Carbazole-modified thiazolo[3,2-: C] [1,3,5,2]oxadiazaborinines exhibiting aggregation-induced emission and mechanofluorochromism

Article abstract of DOI:10.1039/d0ob02225j

Two highly emissive carbazole-containing thiazole-fused oxadiazaborinines were designed and synthesized. These N,O-chelated organoboron dyes displayed large Stokes shifts and remarkable solvatofluorochromism in solutions, as well as good thermal stability

Full text of DOI:10.1039/d0ob02225j

Organic &
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Carbazole-modified thiazolo[3,2-c][1,3,5,2]
oxadiazaborinines exhibiting aggregation-induced
emission and mechanofluorochromism†
Cite this: Org. Biomol. Chem., 2021,
19, 406
Mykhaylo A. Potopnyk,
Dmytro Volyniuk, b Volodymyr Sashuk c and Juozas Vidas Grazulevicius
*
a,b Mykola Kravets, c Roman Luboradzki,c
b
*
Two highly emissive carbazole-containing thiazole-fused oxadiazaborinines were designed and syn-
thesized. These N,O-chelated organoboron dyes displayed large Stokes shifts and remarkable solvatofluor-
ochromism in solutions, as well as good thermal stability and comparatively high photoluminescence
quantum yields (up to 34%) in the solid state. The presence of a carbazole donor unit, linked with the oxa-
diazaborinine acceptor via a phenyl linker, restricted intramolecular rotation, leading to enhanced aggrega-
tion-induced emission properties of the compounds: in THF/water mixtures with a large water percentage,
they demonstrated the formation of emissive nanoaggregates with an average size of 79 and 89 nm for
complexes 2 and 3, respectively. The introduction of bulky tert-butyl groups attached to the carbazole
moiety induced significant mechanofluorochromic properties of the compounds.
Received 8th November 2020,
Accepted 1st December 2020
DOI: 10.1039/d0ob02225j
rsc.li/obc
triphenylamine,2b–e,3a,d,9
carbazole,3e,4d,9c,10
9,10-
Introduction
dihydroacridine,4c,d phenoxazine,4d or phenothiazine,4a have
The development of organic compounds exhibiting aggregation- been utilized to enhance the AIE ability of fluorophores.
induced emission (AIE) has become a very active research field Among luminescent compounds, organoboron complexes are
in the bioimaging area, materials sciences and supramolecular especially valuable due to their advantages such as strong absorp-
chemistry.1 Numerous AIE-active luminogens (AIEgens) have tion bands in the UV-vis region, intensive emission, high PLQYs
been widely applied as photosensitisers in photodynamic in solutions, photochemical stability, etc.11 To date, the most
therapy,2 fluorescent probes,3 emitters in optoelectronic famous organoboron complexes have been boron-dipyrro-
devices,4 components of liquid crystalline materials,5 supramo- methenes (BODIPYs).11a–d However, due to the C2-symmetry of
lecular systems,6 fluorescent gels,7 etc. The most efficient strat- the BODIPY core, such dyes usually exhibit small values of Stokes
egy to achieve the AIE properties is the introduction of propel- shifts, which in turn lead to the low solid-state emissive ability.
ler-shaped or bulky functional groups to the fluorophore struc- This substantially restrains the elaboration of BODIPY-based
tures, which moderately restricts their intramolecular motion in AIEgens.9b,12 The effective strategies to design AIE-active organo-
the aggregated state. This in turn results in an increased photo- boron complexes include desymmetrisation of the fluorophore
luminescence quantum yield (PLQY) in the aggregated state scaffold.12,13 From this point of view, N,O-coordinated organo-
caused by the reduction of nonradiative energy dissipation.1 boron dyes, because of their unsymmetrical nature, look very
Currently, the most well-known AIEgens are tetraphenylethylene promising.3a,9a,13a–h However, to date, it remains challenging to
derivatives.1,2a,c,3b,c,4e,6a,b,7,8 Meanwhile, in the case of donor– design AIE-active organoboron dyes with high solid-state PLQYs.
acceptor type dyes, the bulky donor moieties, such as
Recently, we have described the new highly emissive
difluoroboron complex 1 (Fig. 1), which is based on a thiazolo
[3,2-c]oxadiazaborinine core conjugated with a dimethylamino
donor group via a rotatable phenyl linker.14
aInstitute of Organic Chemistry, Polish Academy of Sciences, Kasprzaka 44/52,
01-224 Warsaw, Poland. E-mail: potopnyk@gmail.com,
Compound 1 exhibited high chemical stability and could be
easily modified in the thiazole unit.14d However, despite the
AIE activity of complex 1, the thiazole-modified analogues
exhibited aggregation-caused quenching (ACQ) of emission.14
Meanwhile, the design of thiazolo[3,2-c]oxadiazaborinines
with enhanced AIE properties could open the way to the wide-
spread application of such dyes.
