Novel fluorophores: Syntheses and photophysical studies of boron-aminotroponimines

Article abstract of DOI:10.1016/j.dyepig.2016.10.051

The syntheses and photophysical study of novel fluorescent boron-aminotroponimine complexes are described. The chemical structure of one of the boron complexes was confirmed by single crystal X-ray analysis, which shows the appearance of the distorted tetrahedral geometry at boron. The photophysical studies of these complexes revealed that the boron-aminotroponimines are fluorescent molecules with quantum yields ca. 0.17–0.28.

Full text of DOI:10.1016/j.dyepig.2016.10.051

Accepted Manuscript
Novel fluorophores: Syntheses and photophysical studies of boron-
aminotroponimines
Chenikkayala Balachandra, Nagendra K. Sharma
PII:
S0143-7208(16)30692-1
10.1016/j.dyepig.2016.10.051
DYPI 5567
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 9 September 2016
Revised Date: 31 October 2016
Accepted Date: 31 October 2016
Please cite this article as: Balachandra C, Sharma NK, Novel fluorophores: Syntheses and
photophysical studies of boron-aminotroponimines, Dyes and Pigments (2016), doi: 10.1016/
j.dyepig.2016.10.051.
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ACCEPTED MANUSCRIPT
Novel Fluorophores: Syntheses and Photophysical studies of Boron-Aminotroponimines
Chenikkayala Balachandra and Nagendra K. Sharma*
School of Chemical Sciences, National Institute of Science Education and Research (NISER), Jatani-752050, Odisha, India.
Corresponding author. Tel.: 0674-249-4141; fax: +91 (674) 2494004; e-mail: nagendra@niser.ac.in
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Novel Fluorophores: Syntheses and Photophysical Studies of
Boron-Aminotroponimines
Chenikkayala Balachandra and Nagendra K. Sharma*
School of Chemical Sciences, National Institute of Science Education and Research (NISER), Jatani-752050,
Odisha, India. Corresponding author. Tel.: 0674-249-4141; fax: +91 (674) 2494004.
e-mail: nagendra@niser.ac.in
Abstract: The syntheses and photophysical study of novel fluorescent boron-
aminotroponimine complexes are described. The chemical structure of one of the boron
complexes was confirmed by single crystal X-ray analysis, which shows the appearance of
the distorted tetrahedral geometry at boron. The photophysical studies of these complexes
revealed that the boron-aminotroponimines are fluorescent molecules with quantum yields ca.
0.17-0.28.
1. Introduction
Tropone and tropolone chemical moieties are unique seven-membered ring non-benzenoid
aromatic system present in many biologically active natural products.[1] The electronic
structure of the tropolonoids is distinctive from the benzenoid aromatic compounds and have
characteristic photophysical properties (absorption and emission).[2, 4a-d] However, notable
fluorescence was observed with the bioactive tropolonoid natural product colchicine,
comprising a methoxytropone moiety, which has shown fluorescence after binding with the
tubulin protein.[3] Later, fluorescent properties of tropolone were also explored, quantum
yields of both tropolone and colchicine are very low at room temperature.[4] Moreover,
substituted tropolone derivatives have shown fluorescence with enhanced quantum yields.[5-
6]
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Aminotroponimines (Figure 1A) are the synthetic aza-derivatives of tropolone, which are
bidentate nitrogen chelating ligands.[7a] The aminotroponimine metal complexes are
synthesized extensively and a few have been used as catalysts in selective organic
transformation reactions.[7,12] Additionally, the synthesis of tropocoronands has also been
elaborated, which are derived from cyclic aminotroponimines and metals.[8] However, the
synthesis and hydrolytic stability of a few 2-bora-l,3-diazaazulene complexes have been
reported in early 1962 but the fluorescent properties of these molecules are not explored so
far.[9] On the other hand, Boron-Dipyrromethene (BODIPY, Figure 1B) fluorescent dyes are
widely explored.[11] Importantly, organic fluorescent dyes are enormously useful in different
fields for many applications such as labeling agents,[10a] chemical sensors,[10b,c] cell
imaging agents[10d] and energy related cassette entities.[10fg]
Overall, fluorescent molecules consisting of tropolonoid core structure are yet to be fully
explored. Hence, it is important to develop the fluorescent molecules containing tropolonoid
aromatic system.
B
C
A
R2
R2
N
N
F
F
B
N
N
NH
R1
N
B
F F
R1
4,4-difluoro-4-bora-
3a,4a-diaza-s-indacene
2,2-difluoro-2-bora-
1,3-diaza-azulene
aminotroponimine ligand
C3N2 core,
C2N2 core
Figure. 1 Chemical structures of A) aminotroponimine ligand. B) boron dipyrromethene core. C) boron-
aminotroponimine core (this work).
