Photoacid as ICT probe for ground state proton transfer process from solute to solvents

Article abstract of DOI:10.1016/j.molliq.2019.112194

In present work, 2-(4-Hydroxybenzylidene)malononitrile (HyDC) acts as a photoacid and intramolecular charge transfer (ICT) probe for monitoring solvent specific ground state proton transfer (GSPT) process from solute to solvents. Presence of electron accepting group in HyDC, photoacidic nature and GSPT process are enhanced and observed by UV–Vis spectroscopy. HyDC shows microsolvation behavior with protic solvents at ground state. Microsolvated clusters of HyDC with solvents are formed and supported by experimental and theoretical results.

Full text of DOI:10.1016/j.molliq.2019.112194

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Photoacid as ICT probe for ground state proton transfer process
from solute to solvents
Sumit Kumar Panja
PII:
S0167-7322(19)35448-0
DOI:
https://doi.org/10.1016/j.molliq.2019.112194
MOLLIQ 112194
Reference:
To appear in:
Journal of Molecular Liquids
Received date:
Revised date:
Accepted date:
1 October 2019
19 November 2019
21 November 2019
Please cite this article as: S.K. Panja, Photoacid as ICT probe for ground state proton
transfer process from solute to solvents, Journal of Molecular Liquids(2019),
https://doi.org/10.1016/j.molliq.2019.112194
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Photoacid as ICT Probe for Ground State Proton Transfer
Process from Solute to Solvents
Sumit Kumar Panja,*
Department of Chemistry, Uka Tarsadia University, Maliba Campus, Gopal Vidyanagar,Bardouli,
Mahuba Road, Surat-394350, Gujrat, India
KEYWORDS: Photoacid, Solute-solvent Interaction, Ground State Proton Transfer (GSPT),
Intramolecular Charge Transfer (ICT), Donor-acceptor.
Abstract: In present work, 2-(4-Hydroxybenzylidene)malononitrile (HyDC) acts as a
photoacid and intramolecular charge transfer (ICT) probe for monitoring solvent specific ground
state proton transfer (GSPT) process from solute to solvents. Presence of electron accepting
group in HyDC, photoacidic nature and GSPT process are enhanced and observed by UV-vis
spectroscopy. HyDC shows microsolvation behavior with protic solvents at ground state.
Microsolvated clusters of HyDC with solvents are formed and supported by experimental and
theoretical results.
Introduction
Intermolecular interactions involving aromatic rings play an important role not only on chemical
and biological recognition processes but also in rational designs of drugs and other new
functional materials [1]. Therefore, study of fundamental intermolecular interactions is of great
importance for designing the new valuable materials and better understanding mechanism of
cluster formation in solution as well [2]. Among several intermolecular interactions, H-bonding
interaction has been extensively studied on a variety of small molecular clusters to probe the
solvation phenomena, as solvation effects plays an important role in defining structural and
functional aspects of biological macromolecules [2-3].

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So, significant contribution and most important characteristic feature of intermolecular
interaction between solvent and solute are described by intermolecular hydrogen bonding
interaction [4-5]. Generally, photoacid exhibits microsolvation behavior with protic solvent via
intermolecular hydrogen bonding interactions. Photosensitive nature of photoacids can be
explained in terms of difference in pKa value of ground and excited states [6]. The excited state
pKa* of a photoacid is generally determined from the F rster cycle, based on absorption and
emission data [7]. Photoacids undergo significant enhancement in their acidity upon electronic
excitation [8]. These intermolecular hydrogen bonding interaction and degree of photoacidity of
photoacids lead to ground state proton transfer process from solute to solvent [6]. Significant
change in acidity of photoacids make them an ideal system and can be used for probing solute-
solvent interactions and ground state proton transfer process from solute to solvent [8-9]. Studies
on microhydration of photoacids can provide better insight into hydration process and proton
transfer phenomena, considering a solute molecule surrounded by a small number of H2O solvent
molecule [10]. Electronic structure calculations of such a hydrated cluster of photoacid can help
to have molecular level understanding about the hydration process [11-12]. Photoacids can be
used as molecular probe for determining the structural transitions of proteins under various
conditions [13-14]. Water accessibility in biological surfaces can also be probed using photo
acids [15].
Phenol is the simplest photoacid which involves the aromatic rings and works as a chromophore
in aromatic amino acid (e.g. tyrosine) [16-17]. Interaction of phenol with interacting solvent
molecule can be employed as model system for studying microsolvated phenol clusters [3, 10,
18]. Theoretical calculations have been reported to explain excited state proton transfer process
from phenol to solvent [12]. It is also reported that enhancement of charge transfer process (ICT)

