Optical properties of 3-substituted indoles

Article abstract of DOI:10.1039/d0ra05405d

The optical properties of various donor or acceptor p-phenyl substituted ethenyl indoles were studied in solvents of varying polarity using absorption, fluorescence and TDDFT methods. Ethenyl indole exhibits non-linear optical properties (NLO) in a substi

Full text of DOI:10.1039/d0ra05405d

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Optical properties of 3-substituted indoles†
*
Jagdeep Kumar, Naresh Kumar and Prasanta Kumar Hota
Cite this: RSC Adv., 2020, 10, 28213
The optical properties of various donor or acceptor p-phenyl substituted ethenyl indoles were studied in
solvents of varying polarity using absorption, fluorescence and TDDFT methods. Ethenyl indole exhibits
non-linear optical properties (NLO) in a substituent dependent manner. Compound with a strong
electron-attracting substituent, shows large NLO properties with charge transfer behavior, whereas
ethenyls with moderate electron withdrawing or electron donating substituent exhibit lower NLO
properties with non polar excited state. A highly dipolar excited state for p-nitro phenyl substituted
ethenyl indoles (me: 18.2–27.1 debye; Dm: 9.4–17.8 debye) is observed as compared to other ethenyls (me:
6.6–9.5 debye; Dm: 4.2–6.2 debye). From TDDFT study, it is shown that the HOMO–LUMO energy of
ethenyl is increased with increasing the electron donating ability of the p-phenyl substitution. The optical
band gap of ethenyl 3 without substitution, is decreased upon p-phenyl substitution either with an
electron withdrawing (Cl, NO2) or an electron donating (OCH3, OH, NH2) substituent. The compound
with a strong electron accepting, p-nitrophenyl ethenyl indole 1 shows 12 times better NLO response as
compared to the reference ethenyl indole 3 (b: 1: 115 ꢀ 10ꢁ30 esuꢁ1 cm5, 3: 9 ꢀ 10ꢁ30 esuꢁ1 cm5).
Ethenyls 2–6 bearing a weak or moderately electron withdrawing or electron accepting substituent,
exhibit lower NLO response. The b of ethenyl is increased with increasing the order of electron
withdrawing nature of phenyl ring. Overall, a correlation of b with the optical band gap, ground state
dipole moment, % of charge transfer in the ground and excited state is found.
Received 20th June 2020
Accepted 23rd July 2020
DOI: 10.1039/d0ra05405d
rsc.li/rsc-advances
molecules and studied their thermal and electro-optic properties.
It is shown that indole can act as a donor in developing nonlinear
1. Introduction
optical material. In such system, increasing the electron donating
ability of indole moiety leads to decrease in thermal stability and
increased in NLO responsive electro-optical properties.47 Simi-
larly, as compared to aniline, the 6-(pyrrolidin-1-yl)-1H-indole
based donor system exhibits enhance in its macro NLO response
electro-optic properties.48 In such system, the NLO properties is
increased with increasing the electron donating ability of indole
moiety. This could be due to favorable intermolecular dipole
interaction forces.49 In comparison to earlier report, the 3-
substituted indole derivatives are more sensitive to medium
polarity due to (i) the formation of stable resonating structure at
indolyl-3-position and (ii) the donating ability of indole moiety
can also be tuned through various N-substitution.50,51
The advantage of these molecules is due to their substituent
induced varied optical properties such as absorption, uores-
cence, extinction coefficient, HOMO–LUMO energy gap, excited
state dipole moment and transition energy, which provide most
valuable information in designing future molecules. In order to
gain more insight into the NLO response of 3-substituted indole
based conjugated molecules, we have studied the substituent
dependent rst hyperpolarizability (b) of various electron
donor/acceptor substituted p-phenyl and N-substituted ethenyl
indoles (1–9) using solvatochromic method. The ethenyls (1–9)
with varied electron withdrawing and donating p-phenyl
substitution (NO2, Cl, H, OCH3, OH, NH2, N–SO2C6H5, N–
Donor–acceptor substituted conjugated molecules have a wide
range of applications in chemistry, biology1–6 and organic elec-
tronics7,8 like photoswitches,9–12 organic light-emitting diodes
(OLEDS),13,14 dye-sensitized solar cell (DSSC),15–25 nonlinear optics
(NLO).7,13,14,26–37 In particular, NLO materials with varied shape and
size (e.g. dipolar, quadrupolar, octupolar) were developed and
extensively studied using various testing methods such as electric
eld induced second harmonic (EFISH) generation, hyper Ray-
leigh scattering (HRS),26–37 solvatochromic methods38–46 and
computational based density functional theory (DFT).39,42 Among
all the methods, the solvatochromic method is most suitable, easy
and cost effective method, in which the NLO response, rst
hyperpolarizability coefficient (b) of small dipolar molecule can be
obtained accurately.38–46 The large second order NLO response,
rst hyperpolarizability can be achieved through extended
conjugation as well as by tuning the donor–acceptor length in
electron donor–acceptor substituted molecule.7,8 NLO response of
6-substituted indole derivatives were tested previously.47,48 These
includes indole with tricyano furan acceptor based conjugated
Department of Chemistry, School of Sciences, Hemvati Nandan Bahuguna Garhwal
University, Srinagar (Garhwal), Uttarakhand 246174, India. E-mail: p.hota@hnbgu.
ac.in
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d0ra05405d
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Scheme 1 Structure of indole compounds 1–9.
COCH3, N–Et) were synthesized (Scheme 1) and the effect of (PMT) detector system. FTIR spectra are recorded on a Impact
1
substitution on the optical properties of indole compounds Nicolet-400 spectrophotometer using KBr discs. The H and 13C
were evaluated.
NMR spectra are recorded on a JEOL 500 MHz FTNMR instrument
The excited state of nitro substituted ethenyls (1, 7, 8, 9) is in CDCl3 as solvent and using tetramethylsilane (TMS) as an
highly dipolar and exhibit charge transfer excited state. A higher internal standard. MS spectra are measured on a GCD 1800A
b value is found for these ethenyls bearing a strong electron Hewlett Packard gas chromatography (GC)–mass spectrometer.
withdrawing substituent. On the other hand, excited state of CHNS analyses are recorded on a Theoquest CE instrument 1112
compounds 2–6 is non-polar and exhibit a lower b value. Overall, series CHNS auto analyzer. Melting points are recorded on a Lab
the 2nd order NLO properties is proportional to the ground state India make melting point apparatus.
dipole moment, polarizability, ionic character and % of charge
separation in the molecule. On the other hand, b value is
2.2 Synthesis of compounds 1–9
inversely proportional to the optical band gap of the ethenyl
The substituted p-phenyl ethenyl-E-indoles (1–5) are synthe-
indole. The above substituent dependent studies on p-phenyl and
sized through condensation of 2 molar ratio of p-substituted
N-substituted ethenyl indole provide most valuable information
phenyl acetic acid with respect to 3-formylindole in pyridine–
piperidine mixture as described earlier6,45,46,50,51 and the routes
in understanding the optical properties in conjugated molecules.
