra as well as the effect of the solvent.

Full text of DOI:10.1039/d0nj03525d

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Chechia Hu, Tzu-Jen Lin et al.
Yellowish and blue luminescent graphene oxide quantum dots
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Synthesis, Photophysical and Electrochemical Properties of Novel
DOI: 10.1039/D0NJ03525D
and Highly Fluorescent Difluoroboron Flavanone β-Diketonate
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Complexes
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Elida Betania Ariza Paeza, Sergio Curciob, Natália P. Nemebc, Matheus J.S. Matosb,
Rodrigo S. Correaa, Fabio Junio Pereiraa, Flaviane Francisco Hilárioa, Thiago Cazatib and
Jason Guy Taylora
a Chemistry Department, ICEB, Federal University of Ouro Preto, Campus Universitário Morro
do Cruzeiro, 35400-000, Ouro Preto-MG, Brazil.
b Physics Department, ICEB, Federal University of Ouro Preto, Campus Universitário Morro do
Cruzeiro, 35400-000, Ouro Preto-MG, Brazil.
cUniversity of Groningen, Zernike Institute for Advanced Materials, Nijenborgh 4, 9747 AG
Groningen, The Netherlands
* e-mail address of the corresponding author: jason@iceb.ufop.br or jason@ufop.edu.br
Abstract
Difluoroboron β-diketonates complexes are highly luminescent with extensive
properties such as their fluorescence both in solution and in solid state and their high
molar extinction coefficients. Due to their rich optical properties, these compounds have
been studied for their applications in organic electronics such as in self-assembly and
applications in biosensors, bio-imaging and optoelectronic devices. The easy and fast
synthesis of difluoroboron β-diketonate (BF2dbm) complexes makes their applications
even more attractive. Although many different types of difluoroboron β-diketonates
complexes have been studied, the cyclic flavanone analogues of these compounds have
never been reported in the literature. Therefore, the present work aims to synthesize
difluouroboron flavanone β-diketonate complexes, study their photophysical and
electrochemical properties and assess their suitability for applications in optoelectronic
devices. The synthesis was based on a Baker–Venkataraman reaction which initially
provided substituted diketones, which were subsequently reacted with aldehydes to
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afford the proposed flavanones. The complexation was achieved by reacting flDaOvIa: 1n0o.10n3e9s/D0NJ03525D
and BF3. Et2O and in total 9 novel compounds were obtained. A representative
difluoroboron flavanone complex was subjected to single crystal X-ray diffraction to
unequivocally confirm the chemical structure. A stability study indicated only partial
degradation of these compounds over a few days in a protic solvent at elevated
temperatures. Photophysical studies revealed that the substituent groups and the
solvent media significantly influence the electrochemical and photophysical properties
of the final compounds, especially the molar absorption coefficient, fluorescence
quantum yields, and the band gap. Moreover, the compounds exhibited a single excited-
state lifetime in all studied solvent. Computational studies were employed to evaluate
ground and excited states properties and carry out DFT and TDDFT level analysis. These
studies clarify the role of each state in the experimental absorption spectra as well as the
effect of the solvent.
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Keywords: Flavanone; Chromanone; Flavonoid; Difluoroboron; Diketonates; UV-VIS,
Fluorescence; Luminescence; Quantum Yield; X-ray; Photovoltaic; DFT Studies;
Optoelectronic.
Introduction
Organic luminophores stand out as important compounds within a diverse set of
different areas of science because of their applications in technology as part of chemical
detection devices, biological identification applications, bio-imaging and
photoelectronic devices.1,2 The last decade has seen continued efforts to develop a new
class of compounds known as organoboronic complexes.3,4 The difluoroboron β-
diketonates (BF2dbm) belonging to the tetracoordinate organoboron complexes are
highly fluorescent and can present very useful photophysical properties,1-6 with
potential technological applications. The first compound of this type was synthetized by
Morgan and Tunstall in 1924 through Claisen condensation reaction using only aromatic
carbonyl compounds to obtain fluorescent organoboronic compounds.5 The
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difluoroboron β-diketonate complexes are increasingly attracting more attention as
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fluorescent organic materials because they have so many advantages including easy
preparation, high molar extinction coefficient, high quantum fluorescence yields and
important photo physical properties.4 Most of BF2dbm complexes reported showed a
strong emission in solution6 and further chemical modification at the aromatic ring
substituents of the molecule and conjugation expansion allows for adjustments in
emission color throughout the visible region up until NIR region. 7
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Although the synthesis and photophysical characterization of many BF2dbm
8-13
complexes has been described in the literature,3,
only a few notable examples
demonstrating their potential applications have been reported. For example, the
BF2dbm-PLA polymer synthetized by Zhang and collaborators was found in the solid
state to display unique optical properties and good stability of nanoparticles in aqueous
suspension and thus making them suitable for biological imaging, biological detection
and photodynamic therapies.14 The NIR emission characteristic of the curcuminoid
based BF2dbm complexes made them suitable candidates for NIR imaging. Their
capacity to penetrate deeply into the tissues, low auto-fluorescence background and low
toxicity to biological samples allowed Ran and collaborators to use the curcuminoid
complexes as NIR imaging probe for detecting insoluble aggregates of β-amyloid in vitro,
an pathological marker of Alzheimer disease and differentiate in vivo mice and 19 month
old transgenic mice.15 Mo and collaborator reported the use of a BF2dbm complexes as
an organic light-emitting diode emission material as part of a device capable of
exhibiting high quantum yields of 12.8%.16 A BF2dbm derivative containing a
triphenylamine moiety was synthetized as a new type of π electron donor-acceptor
system. The compound showed long wavelength absorption in the UV/visible spectra
and redox properties ideal for applications in sensitized solar cells. The fabricated device
showed solar-to-electric power conversion efficiencies of 2.7– 4.4% under 1.5 AM solar
light.17
Almost all BF2dbm complexes described in the literature have the same basic
structural scaffold with substituents mainly being introduced on the benzenoid rings.
Structural modifications of BF2dbm complexes at the α-carbon of the dicarbonyl moiety
or variants of the 1,3-diketone as cyclic ketone, have up until to now, not been reported.
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The flavanone structural motif is an interesting option for complexation with boron
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difluoride given that the extra aromatic ring at the C2 position of the dihydropyrone
could contribute to the absorption and fluorescent properties. Therefore, in order to
discern how the aforementioned structural modifications affect the photophysical and
electrochemical properties of BF2dbm complexes, we describe the synthesis and evaluate
the photophysical properties of novel difluoroboron flavanone β-diketonates complexes
in different media.
