2-(2′-Hydroxyphenyl)benzothiazole derivatives: Emission and color tuning

Article abstract of DOI:10.1016/j.dyepig.2020.108198

Benzothiazole based molecular species are among the important heterocyclic compounds which are usually characterised by fluorescence that covers a wide spectral range. Herein, we present synthesis and luminescent behaviour of benzothiazole based derivativ

Full text of DOI:10.1016/j.dyepig.2020.108198

Journal Pre-proof
2-(2′-Hydroxyphenyl)benzothiazole derivatives: Emission and color tuning
Ishpreet Kaur, Shivani, Paramjit Kaur, Kamaljit Singh
PII:
S0143-7208(19)32742-1
DOI:
https://doi.org/10.1016/j.dyepig.2020.108198
DYPI 108198
Reference:
To appear in: Dyes and Pigments
Received Date: 25 November 2019
Revised Date: 9 January 2020
Accepted Date: 9 January 2020
Please cite this article as: Kaur I, Shivani , Kaur P, Singh K, 2-(2′-Hydroxyphenyl)benzothiazole
derivatives: Emission and color tuning, Dyes and Pigments (2020), doi: https://doi.org/10.1016/
j.dyepig.2020.108198.
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Author Statement:
Paramjit Kaur: Conceptulisation, Methdology, Writing original Draft
Kamaljit Singh: Validation of data, Supervision, Editing original Draft
Ishpreet Kaur: Experimental and data collection
Shivani: Experimental and data collection

2-(2’-Hydroxyphenyl)benzothiazole Derivatives: Emission and Colour tuning
Ishpreet Kaur,a Shivani,a Paramjit Kaur,a* and Kamaljit Singha*
Department of Chemistry, UGC Centre of Advanced Study, Guru Nanak Dev University,
Amritsar – 143 005, India (E-mail: kamaljit.chem@gndu.ac.in , paramjit19in@yahoo.co.in)