mykhaylo.potopnyk@icho.edu.pl
bDepartment of Polymer Chemistry and Technology, Kaunas University of Technology,
Radvilenu pl. 19, LT-50254 Kaunas, Lithuania. E-mail: juozas.grazulevicius@ktu.lt
cInstitute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52,
01-224 Warsaw, Poland
†Electronic supplementary information (ESI) available. CCDC 2013916 for 2. For
ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/
d0ob02225j
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tert-butylcarbazole (5), which was obtained by standard tert-
butylation15 of substrate 4 in 85% yield (Scheme 1). Carbazoles
4 and 5 were coupled with ethyl 4-iodobenzoate (6) in the pres-
ence of CuI/1,10-phenanthroline/K2CO3 or Cu2O in refluxed di-
methylacetamide (DMA) giving products 7 and 8 in 80 and
95% yields, respectively. Next, after hydrolysis of the ester
group, 4-(9H-carbazol-9-yl)benzoic acid (9) and 4-(3,6-di-tert-
butyl-9H-carbazol-9-yl)benzoic acid (10) were obtained in
nearly quantitative yields (96 and 99%). Then, treatment of
acids 9 and 10 with thionyl chloride in a hot toluene medium
gave the corresponding chlorides, which were introduced in
the acylation reaction with thiazole-2-amine 11 under basic
conditions affording amides 12 and 13 in very good yields (79
and 77%). Finally, transformation of these compounds into
organoboron complexes 2 and 3 was realized by condensation
with boron trifluoride in the presence of N,N-diisopropyl-
ethylamine (DIPEA), giving the final products in 37 and 40%
yields, respectively. The chemical structures of the obtained
compounds were fully confirmed by 1H, 13C and 19F NMR
spectroscopy and high-resolution mass spectrometry (HRMS).
Additionally, the structure of complex 2 was verified by
single-crystal X-ray diffraction (Fig. 2, Fig. S1, Tables S1 and S2
in the ESI†). In the solid crystals of the compound, the carba-
zole moiety is twisted against the phenyl linker plane owing to
the intramolecular interactions between the ortho-hydrogen
atoms of both units. The corresponding dihedral angle (Θ1) is
ca. 45.5°. Meanwhile, the position of the oxadiazaboronine
ring plane is almost coplanar with the phenyl linker (Θ2 = 2.1°,
Fig. 2a). Complex 2 exhibits molecular packing, characterized
by a monoclinic system with four molecules in the unit cell
and the P21/c space group (Table S1†). In the structure, the
neighboring molecules interact by numerous CH⋯F and
CH⋯O hydrogen bonds, as well as by weak n–π and π–π inter-
actions (Fig. 2c and d), which contributes a weak ACQ effect,
thereby leading to the preservation of the emissive properties
of the compounds in the solid state.
Fig. 1 (a) Previous work: design of thiazole annulated oxadiazaborinine
1; (b) structures of carbazole-containing thiazolo-oxadiazaborinines 2
and 3.
Herein, we demonstrate that the replacement of the Me2N-
group by the carbazole moiety (dyes 2 and 3) which results in
the remarkable increase of solvatofluorochromism, and, due
to the restriction of intramolecular rotation, in enhanced AIE
activity. Strikingly, the attachment of two tBu groups to the car-
bazole unit (dye 3) induced significant mechanofluorochromic
properties. The important advantages of compounds 2 and 3,
compared with other N,O-coordinating difluoroborons, are
synthetic availability, high thermal stability, large values of
Stokes shifts and relatively high solid-state PLQYs.
Results and discussion
Synthesis and characterization
Complexes 2 and 3 were successfully synthesized in four steps,
starting from commercially available carbazole (4) and 3,6-di-
Scheme 1 Synthesis of organoboron complexes 2 and 3.
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Fig. 3 Frontier molecular orbitals of compounds 2 and 3.
Fig. 2 Single crystal analysis of compound 2: (a) top and (b) front views;
intermolecular interactions: (c) hydrogen bonds and (d) n–π and π–π
interactions.
Photophysical properties in solutions
The photophysical properties of the dilute solutions of organo-
boron dyes 2 and 3 in five solvents with different polarities
(toluene, THF, DCM, acetone, and acetonitrile) were studied by
UV-vis absorption and photoluminescence (PL) spectrometries.
The UV-vis absorption spectra of the organoboron com-
plexes were hardly affected by the solvent polarity. In all the
investigated solvents, these dyes exhibited low-energy absorp-
tion (centred at 370–384 nm for dye 2 and at 387–399 nm for
analogue 3) (Fig. 4 and Table 1). Thus, the experimentally
obtained results are in good agreement with the compu-
tational data (Fig. S5 and S6 in the ESI†). For comparison,
The thermal properties of complexes 2 and 3 were investi-
gated by differential scanning calorimetry (DSC) and
thermogravimetric analysis (TGA). DSC experiments indi-
cated that during the heating scans, compounds 2 and 3
showed single melting peaks at 245 and 315 °C, respectively.
Noticeably, no obvious glass transition temperature was
observed for these compounds (Fig. S2 in the ESI†). TGA
(Fig. S3 in the ESI†) performed under a nitrogen atmosphere
showed that organoboron dyes 2 and 3 had high thermal
stability with 5% weight loss temperatures (Td) of 313 and
316 °C, respectively.
Quantum chemical calculations
To predict the electronic properties and molecular geometries
of compounds 2 and 3 at the molecular level, density func-
tional theory (DFT) and time-dependent DFT (TD-DFT) calcu-
lations at the M06/def2tzvp level of theory were carried out. In
the ground state, the lowest-energy conformations of the two
compounds have twisted geometry with torsion angles
between the carbazole unit and the phenylene linker of ca.
47.6 and 48.9° for 2 and 3, respectively (Fig. S4 in the ESI†).
