Viscosity sensitive semisquaraines based on 1, 1, 2-trimethyl-1H-benzo[e]indole: Photophysical properties, intramolecular charge transfer, solvatochromism, electrochemical and DFT study

Article abstract of DOI:10.1016/j.molliq.2019.03.173

1, 1, 2-Trimethyl-1H-benzo[e]indole based D-π-A type semisquaraines denoted as, SSK 1–3 were synthesized to investigate their primary photophysical properties, intramolecular charge transfer, solvatochromism, electrochemical properties and viscosity dependent enhanced emission. These semisquaraines contain 1, 1, 2-trimethyl-1H-benzo[e]indole and squaryl ring as donor and acceptor respectively. These dyes displayed absorption between 437 and 444 nm and emission in the range of 469–493 nm. Stokes shift of the synthesized compounds were in the range of 32–54 nm. The SSK 1–3 exhibited intense yellow-orange colored solid state fluorescence (F_max = 520–549 nm). These dyes exhibited intramolecular charge transfer characteristics which were confirmed from their oscillator strengths, transition dipole moments, ground and excited state dipole moments and generalized Mulliken-Hash analysis (GMH). Moreover, the positive emission solvatochromic behaviour (Δλ_em = 21–24 nm) of these dyes was quantitatively analyzed from different solvent polarity plots. More interestingly, these D-π-A type FMRs displayed viscosity sensitive emission enhancement (23 to 32 fold increase in fluorescence intensity) in 99% methanol:glycerol system at the visible region of electromagnetic spectrum (450–650 nm). The molecular structures of SSK 1–3 were correlated with photophysical properties using density functional theory (DFT) and theoretical absorption spectra in eight different solvents were obtained from time dependent density functional theory (TD-DFT).

Full text of DOI:10.1016/j.molliq.2019.03.173

Journal of Molecular Liquids 285 (2019) 123–135
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Viscosity sensitive semisquaraines based on 1, 1, 2-trimethyl-1H-benzo
[e]indole: Photophysical properties, intramolecular charge transfer,
solvatochromism, electrochemical and DFT study
Sushil Khopkar, Mahesh Jachak, Ganapati Shankarling ⁎
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:
1, 1, 2-Trimethyl-1H-benzo[e]indole based D-π-A type semisquaraines denoted as, SSK 1–3 were synthesized to
investigate their primary photophysical properties, intramolecular charge transfer, solvatochromism, electro-
chemical properties and viscosity dependent enhanced emission. These semisquaraines contain 1, 1, 2-
trimethyl-1H-benzo[e]indole and squaryl ring as donor and acceptor respectively. These dyes displayed absorp-
tion between 437 and 444 nm and emission in the range of 469–493 nm. Stokes shift of the synthesized com-
pounds were in the range of 32–54 nm. The SSK 1–3 exhibited intense yellow-orange colored solid state
fluorescence (Fmax = 520–549 nm). These dyes exhibited intramolecular charge transfer characteristics which
were confirmed from their oscillator strengths, transition dipole moments, ground and excited state dipole mo-
ments and generalized Mulliken-Hash analysis (GMH). Moreover, the positive emission solvatochromic behav-
iour (Δλem = 21–24 nm) of these dyes was quantitatively analyzed from different solvent polarity plots. More
interestingly, these D-π-A type FMRs displayed viscosity sensitive emission enhancement (23 to 32 fold increase
in fluorescence intensity) in 99% methanol:glycerol system at the visible region of electromagnetic spectrum
Received 8 January 2019
Received in revised form 8 March 2019
Accepted 30 March 2019
Available online 16 April 2019
Keywords:
1, 1, 2-Trimethyl-1H-benzo[e]indole
Semisquaraines
Positive emission solvatochromism
Intramolecular charge transfer (ICT)
Fluorescent molecular rotors (FMRs)
Viscosity sensitivity
(450–650 nm). The molecular structures of SSK 1–3 were correlated with photophysical properties using density
functional theory (DFT) and theoretical absorption spectra in eight different solvents were obtained from time
dependent density functional theory (TD-DFT).
©
2019 Elsevier B.V. All rights reserved.
1
. Introduction
, 1, 2-Trimethyl-1H-benzo[e]indole, a widely used electron donor
Donor-acceptor compounds (D-π-A) are referred as push-pull com-
pounds having an electron donating aromatic or heterocyclic moiety
(D) linked to electron accepting moiety (A) through the pi-conjugated
system (spacer). Such type of compounds allow the intramolecular
charge transfer (ICT) transition from donor to acceptor, reduces
HOMO-LUMO energy gap and consequently red shift the absorption/
emission wavelength. In addition, ICT is the main reason for
solvatochromic properties of D-π-A type compounds. These charge
transfer compounds are valuable in organic photovoltaics [20], organic
light emitting diodes (OLEDs) [21,22], viscosity sensitive probes [23]
and chemosensors [24]. The photophysical properties of D-π-A system
can be altered by (i) modification at donor (ii) modification at acceptor
(iii) increasing the pi-conjugation length between donor and acceptor
(spacer). Semisquaraines are important intermediates for the prepara-
tion of unsymmetrical squarylium dyes which have potential applica-
tion as sensitizes in solar cells [25] and metal sensors [26,27]. The
modification at donor moiety, use of squaryl moiety as an acceptor or
anchoring group and intramolecular charge transfer properties render
them suitable for these applications.
1
heterocyclic moiety, applied to synthesize various functional dyes com-
prising styryl dyes [1], squarylium dyes [2], cyanine dyes [3], photochro-
mic dyes [4] and organoboron complexes [5]. Their most interesting
features are planar aromatic structure, extended pi-electron conjuga-
tion, and intramolecular charge transfer leading to intense absorption
spectra. Owing to these properties 1, 1, 2-trimethyl-1H-benzo[e]indole
based dyes have potential application as fluorescent probes [6], sensi-
tizes in solar cells [7], protein labeling [8] and photosensitizers in photo-
dynamic therapy [9]. Squaryl moiety is a commonly used acceptor for
synthesis of symmetrical squarylium dyes [10], unsymmetrical
squarylium dyes [11], semisquarylium dyes [12], bis-squarylium dyes
[
[
13], pi-extended squarylium dyes [14], dianionic squarylium dyes
15], acceptor substituted squarylium dyes [16], cationic squarylium
dyes [17] and 1, 2-squarylium dyes [18]. Squaryl ring of semisquaraine
also acts as an anchoring group for application as a sensitizer in dye sen-
sitized solar cells [19].
Fluorescence molecular rotors (FMRs) are mostly donor-acceptor
compounds which can freely rotate in low viscous media resulting in
low fluorescence intensity. However, in highly viscous media, these
⁎
Corresponding author.
E-mail address: gsshankarling@gmail.com (G. Shankarling).
https://doi.org/10.1016/j.molliq.2019.03.173
167-7322/© 2019 Elsevier B.V. All rights reserved.
0

