Fluorescent 7-Substituted Coumarin Dyes: Solvatochromism and NLO Studies

Article abstract of DOI:10.1007/s10895-018-2316-2

The effect of three substituents N,N-diethylamine, carbazole and diphenylamine at the 7 position of coumarin on linear and nonlinear optical properties are studied using absorption and emission solvatochromism, and DFT. By varying the substituent 53?nm red shift is achieved in emission. The polarity plots with regression close to unity revealed good charge transfer in the system. Solvent polarizability and dipolarity are mainly responsible for solvatochromic shift as proved by multilinear regression analysis. General Mulliken Hush analysis shows diphenylamine substituent leads to more charge separation in compound 6c. The hyperpolarizabilities are evaluated by quantum mechanical calculations. Structure of the compounds are optimized at B3LYP/6-31G(d) level and NLO computations are done using range separated hybrid functionals with large basis sets. Second order hyperpolarizability (γ) found 589.27 × 10^?36, 841.29 × 10^?36 and 1043.00 × 10^?36 e.s.u for the compounds 6a, 6b and 6c respectively.

Full text of DOI:10.1007/s10895-018-2316-2

Journal of Fluorescence
https://doi.org/10.1007/s10895-018-2316-2
ORIGINAL ARTICLE
Fluorescent 7-Substituted Coumarin Dyes:
Solvatochromism and NLO Studies
Archana A. Bhagwat1 & Nagaiyan Sekar1
Received: 28 August 2018 /Accepted: 14 October 2018
#
Springer Science+Business Media, LLC, part of Springer Nature 2018
Abstract
The effect of three substituents N,N-diethylamine, carbazole and diphenylamine at the 7 position of coumarin on linear and
nonlinear optical properties are studied using absorption and emission solvatochromism, and DFT. By varying the substituent
53 nm red shift is achieved in emission. The polarity plots with regression close to unity revealed good charge transfer in the
system. Solvent polarizability and dipolarity are mainly responsible for solvatochromic shift as proved by multilinear regression
analysis. General Mulliken Hush analysis shows diphenylamine substituent leads to more charge separation in compound 6c. The
hyperpolarizabilities are evaluated by quantum mechanical calculations. Structure of the compounds are optimized at B3LYP/6-
31G(d) level and NLO computations are done using range separated hybrid functionals with large basis sets. Second order
hyperpolarizability (γ) found 589.27 × 10−36, 841.29 × 10−36 and 1043.00 × 10−36 e.s.u for the compounds 6a, 6b and 6c
respectively.
.
.
.
Keywords Coumarin Solvatochromism DFT Hyperpolarizability
Introduction
coumarin chromophore can be tuned by changing the substit-
uents at seventh position. 7-Substituted coumarin shows a
significant change in emission intensity, Stokes shift and
quantum yield in varying polarity solvents [9–11].
Solvent polarity dependent change in intensity and position in
spectra is termed as solvatochromism. It is divided into sub-
type, positive and negative solvatochromism [1].
Bathochromic shift in emission is known as positive
solvatochromism and a hypsochromic shift in emission is
known as negative solvatochromism. Solvatochromism is
governed by the structural changes of the compounds and
solvent environment. The effect of solvent medium on
photophysical properties of the coumarin dyes are well ex-
plored [2, 3]. 7-Aminocoumarins are applied in electronics
and biological field [4–7]. Solvatochromic Push-Pull (D-π-
A) chromophores show intramolecular charge transfer (ICT)
[8]. The absorption-emission properties of 7-substituted
The photochemistry of the solvatochromic compounds is
studied by different solvatochromic models. Lippert, Dimroth,
Catalan and Kamlet Taft model are well known and widely
used [12–15]. Lippert model useful for estimation of excited
state dipole moment. Knowledge of excited state dipole mo-
ment is important to design nonlinear optical materials [16].
Coumarin dyes found applicable in optoelectronics [17, 18].
The highly fluorescent, solvatochromic, large Stokes shifted
compounds are attracted our attention. Among dye class, cou-
marins are known as highly fluorescent chromophore so we
have chosen chromophore core for our study. To get
solvatochromism and high Stokes shift we have altered sub-
stituent at the C7 position.
In this study, we have shown the effect of three dif-
ferent 7-substituted coumarins on photophysical proper-
ties. Solvatochromic study of synthesized dyes is done
using Lippert-mataga solvent polarity scale, multilinear
regression analysis which involves Kamlet-Taft and
Catalan method. Excited state dipole moment and
hyperpolarizabilies are derived using solvatochromic
and computational approach.
Electronic supplementary material The online version of this article
(https://doi.org/10.1007/s10895-018-2316-2) contains supplementary
material, which is available to authorized users.
* Nagaiyan Sekar
nethi.sekar@gmail.com
1
Department of Dyestuff Technology, Institute of Chemical
Technology, Matunga, Mumbai 400 019, India