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2. Results and Discussion
2.1 Synthesis of Difluoroboron Flavanone β-Diketonates Complexes
The synthetized β-diketones were obtained by esterification of 2-hydroxyacetophenones
with substituted benzoyl chlorides in the first step. Esters 1a-e were subjected to a Baker-
Venkataraman rearrangement in the second step to afford diketones 2a-e in excellent
yields (Scheme 1).18-19
O
OH
O
O
OH
O
O
1. Pyridine
RT
CH3
R1
1. Pyridine
CH3
R1
R1
+ Cl
O
2. KOH, 50oC
O
2a-e
1a-e
O
OH
O
O
OH
O
OH
O
O
2b
97% yield
2c
83% yield
2a
94% yield
OCH3
CH3
OH
O
O
OH
O
O
O
S
2d
2e
88% yield
91% yield
Scheme 1. Synthesis of β-diketones.
All the synthetized β-diketones are known compounds in the literature and therefore
were confirmed by comparison of their physical and spectroscopy data with the reported
data in the literature (see experimental section).
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Next, with diketones 2a-e in hands, flavanones 3a-i were synthetized via an
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Aldol-Oxi domino reaction between β-diketones 2a-e and substituted benzaldehydes
(Scheme 2) according to the procedure describe in the literature.20
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O
O
R1
OH O
O
N
H
H
O
R1
R2
2a-e
Ethanol, reflux
O
3a-i
R2
Scheme 2. Synthesis of flavanones 3a-i
The synthetized flavanones were purified by recrystallization to provide crystalline
compounds in good yield (46-95 %). Flavanones 3a-i are all known compounds and
therefore confirmed by comparison of their melting points and spectroscopic data with
those reported in the literature.20
Table 1. Product yield for compounds 3a-i
Melting
Flavanone
R1
R2
Yield (%)
point (°C)
142-144
160-162
167-168
175-176
118-120
175-176
171-172
144-146
173-175
3a
3b
3c
3d
3e
3f
3g
3h
3i
Phenyl
p-anisyl
p-tolyl
2-furyl
2-thienyl
p-tolyl
2-furyl
2-thienyl
p-anisyl
H
H
H
H
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80
95
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97
68
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79
H
2-Cl
4-F
4-F
2-Cl
The difluroboron flavanone β-diketonate complexes 4a-i were obtained by reacting
flavanones 3a-i with BF3.Et2O in an inert atmosphere of nitrogen (Scheme 3). The
methodology was optimized to obtain the best possible yields. Carrying the reaction out
initially in dichloromethane (DCM) as solvent and at room temperature for two hours
provided a final product yield of only 27 % after purification. Increasing the temperature
to 80 °C and increasing the stoichiometric excess of BF3.Et2O to 8 equivalents not only
permitted to elimination of DCM as solvent but also improved the reaction yield to 78%.
The optimized reaction conditions established for the synthesis of 4a were applied to the
synthesis of compounds 4b-i.
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DOI: 10.1039/D0NJ03525D
F
F
B
O
O
O
O
R1
BF3.Et2O
80ºC/ N2
R2
R1
O
O
R2
3a-i
4a-i
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Scheme 3. Synthesis of difluoroboron flavanones complexes 4a-i.
In total, 9 novel compounds were prepared in good yield (Table 2). Amongst the
flavanone compounds prepared, only one flavanone containing a 2-pyridyl group at R2
position (structure not shown) failed to react with BF3.Et2O and form the corresponding
difluoroboron flavanone β-diketonate complex. This may possibly be due to a
competitive complexation between the pyridine nitrogen and boron trifluoride. Only
compounds 4e, 4g and 4h bearing a 2-furyl or 2-thienyl at R1 were obtained as solids
whereas in comparison the other compounds formed yellow-greenish oils.
Table 2. Product yield for compounds 4a-i
Melting
point (°C)
Compound
R1
R2
Yield (%)
4a
4b
4c
4d
4e
4f
4g
4h
4i
Phenyl
p-anisyl
p-tolyl
2-furyl
2-thienyl
p-tolyl
2-furyl
2-thienyl
p-anisyl
H
H
H
H
-
-
-
-
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188-190
-
165-167
172-174
-
H
2-Cl
4-F
4-F
2-Cl
The infrared spectrum presented some weak bands between 3100-2900 cm-1 attributed
to stretching vibrations of saturated and unsaturated C-H bonds. More intense
absorptions bands were observed between 1600-1540 and 1500-1400 cm-1 corresponding
to C=O and C=C (enolic tautomerism) stretching vibrations. The bands observed in the
region of 1180-1170 cm-1 were attributed to the B-O stretching vibration and the bands
at 1030-1050 cm-1 were attributed to the B-F stretching vibrations.21-22 In order to describe
vibrational modes that are infrared active we performed calculations using the ORCA
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program with B3LYP functional (see Computational Details) for compound 4a and
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compared them with experimental results. Figure 1S (see supporting information) shows
the infrared spectrum of compound 4a theoretically calculated including the main
vibrational normal modes. Weak bands between 3207-3094 cm-1 are formed by fifteen
infrared active modes where fourteen are due to stretching of C-H bonds of the
benzenoid rings. The inset in Figure 1S shows the vibration at 3186 cm-1 (see movies in
the SI). There is a vibration at 3094 cm-1 that is due to C-H stretching from the γ-
dihydropyrone ring. The next main band with a maximum at 1632 cm-1 is formed by six
vibrations, where the first two are due to C-C bond stretching within the benzenoid rings
and rocking of their H atoms. The last two have, in addition, a weak contribution from
C-O stretching. The third band at 1580 cm-1 is attributed to a single vibration from C-O
stretching of the dicarbonyl fragment. The most intense band of the spectrum at 1520
cm-1 is a result of seven vibrations. These modes correspond to C-C bond stretching of
the benzenoid rings and from rocking of H atoms. The most intense peak at 1519 cm-1
also has some weak contributions from C-O bond stretching of dihydropyrone moiety.
The stretching of the B-F bonds occur at 1076 and 1070 cm-1. The proton NMR displayed
a characteristic simplet resonance at approximately 6.60 ppm which corresponded to the
methine hydrogen on the γ-dihydropyrone ring and thus strongly suggesting the
formation of a difluoroboron β-diketonate complex. Likewise, the 13C NMR showed only
one ketone signal at approximately 181 ppm which further supports this conclusion.
Furthermore, a 19F NMR analysis was carried out and the spectrum revealed non-
binomial quartets in the range of -141.4 and -143.6 ppm which is an agreement with
similar BF2dbm complexes reported in the literature.23-26
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2.2 Single Crystal X-Ray Diffraction
The crystal structure of compound 4e is represented in Figure 1. The difluoroboron
coordination with both oxygens of the 1,3-dicarbonyl moiety was confirmed, forming a
six-membered chelate ring. The compound crystalizes in a centrosymmetric P21/c space
group, existing an inverted center inside a unitary cell with an equal 50:50 ratio of the S
and R isomer. (Only the S isomer was presented in Figure 1). Bonding to carbon C14
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exist at the phenyl group that is almost perpendicular (~85°) in comparison with the rest
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of molecule and presented a planar conformation formed by the phenyl, thiophene and
chelate ring (Figure 1). The principal parameters showing the intermolecular distances
and angles are summarized in table 3.
