An insight into the dual fluorescence of 3,6-dihydroxybenzene-1,2,4,5-tetracarboxylic acid tetraethyl ester – An experimental and theoretical study

Article abstract of DOI:10.1016/j.saa.2021.119496

The Excited State Intramolecular Proton Transfer (ESIPT) phenomenon involving photo-induced keto-enol tautomerization is known to cause significant variations in the excited state structures and photophysical properties of certain molecules. Here, the dua

Full text of DOI:10.1016/j.saa.2021.119496

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 252 (2021) 119496
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Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
An insight into the dual fluorescence of 3,6-dihydroxybenzene-1,2,4,5-
tetracarboxylic acid tetraethyl ester – An experimental and theoretical
study
⇑
⇑
L.B. Aswathy, Ani Deepthi , E.G. Jayasree
Department of Chemistry, University of Kerala, Kariavattom Campus, Thiruvananthapuram 695581, Kerala, 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
ꢀ
ꢀ
ꢀ
ESIPT is the reason for dual emission
in compound 2 in polar protic
solvents.
The dianion formation is the reason
for emission in solvents like DMSO
and DMF.
TD-DFT calculations support the
experimental data in this regard.
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 (ESIPT) phenomenon involving photo-induced keto-
enol tautomerization is known to cause significant variations in the excited state structures and photo-
physical properties of certain molecules. Here, the dual emission exhibited by 3,6-dihydroxybenzene-1
Received 13 October 2020
Received in revised form 1 January 2021
Accepted 12 January 2021
Available online 22 January 2021
,
2,4,5-tetracarboxylic acid tetraethyl ester has been studied both experimentally and theoretically and
it is concluded that the second emission is due to ESIPT in polar protic solvents, while it is due to dianion
formation in solvents like DMSO and DMF.
Keywords:
Ó 2021 Elsevier B.V. All rights reserved.
Excited state intramolecular proton transfer
Dual emission
3
,6-dihydroxybenzene-1,2,4,5-tetracar
boxylic acid tetraethyl ester
Cerium (IV) ammonium nitrate
Diethyl-1,3-acetone dicarboxylate
1
. Introduction
and since then it has been a topic of intensive discussions [3–10].
Weller explained the presence of two bands at 350 nm and
The process of excited-state intramolecular proton transfer
ESIPT) was first reported by Weller during his study of the dual
fluorescence of salicylic acid (SA) and methyl salicylate (MS) [1,2]
440 nm in the emission spectrum of MS as arising due to the keto
form (i) and enol form (ii) respectively; the latter formed by the
excited state intramolecular proton transfer (ESIPT) of the keto
form (Fig. 1). Later studies [11–14], proposed that the higher wave-
length emission of MS is due to the hydrogen bonded conformer (i)
in gas phase, while in hydrogen bonding solvents conformer (iv)
predominates compared to (i) and (iii) (Fig. 1). The existence of
two rotamers (i) and (iii) in both gas phase and solution phase
(
⇑
Corresponding authors at: Department of Chemistry, School of Physical and
Mathematical Sciences, University of Kerala, Thiruvananthapuram 695581, Kerala
State, India.
E-mail addresses: anideepthi@gmail.com (A. Deepthi), jelambal@gmail.com (E.G.
Jayasree).
https://doi.org/10.1016/j.saa.2021.119496
1
386-1425/Ó 2021 Elsevier B.V. All rights reserved.

L.B. Aswathy, A. Deepthi and E.G. Jayasree
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 252 (2021) 119496
Fig. 1. Different conformers of MS.
Scheme 1. Synthesis of 3,6-dihydroxybenzene-1,2,4,5-tetracarboxylic acid tetra-
ethyl ester.
were also confirmed by several supersonic jet experiments [15–
7]. Over the past decade detailed theoretical studies on the ESIPT
process occurring in SA and MS have thrown more light into the
photophysics of these molecules [18–24].
1
were recorded on a Bruker Avance DPX-500 MHz spectrometer.