The phenyl linker and the oxadiazaboronine ring are in an
almost coplanar position in each molecule. The calculated
parameters show good correlation with the X-ray single crystal
analysis data obtained for dye 2 (Fig. 2). Consequently, signifi-
cant HOMO–LUMO separation is observed for both com-
pounds: the HOMOs are mainly located on the carbazole
moiety, whereas the LUMOs are dispersed on the planar thia-
zolo[3,2-c]oxadiazaborinine unit and the conjugated phenyl
linker ring (Fig. 3).
The calculated energy gap between the HOMO and LUMO
of compounds 2 and 3 is 3.76 and 3.62 eV, respectively, indu-
cing the violet light absorption ability. TD-DFT calculations
(Table S3 in the ESI†) show that low-lying transition occurs
from the HOMO to the LUMO, demonstrating large oscillator
strengths ( f ≈ 0.69) for both dyes. These results show that the
emission can be attributed to intramolecular charge transfer
(ICT).
Fig. 4 Normalized absorption (solid lines) and emission (dashed lines)
spectra of the solutions of compounds 2 (a) and 3 (b) in toluene (black),
THF (blue), DCM (red), acetone (green) and acetonitrile (magenta) (C =
1.0 × 10−5 M; λex = 374 nm).
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Table 1 Photophysical properties of the solutions of complexes 1, 2,
and 3 in different solvents
that the incorporation of the carbazole donor unit leads to
much higher solvatofluorochromic properties and a decrease
of the fluorescence quantum yield, especially in the polar
medium. These changes could be caused by the increase of
the ICT character in the excited state of compounds 2 and 3.
Dye Solvent
λabs, nm ε, M−1 cm−1 λem, nm Δυ, cm−1 PLQY
1
Toluene
THF
DCM
Acetonitrile 402
Toluene
THF
DCM
Acetone
Acetonitrile 370
Toluene
THF
405
407
402
56 600
54 400
55 400
49 700
23 600
23 300
21 700
23 100
20 600
22 300
25 100
23 300
25 300
24 900
439
453
454
469
462
514
532
560
594
489
546
578
605
631
1912
2495
2849
3553
4397
7211
7518
8953
10 192
4613
7260
7895
9178
9992
>0.99
0.86
0.99
0.10
0.77
0.57
0.48
0.08
0.02
0.70
0.31
0.13
0.02
0.01
Solid-state fluorescence properties
2
384
375
380
373
The solid-state emissive properties of compounds 2 and 3 were
investigated both in crystalline and thin film samples. In the
crystalline state, compounds 2 and 3 demonstrated intense
emission bands with the maxima at 514 and 496 nm, respect-
ively (Fig. 5 and Table 2), and relatively high quantum yields of
26 and 34%, respectively. The thin film of complex 2 exhibited
an almost identical emission spectrum (λem = 514 nm) to that
of the crystalline sample (Fig. 5a). The fluorescence efficiency
(PLQYsolid = 24%) was also comparable, indicating little
change of the fluorescence properties with the change of the
morphology. In contrast, the thin film of dye 3 demonstrated
bathochromically shifted emission bands (λem = 521 nm) with
respect to those of the crystalline sample. This observation
3
399
391
398
389
DCM
Acetone
Acetonitrile 387
compound 1 exhibited an absorption peak maximized at
402–407 nm (Table 1).14a The molar absorption coefficient (ε)
of the solutions of the dyes ranged from 20 600 to 25 300 M−1
cm−1, which is considerably lower than the analogous para-
meter of previously described analogue 1 (ε = 49 700–56 600
M−1 cm−1),14a indicating the enhancement of ICT with the
structural change of the Me2N donor group on carbazole or
3,6-di-tert-butylcarbazole moieties.
More importantly, the solutions of the investigated dyes
exhibited single emission peaks, mirroring the low-energy
absorption peaks (Fig. 4). The emission bands displayed
bathochromic shifts when the solvent was changed from non-
polar toluene (λem of 462 and 489 nm for compounds 2 and
3, respectively) to polar acetonitrile (λem of 594 and 631 nm
for dyes 2 and 3, respectively). Analogically to the absorption
t
spectra, the attachment of Bu groups to the carbazole units
caused bathochromic shifts of the emission bands, indicating
the relatively lower energy level of low-lying singlet excited
states. The fluorescence intensity of the solutions of com-
plexes 2 and 3 decreased with the increase of the solvent
polarity due to the dissipation of energy with the rotation
motions of carbazole and thiazolo[3,2-c]oxadiazaborinine
moieties. Thereby, in the toluene solution, dyes 2 and 3
exhibit PLQYs of 77 and 70%, respectively. Meanwhile, the
emission of acetonitrile solutions is dramatically quenched
(PLQY = 1–2%). The solvent polarity-dependent fluorescence
quenching is more significant for complex 3 than for ana-
logue 2. Thus, for THF and DCM solutions of compound 2,
the PLQY is 57 and 48%, respectively, while the values of this
parameter for the corresponding solutions of complex 3 are
31 and 13% (Table 1). This observation confirms the
t
enhanced ICT emission of the compound with Bu groups
attached to the carbazole unit.
The excited-state lifetimes (τ) of the solutions in toluene of
dyes 2 and 3 were determined as 3.7 and 4.8 ns, demonstrating
the short-lived fluorescence nature of the emission.
Comparison of these emission data with the previously
reported data for Me2N-containing analogue 1 demonstrates
Fig. 5 Normalized emission spectra of dyes 2 (a) and 3 (b) in different
solid states (λex = 374 nm). Reversible switching of solid-state emission
of dye 3 by repeated grinding/DCM-fuming cycles (c).
