Substituent Dependent Optical Properties of p-phenyl Substituted ethenyl-E-thiophenes

Article abstract of DOI:10.1007/s10895-018-2284-6

Various electron donor and acceptor substituted (NO₂, CN, Cl, H, OCH₃, NH₂) p-phenyl ethenyl-E- thiophenes (1–6) were synthesized and substituent dependent optical properties (dipole moment, transition dipole moment, oscillator strength, optical band gap, hyperpolarizability) were studied using Solvatochromism and Density functional theory. It is shown that thiophene acts as a weak electron donor in presence of an electron withdrawing p-phenyl substituent (NO₂, CN, Cl), whereas thiophene acts as a weak electron acceptor in presence of an electron donating p-phenyl substituent (OCH₃, NH₂). In comparison to ethenyl thiophene 4, the HOMO-LUMO energy band gap is decreased upon increasing the electron donating or electron withdrawing capacity of p-phenyl substituent. From the excited state dipole moment calculation, it is shown that the excited state is highly dipolar for nitro and amino compounds 1 and 6, whereas compounds 2–5 show a non-polar excited state. As compared to the ethenyl thiophene 4, the first hyperpolarizability (β) increases upon substitution either with a strong electron withdrawing or strong electron donating p-phenyl substituent. A large β value is found for p-nitro phenyl ethenyl-E-thiophene and p-amino phenyl ethenyl-E- thiophene. Overall, these studies provide useful information in understanding the optical properties of phenyl and heterocyclic based ethenyl systems.

Full text of DOI:10.1007/s10895-018-2284-6

Journal of Fluorescence
https://doi.org/10.1007/s10895-018-2284-6
ORIGINAL ARTICLE
Substituent Dependent Optical Properties of p-phenyl
Substituted ethenyl-E-thiophenes
Naresh Kumar1 & Mahalingavelar Paramasivam2 & Jagdeep Kumar1 & Anamika Gusain1 & Prasanta Kumar Hota1
Received: 7 May 2018 /Accepted: 20 August 2018
#
Springer Science+Business Media, LLC, part of Springer Nature 2018
Abstract
Various electron donor and acceptor substituted (NO2, CN, Cl, H, OCH3, NH2) p-phenyl ethenyl-E- thiophenes (1–6) were
synthesized and substituent dependent optical properties (dipole moment, transition dipole moment, oscillator strength, optical
band gap, hyperpolarizability) were studied using Solvatochromism and Density functional theory. It is shown that thiophene acts
as a weak electron donor in presence of an electron withdrawing p-phenyl substituent (NO2, CN, Cl), whereas thiophene acts as a
weak electron acceptor in presence of an electron donating p-phenyl substituent (OCH3, NH2). In comparison to ethenyl
thiophene 4, the HOMO-LUMO energy band gap is decreased upon increasing the electron donating or electron withdrawing
capacity of p-phenyl substituent. From the excited state dipole moment calculation, it is shown that the excited state is highly
dipolar for nitro and amino compounds 1 and 6, whereas compounds 2–5 show a non-polar excited state. As compared to the
ethenyl thiophene 4, the first hyperpolarizability (β) increases upon substitution either with a strong electron withdrawing or
strong electron donating p-phenyl substituent. A large β value is found for p-nitro phenyl ethenyl-E-thiophene and p-amino
phenyl ethenyl-E- thiophene. Overall, these studies provide useful information in understanding the optical properties of phenyl
and heterocyclic based ethenyl systems.
.
.
.
.
.
.
Keywords Absorption Fluorescence Dipole moment Transition dipole moment Oscillator strength Hyperpolarizability
.
