Benzo[4,5]thiazolo[3,2-c][1,3,5,2]oxadiazaborinines: Synthesis, Structural, and Photophysical Properties

Article abstract of DOI:10.1021/acs.joc.8b02098

A family of highly emissive benzo[4,5]thiazolo[3,2-c][1,3,5,2]oxadiazaborinines, conjugated with the donor 4-dimethylaminophenyl group, was designed and synthesized. Their photophysical, both in solution and in the solid state, and structural properties were investigated. The influence of donor and acceptor substituents (R) in the benzothiazole unit on photophysical properties of complexes was found out. The tetrafluorobenzothiazole analogue exhibits nonbonded nuclear spin-spin coupling between fluorines from the BF₂ group and α-fluorine atom at the benzene ring. Additionally, this boron complex demonstrates a comparatively high solid-state fluorescence quantum yield (φ = 0.34).

Full text of DOI:10.1021/acs.joc.8b02098

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Article
Benzo[4,5]thiazolo[3,2-c][1,3,5,2]oxadiazaborinines:
Synthesis, Structural and Photophysical Properties
Mykhaylo A. Potopnyk, Dmytro Volyniuk, Magdalena Ceborska, Piotr
Cmoch, Iryna Hladka, Yan Danyliv, and Juozas Vidas Grazulevicius
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.8b02098 • Publication Date (Web): 02 Sep 2018
Downloaded from http://pubs.acs.org on September 2, 2018
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Benzo[4,5]thiazolo[3,2ꢀc][1,3,5,2]oxadiazaborinines: Synthesis, Structural and
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Photophysical Properties
Mykhaylo A. Potopnyk,*,† Dmytro Volyniuk,‡ Magdalena Ceborska,# Piotr Cmoch,† Iryna
Hladka,‡ Yan Danyliv,‡ and Juozas Vidas Gražulevičius*,‡
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†Institute of Organic Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01ꢀ224, Warsaw, Poland,
potopnyk@gmail.com
‡Department of Polymer Chemistry and Technology, Kaunas University of Technology, Radvilenu pl. 19, LTꢀ50254
Kaunas, Lithuania, juozas.grazulevicius@ktu.lt
#Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01ꢀ224, Warsaw, Poland
Abstract: A family of highly emissive benzo[4,5]thiazolo[3,2ꢀc][1,3,5,2]oxadiazaborinines,
conjugated with donor 4ꢀdimethylaminophenyl group, was designed and synthesized. Their
photophysical, both in solution and in the solid state, and structural properties were investigated.
The influence of donor and acceptor substituents (R) in the benzothiazole unit on photophysical
properties of complexes was found out. Tetrafluorobenzothiazole analogue exhibits nonbonded
nuclear spinꢀspin coupling between fluorines from BF2 group and αꢀfluorine atom at benzene
ring. Additionally, this
boron
complex
demonstrates
comparatively high solid
state fluorescence quantum
yield (Ф = 0.34).
Introduction
Organoboron complexes belong to the one of the most important and widely used class of
fluorophores.1 These dyes demonstrate practically useful electronic and optical properties (e.g.
high fluorescence quantum yields, relatively long lifetime of the excited state, photostability,
insensitivity to the environment, good solubility in organic solvents).2 As a result they can serve
many application areas, such as fluorescent probes,3,4 photosensitizer in photodynamic therapy,5
photoactive element in organic solar cells (OSC),6ꢀ9 and supramolecular systems10ꢀ12, organic
lightꢀemitting devices (OLEDs), and cholesteric liquid crystal laser13 etc.
The most representative organoboron complexes are boronꢀdipyrromethenes (BODIPYs),
which are based on dipyrrolo annulated 1,3,2ꢀdiazaborine scaffold.14,15 Their fluorescence in
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solutions is usually intensive. However, they demonstrate narrow Stokes shifts, what limits their
optoelectronic applications.1 To tackle this scientific challenge, in the last few years,
modification of BODIPY core was the topic of high research interest.1 Therefore, organoboron
compounds, based on nonꢀpyrrole heterocyclic building blocks annulated to 1,3,2ꢀdiazaborinine
(II),16ꢀ18 as well as, 1,2ꢀazaborinine (I),19 and 1,3,5,2ꢀtriazaborinine (III)20ꢀ24 ring have been
developed (Figure 1a). Another class of sixꢀmembered ring organoboron complexes has been
synthesized from O,Oꢀchelating ligands, forming 1,3,2ꢀdioxaborinine derivatives (IV).25ꢀ28
One of the factors that leads to increasing the Stokes shift of the organoboron complexes is
desymmetrisation of the ligand structures.1 In this context, complexes with hybrid ligands,
having simultaneously nitrogen and oxygen coordinating centres, are looking very attractive.
Organoboron dyes, based on 1,3,2ꢀoxazaborinine (V) scaffold, have been relatively widely
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investigated as complexes formed from
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ꢀketoiminate29ꢀ36 and phenolic37ꢀ44 ligands.
Nevertheless, the 1,3,5,2ꢀoxadiazaborinine (VI) derivatives, formed from amide ligands, have
been reported a little; they are mostly based on nitrogenꢀcontaining sixꢀmembered heterocycles
(pyridine,36,45ꢀ47 pyrazine,47,48 pyridazine,47 or naphthyridine35,49ꢀ52) or 1,3,4ꢀthiadiazole53 unit as
Nꢀcoordinating centres and demonstrate low to weak fluorescence quantum yields. These studies
have shown that oxadiazaborinine ring demonstrates electronꢀacceptor properties. The
conjugation of oxadiazaborinine moiety with strong donor group gives donor–acceptor (D–A)
molecular systems, which demonstrate the intramolecular charge transfer (ICT) character.45,49
Our approach is based on the use of electronꢀrich heterocyclic unit as a building block in the
construction of oxadiazaborinine dyes. This should decrease the acceptor character of
oxadiazaborinine core. In previous work we have demonstrated, that presence of the thiazole
ring, annulated to oxadiazaborinine core, promotes obtaining complexes with high fluorescence
quantum yields (Figure 1b).54
Continuing this research, we have decided to investigate the influence of annulation of
aromatic unit to 1,3ꢀthiazole ring on the photophysical properties of such type of complexes. To
realize this goal we have designed N,Oꢀcoordinated BF2 complexes using benzo[d]thiazole as a
heterocyclic Nꢀcoordinating center. Retrosynthetic analysis leads to simple building blocks:
(paraꢀdimethylamino)benzoyl chloride, 2ꢀaminobenzo[d]thiazole and boron trifluoride (Figure
1c). Herein we report our recent progress on this research topic.
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Figure 1. (a) Introduction to organoboronine sixꢀmembered ring complexes, (b) previous work,
(c) concept of benzo[d]thiazoleꢀbased BF2ꢀcomplexes.
Results and Discussion
Synthesis and Characterization
Unsubstituted benzo[d]thiazolꢀ2ꢀamine (2a) is commercially available. To provide the best πꢀ
conjugation between Nꢀcoordinating center of ligand (following boron unit) and substituents at
the benzo[d]thiazole unit in the final complexes, we decided to modify the 6ꢀposition of this
heterocyclic part. To prepare 6ꢀsubstituted benzo[d]thiazolꢀ2ꢀamines (2b–e) we used oneꢀstep
synthesis method, starting from 4ꢀRꢀanilines (3b–e). Using two equivalents of ammonium
thiocyanate and one equivalent of bromine (Scheme 1) in glacial acetic acid medium we obtained
products 2b–e in good yields (71–91%). Next, acylation of 6ꢀRꢀbenzo[d]thiazolꢀ2ꢀamines (2a–e)
with 4ꢀ(dimethylamino)benzoyl chloride (4) in basic conditions gave N,O ligands 5a–e in fair to
good yields (57 – 66%). Finally, amides 5a–e, were treated with boron trifluoride diethyl
etherate in the presence of diisopropylethylamine (DIPEA) to give complexes 6a–e in 61−68%
yields (Scheme 1). Synthesized benzothiazoleꢀBF2 complexes are modified at the 6ꢀposition of
the benzo[d]thiazole unit, having different substituents, including donor (OMe), weak acceptor
(F, Cl), and strong acceptor (CF3) groups.