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Figure 1. Crystallographic structure of compound 4e with no-hydrogen atoms
labeled. Ellipsoids are represented at 30% probability.
Table 3. Bond distance and angles selecting to molecule 4e.
Angle
(°)
Bond
Distance (Å)
Bond
O(2)-C(3)
O(1)-C(1)
O(2)-B(1)
O(1)-B(1)
F(2)-B(1)
1.307(3)
1.316(3)
1.476(5)
1.458(5)
1.371(5)
1.361(4)
1.380(4)
1.439(4)
1.402(4)
1.513(3)
1.518(3)
1.598(10)
1.635(12)
1.572(16)
C(3)-O(2)-B(1)
C(1)-O(1)-B(1)
F(1)-B(1)-F(2)
F(1)-B(1)-O(1)
F(2)-B(1)-O(1)
F(1)-B(1)-O(2)
F(2)-B(1)-O(2)
O(1)-B(1)-O(2)
O(2)-C(3)-C(2)
O(2)-C(3)-C(4)
C(2)-C(3)-C(4)
C(3)-C(2)-C(1)
C(3)-C(2)-C(14)
C(10)-S(1)-C(13)
120.6(3)
122.2(2)
112.1(3)
109.0(3)
108.3(4)
108.4(4)
109.4(3)
109.7(2)
122.1(3)
115.3(3)
122.6(2)
118.4(2)
119.4(2)
95.5(8)
F(1)-B(1)
C(3)-C(2)
C(3)-C(4)
C(2)-C(1)
C(2)-C(14)
C(14)-C(15)
C(10)-S(1)
C(10)-C(11)
C(12)-C(11)
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C(4)-C(9)
C(4)-C(5)
1.317(8)
1.418(4)
C(4)-C(9)-O(3)
C(4)-C(9)-C(8)
129.D0O(6I:)10.1039/D0NJ03525D
115.4(7)
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Analyzing the bond distance of the compound 4e (Figure 1), some interesting
information was obtained involving principally the 6 member planar ring formed by B1-
O1-C1-C2-C3-O2 atoms. The bond distance of C1-O1 and C3-O2 are 1.316(3) and 1.307(3)
Å, respectively, which is remarkable given that bond lengths for C-O and C=O bonds
according to Mogul analysis 1.41(3) and 1.22(2) Å, respectively were expected.
Moreover, the bond length of C1-C2 was 1.402(4) Å and C2-C3 was 1.380(4) Å, where
typically C=C bonds lengths are 1.33(3) Å. These aspects suggest electronic
delocalization density along the molecular fragment O1-C1-C2-C3-O2 (resonance
hybrid). Therefore, it is suggested that one way to represent the chelate ring is
represented in the Figure 2, which is akin to some structurally related 3-
aroylchromanones that exist exclusively in their keto-enol tautomeric form, stabilized by
intramolecular hydrogen bonding.27
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F
F
B
O
O
Figure 2. Representation of resonate chelate ring formed in the actual structure
The B-O distance bond was 1.476(5) and 1.458(5) for B1-O2 and B1-O1, respectively,
which are similar with reported values recently reported in the literature.26 The B1-F1
distance bond [1.361(4)] and B1-F2 [1.371(5)] agree with B-F bond lengths (1.36-1.37)
reported for structures containing the BF2 fragment coordinated to acetyl-ketones as O-
O donors. The boron atom with sp3 hybridization orbitals presented a tetrahedral
geometry distorted with O-B-O and O-B-F angles very close of 109.5° and the F1-B1-F2
angle (121.3(3)°) is a little bigger because repulsion between electron clouds of fluorine
atoms (F··F). The crystalline structure is stabilized by weak intermolecular interactions,
such as the C-H··F contacts with atoms distance of 2.402 Å (Figure 3). These interactions
form infinite chains along the b axis with the molecules related by the 21 screw axis. A
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better packing visualization is shown the Figure 3b where it can be observed tDhOaIt: 1e0v.10e3r9y/D0NJ03525D
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molecule forms a kind of zig zag.
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Figure 3. (a) Representation of the C-H··F intermolecular interactions observed in
compound 4e with one-dimensional chain along the b crystalline axis. (b) View along
the c axis of the crystal packing between molecules of 4e.
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2.3 Stability of Difluoroboron Flavanone β-Diketonate Complexes
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9
The β-diketonate complex 4e was reacted with methanol at 60 °C for 3 days under
refluxing conditions (Scheme 4). The mixture of products was characterized by proton
NMR which confirmed the partial conversion of 4e to the flavanone starting material 3e.
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F
F
B
O
O
O
O
O
O
CH3OH
60ºC-72 h
S
R2
R2
R2
R2=
O
O
O
3e-cis
3e-trans
4e
<20% conversion
trans:cis ratio = 80:20
Scheme 4. Reaction of 4e with methanol.
The proton NMR revealed that 4e mostly remained unreacted but 20% of the mixture
as determined from the NMR peak integrals was comprised of flavanones 3e-trans and
3e-cis indicated by the presence of 4 sets of doublets. Two smaller doublets
corresponded to cis isomer at 4.61 and 5.82 ppm with a coupling constant of 3Hz and the
other two doublets at 4.94 and 5.99 ppm with a coupling constant of 12 Hz corresponded
to the trans isomer were observed. The ratio between the diastereoisomers (80:20) was
calculated by integration of the signal of both isomers. Thus, based on this result, the
complexes would be expected to slowly lose their fluorescent properties in protic
solvents or in the presence of nucleophilic species.
2.4 Photophysical Properties
The photophysical properties of the final compounds, 4a-i are summarized in Table 4
and normalized absorption and fluorescence spectra of 4a-i in chloroform as solvent at
a concentration of 10-4 M are also shown in Figure 4.
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Table 4. Photophysical properties of the compounds 4a–i in chloroform
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8
Absorption
max (nm) /
Emission
em (nm)
Compound
ST (cm-1)
ΦPL
f (ns) c
krd (ns)-1
knre (ns)-1
b
(ε/ 104)a
9
4a
4b
4c
4d
4e
4f
4g
4h
4i
346;404/(0.5)
360;417/(0.3)
359;418/(0.8)
364;430/(0.4)
364;430/(0.7)
351;410/(0.2)
364;432/(1.8)
364;429/(0.8)
362;421/(0.3)
489
484
485
494
495
489
495
496
487
4303
3262
3191
3013
3054
3940
2946
3149
3163
0.33
0.51
0.32
0.75
0.48
0.36
0.35
0.30
0.28
4.20
3.56
3.69
4.78
4.16
3.91
4.88
4.15
3.69
0.079
0.143
0.087
0.157
0.115
0.092
0.072
0.072
0.076
0.160
0.138
0.184
0.052
0.125
0.164
0.133
0.169
0.195
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a Units = M−1cm−1, the molar absorption coefficients (ε) of the compounds were evaluated from slopes of the curves from
inset of Fig. 29S by Beer-Lambert law; b Fluorescence quantum yields; c Excited-state lifetimes; d Radiative rate constant;
e Non-radiative rate constant.