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Table 2 Fluorescence properties of complexes 2 and 3 in the solid
state
that the investigated compounds should demonstrate AIE
ability.
To verify this presumption, we studied the emission pro-
perties of equimolar solutions of the complexes in THF/water
mixtures with various water fractions ( fw) from 0 to 95%
(Fig. 6). In the cases of both compounds, the initial growth of
fw resulted in the decrease of the emission intensity due to the
increase of the polarity of the media. However, for dye 2, when
fw exceeded 80%, a significant increase of the emission inten-
sity was observed (Fig. 6a and c). This observation indicates
that solute molecules form nanoscopic aggregates (nano-
particles) in water-dominant dispersions due to their hydro-
phobic effect. Indeed, the dynamic light scattering (DLS)
experiments confirmed the formation of nanoaggregates in
the dispersions with an average size of 79 nm (Fig. 6e).
Complex 3 demonstrated even enhanced AIE activity. In
this case, the emission intensity was boosted dramatically
when fw exceeded 70%, and was more than 25-fold enhanced
for the dispersion containing 95% of water (Fig. 6b and d).
DLS measurements showed the formation of nanoaggregates
of dye 3 under such conditions with an average size of 89 nm
(Fig. 6f).
Comp. State
λem, nm PLQYsolid τ, ns
α1/α2
2
3
Crystalline
Film
Crystalline
Ground
514
514
496
534
0.26
0.24
0.34
0.31
0.30
0.32
0.99, 4.28 57.65/42.35
2.30, 7.42 33.83/66.17
1.17, 3.81 60.12/39.88
1.46, 6.53 31.17/68.83
0.99, 4.25 65.03/34.97
2.64, 8.31 57.02/42.98
DCM-fumed 491
Film 521
indicates the morphology-dependent solid-state fluorescence
of complex 3.
In order to shed more light on this phenomenon, we inves-
tigated the fluorescence properties of the solid sample after
mechanical stimuli. The ground sample of 3 exhibited bluish-
green emissions centered at 534 nm, which were even more
bathochromically shifted than those of the thin film.
Afterwards, DCM-fuming of this sample resulted in hypsochro-
mic shifts of the emission spectra (λem = 491). The grinding
and DCM-fuming procedures were repeated, and the obtained
emission spectra were almost identical (Fig. 5c). In compari-
son, the ground sample of complex 2 demonstrated only a
little change in the emission spectrum (λem = 500 nm) com-
pared with that of the pristine (as-synthesized) crystalline
sample (λem = 514 nm). Moreover, the following DCM-fuming
of this ground sample did not have any significant influence
on the emissive properties (λem = 500 nm, Fig. S7 and Table S4
in the ESI†).
The value of the solid-state photoluminescence quantum
yield to a great extent depends on interactions between the
neighboring molecules in this state. In this context, as was
aforementioned (Fig. 2c and d), in the crystalline state, twisted
molecules of 2 interacted largely by hydrogen bonds, while the
π–π/n–π interactions were weak, contributing a small ACQ
effect. In all solid samples of compound 3, the values of the
fluorescence quantum yield were preserved at 30–34%, which
were even higher than those for analogue 2. This phenomenon
was apparently caused by the presence of two bulky tert-butyl
groups in molecules of 3, which additionally decreased the
solid-state π–π/n–π interactions, thus guaranteeing a higher
fluorescence efficiency.
Conclusions
In conclusion, we developed an efficient synthesis of two new
highly thermally stable carbazole-modified thiazolo[3,2-
c][1,3,5,2]oxadiazaborinines 2 and 3. The incorporation of a
carbazole donor unit into the structure of such organoboron
complexes leads to much higher solvatofluorochromism and
larger decrease of the fluorescence quantum yield in polar
solutions. Complex 3 with two tert-butyl substituents at the
positions C-3 and C-6 of the carbazole moiety demonstrated
morphology-dependent solid-state fluorescence and significant
mechanofluorochromic properties (Δλem = 43 nm), probably
owing to the decrease of n–π/π–π-stacking interactions. Both
dyes 2 and 3 demonstrated aggregation-induced emission
enhancement. Taken together, through this work, new insights
are shown for the preferred design strategies for prospective
AIE-active organoboron emitters.
The lifetimes of the excited state of the investigated solid
samples established from double exponential fits of photo-
luminescence decay curves were in the range of 0.99–8.31 ns
(Table 2). These values were comparative with the corres-
ponding values established for the solutions.
Experimental
General
Chemicals and solvents were obtained from commercial
sources (TCI, Acros Organics, FluoroChem, or Aldrich) and
were used without further purification. Column chromato-
Aggregation-induced emission (AIE) properties
Organoboron complexes 2 and 3 demonstrated good solubility graphy was carried out using silica gel (Merck, 230–400 mesh)
in common organic solvents, such as toluene, tetrahydrofuran, as the stationary phase. The NMR spectra were recorded on
chloroform, dichloromethane, acetone and acetonitrile, but Bruker Avance II 400 MHz (at 400 and 100 MHz for 1H and 13
C
were not soluble in water. On the other hand, as aforemen- NMR spectra, respectively) and Varian VNMRS 500 MHz (at
tioned, they exhibited low photoluminescence quantum yields 500, 125 and 470 MHz for 1H, 13C and 19F NMR spectra,
in polar solvents (acetone and acetonitrile), and comparatively respectively) spectrometers for solutions in CDCl3 or DMSO-d6,
high quantum yields in the solid state. These data indicate referenced to tetramethylsilane or solvent peaks. NMR data
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Fig. 6 Emission spectra of the dispersions of dyes 2 (a) and 3 (b) in THF/water mixtures (C = 1.0 × 10−5 M; λex = 374 nm). Plots of the absorption
intensity of 2 (c) and 3 (d) versus fw. Size distribution of complexes 2 (e) and 3 (f) in THF/water medium with water fractions from 0 to 95%.