NMR spectra of compound 3 were recorded on a Bruker Avance
III HD 400 MHz spectrometer. Chemical shifts were reported rela-
tive to TMS as the internal standard. IR spectra were recorded on
an Agilent Cary 630 FTIR spectrometer. Mass spectra were
recorded under ESI/HRMS using JEOL JMS 600H mass spectrome-
ter. Absorption and emission spectra were recorded using Perkin
Elmer UV/Vis Lambda 365 spectrometer and JASCO FP-8300 spec-
trofluorimeter respectively. Fluorescence lifetime decays were
measured by using a picosecond single photon counting system
The photophysics of ESIPT has been extensively studied and
applied [25–28]. Widely known ESIPT fluorophores include ana-
0
0
0
logues of 2-(2 -hydroxyphenyl)benzazole (A-C) [29], 2-(2 ,6 -dihy
droxyphenyl)benzoxazole (DHBO) [30] and 2-(benzothiazol-2-yl)
naphthalene-1,3-diol (DHT) [31] (Fig. 2). Apart from these, certain
derivatives of quinolines, benzophenones, flavones, anthraqui-
nones, benzotriazoles, N-salicylidene anilines and quinoxalines
are also known to display ESIPT properties [32]. The significance
of molecular systems with ESIPT characteristics as fluorescent
probes for sensing and imaging applications has been reviewed
recently [33].
(
Horiba, DeltaFlex) employing 274, 331 and 378 nm LED as excita-
tion sources and picosecond photon detection module (PPD-850)
as a detector. The decay of the fluorescence intensity (I) with time
(
t) was fitted either by a mono or double-exponential function:
Although ESIPT of methyl salicylate (MS) has been widely stud-
ied by many research groups, there has been no attempt as far as
our knowledge to study the ESIPT of its analogues. Herein we pre-
sent the analysis of the experimental and theoretical results
obtained by studying the dual fluorescence of 3,6-dihydroxyben
zene-1,2,4,5-tetracarboxylic acid tetraethyl ester (compound 2,
Scheme 1) which can be considered as an analogue of methyl sal-
icylate with the possibility of intramolecular hydrogen bonding.
The said compound was synthesized in our lab by the cerium
ꢁt=
s
I ¼ A
1
e
1
ꢁ
t=s
ꢁt=
s
I ¼ A
1
e
2
þ A e
1
2
where
1 2 1 2
s and s are the lifetimes of different species, and A and A
are their respective amplitudes.
Computational studies were carried out using Gaussian 09
package. Ground state geometries were calculated using
B3LYP/6-31+G(d) level of theory. Time dependent-DFT calculations
were employed to calculate excited states, excitation and emission
energies at the same level of theory. The calculations were carried
out in gas phase, bulk solvent and microsolvation. For the solvent
calculations, methanol and DMSO were employed. Vibrational fre-
quency calculations were employed to confirm the nature of calcu-
lated stationary points.
(
IV) ammonium nitrate (CAN) mediated oxidation of diethyl-1,3-
acetone dicarboxylate [34]. The fluorescence of this molecule is
due to the rigidity associated with the intramolecularly hydrogen
bonded structure which is further evident when its dimethoxy
derivative namely, 3,6-dimethoxybenzene-1,2,4,5-tetracarboxylic
acid tetraethyl ester does not exhibit any fluorescence behaviour.
2
. Materials and methods
2.1. Synthesis of 3,6-dihydroxybenzene-1,2,4,5-tetracarboxylic acid
tetraethyl ester 2
Solvents used for the synthesis and studies are of HPLC grade
and were purchased commercially and used as such without fur-
ther purification. Cerium(IV) ammonium nitrate (CAN) was pur-
chased from Merck specialties Pvt. Ltd. Diethyl 1,3-acetone
dicarboxylate was purchased from Sigma-Aldrich and triethyl
amine (TEA) was purchased from Spectrochem Pvt. Ltd. Analytical
thin layer chromatography was performed on silica gel coated on
aluminium sheets and was monitored using UV light of wave-
length 366 nm. Gravity column chromatography was performed
using 100–200 mesh silica gel and mixtures of hexane and ethyl
acetate were used for elution. NMR spectra of the compound 2
Based on a developed protocol [34], diethyl 1,3-acetone dicar-
boxylate 1 was treated with 30 mol% CAN in dry acetonitrile sol-
vent. After completion of the reaction, work-up was done
followed by column chromatography which afforded the com-
pound 2 in 47% yield (59% yield based on recovered starting mate-
rial) (Scheme 1).