J Fluoresc
Experimental
was treated with hexane and left overnight with constant mag-
netic stirring to elute impurities. The hexane layer was
discarded and the residue was purified by column using hexane:
ethyl acetate (10:1) as an eluent. Yield 46%. Mp. 138-140 °C.
1H NMR (500 MHz, CdCl3) δ 10.51 (s, 1H), 8.14 (d, J =
7.7 Hz, 2H), 8.07 (d, J = 8.2 Hz, 1H), 7.52 (d, J = 8.2 Hz, 2H),
7.45–7.42 (m, 2H), 7.34–7.30 (m, 2H), 7.29 (dd, J = 8.2,
1.2 Hz, 1H), 7.23 (d, J = 1.7 Hz, 1H), 3.96 (s, 3H).
Materials and Methods
All the starting materials were obtained from S.D. Fine
Chemical and Sigma-Aldrich and used without further purifi-
cation. Synthetic grade solvents used. Anton Paar monowave
400 microwave used for the coupling reaction. Precoated 0.25
mmE.Merck silica gel 60 F254 thin layer chromatography
plates were used for monitoring the reaction. Instrument from
Sunder industrial product was used in measuring the melting
points, and are uncorrected. NMR spectra were obtained from
Agilent 500 MHz instrument using CDCl3 and DMSO-d6. The
chemical shifts are reported in parts per million (ppm) relative
to internal standard tetramethyl silane (TMS) (0 ppm) and cou-
pling constants in Hz. Perkin-Elmer Lambda 25 UV-Visible
spectrophotometer was used for recording visible absorption
spectra at the fixed concentration on. Emissions were recorded
on Cary Eclipse spectrophotometer at 5 slit width. Quantum
yield was calculated by using fluorescein as standard.
13C NMR (126 MHz, CdCl3) δ 188.66, 162.90, 144.68,
140.02, 130.21, 126.27, 123.90, 123.39, 120.72, 120.49,
118.73, 109.89, 109.67, 56.02.
Synthesis of compound 5b: 4-(9H-carbazol-9-yl)-2-
hydroxybenzaldehyde
4-(9H-Carbazol-9-yl)-2-methoxybenzaldehyde 4
(0.16 mmol) was dissolved in 0.5 g of pyridine. HCl and
heated to 185 °C for 1.5 h. On cooling to room temperature
water was added in the reaction mass and solid precipitated
out was filtered, and dried. The compound obtained was pu-
rified by column chromatography using hexane: ethyl acetate
(10:1) as the eluent to get the pure product.
Synthesis
Yield 50%. Mp. 140-144 °C.
Aldehyde 5a and 5c are synthesized according to a literature
procedure [19, 20]. Aldehyde 5b is synthesized as shown in
synthetic scheme 1.
Compound 2 is obtained by the known method [21].
Methylation is carried out using DMS as follows,
1H NMR (500 MHz, DMSO-d6) δ 11.10 (s, 1H), 10.31 (s,
1H), 8.22 (d, J = 7.8 Hz, 2H), 7.89 (d, J = 8.8 Hz, 1H), 7.53 (d,
J = 8.3 Hz, 2H), 7.45–7.41 (m, 2H), 7.29 (t, J = 7.4 Hz, 2H),
7.22 (dd, J = 4.4, 2.4 Hz, 2H).
1H NMR (500 MHz, DMSO-d6 and D2O) δ 10.29 (s, 1H),
8.21 (d, J = 7.7 Hz, 2H), 7.89 (d, J = 8.8 Hz, 1H), 7.52 (d, J =
8.2 Hz, 2H), 7.45–7.41 (m, 2H), 7.28 (t, J = 7.4 Hz, 2H), 7.22
(dq, J = 3.6, 1.8 Hz, 2H).
Synthesis of compound 3: 4-iodo-2-methoxybenzaldehyde
Under nitrogen, 2-hydroxy-4-iodobenzaldehyde
(0.093 mmol) and sodium carbonate (2.8 mmol) were taken
in RB flask with acetone to it DMS (1.8 mmol) was added.
The reaction mixture was refluxed for 12 h. After completion
of reaction, sodium carbonate was filtered out. The remaining
solution was concentrated on rota evaporator. Yield: 40%.
Mp. 84-86 °C.
13C NMR (125 MHz, DMSO-d6) δ 190.37, 162.37,
144.01, 139.64, 131.04, 126.94, 123.68, 121.65, 121.21,
121.09, 117.49, 114.39, 110.53.
Synthesis of 6a, 6b and 6c (Fig. 1)
Aldehydes 5a-5c (1 mmol) and 7-(diethylamino)-2-oxo-
benzo[d]thiazole-2-carbonitrile (1 mmol) were stirred in absolute
ethanol (5 mL) for a period of 4–5 h in presence of piperidine
(0.01 ml). Reaction was monitored by TLC. On completion of
reaction, solution was mixed with ice water (20 mL) containing
0.025 mL of conc. HCl. The precipitate was filtered and washed
with water, dried under vacuum to give compounds 6a-6c.
1H NMR (500 MHz, CdCl3) δ 10.41 (s, 1H), 7.51 (d, J =
8.1 Hz, 1H), 7.42 (ddd, J = 8.2, 1.3, 0.7 Hz, 1H), 7.37 (d, J =
1.3 Hz, 1H), 3.93 (s, 3H).
Synthesis of compound 4: 4-(9H-carbazol-9-yl)-2-
methoxybenzaldehyde
To a magnetically stirred solution of carbazole (0.89 mmol)
i n 1 , 2 - d i c h l o r o b e n z e n e ( 0 . 2 m l ) 4 - i o d o - 2 -
methoxybenzaldehyde 3 (0.89 mmol), K2CO3 (2.67 mmol),
Cu (0.44 mmol) and 18-crown-6 (1.78 mmol) were added in
10 ml microwave tube. The mixture was degassed with nitro-
gen for about 30 min and then heated at 200 °C for 20 min in
the microwave. The temperature of the reaction mixture was
brought down, filtered and evaporated giving a brown oil which
6a: 3-(Benzo[d]thiazol-2-yl)-7-(diethylamino)-2H-chro-
men-2-one: Red solid, mp obtained 198-202 °C.
Reported 208-210 °C.
Elemental Analysis Found: C, 68.59; H, 5.18; N, 7.98%; mo-
lecular formula C20H18N2O2S requires C, 68.55; H, 5.18; N,
7.99%.

