Photophysical study of some 3-benzoylmethyleneindol-2-ones and estimation of ground and excited states dipole moments from solvatochromic methods using solvent polarity parameters

Article abstract of DOI:10.1016/j.molstruc.2011.12.043

3-Benzoylmethyleneindol-2-ones, isatin based chalcones containing donor and acceptor moieties that exhibit excited-state intramolecular charge transfer, have been studied in different solvents by absorption and emission spectroscopy. The excited state beh

Full text of DOI:10.1016/j.molstruc.2011.12.043

Journal of Molecular Structure 1012 (2012) 73–86
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Photophysical study of some 3-benzoylmethyleneindol-2-ones and estimation
of ground and excited states dipole moments from solvatochromic methods
using solvent polarity parameters
⇑
Manju K. Saroj, Neera Sharma, Ramesh C. Rastogi
Department of Chemistry, University of Delhi, Delhi 110 007, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 13 June 2011
Received in revised form 26 December 2011
Accepted 27 December 2011
Available online 4 January 2012
3-Benzoylmethyleneindol-2-ones, isatin based chalcones containing donor and acceptor moieties that
exhibit excited-state intramolecular charge transfer, have been studied in different solvents by absorp-
tion and emission spectroscopy. The excited state behavior of these compounds is strongly dependent
on the nature of substituents and the environment. These compounds show multiple emissions arising
from a locally excited state and the two states due to intramolecular processes viz. intramolecular charge
transfer (ICT) and excited state intramolecular proton transfer (ESIPT). Excited-state dipole moments
have been calculated using Stoke-shifts of LE and ICT states using solvatochromic methods. The higher
values of dipole moments obtained lead to support the formation of ICT state as one of the prominent
species in the excited states of all 3-benzoylmethyleneindol-2-ones. The correlation of the solvatochro-
mic Stokes-shifts with the microscopic solvent polarity parameter (ENT ) was found to be superior to that
obtained using bulk solvent polarity functions. The absorption and florescence spectral characteristics
have been also investigated as a function of acidity and basicity (Ho/pH) in aqueous phase.
Ó 2012 Elsevier B.V. All rights reserved.
Keywords:
Excited state
Dipole moment
Stokes shift
ESIPT
ICT
1. Introduction
The biological activities of any compound are primarily a func-
tion of its molecular structure. It is known that the spectral behav-
Chalcones, the bichromophoric molecules separated by a keto-
vinyl chain, form a remarkable class of compounds that are known
to be effective photosensitive materials and have been of consider-
able photophysical and photochemical interest. Chalcones, both
natural and synthetic, are known to exhibit various biological
activities. They have been reported to possess antioxidant [1,2],
antimalarial [3,4], antileishmanial [5], antitumor [6], antitubercu-
losis [7] and antibacterial [8,9] activities. The presence of a reactive
ior of an organic molecule is strongly related to its structure in
both the ground and excited states. Considering that the solvent
can significantly influence the chemical and physical properties
of the solute, investigation of solvent–solute interactions is very
important [18,19]. The solvent-induced changes in the electronic
transitions of solutes are related to the nature and extent of sol-
ute–solvent interactions developed locally in the immediate vicin-
ity of solutes.
a
,b-unsaturated keto functional group in chalcones is found to be
Solvatochromism is caused by solvation of the ground and ex-
cited states of the light-absorbing molecule and thus provides a
convenient tool to study the photophysical properties and deter-
mine the excited state dipole moments [20–26]. The knowledge
of excited state charge distributions and dipole moments is impor-
tant in understanding photochemical [27,28] and biological pro-
cesses [29].
In this study, we present and discuss photophysical properties
of benzoylmethyleneindol-2-one derivatives by means of steady
state absorption and fluorescence spectroscopy. We have applied
solvatochromic shift methods to estimate the excited-state dipole
moments of a series of substituted benzoylmethyleneindol-2-ones
(see Fig. 1). We have also investigated the absorption and fluores-
cence spectral characteristics as a function of acidity and basicity
(Ho/pH) in aqueous phase.
responsible for their biological active properties, which may be al-
tered depending on the type and position of substituent on the aro-
matic rings.
Benzoylmethyleneindol-2-one, an isatin based chalcone, has
been identified as the essential synthetic building block for the
synthesis of a wide variety of spiro derivatives [10,11] and biolog-
ically active compounds [12–14]. Benzoylmethyleneindol-2-one
derivatives have been investigated for their pharmaceutical prop-
erties [15–17].
⇑
Corresponding author. Fax: +91 11 27662618.
E-mail address: rcrastogi@chemistry.du.ac.in (R.C. Rastogi).
0022-2860/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2011.12.043

74
M.K. Saroj et al. / Journal of Molecular Structure 1012 (2012) 73–86
(a)
R2
3'
R3
R1
2'
5
4
H
4'
5'
6
1'
9
3
6'
7
8
O
N
2
1
O
H
R1
R2
R3
Compounds
3-benzoylmethyleneindol-2-one
3-(4′-hydroxybenzoyl)methyleneindol-2-one
3-(4′-methylbenzoyl)methylene-indol-2-one
3-(3′,4′-dimethylbenzoyl)methyleneindol-2-one
3-(4′-methoxybenzoyl)methyleneindol-2-one
3-(2′,4′-dimethoxybenzoyl)-methyleneindol-2-one
3-(4′-chlorobenzoyl)methyleneindol-2-one
3-(2′,4′-dichlorobenzoyl)methyleneindol-2-one
3-(4′-nitrobenzoyl)-methyleneindol-2-one
Abbreviations
H
H
H
H
H
OCH3
H
Cl
H
H
H
H
CH3
H
H
H
H
H
BMI
OH
CH3
CH3
OCH3
OCH3
Cl
OHBMI
MeBMI
DiMeBMI
OMeBMI
DiOMeBMI
ClBMI
Cl
NO2
DiClBMI
NBMI
H
(b)
Fig. 1. (a) Structures of 3-benzoylmethyleneindol-2-ones. (b) Optimized geometry of BMI in ground state.
checked for the absence of absorbing or fluorescent impurities
2. Experimental
within the scanned spectral ranges. Triple distilled water was used.
Analytical grade sulfuric acid (98%, Rankem), hydrochloric acid
(35.4%, S.D. Fine, India) and sodium hydroxide (98%, BDH, India)
and triple distilled water were used to prepare acidic and basic
solutions. HCl–H2O mixture and NaOH–H2O mixture were used
to prepare pH solutions from pH 1–14, and H2SO4–H2O mixture
was used for solutions below pH = 1 according to Hammett’s acid-
ity scale (Ho) [30].
2.1. Materials
Isatin (98%, Spectrochem), acetophenone (99.5%, SRL, India), p-
hydroxyacetophenone (98%, Spectrochem), p-methylacetophenone
(97%, Spectrochem), 3,4-dimethylacetophenone (98%, Spectro-
chem), p-methoxyacetophenone (99%, Spectrochem), 2,4-dime-
thoxyacetophenone (98%, S.D. Fine, India), p-chloroacetophenone
(98%, SRL, India), 2,4-dichloroacetophenone (99%, S.D. Fine, India)
and p-nitroacetophenone (98%, Spectrochem) were used without
further purification for the synthesis of benzoylmethyleneindol-
2-one and its derivatives.
All solvents, methanol (99.7%, Merck), 1-butanol (99.8%, Merck),
2-propanol (99.7%, Merck), acetonitrile (99.8%, Merck), tert-buta-
nol (99.5%, Merck), N,N-dimethylformamide (99.8%, Merck),
dichloromethane (99.5%, Merck), ethyl acetate (99.5%, Merck),
1,4-dioxane (99.8%, Merck), cyclohexane (99.7%, Merck) and n-
Hexane (99.5%, Merck) used were of spectroscopic grade and were
2.2. Instrumentation
Absorption and fluorescence spectra were recorded by using
SHIMADZU UV-2501 PC spectrophotometer and SHIMADZU RF-
5301 PC spectrofluorometer, respectively. All measurements were
done in a 1 cm quartz cuvette. The slit widths used were 1 nm and
5 nm for absorption and fluorescence spectra, respectively. The pH
values in the range of 1–14 were measured on Eutech make cyber-
scan pH meter.