Photographs of the dispersions of 2 (g) and 3 (h) in THF/water mixtures under UV irradiation at 365 nm.
were analysed using Mnova software. Mass spectra were TA Instrument at a heating rate of 10 °C min−1 under a nitro-
acquired with a Synapt G2-S HDMS (Waters Inc.) mass spectro- gen atmosphere. TGA was performed on a Metter TGA/
meter equipped with an electrospray ion source and a quadru- SDTA851e/LF/1100 apparatus at a heating rate of 10 °C min−1
pole time-of-flight type mass analyser. The melting points of under a nitrogen atmosphere.
the synthesized compounds were determined using an
Single crystals of organoboron complex 2 were obtained by
Automatic Melting Point System (OptiMelt, Stanford Research slow evaporation of its solution in hexanes/DCM (1 : 1) at room
Systems). DSC measurements were performed on a DSC Q 100 temperature. X-ray intensity data were collected at 100 K on a
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SuperNova Agilent diffractometer using Cu Kα (λ = 1.54184 Å) and the mixture was extracted with DCM (3 × 20 mL). The
radiation. Data reduction was processed using CrysAlisPro organic phases were washed with water (2 × 15 mL) and a satu-
(Agilent Technologies, Version 1.171.35.21b). The structures rated aqueous solution of ammonium chloride (2 × 15 mL),
were solved by direct methods and refined using SHELXL dried over Na2SO4, and concentrated. The residue was separ-
Software Package.16 DFT and TD-DFT calculations were per- ated by column chromatography on silica gel (hexanes, next
formed using Gaussian 09 software17 with the inclusion of the hexanes/AcOEt = 99 : 1) to afford a pure product 7 in 80%
DCM solvent effect through the conductor-like polarizable con- (1.45 g) yield. Mp. 97.3–98.8 °C (lit.18 94–95 °C), yellowish
tinuum model (CPCM). UV–vis absorption spectra were powder. 1H NMR (500 MHz, CDCl3): δ = 8.30 (2H, d, J = 8.5 Hz,
recorded on a PerkinElmer Lambda 35 spectrometer for ca. 1.0 Ar–H), 8.15 (2H, br d, J = 7.7 Hz, Ar–H), 7.69 (2H, d, J = 8.5 Hz,
× 10−5 M solutions of dyes. Fluorescence spectra were recorded Ar–H), 7.48 (2H, br d, J = 7.8 Hz, Ar–H), 7.43 (2H, ddd, J = 8.0
using an Edinburgh Instruments FLS980 fluorescence spectro- Hz, J = 7.7 Hz, J = 1.2 Hz, Ar–H), 7.32 (2H, ddd, J = 8.0 Hz, J =
meter (λex = 374 nm) for both ca. 1.0 × 10−5 M solutions and 7.8 Hz, J = 1.2 Hz, Ar–H), 4.46 (2H, d, J = 7.1 Hz, CH2), 1.46
solid crystals of the investigated dyes. Absolute photo- (3H, t, J = 7.1 Hz, CH3) ppm; 13C NMR (125 MHz, CDCl3): δ =
luminescence quantum yields of the samples were measured 165.9, 141.9, 140.3 (2C), 131.3 (2C), 129.0, 126.4 (2C), 126.1
on the same machine using a calibrated integrating sphere. (2C), 123.8 (2C), 120.5 (2C), 120.4 (2C), 109.7 (2C), 61.2,
Fluorescence decays of the solutions and of the solid-state 14.4 ppm. HRMS (ESI-TOF) calcd for C21H17NO2 [M]+:
samples were recorded with a PicoQuant PDL 820 ps pulsed 315.1259, found: 315.1254.
diode laser as an excitation source (λex = 374 nm) using a time-
correlated single-photon counting technique. DLS was compound was obtained using a modified literature pro-
measured using a Malvern Zetasizer.
cedure.19 A mixture of 3,6-di-tert-butylcarbazole (5, 1.50 g,
Ethyl 4-(3,6-di-tert-butyl-9H-carbazol-9-yl)benzoate (8). This
The THF/water mixtures of various ratios were prepared by 5.37 mmol), ethyl 4-iodobenzoate (6, 1.00 mL, 5.98 mmol)
slowly adding distilled water into solutions of dye 2 or 3 in and copper(I) oxide (1.61 g, 11.27 mmol) in dimethyl-
THF under sonication, while the concentration was main- acetamide (10 mL) was refluxed for 24 h under argon. After
tained at 1.0 × 10−5 M. The emission spectra and DLS of the cooling, a saturated aqueous solution of ammonium chloride
obtained mixture were measured immediately.