J Fluoresc
Scheme 1 Synthetic scheme for
compound 6a, 6b and 6c
6b: 3-(Benzo[d]thiazol-2-yl)-7-(9H-carbazol-9-yl)-2H-
chromen-2-one: reddish orange solid, mp 194-196 °C.
6c: 3-(Benzo[d]thiazol-2-yl)-7-(diphenylamino)-2H-
chromen-2-one: yellowish orange solid, mp 244-246 °C.
1H NMR (500 MHz, DMSO-d6) δ 9.37 (d, J = 1.8 Hz, 1H),
8.34 (d, J = 8.3 Hz, 1H), 8.27 (d, J = 7.8 Hz, 1H), 8.20 (d, J =
8.0 Hz, 1H), 8.12 (d, J = 8.1 Hz, 1H), 7.89 (s, 1H), 7.79 (d, J =
8.3 Hz, 1H), 7.60 (dd, J = 9.8, 4.7 Hz, 1H), 7.50 (qd, J = 8.1,
1.0 Hz, 1H), 7.35 (t, J = 7.5 Hz, 1H).
1H NMR (500 MHz, CdCl3) δ 8.95 (s, 1H), 8.05 (d,
J = 8.2 Hz, 1H), 7.97 (d, J = 8.1 Hz, 1H), 7.51 (dd, J =
8.2, 7.1 Hz, 1H), 7.47 (d, J = 8.7 Hz, 1H), 7.44–7.35
(m, 5H), 7.23 (d, J = 7.3 Hz, 6H), 6.93 (dd, J = 8.7,
0.9 Hz, 1H), 6.86 (s, 1H).
13C NMR (125 MHz, DMSO-d6) δ 160.18, 159.85,
154.90, 152.39, 141.96, 141.76, 139.84, 136.37, 132.31,
127.30, 127.13, 126.06, 123.78, 123.50, 123.02, 122.75,
121.51, 121.13, 119.56, 118.11, 114.06, 110.47.
13C NMR (125 MHz, CdCl3) δ 160.97, 160.44, 155.81,
152.93, 152.47, 145.44, 141.41, 136.48, 129.92, 126.63,
126.24, 125.87, 124.82, 122.40, 121.64, 121.21, 116.59,
115.39, 112.15, 105.18.
Elemental Analysis Found: C, 75.69; H, 3.63; N, 6.31%; mo-
lecular formula C28H16N2O2S requires C, 75.66; H, 3.63; N,
6.30%.
Elemental Analysis Found: C, 75.36; H, 4.06; N, 6.28%; mo-
lecular formula C28H16N2O2S requires C, 75.32; H, 4.06; N,
6.27%.
Fig. 1 Structure of the
compounds synthesized

J Fluoresc
Result and Discussion
Solvent Polarity Parameter
Photophysical Properties
The Stoke shift and emission correlation of 6a, 6b and 6c were
studied using Lippert Mataga, and Weller’s polarity plots [12,
24]. Lippert Mataga plot gives the relation between Stokes
shift and orientation polarizability, Eq. (5),
Synthesized compounds solvatochromic response was studied
in nine different polarity solvents. Compounds 6a, 6b and 6c
absorb in range of 406 nm - 460 nm and emission in the range
of 480 nm - 598 nm (Fig. 2). In chloroform compounds 6a, 6b
and 6c absorbed at 444 nm, 408 nm and 453 nm respectively
(Table 1). Among three compounds compound 6c having
diphenyl amine substituent shows the highest molar absorp-
tivity. All the compounds show positive solvatochromism in
emission. Compounds 6a, 6b and 6c show emission maxima
in 492 nm, 511 and 544 nm chloroform. Compound 6a shows
twisted intramolecular charge transfer transition (TICT) in po-
lar protic solvents [22, 23]. Emission intensity decreases as
solvent polarity increases which indicates that intramolecular
charge transfer (ICT) state stabilized by polar solvents.
Compound 6b shows larger FWHM 129 nm and Stokes shift
153 nm in acetonitrile. Large stokes shift for 6b and 6c indi-
cates the ground state and excited state geometry could be
different in these compounds. Quantum yield found more in
nonpolar solvents. Compound 6a shows the highest quantum
yield. Radiative and non-radiative decay in different solvent
studied using Strickler Berg Eq. [25].
ꢀ
ꢁ
2
2 μe−μg
Δϑ ¼
f 1 þ constant
ð5Þ
hca30
where Δϑ, μg, μe, h, c and a0 are Stokes shift in cm−1, dipole
moment at the ground state, dipole moment at the excited
state, Planck constant, speed of light and the d Onsagar radii,
f1 is obtained as,
ϵ−1
n2−1
f 1 ¼
−
ð6Þ
2ϵ þ 1 2n2 þ 1
Mac Rae model [25] is the slightly improvised version of
the Lippert Mataga model which considers solute polarizabil-
ity along with the solvent polarizability.
ꢀ
ꢁ
2
2 μe−μg
Δϑ ¼
f 2 þ constant
ð7Þ
hca30
where f2,
ϵ−1
g1
g2
−9
2
ꢀ−3
ꢀ
Av−1
>
Kr ¼ 2:88 x 10 n < υf
∫εd lnυ
ð1Þ
n2−1
f 2 ¼
−
ð8Þ
ð9Þ
ϵ þ 2 n2 þ 1
ꢀ−3
Av−1
are the refractive index and integral.
where n and < υf
>
Weller equation account frequency of emission.
∫ εd lnῡ was determined from the plot of the absorptivity
against the logarithm of emitted energy. Quantum yield Φ
bears the following relation with radiative and non-radiative
rate constants,
2
2μe
ϑf ¼
f 3 þ constant
hca30
where f3,
ϵ−1
Kr
Φ ¼
ð2Þ
n2−1
ðKnr þ KrÞ
f 3 ¼
−
ð10Þ
2ϵ þ 1 4n2 þ 2
The non-radiative rate constant is given by Eq. (3),
Lippert-Mataga equation shows poor linear fit for 6a with
regression 0.18. Compound 6b and 6c show regression 0.94
and 0.90 respectively. The same trend was observed in Mc
Rae plots. Regression observed 0.19, 0.96 and 0.87 for 6a,
6b and 6c. Good regression coefficient observed for Weller
plots are 0.75, 0.97 and 0.88 for 6a, 6b and 6c respectively. It
suggests good charge transfer occur from donor to acceptor in
all compounds (Fig. 3).
ð1−ΦÞ
Knr
¼
ð3Þ
τ
The following equation is used to get oscillator strength f,
4:32 Â 10−9
f ¼
∫εðϑÞdv
ð4Þ
n
where n refractive index and ∫ε(ϑ)dv area of the absorption
coefficient.
Estimation of the Dipole Moment
Fig. S1 and Table S1 shows nonradiative decay found in-
creases from nonpolar to polar solvents. Radiative decay trend
was random with solvent polarity. Fluorescence lifetime
shows decreases in a polar solvent.
The solvatochromic behavior of the compounds can be asso-
ciated with large changes in dipole moment during transitions
between two electronic states S0 and S1. The Stoke shift is