Optical band gap Density functional theory
Introduction
(DSSC) [13–19], nonlinear optics (NLO) [7, 11, 12, 20–29],
etc. In particular, the nonlinear optical properties of organic
molecules with varied shape and size (e.g. dipolar, quadrupo-
lar, octupolar) were developed and extensively studies using
Electric field induced second harmonic (EFISH) generation
and Hyper Rayleigh scattering (HRS) methods [20–29]. The
second-order NLO response is expressed in terms of the first
hyperpolarizability (β). A higher β can be achieved through
extended conjugation in electron donor-acceptor substituted
organic molecules [7, 8]. In EFISH method, however, the β
of a dipolar molecule is calculated in solution state, which has
a known ground state dipole moment [20, 21, 25]. The main
drawback of HRS method is the interference of the molecular
fluorescence during the measurement of β [23, 28]. Hence, an
alternative solvatochromic method is described for many
small dipolar organic molecules [30–36]. Though β obtained
through solvatochromism is based on several approximation,
it will provide an approximate value, but the information ob-
tained through solvatochromism is more quantitative, and
comparable to the data obtained through other methods such
as EFISH, HRS and computational based density functional
Donor-acceptor conjugated molecules have wide range of ap-
plications in chemistry, biology [1–6] and organic electronics
[7, 8] such as memories and switches [9, 10], organic light-
emitting diodes (OLEDS) [11, 12], dye-sensitized solar cell
Naresh Kumar and Mahalingavelar Paramasivam are joint first author and
contributed equally to this work.
Electronic supplementary material The online version of this article
(https://doi.org/10.1007/s10895-018-2284-6) contains supplementary
material, which is available to authorized users.
* Prasanta Kumar Hota
p.hota@hnbgu.ac.in
1
Department of Chemistry, School of Sciences, Hemvati Nandan
Bahuguna Garhwal University (A Central University), Srinagar
Campus, Pauri (Garhwal), Uttarakhand 246 174, India
2
Department of Advanced Materials Chemistry, College of Sciences
and Technology, Korea University, 2511 Sejong-ro, Jochiwon,
Sejong 339700, South Korea

J Fluoresc
theory (DFT) [31, 34]. In order to gain more insight into the
solvatochromism method, we have synthesized various elec-
tron donor-acceptor substituted ethenyl thiophenes (1–6) and
studied the substituent dependent β value using physical pa-
rameters like excited state dipole moment, transition dipole
moment and oscillator strength. The advantage of these mol-
ecules is due to their substituent induced varied absorption,
fluorescence, extinction coefficient, HOMO-LUMO energy
gap, excited state dipole moment and transition energy that
provide valuable information in designing molecule with var-
ied optical properties. The ethenyls (1–6) with substituent
(NO2, CN, Cl, H, OCH3, NH2) in decreasing order of electron
attracting capacity are shown in (Fig. 1). Calculation of excit-
ed state dipole moment suggest that excited state of nitro and
amino substituted ethenyls (1 and 6) is highly dipolar and
exhibit charge transfer excited state. A higher β value is found
for these ethenyls bearing a strong electron withdrawing or a
strong electron donating substituent. On the other hand, excit-
ed state of compounds 2–5 is non-polar and exhibit a lower β
value. Overall, these studies provides useful information in
understanding the excited state properties of p-phenyl
substituted heterocyclic ethenyl systems.
JEOL 500 MHz FTNMR instruments. GC-MS spectra were
recorded on a GCD 1800A Hewlett packard GC-mass spec-
trometer. CHNS analyses were performed on a Theoquest CE
instrument 1112 series CHNS auto analyzer. Melting points
were determined on a Lab India make melting point apparatus.
The starting materials and reagents for the synthesis were
procured from the local suppliers (M/s. Sisco Research
Laboratory, E. Merck, Sigma-Aldrich, Himedia Chemicals).
UV grade solvents were used for spectroscopic studies.
Absorption and Fluorescence Studies
For all absorption and fluorescence measurements, 0.5 ×
10−5 M solution of compounds were prepared in different
solvents and recorded using a 1 cm × 1 cm, light path length
quartz cuvette. Fluorescence spectra were recorded by excit-
ing the sample at their absorption maximum (λabs max). The
ground and excited state energy (E) of the compounds is cal-
culated from their corresponding absorption (λabs max) and
fluorescence wavelength maximum (λem max) and using the
Eqs. 1a and 1b.
where; Eabs ¼ ðhc=λabs maxÞ ¼ ð1:24=λabs maxÞ ðin KeVÞ ð1aÞ
Eem ¼ ðhc=λem maxÞ ¼ ð1:24=λem maxÞ ðin KevÞ
ð1bÞ
Experimental Section
The energy band gap (Eg) of 1–6 is obtained from the
intersection wavelength of excitation and fluorescence spec-
trum of the compounds in methanol.