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Scheme 1. Synthesis of BenzothiazoleꢀBoron Complexes 6a–e
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R = H (a), OMe (b), F (c), Cl (d), CF3 (e)
For comparison of photophysical properties of the boron dyes 6a–e with corresponding
parameters of an analogue, having stronger acceptor strength in benzothiazole unit, we designed
complex based on perfluorinated benzothiazole synthon. To achieve this goal, we developed the
synthetic strategy shown in Scheme 2. Pentafluoroaniline (7) was treated with benzoyl
isothiocyanate (generated in situ from benzoyl chloride and potassium thiocyanate) to furnish Nꢀ
[(perfluorophenyl)carbamothioyl]benzamide (8a). For cleavage of benzoyl group, this
intermediate, without purification, was heated in 10% NaOH followed by acidification to
generate 1ꢀ(perfluorophenyl)thiourea (8b) in 75% yield. Heating of thiourea 8b under strong
basic conditions in dry DMF provided hydrofluoric acid elimination and benzothiazole
cyclization, giving 4,5,6,7ꢀtetrafluorobenzo[d]thiazolꢀ2ꢀamine (9) in 83% yield. Next, amine 9
was transformed into amide 10 in 45% yield via reaction with chloride 4 under standard
acylating conditions. The synthesis of final complex 11 in 68% yield was successfully
accomplished using BF3·Et2O/DIPEA medium.
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Scheme 2. Synthesis of Complex 11
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All the synthesized compounds were characterized by 1H, 13C, 19F NMR, IR spectroscopy, and
highꢀresolution mass spectrometry. Due to the perfluorinated character of thiourea 8b and
benzothiazole 9, its 13C NMR spectra were recorded as protonꢀcoupled/fluorineꢀdecoupled.
While, to determine all carbon signals in the case of amide 10 and complex 11, two different
type (fluorineꢀcoupled/protonꢀdecoupled and protonꢀcoupled/fluorineꢀdecoupled) of the 13C
NMR spectra were measured.
In the 19F NMR spectra of dyes 6a–e, fluorines linked to the boron atom produced multiplets at
a narrow range from –137.71 to –137.44 ppm; while the corresponding signal of BF2 group in
the spectrum of compound 11 was located at –133.08 ppm.
On the other hand, in the oneꢀdimentional 19F NMR spectrum of complex 11, fluorine nuclei at
the benzene ring (F3, F4, F5, and F6) produced four distinctive signals, three doublets of
doublets (dd) and one triplet of doublet of doublets (tdd). For better interpretation of this
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situation, F–19F NMR COSY and NOESY experiments were performed. All adjacent fluorine
atoms in the benzo unit expectedly correlate to each other (correlations F3↔F4, F4↔F5, and
F5↔F6) with spinꢀspin coupling constant (J) around 20 Hz, what is clearly confirmed by both
COSY and NOESY experiments (Figure 2). Additionally, the atoms F3 and F6 demonstrate
paraꢀcorrelation through five bonds with J = 13.1 Hz; corresponding crossꢀpeaks are observed in
the COSY spectrum and missing in the NOESY plot. This type of throughꢀbond spinꢀspin
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coupling is also observed in the F NMR spectra of tetrafluorobenzo[d]thiazole derivatives 9
and 10 (see Experimental Section). However, in contrast to the corresponding doublet of
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doublets for F3 atom in the 19F NMR spectra of compounds 9 and 10, this nucleus (F3) in
complex 11 produces tdd peak, where triplet splitting is characterized by J = 30.3 Hz and
corresponding F3↔F1/F2 interaction. It should be noted that F3↔F1/F2 crossꢀpeaks are
observed in the NOESY spectrum, but missing in the COSY plot. Based on these results, we can
conclude that F3↔F1/F2 spinꢀspin coupling in complex 11 is dominantly or fully caused by
"throughꢀspace" mechanism. This phenomenon, named nonbonded nuclear spinꢀspin coupling,
was previously investigated in another aromatic molecules and coordination complexes.55
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Figure 2. 19F–19F NMR COSY (top) and NOESY (down) spectra of complex 11.
Xꢀray Analysis
The structures of all investigated complexes were further confirmed by singleꢀcrystal Xꢀray
analysis (Figure 3, Figures S1–S6 and Tables S1–S6 in the Supporting Information). In the solid
state, compounds 6a–6e and 11 have nearꢀtoꢀplanar geometry. The boron atom is almost
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tetrahedrally coordinated by two fluorine, nitrogen, and oxygen atoms: the NꢀBꢀF and OꢀBꢀF
bond angles are in the range 108.1–111.6°, the NꢀBꢀO bond angles are a smaller (105.5–107.6°),
the FꢀBꢀF bond angles in all complexes are ranging from 109.8 to 112.8°. The distances between
fluorines from BF2 group and αꢀfluorine atom (F3) of benzothiazole moiety of complex 11 are
2.82 and 2.87 Å (Figure 3), which, in both cases, is less than the sum of the van der Waals radii
for two fluorines (2.94 Å).
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Figure 3. XꢀRay structures of complexes 6a–e, and 11 (top and front views).
Compound 6a crystallizes in a P21/c monoclinic space group with four 6a molecules in the unit
cell (Table S1 in the Supporting Information). The molecules are connected by π–π/n–π
interactions (a = 3.57Å), forming shifted pairs with antiparallel orientation (Figure 4). These
molecular pairs are locked to each other by secondary π–π/n–π interaction (b = 3.37Å), building
a rectilinear chain (two antiparallel molecular lines painted in green and magenta color in Figure
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4). The neighboring chains are oriented in the same direction, but their molecular pairs are
ordered perpendicularly to each other. The connection between different chains is provided by
two hydrogen bonds and perpendicular S–π interactions.
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Figure 4. Molecular packing of compound 6a: (a) top view; (b) front view; (c) side view; (d) list
of intermolecular interactions. Blueꢀdotted lines show hydrogen bonds; greyꢀdotted lines show
S–π interactions; redꢀdotted lines show π–π and n–π interactions.
Complex 6b demonstrates wellꢀordered molecular packing (Figure 5), characterized by an
orthorhombic lattice with eight molecules per unit cell and the space group Pbca (Table S2 in the
Supporting Information). In the top view, the molecules form dimer with π–π and n–π
interactions (3.30Å, distance between the planes of paired molecules). Additionally, there are
observed numerous hydrogen bonds.
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Figure 5. Molecular packing of dye 6b: (a) top view; (b) front view; (c) side view; (d) list of
intermolecular interactions. Blueꢀdotted lines show hydrogen bonds; redꢀdotted lines show π–π
and n–π interactions.
Molecules of dye 6c are packed in a tetragonal unit cell according to the space group P41
(Table S3 in the Supporting Information). As shown in Figure 6, in the top and front views,
molecules 6c are organized in layers. Inside these layers, the molecules are connected by π–π/n–
π interactions (a = 3.70Å; b = 3.43Å) and hydrogen bonds (C1H–F3), forming stairs; they are
orientated parallel to each other and perpendicular to the plane of the layer. Neighboring layers
are connected by numerous hydrogen bonds (C2H–F1’; C2H–F2’; C4H–O1’; C16H–F1’) and
are perpendicularly twisted to each other, forming netꢀtype structure (side view).
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Figure 6. Molecular packing of complex 6c: (a) top view; (b) front view; (c) side view; (d) list of
intermolecular interactions. Blueꢀdotted lines show hydrogen bonds; redꢀdotted lines show π–π
and n–π interactions.
Complexes 6d,e and 11 crystallize in the Pꢀ1 triclinic space group with two molecules per unit
cell (Tables S4–S6 in the Supporting Information). Neighbouring molecules 6d are organized
antiparallel to each other, forming stairs with strong π–π and n–π interactions (Figure 7).
Nucleophilic heteroatoms (fluorine, chlorine, and sulfur) form several hydrogen bonds.
Additionally, S–F interaction was also observed.
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Figure 7. Molecular packing of compound 6d: (a) top view; (b) side view; (c) front view; (d) list
of intermolecular interactions. Blueꢀdotted lines show hydrogen bonds; redꢀdotted lines show π–
π and n–π interactions.
Crystal packing of complex 6e (Figure 8) is characterized by association of molecular dimers
in the top view, with a "headꢀtoꢀtail" orientation with π–π and n−π interactions (a = 3.55Å).
These dimers are shifted to each other in the front view, building stairs by additional π–π and n–
π interactions (b = 3.65Å). All fluorine atoms (from BF2 and CF3 groups) of compound 6e are
involved in intermolecular bonding. BF2 group takes part in hydrogen bonds, and S–F
interactions. Two fluorines from CF3 group make paired F–F interactions with CF3 group of
another molecule, while third fluorine connects with πꢀelectrons of 1,3,2ꢀoxadiazaborinine ring.