M.K. Saroj et al. / Journal of Molecular Structure 1012 (2012) 73–86
75
Table 1
n-Hexane
Cyclohexane
1,4-Dioxane
Ethyl acetate
DCM
1
(a)
Calculated values of solvent polarity parameters.
0.9
ETN
Solvent
F1(D,n)
F2(D,n)
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
n-Hexane
Cyclohexane
1,4-Dioxane
Ethyl acetate
DCM
0.009
0.006
0.164
0.228
0.309
0.386
0.389
0.460
0.546
0.586
0.762
1.000
ꢁ0.003
ꢁ0.003
0.041
0.489
0.590
0.835
0.688
0.859
0.778
0.750
0.854
0.912
0.254
0.287
0.307
0.497
0.583
0.709
0.607
0.663
0.645
0.646
0.651
0.683
DMF
Band A1
BMI
tert-Butanol
Acetonitrile
2-Propanol
1-Butanol
Methanol
Water
DMF
Band A2
tert-Butanol
Acetonitrile
2-Propanol
1-Butanol
Methanol
Water
210
260
310
360
410
460
Wavelength/nm
Freshly prepared solutions were used for all the studies. Con-
centration of all the solutions was kept 4 ꢀ 10ꢁ5
M and
n-Hexane
Cyclohexane
1,4-Dioxane
Ethyl acetate
DCM
1 ꢀ 10ꢁ5 M for absorption and fluorescence spectra, respectively
for all the studies. The absorption band maximum A2 was used
as excitation wavelength to record emission spectra of BMI deriv-
atives (Table 2). Excitation wavelength used to record the fluores-
cence spectra in different acidic and basic media was 350 nm for all
studied derivatives.
PICT
(b)
BMI
DMF
t-Butanol
Acetonitrile
2-Propanol
1-Butanol
Methanol
Water
2.3. Synthesis
The benzoylmethyleneindol-2-one derivatives were synthe-
sized according to the standard procedures from literature
[31,32] and their purity was confirmed by TLC, IR and 1H NMR.
LE
ESIPT
2.3.1. 3-Benzoylmethyleneindol-2-one (BMI)
A
mixture of indoline-2,3-dione (0.01 mol), acetophenone
(0.01 mol), diethylamine (3–4 drops) and ethanol (30 ml) was re-
fluxed on oil bath for 2–3 h at 78 °C and the progress of reaction
was monitored by TLC in 50% ethyl acetate and 50% petroleum
ether solvent. The reaction mixture was kept overnight at room
temperature to give intermediate (3-hydroxy-3-benzoylmethylen-
eindol-2-one), which separated as a precipitate. The compound
thus obtained was recrystallized from chloroform by adding petro-
leum ether. The intermediate product was then refluxed with 1, 4-
dioxane (30 ml) and concentrated H2SO4 (1–2 drops) for 1-2 h on
oil bath at 100 °C. After completion of reaction, the reaction mix-
ture was poured into crushed ice with vigorous stirring. The solid
thus separated was extracted with ethyl acetate and the moisture
was removed by adding Na2SO4(s). After filtration, ethyl acetate
was evaporated. The solid thus obtained was recrystallised from
chloroform by adding petroleum ether. The separated orange–red
Fig. 2. (a) Electronic absorption spectra of BMI in different solvents. (b) Fluores-
cence emission spectra of BMI in different solvents.
crystals were characterized by IR and 1H NMR whose spectral data
are given in the next section. Other derivatives were also synthe-
sized using the same procedure.
2.4. Spectral data
2.4.1. 3-Benzoylmethyleneindol-2-one (BMI)
Yield: 76% M.p.: 192–194 °C IR (KBr, cmꢁ1): 3426
1712, 1655 m(C@O), 1605 m
m
(NH), 3155,
(C@O) (amide), 1460, 1330; 1H NMR
Table 2
Absorption maxima of 3-benzoylmethyleneindol-2-one and its derivatives in different solvents.
Solventa
BMI
OHBMI
MeBMI
DiMeBMI
OMeBMI
DiOMeBMI
ClBMI
DiClBMI
NBMI
kA1/kA2 (nm)
kA1/kA2 (nm)
kA1/kA2 (nm)
kA1/kA2 (nm)
kA1/kA2 (nm)
kA1/kA2 (nm)
kA1/kA2 (nm)
kA1/kA2 (nm)
kA1/kA2 (nm)
n-Hexane
Cyclohexane
1,4-Dioxane
Ethyl acetate
DCM
257
259
259
257
259
–
261
259
260
260
261
257
333
335
335
335
337
337
339
334
339
340
340
336
258
258
260
259
259
–
259
259
259
260
258
257
331
333
334
334
336
337
336
332
338
337
336
338
259
258
260
259
259
265
260
259
260
260
259
259
332
333
334
334
337
338
337
332
337
338
339
338
258
259
261
260
259
–
260
259
260
260
257
261
333
335
335
335
336
338
338
333
338
338
341
338
NSb
NS
259
259
258
–
259
257
259
259
257
254
NS
NS
259
259
260
261
261
266
262
260
262
262
265
265
333
334
336
334
331
338
338
328
338
340
343
330
258
259
261
260
260
266
261
262
261
261
264
258
334
335
335
335
338
337
339
332
339
339
340
341
258
259
259
259
259
–
260
259
260
259
259
259
334
336
337
335
339
340
339
334
339
341
339
340
266
267
267
267
269
–
267
267
266
267
274
NS
340
341
343
342
344
344
347
341
345
346
346
NS
337
339
339
344
345
335
345
345
343
347
DMF
tert-Butanol
Acetonitrile
2-Propanol
1-Butanol
Methanol
Water
a
Solvents are listed in order of increasing dielectric constant values.
Not soluble in this solvent.
b

