Ammonium Arylspiroborate Compounds: Synthesis, Crystal Structure, Fluorescence Properties, and Antibacterial Activity

Article abstract of DOI:10.1021/acs.organomet.7b00435

A series of R-substituted ammonium arylspiroborate compounds (R = CH₃, H, F, Cl, Br) have been synthesized via a facile one-pot method. The structures of these new kinds of boron compounds were characterized by IR, NMR spectra, elemental analys

Full text of DOI:10.1021/acs.organomet.7b00435

Article
pubs.acs.org/Organometallics
Ammonium Arylspiroborate Compounds: Synthesis, Crystal
Structure, Fluorescence Properties, and Antibacterial Activity
Qiaoyun Wang and Hong Zhou*
School of Chemistry and Environmental Engineering, Wuhan Institute of Technology, 693 Xiongchu Avenue, Wuhan 430073,
People’s Republic of China
S
* Supporting Information
ABSTRACT: A series of R-substituted ammonium arylspiroborate
compounds (R = CH3, H, F, Cl, Br) have been synthesized via a facile
one-pot method. The structures of these new kinds of boron
compounds were characterized by IR, NMR spectra, elemental analysis
and X-ray diffraction techniques, and their possible formation
mechanism was proposed. Surprisingly, the as-synthesized boron
compounds showed multifunctional character with not only excellent
fluorescence properties (high quantum yields and large Stokes shift) in
both the liquid and solid state but also high antibacterial activity against
both Gram-negative (E. coli) and Gram-positive (S. aureus) bacteria.
The chlorine-substituted boron compound even possessed antibacterial
activity comparable with that of commercial antibiotics. More
importantly, the fluorescence properties and antibacterial activity of
these boron compounds could be easily mediated by the variation of the substituents, which was proved to regulate the
conjugated system, modify the hydrophobic character, and adjust the electric charge on the surface of boron compounds. In
addition, the influence of different factors including solvent (i.e., polarity, concentration), temperature, and pH on the
fluorescence properties of boron compounds was discussed in detail. This work provides a new strategy for the design of
multifunctional organoboron compounds and the mediation of their properties.
with high quantum yields and large Stokes shifts not only in the
liquid form but also in the solid state.27−30 However, the types
of luminescent organoboron compounds with high efficiency
are still limited and the discussion about different influencing
factors on their fluorescence properties is not systematic at
present, which hinders their development and practical
application in high-tech industry.
On the other hand, boron has been acknowledged as a trace
element with the characteristics of low mammalian toxicity,
high activity, nonflammability, and noncorrosiveness, which
contribute to the significant importance of boron compounds
in a series of biochemical applications such as protecting
groups,31,32 enzyme inhibitors, biosensors,33,34 neutron captur-
ing agents for cancer therapy,35 bioconjugates, and protein
labels. Among various natural and synthetic boron compounds,
some have been proved to exhibit antibacterial, antifungal,
antimalarial, and antiviral activities.36,37 For example, it has
recently been reported that an organo-soluble borate ester
compound with tetrahedral ammonium as the countercation
showed obvious biological activity against wood decay
fungi.38−40 In addition, one boron compound (sodium
boranocarbonate) has been reported as the first water-soluble
and non-transition-metal-containing CO-releasing molecule
1. INTRODUCTION
Luminescent complexes have attracted significant attention
owing to their potential applications in organic light-emitting
diodes (OLEDs), solar cells, laser dyes, photosensitizers,
biomolecular labels, and molecular probes.1−11 Most fluores-
cent materials are highly fluorescent in dilute solution but
weakly luminescent or even nonemissive in the solid state, a
phenomenon known as aggregation-caused quenching
(ACQ).12−15 One factor is that these materials pack tightly in
the amorphous solid phase or crystalline state, and another
main factor is the energy loss via rotational movements, which
results in significant self-quenching of the emission and restricts
their potential application. As far as we know, fluorescent
organic solids are highly sought for applications in many
advanced technologies, including chemical sensing, bioimaging,
information display, and organic photonics.16−20 Therefore, it is
meaningful to develop new kinds of fluorescent materials which
can emit strong fluorescence with high quantum efficiency not
only in the liquid form but also in the solid state.
Organoboron compounds are one of the most important
types of fluorescent materials, which have attracted much
attention due to their excellent optical properties with sharp
absorption bands, high fluorescence quantum yields, high molar
extinction coefficients, and chemical stability.21−26 In recent
years, boron diketonate derivatives have emerged as excellent
fluorescent materials for their promising luminescent properties
Received: June 13, 2017
© XXXX American Chemical Society
A
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then cooled in an ice−water bath. 1-Methylbenzotriazole (MeBt) was
obtained after the precipitate was filtered, washed with 1.00 mol/L
sodium hydroxide (3 × 10.00 mL), and dried at room temperature
(29.402 g, yield 65%) Mp: 61−62 °C (lit.42 mp 65 °C). Anal. Calcd
for C7H7N3: H, 5.51; C, 63.08; N, 31.38. Found: H, 5.30; C, 63.14; N,
31.56. H NMR (d6-DMSO): δ 4.30 (s, 3H), 7.37−7.41 (m, 1H),
7.52−7.56 (m, 1H), 7.81−7.84 (m, 1H), 8.01−8.03 (m, 1H).