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is observed with increasing acidity of photoacid and in presence of weak or strong bases [19-20].
Inhomogeneous clusters include water/alcohol [21-22], water/aromatic alcohol [23-26],
alcohol/aromatic alcohol [27-30], and other more complicated related systems are explored using
infrared (IR) and/or ultraviolet (UV) spectroscopic methods [31-32]. Even subtle perturbations
caused by the hydrogen bonding interactions in these clusters result in redshifts of OH
vibrational stretching energies or change the * electronic transition energies of aromatic
groups [33-34]. These studies indicate that solvation of photoacid can have a dramatic effect on
electronic properties of solute molecule. Experimental and theoretical analysis of solute-solvent
interaction provide greater insight into nature H-bonding interactions [35-36].
Further, Knoevenagel product is obtained from well-known carbon-carbon bond-forming
reaction, it is also widely used in the chemical, pharmaceutical and perfume industries [37]. The
reaction is also well known for the synthesis of a large variety of intermediates useful in the
manufacturing of top selling drugs (such as atorvastatin, pioglitazone, AMG 837, pregabalin,
lumefantrine, entacapone) in the world [38-39]. Recently, Knoevenagel product is used for
evidence of CT complex formation between a probe and unreacted methylimidazole in
imidazolium cation based ionic liquids [40].
In present work, we have shown that 2-(4-Hydroxybenzylidene)malononitrile (HyDC, Chart 1) is
acts as photoacid and used as ICT probe for monitoring ground state proton transfer (GSPT)
process based on solute-solvent interaction.
Chart 1: Structural Formula and Abbreviation for Investigated System

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Experimental Section
Chemicals and Synthesis: All materials were of analytical grade and procured from Sigma
Aldrich including solvents for the synthesis (ultrapure water, acetonitrile, dichloromethane,
ethanol) and CDCl3 for the NMR experiments.
General Procedure
4-Hydroxybenzaldehyde (1 mmol) and malononitrile (1.01 mmol), catalyst (0.05 mmol) were
taken in a round bottom flask stirred at room temperature for the stipulated period of time till the
completion of the reaction (monitored by TLC). After completion of the reaction, solid
precipitated separated out, filtered and washed with water (3x10 mL) followed by ethanol (3x10
mL), dried in high vacuum to afford NMR pure product [40].
Characterization of HyDC: State: Solid; Colour: White; Melting Point:
190.0 °C; FTIR (KBr, cm-1): 3350, 3330, 3090, 3070, 3035, 2937, 2228,
1
1555, 1565, 1495; H-NMR (300 MHz, CDCl3): δ (ppm) 10.38 (s, 1H),
13
7.82 (d, J = 8.7 Hz, 2H), 7.65 (s, 1H), 6.95 (d, J = 8.7 Hz, 2H); C-NMR (75 MHz, CDCl3): δ
(ppm) 163.2, 162.6, 154.6, 133.8, 132.0, 128.6, 122.8, 116.3, 115.8, 97.5, 62.0.
Instrumentation
Steady State UV-Vis and Fluorescence Measurement: Steady state Uv-Vis and fluorescence
spectral measurements were done at ∼10−5 to 10−6 M concentrations range. The steady state
absorption spectra were recorded on Hitachi UV-vis U-3501 spectrophotometer. The
fluorescence spectra were recorded on a Perkin-Elmer LS55 fluorescence spectrophotometer
equipped with a 10 mm quartz cell and a thermostat bath.
Infrared Spectroscopy: FTIR-8900 spectrophotometer was used for recording FTIR in the
4000-400 cm-1 region using KBr.
Raman Spectroscopy. Fourier transform Raman spectra were acquired on a Vertex 70-RAM II
Bruker FT-RAMAN spectrometerwith a Nd:YAG laser (wavelength 1064 nm, maximum power