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2-(2′-Hydroxyphenyl)benzothiazole Derivatives: Emission and Color
tuning
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Ishpreet Kaur,a Shivani,a Paramjit Kaur,a* and Kamaljit Singha*
Department of Chemistry, UGC Centre of Advanced Study, Guru Nanak Dev University,
Amritsar – 143 005, India (E-mail: kamaljit.chem@gndu.ac.in, paramjit19in@yahoo.co.in)
----------------------------------------------------------------------------------------------------------------
Abstract: Benzothiazole based molecular species are among the important heterocyclic
compounds which are usually characterized by fluorescence that covers a wide
spectral range. Herein, we present synthesis and luminescent behaviour of
benzothiazole based derivatives and their rigidified boron(III) N^O chelates.
Significant tuning of emission and absorption behaviour of the compounds led us to
identify compositions of the complimentary colors emitted from these molecular
species both in solution (CIE coordinates: x = 0.35753, y = 0.3315) as well as in the
solid (CIE coordinates: x = 0.34753, y = 0.32062) state that furnished nearly white
light emission. This work involving structurally simpler and easy to synthesise
molecular species with different emission properties, importantly in the solid state,
may find potential application in the context of colored and white-emitting materials.
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Key words: Benzothiazole; Boron(III) rigidified chelates; Multicolour emission; CIE
coordinates; Broad range light emission.
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1. Introduction
Synthesis of π-conjugated materials and tuning of their photophysical properties by structure
modification as well as manipulation by way of supramolecular assembly and/or donor-
acceptor dyads has led to identification of several functional π-systems [1]. Tuning of the
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photophysical properties of such materials is manifested in absorption and emission
behaviour, leading to their potential application in information displays, fluorescent sensors,
optical recording systems [2]. Especially, the luminescent molecular assemblies, which can
be tuned for white-light emission are of major importance because of their potential
applications in lighting devices and display media [3-5]. The most common strategy adopted
to tune the emissions is by combining primary or the dyes emitting complimentary colors [3,
6]. Among these, compounds exhibiting excited state intramolecular proton transfer (ESIPT)
are widely used as these are often characterised by emission and absorption profiles covering
a broad spectral range. Based upon the ESIPT concept, Park et al. [7] reported the first
example of color tuning leading also to white light emission by combining blue and orange
light emitters. Derivatives of 2-(2'-hydroxyphenyl)benzthiazole (HBT) are well known
ESIPT compounds. In one of our recent publications [8], we have reported color-tunable
HBT based chromophores exhibiting wavelength based reversible multicolour emission upon
aggregation in solution as well as in the solid state. In continuation of our interest in boron
containing systems such as 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) dyes [9],
we envisaged, that the rigid systems obtained by locking N,N and N,O donor ligands as boron
chelated complexes, may depict tuneable emission and colors especially in the solid state in
line with the BF2-hydrazine adducts reported in literature [10]. Borate chelation leading to
emission changes has previously been reported in case of phenanthro[9,10-d]imidazole-
quinoline-BF2, naphthyridine-BF2, pyridomethene-BF2 and 2-(6'-hydroxy-5'-benzofuryl)BF2
complexes [11-14].
In this paper, we report the design, synthesis and tuning of the emission colour of
HBT based chromophores. Additionally, we have demonstrated that using binary
combinations of HBT based chromophores, white light emission both in solution as well as in
the solid state can be obtained. Using the additivity of mixing of colours, the current work
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demonstrates the identification of formulations with emission spanning from blue to red
region, including white emission.
2. Experimental
2.1. Materials and reagents
All liquid reagents were dried/purified using recommended drying agents and/or distilled
over 4 Å molecular sieves. DMF, triethylamine and piperidine were dried, distilled and stored
overnight over molecular sieves. Other solvents (analytical grade) used for the analytical
work were purchased from Thomas Baker, while the ones used for the synthetic work were of
synthesis grade. 4-Aminosalicylic acid, 1-bromo-2-(2-bromoethoxy)ethane, propargyl
bromide and malononitrile were purchased from Sigma-Aldrich. Whereas NaBH3CN,
paraformaldehyde, NaH and BF3.OEt2 were purchased from Spectrochem and used as such.
Solid state mixtures (M1-M4) were prepared by mixing 3 (0.001 g) with either 5 (0.001 g for
M1; 0.0005 g for M2) or 6 (0.087 g for M3; 0.00113 g for M4) uniformly, with silica gel
(0.001 g, 60-120 mesh) together with THF (10-20 mL) as annealing solvent.
2.2. Instrumentation
IR spectra were recorded on Cary 630 FTIR spectrophotometer of Agilent Technologies in
the range 650–4000 cm−1. Fluorescence studies were carried out using Perkin Elmer LS 55
and Fluorolog Horiba Fluorescence Spectrometer, having a pulsed xenon flash lamp (50–60
Hz) and a 450 W CW Ozone-free Xenon arc lamp (250 to 2500 nm) respectively. The
fluorescence spectrometer consisted of Monk-Gillieson and Czerny-Turner type
monochromators having 200–800 nm excitation range and 200–900 nm emission range and
zero order R928 photomultiplier. The fluorescence spectroscopic studies were carried out
using ultraviolet (UV) LED with excitation wavelengths of 340 nm, 400 nm and 440 nm
(depending on compounds), focused perpendicularly to one side of the fluorescence quartz
cuvette at excitation slit width of 12 nm. The emission spectrum was recorded by scanning
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the monochromator in the visible region from 200 to 800 nm using emission slit width of 2.5
nm and 12 nm (depending on compounds), and detecting the optical signal using a
photomultiplier tube located at the exit port of the monochromator, which was further
connected to a power meter. Electronic absorption data was recorded on HITACHI U-2910
Spectrophotometers using matched quartz cuvettes with path length of 1 cm. The temperature
of cell holder was maintained at 25±1˚C using a Peltier temperature controller. Mass
spectrum (MS) was recorded on a Bruker HRMS MICROTOF II spectrometer. Melting
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points were determined in open capillaries and are uncorrected. H NMR and 13C NMR
spectra were recorded on Bruker Avance II NMR spectrometer at 400 MHz and Bruker
Biospin Avance III HD at 500 MHz, with TMS as internal standard using CDCl3, DMSO-d6
and D2O as solvents. Data are reported as follows: chemical shift in ppm (δ), integration,
multiplicity (s = singlet, d = doublet, t = triplet, m = multiplet), coupling constant J (Hz) and
assignment.
2.3. Computational details
All theoretical calculations were performed by using Gaussian09 suite of programmes [15].
The molecular geometries of the chromophores were optimised at the density functional
theory (DFT) method employing the hybrid B3LYP functional group and the 6-31G(d) basis
set. The first 30 excited states were calculated by using time-dependent density functional
theory (TD-DFT calculations) in THF as solvent medium (CPCM) model. The molecular
orbital contours were visualized using Gauss view 5.0.9.
2.4. Quantum yield calculations
The fluorescence quantum yields were calculated using 9,10-diphenylanthracene [16] as
standard having quantum yield of 0.95 in cyclohexane, employing the following equation:
−ꢃ
ꢅ
× ꢇꢀ2
× ꢇꢄ2
ꢄ
ꢄ
ꢂ
ꢆ
ꢆ
ꢁ × 1 − 10
ꢀ
ɸꢀ =
× ɸꢄ
−ꢃ
ꢅ
ꢀ
ꢀ
ꢂ
ꢁ × 1 − 10
ꢄ
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where Φu and Φs are the quantum yields of the test and the standard samples, respectively. Au
and As are the absorbance values of the test sample and the standard sample, respectively, Fu
and Fs are the areas of emission bands for the test sample and the standard sample, nu and ns
are the refractive indices of test sample and standard sample solutions in their respective pure
solvents. L is length of cell (1.0 cm for standard and test samples).
2.5. Synthetic procedures
2.5.1. Synthesis of 2-(4-amino-2-hydroxyphenyl)benzothiazole (1) [17]
An equimolar mixture of 4-aminosalicylic acid (18.3 mmol, 2.53 g) and 2-aminobenzenethiol
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(18.3 mmol, 2.00 g) in polyphosphoric acid (10 mL) was stirred at 180 C for 5 h. After
cooling, the mixture was poured onto ice cold water and the precipitate was filtered, washed
with water and dried using vacuum pump. The solid was purified by column chromatography
using 10:90 (ethyl acetate/hexane; v/v) as eluant leading to 2.09 g of the product in 54%
yield. M.p. = 210-213 oC [17]. 1H NMR (300 MHz, DMSO-d6) δ/ppm: 5.0 (2H, s, -NH2, D2O
exchangeable), 5.2 (1H, s, ArH), 5.27 (1H, d, J = 8.7 Hz, ArH), 6.37 (1H, t, J = 7.8 Hz, ArH),
6.5 (1H, t, J = 7.8 Hz, ArH), 6.66 (1H, d, J = 8.7 Hz, ArH), 6.92 (1H, d, J = 8 Hz, ArH), 7.05
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(1H, d, J = 7.8 Hz, ArH), 10.76 (1H, s, -OH, D2O exchangeable). C NMR (75 MHz,
DMSO-d6) δ/ppm: 99.31, 106.3, 107.15, 120.82, 121.94, 124.32, 126.418, 130.03, 132.49,
151.857, 153.797, 158.79, 167.93.
2.5.2. Synthesis of 2-(4-morpholino-2-hydroxyphenyl)benzothiazole (2a)
To a solution of 2-(4-amino-2-hydroxyphenyl)benzothiazole 1 (0.2 g, 0.82 mmol) in DMF
(10 mL) were added triethylamine (l mL) and 1-bromo-2-(2-bromoethoxy)ethane (0.52 mL,
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4.12 mmol). The reaction mixture was heated at 140 C for 5-6 h and then the solution was
diluted with ethyl acetate (100 mL). The solution was washed with saturated aqueous Na2CO3
solution (50 mL), the organic layer dried over anhydrous Na2SO4 and the solvent removed
under reduced pressure. The residue was purified by column chromatography (1:9 ethyl
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acetate/hexane) to afford 2a as a yellow solid in 55% yield. M.p. = 175-180 C. IR (KBr):
νmax 3444.1, 3354.6, 3056.4, 2959.5, 2847.7, 2370.6, 2117.1, 1871.1, 1625.1, 1431.3, 1207.5,
976.6, 760.4, 596.4.1H NMR (500 MHz, CDCl3) δ/ppm: 7.90 (1H, d, J = 10 Hz, ArH), 7.85
(1H, d, J = 10.5 Hz, ArH), 7.54 (1H, td, Ja = 11.5 Hz, Jb = 2 Hz, ArH), 7.46 (1H, dt, Ja = 10.0
Hz, Jb = 1.5 Hz, ArH), 7.34 (1H, dt, Ja = 10 Hz, Jb = 1 Hz, ArH), 6.51 (1H, s, ArH), 6.48 (1H,
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d, J = 3 Hz, ArH), 3.86 (4H, t, J = 6 Hz, CH2), 3.29 (4H, t, J = 6 Hz, CH2). C NMR (125
MHz, CDCl3) δ/ppm: 169.34, 159.63, 154.54, 152.14, 132.23, 129.65, 126.60, 124.89,
121.60, 121.49, 108.94, 106.82, 101.82, 77.36, 66.76, 47.85. HRMS: m/z calculated for
C17H16N2O2S: 313.1005 [M+1] +, Found: 313.0984 [M+1] +.
2.5.3. Synthesis of 2-(4-morpholino-2-(prop-2-yn-1-yloxy)phenyl)benzothiazole (3)
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To the suspension of NaH (0.05 g, 2.0 mmol) in DMF at 10 C, a solution of 2a (0.5 g, 2.1
mmol) dissolved in DMF (10 mL) was added and stirred for 30 min at the same temperature.
Propargyl bromide (0.5 g, 4.2 mmol) was added dropwise to the reaction mixture and stirred
for 2 h for completion (TLC). The reaction mixture was then diluted with cold water and
extracted with ethyl acetate. The organic layer was dried over anhydrous Na2SO4 and solvent
was removed under reduced pressure to obtain crude 3, which was further purified by column
chromatography using 10:90 (ethyl acetate/hexane; v/v) as eluents to yield pure 3 in 63%
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yield. M.p. = 150-155 C. IR (KBr): νmax 3265.1, 3056.4, 2967.0, 2847.7, 2318.4, 2117.1,
1595.3, 1431.3, 1237.5, 1185.3, 1118.2, 641.1. 1H NMR (500 MHz, CDCl3) δ/ppm: 8.40 (1H,
d, J = 11 Hz, ArH), 8.00 (1H, qd, Ja = 10.5 Hz, Jb = 1 Hz, ArH), 7.86 (1H, qd, Ja = 10.0 Hz,
Jb = 1 Hz, ArH), 7.42 (1H, dt, Ja = 10.0 Hz, Jb = 1.5 Hz, ArH), 7.29 (1H, dt, Ja = 9.5 Hz, Jb =
1.5 Hz, ArH), 6.67 (1H, Ja = 11.0 Hz, Jb = 3 Hz, ArH), 6.64 (1H, d, J = 3 Hz, ArH), 4.92 (2H,
d, J = 3 Hz, CH2), 3.87 (4H, t, J = 6 Hz, CH2), 3.28 (4H, t, J = 6 Hz, CH2), 2.6 (1H, t, J = 3
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Hz, C≡C−H). C NMR (125 MHz, CDCl3) δ/ppm: 163.23, 156.61, 153.83, 152.27, 135.62,
130.59, 125.76, 124.06, 122.19, 121.12, 114.48, 108.66, 99.38, 78.05, 76.47, 66.64, 56.61,
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48.10. HRMS: m/z calculated for C20H18N2O2S: 373.0981 [M+Na]+, Found: 373.0867
[M+Na]+.
2.5.4. Synthesis of 2-(4-dimethylamino-2-hydroxyphenyl)benzothiazole (2b)
To the stirred mixture of 1 (0.75 g, 2.60 mmol) and paraformaldehyde (0.39 g, 13.0 mmol) in
glacial acetic acid (10 mL) at 0 °C, NaBH3CN (0.492 g, 7.80 mmol) was added in small
portions. The mixture was allowed to warm up to room temperature and stirred overnight
before being poured into cold water (150 mL). Formed solution was extracted with ethyl
acetate (3x30 mL). Combined organic layer was washed with saturated aqueous NaHCO3
solution and dried with anhydrous Na2SO4. Organic solvent was removed under reduced
pressure and the obtained residue was purified by crystallization from ethanol to give the
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product 2b in 80% yield. M.p. = 175-180 C; Anal. Found: C, 66.46; H, 5.32; N, 10.34; S,
11.74%, molecular formula C15H14N2OS requires: C, 66.64; H, 5.22; N, 10.36; S, 11.86%; IR
(KBr): νmax 3056.4, 2922.2, 2117.1, 1863.7, 1625.1, 1568.0, 1237.5, 969.1, 887.1, 745.5,
581.5, 454.7. 1H NMR (500 MHz, CDCl3) δ/ppm: 12.6 (1H, s, -OH, D2O exchangeable), 7.86
(1H, d, J = 8 Hz, ArH), 7.81 (1H, d, J = 8 Hz, ArH), 7.48 (1H, d, J = 8 Hz, ArH), 7.42 (1H, t,
J = 7.5 Hz, ArH), 7.29 (1H, t, J = 7.5 Hz, ArH), 6.31 (1H, d, J = 8.5 Hz, ArH), 6.3 (1H, s,
ArH), 3.05 (6H, s, N-CH3). 13C NMR (125 MHz, CDCl3) δ/ppm: 169.75, 159.53, 153.74,
152.30, 132.08, 129.72, 126.41, 124.43, 121.37, 121.24, 106.53, 104.64, 98.79, 40.26.
2.5.5. Synthesis of 2-(4-dimethylamino-3-formyl-2-hydroxyphenyl)benzothiazole (4)
A mixture of DMF (3 mL, 0.0257 mol) and POCl3 (3 mL, 0.0257 mol) was cooled in an ice
bath and stirred under nitrogen atmosphere for 5 min. After warming to room temperature,
the reaction mixture was further stirred for 30 min. Then, 2b (1.4 g, 0.0052 mol) in 1,2-
dichloroethane (140 mL) was added to the reaction mixture. After rising the temperature to
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80 C, the reaction mixture was further stirred for 10 h, cooled to room temperature and
slowly poured into a saturated aqueous solution of K2CO3 (200 mL) cooled in an ice bath.
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After warming to room temperature, the reaction mixture was further stirred for 1 h and
extracted with DCM. The organic layers were combined, washed with water, dried with
anhydrous NaSO4, and the solvent was removed under reduced pressure. The crude product
was purified by silica gel (60-120 mesh) column chromatography using 30:70 (ethyl
acetate/hexane; v/v) as the eluent to give the formylated product 4 as yellowish-brown
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powder. Yield 23%. M.p. = 105-110 C. IR (KBr): νmax 3175.7, 2996.8, 2877.5, 2795.5,
2102.2, 1871.1, 1580.4, 1476.0, 1401.5, 1267.3, 1051.1, 760.4, 603.8, 544.2, 477.1. 1H NMR
(500 MHz, CDCl3) δ/ppm: 13.52 (1H, s, -OH, D2O exchangeable), 10.28 (1H, s, -CHO), 8.16
(1H, s, ArH), 7.96 (1H, d, J = 8 Hz, ArH), 7.88 (1H, d, J = 8 Hz, ArH), 7.45 (1H, t, J = 7.5
Hz, ArH), 7.34 (1H, t, J = 7.5 Hz, ArH), 6.56 (1H, d, J = 8.5 Hz, ArH), 3.06 (6H, s, N-CH3).
13C NMR (125 MHz, CDCl3) δ/ppm: 186.52, 151.83, 151.80, 126.22, 124.52, 121.81, 121.34,
111.28, 107.99, 44.98. HRMS: m/z calculated for C16H14N2O2S: 299.0849 [M+H]+, Found:
299.0818 [M+H]+.
2.5.6. Synthesis of 2-(3-(benzothiazole-2-yl)-6-(dimethylamino)-2-hydroxybenzylidene)
malononitrile (5)
A solution of 4 (0.1 g, 0.33 mmol), piperidine (0.038 mL, 0.39 mmol) anhydrous THF (20
mL) under inert atmosphere was cooled to 0 oC and a solution of malononitrile (0.048 g, 0.13
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mmol) in anhydrous THF (1 mL) was added dropwise and the reaction stirred at 0 C until
completion (TLC). After the completion of reaction, THF was removed under reduced
pressure. And crude was washed with water (2x20 mL) and DCM, the organic layer was
dried over anhydrous sodium sulphate. The solvent removed under reduced pressure to obtain
crude 5, which was purified by column chromatography using 35:65 (ethyl acetate/hexane;
v/v) as eluents to isolate analytically pure 5 as yellowish orange solid. Yield 60%. M.p. = 200
oC. IR (KBr): νmax 3302.4, 3063.9, 2862.6, 2117.1, 1990.4, 1900.9, 1736.9, 1654.9, 1580.4,
1289.7, 1192.7, 939.3, 760.4, 626.2, 484.64. 1H NMR (500 MHz, CDCl3) δ/ppm: 8.54 (1H, d,
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J = 8 Hz, ArH), 8.08 (1H, s, C=C−H), 8.068 (1H, d, J = 8.5 Hz, ArH), 7.96 (1H, s, -OH, D2O
exchangeable), 7.95 (1H, d, J = 8 Hz, ArH), 7.52 (1H, t, J = 7.5 Hz, ArH), 7.42 (1H, t, J = 7.5
Hz, ArH), 6.88 (1H, t, J = 9 Hz, ArH), 3.01 (6H, s, N-CH3). 13C NMR (125 MHz, DMSO-d6)
δ/ppm: 160.25, 155.04, 152.54, 152.07, 135.64, 134.47, 126.35, 125.11, 122.84, 121.42,
118.71, 114.87, 113.33, 77.23, 44.95. HRMS: m/z calculated for C19H14N4OS: 347.0961
[M+H]+, Found: 347.0774 [M+H]+.
2.5.7. Synthesis of (E)-2-(3-(3-(benzothiazole-2-yl)-6-(dimethylamino)-2-hydroxystyryl)-
5,5-dimethylcyclohex-2-en-1-ylidene)malononitrile (7)
Similarly, using 4 (0.16 g, 0.33 mmol), piperidine (0.038 mL, 0.39 mmol) and 2-(3,5,5-
trimethyl-2-cyclohexenylidene)malononitrile (0.13 g, 0.7 mmol) in THF (10 mL) and stirring
the reaction at 40 oC for 24 h furnished 7 as red solid, after extractive work up of the reaction
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followed by purification as described above. Yield 30%. M.p. = 175-180 C. IR (KBr): νmax
3071.3, 3011.7, 2922.2, 2788.0, 2221.5, 2109.7, 1908.4, 1848.8, 1610.2, 1550.6, 1326.9,
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1103.3, 998.9, 909.5, 797.7, 492.0. H NMR (500 MHz, CDCl3) δ/ppm: 13.83 (1H, s, -OH,
D2O exchangeable), 7.96 (1H, d, J = 8 Hz, ArH), 7.89 (1H, d, J = 8 Hz, ArH), 7.64 (1H, d, J
= 16.5 Hz, C=CH), 7.59 (1H, d, J = 8.5 Hz, ArH), 7.52 (1H, t, J = 8 Hz, ArH), 7.41 (1H, t, J
= 8 Hz, ArH), 7.35 (1H, d, J = 16.5 Hz, C=CH), 6.91 (1H, s, ArH), 6.66 (1H, d, J = 8.5 Hz,
ArH), 2.90 (6H, s, N-CH3), 2.64 (2H, s, CH2), 2.57 (2H, s, CH2), 1.13 (6H, s, -CH3). 13C
NMR (125 MHz, CDCl3) δ/ppm: 169.38, 158.75, 157.41, 156.33, 151.62, 132.44, 132.20,
132.03, 129.61, 126.86, 125.35, 122.92, 122.74, 121.77, 121.49, 115.39, 114.06, 111.13,
109.46, 44.45, 43.20, 38.95, 32.15, 28.22. HRMS: m/z calculated for C28H26N4OS: 467.1900
[M+H]+, Found: 467.1735 [M+1] +.
2.5.8.
Synthesis
of
2-((3-(dimethylamino)-6,6-difluoro-6H-6λ4,7λ4-
benzo[e]benzo[4,5]thiazolo[3,2-c][1,3,2]oxazaborinin-4-yl)methylene)malononitrile (6)
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To the solution of 5 (0.050 g) in DCM (4 mL) was added Et3N (52 µL) at room temperature.
The resulting mixture was stirred for 30 min. After that BF3·OEt2 (54 µL) was added to the
reaction mixture over a period of 15 min and the reaction was stirred until completion (TLC).
The reaction mixture was then diluted with cold water and extracted with DCM. The organic
layer was dried over anhydrous Na2SO4 and solvent was removed under reduced pressure to
obtain crude 6, which was further purified by column chromatography using 15:85 (ethyl
acetate/hexane; v/v) as eluents to yield pure 6 as orange solid in 90% yield. M.p. = 220 oC. IR
(KBr): νmax 3324.8, 2228.9, 1729.5, 1610.2, 1520.8, 1416.4, 1312.0, 1215.1, 1028.7, 767.8,
641.1, 521.8, 432.4. 1H NMR (500 MHz, CD3CN) δ/ppm: 8.72 (1H, s, C=CH), 8.24 (1H, d, J
= 10 Hz, ArH), 8.20 (1H, d, J = 8.5 Hz, ArH), 8.06 (1H, d, J = 8.5 Hz, ArH), 7.79 (1H, t, J =
8 Hz, ArH), 7.68 (1H, t, J = 8 Hz, ArH), 7.06 (1H, d, J = 9.5 Hz, ArH), 3.32 (6H, s, N-CH3).
13C NMR (125 MHz, DMSO-d6) δ/ppm: 160.18, 156.14, 155.17, 153.39, 151.61, 151.06,
134.96, 134.59, 126.53, 125.01, 122.31, 122.14, 115.15, 112.79, 110.78, 107.76, 96.02,
44.40. HRMS: m/z calculated for C19H13BF2N4OS: 433.0506 [M+nK]+, Found: 433.0397
[M+nK]+.
2.5.9.
Synthesis
of
(E)-2-(3-(2-(3-(dimethylamino)-6,6-difluoro-6H-6λ4,7λ4-
benzo[e]benzo[4,5]thiazolo[3,2-c][1,3,2]oxazaborinin-4-yl)vinyl)-5,5-dimethylcyclohex-
2-en-1-ylidene)malononitrile (8)
Similarly, using 7 (0.050 g), Et3N (52 µL) and BF3·OEt2 (100 µL) in CH2Cl2 (4 mL) and
stirring the reaction at room temperature for 3-4 h furnished 8 as red solid, after extractive
work up of the reaction followed by purification as described above. Yield 90%. M.p. = 270-
o
275 C. IR (KBr): νmax 2922.2, 2214.0, 2117.1, 1871.1, 1736.9, 1483.5, 1326.9, 1110.7,
1
797.7, 626.2, 521.8. H NMR (500 MHz, CDCl3) δ/ppm: 8.28 (1H, d, J = 10.5 Hz, ArH),
7.84 (1H, d, J = 9.5 Hz, ArH), 7.62 (1H, dt, Ja = 9.5 Hz, Jb = 1.5 Hz, ArH), 7.51 (1H, dt, Ja =
9.0 Hz, Jb = 1.5 Hz, ArH), 7.48 (1H, d, J = 11 Hz, ArH), 7.40 (1H, d, J = 20.5 Hz, C=CH),
10