(15 mL) was added and the mixture was extracted with DCM
(3 × 20 mL). The organic phases were washed with water (2 ×
15 mL) and a saturated aqueous solution of ammonium
Synthesis
3,6-Di-tert-butyl-9H-carbazole (5). This compound was chloride (2 × 15 mL), dried over Na2SO4, and concentrated.
obtained using a modified literature procedure.15 The reaction The residue was separated by column chromatography on
was conducted under an argon atmosphere. tert-Butyl chloride silica gel (hexanes, next hexanes/AcOEt = 99 : 1) to afford a
(8.40 mL, 76.25 mmol) was added to a solution of carbazole (4, pure product 8 in 95% (2.19 g) yield. Mp. 180.9–182.0 °C,
1
5.00 g, 29.90 mmol) and zinc chloride (10.19 g, 74.75 mmol) white powder. H NMR (400 MHz, CDCl3): δ = 8.28 (2H, d, J =
in nitromethane (40 mL). The solution was stirred at room 8.6 Hz, Ar–H), 8.16 (2H, d, J = 1.8 Hz, Ar–H), 7.68 (2H, d, J =
temperature for 12 h. After that, water (50 mL) was added and 8.6 Hz, Ar–H), 7.49 (2H, dd, J = 8.7 Hz, J = 1.8 Hz, Ar–H), 7.43
the mixture was extracted with dichloromethane (3 × 60 mL). (2H, d, J = 8.7 Hz, Ar–H), 4.47 (2H, d, J = 7.1 Hz, CH2), 1.49
The combined organic layer was washed with water (50 mL), [18H, s, 2 × C(CH3)3], 1.47 (3H, t, J = 7.1 Hz, CH3) ppm; 13C
saturated sodium carbonate solution (30 mL), and brine NMR (100 MHz, CDCl3): δ = 166.0, 143.5 (2C), 142.5, 138.6
(30 mL), dried over anhydrous Na2SO4, filtered and concen- (2C), 131.2 (2C), 128.5, 125.8 (2C), 123.8 (2C), 123.8 (2C),
trated. The obtained solid was purified by column chromato- 116.4 (2C), 109.2 (2C), 61.1, 34.7 (C2), 32.0 (6C), 14.4 ppm.
graphy on silica gel (hexanes/ethyl acetate = 100 : 0 to 99 : 1, HRMS (ESI-TOF) calcd for C29H34NO2 [M + H]+: 428.2590,
v/v) to give product
5
in 85% (7.14 g) yield. found: 428.2587.
4-(9H-Carbazol-9-yl)benzoic acid (9). A mixture of ester 7
Mp. 225.3–227.4 °C, white powder. 1H NMR (400 MHz, CDCl3):
δ = 8.11 (2H, d, J = 1.9 Hz, Ar–H), 7.78 (1H, br s, N–H), 7.49 (1.20 g, 3.80 mmol), THF (20 mL), EtOH (4 mL), H2O (4 mL)
(2H, dd, J = 8.6 Hz, J = 1.9 Hz, Ar–H), 7.32 (2H, d, J = 8.6 Hz, and KOH (0.64 g, 11.42 mmol) was refluxed for 1 h. After
Ar–H), 1.48 [18H, s, 2 × C(CH3)3] ppm; 13C NMR (100 MHz, cooling, the solvents were evaporated and water (5 mL) and 5
CDCl3): δ = 142.2 (2C), 138.0 (2C), 123.5 (2C), 123.3 (2C), 116.2 N HCl solution were added to adjust the pH to ∼5. The
(2C), 110.0 (2C), 34.7 (C2), 32.0 (6C) ppm. HRMS (ESI-TOF) mixture was left in a fridge for 4 h. The precipitate was filtered,
calcd for C20H26N [M + H]+: 280.2065, found: 280.2056.
washed with water, and dried to give product 9 in 96% (1.05 g)
1
Ethyl 4-(9H-carbazol-9-yl)benzoate (7). This compound was yield. Mp. 256.8–259.6 °C, white powder. H NMR (500 MHz,
obtained using a modified literature procedure.18 A mixture of DMSO-d6): δ = 8.21–8.27 (4H, m, Ar–H), 7.77 (2H, d, J = 8.4 Hz,
carbazole (4, 1.00 g, 5.98 mmol), ethyl 4-iodobenzoate (6, Ar–H), 7.48 (2H, d, J = 8.1 Hz, Ar–H), 7.44 (2H, dd, J = 8.1 Hz, J
1.00 mL, 5.98 mmol), copper(I) iodide (114 mg, 0.60 mmol), = 7.1 Hz, Ar–H), 7.31 (2H, dd, J = 7.5 Hz, J = 7.1 Hz, Ar–H)
potassium carbonate (2.46 g, 17.94 mmol) and 1,10-phenan- ppm; 13C NMR (125 MHz, DMSO-d6): δ = 166.7, 140.9, 139.6
throline (108 mg, 0.60 mmol) in dimethylacetamide (10 mL) (2C), 131.3 (2C), 129.4, 126.4 (2C), 126.3 (2C), 123.1 (2C), 120.6
was refluxed under argon for 24 h. After cooling, a saturated (2C), 120.6 (2C), 109.8 (2C) ppm. HRMS (ESI-TOF) calcd for
aqueous solution of ammonium chloride (15 mL) was added C19H12NO2 [M − H]−: 286.0868, found: 286.0861.
412 | Org. Biomol. Chem., 2021, 19, 406–415
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4-(3,6-Di-tert-butyl-9H-carbazol-9-yl)benzoic acid (10).