All compounds exhibited two main absorption bands between 346 nm and 432 nm
(Figure 4a-c) whereas molar absorption coefficient of maximum absorption wavelength
ranges of 0.2 × 104 M−1cm−1 (for compound 4f) to 1.8 × 104 M−1cm−1 (for compound 4g).
The two bands for 4a in CHCl3 correspond to π-π* transitions of the conjugated systems
that involve the aromatic rings.28-29 The maxima emission wavelength of most
compounds in chloroform roughly take place at 486 nm, except for compounds 4d, 4e,
4g and 4h where their maxima emission wavelength occur around 495 nm (see Table 4).
The Stokes shifts (ST) of the compounds in chloroform are in the range of 2946 to 4303
cm-1, respectively, for 4g and 4a. The compounds in chloroform solution exhibited
fluorescence quantum yields (ΦPL) between 0.28 and 0.75 (see Table 4). Although no
clear correlations between the electronic properties of the different substituents present
in compounds 4a-i with the quantum yields could be established, a few standout
compounds with promising photophysical properties were identified. Specifically,
compound 4d bearing a 2-furyl moiety achieved up to 0.75 in chloroform as solvent
making it a lead compound for further studies in the solid state and potential
applications in optoelectronic devices. Furthermore, compound 4b bearing a p-anisyl
moiety displayed a significant quantum yield of 0.51 and thus indicates that the
introduction of a methoxy group improved this property of the difluoroboron β-
diketonate complexes. The influence of methyl groups and halogens on the benzenoid
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rings can assessed by comparison of 4c and 4f with 4a. In this case, quantum yields were
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very similar which suggests that these functional groups do not increase quantum yields
of the difluoroboron β-diketonate complexes. Comparison of compounds 4g-i with their
non halogen bearing counterparts 4d and 4e suggests that the introduction of halogens
does not favor larger quantum yields. All difluoroboron flavanone complexes emitted
in the green region. The fluorescent boron-difluoride o-hydroxybenzoylmethanate was
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30
studied by Fedorenko and co-workers presented a smaller quantum yield (0.26 in
CHCl3) than the analogous cyclic flavanone equivalent 4a reported here (0.33 in CHCl3).
(a)
(b)
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
in CHCl3
in CHCl3
4d
4e
4f
4a
4b
4c
300
400
500
600
300
400
500
600
Wavelength /nm
Wavelength /nm
in CHCl3
(d)
(c)
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
4a
4b
4c
4d
4e
4f
4g
4h
4i
in CHCl3
4g
4h
4i
300
400
500
600
400
450
500
550
600
Wavelength /nm
Wavelength /nm
Figure 4. Absorption (a-c) and fluorescence (d) spectra for compounds 4a-i in
chloroform solution at a concentration of 10-4 M
All compounds in chloroform solution at a concentration of 10-4 M showed a mono-
exponential fluorescence decays with lifetimes in the nanosecond timescale (see Table
4), which is a feature of monomeric emission from difluoroboron β-diketonate dyes in
diluted solutions.3 The fluorescence decay curves of the compounds in chloroform are
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shown in the supporting information (See Figure. 32S). The singlet excited-state lifetimes
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(f) of compounds 4a-i varied from 4.88 ns (4g) to 3.56 ns (4b) in chloroform. Moreover,
compound 4d showed the highest radiative rate constants and the lowest non-radiative
rate constants, which explains the higher fluorescence efficiency observed for 4d (0.75)
in chloroform. Compounds 4g and 4h containing 4-F group at R2 position displayed the
lowest radiative rate constants, whilst compound 4i exhibited the highest non-radiative
rate constants. The photophysical properties of the final compounds, 4a-i, in acetonitrile
solution are summarized in Table 5. Normalized absorption and fluorescence spectra of
4a-i in acetonitrile solution at concentration of 10-4 M are showed in Figure 5.
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Table 5. Photophysical properties of the compounds 4a–i in acetonitrile
Absorption
Emission
em (nm)
b
Compound
max (nm) /
ST (cm-1)
ΦPL
f (ns) c
krd (ns)-1
knre (ns)-1
(ε/ 104)a
4a
4b
4c
4d
4e
4f
4g
4h
4i
344;404/(0.4)
360;417/(0.3)
358;413/(0.3)
362;433/(0.3)
362;430/(0.8)
352;410/(0.2)
361;432/(1.8)
362;428/(0.6)
359;418/(0.4)
502
496
497
507
513
499
495
512
496
4832
3820
3689
3371
3763
4350
2946
3833
3762
0.20
0.46
0.22
0.34
0.36
0.19
0.35
0.32
0.48
6.56
4.47
4.54
5.78
5.35
6.26
4.88
5.32
4.44
0.030
0.103
0.048
0.059
0.067
0.030
0.072
0.060
0.108
0.122
0.121
0.172
0.114
0.120
0.129
0.133
0.128
0.117
a Units = M−1cm−1, the molar absorption coefficients (ε) of the compounds were evaluated from slopes of the curves
from inset of Fig. 30S by Beer-Lambert law; b Fluorescence quantum yields; c Excited-state lifetimes; d Radiative rate
constant; e Non-radiative rate constant.
Compounds 4a-i also exhibited two main absorption bands between 344 nm and 433
nm in acetonitrile solution (Figure 4a-c) corresponding to π-π* transitions of the
conjugated systems that involve the aromatic rings29 whereas molar absorption
coefficient of maximum absorption wavelength ranges were 0.2 × 104 M−1cm−1 (for
compound 4f) to 1.8 × 104 M−1cm−1 (for compound 4g). The maxima emission wavelength
in acetonitrile occurs between 495 nm (for the compounds 4b) and 513 nm (compound
4e) as showed in Table 5. The Stokes shifts (ST) of the compounds in acetonitrile are
in the range of 2946 to 4832 cm-1, respectively, for 4g and 4a. The fluorescence quantum
yields (ΦPL) of compounds 4a-i in acetonitrile are shown in Table 5. The compounds in
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acetonitrile solution exhibited fluorescence quantum yields (ΦPL) between 0.19DaOnI:d100.10.4398/.D0NJ03525D
The fluorescence quantum yields of compounds 4a-i in acetonitrile were smaller
compared to quantum yields of the compounds in chloroform solution. Compound 4i
containing p-anisyl group at R1 position and 2-Cl at R2 position exhibited the highest
fluorescence efficiency (0.48). It is worth noting that the fluorescence efficiency of
compound 4d in acetonitrile solution (0.34) was significantly lower than in chloroform
(0.75).
