ESIPT-rhodol derivatives with enhanced Stokes shift: Synthesis, photophysical properties, viscosity sensitivity and DFT studies

Article abstract of DOI:10.1016/j.molliq.2019.111626

Three rhodol derivatives, namely spirolactamized phenanthrene-imidazole, diphenyl-imidazole and benzothiazole substituted N, N-diethylamine rhodols were designed, synthesized and characterized. Their photophysical properties were studied in spirocyclic and open form from non-polar to polar solvents. The spirocyclic form exhibited larger Stokes shifts (50–260 nm) due to presence of ESIPT process whereas, open form showed small Stokes shift (10–40 nm) due to the lack of protons. Positive solvatochromism was observed for both the spirocyclic and open forms which is well supported by the linear (Lippert-Mataga and Mac-Rae) and multi-linear (Kamlet-Taft and Catalan parameters) analysis. Solvent polarizability (d_SP) is the major factor responsible for red shift in absorption/emission and larger Stokes shift for spirocyclic form. Polarity graphs and charge transfer descriptors are in good relation with Generalized Mulliken-Hush (GMH) parameters. From the solvatochromic data we observed that open forms show good TICT characteristics as compared to their respective spirocyclic forms. Open form is highly sensitive to viscosity as compared to spirocyclic form in a mixture of polar-protic solvents (EtOH:PEG 400). The experimental results are well correlated theoretically using Density Functional Theory (DFT) computations.

Full text of DOI:10.1016/j.molliq.2019.111626

Journal of Molecular Liquids 294 (2019) 111626
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
ESIPT-rhodol derivatives with enhanced Stokes shift: Synthesis,
photophysical properties, viscosity sensitivity and DFT studies
Sagar B. Yadav, Shantatram Kothavale, Nagaiyan Sekar ⁎
Dyestuff Technology Department, Institute of Chemical Technology, N. P. Marg, Matunga, Mumbai 400019, Maharashtra, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 23 May 2019
Received in revised form 21 July 2019
Accepted 24 August 2019
Available online 29 August 2019
Three rhodol derivatives, namely spirolactamized phenanthrene-imidazole, diphenyl-imidazole and
benzothiazole substituted N, N-diethylamine rhodols were designed, synthesized and characterized. Their
photophysical properties were studied in spirocyclic and open form from non-polar to polar solvents. The
spirocyclic form exhibited larger Stokes shifts (50–260 nm) due to presence of ESIPT process whereas, open
form showed small Stokes shift (10–40 nm) due to the lack of protons. Positive solvatochromism was observed
for both the spirocyclic and open forms which is well supported by the linear (Lippert-Mataga and Mac-Rae) and
multi-linear (Kamlet-Taft and Catalan parameters) analysis. Solvent polarizability (dSP) is the major factor re-
sponsible for red shift in absorption/emission and larger Stokes shift for spirocyclic form. Polarity graphs and
charge transfer descriptors are in good relation with Generalized Mulliken-Hush (GMH) parameters. From the
solvatochromic data we observed that open forms show good TICT characteristics as compared to their respective
spirocyclic forms. Open form is highly sensitive to viscosity as compared to spirocyclic form in a mixture of polar-
protic solvents (EtOH:PEG 400). The experimental results are well correlated theoretically using Density Func-
tional Theory (DFT) computations.
Keywords:
ESIPT-rhodols
Positive solvatochromism
TICT
Viscosity sensitivity
Density Functional Theory (DFT)
©
2019 Elsevier B.V. All rights reserved.
1
. Introduction
In the past few years, excited-state intramolecular proton transfer
ESIPT) in the molecules has emerged as a useful tool for sensing appli-
(
Xanthene dyes such as rhodamine, fluorescein with reactive linker
cations [35–40]. The ESIPT-active molecules give large Stokes shifts and
change in fluorescence from excited enol and keto tautomers [41].
These properties of ESIPT molecules assist in designing numerous mo-
lecular probes for detection of targeted species [42–48]. 2-(2′-
Hydroxyphenyl)-benzoxazole undergoes ESIPT process on photo-
excitation in which rapid photo-induced proton transfer leads to
tautomerization [49–51]. The blockage of ESIPT process in 2-(2′-
hydroxyphenyl)-benzothiazole (HBT) mainly results in enol-like emis-
sion and is used as ratiometric sensing for the variety of analytes [52].
In the absence of analyte, the keto-form exhibits stronger fluorescence
at longer wavelength on photo-excitation [53–62]. This unusual red
shift in emission obtained with ESIPT molecules is useful for their appli-
cations in photochromic dyes [63], laser dyes [64], photo-stabilizers [65]
and fluorescence recording techniques [66,67].
Rhodol is a hybrid form of fluorescein and rhodamine which shows
excellent photophysical properties such as good solubility, photo- sta-
bility and high fluorescence quantum yield [68–71]. Rhodol has an
acidic phenol moiety in their spirocyclic form that could be readily ion-
ized to the more nucleophilic phenoxide ion in open form. This molecu-
lar configuration is the basis for the ESIPT process between the phenolic
hydroxyl group and N-atom of the benzothiazole ring which exhibits
similar emission properties like HBT [72,73]. This ESIPT process
functionalities are extensively used in various applications as they are
biocompatible, excite with low energy and emit with high fluorescence
quantum yield [1–7]. The conformational changes in xanthenes (from
spiro-lacton to open form) are useful in sensing the heavy metal ions
8–19]. The photophysical properties of xanthene derivatives can be al-
tered by structural modifications using different synthetic strategies
20]. In addition, a new strategy to obtain red shifted absorption is re-
[
[
ported where the xanthene oxygen is substituted by other atoms such
as silicon and carbon [21,22].
Though xanthene derivatives are widely applied in numerous fields,
they suffer a major drawback of small Stokes shift leading to self-
quenching and errors in fluorescence detection [23,24]. Hence, a num-
ber of researchers in the past few years contributed to overcome the
above drawbacks by synthesizing hybrid dyes such as ESIPT-BODIPY
25,26], ESIPT-coumarin [27,28], BODIPY-coumarin [29,30], BODIPY-
rhodamine [31,32], and coumarin-rhodamine [33,34] with unique
photophysical properties.
[
⁎
Corresponding author.
E-mail address: n.sekar@ictmumbai.edu.in (N. Sekar).
https://doi.org/10.1016/j.molliq.2019.111626
167-7322/© 2019 Elsevier B.V. All rights reserved.
0

2
S.B. Yadav et al. / Journal of Molecular Liquids 294 (2019) 111626
Fig. 1. Structures of the ESIPT-rhodol derivatives 10, 11 and 12 (spirocyclic and open form) with their photographs under UV light in chloroform.
1
generates photo induced structural transformation of rhodols from phe-
nol (enol) to keto form [74].
using silica of 100–200 mesh size. H and 13C NMR spectra were recorded
on 500 MHz and 100 MHz respectively on Agilent Technologies instru-
ment (TMS as an internal standard). Perkin Elmer UV–visible spectropho-
tometer (Lambda 25) was used for absorption studies. Varian Cary Eclipse
fluorescence spectrophotometer was used for emission studies. Melting
points were recorded on melting point equipment from Sunder Industrial
Products, Mumbai. Fluorescence quantum yield were calculated by using
coumarin 6 (ϕ = 0.94 in chloroform) and rhodamine 101 (ϕ = 0.94in
ethanol) as reference standards for spirocyclic and open form respectively.
Herein, we report three rhodol derivatives 10, 11, and 12 (Fig. 1). In
these compounds, benzimidazole and benzothiazole units were strategi-
cally introduced adjacent to oxygen atom of phenolic group of rhodol
dyes. It is expected that the “phenol-quinone” transmission in these
rhodol dyes switch the ESIPT process existing between the phenolic OH
group and N atom of benzimidazole/benzothiazole ring in spirocyclic
form. The ESIPT-rhodol derivatives were reported by Yang et al. as
2+
ratiometric probes for Cu [75], Goswami et al. reported rhodol-based
chemodosimeter for both Hg2 and OCl [76], Zhang et al. reported
rhodol-based far-red fluorescent probe for the detection of cysteine, ho-
mocysteine and glutathione [77] and the Strongin et al. rhodol-thioester
fluorescent probe for the detection of glutathione and cysteine/homocys-
teine [78]. However, their photophysical properties in closed and open
form, detailed solvatochromic properties, intramolecular-charge transfer
+
−
2.2. Computational methods
DFT [79] computations of the synthesized rhodols were carried on
Gaussian 09 [80] program using B3LYP hybrid functional. The B3LYP is
combination of Becke's three exchange functional parameter (B3) [81]
and the functional by Lee, Yang and Parr [82]. B3LYP/6-31 G
(d) method was used for DFT and TD-DFT studies. Vertical excitation en-
ergies and oscillator strengths were obtained for lowest 10 singlet-
singlet transitions at the optimized ground state equilibrium geometries
by using TD-B3LYP/6-31 G (d) [83].
(ICT) and twisted-intramolecular charge transfer (TICT), viscosity sensi-
tivity and computational studies are not yet reported. Accordingly, herein
we wish to report the solvatochromism of synthesized rhodols 10, 11 and
12, their relation with linear (i.e., Lippert-Mataga, Mac-Rae, Rettig's, and
Generalized Mulliken-Hush polarity functions) and multilinear (i.e.
Kamlet-Taft and Catalan equations) polarity functions, viscosity sensitiv-
ity in mixtures of polar-protic (EtOH:PEG 400) media. The observed ex-
perimental results correlated theoretically using Density Functional
Theory (DFT) and Time Dependant-Density Functional Theory (TD-DFT).
2.3. Synthesis and characterization
2-(4-(Diethyl amino)-2-hydroxybenzoyl) benzoic acid 2 was syn-
thesized by using reported protocol [33]. 2,4-Dihydroxybenzaldehyde
4
was synthesized from resorcinol following the reported method
2
. Experimental
[84]. Intermediate 4 was reacted with 9,10 phenanthrene-dione and
benzil in a mixture of ammonium acetate and acetic acid to afford hy-
droxyl imidazole derivatives 5 and 7 respectively. Similarly, 4 was
treated with 2-aminobenzenethiol 8 in sodium metabisulfite and DMF
as solvent to afford 4-(benzo[d]thiazol-2-yl) benzene-1, 3-diol 9 in
good yields (Scheme 1). All these intermediates 5, 7 and 9 were reacted
2
.1. Materials and instruments
The required chemicals were purchased from the regional distributor,
S. D Fine chemical limited, Mumbai, India. The samples were purified by

