Comprehensive study on excited state intramolecular proton transfer in 2-(benzo[d]thiazol-2-yl)-3-methoxynaphthalen-1-ol and 2-(benzo[d]thiazol-2-yl)naphthalene-1,3-diol: Effect of solvent, aggregation, viscosity and TDDFT study

Article abstract of DOI:10.1016/j.jphotochem.2016.12.033

Three compounds DMT, MMT and DHT having dimehtoxy, mono-hydroxy and di-hydroxy groups respectively were prepared. Environmental interactions dependent photophysical properties especially excited state intramolecular proton transfer (ESIPT), solvatochromism, aggregation induced emission enhancement (AIEE) and viscosity dependent emission characteristics were studied. The effect of solvent polarity on ESIPT dynamics were studied using UV–vis and emission spectroscopy along with aggregation induced enhanced emission and viscosity effect. MMT and DHT showed unexpected and contrasting behavior in the solvents which are in good agreement with the density functional theory and time dependent density functional theory findings. Aggregation and viscosity study showed that cis-keto emission increased drastically in the aggregate state and highly viscous state due to restricted intramolecular rotation increasing the population of required cis enol rotamer (E). The study directs the potential applications of such molecules for advanced optoelectronics and viscosity based investigations.

Full text of DOI:10.1016/j.jphotochem.2016.12.033

Accepted Manuscript
Title: Comprehensive study on excited state intramolecular
proton transfer in
2-(benzo[d]thiazol-2-yl)-3-methoxynaphthalen-1-ol and
2-(benzo[d]thiazol-2-yl)naphthalene-1,3-diol: Effect of
solvent, aggregation, viscosity and TDDFT study
Author: <ce:author id="aut0005"
author-id="S1010603016308796-
fbbeea3285c1f666041514ff2063ea84"> Umesh
Warde<ce:author id="aut0010"
author-id="S1010603016308796-
106a0fc3ee60a5e9e1e3f7a4ba3d8615"> Sekar
Nagaiyan
PII:
S1010-6030(16)30879-6
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.jphotochem.2016.12.033
JPC 10488
To appear in:
Journal of Photochemistry and Photobiology A: Chemistry
Received date:
Revised date:
Accepted date:
6-10-2016
29-12-2016
30-12-2016
Please cite this article as: Umesh Warde, Sekar Nagaiyan, Comprehensive
study on excited state intramolecular proton transfer in 2-(benzo[d]thiazol-2-yl)-3-
methoxynaphthalen-1-ol and 2-(benzo[d]thiazol-2-yl)naphthalene-1,3-diol: Effect of
solvent, aggregation, viscosity and TDDFT study, Journal of Photochemistry and
Photobiology A: Chemistry http://dx.doi.org/10.1016/j.jphotochem.2016.12.033
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Comprehensive study on Excited State Intramolecular Proton
Transfer in 2-(benzo[d]thiazol-2-yl)-3-methoxynaphthalen-1-ol and
2-(benzo[d]thiazol-2-yl)naphthalene-1,3-diol : Effect of solvent,
aggregation, viscosity and TDDFT study.
Umesh Warde, Nagaiyan Sekar*
Department of Dyestuff Technology, Institute of Chemical Technology
N. P. Marg, Matunga, Mumbai (MH) India-400019
Email: n.sekar@ictmumbai.edu.in, nethi.sekar@gmail.com
Tel.: +91 22 3361 1111/ 2707; Fax.: +91 22 3361 1020
1

Graphical abstract
2

Highlights
 Substituents and solvents played major role in ESIPT outcome for compound MMT and
DHT.
 Solvent acidity and basicity are the governing factors for variation in the emission
properties.
 Restricted intramolecular motion is the main cause for drastic emission enhancement for
MMT as proved by AIEE and viscosity experiment.
3

Abstract
Three compounds DMT, MMT and DHT having dimehtoxy, mono-hydroxy and di-hydroxy
groups respectively were prepared. Environmental interactions dependent photophysical
properties especially excited state intramolecular proton transfer (ESIPT), solvatochromism,
aggregation induced emission enhancement (AIEE) and viscosity dependent emission
characteristics were studied. The effect of solvent polarity on ESIPT dynamics were studied
using UV-visible and emission spectroscopy along with aggregation induced enhanced emission
and viscosity effect. MMT and DHT showed unexpected and contrasting behavior in the solvents
which are in good agreement with the density functional theory and time dependent density
functional theory findings. Aggregation and viscosity study showed that cis-keto emission
increased drastically in the aggregate state and highly viscous state due to restricted
intramolecular rotation increasing the population of required cis enol rotamer (E). The study
directs the potential applications of such molecules for advanced optoelectronics and viscosity
based investigations.
Keywords: ESIPT, DFT, TDDFT, Aggregation, Viscosity.
1. Introduction
(Hydroxyphenyl)benzazoles (HBXs, Fig. 1) are a class of compounds that are known to exhibit complex
photochemical and photophysical properties and have thus been studied extensively . Specifically, these
molecules undergo very fast (1012 s-1) excited-state intramolecular proton transfer (ESIPT) to form
phototautomers [1–5]. The photophysical behavior of the HBXs is shown to be different in polar, protic
solvents as compared to that observed in aprotic, nonpolar media. The phototautomers of the molecules
have been demonstrated to be more efficiently produced in hydrocarbon solvents as compared to alcohols
4