124
S. Khopkar et al. / Journal of Molecular Liquids 285 (2019) 123–135
2. Experimental
2.1. Chemicals and instruments
All chemicals and reagents were procured from SD Fine chemical
Ltd. and Finar Ltd. 1H (500 MHz) and C (101 MHz) NMR spectra
were recorded on Agilent instrument with CDCl as a solvent and TMS
3
13
Fig. 1. Molecular structures of synthesized dyes.
as an internal standard. ESI-MS analyses were performed on Agilent
6550 mass spectrometer. FT-IR spectra were recorded on Bruker alpha
spectrometer using KBr pellets. UV–visible spectra were performed
with a Jasco spectrophotometer at room temperature. Fluorescence
spectra were performed with a Jasco fluorescence spectrofluorometer
at room temperature. Solid state fluorescence spectra were recorded
on Varian spectrofluorometer at room temperature.
intramolecular rotations get restricted (RIR) and fluorescence intensity
become enhanced. Due to this spectral property, FMRs are useful as
microviscosity sensor [28], bulk viscosity sensor [29], protein sensor
and protein aggregation probe [30]. The requirements for good FMRs
are (i) rotatable single bond between donor and acceptor (ii) good in-
tramolecular charge transfer characteristics (iii) good stokes shift
which separates excitation and emission wavelengths (iv) tunable
emission (v) good quantum yield in highly viscous medium (vi) high
sensitivity and (vii) high brightness. To date, various dyes including
styryl dyes [31], rhodamines [32], symmetrical squaraines [33],
BODIPY [34] and organoboron complexes [35] have been used as
FMRs for viscosity sensor. However, they have some disadvantages
such as (i) complicated synthesis (ii) small Stokes shift which suffers
from self-quenching (iii) low sensitivity (low viscosity sensitivity pa-
rameter) (iv) low quantum yield in a viscous medium. Consequently,
it is crucial to develop new FMRs with good absorption-emission and
Stokes shift compared to reported FMRs. Recently, we have reported
the viscosity sensitivity properties (viscosity sensitivity parameter =
2
2
.2. Synthesis
.2.1. Synthesis of N-alkyl, 1, 1, 2-trimethyl-1H-benzo[e]indol-3-ium iodide
(2, 3)
The quaternization of 1, 1, 2-trimethyl-1H-benzo[e]indole was per-
formed according to reported procedure [38]. To 1, 1, 2-trimethyl-1H-
benzo[e]indole 1 (1.0 g, 4.77 mmol) in 20 ml acetonitrile, iodoalkane
(
iodomethane for 2, 1.69 g, 11.9 mmol:iodoethane for 3, 1.86 g,
1
1
1.9 mmol) was added. The reaction mixture was then refluxed for
0 h. The reaction mass was then filtered and the crude product (resi-
due) obtained was washed with diethyl ether to give purified product
2, 1.34 g, 80%:3, 1.36, 78%).
0
.55 to 0.61) of pyrrolo quinaldine based A-D-A type semisquaraines
36].
Though there are benzo-indolenine based semisquaraines reported
(
[
in the literature [37], their solvatochromic, intramolecular charge trans-
fer, electrochemical and viscosity sensitivity properties are not yet
discussed. Accordingly, herein, we wish to report the solvatochromic,
intramolecular charge transfer and viscosity sensitive emission proper-
ties of semisquaraines consisting of 1, 1, 2-trimethyl-1H-benzo[e]indole
as an electron donor and squaryl ring as an electron acceptor linked
through pi-bridge. Fig. 1 represents the molecular structures of dyes
under study having D-A type configuration.
2.2.2. Synthesis of semisquaraines (SSK 1–3)
A mixture of an alcoholic solution (SSK 1, SSK 3:ethanol, SSK 2:n-bu-
tanol) of N-alkyl, 1, 1, 2-trimethyl-1H-benzo[e]indol-3-ium iodide 2–3
(1.4 mmol), 3, 4-dialkoxycyclobut-3-ene-1, 2-dione 4–5 (1.4 mmol)
and triethylamine (2.8 mmol) was refluxed for 3 h. After completion
of the reaction, water was added to reaction mixture and extracted
with ethyl acetate. The ethyl acetate layer was evaporated under
vacuum to give the crude product which was then purified by column
Scheme 1. Synthesis of semisquaraines SSK 1, SSK 2 and SSK 3.

