Intriguing photophysical aspects of nitro substituted 2-(2′-hydroxyphenyl)benzothiazole (HBT) derivative: AIE in non-polar media?

Article abstract of DOI:10.1016/j.chemphys.2020.111032

Excited photophysics of a synthesized molecule namely 2-(benzo[d]thiazol-2-yl)-5-nitrophenol (BTNP) have been studied using steady state and time resolved techniques. BTNP exhibits ESIPT exclusively in non-polar media with exceptional Stokes’ shift. In po

Full text of DOI:10.1016/j.chemphys.2020.111032

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Intriguing Photophysical Aspects of Nitro Substituted 2-(2'-hydroxyphen‐
yl)benzothiazole (HBT) Derivative: AIE in Non-Polar Media?
Arghyadeep Bhattacharyya, Nikhil Guchhait
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DOI:
Reference:
https://doi.org/10.1016/j.chemphys.2020.111032
CHEMPH 111032
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Please cite this article as: A. Bhattacharyya, N. Guchhait, Intriguing Photophysical Aspects of Nitro Substituted
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Intriguing Photophysical Aspects of Nitro Substituted 2-(2'-hydroxyphenyl)benzothiazole
(HBT) Derivative: AIE in Non-Polar Media?
Arghyadeep Bhattacharyya, Nikhil Guchhait*
Department of Chemistry, University of Calcutta, 92, A.P.C. Road, Kolkata-700009, India
*Corresponding author. Tel.: +913323508386; fax: +913323519755.
E-mail address: nguchhait@yahoo.com (N. Guchhait).
Abstract: Excited photophysics of a synthesized molecule namely 2-(benzo[d]thiazol-2-yl)-5-
nitrophenol (BTNP) have been studied using steady state and time resolved techniques. BTNP
exhibits ESIPT exclusively in non-polar media with exceptional Stokes’ shift. In polar solvents
anion formation of BTNP hampers the ESIPT process. Solid state emission spectral pattern
matches reasonably well with that in non-polar medium. Dynamic light scattering showed the
increment of particle size in heptane in comparison to water, a trend reversed to conventional
trait of Aggregation Induced Emission (AIE) active molecules. Nitro substituent in BTNP
reverses the conventional definition of a ‘good solvent’ and ‘bad solvent’, thereby promoting
ESIPT in non-polar medium via aggregation. As all reports on AIE active compounds claim
water to be the solvent capable of producing the effect, we have reversed the effect by simple
change in substitution. Reversal of AIE phenomenon would certainly open up new avenues in
future research on AIE active fluorophores.
Keywords: Substituted hydroxy benzothiazole, Fluorescence, ESIPT, AIE

Introduction
The discovery of aggregation induced emission (AIE) by Ben Zhong Tang and coworkers [1-4]
has revolutionized the design of various analytes capable of sensing, bioimaging and other
optoelectronic applications. The concept of AIE has been very well depicted by Zhou et al [5] in
their article regarding demonstration of AIE phenomenon to undergraduate chemistry students.
The aforesaid group suggested the enhanced luminescence of AIEgens in water due to restriction
of intramolecular rotation (RIR) [6]. This RIR has been regarded as the preliminary and
necessary phenomenon behind a molecule to be treated as an AIEgen. Till date, a large number
of groups have successfully designed and applied AIE active compounds [7-11]. However, the
entire phenomenon of AIE has been observed in water as revealed by our literature survey. Due
to this reason, any compound is treated as AIE active based on its luminescence aptitude in water
and this has become the usual definition of the mentioned process. We came across a report by
Ohshita et al [12] where sulfonated TPE derivative showed AIE characteristic in organic media.
Inspired by this report, we decided to generalize the definition of an AIE active compound itself.
For this purpose, a nitro substituted compound 2-(benzo[d]thiazol-2-yl)-5-nitrophenol (BTNP)
was synthesized and its photophysical properties were explored. BTNP belongs to the class of
hydroxyl benzothiazole (HBT) derivatives, a class well explored by chemists for intramolecular
excited state proton transfer reaction [13-15]. BTNP has a nitro group attached para to the
phenolic –OH group present in the molecule. It was observed that BTNP shows intense emission
intensity in the non- polar solvents. However, the emission intensity drops upon introduction of
polar solvents which could be visible through naked eye color of the solutions of BTNP under
UV light. The emission of BTNP was attributed to ESIPT operative in the same. The poor
emission could also be rationalized from the well-known inhibition of ESIPT in polar solvents