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1.5 W). The instrument is equipped with a liquid-nitrogen cooled Ge detector.The spectra (200-
2500 cm-1) were collected with 1 cm-1 nominal resolution by co-adding 128 scans.
Quantum Chemical Calculation:
DFT calculations with a hybrid functional CAM-B3LYP (Becke’s three parameter hybrid
functional using the LYP correlation functional) at 6-311G (d, p) basis set were performed with
the Gaussian 09W software package [42]. The Gaussian 09 suite of programs was used for all
the computations, and the GaussView 5.1 program was used to visualize the structures [42]. The
electronic absorption spectra were calculated using the time-dependent density functional
theory (TD-DFT) and CAM-B3LYP functional with 6-311G++ (d, p) basis and polarizable
continuum model (PCM) with CAN, H2O, EtOH and DMSO as solvent. DFT calculations were
employed to calculate the optimized energies and to compute possible different geometries of
solute-solvent clusters using CAM-B3LYP functional with 6-311G++ (d, p) basis and
polarizable continuum model (PCM) with ACN, H2O, EtOH and DMSO as solvent.
Results and Discussion
UV-Visible studies of HyDC in different solvents
UV-Vis spectra of HyDC molecule are measured in different solvents and normalized UV-Vis
spectrum of HyDC is shown in Figure 1. From detailed analysis of UV-Vis spectra,
intramolecular charge transfer (ICT) band and π π* electronic transition band are observed for
HyDC in solution. In polar aprotic solvent strong π π* electronic transition is observed at 347
nm. But in polar protic solvent, strong π π* electronic transition (at 347 nm) along with a
weak ICT process at 430 nm also observed. Intensity of ICT band varies depending upon nature
and polarity of solvent (Figure 1). Interesting factor is that molar extinction coefficient of ICT
band is very low in ethanol but higher in 2-propanol, H2O and DMSO (Figure 1). From above

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experimental results, it is clear that solute-solvent interaction is very specific and depends on
nature of solvents.
Figure 1: Normalized UV-Vis spectra of HyDC in different solvents
In ACN, UV-vis spectrum of HyDC is altered after gradually addition of H2O (ESI-Figure 1).
During experiment, absorption band at 343 nm is shifted to 348 nm after addition of H2O from 0
ml to 0.6 ml along with appearance of new absorption band at 435 nm. Spectral shift is found to
be 5 nm and presented in Figure 2. Further, normalized spectrum of above experiment is
presented for better understanding the experimental facts (in Figure 2 and onset).

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Figure 2: Normalized absorption spectra of HyDC in ACN after addition of H2O (Conc:1.1x10-6
M)
The appearance of new absorption band at 435 nm is due to specific HyDC-H2O interaction.
Similarly, strong solute-solvent interaction is observed in DMSO solvent (Figure 1and 3). As
strong solute-solvent interaction is observed in DMSO and H2O solvents at room temperature,
(Figure 1, 3), effect of temperature on solute-solvent interaction in H2O and DMSO solvents has
been investigated by UV-vis study. Molar extinction coefficient of ICT band and π π*
electronic band is not affected by temperature from 25 C to 75 C in DMSO (Figure 3). But
molar extinction coefficient of ICT band increases and subsequently π π* electronic band
decreases from 25 C to 75 C in H2O (Figure 4). From above experimental results, it is clear
that strong solute-solvent interaction is observed in DMSO and H2O. The dissociation of solute-
solvent complex by temperature is insignificant in DMSO but prominent in H2O. The
equilibrium between solute and solvent in solute-solvent complex is observed in H2O due to
alternation of orientation and intermolecular interaction by temperature.