are shown in Scheme 2, e.g. for obtaining compound 1, the
typical synthetic protocol is as follows: 2-formyl indole (1.45 g,
0.01 M) is taken with p-nitrophenyl acetic acid (3.62 g, 0.02
mol) along with in freshly distilled pyridine (10 mL) and
2. Experimental section
2.1 Materials and analytical equipments
The starting materials and reagents for the synthesis of ethenyl piperidine (0.6 mL) in a roꢂund bottom ask. The reaction
indoles were purchased from the local suppliers (Ms E. Merck, mixture was reuxed at 100 C for six hours. The progress of
Sisco Research Laboratory, Sigma-Aldrich). UV grade solvents are the reaction is monitored by thin layer chromatography (TLC).
used for spectroscopy study. Compounds are synthesized using Aer cooling the reaction mixture, the excess of pyridine was
Carousel 6 plus reaction station, Radleys make. Synthetic remove from the reaction mixture by treating with 100 mL of
1
compounds are characterized by H and 13C nuclear magnetic diluted hydrochloric acid. A red colored product is collected
resonance (NMR), Fourier-transform infrared (FTIR), mass spec- aer extracting the crude product in dichloromethane. The
trometry (MS) using electron impact (EI) method and carbon, product was further puried by column chromatography using
hydrogen, nitrogen and sulfur (CHNS) analysis. The absorption 2% ethyl acetate in petroleum ether as the eluting solvent. The
spectra are recorded on a PerkinElmer Lambda 25 UV-Vis and yield of the desired compound is obtained in 31%. Similarly,
Lambda 750 UV/VIS/near infrared (NIR) spectrophotometer. The compounds 2–5 were obtained with yield 56%, 45%, 34% and
uorescence spectra are recorded on a PerkinElmer LS-55 uo- 33% respectively. Compound 6 was prepared with 47% yield
rescence spectrophotometer using a red photomultiplier tubes through reduction reaction of 1. For this purpose, the
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Scheme 2 Synthetic routes and reaction conditions for obtaining indole compounds, (a) pyridine, piperidine, 100 ꢂC, reflux, 6 h; (b) FeSO4, aqu.
NH3, ethanol, 100 ꢂC, 3 h; (c) ethyl bromide, potassium-t-butoxide, t-butyl alcohol, reflux, 4 h; (d) acetic anhydride, sodium acetate, reflux, 1 h; (e)
benzene sulphonyl chloride, potassium carbonate, acetone, 0 ꢂC, 2 h.
alcoholic solution of ethenyl indole 1 (0.2 g, 0.001 M) is
reuxed in ferrous sulfate (1.5 g, 0.01 M) and aqueous
ammonia solution at 100 ꢂC for 3 h. Compound 7 was obtained
with 80% yield through N-alkylation of compound 1 (0.5 g,
0.002 M) with ethyl bromide (2 mL, 0.01 mol) in presence of
potassium-t-butoxide (0.2 g, 0.002 M) and t-butyl alcohol (20
mL).52 Compound 8 was obtained with 83% yield through N-
acetylation of compound 1 (0.1 g, 0.0004 M) using acetic
anhydride (10 mL) in presence of sodium acetate (0.1 g, 0.0012
M).53 Compound 9 was obtained with 80% yield through N-
sulphonation of compound 1 (0.2 g, 0.001 M) using benzene
sulphonyl chloride (1 mL, 0.01 M) and potassium carbonate
(1.0 g, 0.01 M) in acetone54 as shown in Scheme 2. The prod-
ucts are puried by column chromatography using 2–5% ethyl
acetate in petroleum ether (60–80 ꢂC) as the eluting solvent.
The characterization of compounds were carried out satisfac-
2.3 Absorption and uorescence studies
For all absorption and uorescence measurements, UV grade
solvents were used. For absorption, (1–4) ꢀ 10ꢁ5 M solution and
for uorescence studies, 0.5 ꢀ 10ꢁ5 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 exciting the sample at their absorption
maximum (labs max). The ground and excited state energy (E) of
the compounds are calculated using absorption wavelength
(labs max), uorescence wavelength (lem max) maximum and eqn
(1). Where,
Eabs ¼ (hc/labs max) ¼ (1.24/labs max) (in KeV)
Eem ¼ (hc/lem max)¼ (1.24/lem max) (in Kev)
(1a)
(1b)
1
tory using H and 13C NMR, MS, FTIR, CHNS analysis.
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The energy band gap (DE) of 1–9 is obtained from the
intersection of excitation and uorescence spectrum, Tauc plot
and TDDFT computation method.
2.4 Dipole moment calculation
Change of excited state dipole moment of compounds is
calculated using McRay eqn (2).41,55
Table 1 UV-Vis absorption and fluorescence data of 1–9 in different solvents
labs
lem
Quantum yield
Ff
max
Solvents
(nm)
3 (Mꢁ1 cmꢁ1
)
max (nm)
lex max (nm)
Stokes' shi (va ꢁ vf) nm (cmꢁ1
)
1
2
3
4
5
6
7
8
9
n-Hexane
1,4-Dioxan
THF
MeOH
AcCN
403
407
416
413
411
427
338
336
333
335
340
338
328
329
331
327
325
332
328
330
331
327
326
333
310
318
319
322
323
331
320
324
325
324
325
333
402
415
420
415
419
430
362
378
375
375
376
382
360
369
374
370
369
377
15 423
21 011
22 603
23 900
24 924
17 068
17 900
14 750
18 600
18 800
19 475
18 075
4675
14 575
14 200
10 800
14 925
14 975
15 496
18 229
17 101
17 800
15 020
18 530
14 987
18 333
21 754
24 400
16 741
15 588
8467
497
554
607
590
642
648
384
388
387
395
396
398
384
384
397
396
407
404
389
397
403
404
409
411
389
384
385
390
395
398
389
394
397
405
397
400
508
566
590
556
649
640
423
533
537
564
602
592
403
529
576
588
593
605
408
372
443
370
380
446
351
341
345
354
355
350
335
325
347
323
333
346
340
329
324
344
349
349
308
330
332
330
335
347
333
354
355
339
337
365
395
406
420
417
415
426
363
370
375
374
374
380
356
371
377
371
370
369
94 (4693)
147 (6520)
191 (7564)
177 (7264)
231 (8754)
221 (7987)
46 (3544)
52 (3988)
54 (4191)
60 (4534)
56 (4159)
60 (4460)
56 (4446)
55 (4354)
66 (5023)
69 (5329)
82 (6199)
72 (5368)
61 (4781)
67 (5115)
72 (5398)
77 (5829)
83 (6225)
78 (5700)
79 (6552)
66 (5405)
66 (5373)
68 (5414)
72 (5643)
67 (5086)
69 (5544)
70 (5484)
72 (5581)
81 (6173)
72 (5581)
67 (5030)
106 (5190)
151 (6429)
170 (6860)
141 (6111)
230 (8482)
210 (7630)
61 (3984)
155 (7694)
162 (8045)
189 (8936)
226 (9984)
220 (9287)
43 (3197)
160 (8196)
202 (9376)
218 (10 020)
224 (10 236)
228 (9996)
0.0104
0.0216
0.0418
0.0013
0.0118
0.0052
0.0026
0.0037
0.0020
0.0016
0.0015
0.0024
0.0066
—
DMF
n-Hexane
1,4-Dioxan
THF
MeOH
AcCN
DMF
n-Hexane
1,4-Dioxan
THF
MeOH
AcCN
0.0083
0.0072
—
DMF
0.0110
0.0027
0.0209
0.0163
0.0137
0.0176
0.0196
0.0114
0.0035
0.0058
0.0030
0.0042
0.0081
0.0361
0.0041
0.0334
0.0301
0.0745
0.0056
0.0005
0.0006
0.0007
0.0006
0.0008
0.0007
0.0003
0.0007
0.0008
0.0009
0.0010
0.0009
0.0003
0.0008
0.0008
0.0009
0.0010
0.0009
n-Hexane
1,4-Dioxan
THF
MeOH
AcCN
DMF
n-Hexane
1,4-Dioxan
THF
MeOH
AcCN
DMF
n-Hexane
1,4-Dioxan
THF
MeOH
AcCN
14 522
2989
12 200
12 814
12 185
24 026
27 090
23 141
22 823
21 238
27 318
13 975
12 655
11 800
11 464
10 900
11 200
26 722
27 742
16 773
20 000
14 065
14 178
DMF
n-Hexane
1,4-Dioxan
THF
MeOH
AcCN
DMF
n-Hexane
1,4-Dioxan
THF
MeOH
AcCN
DMF
n-Hexane
1,4-Dioxan
THF
MeOH
AcCN
DMF
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nabs ꢁ nem ¼ (dabs + dem) + {(2Dm2/hca3)F(3,n)} ¼ constant +
Overall the yield of reaction is obtained satisfactory. All the
compounds were characterized through 1H NMR, 13C NMR,
FTIR, MS (EI + method) and CHN analysis. In 1H NMR, the two
doublet peaks correspond to trans olen protons of compound
1–9 appear near d 7.2 and d 7.4 (each of 1H, J ¼ 15.8–16.5 Hz).