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(a)
1.0
1.0
0.8
0.6
0.4
0.2
0.0
(b)
0.8
0.6
0.4
0.2
0.0
in CH3CN
in CH3CN
4a
4b
4c
4d
4e
4f
300
400
500
600
300
400
500
600
Wavelength /nm
Wavelength /nm
in CH3CN
(d)
(c)
1.0
0.8
0.6
0.4
0.2
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1.0
0.8
0.6
0.4
0.2
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4b
4c
4d
4e
4f
4g
4h
4i
in CH3CN
4g
4h
4i
300
400
500
600
400
450
500
550
600
Wavelength /nm
Wavelength /nm
Figure 5. Absorption (a-c) and fluorescence (d) spectra for compounds 4a-i in
acetonitrile solution at a concentration of 10-4 M
The fluorescence decays of all compounds in acetonitrile solution at concentration of
10-4 M were also best fitted using a mono-exponential with lifetimes in the nanosecond
timescale (see Table 5). The fluorescence decay curves of the compounds in acetonitrile
are shown in the supporting information (See Figure 33S). The excited-state lifetimes (f)
of compounds 4a-i in acetonitrile were slightly longer compared to lifetimes of the
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compounds in chloroform solution. The lifetimes of the compounds 4a-i in acetonitrile
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range from 6.56 ns (4a) to 4.44 ns (4i), as shown in Table 5. The highest radiative rate
constant was exhibited by compound 4i, which also showed the higher fluorescence
efficiency (0.48) in acetonitrile. In addition, the non-radiative rate constant of the
compound 4d in acetonitrile solution was higher than the radiative rate constant, which
explains the decreased of fluorescence efficiency of the compound 4d in acetonitrile
compared to its efficiency in chloroform. Compounds 4a and 4f showed the lowest
radiative rate constants, whilst compound 4c exhibited the highest non-radiative rate
constant. The photophysical properties of compounds, 4a-i, in tetrahydrofuran (THF)
solution are summarized in Table 6. Figure 6 shows the normalized absorption and
fluorescence spectra of 4a-i in THF solution at concentration of 10-4 M.
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40
41
42
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46
47
48
49
50
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52
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58
59
60
Table 6. Photophysical properties of the compounds 4a–i in tetrahydrofuran
Absorption
Emission
em (nm)
b
Compound
max (nm) /
ST (cm-1)
ΦPL
f (ns) c
krd (ns)-1
knre (ns)-1
(ε/ 104)a
4a
4b
4c
4d
4e
4f
4g
4h
4i
323;348;/(0.3)
358;415/(0.3)
358;414/(0.8)
362/(0.1)
366;387/(0.5)
328;352/(0.1)
366;398/(1.1)
364;402/(0.5)
361;414/(0.2)
488
482
483
490
495
487
491
496
483
6608
3408
3335
2740
2786
6204
2835
3149
3106
0.21
0.56
0.36
0.40
0.08
0.27
0.10
0.11
0.25
4.58
3.91
3.96
5.25
4.27
4.46
4.88
4.28
3.98
0.046
0.143
0.091
0.076
0.019
0.061
0.020
0.026
0.063
0.172
0.113
0.162
0.114
0.215
0.141
0.184
0.208
0.188
a Units = M−1cm−1, the molar absorption coefficients (ε) of the compounds were evaluated from slopes of the curves
from inset of Fig. 31S by Beer-Lambert law; b Fluorescence quantum yields; c Excited-state lifetimes; d Radiative rate
constant; e Non-radiative rate constant.
In THF, the absorption spectra of the compounds 4a-i exhibited peaks and shoulders
ranging of 323 nm to 415 nm (Figure 6a-c). The main absorption bands of the compounds
are given in Table 6. THF bears a sp3 hybridized oxygen atom with lone pairs of electrons
and as a result is capable of different interactions with compounds 4a-i that cause the
observed changes in the spectra. Based on theoretical and experimental demonstrations,
Gelfand et al suggested that the deformation of the spectral shape observed in THF is
due to numerous vibrations outside the plane and explicit solvation which results in
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several low-frequency intermolecular vibrations and vibrations from the solvent
molecule itself.21 The molar absorption coefficients of main absorption wavelength are
between 0.1 × 104 M−1cm−1 (for compound 4d and 4f) to 1.1 × 104 M−1cm−1 (for compound
4g). The maxima emission wavelength in THF occurs between 482 nm and 496 nm
(compound 4h) as shown in Table 5. The Stokes shifts (ST) of the compounds are in
the range of 2740 to 6608 cm-1, respectively, for 4d and 4a. The fluorescence quantum
yields (ΦPL) of compounds 4a-i in THF were between 0.08 and 0.56 (Table 6) and
compound 4b exhibited the highest fluorescence efficiency (0.56) in THF. Compound 4d
in THF exhibited fluorescence efficiency (0.40) that was close to its efficiency recorded
in acetonitrile (0.34).
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(a)
(b)
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
in THF
in THF
4a
4b
4c
4d
4e
4f
300
400
500
600
300
400
500
600
Wavelength /nm
Wavelength /nm
in THF
4a
(d)
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
(c)
4b
4c
4d
4e
4f
4g
4h
4i
in THF
4g
4h
4i
300
400
500
600
400
450
500
550
600
Wavelength /nm
Wavelength /nm
Figure 6. Absorption (a-c) and fluorescence (d) spectra for compounds 4a-i in
tetrahydrofuran solution at a concentration of 10-4 M
The fluorescence decays of all compounds in THF solution at concentration of 10-4 M
were also best fitted using a mono-exponential with lifetimes in the nanosecond
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timescale (see Table 5). The fluorescence decay curves of the compounds inDTOIH: 10F.10a3r9e/D0NJ03525D
shown in the supporting information (See Figure 34S). The excited-state lifetimes (f) of
the compounds 4a-i in THF were similar to those of the compounds in chloroform
solution. The lifetimes of the compounds 4a-i, showed in Table 6, are between 5.25 ns
(4d) and 3.91 ns (4b). Compound 4b showed the highest radiative rate constants and the
lowest non-radiative rate constants, explaining the higher fluorescence efficiency
observed for 4b (0.56) in THF. In contrast, compound 4e exhibited the lowest radiative
rate constant and the highest non-radiative rate constant. This explains the low
fluorescence quantum yields of compound 4e (0.08) in THF. Similarly as observed in
acetonitrile, compound 4d in THF exhibited a non-radiative rate constant higher than
the radiative rate constant leading to a decrease in fluorescence efficiency of compound
4d when compared to its efficiency in chloroform.
4
5
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2.5 Solvatochromic Properties
In order to study in detail the solvatochromic effect, the absorption and fluorescence
spectra of 4a were taken in solutions of chloroform, acetonitrile and tetrahydrofuran.