J Fluoresc
Fig. 2 Absorption and emission graphs of 6a, 6b and 6c

J Fluoresc
Table 1 Optical properties of 6a, 6b and 6c
λabs
(nm)
λabs
FWHM
(nm)
FWHM
λems
(nm)
λems
Stokes shift
(nm)
Stokes shift
εmax
ɸ
(cm−1
)
(cm−1
)
(cm−1
)
(cm−1
)
6a
Toluene
Chloroform
DCM
438
444
464
456
442
467
467
457
460
22,831
22,523
21,552
21,930
22,624
21,413
21,413
21,882
21,739
62
68
61
65
62
60
63
63
59
3218
3469
3033
3277
3189
2860
3011
3147
2906
484
492
496
500
491
508
513
502
504
20,661
20,325
20,161
20,000
20,367
19,685
19,493
19,920
19,841
46
48
32
44
49
41
46
45
44
2170
2197
1390
1930
2258
1728
1920
1962
1898
72,180 0.32
67,430 0.24
78,490 0.28
74,480 0.11
74,450 0.09
71,370 0.27
76,370 0.29
68,380 0.05
68,590 0.04
Acetone
EA
DMF
DMSO
MeCN
MeOH
6b
Toluene
418
408
417
406
410
413
408
408
409
23,923
24,510
23,981
24,631
24,390
24,213
24,510
24,510
24,450
77
4610
4073
4232
7507
4205
4678
3756
9324
4628
480
511
531
551
515
559
558
561
558
20,833
19,569
18,832
18,149
19,417
17,889
17,921
17,825
17,921
62
3090
4940
5148
6482
4973
6324
6589
6684
6529
7550
6327
7445
6525
0.21
0.11
0.09
0.07
Chloroform
DCM
70
103
114
145
105
146
150
153
149
72
Acetone
EA
111
69
11,745 0.02
DMF
77
6680
5357
7985
6640
0.01
0.02
0.01
0.01
DMSO
MeCN
MeOH
64
129
76
6c
Toluene
Chloroform
DCM
445
453
454
445
445
451
457
447
452
22,472
22,075
22,026
22,472
22,472
22,173
21,882
22,371
22,124
68
72
69
74
68
71
72
73
69
3454
3563
3405
3832
3501
3568
3578
3754
3466
511
544
561
572
544
586
573
593
598
19,569
18,382
17,825
17,483
18,382
17,065
17,452
16,863
16,722
66
2902
3693
4201
4989
4090
5108
4430
5508
5401
85,860 0.27
91,950 0.19
95,190 0.16
90,690 0.11
83,340 0.02
81,390 0.04
78,280 0.01
116,580 0.03
89,230 0.02
91
107
127
99
Acetone
EA
DMF
135
116
146
146
DMSO
MeCN
MeOH
λ
abs, Absorption wavelength
ems, Emission wavelength
λ
FWHM, Full Width Half Maxima
ε, Molar absorptivity (M−1 cm−1
)
ɸ, Quantum Yield
indicative of a change in dipole moment in the excited state. In
solvatochromic method absorption and emission maxima are
used to estimate the excited state dipole moments. By using
the relations below, a change in dipole moment was obtained
[26, 27].
where m1, m2 are slopes obtained by plot of Sthe tokes shift
Δϑ and ϑa + ϑf /2 versus f4 and f5 respectively.
ꢀ
ꢁ
2
2 μe−μg
m1 ¼
m2 ¼
ð13Þ
ð14Þ
hca30
Δϑ ¼ m1 f 4ðϵ; nÞ þ const
ð11Þ
ð12Þ
ꢀ
ꢁ
2 μ2e−μ2g
ϑa þ ϑf
¼ m2 f 5ðϵ; nÞ þ const
hca30
2

J Fluoresc
Fig. 3 Lippert-mataga (left), Mc Rae (middle) and Weller (right) Plots for 6a, 6b and 6c
ϑa and ϑf are absorption and emission frequency in cm−1
respectively. f4 and f5 Bakshiev [28] and Kawski [29] polarity
functions which given as follows,
Reichardt proposed ENT parameter by following Eq.,
ET ðSolventÞ−ET ðTMSÞ Esolvent−30:7
ETN
¼
¼
ð20Þ
ð21Þ
ꢂ
ꢃ
ET ðWaterÞ−ET ðTMSÞ
32:4
2n2 þ 1 ϵ−1 n2−1
f 4ðϵ; nÞ ¼
f 5ðϵ; nÞ ¼
−
ð15Þ
ð16Þ
n2 þ 2 ϵ−2 n2 þ 2
Stokes shift variation with ETN is expressed as [31],
"
#
ꢂ
ꢃ
2n2 þ 1 ϵ−1 n2−1
3ðn4−1Þ
−
þ
2
2ðn2 þ 2Þ ϵ−2 n2 þ 2
"
#
2ðn2 þ 2Þ
ꢄ
ꢅ ꢄ
ꢅ
2
3
Δμ
ΔμB
aB
a0
Δϑ ¼ 11307:6
ENT þ constant
Ground state dipole moment obtained by the following Eq.
1
ꢄ
ꢅ
m2−m1 hca30
2
μg ¼
μe ¼
ð17Þ
ð18Þ
where ΔμB and a0 are 9D and 6.2 Å respectively for betaine
dye.
From the above equation change in dipole moment, Δμ
obtained as follow,
2
2m1
1
2
ꢄ
ꢅ
m2 þ m1 hca30
2
2m1
vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
The ratio of dipole moments obtained by the following Eq.
u
m3 X 81
u
Δμ ¼ μ −μ ¼
ð22Þ
ꢀ
ꢁ
t
e
g
=
3X 11307:6
μe m2 þ m1
6:2
¼
ð19Þ
a0
μe
m2−m1
À
Á
Reichardt polarity scale ETN:
where m3 is the slope obtained from the linear plot of Stokes
shift (Δϑ) versus ENT function.
Ground state and excited state dipole moment, the ratio (μe/
μg) and excess (Δμ) obtained by the above methods and by
The excited state dipole moment can be obtained by
solvatochromic Stoke shift and ENT parameter correla-
tion [30]

