A spectroscopic exploration of the influence of charge donor group on ESIPT process and its consequences in a salicylimine

Article abstract of DOI:10.1016/j.jphotochem.2018.03.010

Photophysical properties of a newly designed synthetic compound DYMDAP have been investigated in combination with computational calculations. DYMDAP shows a solvent dependent emissive behavior which in non-polar solvents shows two distinct peaks along wit

Full text of DOI:10.1016/j.jphotochem.2018.03.010

Accepted Manuscript
Title: A spectroscopic exploration of the influence of charge
donor group on ESIPT process and its consequences in a
salicylimine
Authors: Subhash Chandra Makhal, Arghyadeep
Bhattacharyya, Soumen Ghosh, Nikhil Guchhait
PII:
S1010-6030(18)30012-1
DOI:
Reference:
https://doi.org/10.1016/j.jphotochem.2018.03.010
JPC 11179
To appear in:
Journal of Photochemistry and Photobiology A: Chemistry
Received date:
Revised date:
Accepted date:
5-1-2018
27-2-2018
7-3-2018
Please cite this article as: Subhash Chandra Makhal, Arghyadeep Bhattacharyya,
Soumen Ghosh, Nikhil Guchhait, spectroscopic exploration of the
influence of charge donor group on ESIPT process and its consequences
in salicylimine, Journal of Photochemistry and Photobiology A:
A
a
Chemistry https://doi.org/10.1016/j.jphotochem.2018.03.010
This is a PDF file of an unedited manuscript that has been accepted for publication.
As a service to our customers we are providing this early version of the manuscript.
The manuscript will undergo copyediting, typesetting, and review of the resulting proof
before it is published in its final form. Please note that during the production process
errors may be discovered which could affect the content, and all legal disclaimers that
apply to the journal pertain.

A spectroscopic exploration of the influence of charge donor group on ESIPT process and
its consequences in a salicylimine
Subhash Chandra Makhal, Arghyadeep Bhattacharyya, Soumen Ghosh, Nikhil Guchhait*
Department of Chemistry, 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).
Graphical Abstract

Research highlights
 Newly designed Schiff bases capable for excited state proton donor and charge donor
reaction
 Hydrogen bonding interaction from single crystal analysis
 Study the influence of charge donor on Keto-enol tautomerisation using spectroscopically
 Study the effect of the nature of solvents on ESIPT reaction
 Theoretical DFT calculations on ESIPT process corroborates experimental results
Abstract
Photophysical properties of a newly designed synthetic compound DYMDAP have been
investigated in combination with computational calculations. DYMDAP shows a solvent
dependent emissive behavior which in non-polar solvents shows two distinct peaks along with a
shoulder but with increasing polarity as also proticity of the solvents the intensity of each band
varies significantly. Along with the two bands, a new band is observed in polar protic solvent
due to the proton transfer (PT) in the excited state of the compound. Only in polar protic
solvents, intensity of the PT band increases significantly whereas in non-polar or polar aprotic
medium intensity is sparse. By comparing the spectral observations of DYMDAP with two
control compounds DYMP and DYMDAB the proton transfer process is assigned to be due to
solvent mediated and the proton transfer process is enhanced by the presence of charge transfer
moiety.

Keywords:
1-[1-(4,6-dimethylpyrimidine-2-yl)-hydrazonomethyl]-4-diethylamino-2-phenol
(DYMDAP), ESIPT, Fluorescence spectroscopy, DFT
1. Introduction
Since the epoch making report by Weller in 1956 [1], the phenomenon of excited state
intramolecular proton transfer (ESIPT) process still fascinates physicists and chemists alike.
With each passing year, researchers are trying to device molecular entities to tune the rate of
proton transfer process as well as to get a better insight about the ESIPT process compared to
reports available in literature [2-3]. Similarly fascinating, if not more, is the phenomenon of
excited state charge transfer (CT) which was first reported by Grabowski et al [4] in the
celebrated molecule dimethyl aminobenzonitrile (DMABN). The charge transfer phenomenon is
assisted in polar solvents, unlike ESIPT processes where increment in polarity/hydrogen bonding
ability of the medium damps the aptitude of ESIPT probes to display dual emission bands
barring a few exceptions. Thus, charge transfer probes are better candidates to probe bio-
molecular assemblies under physiological conditions. But, due to their dual emission behavior,
proton transfer probes are more likely to show ratiometric type of interaction with analytes,
thereby decreasing the chance of measurement error, compared to single emission type probes,
where monitoring the same band leads to considerable measurement errors. Taking all these facts
in mind, chemists are engaged in developing probes which show intramolecular charge transfer
(ICT) coupled ESIPT or vice versa [5]. The numbers of complex molecular entities where both
charge transfer and proton transfer processes occur in the excited state are plentiful. However,
reports where the sole effect of a potential ICT group is to only enhance the ESIPT process in the
same molecular framework are sparse. Our group has been involved in exploring the various
types of interplay possible between ESIPT and ICT processes in purposefully designed

