Photophysical properties of ESIPT inspired fluorescent 2-(2-hydroxyphenyl)- 6-methylimidazo[4,5-f]isoindole-5,7(1H,6H)-dione and its derivative: Experimental and DFT based approach

Article abstract of DOI:10.1016/j.saa.2014.07.021

The excited-state intramolecular proton transfer chromophores 2-(2-hydroxyphenyl)-6-methylimidazo[4,5-f]isoindole-5,7(1H,6H)-dione and 2-(4-(diethylamino)-2-hydroxyphenyl)-6-methylimidazo[4,5-f]isoindole-5,7(1H,6H) -dione are synthesized from 4,5-diamino-

Full text of DOI:10.1016/j.saa.2014.07.021

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 135 (2015) 457–465
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Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Photophysical properties of ESIPT inspired fluorescent
2-(2-hydroxyphenyl)-6-methylimidazo[4,5-f]isoindole-5,7(1H,6H)-
dione and its derivative: Experimental and DFT based approach
⇑
Mininath S. Deshmukh, Nagaiyan Sekar
Tinctorial Chemistry Group, Dyestuff Technology Department, Institute of Chemical Technology (Formerly UDCT), N.P. Marg, Matunga, Mumbai 400 019, Maharashtra, India
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
ꢀ
ꢀ
ꢀ
Synthesis of phthalimide based ESIPT
molecules.
Spectroscopic investigations of a
series of new benzimidazole.
Existence of ESIPT has been
experimentally and is
computationally supported.
Good thermal stability.
ꢀ
a r t i c l e i n f o
a b s t r a c t
Article history:
The excited-state intramolecular proton transfer chromophores 2-(2-hydroxyphenyl)-6-methylimi-
dazo[4,5-f]isoindole-5,7(1H,6H)-dione and 2-(4-(diethylamino)-2-hydroxyphenyl)-6-methylimi-
dazo[4,5-f]isoindole-5,7(1H,6H)-dione are synthesized from 4,5-diamino-N-methylphthalimide. The
photophysical behavior of the synthesized chromophores was studied using UV–visible and fluorescence
spectroscopy in the polar and non-polar solvents. The synthesized o-hydroxyphenyl benzimidazole deriv-
atives are fluorescent and very sensitive to the solvent polarity. These dyes are thermally stable up to
317 °C. Density Functional Theory computations have been used to understand the structural, molecular,
electronic and photophysical properties of the chromophores. The experimental absorption and emission
wavelengths are in good agreement with the computed vertical excitation and theoretical emission
obtained by Density Functional Theory and Time Dependant Density Functional Theory.
Ó 2014 Elsevier B.V. All rights reserved.
Received 27 January 2014
Received in revised form 3 July 2014
Accepted 12 July 2014
Available online 22 July 2014
Keywords:
o-Hydroxyphenyl benzimidazole
ESIPT
Fluorescence
Solvatochromism
Thermal stability
TD-DFT
Introduction
[7–10] and molecular logic gates [11]. The most significant photo-
physical property of the ESIPT chromophores is the large Stokes
Excited state intramolecular proton transfer (ESIPT) is of prime
importance in molecular probes [1], luminescent materials [2,3],
metal ion sensors [4–6], organic light emitting devices (OLEDs)
shift, compared to the normal fluorophores such as fluorescein,
boron-dipyrromethene (BODIPY) or rhodamine [12,13]. The large
Stokes shift is a distinct characteristic for fluorophores because
the self-absorption, or the inner filter effect, can be avoided and
the fluorescence can be improved with this kind of fluorophores.
It is difficult to increase the Stokes shift of the conventional fluoro-
phores by chemical modification [14].
⇑
Corresponding author. Tel.: +91 22 3361 1111/2707; fax: +91 22 3361 1020.
E-mail addresses: n.sekar@ictmumbai.edu.in, nethi.sekar@gmail.com (N. Sekar).
http://dx.doi.org/10.1016/j.saa.2014.07.021
1386-1425/Ó 2014 Elsevier B.V. All rights reserved.

458
M.S. Deshmukh, N. Sekar / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 135 (2015) 457–465
Upon photoexcitation to the first excited state, the acidity of the
instrument (USA) using TMS as an internal standard. Mass spectra
were recorded on Finnigan mass spectrometer. The visible absorp-
tion spectra of the compounds were recorded on a Perkin Elmer
Lambda 25 spectrometer; fluorescence emission spectra and fluo-
rescence quantum yields were recorded by using quinine sulfate
(0.54 in 0.1 M H2SO4) as reference [21] on Varian Cary Eclipse
fluorescence spectrophotometer. Simultaneous DSC–TGA mea-
surements were performed on SDT Q 600 v8.2 Build 100 model
of TA instruments Waters (India) Pvt. Ltd.
acidic center and the basicity of the basic center increase simulta-
neously because of the redistribution of the charge density of the
chromophore realized in the first singlet excited state. This leads
to the migration of the acidic proton of the acidic center to the
basic nitrogen unit via the already existing hydrogen bond coordi-
nate to give the phototautomer. The ESIPT process is a very fast
process occurring within subpicosecond time scale, and on excita-
tion the molecule passes to the potential well and then relaxes vib-
rationally [15,16]. Generally the ESIPT chromophores show dual
emission, one being to the normal emission from the local excita-
tion and the second which is largely Stokes shifted (up to
10,000 cmꢁ1), due to the tautomer formed by proton transfer [14].