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Figure 8. Molecular packing of dye 6e: (a) top view; (b) front view; (c) side view; (d) list of
intermolecular interactions. Blueꢀdotted lines show interactions of BF2 group (hydrogen bonds
and S–F interactions); greyꢀdotted lines show interaction of CF3 group (F–F and F–π
interactions); redꢀdotted lines show π–π and n–π interactions.
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Specificity of molecular packing of complex 11 (Figure 9) owns in a large part due to the
presence of four fluorine atoms in the benzothiazole unit. Antiparallel oriented molecules form
stacking columns by connection via π–π/n–π coupling. The interactions between these columns
are provided by numerous hydrogen bonds and F–F interactions.
Figure 9. Molecular packing of complex 11: (a) top view; (b) front view; (c) side view; (d) list
of intermolecular interactions. Blueꢀdotted lines show hydrogen bonds and F–F interactions; redꢀ
dotted lines show π–π and n–π interactions.
Photophysical Properties of Solutions
The normalized absorption and emission spectra of the dilute solutions of
benzo[4,5]thiazolo[3,2ꢀc][1,3,5,2]oxadiazaborinines 6a–e, 11 are shown in Figure 10. The
spectroscopic characteristics of dye solutions in toluene are summarized in Table 1 (the
corresponding data for THF, DCM, acetone and acetonitrile solutions are given in Table S7 in
the Supporting Information). UV/Vis spectra show broad absorption bands with maxima well
above 400 nm.
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Most of the synthesized complexes demonstrate one characteristic absorption peak,
corresponding to the S0→S1 transition, with the maximum absorption (λabs) at 421–426 nm for
6a,c; 426–431 nm for 6b,d,e; and 431–434 nm for 11, and show almost no variation with the
change of solvent polarity. The corresponding molar absorption coefficient (ε) values of the
solutions in toluene are in the range 62600–88400 Mꢀ1cmꢀ1, and deсrease for the solutions in
acetonitrile to 44500–61300 Mꢀ1cmꢀ1. The absorption spectrum of the solution of compound 6b
in toluene has the second blueꢀshifted absorption peak (λabs = 411 nm, ε = 82500 Mꢀ1cmꢀ1), which
is probably induced by vibrational transition. It disappeared in the spectra of the solutions in
polar solvents due to the interaction between dye 6b and solvent molecules.
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Emission spectra of the dilute solutions of complexes 6a–e, 11 typically have one similar band,
a little deformed for the toluene solution of 6b. This observation is in good agreement with the
corresponding absorption spectra. All the synthesized organoboron dyes demonstrate
solvatofluorochromism; their emission maxima (λem) are batochromically shifted with increasing
the solvent polarity from toluene to acetonitrile: 450→486 nm for 6a, 454→481 nm for 6b,
452→487 nm for 6c, 456→491 nm for 6d, 460→500 nm for 6e, 464→503 nm for 11.
The comparison of emission spectra of 6a–e, 11 clearly demonstrates the influence of the
electronic structure of the benzothiazole residue on the photophysical properties. The more
strongly electronꢀaccepting substituents attached to benzothiazole moiety, the more redꢀshifted
the emission band: λem (6a) < λem (6c) < λem (6d) < λem (6e) < λem (11) for the solutions in all
solvents used. Dye 6b with donor (OMe) substituent demonstrates slightly different behaviour.
The wavelengths of emission maxima of compounds 6a and 6b change with the solvent polarity:
in toluene λem (6a) < λem (6b), in DCM λem (6a) = λem (6b), but in THF, acetone, and acetonitrile
λem (6a) > λem (6b).
As a result of solvatofluorochromism is the increase of the Stokes shift (ꢁυ) of the investigated
dyes with the solvent polarity increasing. This observation indicates that fluorescence of
compounds 6a–e, 11 partially or fully originates from photoinduced ICT state.
Comparison of these results with the previously reported for unfused thiazole derivative 1 (λabs
= 402–407 nm, λem = 439–469 nm, ε = 49700–56600 Mꢀ1cmꢀ1 in different solvents)54 shows that
annulation of phenyl ring (complex 6a) results in batochromic shift of the absorption and
emission bands, and increase of molar absorption coefficient (λabs = 421–425 nm, λem = 450–486
nm, ε = 53800–62600 Mꢀ1cmꢀ1).
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Figure 10. Absorption (solid lines) and emission (dashed lines) spectra of the solutions of 6a–e,
11 in different solvents.
The solutions of compounds 6a–e in nonꢀpolar solvents showed high fluorescence quantum
yield (Ф = 0.84–0.93 in toluene, Table 1), while the solution of dye 11 exhibited a little less
efficient emission (Ф = 0.65 in toluene). The fluorescence quantum yields of all the synthesized
boron complexes in solutions decreased with the increasing solvent polarity (Table S7 in the
Supporting Information). The solutions in acetonitrile of the dyes with strong electron
withdrawing substituents at benzo[d]thiazole unit exhibited very low emission quantum yields
(Ф = 0.05 for 6e, and Ф = 0.02 for 11). The solvent polarity influence was reduced with the
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decrease of electron withdrawing strength of benzothiazole part (in acetonitrile Ф = 0.12; 0.13;
and 0.17 for 6d, 6c and 6a, respectively) and was considerably smaller in the case of complex
with electron donating group at benzothiazole unit (for dye 6b: Ф = 0.83, 0.80, 0.56, and 0.38 in
DCM, THF, acetone, and acetonitrile, respectively). This observation clearly indicates that ICT
character is less expressed in the case of compound 6b and increases with the incorporation of
electronꢀaccepting substituents into the benzothiazole moiety (6a < 6c < 6d < 6e < 11).
The lifetimes of excited state (τ) were measured for the toluene solutions of the investigated
dyes (Table 1). The values of lifetime of complexes 6a,c–e were observed in a narrow range
from 1.68 to 1.78 ns; while it was higher for complex 11 (2.00 ns), and lower for complex 6b
(1.59 ns).
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Table 1. Photophysical Data for the Solutions in Toluene of Complexes 6a–e, and 11
Compound
λabs, nm
421
ε, Mꢀ1cmꢀ1
62600
λem, nm
450
Ф
ꢁυ, cmꢀ1
1531
1392
1516
1489
1625
1650
τ, ns
1.74
1.59
1.68
1.71
1.78
2.00
0.84
0.88
0.85
0.91
0.93
0.65
6a
6b
6c
6d
6e
11
411/427 82500/88400
454
423
427
428
431
71700
73200
86000
84900
452
456
460
464
Electrochemical Properties
The redox behaviour of six synthesized boron complexes was investigated by cyclic
voltammetry. The reduction and oxidation scans were recorded at room temperature for
deoxygenated dichloromethane solutions using tetrabutylammonium hexafluorophosphate
(TBAPF6) as supporting electrolyte. The ionization potentials (IPs) and the electron affinities
(EAs) of compounds 6a–e, 11 are given in Table 2. The corresponding cyclic voltammograms
(Figures S7–S12) and the obtained onset reduction and onset oxidation potentials (Table S8) are
summarized in the Supporting Information.
The IP values of complexes 6a–e, 11 are observed in a narrow range from 5.12–5.18 eV. The
EA values are more dependent on acceptor substituents at the benzo[d]thiazole unit. Thus, the
incorporation of donor (methoxy) group has no influence on the value of electron affinity (2.40
eV for 6a and 2.41 eV for 6b), while the acceptor substituents definitely promote increasing of
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EA, according to electron withdrawing strength of the substituent: from 2.45 eV for 6c to 2.75
eV for 11.
In comparison to previous results for complex 1 (IP = 5.07 eV, EA = 2.29 eV),54 the fusion of
benzene ring to thiazolo[3,2ꢀc][1,3,5,2]oxadiazaborinine moiety enhances both the reduction and
oxidation potentials.
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Table 2. Electron Affinities and Ionization Potentials of Compounds 6a–e, and 11 in DCM
Compound
IP, eV
5.12
5.12
5.13
5.13
5.16
5.18
EA, eV
2.40
Eg, eV
2.72
2.71
2.68
2.65
2.59
2.43
6a
6b
6c
6d
6e
11
2.41
2.45
2.48
2.57
2.75
Quantum Chemical Calculations
For better understanding of the electronic and optical properties of the synthesized
organoboron complexes, we carried out molecular orbital calculations for these compounds on
the density functional theory (DFT) level using the Gaussian 09 software package.56 For
calculation of singlet transitions timeꢀdepended DFT (TDꢀDFT) computation were also
performed. All geometrical structures are optimized and molecular energy levels calculated at
the B3LYP functional and 6ꢀ31G* basic set with the inclusion of dichloromethane solvent effect
through the integralꢀequationꢀformalism polarizable continuum model (IEFPCM).