molecules (Scheme 1). We reported the charge transfer suppressed proton transfer in the excited
state for 4-diethylamino salicylaldehyde [6]. In another report, we also reported proton transfer
assisted charge transfer in an azine linked hetero anil Schiff base, which also involved 4-
diethylamino salicylaldehyde at one end of the azine linkage [7]. Thus, it is obvious that
compounds constituting 4-diethylamino salicylaldehyde are likely to exhibit exciting correlation
between ICT and ESIPT processes. Apart from our reports, various reports dealing with the
interplay of ICT and ESIPT are well documented in literature [8-11].Taking queue of the above
discussed facts, we devised a simple molecule - [1-(4, 6-dimethylpyrimidine-2-yl)-
hydrazonomethyl]-4-diethylamino-2-phenol (DYMDAP) obtained by condensing 3, 5-dimethyl
2-hydrazino pyrimidine with 4-Diethylamino salicylaldehyde. Our rationale in choosing this
molecule is as follows: i) we are interested to check whether introduction of a weak acceptor of
charge in the form of the electron rich pyrimidine moiety on one end of the azine linkage would
damp the charge transfer process due to the presence of diethylamino group and drive the system
towards ESIPT exclusively. ii) As it contains the biologically interesting pyrimidine moiety,
application of DYMDAP in biological systems would be justified after the photophysical
investigation of the same. DYMDAP has been thoroughly investigated by means of monitoring
its absorption, steady state as also time resolved emission profiles in various solvents as also by
observing the effect of acid/base on the same. We also obtained single crystals of DYMDAP to
understand its absolute structure and extended our analysis by conducting Density Functional
Theory (DFT) level optimization of DYMDAP in the ground state as well as in the excited state.
We have concluded that DYMDAP favours proton transfer in the excited state due to the
presence of charge donor. In addition to this, another remarkable property exhibited by
DYMDAP is that it shows a rarely encountered proton transfer in polar protic solvent, which

undoubtedly enhances its potential to be a useful tool to probe biomimetic assemblies. Thus,
rather than being yet another entry to our already existing reports [12,13] on interesting
photophysical phenomena in self-designed molecules, the current report deserves its own merit
on grounds of novelty, structural simplicity and exciting photophysical properties.
2. Experimental details
2.1. Chemicals
4-diethylamino salicylaldehyde, acetyl acetone and hydrazine hydrate have been purchased from
Merck. All solvents were of spectroscopic grade and purchased from Spectrochem India Pvt
Limited. The solvents are abbreviated as n-Hept: n-Heptane; CyH: Cyclohexane; MCH:
Methylcyclohexane; DCM: Dichloromethane; CHCl3: Chloroform; ACN: Acetonitrile; n-BuOH:
n-butanol; i-PrOH: isopropanol; MeOH: Methanol; DMSO: dimethylsulfoxide; TFA:
Trifluoroacetic acid. Triple distilled water was used wherever needed.
2.2. Steady state absorption and emission measurements
All the absorption and emission measurements were carried out using very dilute solution
of DYMDAP ( ~10-6 (M)) to avoid the aggregation of the compound and the reabsorption effect
which can be responsible for the deviation from the Lambert-Beers law in case of absorption
spectra and for self quenching effect for the emission measurements. All the measurements were
done at room temperature using freshly prepared solutions of DYMDAP. The absorption
measurement was done using Hitachi UV-Vis U-3501 spectrometer and the emission spectra were
obtained using Perkin Elmer LS55 luminescence spectrophotometer.

Following equation has been used to measure fluorescence quantum yield of the
compound relative to the Coumarin-153 whose quantum yield is known (=0.38) in ethanol
medium.
ퟐ

(푨풃풔)
s = R× 푨푨
×
×
(푨풃풔) 
푺
푺
푹
푺
ퟐ
푹
푹
Where ‘A’ denotes for area under the curve of the fluorescence spectra, ‘(abs)’
and ‘’ denotes the absorbance and the refractive index of the solvents respectively.’ refers to
the fluorescence quantum yield of the corresponding compounds. Subscripts ‘R’ and ‘S’ denote
reference and sample respectively.
2.3. Time resolved emission spectra measurements
Fluorescence lifetime of the compound was measured by the method of time correlated
single photon counting (TCSPC) technique using HORIBA Jobin Yvon Fluorocube-01-NL
fluorescence lifetime spectrometer. The sample was excited using a picosecond diode laser of 404
nm and the fluorescence signal was recorded keeping the emission polariser at the magic angle
(54.70) with respect to the excitation polariser to eliminate the loss of signal for the contribution of
anisotropy decay and fluorescence decays were deconvoluted using DAS6 software. The goodness
of the fits was judged by 2 criteria. The relative contributions (n) to the fluorescence decay of
the multi exponential decay were obtained using the following relation:
푩
풏
an=∑
푵
푩
풊
풊=ퟏ
Bi is the pre-exponential factor of a single exponential decay. The average lifetime of the
compound was calculated using the following relation:
ퟐ
<t>=∑∑
풂 풕
풊
풊
풂 풕
풊 풊
2.4. Computational details