S. Khopkar et al. / Journal of Molecular Liquids 285 (2019) 123–135
125
+
chromatography (eluent system:n-hexane-ethyl acetate 9:1) to afford
purified product (SSK 1: 76%, SSK 2: 72%, SSK 3: 72%).
MS: m/z calcd for C22
H
21NNaO
3
(M ): 370.1419, found: 370.1414. FT-
−
1
IR (cm ): 3053, 2931, 2872, 2371, 1963, 1766, 1708, 1534, 1471,
1418, 1353, 1310, 1205, 1130, 1093, 1041, 1005, 930, 858, 817, 748.
2
.3. Compounds characterization
2.3.2. 3-Butoxy-4-((1, 1, 3-trimethyl-1, 3-dihydro-2H-benzo[e]indol-2-
2
.3.1. 3-Ethoxy-4-((1, 1, 3-trimethyl-1, 3-dihydro-2H-benzo[e]indol-2-
ylidene) methyl) cyclobut-3-ene-1, 2-dione (SSK 2)
ylidene) methyl) cyclobut-3-ene-1, 2-dione (SSK 1)
Yellow colored solid, yield: 72%, melting point: 216 °C, 1H NMR
Yellow colored solid, yield: 76%, melting point: 210 °C, 1H NMR
3
(500 MHz, CDCl , TMS): δ 8.08 (d, J = 8.5 Hz, 1H), 7.84 (m, 2H), 7.52
(
500 MHz, CDCl
.52 (t, J = 7.3 Hz, 1H), 7.36 (t, J = 7.4 Hz, 1H), 7.26–7.19 (m, 1H),
.39 (s, 1H), 4.91 (q, J = 13.8, 6.7 Hz, 2H), 3.46 (s, 3H), 1.90 (s, 6H),
.54 (t, J = 6.9 Hz, 3H). 13C NMR (101 MHz, CDCl
): δ 192.56, 187.37,
73.30, 170.92, 140.07, 132.30, 130.81, 129.72, 129.55, 128.52, 127.17,
23.71, 122.15, 109.45, 81.14, 69.90, 49.67, 30.18, 26.69, 15.94. ESI-
3
, TMS): δ 8.08 (d, J = 8.0 Hz, 1H), 7.90–7.79 (m, 2H),
(t, J = 7.2 Hz, 1H), 7.36 (t, J = 7.4 Hz, 1H), 7.21–7.24 (m, 1H), 5.39 (s,
1H), 4.87 (t, J = 6.6 Hz, 2H), 3.46 (s, 3H), 1.89 (s, 6H), 1.73–1.59 (m,
7
5
1
1
1
13
2H), 1.57–1.47 (m, 2H), 1.00 (t, J = 7.2 Hz, 3H). C NMR (101 MHz,
CDCl ): δ 192.67, 187.50, 173.08, 170.81, 140.09, 132.25, 130.80,
129.71, 129.54, 128.54, 127.18, 123.70, 122.15, 109.42, 81.08, 73.76,
3
3
−
1
49.66, 32.17, 30.17, 26.68, 18.69, 13.68. FT-IR (cm ): 2959, 2929,
0
0
0
0
0
0
0
0
.7
.6
.5
.4
.3
.2
.1
.0
1.1
Toluene
THF
Dioxane
Chloroform
DMF
DMSO
Acetonitrile
Methanol
21 nm
Toluene
THF
Dioxane
Chloroform
DMF
DMSO
Acetonitrile
Methanol
A
B
1.0
.9
0
0.8
0.7
0
0
0
0
0
0
.6
.5
.4
.3
.2
.1
0.0
3
80
400
400
400
420
440
460
480
500
460
480
500
520
540
560
580
Wavelength (nm)
Wavelength (nm)
1
0
0
0
0
0
0
0
0
0
0
.0
1
.1
Toluene
THF
Dioxane
Chloroform
DMF
DMSO
Acetonitrile
Methanol
24 nm
Toluene
THF
Dioxane
Chloroform
DMF
DMSO
Acetonitrile
Methanol
C
D
.9
.8
.7
.6
.5
.4
.3
.2
.1
.0
1.0
0.9
0
.8
0.7
0
0
0
0
0
0
0
.6
.5
.4
.3
.2
.1
.0
3
80
420
440
460
480
500
4
60
480
500
520
540
560
580
Wavelength (nm)
Wavelength (nm)
0
0
0
0
0
0
0
0
0
.8
.7
.6
.5
.4
.3
.2
.1
.0
1.1
1.0
Toluene
THF
Dioxane
Chloroform
DMF
DMSO
Acetonitrile
Methanol
24 nm
Toluene
THF
Dioxane
Chloroform
DMF
DMSO
Acetonitrile
Methanol
E
F
0
0
.9
.8
0.7
0
0
0
0
0
0
0
.6
.5
.4
.3
.2
.1
.0
3
80
420
440
460
480
500
460
480
500
520
540
560
580
Wavelength (nm)
Wavelength (nm)
Fig. 2. Absorption spectra of (A) SSK 1 (B) emission spectra of SSK 1 (C) absorption spectra of SSK 2 (D) emission spectra of SSK 2 (E) absorption spectra of SSK 3 (F) emission spectra of SSK
−6
3
in various solvents (c = 5 × 10 M).