Similarly, multiplate peaks near d 7.1–7.2 (2H, m –C5–H, C6–H),
one singlet peak near d 7.3–7.4 (1H, s, –C2–H), two doublet
peaks near d 7.9 (1H, d, J ¼ 7.5–8.2 Hz, –C4–H) and d 8.0 (1H, J ¼
6.2–7.5 Hz, –C7–H) correspond to the indole ring protons. In
FTIR, the indole N–Hst is identied near 3370 cmꢁ1. The C–Hst
appear near 3040 cmꢁ1, C]Cst near 1620 cmꢁ1. Similarly, four
peaks correspond to the C]Cst and C–Cst of phenyl ring are
mF(3,n)
(2)
F(3,n) ¼ [(3 ꢁ 1)/(3 + 2) ꢁ (n2 ꢁ 1)/(n2 + 2)], m ¼ (2Dm2/hca3)
where, nabs is absorption maximum wave number, nem uores-
cence maximum wave number, nabs ꢁ nem is the Stokes' shi,
dabs and dem are difference in vibrational energy of molecule
(in cmꢁ1) in excited and ground state for absorption and
emission respectively, me and mg are the excited state and ground
state dipole moments respectively, me ꢁ mg ¼ Dm is the change in
dipole moment, h is the Planck constant (6.62 ꢀ 10ꢁ34 joule s), c
is the velocity of light in vacuum (3 ꢀ 108 meter per s), 3 is the
relative permittivity (i.e. dielectric constant) and n is the
refractive index of the solvent.56,57 The Onsagar cavity radius (a)
can be calculated using eqn (3) as described elsewhere.58
observed near 1520 cmꢁ1, 1488 cmꢁ1, 1455 cmꢁ1, 1400 cmꢁ1
.
For compound 1, the symmetrical and asymmetrical stretching
frequency of nitro group is conrmed at 1330 cmꢁ1 and
1520 cmꢁ1. In compound 4, the methoxy, O–Cst is conrmed at
1244 cmꢁ1. For compound 6, two sharp peaks at 3394 cmꢁ1 and
3341 cmꢁ1 are conrmed as primary amine NHst. For
compound 7–9, the NHst peaks are disappeared upon N-
substitution. For 8, the C]O stretching peak at 1703 cmꢁ1
correspond to N-acetyl group and for 9, the S]O stretching
appears at 1180 cmꢁ1. Thus, olens bearing indole heterocyclic
unit and indole N-substituted ethenyls were synthesized using
mild reaction condition and characterized successfully. The
detail of characterization data is shown in ESI.†
a ¼ (3M/4pdN)1/3
,
(3)
where, M ¼ molecular weight of molecule, N ¼ Avogadro
number ¼ 6.022 ꢀ 1023, d ¼ molecular density of molecule.
2.5 First hyperpolarizability calculation
The rst hyperpolarizability coefficient (b) is related to second order
nonlinear optical (NLO) properties of molecule. The sol-
vatochromism method is used to obtain b in methanol using Oudar
formula59 as reported elsewhere38–44 using eqn (4) and eqn (5).
3.2 Absorption and uorescence studies of indole
compounds 1–9
The absorption and uorescence data of compounds 1–9 in
different solvents of varying polarity are summarized in Table 1.
From the absorption (Fig. 1 and S1a-i†) and uorescence
spectra (Fig. 2 and S2a-i†), it is shown that the absorption
coefficient of ethenyl indoles (1–9) is in between 10 800–
23 900 Mꢁ1 cmꢁ1 (Table 2). On increasing the solvent polarity,
the absorption and uorescence wavelength maximum are red
shied. This suggest, a p–p* nature of electronic transition. On
increasing the solvent polarity from n-hexane to
2
2
(b) ¼ (3/2h2c2) ꢀ {(vabs)2(rg)2(Dm)}/{(vabs ꢁ v2)(vabs ꢁ 4v2)} (4)
where, vabs: absorption maximum wave number and; v: incident
reference wave number, 1064 nm of Nd:YAG laser source to
which the b value is referred;
The transition dipole moment (rg) is calculated using eqn (5).
(rg)2 ¼ [(3e2h)/(8p2mc)] ꢀ (f/vabs) ¼ 2.13 ꢀ 10ꢁ30 ꢀ (f/vabs
) (5)
Ð
where, f is the Oscillator strength, f ¼ 4:32 ꢀ 10ꢁ9 3ðnÞdn,
which is obtained from the plot, molar absorption coefficient (3)
vs. wave number (n).60
2.6 Computed parameters using time dependent density
functional theory (TDDFT)
For computational calculation, the Orca quantum chemical
soware package61–63 with time dependent density functional
theory (TDDFT)64–66 is used. The ground state dipole moment,
absorption wavelength, the vertical excitation energy, oscillator
strength of the optimized ethenyls were obtained using B3LYP
functional with a def2 SVP basis set.67
3. Results and discussion
3.1 Synthesis
The trans-olens of 1–9 were obtained through condensation
reaction as shown in Scheme 2 with reasonable yield (31–83%). Fig. 1 Absorption spectra of 1–9 in methanol.