248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
7.24 (1H, d, J = 20.5 Hz, C=CH), 6.84 (1H, s, ArH), 6.69 (1H, d, J = 11 Hz, ArH), 2.96 (6H,
13
s, N-CH3), 2.61 (2H, s, CH2), 2.56 (2H, s, CH2), 1.10 (6H, s, -CH3). C NMR (125 MHz,
CDCl3) δ/ppm: 169.58, 159.79, 155.88, 154.81, 132.17, 131.08, 128.59, 127.90, 127.26,
126.67, 122.76, 121.84, 110.64, 106.50, 102.83, 92.71, 43.84, 43.18, 39.08, 32.04, 28.11.
HRMS: m/z calculated for C28H25BF2N4OS: 515.1810 [M]+, 537.1707 [M+nNa]+, 553.1447
[M+nK]+, Found: 515.1809 [M]+, 537.1630 [M+nNa] +, 553.1381 [M+nK]+.
(Spectra shown in Figs. S1-S38, See ESI)
3. Results and Discussion
3.1. Synthesis
The synthesis of the target compounds started from reaction of 2-(4-amino-2-
hydroxyphenyl)benzothiazole 1 with 1-bromo-2-(2-bromo ethoxy)ethane to obtain 2-(4-
morpholino-2-hydroxyphenyl)benzothiazole 2a. Compound 1 in turn was prepared from the
reaction of 4-aminosalicylic acid and 2-aminobenzenthiol. Compound 2a was efficaciously
propargylated to 2-(4-morpholino-2-(prop-2-yn-1-yloxy)phenyl)benzothiazole
3
upon
treatment with propargyl bromide under base catalysed reaction conditions (Scheme 1).
Compound 1 was also reductively alkylated to obtain 2-(4-dimethylamino-2-hydroxy
phenyl)benzothiazole 2b in 80% yield. Vilsmeier-Haack formylation of 2b yielded 2-(4-
dimethylamino-3-formyl-2-hydroxyphenyl)benzothiazole 4, which upon piperidine catalysed
Knoevenagel condensation reaction with malononitrile and 2-(3,5,5-trimethylcyclohex-2-en-
1-ylidene)malononitrile furnished compounds 2-(3-(benzothiazole-2-yl)-6-(dimethylamino)-
2-hydroxybenzylidene)malononitrile
5
and
(E)-2-(3-(3-(benzothiazole-2-yl)-6-
(dimethylamino)-2-hydroxystyryl)-5,5-dimethylcyclohex-2-en-1-ylidene)malononitrile
7,
respectively. Finally, treatment of 5 and 7 with borontrifluoride etherate in anhydrous DMF
in the presence of triethylamine gave 2-((3-(dimethylamino)-6,6-difluoro-6H-6λ4, 7λ4-benzo
[e]benzo[4,5]thiazolo[3,2-c] [1,3,2]oxazaborinin-4-yl)methylene) malononitrile 6 and (E)-2-
11

