Imine-Amine Tautomerism vs Keto-Enol Tautomerism: Acceptor Basicity Dominates over Acceptor Electronegativity in the ESIPT Process through a Six-Membered Intramolecular H-Bonded Network

Article abstract of DOI:10.1021/acs.jpca.9b08646

Photophysical properties of a synthesized asymmetric two-way proton transfer molecule 3-(benzo[d]thiazol-2-yl)-2-hydroxy-5-methoxybenzaldehyde (BTHMB) were studied in detail. BTHMB could undergo excited-state intramolecular proton transfer (ESIPT) involvi

Full text of DOI:10.1021/acs.jpca.9b08646

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A: Kinetics, Dynamics, Photochemistry, and Excited States
Imine-amine Tautomerism vs. Keto-enol Tautomrisation: Acceptor
Basicity Dominates Over Acceptor Electronegativity in ESIPT
Process through Six-member Intramolecular H-Bonded Network
Arghyadeep Bhattacharyya, Sujoy Kumar Mandal, and Nikhil Guchhait
J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.9b08646 • Publication Date (Web): 02 Nov 2019
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Imine-amine Tautomerismvs. Keto-enol Tautomrisation:Acceptor Basicity Dominates
Over Acceptor Electronegativity in ESIPT Process through Six-member Intramolecular H-
Bonded Network
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a*
Arghyadeep Bhattacharyya , Sujoy Kumar Mandal , and Nikhil Guchhait
aDepartment of Chemistry, University of Calcutta, 92, A.P.C. Road, Kolkata-700009, India
bDepartment of Physics, University of Calcutta, 92, A.P.C. Road, Kolkata-700009, India
*
Corresponding author. Tel.: +913323508386; fax: +913323519755.
E-mail address: nguchhait@yahoo.com (N. Guchhait).
Abstract: Photophysical properties of a synthesized asymmetric two ways proton transfer
molecule 3-(benzo[d]thiazol-2-yl)-2-hydroxy-5-methoxybenzaldehyde (BTHMB) was studied in
details. BTHMB could undergo excited state intramolecular proton transfer (ESIPT) involving a
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-member H-bonded network towards either N as well as the O atom present in the molecule as
proton acceptors. From tedious spectroscopic dissection, however, it was established that ESIPT
was driven exclusively towards N centre over O center in solid state as well as in the solution
phase. The aforementioned deduction was based on spectral comparison with judicially synthetic
compounds 2-hydroxy-5-methoxybenzaldehyde (HMB) and 2-(benzo[d]thiazol-2-yl)-4-
methoxyphenol (BTMP). In solution, the co-existence of anion and neutral form of BTHMB in
basic solvents DMSO and DMF enables BTHMB to behave as a white light emitter. Thus, apart
from establishment of directionality of ESIPT in double ESIPT probes, current work deserves
special merit as BTHMB can be considered a standard in future designs involving red light and
solvent specific white light emitters.
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. Introduction
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Since the epoch making report by Weller et al regarding excited state intramolecular proton
transfer (ESIPT) in methyl salicylate, the said excited state phenomenon has been enjoying
tremendous attention by chemists till date. With passage of times, several ESIPT active
fluorophores have been synthesized and reported by various groups, providing deeper insight
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into the aforesaid excited state phenomenon . ESIPT may involve through a 6-membered
(salicylic acid), 5-membered (3-hydroxy flavones) or even 4-membered (2-
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hydroxypyridine)hydrogen bonded network between the donor and acceptor groups. However,
on thermodynamic grounds, 6-membered H-bonded network is more favoured over the other
two. Thus, the ESIPT systems where ESIPT occurs through 6-membered hydrogen bonded site
have been studied more rigorously by chemists over the years. Of late, there has been a mounting
interest in the spectroscopic study of organic compounds having double proton transfer sites. For
example, compounds like 2, 2'-Bipyridine-3, 3'-diol have been reported to undergo simultaneous
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ESIPT , whereas bis-Schiff bases like salicylidene phenylenediamine are reported to undergo
single ESIPT involving a single enol to keto photo conversion (Scheme 1). The aforementioned
ESIPT probes contain N atom as acceptor(s) of proton whereas a phenolic group is present as
proton donor site(s). Another important class of double ESIPT probes are those where carbonyl
oxygen is present as the proton acceptor. Some noteworthy examples are 5, 8-dihydroxy-1, 4-
naphtoquinones and anthrarufin9-10, among which the former undergoes simultaneous double
proton transfer whereas the latter undergoes ultrafast single ESIPT process. Some noteworthy
theoretical calculations have also been carried out for some biologically as well as
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spectroscopically interesting molecules such as Deep Red Pigment Alkannin , bisflavonols as
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well as 3-hydroxyisoquinoline . From the above discussions, it is evident that the fundamental
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understanding of double EISPT processes is a problem which has garnered tremendous attention
from experimental as well as theoretical chemists. A scrutiny of the structures of the stated
compounds reveals that both the proton transfer sites are identical, i.e. the systems have exactly
similar sites where ESIPT can occur. On the contrary, very little emphasis has been provided on
synthesized systems where the same molecule has two different proton transfer sites. A report by
Chou et al is worth mentioning at this juncture (Scheme 1) where the double ESIPT of a pyridine
bearing 3-hydroxy flavone derivative, 3-Hydroxy-2-(pyridin-2-yl)-4H-chromen-4-one (HPC)
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were studied (Scheme 1) . The study revealed that in solid state, ESIPT occurred between
pyridine N and the-OH proton, with an emission band at 540 nm. However, in the solution phase
an additional ESIPT process occurred between the carbonyl oxygen with the –OH group.