or water due to less (or absence of) competition between intramolecular and intermolecular hydrogen
bonding with the solvent molecules [6–9]. As a result, in solvents of different polarity and pH, different
emitting forms (e.g., neutral, anion, cation, and tautomers) of the HBXs are observed, suggesting complex
excited-state proton transfer (ESPT) equilibrium. The formation of different keto rotamers (i.e.,cis and
trans) as well as zwitterionic tautomers upon electronic excitation has also been proposed in the literature
[9,10]. Among the HBXs, hydroxyphenyl-benzothiazole (HBT) analogue is believed to possess a lower
rotational energy barrier and is thus considered to rapidly undergo ESIPT to produce cis and trans
phototautomers compared to HBO and HBI. This means that non-radiative decay is significant in case of
HBT and other related derivatives compared to HBO and HBI [2,11].
<< Please insert Fig. 1 here >>
(Hydroxyphenyl)benzothiazole is different from its O and N analogues in ground state as well as in
excited state properties. HBT has red shifted absorption spectra compared to HBO and HBI (around 20
nm). The differences in the photophysical properties of HBO and HBT have been rationalized by several
authors [12,13] . For HBT, both cis and trans isomeric proton transfer (PT) tautomers exist in the excited
state, but there is only a cis conformer in the case of HBO. The authors proposed that the linkage joining
the benzothiazole moiety and the phenol ring has more single-bond character in the proton transfer
tautomer of HBT compared to HBO and HBI. The origin of this difference lies in the lower
electronegativity of the sulfur compared to oxygen and nitrogen. For the azole system in HBT, this means
a better delocalization of the π-electrons, a higher nitrogen basicity [14,15] and less chance of hydrogen
bonding involving the second heteroatom as in trans rotamer [16] (Fig. 2) .
5

In case of the naphthalene analogues HNBT and HNBO (Fig. 1), time resolved fluorescence spectra
revealed dual emission for both compounds in non-polar as well as polar solvents. (Where as HBT and
HBO shows only single emission in non-polar solvents but dual in polar solvents). Two distinct
fluorescence bands originating from K*and the excited rotamer E′* [17–19]. Naphthalene though a
symmetric molecule, positions of hydroxyl groups and azole groups on it make a huge difference in the
photophysical properties of ESIPT molecules as seen in case of 2H3NBT and 1H2NBT (Fig. 1) [20].
2H3NBT shows huge Stokes shift compared to 1H2NBT as explained by nodal plane theory [21].
Considering the major role of substituents and solvents on output of the ESIPT process [22], it is
important to increase the cis-keto emission. The keto emission with high Stokes shift has a great
importance in various applicative fields such as analytical tools and optoelectronics [20,23]. ESIPT is a
unique process which gives Stokes shifted emission without self-absorption. The only draw-back of
majority ESIPT compounds is their low quantum yields which limits their use in advanced new
generation electronics such as OLED, LASER [23]. It is therefore, important to restrict the non-radiative
decay of these molecules either via restricting the fast isomerization in the ground state or use the media
where isomerization can be minimized.
<< Please insert Fig. 2 here – Please keep the color as it is >>
Dual hydrogen bond forming systems reported (Fig. 3) by Yi Pang et.al, explained the use of extra
hydroxyl group to increase the proportion of enol rotamer which is responsible to give ESIPT tautomer to
increase the cis-keto emission. The second hydroxyl group could form the hydrogen bond with oxygen
atom of oxazole ring thus making the system rigid, suitable for ESIPT process thereby decreasing the
proportion of trans enol rotamer which suppresses the keto yield [24]. In case of HBT, only one report is
available as dihydorxy analogue but only regarding with solid state quantum yield [25].
<< Please insert Fig. 3 here >>
6

We intended to study the effect of substituents and solvent on naphthalene based HBT system in order to
understand the ESIPT dynamics to improve its quantum efficiency for the keto emission. We made
naphthalene based three derivatives DMT, MMT and DHT having dimethoxy group, monohydroxy
group and dihydroxy group (Fig. 4 – synthetic scheme) and studied their photophysical properties. Here
we report the comprehensive study of novel naphthalene based ESIPT mono hydroxyl and dihydroxy
analogues of HNBT and comparison of their photophysical properties in solvents of different polarity.
Solvent permittivity, solvent polarizability and multiparameter scales Kamlet-Taft and Catlan were used
to study the solvent effects on ESIPT process. The compound MMT showed poor quantum yield in all
solvents but DHT has considerable quantum yield in non-polar solvents. The extensive intramolecular
rotation may be reason for low quantum yield in solution. The low quantum yield in solution and high
solid state emission of the compound MMT encouraged us to study the aggregation induced emission and
viscosity based enhanced emission as it can be greatly enhanced by restricting the intramolecular motion
[26]. Evaluation of the restricted intramolecular motion which probably can enhance the keto emission is
very necessary for the practical applications of ESIPT molecules as sensors and optoelectronics. This is
because restricted motion will increase the population of necessary enol responsible for keto emission
[23,27]. Considering these factors, we have studied aggregation induced enhanced emission (AIEE),
extent of aggregation and the viscosity effect on the emission intensity to find out the consequences of
restricted intramolecular rotation and the interference of solvent in the ESIPT emission. DFT and TDDFT
study has been done to evaluate the vertical excitation properties on molecular level which can be
complementary and supportive to understand the ESIPT process. The TDDFT computed vertical
excitation properties a are compared with the experimental absorption emission.
<< Please insert Fig. 4 here >>
7