The fluorescent properties of a few reported tropolonoids and boron-dipyrromethene
complexes inspired us to design a set of novel organic fluorescent molecules, boron-
aminotroponimines (Figure 1C). These molecules are synthesized from aminotroponimine
ligand, containing tropolonoid aromatic system, in place of dipyrromethene ligand of
BODIPY complexes. However, boron-aminotroponimine core structure is a bicyclic core
structure with the seven membered troponyl ring and a five membered ring, which is
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distinctive from BODIPY core structure. Herein, we report the syntheses of various boron-
aminotroponimines and examine their photophysical properties.
2. Results and Discussion
Syntheses and characterization: We began the syntheses of boron-aminotroponimine
complexes from commercially available tropolone by following the previously reported
procedures.[13] In scheme 1, tropolone was converted into 2-tosyloxytropone and then
treated with different amines (ca. 3.0 equivalents) which produced N-substituted
aminotropone derivatives (3) in good yield. These aminotropones (3) were further converted
into aminotroponimines (4) by treating sequentially with triethyloxonium tetrafluoroborate
and an amine.[11a]
3a R1 = PhCH2
3b.R1 = Ph
O
(ii) Amine, Et3N,EtOH
reflux 48 h, 80%
O(i) Tosyl chloride, Et3N
DCM, 24 h, 95%
OH
O
3c R1 = p-CH3-Ph
3d R1 = p-Cl-Ph
NH
R
OTs
3
1 3d R1 = p-F-Ph
2
1
3e R1 = p-CH3O-Ph
(iii) Et3OBF4, DCM, 4-12 h
(iv) Amine, EtOH, 12 h
R2
R2
N
(v) BF3OEt2, Et3N
DCM, 30 min
N
B
N
F
F
NH
R1
R1
5
4
4e R1 = p-Cl-Ph, R2 = PhCH2
4f R1 = p-F-Ph, R2 = PhCH2
4g R1 = p-CH3O-Ph, R2 = PhCH2
4h R1 =Ph, R2 = CHCCH2
4a R1 = PhCH2, R2 = PhCH2
4b R1 = Ph, R2 = PhCH2
4c R1 = p-CH3-Ph,R2 = PhCH2
4d R1 = Ph, R2 = Ph
N
N
N
N
N
N
N
N
B
B
B
B
F
F
F
F
F
F
F
F
5d (70%)
5b (73%)
5c (90%)
5a (77%)
N
N
N
N
N
N
F
N
N
B
B
B
B
F
Cl
O
F
F
F F
F
F
F
5g (45%)
5e (81%)
5h
(66%)
5f (76%)
Scheme 1. Synthesis of boron-aminotroponimine complexes
Importantly, the reaction was fast with alkyl amines (benzyl/propergyl) as compared to aryl
amines. The reported conversion of N-phenyl aminotropone (3b) into N-(Phenyl)-2-
(phenylamino)troponimine (4d) was extremely slow (ca. 8.0 days) and yield was approx.
45.0%.[13a] Further, these aminotroponimines were converted into boron-aminotroponimines
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by treating with an excess of boron trifluoride diethyl etherate (10 equiv.) and triethylamine
(10 equiv.) at room temperature within 30.0 minutes. Exceptionally, the synthesis of N,N'-
diaryl substituted boron complex (5d) required 20 equivalents of boron trifluoride diethyl
etherate and triethylamine.
Further, we designed a dimeric complex 5i that contain two monomeric units of 3b
connected by a butyl chain. This dimeric complex (5i) was synthesized from the N-phenyl
aminotropone (3b) and 1,4-diaminobutane (Scheme 2).
Scheme. 2 Synthesis of dimeric aminotroponimine 4i and its boron complex 5i.
First N-phenyl aminotropone (3b) was treated with triethyloxonium tetrafluoroborate
followed by the treatment with 1,4-diaminobutane results in formation of N,N'-(diphenyl)-2-
(1,4-diaminobutyl) ditroponimine with moderate yield (20%). To avoid the formation of
monomeric ligand (4j), only1.0 equivalent of 1,4-diaminobutane was added portionwise, but
still we were unable to control the formation of the monomeric ligand (4j). Next, this dimeric
ligand was subjected to BF2-complexation under the above optimized conditions, which
produced the desired complex (5i) with good yield (40.0%). The synthesized boron-
aminotropnimine complexes are stable at room temperature/in the open air and were
1
characterized by H, 13C, 11B, 19F NMR and HRMS (ESI-MS). Their spectral data are
provided in the Supplementary Data.
Finally, the chemical structure of one of the boron complexes 5c, was established by the
single crystal X-ray analysis and its ORTEP diagram is depicted in Figure 2A.
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Figure. 2 A) ORTEP diagram of 5c with 25% probability of ellipsoids, hydrogen atoms are deleted for
clarity; B) Asymmetric unit of 5c.