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Figure 3: Temperature dependent UV-Vis spectra of HyDC in DMSO (Conc:1.1x10-6 M)
Figure 4: Temperature dependent UV-Vis spectra of HyDC in H2O (Conc:1.1x10-6 M)
Further, effect of base on ICT process of HyDC is also investigated in ACN solvent. In presence
of KOH, weak electron donor group (-OH) is converted to strong electron donating group (-O¯),
results strong electron donor-acceptor system and strong ICT process is observed from strong
electron donor to strong electron acceptor group. Initially, ICT process of HyDC is not observed

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in ACN due to weak electron donor group (-OH) but ICT process is enhanced after addition of
KOH in ACN solvent (Figure 5).
Figure 5: UV-Vis titration of HyDC with different KOH concentration in ACN
The ICT band is appeared at 443 nm and intensity of ICT band is enhanced by gradually addition
of KOH and, but intensity of π π* band at 345 nm decreases significantly in ACN.
Deprotonation of -OH group is taken place in presence base (KOH) and protonation of -O¯ group
in presence of TFA (tetrafluoro acetic acid) has been investigated by monitoring the relative
intensity of ICT band. Deprotonated HyDC (-O¯) is formed linearly with addition of KOH in
ACN which is similar observation from our similar previous work [43].
After completion of KOH addition, TFA solution (trifluoroacetic acid) is gradually added to
previous solution containing KOH. Initially, after addition of TFA, change of intensity of ICT
band is very high. But suddenly, it becomes very low after addition of TFA and (Figure 6). From
these experimental results, formation of protonated HyDC (-O-H) from deprotonated HyDC (-O-
) becomes very fast at initial stage but later on it is formed at very slow rate due to presence of

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small amount of deprotonated HyDC in solution. Protonation process of HyDC anion is highly
favorable and supported by experimental results.
Figure 6: UV-Vis titration of KHyDC at different TFA concentrations in ACN
Further, Benesi-Hildebrand (B-H) method has been employed to determine dissociation constant
(Kd) by UV-Vis spectroscopy [44]. The B-H plot is carried out at 443 nm for HyDC dissociation
in ACN (ESI-equation 1). Linearity of B-H plot indicates that experimental result is acceptable
for 1:1 system (ESI-Figure 2). Dissociation constant (Kd), intercept and slope are obtained from
linear fit curve (Table 1 and ESI-Figure 2). The Kd value of HyDC is 2.59x104 cm3 mol−1 at 443
nm in ACN. B-H plot is carried out at particular wavelength at 443 nm for formation of HyDC
(ESI-Equation 2). The formation constant (Kf), intercept and slope are obtained from linear fit
curve (Table 1 and ESI-Figure 3). The Kf value of HyDC formation is 7.15x105 cm3 mol−1 at
443 nm in ACN.
Table 1: Experimental parameters for Kd and Kf of HyDC in ACN
Kd (dissociation constant) in ACN

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Intercept
(1/[A-Ao])
0.11
Slope
K
2
(1/K[A-Ao])
4.44x10-6
(cm3 mol−1)
2.59 x104 (@443 nm
0.99
Kf (formation constant) in ACN
0.71
1.01x10-6
0.99
7.15 x105 (@443 nm
From above experimental results, it is clear that deprotonation and protonation process of HyDC
in ACN are reversible by monitoring intensity alternation of ICT band (Figure 7) ICT band is
observed from deprotonated HyDC molecule in solution and represents the population of
deprotonated form in ACN.
Figure 7: Schematic presentation of reversible ICT process
Deprotonated HyDC is only formed if proton from O-H bond of protonated HyDC is transferred
to solvent via solute-solvent interaction. Depending upon solute-solvent interaction, intensity of
ICT band population of deprotonated HyDC molecule is altered. From experimental observation
by comparing the relative intensities of ICT bands, it is observed that solute-solvent interaction is
weak in ACN solvent but strong in DMSO and H2O solvents. Above results indicate that ground
state proton transfer is occurring from solute (HyDC) to solvents. The degree of ground state
proton transfer is observed from solute (HyDC) to solvents is higher in DMSO and H2O
compared to ACN.