126
S. Khopkar et al. / Journal of Molecular Liquids 285 (2019) 123–135
2
1
870, 1764, 1705, 1538, 1466, 1418, 1354, 1304, 1210, 1120, 1057,
035, 922, 816, 789, 747, 672, 638.
3.2. Primary photophysical properties
The polarity of solvent can change ground and excited state energy
levels of molecules and consequently can alter the absorption/emission
bands. Therefore, we have determined the primary photophysical prop-
erties of synthesized dyes, SSK 1–3 in different polar and non-polar sol-
vents of different dielectric constants and refractive indices. Fig. 2
represents the photophysical spectra of these dyes in eight different sol-
vents at concentration of 5 × 10 M. The photophysical data of these
dyes including absorption maxima (λmax), molar extinction coefficient
(ε), full width at half maximum (FWHM), optical band gap (ΔEopt),
2
2
.3.3. 3-Ethoxy-4-((3-ethyl-1, 1-dimethyl-1, 3-dihydro-2H-benzo[e]indol-
-ylidene) methyl) cyclobut-3-ene-1, 2-dione (SSK 3)
Yellow colored solid, yield: 72%, melting point: 214 °C, 1H NMR
(
(
1
500 MHz, CDCl
t, J = 7.2 Hz, 1H), 7.35 (t, J = 7.5,1H), 7.25–7.18 (m, 1H), 5.43 (s,
H), 4.91 (q, J = 13.4, 6.3 Hz, 2H), 3.98 (q, J = 10.4, 4.1 Hz, 2H), 1.88
3
,TMS): δ 8.08 (d, J = 7.7 Hz, 1H), 7.83 (m, 2H), 7.52
−6
1
3
(
(
1
6
1
1
s, 6H), 1.54 (t, J = 6.1 Hz, 3H), 1.37 (t, J = 6.0 Hz, 3H). C NMR
101 MHz, CDCl ): δ 192.68, 187.03, 173.27, 169.87, 139.23, 132.60,
30.81, 129.68, 129.59, 128.60, 127.14, 123.73, 122.17, 109.43, 80.48,
3
emission maxima (λem), Stokes shift (λ
s
), quantum yield (ɸ
f
), oscillator
strength (f) and transition dipole moment (μeg) are compiled in Table 1.
All three dyes are yellow colored solids thus they exhibited absorption
wavelength within 437–444 nm and emission in the range of
469–493 nm from non-polar to polar solvents possibly due to intramo-
lecular charge transfer from benzo-indolenine nitrogen to squaryl ring
oxygen of these dyes. The absorption spectra of these dyes showed a
negligible effect of solvent polarity indicating negligible intramolecular
interactions in the ground state of molecule. However, the emission
spectra showed a noticeable effect of solvent polarity on the emission
wavelength. The emission wavelength exhibited red shift from non-
polar to polar solvents (470 nm to 491 nm for SSK 1 from toluene to
methanol, 469 nm to 493 nm for SSK 2 and SSK 3 from toluene to meth-
anol) suggesting positive emission solvatochromic behaviour of studied
dyes. All three dyes showed single emission peak in their emission spec-
tra with maximum emission wavelength observed in methanol
(491 nm for SSK 1, 493 nm for SSK 2 and SSK 3). The red shifted emis-
sion in methanol may be due to the hydrogen bonding of methanol with
solute molecules. All three dyes showed a similar type of absorption and
emission spectra indicating that alkyl and alkoxy groups have a negligi-
ble effect on intramolecular charge transfer mechanism and
−
1
9.85, 49.83, 37.80, 26.59, 15.96, 11.68. FT-IR (cm ): 3054, 2973,
765, 1709, 1610, 1535, 1473, 1414, 1343, 1311, 1215, 1122, 1083,
037, 999, 923, 865, 813, 759, 672.
3
. Results and discussion
3
.1. Synthesis
The synthesis of SSK 1–3 was accomplished by reacting one equiva-
lent of N-alkyl, 1, 1, 2-trimethyl-1H-benzo[e]indolenium iodide with
one equivalent of squaric acid ester in presence of triethylamine in eth-
anol under reflux reaction condition as shown in Scheme 1. The synthe-
sized compounds are well characterized by spectroscopic techniques
(
see the Supporting information). These dyes consist of benzo-
indolenine as an electron donor and squaryl moiety as an electron ac-
ceptor. The SSK 1–3 were modified at (i) N-alkyl group of benzo-
indolenine (ii) position 3 of squaryl ring (alkoxy group) to investigate
their effect on spectral properties.
Table 1
Photophysical data of SSK 1, SSK 2 and SSK 3 in different solvents.
a
b
FWHMc (nm)
d
e
f
g
h
fi
j
Compound
Solvent
λ
max (nm)
Log ε
λ
onset (nm)
ΔEopt (eV)
λ
em (nm)
λ
s
(nm)
ɸ
μ
eg (Debye)
SSK 1
Toluene
THF
438
437
437
442
440
443
437
438
438
437
437
442
441
443
437
439
439
438
438
444
442
444
438
440
4.31
4.32
4.34
4.25
4.33
4.34
4.27
4.27
4.31
4.29
4.38
4.27
4.40
4.48
4.32
4.32
4.36
4.30
4.32
4.26
4.29
4.39
4.29
4.25
43.50
46.68
46.03
50.01
50.24
53.38
44.55
50.09
44.07
47.02
45.31
47.19
47.19
55.96
45.92
50.04
42.87
45.10
45.75
44.15
45.87
53.87
45.32
49.05
460.46
467.07
464.81
481.84
475.93
476.63
470.02
478.11
460.46
467.77
464.81
470.72
473.68
480.37
468.55
476.63
464.11
462.07
462.29
474.46
470.02
478.88
466.29
470.06
2.69
2.65
2.67
2.57
2.61
2.60
2.64
2.59
2.69
2.65
2.67
2.63
2.62
2.58
2.65
2.60
2.67
2.68
2.68
2.61
2.64
2.59
2.66
2.64
470
478
480
488
484
489
482
491
469
477
475
481
484
490
482
493
469
477
474
480
482
490
484
493
32
41
43
46
44
46
45
53
31
40
38
39
43
47
45
54
30
39
36
36
40
46
46
53
˂0.01
˂0.01
˂0.01
0.0121
˂0.01
0.0100
˂0.01
˂0.01
˂0.01
˂0.01
˂0.01
0.0119
˂0.01
0.0118
˂0.01
˂0.01
˂0.01
˂0.01
˂0.01
0.0112
˂0.01
0.0106
˂0.01
˂0.01
1.39
1.58
1.59
1.52
1.79
1.94
1.22
1.38
1.42
1.48
1.71
1.30
1.77
2.79
1.46
1.53
1.57
1.40
1.56
1.08
1.30
2.20
1.35
1.27
11.39
12.11
12.18
11.96
12.97
13.53
10.67
11.35
11.52
11.73
12.61
11.04
12.91
16.24
11.66
11.96
12.10
11.41
12.07
10.12
11.07
14.42
11.24
10.92
1,4-Dioxane
Chloroform
DMF
DMSO
Acetonitrile
Methanol
Toluene
THF
SSK 2
1,4-Dioxane
Chloroform
DMF
DMSO
Acetonitrile
Methanol
Toluene
THF
SSK 3
1,4-Dioxane
Chloroform
DMF
DMSO
Acetonitrile
Methanol
a
Absorption maxima at molar concentration 5 × 10−6 M.
Molar extinction coefficient determined from the absorbance at molar concentration 5 × 10 M.
Full width at half maximum.
Onset absorption edge at a higher wavelength.
Optical band gap, ΔEopt = 1240 × λonset
Emission maxima at molar concentration 5 × 10 M.
Stokes shift.
Quantum yield calculated using quinine sulfate as a standard (ɸ = 0.54 in 0.1 M H
Transition dipole moment (f) and transition dipole moments (μeg).
b
c
d
e
f
−6
.
−6
g
h
i
2 4
SO ) [40].
Oscillator strength.
j

S. Khopkar et al. / Journal of Molecular Liquids 285 (2019) 123–135
127
useful for optoelectronic applications [39]. The Stokes shifts of SSK 1–3
were low in non-polar solvent (30–32 nm in toluene) and high in polar
solvent (53–54 nm in methanol). The highest stokes shift of 54 nm was
obtained for SSK 2 in methanol. The good stokes shift values of these
dyes are indicative of fact that emission occurs from the intramolecular
charge transfer state and wide structural reorganization in the excited
state. The red shift in the Stokes shift from non-polar to polar solvents
demonstrating stabilization of polar excited state in polar solvents. Fur-
thermore, the optical band gaps (ΔEopt) of SSK 1–3 calculated from
onset of absorption spectra (λonset) were found to decrease from non-
polar to polar solvent which supports positive solvatochromic behav-
iour of these dyes. Moreover, low fluorescence quantum yields of SSK
1
–3 were obtained in the studied solvents and among them highest
fluorescence quantum yield was found in chloroform (0.0121 for SSK
, 0.019 for SSK 2, 0.0112 for SSK 3). The low fluorescence quantum
1
yield in polar solvents may be due to the presence of intramolecular
charge transfer (ICT) and twisted intramolecular charge transfer
(
TICT) characteristics in solution which increases the non-radiative
decay [41]. The charge transfer characteristics of these compounds
were evaluated from their oscillator strengths.
Fig. 3. Solid state fluorescence spectra of SSK 1–3.
Oscillator strength provides absorption area in the electromagnetic
spectrum while transition dipole moment gives the measure of proba-
bility of π to π* transition. The values of f and μge can be calculated
using following expressions (Eqs. (1) and (2)) [42],
consequently on the spectroscopic shifts. The SSK 1–3 exhibited high
molar extinction coefficient in the studied solvents and among all sol-
vents highest molar extinction coefficient was observed in DMSO (log
e = 4.34 for SSK 1, 4.48 for SSK 2 and 4.39 for SSK 3). Moreover,
FWHM which is a measure of broadness of peak in the absorption spec-
tra were in between 43 and 55 nm suggesting that present dyes can be
Z
−9
f ¼ 4:32 ꢀ 10
εðvÞdv
ð1Þ
2
2
2
2
1
1
1
600
400
200
000
800
600
400
2600
A
B
2400
2200
2000
1800
1600
1400
2
2
SSK1 R = 0.60
SSK1 R = 0.57
2
2
SSK2 R = 0.68
SSK2 R = 0.68
2
2
SSK3 R = 0.68
SSK3 R = 0.68
0
.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35
f (LM)
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
f (MR)
2
2
2
2
1
1
1
600
400
200
000
800
600
400
C
2
SSK1 R = 0.83
2
SSK2 R = 0.86
2
SSK3 R = 0.78
30
35
40
45
50
55
ET(30)
Fig. 4. (A) Lippert-Mattaga graph (B) McRae graph (C) correlation of Stokes shift of SSK 1–3 with ET(30) function.