A
afford pure amide 13 in 77% (446 g) yield. Mp. 327.6–330.5 °C.
mixture of ester 8 (1.30 g, 3.04 mmol), THF (25 mL), EtOH 1H NMR (500 MHz, CDCl3): δ = 11.24 (1H, br s, N–H), 8.23 (2H,
(5 mL), H2O (5 mL) and KOH (0.51 g, 9.12 mmol) was refluxed for d, J = 8.5 Hz, Ar–H), 8.15 (2H, d, J = 1.9 Hz, Ar–H), 7.78 (2H, d,
1 h. After cooling, the solvents were evaporated and water (5 mL) J = 8.5 Hz, Ar–H), 7.50 (2H, dd, J = 8.6 Hz, J = 1.9 Hz, Ar–H),
and 5 N HCl solution were added to adjust the pH to ∼5. The 7.46 (2H, d, J = 8.6 Hz, Ar–H), 7.35 (1H, d, J = 3.6 Hz, thiazole–
mixture was left in a fridge for 4 h. The precipitate was filtered, H), 7.05 (1H, d, J = 3.6 Hz, thiazole–H), 1.48 [18H, s, 2 ×
washed with water, and dried to give product 10 in 99% (1.20 g) C(CH3)3] ppm; 13C NMR (125 MHz, CDCl3): δ = 164.3, 159.4,
yield. Mp. 273.5–276.8 °C, white powder. 1H NMR (600 MHz, 143.8 (2C), 142.6, 138.5 (2C), 137.4, 130.0, 129.5 (2C), 126.3
DMSO-d6): δ = 8.30 (2H, br s, Ar–H), 8.19 (2H, d, J = 8.0 Hz, Ar–H), (2C), 124.0 (2C), 123.9 (2C), 116.5 (2C), 114.0, 109.2 (2C), 34.8
7.71 (2H, d, J = 8.0 Hz, Ar–H), 7.48 (2H, d, J = 8.6 Hz, Ar–H), 7.40 (2C), 32.0 (6C) ppm. HRMS (ESI-TOF) calcd for C30H31N3OSNa
(2H, d, J = 8.6 Hz, Ar–H), 1.41 [18H, s, 2 × C(CH3)3] ppm; 13C [M + Na]+: 504.2086, found: 504.2087.
NMR (150 MHz, DMSO-d6): δ = 167.1, 143.0 (2C), 140.7, 138.1
3-(4-(9H-Carbazol-9-yl)phenyl)-1,1-difluoro-1H-1λ4,8λ4-thia-
(2C), 131.1 (2C), 131.0, 125.5 (2C), 123.8 (2C), 123.2 (2C), 116.8 zolo[3,2-c][1,3,5,2]oxadiazaborinine (2). Distilled N,N-diiso-
(2C), 109.2 (2C), 34.5 (2C), 31.8 (6C) ppm. HRMS (ESI-TOF) calcd propylethylamine (2.64 mL, 15.16 mmol, 20 equiv.) and
for C27H28NO2 [M − H]−: 398.2120, found: 398.2108.
BF3·Et2O (0.94 mL, 7.67 mmol, 10 equiv.) were added to a solu-
4-(9H-Carbazol-9-yl)-N-(thiazol-2-yl)benzamide (12). Thionyl tion of amide 12 (280 mg, 0.76 mmol, 1 equiv.) in dry DCM
chloride (142 μL, 1.95 mmol, 1.5 equiv.) was added to a sus- (30 mL) under an argon atmosphere. The reaction mixture was
pension of acid 9 (374 mg, 1.30 mmol, 1.0 equiv.) in dry stirred for 24 h at room temperature and then washed with
toluene (20 mL). The reaction mixture was heated at 100 °C for water. The organic phase was dried over anhydrous Na2SO4
1 h. After cooling, the solvent and the rest of SOCl2 were evap- and filtered. The solvent was removed under reduced pressure,
orated under vacuum. The obtained benzoyl chloride was dis- and the crude product was purified by column chromato-
solved in 1,4-dioxane (20 mL); amine 11 (196 mg, 1.95 mmol, graphy (hexanes/dichloromethane from 8 : 1 to 1 : 1, v/v) to
1.5 equiv.), dry triethylamine (545 μL, 3.91 mmol, 3.0 equiv.) obtain pure product
2
in 37% yield (117 mg).