Materials and Analytical Equipments
Compounds were synthesized using Radleys make, Carousel
6 plus reaction station. Synthetic compounds were character-
Dipole Moment Calculation
1
ized by UV-Vis, FTIR, H and 13C NMR, GC-MS spectros-
copy. The absorption spectra were measured on a Perkin
Elmer Lambda 25 UV-Vis spectrometer and the fluorescence
spectra were recorded on a Perkin Elmer LS-55 fluorescence
spectrophotometer using a red PMT detector system. FTIR
spectra in KBr discs were recorded on a Impact Nicolet-400
spectrophotometer. The 1H and 13C NMR spectra in CDCl3 as
solvent and TMS as an internal standard were recorded on a
Change of excited state dipole moment is calculated from the
absorption and fluorescence spectral data of the compound
using McRay Eq. 2 [33, 37].
À
Á
É
νabs−νem ¼ ðδabs þ δemÞ þ 2Δμ2=hca3 Fðε; nÞ ¼ constant þ m Fðε; nÞ
Â
À
Á À
ÁÃ
À
Á
Fðε; nÞ ¼ ðε−1Þ=ðε þ 2Þ− n2−1 = n2 þ 2 ; m ¼ 2Δμ2=hca3
ð2Þ
Fig. 1 Structure of ethenyl
thiophenes 1–6
H
NO2
4
1
2
OCH3
CN
5
NH2
Cl
6
3

J Fluoresc
where, νabs is absorption maximum wave number, νem fluores-
cence maximum wave number, νabs-νem is the Stokes’shift, δabs
and δem are difference in vibrational energy of molecule (in
cm−1) in excited and ground state for absorption and emission
respectively, μe and μg are the excited state and ground state
dipole moments respectively, μe-μg = Δμ is the change in dipole
moment, h is the Planck constant (6.62 × 10−34 joule sec), c is the
velocity of light in vacuum (3 × 108 m/s), ε is the relative per-
mittivity (i.e. dielectric constant) and n is the refractive index of
the solvent [38, 39], The Onsagar cavity radius (a) can be cal-
culated using Eq. 3 as described elsewhere [40].
The minimized geometry is further confirmed by vibrational
analysis, resulting in no imaginary frequencies. This geometry
is used as the input for further calculations to obtain the frontier
molecular orbitals (FMOs), single-point TDDFT studies (first
15 vertical singlet–singlet transitions) to obtain the UV-Vis
spectra using non-local B3LYP functional and 6–31G (d, p)
basis set. The effect of solvent polarities have not been taken
into account. The software GaussSum 2.2.5 was employed to
simulate the major portion of the absorption spectrum and to
interpret the nature of transitions.
Synthesis of Compounds 1–6
1=3
a ¼ ð3M=4πδNÞ
;
ð3Þ
The substituted p-phenyl ethenyl-E-thiophenes (1 and 3–5)
were synthesized by the condensation of p-substituted phenyl
acetic acid with the corresponding 2-formylthiophene (2:1 M
ratio) (for 1 and 3–5) in presence of pyridine-piperidine mixture
as described earlier [40, 46], e.g. for obtaining compound 1, the
typical synthetic protocol is as follows: 2-Formyl thiophene
(0.93 mL, 0.01 mol) was taken in freshly distilled pyridine
(10 mL), piperidine (0.6 mL) and p-nitrophenyl acetic acid
(3.62 g, 0.02 mol) in a round bottom flask fitted with a reflux
condenser. The reaction mixture was heated at 100 °C for 8 h.
The progress of the reaction was monitored by thin layer chro-
matography. The reaction mixture was cooled to room temper-
ature, poured in ice-cold water and treated with 100 mL of
diluted hydrochloric acid to remove excess of pyridine from
the reaction mixture. A brick red colored product was extracted
in dichloromethane and purified by column chromatography
using 2–5% ethyl acetate in petroleum ether as the eluting sol-
vent, when the desired compound was obtained in 31% yield.