Light yellow solid (Elution with 15:85 ethyl acetate: hexane);
ꢁ
1
yield: 88 mg (47%), mp: 121.5–122.5 °C, IR (powder,
v
cm ):
H NMR (500 MHz,
1.40 (t, J = 7.5 Hz, 6H), 4.41 (q, J = 7.5 Hz, 4H), 10.65
): d 166.4, 150.1, 120.1, 62.2,
10: 421.1111;
ꢁ1
1
3
378, 2989, 1725, 1498, 1285, 961 cm .
CDCl
3
): d
H
3
1
(
s, 1H). C NMR (125 MHz, CDCl
3
C
+
1
3.9. HRMS (ESI): m/z [M + Na] calcd for C18
H
22
O
1
13
found: 421.1117 ( H NMR, C NMR and mass spectra are provided
in supporting information).
2.2. Synthesis of 3,6-dimethoxybenzene-1,2,4,5-tetracarboxylic acid
tetraethyl ester (3)
To a solution of compound 2 (0.15 mmol) in dry dimethylfor-
mamide (DMF, 10 mL), methyl iodide (0.3 mmol) and anhydrous
potassium carbonate (0.6 mmol) were added. The solution was
Fig. 2. Some examples of molecules other than MS showing ESIPT.
2

L.B. Aswathy, A. Deepthi and E.G. Jayasree
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 252 (2021) 119496
stirred at room temperature for 12 h. After completion of the reac-
tion, the solvent was removed, and reaction mixture was worked-
up with excess ethyl acetate and water. The residue from the ethyl
acetate layers were collected and was subjected to column chro-
matography which afforded compound 3 in 58% yield (Scheme 2).
Yellow liquid (Elution with 5:95 ethyl acetate: hexane); yield:
ton accepting DMF and the compound exhibits a yellow fluores-
cence in DMF.
3.3. Exceptional behaviours in DMF and DMSO
Earlier studies have clearly shown that higher solvation induced
by proton accepting solvents such as DMF and DMSO cause dra-
matic changes in the fluorescence properties of compounds [35].
Compound 2 shows absorption maxima at 325 and 335 nm in
DMF and DMSO respectively and these bands are highly blue
shifted when compared with the spectra of 2 taken in other sol-
vents due to greater solvation provided by these solvents which
disrupts the intramolecular hydrogen bonding in 2 and conse-
quently intermolecular hydrogen bonding with these solvents
ꢁ1
5
1
8%, IR (thin film,
v
cm ): 2917, 2846, 1736, 1605, 1456, 1229,
ꢁ1
1
020 cm
.
H NMR (400 MHz, CDCl d (ppm): 1.37 (t,
3
)
1
3
J = 7.2 Hz, 3H), 3.87 (s, 3H), 4.37 (q, J = 7.2 Hz, 2H). C NMR
100 MHz, CDCl ) d (ppm): 173.3, 157.4, 138.2, 62.2, 50.4, 10.3.
HRMS (ESI): m/z [M + Na] calcd for C20
(
3
+
H
26
O
10: 449.1424; found:
49.1421 ( H NMR, C NMR and mass spectra are provided in sup-
porting information).
1
13
4
(
DMSO and DMF) becomes prominent, which causes exceptional
3
. Results and discussion
blue shift in absorption maximum (Conformer I, Scheme 4).
Regarding emission, the compound shows a yellow fluorescence
in DMSO and DMF (Fig. S6 in SI). From the emission spectrum,
the prominent blue band (k1emiss, Table 1) shows significant
decrease in intensity in DMSO compared to other solvents with a
wavelength maximum at 440 nm. In the case of the second emis-
sion band (yellow band, k2emiss, Table 1), there occurs a red shift
to 570 nm with low intensity. This may be explained along the fol-
lowing lines. On excitation, this intermolecular hydrogen bonding
is retained along with intramolecular hydrogen bonding to form
conformer II which gives rise to a less intensity blue band emission
3
.1. Absorption and emission spectra of 2 in different solvents
Fig. 3 shows the absorption and emission spectra of 2 recorded
at room temperature in various solvents (cyclohexane, hexane,
ethyl acetate, acetone, dichloromethane, methanol, ethanol, carbon
tetrachloride, toluene, chloroform, THF, acetonitrile, DMF and
DMSO). Similar to MS, compound 2 exhibits dual fluorescence
emission in all these solvents. The absorption and emission spectra
of 2 shows strong dependency on solvent polarity, which is very
pronounced with DMSO and DMF. The absorption, emission and
Stokes shift of 2 in different solvents is depicted in Table 1.