128
S. Khopkar et al. / Journal of Molecular Liquids 285 (2019) 123–135
Fig. 5. (A) Wellers plot (B) Rettig solvent polarity plot of SSK 1–3.
3.3. Solid state fluorescence
f
2
eg
μ
¼
ð2Þ
Further, we have investigated the fluorescence properties of SSK 1–3
ð 4:32 ꢀ 10−7 ꢀ vÞ
in solid state. For this purpose, the fluorescence spectra were recorded
with excitation wavelength 471 nm for SSK 1, 469 nm for SSK 2 and
465 nm for SSK 3 keeping slit width and source of light constant.
Fig. 3 shows the fluorescence spectra of SSK 1–3 in crystalline state.
All three compounds exhibited bright yellow to orange fluorescence in
a solid state. The solid state fluorescence maxima (Fmax) of SSK 1 were
observed at 520 nm. Under the same condition, SSK 2 and SSK 3 showed
where ε is molar extinction coefficient and ν is wave number in cm−1.
The maximum oscillator strength and transition dipole moment
were observed in DMSO for all three dyes (f = 1.94, μge = 13.53 for
SSK 1, f = 2.79, μge = 16.24 for SSK 2, f = 2.20, μge = 14.42 for SSK
3
). The oscillator strengths and transition dipole moments
were increased from non-polar to polar solvents signifying
increased intramolecular charge transfer from non-polar to polar
solvents.
Fmax at 549 nm and 523 nm respectively. These results showed that
solid state fluorescence maxima are bathochromic as compared to that
in solution. For example, solid state emission maxima of SSK 1–3 was
Table 2
Correlation of charge transfer characteristics and positive solvatochromism with dipole moment.
b
ac (cm−1) × 10−8
m(LM)d
1474.3
)e (Debye)
f
g
h
Compound
Solvent
fa
μ
eg
(μ
e
− μ
g
μ
g
(Debye)
μ
e
(Debye)
e g
μ /μ
(Debye)
1.11
SSK 1
Toluene
THF
1.39
1.58
1.59
1.52
1.79
1.94
1.22
1.38
1.42
1.48
1.71
1.30
1.77
2.79
1.46
1.53
1.57
1.40
1.56
1.08
1.30
2.20
1.35
1.27
11.39
12.11
12.18
11.96
12.97
13.53
10.67
11.35
11.52
11.73
12.61
11.04
12.91
16.24
11.66
11.96
12.10
11.41
12.07
10.12
11.07
14.42
11.24
10.92
4.201
4.194
4.200
4.197
4.191
4.191
4.191
4.192
4.200
4.194
4.200
4.197
4.191
4.191
4.191
4.191
4.201
4.196
4.202
4.198
4.192
4.192
4.193
4.192
3.2959
3.2841
3.2959
3.2959
3.2841
3.2841
3.2841
3.2841
3.9087
3.8947
3.9087
3.9087
3.8947
3.8947
3.8947
3.8947
4.0427
4.0427
4.0427
4.0427
4.0283
4.0283
4.0283
4.0283
9.5261
11.4430
9.4167
10.8003
12.5942
12.6691
12.6017
12.5509
9.5149
12.822
14.7271
12.7126
14.0962
15.8783
15.9532
15.8858
15.835
13.4236
15.3291
13.3111
14.7015
16.4863
16.5624
16.4933
16.4427
13.7719
15.6916
13.6571
15.0528
16.829
1,4-Dioxane
Chloroform
DMF
DMSO
Acetonitrile
Methanol
Toluene
THF
SSK 2
2073.5
2218.2
1.11
1.10
11.4344
9.4024
1,4-Dioxane
Chloroform
DMF
DMSO
Acetonitrile
Methanol
Toluene
THF
10.7928
12.5916
12.6677
12.5986
12.5480
9.7292
SSK 3
11.6489
9.6144
1,4-Dioxane
Chloroform
DMF
DMSO
Acetonitrile
Methanol
11.0101
12.8007
12.8763
12.8101
12.7569
16.9046
16.8384
16.7852
a
Oscillator strength.
Transition dipole moment.
b
c
Onsager radius calculated from density functional theory.
Slope obtained from Lippert-Mattaga plot.
Difference in ground state and excited state dipole moment calculated using Lippert-Mattaga equation.
Ground state dipole moment calculated using density functional theory.
Excited state dipole moment.
d
e
f
g
h
Dipole moment ratio calculated using Backshiev equation.