The X-ray analytical data as cif file is also deposited in Cambridge Crystallographic Data
Center (CCDC) with CCDC number1400815. The asymmetric unit of complex 5c is depicted
in Figure 2B, which shows well-ordered molecular packing in its unit cell. The crystal
structure analysis revealed that the boron center exists in distorted tetrahedral geometry and
both the seven and five membered rings of boron-aminotroponimine core structure are tilted
in 7.190. N2-B bond is a typical N-B single bond, with the bond length 1.542 Å. The bond
length of N1-B is 1.528 Å, which is lesser than N2-B bond length.[15] Selected bond lengths
and bond angles are given in Table 1.
The crystals of the boron complex 5c were grown in chloroform by slow evaporation method
for single crystal X-ray diffraction studies. Though, we attempted to crystallize the other
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solid boron complexes like 5a, 5d but our trials ended with needle like structures, from which
we could not collect good X-ray diffraction data.
Table 1. Selected bond lengths and bond angles of boron complex 5c in solid state.
selected bond lengths selected bond angles selected bond angles
C1-C7 = 1.463Å
C1-N1 = 1.334Å
C7-N2 = 1.350 Å
N1-B = 1.528 Å
N2-B = 1.542 Å
N1-B-N2 = 98.10˚
F2-B-N2 = 112.26˚
F1-B-F2 = 107.72˚
F2-B-N1 = 113.44˚
F1-B-N1 = 111.36˚
F1-B-N2 = 113.90˚
C1-N1-B =112.87˚
C7-N2-B = 111.64˚
N2-C7-C1 =108.45˚
N1-C1-C7 = 108.61˚
Photophysical Studies: After synthesis, we performed the photophysical studies with the
boron-aminotroponimine complexes (5a-i). First, the absorption and emission of
aminotroponimine ligands (4a-g) were recorded in cyclohexane as control experiments. The
absorption spectra of ligands (4a-g) in cyclohexane are depicted in Figure 3A, while their
absorption spectra in acetonitrile are provided in SD (Figure S2). The UV-Vis spectrum of
1.0
0.8
0.6
0.4
0.2
1.0
0.8
0.6
0.4
0.2
0.0
B
A
5a
5b
5c
5d
5e
5f
5g
5h
5i
4a
4b
4c
4d
4e
4f
4g
250
300
350
400
450
500
300
350
400
450
500
550
Wavelength (nm)
Wavelength (nm)
Figure 3. A) Absorption spectra of aminotroponiminate ligands (4a-g) in cyclohexane. B) Normalized
absorption spectra of boron-complexes (5a-i) in cyclohexane, while their absoption spectra in acetonitrile
provided in SD (Figure S2).
aminotroponimine ligand 4a exhibits three peaks at 416 nm, 359 nm and 345 nm in
cyclohexane. The absorption peaks at 359 and 345 nm has shown partial splitting and
separated by 1130 cm-1. While a clear separation was observed between the absorption peaks
at 416 and 359 nm and separated by 3817.0 cm-1. The absorption peak at 416 nm is a broad
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absorption peak. The appearance of two major absorption peaks are due to transition to two
different electronically excited states.[4a-d] The partial splitting between the two peaks at
359 nm and 345 nm is possibly due to the vibronic progression of the electronic transition to
the same electronically excited state. In literature, similar kind of absorption patterns is
reported for tropolone in cyclohexane.[4a-d] But, a redshift in absorption peaks and changes
in their intensities are observed with aminotroponimine ligand 4a when compared to
tropolone absorption spectrum in cyclohexane. The absorption spectra of the remaining
aminotroponimine ligands 4b-g exhibits a broad absorption peak with the absorption maxima
at ca. 402 (for 4d, λmax=412 nm) and a shoulder peak at 362 nm (Figure. 3A).
We also found a blueshift in absorption maxima for aminotroponimine ligands 4b/c/e/f/g
(λmax=402 nm) over 4a (λmax=416 nm). This blueshift of ligands 4b/c/e/f/g is due to replacing
the benzyl substituent at amine nitrogen atom of the ligand with phenyl and substituted
phenyl substituents. The presence of phenyl ring at N-atom of aminotroponimine decreases
the basicity of nitrogen, and subsequently the delocalization of nitrogen lone pair decreases
towards the troponyl ring. [14b,c] However, the substituents on the phenyl ring does not have
any effect on the absorption maxima of ligands 4b/c/e/f/g. For example, 4b/c/e/f/g have
shown similar absorption pattern though ligand 4e/f contains 4-chlorophenyl/4-fluorophenyl
substituents and ligands 4b/c/g contains phenyl/4-methylphenyl/4-methoxyphenyl
substituents on nitrogen atoms. In contrast, the absorption maxima of aminotroponimine
ligand 4d is redshifted over 4b/c/e/f/g to ca. 412 nm with a shoulder peak at 362 nm. This is
possibly occurring due to replacement of two benzyl substituents with the phenyl
substituents, as a result extension of conjugation towards phenyl ring occurs through the
imine N-atom of aminotroponimines.