[16]. However, the particle size of BTNP in a non-polar solvent n-heptane was found to be twice
of that in water, as evidenced from Dynamic Light Scattering (DLS) experiments. This prompted
us to infer that BTNP portrays the coupling of AIE and ESIPT, however the coupling being
realized in non-polar solvent rather than water. Poor solvation of the very polar BTNP in heptane
due to the presence of nitro group results in the formation of aggregates, whereas moderate to
excellent solvation in polar solvents accompanied with formation of anion of BTNP leads to loss
of AIE as well as ESIPT. Although several other groups had showed the coupling of AIE and
ESIPT [17-18], the aforementioned coupling has been realized in water only. This is the first
time that AIE and ESIPT coupling is being reported in strictly non-polar medium. This work thus
aims to generalize the definition of AIE by demonstrating that solvent in which a compound
would show AIE depends solely on the design of the molecule. Furthermore a corollary of this
report is that the classification of good and bad solvents [19] needs to be treated as a molecule
specific rather than a universal one. Hence, our current report has its own worth as it generalizes
the phenomenon of AIE and would thus encourage researchers to design such compounds to
portray the ‘reverse AIE’ effect in future.
Experimental Section
Materials and methods.
5-nitrosalicylaldehyde and 2-aminothiophenol were purchased from Merck and used as received.
Spectroscopic grade solvents were purchased from Spectrochem. Triple distilled water was used
1
for spectroscopic measurements. H NMR spectra were recorded on a Bruker Advance 300
spectrometer, where chemical shifts ( in ppm) were determined with respect to

Tetramethylsilane (TMS) as the internal standard. IR spectrum was recorded on Perkin Elmer
spectrum-100. ESI-MS was recorded in Waters Xevo G2-S Q TOF mass spectrometer. Steady
state electronic absorption and fluorescence spectra were recorded on a Shimadzu UV–1900
spectrophotometer and HORIBA JobinYvon Fluorolog 3 (Model 3-11), respectively.Time
resolved emission was recorded in Horiba Delta Flex NL-01 fluorimeter. The goodness of fitting
was judged using χ2 criteria [20]. The average lifetime was calculated using standard procedures
[21].
Synthesis of BTNP
BTNP was synthesized using our previously reported method [22] according to Scheme 1 and
characterized by NMR and FTIR spectroscopy (ESI, Figure S1-S4).
S
O2N
NH2
SH
O2N
CHO
OH
N
MeOH, I2
OH
2-(benzo[d]thiazol-2-yl)-4-nitrophenol(BTNP)
Stirring, 65%
Scheme 1: Synthetic route of BTNP.
Results and discussions.
The steady state absorption spectra of BTNP were recorded in various solvents (Figure 1a, Table
1). BTNP showed two prominent absorption bands positioned at ~290 nm and ~340 nm.
Compared to the parent molecule 2-(2’-hydroxyphenyl)benzothiazole (HBT) which shows

absorption bands at ~270 nm and ~325 nm respectively [23], the red shift in absorption maxima
could be an outcome of chromophoric stabilization upon introduction of nitro group. The low
energy band was structured in nature due to the presence of various conformers in solution as
was observed by Chou et al [24]. The high energy band has been assigned due to the open form
of the molecule and the low energy band due to the hydrogen bonded form. In polar solvents, an
additional band appeared at ~425-440 nm depending on the polarity and proticity of the solvent.
In DMSO, the additional band appeared at ~440 nm, whereas that in methanol was at ~410 nm.
This could be accounted on the basis of hydrogen-bonded clusters formed in the ground state in
protic solvents. Upon adding triethylamine as base to an acetonitrile solution of BTNP,
additional band appeared at ~430 nm. Hence, this was assigned due to anion formation in
polar/protic solvents. Once again, although the anion of the parent molecule HBT absorbs at a
wavelength ~380 nm, presence of nitro group accounts for the red shift in the absorption band of
the anion. Following the results of steady state absorption spectra, the emission spectra of BTNP
were recorded (Figure 2a, Table 1). Upon exciting at 340 nm, an intense emission band was
obtained at ~500 nm (Figure 2a) in non-polar solvents. Following literature reports on parent and
similar series of molecule [15], this band was assigned due to emission from the keto form of
BTNP, a consequence of ESIPT. In polar solvents, the emission intensity decreased markedly, as
reflected from the quantum yield values (Table 1). The excitation spectra could only be recorded
for the solvents Heptane, Cyclohexane, Methylcyclohexane, Chloroform and Dichloromethane
where the excitation spectra closely resembled the low energy band of the UV spectrum,
indicating the band having exceptionally high Stokes Shift arises from the band at ~340 nm. For
polar solvents, the intensity was very poor to provide a clean spectrum. In DMSO, a high energy
band appeared at ~430 nm, which was assigned to be due to emission from enol form or local

