TD-DFT Investigation of 2,5-Bis(2-benzothiazolyl)hydroquinone and 2,5-Bis(benzo[d]thiazol-2-yl)-4-methoxyphenol

Article abstract of DOI:10.1007/s10953-017-0628-5

Density functional theory (DFT) and time dependent density functional theory (TD-DFT) calculations of two excited state intramolecular proton transfer (ESIPT) molecules [2,5-bis(2-benzothiazolyl)hydroquinone and 2,5-bis(benzo[d]thiazol-2-yl)-4-methoxyphenol] were performed to study their structural and photo-physical behavior upon excitation. The most stable structure was established by optimizing all possible rotamers. The vertical excitation and emission wavelengths obtained by using TD-DFT show very good correlation with the experimental values. A correlation has been established based on the absorption values to determine the contribution of stable rotamers.

Full text of DOI:10.1007/s10953-017-0628-5

J Solution Chem
DOI 10.1007/s10953-017-0628-5
TD-DFT Investigation of 2,5-Bis-
(2-benzothiazolyl)hydroquinone and
2,5-Bis(benzo[d]thiazol-2-yl)-4-methoxyphenol
Manoj M. Jadhav1 Ibrahim A. Alswaidan2 Lydia Rhyman3,4,5
•
•
•
Ponnadurai Ramasami3,6 Nagaiyan Sekar1
•
Received: 10 July 2016 / Accepted: 1 January 2017
Ó Springer Science+Business Media New York 2017
Abstract Density functional theory (DFT) and time dependent density functional theory
(TD-DFT) calculations of two excited state intramolecular proton transfer (ESIPT)
molecules [2,5-bis(2-benzothiazolyl)hydroquinone and 2,5-bis(benzo[d]thiazol-2-yl)-4-
methoxyphenol] were performed to study their structural and photo-physical behavior upon
excitation. The most stable structure was established by optimizing all possible rotamers.
The vertical excitation and emission wavelengths obtained by using TD-DFT show very
good correlation with the experimental values. A correlation has been established based on
the absorption values to determine the contribution of stable rotamers.
Keywords ESIPT Á DFT Á TD-DFT Á 2,5-Bis(2-benzothiazolyl)hydroquinone Á
2,5-Bis(benzo[d]thiazol-2-yl)-4-methoxyphenol
Electronic supplementary material The online version of this article (doi:10.1007/s10953-017-0628-5)
contains supplementary material, which is available to authorized users.
&
&
1
Ponnadurai Ramasami
p.ramasami@uom.ac.mu
Nagaiyan Sekar
nethi.sekar@gmail.com
Tinctorial Chemistry Group, Department of Dyestuff Technology, Institute of Chemical
Technology, N. P. Marg, Matunga, Mumbai, Maharashtra 400019, India
2
Department of Pharmaceutical Chemistry, College of Pharmacy, King Saud University,
Riyadh 11451, Saudi Arabia
3
Computational Chemistry Group, Department of Chemistry, Faculty of Science, University of
´
Mauritius, Reduit 80837, Mauritius
4
Department of Applied Chemistry, University of Johannesburg, Doornfontein Campus,
Johannesburg 2028, South Africa
5
Department of Chemistry, University of Johannesburg, PO Box 524, Auckland Park,
Johannesburg 2006, South Africa
6
Department of Chemistry, College of Science, Engineering and Technology, University of South
Africa, Pretoria, South Africa
123

J Solution Chem
1 Introduction
Molecules exhibiting excited-state intermolecular proton transfer (ESIPT) phenomenon
have emerged as functional materials after the pioneering work of Weller [1, 2]. The
environmental dependence of the spectral properties and a large Stokes-shifted fluores-
cence associated with these molecules enhance their importance as smart molecules [3].
Upon electronic excitation, these molecules each exhibit a four-level photo-physical state
[4]. Experimental and theoretical investigations have been extensively used to explore this
four-level photo-physical state [5, 6]. ESIPT is a keto-enol tautomeric succession occurring
in the excited state which arises from the transfer of an acidic proton from a hydroxyl or
amino group to a basic acceptor such as a carbonyl oxygen or nitrogen atom in a hete-
rocyclic compound [7, 8]. Upon electronic excitation, abrupt enhancement of the acidity of
the hydroxyl or amino proton and basicity of acceptor lead to extremely fast migration of
the proton to the acceptor [9, 10]. The fast delocalization of the positive charge over the
heterocyclic ring in the excited state results in a large Stokes-shifted keto tautomer
[11, 12]. On relaxation of the keto form, the energetically preferred enol form is regen-
erated in a non-radiative manner by reverse proton transfer [7, 13]. Subsequently, this
process leads to the momentary alternation of electronic properties such as electron density
distribution, energies of electronic states and dipole moments. Emission from the keto state
gives rise to a large Stokes-shifted emission [14, 15] which is of importance in the fields of
laser dyes [16], fluorescent probes [17, 18], photo-chromic dyes [19], sensors [20], organic
electroluminescence [21, 22], pH sensor [23], optical materials and organic triplet photo-
stabilizers [24].
