A highly fluorescent tri-nuclear boron complex with large Stokes shifts based on tripodal Schiff base: synthesis and photophysical properties

Article abstract of DOI:10.1007/s12039-019-1643-4

Abstract: In this study, a new imine-based tripodal ligand and its difluoroboron complex were designed and synthesized. This trinuclear-boron complex was prepared for the first time and fully characterized by common spectroscopic techniques such as ¹

Full text of DOI:10.1007/s12039-019-1643-4

J. Chem. Sci.
(2019) 131:63
© Indian Academy of Sciences
https://doi.org/10.1007/s12039-019-1643-4
REGULAR ARTICLE
A highly fluorescent tri-nuclear boron complex with large Stokes
shifts based on tripodal Schiff base: synthesis and photophysical
properties
PINAR SEN∗
Department of Chemistry, Centre for Nanotechnology Innovation, Rhodes University, PO Box 94,
Grahamstown 6140, South Africa
E-mail: sen_pinar@hotmail.com
MS received 30 December 2018; revised 12 May 2019; accepted 13 May 2019
Abstract. In this study, a new imine-based tripodal ligand and its difluoroboron complex were designed
and synthesized. This trinuclear-boron complex was prepared for the first time and fully characterized by
common spectroscopic techniques such as 1H-NMR, 13C-NMR, FT-IR, UV–Vis and MS analysis. The optical
and fluorescence properties of complex were examined in CHCl3, THF, acetonitrile, EtOAc, DMF and DMSO.
It was revealed that the obtained boron complex showed intense emission with large Stokes shifts in the range
from 83 nm to 96 nm. As a result of quantification of fluorescence quantum yields, it has been observed that it
has high quantum yield values up to 48%. The large Stokes shifts and high efficient emissions in the solutions
of boron complex make it precious fluorophore for potential applications in materials science.
Keywords. Synthesis; Difluoroboron complex; tripodal ligand; fluorescence property; Stokes’ shift;
quantum yield; fluorescence lifetime.
1. Introduction
optical properties with large Stokes shift which is a very
important feature of a fluorophore. Thus, they could
be used as a material in many fields. For this pur-
pose, in addition to dipyrromethene derivatives, benz-
imidazoles,9 hydrazine-bispyrrole(BOPHY),10 anilido-
imine11 ligands have been studied as the N,N-chelated
difluoroboron complexes because of the promising
luminescent properties so far. Among the compounds
studied with this aim, BF2 complexes of Schiff-base
ligands have been presented in a few studies as the N,O-
chelated difluoroboron complexes.12 Schiff base ligands
can form stable complexes with most metals and, there-
fore the design and synthesis of Schiff base ligands in
coordination chemistry are crucial.13
However, while the most fluorophores show intense
fluorescence in solution, they display weak fluorescence
emission in the solid state resulting from the self-
quenching of the fluorophore. This deficiency limiting
its use as a material for practical applications can be
attributed to the planarity of the most dyes.14
Fluorescentdyeshavedrawnsignificantinterestincreas-
ingly for many fields such as fluorescence imag-
ing and labeling,1,2 light-emitting diodes,3 fluorescent
chemosensors for ions and neutral analytes for the detec-
tion of chemically and/or environmentally important
species.4
BF2 complexesofdipyrromethene(BODIPY), known
as one of the most main fluorescent dyes, form the
class of organic fluorine boron complexes. They have
exceptional photophysical properties presenting intense
fluorescence emission peaks with high quantum yields
and chemical and photo-durability.5 In addition to
the use as a fluorescent material, these properties of
BODIPYs have been also found to be valuable for
some applications such as photodynamic therapy as
a photosensitizer,6 solar cells,7 electrochromic display
systems.8 However, BODIPY derivatives mostly show
small Stokes shift.
Recent demands focused on obtaining new boron-
containing fluorescent dyes that can provide promising
In this study, a new trinuclear boron complex was
designed and synthesized in order to obtain an emissive
*
For correspondence
Electronic supplementary material: The online version of this article (https://doi.org/10.1007/s12039-019-1643-4) contains
supplementary material, which is available to authorized users.
0123456789().: V,-vol