76
M.K. Saroj et al. / Journal of Molecular Structure 1012 (2012) 73–86
Table 3
Fluorescence maxima of 3-benzoylmethyleneindol-2-one and its derivatives in different solvents (Ex. wavelength is kA2).
Solventa
BMI
OHBMI
MeBMI
DiMeBMI
OMeBMI
DiOMeBMI
ClBMI
DiClBMI
NBMId
kF1/kF2/kF3
kF1/kF2/kF3
kF1/kF2/kF3
kF1/kF2/kF3
kF1/kF2/kF3
kF1/kF2/kF3
kF1/kF2/kF3
kF1/kF2/kF3
kF1/kF2/kF3
kLE/kICT/kESIPT
kLE/kICT/kESIPT
kLE/kICT/kESIPT
kLE/kICT/kESIPT
kLE/kICT/kESIPT
kLE/kICT/kESIPT
kLE/kICT/kESIPT
kLE/kICT/kESIPT
kLE/kICT/kESIPT
(nm)
(nm)
(nm)
(nm)
(nm)
(nm)
(nm)
(nm)
(nm)
n-Hexane
374 402 486 382 405 541 376 400 545 387
–
–
545 NSb NS NS 385 406 479 382 408 577 384 410 518 NFc NF NF
Cyclohexane 374 402 485 384 411 535 377 398 545 387
1,4-Dioxane 396 430
Ethyl acetate 394 432
DCM
DMF
547 NS NS NS 379 407 487 382 406 577 387 419 518 NF
NF NF
NF NF
NF NF
NF NF
NF NF
NF NF
NF NF
NF NF
NF NF
NF NF
NS NS
–
–
–
–
–
–
–
–
–
–
399 434
398 432
400 434
401 439
406 445
401 441
407 448
407 444
408 446
424 464
–
–
–
–
–
–
–
–
–
–
392 417
398 439
399 441
408 455
412 458
409 458
411 455
412 458
411 459
412 462
–
–
–
–
–
–
–
–
–
–
397 430
395 426
418 449
417 454
400 443
413 448
403 443
401 444
399 443
409 447
–
–
–
–
–
–
–
–
–
–
373 407
375 403
411 439
375 402
381 408
372 402
378 423
378 412
380 419
407 439
–
–
–
–
–
–
–
–
–
–
424 446
425 443
403 432
422 431
416 454
412 464
411 450
411 442
420 457
411 463
–
–
–
–
–
–
–
–
–
–
394 424
395 429
394 431
398 426
405 436
398 434
404 434
405 437
406 439
410 459
–
–
–
–
–
–
–
–
–
–
399 438
403 440
398 444
411 450
414 449
407 457
415 453
415 450
415 458
427 491
–
–
–
–
–
–
–
–
–
–
NF
NF
NF
NF
NF
NF
NF
NF
NF
NS
398 438
404 447
tert-Butanol 407 442
Acetonitrile
2-Propanol
1-Butanol
Methanol
Water
398 444
412 446
411 452
412 454
424 464
a
b
c
Solvents are listed in order of increasing dielectric constant values.
Not soluble in this solvent.
No fluorescence observed in this solvent.
Not used in dipole moment calculation.
d
160
140
120
100
80
BMI in Methanol
ICT
(a)
OHBMI
60
40
R = 0.996
600
20
0
350
LE
400
450
500
550
650
Wavelength/nm
140
120
100
80
BMI in Acetonitrile
ESIPT
60
(b)
40
R = 0.978
600
20
ICT
0
350
400
450
500
550
650
MeBMI
Wavelength/nm
90
80
70
60
50
40
30
20
10
0
BMI in n-Hexane
LE
ESIPT
R = 0.995
600
350
400
450
500
550
650
Wavelength/nm
Fig. 3. (a) Fluorescence spectra of OHBMI in different solvents. (b) Fluorescence
spectra of MeBMI in different solvents.
Fig. 4. Deconvoluted fluorescence emission spectra of BMI in methanol, acetonitrile
and n-hexane.
(300 MHz, DMSO-d6, ppm): d 6.84 (d, J = 6 Hz, 1H, ArAH), 7.03 (t,
1H, ArAH), 7.31 (t, 1H, ArAH), 7.80 (s, 2H, ArAH), 7.49 (d,
J = 9 Hz, 2H, ArAH), 8.04 (d, J = 9 Hz, 2H, ArAH), 8.33 (d, J = 6 Hz,
1H, ArAH), 10.45 (s, 1H, ANH).
2.4.2. 3-(4-Hydroxybenzoyl)methyleneindol-2-one (OHBMI)
Yield: 61% M.p.: 140–142 °C IR (KBr, cmꢁ1): 3441
m(NH), 3200–
2900 m(OH), 1711, 1657 m(C@O), 1608 m(C@O) (amide), 1460, 1329;

M.K. Saroj et al. / Journal of Molecular Structure 1012 (2012) 73–86
77
..
(a)
R
R
HN
HN
O
O
O
O
I (LE)
II (ICT)
R
R
HN
O
HN
O
O
O
II'
(b)
(c)
R
R
HN
N
O
O
OH
O
II
III
Lactim
Lactum
R
R
HN
O
HN
O
O
O
IV
II
Fig. 5. (a) Possible resonance structures of BMI and its derivatives. (b) ESIPT mechanism for the BMI and its derivatives. (c) Charge separated mesomeric form of BMI and its
derivatives.
1H NMR (300 MHz, DMSO-d6, ppm): d 6.78 (s, 1H, AOH), 6.82–7.98
(m, 8H, ArAH), 7.69 (1H, S, ACOACH@), 10.80 (s, 1H, ANH).
7.28 (t, 1H, ArAH), 7.68 (s, 1H, ACOACH@), 7.96 (d, J = 9 Hz, 1H,
ArAH), 8.04 (d, J = 9 Hz, 2H, ArAH), 10.45 (s, 1H, ANH).
2.4.3. 3-(4-Methylbenzoyl)methylene-indol-2-one (MeBMI)
2.4.6. 3-(2,4-Dimethoxybenzoyl)-methyleneindol-2-one (DiOMeBMI)
Yield: 80% M.p.: 204–206 °C IR (KBr, cmꢁ1): 3433
m
(NH), 3200,
Yield: 79% M.p.: 164–166 °C IR (KBr, cmꢁ1): 3423
m
(NH), 3158,
1712, 1656
m
(C@O), 1609
m
(C@O) (amide), 1459, 1329; 1H NMR
1716, 1655 m(C@O), 1615 m
(C@O) (amide), 1464, 1330, 1139; 1H
(300 MHz, DMSO-d6, ppm): d 2.41 (s, 3H, ACH3), 6.86 (d, J = 9 Hz,
1H, ArAH), 6.92 (t, 1H, ArAH), 7.31 (t, 1H, ArAH), 7.40 (d,
J = 9 Hz, 1H, ArAH), 7.70 (s, 1H, ACOACH@), 7.96 (m, 4H, ArAH),
10.80 (s, 1H, ANH).
NMR (300 MHz, DMSO-d6, ppm): d 3.84 (s, 6H, 2xOCH3), 6.65–
6.68 (m, 2H, ArAH), 6.83 (d, J = 9 Hz, 1H, ArAH), 6.88 (t, 1H, ArAH),
7.25 (t, 1H, ArAH), 7.63 (s, 1H, ACOACH@), 7.78 (d, J = 9 Hz, 1H,
ArAH), 8.08 (d, J = 9 Hz, 1H, ArAH), 10.62 (s, 1H, ANH).
2.4.4. 3-(3,4-Dimethylbenzoyl)methyleneindol-2-one (DiMeBMI)
2.4.7. 3-(4-Chlorobenzoyl)methyleneindol-2-one (ClBMI)
Yield: 63% M.p.: 174–176 °C IR (KBr, cmꢁ1): 3434
m
(NH), 3161,
Yield: 81% M.p.: 208–210 °C IR (KBr, cmꢁ1): 3431
m
(NH), 3158,
1709, 1656
m
(C@O), 1609
m
(C@O) (amide), 1461, 1337; 1H NMR
1719, 1659 m(C@O), 1622 m
(C@O) (amide), 1461, 1330, 889; 1H
(300 MHz, DMSO-d6, ppm): d 2.33 (s, 6H, 2xCH3), 6.80–7.98 (m,
7H, ArAH), 7.70 (s, 1H, ACOACH@), 10.81 (s, 1H, ANH).
NMR (300 MHz, DMSO-d6, ppm): d 7.68 (s, 1H, ACOACH@),
6.84–8.10 (m, 8H, ArAH), 10.91 (s, 1H, ANH).
2.4.5. 3-(4-Methoxybenzoyl)methyleneindol-2-one (OMeBMI)
2.4.8. 3-(2,4-Dichlorobenzoyl)methyleneindol-2-one (DiClBMI)
Yield: 82% M.p.: 238–240 °C IR (KBr, cmꢁ1): 3435
m
(NH), 3197,
Yield: 90% M.p.: 224–226 °C IR (KBr, cmꢁ1): 3449
m
(NH), 2924,
1713, 1622
m
(C@O), 1618
m
(C@O) (amide), 1462, 1337, 1173; 1H
1723, 1662 m(C@O), 1602 m
(C@O) (amide), 1461, 1331,744; 1H
NMR (300 MHz, DMSO-d6, ppm): d 3.83 (s, 3H, AOCH3), 6.86 (d,
J = 6 Hz, 1H, ArAH), 6.89 (t, 1H, ArAH), 7.08 (d, J = 9 Hz, 2H, ArAH),
NMR (300 MHz, DMSO-d6, ppm): d 6.83 (d, J = 10 Hz, 1H, ArAH),
7.23 (t, 1H, ArAH), 7.83 (s, 1H, ACOACH@), 7.41 (t, 1H, ArAH),