S.B. Yadav et al. / Journal of Molecular Liquids 294 (2019) 111626
3
Scheme 1. Synthesis of phenanthrene-imidazole (5), diphenyl-imidazole (7) and thiazole-benzene (9) intermediates.
with N, N-diethyl meta amino phenol keto acid 2 in methane sulfonic
acid to isolate respective ESIPT-rhodol derivatives 10, 11 and 12 in
their spirocyclic form and purified by column chromatography. The
spirocyclic form of 10, 11 and 12 were converted into their open form
by the treatment of methanolic HCl (Scheme 2). The newly synthesized
0.618 mmol) in acetic acid (10 ml). The reaction mixture was refluxed
for 4–5 h. On completion of the reaction, the reaction mass was cooled
to room temperature and quenched with water. The reaction mixture
was then neutralized with aq. ammonia. During the neutralization pro-
cess the solid precipitated out, which was filtered and washed with
water. The crude product obtained was purified by column chromatog-
raphy using 10–30% ethyl acetate in hexane to obtain the pure product
as white solid (0.160 g, 80%) (Scheme 1).
1
13
ESIPT-rhodol derivatives were fully characterized by H, C NMR, ele-
mental and mass analysis.
2
.3.1. Synthesis of 4-(1H-phenanthro [9, 10-d] imidazol-2-yl) (5)
The compound 5 was synthesized according to the reported method
85]. 2, 4-Dihydroxy benzaldehyde (0.12 g, 0.72 mmol) was reacted
2.3.3. Synthesis of 4-(benzo[d] thiazole-2-yl)-benzene-1, 3-diol (9) [87]
2-Amino thiophenol 8 (0.33 ml, 3.12 mmol), sodium metabisulfite
[
with phenanthre-9, 10-dione (0.13 g, 0.624 mmol) and ammonium ace-
tate (0.86 g, 11.20 mmol) in acetic acid (10 ml). The reaction mixture
was refluxed for 4–5 h. On completion of the reaction, the reaction
mass was cooled to room temperature and quenched with water. The
reaction mixture was then neutralized with aq. ammonia solution. Dur-
ing the neutralization process the solid precipitated out, which was fil-
tered and washed with water and dried under vacuum. The
purification of crude product was done by column chromatography (el-
uent system; ethyl acetate in hexane 10–30%) to obtain white colored
pure product (0.170 g, 85%) (Scheme 1).
2 2 5
(Na S O , 0.610 g) and 2, 4-dihydroxy benzaldehyde 4 (0.446 g,
3.18 mmol) were refluxed in dry DMF (20 ml) for 2 h. The reaction mix-
ture was cooled to room temperature and water (200 ml) was added
slowly to the reaction mixture. After completion of the addition, the
solid precipitated out which was filtered, washed with water (60 ml)
and dried to yield crude product. The crude product was purified by re-
crystallization from methanol to obtain light yellow solid product
(0.634 g, 85%) (Scheme 1).
2.4. Synthesis of rhodol derivatives (10, 11 and 12)
2
.3.2. Synthesis of 4-(4, 5-diphenyl-1H-imidazolyl)-benzene-1, 3-diol (7)
Compound 7 was synthesized according to the reported method
Compound 10 was obtained by refluxing 5 (0.101 g, 0.307 mmol)
with 2 (0.128 g, 0.41 mmol) in methanesulfonic acid (5 ml). Compound
11 was obtained by refluxing 7 (0.101 g, 0.305 mmol) with 2 (0.128 g,
0.41 mmol) in methanesulfonic acid (5 ml). Compound 12 was obtained
[86]. 2, 4-Dihydroxy benzaldehyde 4 (0.1 g, 0.724 mmol) was reacted
with ammonium acetate (0.88 g, 11.4 mmol) and benzil 6 (0.13 g,

4
S.B. Yadav et al. / Journal of Molecular Liquids 294 (2019) 111626
Scheme 2. Synthesis of ESIPT-rhodol derivatives (10, 11 and 12).
1
on refluxing 9 (0.100 g, 0.41 mmol) with 2 (0.128 g, 0.41 mmol) in
methanesulfonic acid (5 ml). The temperature of the reaction mixture
was maintained at 90–95 °C for 24 h (Scheme 2). Then, reaction mass
was cooled to room temperature and poured in ice-water (30 ml).
After sometime the precipitated product was washed three times with
brine water solution (3 × 5 ml) and then dried under vacuum to give
the crude product. The purification was done by column chromatogra-
phy using 1–2% methanol in chloroform as an eluent system to afford
solid product.
255 °C, H NMR (400 MHz, DMSO) δ 13.76 (s, 1H), 9.01 (s, d = 8.6 Hz,
2H), 8.00 (d, J = 8.6 Hz, 2H), 7.82 (s,1H), 7.37 (m, 3H), 7.13 (d, J =
5.3 Hz, 2H), 6.58 (d, J = 6.5 Hz, 2H), 6.41 (s, 1H), 1.19 (t, J = 7.1 Hz,
6H), 13C NMR (126 MHz, DMSO) δ 166.93, 161.23, 155.77, 151.35,
135.57, 133.96, 131.90, 131.24, 130.71, 130.70, 129.67, 129.29, 126.86,
125.57, 122.81, 122.36, 120.39, 120.31, 117.07, 116.98, 116.56, 114.80,
103.74, 97.72, 84.30, 46.63, 12.98. Mass analysis: Calculated for
(C31
(C31
H
H
24
N
N
2
O
O
4
S): 520.1, found: 521.1 (M+1). Elemental analysis
S): Calculated: C – 77.51, H – 4.65, N – 6.16. Found: C –
24
2
4
77.54, H – 4.64, N – 6.17.
2
.4.1. Characterization (10, 11 and 12)
3. Results and discussion
2
.4.1.1. 6′-(Diethyl amino)-3′-hydroxy-2′-(1H-phenanthro [9, 10-d]
imidazol-2-yl)-3H-spiro [isobenzofuran-1, 9′-xanthen]-3-one (10). Yield:
3.1. Photophysical properties
1
7
8
7
6
7%, melting point: 289 °C, H NMR (400 MHz, DMSO) δ 13.75 (s, 1H),
.80 (d, J = 6.7 Hz, 2H), 8.36 (s, 3H), 8.09 (d, J = 7.1 Hz, 1H),
.81–7.67 (m, 5H), 7.62 (d, J = 7.4 Hz, 2H), 7.30 (d, J = 6.6 Hz, 1H),
Absorption and emission spectra of ESIPT-rhodols were investigated
in both spirocyclic as well as open forms in seven organic solvents of dif-
ferent polarities. The spirocyclic forms 10, 11 and 12 are colorless while
in open form they are shiny reddish black colored solids. The spirocyclic
form of rhodols on treatment with methanolic HCl solution transform
into open form-rhodols. After repeating twice the above procedures,
the solvent was evaporated on rotary evaporator to get open form of
10, 11 and 12 as shiny reddish black solids. Due to the ESIPT process,
spirocyclic-rhodols 10, 11 and 12 show larger Stokes shift
.97 (s, 1H), 6.45 (d, J = 12.5 Hz, 3H), 1.06 (t, J = 6.5 Hz, 6H). 13
C
NMR (101 MHz, DMSO) δ 169.62, 159.98, 153.28, 152.10, 149.65,
1
1
4
6
48.74, 136.11, 136.10, 130.51, 128.78, 126.45, 125.44, 125.43, 124.45,
22.80, 122.07, 111.42, 111.04, 109.29, 109.27, 105.37, 104.35, 97.37,
4.22, 12.74. Mass analysis: Calculated for (C39 ): 603.2, found:
H N O
29 3 4
04.1 (M+1). Elemental analysis (C39H N O ): Calculated: C – 77.60,
29 3 4
H – 4.84, N – 6.96. Found: C – 77.63, H – 4.84, N – 6.97.
(
50–260 nm) than the open form-rhodols 10, 11 and 12 (10–40 nm).
2
.4.1.2. 6′-(Diethyl amino)-2′-(4, 5-diphenyl-1H-imidazol-2-yl)-3′-hy-
Fig. 2 shows the absorption and emission spectra of spirocyclic and
open form of 10, 11 and 12 in chloroform. In spirocyclic form, rhodol
10 show red shifted absorption as compared to 11 and 12, but the oppo-
site trend is observed in emission and Stokes shifts. The compounds 11
and 12 show red shifted emission and large Stokes shift as compared to
10. All spirocyclic forms absorbed in the range of 330–370 nm in chloro-
form. Spirocyclic 10 emits at 428 nm while, spirocyclic 11 and 12 emit at
445 nm and 525 nm respectively in chloroform while the open form 10
shows highly red shifted emission at 610 nm compared to open forms
11 and 12. All the compounds in open form absorb at 335–340 nm
and emit at 590–600 nm in chloroform. In their spirocyclic form molar
extinction coefficients were higher in comparison with their open
forms. From the experimental results for spirocyclic and open forms,
the oscillator strength (ƒ) and transition dipole moment (μeg) were cal-
culated by using reported expressions [88]. In addition, the ESIPT-
droxy-3H-spiro [isobenzofuran-1, 9′-xanthen]-3-one (11). Yield: 75%,
melting point: 272 °C, H NMR (400 MHz, DMSO) δ 13.76 (s, 1H), 7.99
1
(
d, J = 7.1 Hz, 2H), 7.70 (dd, J = 18.4, 6.8 Hz, 2H), 7.55 (s, 1H), 7.39
(
m, 7H), 7.23 (dd, J = 56.0, 7.2 Hz, 4H), 6.86 (s, 1H), 6.46–6.36 (m,
3
1
1
1
9
6
H), 1.05 (t, J = 6.8 Hz, 6H). 13C NMR (101 MHz, DMSO) δ 169.55,
59.22, 153.59, 153.58, 152.47, 152.04, 149.55, 145.19, 145.17, 135.93,
35.91, 135.90, 135.87, 130.31, 130.28, 130.27, 129.27, 128.97, 128.82,
26.36, 125.46, 125.21, 125.20, 124.31, 110.98, 109.18, 105.28, 103.97,
7.31, 44.20, 12.74. Mass analysis: Calculated for (C39 ):
H
31
N
3
O
4
05.23, found: 606.1 (M+1). Elemental analysis (C39H N O ): Calcu-
31 3 4
lated: C – 77.68, H – 4.47, N – 6.91. Found: C – 77.66, H – 4.46, N – 6.92.
2
.4.1.3. 2′-(Benzo[d]thiazol-2-yl)-6′-(diethylamino)-3′-hydroxy-3H-spiro
[
isobenzofuran-1, 9′-xanthen]-3-one (12). Yield: 72%, melting point:

S.B. Yadav et al. / Journal of Molecular Liquids 294 (2019) 111626
5
Fig. 2. Normalized absorption and emission spectra of compound 10, 11 and 12 in spirocyclic form and open form in chloroform.
rhodols show higher transition dipole moment and oscillator strength
in open form in comparison with their spirocyclic form (Table 1).
Complete photophysical parameters of dye 10 in their spirocyclic
form and open form are presented in Tables 2 and 3.
of solvents on emission λmax values with the observed solvatochromism
in dyes we used Lippert-Mataga and Mac-Rae equations [91–93]. The
observed linear relationship between the Stokes shift and Lippert-
Mataga function, Mac-Rae function for the dyes in their spirocyclic
and open forms (Fig. 5) suggests that refractive index and dielectric
constant are together responsible for red shift in emission in polar
solvents.
3
.2. Solvatochromism
From Tables S1–S6 it was observed that ESIPT-rhodol derivatives 10,
1 and 12 in spirocyclic form shows red shift in absorption and emission
1
3.3. Generalized Mulliken Hush (GMH) analysis of intramolecular charge
transfer (ICT)
spectra as we go from non-polar solvents to polar solvents suggesting
that highly polar excited state in polar solvents [89,90]. Fig. 3 represents
the normalized absorption and emission spectra of spirocyclic 12 which
shows a red shift in absorption and emission spectra in non–polar to
polar solvents. Interestingly, in their open form 10, 11 and 12 show in-
tense red shift from non-polar to polar solvents in the absorption and
slightly red shift in emission. Fig. 4 shows normalized absorption and
emission spectra of the open form of 12.
Red shifted absorption and emission in solvents of different polarity
are mainly due to the fact that the excited state of the compounds is in-
fluenced by the dipole-dipole interaction, and solute-solvent interac-
tion. To correlate the effect of refractive index and dielectric constant
The strength of electronic intramolecular charge transfer (ICT) in the
dyes, can be measured to predict the solvatochromism in dyes, as
greater charge transfer leads to highly polar excited state resulting in
positive solvatochromism. The equation for two-state GMH analysis of
ICT in molecules is presented as [94],
2
2
2
ge
Δμ ¼ Δμ þ 4μ
ð1Þ
ab
ge
2
2
The, Δμge and μge terms are obtained from Lippert-Mataga equation
and absorption plots respectively. The electronic coupling between the
Table 1
Photophysical properties of compound 10, 11 and 12 for both forms in chloroform solvent.
Compounds
λ
abs (nm)
ε
max × 104 (M−1 cm−1
)
FWHM (nm)
λ
ems (nm)
Stokes shift
ϕ
ƒ
f
μeg (debye)
(nm)
(cm−1)
1
1
1
1
1
1
0 (spirocyclic)
1 (spirocyclic)
2 (spirocyclic)
0 (open)
1 (open)
2 (open)
368
330
342
556
557
544
7.86
4.96
5.46
2.8
1.54
2.91
72
53
64
89
85
66
428
445
525
610
578
590
60
3800
7800
10,200
1600
650
0.154
0.152
0.157
0.321
0.314
0.329
0.19
0.88
0.91
3.42
3.01
3.32
3.79
7.89
8.16
19.76
18.48
19.55
115
183
54
21
46
1400
λ
abs – absorption maximum wavelength. λemm – emission maximum wavelength. εmax – molar extinction coefficient. FWHM – full width half wave maxima. ϕ
f
– Fluorescence quantum
yield. ƒ – Oscillator strength. μ – transition dipole moment.

6
S.B. Yadav et al. / Journal of Molecular Liquids 294 (2019) 111626
Table 2
Within the two-level approximation in Eq. (3), (HDA) is corre-
lated with the vertical excitation energy (ΔEge), the difference be-
tween the adiabatic dipole moments of the ground and excited
states (Δμge). The difference in diabatic state dipole moments and
Photophysical properties of 10 in their spirocyclic form in all solvents.
Solvent
λ
abs
ε
max
4
×
FWHM
(nm)
λ
(nm)
ems Stokes shift
ϕ
ƒ
f
μ
eg
(debye)
(
nm) 10
(nm) (cm−1)
M−
1
(
D
the transition dipole moments (Δμge) is correlated by GMH method
cm−1)
[
95,96] as follows,
Hexane
Toluene 367
366
9.39
3.77
7.86
3.22
1.74
2.86
3.08
74
75
72
71
71
73
75
416
428
428
438
451
454
459
50
61
60
69
82
83
88
3200
3900
3800
4300
4900
4900
5200
0.086 0.18
0.132 0.68
0.154 0.19
0.144 0.58
0.095 0.22
0.327 0.55
0.347 0.61
3.79
7.33
3.79
6.72
4.12
6.59
6.95
ΔEgeμge
ΔEge∙μ
ge
HDA ¼
¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ð4Þ
CHCl
EA
3
368
369
369
D
Δμ
2
2
ge
Δμ þ 4μ
ge
ge
MeOH
Dioxane 371
DMSO
The charge transfer D-π-A coupling distance (RDA) [96] can be deter-
371
mined by using Eq. (5),
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ΔEgeεmaxΔν1=2
−2
Table 3
RDA ¼ 2:06 ꢀ 10
ð5Þ
HDA
Photophysical properties of 10 in their open form in all solvents.
where, ‘εmax’ and ‘Δυ1/2’ are the molar absorptivity (L·M−1·cm−1) and
bandwidth (cm ) respectively.
Solvent
λ
abs
ε
max
4
×
FWHM
(nm)
λ
(nm)
ems Stokes shift
ϕ
ƒ
f
μ
eg
(debye)
(
nm) 10
−1
M−
(
nm) (cm−1)
1
(
cm 1)
−
The observed results of GMH analysis show that there is a good
charge transfer in spirocyclic and open form of 10, 11 and 12 as degree
of delocalization (C b) and D-π-A coupling value (HDA) increases with
solvent polarity. Similarly 10, 11 and 12 showed positive
solvatochromism as D-π-A coupling distance (RDA) values decreased
with solvent polarity for 10, 11 and 12 in their spirocyclic as well as
open form (Tables 4 and 5).
Hexane
Toluene 546
536
7.3
1.53
2.8
6.68
1.96
1.38
2.45
141
93
89
82
75
68
89
598
602
610
618
614
618
623
62
56
54
53
48
52
56
1900
1700
1600
1500
1400
1500
1600
0.135 1.01
0.26 1.82
0.321 3.42
0.292 0.59
0.157 2.09
0.592 1.14
0.647 0.21
10.86
14.82
19.76
8.48
15.75
11.72
4.93
2
CHCl
EA
3
556
565
566
MeOH
Dioxane 566
DMSO 567
3.4. Multi-linear regression analysis using Kamlet-Taft and Catalan
equations
donor (D) and acceptor (A) group is an important factor in controlling
the charge transfer for D-π-A system [95]. Hence it is essential to esti-
mate the D-π-A coupling matrix value (Hab) to better understand the
behaviour of long- distance charge transfer, where, ‘a’ and ‘b’ are initial
and final diabatic states, which are characterized as the ground and ex-
cited states in charge transfer process. The coupling matrix in adiabatic
states of D-π-A system can be denoted as HDA, and are assumed to be
composed of three diabatic states (i.e. donor group state (GS), donor lo-
cally excited state (ES) and charge transfer state (CT) transferring
In general linear correlation parameters such as Lippert-Mataga and
Mac-Rae polarity functions are used to understand the effect of solvent
polarity on observed absorption and emission shift, but these parame-
ters are not explaining precisely which solvent parameters such as sol-
vent acidity, basicity, dipolarity or polarizability are responsible for
observed spectral shift. Hence multilinear approach is proposed by
Kamlet and Taft [97,98] as represented in Eq. (6)
ꢁ
charge from donor to the acceptor unit). The degree of delocalization
y ¼ y þ a α þ b β þ c
ꢁ
π
ð6Þ
0
α
β
π
2
(C
b
) and donor-acceptor coupling matrix value (HDA) calculated from
spectroscopic data for 10, 11, and 12 in their spirocyclic as well as
open form by using following equation,
where, ‘y’ is solvent affected physiochemical property (e.g. absorption
maxima (υabs), emission maxima (υemm) and Stokes shift (Δυ) etc.).
0
‘y ’ is studied physiochemical property in gas phase, while a, b, c, and
0
vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1
u
d are adjustable coefficients which reflect the dependence of ‘y’ to the
various solvent parameters (e.g. effect of solvent acidity (α), effect of
solvent basicity (β), collective effect of solvent polarity/dipolarity
2
u
Δμge
1
2
2
b
t
@
A
C
¼
1−
ð2Þ
ð3Þ
2
2
Δμ þ 4μ
ge
ge
⁎
as well as solvent polarizability (π ) etc.). However, if solvent acidity
and basicity are not affecting the value of ‘y’, then among solvent
polarity, dipolarity and polarizability it is difficult to find out which
factor is exactly influencing the studied physiochemical property
ΔEgeμge
D
Δμ
ge
HDA ¼
Fig. 3. Absorption and emission spectra of spirocyclic 12.