273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
(3-(2-(3-(dimethylamino)-6,6-difluoro-6H-6λ4,7λ4-benzo[e]benzo[4,5]thiazolo [3,2-c] [1,3,2]
oxazaborinin-4-yl)vinyl)-5,5-dimethylcyclohex-2-en-1-ylidene)malononitrile 8, respectively
(Scheme 1). All compounds were characterized using spectroscopic (1H NMR, 13C NMR,
HRMS) data (Figs. S1-S38, See ESI).
O
HO
HO
N
S
N
S
N
S
(iii)
(i)
N
O
NH2
N
O
2a
1
3
(ii)
CN
CN
HO
HO
CHO
HO
N
N
S
(iv)
N
(v)
NMe2
NMe2
NMe2
S
S
2b
4
5
(vi)
(vii)
HO
7
CN
CN
CN
CN
F
CN
F
F
B
O
F
B
O
CN
N
S
(vi)
N
S
N
S
NMe2
NMe2
NMe2
8
6
Scheme 1. Reagents, reaction conditions and yield (%): (i) Et3N/DMF (anhyd.)/1-bromo-2-
o
(2-bromoethoxy)ethane/140 C (55%); (ii) NaCNBH3/paraformaldehyde/AcOH/r.t. (80%);
o
(iii) NaH/DMF (anhyd.)/propargyl bromide /r.t. (63%); (iv) DMF/POCl3/80 C (23%); (v)
o
THF (anhyd.)/piperidine/malononitrile/0 C/N2 (60%); (vi) BF3.OEt2/DCM/Et3N/r.t. (90%);
(vii) 2-(3,5,5-trimethylcyclohex-2-en-1-ylidene)malononitrile/THF/piperidine, 40 oC, N2
(30%).
3.2. Optical properties
Compounds 1-8 showed structure dependent absorption as well as emission behaviour (Fig.
1) (For detailed spectra, see Fig. S39 in ESI). Thus, replacing NH2 group in 1 with
morpholino or NMe2 in 2a and 2b saw low energy shift in the absorption band of 1 at 358 nm
12