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From concentration dependent fluorescent experiment, self-quenching is observed in solvents
(ESI-Figure 4-5). The self-quenching may happen due to intermolecular interactions and parallel
arrangement of two dipole molecule in opposite charge site in solution.
Quantum Chemical Calculation
Microhydration at Ground State:
For optimization of geometry, DFT method is adopted using hybrid functional CAM-B3LYP
(Becke’s three parameter hybrid functional using the LYP correlation functional) at 6-311G++
(d, p) basis and polarizable continuum model (PCM) with respective solvents for individual
solute-solvent cluster. At ground state, optimized geometry of HyDC is presented in figure 8a
and ESI-figure 5. In optimized structure of HyDC, bond length of O-H bond is found to be 0.96
Å. For most stable conformer of HyDC with single DMSO molecule, H atom of -OH group from
HyDC forms H-bond with O atom of the DMSO (Figure 8b). In optimized structure of HyDC-
DMSO complex, bond length of O-H bond is found to be 1.00 Å which is longer than its
optimized geometry and H-bonded H--DMSO distance is found to be 1.59Å. In HyDC-EtOH
complex (1:1), the bond length of O-H bond is also found to be 0.99 Å which is longer than its
optimized geometry and H-bonded H--EtOH distance is found to be 1.69Å (Figure 8c).
Similarly, with addition of a single H2O molecule, most stable conformer of HyDC forms
hydrogen bond between H-O group and O atom of H2O molecule (Figure 8c). From optimized
structure of HyDC-H2O complex (1:1), bond length of O-H bond is 0.98 Å, which is shorter than
HyDC-DMSO complex and longer than its optimized geometry and supported by previously
reported work [45]. The H-bonded H--OH2 distance is found to be 1.70 Å (Figure 8d). The O-H
bond length of HyDC molecule increases to 1.00 Å (mono HyDC-DMSO complex) from 0.96 Å
(HyDC alone) and the C-O bond length of HyDC decreases to 1.33 Å (mono HyDC-DMSO

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complex, HyDC-EtOH and HyDC-H2O) from 1.35Å (HyDC alone). The H-bond distance
between oxygen atom of O-H in HyDC and O atom of DMSO, EtOH and H2O molecule is
calculated to be 2.59 Å, 2.67 Å and 2.69 Å respectively.
Figure 8a. Optimized structure of HyDC
Figure 8b. Optimized structure of HyDC-
Figure 8c. Optimized structure of HyDC-EtOH
DMSO complex
complex
Figure 8e. Optimized structure of HyDC-(H2O)3
Figure 8d. Optimized structure of HyDC
complex
H2O complex
With three H2O molecules, most stable conformer (Figure 8e) shows that O-H bond of HyDC is
hydrogen bonded to oxygen atom of one H2O molecule and hydrogen atom of another H2O

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molecule formed with oxygen atom of O-H bond of HyDC. Third H2O molecule forms H-bond
to previous two H-bonded H2O molecules with HyDC molecule in cyclic way (Figure 8e). Four
H-bonding interaction sites in HyDC-(H2O)3 complex (1:3) is different and found to be different
hydrogen bonding interaction energy from DFT calculation. The O−H and C−O bond lengths in
most stable HyDC-(H2O)3 complex (1:3) is 0.99 and 1.33 Å respectively which has closed ring
geometry [46-47]. Further, optimization of all solute-solvent clusters in gaseous phase and
detailed information regarding bond lengths are presented in ESI-6a-6d.
From theoretical studies, the O-H bond length of HyDC molecule and internuclear distance via
H-bonding interaction between oxygen atom of O-H in HyDC and O atom of DMSO, EtOH and
H2O molecule is different but the C-O bond length of HyDC is similar value for HyDC-solvent
complex (1:1). From experimental UV-Vis results, the molar extinction coefficient of ICT band
in EtOH is very low and different from other solvents like DMSO and H2O. Hydrophobic and
hydrophilic parameters play an important role on solvation and solute-solvent cluster formation
with EtOH. Above experimental result can be only explained on the basis of solvation nature and
formation of solute-solvent cluster. So, Cluster formation in EtOH is different from DMSO and
H2O solvents and shows low molar extinction coefficient of ICT band.
The O-H Potential Energy Profile
The dissociating O-H bond potential energy profile of HyDC-solvent complex gives the barrier
for proton transfer from HyDC to the neighboring solvent molecules. The potential energy
profile for dissociating O-H bond of HyDC-DMSO and protonated HyDC-H2O complex, at
ground, is presented in figure 9 from DFT calculation using hybrid functional CAM-B3LYP with
6-311G++ (d, p) basis. The equilibrium bond length of O-H in HyDC is altered from 0.98 Å to
1.00 Å for HyDC-DMSO and HyDC-H2O complex respectively at ground state.