(CO-RM); its biological properties has also been thoroughly
investigated.41 All these facts indicate that boron compounds
can be considered as potential agents for antibacterial,
antimalarial, and antiviral applications.
Due to the attraction of the many advantages of boron
compounds, a series of R-substituted ammonium arylspirobo-
rate compounds (R = CH3, H, F, Cl, Br) have been designed
and synthesized via a facile one-pot method. The structures of
these new kinds of boron compounds were characterized by IR,
NMR spectra, elemental analysis, and X-ray diffraction
techniques, and their possible formation mechanism was
proposed. Surprisingly, the as-synthesized boron compounds
showed multifunctional character with not only excellent
fluorescence properties (high quantum yields and large Stokes
shift) in both the liquid and solid state but also high
antibacterial activity against both Gram-negative (E. coli) and
Gram-positive (S. aureus) bacteria. The chlorine-substituted
boron compound even possessed antibacterial activity com-
parable to that of commercial antibiotics. More importantly, the
fluorescence properties and antibacterial activity of these boron
compounds could be easily mediated by the variation of the
substituents, which was proved to regulate the conjugated
system, modify the hydrophobic character, and adjust the
electric charge on the surface of boron compounds. In addition,
the influence of different factors including solvent (i.e., polarity,
concentration), temperature, and pH on the fluorescence
properties of boron compounds was examined systematically.
The methyl-substituted boron compound exhibited the best
fluorescence properties in all cases. This work provides a new
strategy for the design of multifunctional organoboron
compounds and the mediation of their properties.
1
To synthesize MeBtb, the obtained MeBt (1.331 g, 10.00 mmol)
was dissolved in dried THF (10.00 mL) in a 50.00 mL round-bottom
flask. Then BH3·THF (11.00 mL) was added dropwise by a syringe
with continuous stirring in an ice−water bath for 2 h. After the
completion of the reaction, MeBtb (1.233 g, yield: 84%) was obtained
by filtration as a white solid. Mp:187−188 °C (lit.43 mp 187.2−188.9
°C). Anal. Calcd for C7H10BN3: H, 6.66; C, 57.39; N, 28.46. Found:
1
H, 6.88; C, 57.20; N, 28.59. H NMR (300 MHz, CDCl3): δ 8.17−
8.15 (m, 1H), 7.68−7.64 (m, 2H), 7.63−7.58 (m, 1H), 4.37 (s, 3H),
2.73−2.54 (br m, 3H).
2.2.2. Synthesis of Boron Compounds. Compound 1 was prepared
as follows (Scheme 2): 5-methylsalicylaldehyde (1.365 g, 10.00 mmol)
Scheme 2. Synthesis of Boron Compounds
2. EXPERIMENTAL SECTION
2.1. Materials. The various aldehydes, dimethyl sulfate, benzo-
triazole, BH3·THF, and ciprofloxacin were purchased from Aladdin
(Shanghai, China). 1,2-Dichloroethane (DCE), sodium hydroxide,
hydrochloric acid, diethylenetriamine (DETA), cyclohexane (CYH),
toluene (TL), tetrahydrofuran (THF), acetonitrile (AN), and methyl
alcohol (MeOH) were obtained from Sinopharm Chemical Reagent
Co. (China). The strains of S. aureus, B. subtilis, E. coli and P.
aeruginosa were purchased from Southern Biological. All reagents were
analytical grade and were used as received.
2.2. Synthesis. 2.2.1. Synthesis of 1-Methylbenzotriazole−
Borane (MeBtb). 1-Methylbenzotriazole was synthesized according
to a previously reported method with some modifications27 (Scheme
1). First, benzotriazole (40.810 g, 342.50 mmol) was dissolved in
sodium hydroxide solution (14.00 mol/L). Then dimethyl sulfate
(48.020 g, 380.70 mmol) was dropped into the above solution with
vigorous stirring for 2.5 h at room temperature. After that, the mixture
was washed with 2 N hydrochloric acid until the pH reached 3 and
was dissolved in 20.00 mL of DCE. Then N-1-methyl-3-boranebenzo-
triazole (1.023 g, 7.00 mmol) was added to this mixture with stirring
for 30 min to form a clear solution. After that, diethylenetriamine
(0.515 g, 5.00 mmol) was dropped into this solution, followed by
stirring for 6 h at room temperature. The mixture was then filtered and
wished with DCE (3 × 10.00 mL). The final products were obtained
by crystallization in ethanol to form flaky crystals (2.094 g, yield 97%).