S.B. Yadav et al. / Journal of Molecular Liquids 294 (2019) 111626
7
Fig. 4. Absorption and emission spectra of open 12.
Fig. 5. Lippert-Mataga and Mac-Rae plots for compound 10, 11 and 12.
‘
y’, hence Catalan [99–101] introduced another expression as repre-
analysis method indicates the negative solvatochromism, whereas,
the negative values for factors in multi-linear analysis methods indi-
cate positive solvatochromism. The details of analysis employing
Kamlet-Taft (Eq. (6)) and Catalan (Eq. (7)) parameters together for
multi-linear analysis of vabs (absorption maxima), vemn (emission
maxima) and Δv (the Stokes shift) for spirocyclic 10 in seven sol-
vents of various polarity are shown in Table 6.
Catalan parameters (0.86, 0.97, and 0.71) show higher values for ab-
sorption maxima (υabs), emission maxima (υemi) and Stokes shift (Δυ)
respectively than Kamlet-Taft parameters (0.54, 0.87, and 0.21). From
Table 6 it is clear that solvent polarizability factor (dSP) is the major fac-
tor responsible for red shift in absorption and emission spectra of
spirocyclic 10.
sented in Eq. (7)
y ¼ y þ aSASA þ bSBSB þ cSPSP þ d
SdP
ð7Þ
0
SdP
where, Catalan used same solvent parameters (e.g. solvent acidity
SA), solvent basicity (SB) etc.), which is already utilized by Kamlet
(
⁎
and Taft. Here, he divided the last parameter ‘π ’ into two separate
parameters such as solvent polarizability (SP) and solvent dipolarity
(
parameters, here we used both the parameters simultaneously to
understands the exactly which factor is influence the observed spec-
tral shift in absorption and emission spectra from non-polar to polar
solvents in all dyes 10, 11, and 12. Taft and Kamlet [102] and Catalán
SdP). The Catalan parameters are modified version of Kamlet-Taft
Tables S7–S12 show multi-linear analysis in solvents of different po-
larity by using Kamlet-Taft and Catalan parameters together for 10, 11
and 12 in spirocyclic and open form. Similarly, for spirocyclic 11 and
12, solvent polarizability (dSP) mainly responsible for red shift in
[
103] functions are used to find out the main factor influence the ab-
sorption spectra, emission spectra and the Stokes shifts in solvents of
various polarity. The positive value for various factors in multi-linear
Table 4
2
Degree of delocalization (C b), donor-acceptor coupling value (HDA) and donor-acceptor coupling distance (RDA) values for spirocyclic form 10, 11 and 12.
Solvent
Spirocyclic - 10
Spirocyclic - 11
Spirocyclic - 12
2
DA (cm−1
2
DA (cm−1
2
DA (cm−1
C b
H
)
RDA (Å)
C b
H
)
RDA (Å)
C b
H
)
RDA (Å)
Hexane
Toluene
0.4906
0.4909
0.4911
0.4944
0.4946
0.4948
0.4951
13,476.3
13,476.4
13,547.7
13,549.2
13,586.1
13,621.8
13,658.9
28.1
26
0.486
0.495
15,059.7
15,151.2
15,243.4
15,237.9
15,290
33.2
25.5
21.9
22.9
17.7
21.9
15.3
0.4921
0.4961
0.496
0.4967
0.4963
0.4978
0.4976
14,490.9
14,577
34.9
30.6
22.4
22.7
21.7
22.2
11.4
CHCL
EA
3
16.8
17.7
12.3
15.6
16
0.4961
0.4959
0.4963
0.4955
0.497
14,619.5
14,662.6
14,619.4
14,662.3
14,748.7
MeOH
Dioxane
DMSO
15,243.2
15,575.5

8
S.B. Yadav et al. / Journal of Molecular Liquids 294 (2019) 111626
Table 5
2
Degree of delocalization (C b), donor-acceptor coupling value (HDA) and donor-acceptor coupling distance (RDA) values for open form 10, 11 and 12.
Solvent
Open - 10
Open - 11
Open - 12
2
DA (cm−1
2
DA (cm−1
2
DA (cm−1
C b
H
)
RDA (Å)
C b
H
)
RDA (Å)
C b
H
)
RDA (Å)
Hexane
Toluene
0.4933
0.4976
0.4968
0.4978
0.497
8817.5
8833.8
8833.7
8992.7
8849.2
9157.3
9328.3
53.5
39.7
49.6
44
21.9
27.9
16.2
0.4734
0.4935
0.4935
0.4943
0.4977
0.4982
0.4982
8805.9
8896
8960
8975.9
9345.7
8976.6
9363.2
60.4
57
0.4841
0.4926
0.4956
0.4964
0.4983
0.4968
0.4983
8710.6
8976.3
9190.9
8959.6
9276.4
9169.7
9310.9
63.4
60.1
43.7
50.4
15.6
28.8
15.9
CHCL
EA
3
39.9
40.5
28.1
35.5
13.3
MeOH
Dioxane
DMSO
0.4959
0.4982
Table 6
Multi-linear analysis of (υabs), (υemi) and (Δυ) of compound spirocyclic 10 by using Kamlet-Taft and Catalan parameters.
Kamlet-Taft
y
0
× 103
a
α
b
β
c
π∗
r
υabs
υemi
Δυ
18.08 ± 0.32
16.53 ± 0.11
1.55 ± 0.22
0.59 ± 0.48
0.31 ± 0.16
0.27 ± 0.33
−1.32 ± 0.75
−1.0 ± 0.25
−0.33 ± 0.51
0.41 ± 0.62
0.21 ± 0.21
0.21 ± 0.43
0.54
0.87
0.21
Catalan
y
0
× 103
a
SA
b
SB
c
SdP
d
SP
r
υabs
υemi
Δυ
19.00 ± 1.03
16.92 ± 0.28
2.08 ± 0.78
−0.43 ± 0.71
−0.13 ± 0.19
−0.31 ± 0.54
−2.34 ± 0.69
−1.45 ± 0.19
−0.90 ± 0.52
3.36 ± 1.33
−1.32 ± 0.37
2.04 ± 1.01
−3.87 ± 2.00
−1.48 ± 0.55
−2.39 ± 1.52
0.86
0.97
0.71
absorption and emission spectra, whereas, solvent polarizability (dSP) is
3.5. Twisted intramolecular charge transfer (TICT) characteristics
the main factor responsible for observed Stokes shift in spirocyclic 10,
1
1 and 12.
As there is a possibility of rotation between the benzimidazole and
benzothiazole on one part and N,N- diethyl amine substituent on an-
other part of compound 10, 11 and 12 in their spirocyclic as well as
open form, we studied the TICT characteristics of these compounds. In
addition, the free rotating phenyl group at xanthene core suggests the
possibility of twisted intramolecular charge transfer (TICT) characteris-
tic in both spirocyclic as well as in open form. The TICT characteristics of
The graph plotted for the predicted emission data collected by
Kamlet-Taft and Catalan parameters with the experimental data
for all the compounds showed remarkable linear fit with the re-
gression of 0.97, 0.73, and 0.95 for spirocyclic form 10, 11 and 12
respectively whereas,0.99, 0.88, 0.82 for open form 10, 11 and 12
(
Fig. 6).
Fig. 6. Correlation between experimental and predicted emission wave numbers for spirocyclic - (a) 10, (b) 11, (c) 12 and open- (d) 10, (e) 11 and (f) 12 by multi-linear analysis method.

S.B. Yadav et al. / Journal of Molecular Liquids 294 (2019) 111626
9
From above results it is seen that 10, 11 and 12 exhibit TICT in the
molecule from substituents such as benzimidazole and benzothiazole
on one part and N, N-diethyl amine on the other part in their spirocyclic
form.
3.6. Viscosity sensitivity of spirocyclic and open form rhodols
From Rettig's plot we concluded that 10, 11 and 12 show TICT char-
acteristics in their both spirocyclic and open form. To correlate these ex-
perimental results, we examined the viscosity sensitivity of these ESIPT-
rhodol derivatives in ethanol: PEG 400 mixture. The possible intramo-
lecular rotation of donor and acceptor substituents around single bond
can be restricted by the rigidification of chemical structures [84,105].
In case of rhodols 10, 11 and 12 there are possibilities of rotation for
benzimidazole and benzothiazole substituents on one side and N,N–
diethyl amine on the another side and the free rotating phenyl group
on middle xanthene part. So, we were interested in the fluorescent mo-
lecular rotors (FMR) properties of these dyes.
Fig. 7. Rettig's plot for spirocyclic as well as open form - 10, 11 and 12.
1
0, 11 and 12 were investigated by using Rettig's [104] Eq. (8)
For the viscosity sensitivity study we prepared 10 μM sample solu-
tions in ethanol, by increasing the percentage of polyethylene glycol
(PEG 400) from 0 to 100% in ethanol sample solution. Compounds 10,
11 and 12 were investigated for viscosity sensitivity in their both
spirocyclic and open form. Both the forms show enhancement in emis-
sion intensity with the increase in percentage of polyethylene glycol
(PEG 400) in ethanol. Interestingly, we observed very good enhance-
ment in emission intensity for open form of 10, 11 and 12 as compared
to their spirocyclic form. Hence, it can be concluded that the availability
of free rotating phenyl group at xanthene core in the open form is
mainly responsible for highly sensitivity to the viscous medium. Fig. 8
shows the availability of free rotating phenyl ring in the open form com-
pared to the spirocyclic form. Two possible rotating sites are present in
the spirocyclic form of 10, 11 and 12 whereas, in the open form of 10, 11
and 12 there are three possible rotating sites.
2
υƒ ¼ 2 3Δf R þ C
μ
ð8Þ
e
hca
Here υ , c, a, C and μ
e
are emission, velocity of light, Onsager radii,
ƒ
constant and dipole moment in excited state respectively. ‘Δƒ
Rettig's polarizability orientation which correlated with the emission
wave number (υ ) in Eq. (9),
R
’ is
ƒ
ꢀ
ꢁ
2
ε−1
η −1
ΔƒR
¼
−
ð9Þ
2
ε þ 2 2η þ 4
Herein, we observed linear relation in 10, 11 and 12 from non–polar
to polar solvents with the Rettig's function (Fig. 7) with regression
values for the dyes 0.727, 0.811 and 0.845 for spirocyclic form 10, 11
and 12 respectively. Similarly 0.954, 0.911 and 0.924 for open form 10,
We observed 4.8, 4 and 6 fold enhancement in emission intensity for
the spirocyclic form of 10, 11 and 12 while 12.41, 33.44 and 7.09 fold en-
hancement in emission intensity for the open forms of 10, 11 and 12
1
1 and 12 respectively.
Fig. 8. Open form (highly viscosity sensitive) and spirocyclic form (viscosity sensitive) of compound 12.
Table 7
Photophysical data for 10, 11 and 12 in ethanol and (2:98) ethanol:PEG 400.
(
α)
(b)
R2
Compound
λ
abs (EtOH)
λ
emm(EtOH)
λ
abs (98%PEG 400)
λ
emm (98% PEG 400)
ϕ
ϕ
x
Spirocyclic - 10
Spirocyclic - 11
Spirocyclic - 12
Open - 10
Open - 11
Open - 12
372
338
352
545
562
542
438
435
589
602
585
592
377
349
358
552
569
551
450
436
600
599
616
599
0.115
0.127
0.147
0.172
0.185
0.197
0.552
0.611
0.706
0.826
0.742
0.788
0.091
0.093
0.061
0.229
0.26
0.822
0.906
0.876
0.993
0.953
0.976
0.24
a
b
ϕ
– Fluorescence quantum yield for compound in ethanol. ϕ – Fluorescence quantum yield for compound in 98% PEG 400.