1
and DMAP (8 mg, 0.07 mmol, 0.05 equiv.) were added. Next, Mp. 244.7–246.8 °C. H NMR (500 MHz, CDCl3): δ = 8.58 (2H,
the reaction mixture was refluxed for 24 h. Water (30 mL) and d, J = 8.6 Hz, Ar–H), 8.15 (2H, d, J = 8.0 Hz, Ar–H), 7.76 (2H, d,
a saturated aqueous solution of NaHCO3 (30 mL) were added J = 8.6 Hz, Ar–H), 7.64 (1H, d, J = 4.3 Hz, thiazole–H), 7.52 (2H,
and the mixture was extracted with DCM (3 × 50 mL). The com- d, J = 8.0 Hz, Ar–H), 7.45 (2H, ddd, J = 8.0 Hz, J = 7.6 Hz, J = 1.3
bined organic layers were dried over anhydrous Na2SO4, fil- Hz, Ar–H), 7.33 (2H, ddd, J = 8.0 Hz, J = 7.6 Hz, J = 1.0 Hz, Ar–
tered and concentrated. The crude product was purified by H), 7.19 (1H, d, J = 4.3 Hz, thiazole–H) ppm; 13C NMR
column chromatography on silica gel (hexanes/dichloro- (125 MHz, CDCl3): δ = 173.4, 165.9, 143.0, 140.1 (2C), 131.8
methane from 4 : 1 to 0 : 1, v/v) to afford pure amide 12 in 79% (2C), 130.3, 129.5, 126.3 (2C), 126.3 (2C), 124.0 (2C), 120.7 (2C),
1
(380 g) yield. Mp. 286.0–287.2 °C. H NMR (500 MHz, DMSO- 120.5 (2C), 114.7, 109.9 (2C) ppm; 19F NMR (470 MHz, CDCl3):
d6): δ = 12.82 (1H, br s, N–H), 8.41 (2H, d, J = 8.5 Hz, Ar–H), δ = −135.18 (2F, m, BF2) ppm. HRMS (ESI-TOF) calcd for
8.27 (2H, d, J = 7.7 Hz, Ar–H), 7.83 (2H, d, J = 8.5 Hz, Ar–H), C22H15BN3OF2S [M + H]+: 418.0997, found: 418.0995.
7.60 (1H, d, J = 3.6 Hz, thiazole–H), 7.44–7.52 (4H, m, Ar–H),
3-(4-(3,6-Di-tert-butyl-9H-carbazol-9-yl)phenyl)-1,1-difluoro-
(3).
7.29–7.36 (3H, m, Ar–H, thiazole–H) ppm; 13C NMR (125 MHz, 1H-1λ4,8λ4-thiazolo[3,2-c][1,3,5,2]oxadiazaborinine
DMSO-d6): δ = 164.4, 158.9, 140.6, 139.6 (2C), 137.5, 130.8, Distilled N,N-diisopropylethylamine (2.24 mL, 12.87 mmol, 20
130.2 (2C), 126.5 (2C), 126.3 (2C), 123.1 (2C), 120.6 (2C), 120.6 equiv.) and BF3·Et2O (0.79 mL, 6.43 mmol, 10 equiv.) were
(2C), 113.9, 109.8 (2C) ppm. HRMS (ESI-TOF) calcd for added to a solution of amide 13 (310 mg, 0.64 mmol, 1 equiv.)
C22H15N3OSNa [M + Na]+: 392.0834, found: 392.0827.
4-(3,6-Di-tert-butyl-9H-carbazol-9-yl)-N-(thiazol-2-yl)benz-
in dry DCM (30 mL) under an argon atmosphere. The reaction
mixture was stirred for 24 h at room temperature and then
amide (13). Thionyl chloride (131 μL, 1.80 mmol, 1.5 equiv.) washed with water. The organic phase was dried over anhy-
was added to a suspension of acid 10 (480 mg, 1.20 mmol, 1.0 drous Na2SO4 and filtered. The solvent was removed under
equiv.) in dry toluene (20 mL). The reaction mixture was reduced pressure, and the crude product was purified by
heated at 100 °C for 1 h. After cooling, the solvent and the rest column chromatography (hexanes/dichloromethane from 8 : 1
of SOCl2 were evaporated under vacuum. The obtained benzoyl to 1 : 1, v/v) to obtain pure product 3 in 40% yield (138 mg).
1
chloride was dissolved in 1,4-dioxane (20 mL); amine 11 Mp. 313.1–315.2 °C. H NMR (500 MHz, CDCl3): δ = 8.55 (2H,
(180 mg, 1.80 mmol, 1.5 equiv.), dry triethylamine (503 μL, d, J = 8.6 Hz, Ar–H), 8.14 (2H, br s, Ar–H), 7.74 (2H, d, J = 8.6
3.60 mmol, 3.0 equiv.) and DMAP (7 mg, 0.06 mmol, 0.05 Hz, Ar–H), 7.63 (1H, d, J = 4.3 Hz, thiazole–H), 7.45–7.51 (4H,
equiv.) were added. Next, the reaction mixture was refluxed for m, Ar–H), 7.18 (1H, d, J = 4.3 Hz, thiazole–H), 1.47 [18H, s, 2 ×
24 h. Water (30 mL) and a saturated aqueous solution of C(CH3)3] ppm; 13C NMR (125 MHz, CDCl3): δ = 173.5, 166.0,
NaHCO3 (30 mL) were added and the mixture was extracted 143.8 (2C), 143.6, 138.4 (2C), 131.8 (2C), 130.2, 128.8, 125.7
with DCM (3 × 50 mL). The combined organic layers were (2C), 124.0 (2C), 123.9 (2C), 116.4 (2C), 114.6, 109.4 (2C), 34.8
dried over anhydrous Na2SO4, filtered and concentrated. The (2C), 32.0 (6C) ppm; 19F NMR (470 MHz, CDCl3): δ = −135.27
crude product was purified by column chromatography on (2F, m, BF2) ppm. HRMS (ESI-TOF) calcd for C30H31BN3OF2S
silica gel (hexanes/dichloromethane from 4 : 1 to 0 : 1, v/v) to [M + H]+: 530.2249, found: 530.2258.
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Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was supported by Poland’s National Science Centre
(Grant No. UMO-2019/03/X/ST4/00037). The calculation
research was supported by PLGrid Infrastructure. Jurate
Simokaitiene is gratefully acknowledged for the help with the
DSC and TGA measurements.
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