The cyano compound 2 was prepared by the condensation
of corresponding 2-formylthiophene with p-cyano benzyl
phosphonate (NC–C6H4–CH2–P:O(OEt)2) (1:2 M ratio) using
Emmons-Horner reaction condition [47, 48] as shown in
Scheme 1. For this purpose, p-Cyano benzyl phosphonate
was prepared by refluxing p-cyano benzyl bromide
(0.01 mol, 1.96 g) and triethyl phosphite (0.02 mol, 3.2 mL)
for a period of 3 h at 150 °C in situ. The p-cyano benzyl
phosphonate was then syringed to the suspension of sodium
methoxide (0.02 mol, 1.08 g) solution in freshly distilled dry
dimethylformamide (15 mL) at 0 °C. In the second part of this
experiment, the 2-formyl thiophene (0.01 mol, 0.9 mL) in
10 mL dry dimethylformamide was syringed slowly into the
reaction flask and continued stirring at 0 °C for 2 h. The
progress of the reaction was monitored through thin layer
chromatography. The crude product was extracted with di-
chloromethane, and purified by column chromatography
using 2–5% ethyl acetate in petroleum ether (60–80 °C).
Compound 6 was prepared through reduction reaction of 1.
For this purpose, the ethanolic solution of ethenyl thiophene 1
is refluxed in presence of aqueous ferrous sulfate and ammo-
nia solution at 100 °C for 3 h. The products were purified by
where, M = Molecular weight of molecule, N = Avogadro num-
ber = 6.023 × 1023, δ = Molecular density of molecule.
First Hyperpolarizability Calculation
The first hyperpolarizability (β) is related to second order non-
linear optical properties of molecule. The solvatochromism
method is used for obtaining β in methanol using Ouder for-
mula [41] (Eqs. 4 and 5) as reported elsewhere [30–36].
À
Á
ðβÞ ¼ 3=2h2c2
n
o
À Á
ÈÀ
ÁÀ
ÁÉ
2
2
Â
ðvabsÞ rg ðΔμÞ = vabs2−v2 vabs2−4v2 ð4Þ
where, vabs: absorption maximum wave number and; v: inci-
dent reference wave number (1064 nm of Nd: YAG laser
source) to which the β value referred; rg: Transition dipole
moment;
À Á
ÂÀ
Á À
ÁÃ
2
rg
¼
3e2h = 8π2mc  ðf =vabs
Þ
¼ 2:13 Â 10−30 Â ðf =vabs
Þ
ð5Þ
and ƒ: Oscillator strength, ƒ = 4.32 × 10−9 ∫ε (ν)dν which is
obtained from the area under the absorption band (ε vs wave
number), ε: molar absorption coefficient [42].
Computed Parameters Using Density Functional
Theory (DFT)
The ground state dipole moment is computed using the Density
functional theory (DFT) in gas phase and in methanol. For this
purpose, Gaussian 09 ab initio quantum chemical software
package is used. The Time-dependent density functional theory
(TDDFT) [43, 44] has been used to calculate the absorption
wavelength maximum (λabs max), the vertical excitation energy
and oscillator strength (ƒ) on the ground state optimized geom-
etries at 6–31 g(2d,p) level [45]. The geometry optimization of
the molecule is obtained using non-local functional B3LYP
with 6–31G (d, p) basis set without any symmetry constraints.