Therefore, taking into consideration the wide applications and
importance of ESIPT inspired molecules and in continuation of
our research work on ESIPT materials [17–19] herein, we report
the synthesis, photophysical properties and the computational
study of novel o-hydroxyphenyl benzimidazole (HPBI) derivatives,
wherein the phthalimide–imidazole fused heterocycle. It may pos-
sess good electron-accepting ability. So the substitution by amine
leads to push–pull dipolar compounds. Prominent red-shifted
absorption of 6a is due to the ICT present in this compound.
Phthalimide and naphthalimide-based dyes are interesting due to
their excellent electron accepting properties [20].
Computational methods
The compounds 6a–6b and their tautomers involved are illus-
trated in Fig. 1. The ground state (S0) geometry of the tautomers
of compounds 6a and 6b in their Cs symmetry was optimized using
the tight criteria in the vacuum phase using Density Functional The-
ory (DFT) [22]. The functional used was B3LYP. The B3LYP method
combines Becke’s three parameter exchange functional (B3) [23]
with the nonlocal correlation functional by Lee, Yang, and Parr
(LYP) [24]. The basis set used for all atoms was 6-31G(d), the latter
has been justified in the literature [25–27] for the current investiga-
tion. The vibrational frequencies at the optimized structures were
computed using the same method to verify that the optimized
structures correspond to local minima on the energy surface [28].
The vertical excitation energy and oscillator strengths at the ground
state equilibrium geometries were calculated using TD-DFT with
the same hybrid functional and basis set [29–31]. The low-lying
first singlet excited state (S1) of the dyes was relaxed using TD-
DFT to obtain its minimum energy geometry. The difference
between the energies of the optimized geometries at the first sin-
glet excited state and the ground state was used in computing the
emissions [32,33]. Frequency computations were also carried out
on the optimized geometry of the low-lying vibronically relaxed
first excited state of the conformers. All the computations in sol-
vents of different polarities were carried out using the Polarizable
Continuum Model (PCM) [34]. All the electronic structure computa-
tions were carried out using the Gaussian 09 program [35].
Experimental section
Materials and equipments
All the commercial reagents and the solvents were purchased
from S. D. Fine Chemicals Pvt. Ltd. Mumbai and were used without
purification. The reaction was monitored by TLC using 0.25 mm sil-
ica gel 60 F254 precoated plates, which were visualized with UV
light. Melting points were measured on standard melting point
apparatus from Sunder industrial product Mumbai, and are uncor-
rected. FT-IR spectra were recorded on Jasco 4100 using ATR acces-
sory. 1H NMR spectra were recorded on VARIAN 400/500-MHz
Fig. 1. Excited state intramolecular proton transfer (ESIPT) pathway.

M.S. Deshmukh, N. Sekar / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 135 (2015) 457–465
459
Scheme 1. Synthesis of ortho-hydroxybenzimidazole (HPBI) derivatives (6a–6b).
Synthetic strategy
FT-IR: 3115 (ANH), 1711, 1697 (AC@O), 1612, 1517 (C@C ring
stretching), 1545, 1352 (NAO) cmꢁ1
.
They were synthesized by the conventional condensation of
5,6-diamino-2-methylisoindoline-1,3-dione 5 with the aromatic
hydroxy aldehydes a and b as shown in scheme 1. In the first step,
N-methyl-4-chlorophthalimide 2 was synthesized using Sandmey-
er reaction on 4-amino-N-methylphthalimide 1. Nitration of the
compound 2 with fuming HNO3 and H2SO4 gave the compound
3, which on fusion with urea gave 4. The reduction of the
compound 4 using H2–10% Pd/C to gave 5. Finally, diamino 5 and
aromatic hydroxy aldehydes a or b were refluxed in absolute
ethanol containing a catalytic amount of PCl3 to yield the desired
compounds 6a and 6b.
1H NMR (500 MHz, DMSO-d6): d 2.99 (s, 3H, ACH3), 7.43 (s, 1H,
AAr), 8.23 (s, 1H, AAr), 8.36 (brs, 2H, ANH2).
4,5-Diamino-N-methylphthalimide (5)
A solution of N-methyl-4-amino-5-nitrophthalimide (4) 1.8 g,
(8 mmol) in methanol (10 mL) was hydrogenated in presence of
10% Pd–C (20 mg) for 4 h at room temperature; the catalyst was
removed by filtration. The filtrate was concentrated under reduced
pressure to obtain the crude product, which was further recrystal-
ized from methanol to give 5.
Yield: 1.3 g (84%), m.p.: 258–260 °C (decomposed).
The structures of the compounds were confirmed by FTIR, 1H
NMR, and Mass spectral analysis. In 1H NMR spectra of the com-
pound 6a, the AOH and ANH protons are not observed, it is
because of the exchange of proton with the deuterated solvent.
But the structure is confirmed by mass spectral analysis. The
M+1 peak was found to be at 365.13. In the case of the compound
6b the proton exchange with the solvent was not observed during
1H NMR scanning. Their AOH, ANH proton signal was observed at
13.588 ppm, 12.222 ppm and its (M) peak was found to be at
293.13, which is in good agreement with the molecular weight.
To find out the effect of solvent polarities on the absorption–emis-
sion properties, photophysical properties were studied in solvents
of different polarity.
Mass m/z: 214.06 (M+Na)+.
FT-IR: 3121 (ANH), 1702, 1695 (AC@O), 1607, 1527 (C@C ring
stretching) cmꢁ1
.
1H NMR (500 MHz, DMSO-d6): d 2.88 (s, 3H, ACH3), 5.51 (s, 4H,
ANH2), 6.84 (s, 2H, AAr).