Theoretical calculations were performed using Gaussian 09W suite program [14]. The
optimized structure of the compound at the global minima was obtained using 6-311G++ (d,p)
basis set at Density functional theory (DFT) level using B3LYP (which is Backe’s three parameter
hybrid exchange functional in conjunction with nonlocal correlation function of Lee, Yong and
Parr) hybrid orbital. 6-311G++(d,p) basis set was used because it is based on the triple- function
for valence electron with diffusion function which is very useful for the calculation of anionic
structures or for the compounds containing lone pair electrons. To get optimized structure of the
compound in solvents of different polarity, Polarisable Continuum Model (PCM) was used. In this
method the target compound is surrounded by the solvent molecules, which treated quantum
mechanically, and solvents are characterized by its bulk properties like dielectric constant or
polarity.
2.5. Syntheses
The short description of synthetic procedures for synthesis (Scheme 2) of the title
compound DYMDAP and two other relevant compounds- [1-(4, 6-dimethylpyrimidine-2-yl)-
hydrazonomethyl]-4-diethylamino-2-benzene (DYMDAB) and o-[1-(4, 6-dimethylpyrimidine-2-
yl)-hydrazonomethyl]-phenol (DYMP) is as follows:
I) The title compound DYMDAP was synthesized as follows (Scheme 2):
a) Synthesis of 3, 5-dimethyl 2-hydrazino pyrimidine
It was prepared by the condensation of acetyl acetone with urea followed by sequential
treatment with POCl3 and Hydrazine hydrate as per literature report [15].
b) Synthesis of DYMDAP
1.0 mM of 4-diethylamino salicylaldehyde was dissolved in methanol (5 mL) and a hot
solution of 1.0 mM 3, 5-dimethyl 2-hydrazino pyrimidine in 2 mL methanol was added drop wise

to it. After stirring overnight and then reducing the volume to ~3 mL, deep yellow crystals were
obtained. It was washed with cold methanol and dried in vacuum overnight. Yield: 68%.
II) The control compound DYMDAB was obtained by refluxing an equimolar mixture of 4-
dimethylaminobenzaldehyde with 3, 5-dimethyl 2-hydrazino pyrimidine (Scheme 2) in 10 mL
methanol for 6 hours, leading to light yellow shiny particles, which were filtered, washed with
methanol and dried in vacuum.
III) The control compound DYMP was synthesized (Scheme 2) by report available in the
literature [16].
Scheme 1: List of previously reported compounds with the current one. DDBHP: 5-diethylamino-
2-[(4-diethylamino-benzylidene)-hydrazonomethyl]-phenol,
DEASH:
N,
N′-bis(4-
N,Ndiethylaminosalisalidene)hydrazine, DDEAHB: 4-(diethylamino)-2-hydroxybenzaldehyde

Scheme 2: Synthetic scheme of a) DYMDAP, b) DYMDAB, c) DYMP.
3. Results and discussion
The compounds DYMDAP, DYMDAB and DYMP have been synthesized following
simple condensation reaction between hydrazono pyrimidine and substituted benzaldehyde
13
(Scheme 2). All compounds have been characterized by 1H, C NMR, IR and mass (Fig. S1-S8)
spectroscopic methods. The main purpose of synthesizing DYMDAB and DYMP was to ascertain
the influence of each of the dialkylamino and –OH group present in DYMDAP upon comparison.
The crystals of DYMDAP were obtained by slow evaporation of a saturated solution of the same
in a 1:1 (V: V) binary solvent system comprising of methanol and water. Single crystal X-ray
diffraction studies indicate that compound DYMDAP crystallizes in P2/c space group (Table S1).
The ORTEP diagram of DYMDAP with 30% atomic probability has been shown in ESI (Fig. S9).
The crystal structure of DYMDAP shows that two aromatic rings are not in same plane, tiled by

an angle ~ 24o where an intramolecular H-bonding is present between the phenolic –OH (O12) and
imine nitrogen (N7) having distance 2.563 Å (Fig. 1, Table S2). Beside this intramolecular H-
bond, an intermolecular H-bonded network is formed based on water molecule. It has been found
that one water molecule is H-bonded with three DYMDAP molecules through intermolecular H-
bond where two DYMDAP molecules act as H-bond acceptor (O13….N113, 2.85 Å and
O13….N113, 2.855 Å) and one molecule acts as H-bond donor (O13….N113, 2.867 Å, (Fig. 2)
to form an one dimensional layer (Fig. S10, Table S2). This 1D layer is connected with another
almost perpendicular 1D layer via CH…. interaction (distance 3.157 Å, Fig. S11, Table S3) to
form a 3D polymeric structure.
Fig. 1: Molecular structure of DYMDAP.
N113
H111
N6
O13
N114