All aminotroponimine ligands, other than 4a, have shown broad absorption peak with
absorption maxima at 402 nm and 412 nm (4d) and a shoulder peak at 362 nm. The
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absorption spectra of aminotroponimine ligands (4b-g) in acetonitrile has shown more
separation between 363 nm and 405-410 nm absorption peaks (SD, Figure S2), unlike
absorption spectra in cyclohexane. Interestingly, in case of ligand 4a, partial merging
of absorption peaks at 346 and 358 nm was observed in acetonitrile. These studies
supports that the appearance of shoulder peak at ~362 nm is most probably due to
another electronic transition.[14a-d] Overall, two electronic transitions were observed
for all aminotroponimine ligands in cyclohexane and acetonitrile, and substituents on
the nitrogen atoms of ligands have significant effect on their absorption properties.
However, the emission spectra of these ligands (4a-g) did not exhibit any significant
emission peaks. These studies supports the feature that the ligands are non-fluorescent
molecules in cyclohexane solvent.
Then, the absorption and emission spectra of boron-aminotroponimine complexes (5a-
i
) were recorded in cyclohexane. Their spectra are provided in Figure. 3B. The
absorption spectra of these boron-aminotroponimine complexes exhibits only two
major absorption bands at ~350 nm and 430 nm, with absorption maxima at ~430 nm
(approx.) in cyclohexane. The occurrence of two absorption peaks are due to two
different electronic transitions. The separation between the two absorption peaks are
more or less similar for all boron complexes. Significant redshift in the absorption
maxima is observed from aminotroponimines to boron-aminotroponimines. The
summary of UV-vis absorption data of boron-aminotroponimines (5a-i) are provided
in Table 2 (column 2). The extinction coefficients (ε) of 5a-i are determined at λabs,max
430 nm from the respective absorption spectra and their values are described in Table
2
(column 3). Unlike aminotroponimine ligands, absorption maxima for all boron
complexes 5a-i are observed at ~430 nm. Importantly, sharp absorption peaks were
observed in case of boron-aminotroponimines in cyclohexane, ethanol and acetonitrile.
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The appearance of sharp absorption peaks indicates the rigidification of the structure
after BF2-complexation. The absorption spectra of boron complex 5a in cyclohexane
has shown sharp absorption peaks at 433, 421, and 408 nm and in case of other
complexes (5b-i) these peaks were appeared as shoulder peaks. The appearance of
these peaks are due to vibronic transition to the same electronically excited state. This
was further supported by the absorption spectra in ethanol and acetonitrile, where
these peaks are merged and single absorption peak was observed at ~425 nm (SD,
Figure S1/S2). From the literature, it has been learnt that the occurrence of vibronics
are indication of less conformational disorder.[14a] In case of boron complex 5a, the
occurrence of well-resolved vibronic peaks are possibly due to less conformational
disorder. Whereas, in boron complex 5d the vibronic peaks are merged and single
absorption peak was observed, this is probably due to more conformational disorder.
The emission spectra of boron complexes (5a-i) were then recorded at excitation wavelength
350 nm in cyclohexane. These spectra are depicted in Figure. 4B and emission maxima
(λem.), Stokes shifts and quantum yields are summarized in Table 2. Boron-
aminotroponimines have shown remarkable fluorescence character. It is important to discuss
the emission properties of these complexes. First of all, unlike absorption, tremendous
variation in the emission properties of boron-aminotroponimines were observed. Nature of
the substituents on the nitrogen atoms of the boron-aminotroponimines have dramatic effect
on emission properties such as emission maxima and quantum yield. Boron complex 5a
exhibits strong emission at 445 nm (blue fluorescence) with 12 nm Stokes shift. Whereas the
boron complexes 5b/5h, containing one N-phenyl substituent at nitrogen atom, have shown
emission maxima (λem) at 478 nm (green fluorescence) with larger Stokes shift (44 nm).
Interestingly, complex 5d, containing N,N'-diphenyl substituents on nitrogen atoms, emits at
wavelength 487 nm (λem) with further enhancement in Stokes shift (59 nm). Thus, N-phenyl
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substituted boron complexes shows noticeable redshift in their emission maxima with a
corresponding increase in the Stokes shift. The emission maxima of boron complexes 5e/f is
more redshifted when compare to 5b and their Stokes shifts are 50 and 47 nm, respectively
(chlorophenyl/fluorophenyl substituents). Moreover, boron complexes 5c has shown further
redshift in emission maxima when compare to 5b/e/f. Overall, presence phenyl and
substituted phenyl substituents on nitrogen atoms of boron complexes leads to larger Stokes
shifts. Furthermore, the electron donating groups containing phenyl substituents on nitrogen
atom of boron complexes causes more redshift.