emission of BTNP as the emission from the keto form is observed at 500nm . Addition of base
resulted in diminishing of the ESIPT band (Figure 2b). Although the anionic form of HBT
derivatives are reported to be emitting at ~462-470 nm [15], presence of the electron
withdrawing nitro group probably leads to the non-emissive nature of the anion itself. In protic
solvents, although the ESIPT band was found, the intensity was very poor as in the case of polar
solvents (ESI, Figure S5). As already mentioned in the introduction section, this decrement in
ESIPT emission band intensity was considered to be due to hampering of ESIPT in polar and
protic solvents. The equilibrium operative between the anion and neutral form of BTNP is
particularly responsible for the poor emission of BTNP in polar solvents, since the anion of the
molecule is practically non-emissive. Our claims of anion formation in polar aprotic as well as
protic solvents were further fortified from the literature reports [25-27]. The solid state emission
spectra of BTNP showed high quantum yield efficiency with maxima at ~500 nm (Figure 3a).
This was due to ESIPT being highly favorable in the solid state. Moreover, the color of solid
BTNP under UV-light resembled exactly the color of a solution of BTNP in non-polar solvents
(Figure 3b). No color was observed under UV light for solutions of BTNP in polar solvents.
Since AIE active molecules in their aggregated form in solution emit at the same wavelength as
in the solid state, we considered examining whether non-polar solvents are favoring aggregation
of BTNP. For this purpose, chloroform was added gradually to an acetonitrile solution of BTNP,
which resulted in gradual intensity enhancement of BTNP (Figure 2c). Hence, in non-polar
medium, BTNP experiences an enhancement in emission intensity. This could be rationalized by
considering the poor solvation of the nitro group in non-polar solvents, which could prompt the
aggregation of BTNP in non-polar solvents. To gain

Table 1. Steady State spectral properties of BTNP
Solvent
Stokes Shift
(cm-1)×105
9.94
ε340(moll-1cm-
1)×104
2.08
ΦF
maxabs (nm)
maxem (nm)
H
280, 334
285, 340
500
500
0.87
0.90
0.85
0.14
0.08
0.02
0.11
0.02
0.03
0.01
Cy
9.41
2.16
MCH/Mcy
CHCl3
DCM
288, 342
500
9.24
1.66
287, 332
500
10.12
9.85
2.44
287, 335
500
2.64
CAN
290, 345, 431
280, 340, 450
280, 360, 405
280, 360, 410
280, 360, 410
500
8.98
0.90
DMSO
MeOH
iPrOH
W
430
9.41
0.56
428, 498
430, 530
435, 500
7.78
1.56
7.78
1.90
7.78
1.10
* Quantum yield was calculated using anthracene as standard. Solvent abbreviation Index is
provided in Table S1 of Supporting Information Section. Quantum yield is calculated for PT
band using Anthracene in Cyclohexane as standard, whereas LE band was calculated for DMSO
only.
stronger support in favor of aggregation in non-polar solvent, DLS measurements were carried
out in heptane and water (Figure 4). Interestingly, in heptane, the particle size was ~680 nm,
whereas in water the same was reduced to ~218.5 nm. As mentioned earlier, the poor solvation
of the nitro group in heptane causes aggregation in the mentioned solvent, whereas anion
formation along with the excellent solvation of nitro functionality provided by polar and protic
solvents restricts BTNP from forming aggregates. This strongly indicated aggregation formation

in heptane due to increase of particle size, a necessary condition for a fluorophore to be treated as
AIE active [28]. Hence, it could be stated from steady state spectral studies and DLS
measurements that BTNP portrays coupling of AIE and ESIPT. However, the coupling occurs in
non-polar medium which is a unique feature. To gain further insight in to the excited state
lifetime of BTNP, time resolved fluorescence experiments were performed (Figure 5a, Table 2).
In non-polar solvents, the decay of BTNP was strictly mono-exponential with a lifetime of ~3.2
ns. Such high lifetime value is atypical of normal HBT derivatives in solution since the lifetime
of the keto tautomer has been reported to be in the order of a few hundred picoseconds [29].
However, Padalkar et al reported similar lifetime values in the solid state of some HBT
derivatives [30]. Thus, it could be concluded that formation of aggregate in non-polar solvents
enhances the emission lifetime of BTNP due to arrest of non-radiative decay pathways.
However, in polar solvents the lifetime was very short compared to that in non-polar solvents.
This could be due to ease of non-emissive anion formation coupled with the well-known
decrement of ESIPT lifetime in polar solvents. In water and methanol, no fitting could be
provided owing to the decay falling beyond the instrumental response function. Fluorescence
lifetime of the local emission form was considerably high in DMSO and isopropanol, which
could be a consequence of the viscosity of the solvents. Support in favor of aggregate formation
was also provided by recording the anisotropy of BTNP as aggregated form of BTNP is
expected to show slower correlation time compared to its free form. Keeping this in mind, time
resolved anisotropy measurements were carried out (Figure 6). In heptane, the initial anisotropy
value was ~0.1 and the decay was monoexponential in nature (Figure 6a). The rotational
correlation time was ~0.60 ns. However, in isopropanol the anisotropy decay pattern could not
be fitted into any acceptable exponential forms, mainly due to very fast decay of anisotropy