Fig. 2: H-bonding interactions via water molecule.
3.1. Absorption spectra
The absorption spectra of DYMDAP were measured in different solvents and the
corresponding absorption maxima are listed in Table 1. From Fig. 3a and 3b it is clear that the
absorption spectra of DYMDAP show two bands at 365nm (strong, 365 = 4.3 LMol-1 cm-1) and
425nm (weak, 425 = 0.1253 LMol-1 cm-1) and the band structures are to some extent solvent
polarity dependent. In case of non-polar solvent there is a clear bifurcation in both the bands, but
with increasing polarity of solvent the same gets disappeared as shown in Fig. 3a and 3a. As per
the structure of the compound the absorption bands have been assigned to the π-π* transitions of
the compound [17]. These bifurcated bands in solution could be due to existence of two
conformers with respect to the orientation of OH group (i.e. open form and intramolecularly
hydrogen bonded close form (Scheme 3). To fortify our assumptions, the absorption spectra of the
two other similar compounds DYMDAB and DYMP were then recorded. In case of DYMP,
which contains only –OH group shows two distinct bands (Fig. 4a), one at ~296nm and other at
~330nm. In case of DYMDAB which contains only –NMe2 group shows a strong band with a
shoulder at the higher energy side (Fig. 4b) unlike DYMDAP and DYMP, which may support the
existence of open and closed form with respect to –OH group of DYMDAP. Therefore bifurcated
bands at 407nm and at428nm are assigned to S0S1 transitions the open and closed form of
DYMDAP and bands at 355nm and at 370nm are assigned their higher energy states (S0S2
transitions). Structurally the ground state stability difference between the open and closed forms
(difference in the orientation of OH group) should be small and hence the expected π-π*
transitions energies should also be small.

The disappearance of the structured bands in DYMDAP upon increasing the solvent
polarity can be explained from the point of view of the effect of solvent polarity and hydrogen
bonding effect. With increasing polarity and hydrogen bonding effect of solvent, a solvated form
may be generated for both the forms, thereby leading to the aforesaid observation.
Fig. 3: Absorption spectra of DYMDAP (~10-6M) in solvents of varying polarity.

Scheme 3: (a) Closed and (b) Open form of DYMDAP.
Fig. 4: Absorption spectra of a) DYMP (~10-6M) and in b) DYMDAB (~10-6M) solvents of
varying polarity.
Table1. Spectroscopic parameters obtained from absorption and emission spectral bands of
DYMDAP, DYMP and DYMDAB in different solvents.

Solvents DYMDAP
abs(nm) em (nm)
DYMP
DYMDAB
f
abs(nm) em(nm) f
abs(nm) em(nm)
313, 345 431
318, 345 431
321, 346 431
320, 350 433
323, 358 433
322, 359 433
365,425 458,481, 515 0.033
365,425 458,481, 515 0.034
365,425 459,482, 515 0.050
365,425 460,487,515 0.098
365,425 458,480, 520 0.092
--
--
--
--
n-Hept
MCH
295, 334 --
0.000
0.003
CyH
295, 332 509
DOX
--
--
--
CHCl3
DMSO
ACN
365,425 447,485, 520 0.0614 --
365,425 456,480, 516 0.088
365,425 458, 483,514 0.126
365,425 458,482, 514 0.094
363,428 458,482, 514 0.170
295, 332 509
0.0023 317, 351
--
--
318, 351 421
318, 352 --
n-BuOH
i-PrOH
MeOH
Water
295, 332 406, 509 0.001
295, 332 406, 509 0.006
295, 332 406, 509 0.0003
361
452,483,515 0.002
3.2. Steady state emission and excitation spectra
Due to a rather complex absorption profile and its dependence on the nature of the
solvents, we have chosen to excite DYMDAP at all the wavelengths at which prominent or
considerable absorption was found according to the solvents instead of exciting at a common
wavelength. Upon low energy excitation (~425nm) the emission profiles in non-polar solvents
clearly show two distinct emission bands at ~460nm, ~485nm along with a shoulder at ~515nm
(Fig. 5a). Emission spectra in non-polar solvents show the band gap between two bands
at~460nm and ~485nm is almost same as the band gap in the absorption spectra (407nm and