The compounds were found to be insoluble in nonpolar solvents such as hexane,
cyclohexane and heptane. In addition, alcohols such as methanol or ethanol caused an
immediate loss in the fluorescent properties of compounds 4a-i. The absorption and
emission spectra of 4a in those solvents are shown in Figure 7. The photophysical
properties of compound 4a in solution of several solvents at 10-4 M are summarized in
Table 7.
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
CHCl3
CH3CN
THF
CHCl3
CH3CN
THF
400
450
500
550
600
300
400
500
600
Wavelength / nm
Wavelength / nm
Figure 7. Normalized absorption (a) and fluorescence (b) spectra of compound 4a in
CHCl3, CH3CN and THF
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Concerning the polarities of each solvent, CHCl3 and THF both have similar
polarity whereas CH3CN is a more polar solvent. The absorption spectra of compound
4a in CHCl3 and CH3CN were very similar. However, the shape of absorption spectrum
of 4a in THF was totally modified with a remarkable hypsochromic shift. Moreover, the
maximum emission wavelength of 4a in CH3CN is shifted to longer wavelengths from
488-489 nm (THF and CHCl3) to 502 nm (CH3CN). This implies that the excited state of
4a is more polar than the ground state. Our TD-DFT/B3LYP calculations (see
Computational details) show a change in the dipole moment of the ground state from
μGS = 7.28 D to μES = 10.60 D for the first excited state. The fluorescence quantum yields
of 4a in CH3CN and THF (Table 7) were very similar whereas the quantum yield of 4a
in CHCl3 was slightly higher.
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35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Table 7. Photophysical properties of the compounds 4a in different solvents
Absorption
e
Emission
em (nm)
ST
(cm-1)
krd
knr
b
Solvent
max (nm) /
ΦPL
f (ns) c
(ns)-1
(ns)-1
(ε/ 104)a
CHCl3
CH3CN
THF
346;404/(0.5)
344;404/(0.4)
323;348;/(0.3)
489
502
488
4303
4832
6608
0.33
0.20
0.21
4.20
6.56
4.58
0.079
0.030
0.046
0.160
0.122
0.172
a Units = M−1cm−1; b Fluorescence quantum yields; c Excited-state lifetimes; d Radiative rate constant; e Non-
radiative rate constant.
The excited state lifetimes of 4a in different solvents are shown in Table 7. In polar
solvent such as CH3CN compound 4a showed a longer excited state lifetime compared
to it excited state lifetimes in CHCl3 and THF. The fluorescence decay curves of the
compound 4a in acetonitrile and tetrahydrofuran solution is shown in Figure 8.
Compound 4a exhibited the highest radiative rate constants in CHCl3 and the lowest
radiative rate constants in CH3CN (Table 7). The highest non-radiative rate constant of
4a was observed in THF as solvent.
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8
103
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100
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52
53
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58
59
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CHCl3
CH3CN
THF
0
10
20
30
Lifetime / ns
Figure 8. The fluorescence decay curves of the compound 4a in acetonitrile,
chloroform and tetrahydrofuran solution
2.6 Electrochemical Characterization
The electrochemical properties of the final compounds were investigated by cyclic
voltammetry (CV). The data from the electrochemical study and optical properties of 4a-
i are summarized in Table 8. The HOMO was estimated according to the reduction onset
퐸푟푒푑
퐸푟푒푑
(
표푛푠푒푡) potentials from voltammograms using the following equation: HOMO= −(
표푛푠푒푡
퐸표푝푡
+ 4.44) eV. The LUMO was estimated from optical band gaps ( 푔푎푝) previously
determined from the absorption edge of compounds in solution, according to equation:
퐸표푝푡
LUMO = (HOMO - 푔푎푝) eV. The cyclic voltammograms of the compounds are provided
in the supporting information (See Figures 35S – 43S).
Table 8. Electrochemical and optical properties of compounds 4a-i
풐풑풕
푬
품풂풑
풐풏풔풆풕
푬
HOMO LUMO
onset
(nm)e
풓풆풅
(V)a
Compounds
4a
(eV)
(eV)b
(eV)c
d
3.07
404
-1.09
5.53
2.67
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2.97
2.97
2.88
2.88
3.02
2.87
2.89
2.95
417
418
430
430
410
432
429
421
4b
4c
4d
4e
4f
4g
4h
4i
-1.10
-0.95
-1.23
-1.26
-0.89
-1.28
-1.19
-0.89
5.54
5.39
5.67
5.70
5.33
5.72
5.63
5.33
2.72
2.58
2.97
2.96
2.50
3.02
2.88
2.58
10
11
12
13
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60
표푛푠푒푡 is the onset potential of reduction, HOMO = – (
a
b
c
퐸푟푒푑
퐸표푛푠푒푡
- 4.44) eV;
푟푒푑
표푝푡
표푝푡
d
(
)
LUMO = HOMO – 퐸푔푎푝 eV 퐸푔푎푝
;
is optical band gap calculated from the low
퐸표푝푡
e
energy edge of the absorption spectrum (
absorption onsets wavelength.
= 1240 / onset); onset is the
푔푎푝
The cyclic voltammograms of the compounds (see ESI) exhibit reduction processes with
표푛푠푒푡
퐸
varying from – 0.9 to –1.3 V vs SCE (see Table 8). Moreover, the substituent group
푟푒푑
changes the HOMO and LUMO values as well as the width of the optical band gaps. As
shown in Table 8, HOMO of the compounds 4a-i was estimated to be between 5.33 eV
and 5.72 eV whereas the LUMO varied from 2.50 eV to 3.02 eV. The HOMO and LUMO
energy levels of the present compounds are comparable to other boron difluoride
derivatives applied as dye sensitizers material in dye-sensitized solar (DSC´s).17
Comparing these results obtained with (Dibenzoylmethanato) boron difluoride
derivatives containing triphenylamine moieties, the values obtained for the LUMO and
HOMO energy levels energies were similar to values found for compounds 4a-i.17
Moreover, the optical band gaps of the compounds 4a-i are between 2.70 and 2.86 eV,
which are close to the other emitting materials applied in the construction of the organic
light-emitting diode (OLED).16 Thus, these new compounds have potential for
applications in optoelectronic devices.
2.7 Theoretical Studies
In order to explore the theoretical photophysical properties, compounds 4a and
4e were chosen as representative examples. We performed first-principle calculations
based on DFT and TDDFT formalisms (see Computational details) to determine UV-VIS
spectra, TD-DFT difference densities and ground state properties. In order to find out
which approximation to the exchange-correlation functional best describes the
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experimental results, we studied different properties with six different functionals:
5
6
7
8
B3LYP, PBE0, CAM-B3LYP, BHLYP, M06-2X, and PBE. The first set of results were
analyzed in detail for the B3LYP functional. The ground state geometry optimization of
molecules 4a and 4e determined by DFT/B3LYP can be seen in Figures 9 and 10 and 41S.