2-(20-Hydroxyphenyl)benzothiazole (HBT) [25–29] and its derivatives [30–32] are
among the most investigated ESIPT molecules due to their applications in the fields of bio-
imaging, medicine, agrochemicals and sensors [17, 30, 33]. DFT and TD-DFT methods
[34, 35] have been used for ground state geometry optimization and for understanding the
excited state behavior of ESIPT-based benzoxazole molecules. However, there are few
papers concerning 2,5-bis(2-benzothiazolyl)hydroquinone [36, 37] and its derivatives.
Therefore, taking note of the potential application of these ESIPT molecules and in con-
tinuation of our research work on fluorescent materials, herein we report a TD-DFT study
to understand the geometry of the different conformers of 2,5-bis(2-benzothiazolyl)
hydroquinone and 2,5-bis(benzo[d]thiazol-2-yl)-4-methoxyphenol along with the depen-
dence of their absorption and fluorescence maxima on the solvent.
2 Methodology
2.1 Experimental Section
2.1.1 Chemicals and Instruments
All the chemicals were procured from S. D. Fine Chemicals and used without further
purification. The synthetic grade solvents were taken from S. D. Fine chemicals and used
as supplied. The UV–Visible spectra were recorded on a Lab India UV–Vis with Model No
3000? instrument. The emission spectra was recorded on a Varian Cary Eclipse fluores-
cence spectrophotometer.
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J Solution Chem
2.1.2 Synthesis Strategy
The detailed routes for the synthesis of 2,5-bis(2-benzothiazolyl)hydroquinone and 2,5-
bis(benzo[d]thiazol-2-yl)-4-methoxyphenol are replicated in Scheme 1. 2,5-Dibromo-1,4-
dimethylbenzene was oxidized according to the reported protocol by using potassium
permanganate in a hot pyridine:water (1:1) mixture to obtain compound 2 with 81% yield
[38, 39]. Purification of compound 2 was carried by recrystallization from a dichlor-
omethane:methanol (7:3) mixture. Compound 3 was obtained with 60% yield by treating
compound 2 with sodium carbonate in the presence of a catalytic amount of pyr-
idine ? cuprous bromide complex in water [40]. Condensation of compound 3 was carried
according to the reported protocol by using 2 equivalents of 2-aminothiophenol and ten
equivalents PPA to obtain 2,5-bis(2-benzimidazolyl)hydroquinone [37] (compound 4) with
47% yield. Controlled mono-methylation, using the reported procedure, with methyl iodide
in DMF in the presence of potassium carbonate, gave 2,5-bis(benzo[d]thiazol-2-yl)-4-
methoxyphenol [41] (compound 5) with 58% yield. Purification of compounds 4 and 5
were carried out by performing repetitive crystallization from DMF [37].
2.2 Computational Methods
The B3LYP functional [42] was employed to optimize the ground state of all possible
keto–enol rotamers of compounds 4 and 5 in the gas phase and solvent. The B3LYP
functional is a combination of Becke’s three parameter exchange functional (B3) [43]
with the nonlocal correlation functional of Lee, Yang, and Parr (LYP) [44]. The basis set
6-31G(d) [45] was used for all atoms in the ground and excited states. The local min-
imum and the vibrational frequency of each enol rotamer and its corresponding keto
tautomer were computed using TD-DFT [46, 47] calculations using the B3LYP func-
tional and the 6-31G(d) basis set and to obtain the optimized structure of the low-lying
first singlet excited state (S1) of each conformer and its minimum energy geometry. The
excited state geometries obtained from TD-DFT calculation were also used to obtain
vertical excitation energies, which correspond to the emission values, using the same
H
O
O
H
O
O
r
B
r
B
H
O
a
b
c
r
B
r
B
H
O
H
O
O
H
O
O
O
1
2
3
H
O
N
S
S
N
N
S
N
d
S
H
O
H
O
4
5
Scheme 1 Synthesis of 2,5-bis(2-benzothiazolyl)hydroquinone (compound 4). Reactions and conditions of
Scheme 1: a KMnO4, 6 equivalents (two equivalents first addition ? 0.5 equivalents each in eight more
additions), pyridine, H2O, 81%. b Na2CO3, pyridine, CuBr, H2O, 60%. c 2-aminothiophenol, two equiv-
alents, PPA 10 equivalents, 190 °C, 24 h, 47%. d MeI (1 eq), NaOH (1.05 eq), DMF, 80 °C, 58%
123

J Solution Chem
basis set. Frequency computations were also performed on the optimized geometry of the
low-lying vibronically relaxed first excited state of conformers. The polarizable contin-
uum model (PCM) [48, 49] was used for optimizations in solvents of different polarities.
All electronic structure computations were performed by using the Gaussian 09 program
[50].