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Figure 9: Potential energy curve of HyDC-DMSO and HyDC-H2O complexes (1:1 complex)
In these cases, it is seen that when the O-H bond distance is increased, the energy decreases and
the system become stable up to 1.40 Å. The potential energy profile of the dissociating O−H
bond of HyDC at ground state shows a minimum at O-H bond distance ∼1.00 Å. Further
increasing O-H bond length leads to destabilization of the system. However, rigid potential
energy scan of the acidic O-H bond of HyDC-DMSO and HyDC-H2O complex at ground state
reveals that in the presence of solvent molecules, the acidic proton transfer from HyDC to the
neighboring solvent molecules becomes a barrierless process, as shown in Figure 9. The
potential energy profiles show that a barrier exists for proton transfer from HyDC to solvent
molecules from the DFT calculation.
Electronic Properties Calculation
The electronic absorption spectra are calculated using the time-dependent density functional
theory (TD-DFT) using hybrid functional CAM-B3LYP with 6-311G++ (d, p) basis and
polarizable continuum model (PCM) with ACN as solvent.

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Figure 10: Experimental and theoretical electronic transition of HyDC and HyDC-OK in ACN
solvent
It shows the existence of π π* transition in HyDC and ICT transition in HyDC-OK. The π
π* transition is observed at 354 nm for HyDC due to HOMO to LUMO transition (f = 0.81 at
343 nm) (Figure 10). Similarly, ICT transitions are appeared at 443 nm for HyDC-OK due to
HOMO to LUMO transition (f = 0.96 at 383 nm) respectively and presented in figure 10.
Further, time-dependent density functional theory (TD-DFT) using hybrid functional CAM-
B3LYP with 6-311G++ (d, p) basis and polarizable continuum model (PCM) with EtOH, H2O
and DMSO as solvent is performed and observed insignificant change of ICT and π π*
transition for both cases. From involvement of orbital in ICT transition process, it can be stated
that involvement of benzene π electron with cyano group plays the crucial role for ICT process.
Conclusion
In conclusion, HyDC acts as a strong photoacid and shows highly solvent specific ground state
proton transfer process from solute to solvents especially in DMSO and H2O exclusively.
Solvent specific intensity alternation of ICT band in solutions is an indication for ground state
proton transfer process. Further, the ground state proton transfer process depends on H-bonding

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ability of the solvents which is supported by experimental and theoretical studies. In presence of
cyano groups (electron withdrawing), the photoacidic nature of HyDC is enhanced and favors the
ground state proton transfer (GSPT) process. The solute-solvent interaction is relatively strong
and capable of forming microsolvated HyDC clusters with solvents. Strong ICT band in H2O and
DMSO solvents indicates that the ground state proton transfer process is highly feasible due to
presence of strong solute-solvent interaction. From above experimental and theoretical results, it
can be stated that HyDC acts as a highly active photoacid and ICT probe for monitoring GSPT
process.
AUTHOR INFORMATION
Corresponding Author
E-mail: sumit.panja@utu.ac.in; sumitkpanja@gmail.com;
Department of Chemistry, Uka Tarsadia University, Maliba Campus, Gopal Vidyanagar,Bardouli,
Mahuba Road, Surat-394350, Gujrat, India.
ORCID of SKP: orcid.org/0000-0001-5658-0452
ACKNOWLEDGMENT
SKP acknowledges to Department of Chemistry, Uka Tarsadia University, Surat-394350, Gujrat,
India for providing infrastructure and instrument facilities.
Notes
The authors declare no competing financial interest
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Graphical Abstract

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Photoacid as ICT Probe for Ground State Proton Transfer
Process from Solute to Solvents
Sumit Kumar Panja,*
Solvent specific ground state proton transfer (GSPT) process is
observed from solute (protoacid; HyDC) to solvents. Protoacid HyDC
has shown strong ICT process in solution and used as ICT probe for
GSPT process.
Highlights
 Solvent specific ground state proton transfer (GSPT) process is observed from solute
(protoacid; HyDC) to solvents.
 Protoacid (HyDC) has shown strong ICT process in solution.
 Protoacid (HyDC) is used ICT probe for GSPT process.

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7