78
M.K. Saroj et al. / Journal of Molecular Structure 1012 (2012) 73–86
A*(FC)
S1
A*(LE)
S1'
A*(ICT)
S1''
A*(ESIPT)
S1'''
hvabs
hv
F1
hv
F2
hv
F3
Ao
(ESIPT)
Ao
'''
So
(ICT)
So''
Ao
'
(LE)
So
So
A
R
R
R
N
O
N
N
O
O
O
O
O
H
H
H
A* (ICT)
A* (ESIPT)
A*(LE)
Fig. 6. Energy-reaction coordinate diagram of ground and excited state processes proposed for the interpretation of the photophysical behavior of donor–acceptor BMI and its
derivatives, where FC = Franck Condon state, LE = locally excited state, ICT = intramolecular charge transfer, ESIPT = excited state intramolecular proton transfer and A = BMI
and its derivatives.
ꢀ
ꢀ
ma
ꢁ
mf ¼ S1F1ðD; nÞ þ C1
ð1Þ
ð2Þ
7.60–7.63 (m, 1H, ArAH), 7.80–7.84 (m, 2H, ArAH), 8.31 (d,
J = 9 Hz, 1H, ArAH), 10.85 (s, 1H, ANH).
ꢀ
ꢀ
1=2ðma
þ
mf Þ ¼ S2F2ðD; nÞ þ C2
ꢀ
ꢀ
where ma and mf are the absorption and fluorescence maxima, n and
D are the refractive index and the dielectric constant of the solvents,
respectively. The slopes S1 and S2 are expressed as
2.4.9. 3-(4-Nitrobenzoyl)-methyleneindol-2-one (NBMI)
Yield: 79% M.p.: 246–248 °C IR (KBr, cmꢁ1): 3326
m
(NH), 3170,
(C@O) (amide), 1518, 1461, 1334,
1145; 1H NMR (300 MHz, DMSO-d6, ppm):
7.74 (s, 1H,
1722, 1666
m(C@O), 1605 m
2
2ðlꢂ
hca3
where lꢂ and
ꢁ
l
Þ
ꢁ2ðlꢂ2
hca3
are respectively excited and ground state dipole
ꢁ
l2
Þ
d
S1 ¼
and S2 ¼
ð3Þ
ACOACH@), 6.89–8.45 (m, 8H, ArAH), 10.83 (s, 1H, ANH).
l
2.5. Methods: dipole moment calculation
moments of the solute molecule and ‘a’ is the Onsager cavity radius.
F1(D,n) and F2(D,n) are taken as
The dipole moment of a molecule in the excited state is deter-
mined by the effect of electric field (internal or external) on the po-
sition change of the absorption and fluorescence bands. Two
methods depending on the internal electric field (solvatochro-
mism) were employed in the present study.
In the first method, by employing the simplest quantum-
mechanical second order perturbation theory and taking into ac-
count Onsager’s model, Kawski et al. [33–40] have obtained the
ꢀ
ꢁ
D ꢁ 1 n2 ꢁ 1
2n2 þ 1
n2 þ 2
F1ðD; nÞ ¼
ꢁ
ꢀ
ð4Þ
ð5Þ
D þ 2 n2 þ 2
1
3=2ðn4 ꢁ 1Þ
F2ðD; nÞ ¼ F1ðD; nÞ þ
2
ðn2 þ 2Þ
2
ꢀ
ꢀ
ꢀ
ꢀ
f ) against the bulk
Plots of the Stokes-shifts (ma
ꢁ
m
f ) and 1/2(ma
þ
m
solvent polarity functions (F1(D,n) and F2(D,n)) for different sol-
ꢀ
ꢀ
m
ꢀ
ꢀ
m
following expressions for (ma
ꢁ
f ) and 1/2(ma
þ
f ):
vents yield the slopes S1 and S2, respectively. Assuming that the

M.K. Saroj et al. / Journal of Molecular Structure 1012 (2012) 73–86
79
Table 4
Absorbance and fluorescence maxima assignment of different prototropic species in Ho/pH range ꢁ5 to 14.
Ho/
pH
BMI
OHBMI
ClBMI
Absorbance
Fluorescenceb
Absorbance
Fluorescence
Absorbance
Fluorescence
kA1/kA2
(nm)
Assigned
species
kLE/kICT
(nm)
Assigned
speciesa
kA1/kA2
(nm)
Assigned
species
kLE/kICT
(nm)
Assigned
species
kA1/kA2
(nm)
Assigned
species
kLE/kICT
(nm)
Assigned
species
ꢁ5
ꢁ4
ꢁ3
ꢁ2
ꢁ1
1
258, 369
259, 362
261, 360
265, 358
265, 358
263, 350
265, 346
C
C
C
C
C
C
C
470
466
466
466
C
C
C
C
256, 366
254, 363
256, 360
257, 357
258, 357
261, 352
262, 349
C
C
C
C
C
C
C
480
478
478
476
476
472
472
C
C
C
C
C
C
C
265, 359
265, 359
263, 354
266, 352
265, 345
266, 344
267, 344
C
C
C
C
C
C
C
485
472
472
468
460
460
C
C
C
C
C
C
466
C
424sh, 502
424sh, 502
NLE + HB
NLE + HB
2
415sh
460
,
,
NLE + C
3
4
266, 344
264, 340
C
424sh, 502
NLE + HB
262, 349
262, 340
C
424sh
470
424, 455 NLE + NICT
,
NLE + C
267, 344
266, 340
C
415sh
460
415, 460 NLE + NICT
NLE + C
N
424, 454,
500sh
NLE + NICT
N
N
5
6
7
8
9
10
11
12
13
14
266, 340
266, 340
264, 340
267, 340
266, 340
266, 340
266, 340
265, 340
270, 346
274, 350
N
N
N
N
N
N
N
N
A
424, 454
424, 454
424, 454
424, 454
424, 454
424, 454
424, 454
424, 458
406
NLE + NICT
NLE + NICT
NLE + NICT
NLE + NICT
NLE + NICT
NLE + NICT
NLE + NICT
NLE + NICT
A
262, 340
262, 340
260, 340
262, 340
261, 340
262, 340
261, 340
261, 340
273, 344
275, 344
N
N
N
N
N
N
N
N
A
424, 456 NLE + NICT
424, 456 NLE + NICT
424, 456 NLE + NICT
424, 456 NLE + NICT
424, 456 NLE + NICT
424, 456 NLE + NICT
424, 456 NLE + NICT
424, 456 NLE + NICT
266, 340
266, 340
266, 340
266, 340
266, 340
265, 340
261, 340
264, 340
264, 366
266, 377
N
N
N
N
N
N
N
N
A
415, 450 NLE + NICT
415, 450 NLE + NICT
415, 450 NLE + NICT
415, 450 NLE + NICT
415, 450 NLE + NICT
415, 450 NLE + NICT
415, 450 NLE + NICT
415, 450 NLE + NICT
407
407
A
A
395
395
A
A
A
406
A
A
A
a
Species assigned as N = neutral, C = cation, A = anion and HB = intramolecular hydrogen bonding in cationic species.
Excitation wavelength used to record the fluorescence spectra is 350 nm.
b
1
BMI
OHBMI
(b) 1.4
(a)
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
Ho = -5
1.2
pH = 7
Ho = -5
pH = 7
pH = 14
1
pH = 14
0.8
0.6
0.4
0.2
0
240
340
440
540
640
240
340
440
540
640
Wavelength/nm
Wavelength/nm
1
ClBMI
(c)
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
Ho = -5
pH = 7
pH = 14
240
340
440
540
640
Wavelength/nm
Fig. 7. (a) Absorbance spectra of BMI in water at selected Ho/pH. (b) Absorbance spectra of OHBMI in water at selected Ho/pH. (c) Absorbance spectra of ClBMI in water at
selected Ho/pH.