3 Results and Discussion
3.1 Structural Parameters
Compound 4 has two hydroxyl groups at the 1 and 4 positions whereas compound 5
contains one hydroxyl and one methyl group at the 1 and 4 positions. These groups can
interact with the two benzothiazole rings in the excited state by absorption of light, which
leads to the possibility of having different conformer arrangements in the excited state via
the rearrangement of a proton leading to excited state intramolecular proton transfer
(ESIPT). Both compounds 4 and 5 can exist in enol or keto forms or as a mixture of enol-
keto tautomers. Due to the rotation around a single bond, compounds 4 and 5 can have
interconvertible conformational isomers that may co-exist. To obtain the lowest energy of
the stable conformer of compounds 4 and 5, all the possible keto-enol rotamers of com-
pounds 4 and 5 were optimized in the gas phase and in the solvent phase using the B3LYP/
6-31G(d) method. Compound 4 shows the possibility of having nine enol rotamers and
their nine respective keto rotamers, whereas compound 5 can exist in sixteen enol rota-
meric forms and sixteen keto rotameric forms. Figure 1 shows all the nine enol-keto
rotamers of compound 4 considered for optimization while all sixteen enol–keto rotamers
of compound 5 are shown in Fig. 2. The relative energies in both kJÁmol-1 and kcalÁmol-1
of all the optimized planar enol-keto rotamers for compounds 4 and 5 in the gas phase are
presented in Tables 1 and 2, respectively. The relative energies of all enol and keto
tautomers were obtained by subtracting the energy of the most stable rotamer from the
actual energy of the each enol and keto rotamer obtained for the gas phase optimized
geometries of each rotamer, using the B3LYP/6-31G(d) method.
The relative energies listed in Table 1 indicate that the R1 rotamer of compound 4 in its
enol form, where the two hydroxyl groups at 1,4-positions are in close vicinity of the N=C
bond of the benzothiazole ring, is the thermodynamically most stable rotamer in the gas
phase. The same trend is observed in polar and non-polar solvents, while its keto tautomer
has the lowest energy among all the keto conformers. The optimized enol-keto geometry of
the stable conformer R1 in DMF is shown in Fig. 3 while the structural parameters are
presented in Table 3. The rotamer R1 in its enol form shows the possibility of having two
intermolecular hydrogen bonds between the two acidic protons of the hydroxyl groups
present at the 1,4-positions and the basic nitrogen atoms of two benzothiazole rings.
The possibility of intermolecular hydrogen bonding and structural orientation of the R1
enol conformer facilitates the migration of a proton from a hydroxyl group to a nitrogen
atom upon photo-excitation exhibiting the ESIPT phenomenon. The presence of ESIPT is
confirmed from the structural parameters and the changes in Mulliken charges of the
optimized geometries of the R1 conformer at the ground and excited state. Comparison of
the relative energies confirms the enol R1 conformer as the preferred geometry of com-
pound 4 in the ground state as compared to the other conformers.
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J Solution Chem
Compound 4 (Enol)
Compound 4 (Keto)
H
H
O
O
S
N
N
S
S
N
S
N
H
H
H
O
O
O
O
R1
R1
H
H
O
O
S
N
S
N
S
S
N
N
H
R2
R2
H
H
H
H
O
O
O
O
H
N
N
S
N
S
N
S
S
H
O
O
O
O
R3
R3
S
N
N
S
N
S
S
N
H
H
R4
H
R4
H
O
O
N
S
S
N
N
S
S
N
H
O
O
H
R5
R5
H
H
O
O
S
N
S
N
S
S
N
N
H
O
O
H
R6
R6
H
H
O
O
O
O
H
N
N
S
S
N
S
N
S
O
O
O
H
R7
R7
H
H
H
N
N
S
N
S
N
S
S
O
H
R8
R8
H
H
O
O
H
N
N
S
N
S
N
S
S
O
O
H
R9
R9
Fig. 1 Rotameric and tautomeric species of 2,5-bis(2-benzothiazolyl)hydroquinone (compound 4)
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J Solution Chem
Compound 5 (Enol)
Compound 5 (Keto)
H
H
O
O
N
S
N
S
S
N
S
N
O
O
O
O
R1
O
R1
O
H
S
N
S
N
S
N
S
N
H
R2
R2
H
H
O
O
N
S
N
S
S
N
S
N
O
O
H
O
O
O
R3
O
R3
O
H
N
N
S
N
S
N
S
S
R4
R4
O
H
N
H
O
S
N
N
S
S
N
S
R5
O
O
O
O
R5
H
H
N
O
S
N
N
S
S
N
S
O
R6
O
R6
H
H
N
O
N
S
S
N
S
N
S
O
O
R7
R7
O
H
O
S
N
S
N
S
N
S
N
H
O
R8
R8
Fig. 2 Rotameric and tautomeric species of 2,5-bis(benzo[d]thiazol-2-yl)-4-methoxyphenol (compound 5)
123

J Solution Chem
H
O
O
S
S
N
S
N
S
N
N
H
O
O
O
R9
O
R9
O
H
S
N
S
N
S
N
S
N
H