General procedure for the preparation of benzimidazole. Phosphorus
trichloride 0.22 mL (2.6 mmol) was added drop wise to a solution
of 4,5-diamino-N-methylphthalimide
5 0.5 g (2.6 mmol), and
substituted o-hydroxybenzaldehyde (a or b) (2.6 mmol) in ethanol
(5 mL), maintaining the temperature at 40–45 °C. The mixture was
refluxed for 6 h. On completion of the reaction (checked by TLC)
the product obtained was filtered, dried and recrystallized from
ethanol to give the desired product 6a and 6b. On completion of
the reaction the reaction mass was precipitated during the reaction
or room temperature to obtained 6a–6b. The separated product
was recrystallized from ethanol.
Synthesis and characterization
The synthetic scheme for the preparation of the o-hydroxybenz-
imidazole derivatives are shown in Scheme 1. N-Methyl-4-chloro-
5-nitrophthalimide (3) was prepared by the reported procedure
from 4-amino-N-methylphthalimide (1) [36,37].
2-(4-(Diethylamino)-2-hydroxyphenyl)-6-methylimidazo[4,5-
f]isoindole-5,7(1H,6H)-dione 6a
Yield: 0.60 g (63%), m.p.: 178–180 °C.
N-Methyl-4-amino-5-nitrophthalimide (4)
Mass m/z: 365.13 (M+1).
A mixture of N-methyl-4-chloro-5-nitrophthalimide (3) 2.4 g
(10 mmol) and urea 0.9 g (15 mmol) was fused at 135 °C for 2 h.
Then, decrease the temperature up to 60 °C and water was added,
stirred for 1 h. The product obtained was filtered and recrystallized
from ethanol to give (4).
FT-IR: 3312 (AOH), 2977 (ANH), 1769, 1706 (AC@O), 1607,
1509 (C@C, C@N ring stretching), 1412, 1372 (CAN amide stretch-
ing) cmꢁ1
.
1H NMR (400MHz, DMSO-d6): d 1.17 (t, 6H, ACH3), 3.07 (s, 3H,
ANCH3), 3.507 (q, 4H, ANCH2), 6.31 (s, 1H, AArH), 6.51 (d, 1H,
AArH, J = 6.4 Hz), 7.92 (d, 1H, AArH, J = 6.8 Hz), 7.97 (s, 2H, AArH).
Yield: 1.8 g (80%), m.p.: 254–256 °C.

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M.S. Deshmukh, N. Sekar / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 135 (2015) 457–465
Table 1
Observed UV–visible absorption and the computed absorption of the compound 6a in different solvents.a
Solvent
Experimentalb
TD-DFTc
D%g
a
kmax (nm)
Molar absorptivity,
e
(L molꢁ1 cmꢁ1
)
Vertical excitation (nm)
fh
Orbital contributionf
THF
371d
308e
371d
302e
371d
308e
17,108
6443
366
295
1.107
0.351
H ? L + 1 (98%)
H ꢁ 3 ? L (52%)
H ? L + 1 (98%)
H ꢁ 3 ? L (54%)
H ? L + 1 (98%)
H ꢁ 3 ? L (54%)
H ? L + 1 (98%)
H ꢁ 3 ? L (54%)
H ? L + 1 (98%)
H ꢁ 3 ? L (54%)
H ? L + 1 (98%)
H ꢁ 3 ? L (54%)
H ? L + 1 (98%)
H ꢁ 3 ? L (54%)
H ? L + 1 (98%)
H ꢁ 3 ? L (54%)
1.37
4.29
Acetone
DCM
49,431
21,076
367
296
1.101
0.356
1.21
2.11
21,403
8044
367
295
1.114
0.354
1.20
4.18
Ethanol
Methanol
Acetonitrile
DMF
375d
309e
396d
306e
372d
280e
374d
332e
44,954
18,236
367
295
1.075
0.352
2.13
4.53
30,066
6042
366
296
1.093
0.360
7.51
3.35
6443
22,495
367
296
1.098
0.356
1.45
5.65
47,356
28,465
368
296
1.123
0.359
1.61
10.81
DMSO
377d
305e
41,059
15,543
368
296
1.120
0.358
2.42
2.90
a
b
c
Analyses were carried out at room temperature (25 °C).
Experimentally observed kmax
TD-DFT computations were carried out with the use of optimized structures at B3LYP method with 6-31G(d) basis set.
.
d
e
f
kmax
.
Shoulder peak.
Only major contributions are presented.
% Deviation between experimental absorption and the vertical excitation computed by DFT.
Oscillator strength.
g
h
2-(2-Hydroxyphenyl)-6-methylimidazo[4,5-f]isoindole-5,7(1H,6H)-
dione 6b
and a free phenyl group having the ortho-hydroxyl group partici-
pating in the proton transfer at the excited state [38]. In the ground
state the molecule can exist in equilibrium with the different con-
formers arising from rotamerism and their tautomers are summa-
rized in Fig. S1. The usually planar syn form (E-Enol) allows an
intramolecular hydrogen bond between the acidic AOH group
and the basic nitrogen atom of the N-methylphthalimide ring.
The enol (E) conformer undergoes proton transfer to form keto
(K) tautomer. The conformer enol (E) can undergo rotamerization
around the single bond of ortho-hydroxyphenyl group generating
the anti enol (E1) conformer, where the probability of proton
transfer happens to be very remote. The compounds 6a and 6b
can coexist in various conformers with their tautomers in solution.
DFT computations showed that in all the solvents, the syn enol
form is more stable than the keto and anti enol form, which is
preferred in the ground state (Tables S1 and S2). The keto forms
are prohibitively higher in energy such that they do not exist in
the ground state. The absence of the long wavelength absorption
in the absorptive species is also indicative of the non-existence
of the keto form.
Yield: 0.55 g (65%), m.p.: 172–174 °C.