63 Page 2 of 7
J. Chem. Sci.
(2019) 131:63
molecule possessing large Stokes shift to overcome the dissolved in 20 mL ethanol at 50 ◦C. 3-tert-butyl-2-hydroxy-
5-methylbenzaldehyde (1) (0.39 g, 2.043 mmoL) was dis-
solved in 20 mL of ethanol and added into mixture dropwise.
The reaction mixture was degassed by argon-vacuum system.
The reaction temperature was adjusted to boiling temper-
ature and stirred under reflux for 6 h. The course of the
reaction was checked by TLC (chloroform-EtOAC 10/1).
After the reaction was completed, the solution was evapo-
rated to 1/3 volume. The precipitated product was filtered off,
washed with ethanol and dried in a vacuum desiccator. Yield:
ν
mentioned challenge.
The condensation of an aromatic aldehyde deriva-
tive with tris(2-aminoethyl)amine (tren) in a 3:1 ratio
resulted in a Schiff base structure called tripodal back-
bone with ternary imine group. The synthesis and
characterization of its boron complex (BF2)3L (3) as
flexible fluorophore with three BF2 units was performed.
To the best knowledge, there are a few examples on the
tripodal ligand-like and their metal complexes in the lit-
erature.15–17
92% (0.415 g). FT-IR (UATR-TWOTM
)
max/cm−1: 3040
(Ar, C-H), 2947–2810 (Aliph., C-H), 1631 (C = N), 1594
The optical and fluorescence properties of the pre-
(Ar, C = C), 1437–1357 (Aliph., C-C), 1264, 1046, 842,
pared tris-metallic boron complex (3) were also inves- 783. 1H-NMR (CHCl3) δ (ppm): 7.63 (s, 3H), 7.09 (d, 3H),
5.41 (s, 3H), 3.49–3.47 (t, 6H), 2.84–2.81 (t, 6H), 2.03 (s,
9H), 1.43 (s, 27H). 13C-NMR (CHCl3) δ (ppm): 167.07,
158.27, 136.68, 130.47, 130.06, 126.42, 118.64, 58.38, 56.38,
tigated. It was observed that the obtained chromophore
showed high blue fluorescence in solution with a large
Stokes shift.
ε
34.90, 29.64, 20.54. UV–Vis (DMSO): max (nm) (log ) 266
λ
(4.25), 330(3.83). MS(MALDI-TOF):m/z670.128[M+1]+,
692.155 [M + Na]+.
2. Experimental
2.1 Chemicals and instruments
2.2b Synthesis of L(BF2)3 Complex (3): In a 100 mL
round bottom flask, DIPEA (0.8 mL) was added to a stirred
solution of ligand (2) (0.2 g, 0.3 mmoL) in dry dichloroethane
(30 mL) and degassed with argon at room temperature. After
the mixture was stirred for 15 min at 60 ◦C, BF3.O(Et)2 (0.6
mL, 4.5 mmol) was◦added slowly to the resulting mixture and
kept for 3 h at 85 C under argon atmosphere. The course
of the reaction was monitored with TLC (chloroform-EtOH
5/1). The completed reaction was diluted with CHCl3 (40
mL) and quenched by adding NaHCO3 solution. The mixture
was extracted with CHCl3 (3 × 15 mL). The CHCl3 phase
was dried over Na2SO4 and the solvent was evaporated to
dryness. The obtained crude product was purified by column
chromatography on silica gel eluting with CHCl3/methanol-
100/1 to obtain pure boron complex (3). Yield: 42% (0.102
ν
2-tertbutyl-4-methylphenol, urotropin, glacial acetic acid,
tris (2-aminoethyl), chloroform (CHCl3), ethylacetate
(EtOAc), methanol, ethanol (EtOH), dichloroethane, N,N-
diisopropylethylamine
(DIPEA),
dimethylformamide
(DMF), dimethyl sulfoxide (DMSO), tetrahydrofuran (THF),
acetonitrile, boron trifluoride diethyl etherate BF3.O(Et)2,
NaHCO3 were obtained from Sigma-Aldrich. All solvents
were dried and purified according to the procedures given by
Perrin and Armarego18 before using. Thin layer chromatog-
raphy (TLC) based on silica gel 60-HF254 as an adsorbent
was applied for monitoring the progress of the reactions and
chemicalpurityofthecompounds. Silicagel(Merckgrade60)
was used at column chromatography. Infrared spectra were
acquired on a PerkinElmer UATR-TWO diamond attenuated
total reflectance (ATR) spectrophotometer. Electronic spectra
were obtained on a Hitachi U-2900 UV-Vis spectrophotome-
ter. Fluorescence spectra were recorded from Hitachi F-2710
Fluorescence spectrofluorometer with quartz cell of 1 cm at
g). FT-IR (UATR-TWOTM
)
max/cm−1: 3012 (Ar, C-H),
2955–2857 (Aliph., C-H), 1642 (C = N), 1568 (Ar, C = C),
1
1446–1363 (Aliph., C-C), 1245, 1120, 1024, 945, 823. H-
NMR (DMSO) δ (ppm): 8.43 (s, 3H), 7.41 (d, 3H), 6.58 (s,
3H), 3.77 (t, 6H), 2.96 (t, 6H), 2.13 (s, 9H), 1.36 (s, 27H).
13C-NMR (DMSO) δ (ppm): 167.17, 155.20, 138.40, 136.62,
129.57, 128.81, 115.81, 52.89, 50.82, 34.85, 29.42, 20.38. 11B
1
room temperature. H and 13C NMR spectra were carried
out with Bruker AVANCE 600 MHz NMR Spectrometer and
Varian Mercury Plus 300 MHz spectrometer. Mass analysis
was performed on Bruker microTOF. Fluorescence lifetimes
were measured using a time-correlated single photon count-
ing setup (TCSPC) (FluoTime 200, Picoquant GmbH) with a
diode laser (LDH-P-670 with PDL 800-B, Picoquant GmbH,
670 nm, 20 MHz repetition rate, 44 ps pulse width).
(BF .O(Et) , CDCl3): δ 0.382 (t). UV–Vis (DMSO): max
λ
3
2
ε
(nm) (log ) 276 (4.72), 362 (4.19). MS (MALDI-TOF): m/z
794.336 [M − F]+, 835.372 [M + Na]+, 852.356 [M + Na +
H2O]+.
2.2 Synthesis
3. Result and Discussion
The synthesis of 3-tert-Butyl-2-hydroxy-5-methylbenzalde-
hyde (1) was accomplished according to the reported work.17
The spectroscopic results are consistent with the literature.
3.1 Synthesis and spectroscopic characterization
As a first step, 3-tert-Butyl-2-hydroxy-5-methylben-
zaldehyde (1) was obtained from the reaction of 2-
2.2a Synthesis of Ligand (2): In a 100 mL flask,
tris (2-aminoethyl) amine (0.102 mL, 0.68 mmoL) was tert-butyl-4-methylphenol with urotropine in glacial