1
(k ). However, owing to the high proton affinity of DMSO, inter-
molecular proton dislocation is observed by breaking the
intramolecular hydrogen bond in species II and results in dianion
3
.2. The ESIPT process
III which might be responsible for the yellow band emission (k
35].
The excitation spectra of 2 (recorded by setting the monochro-
2
)
[
The compound exhibits a blue fluorescence in all solvents
except in DMSO and DMF. The absorption maximum is probably
due to the closed hydrogen bonded species K in the ground state
as shown in Scheme 3. From the Fig. 3, the emission spectra of
compound 2 shows a prominent blue emission band k
a lower intensity yellow band k (excitation wavelength 380 nm).
Based on analogy with methyl salicylate, the short wavelength
blue band corresponds to the excited keto form (K*) and long
wavelength yellow band corresponds to the enol form (E*) proba-
bly formed by ESIPT from the keto form (Scheme 3). The latter
exhibits larger Stokes shift both in polar and non-polar solvents
when compared to the short wavelength blue band. In polar protic
mator to two emission wavelengths) in DMF and DMSO show
bands which were distinct from their corresponding absorption
spectra (Fig. S7 in SI) while the excitation spectra of 2 in other sol-
vents shows a single band that correlates with their absorption
spectra (as expected). In the case of DMF, the excitation spectra
show a prominent band at 367 nm and a small shoulder at
1
along with
2
2
70 nm for both emission at 457 and 540 nm (kabs in DMF is
3
25 nm). Similarly, the excitation spectra in DMSO, monitored at
two emission maxima at 440 and 570 nm, shows bands at 368
and 358 nm respectively (kabs in DMSO is 335 nm). The difference
in excitation spectra from absorption spectra indicates that the
dual emission (blue band and yellow band) is an excited state phe-
nomenon and both emissions might be originated in the excited
state due to specific solvation interaction of 2 in highly polar apro-
tic environments. The emission intensity of both bands (blue and
yellow) in 2 depends on excitation energy in DMSO which is again
an indication of more than one species in the excited state (Fig. 4).
(
3
methanol and ethanol) and aprotic solvents (DMF and CH CN) the
intensity of blue band tends to decrease when compared with non-
polar solvents which is explained in the following lines. In polar
protic solvents intermolecular hydrogen bonding ability of sol-
vents play a crucial role in the ESIPT mechanism. Such solvents
interrupt intramolecular hydrogen bond formed by the compound
which results in decrease in intensity of blue band, and such inter-
ruption also decreases the ESIPT emission. In the case of polar
3.4. Effect of base and acid on the emission spectra of 2 in DMSO and
3
aprotic solvents such as CH CN, the solvation effect predominates
DMF
and hinders the intramolecular hydrogen bond formation which
again decreases the ESIPT emission. Additionally, in the case of
DMF (a proton accepting polar aprotic solvent), the ESIPT process
is hindered due to the partial dislocation of –OH proton to the pro-
The effect of acid and base on solute–solvent hydrogen bonding
interactions were studied by adding increasing amounts of acid
and base to 2 in DMSO and DMF. On addition of triethyl amine
(
TEA) to the compound in DMSO the yellow band (570 nm)
increases in intensity at the expense of blue band (440 nm) whose
intensity decreases slightly (Fig. S8 in SI). Increasing the concentra-
tion of TEA, an isosbestic point at 476 nm is observed indicating co-
existence of species II and III. On addition of TEA the intensity of
yellow band increases which may be due to the higher deprotona-
tion in 2 which increases the concentration of III (Scheme 4). At the
same time, when 2 is taken in DMF, emission spectrum indicates
that increasing concentration of TEA causes a decrease of intensity
of both blue and yellow bands (Fig. S9 in SI). This means that only
Scheme 2. Methylation of compound 2.