1H NMR (400 MHz, CD-DMSO): δ 6.74−6.72 (m, 1H), 6.61(d, 1H),
6.37 (d, 1H), 4.54 (s, 2H), 2.74 (t, 1H), 2.58 (t, 1H), 2.15 (s, 3H). 13
C
NMR (100 MHz, CD-DMSO): δ 155.3, 127.6, 126.3, 125.2, 124.4,
116.7, 60.7, 20.8. Anal. Calcd for 1, C36H45B2N3O8: H, 6.56; C, 64.61;
N, 6.34. Found: H, 6.78; C, 64.59; N, 6.28. Mp: 223−224 °C. IR
(cm−1, KBr disk): 2920 m, 2833 w, 1619 w, 1499 s, 1456 m, 1359 w,
1275 s, 1232 w, 1128 s, 1091 m, 1039 m, 941 m, 876 m, 820 w, 764 s.
The other four compounds (2−5) were synthesized in the same
way as for compound 1 except that 5-methylsalicylaldehyde was
replaced by salicylaldehyde, 5-fluorosalicylaldehyde, 5-chlorosalicylal-
dehyde, and 5-bromosalicylaldehyde, respectively. The other synthetic
steps were totally identical. The yields of compounds 2−5 were 99%,
93%, 95%, and 95%, respectively, and the melting points were 221−
222, 234−235, 232−233, and 229−230 °C for compounds 2−5,
respectively. The other characterization data for compounds 2−5 can
be found in the Supporting Information.
Scheme 1. Synthesis of 1-Methylbenzotriazole−Borane
(MeBtb)
Crystals of boron compounds 1−5 suitable for X-ray analysis were
obtained by slow diffusion of diethyl ether into a saturated methanol
solution of the as-prepared compounds for several weeks.
2.3. Characterization. The FT-IR spectra were measured on a
Nicolet Magna 750 FT-IR spectrometer in the range of 4000−400
cm−1 by using KBr pellets. Elemental analyses were recorded with a
PerkinElmer 240 analyzer. 1H NMR spectra were measured on Bruker
400 MHz and Varian 300 M instruments for solutions in CDCl3 and
B
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CD-DMSO. 13C NMR spectra were recorded on a Bruker 100 MHz
instrument with complete proton decoupling for solutions in CD-
DMSO. Chemical shifts are reported in ppm from tetramethylsilane
with the solvent resonance as internal standard. The absorption
spectra were recorded on a PerkinElmer Lambda 35 UV/vis
spectrophotometer at room temperature. The emission spectra were
recorded on a PerkinElmer LS55 spectrofluorimeter equipped with a
commercial low-temperature accessory and a special commercial
cuvette. The fluorescence spectra in the solid phase were measured
from the surface of the pressed powder in the special cuvette. The
spectra were corrected for the characteristics of the emission
monochromator and for the photomultiplier response and by
excitation at a wavelength of 370 nm. The fluorescence spectra in
the liquid phase were measured at room temperature. The static
contact angles (CAs) were recorded by a Dataphysics OCA 20
(Germany) contact angle measuring instrument, using 3 μL of a water
or oil droplet at ambient temperature. The fluorescence images were
acquired with an inverted fluorescence microscope (Leica DMIL)
equipped with a Nikon digital camera. A 360 nm laser was used to
excite the samples during imaging.
2.4. Crystallography. The X-ray single-crystal diffraction measure-
ments of the obtained boron compounds were performed in sequence
on a Super Nova, Dual, Cu at zero, Atlas diffractometer. Crystallo-
graphic data and structure refinement data were obtained by using the
SMART and SAINT programs. All structures were solved by direct
methods using SHELXS-2014 and refined on F2 by full-matrix least
squares (SHELXS-2014) in the WINGX environment. The non-H
atoms were treated as anisotropic. The H atoms were placed in
calculated positions and refined isotropically using the riding mode.
2.5. Antibacterial Activities of Compounds. To evaluate the
antibacterial activities of the as-prepared boron compounds, inhibition
zone, inhibition curves, colony counting and leakage of reducing sugars
were preformed against Gram-positive S. aureus and B. subtilis and
Gram-negative E. coli and P. aeruginosa. The quantity of the bacteria
and the sample used in the antibacterial test are given in Table 1.
(hydroxymethyl)-4-R-phenol (L) and one B3+ ion, where B3+
covalently connects two phenoxide and two hydroxymethyl
groups (from L), resulting in tetrahedral coordination. The B−
O bond lengths in compound 2 are in the range of 1.42−1.52
Å, which are typical for such tetrahedral borates and consistent
with the values in the Cambridge Structural Database (CSD).44
In the boron tetrahedral configuration, the angles of boron
atoms bonded with two oxide atoms from the same L (i.e.,
angles between planes C and D and planes C′ and D′) are
almost vertical (87.28 and 89.88°) but are slightly different,
leading to a slightly distorted tetrahedral configuration (Table
S3 in the Supporting Information). The protonated dieth-
ylenetriamine cation in compound 2 serves mainly as a charging
balance counterpart and is located in the middle of the two
boron structure units with B1−N1 and B2−N1 distances of
3.737(4) and 5.707(4) Å, respectively.