(Figure 6b). Hence, anisotropy studies indicated some motional restriction present in BTNP in n-
heptane [28]. At this stage, it could be collectively stated from the experimental results that
BTNP forms aggregates in non-polar solvents, which in turn promotes the ESIPT process in the
compound. The poor solvation of the nitro group in non-polar medium triggers the aggregation
process, whereas in polar/protic solvents, the equilibrium operative between solvated BTNP and
its anion renders the compound practically non-emissive. Thus, by judicious design of BTNP,
we could reverse the circulated definition of ‘good’ and ‘bad’ solvents as well as the reversal of
AIE effect were achieved.
Fig. 1 (a) Absorption profile of BTNP (10 µM) in various solvents, (b) Effect of addition of base
(triethylamine, 0-2 equivalents) on the absorption profile of BTNP (10 µM) in acetonitrile.

Fig. 2 (a) Emission profile of BTNP (5 µM) in various solvents (ex=340 nm), (b) Effect of
addition of base (triethylamine, 0-2 equivalents) on the emission profile of BTNP (5 µM) in
acetonitrile, (c) Effect of increasing percentage volume of chloroform of in acetonitrile solution
of BTNP.

Fig. 3 (a) Solid state emission profile of BTNP (ex=340 nm),(inset) color of BTNP under (i)
ambient light and (ii) UV-light; (b) Color of solutions of BTNP (20 µM) in (from left) Heptane,
Cyclohexane, Methylcyclohexane, Chloroform and Dichloromethane under UV-light, (c) Color
of solutions of BTNP (20 µM) in (from left) Heptane, Chloroform, Acetonitrile, DMSO,
Methanol, Isopropanol and Water under UV-light.

Fig. 4 DLS profiles of BTNP (5 µM) in; a) n-heptane and b) water.
Fig. 5 Decay profiles of BTNP (5 µM) in various solvents (ex=336 nm).

Fig. 6 Anisotropy decay profiles of BTNP (5 µM) in (a) n-heptane and (b) Isopropanol.
Table 2. Excited State decay parameters of BTNP (mon = 500nm).
Solvent
τ1(ns)/α1(%)
3.14
τ2(ns) /α2(%)
χ2
H
-
1.09
1.05
1.01
1.06
1.10
1.17
1.02
Cy
3.09
-
MCH/Mcy
CHCl3
3.06
-
0.71
-
DCM
0.68
-
ACN
0.12 (95)
1.21 (75)
1.63 (5)
8.34 (25)
DMSO (mon
430 nm)
MeOH
iPrOH
=
-
-
-
1.12
-
0.3 (94)
-
2.59 (6)
-
W

CONCLUSIONS
A nitro substituted HBT derivative BTNP was synthesized and its photophysical properties were
explored. BTNP shows ESIPT in non-polar medium associated with very high quantum yield. In
non-polar medium, BTNP shows an increment in particle size as evidenced by DLS studies. In
non-polar solvents, the poor solvation of nitro group leads to AIE, which in turn promotes ESIPT
process. However, in polar solvents, the competing anion formation of BTNP and solvation of
nitro group contribute to low quantum yield and hampering of ESIPT. Thus, judicious design has
led to achievement of AIE coupled ESIPT in non-polar medium. Therefore the current report is
expected to shed new light on the design of molecular probes for AIE research.
Declaration of interests
The authors declare that they have no known conflict of interests reported in this paper.
ACKNOWLEDGMENTS
AB would like to thank CSIR for granting fellowships. NG thanks DST, India (Project No.
EMR/2016/004788) and CSIR, India (Project No. 01(2920)18/EMR-II) for financial support.
Appendix A. Supplementary information
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TOC
Photophysical investigation of synthesized probe BTNP shows it undergoes coupling of AIE and
ESIPT in strictly non-polar medium, as opposed to conventional trend of AIEgens forming
aggregates in water only. Hence, BTNP was used to generalize the definition of AIE
phenomenon itself.
Arghyadeep Bhattachrayya: Data collection, Formal analysis of data, Writing draft.
Nikhil Guchhait: Supervision, Conceptualization, data analysis, correction and finalized the
draft, Acquisition of Financial support.
Declaration of interests

The authors declare that they have no known conflict of interests reported in this paper.
Graphical Abstract
Photophysical investigation of synthesized probe BTNP shows it undergoes coupling of AIE and
ESIPT in strictly non-polar medium, as opposed to conventional trend of AIEgens forming
aggregates in water only. Hence, BTNP was used to generalize the definition of AIE
phenomenon itself.