80
M.K. Saroj et al. / Journal of Molecular Structure 1012 (2012) 73–86
dye [48], whereas
D
l
and ‘a’ are the corresponding quantities for
250
200
150
100
50
Ho = -5
Ho = -4
Ho = -3
Ho = -2
Ho = -1
pH = 1
pH = 2
pH = 3
pH = 4
pH = 5
pH = 7
pH = 13
BMI
(a)
ꢀ
maf
the molecule of interest (isatin chalcones). Thus, the plot of
D
(Stokes-shifts) versus ENT (Eq. (7)) can be used to obtain the change
in the dipole moment ( ) upon excitation.
Dl
The Onsager cavity radii of all molecules have been taken to be
40% of the longest axis in the optimized geometry of the molecule
[49], obtained by using semiempirical calculations. Thus, by know-
ing ground state dipole moment ‘l’ from theoretical calculations,
excited state dipole moment, lꢂ can be determined.
2.6. Theoretical calculations
Parametric Method 3 (PM3) semiempirical molecular-orbital
calculations were carried out to estimate the ground state and ex-
cited-state dipole moments. The Onsager cavity radii of all the mol-
ecules were determined theoretically using the optimized
geometry. The quantum chemical package HyperChem Release
5.1 Pro [50] was used for the theoretical calculations reported here.
0
360
410
460
510
560
610
660
Wavelength/ nm
500
450
400
350
300
250
200
150
100
50
Ho = -5
OHBMI
(b)
Ho = -4
Ho = -3
Ho = -2
Ho = -1
pH = 1
pH = 2
pH = 3
pH = 4
pH = 5
pH = 7
pH = 13
3. Results and discussion
3.1. Absorption and emission properties
Absorption and fluorescence spectra of benzoylmethyleneindol-
2-ones were studied in twelve solvents of different polarity.
Absorption and fluorescence spectra of BMI in different solvents
are shown in Fig. 2 and the corresponding spectral data of BMI
and its derivatives are summarized in Tables 2 and 3.
0
3.1.1. Absorption spectra
360
410
460
510
560
610
660
Absorption spectra of BMI and its derivatives exhibit two main
bands in all the solvents (Table 2). The first band (band A1) is lo-
cated in the range 250–270 nm and does not show any change to-
wards the polarity of the solvent as well as the various substituents
on the phenyl ring. The second band (band A2, 330–350 nm) is
slightly structured and shows moderate sensitivity (3–6 nm) to-
wards both solvent polarity and the substituents as compared to
band A1. From this it can be concluded that the substitution as well
as solvent effects are almost negligible in the ground state of the
molecules.
Apart from the two strong absorption peaks (A1 and A2), one
low intensity broad band has also been observed around 420 nm
(Fig. 2a). For the assignment of absorption bands, the absorption
spectrum of BMI was compared with the absorption spectra of isat-
in. This low-intensity broad band (ꢄ420 nm) may be assigned to
the characteristic band of ANHACOA functional group, as this is
similar in shape and position to isatin absorption spectra, and
stems from the transition in the ANHACOA functional group. Evi-
dently, isatin displays three bands, ꢄ241 nm and ꢄ311 nm due to
the presence of benzene ring, and ꢄ417 nm due to NHACOACOA
functional group [51].
Wavelength/ nm
Fig. 8. (a) Fluorescence spectra of BMI in water at selected Ho/pH (Ex. 350 nm). (b)
Fluorescence spectra of OHBMI in water at selected Ho/pH (Ex. 350 nm).
angle between
l
and lꢂ is small and the cavity radius ‘a’ is same in
both the ground and the excited states and using the Eqs. (1)–(3),
the ratio of the dipole moments in the excited and ground states
is given by the equation:
l
l
ꢂ
jS1 ꢁ S2j
jS1 þ S2j
¼
ð6Þ
It may be noted that most theories [41–46] of the solvent effect on
ꢀ
m
ꢀ
m
the location of the absorption (
a) and fluorescence (
f ) bands lead,
ꢀ
ꢀ
mf
in spite of different assumptions, to similar expressions for ma
ꢁ
ꢀ
ꢀ
f ) (Eqs. (1) and (2)) with the difference, however, that
and 1/2(ma
þ
m
the applied solvent polarity parameters (F1(D,n) and F2(D,n)), differ
significantly.
In the second method [47], the problem associated with the er-
ror in estimating the Onsager radius ‘a’ is minimized, since the ra-
tio of two Onsager radii (aB/a) is involved Eq. (7). The excited-state
dipole moment is determined by
Attention was focused on the absorption band A2 showing a
slight bathochromic shift from nonpolar to polar solvent. This band
has been assigned to a charge transfer from the high-energy, occu-
pied orbital of the substituted phenyl-donor group to the low-en-
ergy, unoccupied orbital of the acceptor carbonyl group (i.e.
2
3
ꢀ
D
maf ¼ 11307:6½ð
D
l=
D
lBÞ ðaB=aÞ ꢃEN þ constant
ð7Þ
T
ꢀ
where
D
maf is the Stokes-shifts and ENT is the solvent-polarity param-
?
pꢂC@O, [52,53]) of the molecule.
eter proposed by Reichardt [48] that correlates better than the tra-
ditionally used bulk solvent polarity functions. ENT is based on the
absorption wavenumber of a standard betaine dye in the solvent.
Its value for different solvents is given in Table 1 and is expressed as
pPh
3.1.2. Fluorescence spectra
The wavelength of absorption band (A2) was used as excitation
wavelength to record the emission spectra of BMI and its deriva-
tives. Fluorescence spectra of BMI and its derivatives split into
two bands in all the solvents (Fig. 2b, Fig. 3a and b). Exceptionally,
along with the dual emissive bands a long wavelength band has
also been observed to exist in cyclohexane and n-hexane. Hence,
ET ðsolventÞ ꢁ ET ðTMSÞ
ETN
¼
ð8Þ
ET ðwaterÞ ꢁ ET ðTMSÞ
D
lB (=9 D) and RB (=6.2 Å) are the dipole-moment changes upon
excitation and the Onsager cavity radius, respectively, of betaine