J. Chem. Sci.
(2019) 131:63
Page 3 of 7
63
Scheme 1. Synthesis route: (i) glacial CH3COOH, urotropin, 120 ◦C (ii) methanol, reflux temperature (iii)
TEA, BF3.Et2O, dichloroethane.
CH3COOH by applying Duff reaction method which is
the procedure to obtain aldehyde in the ortho-position of factory information confirming the proposed structure.
1H-NMR and 13C-NMR spectra have provided satis-
1
aromatic phenols.19 In the second step, the condensation
When the H-NMR spectra of compounds 1 and 2
reaction was performed between the aldehyde derivative are compared, the disappearances of HC = O proton
(1) and tris (2-aminoethyl) amine in anhydrous ethanol signal of 1 shows that the imine condensation reaction
1
at reflux temperature to give the ligand (2) providing has taken place. The H-NMR spectra obtained for 2
N,O-chelating. Finally, the formation of the trinuclear and 3, the absence of OH proton signal of ligand (2) was
fluoroborate complex was carried out with the reaction the evidence that the boron complexation was occurred
of 2 and BF3.O(Et)2 in dry dichloroethane at 85 ◦C by to give triboron complex 3. In the 1H-NMR spectra of
using DIPEA as a base to afford 3.
the newly synthesized compounds, the aromatic protons
The structural characterization of the achieved pure were observed between 7.63–7.09 ppm for compound
products was performed by spectroscopic methods such 2 and 7.41–6.58 ppm for compound 3. The azomethine
as 1H-NMR, 13C-NMR, UV/Vis and FT-IRand MSanal- –CH = N– proton of complex (3) arose at 8.43 ppm.
ysis. The spectroscopic results confirmed the expected The aliphatic protons belonging to the −CH3 groups on
structures.
the benzene ring were in the range of 2.03–1.43 ppm for
In the FT-IR spectrum of 2, the formation of tris imine compound 2 and 2.13–1.36 ppm for compound 3. The
was verified the disappearance of the carbonyl band 11B-NMR signals were obtained at 0.382 ppm that is in
at 1645 cm−1 originating from 1 and the existence of accordance with sp3 boron.
– C = N vibration band at 1631 cm−1. The FT-IR spectra
When the 13C-NMR spectra of the obtained structures
of triboron complex (3), the most significant differences are examined, the formation of imine condensation was
were that the imine (C = N) and Ar-O-H vibrations at approved by the absence of HC = O carbon signal of
1631 cm−1 and 1264 cm−1 arising from free ligand 2 1. The observed signals for carbon atoms of the imine(–
shifted to 1642 cm−1 and 1245 cm−1 upon complex- CH = N–) groups were 167.07 ppm and 167.17 ppm
ation with boron. These results indicate that phenolic for compounds 2 and 3, respectively.
protons and imines are involved in coordination with
The mass spectrum of 2 and 3 was obtained by the
boron. Besides, the presence of B-O and B-N vibration MALDI-TOF Mass spectrometer confirming the pro-
peaks at 1180 cm−1 and 1024 cm−1 are evidence for the posed structures. In the mass spectrum of 2 the molec-
formation of boron complex (3).
ular ion peak and Na-adducted peak were observed at