Mass m/z: 293.13 (M).
FT-IR: 3332 (AOH), 2982 (ANH), 1760, 1687 (AC@O), 1618,
1535 (C@C, C@N ring stretching), 1426, 1374 (CAN amide stretch-
ing) cmꢁ1
.
1H NMR (400 MHz, DMSO-d6): d 3.07 (s, 3H, ANCH3), 7.08 (t, 1H,
AArH, J = 6 Hz), 7.12 (d, 1H, AArH, J = 6.8 Hz), 7.45 (d, 1H, AArH,
J = 6.4 Hz), 8.07 (brs, AArH, 2H,), 8.18 (d, 1H, AArH, J = 6.4 Hz),
12.22 (brs, 1H, ANH), 13.59 (brs, 1H, AOH).
Result and discussion
ESIPT phenomenon
The synthesized compounds 6a and 6b contain an acidic AOH
group at the o-position with respect to the benzimidazole ring.
The location of the acidic proton of the AOH group and the benz-
imidazole @NA groups is such that there exists an intramolecular
hydrogen bonding in the ground state. On excitation, the @NA
moiety of the benzimidazole ring becomes strongly basic and the
proton of the AOH group becomes strongly acidic as shown by
the changes in the Mulliken charge distribution in the ground
and the excited states. Absorption of the radiation leads to the
excitation of the ground state enol (E-Enol) to the excited enol
(E-Enol*). This E-Enol* form undergoes a fast proton transfer
(ESIPT) to form the excited state keto (K-Keto*) form, the tautomer
in a subpicosecond time scale or emits a radiation and returns to
the ground state enol form (E-Enol). The excited state keto form
(K-Keto*) emits radiation and comes to the ground state keto form
(K-Keto). A schematic diagram for ESIPT is shown in Fig. 1.
Photophysical properties
The UV–Vis absorption and fluorescence emission spectra of the
benzimidazole derivatives (6a and 6b) in the solvents of varying
polarities are reported in Tables 1 and 2. The absorption spectra
of the compounds in all the studied solvents are almost nearly
the same; their absorption property is independent of the solvent
polarity except for the compound 6a in methanol (Fig. S2). How-
ever, the absorption intensity is very sensitive toward the solvent
polarity. The intensity of transition in the compounds 6a and 6b
(both computed and observed) of the short wavelength absorption
is always nearly twice and half the intensity of long wavelength
absorption respectively. The decreasing order of the absorption
intensity in the short wavelength region is DMF > ACN > Ace-
tone > EtOH > DMSO > DCM > THF > MeOH for the compound 6a
and THF > MeOH > EtOH > DMF > ACN > Acetone > DMSO for the
Rotamers and conformers and their ground state energy
The compounds 6a and 6b have a fused imidazole ring (at
4- and 5-position of the N-methylphthalimide ring), which is rigid

M.S. Deshmukh, N. Sekar / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 135 (2015) 457–465
461
Table 2
Observed UV–visible absorption and computed absorption of the compound 6b in different solvents.a
Solvent
Experimentalb
TD-DFTc
D%g
a
kmax (nm)
Molar absorptivity,
e
(L molꢁ1 cmꢁ1
)
Vertical excitation (nm)
fh
Orbital contributionf
THF
337d
271e
343d
268e
27,073
59,655
329
277
0.457
0.852
H ? L + 1 (90%)
H ꢁ 1 ? L + 1 (57%)
H ? L + 1 (90%)
H ꢁ 1 ? L + 1 (58%)
2.37
2.23
Acetone
18,869
42,632
328
277
0.455
0.861
4.26
3.37
DCM
340d
28,978
329
0.463
H ? L + 1 (90%)
3.20
Ethanol
340d
271e
338d
272e
340d
271e
24,846
52,330
328
277
0.453
0.859
H ? L + 1 (90%)
H ꢁ 1 ? L + 1 (58%)
H ? L + 1 (90%)
H ꢁ 1 ? L + 1 (58%)
H ? L + 1 (90%)
H ꢁ 1 ? L + 1 (58%)
H ? L + 1 (90%)
H ꢁ 1 ? L + 1 (58%)
3.47
2.21
Methanol
Acetonitrile
DMF
26,253
55,055
328
277
0.449
0.861
2.94
1.78
22,883
47,554
328
277
0.453
0.863
3.47
2.21
343d
274e
334d
277e
25,403
48,433
329
278
0.474
0.869
4.11
1.34
DMSO
23,821
39,877
329
278
0.472
0.869
H ? L + 1 (90%)
H ꢁ 1 ? L + 1 (58%)
1.56
0.22
a
b
c
Analyses were carried out at room temperature (25 °C).
Experimentally observed kmax
TD-DFT computations were carried out with the use of optimized structures at B3LYP method with 6-31G(d) basis set.
More intense peak.
Less intense peak.
Only major contributions are presented.
% Deviation between experimental absorption and the vertical excitation computed by DFT.
Oscillator strength.
.
d
e
f
g
h
compound 6b. In the case of the long wavelength transition the
order is acetone > DMF > EtOH > DMSO > ACN > MeOH > DCM >
THF and DCM > THF > MeOH > DMF > EtOH > DMSO > ACN > ace-
tone for the compounds 6a and 6b respectively (Tables 1 and 2,
Fig. S2a and b). The compounds 6a and 6b have two absorption
maxima. Among these two transitions, first one is higher energy
absorptions between 268 nm and 332 nm probably due to the aryl
6b shows dual emission only in ethanol, DMF (Table 4, Fig. S5b).