3

L.B. Aswathy, A. Deepthi and E.G. Jayasree
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 252 (2021) 119496
Fig. 3. Absorption and emission spectra of 2 in different solvents.
Table 1
Absorption and emission wavelengths in different solvents.
Stokes shift (in cm 1) for blue emission
ꢁ
Stokes shift (in cm ) for yellow emission
ꢁ1
Solvent
k
abs (nm)
k1emiss (nm)
k
2emiss (nm)
Cyclohexane
Hexane
Ethyl Acetate
Acetone
Dichloromethane
Methanol
Ethanol
Carbon tetrachloride
Toluene
Chloroform
THF
Acetonitrile
DMF
391
392
383
380
385
377
380
391
392
387
383
381
325
335
455
457
455
453
459
454
452
459
461
458
456
453
457
440
526
532
531
535
530
535
533
531
531
535
532
533
540
570
3597
3628
4131
4240
4187
4498
4191
3788
3818
4005
4179
4171
8887
7123
6564
6713
7277
7624
7106
7833
7554
6743
6677
7148
7312
7484
12,250
12,306
DMSO
Scheme 3. Probable species responsible for the dual emission in all solvents except DMSO and DMF.
at higher concentration of TEA, significant deprotonation of com-
pound takes place in DMF. Since DMSO is more polar than DMF
with higher dielectric constant (Dielectric constant for DMSO is
4
6.7 F/m whereas for DMF is 36.7 F/m), the solvation effect offered
by DMSO is more pronounced than that by DMF, so that the dian-
ion (III, Scheme 4) is formed more quickly in DMSO compared to
DMF. On account of the above facts, it is clear that the red-shift
observed when compound is taken in DMSO in base independent
while in DMF is base dependant.
Scheme 4. Probable species responsible for dual emission in DMSO and DMF.
Fig. 4. Emission spectra of 2 in DMSO (i) excited at 380 nm and (ii) excited at 340 nm.
4

L.B. Aswathy, A. Deepthi and E.G. Jayasree
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 252 (2021) 119496
In order to check the reversibility, a small amount of acetic acid
was added to the compound in DMSO containing TEA and emission
spectrum was recorded. The intensity of yellow band was found to
decrease on addition of increasing amounts of acid which indicates
reversibility in DMSO (Fig. S10 in SI). Similar experiment con-
ducted with compound in DMF indicates increase in intensity of
both bands as expected.
decreases on further increase in pH). At pH 11 the band at
425 nm vanishes and a new band appears at 568 nm, which is
probably due to the dianion formation as shown in Scheme 5. At
strong alkaline condition only the 568 nm band is seen promi-
nently (Fig. S13 in SI). From Table 1, compound 2 in methanol
shows dual emission at 454 and 535 nm. Here also at acidic pH
intensity of the two bands decreases but as methanol can form
intermolecular hydrogen bond with the compound the red shift
of the 535 nm band to 552 nm occurs at pH 5; the intensity of
3.5. Effect of base and acid on the excitation spectrum of 2 in DMSO
4
54 nm band now becomes comparable with the new 552 nm
band. At pH 8 the 454 nm band vanishes and upto pH 11 only
52 nm exists (Fig. S14). Further increase in pH will probably cause
complete deprotonation and makes the solution colourless
Fig. S15).
and DMF
5
The changes in excitation spectra by the addition of base/acid to
the compound in DMSO and DMF were recorded (Figs. S11 and
S12, SI) by monitoring their respective emission wavelengths. As
mentioned in Section 3.3, the excitation maxima of compound 2
in DMSO is found to be 358 and 368 nm (emission monitored at
(
3.7. Fluorescence lifetime decay
5
70 and 440 nm). Addition of TEA causes a red shift in wavelength
from 358 to 410 nm with an increase in intensity. Thus the 358 nm
band may be due to the conformer III (Scheme 4). Reversibility was
checked by addition of acetic acid which causes a blue shift of
wavelength from 410 to 367 nm with decrease in intensity. On
the other hand, no considerable change in excitation maximum
at 368 nm was observed by the addition of base and acid to the
compound in DMSO (Fig. S11). This indicates that the 440 nm band
may be due to structure II (Scheme 4).