2. Materials and methods
All the commercial reagents were procured from SD Fine Chemicals (Mumbai) and were used
without further purification. Laboratory reagent grade solvents were purchased from Rankem,
Mumbai. The reactions were monitored by TLC using on 0.25 mm EMerck silica gel 60 F254
precoated plates, which were visualized using UV light (254 nm and 344 nm). Melting points
were measured on standard melting point apparatus from Sunder Industrial products, Mumbai
and are uncorrected. Proton and Carbon NMR spectra were recorded on Varian 500 MHz
instrument using TMS as an internal standard. Mass spectra were recorded on FINNIGAN LCQ
ADVANTAGE MAX instrument from Thermo Electron Corporation (USA). The absorption
spectra of the compounds were recorded on a Perkin Elmer Lambda 25 UV-Visible
spectrophotometer; emission spectra were recorded on Varian Inc. Cary Eclipse
spectrofluorometer. The relative quantum yields were calculated using Quinine Sulphate as
standard [28,29].
All the computations were performed using the Gaussian 09 revision D.01 program package
[30]. The ground state (S0) geometry of the compounds under investigation was optimized in gas
phase using DFT method [31]. The functionals used was B3LYP [32,33]. The basis set used for
all atoms was the popular Pople’s split-valence basis set 6−31+G(d) [34]. The vibrational
frequencies of the optimized structures were computed using the same method to verify the
nature of the stationary points. The computations in solvents of different polarities were carried
out using the self-consistent reaction field (SCRF) incorporated in the Polarisable Continuum
Model (PCM) [35,36]. The vertical excitation energies for absorption [31,37,38] and emission
[29,39] along with oscillator strengths were obtained for the lowest 20 singlet-singlet transitions
at the optimized ground state equilibrium geometries by using the TD-B3LYP/6-31+G(d)
8

3. Synthesis
The intermediate ethyl 1,3-dihydroxy-2-naphthoate was synthesized using reported procedure
[40]. The intermediate 1,3-di-methoxy-2-naphthaldehyde and 1-hydroxy-3-methoxy-2-
naphthaldehyde were synthesized using reported method [41] starting from phenylacetic acid.
The synthesis of 2-(1,3-dimethoxynaphthalen-2-yl)benzo[d]thiazole (DMT), 2-(benzo[d]thiazol-
2-yl)-3-methoxynaphthalen-1-ol (MMT) and 2-(benzo[d]thiazol-2-yl) naphthalene-1, 3-diol
(DHT) are as follows
General procedure for preparation of 2-(1,3-dimethoxynaphthalen-2-yl)benzo[d]thiazole (DMT)
To a solution of 1,3-di-methoxy-2-naphthaldehyde in DMSO (1 g, 0.0031 mol) , 2-amino-
thiophenol (0.39 g, 0.0031 mol) was added and heated at 80 ° C overnight for 12 hours. Reaction
mass cooled to room temperature and quenched in ice water under stirring. Solid filtered and
purified using ethanol to give off white tile compound. Yield, 1.1 g (73 %) ; M.P.: 187-190 ° C;
1H NMR (500 MHz, dmso [D6] ) δ 8.17 – 8.12 (m, 1H), 8.11 – 8.06 (m, 1H), 8.02 (d, J = 8.3 Hz,
1H), 7.91 (d, J = 8.2 Hz, 1H), 7.59 – 7.56 (m, 1H), 7.56 – 7.53 (m, 1H), 7.48 (ddd, J = 8.3, 7.3,
13
1.2 Hz, 1H), 7.44 (ddd, J = 8.2, 6.9, 1.1 Hz, 1H), 7.33 (s, 1H), 3.83 (s, 3H), 3.75 (s, 3H). C
NMR (126 MHz, dmso [D6]) δ 161.27, 156.17, 155.40, 153.15, 136.41, 135.93, 128.66, 127.59,
126.56, 125.83, 124.87, 123.45, 123.01, 122.93, 122.44, 116.77, 102.94, 63.54, 56.44; HRMS
(ESI): m/z calculated for C19H15NO2S+H+:322.0902 [M+H]+ ; found: 322.0930.
General procedure for preparation of 2-(benzo[d]thiazol-2-yl)-3-methoxynaphthalen-1-ol
(MMT)
9