The emission property of the compound 6a is red shifted in polar
solvent like DMF (588 nm) and DMSO (583 nm) as compared to
the other solvents, while compound 6b red shifted in DMF
(538 nm), ethanol (543 nm) and acetonitrile (448 nm) than the
other solvents. It is well known that in the ESIPT molecules, the
long wavelength emission is attributed to the excited state keto
tautomer originating from the excited state enol tautomer
[14,25]. The driving force after the proton transfer is associated
with an intrinsic extra stabilization of the keto tautomer at the
excited state. This aspect is confirmed by the TD-DFT computations
that the excited state keto form has lower energy in non-polar as
well as high polar solvents (Table 5) and this reveals that the
stabilization is exclusively intrinsic and not due to the solvent
(non polar or polar) environment. The TD-DFT calculation at the
excited state reveals that there is hardly any difference between
the energy of the optimized enol and keto tautomers. The differ-
ence between the energies of the optimized geometries at the first
singlet excited state and the ground state are used to calculate the
emissions in all the solvents are summarized in Tables S3–S6.
In highly polar solvent the emissive S1 state of intramolecular
charge transfer (ICT) character is strongly solvated and its energy
is hence significantly lowered. As a consequence, the energy gap
core and
p–
p* transition while the lower energy band between
334 nm and 396 nm attributed to the intramolecular charge trans-
fer transition (ICT) between the donor group and the acceptor moi-
ety. The excitation spectra illustrated in Figs. S3 and S4 were
obtained by using shorter (439–510 nm) and longer wavelength
(538–588 nm) emissions of the respective solvents.
The red shifted absorption maxima indicates that the o-hydroxy
and N,N-diethylamino moiety present in 6a (396 nm) has a stron-
ger electron donating effect than the compound 6b (343 nm)
devoid of N,N-diethylamino group. These results also showed that
the compound 6a exhibited strong solvatochromic properties in
emission as compared to the compound 6b. The introduction of
electron donor group in the positions induced intramolecular
charge transfer and mesomeric dipole moment. When compared
with unsubstituted analogues [19] of the molecules 6a and 6b later
have shown a red shifted absorption. The electron withdrawing
effect of cyclic amide group has induced efficient donor to acceptor
charge transfer. The molecule 6a absorbs at 396 nm and 6b at
338 nm in methanol as compared to blue shifted absorption of
their unsubstituted analogue at 224 nm and 243 nm respectively.
It is also observed that molecule 6a absorbs at longer wavelength
than the nitro substituted analogue. Nitro group being strong elec-
tron withdrawer than amide was expected to show a red shift in
absorption [19]; however, the cyclic amide group gives rigidity
and planarity to the molecules and proved to be a efficient electron
withdrawer.
D
E (S1, S2) is enlarged so that the coupling of the S1 state directly
to the ground state stays opened and the intersystem crossing from
S1 to T state is enhanced. The high electron-donating character of
N,N-diethylamino moiety leads to a red shift (lower energy) of the
emission relative to the absorption is caused by the energy losses
due to the dissipation of vibrational energy during the decay and
is influenced by the interaction between the fluorophore and the
solvent molecules around the excited dipole, hydrogen bonding
and formation of charge complexes.
The fluorescence emission study of the compound 6a reveals
that a dual emission is observed in non-polar as well as highly
polar solvents (THF, dichloromethane, acetonitrile, DMF and
DMSO) and in the polar protic solvents (methanol, ethanol,
acetone) a single emission (Table 3, Fig. S5a), while the compound
Relative quantum yield of compounds
The fluorescence quantum yields of the synthesized compounds
6a and 6b were determined in different solvents and tabulated in
Tables 3 and 4. The fluorescence quantum yield mostly depends

462
M.S. Deshmukh, N. Sekar / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 135 (2015) 457–465
Table 3
Observed and the computed emission of the compound 6a in various solvents along with its quantum yield.a
Solvent
Experimental (short emission)
TD-DFT
Experimental (long emission)
TD-DFT (long
Excitation
(nm)
Total
(short emission)c
(nm)
emission)c (nm)
quantum yielde
Emission (nm)
D
k (nm)
Ub
Emission (nm)
D
k (nm)
Ud
THF
Acetone
DCM
Ethanol
Methanol
Acetonitrile
DMF
436
429
436
510
507
435
436
429
65
58
65
135
111
63
0.06
0.03
0.04
0.06
0.06
0.03
0.04
0.02
479
486
481
487
488
488
488
489
553
570
553
–
182
199
182
–
0.09
0.04
0.07
–
570
570
568
–
376
377
382
276, 369
282, 372
280, 346
282, 380
282, 381
0.15
0.07
0.11
0.06
0.06
0.07
0.10
0.05
–
–
–
–
539
588
583
167
214
206
0.04
0.06
0.03
561
560
560
62
52
DMSO
a
b
c
Analyses were carried out at room temperature (25 °C).
Quantum yield for short wavelength emission, quinine sulfate was used as reference standard.
Computed using TD-DFT method with the B3LYP/6-31G(d).
Quantum yield for long wavelength, quinine sulfate was used as reference standard.
Summation of short wavelength and long wavelength quantum yield.
d
e
Table 4
Observed and computed emission of compound 6b in various solvents along with its quantum yield.a
Solvent
Experimental (short emission)
TD-DFT
Experimental (long emission)
TD-DFT
Excitation
(nm)
Total quantum
yielde
(short emission)c
(nm)
(long emission)c
(nm)
Emission (nm)
D
k (nm)
Ub
Emission (nm)
D
k (nm)
Ud
THF
Acetone
DCM
Ethanol
Methanol
Acetonitrile
DMF
436
434
435
431
445
448
440
431
97
91
95
0.10
0.06
0.08
0.04
0.06
0.07
0.05
0.06
395
398
396
398
397
399
399
399
–
–
–
543
–
–
538
–
–
–
–
203
–
–
198
–
–
–
–
0.04
–
–
0.04
–
–
–
–
545
–
–
546
–
352
348
336
357
372
350
377
335
0.10
0.06
0.08
0.08
0.06
0.07
0.09
0.06
91
107
108
100
97
DMSO
a
b
c
Analyses were carried out at room temperature (25 °C).