As mentioned in Section 3.3, the excitation maxima of com-
pound 2 in DMF is found to be at 367 nm (recorded by monitoring
emission at 457 and 540 nm). Addition of TEA causes decrease in
intensity of the 367 nm band which is very prominent at higher
TEA concentrations (Fig. S12 in SI). Addition of acetic acid indicates
irreversibility which indicates that 367 nm band is probably due to
species II (Scheme 4).
Fluorescence lifetime decay of 2 in three different solvents
(hexane, methanol and DMSO) at two emission wavelengths were
recorded using a picosecond single photon counting system
(Fig. S16). The decay of the shorter wavelength component can
be explained using a bi-exponential function (compound in hex-
ane/methanol) or a tri-exponential function (compound in DMSO)
and the results are given in Table 2. This indicates a different decay
process in DMSO compared to decay in hexane/methanol indicat-
ing that the dual emission of 2 is strongly dependent on the solva-
tion effect provided by DMSO. The decay of the longer wavelength
component in methanol and DMSO can be explained using a single
exponential function while in hexane a bi-exponential function is
needed. This may be due to the strong intermolecular hydrogen
bonding effects when the compound 2 is taken in methanol and
DMSO.
3.6. Acidochromism
3.8. Computational results
Acidochromic behaviour of 2 in hexane and methanol has been
The absorption and emission spectra were calculated with the
time-dependent density functional theory (TD-DFT) as imple-
mented in the Gaussian 09 program code. B3LYP/6-31+G(d) level
of theory was employed for all the computations. Earlier reports
on methyl salicylate ruled out the possibility of existence of struc-
ture analogs of (iii, Fig. 1) where the intramolecular H-bonding
occurs with alkoxy oxygen. Therefore, computations have been
carried out for the conformer of 2 where intramolecular H-
bonding exists with carbonyl oxygen (Fig. 5). All ethoxy groups
investigated at different pH (Figs. S13 and S14 in SI). From Table 1,
compound 2 in hexane shows dual emission at 457 and 532 nm
which corresponds to the blue and yellow bands respectively. At
acidic pH the intensity of blue band decreases probably due to pro-
tonation of ester carbonyl of 2 which interrupts intramolecular
hydrogen bonding (similarly yellow band intensity also dimin-
ishes). At pH 7 a sharp decrease in intensity of blue band is noticed
with a new band appearing at 425 nm (the intensity of which
Scheme 5. Effect of pH on the dual emission properties of 2.
Table 2
Fluorescence lifetime data (in ns) of 2 in hexane, methanol and DMSO from time-correlated single photon counting (TCSPC) data.
Solvent
Emission maxima (k
1
/k
2
) nm
Lifetime in ns
Lifetime in ns
Lifetime in ns
s
1
(amplitude %)
s
2 (amplitude %)
s
3 (amplitude %)
Hexane
MeOH
DMSO
457
2.3245 (1.16)
2.0304 (1.04)
2.2205 (4.06)
7.1277 (100)
0.9616 (43.79)
7.7861 (100)
8.7925 (98.84)
8.7830 (98.96)
7.0622 (95.94)
–
0.2719 (46.03)
–
–
–
–
–
5
32
454
32
440
70
5
6.1718 (10.19)
–
5
5

L.B. Aswathy, A. Deepthi and E.G. Jayasree
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 252 (2021) 119496
(
448 nm). These values are in agreement with experimental k1emiss
values. The optimized excited states were found to have both kind
of hydrogen bonding interactions with intramolecular H-bonding
being the more stable one (in the range 1.65–1.71 Å) over the
intermolecular interactions (2.25–2.35 Å). The intramolecular H-
bonding is found to be stronger in methanol (by 0.05 Å) while
the intermolecular hydrogen bonding has been calculated to be
stronger with DMSO than with methanol (by 0.08 Å) which is evi-
dent from the corresponding bond lengths. This makes the DMSO
excited state to be less stable leading to slightly small k1emiss value.