63 Page 4 of 7
J. Chem. Sci.
(2019) 131:63
and 364 nm in acetonitrile, 268 and 364 nm in EtOAc,
280 and 362 nm in DMF, 276 and 362 nm in DMSO at
concentration of 1 × 10−5 M solutions (Table 1). When
the obtained results among the studied solvents are com-
pared for boron complex (3), it was observed that the
increased solvent polarity led to absorption in the higher
energy region (2–4 nm) in accordance with the natural
behavior of BF2 complexes of NˆO-bidentate ligands.21
This could be attributed that the improved dipolar char-
acteristics reduce the transition energy of the species.22
However, the maximum absorption wavelength values
that are not very different can be explained by the fact
that the molecule is not much affected by the solvent
polarity and the dipole moments of the molecules in
their ground and excited states were almost equal.23
The molar absorption coefficient of 3 was observed
withminordifferencesindifferentsolventsandthehigh-
est value was observed in the EtOAc solution.
Figure 1. Absorption spectra of 2 and 3 in DMSO (Con-
centration 1 × 10−5 molL−1).
3.2 Photophysical properties
The fluorescence behavior of the boron complex (3) was
measured at room temperature in CHCl3, THF, acetoni-
trile, EtOAc, DMF and DMSO to observe the effect
of solvents on fluorescence intensity depending on the
polarity of molecular medium, since the sensitivity of
the fluorophores to solvent polarity was very important
in the fluorescence field.
Boron difluoride complex (3) showed similar spectral
action in terms of emission wavelength in all studied
solutions and, exhibited blue fluorescence with emission
maximaof450nminEtOAc, 451nminCHCl3 andTHF,
455 nm in acetonitrile, 456 nm in DMSO when excited
at their absorption maxima. As shown in Figure 3, the
complex 3 showed the longest emission band with a
Figure 2. UV-Vis spectra of the compound 3 in CHCl3,
THF, acetonitrile, EtOAc, DMF, DMSO (Concentration
1 × 10−5 mol.L−1).
670.128 [M + 1]+, 692.155 [M + Na]+. The observed peak at 458 nm in DMF.
main peaks for compound 3 were determined at 794.336
The emission wavelength of this complex (3) signif-
[M−F]+, 835.372[M+Na]+, 852.356[M+Na+H2O]+. icantly shifted to the red region of the spectrum when
The UV-Vis absorption spectrum of 2 was recorded compared to the absorption wavelength. As a result of
together with the triboron complex (3) at a concentra- the coordination of the boron to the ligand, the bonding
tion of 1 × 10−5 M DMSO solution, as comparatively of the lone-pair of electrons of N atom on the ligand
π
(Figure 1). In the absorption spectrum of the 2, two main to the B atom reduces the energy gap between the
π
absorption bands appeared at 266 and 330 nm which and * of the ligand. This phenomenon makes the com-
20
π
π
π
arises from − */n- * transitions.
plex more stable and increases emission efficiency, as
The triboron complex (3) showed good solubility in indicated in previous studies for boron difluoride com-
common organic solvents. For this reason, the elec- plexes.24
tronic spectrum of the synthesized boron complex (3)
This compound also demonstrated large Stokes shifts
was recorded in solutions prepared with different sol- up to 96 nm. Stokes shifts were calculated by consid-
vents (CHCl3, THF, acetonitrile, EtOAc, DMF, DMSO) ering the difference between absorption and emission
toseetheeffectofsolventdifferenceonitsspectroscopic wavelength. The observed Stokes shifts are 83 nm in
properties at room temperature (Figure 2).
CHCl3, 86 nm in EtOAc, 87 nm in THF, 91 nm in
Absorptions of triboron complex (3) were observed at acetonitrile, 94 nm in DMSO and 96 nm in DMF.
270 and 368 nm in CHCl3, 288 and 364 nm in THF, 270 It was recorded as very high compared to existing