80
A
1.0
B
70
60
0.8
5a
5a
50
40
30
20
10
5b
5c
5d
5e
5f
5b
5c
5d
5e
5f
0.6
0.4
0.2
0.0
5h
5i
5h
5i
0
400
500
600
400
450
500
550
600
650
Wavelength (nm)
Wavelength (nm)
Figure 4. A) Emission spectra of boron-aminotroponimine complexes (5a-i) in cyclohexane at 20.0 ꢀM
concentration. B) Normalized emission spectra of boron complexes (5a-i) in cyclohexane.
Table 2. Photophysical parameters of boron-aminotroponimines
entry
λabsa (nm)
εa (M-1 cm-1)
λemac (nm)
Φfb
Stokes
shift (nm)
Stokes
shift (cm-1)
5a
433, 421, 408,
352
36881
445
0.17
12
622
5b
5c
434, 421, 355
434, 421, 355
16709
24651
478
487
0.17
0.07
44
53
2120
2508
5d
5e
435, 362
18342
12789
11571
15155
494
482
479
nd
0.05
0.17
0.15
nd
59
50
47
nd
2746
2402
2272
nd
432, 421, 355
432, 421, 355
433, 355
5f
5gd
5h
431, 419, 354
16670
478
0.15
47
2281
5i
435, 421, 357
18042
478
0.28
43
2068
aAll measurements were carried out in cyclohexane, bQuantum yields were determined
by considering quinine sulfate in 0.1 M H2SO4 as standard reference. cλex = 350 nm.
dboron complex 5g has shown negligible fluorescence in cyclohexane.
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Here, we also attempted to investigate the effect of excitation wavelength on emission
maxima. The boron complexes 5a/b/c/e/f were excited at a wavelength 433 nm in
cyclohexane. Their emission spectra are given in Supplementary Data. These results indicate
that the emission wavelength is independent of excitation wavelength.
Further, the quantum yields of boron-aminotroponimines (5a-i) were determined in
cyclohexane by following the well-known relative method.[16] Quinine sulfate in H2SO4 (0.1
M) was used as reference standard. All the measurements were carried out under same
instrumental parameters (λex = 350 nm, for reference and samples to be analyzed) and at 20
oC. The obtained emission spectra are depicted in Figure. 4B and their quantum yields are
given in Table 2. As mentioned above, nature of the substituents on nitrogen atoms have
significant effect on the quantum yield of boron-aminotroponimines 5a-i. The maximum
quantum yield achieved with the monomeric boron-aminotroponimines 5a/b/e/f/h is 0.15-
0.17 (Φf). The quantum yield of remaining monomeric boron-aminotroponimines such as
5c/d is 0.07 and 0.05, respectively. However, as envisioned, the quantum yield of the dimeric
boron complex 5i was remarkably increased, almost twofold (Φf=0.28). Interestingly, the
calculated quantum yield for boron complexes 5a and 5b was same irrespective of their
substituents. In case of other complexes, we found that the presence of electron donating
groups on N-atoms of their ligands such as 4-methylphenyl (5c, Φf=0.07), 4-methoxyphenyl
(5g, negligible fluorescence) leads to dramatic decrease in the quantum yield, while presence
of electron withdrawing substituents such as 4-chlorophenyl (5e, Φf=0.17) and 4-
fluorophenyl (5f, Φf=0.15) does not lead to significant changes in quantum yield of respective
boron complexes when compared to 5b (Φf=0.17). Overall, substituents at the nitrogen atoms
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of the boron-aminotroponimines have significant effect on emission maxima and quantum
yield, unlike absorption.
We also recorded the absorption and emission spectra of boron-aminotroponimines (5a-i) in
ethanol. Their spectra are provided in Supplementary Data. Unlike in cyclohexane, only two
absorption peaks at ~425 and 350 nm are observed with all boron-aminotroponimines.
However, the blue shifts (5-10 nm) in absorption and emission maxima was observed in
ethanol and the larger Stokes shifts were also observed in ethanol. Except for 5a, their
quantum yields were significantly diminished in ethanol (Table S3, SI). Overall, our
spectroscopic studies strongly suggest that boron-aminotroponimine complexes are
fluorescent molecules like BODIPY analogue, but aminotroponimine precursors are non-
fluorescent molecules.
3. Conclusions
In summary, we have successfully synthesized various boron-aminotroponimine
complexes containing tropolonoid aromatic system and their initial photophysical properties
are explored. The photophysical properties are demonstrated in two solvent systems. We also
established the chemical structure of one of the boron-complex 5c by single crystal X-ray
analysis. Our initial studies revealed that only boron-aminotroponimine complexes are
fluorescent molecules. No emission properties were observed with the aminotroponimine
ligands. These observations provide enormous opportunities to construct novel fluorophores
containing boron-aminotroponimine structure by tuning the chemical structure.