O
O
R10
O
R10
O
H
N
S
S
N
N
S
S
N
H
O
R11
O
R11
H
O
N
S
S
N
N
S
S
N
O
H
O
O
R12
O
R12
O
H
N
S
N
S
S
N
S
N
H
O
O
R13
R13
O
H
N
H
O
N
S
N
S
N
S
S
O
R14
O
R14
H
H
N
O
N
S
N
S
N
S
S
O
O
O
O
R15
O
R15
O
H
H
N
S
N
S
N
N
S
S
R16
R1
6
Fig. 2 continued
123

J Solution Chem
Table 1 Relative energies of all
the rotamers of compound 4 in
the gas phase
Rotamer Relative energy of compound 4 in kJÁmol-1 (kcalÁmol-1
)
Enol
Keto
R1
R2
R3
R4
R5
R6
R7
R8
R9
0.0 (0.0)a
53.6 (12.8)
53.7 (12.8)
105.5 (25.2)
57.9 (13.9)
114.3 (27.3)
91.5 (21.9)
67.8 (16.2)
39.4 (9.4)
84.9 (20.3)
84.9 (20.3)
107.9 (25.8)
107.9 (25.8)
142.3 (34.0)
126.4 (30.2)
142.3 (34.0)
84.9 (20.3)
a
SN first number is in units of
kJÁmol-1; the second number (in
parentheses) is in units of
kcalÁmol-1
91.5 (21.9)
Table 2 Relative energies of all
the rotamers of compound 5
Rotamer Relative energy of compound 5 in kJÁmol-1 (kcalÁmol-1
)
Enol
Keto
R1
19.7 (4.7)a
18.1 (4.3)
19.7 (4.7)
19.7 (4.7)
76.2 (18.2)
77.9 (18.6)
20.8 (5.0)
76.2 (18.2)
53.1 (12.7)
18.1 (4.3)
76.2 (18.2)
29.6 (7.1)
18.1 (4.3)
76.2 (18.2)
73.5 (17.6)
0.0 (0.0)
68.6 (16.4)
R2
104.0 (24.9)
85.4 (20.4)
83.7 (20.0)
83.7 (20.0)
72.8 (17.4)
72.8 (17.4)
85.4 (20.4)
98.1 (23.4)
98.1 (23.4)
85.4 (20.4)
98.1 (23.4)
98.1 (23.4)
83.7 (20.0)
68.5 (16.4)
64.4 (15.4)
R3
R4
R5
R6
R7
R8
R9
R10
R11
R12
R13
R14
R15
R16
a
The first number is in units of
kJÁmol-1; the second number (in
parentheses) is in units of
kcalÁmol-1
Table 3 summarizes the data, showing the variation in bond distance and bond angle of
the rotamer R1 in DMF in the ground and excited states. The variation in bond angle and
bond distance of the ground and excited states suggests the existence of the keto tautomer
of R1. The existence of the keto tautomer is further confirmed from the emission spectra,
where a major red-shifted emission peak is obtained due to the keto tautomer. The vari-
ation in Mulliken charges on atoms N31, H36, and O35 of the ground and excited state
optimized geometries of the enol form and the keto form are given in Table S1 (supple-
mentary material). The planarity of the molecule contributes substantially to ESIPT,
facilitating the transfer of proton H36 in the excited state from O35 to N31.
In the case of compound 4 in DMF, the reduction in bond distance between N31 to H36
˚
˚
˚
˚
by 0.110 A from enol (1.753 A) to enol* (1.643 A), which is further reduced by 0.609 A
for the enol* to keto* transition space between number and units, supports the existence of
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J Solution Chem
Fig. 3 Optimized structures of compound 4: R1 conformer in enol-keto forms
˚
ESIPT. There is an enhancement in the bond distance between H36 and O35 by 0.027 A
˚
for enol to enol* and by 0.776 A for enol* to keto*, respectively. This substantial incre-
mental change in bond length between H36 and O35 from the enol to keto states suggests
that in the excited state (enol*), proton H36 approaches near to N31 via intramolecular
hydrogen bonding for the transfer of proton. The reduction in bond distance of the C4–C20
˚
bond for enol (1.459 A) to enol* (1.426 A) by 0.033 A, and O35–C5 bond for enol
˚
˚
˚
(1.353 A) to enol* (1.326 A) by 0.027 A, which is further reduced by 0.057 A in the case
˚
˚
˚
˚
of keto* (1.269 A), suggests the occurrence of the ESIPT phenomenon. In addition to
these, the bond angle H36–O35–C5 increases by 0.3° from enol (107.7°) to enol* (108.0°).
This confirms that H36 approaches nearer to N31 in enol*, which leads to the transfer of
˚
proton to N31 in the keto excited state (keto*) with N31–H36 bond distance 1.034 A and
˚
O35–H36 hydrogen bonding distance of 1.796 A. The keto* goes to the ground state keto
˚
conformer which exhibits a N31–H36 bond distance of 1.058 A and O35–H36 hydrogen
˚
bond distance of 1.625 A. As H36 approaches N31, in the excited state the hydrogen bond
˚
˚
distance for H36-O35 decreases by 0.171 A from 1.796 (keto*) to 1.625 A (enol) and it
converts to the keto conformer but then goes back to the enol ground state through non-
radiative decay to complete the ESIPT process. These changes in bond distances and bond
angles confirm the existence of a single ESIPT process which is supported by the red-
shifted emission Table 5.