298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
(1 x 10-5 M in THF) to appear at 376 and 366 nm, respectively. Compound 5 shows two
bands: at 338 nm (εmax: 52400 molcm-1L-1) and 418 nm (εmax: 9000 molcm-1L-1) attributed to
π-π* (H→L+1 and H→L) transitions, respectively. The latter observed low energy shift to
appear at 442 nm (εmax: 6200 molcm-1L-1) in the oxazaborinin compound 6.
2b
1
0.8
0.6
0.4
0.2
0
1
3
5
6
7
8
0.8
0.6
0.4
0.2
0
2b
3
5
6
7
8
(b)
270
370
470
570
670
Wavelength (nm)
(a)
Fig. 1 (a) UV-visible absorption spectra (dotted line) of 2b, 3, 5-8 (1 x 10-5 M in THF) and
fluorescence emission spectra (solid line) of 2b, 3, 5-8 (1 x 10-5 M in THF); (b) Photographs
of respective solutions under illumination at 365 nm.
313
314
315
316
317
318
319
320
321
Likewise, compound 7 showed major absorption band at 370 nm (εmax: 62,100 molcm-1L-1)
(H-2→L, π-π*), the corresponding oxazaborinin derivative 8 showed intense band at 408 nm
(εmax: 1,56,900 molcm-1L-1; H-1→L, π-π*) attesting to significant low energy shift as well as
increase in extinction coefficient in the oxazaborinin. The bathochromic shifts observed in
oxazaborinin 6 and 8 relative to corresponding non-chelated 5 and 7 respectively, are well
supported by the TD-DFT calculations (Fig. 2) (Fig. S40, see ESI). The π-π* bands of these
compounds did not show a regular trend in solvent dependence in the absorption spectra
recorded in solvents of different polarity, nor was any solvatofluorism trend observed (Fig.
S41-45, Tables S7, S8, See ESI).
13