J Fluoresc
Lippert Mataga, Mc Rae are summarized in same Table 2. The
dipole moment difference and ratio are found positive in all
models indicating that dipole moment increases in the excited
state. Dipole moment difference found large by Lippert
Mataga model because this method does not account polariz-
ability of the solute. Compound 6a shows small dipole mo-
ment difference than 6b and 6c, which may be due to the small
effect of 7 substituted substituent on energy levels of mole-
cules. The greater dipole moment in the excited state than
ground state supports the intramolecular charge transfer in
molecules.
describe solute-solvent interactions by using Kamlet-
Taft and Catalan which involve the dipolarity/
polarizability and acidity and basicity parameters of
the solvent [14, 15]. Effect of solvent on ϑabs, ϑemm
and Stokes shift Δϑ is given as,
y ¼ y0 þ aA þ bB þ cC þ dD
ð23Þ
where y0 is property in the gas phase, a, b and c are the
adjustable regression coefficients which indicate the de-
pendency of y to the various solvent parameters (A, B,
C, and D)
The polarity/polarizability (π*), acidity (α) and basicity (β)
parameters of the solvent proposed by Kamlet and Taft in
form of Eq. (24)
Linear Solvation Energy Relationships
and Solvatochromism
As compounds are shown emission solvatochromism,
linear solvation energy relationships (LSERs) are to
*
*
y ¼ y0 þ aαα þ bββ þ cπ π
ð24Þ
Table 2 Dipole moment difference and ratio of 6a, 6b and 6c
Bilot Kawski
Transition Dipole Moment Dipole moment difference (Δμ)
μe/μg
Bakshiev Bilot Kawski
GS
ES
Bakshiev Lippert Mataga Mc Rae Bilot Kawski ETN
6a
6b
6c
Toluene 1.04E-17 1.3E-17 10.260
CHCl3 1.02E-17 1.28E-17 11.157
CH3CN 9.71E-18 1.22E-17 10.257
2.799
2.747
2.610
2.821
4.870
4.781
4.541
4.908
3.052
2.996
2.846
3.076
2.617
2.569
2.441
2.638
1.30 1.29
1.25
12.43
4.29
1.25
1.13
1.32
1.05E-17 1.31E-17 11.091
Acetone
EA
1.01E-17 1.27E-17 10.160
9.84E-18 1.23E-17 9.539
2.725
2.646
2.747
4.743
4.604
4.781
2.972
2.885
2.996
2.549
2.474
2.569
1.23
1.16
1.25
DMF
DMSO 1.02E-17 1.28E-17 10.660
Toluene 7.29E-19 9.06E-18 4.340
CHCl3 7.36E-19 9.16E-18 3.311
CH3CN 7.83E-19 9.75E-18 4.846
8.300
8.386
8.926
8.450
15.774
15.936
16.963
16.058
9.989
8.334
6.18 11.85
6.31
10.091 8.419
10.742 8.962
10.169 8.484
7.15
7.42E-19 9.23E-18 4.059
Acetone
6.41
EA
7.47E-19 9.3E-18 4.992
7.47E-19 9.3E-18 4.050
8.514
8.514
8.407
16.181
16.181
15.977
10.246 8.548
10.246 8.548
10.117 8.441
6.51
6.51
6.34
DMF
DMSO 7.38E-19 9.18E-18 2.912
Toluene 2.12E-18 9.11E-18 13.391
CHCl3 2.14E-18 9.2E-18 17.666
CH3CN 2.08E-18 8.95E-18 22.576
6.992
7.062
6.870
6.922
11.929
12.049
11.721
11.810
7.744
7.822
7.609
7.667
6.992
7.062
6.870
6.922
12.19 5.61
12.43
11.77
2.1E-18 9.02E-18 13.719
Acetone
11.95
EA
2.05E-18 8.8E-18 14.688
2.12E-18 9.11E-18 13.222
6.749
6.992
6.853
11.514
11.929
11.692
7.475
7.744
7.590
6.749
6.992
6.853
11.36
12.19
11.71
DMF
DMSO 2.08E-18 8.93E-18 16.008

J Fluoresc
where α, β and π* are respectively solvent acidity, solvent
basicity and solvent polarity/polarizability.
Further Catalan used four empirical solvent scales and is
expressed in form of Eq. (13),
where, e, h, m, c, f, and ϑ are ca harge on the electron, Planks
constant, the mass of the electron, speed of light, oscillator
strength, and frequency of the absorption.
The degree of delocalization (Cb2) and electronic coupling
matrix (HDA) between charge transfer excited states are impor-
tant expressed as,
y ¼ y0 þ aSASA þ bSBSB þ cSPSP þ dSdPSdP
ð25Þ
vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
where, SA, SB, SP, and SdP are respectively solvent acidity,
solvent basicity, solvent polarizability and solvent
dipolarizability.
0
1
u
u
t
Δμ2ge
Δμ2ge þ 4μ2ge
1
2
C2b ¼
1−
ð28Þ
ð29Þ
@
A
Multilinear regression analysis was done with nine
solvents. From results (Table S2-S7), it is seen that in
emission correlation, the regression coefficient is larger
in the case of Catalan parameter than Kamlet-Taft pa-
rameter. For absorption, emission, and Stokes shift cor-
relation high standard errors were observed. Compound
6a exhibits better regression coefficient for absorption,
emission and Stokes shift (r = 0.94, 0.94, 0.74 respec-
tively for Kamlet Taft parameter). The negative sign of
coefficient c and dexplains that solvent polarizability
and dipolarity are the key factors for the observed
solvatochromic shift. The plot of experimental and pre-
dicted emission values (Fig. 4 and S2) shows good cor-
relation with regression coefficient r = 0.94, 0.95 for 6a,
0.76, 0.88 for 6b and 0.73 and 0.82 for 6c respectively
in Kamlet-Taft and Catalan parameter.
ΔEgeμge
¼
Δμab
ΔEgeμge
Δμ2ge þ 4μ2ge
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
HDA
¼
where, ΔEge vertical excitation energy, Δμge the excess
dipole moments, Δμab depends on the difference in adi-
abatic state dipole moment and the transition dipole
moment.
The assumption here is that the adiabatic states con-
tain three diabatic states viz, ground (S0), local excited
(LE), and charge transfer (CT) state with the transfer of
localized electron. Thus, the extent of charge separation
generated because of the interactions through π-bridge
is expressed as,
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ΔEgeεmaxΔν1=2
RDA ¼ 2:06 Â 10−2
ð30Þ
General Mulliken Hush Analysis (GMH)
HDA
Donor to acceptor charge transfer characteristics is evaluated
by General Mulliken Hush (GMH) analysis given by Coe
et al. [32]. GMH analysis in its two-state analysis of ICT
states,
where, RDA, εmax, Δν1/2, HDA are distance between the
centroids of the donor-acceptor orbitals (Å), absorptivity
(M−1 cm−1), bandwidth (cm−1), electronic coupling ma-
trix (cm)−1, and ΔEge same as above.
Δμ2ab ¼ Δμ2ge þ 4μ2ge
ð26Þ
The transition dipole moment (Δμeg), the degree of
delocalization (Cb2), electron coupling matrix (HDA) and
donor-acceptor separation (RDA) are summarized in
Table 3. It reveals that the values of the degree of
where μge is the transition dipole moment, expressed as,
3e2h
f
delocalization (C2 ) tend towards zero suggested their
μ2ge
¼
Â
ð27Þ
b
8π2mc ϑeg
significant charge transfer occurred. The electron
Fig. 4 Plot of experimental and predicted emission wavenumber