There are abundant hydrogen bonds in the obtained boron
compounds. Here we use compound 2 as an example. As
shown in the hydrogen bond topology diagrams of compound
2 (Figure 1b), the protonated diethylenetriamine cation is
located in the middle of the two anions, acting as a hydrogen
bond donor that connects two anionic boron coordination
structure units through seven N−H···O interactions. The N···O
distances are in the range of 2.220−3.244 Å, and the N−H···O
angles are in the range 116.6−169.4°. Two anionic boron
coordination structure units connect with each other through
C(7)−H(7B)···O(5) hydrogen bond interactions with C···O
distances of 3.438 Å. Every chain in compound 2 connects with
two adjacent chains through the two hydrogen bond
interactions N(2)−H(2A)···O(4) and N(3)−H(3C)···O(6).
Furthermore, the hydrogen bond interactions through N(2)−
H(2C)···O(8) lead to the construction of the 2D network.
The molecular structures and hydrogen bond topology
diagrams of the other four boron compounds are shown in
Figure S14a−d and Figure S14e−h in the Supporting
Information, respectively. Obviously, other than the difference
in substituents in the benzene groups, the other four boron
compounds possess molecular structures similar to that of
compound 2. In addition, their hydrogen bonds are basically
the same as those of compound 2, except for slight variations in
the lengths and angles of the bonds, which lead to slight
differences in the three-dimensional network (the hydrogen
bond data are given in Table S5 in the Supporting
Information).
Table 1. Quantity of the Bacteria and the Sample Used in the
Antibacterial Test
antibacterial activity of boron compounds
growth
inhibition
curve
colony
counting
test
inhibition
zone test
reducing
sugars
a
bacteria
sample
V (μL)
20
20
2 × 103
104
108
C
108
108
105
(CFU/
mL)
V (μL)
10
5
20
2 × 103
b
104
102
C (μg/
mL)
b
3.2. Possible Formation Mechanism. As described in
Scheme 3, the reaction is initiated from the reduction of
triazole−boranes caused by salicylic aldehyde derivatives with
different substituents (CH3, H, F, Cl, and Br), which involves
hydride ion transfer from the triazole−boranes to the carbonyl
carbon of salicylic aldehyde derivatives to form O− and BH2.
Then the unsaturated boron in BH2 binds with oxygen atoms
to become negatively charged and thus form the octet structure,
which undergoes a hydrolysis process afterward to generate a
secondary alcohol and boric acid (step 1).45−47 After that, the
presence of DETA can trigger dehydrogenation at phenolic p-
hydroxy groups of alicylic alcohol derivatives. As illustrated in
step 2, the deprotonated phenolic hydroxy groups can not only
condense with the hydroxy group of the boric acid but also
nucleophilically attack the empty orbital of boric acid, so that
boric acid can form the 1:1 monochelate first and then
eventually form 1:2 bidentate complexes with the correspond-
ing salicyl alcohol derivatives.48 Specifically, the procedure of
this reaction can be divided into the following: first, boric acid,
a
Bacterium: incubation in the LB liquid medium at 37 °C for 24 h
with an initial concentration of 108 CFU/mL (optical density 0.068−
0.075 at 660 nm). Sample with a different concentration.
b
3. RESULTS AND DISCUSSION
3.1. Structural Descriptions. The five boron compounds
obtained were characterized by IR spectra (Figure S13 in the
Supporting Information) and X-ray technology, and their
crystallographic data and details about the data collection and
refinement are summarized in Table S1 in the Supporting
Information. Figure 1a shows the molecular structure of
compound 2, which crystallized in the monoclinic phase with
space group C12 and is composed of two anionic boron
coordination structure units and one ammonium countercation
(diethylenetriamine with two primary amine protonations).
Each of the two borate anions consists of two deprotonated 2-
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Figure 1. (a) Molecular structure of compound 2 with atom labels (the other four compounds have the same atom labels with compound 2 except
for different substituents at C4, C11, C17, and C25). Hydrogen atoms have been omitted for clarity. (b) Hydrogen bond topology diagrams of
compound 2. The hydrogen atoms and solvent molecules which are not involved in hydrogen bond interactions are omitted for clarity.
which is one product in the first step, would accept an electron
pair through the nucleophilic attack of the deprotonated
phenolic hydroxy groups, followed by a condensation reaction
with the alcoholic hydroxy group of the ligand to form the 1:1
monochelate complex; then it reacts with the protonated salicyl
alcohol moieties. This condensation reaction results in the
generation of a 1:2 bis-chelate complex and two water
molecules. Because one DETA can deprotonate two hydroxy
groups, there are two anionic boron coordination compounds
and a protonated DETA countercation with a positive charge of
+2 in the final product.