428nm) of DYMDAP. This hinted to the fact that these two bands contribute to the local
emission and indirectly shows the existence of open and closed forms of DYMDAP in the
excited state, but these facts demanded verification. For this purpose, we recorded the excitation
spectra (Fig. 6) of DYMDAP in various solvents which furnished results closer to those obtained
from UV-Vis absorption studies (Fig. 3b). Thus, the humps at ~460nm and ~485nm could be
attributed to be due to local emission, which indirectly supports the existence of open and closed
forms of DYMDAP in the excited state also. Thus, the peak at 515nm could be due to some
excited state photophysical phenomenon. DYMDAP has both proton transfer site (i.e. a
probability of transfer of proton from –OH group to –CH=N- group) along with a charge transfer
moiety (i.e. probability of transfer of electron density from –NEt2 group to –CH=N- group). So
there is a probability of ESIPT or excited state ICT process or both. Table 1 show that the
Stokes’ Shift (~90nm) between the lowest energetic band and the excitation wavelength is
unchanged upon solvent variation. Thus, such a large value of Stoke’s Shift could be due to some
photophysical processes operative in DYMDAP in the excited state. It is to point out here that
excitation at the high energy band at ~365nm did not show any emission. The excited molecules
dissipated energy through some non-radiative processes for higher energy excitation. This also
supports the fact that 365nm absorption band is not the S0S1 transitions for the open form but a
transition to the higher energy excited state.
Herein, our judicially synthetic compounds DYMP (only proton transfer possible) and
DYMDAB (exclusively a charge transfer group present) proved useful information about this.
Compound having only –OH group shows two emission peaks one at 408nm (weak) and other at
~512nm (strong) only in polar protic and aprotic medium, but in nonpolar medium there was no
fluorescence as shown in Fig. 5b. The observed weak high energy emission band may be due to

local emission and the reddest shifted emission is nothing but the ESIPT band which is
dominated in polar protic solvents [19, 20]. DYMDAB group shows only one broad band at
~430nm as shown in Fig. 5c and there is less solvent dependence and hence this band is assigned
to the local emission (no CT emission is observed) of DYMDAB. From all the above
observations extracted from the control compounds it becomes apparent that the presence of both
–OH and -NEt2 groups lead to the emission profile of DYMDAP. Having ascertained so, further
experiments were carried out to unravel the excited state photophysical phenomenon which is
supposed to be occurring in DYMDAP.
Fig. 5: Emission of a) DYMDAP (ex=430nm), b) DYMP (ex= 330nm) and c) DYMDAB
(ex=350 nm) in various solvents.

Fig. 6: Fluorescence excitation spectra of DYMDAP (em=515nm).
3.3. Analysis of fluorescence quantum yield
The observed fluorescence quantum yield of DYMDAP was measured at room
temperature with variation of polarity and hydrogen bonding ability of the solvents and the result
is presented in Table 1. DYMDAP shows high quantum yield in polar protic solvents compared
to that of polar aprotic and non-polar solvents. The comparative study of quantum yield with
DYMP (Table 1) strongly suggests that incorporation of a charge donor group to DYMP
increases the proton transfer ability of the compound, which supports our conclusion that there is
a significant effect of charge donor group to the excited state proton transfer process. In case of
polar aprotic solvents observed fluorescence quantum yield is higher than that of non-polar
solvents, this due to the stabilisation of excited state dipole by means of solvent-dipole
interaction [21]. High quantum yield in polar protic solvents reveals the effect of H-bonding on

the fluorescence spectra which is also evident from the generation of a new band in fluorescence
spectra only in polar protic solvents.
3.4. Effect of Acid and base on Absorption and emission spectra
As the special characteristics in the emission profile of DYMDAP is solely due to the
presence of -NEt2 and –OH groups, acid and base effects were observed as rendering the –NEt2
and –OH groups ineffective would be useful to understand the photophysical process operative
in DYMDAP. Absorbance at ~420nm increases with addition of trifluoroacetic acid (TFA) in the
neat solution of DYMDAP in methanolic medium as shown in Fig. 7. There is similar
observation for adding base (Fig. 7). Isosbestic points at 378 (acid addition) and 382 nm (base
addition) indicate that acid effect and base effect are equilibrium driven [22]. Addition of acid
results in protonation of –NEt2 group leading to the formation of cationic species which has
absorption peak different from the neutral species leading to arrest of charge transfer from
diethyl amino (-NEt2) group to imine (-CH=N-) moiety. However, concerted participation from
the secondary amine functionality to the imine moiety results in attainment of a partially double
bond character along the azine linkage (-N-NH-), thereby imparting planarity to the otherwise
non-planar structure of DYMDAP, as evident from its crystal structure.
In case of fluorescence spectra, intensity of emission decreases with addition of both acid
and bases. In both cases, intensity of the band at 515nm decreases uniformly. However, the
decrease in intensity of the band at 460nm was differentially affected by addition of acid and
base. Upon adding base, the intensity of the band at 515nm decreases to a much greater extent
compared to the same in 460nm (Fig. 8a). However, upon acid addition the intensity of both
bands at 460nm and 515nm decreased uniformly (Fig. 8b). As diethyl amino group of DYMDAP