S. Khopkar et al. / Journal of Molecular Liquids 285 (2019) 123–135
129
red shifted to 29 nm, 56 nm and 30 nm respectively as compared to
Table 3
Charge transfer characteristics of SSK 1–3 by Mulliken-Hush analysis.
their emission maxima (λem) in methanol. This bathochromic emission
in solid state can be attributed to the intermolecular pi-pi interactions of
SSK 1–3 in solid state [43].
−8
fb
c
2d
HDAe
RDAf
Compound Solvent
aa × 10
μeg
C
−
1
(cm
)
SSK 1
SSK 2
SSK 3
Toluene
THF
Dioxane
Chloroform
DMF
4.201
4.194
4.200
4.191
4.191
4.197
4.191
4.192
4.200
4.194
4.200
4.191
4.191
4.197
4.191
4.191
4.201
4.196
4.202
4.192
4.192
4.198
4.193
4.192
1.38 11.35 0.428 11,206 18.82
1.39 11.39 0.428 11,273 18.44
1.22 10.67 0.424 11,204 18.92
1.79 12.97 0.437 11,338 16.73
1.59 12.18 0.433 11,308 17.73
1.94 13.53 0.440 11,338 16.56
1.52 11.96 0.432 11,298 17.94
1.58 12.11 0.433 11,298 17.85
1.42 11.52 0.416 11,206 20.15
1.46 11.66 0.418 11,211 20.14
1.30 11.04 0.413 11,140 20.27
1.77 12.91 0.425 11,287 17.71
1.71 12.61 0.423 11,285 17.78
2.79 16.24 0.440 11,307 17.41
1.48 11.73 0.418 11,241 18.78
1.53 11.96 0.420 11,254 18.55
1.27 11.07 0.409 11,129 18.78
1.30 11.24 0.410 11,153 18.73
1.08 10.12 0.402 11,042 20.32
1.57 12.10 0.418 11,241 17.91
1.56 12.07 0.417 11,236 18.05
2.20 14.42 0.431 11,258 16.77
1.35 11.41 0.412 11,175 18.46
1.40 11.43 0.413 11,234 18.25
3
.4. Solvatochromism
The emission spectra of SSK 1–3 showed positive solvatochromic
properties i.e. emission wavelength increases with increase in solvent
polarity (Fig. 2). This positive solvatochromic behaviour was quantita-
tively determined using different solvent polarity plots viz. Lippert-
Mattaga plot, MacRae plot and ET (30) solvent polarity parameter.
Lippert-Mattaga plot provides the correlation of physical parameters
DMSO
Acetonitrile
Methanol
Toluene
THF
Dioxane
Chloroform
DMF
(
dielectric constant and refractive index) of solvent with Stokes shift of
compound [44]. It is plot of Stokes shift in wave number versus orienta-
tion polarizability (ΔfLM). It does not account for the specific solute-
solvent interactions such as hydrogen bonding and molecular com-
plexes. Fig. 4A represents the graph of Stokes shift of SSK 1–3 versus ori-
entation polarizability. A good linear correlation with good regression
DMSO
Acetonitrile
Methanol
Toluene
THF
Dioxane
Chloroform
DMF
DMSO
Acetonitrile
Methanol
2
coefficient (R = 0.60 for SSK 1, 0.68 for SSK 2 and 0.68 for SSK 3) ob-
served in Fig. 4A suggesting that physical parameters of solvents are col-
lectively responsible for positive emission solvatochromic shift in polar
solvents.
McRae solvent polarity plot considers solute polarizability along
with the solvent polarizability [45]. Fig. 4B shows a graph of Stokes
a
Onsager radii calculated from DFT.
Oscillator strength.
Transition dipole moment.
The degree of delocalization or fractional degree of localization of the excess charge.
Electronic coupling matrix.
Donor-acceptor coupling matrix distance.
−
1
b
c
d
e
f
shift of SSK 1–3 in cm
versus McRae function, ΔfMR. The linear
relationship observed in Fig. 4B suggests that the dipole created be-
tween benzo-indolenine and squaryl ring of these dyes also contributed
to the emission solvatochromic shift. ET (30), empirical solvent
polarity parameter introduced by Dimroth-Reichardt correlates the
solvatochromic stokes shift of compound with ET (30) solvent polarity
parameter [46]. The relationship between stokes shift and ET (30) can
be explained by parameter which represents acidity, dipolarity or polar-
excited state dipole moment (μ
e g e
− μ ) and dipole moment ratio μ /
izability but not the basicity of solvent. The good regression coefficients
μ of SSK 1–3. The Onsager radii of SSK 1–3 were calculated from
DFT optimized geometries considering the charge transferred from
benzo-indolenine nitrogen to squaryl oxygen. The ground state di-
g
2
(
R = 0.78–0.86) and good linearity observed in Fig. 4C indicates that
dielectric solute-solvent interaction also contributed for the positive
emission solvatochromic shift.
pole moments (μ
optimized ground state geometries in different polar and non-polar
solvents. From the ground state dipole moments (μ ) and slope of
g
) of these compounds were calculated from DFT
3
.5. Intramolecular charge transfer and dipole moment
g
Lippert-Mattaga plot (m), the excited state dipole moments (μ
e
)
Positive solvatochromism is a strong evidence of stabilization of ex-
were calculated.
cited state and increased intramolecular charge transfer characteristics
of SSK 1–3 from non-polar to polar solvents. The intramolecular charge
transfer of SSK 1–3 occurs from benzo-indolenine donor to squaryl ac-
ceptor. To correlate the positive solvatochromism with intramolecular
charge transfer (ICT) and twisted intramolecular charge transfer
The positive difference in dipole moment of S
suggests that excited state is more stabilized in polar solvents and con-
firms the increased intramolecular charge transfer of excited state in
0
and S
1
state (S
1
˃ S
0
)
(
TICT), we studied the Wellers and Rettig solvent polarity plots. Weller's
plot is a plot of emission wave number of compound versus Weller's
function, Δf [47]. This plot gives the evidence of intramolecular charge
transfer within D-π-A type dyes. Fig. 5A shows the graph of emission
wave number of SSK 1–3 in cm versus Weller's function. The good
linearity with good regression coefficients in Wellers plot demonstrates
that effective intramolecular charge transfer phenomenon responsible
for positive emission solvatochromism. Rettig plot gives the evidence
of twisted intramolecular charge transfer in the molecules [48]. The lin-
ear relationship in Rettig plot (Fig. 4B) indicates that there is TICT in ad-
dition with ICT within SSK 1–3.
0
0
0
0
.000025
.000020
.000015
.000010
w
SSK1
SSK2
SSK3
−
1
0.000005
0.000000
0.000005
Although regression coefficients of Wellers and Rettig plots were
close to unity, it does not give the order of charge transfer character-
istics. The order of charge transfer characteristics can be determined
using difference in dipole moment of ground state (μ
g
) and excited
-
state (μ ). The ground state and excited state dipole moments give
e
0.0
0.2
0.4
Potential / V
0.6
0.8
1.0
an idea about dipolar and polarizability character of molecule in
the ground state and excited state. Table 2 shows the oscillator
strength (f), transition dipole moment (μeg), Onsager radius (a),
slope of Lippert-Mattaga plot (m), difference in ground state and
4 4
Fig. 6. Cyclic voltammograms of SSK 1–3 (1 mM) in chloroform containing n-Bu NClO
(0.1 M).