Quantum yield for short wavelength emission, quinine sulfate was used as reference standard.
Computed using TD-DFT method with the B3LYP/6-31G(d).
Quantum yield for long wavelength, quinine sulfate was used as reference standard.
Summation of short wavelength and long wavelength quantum yield.
d
e
on both the nature of the substituent and the solvent polarity.
Here, it has been observed that the quantum yield is higher in
the non-polar solvents than in the polar solvents and this can be
attributed to the contribution coming from the long wavelength
emission. The quantum efficiency of the compounds 6a and 6b
studies have been carried out using thermo gravimetric analysis
(TGA) in the temperature range 40–600 °C under nitrogen gas at
a heating rate of 10 °C minꢁ1. The TGA results indicated that the
synthesized compounds are thermally stable up to 317 °C. TGA
revealed the onset decomposition temperature (Td) of 6a and 6b
are 317 °C (95%) and 375 °C (91%), respectively. In general the
backbones of these compounds are stable up to 317 °C, above
317 °C and 375 °C for compound 6a and 6b, the thermogravimetric
curve showed a major loss in the weight respectively. The compar-
isons of the Td (decomposition temperature) show that the thermal
stability of the compound 6b is more than that of the compound
6a. The compound 6a showed a sharp decomposition after a tem-
perature of 317 °C and completely decomposed beyond 450 °C.
However, the compound 6b showed sluggish decomposition nat-
ure and it does not decompose completely up to 600 °C (Fig. S8).
was much higher in the DMF (
U = 0.10 and 0.09) as compared to
the other polar solvents. The short wavelength emission of the
compound 6a occurs in all cases with a narrow range of Stokes
shift (72–76 nm) except in acetonitrile (106 nm). It is observed
that the short wavelength emission is much red shifted in acetoni-
trile. In the case of the compound 6a, the solvent polarity effect on
the fluorescence emission is significant; it shows a red shifted
emission from non polar to polar solvents for both the shorter
and the longer wavelength regions (Fig. S6). However, for the com-
pound 6b, the long wavelength emission is independent of the sol-
vent polarity but the short wavelength emission is dependent on
the solvent polarity (Fig. S7).
Electronic vertical excitations (TD-DFT)
Thermal stability
The electronic vertical excitations were calculated using TD-
B3LYP/6-31G(d) method. The experimental absorption wave-
lengths and the computed vertical excitation spectra associated
with their oscillator strengths, orbital composition, and their
Intramolecular interaction from compact aggregates enhances
the thermal stability of the compounds [39]. Thermal stability
Table 5
Excited state energy difference between K1-Enol* and K1-Keto* in kcal molꢁ1 a
.
Solvents
THF
1.23
12.52
Acetone
DCM
Ethanol
Methanol
Acetonitrile
DMF
DMSO
6a-E*–K* (kcal molꢁ1
)
)
0.13
11.71
0.94
12.31
0.03
11.63
0.09
11.76
0.13
11.5
0.14
11.49
0.21
11.44
6b-E*–K* (kcal molꢁ1
a
Excited state energy were computed using TD-DFT with B3LYP method and 6-31G(d) basis set.

M.S. Deshmukh, N. Sekar / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 135 (2015) 457–465
463
Table 6
corresponding assignments for the compounds 6a and 6b are sum-
Frontier molecular orbital’s of the compounds 6a and 6b in DMF.
marized in Tables 1 and 2. The absorption band occurring at the
lower energy with higher oscillator strength is due to the intramo-
lecular charge transfer (ICT) and it is the characteristic of the
donor–acceptor chromophore. These ICT bands of the compounds
6a and 6b were mainly due to the electronic transition from the
highest occupied molecular orbital HOMO and HOMO ꢁ 1 to the
lowest unoccupied molecular orbital (LUMO + 1) respectively.
The experimental absorption and the computed vertical excitation
of the compounds 6a and 6b are independent of the solvent
polarity.
Orbital
6a
6b
LUMO+1
LUMO
The percent deviation between the experimental and the com-
puted absorption at the shorter and longer wavelength of the com-
pound 6a is 10.8% in DMF and 7.5% in methanol while compound
6b in acetone is 3.27% and 4.26% respectively. As an example of
the compound 6a, in the case of non polar solvent like acetone
HOMO to LUMO + 1 (98%) transition is responsible for the vertical
excitation located at 367 nm with oscillator strength (f) 1.1007,
which corresponds to the experimentally observed prominent
absorption peak at 371 nm. The HOMO ꢁ 3 to LUMO (54%) (Table 1)
transition is responsible for the vertical excitation at 296 nm with
oscillator strength (f) 0.3561, which corresponds to the experimen-
tally observed absorption (shoulder peak) at 302 nm. For polar sol-
vents like DMF, HOMO to LUMO + 1 (98%) transition is responsible
for the vertical excitation located at 368 nm with oscillator
strength (f) 1.1232, which corresponds to the experimentally
observed prominent absorption peak at 374 nm. The HOMO ꢁ 3
to LUMO (54%) transition is responsible for the vertical excitation
at 296 nm with oscillator strength (f) 0.3586, which corresponds
to the experimentally observed absorption (shoulder peak) at
332 nm (Table 1). For the compound 6b in non-polar solvent like
acetone HOMO to LUMO + 1 (90%) transition is responsible for
the vertical excitation located at 328 nm with oscillator strength
(f) 0.4548, which corresponds to the experimentally observed
absorption (shoulder peak) at 343 nm. The HOMO ꢁ 1 to LUMO + 1
(58%) (Table 2) transition is responsible for the vertical excitation
at 277 nm with oscillator strength (f) 0.8607, which corresponds
to the experimentally observed prominent absorption peak
268 nm. For the polar solvents like DMF, HOMO to LUMO + 1
(90%) transition is responsible for the vertical excitation located
at 329 nm with oscillator strength (f) 0.4741, which corresponds
to the experimentally observed absorption (shoulder peak) at
343 nm. The HOMO ꢁ 1 to LUMO + 1 (58%) transition is responsi-
ble for the vertical excitation at 278 nm with oscillator strength
(f) 0.8685, which corresponds to the experimentally observed
prominent absorption peak at 274 nm (Table 2).