4. Experimental Section:
Materials, instrumentation and methods: All required materials and solvents were
purchased from commercial suppliers and used without any further purification unless noted.
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Anhydrous dichloromethane was freshly prepared by distilling over Calcium hydride.
Reactions were monitored by thin layer chromatography, visualized by UV and Ninhydrin.
Column chromatography was performed in 230-400 and 100-200 mesh silica. Mass spectra
were obtained from Bruker micrOTOF-Q II Spectrometer and the samples were prepared in
acetonitrile and injected in acetonitrile and water mixture. NMR spectra were recorded on
1
Bruker AV-400 at room temperature (1H: 400 MHz, 13C: 100.6 MHz). H and 13C NMR
chemical shifts were recorded in ppm, downfield from tetramethyl silane. Splitting patterns
are abbreviated as: s, Singlet; d, doublet; dd, doublet of doublet; app d, apparent doublet; app
dd, apparent doublet of doublet; t, triplet; q, quartet; dq, doublet of quartet; m, multiplet. FT-
IR spectra were obtained from Bruker Tensor 27 spectrometer from 4000-600 cm-1.
Absorption spectra were obtained using Perkin-Elmaer λ-750. Fluorescence spectra were
obtained from Perkin-Elmer LS-55 using Xenon lamp. All spectroscopic measurements were
carried out with spectroscopic grade, non-degassed solvents and at 20 oC.
Relative fluorescence quantum yields were determined by comparing with quinine sulfate
quantum yield in 0.1M H2SO4 (0.54). The obtained values were substituted in the equation
given below.
The subscripts x and r refers sample to be measured and reference. F denotes the area
under curve of the integrated fluorescence spectra, A stands for optical density at excited
wavelength, η represents refractive index of the solvent used.
General procedure for the synthesis of boron-aminotroponimines: 0.1 gm (0.33 mmol) of
Aminotroponimine (4a) was dissolved in anhydrous dichloromethane (6 mL) and to this 10
equivalents (465 mL, 3.33 mmol) of anhydrous triethylamine was added under nitrogen
14

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atmosphere. To the resultant reaction mixture 10 equivalents of BF3.OEt2 (411 mL, 3.33
mmol) was added dropwise and mixture left to stir at room temperature until reaction
completion, as judged by TLC, all reactions were completed within half an hour (For the
synthesis of 5d, 20 equivalents of triethylamine and BF3.OEt2 was used, whereas for the
synthesis of 5i, 25 equivalents of triethylamine and BF3.OEt2 was used). All volatiles were
evaporated under reduced pressure and to the crude residue, water was added to quench the
unreacted BF3.OEt2 and extracted with ethyl acetate (thrice). The combined organic layers
were dried over sodium sulfate and concentrated. The crude product was purified through
silica gel column chromatography by using 100% DCM as the mobile phase. Boron complex
5a was obtained in 77.0% yield. Remaining all boron complexes were synthesized by
following this general procedure.
Boron complex 5a: The pure product was obtained as light green solid. (90 mg, isolated yield
= 77%, mp = 159-161 oC). FT-IR (KBr plate) ν = 2922, 1599, 1546, 1495, 1469, 1452, 1439,
1415, 1351, 1298, 1248, 1144, 1096, 1075, 1056, 995, 733, 698. 1H NMR (400 MHz,
CDCl3) δ 7.40 (d, J = 7.6 Hz, 4H), 7.33 (t, J = 7.5 Hz, 4H), 7.26 – 7.21 (m, 4H), 6.81 – 6.65
(m, 3H), 4.85 (s, 4H). 13C NMR (101 MHz, CDCl3) δ 156.60, 138.50, 136.77, 128.71,
11
127.23, 127.18, 122.92, 113.93, 46.85. B NMR (128 MHz, CDCl3) δ 5.57 (t, J = 30.8 Hz).
19F NMR (376 MHz, CDCl3) δ -138.32 (dd, J = 61.6, 30.5 Hz). HRMS (ESI-TOF) m/z:
[M+H]+ calcd. for C21H19BF2N2 349.1686, found 349.1723.
Boron complex 5b: The pure compound was obtained as yellow thick liquid. (85 mg, isolated
yield = 73%) FT-IR (KBr plate) ν = 2920, 2850, 1600, 1545, 1494, 1469, 1452, 1436, 1412,
1251, 1101, 1076. 1H NMR (400 MHz, CDCl3) δ 7.49 (t, J = 7.7 Hz, 2H), 7.42 (d, J = 5.0 Hz,
4H), 7.37 - 7.29 (m, 4H), 7.27 - 7.24 (m, 2H), 7.02 (app d, J = 10.7 Hz, 1H), 6.84 - 6.76 (m,
13
2H), 4.86 (s, 2H). C NMR (101 MHz, CDCl3) δ 156.55, 156.26, 139.05, 138.60, 138.55,
136.64, 129.68, 128.71, 127.27, 127.19, 126.15, 114.89, 114.70, 46.88. 11B NMR (128 MHz,
15

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CDCl3) δ 5.41 (t, J 29.8). 19F NMR (376 MHz, CDCl3) δ -135.68 – -136.02 (m). HRMS (ESI-
TOF) m/z: [M+H]+ calcd. for C20H17BF2N2 335.1529, found 335.1546.