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Table 3 Structural parameters
of conformer R1 of compound 4
in DMF obtained by optimization
using the B3LYP/6-
Enol E0
Enol* E1
Keto K0
Keto* K1
˚
Bond length (in A)
N31–H36
H36–O35
O35–C5
C5–C4
1.753
0.993
1.353
1.425
1.459
1.310
1.643
1.020
1.326
1.460
1.426
1.338
1.058
1.625
1.282
1.457
1.419
1.341
1.034
1.796
1.269
1.479
1.421
1.355
31G(d) method
C4–C20
C20–N31
˚
Bond angle (in )
N31–H36–O35
H36–O35–C5
O35–C5–C4
146.6
107.7
125.3
119.9
123.0
99.6
149.5
108.0
120.9
119.7
122.8
99.1
140.4
133.9
106.3
121.5
119.7
127.6
114.9
104.0
123.5
117.8
121.7
112.6
C5–C4–C20
C4–C20–N31
C20–N31–H36
Comparison of the relative energies of all 16 enol–keto tautomers of compound 5
demonstrates that the R16 enol conformer is the thermodynamically most stable conformer
in the ground state and its respective keto tautomer has the lowest energy among all the
keto conformers. The optimized enol-keto geometry obtained for the stable conformer of
R16 in DMF is shown in Fig. 4. The bond lengths, bond angles and dihedral angles for
compound 5 are presented in Table 4. In the case of compound 5, rotamer R16, which is
the most stable rotamer, has a hydroxyl group at the 1-position which lies in close vicinity
to the N=C bond of the benzothiazole ring and this increases the possibility of
intramolecular hydrogen bonding with the basic nitrogen atom (OH…N=C). The methyl
group at the four position moves away from the other benzothiazole ring to reduce the
steric hindrance. Due to the presence of only one hydroxyl group, R16 can have only one
intramolecular hydrogen bond. This intramolecular hydrogen bonding in R16 promotes the
migration of the proton through enol-keto tautomerism upon photo-excitation to give
ESIPT. The structural parameters and Mulliken charges of the ground and excited states for
R16 confirm the existence of ESIPT.
The alteration in Mulliken charges on N32, H37, and O36 atoms of compound 5 upon
photo excitation for the keto-enol tautomers of R16 are given Table S2 (supplementary
material). The most stable rotamer R16 of compound 5 shows planarity in the region where
the acidic hydrogen atom is in close proximity to the basic nitrogen, which facilitates
hydrogen transfer from O36–H37 to N32 of the benzothiazole ring upon photo excitation.
The proton H37 is close to N32 in the enol ground state which promotes the migration of
the proton from O36 to N32. The changes in the bond distances in the optimized keto-enol
tautomeric structures in the ground and excited states proves the successful transfer of
acidic proton H37 to N32 to form the keto tautomer upon photo excitation. The reduction
˚
in the N32–H37 bond distance by 0.029 A from enol (1.750 A) to enol* (1.671 A),
˚
˚
˚
compared with the increase in the bond distance between H37–O36 by 0.019 A from enol
to enol*, suggests the approach of H37 to N32 via hydrogen bonding for the transfer of the
proton during photo excitation. The reduction in the C1–C9 and O36–C2 bond distances
from enol to enol*, and further decrease from enol* to keto*, confirm the possibility of
ESIPT. In addition to this, the increase of the H37–O36–C2 bond angle by 0.4° from enol
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J Solution Chem
Fig. 4 Optimized structure of compound 5: R16 conformer in the enol-keto forms
(107.7°) to enol* (108.1°) suggests that the proton H37 approaches nearer to N32 in enol*
and further transfers to N32 to give keto* with N32–H37 bond distance 1.035 A and O36–
˚
˚
H37 hydrogen bonding distance of 1.788 A. The keto* returns to ground state keto con-
˚
former with N32–H37 bond distance of 1.051 A and O36–H37 hydrogen bonding distance
˚
of 1.660 A. The potential energy surfaces for compound 4 and compound 5 were obtained
by changing the torsional angle (N32–C9–C1–C2) by 10°. The potential energy scans for
compounds 4 and 5 are given in supporting information Figs. S9 and S10. From the
potential energy scan it is evident that rotamer R1 of compound 4 and R16 of compound 5
are thermodynamically the most stable rotamers.
3.2 Photophysical Properties
Tables 5 and 6 report the correlation between the observed absorption and emission data of
compounds 4 and 5 with vertical excitation and emission results obtained from the TD-
DFT calculations. Compound 4 shows solvent polarity dependant absorption and emission
maxima, with negative solvatochromism and negative solvatofluorism in going from non-
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Table 4 Structural parameters
of conformer R16 of compound 5
obtained by optimization using
B3LYP/6-31G(d) method in
DMF
Enol E0
Enol E1
Keto K0
Keto K1
˚
Bond length (in A)
N32–H37
H37–O36
O36–C2
C2–C1
1.750
0.993
1.351
1.420
1.457
1.310
1.671
1.012
1.333
1.463
1.420
1.341
1.051
1.660
1.278
1.454
1.416
1.357
1.035
1.788
1.270
1.478
1.420
1.357
C1–C9
C9–N32
˚
Bond angle (in )
N32–H37–O36
H37–O36–C2
O36–C2–C1
C2–C1–C9
146.6
107.7
123.3
120.0
122.9
99.5
149.2
108.1
120.8
120.1
123.1
98.6
134.4
134.4
106.1
121.5
119.9
123.3
114.7
106.1
121.5
119.9
123.3
114.7
C1–C9–N32
C9–N32–H36
polar to polar solvents; whereas, compound 5 shows negative solvatochromism and pos-
itive solvatofluorism in going from non-polar to polar solvents. Compound 4 shows five
absorption maxima at 426, 392, 342, 333 and 317 nm in n-hexane. The absorption spec-
trum of compound 4 in n-hexane is given in Fig. S1 (supplementary information). This
trend is observed in all the polar and non-polar solvents for compound 4. There is a good
agreement between experimental absorption values and theoretical vertical excitation
values. In DMF, the largest difference in wavelengths between the computed and exper-
imental absorption maxima is 30 nm with 7.2% deviation.