10
S.B. Yadav et al. / Journal of Molecular Liquids 294 (2019) 111626
respectively. The viscosity sensitivity data for 10, 11 and 12 in their both
spirocyclic and open form are shown in Table 7.
3.7. TD-DFT studies
The synthesized rhodol derivatives show viscosity sensitivity in
order of 11 N 10 N 12 in spirocyclic form whereas, open form follow
the viscosity sensitivity order as 11 N 12 N 10 (Fig. 9).
The relation between the viscosity of solvent mixture and change in
emission intensity is illustrated by using Forster-Hoffmann Eq. (10).
3.7.1. Optimization of molecular structure
Computational studies were carried by using Density Functional
Theory (DFT) to understand geometries of rhodol derivatives 10, 11
and 12 with the method B3LYP/6-31 G (d). There is a variation in dihe-
dral angles of the dyes in the geometries at the ground state as shown
(Fig. 11).
All these dyes show significant variation in the dihedral angles at C3
log I ¼ C þ x log η
ð10Þ
and C6 position in xanthene part and N, N-diethyl amine part (137.73°
to 176.68°). The compound 10 shows dihedral angle
C6\\C3\\C17\\C19 137.73° in spirocyclic form whereas in open form
the dihedral angle in C6\\C3\\C17\\C19 is 173.18°. Bond lengths are
N27\\C6 1.38 Å, C17\\C19 1.52 Å, O45\\C11 1.34 Å and C10\\C49
.46 Å in spirocyclic form of 10 whereas they are N25\\C6 1.37 Å,
C17\\C18 1.50 Å, O43\\C11 1.26 Å and C10\\C46 1.46 Å in the open
form of 10. The compound 12 shows dihedral angle C6\\C3\\C17\\C19
In the above equation, I, C, x and η are the maximum emission
intensity, constant, viscosity sensitivity parameter and viscosity of
2
solvent system respectively. The ‘x’ values of 0.091 (R = 0.822),
1
2
0
.093 (R = 0.906) and 0.061 (R2 = 0.876) were observed for
spirocyclic form 10, 11 and 12 respectively (Fig. 10). Similarly, vis-
cosity sensitivity parameter for the open forms 10, 11 and 12 were
1
38.16° in spirocyclic form whereas in open form dihedral angle in
C6\\C3\\C17\\C19 is 176.68°. The bond lengths are N27\\C6 1.38 Å,
C17\\C19 1.52 Å, O45\\C11 1.34 Å and C10\\C49 1.46 Å in spirocyclic
2
2
2
0
.229 (R = 0.993), 0.260 (R = 0.953) and 0.240 (R = 0.976) re-
spectively (Fig. 10).
Fig. 9. Emission enhancement spectra of spirocyclic form - (a) 10, (b) 11 and (c) 12 and open form - (d) 10, (e) 11 and (f) 12 in different ratio of PEG 400 (0 to 100%) in ethanol at room
temperature.

S.B. Yadav et al. / Journal of Molecular Liquids 294 (2019) 111626
11
Fig. 10. Comparative Forster-Hoffman plot for compound 10, 11 and 12.
Fig. 11. Optimized structure of 10 in their spirocyclic and open form at B3LYP-6-31G (d) level in chloroform.
Table 8
Experimental and computational photophysical properties of 12 in their spirocyclic and open form.
Solvents
Experimental/computational (spirocyclic form)
Experimental/computational (open form)
a
Vertical excitationb
c
Majord orbital contribution
e
f
g
Majorh orbital contribution
λ
abs (nm)
ƒ
λ
abs (nm)
λ
abs (nm)
ƒ
Hexane
Toluene
339
340
342
341
341
342
343
345.38
346.44
345.17
344.11
342.82
345.73
343.84
1.11
2.14
0.91
2.65
0.19
0.78
0.75
H → L (95.22%)
H → L (95.22%)
H → L (95.22%)
H → L (95.22%)
H → L (95.22%)
H → L (95.22%)
H → L (95.22%)
537
539
544
545
557
558
574
535.18
539.96
540.18
545.99
550.89
550.32
559.48
0.41
0.72
3.32
0.16
3.52
0.04
0.86
H → L (98%)
H → L (98%)
H → L (98%)
H → L (98%)
H → L (98%)
H → L (98%)
H → L (98%)
CHCl
EA
3
MEOH
Dioxane
DMSO
a
Experimental absorption λmax of Spirocyclic form.
Vertical excitation for spirocyclic form.
Experimentally calculated oscillator strength for spirocyclic form.
Major electronic transition from ground state to excited state for spirocyclic form.
Experimental absorption λmax of open form.
Vertical excitation for open form.
Experimentally calculated oscillator strength for open form.
Major electronic transition from ground state to excited state for open form.
b
c
d
e
f
g
h

12
S.B. Yadav et al. / Journal of Molecular Liquids 294 (2019) 111626
form of 12 whereas N26\\C6 1.37 Å, C17\\C19 1.50 Å, O44\\C11 1.25 Å
and C10\\C45 1.46 Å in open form of 12.
LUMOs were located on lactone part and extended over the benzimid-
azole and benzothiazole substituents in rhodol derivatives 10, 11 and
12 in their both the forms. Fig. 12 shows the frontier molecular orbital
(FMO) diagram of 10, 11 and 12 in their spirocyclic and open forms in
chloroform. Comparatively lower HOMO-LUMO energy gap is observed
for the open form 10, 11 and 12 than their respective spirocyclic forms
which suggests a better ICT and red shifted emission in the open
forms 10, 11 and 12.
The observed changes in dihedral angle from spirocyclic form to
open form lead to the co-planarity in the molecules. There are signifi-
cant changes in bond length O45-C11 compared to the N- substituent
compound which clearly indicates that H atom from OH itself modulate
quinoid form. Hence, ESIPT process is inhibited in open form due to lack
of protons in close proximity of benzimidazole and benzothiazole units
during quinoid form.
We also studied separation of HOMO and LUMO energy levels in 10,
11 and 12 in their spirocyclic and open forms. Fig. 13 shows the HOMO-
LUMO energy gap diagram for 10, 11 and 12.
In order to understand the solvatochromism in rhodol derivatives
the ground state optimized geometries of the dyes at B3LYP/6-31 G
(d) level were subjected to TD for the first 10 states in the solvents of dif-
ferent polarities to get the vertical excitations and results are tabulated
in Table 8. The results obtained by TD-DFT suggested that there is a pro-
nounced effect of solvent polarities on solvatochromism in rhodols.
Electron density in HOMOs is located on the xanthene part, benz-
imidazole, benzothiazole substituents and nitrogen atom, whereas in
3.7.2. Molecular electrostatic potential plot (MEP)
The optimization of rhodol derivatives 10, 11 and 12 in their
spirocyclic and open form at B3LYP/6-31 G (d) level. We observed the
exact reactive sites in molecules by molecular electrostatic potential
(MEP) analysis [106]. In MEP surface plot the red colored area indicates
Fig. 12. HOMO-LUMO FMO diagrams for 10, 11 and 12 (spirocyclic and open form) in chloroform.

S.B. Yadav et al. / Journal of Molecular Liquids 294 (2019) 111626
13
acceptor part have negative electron potential. The carboxyl
\\COOH) group and N, N-diethyl amine substituent in open forms of
0, 11 and 12 act as electron donor and have a positive electron poten-
tial. MEP observed for 10, 11 and 12 in spirocyclic form are ± 0.136, ±
.123, ± 0.0999 and in open form are ± 0.112, ± 0.117, ± 0.123
respectively.
(
1
0
4. Conclusion
Three rhodol compounds, namely spirolactamized phenanthrene-
imidazole, diphenyl-imidazole and benzothiazole substituted N, N-
diethylamine rhodols were synthesized to investigate their ESIPT phe-
nomena. Detailed photophysical investigations of these dyes in their
spirocyclic and open form are presented in a correlative manner. Due to
ESIPT process, a large Stokes shift (50–260 nm) is observed in their
spirocyclic form while no ESIPT process [Stokes shift 10–40 nm] is ob-
served in their open form due to the lack of protons. Both Spirocyclic
and open form shows positive solvatochromism which is supported by
the linear analysis (Lippert-Mataga and Mac-Rae functions) as well as
by using multi-linear regression analysis (Kamlet-Taft and Catalan pa-
rameters). The intramolecular charge transfer (ICT) and twisted intramo-
lecular charge transfer (TICT) of synthesized rhodols were investigated
using polarity graphs and Generalized Mulliken-Hush parameters
(GMH) calculations. Moreover, open form of synthesized rhodols shows
a high sensitivity towards viscous medium (mixture of EtOH:PEG 400)
as compared to their spirocyclic form. The computational results obtained
from DFT are in good agreement with the experimental results.
Fig. 13. HOMO-LUMO energy level diagram of 10, 11 and 12 (spirocyclic and open form) in
chloroform.
electron acceptor part whereas, the blue colored area indicates electron
donor part in molecule. Fig. 14 shows the electrostatic potential character-
istics through plots for 10, 11 and 12 in their spirocyclic and open forms.
In spirocyclic form of 10 and 11 lactone group, and benzimidazole
substituent act as electron acceptors, where as in 12 lactone group,
and benzothiazole substituent act as electron acceptors. Therefore, in
all spirocyclic form of 10, 11 and 12 acceptor part have negative electron
potential. The N, N-diethyl amine substituent in spirocyclic form of 10,
Acknowledgment
1
1 and 12 acts as electron donors and has a positive electron potential.
Similarly, in the open form of 10 and 11 the quinoidal part and adja-
The authors Sagar B. Yadav and Shantaram Kothavale are grateful to
UGC-SAP, New Delhi, India for financial support by way of Junior Research
and Senior Research Fellowships. We are thankful to Mrs. Pallavi Khapre
and Mrs. Priyanka Jain, Department of SAIF, IIT Bombay for mass analysis.
cent benzimidazole substituent acts as electron acceptors, whereas in
2 the quinoidal part and adjacent benzothiazole substituent act as an
electron acceptors. Therefore, in all the open forms of 10, 11 and 12
1
Fig. 14. MEP surface plot for 10, 11 and 12 (spirocyclic and open form).