J Fluoresc
Scheme 1 Synthetic routes and
reaction conditions for obtaining
ethenyl thiophenes. (a) Pyridine,
Piperidine, 100 °C, reflux 8 h; (b)
N-Bromo succinimide, CCl4,
Benzoyl peroxide, 100 °C, reflux
3 h; (c) Triethyl phosphite,
150 °C, reflux 3 h; (d) Sodium
hydride, DMF, 0 °C, 1 h; (e)
FeSO4, aqu. NH3, Ethanol,
100 °C, 3 h
R
R
CHO
a
COOH
1: R = NO2, 3: R = Cl,
4: R = H, 5: R = OCH3
CN
c
CN
CN
b
Br
OC2H5
OC2H5
CHO
d
CN
2
6
NH2
NO2
e
1
column chromatography using 2–10% ethyl acetate in petro-
leum ether (60–80 °C) as the eluting solvent. All compounds
1–6 show satisfactory physico-chemical data (UV-Vis, FTIR,
1H and 13C NMR, GC-MS and CHNS analysis). The detail of
characterization data is provided as a supporting material.
summarized in Table 1 and Table S1 respectively. The absorp-
tion and fluorescence spectra of 1–6 are shown in Figs. 2 and 3
and in Supporting Figures S1-S2. All these ethenyl thiophenes
show absorption coefficient in between 21,000 and
44,500 l mol−1 cm−1, in methanol, which suggest, a π- π*
nature of electronic transition. In case of a strong electron
withdrawing nitro substituent (in case of 1) and strong elec-
tron donating amino substituent (in case of 6), the absorption
maximum (λabs max) is moderately red shifted by 15 and 17 nm
respectively on increasing the solvent polarity from n-hexane
to dimethylformamide (DMF). On the other hand, the λabs max
of ethene 2–5 is moderately sensitive to solvent polarity and a
red shift of only 3–6 nm is observed. In contrast to λabs max, the
fluorescence wavelength maximum (λem max) of 1 and 6 is
Results and Discussion
Absorption and Fluorescence Studies of Ethenyl
Thiophene 1–6
The absorption and fluorescence data of compounds 1–6 in
methanol and in different solvents of varying polarity are
Table 1 Solvatochromism data of ethenyl thiophenes (1–6) in methanol
Compound λabs max λem max Extinction coefficient Oscillator
S0-S1
(rg) Debye (Δμ)
(μe)
Hyper-polarizability
(nm)
(nm)
(ε) (Mol−1L. cm−1
)
strength (ƒ) (ΔE) nm (eV)
Debye Debye (β) (10−30) Esu−1.cm5
1
2
3
4
5
6
372
340
327
321
332
343
614
412
384
385
387
435
21,500
27,500
21,000
30,400
34,500
39,000
0.639
0.844
0.581
0.458
0.670
0.716
394 (3.14)
374 (3.31)
355 (3.49)
349 (3.55)
360 (3.44)
388 (3.19)
7.11
7.81
6.36
5.59
6.88
7.23
13.85
2.54
2.47
4.41
1.91
6.52
20.46
8.63
4.98
5.08
3.1
82
12
7
9
7
7.2
29
Onsagar cavity radius Ba^ (in Ao ); 1: 4.27; 2: 4.22; 3: 4.20; 4: 4.09; 5: 4.29; 6: 4.18; μe = Δμ + μg
(DFT)
Absorption wavelength maximum (λabs max), fluorescence wavelength maximum (λem max), oscillator strength (ƒ), S0-S1 transition state energy (ΔE, eV)
and transient dipole moment between ground and excited states (rg)

J Fluoresc
45000
The singlet state energy of 1–6 is calculated from their ab-
sorption and fluorescence emission maximum (Table S2).