130
S. Khopkar et al. / Journal of Molecular Liquids 285 (2019) 123–135
Table 4
polar solvents [49]. Moreover, the ratio μ
g e
/μ is more than unity, which
Electrochemical and computational data for SSK 1–3 in chloroform.
entails that the excited state S is more polar than ground state S
1
0
a
Theoreticale (eV)
Compound
E
pa
Experimental
[50]. This can be attributed to the stabilization of excited state of SSK
1–3 by polar solvents via solvation or hydrogen bonding. In order to
get more insights into charge transfer characteristics of SSK 1–3 with
varying solvent polarity, the generalized Mulliken-Hash analysis
b
c
d
E
HOMO
E
LUMO
E
g
E
HOMO
E
LUMO
E
g
SSK 1
SSK 2
SSK 3
0.66
0.66
0.64
−5.37
−5.37
−5.35
−2.74
−2.76
−2.78
2.63
2.61
2.57
−5.30
−5.30
−5.29
−1.98
−1.98
−1.98
3.32
3.32
3.31
(
GMH) was carried out [51,52] and results including the electronic cou-
pling matrix (HDA), degree of delocalization or fractional degree of local-
ization of excess charge (C2
), donor-acceptor coupling matrix distance
(RDA) are summarized in Table 3. The electronic coupling matrix (HDA
a
Peak anodic oxidation potential.
b
c
b
EHOMO = −e (Eox onset + 4.71) eV.
ELUMO = EHOMO − Eo-o, Eo-o = zero-zero excitation energy.
)
d
e
Energy gap obtained from CV data.
Density functional theory.
gives the rate of charge transfer within donor-acceptor type molecule
while donor-acceptor coupling matrix (RDA) distance gives the degree
Fig. 7. Emission spectra of (A) SSK 1 (C) SSK 2 (E) SSK 3 in methanol: glycerol mixtures and corresponding plot of the dependence of emission intensity of (B) SSK 1 (D) SSK 2 (F) SSK 3 on
viscosity of solvent system at room temperature.

S. Khopkar et al. / Journal of Molecular Liquids 285 (2019) 123–135
131
Table 5
Viscosity sensitivity of SSK 1–3.
a
b
R2c
Compound
λ
abs methanol
λ
(nm)
em methanol
λ
(nm)
abs 99% glycerol
λ
(nm)
em 99% glycerol
ɸf
ɸf 99%
Viscosity sensitivity parameter
(x)
(
nm)
methanol
glycerol
SSK 1
SSK 2
SSK 3
438
439
440
491
493
493
447
448
449
503
503
502
0.0023
0.0026
0.0028
0.0892
0.0874
0.0812
0.60
0.49
0.57
0.94
0.83
0.96
a
Quantum yield measured in methanol.
Quantum yield measured in 99% glycerol using quinine sulfate in 0.1 N H
Regression coefficient.
b
c
2 4
SO
f
as standard (ɸ = 0.54).
of charge separation due to interaction between donor and acceptor.
The values of C , HDA and RDA were calculated using Eqs. (3)–(5),
b
voltammograms (CV) of SSK 1–3 in chloroform. Table 4 shows the rele-
vant electrochemical data of these dyes. Semisquaraines generally show
poorly defined redox peaks in their cyclic voltammograms due to shifting
of electron density from heterocyclic or aromatic nitrogen to squaryl ox-
ygen [54]. Similarly, the dyes SSK 1–3 showed poor redox peaks in Fig. 6.
From cyclic voltammograms of SSK 1–3, the HOMO-LUMO energy
levels were calculated and the obtained results are compiled in
Table 4. The onset value of oxidation potentials (Eox) for SSK 1–3 were
found to be 0.66 V, 0.66 V and 0.64 V respectively, corresponds to
HOMO energy levels of −5.37 eV for SSK 1, −5.37 eV for SSK 2 and
−5.35 eV for SSK 3. No electrochemical reduction potential (Ered) was
observed in the range of 0.0 to 1.0 eV. Consequently, the energy of
LUMOs of these dyes were calculated from values of Eox and zero-zero
excitation energies (Eo-o) were found to −2.74 eV, −2.76 eV and
−2.78 eV for SSK 1, SSK 2 and SSK 3 respectively. These experimentally
obtained HOMO-LUMO energy levels were in good agreement with the-
oretically obtained HOMO-LUMO levels calculated from DFT (Table 4).
2
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
2
1
2
Δμge
2
b
c
¼
ð1−
ð3Þ
ð4Þ
Δμge þ 4μge2
2
ΔEge μge
ΔEgeμge
HDA ¼
¼ qffiffiffiffiffiffiffiffiffiffi
Δμab
Δμ þ 4μ2
2
ge
ge
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ΔEge εmax Δv1=2
−2
RDA ¼ 2:06 ꢀ 10
ð5Þ
HDA
where, μge = transition dipole moment, Ege = vertical excitation en-
ergy, Δμab = difference between diabatic dipole moment and transition
dipole moment, εmax is molar extinction coefficient at maximum ab-
sorption, Δv1/2 is full width at half maximum (FWHM) of charge trans-
fer band. The values of RDA and HDA are inversely proportional to each
g
The electrochemical band gap (E ) values of SSK 1–3 were found to
2.63 eV, 2.61 eV and 2.57 eV respectively which were consistent with
theoretically obtained HOMO-LUMO band gap.
other.
2
The value of C
b
tending towards zero implies the high degree of charge
delocalization while tending towards unity indicates charge localization.
3.7. Viscosity sensitivity
From Table 3, it is observed that the values of C2
are less than unity, there-
b
fore delocalization of charge occurs from benzo-indolenine nitrogen to
squaryl ring oxygen leading to effective intramolecular charge transfer.
Under these circumstances, the values of HDA and RDA also displays the ef-
fective charge transfer properties of these compounds. The HDA values in-
creased from non-polar to polar solvents while RDA values decreased from
non-polar polar solvents confirming increased intramolecular charge
transfer from non-polar to polar solvents and positive solvatochromic be-
haviour of studied compounds [53].
Wellers and Rettig plots revealed that SSK 1–3 have ICT and TICT
characteristics in a solution state. These ICT as well as TICT characteris-
tics in low viscous solvents and possible rotations sites on SSK 1–3 pro-
voked us to study viscosity sensitivity properties of these dyes. For this
purpose, we have studied the viscosity sensitivity of SSK 1–3 in metha-
nol: glycerol system with increasing percentage of glycerol in methanol
(0.80 to 500 cP) as presented in Fig. 7. The related viscosity sensitivity
data is summarized in Table 5. From Fig. 7, it appears that with increas-
ing viscosity of methanol:glycerol system, the fluorescence intensity
gets increases. For example, on increasing percentage of glycerol in
methanol, SSK 1 showed 32 fold increase in emission intensity. Under
the same condition, SSK 2 and SSK 3 showed 22 fold and 26 fold en-
hancement in emission intensity. The quantum yield of SSK 1–3 in
methanol were calculated as 0.0023, 0.0026 and 0.0028 respectively.
Upon addition of glycerol, it increased to 0.0892, 0.0874 and 0.0812 in
99% glycerol.
3
.6. Electrochemical properties
The electrochemical properties of SSK 1–3 were explored from cyclic
voltammograms (CV) in chloroform using tetrabutyl ammonium per-
+
chlorate (TBAP) as supporting electrolyte, Fe/Fe , glassy carbon and Ag/
AgCl as external reference, working and reference electrode respectively
−1
with scan rate of 100 mV s . Fig. 6 shows the overlaid cyclic
A
B
C
D
E
F
SSK 1
SSK 2
SSK 3
Fig. 8. UV light photographs of SSK 1–3 in 0% glycerol (A), (C), (E) and 99% glycerol (B), (D), (F).