The major intense absorption peaks in all the solvents can be assigned to HOMO to
LUMO, HOMO to LUMO?1 and HOMO to LUMO?2 transitions of the R1 enol rotamer
of compound 4. The frontier molecular orbital (FMO) 97 is the HOMO and FMO 98 is the
LUMO for all the rotamers of compound 4 in all of the solvents. From the frontier orbital
diagram (supplementary material, Fig. S3), the electron density of the HOMO is mainly
concentrated on the phenyl ring and two 1,4-hydroxy groups attached to the phenyl ring
and upon excitation, is redistributed onto the two benzothiazole rings in the LUMO. The
two benzothiazole rings share more electron density in the LUMO state, so that these two
rings act as an acceptor in the excited state and transfer of an acidic proton is promoted
from the oxygen to basic nitrogen atoms. In chloroform the calculated vertical excitation
for compound 4 is at 448 nm with an oscillator strength (f) of 0.6958, corresponding to a
HOMO–LUMO electronic transition (98.7%) with the experimentally observed absorption
being at 420 nm with a 6.7% deviation. The HOMO to LUMO?1 (97.7%) transition
accounts for the vertical excitation at 354 nm with oscillator strength (f) 0.8389. This
transition corresponds to the experimentally observed absorption peak at 341 nm with
3.8% deviation. A HOMO to LUMO?2 (95.3%) electronic transition is observed at
326 nm with oscillator strength (f) 0.1264. This transition corresponds to the experimen-
tally obtained absorption peak at 321 nm with 1.6% deviation. In polar solvents, a similar
trend is observed. This confirms that the experimentally observed short wavelength
absorption in the range 317–321 nm is due to the HOMO to LUMO?2 excitation and that
at 338–347 nm is due to the HOMO to LUMO?1 transition, whereas the longer wave-
length absorption in the range of 426–414 nm is due to the HOMO–LUMO transition.
123

J Solution Chem
Table 5 Observed UV–Visible photo-physical data and computed photo-physical data for compound 4 in
different solvents
Solvent
Experimental
Theoretical
kabs
nm
kem
nm
Vertical excitation
Emission
nm (oscillator
strength)
% Orbital
contribution
%D nm (oscillator
strength)
%D
n-Hexane
426
392
342
625
510
489
453 (R1-enol)
H ? L (0.6085)
99.0
6.3
689 (R1-keto)
L ? H (0.3153)
9.3
3.3
527 (R1-enol)
L ? H (0.5397)
353 (R1-enol)
H-1 ? L (0.8780)
96.7
3.2
333
317
322 (R1-enol)
H-2 ? L (0.1215)
95.3
99.0
1.6
6.6
n-Heptane
424
392
338
622
510
480
452 (R1-enol)
H ? L (0.6125)
689 (R1-keto)
L ? H (0.3174)
10.8
3.2
527 (R1-enol)
L ? H (0.5437)
353 (R1-enol)
H-1 ? L (0.8787)
96.7
4.4
329
317
321 (R1-enol)
H-2 ? L (0.1218)
95.3
98.7
1.3
6.7
Chloroform 420
617
514
480
448 (R1-enol)
H ? L (0.6620)
674 (R1-keto)
L ? H (0.3463)
9.2
1.2
390
341
520 (R1-enol)
L ? H (0.6140)
354 (R1-enol)
H-1 ? L (0.8389)
97.7
3.8
334
321
326 (R1-enol)
H-2 ? L (0.1264)
95.5
98.8
1.6
6.5
Methanol
415
389
343
615
516
474
442 (R1-enol)
H ? L (0.6589)
657 (R1-keto)
L ? H (0.3478)
6.8
0.8
512 (R1-enol)
L ? H (0.6341)
352 (R1-enol)
H-1 ? L (0.7878)
95.8
2.6
331
316
326 (R1-enol)
H-2 ? L (0.1281)
95.2
98.9
3.2
7.2
DMF
414
394
347
612
509
475
444 (R1-enol)
H ? L (0.6876)
661 (R1-keto)
L ? H (0.3625)
7.4
1.2
515 (R1-enol)
L ? H (0.6627)
354 (R1-enol)
H-1 ? L (0.7983)
96.2
95.4
2.0
2.2
334
319
326 (R1-enol)
H-2 ? L (0.1297)
123

J Solution Chem
Table 5 continued
Solvent
Experimental
Theoretical
kabs
nm
kem
nm
Vertical excitation
Emission
nm (oscillator
strength)
% Orbital
contribution
%D nm (oscillator
strength)
%D
DMSO
414
392
345
610
508
475
443 (R1-enol)
H ? L (0.6851)
98.9
7.0
661 (R1-keto)
L ? H (0.3613)
8.4
1.2
514 (R1-enol)
L ? H (0.6611)
353 (R1-enol)
H-1 ? L (0.7956)
96.1
95.4
2.3
2.5
331
318
326 (R1-enol)
H-2 ? L (0.1296)
The solvatofluorism study of compound 4 reveals the presence of two major emission
peaks in all of the polar and non-polar solvents. The emission spectrum of compound 4 in
n-hexane is given in the supplementary material (Fig. S2). The overlay of the experimental
absorption spectrum and vertical excitation of compound 4 in n-hexane is given in Fig. S7
of the supplementary material. The long wavelength emission in all ESIPT molecules is
due to the excited state keto tautomer obtained from the migration of the acidic proton to a
basic nitrogen atom in the excited enol state. The transfer of the acidic proton is due to
extra stabilization of the keto tautomer in the excited state compared with the excited enol
tautomer, which is confirmed by the computations. The energy of the excited keto state
obtained from TD-DFT calculations is lower than the corresponding excited enol state in
solvents; this suggests that the stabilization is intrinsic. TD-DFT calculations suggest that
the longer wavelength emission in the range 606–631 nm is due to a LUMO–HOMO
transition from excited state keto* to ground state keto in all solvents, which corresponds
with the observed emissions in the range 610–625 nm with 10.8% of maximum deviation.