than that of compound 2 even though fluorine is an electron-
withdrawing group. This is probably caused by the fact that
fluorine with a small van der Waals radius can easily form a
strong emissive fluorescence state (F2−) to enhance the
fluorescence intensity. This also explains why the φf value of
compound 3 (0.31) is larger than that of compound 2 (0.17).55
It is intriguing that all of the boron compounds exhibit
intense fluorescence in the solid state (Table 2 and Figure 2b),
while their optoelectronic properties are greatly influenced by
the nature of the substituents. Similarly to the solution behavior
(in MeOH), the emission is red-shifted when an electron-
donating (CH3) or electron-withdrawing (F, Cl, or Br) group is
incorporated in the benzene group, in comparison to the
unsubstituted fluorophore compound 2. Surprisingly, the solid-
state emission of every boron compound is located at a more
red shifted wavelength in comparison to the emission obtained
in MeOH (Table 2). According to earlier reports, the reason
for this phenomenon can possibly be ascribed to the following
aspects: (1) the process of solution equilibrium decreases the
ground state energy level of solute molecules and (2) the larger
molecular interaction in solid state in comparison to that in the
liquid state increases the π-conjugation length.4 This result also
indicates that the variation of solution system (e.g., type,
concentration) possibly has a great influence on the
fluorescence properties of as-synthesized boron compounds.56
To further verify the influence of the solution system on the
fluorescence properties of boron compounds, we measured the
absorption and emission spectra of compound 1 in different
solution systems. First, the optoelectronic properties of
compound 1 depend on the solvent, as shown in Figure 2c.
A significant shift in the emission wavelength was observed
when the solvent was changed among CYH (cyclohexane), TL
(toluene), THF (tetrahydrofuran), AN (acetonitrile), and
MeOH (methanol), and the detailed optical properties of
compound 1 in different solvents are given in Table S6 in the
Supporting Information. Specifically, the degree of red shift in
the emission spectra of compound 1 deepens when the polarity
of the solvent increases. For example, the maximum emission of
compound 1 red-shifts from 429 nm in CYH to 471 nm in
MeOH. However, the variation of solvent polarity produces a
negligible shift in the absorption spectrum in comparison to the
emission spectrum, indicating that the dipole moment of the
excited state is higher than that of the ground state, which
contributes to the larger Stokes shift.57 Furthermore, the effect
of solution concentration on the fluorescence properties of
3.3. Fluorescence Properties. The absorption and
fluorescence emission spectra of as-prepared boron compounds
1−5 were recorded in MeOH with a concentration of 1 × 10−4
mol/L, and the relevant data are included in Table 2. As shown
in Figure 2a, all of the absorption bands are mainly located in
the range of 300−400 nm, which could be derived from S0−S1
transitions.49 However, the emission peaks of the boron
compounds basically range from 380 to 580 nm, owing to
the π-electron delocalization of B3−, which are symmetric as
mirror images of the absorption spectra.50 In addition,
compounds 1−5 exhibit large Stokes shifts of 103, 95, 86, 83,
and 76 nm, respectively, indicating that the as-prepared boron
compounds possess the advantage of a relatively wide
adjustable range of fluorescence color. It is obvious that the
absorption and emission spectra of compounds 1 and 3−5
assume a red shift in comparison to that of compound 2, which
is due to the push−pull character effect.51,52 This can be
explained as follows: the electron-withdrawing (F, Cl, or Br)
and -donating (CH3) groups attached on the benzene groups of
boron compounds could result in a bathochromic shift by
decreasing the electron transition energy and enlarging the
conjugation effect, separately. Meanwhile, the steric hindrance
of the electron donor methyl could effectively inhibit the close
stacking of molecules, thus preventing spectrum broadening,
which contributes to the largest red shift of compound 1.50 In
addition, compound 1 displays the highest absorption and
emission intensities, as well as the largest quantum yield (φf =
0.63), which are due to the fact that methyl could donate
electrons to the conjugated system and therefore increase the
amount of transition electrons.53,54 In contrast, compounds 4
and 5 show a relatively lower intensity in both absorption and
emission spectra in comparison to compound 2 due to their
electron-withdrawing character. Inconsistent with this effect,
the fluorescence spectral intensity of compound 3 is higher
D
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Scheme 3. Mechanism for the Formation of Boron Compounds
Table 2. Optical Properties of Compounds 1−5
in methanol
solid state
a
b
compound
λmax (nm) (log εmax
)
Fmax (nm)
Stokes shift (nm)
quantum yield Φ
τ (ns)
exmax (nm)
emmax (nm)
1
2
3
4
5
368 (3.96)
330 (3.85)
363 (3.78)
352 (3.56)
348 (3.42)
471
425
449
435
424
103
95
0.63
0.17
0.31
0.09
0.10
4.05
2.55
2.88
2.01
1.07
375
375
375
375
375
498
431
468
459
443
86
83
76
a
b
Molar absorption (log εmax 0.1). The fluorescence quantum yield was measured with a concentration of 5 × 10−6 mol/L in MeOH by using
perylene (QY = 0.71 in toluene) as the standard.61,62 The Φ values were corrected for differences in refractive index between the solvents used for
sample and reference measurements.
compound 1 was also investigated. Figure 2d illustrates the
fluorescence emission spectra of compound 1 with different
concentrations in MeOH. It is obvious that the emission peak
displays a red shift when the concentration increases, with the
E
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Figure 2. Absorption and florescence emission spectra of compounds 1−5 (a) in MeOH solution (1 × 10−4 mol/L) and (b) in the solid state. (c)
Spectroscopic properties of compound 1 in MeOH, AN, THF, TL, and CYH (1 × 10−4 mol/L). (d) Flurescence emission spectra of compound 1
with different concentrations in MeOH (inset: the linear curve of concentration and maximum emission wavelength).