The calculated highest occupied molecular orbital (HOMO) and the lowest unoccupied
molecular orbital (LUMO) level energies (Figure 11) are in good agreement with the
electrochemically determined ionization potentials and the electron affinities (Table 2).
The ICT character of the D–A fluorophores depends on the spatial overlap between HOMO
and LUMO. The computation results show that the HOMOs of all synthesized boron complexes
are somewhat more localized on the the (N,Nꢀdimethylamino)phenyl unit, what is obvious due to
the strong electron donating effect of this group; whereas the LUMOs are delocalized along the
entire backbone (Figure 11). The HOMO density on the benzothiazole moiety, decreases
depending on the electronꢀwithdrawing strength of linked substitute: 6b > 6a > 6c > 6d > 6e >
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11. The LUMO density on this part of molecule clearly increases in the case of compounds with
electron acceptor substituents.
Based on this observation, we can conclude that the donor substituent (OMe) at the
benzothiazole unit causes the larger spatial overlap between HOMO and LUMO, while acceptors
contribute to the decrease of this overlapping. This is the reason of lower ICT properties of
complex 6b, what is consistent with its much higher fluorescence quantum yields in the polar
solvents in comparison with corresponding values for the rest of the complexes.
TDꢀDFT calculation showed that the excitation of all the investigated benzothiazole boron
complexes are preferable going to the first singlet excited state (S0→S1, oscillator strength values
are higher than 1.00), and theoretical absorption maxima (Table S9 in the Supporting
Information) are in very good agreement with the corresponding experimentally obtained ones
(Tables S7).
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Figure 11. Frontier molecular orbitals (HOMO and LUMO) of complexes 6a–e, and 11.
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Fluorescence Properties of Solid Samples
Fluorescence properties of the synthesized benzo[4,5]thiazolo[3,2ꢀc][1,3,5,2]oxadiazaborinines
were also investigated in the solid state using their thin films. The normalized emission spectra
are shown in Figure 12 (the corresponding data are given in Table S10 in the Supporting
Information). The solid samples of all the studied dyes exhibited batochromically shifted
emission maxima, comparing to the corresponding data in the solutions. Most of the synthesized
complexes demonstrated small solid state fluorescence quantum yields (Ф = 0.02–0.06) and
relatively short lifetimes of excited states (τ1 = 0.66–1.04 ns; τ2 = 2.62–5.90 ns). These low solid
state emissions can apparently be explained by strong aggregation quenching effect, caused by
molecular packing of complexes 6a–e (Figures 4–8).
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The exception is compound 11, which exhibited solid state emission with enhanced
fluorescence quantum yield (Ф = 0.34) and elongated excitedꢀstate lifetimes (τ1 = 1.77 ns; τ2 =
7.56 ns). The relatively higher quantum yield of the thin film of complex 11 should be caused by
specific intermolecular interaction in the molecular packing (Figure 9). In comparison to other
studied benzo[4,5]thiazolo[3,2ꢀc][1,3,5,2]oxadiazaborinines, complex 11 in the solid state forms
stacking columns, where the molecules are antiparallel oriented with strong donorꢀacceptor π–
π/n–π interaction. Additionally, all fluorine atoms from the tetrafluorobenzo[d]thiazole moiety
take part in hydrogen bonding, what provide to decreasing of their acceptor strength. It causes
the prolonged excitedꢀstate lifetime, which in turn induces the higher fluorescence quantum
yield, compared to complexes 6a–e.
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Figure 12. Overlaid normalized emission spectra of solid nonꢀdoped samples of 6a–e and 11.
Conclusions
In conclusion, we developed the shortꢀsteps synthesis of the family of benzo[4,5]thiazolo[3,2ꢀ
c][1,3,5,2]oxadiazaborinines conjugated with donor 4ꢀdimethylaminophenyl group, starting from
commercially available reagents. The structural and photophysical properties of the obtained
organoboron complexes have been investigated. All the synthesized oxadiazaborinines exhibited
high fluorescence quantum yields in nonpolar solvents, which decreased in the polar medium due
to the ICT effect. The incorporation of donor substituent into the benzothiazole moiety caused
the ICT reducing and the emission increasing of the corresponding complex in polar solvents.
The steric interaction between two fluorines from BF2 group and αꢀfluorine atom of
benzothiazole moiety in compound with perfluorinated benzothiazole unit (11), confirmed by Xꢀ
ray analysis, caused nonbonded nuclear spinꢀspin coupling (J = 30.3 Hz) in the corresponding
19F NMR spectrum. Additionally, due to expanded hydrogen bonding capability, dye 11
demonstrated different than complexes 6a–e molecular packing, causing comparatively higher
solid state fluorescence quantum yield (Ф = 0.34).
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Experimental Section
General
7
All chemicals [including benzo[d]thiazolꢀ2ꢀamine (2a), pꢀanisidine (3b), 4ꢀfluoroaniline (3c),
4ꢀchloroaniline (3d), 4ꢀ(trifluoromethyl)aniline (3e), (paraꢀdimethylamino)benzoyl chloride (4),
2,3,4,5,6ꢀpentafluoroaniline (7)] were received from commercial sources (TCI, Aldrich, Alfa
Aesar, or Acros Organics) and used without further purification. Melting points of all
synthesized compounds were measured on Automatic Melting Point System (OptiMelt, Stanford
Research Systems). Column chromatography was performed on silica gel (Merck, 230ꢀ400 mesh).
The NMR spectra were recorded with Bruker Avance II 400 MHz (at 400 MHz, 100 MHz, and
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1
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375 MHz for H, C, and F NMR spectra, respectively), Varian VNMRS 500 MHz (at 500
1
MHz, 125 MHz, and 470 MHz for H, 13C, and 19F NMR spectra, respectively) or Varian
VNMRS 600 MHz (at 150 MHz for 13C NMR spectra) spectrometers for solutions in DMSOꢀd6 or
CDCl3 and TMS as the internal standard. Infrared (IR) spectra were recorded in KBr pallets with a
Jasco FT/IRꢀ6200 spectrometer in the 4000–400 cmꢀ1 region. Mass spectra were measured with
Synapt G2ꢀS HDMS (Waters Inc) mass spectrometer equipped with an electrospray ion source
and qꢀTOF type mass analyzer.
UVꢀvis absorption spectra of ca. 10ꢀ5 M solutions of complexes were recorded in quartz cells
using Perkin Elmer Lambda 35 spectrometer. Emission spectra of ca. 10ꢀ5 M solutions and the
solid state samples of complexes were recorded using Edinburgh Instruments' FLS980
Fluorescence Spectrometer (λex = 375 nm). Thin solid films were prepared by using spinꢀcoating
technique utilizing SPSꢀEurope Spin150 Spin processor using 2.5 mg/ml solutions of the
compounds in DCM on the preꢀcleaned quartz substrates. Fluorescence quantum yields of the
samples were estimated using the integrating sphere method. An integrating sphere (Edinburgh
Instruments) coupled to the FLS980 spectrometer was calibrated with two standards: quinine
sulfate in 0.1 M H2SO4 and rhodamine 6G in ethanol. Fluorescence decay curves of the solutions
and of the solid films were recorded using a timeꢀcorrelated single photon counting technique
utilizing the PicoQuant PDL 820 picosecond pulsed diode laser as an excitation source (λex = 375
nm). Electrochemical experiments were carried out using mAUTOLAB Type III apparatus,
glassy carbon, silver wire and platinum coil as working, reference and auxiliary electrode,
respectively and the scan rate at 100mV/s. Ferrocene was used as the internal standard. Cyclic
voltammetry measurements were conducted using deaerated 0.1M DCM solutions of complexes
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in a standard oneꢀcompartment cell, under argon atmosphere with 0.1M Bu4NPF6 as the
supporting electrolyte. Ionization potentials and electron affinities were calculated by the
onset
onset
onset
equations IP = Eox
+ 4.4 and EA = Ered
+ 4.4, where Eox
is the onset oxidation
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onset
potential, and Ered
is the onset reduction potential (Table 1 and Table S8 in the Supporting
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Information).