J Fluoresc
Table 3 Linear optical and charge transfer characteristics of 6a, 6b and 6c
a0 a (Å) IACb(M−1 cm−1 fc ΔμCT d Debye Δμge e Debye Δμab f Debye Cb2 g HDA
h
i
)
RDA
6a
6b
6c
Toluene
CHCl3
CH3CN
Acetone
EA
5.71
5.64
5.45
5.74
5.61
5.5
257,195,054
299,999,276
246,348,951
288,660,039
249,930,412
208,508,252
260,373,896
1.129 4.870
1.317 4.781
1.081 4.541
1.267 4.908
1.097 4.743
0.915 4.604
1.143 4.781
10.260
11.157
10.257
11.091
10.160
9.539
21.090
22.820
21.011
22.718
20.867
19.626
21.849
0.385 11,107.014 4.271
0.395 11,011.361 4.294
0.392 10,682.304 4.185
0.392 10,705.925 4.452
0.386 11,016.162 4.334
0.383 10,407.885 4.138
0.391 10,447.188 4.375
DMF
DMSO
5.64
10.660
Toluene
CHCl3
CH3CN
Acetone
EA
5.84
5.88
6.13
5.91
5.94
5.94
5.89
48,213,798
28,747,241
61,593,878
43,419,636
65,032,933
42,508,238
22,247,982
0.212 15.774
0.126 15.936
0.270 16.963
0.191 16.058
0.285 16.181
0.187 16.181
0.098 15.977
4.340
3.311
4.846
4.059
4.992
4.050
2.912
18.004
17.257
19.537
17.993
19.013
18.095
17.005
0.062 5766.552
0.038 4702.106
0.066 6079.585
0.054 5555.919
0.074 6403.540
0.053 5419.831
0.030 4197.719
3.260
3.482
4.577
4.073
3.531
3.306
3.446
DMF
DMSO
Toluene
CHCl3
CH3CN
Acetone
EA
6
431,196,840
737,199,594
1,220,151,600
452,624,837
518,781,700
414,799,279
600,013,407
1.893 11.929
3.236 12.049
5.356 11.721
1.987 11.810
2.277 11.514
1.821 11.929
2.634 11.692
13.391
17.666
22.576
13.719
14.688
13.222
16.008
29.318
37.329
46.649
29.873
31.552
29.010
34.083
0.297 10,263.754 5.181
0.339 10,446.746 5.303
0.374 10,826.816 5.953
0.302 10,320.527 5.578
0.318 10,461.036 5.043
0.294 10,105.729 5.173
0.328 10,277.054 4.963
6.04
5.93
5.96
5.86
6
DMF
DMSO
5.92
a Onsagar Radii
b Integrated Absorption Coefficient
c Oscillator strength
d Difference between ground and excited state dipole moments evaluated using Lippert-Mataga solvent polarity parameter
e Transition dipole moment
f Difference in diabatic state dipole moments and the transition dipole moment
g Degree of delocalization
h Electronic coupling matrix
i Donor-acceptor separation
coupling matrix (HDA) has the trend as 6a > 6c > 6b.
Charge separation (RDA) is more in 6c as compared to
6a and in 6b. It indicates that diphenyl substituent leads
to more charge separation than N,N-diethylamine and
carbazole substituent.
to coumarin ring in an excited state. Same observation
for 6c. Two phenyl rings are perpendicular to the cou-
marin ring (Fig. 5). Single and double bond from C1 to
C30 bond lengths in the ground state, decreases and
increases respectively in an excited state (Table 4)
shows electron delocalization.
Vertical excitation, oscillator strength, orbital contri-
bution are listed in Table 5. Vertical excitation of the
compounds is in good agreement to experimental ab-
sorption values. In all the compounds and solvents
HOMO to LUMO transition takes place which corre-
sponds to π-π* transition.
Electron density distributions of molecules are stud-
ied by frontier molecular orbital approach (FMO). Fig 6
indicates HOMO- LUMO of the compounds in chloro-
form at the ground state. From Fig. 6 it is observed that
in 6a, electron density located all over molecule in
HOMO while in LUMO electron density concentrated
on coumarin ring. In 6b and 6c, electron density located
Time Dependent Density Functional Theory (TD-DFT)
Study
The geometry of the compounds are optimized at the
ground and excited state with global hybrid B3LYP
and basis set 6-31G(d). The bond lengths bond angles
and dihedral angles are summarized in the table. From
the ground state and excited state geometry, it was ob-
served that coumarin ring and benzothiazole ring are
planar, but the geometries differ. In 6a dihedral angle
N, N-diethyl amine group and coumarin benzene ring
change from 2.150 to 23.450. In 6b dihedral angle from
49.21 to 89.78 carbazole ring completely perpendicular