Correlation of Hammett substituent constant [51] with the
ground and excitation energy provides interesting information
(Fig. 4a). The absorption energy of compound 2–6 is 0.28–
0.43 eV higher than 1, whereas, the emission state energy of
2–6 is 0.84–1.21 eV higher than compound 1. The energy band
gap of first singlet excited state energy for 1–6 is obtained in
between 3.14–3.55 eV and the order is 4 > 3 > 2 > 1 and 4 >
5 > 6. Interestingly, thiophene is acting as a weak electron do-
nor in presence of an electron withdrawing p-phenyl substituent
(NO2, CN, Cl in case of 1–3), whereas thiophene acts as a weak
electron acceptor in presence of an electron donating p-phenyl
substitution (OCH3, NH2 in case of 5–6) (Fig. 4b). In order to
understand the effect of substituent and solvent polarity on the
nature of excited state, the Stokes’ shift (va-vf) vs solvent
polarity parameter, F(ε,n) (McRay Eq. 2), (Fig. 5) is drawn
1
2
3
4
5
6
b
36000
27000
18000
9000
0
e
f
c
a
d
250
290
330
370
410
450
Wavelength (nm)
Fig. 2 Absorption spectra of compounds in methanol, (a) 1, (b) 2, (c) 3,
(d) 4, (e) 5, (f) 6
significantly red shifted by 156 and 45 nm respectively on
increasing the solvent polarity from n-hexane to DMF. For
2–5, λem max is moderately red shifted by 6, 8, 13 and 9 nm
respectively. The excited state of 1 and 6 is further stabilized in
protic polar solvent, methanol (for 1) and in aprotic polar
solvent, DMF (for 6) and a λem max of 614 nm and 444 nm
is observed for 1 and 6 respectively. A net stabilization of the
excited state for 1 and 6, suggest a type of coulomb interaction
between the charged solute and solvent molecules [49, 50]. A
red-shifted λem max in ethenyl thiophene is due to electron
delocalization from electron donor-to-the electron acceptor
group. It is shown that strong electron acceptor -NO2 group
(σp + 0.81, for 1) or strong electron donor -NH2 group (σp −
0.57, for 6) at the p- phenyl ring induces large electron delo-
calization in the excited state. The presence of weak electron
acceptor group -Cl (σp + 0.24, in case of 3), -H (σp 0.0 in case
of 4) and weak electron donor group -OCH3, (σp - 0.28, in
case of 5), however, do not facilitate the electron delocaliza-
tion as compared to 1 and 6.
(a)
4
3.5
3
Absorption
Emission
2.5
2
1.5
1
1
2
3
4
5
6
Compound
(b)
4
Solvatochromism
DFT
350
1
c
2
3
4
5
6
3.5
3
280
210
140
70
e
2.5
2
b
a
d
1.5
1
f
0
1
2
3
4
5
6
325
405
485
565
645
725
Compound
Wavelength (nm)
Fig. 4 Comparison of (a) Absorption and fluorescence excited state
energy of 1–6 in methanol, (b) First excited state energy band gap of 1–
6 in methanol
Fig. 3 Fluorescence spectra of compounds in methanol, (a) 1, (b) 2, (c) 3,
(d) 4, (e) 5, (f) 6

J Fluoresc
12000
10000
8000
6000
4000
2000
0
1
2
3
4
5
6
excited state in 1 and 6 could be due to twist over the single
bond attached to p-phenyl ring as suggested in other donor-
acceptor substituted ethenyl systems [46, 52–60].
Density Functional Theory (DFT) Studies on 1–6
Absorption and Excited State Properties
The DFT computed data for 1–6 is shown in Table 2. The
trend of DFT data with respect to the substituent is comparable
to the experimental results. For compounds with strong elec-
tron withdrawing substituents, it is shown that the ground state
is stabilized more due to the efficient delocalization of π elec-
tron, whereas, there is a hindrance in delocalization of π elec-
tron, in presence of an electron donating methoxy substituent.
The stabilization in the ground state is due to mutual pushing
and pulling of π electrons from both the ends [e.g. from thio-
phene to phenyl in case of electron withdrawing p-phenyl-
substitution, NO2, CN, Cl, H (1–4) and phenyl to thiophene
in case of electron donating p-phenyl substitution, NH2,
OCH3(5–6). This leads to the stabilization of the ground state
energy. The optimized geometry of these molecules is shown
to have planar structure.
In order to understand the substituent induced charge trans-
fer character of the molecule, the HOMO-LUMO energy gap
is computed using Frontier Molecular Orbitals,. All these
compounds show one intense band with λabs max between
300 and 400 nm (Table 2, Fig. 6). Compound 1 exhibits lon-
gest λabs max of 358 nm and 4 has the lowest λabs max of
316 nm. This absorption is due to the HOMO→LUMO
(So → S1) transition with a contribution greater than 94%.