1
4
S.B. Yadav et al. / Journal of Molecular Liquids 294 (2019) 111626
experimental and theoretical study, Dyes Pigments 92 (2012) 1361–1369,
Appendix A. Supplementary data
https://doi.org/10.1016/j.dyepig.2011.09.023.
[
28] S. Barman, S.K. Mukhopadhyay, M. Gangopadhyay, S. Biswas, S. Dey, N.D.P. Singh,
Coumarin-benzothiazole-chlorambucil (Cou-Benz-Cbl) conjugate: an ESIPT based
pH sensitive photoresponsive drug delivery system, J. Mater. Chem. B 3 (2015)
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.molliq.2019.111626.
3490–3497, https://doi.org/10.1039/c4tb02081b.
[
29] A.Y. Bochkov, I.O. Akchurin, O.A. Dyachenko, V.F. Traven, NIR-fluorescent
coumarin-fused BODIPY dyes with large Stokes shifts, Chem. Commun. 49
(2013) 11653–11655, https://doi.org/10.1039/c3cc46498a.
References
[
30] I. Esnal, G. Duran-Sampedro, A.R. Agarrabeitia, J. Bañuelos, I. García-Moreno, M.A.
Macías, E. Peña-Cabrera, I. López-Arbeloa, S. De La Moya, M.J. Ortiz, Coumarin-
BODIPY hybrids by heteroatom linkage: versatile, tunable and photostable dye la-
sers for UV irradiation, Phys. Chem. Chem. Phys. 17 (2015) 8239–8247, https://doi.
org/10.1039/c5cp00193e.
31] H. Yu, L. Jin, Y. Dai, H. Li, Y. Xiao, From a BODIPY–rhodamine scaffold to a
ratiometric fluorescent probe for nitric oxide, New J. Chem. 37 (2013)
[
[
1] H.M. Kim, B.R. Cho, Two-photon fluorescent probes for metal ions, Chem. - An
Asian J. 6 (2011) 58–69, https://doi.org/10.1002/asia.201000542.
2] H.M. Kim, H.-J. Choo, S.-Y. Jung, Y.-G. Ko, W.-H. Park, S.-J. Jeon, C.H. Kim, T. Joo, B.R.
Cho,
A two-photon fluorescent probe for lipid raft imaging: C-laurdan,
[
ChemBioChem 8 (2007) 553–559, https://doi.org/10.1002/cbic.200700003.
3] S. Sumalekshmy, C.J. Fahrni, Metal-ion-responsive fluorescent probes for two-
photon excitation microscopy, Chem. Mater. 23 (2011) 483–500, https://doi.org/
[
1688–1691, https://doi.org/10.1039/C3NJ41127C.
1
0.1021/cm1021905.
[32] C. Ram, S. Sivamani, T. Micha Premkumar, V. Hariram, Computational study of lead-
ing edge jet impingement cooling with a conical converging hole for blade cooling,
ARPN J. Eng. Appl. Sci. 12 (2017) 6397–6406, https://doi.org/10.1039/b000000x.
[33] J. Chen, W. Liu, B. Zhou, G. Niu, H. Zhang, J. Wu, Y. Wang, W. Ju, P. Wang, Coumarin-
and rhodamine-fused deep red fluorescent dyes: synthesis, photophysical proper-
ties, and bioimaging in vitro, J. Org. Chem. 78 (2013) 6121–6130, https://doi.org/
[
[
4] Z. Liu, W. He, Z. Guo, Metal coordination in photoluminescent sensing, Chem. Soc.
Rev. 42 (2013) 1568, https://doi.org/10.1039/c2cs35363f.
5] L.E. Kreno, K. Leong, O.K. Farha, M. Allendorf, R.P. Van Duyne, J.T. Hupp, Metal–
organic framework materials as chemical sensors, Chem. Rev. 112 (2012)
1105–1125, https://doi.org/10.1021/cr200324t.
10.1021/jo400783x.
[
[
[
6] Y. Jeong, J. Yoon, Recent progress on fluorescent chemosensors for metal ions,
Inorganica Chim. Acta. 381 (2012) 2–14, https://doi.org/10.1016/j.ica.2011.09.011.
7] Y. Cui, Y. Yue, G. Qian, B. Chen, Luminescent functional metal–organic frameworks,
Chem. Rev. 112 (2012) 1126–1162, https://doi.org/10.1021/cr200101d.
8] X. Chen, T. Pradhan, F. Wang, J.S. Kim, J. Yoon, Fluorescent chemosensors based on
spiroring-opening of xanthenes and related derivatives, Chem. Rev. 112 (2012)
[34] S. Kothavale, N. Sekar, Novel triphenylamine based rhodamine derivatives: synthe-
sis, characterization, photophysical properties and viscosity sensitivity, RSC Adv. 6
(2016) 100271–100280, https://doi.org/10.1039/c6ra19899f.
[35] B.K. Paul, N. Guchhait, 1-Hydroxy-2-naphthaldehyde: a prospective excited-state
intramolecular proton transfer (ESIPT) probe with multi-faceted applications, J.
Lumin. 132 (2012) 2194–2208, https://doi.org/10.1016/j.jlumin.2012.03.036.
[36] L. Fan, X. Jiang, B. Wang, Z. Yang, 4-(8′-Hydroxyquinolin-7′-yl)methyleneimino-1-
phenyl-2,3-dimethyl-5-pyzole as a fluorescent chemosensor for aluminum ion in
acid aqueous medium, Sensors Actuators B Chem. 205 (2014) 249–254, https://
doi.org/10.1016/j.snb.2014.08.085.
[37] L. Tang, Z. Zheng, K. Zhong, Y. Bian, A 2,5-diaryl-1,3,4-oxadiazole-based fluorescent
probe for rapid and highly selective recognition of hydrogen sulfide with a large
Stokes shift through switching on ESIPT, Tetrahedron Lett. 57 (2016) 1361–1364,
https://doi.org/10.1016/j.tetlet.2016.02.056.
1910–1956, https://doi.org/10.1021/cr200201z.
[
9] Y. Yang, Q. Zhao, W. Feng, F. Li, Luminescent chemodosimeters for bioimaging,
Chem. Rev. 113 (2013) 192–270, https://doi.org/10.1021/cr2004103.
[
[
[
[
[
10] J.Y. Kwon, Y.J. Jang, Y.J. Lee, K.M. Kim, M.S. Seo, W. Nam, J. Yoon, A highly selective
fluorescent chemosensor for Pb2+, J. Am. Chem. Soc. 127 (2005) 10107–10111,
https://doi.org/10.1021/ja051075b.
11] Y.-K. Yang, K.-J. Yook, J. Tae, A rhodamine-based fluorescent and colorimetric
chemodosimeter for the rapid detection of Hg2+ ions in aqueous media, J. Am.
Chem. Soc. 127 (2005) 16760–16761, https://doi.org/10.1021/ja054855t.
12] M.H. Lee, J.-S. Wu, J.W. Lee, J.H. Jung, J.S. Kim, Highly sensitive and selective
chemosensor for Hg2+ based on the rhodamine fluorophore, Org. Lett. 9 (2007)
[
38] B. Gu, L. Huang, W. Su, X. Duan, H. Li, S. Yao, A benzothiazole-based fluorescent
probe for distinguishing and bioimaging of Hg2+ and Cu2+, Anal. Chim. Acta
954 (2017) 97–104, https://doi.org/10.1016/j.aca.2016.11.044.
2
501–2504, https://doi.org/10.1021/ol0708931.
13] S.-K. Ko, Y.-K. Yang, J. Tae, I. Shin, In vivo monitoring of mercury ions using a
rhodamine-based molecular probe, J. Am. Chem. Soc. 128 (2006) 14150–14155,
https://doi.org/10.1021/ja065114a.
14] H. Zheng, Z.-H. Qian, L. Xu, F.-F. Yuan, L.-D. Lan, J.-G. Xu, Switching the recognition
preference of rhodamine B spirolactam by replacing one atom: design of rhoda-
mine B thiohydrazide for recognition of Hg(II) in aqueous solution, Org. Lett. 8
[39] D. Xu, L. Tang, M. Tian, P. He, X. Yan, A benzothizole-based fluorescent probe for
Hg2+ recognition utilizing ESIPT coupled AIE characteristics, Tetrahedron Lett.
58 (2017) 3654–3657, https://doi.org/10.1016/j.tetlet.2017.08.016.
[40] S. Chen, W. Wang, M. Yan, Q. Tu, S.-W. Chen, T. Li, M.-S. Yuan, J. Wang, 2-Hydroxy
benzothiazole modified rhodol: aggregation-induced emission and dual-channel
fluorescence sensing of Hg2+ and Ag+ ions, Sensors Actuators B Chem. 255
(2018) 2086–2094, https://doi.org/10.1016/j.snb.2017.09.008.
(2006) 859–861, https://doi.org/10.1021/ol0529086.
[
[
[
15] Y. Xiang, A. Tong, P. Jin, Y. Ju, New fluorescent rhodamine hydrazone chemosensor
for Cu(II) with high selectivity and sensitivity, Org. Lett. 8 (2006) 2863–2866,
https://doi.org/10.1021/ol0610340.
[41] J. Wu, W. Liu, J. Ge, H. Zhang, P. Wang, New sensing mechanisms for design of fluo-
rescent chemosensors emerging in recent years, Chem. Soc. Rev. 40 (2011) 3483,
https://doi.org/10.1039/c0cs00224k.
16] X. Zhang, Y. Shiraishi, T. Hirai, Cu(II)-selective green fluorescence of a rhodamine–
diacetic acid conjugate, Org. Lett. 9 (2007) 5039–5042, https://doi.org/10.1021/
ol7022714.
17] Y. Xiang, A. Tong, A new rhodamine-based chemosensor exhibiting selective FeIII-
amplified fluorescence, Org. Lett. 8 (2006) 1549–1552, https://doi.org/10.1021/
ol060001h.
[42] T. Mutai, H. Sawatani, T. Shida, H. Shono, K. Araki, Tuning of excited-state intramo-
lecular proton transfer (ESIPT) fluorescence of Imidazo[1,2-a]pyridine in rigid ma-
trices by substitution effect, J. Org. Chem. 78 (2013) 2482–2489, https://doi.org/10.
1021/jo302711t.
[
43] Y. Liu, D. Yu, S. Ding, Q. Xiao, J. Guo, G. Feng, Rapid and ratiometric fluorescent de-
tection of cysteine with high selectivity and sensitivity by a simple and readily
available probe, ACS Appl. Mater. Interfaces 6 (2014) 17543–17550, https://doi.
org/10.1021/am505501d.
[
18] S. Bae, J. Tae, Rhodamine-hydroxamate-based fluorescent chemosensor for FeIII, Tet-
rahedron Lett. 48 (2007) 5389–5392, https://doi.org/10.1016/j.tetlet.2007.06.014.
19] X. Zhang, Y. Shiraishi, T. Hirai, A new rhodamine-based fluorescent chemosensor
for transition metal cations synthesized by one-step facile condensation, Tetrahe-
dron Lett. 48 (2007) 5455–5459, https://doi.org/10.1016/j.tetlet.2007.05.171.
20] M. Beija, C.A.M. Afonso, J.M.G. Martinho, Synthesis and applications of Rhodamine
derivatives as fluorescent probes, Chem. Soc. Rev. 38 (2009) 2410–2433, https://
doi.org/10.1039/b901612k.
[
[44] K. Kanagaraj, K. Pitchumani, Highly selective “turn-on” fluorescent and colorimet-
ric sensing of fluoride ion using 2-(2-hydroxyphenyl)-2,3-dihydroquinolin-4(1H)-
one based on excited-state proton transfer, Chem. - An Asian J. 9 (2014) 146–152,
https://doi.org/10.1002/asia.201300816.
[45] Y. Zhang, J.-H. Wang, W. Zheng, T. Chen, Q.-X. Tong, D. Li, An ESIPT fluorescent dye
based on HBI with high quantum yield and large Stokes shift for selective detection
[
[
21] Y. Koide, Y. Urano, K. Hanaoka, T. Terai, T. Nagano, Evolution of group 14 rhoda-
mines as platforms for near-infrared fluorescence probes utilizing photoinduced
electron transfer, ACS Chem. Biol. 6 (2011) 600–608, https://doi.org/10.1021/
cb1002416.
of Cys, J. Mater. Chem.
C4TB00190G.
B 2 (2014) 4159–4166, https://doi.org/10.1039/
[
46] B. Liu, H. Wang, T. Wang, Y. Bao, F. Du, J. Tian, Q. Li, R. Bai, A new ratiometric ESIPT
sensor for detection of palladium species in aqueous solution, Chem. Commun. 48
(2012) 2867, https://doi.org/10.1039/c2cc17677g.
[
[
22] J.B. Grimm, A.J. Sung, W.R. Legant, P. Hulamm, S.M. Matlosz, E. Betzig, L.D. Lavis,
Carbo fl uoresceins and carborhodamines as scaffolds for high-contrast fluorogenic
probes, (2013).
23] X. Zhang, Y. Xiao, X. Qian, A ratiometric fluorescent probe based on FRET for imag-
ing Hg2+ ions in living cells, Angew. Chemie - Int. Ed. 47 (2008) 8025–8029,
https://doi.org/10.1002/anie.200803246.
[47] W.-H. Chen, Y. Xing, Y. Pang, A highly selective pyrophosphate sensor based on
ESIPT turn-on in water, Org. Lett. 13 (2011) 1362–1365, https://doi.org/10.1021/
ol200054w.
[48] X.-F. Yang, H. Qi, L. Wang, Z. Su, G. Wang, A ratiometric fluorescent probe for fluo-
ride ion employing the excited-state intramolecular proton transfer, Talanta 80
(2009) 92–97, https://doi.org/10.1016/j.talanta.2009.06.030.
[24] A. Thorne, An introduction to laser spectroscopy, Meas. Sci. Technol. 7 (1996)
http://stacks.iop.org/0957-0233/7/i=11/a=022.
[
49] F. Rodriguez Prieto, M.C. Rios Rodriguez, M. Mosquera Gonzalez, M.A. Rios
Fernandez, Ground- and excited-state tautomerism in 2-(3′-hydroxy-2′-pyridyl)
benzimidazole, J. Phys. Chem. 98 (1994) 8666–8672, https://doi.org/10.1021/
j100086a014.
[
25] P. Yang, J. Zhao, W. Wu, X. Yu, Y. Liu, Accessing the long-lived triplet excited states
in bodipy-conjugated 2-(2-hydroxyphenyl) benzothiazole/benzoxazoles and ap-
plications as organic triplet photosensitizers for photooxidations, J. Org. Chem. 77
(2012) 6166–6178, https://doi.org/10.1021/jo300943t.
[50] M. Mosquera, M.C. Ríos Rodríguez, F. Rodríguez-Prieto, Competition between pro-
tonation and deprotonation in the first excited singlet state of 2-(3′-hydroxy-2′-
[
[
26] Y. Houari, S. Chibani, D. Jacquemin, A.D. Laurent, TD-DFT assessment of the excited
state intramolecular proton transfer in hydroxyphenylbenzimidazole (HBI) dyes, J.
Phys. Chem. B 119 (2015) 2180–2192, https://doi.org/10.1021/jp505036d.
pyridyl)benzimidazole in acidic solutions, J. Phys. Chem.
A 101 (1997)
2766–2772, https://doi.org/10.1021/jp9626834.
27] L. Xie, Y. Chen, W. Wu, H. Guo, J. Zhao, X. Yu, Fluorescent coumarin derivatives with
[51] S. Li, Q. Wang, Y. Qian, S. Wang, Y. Li, G. Yang, Understanding the pressure-induced
large stokes shift, dual emission and solid state luminescent properties: an
emission enhancement for triple fluorescent compound with excited-state