To a solution of 1-hydroxy-3-methoxy-2-naphthaldehyde in DMSO ( 1 g, 0.0049 mol ) , 2-
amino-thiophenol ( 0.62 g , 0.0049 mol ) was added and heated at 80 ° C overnight for 12
hours. Reaction mass cooled to room temperature and quenched in ice water under stirring. Solid
filtered and recrystallized using ethanol to give greenish-yellow title compound. Yield, 1.2 g (80
1
%); M.P.: 168-170 ° C; H NMR (500 MHz, DMSO [D6]) δ 8.29 – 8.24 (m, 1H), 8.18 – 8.14
(m, 1H), 8.11 (d, J = 8.0 Hz, 1H), 7.77 (d, J = 8.2 Hz, 1H), 7.60 – 7.53 (m, 2H), 7.47 (dd, J =
13
7.9, 0.9 Hz, 1H), 7.42 – 7.36 (m, 1H), 7.02 (s, 1H), 4.09 (s, 3H). C NMR (126 MHz, DMSO
[D6]) δ 165.98, 157.72, 154.36, 148.79, 135.69, 133.11, 129.85, 127.29, 126.96, 125.77, 124.10,
123.86, 122.19, 121.40, 120.73, 103.76, 97.52, 56.30; HRMS (ESI): m/z calculated for
C18H13NO2S+H+:308.0745 [M+H]+ ; found: 308.0712.
General procedure for preparation of 2-(benzo[d]thiazol-2-yl) naphthalene-1, 3-diol (DHT)
Mixture of 2-(1, 3-dimethoxynaphthalen-2-yl)benzo[d]thiazole ( 2 g, 0.0062 mol) and pyridine
°
hydrochloride (4 g 0.0346 mol) heated neat in 2-necked round bottom flask at 180 C for 2
°
hours. Reaction mass cooled to 90 C and added methanol and stirred till the temperature
reaches at room temperature. Reaction mass then quenched in water and filtered to give the off
1
white title compound. Yield, 1.7 g (93 %) without purification; M.P.: 290-293 ° C; H NMR
(500 MHz, DMSO [D6]) δ 11.49 (s, 1H, -OH (Hydrogen Bonding) ), 8.21 (d, J = 8.3 Hz, 1H),
8.16 (d, J = 7.9 Hz, 1H), 8.10 (d, J = 8.1 Hz, 1H), 7.64 (d, J = 8.1 Hz, 1H), 7.56 (t, J = 7.6 Hz,
1H), 7.47 (dd, J = 15.8, 7.9 Hz, 2H), 7.29 (t, J = 7.5 Hz, 1H), 6.81 (s, 1H); 13C NMR (126 MHz,
DMSO [D6]) δ 166.36, 157.77, 153.11, 148.74, 136.01, 133.44, 129.45, 127.19, 126.15, 125.59,
10

123.82, 123.08, 122.25, 121.33, 119.86, 103.98, 100.24. HRMS (ESI): m/z calculated for
C17H11NO2S+H+:294.0589 [M+H]+ ; found: 294.0672
1-Hydroxy-3-methoxy-2-naphthaldehyde.
1
M.P.: 152-155 ° C; H NMR (500 MHz, DMSO [D6]): δ 13.56 (s, 1H), 10.28 (s, 1H), 8.17 –
8.13 (m, 1H), 7.78 – 7.73 (m, 1H), 7.63 (ddd, J = 8.2, 6.9, 1.2 Hz, 1H), 7.39 (ddd, J = 8.3, 6.9,
13
1.1 Hz, 1H), 6.87 (s, 1H), 3.94 (s, 3H); C NMR (126 MHz, DMSO [D6]): δ 194.98, 163.02,
156.72, 138.94, 131.75, 127.36, 124.48, 124.08, 119.94, 107.93, 97.36, 56.37; elemental analysis
calcd (%) for C12H10O3 (202.0630): C 71.28, H 4.26, N 4.98, O 23.74; found: C 71.31, H 4.15, O
23.78.
1,3-Di-methoxy-2-naphthaldehyde
M.P.: 140-142 ° C; 1H NMR (500 MHz, DMSO [D6]): δ 10.46 (s, 1H-aldehyde proton), 8.06 (d,
J = 8.3 Hz, 1H), 7.84 (d, J = 8.3 Hz, 1H), 7.61 – 7.57 (m, 1H), 7.44 – 7.41 (m, 1H), 7.25 (s, 1H),
3.94 (s, 3H), 3.92 (s, 3H); 13C NMR (126 MHz, DMSO [D6]): δ 189.64, 160.06, 157.33, 137.70,
130.13, 127.48, 125.06, 123.77, 123.24, 117.90, 103.15, 64.59, 56.49; elemental analysis calcd
(%) for C13H12O3 (216.0786): C 72.21, H 5.59, O 22.20; found: C 72.20, H 5.59, O 22.31.
(1,3-Dimethoxynaphthalen-2-yl)methanol
1
M.P.: 105-108 ° C; H NMR (500 MHz, DMSO [D6]): δ 7.94 (d, J = 8.3 Hz, 1H), 7.81 (d, J =
8.1 Hz, 1H), 7.45 (dd, J = 7.9, 7.0 Hz, 1H), 7.40 – 7.32 (m, 1H), 7.14 (s, 1H), 4.71 (t, J = 5.4 Hz,
13
1H), 4.63 (d, J = 5.3 Hz, 2H), 3.92 (s, 3H), 3.89 (s, 3H); C NMR (126 MHz, DMSO [D6]): δ
11

157.08, 155.48, 134.80, 127.30, 127.12, 124.02, 123.26, 123.04, 122.45, 102.23, 63.75, 56.01,
52.98; elemental analysis calcd (%) for C13H12O3 (218.0943): C 67.23, H 5.21, O 27.56; found:
C 67.22, H 5.20, O 27.58.
1,3-Dimethoxy-2-naphthoate
M.P.: 130-132 ° C; 1H NMR (500 MHz, CDCl3 [D1]): δ 8.04 (d, J = 8.4 Hz, 1H), 7.74 (d, J = 8.1
Hz, 1H), 7.49 (dd, J = 8.2, 6.9 Hz, 1H), 7.39 (dd, J = 8.2, 7.0 Hz, 1H), 6.96 (s, 1H), 4.48 (q, J =
7.1, 1.1 Hz, 2H), 4.03 (s, 3H), 3.94 (s, 3H), 1.43 (t, J = 7.1, 1.1 Hz, 3H).13C NMR (126 MHz,
CDCl3 [D1]): 166.44, 154.37, 154.24, 135.40, 127.70, 126.98, 124.07, 123.04, 122.60, 117.69,
101.98, 63.14, 61.60, 55.93, 14.27; elemental analysis calcd (%) for C13H12O3 (260.1049): C
69.22, H 6.20, O 24.59; found: C 69.23, H 6.20, O 24.58.
4. Result and discussion.
Photophysical properties including TDDFT vertical excitation and emission are mentioned in the
table 1, 2 and 3 respectively and Fig. 5, 6, 7 (Same concentration samples used to record
absorption and emission in order to omit concentration oriented intensity differences).
<< Please insert Fig. 5, 6, 7 here >>
4.1 Absorption and emission spectra
Absorption maxima for MMT in chloroform appeared at 343 nm along with shoulder peak at 330 nm and
hump at 380nm along with basic azole molecular peak at 296 nm similar to compound HNBT ( Fig. 1)
reported by Arai et.al. [17]. Absorption maxima for DHT appeared at 370 nm and small hump at 342 nm.
Emission properties are quite different for the two compounds. MMT shows dual emission at 407 nm and
12