maximum emission shifting from 465 nm (CMeOH = 1 × 10−6
mol/L) to 491 nm (CMeOH = 1 × 10−2 mol/L). From the linear
relationship between the concentration and the fluorescence
emission (inset of Figure 2d), it can be seen clearly that the
emission peak red-shifted gradually with an increase in the
concentration. The reason is that boron-containing fluoro-
phores are known to form excimers in concentrated solution
and in the solid state, and their emitting properties are greatly
influenced by the intermolecular interactions present in the
crystal structure.58−60
The fluorescence microscopy images of boron compound
crystals are shown in Figure 3a−e. We can easily find that all of
these crystals display blue luminescence upon the excitation
wavelength of 360 nm, which is consistent with the result of
fluorescence spectra (both in the liquid state and in the solid
state). Furthermore, the fluorescent crystals present different
shapes, with rectangles for compound 1, hexagons for
compound 2, and octagons for compounds 3−5. This is
caused by the difference in bond lengths and angles in different
boron crystal structures, which affects the orientation of crystal
growth.
In addition, we also performed time-resolved fluorescence
spectroscopy to estimate the fluorescence lifetime of the five
boron compounds. As depicted in Figure 3f−j, the obtained
compounds show the signal of decay profiles with an
adequately fitted double-exponential decay curve, and their
average fluorescence lifetimes (τ) are in the range of 1.07−4.05
ns. Importantly, the methyl-substituted boron compound
possesses the longest average fluorescence lifetime, indicating
that it has the best fluorescence properties, which is consistent
with the result of absorption and emission spectra. For the
halogen-substituted boron compounds, the fluorescence life-
time is shortened with the increase in the atomic mass, which is
due to the halogen effect.54
Finally, temperature- and pH-dependent fluorescence
properties were investigated in MeOH. Figure 4a exhibits the
emission spectra of compound 1 recorded in the temperature
ranging from 193 to 293 K. Obviously, the intensity of
fluorescence becomes stronger on cooling, especially when the
temperature approaches the freezing point of the solvent. In
addition, over the same temperature range, the peak shifts
sharply toward higher energy. This situation is consistent with
freezing of the solvent, leading to a significant decrease in the
magnitude of solvent polarity, which is due to the precipitous
change in the dielectric constant as the density of the solvent
increases.63,64 The effect of pH on the fluorescence properties
of compound 1 is shown in Figure 4b, which implies that the
emission spectrum is slightly influenced by the variation of pH.
Both peracid and perbasic treatments could slightly diminish
the fluorescence, possibly due to the fact that the boron
compounds are unstable under peracid or perbasic conditions.
3.4. Antibacterial Activities. The multifunctional charac-
ter of the obtained boron compounds was demonstrated by
antibacterial experiments. To comprehensively understand the
antibacterial activities of as-prepared boron compounds, both
Gram-positive (S. aureus and B. subtilis) and Gram-negative (E.
coli and P. aeruginosa) bacteria were selected as research targets.
Control experiments were conducted in the absence of
compounds. The inhibition zones of boron compounds with
different substituents toward S. aureus, B. subtilis, E. coli, and P.
aeruginosa are shown in Figure 5 (diameters of inhibition zones
are given in Table S7 in the Supporting Information). It is
found that the diameters of inhibition zones over different
compounds varies with the substituent. For instance, the
diameters of the inhibition zone for compounds 1−5 against S.
aureus were 12.4, 11.6, 10.8, 15.2, and 13.2 mm, respectively.
Moreover, the sizes of inhibition zones over compounds 1−5
against Gram-positive S. aureus and B. subtilis are evidently
larger than those of Gram-negative E. coli and P. aeruginosa,
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Figure 5. Typical inhibition zones of obtained boron compounds
against (a) S. aureus, (b) B. subtilis, (c) E. coli, and (d) P. aeruginosa (5
μg/mL, 10 μL). Note that (0) is the blank control in each disk.
selected Gram-positive S. aureus and Gram-negative E. coli to
further examine the antimicrobial activities of boron com-
pounds 1 (CH3), 2 (H), and 4 (Cl) through their
corresponding minimum inhibitory concentration (MIC) and
minimum bactericidal concentration (MBC) values. The
growth inhibition curves of S. aureus and E. coli over
compounds 1, 2, and 4 at different concentrations are shown
in Figure 6. When there is no antibacterial agent in the system,
the bacteria rapidly breed and reach the maximum peak after
6.5 h for S. aureus or 6 h for E. coli. After the boron compound
is introduced, the reproduction of the bacteria is restrained, and
the reproduction ability of the bacteria is negatively correlated
to the concentration of the boron compounds. For instance,
compound 4 could completely inhibit the S. aureus growth at a
concentration of 0.25 μg/mL, whereas it only delayed less than
40% of the exponential phase at a concentration of 0.125 μg/
mL. The MIC and MBC values for all compounds are shown in
Table 3. It is observed that the MIC and MBC values of boron
compounds 1, 2, and 4 toward the two bacteria varies with their
substituents. Specifically, compound 4 with a chlorine
substituent possesses the lowest MIC and MBC values,
indicating that it has the best antibacterial performance,
which is even equivalent in S. aureus inhibition to that of
commercial antibiotics (the fluoroquinolone derivative cipro-
floxacin).65 On the other hand, the above experiments indicate
that the as-prepared boron compounds possess excellent
antibacterial properties against both Gram-positive bacteria
and Gram-negative bacteria and the antibacterial effect of the
boron compounds against the Gram-positive bacteria S. aureus
is better than that against the Gram-negative bacteria E. coli.