S.B. Yadav et al. / Journal of Molecular Liquids 294 (2019) 111626
15
intramolecular proton transfer, J. Phys. Chem. A 111 (2007) 11793–11800, https://
doi.org/10.1021/jp075301a.
52] R. Hu, J. Feng, D. Hu, S. Wang, S. Li, Y. Li, G. Yang, A rapid aqueous fluoride ion sen-
sor with dual output modes, Angew. Chemie Int. Ed. 49 (2010) 4915–4918, https://
doi.org/10.1002/anie.201000790.
53] T.-I. Kim, H.J. Kang, G. Han, S.J. Chung, Y. Kim, A highly selective fluorescent ESIPT
probe for the dual specificity phosphatase MKP-6, Chem. Commun. (2009) 5895,
https://doi.org/10.1039/b911145j.
54] S. Goswami, S. Das, K. Aich, B. Pakhira, S. Panja, S.K. Mukherjee, S. Sarkar, A
chemodosimeter for the ratiometric detection of hydrazine based on return of
ESIPT and its application in live-cell imaging, Org. Lett. 15 (2013) 5412–5415,
https://doi.org/10.1021/ol4026759.
55] Z. Xu, L. Xu, J. Zhou, Y. Xu, W. Zhu, X. Qian, A highly selective fluorescent probe for
fast detection of hydrogen sulfide in aqueous solution and living cells, Chem.
Commun. 48 (2012), 10871. https://doi.org/10.1039/c2cc36141h.
56] L. Cui, W. Zhu, Y. Xu, X. Qian, A novel ratiometric sensor for the fast detection of
palladium species with large red-shift and high resolution both in aqueous solu-
tion and solid state, Anal. Chim. Acta 786 (2013) 139–145, https://doi.org/10.
[78] X.-F. Yang, Q. Huang, Y. Zhong, Z. Li, H. Li, M. Lowry, J.O. Escobedo, R.M. Strongin, A
dual emission fluorescent probe enables simultaneous detection of glutathione
and cysteine/homocysteine, Chem. Sci. 5 (2014) 2177, https://doi.org/10.1039/
c4sc00308j.
[79] W. Kohn, L.J. Sham, Self-consistent equations including exchange and correlation
effects, Phys. Rev. 140 (1965) A1133–A1138, https://doi.org/10.1103/PhysRev.
140.A1133.
[80] J.H.M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman,
G. Scalmani, V. Barone, B. Mennucci, G.A. Petersson, H. Nakatsuji, M. Caricato, X. Li,
H.P. Hratchian, A.F. Izmaylov, J. Bloino, G. Zheng, J.L. Sonnenberg, M. Had, Gaussian
09, Gaussian, Inc., Wallingford CT, 2009.
[81] A.D. Becke, A new mixing of Hartree–Fock and local density-functional theories, J.
Chem. Phys. 98 (1993) 1372–1377, https://doi.org/10.1063/1.464304.
[82] C. Lee, W. Yang, R.G. Parr, Development of the Colle-Salvetti correlation-energy for-
mula into a functional of the electron density, Phys. Rev. B 37 (1988) 785–789,
https://doi.org/10.1103/PhysRevB.37.785.
[83] G. Scalmani, M.J. Frisch, B. Mennucci, J. Tomasi, R. Cammi, V. Barone, Geome-
tries and properties of excited states in the gas phase and in solution: theory
and application of a time-dependent density functional theory polarizable con-
tinuum model, J. Chem. Phys. 124 (2006), 094107. https://doi.org/10.1063/1.
2173258.
[84] A.G. Jadhav, S. Kothavale, N. Sekar, Red emitting triphenylamine based rhodamine
analogous with enhanced Stokes shift and viscosity sensitive emission, Dyes Pig-
ments 138 (2017) 56–67, https://doi.org/10.1016/j.dyepig.2016.11.021.
[85] W. Lin, Y. Liu, F. Meng, Q. Shi, Fluorescent Probe for Specific Identification of Diva-
lent Copper Ions in Water Phase, 2015.
[
[
[
[
[
1016/j.aca.2013.05.011.
[
[
57] M. Santra, B. Roy, K.H. Ahn, A “reactive” ratiometric fluorescent probe for mercury
species, Org. Lett. 13 (2011) 3422–3425, https://doi.org/10.1021/ol2011693.
58] C. Huang, T. Jia, C. Yu, A. Zhang, N. Jia, An ESIPT based fluorescent probe for highly
selective and ratiometric detection of periodate, Biosens. Bioelectron. 63 (2015)
513–518, https://doi.org/10.1016/j.bios.2014.08.005.
[
[
[
[
[
59] D.P. Murale, H. Kim, W.S. Choi, D.G. Churchill, Highly selective excited state intra-
molecular proton transfer (ESIPT)-based superoxide probing, Org. Lett. 15 (2013)
3
946–3949, https://doi.org/10.1021/ol4017222.
[86] H.R. Shaterian, M. Ranjbar, An environmental friendly approach for the synthesis of
highly substituted imidazoles using Brønsted acidic ionic liquid, N-methyl-2-
pyrrolidonium hydrogen sulfate, as reusable catalyst, J. Mol. Liq. 160 (2011)
40–49, https://doi.org/10.1016/j.molliq.2011.02.012.
[87] V.S. Patil, V.S. Padalkar, A.B. Tathe, V.D. Gupta, N. Sekar, Synthesis, photo-physical
and DFT studies of ESIPT inspired novel 2-(2′,4′-dihydroxyphenyl) benzimidazole,
benzoxazole and benzothiazole, J. Fluoresc. 23 (2013) 1019–1029, https://doi.org/
10.1007/s10895-013-1228-4.
[88] S. Kothavale, N. Sekar, Novel pyrazino-phenanthroline based rigid donor-π-
acceptor compounds: a detail study of optical properties, acidochromism,
solvatochromism and structure-property relationship, Dyes Pigments 136 (2017)
31–45, https://doi.org/10.1016/j.dyepig.2016.08.032.
[89] S. Kothavale, A.G. Jadhav, N. Sekar, Deep red emitting triphenylamine based
coumarin-rhodamine hybrids with large Stokes shift and viscosity sensing: syn-
thesis, photophysical properties and DFT studies of their spirocyclic and open
forms, Dyes Pigments 137 (2017) 329–341, https://doi.org/10.1016/j.dyepig.
2016.11.010.
60] G. Li, D. Zhu, Q. Liu, L. Xue, H. Jiang, Rapid detection of hydrogen peroxide based on
aggregation induced ratiometric fluorescence change, Org. Lett. 15 (2013)
9
24–927, https://doi.org/10.1021/ol4000845.
61] D. Maity, V. Kumar, T. Govindaraju, Reactive probes for ratiometric detection of Co2
and Cu+ based on excited-state intramolecular proton transfer mechanism,
+
Org. Lett. 14 (2012) 6008–6011, https://doi.org/10.1021/ol302904c.
62] L. Xiong, J. Feng, R. Hu, S. Wang, S. Li, Y. Li, G. Yang, Sensing in 15 s for aqueous fluo-
ride anion by water-insoluble fluorescent probe incorporating hydrogel, Anal.
Chem. 85 (2013) 4113–4119, https://doi.org/10.1021/ac400252u.
63] S. Jana, S. Dalapati, N. Guchhait, Proton transfer assisted charge transfer phenom-
ena in photochromic Schiff bases and effect of –NEt 2 groups to the anil Schiff
bases, J. Phys. Chem. A 116 (2012) 10948–10958, https://doi.org/10.1021/
jp3079698.
[
[
[
[
64] J.E. Kwon, S.Y. Park, Advanced organic optoelectronic materials: harnessing
excited-state intramolecular proton transfer (ESIPT) process, Adv. Mater. 23
(
2011) 3615–3642, https://doi.org/10.1002/adma.201102046.
65] L. Giordano, V. V Shvadchak, J.A. Fauerbach, E.A. Jares-Erijman, T.M. Jovin, Highly
[90] C. Reichardt, Solvatochromic dyes as solvent polarity indicators, Chem. Rev. 94
solvatochromic 7-aryl-3-hydroxychromones, J. Phys. Chem. Lett. 3 (2012)
1
(1994) 2319–2358, https://doi.org/10.1021/cr00032a005.
011–1016. doi:https://doi.org/10.1021/jz3002019.
[91] S.K. Lanke, N. Sekar, Aggregation induced emissive carbazole-based push pull
NLOphores: synthesis, photophysical properties and DFT studies, Dyes Pigments
124 (2016) 82–92, https://doi.org/10.1016/j.dyepig.2015.09.013.
[92] E.G. McRae, Theory of solvent effects on molecular electronic spectra. Frequency
shifts, J. Phys. Chem. 61 (1957) 562–572, https://doi.org/10.1021/j150551a012.
[93] S.B. Yadav, S. Kothavale, N. Sekar, Triphenylamine and N-phenyl carbazole-based
coumarin derivatives: synthesis, solvatochromism, acidochromism, linear and
nonlinear optical properties, J. Photochem. Photobiol. A Chem. 382 (2019),
111937. https://doi.org/10.1016/j.jphotochem.2019.111937.
[94] B.J. Coe, J. Fielden, S.P. Foxon, J.A. Harris, M. Helliwell, B.S. Brunschwig, I.
Asselberghs, K. Clays, J. Garín, J. Orduna, Diquat derivatives: highly active,
two-dimensional nonlinear optical chromophores with potential redox
switchability, J. Am. Chem. Soc. 132 (2010) 10498–10512, https://doi.org/10.
1021/ja103289a.
[95] S.M. Abernathy, R.R. Sharp, Spin dynamics calculations of electron and nuclear spin
relaxation times in paramagnetic solutions, J. Chem. Phys. 106 (1997) 9032–9043,
https://doi.org/10.1063/1.474035.
66] S. Kim, S.Y. Park, Photochemically gated protonation effected by intramolecular hy-
drogen bonding: towards stable fluorescence imaging in polymer films, Adv.
Mater. 15 (2003) 1341–1344, https://doi.org/10.1002/adma.200305050.
67] S. Kim, S.Y. Park, I. Yoshida, H. Kawai, T. Nagamura, Amplified spontaneous emis-
sion from the film of poly(aryl ether) dendrimer encapsulating excited-state intra-
molecular proton transfer dye, J. Phys. Chem. B 106 (2002) 9291–9294, https://doi.
org/10.1021/jp021048x.
[
[
68] P. Osa, vol. 3. Osnovni principi solarne geometrije, (n.d).
69] S. Kenmoku, Y. Urano, H. Kojima, T. Nagano, Development of a highly specific
rhodamine-based fluorescence probe for hypochlorous acid and its application to
real-time imaging of phagocytosis, J. Am. Chem. Soc. 129 (2007) 7313–7318,
https://doi.org/10.1021/ja068740g.
70] J. Wu, Z. Ye, F. Wu, H. Wang, L. Zeng, G.-M. Bao, A rhodamine-based fluorescent
probe for colorimetric and fluorescence lighting-up determination of toxic
thiophenols in environmental water and living cells, Talanta 181 (2018)
[
2
39–247, https://doi.org/10.1016/j.talanta.2018.01.028.
[
71] K. Wechakorn, S. Prabpai, K. Suksen, P. Piyachaturawat, P. Kongsaeree, Rhodol-
based fluorescent probe for Au3+ detection and its application in bioimaging,
RSC Adv. 6 (2016) 24752–24755, https://doi.org/10.1039/C6RA02342H.
72] X.-F. Yang, Q. Huang, Y. Zhong, Z. Li, H. Li, M. Lowry, J.O. Escobedo, R.M. Strongin, A
dual emission fluorescent probe enables simultaneous detection of glutathione
and cysteine/homocysteine, Chem. Sci. 5 (2014) 2177, https://doi.org/10.1039/
c4sc00308j.
[96] C. Creutz, M.D. Newton, N. Sutin, Metal—lingad and metal—metal coupling ele-
ments, J. Photochem. Photobiol. A Chem. 82 (1994) 47–59, https://doi.org/10.
1016/1010-6030(94)02013-2.
[97] M.J. Kamlet, R.W. Taft, The solvatochromic comparison method. I. The .beta.-scale
of solvent hydrogen-bond acceptor (HBA) basicities, J. Am. Chem. Soc. 98 (1976)
377–383, https://doi.org/10.1021/ja00418a009.
[98] M.J. Kamlet, J.L. Abboud, R.W. Taft, The solvatochromic comparison method. 6. The .
pi.* scale of solvent polarities, J. Am. Chem. Soc. 99 (1977) 6027–6038, https://doi.
org/10.1021/ja00460a031.
[99] J. Catalán, V. López, P. Pérez, R. Martin-Villamil, J.-G. Rodríguez, Progress towards a
generalized solvent polarity scale: the solvatochromism of 2-(dimethylamino)-7-
nitrofluorene and its homomorph 2-fluoro-7-nitrofluorene, Liebigs Ann. 1995
(1995) 241–252, https://doi.org/10.1002/jlac.199519950234.
[100] J. Catalán, C. Díaz, Extending the solvent acidity scale to highly acidic organic sol-
vents: the unique photophysical behaviour of 3,6-diethyltetrazine, European J.
Org. Chem. (1999) (1999) 885–891, https://doi.org/10.1002/(SICI)1099-0690
(199904)1999:4b885::AID-EJOC885N3.0.CO;2-W.
[101] J. Catalán, H. Hopf, Empirical treatment of the inductive and dispersive compo-
nents of solute−solvent interactions: the solvent polarizability (SP) scale,
European J. Org. Chem. 2004 (2004) 4694–4702, https://doi.org/10.1002/ejoc.
200400311.
[
[
73] H. Wen, Q. Huang, X.-F. Yang, H. Li, Spirolactamized benzothiazole-substituted N,
N-diethylrhodol: a new platform to construct ratiometric fluorescent probes,
Chem. Commun. 49 (2013) 4956, https://doi.org/10.1039/c3cc41343h.
74] D. Raghavender Goud, A.K. Purohit, V. Tak, D.K. Dubey, P. Kumar, D. Pardasani, A
highly selective and sensitive “turn-on” fluorescence chemodosimeter for the de-
tection of mustard gas, Chem. Commun. 50 (2014) 12363–12366, https://doi.
org/10.1039/C4CC04801F.
[
[
75] H. Wen, Q. Huang, X.-F. Yang, H. Li, Spirolactamized benzothiazole-substituted N,
N-diethylrhodol: a new platform to construct ratiometric fluorescent probes,
Chem. Commun. 49 (2013) 4956, https://doi.org/10.1039/c3cc41343h.
76] A. Manna, D. Sarkar, S. Goswami, C.K. Quah, H.-K. Fun, Single excited state intramo-
lecular proton transfer (ESIPT) chemodosimeter based on rhodol for both Hg2+
and OCl−: ratiometric detection with live-cell imaging, RSC Adv. 6 (2016)
[
57417–57423, https://doi.org/10.1039/C6RA11009F.
[
77] Y. Liu, K. Xiang, B. Tian, J. Zhang, Rhodol-based far-red fluorescent probe for the de-
[102] R.W. Taft, M.J. Kamlet, The solvatochromic comparison method. 2. The .alpha.-scale
of solvent hydrogen-bond donor (HBD) acidities, J. Am. Chem. Soc. 98 (1976)
2886–2894, https://doi.org/10.1021/ja00426a036.
tection of cysteine and homocysteine over glutathione, Luminescence 32 (2017)
78–85, https://doi.org/10.1002/bio.3152.