322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
These compounds showed contrasting yet interesting emission behaviour (Fig. 1). The
compound 1 showed emission bands at 402 nm and 520 nm, corresponding to enolic and keto
emission of the typical ESIPT chromophores.
(5)
(6)
(7)
(8)
L+1 (-1.87296 eV)
3.84987 eV
L (-2.70633 eV)
L (-2.7157 eV)
L (-2.83325 eV)
3.6866 eV
3.25405 eV
3.40469 eV
H (-5.72283 eV)
H (-5.96038 eV)
H-1 (6.12039 eV)
H-2 (-6.51985 eV)
Fig. 2 The contour surfaces of the frontier molecular orbitals HOMO (H) and LUMO (L)
contributing to the electronic transitions of 5-8 obtained by TD-DFT calculations using
B3LYP/6-31G (d)/CPCM/ THF phase at an isovalue of 0.02. (Fig. S40, See ESI)
339
340
341
342
343
344
Compounds where NH2- group was replaced with a strong electron donor: morpholino
(
2a) or Me2N (2b) and when the OH was converted into
only enolic emission at 408 nm (2a 2b) and 420 nm ( ). Owing to structural features,
chromophores appended with electron withdrawing groups: dicyanovinylidene in
5,5-dimethylcyclohex-2-en-1-ylidene malononitrile in or the chelated counterparts:
o-propargyl group (3) showed
,
3
5
,
7
oxazaborinins
6
and
8
showed emission at unexpectedly lower energy (Fig. 1)
14