HOMO
HOMO-1
Molecular orbital diagrams of the compounds 6a and 6b are
shown in Table 6. From the pictorial diagram the compounds 6a
and 6b were found that HOMO ? LUMO + 1 orbitals fully delocal-
ized throughout the molecule. The electron densities of the com-
pounds 6a and 6b in the HOMO were slightly more located on
the donor core and the electron densities on the LUMOs were
shifted toward the acceptor end (imidazole core). The cyclic four
level photophysical process involving the excited state proton
transfer of the compounds 6a and 6b (E-Enol/E-Enol*/K-Keto*/K-
Keto) is exemplified in Figs. S12 and S13.
Structural properties of the compounds
During the photoexcitation of the enol form the first excited
singlet state undergoes redistribution of charge densities leading
to a different geometry in terms of bond angles and bond distances
favoring a proton transfer and ultimately the excited state keto
form, which becomes an emissive species. In the excited state,
the hydrogen attached to oxygen approaches near to the basic
nitrogen of imidazole unit via six member hydrogen bonding for
transfer of proton. Molecular planarity is also the most important
factor for ESIPT facilitating the proton transfer in the excited state.
This ESIPT aspect is evident from the TD-DFT computation. The dif-
ferences between the ground state and excited state of the opti-
mized geometries of the enol as well as the keto form (bond
angles, bond lengths, Mulliken charges) are explained with the
help of TD-DFT. This aspect is evident from the TD-DFT
computations.
Frontier molecular orbital
The different frontier molecular orbitals were studied to under-
stand the electronic transition and the charge delocalization within
the molecules. The comparative increase and decrease in the
energy of the HOMO’s and LUMO’s gives a qualitative idea of the
excitation properties and the ability of the hole or electron injec-
tion [40]. The first allowed and the strongest electron transitions
usually correspond almost exclusively to the ICT of an electron
from HOMO ? LUMO + 1 energies of different molecular orbitals
involved in the electronic transitions of these compounds in differ-
ent solvents are illustrated in Tables S7 and S8. In the case of the
entire solvents energy gap between the HOMO ? LUMO + 1 orbital
nearly same as the solvent polarity was increased or decreased
(Figs. S9 and S10). The HOMO and LUMO energy level of the com-
pounds 6a and 6b in DMF remains same, and it may be due to the
very similar reduction potential (Fig. S11). This is understandable
because of these structural isomer having same reduction site [41].
As a representative example, the structural aspects of the com-
pounds 6a and 6b in the ground and the excited state of the enol as
well as the keto form in acetone using Gauss View 5.0 software
[42] are summarized in Figs. S14 and S15. The bond distances
N13AH37, O23AH37, O23AC17, C14AC16 and Mulliken charges on
N13, H37, and O23 of the ground and the excited state of the opti-
mized geometries of the enol form as well as the excited state
geometry of keto form are summarized in Tables S9–S11. The com-
pound 6a in acetone, the N13AH37 hydrogen bonding distance
decreases by 0.049 Å from the ground state enol (1.732 Å) to the
excited state enol (1.683 Å) while the Mulliken charges on H37
and O23 were decreased by 0.001|e| and 0.005|e| from the ground
state enol (E) to the excited state enol (E*), respectively. The

464
M.S. Deshmukh, N. Sekar / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 135 (2015) 457–465
distance between O23AH37 bond increases by 0.009 Å from the
E-enol (0.997 Å) to the E*-enol (1.006 Å), which means that in the
excited state (E-Enol) the H37 approaches near the N13 via hydro-
gen bonding for the proton transfer. The distance between
Here, we report the ground and excited state dipole moments of
the compounds 6a–6b using Bakhshiev [43] and Kawski–Cham-
ma–Viallet correlations [44,45]. This method is based on a linear
relation between the absorption, emission maxima and solvent
polarity functions (ENT ) [46–48] which is dependent on both the rel-
O23AC17 bond decreases by 0.015 Å from the ground state enol
(1.351 Å) to the excited state enol (1.336 Å), which is further
decreased by 0.071 Å in the excited state keto form (1.265 Å). In
addition to this, the bond angle H37AO23AC17 increases by 0.28°
from the ground state enol (107.96°) to the excited state enol
(108.24°) (Table S12). In this pathway, H37 approaches closer to
ative permittivity (e) and refractive index (g) of the solvent. The
polarity function of solvent has been taken from literature [49].
By using the solvatochromic data and polarity function of sol-
vents, we calculated the dipole moment ratio of the excited state
to the ground state of the compounds 6a and 6b. The experimental
dipole moments were compared with the dipole moments in the
vacuum phase computed by DFT and TD-DFT computation
(Table 7). The dipole moment obtained from short wavelength
emission data compared with the ratio of the dipole moment of
excited state enol and ground state enol form in vacuum phase.