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constant,70 it is shown that strong electron acceptor –NO2 group
(sp +0.81, for 1, 7–9) at the p-phenyl ring induces large electron
delocalization in the excited state as compared to the weak
electron acceptor chloro group –Cl (sp +0.24, in case of 2), –H (sp
0.0 in case of 3) and weak electron donor methoxy group –OCH3,
(sp ꢁ0.28, in case of 4).
The singlet state energy of 1–9 is calculated from their
absorption and uorescence wavelength maximum (Table 2).
The rst singlet excited state energy band gap for 1, 7–9 is 2.47–
2.87 eV, whereas it is 3.34–3.40 eV for 2–6. As per Tauc plot and
TDDFT calculation, the order is 3 > 2 > 9 > 8 > 1 > 2 and 3 > 4 ¼ 5
> 6 (Fig. S3†). The optical band gap of 1 and 7–9 is 0.70–0.94 eV
lower than compound 2–6. Interestingly, indole is acting as
a strong electron donor in presence of an electron withdrawing
p-phenyl nitro substituent (for 1, 7–9), whereas indole acts as
a weak electron acceptor in presence of an electron donating p-
phenyl methoxy and amine (OCH3, NH2) substituent. In order to
understand the effect of substituent and solvent polarity on the
Fig. 2 Fluorescence spectra of 1–9 (0.5 ꢀ 10ꢁ5 M) in methanol.
dimethylformamide (DMF), the absorption maximum (labs max
)
is moderately red shied by 24 nm, 29 nm, 22 nm, 17 nm and
13 nm respectively for strong electron withdrawing nitro
ethenyls (1, 7–9) and for strong electron donating amino
compound 6. The labs max of compound 5 with phenolic group is
also moderately red shied by 11 nm from n-hexane to polar
solvent, DMF. On the other hand, the labs max of other ethenyls
2–4 (Cl, H, OCH3) is not much sensitive to solvent polarity and
a red shi of 0–5 nm is observed. In contrast to labs max, the
uorescence maximum (lem max) is signicantly red shied by
151 nm, 133 nm, 172 nm, 202 nm for 1, 7–9 respectively from n-
hexane to DMF. For 2–6, lem max is moderately red shied by
14 nm, 20 nm, 22 nm, 09 nm and 11 nm respectively. This
suggest the drastic stabilization of the excited state of 1, 7–9 due
to coulomb interaction between the dipolar solute and solvent
molecules.68,69 A red-shied lem max in ethenyl indole (1, 7–9) is
due to the electron delocalization from the indole moiety-to-the
electron acceptor nitro group. As the transition is p / p*
nature, the more stabilization of excited state with respect to the
ground state leads to a red shied in absorption and uores-
cence wavelength. From the correlation of Hammett substituent
Fig. 3 McRay plot, Stokes' shift vs. F(3,n) of 1–9. Solvents used are n-
hexane, 1,4-dioxane, THF, MeOH, AcCN and DMF.
Table 2 Comparison of absorption wavelength maximum (labs max), fluorescence wavelength maximum (lem max), extinction coefficient,
oscillator strength (f), S0–S1 transition state energy (DE, eV), transient dipole moment between ground and excited states (rg), change of excited
state dipole moment (Dm), optical band gap (DE), first hyperpolarizability (b) of ethenyl indoles 1–9 in methanola
labs
lem
(3)
S0–S1 (DE)
(eV)
(rg)
debye
(b)
max (nm)
max (nm)
(Mꢁ1 cmꢁ1
)
f
(Dm) debye
(me) debye
(in 10ꢁ30) esuꢁ1 cm5
1
2
3
4
5
6
7
8
9
413
335
327
327
322
324
418
374
370
590
395
396
404
390
405
558
563
588
23 900
18 800
10 800
17 800
24 400
12 200
22 823
11 464
20 000
0.69
0.57
0.34
0.58
0.81
0.44
0.61
0.32
0.56
2.50
3.34
3.41
3.40
3.40
3.34
2.47
2.83
2.87
7.79
6.37
4.86
6.35
7.45
5.51
7.36
5.03
6.64
9.86
5.52
5.19
5.77
6.29
4.18
9.44
14.21
17.78
18.25
9.51
7.76
6.82
7.66
115
20
9
17
24
9
106
43
90
6.62
18.38
18.55
27.10
a
˚
Onsagar cavity radius “a” (in A); 1: 4.53; 2: 4.47; 3: 4.38; 4: 4.55; 5: 4.43; 6: 4.45; 7: 4.74; 8: 4.75; 9: 5.12; ground state dipole moment mg (in debye): 1:
8.39; 2: 3.99; 3: 2.57; 4: 1.05; 5: 1.37; 6: 2.44; 7: 8.94; 8: 4.34; 9: 9.32; excited state dipole moment me ¼ Dm(McRay method) + m(gTDDFT)
.
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(Table S1†). Thus, the large solvatochromic shi in 1 and 7–9 is
due to charge transfer excited state. On the other hand, ethenyl
indoles 2–6 with a weak electron acceptor or donor group (–Cl,
–H, –OCH3, –OH, –NH2) show small change in excited state
dipole moment (4.18–6.29 D) and exhibit non polar excited state
as compared to 1 and 7–9.
From Kamlet–Ta plot71 (Fig. 4 and Table S1†), it is shown
that nitro compounds (1, 7–9) are highly polarized in the excited
state. A large slope is observed for 1 and 7–9 as compared to 2–6
(slope: ꢁ5.14 ꢀ 10ꢁ3, R ¼ 0.98 for 1; ꢁ0.93 ꢀ 10ꢁ3, R ¼ 0.85 for
2; ꢁ1.32 ꢀ 10ꢁ3, R ¼ 0.99 for 3; slope ꢁ1.43 ꢀ 10ꢁ3, R ¼ 0.99 for
4; slope ꢁ2.69 ꢀ 10ꢁ3, R ¼ 0.91 for 5; slope ꢁ0.71 ꢀ 10ꢁ3, R ¼
0.96 for 6; ꢁ4.03 ꢀ 10ꢁ3, R ¼ 0.94 for 7; ꢁ7.38 ꢀ 10ꢁ3, R ¼ 0.98
for 8; ꢁ9.22 ꢀ 10ꢁ3, R ¼ 0.97 for 9). The formation of charge
transfer excited state in 1 and 7–9 could be due to twist over the
single bond attached to the p-phenyl ring and such phenomena
is also suggested in other donor–acceptor substituted ethenyl
systems.45,46,72–79
Fig. 4 Kamlet–Taft plot, emission wave number vs. solvent polariz-
ability parameter p* of 1–9 in solvents, n-hexane, 1,4-dioxane, THF,
MeOH, AcCN and DMF.
3.3 Time dependent density functional theory (TDDFT)
studies
excited state, McRay plot, the Stokes' shi (va ꢁ vf) vs. solvent
polarity parameter, F(3,n) is drawn, (Fig. 3) for 1–9 and the
excited state dipole moment is calculated (Tables 2 and S1†).