Crystals of compounds 6a–e, and 11 were obtained by the slow evaporation of their solution in
hexanes/DCM (1:1). The Xꢀray measurements were carried out at 100 K on SuperNova Agilent
diffractometer using CuKα (λ = 1.54184 Å) radiation. Data reduction was done with CrysAlisPro
(Agilent Technologies, Version 1.171.35.21b). The structures were solved by direct methods and
refined using SHELXL Software Package.57 Crystallographic data of all investigated complexes
have been deposited with the Cambridge Crystallographic Data Centre (CCDC). These data can
be obtained, free of charge, from CCDC [eꢀmail: deposit@ccdc.cam.ac.uk or fax: +44(0)ꢀ1223ꢀ
336033].
Synthesis
Synthesis of 2ꢀaminobenzothiazoles 2b–e (General procedure A). Compounds 2b–e were
prepared using a modified literature procedure.58 A mixture of aniline 3bꢀe (6–9 mmol, 1 eq.)
and ammonium thiocyanate (12–18 mmol, 2 eq.) in glacial acetic acid (20 mL) was cooled to
10 °C and stirred. A solution of bromine (6–9 mmol, 1 eq.) in glacial acetic acid (3 mL) was
added dropwise at such a rate to keep the temperature below 10 °C throughout the addition.
Stirring was continued for an additional 12 h, and then the reaction mixture was poured into cold
water and an aqueous solution of ammonium hydroxide (25%) was added to reach pH ~ 9. The
precipitate was filtered, washed with water, dried, and recrystallized from ethanol.
6ꢀMethoxybenzo[d]thiazolꢀ2ꢀamine (2b) was obtained in 75% yield (1.10 g) using general
procedure A from pꢀanisidine (3b, 1.00 g, 8.12 mmol), ammonium thiocyanate (1.24 g,
16.24 mmol) and bromine (419 ꢂL, 8.12 mmol). Mp. 162.6–163.7 °C (lit.59 158−159 °C), black
powder. 1H NMR (400 MHz, DMSOꢀd6): δ = 7.28 (1H, d, J = 2.6 Hz, ArꢀH), 7.23 (1H, d, J = 8.7
Hz, ArꢀH), 7.19 (2H, s, NH2), 6.80 (1H, dd, J = 8.7 Hz, J = 2.6 Hz, ArꢀH), 3.73 (3H, s, OMe)
ppm; 13C NMR (100 MHz, DMSOꢀd6): δ = 164.7, 154.3, 146.8, 131.9, 118.0, 112.8, 105.5, 55.5
ppm. IR: 3386 (NꢀH), 3087 (NꢀH) cmꢀ1. HRMS (ESIꢀTOF) calcd for C8H9N2OS [M + H]+:
181.0436, found: 181.0429.
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6ꢀFluorobenzo[d]thiazolꢀ2ꢀamine (2c) was obtained in 71% yield (1.08 g) using general
procedure A from 4ꢀfluoroaniline (3c, 1.00 g, 9.00 mmol), ammonium thiocyanate (1.37 g,
18.00 mmol) and bromine (464 ꢂL, 9.00 mmol). Mp. 182.3–183.0 °C (lit.59 183−184 °C),
yellowish powder. 1H NMR (400 MHz, DMSOꢀd6): δ = 7.56 (1H, dd, JHꢀF = 8.7 Hz, J = 2.6 Hz,
ArꢀH), 7.44 (2H, s, NH2), 7.30 (1H, dd, J = 8.8 Hz, JHꢀF = 4.9 Hz, ArꢀH), 7.02 (1H, ddd, J = 8.8
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Hz, JHꢀF = 9.0 Hz, J = 2.6 Hz, ArꢀH) ppm; 13C NMR (100 MHz, DMSOꢀd6): δ = 166.3 (d, JCꢀF
=
1.7 Hz), 157.1 (d, JCꢀF = 236.2 Hz), 149.4 (d, JCꢀF = 1.3 Hz), 131.9 (d, JCꢀF = 11.1 Hz), 118.1 (d,
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JCꢀF = 8.8 Hz), 112.7 (d, JCꢀF = 23.6 Hz), 107.7 (d, JCꢀF = 27.1 Hz) ppm; F NMR (375 MHz,
DMSOꢀd6): δ = –122.41 ppm. IR: 3385 (NꢀH), 3073 (NꢀH) cmꢀ1. HRMS (ESIꢀTOF) calcd for
C7H6N2FS [M + H]+: 169.0236, found: 169.0237.
6ꢀChlorobenzo[d]thiazolꢀ2ꢀamine (2d) was obtained in 91% yield (1.32 g) using general
procedure A from 4ꢀchloroaniline (3d, 1.00 g, 7.84 mmol), ammonium thiocyanate (1.19 g,
15.68 mmol) and bromine (404 ꢂL, 7.84 mmol). Mp. 196.8–198.0 °C (lit.59 200−201 °C),
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yellowish powder. H NMR (400 MHz, DMSOꢀd6): δ = 7.76 (1H, s, ArꢀH), 7.58 (2H, s, NH2),
13
7.30 (1H, d, J = 8.4 Hz, ArꢀH), 7.21 (1H, d, J = 8.4 Hz, ArꢀH) ppm; C NMR (100 MHz,
DMSOꢀd6): δ = 167.1, 151.7, 132.5, 125.5, 124.5, 120.5, 118.5 ppm. IR: 3456 (NꢀH), 3091 (Nꢀ
H) cmꢀ1. HRMS (ESIꢀTOF) calcd for C7H6ClN2S [M + H]+: 184.9934, found: 184.9942.
6ꢀ(Trifluoromethyl)benzo[d]thiazolꢀ2ꢀamine (2e) was obtained in 90% yield (1.19 g) using
general procedure A from 4ꢀ(trifluoromethyl)aniline (3e, 1.00 g, 6.21 mmol), ammonium
thiocyanate (0.95 g, 12.42 mmol) and bromine (320 ꢂL, 6.21 mmol). Mp. 113.7–115.5 °C (lit.59
1
109−110 °C), yellowish powder. H NMR (400 MHz, DMSOꢀd6): δ = 8.09 (1H, d, J = 1.6 Hz,
ArꢀH), 7.86 (2H, s, NH2), 7.50 (1H, dd, J = 8.4 Hz, J = 1.6 Hz, ArꢀH), 7.44 (1H, d, J = 8.4 Hz,
13
ArꢀH) ppm; C NMR (100 MHz, DMSOꢀd6): δ = 169.3, 155.8, 131.5, 124.8 (q, J = 271.3 Hz,
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CF3), 122.5 (q, J = 3.7 Hz), 121.0 (q, J = 31.8 Hz), 118.5 (q, J = 4.1 Hz), 117.5 ppm; F NMR
(375 MHz, DMSOꢀd6): δ = –59.22 (3F, s, CF3) ppm. IR: 3493 (NꢀH), 3107 (NꢀH) cmꢀ1. HRMS
(ESIꢀTOF) calcd for C8H6N2F3S [M + H]+: 219.0204, found: 219.0197.
Synthesis of ligands 5a–e (General procedure B). To a solution of amine 2a–e, or 9 (0.50–
0.70 mmol, 1 eq.) in 1,4ꢀdioxane (20 mL) were added 4ꢀ(dimethylamino)benzoyl chloride (4,
0.50–0.70 mmol, 1 eq.), distillated trimethylamine (1.50–2.10 mmol, 3 eq.) and DMAP (0.025–
0.075 mmol, 0.05 eq.). The reaction mixture was refluxed for 24h. After cooling, a saturated
aqueous solution (50 mL) of NaHCO3 were added and the mixture was extracted with DCM (3 ×
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40 mL). The combined organic layers were dried over Na2SO4, filtered and concentrated. The
product was purified by column chromatography (hexanes/DCM = 1:1 to 0:1, and next,
DCM/MeOH = 100:0 to 99:1, v/v).