at 509 nm in toluene. This is definitely the red shifted keto emission compared to reported compound
(keto = 500 nm) [17] . It was suggested that due to naphthalene ring, energy gap between excited state
keto and ground state keto form is increased and hence the blue shifted emission of keto tautomer is
observed. But in the present case methoxy group next to benzothiazole group is overcoming this
deficiency and giving considerable red shifted emission (Effect of position of ESIPT group can also be
considered for this observation). In case of DHT, only one emission is observed at 448 nm in toluene,
which most likely of the K* exclusively from ESIPT process with higher quantum yield. No enol
emission is observed. This emission is about 60 nm blue shifted compared to MMT keto emission which
probably due to the increased energy gap due to the naphthalene ring and the absence of methoxy group
[17] . It indicates that the charge transfer from methoxy group is necessary for red shifted emission which
making the energy gap low in the excited state. Interesting thing is that, though keto emission for DHT is
blue shifted, its quantum yield is improved significantly. Similar results are also observed in solvent
chloroform also. This suggests that two distinct rotamers are present in solution in the ground state in case
of compound MMT. One responsible for formation of K* and other for normal enol emission. It is
known that mostly rotamer E’* is responsible for normal emission and rotamer E for longer wavelength
emission after conversion into K* [17] . Low solvation energy of non-polar and slightly polar solvent
makes rotamer E stable than E’. Thus the relative population of rotamer giving keto emission will be
higher compared to other rotamer E* [17] . Compound MMT seems complementing the above
observation but compound DHT does not. Only one emission having Stokes shift 85 nm is observed in
case of DHT in solvent toluene and chloroform. This is really interesting because of more planar
geometry than MMT, this should give dominating keto emission but giving only one emission in high
efficiency.
<< Please insert Table 1, 2, 3 here >>
13

Now the question arises, this is due to enol emission or keto? As non-polar and aprotic solvents, the
solvent interacting species can be neglected. The only two possibilities are rotamer E and E’. In both
cases there is a hydrogen bonding between nitrogen and hydroxyl group from naphthalene 1st position and
3rd position respectively as shown in structures (Fig. 1). This means that dual emission must be observed
in any case. But there is neither a long wavelength emission at about 600 nm [21] nor emission similar to
MMT. This proves that emission is definitely not coming out of rotamer E’ but out of rotamer E. The
reason for blue shift may be the increased band gap between the excited and ground state as in the case of
HNBO [17] . Another reason of ESIPT emission for DHT can be understood from the emission of
compound DMT. In DMT, there is no proton transfer so no ESIPT. Whatever emission observed is only
because of charge transfer from naphthalene ring assisted by methoxy group to thiazole ring which acting
as a charge acceptor. Emission for DMT in toluene is at 425 nm which is blue shifted compared to DHT
and also very weak. This proves that emission in case of the DHT is the ESIPT keto emission and not the
charge transfer due to naphthalene ring to benzothiazole ring because if it is not the ESIPT emission it
must be blue shifted compared to DMT.
Observations are different in 1,4-dioxane (dioxane). Compound MMT has similar absorption profile in
dioxane but different emission properties compared to toluene and chloroform. Shorter wavelength
emission is greatly increased and longer wavelength emission get lowered in the intensity suggesting the
interaction of molecule with the solvent similar to previous results [42] . Two reasons can be there for
this behavior. E’ which is present and second is the intermolecular hydrogen bonding in competition with
intramolecular hydrogen bonding due to the highly acidic hydroxyl proton (16 ppm in H1 NMR) which is
responsible for open form of rotamer E and thus increasing the overall enol emission. Compound DHT
14