J. Chem. Sci.
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Page 5 of 7
63
Table 1. Absorption, excitation, emission spectral data for compound 3.
ε
τT
(μs)
Solvent
max . (nm)
λ
log
Excitation
Emission Stokes Shift
ꢁF
ꢀ
(nm)
λ
λ
Ex (nm)
Em(nm)
Stokes
CHCl3
THF
270, 368 4.71, 4.16
288, 364 4.35, 4.02
369
367
367
367
368
367
451
451
455
450
458
456
83
87
91
86
96
94
0.49 3.02
0.42 2.52
0.47 2.58
0.48 2.72
0.35 2.25
0.30 2.14
Acetonitrile 270, 364 4.67, 4.17
EtOAc
DMF
268, 364 4.66, 4.22
280, 362 4.56, 4.06
276, 362 4.72, 4.19
DMSO
Figure 3. (a) Emission spectrum of compound 3 in different solvents (Concentration: 1 × 10−5 M), (b)
Normalized emission spectrum of compound 3 in different solvents.
commercial fluorescent dyes.25 The enlarged large
Stokes shifts could be attributed to the presence of bulky
π
π
stack-
side groups reducing the intermolecular
−
ing.26
The absorption and fluorescence emission, the exci-
tation spectrum of boron complex (3) in DMSO is given
as an example in Figure 4. The excitation spectrum
of compound 3 was similar to absorption spectra and
both were mirror images of the fluorescence emission
spectra (Figure 4). However, the excitation spectra of
compound 3 are slightly red-shifted (1–5 nm) which is
an inconsiderable shift in relation to its absorption spec-
tra (Figure 4). Thus, it could be proposed that the boron
complex (3) is not affected by the excitation in studied
solutions.
Figure 4. Absorption, excitation and emission spectra of 3
in DMSO. (Excitation wavelength = 362 nm).
3.2a Determination of fluorescence quantum yields
and lifetimes: Fluorescence quantum yields (ꢁF) of
boron complex 3 have been measured in various sol-
vents by the comparative method using Eq. (1).27
where F and FStd are the areas under the emission profile
of the boron complex (3) and the standard, respectively.
A and AStd are the absorbances of the sample and stan-
dard corresponding to excitation wavelengths, and n2
and n2Std are referred to refractive indices of utilized
F.AStd.n2
ꢁF = ꢁF(Std)
(1)
FStd.A.n2
Std

63 Page 6 of 7
J. Chem. Sci.
(2019) 131:63
fluorescence decay curves demonstrate an example for
complex 3 in Figure 5.
4. Conclusions
In this study, the newly synthesized trinuclear diflu-
oroboron complex (3) of Schiff base tripodal ligand
(2) was reported. The boron complex (3) was well-
characterized by spectroscopic methods and the absorp-
tion and photophysical properties were investigated in
different solvents in order to determine the appropri-
ate solvent for future applications. The measurements
depending on the variety of solvents showed that com-
plex 3 is very emissive fluorophore with an emission
within the range of 450–458 nm in the solutions with
large Stokes shifts (83–96 nm). Boron complex (3) dis-
played relatively high quantum yields of 30–49% in
organic solvents. In the light of these results, the pre-
sented new boron complex (3) could be employ as a new
generation blue-emitting molecule with large Stokes
shift and high efficient emission in the field of fluores-
cence materials.
χ
Figure 5. Fluorescence decay (blue), 2 fitting (black) and
IRF (red) curves for complex 3 in DMSO.
solvents
(nacetonitrile : 1.34, nchlorof orm: 1.44, nEt O Ac
: 1.37, nDM F : 1.43, nDMSO : 1.48, nT H F :1.41)insolutio-
ns for the sample and standard, respectively. Quinine
sulfate (ꢁF = 0.54 in 0.1 M H2SO4) was employed as
standard.28
Supplementary Information (SI)
FT-IR (S1 and S5), 1H NMR (S2 and S6), 13C NMR (S3 and
S7) and mass spectra (S4 and S8) applied to characterize the
structure of new molecules which are synthesized are pre-
sented in supporting information at www.ias.ac.in/chemsci.
The fluorescence quantum yields of 3 were deter-
mined in distinct organic solvents at room temperature
and the obtained data were summarized in Table 1.
It was determined that the values of quantum yield
were in the range of 0.30–0.49 in the studied solvents; in
CHCl30.49, in THF 0.42, in acetonitrile 0.47, in EtOAc
0.48, in DMF 0.35 and in DMSO 0.30 (Table 1).
The energy of the emitted state is known to depend on
the solvent feature. The significantly increased quantum
efficiency of complex 3 was obtained in CHCl3 solu-
tion. Lowest fluorescence quantum yield value of boron
complex 3 was obtained in DMSO solution due to the
intramolecularrotationinducednon-radiativeprocess.29
This phenomenon is consistent with the fluorophores
which exhibit diminished quantum yield in solvents of
high polarity.30
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