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Nꢀ(Benzo[d]thiazolꢀ2ꢀyl)ꢀ4ꢀ(dimethylamino)benzamide (5a) was obtained in 61% yield (123
mg) using general procedure B from benzo[d]thiazolꢀ2ꢀamine (2a, 102 mg, 0.68 mmol),
compound 4 (125 mg, 0.68 mmol), trimethylamine (284 ꢂL, 2.04 mmol) and DMAP (4 mg,
0.03 mmol). Mp. 211.7–213.6 °C, yellowish powder. 1H NMR (400 MHz, DMSOꢀd6): δ = 12.40
(1H, s, NH), 8.07 (2H, d, J = 8.9 Hz, ArꢀH), 7.97 (1H, d, J = 7.8 Hz, ArꢀH), 7.75 (1H, d, J = 8.0
Hz, ArꢀH), 7.44 (1H, dd, J = 8.0 Hz, J = 7.2 Hz, ArꢀH), 7.30 (1H, dd, J = 7.8 Hz, J = 7.2 Hz, Arꢀ
H), 6.77 (2H, d, J = 8.9 Hz, ArꢀH), 3.02 (6H, s, NMe2) ppm; 13C NMR (100 MHz, DMSOꢀd6): δ
= 165.2, 159.2, 153.1, 148.6, 131.6, 129.9 (2C), 125.9, 123.3, 121.5, 120.1, 117.5, 110.8 (2C),
39.6 (2C) ppm. IR: 2895 (NꢀH), 1661 (C=O) cmꢀ1. HRMS (ESIꢀTOF) calcd for C16H15N3OSNa
[M + Na]+: 320.0834, found: 320.0829.
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4ꢀ(Dimethylamino)ꢀNꢀ(6ꢀmethoxybenzo[d]thiazolꢀ2ꢀyl)benzamide (5b) was obtained in 60%
yield (121 mg) using general procedure B from 6ꢀmethoxybenzo[d]thiazolꢀ2ꢀamine (2b, 110 mg,
0.61 mmol), compound 4 (112 mg, 0.61 mmol), trimethylamine (255 ꢂL, 1.83 mmol) and
DMAP (4 mg, 0.03 mmol). Mp. 203.5–204.7 °C, yellow powder. 1H NMR (400 MHz, CDCl3): δ
= 11.14 (1H, s, NH), 7.87 (2H, d, J = 9.0 Hz, ArꢀH), 7.31 (1H, d, J = 8.8 Hz, ArꢀH), 7.29 (1H, d,
J = 2.3 Hz, ArꢀH), 6.88 (1H, dd, J = 8.8 Hz, J = 2.3 Hz, ArꢀH), 6.58 (2H, d, J = 9.0 Hz, ArꢀH),
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3.85 (3H, s, OMe), 2.99 (6H, s, NMe2) ppm; C NMR (100 MHz, CDCl3): δ = 165.2, 158.2,
156.5, 153.3, 142.4, 133.3, 129.6 (2C), 121.3, 118.1, 114.8, 111.1 (2C), 104.1, 55.8, 40.0 (2C)
ppm. IR: 3170 (NꢀH), 1663 (C=O) cmꢀ1. HRMS (ESIꢀTOF) calcd for C17H17N3O2SNa [M +
Na]+: 350.0939, found: 350.0933.
4ꢀ(Dimethylamino)ꢀNꢀ(6ꢀfluorobenzo[d]thiazolꢀ2ꢀyl)benzamide (5c) was obtained in 66%
yield (125 mg) using general procedure B from 6ꢀfluorobenzo[d]thiazolꢀ2ꢀamine (2c, 101 mg,
0.60 mmol), compound 4 (110 mg, 0.60 mmol), trimethylamine (251 ꢂL, 1.80 mmol) and
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DMAP (4 mg, 0.03 mmol). Mp. 236.5–237.8 °C, yellowish powder. H NMR (400 MHz,
DMSOꢀd6): δ = 12.43 (1H, s, NH), 8.04 (2H, J = 9.0 Hz, ArꢀH), 7.87 (1H, dd, JHꢀF = 8.7 Hz, J =
2.7 Hz, ArꢀH), 7.74 (1H, dd, J = 8.9 Hz, JHꢀF = 4.8 Hz, ArꢀH), 7.27 (1H, ddd, J = 8.9 Hz, JHꢀF
=
13
9.0 Hz, J = 2.7 Hz, ArꢀH), 6.76 (2H, J = 9.0 Hz, ArꢀH), 3.01 (6H, s, 2 × CH3) ppm; C NMR
(100 MHz, DMSOꢀd6): δ = 165.2, 159.2, 158.6 (d, JCꢀF = 239.6 Hz), 153.1, 145.4, 132.9 (d, JCꢀF
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= 11.1 Hz), 130.0 (2C), 121.1 (d, JCꢀF = 9.3 Hz), 117.4, 114.0 (d, JCꢀF = 24.5 Hz), 110.8 (2C),
107.9 (d, JCꢀF = 27.8 Hz), 39.6 (2C) ppm; 19F NMR (375 MHz, DMSOꢀd6): δ = –118.77 ppm. IR:
3427 (NꢀH), 1656 (C=O) cmꢀ1. HRMS (ESIꢀTOF) calcd for C16H14N3OFSNa [M + Na]+:
338.0739, found: 338.0743.
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Nꢀ(6ꢀChlorobenzo[d]thiazolꢀ2ꢀyl)ꢀ4ꢀ(dimethylamino)benzamide (5d) was obtained in 57%
yield (109 mg) using general procedure B from 6ꢀchlorobenzo[d]thiazolꢀ2ꢀamine (2d, 106 mg,
0.57 mmol), compound 4 (105 mg, 0.57 mmol), trimethylamine (240 ꢂL, 1.72 mmol) and
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DMAP (4 mg, 0.03 mmol). Mp. 219.8–221.0 °C, yellowish powder. H NMR (400 MHz,
CDCl3): δ = 11.34 (1H, s, NH), 7.84 (2H, d, J = 9.0 Hz, ArꢀH), 7.77 (1H, d, J = 2.0 Hz, ArꢀH),
7.28 (1H, d, J = 8.6 Hz, ArꢀH), 7.20 (1H, dd, J = 8.6 Hz, J = 2.0 Hz, ArꢀH), 6.56 (2H, d, J = 9.0
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Hz, ArꢀH), 3.00 (6H, s, NMe2) ppm; C NMR (100 MHz, CDCl3): δ = 165.4, 160.6, 153.4,
146.7, 133.4, 129.7 (2C), 128.9, 126.4, 121.4, 120.8, 117.7, 111.1 (2C), 39.9 (2C) ppm. IR: 3156
(NꢀH), 1660 (C=O) cmꢀ1. HRMS (ESIꢀTOF) calcd for C16H15N3OSCl [M + H]+: 332.0624,
found: 332.0616.
4ꢀ(Dimethylamino)ꢀNꢀ(6ꢀ(trifluoromethyl)benzo[d]thiazolꢀ2ꢀyl)benzamide (5e) was obtained
in 64% yield (118 mg) using general procedure B from 6ꢀ(trifluoromethyl)benzo[d]thiazolꢀ2ꢀ
amine (2e, 110 mg, 0.50 mmol), compound 4 (93 mg, 0.50 mmol), trimethylamine (210 ꢂL,
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1.50 mmol) and DMAP (4 mg, 0.03 mmol). Mp. 223.2–225.0 °C, white powder. H NMR (400
MHz, DMSOꢀd6): δ = 12.64 (1H, s, NH), 8.46 (1H, br s, ArꢀH), 8.06 (2H, d, J = 9.0 Hz, ArꢀH),
7.89 (1H, d, J = 8.4 Hz, ArꢀH), 7.72 (1H, dd, J = 8.4 Hz, J = 1.5 Hz, ArꢀH), 6.76 (2H, d, J = 9.0
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Hz, ArꢀH), 3.02 (6H, s, NMe2) ppm; C NMR (100 MHz, DMSOꢀd6): δ = 165.3, 162.4, 153.2,
151.5, 132.2, 130.1 (2C), 124.6 (q, J = 271.8 Hz, CF3), 123.4 (q, J = 31.8 Hz), 122.7 (q, J = 3.6
Hz), 120.4, 119.6 (q, J = 4.1 Hz), 117.1, 110.8 (2C), 39.5 (2C) ppm. 19F NMR (375 MHz,
DMSOꢀd6): δ = –59.40 (3F, s, CF3) ppm. IR: 3175 (NꢀH), 1675 (C=O) cmꢀ1. HRMS (ESIꢀTOF)
calcd for C17H14N3OF3SNa [M + Na]+: 388.0707, found: 388.0706.
Synthesis of complexes 6a–e, 11 (General procedure C). To a solution of compound 5a–e, or
10 (0.19–0.29 mmol, 1 eq.) in dry DCM (20 mL) were added BF3·Et2O (10 eq.) and distillated
N,Nꢀdiisopropylethylamine (20 eq.) under an argon atmosphere. The reaction mixture was stirred
for 24h at room temperature and then washed with water. The organic phase was dried over
anhydrous Na2SO4, and filtered. The solvent was removed under reduced pressure and the crude
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product was purified by column chromatography (hexanes/dichloromethane from 1:1 to 0:1,
v/v).