For example, compound 2 shows an MIC value of 2.0 μg/mL
against S. aureus, while this value s 16 μg/mL toward E. coli.
This is probably due to a special cell structure named
lipopolysaccharide that only exists in the outer membrane of
Gram-negative bacteria, which was reported to provide an
effective resistive barrier against boron compounds.66
Figure 3. (a−e) Fluorescence microscopy images (at the excitation
wavelength of 360 nm) and (f−j) fluorescence decay of crystals of 1−5
obtained by diffusing at room temperature.
Figure 4. Effects of the (a) temperature and (b) pH on the emission
spectrum recorded for compound 1 in MeOH solution (1 × 10−4
mol/L).
which demonstrates that all of the boron compounds exhibit
selective antibacterial activities. Remarkably, all compounds
strongly inhibit S. aureus, B. subtilis and E. coli, whereas no
compounds show obvious antibacterial activity against P.
aeruginosa. In addition, we can easily find that the inhibition
zone diameters of compound 4 (Cl-substituted) are larger than
those of other compounds in all antibacterial experiments,
indicating that it has the best antibacterial performance.
From the results of the inhibition zone experiments, the as-
prepared boron compounds exhibited obvious antibacterial
activities toward S. aureus, B. subtilis, and E. coli. Thus, we
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Figure 6. Time-dependent growth curve of different bacteria in the presence of compounds 1 (CH3), 2 (H), and 4 (Cl).
Table 3. MIC and MBC Values of Compounds 1, 2, and 4
against S. aureus and E. coli
S. aureus
E. coli
a
b
MIC
MBC
MIC
(μg/mL)
MBC
(μg/mL)
compound
(μg/mL)
(μg/mL)
1
2
4
1.0
4.0
16
8.0
16
64
128
32
2.0
0.25
0.25
0.5
0.5
2.0
0.5
c
ciprofloxacin
1.0
a
b
MIC: minimum inhibitory concentration. MBC: minimum bacter-
c
icidal concentration. Ciprofloxacin: a commercial antibiotic was used
as a standard drug.
In addition, the antibacterial sensitivity of the obtained boron
compounds against two Gram-positive bacteria (S. aureus and
B. subtilis) was also investigated. Here we performed the colony
counting test of compounds 1 (CH3), 2 (H), and 4 (Cl) against
the mixed cultures S. aureus and B. subtilis in the same agar
plate. Control experiments were conducted in the absence of
compounds, and the experiments were performed in triplicate.
Figure 7 displays the inhibition rates of compounds 1, 2, and 4
against the cocultured bacteria. Under the same culture
conditions, the inhibition rate of compounds 1, 2, and 4
against S. aureus is higher than that against B. subtilis. For
example, when the sample concentration is 0.125 μg/mL, the
inhibition rates for compounds 1, 2, and 4 are 49.2%, 40.2%,
and 65.1% against S. aureus, while they are 40.5%, 35.5%, and
51.2% against B. subtilis, respectively. With an increase in
concentration to 0.5 μg/mL, the inhibition rates increase to
85.2%, 71.2%, and 100% against S. aureus, while those against B.
subtilis are only 78.1%, 62.5%, and 82.2%, respectively.
Definitely, S. aureus could be completely inhibited with
concentrations of 4.0, 16, and 0.5 μg/mL over compounds 1,
Figure 7. Inhibition rates of compounds 1 (CH3), 2 (H), and 4 (Cl)
against cultured bacterias (S. aureus, B. subtilis).
2, and 4, respectively. However, B. subtilis could be completely
inhibited over compounds 1, 2, and 4 when the concentrations
reach 8.0, 16, and 2.0 μg/mL, respectively. These results further
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verify that the boron compounds are more sensitive to S. aureus
than to B. subtilis.
To further prove the bacteria-dependent antibacterial
activity, it is necessary to investigate the bacterial cell
decomposition. Thus, the leakages of reducing sugars as a
measure of cell disruption after treatment with boron
compounds 1 (CH3), 2 (H), and 4 (Cl) were estimated.
Figure 8 reveals that an appreciable amount of reducing sugar
Figure 9. (a−c) Contact angles and (d) ζ potentials of compounds 1,
2, and 4.
(H) both show superior hydrophobic character in comparison
to unsubstituted compound 2. Interestingly, the incorporation
of a hydrophobic substituent (chlorine) could improve the
hydrophobicity of boron compounds, thus leading to
compound 4 having the best hydrophobic properties.