345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
compared to CHO appended
465 nm and 517 nm, respectively (For detailed spectra see Fig. S39 in ESI).
Compounds and show green and yellowish-orange emission at 514 nm and 600 nm
when excited at 340 nm and 400 nm, respectively (Fig. 1). The blue shift of 6 nm in
the emission band of in comparison to the compound is believed to be due to the
deviation of the benzothiazole unit from coplanarity with the remainder of the
4, which showed both enolic as well as keto emission at
5
7
5
1
molecule in compound
oxazaborinin derivatives
5
as predicted by the dihedral angles (Fig. S46). The
and show emission bands at 557 nm and 582 nm (λexc:
6
8
340 nm and 440 nm, respectively), corresponding to yellow and orange colours,
respectively. The blue shift observed in the emission band of 8 in comparison to 7 is
believed to be due to the perturbation in the π-conjugation on complexation with
boron(III). The quantum yields in THF are given in the Table 1 (For the fluorescence
quantum yields in various solvents see Table S9 in ESI).
In combination with a blue emitting compound such as
3 (λem: 420 nm; λexc: 300 nm),
chromophores were expected to yield a complementary combination covering
5-7
emission in the range 420-600 nm, yielding nearly white overall emission. White light
emission from a single molecular system has been very challenging and is scarcely
reported [18-22], while the independent emission of red, blue and green fluorophores
grafted in a covalent conjugate, is often hampered by the operation of either
intramolecular Foster Resonance Energy Transfer (FRET) and/or Through Bond
Energy Transfer (TBET) [23-26]. Thus, identification of white light emitting binary or
ternary combination of materials is highly sought after and challenging task. Literature
records several instances of white light emission wherein combination of materials
emitting different colours have been combined as represented by supramolecular
assemblies [4a], polymers [27,28], dye-doped soft materials such as organogels
15

370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
[6b,29-31] or even composites [32,33]. Many such systems are based on photo-
physical processes such as ESIPT [34], intramolecular charge transfer (ICT) [35,36] or
operation of tautomerization [37-39] by pH control and aggregation leading to
existence of molecules in more than one form, each displaying its discrete emission
properties. This prompted us to calculate chromaticity coordinates of these compounds
and to formulate a suitable combination, which upon mixing in a specific proportion
would yield white emission. The chromaticity coordinates were calculated using the
method recommended by CIE by summation (360-780 nm) of the emission data,
relative energy distribution of the CIE standard D65 illuminant and colour matching
functions corresponding to CIE 1931 standard (2o) observer. While the emission
behaviour of the solutions of the chromophores and their representative combinations,
relevant to white emission production are shown in Fig. 3a,b (Fig. S47, See ESI) their
placement in the CIE chromaticity diagram, plotted using chromaticity coordinates (x
and y) corresponding to D65 is shown in Fig. 3c. Since the line joining the points
corresponding to
3 (x = 0.1636; y = 0.0418) and 6 (x = 0.3887; y = 0.5368) in the
chromaticity diagram passes through the white region corresponding to the illuminant
D65, these colours constitute complementary spectral colours and their combination,
M7 (3:6, 1.5:8.5 v/v) corresponded to white emission (x = 0.35753, y = 0.3315) (Table
1). Interestingly, emission of this combination covered full spectral region ranging
from 400-700 nm with prominent dual emission at 398 nm and 564 nm (Fig. 3b).
Further, since the absorption and emission behaviour of
(no energy transfer) the dual emission bands of the combination M7 represent a
superposition of the individual emission bands of (420 nm) and (557 nm) (Fig.
S48, See ESI) pointing to the production of a stable white emission from M7
3 and 6 operate independently
3
6
.
Similarly, wavelength of maximum absorbance of M7 in THF was at 338 nm, which is
16

395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
comparable to both
3 (346 nm) and 6 (338 nm). On the contrary, combination of 3, 5
and M1), consisting of chromophores lacking oxazaborinin ring did not produce as
7
(
white emission as M7, as it lacked significant red emission. Thus, oxazaborinin ring is
seen to enhance red emission, leading to white light emission by the mixture M7 (Fig.
3d) (For the Stokes shifts see the Table S 10 in ESI).
n
3
(a)
(b)
n
3
1
0.8
0.6
0.4
0.2
0
1a
2a
1
0.8
0.6
0.4
0.2
0
5
1b
6
7
5y2
M7
3m
M1
360
460 560
Wavelength (nm)
660
360
460 560
Wavelength (nm)
660
M7
M1
(d)
(c)
Fig. 3 Fluorescence emission spectra of (a) 3, 5 and 7 (1 x 10-5 M) and their mixture M1
(3:5:7, 1:1:3, v/v; 5 x 10-5 M); (b) 3 and 6 (1 x 10-5 M) and their mixture M7 (3:6, 1.5:8.5 v/v;
5 x 10-5 M) in THF; (c) placement of the colors in the CIE chromaticity diagram (1931,
D65/2o observer) based on the emission of respective solutions. The line connecting 3 and 6
passes through white region of standard illuminant; (d) Photographs of M1 and M7 in THF
under illumination at 365 nm.
In order to probe the white light emission in the solid state, we recorded fluorescence
emission spectra of solid admixture of the compounds (0.001 g) as well as their
combinations, mixed uniformly, with silica gel (0.001 g, 60-120 mesh) using THF as
annealing solvent. The emission profile of the compounds (Fig. 4, Fig. S49) reveals
17