In order to see if k2emiss could be explained by enol formation,
both excitation and emission energies were calculated for enol as
well. It was found that the absorbance and k1emiss value closely
parallel the experimental value in methanol but differed to a large
extent in DMSO while k2emiss could not be obtained in both cases.
However, it has been found that the calculated emission value of
dianions (of the type III) lie at a higher wavelength region
Fig. 5. Stable trans isomer 4 selected for the computational study.
were replaced by methoxy groups for computational simplicity.
Among the various conformers calculated, one with the trans ori-
entation of two hydroxyl groups has been obtained as the stable
ground state geometry and has been taken for TD-DFT calculations
in selected solvents, methanol and DMSO.
Adding bulk solvation without incorporating explicit solvent
interactions shows stronger intramolecular H-bonding in the
excited state and the excitation energy so calculated was 379 nm
in DMSO (Table 3) which does not correlate with the experimental
value of 335 nm. However, computed and experimental values are
in well agreement for methanol. Upon adding explicit solvation,
this intramolecular H-bonding has been found to be retained along
with certain intermolecular H-bonding due to methanol molecule
(~600 nm) which indicates that the k2emiss value may be coming
from the dianion. This is in agreement with the experimental
observation of increase in intensity of yellow band on base
addition.
4. Conclusion
in both ground S
change in its absorbance from bulk solvation. On the other hand,
the ground S of 2 has only intermolecular H-bonding interaction
with DMSO, the OH. . .O(DMSO) distance being 1.694 Å, which
destabilizes the vertically excited state and the excitation energy
is reduced to 331 nm which matches well with the experimental
value (Fig. 6). It can be stated that while the intramolecular H-
bonding determines the excitation energy in methanol, the inter-
molecular H-bonding due to solvent determines the corresponding
value in DMSO. The calculated emission wavelength in DMSO is
0
and excited S
1
states, thus showing no significant
Thus, the cause of dual emission in 3,6-dihydroxybenzene-1,2,
4,5-tetracarboxylic acid tetraethyl ester 2 was investigated both
experimentally and theoretically and it was found that intramolec-
ular hydrogen bonding between –OH proton and carbonyl group is
responsible for the fluorescence of 2 as the substitution of –OH
with –OCH3 group makes the compound non-emissive. The
observed dual emission of 2 in solvents (except in DMSO and
DMF) were attributed to the intramolecularly hydrogen bonded
keto form K* (shorter wavelength blue band) and enol form E*
(longer wavelength yellow band), the latter formed via ESIPT from
keto form. However, in DMSO and DMF the emissions at shorter
and longer wavelength arises due to the structures II and III
respectively (Scheme 4). The molecule was earlier reported by us
to act as an efficient fluorescent probe for detection of picric acid
[34] and further exploration of its applications by combining to
molecules which fluoresce by other mechanisms like AIE, PET or
ICT is underway.
0
4
39 nm which is lower than that calculated in methanol
Table 3
Computed vertical excitation (absorbance) and emission values for structure 4.
Trans/medium
Vertical Excitation (nm)
Emission (nm)
Bulk DMSO
Bulk Methanol
Bulk + 2 explicit DMSO
Bulk + 2 explicit MeOH
379.49
376.59
330.73
373.98
457.64
453.73
438.61
448.40
CRediT authorship contribution statement
L. B. Aswathy: Data curation, Formal analysis, Writing - original
draft. Ani Deepthi: Conceptualization, Investigation, Methodology,
Project administration, Resources, Supervision, Writing - review &
editing. E. G. Jayasree: Software, Validation.
Declaration of Competing Interest
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared
to influence the work reported in this paper.
Acknowledgements
ALB thank UGC for Senior Research Fellowship. All authors
thank CLIF, University of Kerala for the spectroscopic analysis
and CSIR-NIIST, Thiruvananthapuram for TCSPC analysis.
Appendix A. Supplementary material
Fig. 6. Computed ground and first excited state geometries of structure 4 in DMSO
incorporating explicit solvent interactions at B3LYP/6-31+G(d) and TD-B3LYP/6-31
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.saa.2021.119496.
+G(d) methods respectively.
6

L.B. Aswathy, A. Deepthi and E.G. Jayasree
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 252 (2021) 119496
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