was protonated with addition of acid, thereby local emission of the neutral species decrease. A
schematic representation of effect of acid and base on DYMDAP has been provided in Scheme
5. At this stage it was assumed that a proton transfer process could be operative in DYMDAP as
decrease in fluorescence intensity upon loss of phenol –OH (addition of base) strongly indicates
an ESIPT process in accordance to literature report [23]. As far as acid effect is concerned,
protonation of the donor in DYMDAP causes immediate protonation of the NEt2 group, thereby
causing a loss in charge transfer. This also resulted in quenching of fluorescence. Thus, it was
now evident that presence of NEt2 group in its neutral form is indispensable for DYMDAP to
exhibit its dual emission in polar solvents. But the main concern in assigning an ESIPT process
operative in DYMDAP was that it was observed only in polar protic solvents, in contrast to a fair
number of literature reports [21-24] where ESIPT occurs only in non-polar/polar aprotic solvents
having similar cavity designed for proton transfer. Thus, even if ESIPT occurs in DYMDAP it
was assisted by the diethyl amino group (charge transfer moiety) which lead to assuming
DYMDAP shows the very rare phenomenon of charge transfer assisted proton transfer in polar
protic medium, which is a merit in itself. Steady state spectral measurements could thus far
indicate the aforementioned photophysical phenomenon which needed even further analysis.
With this aim in mind, TCSPC measurements were next done.
H
H
H
N
N
N
N
N
N
N
N
N
N
N
N
Et2N
OH
Et2NH
OH
Et2N
O
protonated form, donation
from 20-amine
DYMDAP
anionic form, resonance stablised
operative

Scheme 5: Acid and Base effect on DYMDAP
Fig. 7: Absorption spectral changes on addition of a) TFA and b) Et3N to DYMDAP in methanol
medium.
Fig. 8: Effect of a) addition of base to DYMDAP and b) addition of TFA to DYMDAP on the
emission spectra in methanol.
3.5. Analysis of Fluorescence lifetime
To ascertain the various species present in the excited state, fluorescence lifetime of
DYMDAP in different solvents were measured (Fig 10) and the corresponding residuals

regarding to the goodness of the exponential fitting have been given in S9. Irrespective of
solvent polarity or proticity, all decay profiles well fitted into triexponential decay and the time
resolved data are given in Table 2 (λex=404nm, λem=465nm, 515nm). Decay analysis suggested
the open and closed forms collectively contributed to the local emission and the proton transfer
form was responsible for the third component in the excited state. Corroborating to results of
steady state emission, the three components undergo alteration in their relative contribution upon
varying the H-bonding ability of the solvents. As the open form of the molecule cannot take part
in proton transfer process so there should be at least one component whose lifetime and
contribution to average life time would be almost unperturbed. Since τ2 and α2 do not change
with changing the solvent polarity, species corresponding to life time τ2 is nothing but the open
form of DYMDAP in the excited state. Decay component corresponding to life time τ1 suffers
decrease in lifetime as well as in α1 upon solvent variation whereas the third component (with
life time τ3) picks up in both τ3 and α3. At this juncture, it was possible to assign components
lifetimes τ2, τ1 and τ3 to be open, closed and proton transferred forms of DYMDAP respectively.
Switching of average contribution between second and third form resulted upon varying the
polarity as well as proticity of the solvent. However, the values of lifetime of the proton
transferred form in various solvents were way below that observed in literature [14], which
needed to be rationalized. This could be explained by considering the proton transferred form of
DYMDAP, where the unrestricted bond rotation about the hydrazine linkage affects the excited
state lifetime of the keto form in DYMDAP (Fig. 9). Apart from the primary effect of
diethylamino group in assisting the ESIPT phenomenon in DYMDAP the auxiliary effect of
polarity as well as proticity of the medium, if any, still needed to be analyzed. The participation
of protic solvent to relay the proton transfer process was also analyzed by lifetime

measurements. This was achieved by plotting relative contribution to the fluorescence decay to
the emission wavelength profiles in two extreme environments- one in n-hexane and in methanol
(Fig. 10 and Fig. 11). The plots clearly reflected that undergoing ESIPT is an innate property of
DYMDAP; otherwise there would have been no contribution of the third component to the
overall profile in n-heptane. However, the contributions from each component barely converged
in n-heptane. However, excellent convergence followed by crossover of emissive species was
observed in methanol medium. However, component with lifetime τ2 did not show any
convergence or crossover, irrespective of any medium. Thus, the role of solvent is a key factor
apart from the essential condition of -NEt2 group to be present in DYMDAP to obtain observable
ESIPT. Lifetime data of DYMDAP in n-Heptane and methanol medium have been represented in
Table S4 and Table S5 respectively.
Fig. 9: Fluorescence lifetime decay pattern in different solvents .