9
10
11
12
13
14
15
16
17
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40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 9: Molecular Orbital plots and TDDFT difference densities and main
excitation states for full TDDFT calculations for molecule 4a. In the TDDFT difference
densities, red indicates a loss of electron upon excitation and blue indicates a gain of
electron density. The colour difference in MO of the isosurfaces indicates a difference
in sign.
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41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 10: Molecular Orbital plots and TDDFT difference densities and main
excited states for full TDDFT calculations for molecule 4e. In the TDDFT difference
densities, red indicates a loss of electron upon excitation and blue indicates a gain of
electron density. The colour difference in MO of the isosurfaces indicates a difference
in sign.
Tables 3S and 4S show the optimized parameters for bonds and angles between
the atoms in the gas phase for molecules 4a and 4e. The labeling scheme is presented in
Figure 41S for molecule 4e. Comparing these results with those obtained by X-ray
crystallography for compound 4e (see Figure 1 and Table 3) it is possible to see small
differences for some bonds and angles. For C1-O1 and C3-O2 the calculations show bond
distances only 1.3% and 1.2% shorter than the experimental values. For C1-C2 bond the
differences only appear in the third decimal place and we have a C2-C3 distance 2%
longer than experimental results. B-O distance bond were 2.2% and 3.1% longer for B1-
O2 and B1-O1, respectively. The B-F bond distances do not show any significant
differences. The largest deviations appear for atoms in the 2-thienyl group. While C10-
S1 bond is almost 9% longer, C10-C11 and C12-C11 are 15.4 and 10.5% shorter,
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respectively. The angles obtained with the DFT calculations do not show deviations
5
6
7
8
greater than 6%. In general, geometry optimization with solvation effects can slightly
change the bonding distances of some groups in the molecule. F-B (O-B) distance bonds
can be shorter or longer by up to 1.7% in relation to the gas phase using C-PCM model
with chloroform, THF and acetonitrile. This same effect occurs for molecule 4a with
differences of 1.6% being observed.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
In order to understand the experimental absorption spectra, the four lowest
excited states responsible for the main absorption bands observed were investigated.
Figures 9 and 10 present the main molecular orbitals (MO) and the TDDFT difference
densities of many excited states with the highest oscillator strength (fosc) for a full
TDDFT/B3LYP calculation. Side and top view are shown in the figures. HOMO and
LUMO plots are mainly localized at the flavanone framework and benzoyl (4a) or 2-
thienyl (4e) moieties.
Tables 5S and 6S provide the main transitions for each excited state and the
degree of each individual MO contribution. Excited state 1 represents 95% of the
HOMO/LUMO transition for 4a and 93% for 4e. An excitation energy of 3.210 eV for
molecule 4a and 3.066 eV for compound 4e was also calculated, and a small contribution
from the HOMO-1/LUMO transition was noted. This transition is mostly responsible for
the excited state 2 in both molecules contributing 94% for molecule 4a and 87% for 4e.
Excitation occurs for energies at 3.693 eV and 3.524 eV for molecules 4a and 4e,
respectively. These two states have the same main set of excitations with different
contributions and similar TDDFT difference densities. Therefore, Figures 9 and 10 show
difference densities only for excited state 1 for both molecules. The excited state 3 has an
excitation energy of 3.900eV (3.671 eV) contributing 98% (87%) for HOMO-2/LUMO
transition for molecule 4a (4e) and a small HOMO-3/LUMO transition contribution. By
contrast, state 4 has a greater contribution from HOMO-3/LUMO with 96% for both
molecules and small contribution in the HOMO-2/LUMO excitation, with energy of
3.914 and 3.714 eV for molecules 4a and 4e, respectively. For states 5 and 6, the transitions
follow the same logic as shown in Tables 5S and 6S.
The absorption spectra for molecules 4a and 4e in gas phase are presented in
Figure 11 (a) and (b), respectively.
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43
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45
46
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48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 11. Theoretical absorption spectra for molecules 4a and 4e. (a) and (b) show the
spectra for gas phase for 30 excited states. Red stick shows the value of oscillator
strength (fosc). On the right (c) and (d) the spectra for different types of solvents was
presented.
The 30 excited states obtained from a full TDDFT/B3LYP/def2-TZVP are shown
and the resulting spectra display four peaks, where the first two are formed by the first
four excited states. These excited states might be responsible for the experimental
absorption bands between 346 nm and 432 nm for the compounds 4a and 4e. The first
peak consists of only the state 1, while the second consists of states 2 to 4. It is important
to note that, for the first two peaks, only 3 states have relevant values for the oscillator
strength. Results in gas phase and with inclusion of solvent effects for the main excited
states are summarized in Table 7S. For molecule 4a, state 1 has a fosc = 0.17 with 휆max in
386 nm (see Figure 11 and Supporting Information) which results in an intense peak.
Using C-PCM model to consider the solvent effects a bathochromic shift of 46 nm for all
solvents used was observed: acetonitrile, chloroform and THF. The peak shifts to max
=
433 nm and the oscillator strength decreases to approximately 0.14 (see Figure 11c). A
similar effect is observed for the molecule 4e with a bathochromic shift of approximately
47 nm for acetonitrile and THF and 38 nm for chloroform (see Figure 11d). Solvents
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hardly changed the oscillator strength in this case. If we directly compare the
5
6
7
8
experimental and theoretical spectra, it is possible to see differences between 휆max values.
The deviations come from the well-known shift between vertical excitations and band
maximum and for the non-description of vibrational effects.31-34 The difference in
electron densities for the two molecules for state 1 are similar (Figure 10 and 11) showing
that the electron density decreases in part of the dihydropyrone moiety and benzenoid
rings while it increases in the R1 group and the dicarbonyl fragment (see Figure 1).