Although the two proton transfer sites are genuinely asymmetric in nature, one site involved 5-
member hydrogen bonded network whereas 6-membered hydrogen bonded network is involved
for ESIPT between N atom and OH group. Our group also came up with a report where lactim-
lactam tautomerism was favoured over enol-keto tautomerism in a nicotinic acid derivative2-
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hydroxy-5-(4-Fluorophenyl) Nicotinic Acid (HFPNA) , which had the same drawback of the
differing H-bonded ground state geometries. In order to have a better model for ascertaining the
directionality of ESIPT, we decided to synthesize a molecule containing two asymmetric proton
transfer sites and involving 6-member H-bonded network for the two potential ESIPT processes,
namely3-(benzo[d]thiazol-2-yl)-2-hydroxy-5-methoxybenzaldehyde
(BTHMB)
(Scheme
1).BTHMB has two ESIPT sites, one involving keto-enol tautomerism, whereas the other
involves imine-amine tautomerism. Detailed steady state and time resolved studies revealed that
ESIPT occurs exclusively towards the N centre than the O centre in solid state as well as in
solution phase. The current report holds particular importance as we could state that the basicity
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of N atom in BTHMB dominates the ESIPT process, overruling the electronegativity of the O
atom.Another impact of the current report is the influence of the methoxy substitution in
BTHMB (Scheme 2). Goswami et al reported a structurally similar compound 5(Benzothiazol-2-
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yl)-4-hydroxyisophthalaldehyde (BHI)as an amino acid sensor . BHI, equipped with a Formyl
group para to the phenolic –OH functionalityshowed ESIPT emission at ~520 nm and yellow
green color could be observed in solution. BTHMB containing an electron releasing methoxy
functionality showed ESIPT emission at ~600 nm and orange red emission is observed. That the
aforementioned band occurs due to N being the acceptor was established by spectral comparison
with control compounds 2-hydroxy 5-methoxy benzaldehyde (HMB)and 2-(benzo[d]thiazol-2-
yl)-4-methoxyphenol (BTMP).As an added advantage, it was seen that BTHMB was a white
light emitter in DMSO as the mentioned basic solvent affords the anion and neutral form of
BTHMB in solution. The design of BTHMB could be treated as a simple starting point for
construction of white light emitters in solution. Hence, our current report holds importance as far
as designing, exploring and tuning of the properties of ESIPT active probes are concerned.
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Scheme 1.Earlier reported double proton transfer probes and our current report.
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. Experimental section
.1.Chemicals and apparatus
All chemicals were purchased from Merck and used without any further purification. Solvents
were of UV-grade and purchased from Spectrochem. NMR spectra were recorded in Bruker
Advanced Supercon 300 MHz NMR spectrophotometer using TMS as internal standard. Steady
State UV-Vis absorption profiles were recorded on a Shimadzu UV-1900 spectrophotometer.
Steady state emission and excited state decay profiles were recorded on HORIBA Jobin Yvon
Flurolog 3 (Model 3-11) and Horiba Delta Flex-01-NL time resolved fluorimeter
respectively.Mass spectrometrywas carried out inWatersXevo G2-S Q TOF mass spectrometer.