Boron complex 5c: The pure product was obtained as yellow solid. . (100 mg, isolated yield =
o
90%, mp = 149-149 C) FT-IR (KBr plate) ν = 2923, 2852, 1599, 1544, 1512, 1468, 1437,
1414, 1350, 1251, 1099, 1075, 1000, 727. 1H NMR (400 MHz, CDCl3) δ 7.43 (d, J = 7.4 Hz,
2H), 7.38 – 7.22 (m, 9H), 7.03 (app d, J = 10.7 Hz, 1H), 6.82 - 6.75 (m, 2H), 4.86 (s, 2H),
2.41 (s, 3H). 13C NMR (101 MHz, CDCl3) δ 156.63, 156.22, 138.52, 138.42, 137.02, 136.73,
136.32, 130.31, 128.70, 127.27, 127.25, 125.87, 123.79, 114.74, 114.63, 46.89, 21.13. 11B
NMR (128 MHz, CDCl3) δ 5.37 (t, J 29.7). 19F NMR (376 MHz, CDCl3) δ -135.88 – -136.31
(m). HRMS (ESI-TOF) m/z: [M+Na]+ calcd. for C21H19BF2N2 371.1505, found 371.1540.
Boron complex 5d: The pure product was obtained as yellow fine solid. (90 mg, isolated yield
= 70%, mp = 165-167 oC) FT-IR (KBr plate) ν = 2919, 2850, 1592, 1542, 1493, 1474, 1453,
1434, 1405, 1250, 1097, 1060, 751, 694. 1H NMR (400 MHz, CDCl3) δ 7.55 – 7.47 (m, 4H),
7.44 (d, J = 7.2 Hz, 4H), 7.38 - 7.30 (m, 4H), 7.10 (app d, J = 10.6 Hz, 2H), 6.87 (t, J = 9.6
Hz, 1H). 13C NMR (101 MHz, CDCl3) δ 156.24, 138.93, 138.64, 129.73, 127.30, 126.24,
125.07, 115.66. 11B NMR (128 MHz, CDCl3) δ 5.21 (t, J 28.7). 19F NMR (376 MHz, CDCl3)
δ -133.38 (dd, J 57.4, 28.6). HRMS (ESI-TOF) m/z: [M+Na]+ calcd. for C19H15BF2N2
343.1192, found 343.1048.
Boron complex 5e: The pure product is obtained as yellow viscous liquid. (75 mg, isolated
yield = 81%) FT-IR (KBr plate) ν = 2923, 2852, 1600, 1544, 1492, 1468, 1452, 1436, 1415,
1
1251, 1100. H NMR (400 MHz, CDCl3) δ 7.51 – 7.32 (m, 9H), 7.32 – 7.23 (m, 2H), 7.02
(app d, J = 10.7 Hz, 1H), 6.87 - 6.78 (m, 2H), 4.87 (s, 2H). 13C NMR (101 MHz, CDCl3) δ
156.35, 147.07, 138.86, 138.70, 137.63, 136.43, 132.71, 129.85, 128.72, 127.48, 127.32,
11
127.21, 124.42, 115.33, 114.49, 46.88. B NMR (128 MHz, CDCl3) δ 5.32 (t, J = 29.8 Hz).
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19F NMR (376 MHz, CDCl3) δ -135.49 – -135.93 (m). HRMS (ESI-TOF) m/z: [M+H]+ calcd.
for C20H16ClBF2N2 369.1139, found 369.1163.
Boron complex 5f: The pure product is obtained as yellow viscous thick liquid. (70 mg,
isolated yield = 76%) FT-IR (KBr plate) ν = 2920, 2850, 1641, 1600, 1545, 1508, 1469,
1438, 1253, 1229, 1213, 1102, 815, 796, 732, 706. 1H NMR (400 MHz, CDCl3) δ 7.45 – 7.36
(m, 4H), 7.36 – 7.30 (m, 3H), 7.31 – 7.23 (m, 2H), 7.22 – 7.14 (m, 2H), 6.96 (app d, J = 10.7
13
Hz, 1H), 6.87 - 6.79 (m, 2H), 4.86 (s, 2H). C NMR (101 MHz, CDCl3) δ162.77, 160.32,
156.63, 156.33, 138.75, 138.69, 136.53, 128.74, 127.97, 127.89, 127.33, 127.25, 124.17,
11
116.72, 116.49, 115.13,114.47, 46.91. B NMR (128 MHz, CDCl3) δ 5.33 (t, J = 29.8 Hz).