For all compounds, the Stokes' shi values are increased line-
arly with increasing the solvent polarity. This further improved
by the deletion of two solvents, 1,4-dioxan and acetonitrile. In
order to get a good correlation factor, these two solvents were
excluded from our calculation and the following correlation are
obtained (1: m1 ¼ 4245, R ¼ 0.91; 2: m1 ¼ 1384, R ¼ 0.99; 3: m1 ¼
1304, R ¼ 0.99; 4: m1 ¼ 1434, R ¼ 0.99; 5: m1 ¼ ꢁ1847, R ¼ 0.92;
6: m1 ¼ 805, R ¼ 0.85; 7: m1 ¼ 3397, R ¼ 0.92, 8: m1 ¼ 7645, R ¼
0.97, 9: m1 ¼ 9563, R ¼ 0.95). It is shown that a large change in
the excited state dipole moment is observed for 1, 7, 8 and 9
(9.44–17.78 debye) as compared to 2, 3, 4, 5 and 6 (4.18–6.29
debye). Similarly, the ground state dipole moment (mg) is
computed for 1–9 using TDDFT. The mg for 2–6 is in between
1.05–3.99 debye, whereas mg for 1, 7, 8, 9 is 4.34–9.32 debye
The geometry of the molecules are optimized and the parame-
ters such as absorption, oscillator strength, HOMO–LUMO
energy, optical band gap is computed for 1–9 using TDDFT
method (Table S2†). The parameters obtained through
computation methods are also followed the similar trend as
compared to the experimental results. Compound with strong
electron withdrawing p-phenyl substituent, the ground and
excited states are stabilized, whereas, the ground and excited
state are destabilized in presence of electron donating substit-
uent compared to the unsubstituted ethenyl indole 3. This is
due to the pushing or pulling of p electrons from indole to the
p-phenyl substituted ring, which leads to the stabilization or
destabilization of the ground or excited state.
All these compounds show one intense band (labs max 300–
440 nm). Compound 1 exhibits longest labs max of 434 nm and 3
has the lowest labs max of 341 nm. This absorption is due to the
HOMO / LUMO (S0 / S1) transition (Fig. S4†). From
Fig. 5 TDDFT computed molecular orbitals, optical band gap and HOMO–LUMO energy of 1–9.
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Fig. 6 Comparative chart of first hyperpolarizability (bCT), excited state dipole moment, change of excited state dipole moment, ground state
dipole moment and optical band gap of 1–9.
solvatochromism data, these absorption is due to the p / p* compared to 3, the b of other ethenyl increases slightly with
transition. As compared to, ethenyl indole (3), the HOMO– increasing the electron donating nature of p-phenyl substitu-
LUMO optical band gap is decreased upon increasing the tion. From previously report on 6-substituted indole based NLO
electron withdrawing or electron donating p-phenyl substitu- materials, NLO response is also increased with increasing the
tion. The energy band gap is decreased by 0.81 eV from phenyl donor ability of indole moiety through pyrrolidine ring.47,48
ethenyl indole (3) to p-nitro phenyl ethenyl indole (1) [3.85 eV Compounds with weak electron donating or weak electron
(for 3) to 3.05 eV (for 1)] (Fig. 5 and S5†). Similarly, the energy withdrawing substituent (Cl, OCH3, OH) (2, 4–5), however,
band gap is slightly decreased by 0.07 eV from phenyl ethenyl exhibit a low b value as compared to nitro compounds, 1 and 7–
indole (3) to p-hydroxy phenyl ethenyl indole (5) [3.86 eV for 3 to 9. The order of b obtained in ethenyl indoles with electron
3.79 eV for 5], whereas the band gap is comparable for amino withdrawing group is NO2 (1, 7–9) > Cl (2) > H (3) and electron
and methoxy substituent [3.86 eV for 3 to 3.84 eV for 4, 3.85 eV donating OH (5) > OCH3 (4) > NH2 (6) (b; 1: 115, 2: 20, 3: 9, 4: 17,
for 6]. Interestingly, N-ethyl substitution destabilized the 5: 24, 6: 9, 7: 106, 8: 43, 9: 87) (in 10ꢁ30 esuꢁ1 cm5).
ground and excited state of 1, whereas the ground and excited
The b value of some of the donor–acceptor nitro compounds,
state is stabilized upon N-acetyl and N-sulfonyl substitution. such as p-nitro aniline, 4-amino-40-nitro stilbene, it is shown
Overall, the HOMO and LUMO energy of ethenyl indole, is that the b value is increased from 20 to 100 (in 10ꢁ30 esuꢁ1, cm5)
progressively stabilized for electron withdrawing group due to with increasing the conjugation length and charge transfer
the delocalization of p electron from indole to the p-phenyl ring nature of the molecule.28 From our previous report on thio-
(for NO2, Cl) (Fig. S6†). In case of electron donating substituents phene and furan based conjugated compounds, similar results
(methoxy, hydroxy and amino) (4–6), the destabilization of are also found.45,46 On the other hand, molecule with moderate
HOMO and LUMO energy level could be due to the hindrance in or weak electron donor/acceptor substituent (cyano, chloro,
effective p conjugation.
methoxy, hydroxy), the effect on the b is very small as compared
to the nitro substitution.45,46 For compound 7–9 with N-ethyl, N-
acetyl and N-sulfonyl substituent, there is a hindrance in
effective p conjugation, which leads to lower b value as
compared to 1. The mg for 2–6 is in between 1.05–3.99 debye,
whereas mg for 1, 7–9 is 4.34–9.32 debye (Tables S1–S3†). Thus,
molecule with charge transfer behavior exhibits large ground
state dipole moment, lower optical band gap and larger b value.
Overall, the b value is increased with (i) increasing the dipole
moment, (ii) increasing the % of charge transfer behavior (iii)
increasing the polarizability and (iv) with increasing the change
of excited state dipole moment of 3-substituted indole
compounds, whereas b value is decreased with (v) increasing
the optical band gap of the molecule. It is the combination of all
ve factors involved in deciding the NLO response of the
molecule. Mostly, for withdrawing substituent, the order of mg,
me, Dm, p* and charge separation in 3-substituted indoles is NO2
3.4 Second order non-linear optical properties
The rst hyperpolarizability coefficient (b) is related to the
second order non-linear optical properties of the molecule.
Thus, b of the compounds 1–9 is calculated in solvent, meth-
anol using solvatochromism method. In general, non linear
optical properties of the molecule is inuenced by the delocal-
ization of p electron. Thus, the effect of p-phenyl and N-
substitution on the NLO response of the ethenyl indole 3 is
studied. For this purpose, the absorption, oscillator strength (f),
dipole moment (Dm) and transition dipole moment (rg) of the
molecules were calculated. It is shown that as compared to the
reference compound 3, the b increases for strong electron
withdrawing p-nitro phenyl substituent (Table 2, Fig. 6 and S7†).
Compound 1 and 7–9, which have strong electron withdrawing
p-nitro phenyl substituent, exhibit large b value. Similarly, as
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(1, 7–9) > Cl (2) > H (3); and the order of b: NO2 (1, 7–9) > Cl (2) >
H (3), where as for electron donating substituent, the order of
me, Dm, p* and charge separation is OH (5) > OCH3 (4) > NH2 (6);
and the order of b: OH (5) > OCH3 (4) > NH2 (6). On the other
hand higher optical band gap reduced the b value. The order of
optical band gap (DE) for electron withdrawing substituent: H
(3) > Cl (2) > NO2 (1, 7–9) and the order of b is NO2 (1, 7–9) > Cl
(2) > H (3). Similarly, the order DE for donating substituent: NH2
(6) > OCH3 (4) > OH (5) and the order of b: OH (5) > OCH3 (4) >
NH2 (6). The optical band gap of N-substituted nitro compound
(7–9) is little larger as compared to 1 and thus, NLO response of
7–9 is slightly lower as compared to compound 1.