In gas phase the second peak is formed mainly by states 2 ( fosc = 0.27) and 4 ( fosc
= 0.12) for molecule 4a and by states 2 ( fosc = 0.29) and 3 ( fosc = 0.11) for molecule 4e. State
2 has the highest oscillator strength of the spectra with wavelength of 336 and 352 nm
for molecules 4a and 4e, respectively. The inclusion of solvents results in a bathochromic
shift between 44-52 nm for 4a and 27-40 nm for 4e (see Table 7S) with a oscillator strength
decrease of 36% for molecule 4a and 20% for 4e. States 3 and 4 for molecules 4a and 4e,
respectively, have the largest shift, but a small value of oscillator strength. State 4 of 4a
presents a excitation in wavelength of 316.8 nm. All solvents used presented,
approximately, a 30 nm bathochromic shift and the oscillator strength increases between
90 and 115%. Because of the bathochromic shifts together with the decrease and increase
of fosc for states 2 and 4, respectively, the spectra shows a third peak around 346 nm. For
molecule 4e the oscillator strength of state 3 increased between 90 and 98% with also a
shift around 30 nm with solvation effects. In this case, the solvation effects only changes
the shape of the spectra for THF and acetonitrile with the appearance of a small shoulder
around 388 nm.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figures 44S and 45S compare the absorption spectra calculated for different
functionals for compounds 4a and 4e in the gas phase or in acetonitrile. We used 10
excited states in these cases. Unlike B3LYP, the other hybrid functionals presented one
well-defined excited states for each of the first two bands (states 1 and 2) for gas phase
calculations. Furthermore, except for the B3LYP, all hybrid functionals showed an
increase in intensity for the first peak in the calculations performed with the C-PCM
model. The bathochromic shift is observed for all functionals. It is important to note that
each excited state has a different shift. For the other hybrid functionals we observed a
bigger shift for excited states 2 and a smaller shift for excited state 1. Consequently, the
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first two excited states stay together, increasing the intensity of the band. For thDeOsIe: 1c0a.1s0e39s/,D0NJ03525D
no significant differences in the spectra were observed for the three solvents used. The
smallest deviation for max is obtained for PBE0 and B3LYP functionals for both
molecules. Using the C-PCM model with acetonitrile, the absolute deviation using PBE0
for compound 4a is only 3 nm for the two absorption bands, whilst B3LYP presents a
deviation of 30 and 41 nm for the first and second bands, respectively. For compound
4e, PBE0 (B3LYP) underestimates (overestimates) the first band 휆max by approximately
22 nm.
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
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30
31
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41
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45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
The HOMO and LUMO energies as well as the molecular energy gap were also
calculated. We estimated the energy gap using the excitation energy of the first singlet
33-35
excited state calculated by TDDFT
and the difference between HOMO and LUMO
energies were calculated by DFT. These results are presented in Table 9. First, the HOMO
energies, obtained from B3LYP/DFT calculation, showed an absolute deviation of 1.13
eV compared to the experimental results which is consistent with other reports for the
exchange-correlation functional used.32-35 For CAM-B3LYP a maximal absolute deviation
of 2.63 eV was observed. For HOMO energies, the best agreement with experimental
data was obtained using the PBE functional. The differences are close to 7% for molecule
4a and less than 3% for the molecule 4e. It is noteworthy that there is a greater difficulty
in theoretically describing virtual orbitals with DFT as pointed out in other reports.34-36
B3LYP functional LUMO energies show differences of 7% in relation to the experimental
data for 4e. In the case of 4a, B3LYP overestimates the LUMO by almost 18%. PBE
functional overestimates the LUMO energies with maximal absolute deviation of 1.10
eV. The best estimate for LUMO energy for compound 4a was obtained with M06-2X
functional, with maximal absolute deviation of 0.21 eV. On the other hand, for
compound 4e, PBE0 shows the best estimate for LUMO energy with a maximal absolute
deviation of 0.10 eV.
The calculated energy gap using PBE/DFT shows an underestimation of about
0.7 eV for the both 4a and 4e. DFT with hybrid functionals overestimates the energy gap.
B3LYP/DFT overestimates the gap by approximately 0.70 eV and the maximal deviation
reaches 3.27 eV using CAM-B3LYP. Among the hybrid functionals, B3LYP has the
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smallest deviation. In addition to the intrinsic problems of DFT calculationDsOfI:o1r0.1g03a9p/D0NJ03525D
energy,32-35 the lack of solvent model in this calculation brings an additional difference
between theoretical and experimental values. We reduced this difference using TD-DFT
calculations with the C-PCM model. The lowest excitation energy is a good estimation
to the HOMO-LUMO gap as can be seen in Table 9. The gas phase calculation shows
differences of less than 7% in relation to the experimental data. An overestimation of 0.14
and 0.19 eV for molecules 4a and 4e, respectively was observed. The inclusion of solvent
effects decreases the excitation energy between 0.27 and 0.35 eV due to the bathochromic
shift. These results underestimate the energy gap between 3 and 7% in relation to the
experimental data. The use of C-PCM model gives good agreement between the
experimental and theoretical energy gap. TD-DFT with PBE0 shows better estimation
for energy gap for compound 4a when calculated with the C-PCM model. In contrast,
for compound 4e, B3LYP and PBE0 show similar performance when compared with
experimental results. With solvent effects B3LYP always underestimates the energies'
gap with maximal deviation of 0.21 eV and 0.08 eV for compound 4a and 4e, respectively.
PBE0, on the other hand, overestimates with maximal deviation of 0.03 eV and 0.13 eV
for compound 4a and 4e, respectively. Hybrid functionals containing more exact
exchange (BHLYP and M06-2X) and the range-separated hybrid CAM-B3LYP always
overestimate the energies' gap. Calculations with C-PCM model, for the three solvents
used, show similar deviation. For example, M06-2X shows deviations of 0.37 eV and
0.51 eV for compound 4a and 4e, respectively. This analyzes for hybrid functionals are
in agreement with previous work on singlet-excited states of organic molecules.33 Our
calculations show that most accurate estimates are obtained for hybrid functionals with
less exact exchange like PBE0 and B3LYP.
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
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Table 9. Orbital Energies and energy gap (in eV) using different methodologies for
molecules 4a and 4e. ΔEgap (TDDFT) was estimated using the lowest excitation energy
from the TDDFT calculations. For ΔEgap (DFT) we use the difference between HOMO
and LUMO energies from DFT calculations.
ΔEgap (TDDFT)
ΔEgap
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Functional HOMO LUMO
(DFT)
Gas Phase CH3C
N
CHCl THF
3
Compound 4a
B3LYP
PBE0
-6.65
-6.96
-8.16
-7.82
-8.13
-5.94
-2.89
-2.81
-1.82
-1.85
-2.25
-3.56
3.77
4.15
6.34
5.97
5.88
3.21
3.35
3.69
3.85
3.62
2.86
3.09
3.53
3.70
3.44
2.87
3.10
3.55
3.71
3.44
2.87
3.10
3.54
3.71
3.44
CAM-B3LYP
BHLYP
M06-2X
PBE
2.38
Compound 4e
B3LYP
PBE0
-6.57
-6.86
-3.00
-2.93
3.57
3.94
3.07
3.21
2.75
3.01
2.80
3.00
2.75
3.01
CAM-B3LYP
BHLYP
M06-2X
PBE
-8.05
-7.70
-8.07
-5.86
-1.97
-2.01
-2.22
-3.63
6.08
5.69
5.85
2.24
3.54
3.67
3.56
3.42
3.50
3.39
3.43
3.50
3.40
3.42
3.50
3.40
3 Conclusion
In conclusion, we describe the synthesis of nine novel difluoroboro flavanone complexes
by a simple and efficient synthetic route that allows for easy structural variation and
inclusion of electron donating and withdrawing groups. The obtained compounds were
confirmed and characterized by infrared spectroscopic, 1H, 19F and 13C NMR
spectroscopy and X-ray diffraction. The photophysical properties of the final
compounds were studied in different solvents. The substituents groups and the solvent
29