Usually, in the case of ESIPT compounds a single shorter wavelength emission in polar
solvents is observed, attributed to a LUMO–HOMO transition from the excited state enol*
to the ground state enol, implying the exclusive presence of an enol state. But, for com-
pound 4, a longer wavelength emission is also observed in polar solvents irrespective of
their polarity. The existence of this longer wavelength emission suggests the presence of a
keto tautomer in polar solvents, which is further confirmed by the lower energies of the
excited state keto conformers over their respective excited state enol conformer. The
second emission of compound 4 in the range of 472–483 nm in different solvents corre-
sponds to a LUMO–HOMO transition from an excited state enol* to the ground state enol,
which corresponds to the observed emission in the range of 510–516 nm with maximum
deviation of 3.2%; the third observed emission in the range of 475–489 nm may be a
vibronic emission from the enol excited state to the enol ground state.
Compound 5 shows blue shifts in the absorption maxima and emission maxima in all the
solvents compared with compound 4. Compound 5 shows only three prominent absorption
peaks in comparison with compound 4, where 5 absorption peaks are observed in all the
solvents. In n-hexane it shows absorption maxima at 404 nm, 371 nm and 322 nm. The
absorption and emission spectra of compound 5 in n-hexane are given in the supplementary
material (Figs. S4 and S5, respectively), the overlay spectra of observed absorption and vertical
excitation of compound 5 in n-hexane are given in Fig. S8. The theoretical values of absorption
123

J Solution Chem
Table 6 Observed and computed UV–visible photo-physical data of compound 5 in different solvents
Solvent
Experimental Theoretical
kabs
nm
kem
nm
Vertical excitation
Emission
nm (oscillator
strength)
Orbital
contribution
%
D
nm (oscillator
strength)
%
D
n-Hexane
404
371
322
404
372
320
571
449
420
571
448
421
575
452
418
578
457
416
581
457
415
583
459
414
421 (R16-enol)
H ? L (0.8820)
98.4
95.7
96.7
98.4
95.7
96.7
98.5
95.6
96.5
98.2
95.3
96.4
98.4
95.5
96.4
98.4
95.5
96.4
4.20 592 (R16-keto)
L ? H (0.4127)
3.6
6.0
354 (R16-enol)
H-1 ? L (0.5669)
4.6
5.9
4.5
4.8
6.5
5.8
2.2
6.9
6.3
2.8
6.9
8.2
0.8
7.3
8.4
0.8
7.6
476 (R16-enol)
L ? H (0.8708)
341 (R16-enol)
H-2 ? L (0.0295)
n-Heptane
422 (R16-enol)
H ? L (0.8863)
592 (R16-keto)
L ? H (0.4186)
3.6
6.3
354 (R16-enol)
H-1 ? L (0.5678)
476 (R16-enol)
L ? H (0.8767)
341 (R16-enol)
H-2 ? L (0.0294)
Chloroform 400
423 (R16-enol)
H ? L (0.9014)
596 (R16-keto)
L ? H (0.4263)
3.6
6.2
361
317
353 (R16-enol)
H-1 ? L (0.5958)
480 (R16-enol)
L ? H (0.9529)
339 (R16-enol)
H-2 ? L (0.0266)
Methanol
395
361
315
391
355
314
389
355
313
420 (R16-enol)
H ? L (0.8604)
600 (R16-keto)
L ? H (0.4330)
3.8
4.6
351 (R16-enol)
H-1 ? L (0.6092)
478 (R16-enol)
L ? H (0.9470)
337 (R16-enol)
H-2 ? L (0.0229)
DMF
423 (R16-enol)
H ? L (0.8955)
604 (R16-keto)
L ? H (0.4396)
3.9
5.5
352 (R16-enol)
H-1 ? L (0.6120)
482 (R16-enol)
L ? H (0.9864)
337 (R16-enol)
H-2 ? L (0.0238)
DMSO
422 (R16-knol)
H ? L (0.8912)
605 (R16-keto)
L ? H (0.3407)
3.8
4.8
352 (R16-enol)
H-1 ? L (0.6121)
481 (R16-enol)
L ? H (0.9829)
337 (R16-enol)
H-2 ? L (0.0236)
show very good agreement with the observed values in all solvents. The maximum deviation in
the theoretical vertical excitation from observed value is 8.4% in DMSO. In all the solvents, for
compound 5, the major intense absorption peaks can be assigned to HOMO to LUMO, HOMO
to LUMO?1 and HOMO to LUMO?2 transitions of the R16-enol rotamer, which is the
thermodynamically most stable rotamer. The frontier molecular orbital (FMO) 101 is HOMO
and FMO 102 is LUMO for all the rotamers in all the solvents. The two benzothiazole rings act
as acceptors that, upon excitation, undergo HOMO to LUMO transitions. The electron density
123

J Solution Chem
ismainlydistributedonthephenyl ringalongwiththe hydroxyl andmethoxygroupsattachedto
the phenyl ring in the HOMO that, upon excitation, redistribute the electron density on the two
benzothiazole rings that are in one plane in the excited state.