References
1 K. Palczewski, Chemistry and biology of vision, J. Biol. Chem.,
2012, 287, 1612–1619.
2 C. Dugave and L. Demange, Cis-trans isomerisation of
organic molecules and biomolecules: implications and
applications, Chem. Rev., 2003, 103, 2475–2532.
3 A. Wand, I. Gdor, J. Zhu, M. Sheves and S. Ruhman,
Shedding new light on retinal protein photochemistry,
Annu. Rev. Phys. Chem., 2013, 64, 437–458.
4 A. K. Singh and P. K. Hota, Development of
bacteriorhodopsin analogues and studies of charge
separated excited states in the photoprocesses of linear
polyenes, Photochem. Photobiol., 2007, 83, 50–62.
5 H. Kobayashi, M. Ogawa, R. Alford, P. L. Choyke and
Y. Urano, New strategies for uorescent probe design in
medical diagnostic imaging, Chem. Rev., 2010, 110, 2620–
2640.
6 A. K. Singh and P. K. Hota, Ethenyl indoles as neutral
hydrophobic uorescence probes, J. Phys. Org. Chem., 2007,
20, 624–629.
7 B.-K. An, J. Gierschner and S. Y. Park, p-Conjugated
cyanostilbene derivatives: a unique self-assembly motif for
molecular nanostructures with enhanced emission and
transport, Acc. Chem. Res., 2012, 45, 544–554.
8 H. Meir, The photochemistry of stilbenoid compounds and
their role in materials technology, Angew. Chem., Int. Ed.
Engl., 1992, 31, 1399–1420.
9 M. Irie, Diarylethenes for memories and switches, Chem.
Rev., 2000, 100, 1685–1716.
10 R. Klajn, Spiropyran-based dynamic materials, Chem. Soc.
Rev., 2014, 43, 148–184.
11 M. Dudek, Z. Pokladek, M. Deiana and K. Matczyszyn,
Molecular design and structural characterization of
photoresponsive azobenzene-based polyamide units, Dyes
Pigm., 2020, 180, 108501.
12 A. Georgiev, D. Yordanov, D. Dimov, I. Zhivkov, D. Nazarova
and M. Weiter, Azomethine phthalimides uorescent E-Z
photoswitches, J. Photochem. Photobiol., A, 2020, 393,
112443.
13 L. Beverina and G. A. Pagani, p-Conjugated zwitterions
paradigm of donor-acceptor building blocks in organic
based materials, Acc. Chem. Res., 2014, 47, 319–329.
14 B. Wex and B. R. Kaafarani, Perspective on carbazole-based
organic compounds as emitters and hosts in TADF
applications, J. Mater. Chem. C, 2017, 5, 8622–8653.
15 H. Hug, M. Bader, P. Mair and T. Glatzel, Biophotovoltaics:
natural pigments in dye- sensitized solar cells, Appl.
Energy, 2014, 115, 216–225.
4. Conclusion
In summary, it is shown that the excited state of ethenyl indole
is highly sensitive to the solvent and the substituent present on
it. Compound 1 and 7–9 with strong electron-attracting
substituent exhibits charge transfer and highly dipolar excited
state as compared to other ethenyls. Compound 2–6 with
a moderate electron donating substituent or weak electron
withdrawing or weak electron donating substituent exhibit non
polar excited state and insensitive to solvent polarity. The
energy band gap of 3 (phenyl ethenyl indole) is decreased by
substituting either with an electron withdrawing (Cl, NO2) or an
electron donating (OCH3, OH, NH2) substituent at the p-phenyl
position. The compound with a strong electron accepting, p-
nitrophenyl ethenyl indole shows 12 times better NLO response
as compared to the reference ethenyl indole 3, whereas, for
ethenyls 2–6 bearing a weak or moderately electron withdrawing
or electron accepting substituent, exhibit lower NLO response.
The b of ethenyl is increased with increasing the order of elec-
tron withdrawing nature of phenyl ring. On the other hand, in
case of compounds bearing electron donating substituent
shows comparable b value. The NLO response is also propor-
tional to the ground state dipole moment, polarizability, dipolar
nature and ionic character of the molecule, whereas it is
inversely proportional to the optical band gap of the molecule.
Overall, the optical properties of indole compound is highly
dependent upon the substituent present in phenyl ring and N-
substitution. In addition, studies on the macroscopic NLO
properties of indole compound is an interesting aspect and
a future prospective to look into. Thus, the above studies will
help in designing and developing optical material for various
electronic applications.
Conflicts of interest
There is no conicts of interest to declare.
16 J. A. Mikroyannidis, D. V. Tsagkournos, S. S. Sharma,
Y. K. Vijay and G. D. Sharma, Conjugated small molecules
with broad absorption containing pyridine and pyran
units: synthesis and application for bulk heterojunction
solar cells, Org. Electron., 2010, 11, 2045–2054.
Acknowledgements
PKH, JK, NK are thankful to University Grants Commission,
New Delhi for research grant (No. F.30-72/2014-BSR) and
research fellowship. Authors acknowledged AMRC, IIT Mandi
for 1H and 13C NMR facility.
17 T. Wang, Y. Chen, X. Bao, Z. Du, J. Guo, N. Wang, M. Sun and
R. Yang, A new isoindigo-based molecule with ideal energy
This journal is © The Royal Society of Chemistry 2020
RSC Adv., 2020, 10, 28213–28224 | 28221

View Article Online
RSC Advances
Paper
levels for solution-processable organic solar cells, Dyes
Pigm., 2013, 98, 11–16.
nitropyridine 1-oxide (POM) single crystal, J. Cryst. Growth,
2004, 261, 55–62.
18 S. Paek, J. K. Lee and J. Ko, Synthesis and photovoltaic 32 S. R. Forrest and M. E. Thompson, Introduction: organic
characteristics of push–pull organic semiconductors
containing an electron-rich dithienosilole bridge for
electronics and optoelectronics, Chem. Rev., 2007, 107,
923–925.
solution-processed small-molecule organic solar cells, Sol. 33 G. S. He, L.-S. Tan, Q. Zheng and P. N. Prasad, Multiphoton
Energy Mater. Sol. Cells, 2014, 120, 209–217.
absorbing materials: molecular designs, characterizations
19 L. Beverina and G. A. Pagani, p-Conjugated zwitterions as
and applications, Chem. Rev., 2008, 108, 1245–1330.
paradigm of donor-acceptor building blocks in organic 34 J. Campo, A. Painelli, F. Terenziani, T. V. Regemorter,
based material, Acc. Chem. Res., 2014, 47, 319–329.
20 I. Benesperi, H. Michaels and M. Freitag, The researcher's
guide to solid-state dye-sensitized solar cells, J. Mater.
Chem. C, 2018, 6, 11903–11942.