J Fluoresc
Fig. 5 optimized geometry at the ground and excited state
at benzthiazole group and in LUMO it shifted to the
coumarin ring. Trend of HOMO-LUMO gap is, 6a >
6b > 6c.
Spectroscopic Approach
The chromophores having D-A systems possess first excited
state as a low lying CT state. The dominant component of the
first order polarizability, αCT and is therefore obtained by [33],
Nonlinear Optical Properties (NLO)
μ2eg
μ2egλeg
Nonlinear optical properties of the compounds were obtained
from DFT and solvatochromic methods.
αCT ¼ αXX ¼ 2
¼ 2
ð31Þ
Eeg
hc
where X, μeg, Eeg, λeg, h, c are charge transfer direction,
transition dipole moment, ground to excited transition
state energy, the ground to excited state transition ener-
gy, Planck’s constant and velocity of light in vacuum
respectively.
Table 4 Bond length and dihedral angle obtained from the optimized
geometry of 6a, 6b and 6c
6a
6b
6c
GS
ES
GS
ES
GS
ES
The first order polarizability αCT found 46.36 × 10−24 e.s.u,
7.52 × 10−24 e.s.u and 80.23 × 10−24 e.s.u for 6a, 6b and 6c
respectively in toluene.
Bond Length (Å)
C1-N14
1.371 1.376 1.411 1.442 1.395 1.453
1.433 1.431 1.413 1.402 1.423 1.400
1.376 1.377 1.383 1.384 1.378 1.387
1.414 1.427 1.411 1.427 1.414 1.425
1.414 1.403 1.426 1.402 1.418 1.406
1.377 1.430 1.369 1.418 1.375 1.415
1.460 1.414 1.465 1.439 1.462 1.439
1.306 1.342 1.305 1.316 1.306 1.317
1.381 1.354 1.380 1.378 1.380 1.377
1.416 1.434 1.417 1.418 1.416 1.419
1.786 1.800 1.781 1.803 1.784 1.802
1.753 1.754 1.751 1.758 1.753 1.757
C1-C5
Two level model based on Oudar equation was used to
calculate first order hyperpolarizability [34, 35]. βCT
expressed as Eq. (32)
C5-C4
C3-C4
C3-C8
ϑ2egμ2egΔμCT
ϑ2eg−ϑ2L ϑ2eg−4ϑ2L
3
C8-C10
ꢀ
ꢁꢀ
ꢁ
βCT ¼ βxxx
¼
Â
ð32Þ
2h2c2
C10-C35
C35-N40
C40-C29
C29-C30
C35-S41
S41-C30
where ϑeg, ΔμCT, ϑL, are the frequency, excess dipole moment
and frequency of the incident radiation to which βCT value
would be referred to.
If ϑL = 0 under static conditions, the Eq. (20) will be,
Dihedral angle (Degree)
3μ2 ΔμCT
C15-N14-C1-C42 2.151 2.457 49.214 89.782 23.582 81.170
C8-C10-C35-N40 0.058 0.070 0.085 0.006 0.162 0.397
2ðeEgmax
Þ
βCT ¼ βxxx
¼
ð33Þ
2
0.022 0.032 0.081 0.003 0.126 0.482
C11-C10-C35-C41
In toluene the first order hyperpolarizability βCT observed
37.29 × 10−30 e.s.u, 16.68 × 10−30 e.s.u and 160.61 × 10−30
e.s.u for 6a, 6b and 6c respectively.
C25-C22-N14-C1 80.574 80.205 2.919 0.171 50.641 29.830

J Fluoresc
Table 5 Vertical excitation, oscillator strength, orbital contribution and HOMO-LUMO for 6a, 6b and 6c
λabsa
λabsb
fc
Orbital contribution
H-L
6a
6b
6c
Toluene
CHCl3
MeCN
438
444
457
456
421
423
424
424
1.278
1.275
1.256
1.260
0.70446
0.70426
0.70400
0.70403
92 - > 93
92 - > 93
92 - > 93
92 - > 93
Acetone
EA
442
460
467
467
422
423
426
426
1.257
1.251
1.281
1.278
0.70417
0.70399
0.70404
0.70403
92 - > 93
92 - > 93
92 - > 93
92 - > 93
MeOH
DMF
DMSO
Toluene
CHCl3
MeCN
418
408
408
406
473
473
472
472
0.710
0.689
0.654
0.657
0.70439
0.70442
0.70438
0.70439
115 - > 116
115 - > 116
115 - > 116
115 - > 116
Acetone
EA
410
409
413
408
472
472
472
473
0.670
0.651
0.674
0.671
0.70438
0.70437
0.70444
0.70443
115 - > 116
115 - > 116
115 - > 116
115 - > 116
MeOH
DMF
DMSO
Toluene
CHCl3
MeCN
445
453
447
445
457
459
459
460
1.287
1.274
1.249
1.253
0.70394
0.70397
0.70399
0.70399
116 - > 117
116 - > 117
116 - > 117
116 - > 117
Acetone
EA
445
452
451
457
458
459
462
462
1.255
1.245
1.273
1.269
0.70398
0.70399
0.70398
0.70398
116 - > 117
116 - > 117
116 - > 117
116 - > 117
MeOH
DMF
DMSO
a Experimental absorption
b Vertical excitation
f Oscillator strength
Determination of second-order hyperpolarizability γ
γ, second order hyperpolarizability is given as [36],
g, e and é represent ground, first excited and second excited
states respectively.
Here we consider the only two-state model, therefore, Eq.
(34) is simplified as below.
"
#
Δμ2μ2eg μ4eg
μ2egμ2ee
́
γSD ¼ 24
À
þ ∑
ð34Þ
"
#
E3eg
E3eg
E2egEe
Δμ2μ2eg μ4eg
́
γSD ¼ 24
−
ð36Þ
E3eg
E3eg
where Δμ, μeg, Eeg are the excess dipole moment, transition
dipole moment, and excitation energy,
The second order hyperpolarizabilities γSD were found to
be −91.68 × 10−36 e.s.u, −40.20 × 10−36 e.s.u and − 74.30 ×
10−36 e.s.u for 6a-6c in toluene.
hc
λ
E ¼
¼ hcϑ
ð35Þ