M.K. Saroj et al. / Journal of Molecular Structure 1012 (2012) 73–86
81
A
LE(S1)
N
LE(S1)
C
LE(S1)
A
ICT (S1)
N
ICT (S1)
C
ICT (S1)
C
ICT (So)
N
A
ICT (So)
ICT (So)
A
(So)
C
N
(So)
(So)
Basic
medium
Acidic
medium
Neutral
medium
NLE = Neutral locally excited state,
NICT = Neutral intramolecular charge transfer State,
C
C
LE = Cationic locally excited state ALE = Anionic locally excited state
ICT = Cationic intramolecular charge transfer State,
AICT = Anionic intramolecular charge transfer State
Fig. 9. Protonation and deprotonation phenomena followed by BMI derivatives in ground and excited state in Ho/pH range from ꢁ5 to 14.
the fluorescence spectra seem to result from more than one form of
excited state. In order to quantify these changes, fluorescence spec-
tra have been fitted as a sum of two Gaussian curve-fitting pro-
Assignment of the bands as LE and ICT can be further confirmed
by the fact that the intensity of ICT emission (F2, more polar state)
increases with increase in polarity of the solvent while the LE emis-
sion (F1, less polar state) shows an opposite dependence on polarity
of the solvent. This is because ICT and LE show strong and weak sol-
ute–solvent interactions with the polar solvent, respectively [54].
The structure and nature of the ICT can be clearly depicted from
the possible resonance states, II and II0 (Fig. 5a), where the car-
bonyl group has been considered to be the electron acceptor while
the rest of the molecule to be the donor. The close vicinity of strong
electron donating phenyl group towards electron accepting car-
bonyl group facilitates the formation of resonance structure II in
the excited state, explaining the substantial charge separation
upon excitation.
grams. Fig.
4 displays the deconvoluted spectra of BMI in
methanol, acetonitrile and n-hexane. The wavelengths of the max-
ima from deconvolution are compiled in Table 3.
For ease of explanation of results, emission spectra of BMI and
its derivatives may be divided into three regions. The nearest (band
F1), intermediate (band F2) and the longest wavelength (band F3)
bands are in the range of 370–400 nm, 400–440 nm and 540–
580 nm (Table 3), respectively (Figs. 2b and 3a and b).
The emission maxima of bands F1 and F2 show large bathochro-
mic shifts with increase in solvent polarity. The shorter wavelength
band (F1) has less bathochromic shift (ꢄ20–50 nm, Table 3) as
compared to the band F2 (ꢄ 20–80 nm, Table 3) from nonpolar
to polar solvents and can be assigned to emission from the locally
excited state (LE). The marked red shift of the fluorescence band, F2
is due to large difference in the electron distribution (originating
from charge transfer) of ground and excited states of the molecule
which makes the molecule highly polar in excited state resulting in
significant interaction with polar solvents. Such behavior indicates
that the emission takes place not only from the initially populated
excited state (locally excited state, LE), but also from a more polar
state with a charge transfer character (intramolecular charge
transfer state, ICT). Therefore, bands F1 and F2 arise from two dif-
ferent states termed as locally excited (LE) state and the intramo-
lecular charge-transfer state (ICT), respectively.
However, the possibility of resonance structure II0 to ICT is ruled
out. The ICT state (band F2) is strongly favorable in all BMI deriv-
atives due to the greater electron donating power of phenyl ring at-
tached to the carbonyl group, which leads to the formation of
coplanar (quinoid) structure (resonance structure II in Fig. 5).
Since BMI derivatives contain the essential ACOANHA group
for proton transfer, they can undergo the lactum–lactim tautomer-
ism as in the case of isatin [55–58]. Even though the molecules are
fully conjugated in ICT state (Fig. 5b, resonance structure II), intra-
molecular proton transfer can easily take place in excited state to
provide more energy stabilization (Fig. 5b, resonance structure
III) [59,60]. But the formation of this lactim form (structure III in
Fig. 5b) is possible only in highly nonpolar solvents (n-hexane

82
M.K. Saroj et al. / Journal of Molecular Structure 1012 (2012) 73–86
(a)
..
R
R
CGS/LE
CICT
HN
O
HN
O
OH
OH
..
..
R
R
NGS/LE
NICT
HN
O
HN
O
O
O
..
..
R
R
AGS/LE
N
O
AICT
N
O
O
O
(b)
H
H
Intramolecular
hydrogen
bonding
HN
O
HN
O
O
H
O
H
Fig. 10. (a) Possible tautomerization, protonation and deprotonation mechanism of BMI derivatives in ground and excited states (LE + ICT) in Ho/pH range from ꢁ5 to 14
where, NGS = neutral ground state, AGS = anionic species in ground state, CGS = cationic species in ground state, NLE = neutral locally excited state, NICT = neutral intramolecular
charge transfer excited state, ALE = anionic species in locally excited state, CLE = cationic species in locally excited state, AICT = intramolecular charge transfer in anionic
species, CICT = intramolecular charge transfer in cationic species. (b) Intramolecular hydrogen bonding in BMI in excited state in acidic medium.
and cyclohexane) due to absence of strong solute–solvent interac-
tion in comparison to all other solvents.
strongly on the solvent polarity as well as the acceptor strength
of the carbonyl group.
By considering the equilibrium Lactum M Lactim (II M III), in
solvents of varying polarity, it has been found that increase in sol-
vent polarity shifts the equilibrium towards the Lactum form. This
form (Lactum) is more dipolar than hydroxyl-form (Lactim) due to
the contribution of the charge-separated mesomeric form (Fig. 5c,
IV). While in nonpolar solvents such as cyclohexane and n-hexane
both tautomers exist in comparable amounts, the tautomeric equi-
librium is shifted entirely in favor of the lactum form in polar sol-
vents such as water. Thus, it is well-established that the longest
wavelength band, F3 corresponds to emission of the excited state
intramolecular proton transfer (ESIPT) [48].
Therefore, a mechanism followed by BMI derivatives in excited
state can be proposed on the basis of fluorescence results and nat-
ure of the emitting states (shown in Fig. 6) [19,61]. As is clear from
the explanation, excitation leads to population of Frank-Condon
(FC) state that seems to decay by three competing pathways: (1)
fluorescence to the ground state (LE), (2) relaxation of LE state to
a closely lying ICT state and (3) ICT state to ESIPT state (Fig. 6). Con-
sequently, the fluorescence bands (F1, F2 and F3) considered to
arise from these three states are termed as the locally excited
(band F1, LE) state, the intramolecular charge transfer state (band
F2, ICT) [62] and the excited state intramolecular proton transfer
state (band F3, ESIPT) [59] observed only in nonpolar solvents
(see Table 3).
NBMI is non-fluorescent owing to greater electron withdrawing
capacity of nitro group. The lack of fluorescence in nitro derivative
can be explained on the basis of the strong electron-withdrawing
capacity of the nitro group (Table 3). Aromatic nitro compounds
are in general only weakly fluorescent or completely non-fluores-
cent, due to the efficient nonradiative decay processes such as sin-
glet–triplet intersystem crossing and internal conversion [63].
3.1.3. Effect of pH
The effect of pH on absorption and fluorescence spectra of three
selected 3-benzoylmethyleneindol-2-one derivatives namely, BMI,
OHBMI and ClBMI has been studied in water at the Ho/pH range
from ꢁ5 to 14. The relevant data are compiled in Table 4. The
absorption spectra and fluorescence spectra of BMI and OHBMI at
selected Ho/pH are shown in Figs. 7 and 8, respectively.
The outlays of protonation and deprotonation phenomena fol-
lowed by the BMI derivatives and possible tautomerization, pro-
tonation and deprotonation mechanisms in Ho/pH range of ꢁ5 to
14 are shown in Figs. 9 and 10, respectively. Neutral BMI deriva-
tives show the presence of two excited states: the neutral LE and
ICT denoted as NLE and NICT. In acidic and basic medium, BMI deriv-
atives show the formation of cation and anion (CLE, ALE, CICT and
AICT) for both LE and ICT neutral states. In highly acidic condition
CICT state is more dominant in comparison to the CLE state as evi-
dent from their fluorescence spectra (discussed latter).
It is clear from the absorption spectral data (Table 4) that the
spectral characteristics of BMI, OHBMI and ClBMI in the pH range
The foregoing results show that the equilibria among the LE
(band F1), ICT (band F2) and ESIPT (band F3) states determine their
relative contributions to the emission band, which depends