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During steady state absorption and emission measurements, the concentration of probe was
maintained at micro molar range to avoid aggregation, re absorption or self-quenching
respectively. All data were recorded at room temperature. The excited state lifetime
measurements were carried out by exciting the sample using a picoseconds laser diode of 375nm
and 450nm. The signals were obtained by setting the emission polarizer at magic angle (54.7º) to
prevent anisotropy and the decays were deconvoluted using DAS6 software. The goodness of
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fitting was judged using χ criteria. The average lifetime was calculated using standard
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procedures . The quantum yield values were calculated using anthracene as standard .
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.2. Synthetic procedure of BTMP and BTHMB
The synthetic outline has been provided in Scheme 2. A 5 mmol methanolic solution of 2-
hydroxy 5-methoxy benzaldehyde (HMB) was treated with equivalent amount of 2-
aminothiphenol and ~2.5 mmol of molecular iodine and stirred overnight to get pure BTMP in
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0% yield. BTMP was characterized using NMR and ESIMS (ESI, Figures S1-S3). BTMP (2
mmol) was suspended in 10mL glacial acetic acid and treated with 10 mmol
hexamethylenetetramine (HMTA). The reaction mixture was refluxed with stirring for 16 hours,
after which TLC analysis showed complete consumption of starting material. The reaction
mixture was then poured onto 100 mL chilled 2 (M) HCl solution, whereupon a reddish brown
precipitate appeared while being stirred. The solid was filtered, washed with ~50 mL water in
portions and dried overnight in vaccum to obtain BTHMB in 48% yield. BTHMB was
characterized using NMR and ESI-MS (ESI, Figures S4-S6).
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MeO
OMe
OH
S
N
NH2
Methanol, I2
Stirring, 80%
SH
OH
CHO
2
-(benzo[d]thiazol-2-yl)-4-methoxyphenol (BTMP)
2
-hydroxy-5-methoxybenzaldehyde (HMB)
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MeO
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reflux, 48%
S
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N
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OH
OH
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-(benzo[d]thiazol-2-yl)-2-hydroxy-5-methoxybenzaldehyde(BTHMB)
Scheme 2.Synthetic outline of BTMP and BTHMB.
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. Results and discussions
.1. Absorption Spectral Studies. The basic photophysical survey of BTHMB was started by
recording the absorption profiles of a 5 µM solution of BTHMB in various solvents. Two
prominent absorption bands were obtained at ~300 nm and ~385 nm respectively. In polar
solvents like DMSO, DMF and isopropanol an additional band was obtained at ~480-520 nm
depending on the solvent (Figure 1, Table 1). The red sided band also appeared when
triethylamine as a source of base was added to an acetonitrile solution of BTHMB (Figure 1).
Hence, the third band was assigned to be due to the anion of the probe. The higher acidity of the
phenolic –OH proton in BTHMB due to the presence of electron withdrawing Formyl group
facilitates anion formation in basic solvents having high dielectric constants as in DMF, DMSO
and isopropanol. For assigning the bands at ~300 and ~385 nm, absorption profiles of the two
control compounds HMB and BTMP were recorded (Figure 2, ESI, Table S1). HMB, bearing
an aldehyde group and an electron releasing methoxy substituent showed absorption band at
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~
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benzaldehyde , the red shifted band was assigned to be due to π- π transition. On the other
hand, BTMP showed two bands at ~290 nm and ~360 nm (Figure 2, ESI, Table S2). The low
energy band was assigned to be due to the coupling of benzothiazole moiety with the methoxy
phenyl moiety, whereas the high energy band was due to that of the benzothiazole moiety, in
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accordance to the report by Kim et al . It is noteworthy to mention that the anionic band did not
appear in the absorption profiles of any of the control compounds. Thus, the presence of anion
band in polar solvents for BTHMB was due to the higher acidity of the phenolic –OH in the
same. Since the control compound BTMP showed two bands 70 nm apart, the spectral
resemblance of BTHMB absorption profile prompted us to assign the high energy band to be
due to coupling of the phenolic –OH proton with the aldehyde O atom, whereas the low energy
band is due to the coupling with thiazole N atom (Scheme 3). However, the almost similar
absorption maxima of the control compounds HMB and BTMP hinted that in solution phase,
equilibrium may be prevalent between the two forms of BTHMB in the ground state.The red
shift in absorption maxima is due to the greater chromophoric stabilization in BTHMB
compared to either of the two control compounds.
O---H bonded form
N---H bonded form
Scheme 3. Two possible conformers of BTHMB in the ground state.