D. Beljonne, E. Goovaerts and W. Wenseleers, First
hyperpoarizability dispersion of the octupolar molecule
crystal violet: multiple resonances and vibrational and
solvent effects, J. Am. Chem. Soc., 2010, 132, 16467–16478.
21 T. Wang, Y. Chen, X. Bao, Z. Du, J. Guo, N. Wang, M. Sun and 35 D. Dini, M. J. F. Calvete and M. Hanack, Nonlinear optical
R. Yang, A new isoindigo based molecule with ideal energy
levels for solution processable organic solar cells, Dyes
Pigm., 2013, 98, 11–16.
materials for the smart ltering of optical radiation, Chem.
Rev., 2016, 116, 13043–13233.
36 L. R. Mingabudinova, V. V. Vinogradov, V. A. Milichko,
E. Hey-Hawkins and A. V. Vinogradov, Metal-organic
frameworks as competitive materials for non-linear optics,
Chem. Soc. Rev., 2016, 45, 5408–5431.
22 J. K. Lee, B.-S. Jeong, J. Kim, C. Kim and J. Ko, Synthesis and
photochemical characterization of fumaronitrile based
organic semiconductor and its use in solution processed
small molecule organic solar cells, J. Photochem. Photobiol., 37 Z. Liu, T. Lu and Q. Chen, An sp-hybridized all-carboatomic
A, 2013, 251, 25–32.
23 G. D. Sharma, J. A. Mikroyannidis, S. S. Sharma and
T. K. R. Justin Thomas, Bulk heterojunction organic
ring, cyclo[18]carbon: electronic structure, electronic
spectrum and optical nonlinearity, Carbon, 2020, 165, 461–
467.
photovoltaic devices based on small molecules featuring 38 M. S. Paley, J. M. Harris, H. Looser, J. C. Baumert,
pyrrole and carbazole and 2-(4-nitrophenyl)acrylonitrile
acceptor segments as donor and fullerene derivatives as
acceptor, Dyes Pigm., 2012, 94, 320–329.
G. C. Bjorklund, D. Jundt and R. J. Twieg,
A
solvatochromic method for determining second order
polarizabilities of organic molecules, J. Org. Chem., 1989,
54, 3774–3778.
24 J. A. Mikroyannidis, D. V. Tsagkournos, S. S. Sharma,
Y. K. Vijay and G. D. Sharma, Conjugated small molecules 39 C. Bosshard, G. Knope, P. Pretre and P. Gunter, Second
with broad absorption containing pyridine and pyran
units: synthesis and application for bulk heterojunction
solar cells, Org. Electron., 2010, 11, 2045–2054.
order
polarizabilities
of
nitropyridine
derivatives
determined with electric eld induced second harmonic
generation and a solvatochromic method: a comparative
study, J. Appl. Phys., 1992, 71, 1594–1605.
25 J. Solovjova,
T. Malinauskas, M. Daskeviciene,
E. Kasparavicius, A. Ilciukaite, A. Sackus, V. Paulauskas 40 S.-S. P. Chou, G.-T. Hsu and H.-C. Lin, Synthesis and second
and V. Getautis, Triphenylamine-based phenylhydrazone-
indolium cationic dyes for solid state DSSC applications,
Mater. Lett., 2020, 274, 128001.
order nonlinearities of sulfonyl substituted pyrrole imino
dyes, Tetrahedron Lett., 1999, 40, 2157–2160.
41 S. Bruni, E. Cariati, F. Cariati, F. A. Porta, S. Quici and
26 Nonlinear Optical Properties of Organic Molecules and Crystals,
ed. D. S. Chemla and J. Zyss, Academic Press, New York,
1987, vol. 1–2.
27 P. N. Prasad and D. J. Williams, Introduction to Nonlinear
Optical Effects in Molecules and Polymers, Wiley, New York,
1991.
D.
Roberto,
Determination
of
the
quadratic
hyperpolarizability of trans-4-[4-(dimethylamino)styryl]
pyridine and 5- dimethylamino-1,10-phenanthroline from
solvatochromism of absorption and uorescence spectra:
a comparison with electric-eld-induced second harmonic
generation technique, Spectrochim. Acta, Part A, 2001, 57,
1417–1426.
28 D. R. Kanis, M. A. Ratner and T. J. Marks, Design and
construction of molecular assemblies with large second 42 M. E. Reish, A. J. Kay, A. Teshome, I. Asselberghs, K. Clays
order optical nonlinearities. Quantum chemical aspects,
Chem. Rev., 1994, 94, 195–242.
and K. C. Gordon, Testing computational models of
hyperpolarizability in merocyanine dye using
a
29 J. Zyss and I. Ledoux, Nonlinear optics in multipolar media:
theory and experiments, Chem. Rev., 1994, 94, 77–105.
spectroscopic and DFT methods, J. Phys. Chem. A, 2012,
116, 5453–5463.
30 E. Goovaerts, W. Wenseleers, M. H. Garcia and G. H. Cross, 43 R. V. Solomon, P. Veerapandian, S. A. Vedha and
Design and characterization of organic and organometallic
molecules for second order nonlinear optics, in Nonlinear
optical materials, ed. H. S. Nalwa, Academic press, San
Diego, CA, 2001, vol. 9, pp. 127–191.
P. Venuvanalingam, Tuning nonlinear optical and
optoelectronic properties of vinyl coupled triazene
chromophores: a density functional theory and time-
dependent density functional theory investigation, J. Phys.
Chem. A, 2012, 116, 4667–4677.
31 S. Boomadevi, H. P. Mittal and R. Dhansekaran, Synthesis,
crystal growth and characterization of 3-methyl 4-
28222 | RSC Adv., 2020, 10, 28213–28224
This journal is © The Royal Society of Chemistry 2020

View Article Online
Paper
RSC Advances
44 S. B. Chemate and N. Sekar, Novel iminocoumarin 60 J. B. Birks, Photophysics of Aromatic Molecules, Wiley-
derivatives: synthesis, spectroscopic and computational
studies, J. Fluoresc., 2015, 25, 1615–1628.
45 N. Kumar, M. Paramasivam, J. Kumar, A. Gusain and
Interscience, London, 1970.
61 F. Neese, The ORCA program system, Wiley Interdiscip. Rev.:
Comput. Mol. Sci., 2012, 2, 73–78.
P. K. Hota, Substituent dependent optical properties of p- 62 T. Petrenko and F. Neese, A general efficient quantum
phenyl substituted ethenyl-E-thiophene, J. Fluoresc., 2018,
28, 1207–1216.
46 N. Kumar, M. Paramasivam, J. Kumar, A. Gusain and
chemical method for predicting absorption bandshapes,
resonance raman spectra and excitation proles for larger
molecules, J. Chem. Phys., 2007, 127, 164319.
P. K. Hota, Tuning of optical properties of p-phenyl 63 T. Petrenko, O. Krylova, F. Neese and M. Sokolowski, Optical
ethenyl-E-furans:
functional theory, Spectrochim. Acta, Part A, 2019, 206, 396–
404.
a
solvatochromism and density
absorption and emission properties of rubrene: insight by
a combined experimental and theoretical study, New J.