shows to different emission profile at two different excitation wavelength in dioxane. It shows low
intensity dual emission when excited at 342 nm and single emission at 370 nm with good intensity.
Excitation spectrum are also different and exactly opposite to absorption spectrum. Excitation spectrum
shows sharp curve at maximum at 371 nm for 450 nm emission. For emission 510 nm it shows sharp peak
at 373 nm and hump at 341 nm. This indicates that the two emission are coming from two different
species. We assign the enol emission collectively from the rotamer E’ and open form of enol E.
(Excitation spectra are provided for compound MMT and DHT in the supporting information in Fig. S19
and S20)
There is excellent agreement observed between the experimental absorption properties and computed
vertical excitation values (Table 1, 2 and 3). It is clear that there are three major peaks in the experimental
absorption spectra in toluene. One at 394 nm, one at 327 nm and 342 nm and third is at 385 nm.
Computed spectra for optimized geometry of E rotamer shows peaks at 294 nm, 334nm, and at 387 nm
respectively which are exactly coinciding with the excitation spectra for emission at 511 nm. Therefore, it
is clear that the longer wavelength emission is coming out of cis enol rotamer which is the only conformer
giving ESIPT emission. The shorter wavelength emission may come from two conformers. One from
open cis conformer and second is trans closed or trans open conformer. In non-polar solvent like toluene
open cis conformer cannot present. Therefore, we assign the shorter wavelength emission to trans enol
rotamer (E’). In polar and polar protic solvent the absorption profile of MMT is similar as non-polar
solvents. This indicates that there is little effect of polarity on absorption of compound MMT. But
emission is red shifted in acetonitrile, DMF (n,n-dimethylformamide) and DMSO (dimethyl sulfoxide).
Generally, in polar solvents enol emission increases and keto emission decreases for most of the HBXs
molecules due to intermolecular hydrogen bonding interrupting the ESIPT [43,44] . This is contradictory
to present results where keto emission intensity is higher than enol emission (MMT spectra) although the
quantum yield is still low compared to non-polar solvent. The presence of methoxy group may helping
the strong intramolecular hydrogen bonding and running the race for ESIPT process to produce keto
15

emission. In methanol still there is dominating keto emission compared to enol emission but maxima get
blue shifted to 503 nm compared to 520 nm in DMSO.
In case of DHT as polarity increases the absorption peak at 360-370 getting vanished. In polar solvents
absorption profile of DHT is similar to MMT except DMF. In DMF there is prominent absorption peak at
445 nm. No emission is observed at 445 nm indicating the generation of anionic species due to the
basicity of DMF towards highly acidic protons. The emission is quite different in DMSO and MeOH for
DHT. Dual emission is observed in both solvents with highly red shifted emission at 550 nm. The reason
for this anomalous behavior may be attributed to strong intermolecular hydrogen bonding between
hydroxyl group at 3rd position and solvent molecules which stabilizing the main ESIPT core and thus the
keto emission is observed. Though the hydroxyl group at 1st position is also experiencing the
intermolecular interactions but strong intramolecular hydrogen bonding is able to carry forward ESIPT
process.
Table 3 clearly indicates that vertical excitation properties predicted using TDDFT method for compound
MMT are closely resembling to the experimental enol and keto emission. (the rotamer E, for MMT
showed same geometry as E rotamer after optimization. Therefore, vertical excitation properties could not
be obtained for E’). In case of DHT, the vertical excitation values for enol form (E) as well as (E’) in all
the solvents are same. In toluene (491 nm) and 1,4-dioxane (493 nm) the values are underestimated but
still not far away from experimental values. For keto tautomer vertical excitation properties are quite
underestimated and are above 600 nm in non-polar solvents such as toluene, chloroform and dioxane
which are quite contrasting to experimental results. In polar solvents vertical excitation properties are
closer to the experimental emission values for both enol and keto emission.
4.2 Analysis using solvent polarity scales
It is clear from the figure (S21-S24 supporting information) that, solvent permittivity (dielectric
constant - ϵ) and solvent polarizability (refractive index – n) played a minor role in the keto
16

emission of the compounds (plots do not show linear relation- supporting information) MMT and
DHT. Having the acidic protons and donor centers (Nitrogen atom in thiazole ring), the
compounds must interact with the acidity and basicity components of the solvents. To study this,
we used the Kamlet-Taft and Catlan polarity scales to verify our supposition. The above scales
which represents only the diplarity/polarizability are not sufficient to understand the observed
keto emission solvatochromism. Therefore, The effect of solvent dipolarity/polarizability and
hydrogen bonding on the emission spectra is interpreted by means of linear solvation energy
relationship (LSER) using a Kamlet–Taft and Catlan solvent parameters involving
dipolarity/polarizability, solvent acidity and solvent basicity [45,46] .
The solvent effect on νabs, εmax, ƒ and ∆ νabs can also be described on the basis of a general multi-
linear expression (Eq.1):
x = ν0 + aA + bB + cC
(1) [47]
in which ν0 stands for the physicochemical property of interest in the absence of solvent (i.e., in
the gas phase); a, b and c are adjustable coefficients that reflect the dependency of the
physicochemical property (y) in a given solvent on the various (A, B, C) solvent parameters. At
least two different sets of solvent scales can be found in the literature to characterize these
solvent properties. In the present analysis, the polarity/polarizability, the polarizability, the
acidity and the basicity of the solvent are considered. Kamlet and Taft [45,48] put forward the
π*, α and β parameters to characterize respectively, the polarity/polarizability, the acidity, and
the basicity of a solvent (Eq. 2). Conversely, Catalan et al. proposed an empirical solvent scales
for polarity/polarizability (SPP) [49] , acidity (SA) [50] and basicity (SB) [51] to describe the
respective properties of a given solvent .
17