4ꢀ(1,1ꢀDifluoroꢀ1Hꢀ1λ4,10λ4ꢀbenzo[4,5]thiazolo[3,2ꢀc][1,3,5,2]oxadiazaborininꢀ3ꢀyl)ꢀN,Nꢀ
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dimethylaniline (6a) was obtained in 66% yield (65 mg) using general procedure C from ligand
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5a (85 mg, 0.29 mmol). Mp. 262.8–265.0 °C, yellow powder. H NMR (600 MHz, CDCl3): δ =
8.26 (2H, d, J = 9.2 Hz, ArꢀH), 7.98 (1H, d, J = 8.1 Hz, ArꢀH), 7.73 (1H, d, J = 7.6 Hz, ArꢀH),
7.54 (1H, ddd, J = 7.6 Hz, J = 7.8 Hz, J = 1.1 Hz, ArꢀH), 7.41 (1H, ddd, J = 7.6 Hz, J = 7.7 Hz, J
= 1.0 Hz, ArꢀH), 6.70 (2H, d, J = 9.2 Hz, ArꢀH), 3.12 (6H, s, NMe2) ppm; 13C NMR (150 MHz,
CDCl3): δ = 173.8, 168.4, 154.6, 140.4, 133.0 (2C), 127.9, 126.6, 125.6, 121.9, 118.0, 117.0,
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111.0 (2C), 40.1 (2C) ppm; F NMR (470 MHz, CDCl3): δ = –137.53 (2F, m, BF2) ppm. IR:
1617 (C=N) cmꢀ1. HRMS (ESIꢀTOF) calcd for C16H14BN3OF2SNa [M + Na]+: 368.0816, found:
368.0812.
4ꢀ(1,1ꢀDifluoroꢀ7ꢀmethoxyꢀ1Hꢀ1λ4,10λ4ꢀbenzo[4,5]thiazolo[3,2ꢀc][1,3,5,2]oxadiazaborininꢀ3ꢀ
yl)ꢀN,Nꢀdimethylaniline (6b) was obtained in 61% yield (56 mg) using general procedure C
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from ligand 5b (80 mg, 0.24 mmol). Mp. 275.1–277.5 °C, yellow powder. H NMR (500 MHz,
CDCl3): δ = 8.24 (2H, d, J = 9.2 Hz, ArꢀH), 7.87 (1H, d, J = 9.0 Hz, ArꢀH), 7.20 (1H, d, J = 2.5
Hz, ArꢀH), 7.12 (1H, dd, J = 9.0 Hz, J = 2.5 Hz, ArꢀH), 6.69 (2H, d, J = 9.2 Hz, ArꢀH), 3.88 (3H,
s, OCH3), 3.12 (6H, s, NMe2) ppm; 13C NMR (125 MHz, CDCl3): δ = 172.4, 167.8, 158.0, 154.4,
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134.3, 132.7 (2C), 128.0, 118.7, 117.2, 116.2, 111.0 (2C), 105.4, 55.9, 40.1 (2C) ppm; F NMR
(470 MHz, CDCl3): δ = –137.71 (2F, m, BF2) ppm. IR: 1608 (C=N) cmꢀ1. HRMS (ESIꢀTOF)
calcd for C17H16BN3O2F2SNa [M + Na]+: 398.0922, found: 398.0915.
N,NꢀDimethylꢀ4ꢀ(1,1,7ꢀtrifluoroꢀ1Hꢀ1λ4,10λ4ꢀbenzo[4,5]thiazolo[3,2ꢀ
c][1,3,5,2]oxadiazaborininꢀ3ꢀyl)aniline (6c) was obtained in 67% yield (64 mg) using general
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procedure C from ligand 5c (82 mg, 0.26 mmol). Mp. 284.9–287.2 °C, yellow powder. H NMR
(500 MHz, CDCl3): δ = 8.24 (2H, d, J = 9.0 Hz, ArꢀH), 7.93 (1H, dd, J = 9.0 Hz, JHꢀF = 4.4 Hz,
ArꢀH), 7.44 (1H, dd, JHꢀF = 7.7 Hz, J = 2.5 Hz, ArꢀH), 7.27 (1H, ddd, J = 9.0 Hz, JHꢀF = 9.1 Hz, J
= 2.5 Hz, ArꢀH), 6.69 (2H, d, J = 9.0 Hz, ArꢀH), 3.19 (6H, s, NMe2) ppm; 13C NMR (125 MHz,
CDCl3): δ = 173.7, 168.4, 160.3 (d, JCꢀF = 247.4 Hz), 154.7, 136.8, 133.1 (2C), 127.7 (d, JCꢀF
=
=
10.2 Hz), 119.1 (d, JCꢀF = 8.8 Hz), 116.8, 116.2 (d, JCꢀF = 24.4 Hz), 111.0 (2C), 108.8 (d, JCꢀF
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27.4 Hz), 40.1 (2C) ppm; F NMR (470 MHz, CDCl3): δ = –114.21 (1F, m, CꢀF), –137.65 (2F,
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m, BF2) ppm. IR: 1607 (C=N) cmꢀ1. HRMS (ESIꢀTOF) calcd for C16H13BN3OF3SNa [M + Na]+:
386.0722, found: 386.0723.
4ꢀ(7ꢀChloroꢀ1,1ꢀdifluoroꢀ1Hꢀ1λ4,10λ4ꢀbenzo[4,5]thiazolo[3,2ꢀc][1,3,5,2]oxadiazaborininꢀ3ꢀ
yl)ꢀN,Nꢀdimethylaniline (6d) was obtained in 64% yield (59 mg) using general procedure C
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from ligand 5d (81 mg, 0.24 mmol). Mp. 274.4–276.0 °C, yellow powder. H NMR (600 MHz,
CDCl3): δ = 8.24 (2H, d, J = 9.2 Hz, ArꢀH), 7.88 (1H, d, J = 8.8 Hz, ArꢀH), 7.71 (1H, d, J = 2.0
Hz, ArꢀH), 7.50 (1H, dd, J = 8.8 Hz, J = 2.0 Hz, ArꢀH), 6.69 (2H, d, J = 9.2 Hz, ArꢀH), 3.13 (6H,
s, NMe2) ppm; 13C NMR (150 MHz, CDCl3): δ = 173.8, 168.6, 154.8, 138.9, 133.2 (2C), 131.4,
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128.5, 127.8, 121.7, 118.7, 116.7, 111.0 (2C), 40.1 (2C) ppm; F NMR (470 MHz, CDCl3): δ =
–137.56 (2F, m, BF2) ppm. IR: 1610 (C=N) cmꢀ1. HRMS (ESIꢀTOF) calcd for
C16H14BN3OF2SCl [M + H]+: 380.0607, found: 380.0596.
4ꢀ(1,1ꢀDifluoroꢀ7ꢀ(trifluoromethyl)ꢀ1Hꢀ1λ4,10λ4ꢀbenzo[4,5]thiazolo[3,2ꢀ
c][1,3,5,2]oxadiazaborininꢀ3ꢀyl)ꢀN,Nꢀdimethylaniline (6e) was obtained in 68% yield (65 mg)
using general procedure C from ligand 5e (85 mg, 0.23 mmol). Mp. 275.8–278.1 °C, yellow
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powder. H NMR (500 MHz, CDCl3): δ = 8.27 (2H, d, J = 9.3 Hz, ArꢀH), 8.05 (1H, d, J = 9.6
Hz, ArꢀH), 8.01 (1H, s, ArꢀH), 7.78 (1H, d, J = 9.6 Hz, ArꢀH), 6.70 (2H, d, J = 9.3 Hz, ArꢀH),
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3.14 (6H, s, NMe2) ppm; C NMR (125 MHz, CDCl3): δ = 175.0, 169.0, 155.0, 142.7, 133.5
(2C), 127.9 (q, J = 33.3 Hz), 126.8, 125.0 (q, J = 3.6 Hz), 123.7 (q, J = 272.5 Hz, CF3), 119.5 (q,
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J = 4.2 Hz), 118.0, 116.5, 111.1 (2C), 40.1 (2C) ppm; F NMR (470 MHz, CDCl3): δ = –61.83
(3F, s, CF3), –137.44 (2F, m, BF2) ppm. IR: 1609 (C=N) cmꢀ1. HRMS (ESIꢀTOF) calcd for
C17H14BN3OF5S [M + H]+: 414.0871, found: 414.0869.