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Figure 1. Absorption profiles of (a) BTHMB in various solvents (5 µM), (b) BTHMB upon
addition of base in acetonitrile, (c) HMB (10 µM) in various solvents and (d) BTMP in various.
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.2. Steady State Emission spectra. Upon exciting BTHMB at 380 nm, a broad band with high
Stokes’ Shift was observed at ~600nm in non-polar heptanemedium (Table 1, Figure 2) and
solvents of slightly higher polarity like chloroform and cyclohexane. In DMSO and DMF, dual
emission was observed. However, bands appeared at ~420 nm and ~556 nm respectively. Upon
adding base to a solution of BTHMB in acetonitrile, the low energy emission band at ~595 nm
underwent a blue shifting to ~565 nm (Figure 2). The band at ~565 nm was also obtained when
BTHMB was excited at their corresponding anion bands in various solvents where the anion was
found to exist in the ground state along with the neutral form (Figure 2). Hence, the band
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observed in polar solvents between ~556-565 nm was due to the anion of BTHMB formed in
solution. Since the anion of HBT derivatives is reported to be emitting at higher energy
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The shift in emission maxima of the anion could be an outcome of greater stabilization of the
same in solution due to presence of Formyl group, which was also observed in the ground state
while recording the absorption profiles. When BTHMB was excited at 300 nm, a single emission
band was obtained between 390-420 nm, depending on the solvent (Table 1,Figure 2). The
nature of these two emission bands were ascertained once again by comparing steady state
emission spectra of the control compounds HMB and BTMP. HMB when excited at 350 nm
showed emission band with very high Stokes’ Shift at ~500 nm, whereas a high energy band was
obtained at ~435 nm which underwent slight red shift in methanol (Figure 3, ESI, Table S1). The
band at ~500 nm was assigned to be due to ESIPT from phenolic –OH to the aldehyde moiety in
HMB, in accordance with reports regarding salicylaldehyde equipped with electron donating
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groups . Since BTMP showed two prominent absorption bands, it was excited at 295 nm and
3
50 nm individually. Upon exciting BTMP at 350 nm, ESIPT band was obtained at ~570 nm, in
close accordance with the report by Kim et al17 regarding BTMP(Figure 3, ESI Table
S2).Thelocal emission band was obtained at ~410 nm. Only the local emission band of BTMP
was obtained when the molecule was excited at 295 nm. Thus, from spectral comparison of
BTHMB with the other two control compounds, we assigned the ESIPT band in BTHMB
appears due to proton transfer from phenolic –OH to thiazole N atom. Had ESIPT occurred from
–
OH proton to aldehyde O atom, the ESIPT band would have appeared at around 500 nm. If we
revisit Scheme 3, it could thus be stated that the N---H bonded form would be the most important
structure for explaining the above observations.
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Figure 2. Emission profiles of BTHMB in (a) various solvents when excited at 380 nm, (b)
various solvents when excited at 300 nm, (c) presence of base addition in acetonitrile and (d)
i
various solvents when excited at their corresponding anionic absorption band (for PrOH and
MEOH,  =478 nm, DMF and DMSO,  =500 nm, ACN,  =420 nm).
ex
ex
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Figure 3. Emission spectrum of (a) HMB in various solvents ( =350 nm), (b) BTMP in
ex
various solvents ( =290 nm) and (c) BTMP in various solvents ( =350 nm).
ex
ex
Moreover, the emission bands obtained when BTHMB was excited at 300 nm matched
reasonably well with the local emission bands of BTMP in various solvents. Since none of the
emission profiles matched with HMB, it could be well stated that the ESIPT process occurring in
BTHMB involves proton transferred from –OH group to the N atom. Hence, the Formyl group
present in BTHMB could by no means participate in the ESIPT process. Moreover, the solid
state emission spectrum of BTHMB matched with those obtained in solution phase as depicted
in Figure 4. The current work is of particular interest as it establishes ESIPT directed towards N
atom in presence of more electronegative O acceptor. This most probably is an outcome of
greater availability of the lone pairs on N atom of the thiazole moiety, since the pair is not
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involved in any sort of conjugation with the benzothiazole moiety. An interesting optical
property was revealed when solutions of BTHMB in various solvents were observed under UV-
light (Figure 4). In several organic solvents, the color was red to orange. Interestingly, the color
of BTHMB was white in DMSO solvent. This could be due to the fact that the dual emission
bands in DMSO spanned the entire visible range; thereby the color of BTHMB appears to be
white. Thus, this property makes BTHMB one of those rare examples where white light
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2
emission occurs through single molecular probe, as reported by Das et al . Having probed the
solid state properties of BTHMB, exited state lifetime measurements were carried out to gain
further insight into the ESIPT process occurring in the probe (Figure 5, Table 2).