Compound 5 in chloroform shows a HOMO–LUMO electronic transition (98.5%) for
vertical excitation at 423 nm with oscillator strength (f) 0.9014. This vertical transition
corresponds to the experimentally observed absorption at 400 nm with a 5.8% deviation. The
HOMO to LUMO?1 (95.6%) transition corresponds to the vertical excitation at 361 nm with
oscillator strength (f) 0.5959, which corresponds to the experimentally observed absorption
peak at 361 nm with a 2.2% deviation, while the HOMO to LUMO?2 (96.5%) electronic
transition appears at 339 nm, with oscillator strength (f) 0.0266 that corresponds to experi-
mentally observed absorption peak at 317 nm with a 6.9% deviation. The orbital diagram for
compound 5 is given in the supplementary material (Fig. S6). Also, in the case of polar
solvents a similar trend is observed. This confirms that the experimentally observed short
wavelength absorption in the range of 313–322 nm is due to the HOMO to LUMO?2 and at
355–372 nm is due to HOMO to LUMO?1 transition, whereas the longer wavelength
absorption in the range of 389–404 nm is due to a HOMO–LUMO transition.
The solvatofluorism study of compound 5 shows three emissions peaks in all polar and
non-polar solvents. The longer wavelength emission results from migration of an acidic
proton to the basic nitrogen atom, which leads to enol-keto tautomerism. The migration of a
proton for compound 5 occurs due to extra stabilization of the keto tautomer in the excited
state compared to the excited enol tautomer, which is confirmed by computational calcula-
tions. Compound 5 shows a red shift in emission in going from non-polar to polar solvents.
The longer wavelength emission in the range 592–605 nm is due to a LUMO–HOMO
transition from the excited state keto* to ground state keto in all the solvents, which corre-
sponds to the observed emissions range 571–583 nm with a 3.9% maximum deviation. The
presence of longer wavelength emission in compound 5 in all polar and non-polar solvents
shows a blue shift in comparison with the corresponding emissions shown by compound 4.
Usually ESIPT compounds show a single shorter wavelength emission in polar solvents
attributing to the LUMO–HOMO transition from excited state enol* to ground state enol
suggesting the exclusive presence of the enol state. In case of compound 5, as for compound 4,
a longer wavelength emission is also observed in polar solvents irrespective of their polarity.
This indicates the presence of the keto tautomer in polar solvents, which is confirmed by the
lower energies of excited state keto conformers over their respective excited state enol
conformers. The second observed emission of compound 5 in the range of 449–459 nm
resembles the transition from LUMO–HOMO for excited state enol* to ground state enol.
This transition shows the theoretical emission is in the range of 476–482 nm with maximum
deviation of 6.3%, whereas the third emission obtained is in the range of 421–414 nm and is
associated with a vibronic transition from enol excited state to enol ground state.
4 Conclusions
In this paper we report the results of DFT and TD-DFT studies of 2,5-bis(2-benzothia-
zolyl)hydroquinone and 2,5-bis(benzo[d]thiazol-2-yl)-4-methoxyphenol. The structural
parameters of the most stable conformers of 2,5-bis(2-benzothiazolyl)hydroquinone and
2,5-bis(benzo[d]thiazol-2-yl)-4-methoxyphenol were obtained by optimizing all of the
possible conformers. The photo-physical behavior of these compounds was studied in six
different solvents. The vertical excitation and emission wavelengths determined by using
123

J Solution Chem
Density Functional Theory (DFT) and Time-Dependent Density Functional Theory (TD-
DFT) show very good agreement with observed absorption/emission values. The photo-
physical properties of the compounds are sensitive to the solvent’s environment. Com-
pound 4 shows a red shift in absorption and emission in comparison with compound 5.
Acknowledgements The authors thank the anonymous reviewers for their useful comments for improving
the manuscript. M.M.J. is grateful to DST and TEQIP for financial support. L.R. and P.R. acknowledge the
facilities from the University of Mauritius. I.A.A. and P.R. extend their appreciation to the International
Scientific Partnership Program ISPP at King Saud University for funding this research work through ISPP#
0070.
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