1ꢀ(Perfluorophenyl)thiourea (8b). Benzoyl chloride (0.73 mL, 6.30 mmol) and potassium
thiocyanate (0.64 g, 6.61 mmol) were dissolved in dry acetone (10 mL) and the mixture was
refluxed for 1h. 2,3,4,5,6ꢀPentafluoroaniline (7, 1.10 g, 6.01 mmol) was added and the refluxing
was continued for 1h more. After cooling, the solvent was evaporated in vacuo, and then water
(30 mL) was added. The mixture was stirred for 10 min and the precipitate of Nꢀ
[(perfluorophenyl)carbamothioyl]benzamide (8a) was filtered and added to an aqueous solution
of NaOH (10%, 12 mL). The resulting reaction mixture was stirred at 80 °C for 2h. The reaction
mixture was cooled, an aqueous solution of HCl (1M) was added to reach pH ~ 3. Then, an
aqueous solution of NH3 (25%) was added to reach pH ~ 9 to remove benzoic acid from the
solution. The mixture was stirred at 5 °C for 30 min. The precipitate was filtered, washed with
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water, and recrystallized from EtOHꢀH2O (1:1) to obtain the product 8b as a white crystalline
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solid (1.09 g, 75%). Mp. 156.4–157.8 °C, white solid. H NMR (500 MHz, DMSOꢀd6): δ = 9.36
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(1H, s, NH), 8.27 (1H, br s, NH), 7.52 (1H, br s, NH) ppm; C {F} NMR (125 MHz, DMSOꢀ
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d6): δ = 183.4, 144.0 (2C), 139.7, 137.3 (2C), 114.7 ppm; 19F NMR (470 MHz, DMSOꢀd6): δ = –
145.43 (2F, d, J = 20.9 Hz), –157.53 (1F, t, J = 22.8 Hz), –164.11 (2F, br s) ppm. IR: 3349 (Nꢀ
H), 3179 (NꢀH) cmꢀ1. HRMS (ESIꢀTOF) calcd for C7H4N2F5S [M + H]+: 243.0015, found:
243.0010.
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4,5,6,7ꢀTetrafluorobenzo[d]thiazolꢀ2ꢀamine (9). Sodium hydride (60% dispersion in mineral
oil, 139 mg, 3.47 mmol) was added to a solution of 1ꢀ(perfluorophenyl)thiourea (8b, 801 mg,
3.31 mmol) in DMF (5 mL). The reaction mixture was heated up to 80 °C for 4h under argon
atmosphere. After cooling, water (15 mL) was added. The precipitate was filtered, washed with
water, and recrystallized from EtOHꢀH2O (1:1) to obtain the product 9 as a white crystalline
1
solid (612 g, 83%). Mp. 200.9–202.3 °C, white solid. H NMR (500 MHz, DMSOꢀd6): δ = 8.17
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(1H, br s, NH2) ppm; C {F} NMR (125 MHz, DMSOꢀd6): δ = 168.0, 140.4, 138.6, 137.6,
137.1, 134.3, 113.9 ppm; 19F NMR (470 MHz, DMSOꢀd6): δ = –140.80 (1F, dd, J = 22.7 Hz, J =
12.7 Hz), –154.46 (1F, ddd, J = 20.5 Hz, J = 12.7 Hz, J = 3.0 Hz), –161.83 (1F, dd, J = 20.5 Hz,
J = 21.4 Hz), –168.02 (1F, ddd, J = 23.2 Hz, J = 21.9 Hz, J = 3.0 Hz) ppm. IR: 3284 (NꢀH), 3115
(NꢀH) cmꢀ1. HRMS (ESIꢀTOF) calcd for C7H3N2F4S [M + H]+: 222.9953, found: 222.9949.
4ꢀ(Dimethylamino)ꢀNꢀ(perfluorobenzo[d]thiazolꢀ2ꢀyl)benzamide (10) was obtained in 45%
yield (90 mg) using general procedure B from 4,5,6,7ꢀtetrafluorobenzo[d]thiazolꢀ2ꢀamine (9, 112
mg, 0.54 mmol), compound 4 (99 mg, 0.54 mmol), trimethylamine (226 ꢂL, 1.62 mmol) and
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DMAP (3 mg, 0.03 mmol). Mp. 196.9–198.7 °C, white powder. H NMR (500 MHz, DMSOꢀd6,
80 °C): δ = 12.76 (1H, s, NH), 8.04 (2H, d, J = 9.0 Hz, ArꢀH), 6.77 (2H, d, J = 9.0 Hz, ArꢀH),
3.04 (6H, s, NMe2) ppm; 13C NMR (125 MHz, DMSOꢀd6, 80 °C): δ = 165.0, 160.8, 153.3, 140.2
(CꢀF), 138.7 (CꢀF), 138.4 (CꢀF), 135.7 (CꢀF), 133.8, 129.8 (2C), 116.2, 115.6, 110.5 (2C), 39.1
19
(2C) ppm; F NMR (470 MHz, DMSOꢀd6, 80 °C): δ = –141.07 (1F, dd, J = 22.2 Hz, J = 14.5
Hz), –152.47 (1F, dd, J = 19.7 Hz, J = 14.7 Hz), –160.57 (1F, dd, J = 20.6 Hz, J = 19.7 Hz), –
163.47 (1F, dd, J = 22.2 Hz, J = 20.6 Hz) ppm. IR: 3126 (NꢀH), 1641 (C=O) cmꢀ1. HRMS (ESI)
calcd for C16H11N3OF4SNa [M + Na]+: 392.0457, found: 392.0450.
N,NꢀDimethylꢀ4ꢀ(perfluoroꢀ1Hꢀ1λ4,10λ4ꢀbenzo[4,5]thiazolo[3,2ꢀc][1,3,5,2]oxadiazaborininꢀ
3ꢀyl)aniline (11) was obtained in 68% yield (54 mg) using general procedure C from ligand 10
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The Journal of Organic Chemistry
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(70 mg, 0.19 mmol). Mp. 257.1–258.9°C, orange solid. H NMR (500 MHz, CDCl3): δ = 8.25
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(2H, d, J = 9.3 Hz, ArꢀH), 6.70 (2H, d, J = 9.3 Hz, ArꢀH), 3.15 (6H, s, NMe2) ppm; C NMR
(125 MHz, CDCl3): δ = 174.7, 169.0, 155.3, 141.3 (CꢀF), 141.2 (CꢀF), 138.3 (CꢀF), 137.3 (CꢀF),
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133.8 (2C), 125.9, 115.9, 111.1 (2C), 110.7, 40.2 (2C) ppm; F NMR (470 MHz, CDCl3): δ = –
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133.08 (2F, m, BF2), –139.70 (1F, dd, J = 21.0 Hz, J = 13.1 Hz, ArꢀF), –143.18 (1F, tdd, J = 30.3
Hz, J = 19.7 Hz, J = 13.1 Hz, ArꢀF), –153.51 (1F, dd, J = 20.1 Hz, J = 19.7 Hz, ArꢀF), –157.67
(1F, dd, J = 21.0 Hz, J = 20.1 Hz, ArꢀF) ppm. IR: 1607 (C=N) cmꢀ1. HRMS (ESIꢀTOF) calcd for
C16H11BN3OF6S [M + H]+: 418.0620, found: 418.0618.
Keywords: boron complex • benzo[d]thiazole • thiazolo[3,2ꢀc][1,3,5,2]oxadiazaborinine •
fluorescence • dual emission
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge via the Internet at http://pubs.acs.org at
DOI:
ORTEP diagrams and crystal data of complexes 6a–e, 11; photophysical properties of complexes
6a–e, 11; onset reduction/oxidation potentials and cyclic voltammograms of dyes 6a–e, 11; DFT
and TDꢀDFT data; fluorescence decays; NMR and IR spectra (PDF)
Crystal data of 6a (CIF)
Crystal data of 6b (CIF)
Crystal data of 6c (CIF)
Crystal data of 6d (CIF)
Crystal data of 6e (CIF)
Crystal data of 11 (CIF)
AUTHOR INFORMATION
Corresponding Author
*Eꢀmail: potopnyk@gmail.com
ORCID
Mykhaylo A. Potopnyk: 0000ꢀ0002ꢀ4543ꢀ2785
Dmytro Volyniuk: 0000ꢀ0003ꢀ3526ꢀ2679
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