Fig. 10: Plot of rel. contribution to the fluorescence decay (Fl. Decay) to the emission
wavelength in n-Heptane medium (‘a’ in the figure legend denotes ‘α’; having usual meaning).
Fig. 11: Plot of rel. contribution to the fluorescence decay to the emission wavelength in
methanol medium (‘a’ in the figure legend denotes ‘α’; having usual meaning).
Table 2: Time resolved data of DYMDAP in different solvents.
Solvents Emission at 460nm

ns) ns) ns) 


<>ns) 2
n-Hept
CyH
0.062
0.102
0.092
0.043
0.066
0.021
0.525
0.578
0.750
0.869
0.584
0.498
0.685
0.569
0.457
0.020
0.036
0.035
0.046
0.049
0.071
0.069
0.089
0.092
0.621
0.582
0.501
0.485
0.201
0.057
0.042
0.035
0.023
0.021
0.033
0.054
0.005
0.118
0.012
0.056
0.025
0.016
0.358 0.416
0.385 0.324
0.445 0.410
0.510 0.196
0.681 0.181
0.931 0.717
0.902 0.687
0.940 0.895
0.961 0.925
1.12
0.98
1.02
1.25
1.08
1.06
0.89
0.98
1.01
MCH
DCM
CHCl3
ACN
n-BuOH 0.003
i-PrOH
MeOH
0.005
0.002
Emission at 515nm
Solvents ns) ns) ns) 


<>ns) 2
n-Hept
CyH
0.070
0.095
0.135
0.054
0.076
0.086
1.337
2.433
2.338
3.062
2.862
4.123
1.749
2.852
5.471
0.054
0.094
0.072
0.036
0.049
0.035
0.063
0.048
0.055
0.765
0.980
0.453
0.901
0.792
0.783
0.520
0.358
0.446
0.042
0.005
0.094
0.003
0.006
0.003
0.009
0.005
0.006
0.193 0.067
0.015 0.095
0.453 0.110
0.096 0.053
0.202 0.073
0.214 0.081
0.471 0.560
0.637 0.415
0.548 0.592
0.98
1.08
0.87
0.99
0.96
1.01
1.02
1.02
1.12
MCH
DCM
CHCl3
ACN
n-BuOH 0.008
i-PrOH
MeOH
0.008
0.007
3.6. Computational Details

The first insight into the electronic structure reorganization during the proton transfer
processes has been achieved by the aid of theoretical calculations. The intramolecular proton
transfer is achieved by determination of structure calculation both in ground state as well as in
excited state. The ground state structure (Scheme 4) was calculated by using DFT method and
the structural data are given in Table 3. The excited state geometry was calculated by TDDFT
method. Although this method does not lead to optimized structure in the excited state, the data
obtained from this calculation is good enough to support our experimental findings.
Evaluation of the GSIPT curve for intrinsic intramolecular proton transfer was done by
optimizing the geometry at B3LYP/6-311++G** level in Polarisable Continuum (PCM) Model
for a number of structures of DYMDAP generated with O10–H11 distance varying in the range
0.98–1.48Å. The ESIPT curves are obtained by adding the vertical excitation energy to the
GSIPT curve. Fig. 12 shows the plot of variation of potential energy of DYMDAP with
increasing the O-H bond length. It is found that energy of DYMDAP in the S0 state increases
with increasing the O-H bond length and it is also found that enol form is the global minima and
keto form is the local minima in potential energy surface of S0 state. It is found that enol form is
more stable than the keto form by 18kcal/mole and the energy barrier is 28kcal/mole in the
ground state. But in the excited state the case is reversed, the keto form is energetically stable
than the enol form with a significant decrease in the potential energy barrier height. Here in the
excited state the keto the stabilized structure than that of enol form by an amount of
>10kcal/mole and it is in plain sight observation that the energy barrier is ~10 kcal/mole which
quite smaller that of the ground state structure so this theoretical results prove that there is a
certain role of solvent proticity on the ESIPT process.

Table 3. Ground state optimized geometrical parameters of closed form of DYMDAP at DFT
level with B3LYP functional and 6-311++G (d, p) basis set.
Bond
RC1-C2
Calculated values Bond angles/Dihedral angles calculated values
1.409
1.444
1.291
1.353
0.985
1.375
109.15
121.68
122.55
118.84
0.1210
0.5500
C30O10H11
C4C27N29
RC4-C27
RC27-C29
RN29-N30
RO-H
C3C4C27
N30C32N33
C4C3C27N29
C27N29N30C32
RN30-C32
Scheme 4: Optimized ground state structure of the closed form of DYMDAP (DFT/ B3LYP/6-
311++G**).