19F NMR (376 MHz, CDCl3) δ -115.59 (s), -135.93 (dd, J = 59.3, 29.3 Hz). HRMS (ESI-
TOF) m/z: [M+H]+ calcd. for C20H16BF3N2 353.1435, found 353.1437.
Boron complex 5g: The pure product is obtained as yellow viscous thick liquid. (50 mg,
isolated yield = 45%) FT-IR (KBr plate) ν = 2956, 2918, 2849, 1643, 1599, 1544, 1510,
1469, 1454, 1438, 1410, 1245, 1099. 1H NMR (400 MHz, CDCl3) δ 7.43 (d, J = 7.4 Hz, 2H),
7.38 – 7.30 (m, 4H), 7.30 – 7.22 (m, 3H), 7.05 – 6.97 (m, 3H), 6.78 (m, 2H), 4.86 (s, 2H),
3.86 (s, 3H). 13C NMR (101 MHz, CDCl3) δ158.62, 156.93, 156.12, 147.07, 138.52, 138.41,
136.75, 131.68, 130.90, 128.71, 127.28, 127.26, 127.22, 124.45, 123.96, 123.71, 114.96,
114.70, 114.57, 55.51, 46.91. 11B NMR (128 MHz, CDCl3) δ 5.32 (t, J = 29.7 Hz). 19F NMR
(376 MHz, CDCl3) δ -136.24 (dd, J = 59.3, 29.4 Hz). HRMS (ESI-TOF) m/z: [M+Na]+ calcd.
for C20H16BF3N2 387.1454, found 387.1473.
Boron complex 5h: (80 mg, isolated yield = 66%) FT-IR (KBr plate) ν = 2923, 2852, 1600,
1545, 1492, 1463, 1412, 1252, 1154, 1102, 1051, 995, 735, 695. 1H NMR (400 MHz, CDCl3)
δ 7.57 - 7.51 (m, 1H), 7.47 (t, J = 7.7 Hz, 3H), 7.41 – 7.29 (m, 4H), 7.16 (app d, J = 10.7 Hz,
1H), 7.04 (app d, J = 10.8 Hz, 1H), 6.91 (t, J = 9.7 Hz, 1H), 4.38 (s, 2H), 2.29 (t, J = 2.5 Hz,
1H). 13C NMR (101 MHz, CDCl3) δ 156.79, 155.53, 147.05, 138.99, 138.81, 138.74, 129.68,
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127.30, 126.08, 124.60, 115.44, 114.63, 77.66, 72.04, 14.07. 11B NMR (128 MHz, CDCl3) δ
4.93 (t, J = 29.7 Hz). 19F NMR (376 MHz, CDCl3) δ -136.06 – -139.11 (m). HRMS (ESI-
TOF) m/z: [M+H]+ calcd. for C16H13BF2N2 283.1216, found 283.1204.
Boron complex 5i: (30 mg, isolated yield = 40%) FT-IR (KBr plate) ν = 2923, 2852, 1599,
1
1544, 1492, 1468, 1453, 1436, 1415, 1251, 1101, 746, 701. H NMR (400 MHz, CDCl3) δ
7.50 - 7.45 (m, 6H), 7.38 - 7.33 (m, 6H), 7.31 – 7.22 (m, 3H), 7.04 (app d, J = 10.9 Hz, 2H),
13
6.95 (app d, J = 10.7 Hz, 2H), 6.83 (t, J = 9.6 Hz, 2H), 3.72 (s, 4H), 2.00 (s, 4H). C NMR
(101 MHz, CDCl3) δ 156.31, 156.11, 139.10, 138.62, 138.33, 129.66, 127.11, 126.22,
123.71, 113.99, 42.87, 25.94. 11B NMR (128 MHz, CDCl3) δ 5.22 (t, J = 30.5 Hz). 19F NMR
(376 MHz, CDCl3) δ -136.37 (dd, J = 60.8, 30.0 Hz). HRMS (ESI-TOF) m/z: [M+Na]+ calcd.
for C30H28B2F4N4 565.2339, found 565.2302.
ACKNOWLEDGMENT
CB grateful to UGC for a research fellowship. We thank Mr. Marri Subba Reddy for
crystal structure analysis.
Supplementary Data
Experimental procedure and characterization data of aminotroponimines and boron-
13
11
19
aminotroponimines (1H, C, B, F NMR and HRMS) are provided in the Supplementary
Data.
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Research Highlights
•
•
•
•
The syntheses and characterization of monomeric and dimeric Boron-Aminotroponimines.
Exploring the absorption and fluorescence properties of Boron-Aminotroponimines.
Boron-Aminotroponimines core structure is distinctive from Boron-Dipyrromethenes.
The fluorescence properties are tunable.
24