x = Cx0 + Cπ*π* + Cββ + Cαα
( Kamlet – Taft )
(2)
(3)
x = Cx0 + CSASA + CSBSB + CSPPSPP
( Catlan )
It is clear from the table 4 that, solvent acidity and basicity are the two major factors governing
the observed solvatochromism for the compounds MMT and DHT. Basicity is the most
dominating factor compared to acidity. This is obvious because the increasing basicity of the
solvent can interact with the acidic compounds via strong hydrogen bonding and affect the
ESIPT outcome in terms of the emission wavelength maxima. It suggested that keto tautomer
which is the charge transfer outcome of the ESIPT process is being stabilized by the solvent
basicity followed by solvent acidity and minimally due to dipolarity/polarizability. The Catlan
which is the advanced version of the Kamlet-Taft scale proven better as suggested by better
regression coefficient (Fig. S25-S30).
This proved that the intramolecular and the intermolecular hydrogen bonding plays the very
crucial role in ESIPT outcome. Intramolecular hydrogen bonding leads molecule to ESIPT
emission and interruption of the same decreases the ESIPT emission. The important point is to
allow the more intramolecular hydrogen bonding and avoid intermolecular hydrogen bonding. In
this whole process molecule orientation is also important. If the geometry of the molecule (E
rotamer) is good enough for the intramolecular hydrogen bonding then there will be smooth
process of ESIPT. In order to do that there must be a large population of the E rotamer in the
restricted motion which is only possible in the solid, aggregate and viscous media. To understand
and verify this supposition we did the aggregation induced emission enhancement (AIEE) study
and viscosity based fluorescence enhancement study.
<< Please insert Table 4 here >>
18

4.3 Aggregation induced enhanced emission (AIEE)
Aggregates are known to be highly emissive, having quantum yield up to unity [27] .
Mechanistically there are many ways in which aggregation leads to emission (AIE) or enhanced
emission (AIEE).The possible mechanisms are molecular conformational planarity, J-aggregates,
H-aggregates due to pi-pi staking interactions and twisted intramolecular charge transfer (TICT)
[52,53] etc. AIEE effect mainly depends on molecular structure and chromophores present on it.
There is another reason for AIEE which explains the restricted intramolecular rotation (RIR).
This mechanistic way depends upon the enhance emission in the solid state because of restricted
intramolecular motion [27,54] .
<< Please insert Fig. 8(a) and 8(b) here >>
In our case quantum yields are low in solutions for the molecule MMT. But this molecule shows
high solid state emission. This observation intrigued us to study whether the compound MMT
shows AIEE or not. The AIE characteristics of MMT were studied by measuring its PL intensity
changes in a solvent mixture of 1,4-dioxane and water. The results are shown in the Fig. 8 (a)
and 8 (b). The PL intensity (I) at 510 nm remains very low up to water fraction reaches 60%. It
increased at 80% with ratio I/I0 0.22 which is still a low value (I0 is the PL intensity at 95% water
fraction). At 90% water fraction emission increased more up to ratio I/I0 is 0.4 while an intense
emission was observed at 510 nm when the water fraction reaches 95%, which verifies that
MMT has typical AIE properties.
19

There is no strong donor and acceptor in the molecule, thus no typical charge transfer is there
which denies the existence of J-aggregates responsible for AIEE. This also excludes TICT [27] .
There is about 18 and 16 nm red shift in the emission maxima when water fractions are 40% and
60% respectively. Slightly increased intensity having ratio I/I0 0.17 compared to 20% water
fraction with ratio I/I0 0.15 suggests that there is conformational rigidity due to planarity. As
water fraction increased from 60% to 90% there is sudden increased in the intensity which
indicates that not only planarity but other phenomenon is also working. This must be the
restricted intramolecular rotation (RIR). It is evident that continuous rotation of ESIPT molecule
in solution responsible for lot of non-radiative decay [27] . The aggregates formation locks the
rotation and allows aggregates to emit via smooth ESIPT process. Therefore, we hypothesize that
AIEE in case of MMT is collectively due to RIR and conformational rigidity of compound. To
support our hypothesis we performed two experiments. One is aggregation extent study and other
is viscosity study to support the RIR mechanism.
4.4 Aggregation extent study
Because of the AIEE characteristics of MMT, in the aggregated state, the intramolecular
hydrogen bonds of MMT can be protected from the disruption of polar solvents, and the KE */E*
emission ratio will be increased via smooth ESIPT process. However, in the non-aggregated
state, its intramolecular hydrogen bonds can be disrupted by polar solvents, and the KE
*/E*emission ratio will be decreased. To verify that the aggregation extent will affect its
emission spectra shape, we have measured the emission spectra of MMT at different
concentrations in dioxane–water mixture (1/95 v/v) (Fig. S34-supporting information). The
results show that with decreasing concentration of MMT (from 10 μM to 1 μM), there is a
20

decreasing KE */E*emission ratio, because the lower concentration of MMT will lead to lower
aggregation extent and the intramolecular hydrogen bonds can be disrupted by the protic water
molecules. This indicates that at higher concentrations molecular motion is restricted due to
uniform formation of aggregates, thus there will be less chance for water to interrupt the ESIPT
process. Therefore, RIR is at work to enhance the emission.
4.5 Viscosity study
Viscosity is known to restrict the molecular motion and thus has significant influence on the
fluorescence property of the compounds having typical structure as donor-π-acceptor [54–56].
As mentioned in the introduction, formation of TCIT keto tautomer (KZ * Fig. 2) via internal
conversion (IC) is the main cause of loss in the fluorescence yield. This proportion is more in
HBTs compared to other HBXs. It was also suggested that in cases of HBXs, especially HBTs,
there is viscosity dependent radiationless deactivation of cis-keto tautomer (KE *) via IC [57–60].
To verify our hypothesis of RIR we studied the effect of paraffin oil and polyethylene glycol
which are highly viscous solvents to ensure that because of RIR, internal conversion is restricted
and emission intensity increases. Solutions of increasing paraffin (non-hydrogen bonding media)
percentages in toluene and increasing percentage of polyethylene glycol (PEG-400- hydrogen
bonding media) in toluene were subjected to study the viscosity effect on ESIPT fluorescence of
MMT.
<< Please insert Fig. 9(a) and 9(b) here >>
21