Phys., 2009, 11, 015001.
47 J. Liu, M. Zhang, W. Gao, A. A. Fedorchuk and I. V. Kityk, 64 A. Dreuw and M. Head-Gordon, Single reference ab initio
Synthesis and non-linear optical properties of novel
conjugated small molecules based on indole donor, J. Mol.
Struct., 2018, 1165, 223–227.
methods for the calculation of excited states of large
molecules, Chem. Rev., 2005, 105, 4009–4037.
´
ˇ
65 J. Kulhanek, F. Bures, A. Wojciechowski, M. Makowska-
Janusik, E. Gondek and I. V. Kityk, Optical operation by
chromophores featuring 4,5-dicyanoimidazole embedded
within poly(methyl methacrylate) matrices, J. Phys. Chem.
A, 2010, 114, 9440–9446.
48 M. Zhang, G. Qin, J. Liu, Z. Zhen, A. A. Fedorchuk,
G. Lakshminarayana, A. A. Albassam, A. M. El-Naggar,
K. Ozga and I. V. Kityk, Modication of indole by electron-
rich atoms and their application in novel electron donor
materials, Chem. Phys. Lett., 2017, 681, 105–109.
66 T. Yanai, D. P. Tew and N. C. Handy, A new hybrid exchange–
correlation functional using the coulomb-attenuating
method (CAM-B3LYP), Chem. Phys. Lett., 2004, 393, 51–57.
49 C. Ouyang, J. Liu, Q. Liu, Y. Li, D. Yan, Q. Wang, M. Guo and
A. Cao, Preparation of main- chain polymers based on novel
monomers with D-pi-a structure for application in organic 67 F. Weigend and R. Ahlrichs, Balance basis sets of split
second-order nonlinear optical materials with good long-
term stability, ACS Appl. Mater. Interfaces, 2017, 9, 10366–
10370.
valence, triple zeta valence and quadruple zeta valence
quality for H to Rn: design and assessment of accuracy,
Phys. Chem. Chem. Phys., 2005, 7, 3297–3305.
50 J. Kumar, N. Kumar, N. Sati and P. K. Hota, Antioxidant 68 B. S. Neporent and N. G. Bakhshiev, On the role of universal
properties of ethenyl indole: DPPH assay and TDDFT
studies, New J. Chem., 2020, 44, 8960–8970.
51 A. K. Singh and P. K. Hota, Fluorescence and
and specic intermolecular interactions in the inuence of
the solvent on the electronic spectra of molecules, Opt.
Spectrosc., 1960, 8, 408–413.
photoisomerization studies of p-nitrophenyl substituted 69 C. Reichardt, S. Lobbecke, A. M. Mehranpour and G. Schafer,
indolic ethenes, J. Phys. Org. Chem., 2006, 19, 43–52.
52 E. Wenkert, J. H. Udelhofen and N. K. Bhattacharyya, 3-
Hydroxy methyleneoxindole and its derivatives, J. Am.
Chem. Soc., 1959, 81, 3763–3768.
Pyridinium N-Phenoxide betaines and their application to
the determination of solvent polarities synthesis and Uv-
visible spectroscopic properties of new lipophillic tert-
butyl-
and
1-adamantyl
substituted,
negatively
53 G. F. Smith, Indoles. Part I. The formylation of indole and
some reactions of 3-formyl indole, J. Chem. Soc. Part IV,
1954, 3842–3846.
54 D. Nagarathnam, A facile synthesis of 3-substituted indoles,
J. Heterocycl. Chem., 1992, 29, 953–958.
solvatochromic pyridinium N-Phenolate betaine dyes, Can.
J. Chem., 1998, 76, 686–694.
70 M. B. Smith and J. March, in March's Advanced organic
chemistry, reactions, mechanism, and structure, ed. M. B.
Smith, A wiley-interscience publication, New York, 6th edn,
2007.
55 E. G. McRae, Theory of solvent effects on molecular
electronic spectra, frequency shis, J. Phys. Chem., 1957, 71 M. J. Kamlet, J.-L. M. Abboud and R. W. Ta, The
61, 562–572.
solvatochromic comparison method. 6. The p* scale of
solvent polarities, J. Am. Chem. Soc., 1977, 99, 6027–6038.
72 A. K. Singh and P. K. Hota, Photoreactivity of donor-acceptor
ethenes, Indian J. Chem., Sect. B: Org. Chem. Incl. Med. Chem.,
2003, 42, 2048–2053.
73 A. K. Singh and P. K. Hota, Absorption and uorescence
spectral properties of donor acceptor ethenes bearing
indole and p-nitrophenyl substituents, Res. Chem.
Intermed., 2005, 31, 85–101.
74 Y. Sonoda, Y. Shimoi, M. Goto, N. Tohnai and M. Kanesato,
Fluorescence properties of (E,E,E)-1,6-di(n-naphthyl)-1,3,5-
hexatriene (n ¼ 1, 2): effects of internal rotation, J. Phys.
Chem. A, 2013, 117, 566–578.
56 C. Reichardt, Solvatochromic dyes as solvent polarity
indicators, Chem. Rev., 1994, 94, 2319–2358.
57 C. Reichardt, Solvents and solvent effects in organic chemistry,
VCH, Weinheim, Basel, Germany, 2nd edn, 1990, ch. 6, pp.
285–338.
58 N. Kumar, J. Kumar and P. K. Hota, Substituent dependence
charge transfer and photochemical properties of donor-
acceptor substituted ethenyl thiophenes, J. Fluoresc., 2017,
27, 1729–1738.
59 J. L. Oudar and D. S. Chemla, Hyperpolarizabilities of the
nitroanilines and their relations to the excited state dipole
moment, J. Chem. Phys., 1977, 66, 2664.
This journal is © The Royal Society of Chemistry 2020
RSC Adv., 2020, 10, 28213–28224 | 28223

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Paper
75 C. Singh, R. Ghosh, J. A. Mondal and D. K. Palit, Excited state 78 J. Kumar, N. Kumar and P. K. Hota, Excited state and
dynamics of a push-pull stilbene: a femtosecond transient
absorption spectroscopic study, J. Photochem. Photobiol., A,
2013, 263, 50–60.
uorescence
probe
properties
of
donor-acceptor
substituted ethenes: a plausible photochromic material for
organic electronics, Indian J. Chem., Sect. B: Org. Chem.
Incl. Med. Chem., 2018, 57, 301–307.
76 C.-K. Lin, Y.-F. Wang, Y.-C. Cheng and J.-S. Yang, Multisite
constrained model of trans-4-(N,N-dimethylamino)-4- 79 P. K. Hota and A. K. Singh, Donor-acceptor conjugated linear
nitrostilbene for structural elucidation of radiative and
nonradiative excited states, J. Phys. Chem. A, 2013, 117,
3158–3164.
polyenes: a study of excited state intramolecular charge
transfer, photoisomerisation and uorescence probe
properties, J. Fluoresc., 2018, 28, 21–28.
77 N. Kumar, J. Kumar and P. K. Hota, Substituent dependent
photoreactivity of donor-acceptor phenyl ethene, Lett. Org.
Chem., 2018, 15, 479–484.
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