Figure 4. (a) Color of BTHMB in (from left) n-Heptane, Chloroform, Acetonitrile,
Dimethylsulfoxide, Methanol, Isopropanol and Water and (b) Solid state emission profiles of
BTHMB when excited at 300 nm (red profile) and 380 nm (black profile), (inset) color of
BTHMB in solid state under UV-light.
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Table 1. Steady State spectral parameters of BTHMB
Solvent* maxabs
maxem
ε(moll cm )×10
-1
-1
4
Φ
a
b
b
b
b
b
H
300,380
300,380
300,390
300,385
300,380
300,380
300,386
310,515
310,520
410 ,600
1.73
1.27
2.22
1.19
0.86
0.79
0.35
0.96
0.23
0.77
0.37
0.36
0.21
0.18
0.24
0.88
0.69
0.48
0.35
0.26
0.12
0.12
0.14
0.59
0.65
0.03
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
a
Cy
410 ,600
a
CHCl3
DCM
DOX
THF
410 ,600
a
415 ,600
a
b
b
400 ,420 ,600
a
b
b
400 ,420 ,524 ,600
a
b
ACN
DMSO
DMF
MeOH
iPrOH
tBuOH
W
415 , 593
a
b
b
b
b
b
b
b
415 ,420 ,560
a
b
420 ,420 ,553
a
b
300,380,467 415 ,420 ,570
a
b
310,383,473 415 ,420 ,573
a
b
300,390
300,380
415 ,420 ,600
a
b
415 ,420 ,600
*
a
Solvent abbreviation provided in Supplementary Information Section, emission band obtained
b
upon excitation at 300 nm, emission band obtained upon excitation at 380 nm. Quantum yield
were measured using anthracene as standard.
3.3. Excited State Lifetime Measurements. For TCSPC experiments, 336 nm pulsed LASER
diode source was used to probe the band at 420 nm, whereas 375 nm sources was used to probe
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the band at 600 nm. For monitoring the anion band, 375 nm was used. When the ESIPT
wavelength at ~600 nm was probed (Table 2), strictly monoexponential decay having lifetime of
~2.0 ns was observed in non-polar solvents. In more polar solvents like chloroform and
dichloromethane, the lifetime component was ~3.0 ns. The higher lifetime in chloroform and
DCM was in accordance with the higher quantum yield of BTHMB in these two solvents. This
lifetime component was assigned to be due to the keto form of BTHMB.In 1, 4-dioxane
biexponential decay was obtained with a faster component (~0.10 ns) along with a slower
component matching with the lifetime of keto form (1.5 ns). The same pattern was obtained in
acetonitrile. The relatively faster component was thus assigned to be due to the local emission or
enol form of BTHMB. Starting from acetonitrile, the excited state population of the enol form
started exceeding that of the keto form. This could be rationalized from the fact that ESIPT is
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
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3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
2
3
hindered in polar solvents . When the anionic band was monitored (Table 2), lifetime
components of ~6.0 ns were obtained in DMF and DMSO with contributions of the anion of the
enol forms. Thus, the slower lifetime values were assigned to be due to the anion of BTHMB in
those solutions. In THF, apart from the anion and enol forms, another lifetime component faster
than the instrumental resolution was obtained. This could be due to some solvent-solute
interactions between the probe and solvent. Finally, when the enol form emissions were
monitored (Table 2), multiexponential decays were observed in all solvents. In non-polar
solvents the enol lifetime values were very fast. Lifetime of enol form underwent increment upon
increasing the polarity as well as the proticity of the solvents.