Moreover, an understanding of the electric charge on the
surface of boron compounds deserves further investigation in
order to clarify the electrostatic interaction between bacteria
and compounds, which greatly influences the absorption
process and thus causing differences in the inactivation process.
ζ potential analysis was therefore performed in LB liquid
medium with different pH values. As presented in Figure 9d,
the surfaces of compounds 1, 2, and 4 are all negatively charged
in LB liquid medium with pHs ranging from 5 to 9. Because the
bacteria are also negatively charged in LB liquid, the boron
compounds and bacteria would repel each other in the LB
liquid medium in the antibacterial experiments (pH 7).
However, the boron compound 4 with a chlorine substituent
(−6.3 mV) is less negatively charged than compounds 1 and 2
(−13.3 and −17.6 mV), indicating a weaker repulsive force
between compound 4 and bacteria and thus a better absorption
of bacteria, which contributes to its better antibacterial activity.
Figure 8. Leakage of reducing sugars from bacteria cells (S. aureus, E.
coli, and B. subtilis) treated with compounds 1 (CH3), 2 (H), and 4
(Cl).
leakage was found in all cases after the boron compound was
introduced in comparison to the control experiment, indicating
that the obtained reducing sugars must have released due to the
decomposition of the bacterial membrane. Furthermore, the
leakage amount of reducing sugars varies with the substituents
and the kinds of bacteria. For example, after 2 h treatment with
compounds 1, 2, and 4, the leakage amounts of reducing sugars
in S. aureus increase from 33, 26, and 45 μg/mL to 135, 113,
and 172 μg/mL, respectively. However, this value increases
from 12, 11, and 18 μg/mL to 86, 78, and 118 μg/mL in the
case of E. coli and from 29, 20, and 31 μg/mL to 119, 98, and
148 μg/mL in the case of B. subtilis, respectively. This result
implies that compounds 1, 2, and 4 could destroy the structure
of the membrane and facilitate the leakage of reducing sugars
from bacterial cytoplasm. In addition, the leakage amount of
reducing sugars of S. aureus is higher than that of B. subtilis and
E. coli in all cases, indicating a more severe destruction to the
cell membrane of S. aureus. Among the three boron
compounds, compound 4 causes the largest leakage amount
of reducing sugars toward both of the bacteria, demonstrating
that it has the best antibacterial ability, which is in accordance
with the result of antibacterial experiments.
It is important to understand the reason the obtained boron
compounds exhibit different antibacterial activity; thus, the
influence of the substituents on the physicochemical properties
that are related to the antibacterial activity of boron compounds
was then investigated. According to earlier reports, hydro-
phobicity was highly related to the ability to diffuse through the
biological membranes and reach reaction sites, which could
directly affect the antimicrobial activity.67 Therefore, we first
utilized the contact angle to characterize the hydrophobic
properties of compounds 1, 2, and 4. As shown in Figure 9a−c,
the contact angles of compounds 1, 2 and 4 are 58.8, 45.8, and
66.6°, respectively, indicating that compounds 1 (CH3) and 4
4. CONCLUSION
In conclusion, we developed a facile strategy to synthesize a
series of multifunctional boron compounds. Our synthetic
strategy has allowed us to prepare boron compounds
containing different types of substituents (CH3, F, Cl, and
Br) on each benzene ring of salicylic aldehyde, which to the
best of our knowledge is not known in the literature. These
boron compounds exhibit substituent-dependent fluorescence
properties that are highly related to the electron-withdrawing
and electron-donating abilities of the substituent. Interestingly,
the methyl-substituted boron compound exhibited bright
fluorescence in both the liquid and solid state with high
quantum yields, large Stokes shifts, and long lifetimes,
indicating its potential applications in solid-state emitting
materials. In addition, the influence of different factors on the
fluorescence properties of boron compounds, including solvent
(i.e., polarity, concentration), temperature, and pH, was also
discussed in detail. In addition, the as-synthesized boron
compounds also exhibited efficient bacterium-selective and
substituent-dependent antibacterial activities. The chlorine-
substituted boron compound even possessed antibacterial
activity comparable with that of commercial antibiotics. The
contact angle characterization and ζ potential analysis further
proved that the difference in hydrophobic properties and
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ASSOCIATED CONTENT
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S
* Supporting Information
The Supporting Information is available free of charge on the
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Elemental analyses, NMR spectral data, FT-IR, crystal
structure data of boron compounds, molecular structure
and structural descriptions of compounds 1 and 3−5,
photoluminescence in different solvents, and sizes of
inhibition zones of compounds (PDF)
Accession Codes
CCDC 1534451−1534455 contain the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-
ing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
■
Corresponding Author
*Hong Zhou Tel: (+86)13871359786 E-mail: hzhouh@126.
com.
ORCID
Hong Zhou: 0000-0003-2409-784X
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful for financial support from the Natural Science
Foundation of China (21171135), the eighth postgraduate
education innovation fund of Wuhan Institute of Technology,
China (CX2016174), and the support of H NMR and 13C
1
NMR spectra from the Shiyanjia laboratory.
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