132
S. Khopkar et al. / Journal of Molecular Liquids 285 (2019) 123–135
Fig. 9. Frontier molecular orbitals of SSK 1–3 showing electron density distribution at HOMO-LUMO energy levels in chloroform.
Table 6
Comparative experimental and computational photophysical parameters of SSK 1–3.
a
Egb (eV)
VEEc (nm)
fd
Orbital contributione
f
% deviationg
Compound
Solvent
λ
max (nm)
EHOMO (eV)
E
LUMO (eV)
μ
eg (Debye)
SSK 1
Toluene
THF
Dioxane
Chloroform
DMF
DMSO
Acetonitrile
MeOH
Toluene
THF
Dioxane
Chloroform
DMF
DMSO
Acetonitrile
MeOH
Toluene
THF
Dioxane
Chloroform
DMF
DMSO
Acetonitrile
MeOH
438
437
437
442
440
443
437
438
438
437
437
442
441
443
437
439
439
438
438
444
442
444
438
440
−5.30
−5.31
−5.30
−5.30
−5.32
−5.32
−5.32
−5.32
−5.29
−5.31
−5.29
−5.30
−5.32
−5.32
−5.32
−5.32
−5.27
−5.30
−5.27
−5.29
−5.31
−5.31
−5.31
−5.31
−1.94
−2.00
−1.93
−1.98
−2.04
−2.05
−2.04
−2.04
−1.93
−2.00
−1.93
−1.98
−2.04
−2.05
−2.04
−2.04
−1.92
−1.99
−1.92
−1.97
−2.04
−2.04
−2.04
−2.04
3.36
3.31
3.37
3.32
3.28
3.27
3.28
3.28
3.36
3.31
3.36
3.32
3.28
3.27
3.28
3.28
3.35
3.31
3.35
3.32
3.27
3.27
3.27
3.27
410.08
412.57
408.20
412.01
415.80
417.16
413.52
412.97
410.12
412.62
408.26
412.05
415.85
417.20
413.58
413.04
410.30
412.37
408.47
411.94
415.38
416.74
413.14
412.62
0.7539
0.7789
0.7247
0.7766
0.8076
0.8289
0.7783
0.7713
0.7644
0.7917
0.7303
0.7888
0.8202
0.8361
0.7913
0.7849
0.7539
0.7829
0.7231
0.7795
0.8129
0.8290
0.7825
0.7753
H → L (96.7)
H → L (96.8)
H → L (96.3)
H → L (96.8)
H → L (97.0)
H → L (97.2)
H → L (96.7)
H → L (96.6)
H → L (96.7)
H → L (96.8)
H → L (96.3)
H → L (96.8)
H → L (97.0)
H → L (97.2)
H → L (96.7)
H → L (96.6)
H → L (96.3)
H → L (96.6)
H → L (95.9)
H → L (96.6)
H → L (96.9)
H → L (97.0)
H → L (96.5)
H → L (96.4)
8.1092
8.2670
7.9323
8.2496
8.4509
8.5499
8.2711
8.2310
8.1669
8.3354
7.9921
8.3146
8.5173
8.6137
8.3429
8.3036
8.1113
8.2866
7.9259
8.2642
8.4745
8.5723
8.2922
8.2492
6.81
5.92
7.06
7.28
5.82
6.19
5.68
6.06
6.80
5.91
7.04
7.27
6.05
6.18
5.66
6.29
6.99
6.22
7.23
7.78
6.41
6.54
6.02
6.64
SSK 2
SSK 3
a
Experimental absorption wavelength.
b
c
d
e
f
HOMO-LUMO energy band gap determined from density functional theory (DFT).
Computational vertical electronic excitation.
Computed oscillator strength.
Major electronic transition.
Computed transition dipole moment.
g
Percentage deviation from experimental absorption wavelength.

S. Khopkar et al. / Journal of Molecular Liquids 285 (2019) 123–135
133
2
The blocking of twisted intramolecular charge transfer state (TICT)
and increase in intramolecular charge transfer (ICT) in the high viscous
medium may be the reason behind this viscosity induced emission [55]
parameters (x) for SSK 1–3 were determined to be 0.60 (R = 0.94),
0.49 (R2 = 0.83) and 0.57 (R2 = 0.96) respectively, making them a
good alternative viscosity sensor for traditional fluorescent molecular
rotors. As single bonds are slow to quench the emission intensity, the
N-alkyl and alkoxy groups on SSK 1–3 have a negligible effect on viscos-
ity sensitive parameter [57]. As semisquaraines SSK 1–3 showed emis-
sion enhancement within range of 450–650 nm, they can be applied
as viscosity sensor in the visible region.
(Fig. 8).
Forster-Hoffmann equation quantitatively correlates the fluores-
cence intensity of SSK 1–3 with viscosity of the solvent system by
Eq. (6) [56].
logI ¼ C þ x logn
ð6Þ
where I is emission intensity of compound, C is temperature dependent
constant, x is viscosity sensitive parameter and n is viscosity of solvent
system. The higher value of x, more is the sensitive emission intensity
of compound towards viscosity of the medium. The viscosity sensitivity
3.8. Computational study
To correlate the molecular structures of semisquaraines SSK 1–3
with intramolecular charge transfer characteristics, density functional
Fig. 10. HOMO-LUMO energy gap plots for SSK 1–3 in different solvents.

134
S. Khopkar et al. / Journal of Molecular Liquids 285 (2019) 123–135
theory (DFT) computations were carried out with B3LYP [58] functional
and def2-TZVP (valence triple–zeta-plus polarization) [59] as a basis set
using Turbomole-V6.5 program [60]. The HOMO-LUMO energy levels,
vertical electronic excitations (VEE), computed oscillator strengths (f),
computed transition dipole moments (μeg) and orbital contributions
for HOMO-LUMO transition in eight different polar and non-polar sol-
vents were determined by using time dependent DFT (TD-DFT) with
same functional and basis set. Fig. 9 displays the DFT optimized ground
state geometries of SSK 1–3 with their electron density distribution at
HOMO-LUMO energy levels in chloroform. Table 6 shows the computed
photophysical data of SSK 1–3 in different solvents.
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.molliq.2019.03.173.
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