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0
Figure 5. TCSPC decay profiles of (a) BTHMB when monitored at 600 nm ( =375 nm), (b)
ex
BTHMB when monitored at corresponding anion bands ( =375 nm) and (c) BTHMB when
ex
monitored at 600 nm ( =336 nm).
ex
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Table 2. Excited State lifetime parameters of BTHMB in various solvents
a
b
c
χ2
Solvent/Wavelength τ (ns)/B
τ (ns)/B
τ (ns)/B
1
1
2
2
3
3
monitored(nm)
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
H/410
H/600
-/42%
3.23/58%
-
-
-
1.25
1.12
2.05/100%
Cy/410
Cy/600
-/18%
2.15/82%
-
1.06
1.01
1.05
1.07
2.39/100%
0.89/42%
3.49/100%
-
-
CHCl /410
3.43/33%
-
-/25%
-
3
CHCl /600
3
DCM/410
DCM/600
0.75/24%
3.27/40%
-
-/36%
-
1.10
1.16
3.19/100%
DOX/410
DOX/600
1.16/33%
0.10/10%
5.06/55%
1.38/85%
-/22%
1.18
1.05
3.66/5%
THF/410
THF/560
THF/600
1.35/19%
4.02/68%
1.29/75%
4.05/81%
13.36/14%
3.98/25%
-
1.13
1.30
1.22
0.91/18%
-
ACN/560
ACN/600
1.49/72%
1.33/77%
4.73/12%
2.49/23%
3.89/16%
-
1.25
1.12
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DMSO/420
DMSO/560
1.09/46%
0.95/5%
2.61/54%
6.78/95%
-
-
1.03
1.20
0
1
2
3
4
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9
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0
1
2
3
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9
0
DMF/420
DMF/553
1.12/30%
1.05/6%
2.85/60%
6.37/94%
-/10%
-
1.09
1.12
MeOH/420
MeOH/570
1.19/4%
4.97/10%
3.87/85%
-/86%
-
1.06
1.20
1.23/15%
iPrOH/420
iPrOH/573
1.23/4%
3.81/6%
-/90%
-
1.00
1.10
1.84/65%
3.65/35%
tBuOH/420
tBuOH/600
-/7%
2.97/57%
3.15/32%
6.91/36%
-
1.05
1.12
1.82/69%
W/420
0.27/16%
4.71/24%
-/60%
-
1.19
1.14
W/600
1.17/43%
4.75/57%
a
b
c
Standard deviation=5%, Standard deviation=3%, Standard deviation=5%.
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3
.4. Theoretical Calculations
The experimental observations could be explained by considering ESIPT from –OH to N
acceptor atom and overruling the possibility of ESIPT towards the Formyl O atom. The reason
behind the observation has been stated to be basicity of N atom in BTHMB dominating the
electronegativity of O atom. Although O is reported to be more electronegative than N in various
1
1
1
1
1
1
1
1
1
1
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2
2
2
2
2
2
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2
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3
3
3
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9
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2
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8
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4
5
6
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0
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4
scales of electronegativity like Pauling Scale, Mulliken Scale and so on , it was yet to be
verified whether the same trend was present in case of BTHMB also. For this purpose, the
25
structure of BTHMB was optimized using Gaussian 09 software at DFT level and utilizing
B3LYP functionality (ESI, Figure S7). Both conformers as shown above in Scheme 3 were
constructed. A perusal of the Mulliken Charge densities on the N and O atoms in both forms
(ESI, Table S3)ensured the higher electronegativity of O than N in the case of BTHMB as
reflected from the negative values of Mulliken Charge on O atom. Hence, from the results of our
theoretical calculations we could state that although O is more electronegative acceptor in our
probe of interest, the basicity of N truly dominated electronegativity.
4
. Conclusions
The photophysical properties of a potential two ways proton transfer molecule BTHMB, having
asymmetric acceptors O and Nwere investigated by means of steady state and time resolved
emission spectroscopic experiments. BTHMB shows imine-amine tautomerisminvolving the
acceptor N atom present in BTHMB. Although more electronegative O atom is present in the
system, it is not involved in ESIPT process. According to our knowledge, this is by far the first
rational comparison of ESIPT between N and O donors present in the same molecule and
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occurring through 6-memenbered hydrogen bonded network. Furthermore, BTHMB is a white
light emitter in DMSO and DMF, due to dual emission spanning the entire spectral region. Since
BTHMB shows an orange-red color emission under UV solvents in some solvents, the current
report also depicts how insertion of an electron donating group in ESIPT probe enables
harvesting of red emission compared to previous reports where green emission was obtained
from such probes containing electron withdrawing groups.
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Associated Content
Supplementary Information
Contains characterization of the compounds, Spectroscopic information for HMB and BTMP
and solvent abbreviation index.
Acknowledgements
AB and SM would like to thank CSIR for granting their fellowship.NG thanks DST, India
(Project No.EMR/2016/004788) and CSIR, India (Project No. 01(2920)18/EMR-II) for financial
support.
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[9] Palit D. K.; Pal H.; Mukherjee T.; Mittal J. P. Photodynamics of the S1 State of
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