16
S.B. Yadav et al. / Journal of Molecular Liquids 294 (2019) 111626
[
103] J. Catalán, Toward a generalized treatment of the solvent effect based on four em-
pirical scales: dipolarity (SdP, a new scale), polarizability (SP), acidity (SA), and ba-
sicity (SB) of the medium, J. Phys. Chem. B 113 (2009) 5951–5960, https://doi.org/
[105] M.A. Haidekker, E.A. Theodorakis, Molecular rotors—fluorescent biosensors for vis-
cosity and flow, Org. Biomol. Chem. 5 (2007) 1669–1678, https://doi.org/10.1039/
B618415D.
1
0.1021/jp8095727.
[106] M. Zouaoui-Rabah, M. Sekkal-Rahal, F. Djilani-Kobibi, A.M. Elhorri, M. Springborg,
Performance of hybrid DFT compared to MP2 methods in calculating nonlinear op-
tical properties of divinylpyrene derivative molecules, J. Phys. Chem. A 120 (2016)
8843–8852, https://doi.org/10.1021/acs.jpca.6b08040.
[
104] W. Rettig, Charge separation in excited states of decoupled systems?TICT com-
pounds and implications regarding the development of new laser dyes and the pri-
mary process of vision and photosynthesis, Angew. Chemie Int. Ed. English. 25
(1986) 971–988, https://doi.org/10.1002/anie.198609711.