For other compounds, the HOMO→LUMO contribution is
greater than 97%. As shown in the solvatochromism experi-
mental data, these absorptions are due to π→π* transition.
Similarly, as compared to the absorption spectrum, the
fluorescence spectrum of 1–6 is also followed the similar
trend. Compound 1 shows largest λem max at 588 nm, while
4 has the lowest λem max at 378 nm. Upon p-phenyl substitu-
tion, either electron donating (OCH3, NH2) or electron
-0.1
0.05
0.2
0.35
0.5
0.65
0.8
F (ε,n)
Fig. 5 McRay plot, Stokes’ shift vs F(ε,n) of 1–6 in n-hexane, 1,4-
dioxane, THF, MeOH, AcCN and DMF
for 1–6. On increasing the solvent polarity, the Stokes’ shift
values are increased linearly with good correlation factor (in
case of ethenyls, 1 and 6). Compound, 2–5, however, deviate
from linearity in solvents, methanol (for 2), 1,4-dioxan and
acetonitrile (for 4 and 5) and n-hexane and acetonitrile (for
3). In order to get a good correlation factor, these solvents were
excluded from our calculation for respective compounds and
the following correlation are obtained (1: m1 = 10,042, R =
0.98; 2: m1 = 351, R = 0.86; 3: m1 = 337, R = 0.80; 4: m1 =
1157, R = 0.83; 5: m1 = 144, R = 0.88; 6: m1 = 2368, R =
0.88). The calculated change in excited state dipole moment
(Δμ)(in D: Debye) is shown in Table 1 and Table S3. A large
change in the excited state dipole moment is observed for 1
(13.85 D) as compared to 2 (2.54 D), 3 (2.47 D), 4 (4.41 D), 5
(1.91 D) and 6 (6.52 D). Similarly, the ground state dipole
moment (μg) is computed for 1–6 using TDDFT. The μg for
3–5 is in between 0.93–2.79 D, whereas μg for 1 and 2 is 6.38–
6.91 D (Table 2). Thus, the large solvatochromic shift in 1 and
6 is due to change in the excited state dipole moment (13.85
Debye for 1 and 6.52 D for 6). This also suggests the involve-
ment of charge transfer (CT) excited state in 1 and 6. On the
other hand, the excited state of ethenyl thiophenes 2–5, bearing
a weak electron acceptor or donor group (-CN, -Cl, -H, -OCH3)
is less polar (1.91–4.41 D). The formation of charge transfer
Table 2 TDDFT data at the B3LYP/6–31 g (d,p) level for ethenyl thiophenes (1–6) in methanol
Compound
λabs max (nm)
λem max (nm)
ƒ
Major transition
HOMO (eV)
LUMO (eV)
(ΔE) (eV)
(μg) Debye
(HOMO→LUMO) (%)
1
2
3
4
5
6
358
336
321
316
323
318
588
392
382
378
383
419
1.15
1.23
1.12
1.01
1.16
1.07
94
97
98
98
98
98
5.92
5.80
5.50
5.36
5.07
5.30
2.60
2.18
1.70
1.50
1.31
1.47
3.32
3.62
3.80
3.86
3.76
3.83
6.61
6.09
2.51
0.67
1.19
0.68
Absorption wavelength maximum (λabs max), electronic transition energy (ΔE, eV), Oscillator strength (ƒ) and ground state dipole moment (μg)

J Fluoresc
As compared to reference compound, ethenyl thiophene
(4), the HOMO-LUMO band gap is decreased upon increas-
ing the electron donating or electron withdrawing substitution
on the p-phenyl ring (Fig. 7). The energy band gap is de-
creased by 0.54 eV from phenyl ethenyl thiophene (4) to p-
nitro phenyl ethenyl thiophene (1)[3.86 eV (for 4) to 3.32 eV
(for 1)]. Similarly, the energy band gap is slightly decreased
by 0.10 ev from phenyl ethenyl thiophene (4) to p-amino
phenyl ethenyl thiophene (6)[3.86 ev (for 4) to 3.76 ev (for
6)]. This indicates that HOMO and LUMO energy of ethenyl
thiophene, is progressively stabilized on increasing the elec-
tron withdrawing capacity of phenyl ring or electron donating
capacity of phenyl ring. This is due to the delocalization of π
electron from thiophene to the p-phenyl ring (for NO2, CN,
Cl) or p-phenyl to thiophene ring (for NH2). In case of
methoxy substituted ethenyl (5), a more destabilized HOMO
and LUMO energy level could be due to the hindrance in
effective π conjugation.