However, the long wavelength emission data is compared with
ratio of dipole moment of the excited state keto and the ground
state keto form in vacuum phase. These results clearly indicate that
a large difference is observed between the dipole moment obtained
from the solvatochromism data and the computed dipole moment.
Dipole moments of the compounds 6a–6b were computed in
the different solvents by DFT and TD-DFT to investigate the elec-
tronic behavior from the ground state to the excited state in differ-
ent solvents. We observed that the dipole moments of the
compounds 6a and 6b are higher for the excited state enol and
the ground state keto forms and lower for the ground state enol
and the excited state keto; this is due to the excited state of the
enol form and the ground state of the keto form stabilized by sol-
vation or hydrogen bonding with different solvents. The plots of
N13 of the imidazole unit in the excited state enol form and further
transfer to N leads to the excited state keto (K*) form with NAH
bond distance of 1.021 Å and H37AO23 bond distance of 1.901 Å.
The excited state keto returns to the ground state keto (K) con-
former with NAH bond distance of 1.046 Å and H37AO23 hydrogen
bonding distance of 1.690 Å. As H37 approaches to O23, the excited
state keto (K*) hydrogen bonding distance decreases by 0.211 Å
from 1.901 to 1.690 Å and it is immediately converted to the
ground state keto (K) conformer followed by the ground state enol
(E) form. Similar structural behavior is observed for the compound
6b (Tables S13–S16, Fig. S15).
In the case of the excited state enol (E*) of the compound 6a, as
the solvent polarity increases the O23AC17 and N13AH37 bond dis-
tance increases from 1.335 to 1.337 and 1.679 to 1.684 Å, while
O23AH37 and C14AC16 bond length decreases from 1.008 to
1.006 Å and from 1.448 to 1.445 Å, respectively (Fig. S16, Tables
S9–S10). The Mulliken charges on N13 and H37 in all the solvents
increase from the ground state enol (E) to the excited state enol
(E*) (Fig. S17, Table S11) while at the same time decrease for O23
atom indicating that in the excited state enol (E*) form, H37 is in
close proximity of N13 atom, which is favorable for ESIPT. From
the excited state (K*-Keto) to the ground state (K-Keto) Mulliken
charges for O23, H37 and N13 atom increases (Fig. S18, Table S11),
which supports the ESIPT process. Similar observation is observed
in the compound 6b (Figs. S19–S21).
the dipole moment (l
) versus solvent polarity function (ENT ) of
compounds 6a–6b enol and keto form in the ground and the
excited state is shown in Tables S17–S18; Figs. S22–S23.
Conclusion
To conclude, this paper reports the synthesis of the fluorescent
2-(2-hydroxyphenyl)-6-methylimidazo[4,5-f]isoindole-
Effect of solvent polarity on the ground and the excited state dipole
moments
5,7(1H,6H)-dione and its derivative. This compound is well charac-
terized by FTIR, 1H NMR and mass spectroscopy. The thermal sta-
bility was determined by TGA analysis and it was found that the
compounds are thermally stable. The compounds 6a and 6b shows
a single prominent absorption peak but in the case of emission of
compound 6a dual in non-polar, polar aprotic and single emission
in polar protic solvent. While in compound 6b dual emission
observed only in ethanol and DMF. The photophysical properties
of the compounds are sensitive toward the solvent environment.
The highest quantum efficiency was observed in THF (6a = 0.15,
6b = 0.10). Photophysical properties of the synthesized compounds
were supported by DFT and it was observed that computational
results are good agreement with the theoretical observations.
The polarity of the solvents affects the fluorescence properties
due to their solvation or hydrogen bonding with heteroatom pres-
ent in the compounds. The dipole moment also depends on the
polarity as well as the groups present in the molecule. The geom-
etry of the compound at the ground and the excited state decide
their electronic behavior. The effect of the solvent on the absorp-
tion and the emission spectra clearly indicates the change in the
dipolar characteristics of the compounds in the excited state,
which means that the solvatochromic data gives an efficient tool
to understand the change in the dipole moment in the first excited
state.
Table 7
Dipole moment (l in Debye) of the compounds 6a and 6b.
Compounds
aEnol lg
bEnol le
bEnol le
aEnol lg
aKeto lg
5.819
bKeto le
5.612
bKeto le
aKeto lg
cExperimental
Dipole moment
% D
6a
6b
a
6.082
6.731
1.107
0.964
0.44d
0.12e
2.54d
3.8e
60f
88g
4.7002
4.921
1.047
4.139
3.812
0.921
59f
76g
Dipole moment of the compound in ground state.
Dipole moment of the compound in excited state.
b
c
d
e
f
Experimental dipole moment obtained from solvatochromism data using Bakhshiev and Kawski–Chamma–Viallet correlations.
Dipole moment at the short wavelength emission obtained from the solvatochromism data using Bakhshiev and Kawski–Chamma–Viallet correlations.
Dipole moment at the longer wavelength emission obtained from solvatochromism data using Bakhshiev and Kawski–Chamma–Viallet correlations.
(% D)% Deviation between the experimental and the computed dipole moment for short wavelength emission.
g
(% D)% Deviation between the experimental and computed dipole moment for long wavelength emission.

M.S. Deshmukh, N. Sekar / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 135 (2015) 457–465
465
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Acknowledgements
The authors are greatly thankful to TIFR, SAIF-I.I.T. Mumbai for
recording the 1H NMR and Mass spectra. One of the authors Mini-
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