Keeping the parent solvent (toluene) same in the study allowed us to study the effect of two
viscous media. One without hydrogen bonding (paraffin oil) and other with hydrogen bonding
(PEG-400). Results are shown in Fig. 9 (a) and 9 (b). It is clear that viscosity in both the cases
increases the fluorescence intensity of the keto tautomer. There is eight fold increase in case of
the paraffin oil and threefold increase in case of PEG. There is linear increase in the fluorescence
intensity with increasing fractions of either viscous medium (Fig. S31 (a) and S31 (b)). Intensity
increase is less (by five-fold) in case of PEG compared to paraffin oil. It is a clear indication of
the interruption of ESIPT process by hydrogen bonding of the PEG. But still the population of E
rotamer is more at 95% fraction compared to 0% fraction of PEG. Our results are contrary to the
traditional understanding of behavior of ESIPT molecules in viscous media. According to the
ESIPT theory the formation of Z-keto isomer is very facile in case of HBTs, and it gets very fast
if either donor group is present on hydroxyl part (deprotonating species ) or electron withdrawing
group is present on azole ring (proton accepting species) [60]. In our case we have methoxy
group on proton donating species and consequently the formation of z-keto form should be more
easy and fast. This is getting observed in the solvent state where florescence is very low. This is
also supported by the DFT calculated energy barrier between E and E’ (5 kcal/mol from DFT
calculations) which is very less and molecule is free to rotate. This suggests that even though
methoxy group is there, rotation is barrierless and torsional angle required to form Z-keto
tautomer (Fig. 2) can be achieved in the excited state. But in viscous media the fluorescence
increasing with each increasing fraction of viscosity. Following these observations, we propose
that in the highly viscous media the methoxy group prohibiting the formation of Z-keto tautomer
making the torsional angle difficult to achieve. Therefore, the population of cis (E)-keto (Fig. 2)
22

tautomer increases and fluorescence increases. Having the positive results in hand we quantified
the observed effect of viscosity on the fluorescence intensity. We used Forster Hoffman equation
to calculate the x values (viscosity sensitivity). The equation is given as
(3)
Where, I is the emission intensity, η is solvent viscosity, C is the constant and x is the compound
viscosity sensitivity. The values for x are found to be 0.26 and 0.17 for toluene/paraffin oil and
toluene/PEG system respectively (Fig.S33 (a) and S33 (b)). These values suggest that the
compound MMT has potential to act as a fluorescent molecular rotor in non-polar as well as
polar-protic media. Though the values are lower compared to standard rotor DCVJ (0.6) [61] but
as a ESIPT molecule these values can be taken into consideration as no ESIPT compound is
studied as FMR. It is now, well known that the FMRs can be any candidate showing emission
enhancement with viscosity and does not depend upon polarity of the solvent and intramolecular
charge transfer (ICT) always [62]. It depends on what radiationless deactivating paths are there
and how viscosity diminishing these effect. This means that in viscous polar media also ESIPT
compounds can show increased fluorescent intensity.
5. Conclusions
In conclusion, the photochemical behavior of naphthalene fused 2-(benzo[d]thiazol-2-yl)-3-
methoxynaphthalen-1-ol and 2-(benzo[d]thiazol-2-yl)naphthalene-1,3-diol (MMT and DHT
respectively) was studied using steady-state spectroscopy. Two compounds behaved entirely
different in same environment (solvents and solid state). In comparison with the parent
compound HBT, MMT and DHT show absorption at longer wavelengths with quite complex
behavior. In case of emission, behavior is entirely different. MMT emitted the fluorescence in
23

the greenish-yellow region at longer wavelength from E-keto tautomer and at the shorter
wavelength from enol tautomer E* which also confirmed from the TDDFT study. This shows
that the methoxy group is overcoming the energy barrier contrary to the compound HNBT where
naphthalene ring increasing the energy barrier and keto fluorescence found at the shorter
wavelength. DHT emitted the fluorescence in the blue region in the non-polar solvents (single
emission) and emits at the longer wavelengths 545 nm (dual emission) in polar solvents such as
DMSO and MeOH showing the existence of dual hydrogen bonding making molecule rigid in
the polar environment. MMT shows poor quantum yields in all solvents whereas DHT shows
reasonable quantum yields in non-polar solvents but poor in polar solvents. The poor emission in
solution state and bright greenish-yellow emission in the solid state evaluated successfully using
AIEE and viscosity study showing existence of intramolecular motion in the solution state
responsible for the low quantum yield. The high intensity in the aggregate and highly viscous
state indicates the locking of molecular conformation in the desired E’ enol rotamer required for
the enhanced keto emission. The study shows that caging of rotamer E’ in the suitable
environment prevent the other isomer E’* and transient species like Z-keto tautomer, triplet
states from interfering in the ESIPT dynamics. Study shows the potential applications of ESIPT
molecules in the optoelectronics and as FMRs.
Acknowledgements
Umesh Warde is grateful to the UGC-SAP fellowship as SRF, India for the financial support.
(Grant - F. 5-65/2007(BSR)).
[Figures, 1H, 13C and photographs are included in supporting information]
24

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