M.K. Saroj et al. / Journal of Molecular Structure 1012 (2012) 73–86
83
(a)
(b)
9000
7500
6000
4500
3000
1500
0
9000
7500
6000
4500
3000
1500
0
y = 2226.8x + 5504.5
R = 0.908
y = 1544.2x + 3771.2
R = 0.847
0.0
0.2
0.4
0.6
0.8
1.0
0.0
0.2
0.4
0.6
0.8
1.0
F1 (D,n)
F1 (D,n)
30000
28500
27000
25500
24000
30000
28500
27000
25500
24000
y = -3547.4x + 29328
R = 0.940
y = -3376.4x + 28184
R = 0.936
0
0.2
0.4
0.6
0.8
0
0.2
0.4
0.6
0.8
F2 (D,n)
F2 (D,n)
9000
7500
6000
4500
3000
1500
0
9000
7500
6000
4500
3000
1500
0
y = 4400.8x + 5122.2
R = 0.927
y = 2118.7x + 3879.4
R = 0.926
0
0.2
0.4
0.6
0.8
1
0
0.2
0.4
0.6
0.8
1
ETN
ETN
Fig. 11. (a) Stokes-shifts for LE band of 3-benzoylmethyleneindol-2-one versus the solvent polarity functions F1(D,n), F2(D,n) and EN. (b) Stokes-shifts for ICT band of 3-
T
benzoylmethyleneindol-2-one versus the solvent polarity functions F1(D,n), F2(D,n) and EN.
T
shift with the increase of acidity up to Ho = ꢁ5 suggests the com-
Table 5
plete formation of cationic species. Similar behavior is observed for
the other BMI derivatives (OHBMI and ClBMI). Thus in BMI deriva-
tives two equilibria (Monocation M Neutral and Neutral M Mono-
anion) exist in the ground state.
Correlation coefficients for the fits of Stokes-shifts of BMI and its derivatives versus
the solvent polarity functions, EN , F1(D, n) and F2(D, n) for both LE and ICT states.
T
Molecule
Correlation coefficients
ENT ðLEÞ ENT ðICTÞ
F1(D,n)LE F1(D,n)ICT ꢁF2(D,n)LE ꢁF2(D,n)ICT
Fluorescence spectra of BMI, OHBMI and ClBMI were recorded
in water at excitation wavelength, 350 nm in different acidic and
basic media. The fluorescence spectra of various prototropic spe-
cies formed in different pH range are explained by taking into ac-
count the existence of molecule in both LE and ICT states and their
interactions.
In the acidic range Ho = ꢁ5 to ꢁ1, a single band is observed,
which can be attributed to the monocationic species for all the
BMI derivatives studied. The site of protonation of BMI monocation
has two possibilities in the indole ring at carbonyl oxygen and at
the nitrogen atom. The protonated species is formed by the proton-
ation of carbonyl oxygen rather than the nitrogen atom. This is be-
cause if the protonation occurred at nitrogen, a blue shift will be
expected in absorption and fluoresce spectra, as this center repre-
sents amino group. This is also supported by previous studies for
the protonation of the similar derivatives of thiosemicarbazone
[65], 2-indolinone [66] and 2-quinoxalinone [67].
BMI
0.847
0.822
0.916
0.849
0.919
0.908
0.970
0.959
0.867
0.972
0.943
0.882
0.943
0.940
0.902
0.983
0.963
0.811
0.895
0.943
0.908
0.936
0.943
0.965
0.868
0.807
0.895
0.882
0.976
0.926 0.927
0.966 0.976
0.922 0.959
0.979 0.976
0.846 0.987
0.804 0.912
0.979 0.981
0.958 0.993
OHBMI
MeBMI
DiMeBMI
OMeBMI
DiOMeBMI 0.880
ClBMI
0.849
0.962
DiClBMI
ꢄ4–9 is due to the neutral species because the data is similar to
what is obtained in the non-aqueous solvents. The absorption
spectra of BMI, OHBMI and ClBMI show a red shift in band A2 at
pH value 13 and above (Table 4). The red shifted band indicates
the formation of a monoanion because of deprotonation from
>NH moiety in BMI derivatives as in the case of isatin [64].
With decrease in pH there is also a red shift observed in absorp-
tion band maxima, A2 (from 340 nm to 350 nm). This shift indi-
cates the formation of monocation in ground state. Further red
For BMI derivatives in the range pH 1–3, a long wavelength
band at ꢄ502 nm is observed along with
a new band at