10.25Kcal/mole
Enol*
Keto*
54.7Kcal/mole
25.77Kcal/mole
Keto
28.18Kcal/mole
Enol
Fig.12. Potential energy curves for the ground (S0) and first excited state (S1) of DYMDAP with
variation of O10–H11 distance using B3LYP functional and 6-311++G** basis set.
4. Conclusion
In this present work, an asymmetric Schiff base DYMDAP has been synthesized by
simple condensation reaction between 4-diethylamino salicylaldehyde and 5-dimethyl 2-
hydrazino pyrimidine and characterized by NMR, IR and single crystal XRD analysis. The
photophysical properties of DYMDAP have been investigated thoroughly in different solvents.
The spectroscopic data furnished by UV, fluorescence and TCSPC experiments reveal that
DYMDAP exhibits charge transfer assisted excited state proton transfer phenomenon in polar
protic solvents, which is a novelty in itself.
Acknowledgement
NG acknowledges DST, India (Project No. EMR/2016/004788) for financial support. SM
and AB thank UGC and CSIR India for fellowship.

References:
[1]
A. Weller, Z. Elektrochem, Innermolekularer Protoneniibergang im angeregten Zustand
60 (1956) 1144.
[2]
S. Sen, A. Pramanik, N.Guchhait, Excited state intramolecular proton transfer involving a
four-membered system: Photophysical exploration of an isoindole fused imidazole
system, J. Lumin. 187 (2017) 78-84.
[3]
[4]
[5]
E. Heyer, K. Benelhadj, S. Budzak, D. Jacquemin, J. Massue, G. Ulrich, Chem. Eur. J.23
(2017) 1-14.
K. Rotkiewicz, K.H. Grellmaiin, Z.R. Grabowski, Reinterpretation of anomalous
fluorescence of p-N, N-dimethylamino benzonitrile, Chem. Phys. Lett. 193 (1973) 315.
Y. Kim, M. Yoon, D. Kim, Excited-state intramolecular proton transfer coupled-charge
transfer of p-N, N-
-cyclodextrin solutions,
Journal of Photochemistry and Photobiology A: Chemistry, 138 (2001) 167-175.
[6]
[7]
S. Jana, S. Dalapati, Nikhil Guchhait, p-N, N-dimethylaminosalicylic acid in aqueous b-
cyclodextrin solutions Transfer Process in 4- (Diethylamino)-2-hydroxybenzaldehyde, J.
Phys. Chem. A, 117 (2013) 4367−4376.
S. Jana, S Dalapati, N Guchhait. Proton transfer assisted charge transfer phenomena in
photochromic Schiff Bases and effect of −NEt2 groups to the Anil Schiff Bases, J. Phys.
Chem. A, 116 (2012) 10948–10958.

[8] Alexander P. Demchenko, Kuo-Chun Tangb and Pi-Tai Chou, Excited-state proton coupled
charge transfer modulated by molecular structure and media polarization, Chem. Soc.
Rev.42 (2013) 1379.
[9] Cheng-Chih Hsieh, Chang-ming Jiang, Pi-Tai Chou, Recent Experimental Advances on
Excited-State Intramolecular Proton Coupled Electron Transfer Reaction, Acc. Chem.
Res. 43 (2010) 1364.
[10] Femtosecond Dynamics on 2-(2’-Hydroxy-4’-diethylaminophenyl)benzothiazole: Solvent
Polarity in the Excited-State Proton Transfer, Yi-Ming Cheng, Shih-Chieh Pu, Chia-Jung
Hsu, Chin-Hung Lai, Pi-Tai Chou, ChemPhysChem, 7 (2006) 1372.
[11] Cheng-Chih Hsieh, Yi-Ming Cheng, Chia-Jung Hsu, Kew-Yu Chen, Pi-Tai Chou,
Spectroscopy and Femtosecond Dynamics of Excited-State Proton Transfer Induced
Charge Transfer Reaction, J. Phys. Chem. A 112 (2008) 8323–8332.
[12] M.J. Frisch, et al., GAUSSIAN 03, Revision D.01, Gaussian, Inc., Wallingford, CT, 2004.
[13]
G. M. Kosolapoff, C. H. Roy, Synthesis of some pyrimidyl phosphonates, J. Org. Chem.,
26 (1961) 1895.
[14] S. Ray, S. Konar, A. Jana, K. Das, A. Dhara , S. Chatterjee, S. K. Kar, Syntheses, crystal
structure, spectroscopic and photoluminescence studies of mononuclear copper(II),
manganese(II), cadmium(II), and a 1D polymeric Cu(II) complexes with a pyrimidine
derived Schiff base ligand, J. Mol. Struct. 1058 (2014) 213–220.