420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
emission covering blue (3: λem. 445 nm, λexc. 300 nm), yellow (5: λem. 575 nm, λexc. 340
nm), yellowish-orange (6: λem. 601 nm, λexc. 400 nm), orange-red (7: λem. 695 nm, λexc. 440
nm) and red (8: λem. 711 nm, λexc. 400 nm) regions.
Table 1. CIE chromaticity coordinates (x,y) and the quantum yields of 3, 5-8 and various
mixtures in THF.
Code (in CIE
Compound/
Ratio
(v/v)
CIE chromaticity
coordinates (x,y)
0.16365, 0.0418
0.23773, 0.60629
0.38876, 0.53679
0.54435, 0.42603
0.47834, 0.49258
0.24032, 0.35897
0.20122, 0.21411
0.25126, 0.44117
0.18227, 0.13384
0.23027,0.15187
0.26486,0.21156
0.35732,0.3315
0.3737,0.39259
Quantum
yield (Φf)
0.308a
0.394b
0.247b
0.335b
0.163b
0.048a
0.097a
0.134a
0.135a
0.019a
diagram)
mixture
3
3
-
5
5
-
6
6
-
7
7
-
8
8
-
M1
M2
M3
M4
M5
M6
M7
M8
3+5+7
3+5+7
3+5+7
3+5+7
3+6
3+6
3+6
3+6
(1 : 1 : 3)
(2 : 1: 2)
(1: 3 : 1)
(3 : 1: 1)
(1 : 1)
(1 : 4)
(1.5 : 8.5)
(1 : 9)
0.002a
0.0055a
0.0089a
b
437
438
a9,10-Diphenylanthracene was used as a reference dye (Φf = 0.90 in cyclohexane). Fluorescein was used as a
reference dye (Φf = 0.79 in 0.1M aqueous NaOH).
439
440
441
442
443
444
The bathochromic shift in the emission of compounds 5-8 (λem: 575-711 nm) in solid state in
comparison to 3 (λem: 445 nm) is attributed to the stabilisation of the excited state due to the
presence of electron withdrawing groups in analogy with the observation in solution phase
emission pattern. Further, the red tuning of the emission in 6 and 8 in comparison to their
corresponding compounds 5 and 7 respectively, can be attributed to the additional
contribution from the boron(III) atom in rigidifying the chromophore, thereby extending the
18

445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
planarity and charge delocalisation along the major molecular axis. It is well supported by the
molecular geometries and the small dihedral angles calculated by DFT calculations (Fig. S46,
See ESI).
3
3c
(a)
5
5c
(c)
1
0.8
0.6
0.4
0.2
0
6c
6
M1
mM44
n+1 (p)
400
500 600
Wavelength (nm)
700
M4
3
5
6
M1
(b)
Fig. 4 (a) Fluorescence emission of 3, 5, 6 and their mixtures M1 (3:5, 1:1 w/w) and M4 (3:6,
1:1.2 w/w) in solid state; (b) Photographs of respective compounds and mixtures under
illumination at 365 nm and (c) placement of the colours in the CIE chromaticity diagram
(1931, D65/2 degree observer) based on the emission of respective solutions. The line
connecting 3 with 5 and 6 passes through white region of standard illuminant.
461
462
463
464
465
466
467
468
Considering the complementarity of the blue color with orange, we formulated
different combination of
chromaticity diagram revealed that the line connecting the point corresponding to
with or , passes through the white region corresponding to D65. However, the best
white emission was obtained from the 1:1 blend of with , represented by the blend
M1 for which the CIE chromaticity coordinates were x = 0.34753 and y = 0.32062
3 with 5 and 6 in the solid state and placing these in the CIE
3
5
6
3
5
(Fig. 4c) (Table 2). Similarly, M4 representing a combination of
3 and 6 (1:1) yielded
white color corresponding to x = 0.31484 and y = 0.2806.
19

469
470
Table 2. CIE chromaticity coordinates (x,y) of 3, 5-8 and various mixtures in solid state.
Sr.
Code (in CIE
Compound/
mixture
3
Ratio
(w/w)
-
CIE chromaticity
coordinates (x,y)
0.15393, 0.11655
no. diagram)
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
1
2
3
4
5
6
7
8
9
3
5
5
6
-
0.44138, 0.43952
0.48693, 0.4966
0.67264, 0.31961
0.56116, 0.31666
0.34753, 0.32062
0.26585, 0.25242
0.2404, 0.1695
6
-
-
7
7
8
8
-
M1
M2
M3
M4
3+5
3+5
3+6
3+6
(1 : 1)
(2 : 1)
(1.3 : 1)
(1 : 1)
0.31484, 0.2806
4. Conclusion
In summary, here we present a case of arresting ESPIT of benzothiazole based ESIPT
chromophores by substitution of the phenyl ring with electron withdrawing groups
and/or locking the ESIPT sites as oxazaborin in rings. This case is unique and different
from earlier reports as in those reports either ESIPT was employed to circumvent the
energy transfer between two chromophores [26] or through modulation of
photoluminescence quantum yields by forming N^O chelates [40]. Our case presents
an example of additive mixing of solutions as well as solid colours to produce white
light emission.
5. Acknowledgements
20

492
493
494
495
496
497
KS is thankful SERB (DST), New Delhi for the financial support under the project
EMR/2017/000520. IK thanks DST, New Delhi for an INSPIRE fellowship and Guru
Nanak Dev University for providing facilities.
Supporting Information
Spectral data, photophysical studies/data, optimized structures, DFT/TD-DFT calculations,
and complete reference of Gaussian09 [15].
498
References
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
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515
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517
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519
520
521
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523
524
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26

Highlights:
•
•
•
•
Benzothiazole based molecular species and their corresponding rigidified derivatives
with Boron(III) are synthesised.
The rigidified derivatives are characterised by the locked excited state intramolecular
proton transfer based emission behaviour.
Using binary combination of the molecular species with the complimentary color
lights, nearly white light emission, both in solution as well as solid state are obtained.
The CIE coordinates in solution and in solid states are : x = 0.35753, y = 0.3315 and x
= 0.34753, y = 0.32062) respectively.