84
M.K. Saroj et al. / Journal of Molecular Structure 1012 (2012) 73–86
Table 6
a
Comparison of (lꢂ
/l) obtained from the solvatochromic method with the computed values.
a
a
b
b
d
d
e
Molecule
S1(LE)
S1(ICT)
ꢁS2(LE)
ꢁS2
l
c/D
lꢂ c/D
(
lꢂ
/
l
)
LE
(
lꢂ
/
l)
ICT
( /l)
lꢂ
(ICT)
BMI
1544
989
1509
918
2227
1950
2836
702
1367
2424
1393
2016
3547
2025
3136
1887
1353
3692
2309
2246
3376
2761
4206
1035
1830
3228
2095
3011
3.85
2.74
3.86
3.63
4.88
4.11
4.00
3.81
5.44
4.57
5.37
5.17
6.11
5.38
5.79
5.81
2.54
2.91
2.86
2.89
2.96
3.05
2.67
2.83
4.87
5.81
5.14
5.22
6.90
7.02
4.97
5.05
1.41
1.67
1.39
1.42
1.25
1.31
1.45
1.52
OHBMI
MeBMI
DiMeBMI
OMeBMI
DiOMeBMI
ClBMI
669
1867
1052
1073
DiClBMI
a
Calculated from the plots of Stokes-shifts versus F1(D,n) using Eq. (1).
Calculated from the plots of Stokes-shifts versus F2(D,n) using Eq. (2).
Theoretical values of ground-state and excited state dipole moments obtained using PM3 method.
Calculated using Eq. (6) from experimental values of S1 and S2 of LE and ICT states.
Calculated using theoretical values of ground-state and excited state dipole moments.
b
c
d
e
The only difference in their fluorescence behavior as compared
to the BMI is the absence of fluorescence band at 502 nm in pH
range 1–3 as shown by the intermediate species. This is because
of the inductive effect of the OH and mesomeric effect of Cl group,
which make the carbonyl oxygen rich in electron density. This high
electron density enhances the phenomenon of solvation of car-
bonyl oxygen by H+ ions present in acidic solution restricting the
interaction of oxygen with its own proton on AOH group (i.e. intra-
molecular hydrogen bonding).
Table 7
b
Comparison of the changes in dipole moments (
mic method with the computed values.
Dl) obtained from the solvatochro-
d
d
Molecule
Cavity
radiusa
(a/Å)
IPb
Slope,
Slope,
m(ICT)
D
(D)
l(LE)
D
(D)
l(ICT)
D
le
(D)
c
c
(eV) m(LE)
BMI
4.48
4.54
4.71
4.96
5.07
9.24 2119
8.49 2899
8.83 2643
8.56 2298
8.22 2217
4401
3818
4138
3676
4193
4414
3974
4464
2.39
2.86
2.88
2.90
2.94
2.60
2.53
2.83
3.44
3.28
3.61
3.67
4.05
4.12
3.47
3.70
1.59
1.83
1.51
1.54
1.31
1.27
1.79
2.00
OHBMI
MeBMI
DiMeBMI
OMeBMI
DiOMeBMI 5.04
ClBMI
3.2. Excited state dipole moments
–
1761
4.66
4.68
9.07 2104
9.10 2242
DiClBMI
Solvatochromic shifts caused by general (non-specific) solvent
effects are often described by solvatochromic methods, which re-
late the energy difference between absorption and emission max-
ima to the dielectric constant and refractive index.
a
Onsager cavity radius (40% of the longest axis in the optimized geometry of the
molecule) obtained by using semiempirical calculations [49].
b
Ionization energies (IP) of gas-phase molecules of substituted benzene ring.
c
Slope (m) = 11307.6[(Dl/Dl
B)2(aB/a)3], calculated from the plot of Stokes-shifts
For estimation of the excited state dipole moments, Stokes-
shifts have been calculated for all the molecules from their absorp-
tion and fluorescence maxima i.e. the wavelengths corresponding
to the band A2 (Table 2) and for both LE and ICT ( Table 3) states
in different solvents, respectively. The shift in emission peaks with
variation in solvent polarity is more pronounced than that in the
versus ENT using Eq. (7).
d
Calculated using Eq. (7).
Calculated using theoretical values of ground-state and excited state dipole
e
moments.
absorption peaks. This indicates that
all the systems studied here i.e. the dipole moment increases on
D
l
(=
l
ꢂ ꢁ
l) is positive for
ꢄ424 nm. The band at ꢄ502 nm is attributed to intramolecular
hydrogen bonding between OH group (formed due to protonation)
and the carbonyl group (present in chalcone moiety) in cationic
species (Fig. 10b). This band is not observed at pH < 1, because of
the increase in number of H+ ions which solvate the carbonyl oxy-
gen restricting hydrogen bonding with –OH proton.
excitation.
Fig. 11 shows the plots of Stokes-shifts for BMI versus solvent
polarity functions, F1(D,n), F2(D,n) and EN for LE and ICT states.
T
The calculated values of correlation coefficients from the linear
least squares fit analysis of the Stokes-shifts versus bulk solvent
polarity functions F1(D,n) and F2(D,n) as well as microscopic sol-
vent polarity parameter (ENT ), are summarized in Table 5 for both
LE and ICT states.
Two bands are observed at 424 nm and 454 nm in the range of
neutral species from pH = 5 to pH = 12. While the lower wave-
length fluorescence band is due to the LE state, the higher wave-
length fluorescence band is due to the ICT interaction in neutral
BMI. This has been previously confirmed by the pronounced red
shift of ICT maximum on increasing the solvent polarity (see sec-
tion 3.1.2). A large blue shifted band is observed at ꢄ406 nm on
further increasing the pH value to 14, which is attributed to the an-
ionic species formed due to the deprotonation at >NH group.
Almost similar behavior has been observed for OHBMI and
ClBMI because the basic structures are same except for the pres-
ence of OH and Cl groups on the benzene ring, which enhances
the process of ICT making it a dominant state. Similar to BMI in
highly acidic medium (Ho = ꢁ5 to pH = 3), a long wavelength band
is observed at 480 nm and 485 nm, respectively for cationic species
of OHBMI and ClBMI. On increasing pH, the fluorescence from cat-
ionic species disappears and two bands are observed for neutral LE
and neutral ICT in the range of pH value ꢄ 4–12. Formation of an-
ion takes place in basic medium after pH ꢄ 12.
The ground state (l
) and excited state (lꢂ) dipole moments and
Onsager cavity radii ‘a’ have been calculated theoretically for all
the systems and are given in Tables 6 and 7, respectively. Table 7
contains the
Dl values derived from the slopes obtained from
the plots of Stoke-shifts versus ENT using Eq. (7). The slopes S1 and
S2 were obtained from the plots of Eqs. (1) and (2) and the ratios
of dipole moments (lꢂ
/l) calculated using Eq. (6) are summarized
in Table 6. The changes in dipole moment, Dl values derived from
the slopes (m) obtained from the plots of Stokes-shifts versus ETN
using Eq. (7), are given in Table 7.
There is an increase in excited-state dipole moments (
/l values, Tables 6 and 7) in comparison to the theoretically cal-
Dl and
lꢂ
culated excited state. This suggests considerable amount of charge
transfer from the donor to the carbonyl acceptor moieties, which
leads to an increase in the polarity of the excited state and conse-
quently, the intensification of the donor–acceptor interaction.
Dl

M.K. Saroj et al. / Journal of Molecular Structure 1012 (2012) 73–86
85
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are larger than those of LE states for all BMI derivatives, indicating
stronger electronic interaction for the former structures (ICT state,
resonance structure II in Fig. 5). These results are in agreement
with the fact that the band F1 arises from LE state with the similar
electronic arrangement as in ground state and band F2 shows the
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in all BMI derivatives.
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derivatives can be reasonably explained on the basis of resonance
structures as shown in Fig. 5, and the nature of emitting states. All
BMI derivatives studied here promote the resonance structure of
type II in the excited state as ICT, explaining the substantial charge
separation upon excitation.
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4. Conclusions
The study of excited state dipole moments of benzoylmethylen-
eindol-2-one systems has brought into focus the presence of three
emitting states (LE, ICT and ESIPT) in these molecules. These emit-
ting states are affected by the change in solvent polarity as well as
the substituted-phenyl moieties.
The experimentally calculated Dl /l values of probes re-
and lꢂ
veal an increase in the excited state dipole moment. Moreover,
experimentally calculated values are significantly larger than the
semiempirical computational (PM3) estimates justifying the ICT
as the main character of the excited state.
Prototropic reaction has found the existence of three species
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two equilibria exist in these derivatives, Monocation M Neutral
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ground state as well as excited state (LE and ICT). Anion and cation
are formed due to the deprotonation from NH group and proton-
ation on carbonyl group, respectively.
The valuable information so obtained about the nature of the
emitting state opens the possibility of examination of various the-
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These solvatochromic studies provide insights for further applica-
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Acknowledgements
The financial support from University of Delhi under the
Scheme ‘‘To strengthen R&D Doctoral Research Program’’ is grate-
fully acknowledged. Manju K. Saroj is thankful to the University
Grants Commission (UGC), New Delhi for the financial assistance.
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