J Fluoresc
Computational Approach
By using computational approach values obtained from μ,
α0, β0, γ in various solvents are summarized in Table 6. The
static dipole moment and first hyperpolarizability increase
from non-polar to the polar solvent for all compounds.
Polarizability and second hyperpolarizability decreases for
6a and increases for 6b and 6c from nonpolar to polar solvent.
Highest static dipole moment observed is 11.42 D for 6a in
Static dipole moment μ, linear average polarizability α0, the
anisotropy of polarizability Δα and first and second
hyperpolarizability β0 and γ were calculated by a computa-
tional approach.
μ, α0, Δα, β0, γ expressed as following Eqs. (37–42)
[37–39],
DMSO. 6c shows highest α0, β0 and γ are 88.89 X 10−24
,
207.35 X 10−30 and 1043 X 10−36 e.s.u respectively in
DMSO.
Static dipole moment (μ)
1
2
ꢀ
ꢁ
μ ¼ μ2x þ μ2y þ μ2z
ð37Þ
ð38Þ
ð39Þ
Mean polarizability α0,
Conclusion
À
Á
1
α0 ¼ αxx þ αyy þ αzz
We have examined the effect of different substituents on
linear and nonlinear optical properties. By using
solvatochromic models charge transfer characteristics
were evaluated. Ground and excited state geometry of
the compounds were optimized by B3LYP-6/31G(d).
Spectroscopic properties supported by DFT study.
Effect of the substituent on ground and excited state
dipole moment was evaluated. Dipole moment differ-
ence and ratio were calculated using different models
to support charge transfer characteristics. Compound
with diphenylamine substituent at 7 position shows red
shifted absorption, emission, high donor-acceptor charge
separation, low HOMO-LUMO band gap and large
hyperpolarizability compared to other compounds with
N,N-diethylamine and carbazole substituent. In conclu-
sion compound, 6c is a better candidate as a linear and
nonlinear chromophore.
3
Anisotropy of polarizability Δα,
h
i
1
2
À
Á
À
Á
−1
2
2
2
2
Δα ¼ 2
αxx−αyy þ αyy−αzz þ ðαzz−αxxÞ þ 6αx2x
The total first hyperpolarizability β0,
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
β0 ¼ β2x þ β2y þ β2z
ð40Þ
h
i
1
2
À
Á
À
Á
À
Á
2
2
2
β0
¼
βxxx þ βxyy þ βxzz þ βyyy þ βyxx þ βyzz þ βzxx þ βzyy þ βzzz
ð41Þ
h
ꢀ
ꢁi
1
5
γ ¼
γxxx þ γyyyy þ γzzzz þ 2 γxxyy þ γxxzz þ γyyzz
ð42Þ
Fig. 6 FMO diagram of 6a, 6b and 6c

J Fluoresc
Table 6 Measured and calculated spectroscopic and theoretical (CAM-B3LYP/6–311 + G(d,p)) linear and non-linear optical properties of 6a, 6b and
6c
μa
αCT
α0
βCT
β0
γSD
γg
μβCT
μβ0
b
c
d
e
f
h
i
× 10−24
(e.s.u)
x 10−24
(e.s.u)
× 10−30
(e.s.u)
x 10−30
(e.s.u)
× 10−36
(e.s.u)
x 10−36
(e.s.u)
× 10−48
(e.s.u)
x 10−48
(e.s.u)
Debye
6a
6b
6c
Toluene
CHCl3
MeCN
Acetone
EA
5.20
46.36
55.57
48.34
56.40
45.88
42.73
53.36
81.17
71.55
68.47
67.63
63.53
68.52
68.75
37.29
44.48
37.83
47.60
36.27
34.64
44.92
81.14
−91.68
742.32
436.40
582.41
561.90
465.95
583.59
589.27
193.78
459.64
430.20
534.93
383.02
394.17
512.76
421.63
10.33
11.37
11.24
10.56
11.38
11.42
125.58
168.42
162.55
134.42
168.76
170.38
−140.70
−107.94
−146.61
−91.47
1297.76
1915.16
1826.62
1419.66
1920.25
1944.89
DMF
DMSO
−82.21
−133.50
Toluene
CHCl3
MeCN
Acetone
EA
4.57
5.00
5.52
5.45
5.11
5.53
5.54
7.92
4.50
9.63
6.73
10.27
6.81
3.48
72.87
78.51
85.64
84.67
80.01
85.69
85.96
19.68
11.02
25.14
16.53
25.70
17.17
8.55
62.15
75.42
89.58
87.88
78.71
89.67
90.12
40.20
22.99
53.57
33.82
51.70
36.04
18.07
557.79
684.25
834.92
815.39
717.21
836.02
841.29
89.89
55.09
138.82
90.12
131.30
94.85
47.42
283.86
376.85
494.58
479.02
402.19
495.45
499.60
DMF
DMSO
Toluene
CHCl3
MeCN
Acetone
EA
7.57
8.26
9.06
8.96
8.44
9.07
9.10
80.23
142.15
229.08
84.22
96.53
79.28
117.75
74.71
80.77
88.54
87.49
82.39
88.60
88.89
160.61
292.57
452.59
166.91
186.51
160.84
237.24
123.30
159.14
205.32
199.15
159.14
205.62
207.35
−74.30
620.34
795.60
1032.22
999.59
844.49
1033.70
1043.00
1216.41
2418.01
4101.17
1495.68
1574.34
1458.62
2157.69
933.86
−615.30
−2153.72
−102.73
−200.89
−66.25
1315.28
1860.49
1784.60
1343.32
1864.79
1885.90
DMF
DMSO
−371.56
a Total static dipole moment (by DFT)
b Experimental first order polarizability
c First order polarizability (by DFT)
d Experimental first order hyperpolarizability
e First order hyperpolarizability (by DFT)
f Experimental second order hyperpolarizability
g Second order hyperpolarizability (by DFT)
h Quadratic hyperpolarizability by the solvatochromic method
i Quadratic hyperpolarizability by the computational method
Acknowledgments Author Archana A. Bhagwat is thankful to UGC for
research fellowship.
5. Enculescu M, Evanghelidis A, Enculescu I (2018) White-light
emission of dye-doped polymer submicronic fibers produced by
electrospinning. Polymers 10:737. https://doi.org/10.3390/
polym10070737
6. Finke JH, Richter C, Gothsch T, Kwade A, Büttgenbach S, Müller-
Goymann CC (2014) Coumarin 6 as a fluorescent model drug: how
to identify properties of lipid colloidal drug delivery systems via
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7. Pretor S, Bartels J, Lorenz T, Dahl K, Finke JH, Peterat G,
Krull R, Al-Halhouli AT, Dietzel A, Büttgenbach S,
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