First Hyperpolarizability
The first hyperpolarizability (β) is related to the second order
non-linear optical properties of the molecule. Thus, β of the
compounds 1–6 is calculated in solvent, methanol using
solvatochromism method. In general, the non linear optical
properties of the molecule is influenced by the delocalization
of π electron. Thus, the effect of substitution on the β of the
ethenyl thiophenes (1–6) is analyzed using absorption, oscil-
lator strength (ƒ), dipole moment (Δμ) and transition dipole
moment (rg) of molecules as described in the Experimental
Section.
Fig. 6 a TDDFT computed absorption spectra of compounds 1–6 in
methanol. b TDDFT computed fluorescence spectra of compounds 1–6
in methanol
withdrawing (Cl, CN, NO2), the fluorescence λem max is red
shifted with respect to reference compound 4.
It is shown that as compared to the reference ethenyl com-
pound (4), β increases upon substitution either in presence of
Fig. 7 TDDFT computed HOMO-LUMO energy levels of 1–6 in methanol

J Fluoresc
90
75
60
45
30
15
0
NLO response as compared to the reference ethenyl thiophene
4. Similarly, p-amino phenyl ethenyl thiophene shows 5 time
better NLO response as compared to the reference ethenyl
thiophene 4. Upon p-phenyl substitution, the β of the ethenyl
is increased with increasing the order of electron withdrawing
nature of phenyl ring (1–3)(-NO2, -CN, -Cl). Similarly β is
increased with increasing the order of electron donating nature
of phenyl ring (-OCH3, -NH2)(5–6). Compound 1 shows larg-
est β as compared to other ethenyls. For ethenyl thiophene,
the order of β is 1 > 2 > 3 ~ 4 and 6 > 4 ~ 5. Overall, these
studies provide useful information in altering the optical prop-
erties of heterocyclic and phenyl based ethenyl systems. It also
helps in designing and developing material for other electronic
applications.
1
0.81
2
3
0.24
4
0.00
5
-0.28
6
-0.57
0.70
σ:
Compounds with Hammett substituent constant (σ)
Fig. 8 Substituent dependent first hyperpolarizability (βCT) of 1–6 in
methanol, obtained through solvatochromism method
Acknowledgements NK, JK and PKH are grateful to University Grants
Commission, New Delhi, India for providing research fellowship and
research grant (F.30-62/2014-BSR). Authors thankful to Sophisticated
Analytical Instrument Facility (SAIF), IIT Bombay for GC-MS and
an electron withdrawing or electron donating p-phenyl substi-
tution (Table 1, Fig. 8). Compound 1 exhibits large β value,
which has a strong electron withdrawing p-nitro phenyl sub-
stituent (Table 1). Similarly, as compared to compound 4, β of
other ethenyl increases with increasing the electron donating
capability of p-phenyl substituent. The order of β obtained in
ethenyl thiophenes is 1 > 2 > 3 and 6 > 4 > 5 (β; 1: 82, 2: 12, 3:
7, 4: 9, 5: 7, 6: 29) (in 10−30 esu−1 cm5). Compounds with
weak electron donating or weak electron withdrawing substit-
uent (3–5) exhibit a lower β value as compared to 1 and 6.
This indicates that the molecule with more polar excited state
exhibits large β value. Overall, a large delocalization of π
electrons leads to a lower optical band gap and hence, 1 and
6 exhibit better NLO property.
Advanced Material Research Center (AMRC), IIT Mandi for 1H & 13
NMR analysis.
C
Author Contributions PKH, NK, JK and AG designed and carried out the
synthesis and solvatochromism experiments. PKH and MP designed the
computational work and MP carried out the DFT calculation.
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