Experimental and theoretical investigation of ground state intramolecular proton transfer (GSIPT) in salicylideneaniline Schiff base derivatives in polar protic medium

Journal Pre-proof Experimental and theoretical investigation of ground state intramolecular proton transfer (GSIPT) in salicylideneaniline Schiff base derivatives in polar protic medium

Bijoya Das, Amrita Chakraborty, Shamik Chakraborty PII:

S1386-1425(19)30833-9

DOI:

https://doi.org/10.1016/j.saa.2019.117443

Reference:

SAA 117443

To appear in:

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy

Received date:

13 June 2019

Revised date:

22 July 2019

Accepted date:

29 July 2019

Please cite this article as: B. Das, A. Chakraborty and S. Chakraborty, Experimental and theoretical investigation of ground state intramolecular proton transfer (GSIPT) in salicylideneaniline Schiff base derivatives in polar protic medium, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy(2019), https://doi.org/10.1016/ j.saa.2019.117443

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© 2019 Published by Elsevier.

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Experimental and theoretical investigation of ground state intramolecular proton transfer (GSIPT) in salicylideneaniline Schiff base derivatives in polar protic medium Bijoya Das, Amrita Chakraborty, Shamik Chakraborty

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Department of Chemistry, Birla Institute of Technology and Science, Pilani. Pilani Campus, Vidya Vihar, Pilani, Rajasthan – 333031, India.

Abstract

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Ground state intramolecular proton transfer process has been comprehensively investigated in three salicylideneaniline Schiff base derivatives (SB1, SB2, and SB3) using experimental and theoretical methods. It has been confirmed that all the three Schiff base molecules in the ground electronic state exist in the enol form in non-polar and polar aprotic solvents. Keto form is being populated by the polar protic solvent through ground state intramolecular proton transfer (GSIPT) process. Ground state equilibrium between the enol and keto tautomers for SB1 and SB3 is mainly governed by the proton donating ability of the solvent. Ground state equilibria between the enol and keto tautomers of SB2 which is a positional isomer of SB3 is governed by the polarity and proton donating ability of the solvents. Excited state intramolecular proton transfer (ESIPT) process is also evidenced in all the three Schiff base molecules. Theoretical calculations at the B3LYP/cc-pVDZ level in the gas phase and in different solvents using polarisable continuum model (PCM) has been failed to establish the GSIPT process. Microsolvation of individual enol and keto conformers have been investigated considering upto three solvent molecules. The energetics of the individual conformers together with the corresponding transition state have been calculated. It has been confirmed that the keto conformer is more stable compared to the enol conformer in microsolvated cluster of three methanol molecules. Lowering of activation energy for the enol to keto tautomerisation in presence of methanol also support the experimental observation for GSIPT process. TDDFT/B3LYP/cc-pVDZ single point calculations for microsolvated clusters of enol and keto form of the Schiff base molecules exhibit an excellent agreement with the experimentally obtained absorption spectra. Difference in spectral nature of the Schiff base molecules has been explained using natural bond orbital (NBO) analysis. Quantum theory of atoms in molecules (QTAIM) has also been utilised to understand the GSIPT process in detail. Keywords: GSIPT, ESIPT, DFT, Clusters, Schiff base, hydrogen bond

Email address: [email protected] (Shamik Chakraborty) Preprint submitted to Elsevier

July 22, 2019

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Introduction

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The transfer of a proton from one centre to other within the same molecule or between two different molecule is one of the most widely investigated phenomenon in all areas of chemistry and biochemistry. In addition, theoretical studies of proton transfer (PT) process in small model systems is an active area of research to gain fundamental understanding of the process. Intramolecular PT may happen in the ground state (GSIPT) and/or in the excited state after photo-excitation (ESIPT). PT induced enol-to-keto tautomerisation plays central role in pharmaceutical activity, enzyme activity, DNA base pair stabilisation, electrochemical reactions, and self-assembly of bio-molecules [1]. PT process in a small aromatic molecule along the coordinate of intramolecular hydrogen bond network leads to photochromism, i.e., distinctly different optical properties of tautomers. The most widely investigated form of hydrogen bond network that leads to photochromism in aromatic Schiff bases is the one with N and O atoms, those act as hydrogen bond donor and/or acceptor in X-H· · ·Y type arrangement, where X and Y are O and N atoms. Generally two possible tautomers, the enol-imine (OH) and keto-amine (NH), are formed in the proton transfer process due to the manifestation of O-H· · ·N and N-H· · ·O type of hydrogen bonding interaction, respectively (Scheme 1). Commonly, enol-imine form (E) is the most stable at room temperature compared to keto-amine (K) form [2–4]. The equilibrium can be shifted towards the keto-amine form depending on substitutions in aromatic ring and nature of perturbation provided by surrounding medium. Photo-induced enol-to-keto tautomerisation (ESIPT) process is common and has been studied extensively [5–7]. Corresponding PT process in the electronic ground state, i.e., GSIPT process, is not so common [8, 9]. The reason is two fold: (i) extra stability of the E conformer in the electronic ground state compared to the K conformer and (ii) very high energy barrier between the two conformers, i.e., E and K. Ortho-hydroxy Schiff bases, such as, salicylideneaniline (SA) [10–17, 17–27] and their derivatives are a special class of compounds which exhibits both GSIPT and ESIPT processes. The mechanism and dynamics of the ESIPT process for enol-to-keto conversion has been studied well in ortho-hydroxy Schiff base molecules in solid, solutions, and in the gas phase under isolated condition [28–33]. In general, ESIPT process is associated with large Stokes shift in the fluorescence spectrum. The large Stokes shift is attributed to a four energy level photophysical process with the creation of photochromic tautomer in the ground electronic state [34]. Various experimental and theoretical methods, such as, UV-VIS, Fluorescence, Raman, FT-IR, NMR, X-ray crystallography, quantum chemical calculations, wave-packet dynamics, have been explored to confirm and characterise the enol-keto tautomerisation in the electronic excited state [4, 10–17, 17–27]. It has been confirmed that the enol-keto tautomerisation takes place by intramolecular proton transfer process between the ortho hydroxyl group and nitrogen atom. Typically, in all of these molecules intramolecular hydrogen bonding facilitates an ultrafast proton transfer process that produces cis-keto tautomer. The cis-keto (Kc) conformer finally decays to a trans-keto (Kt) form by cis-trans isomerisation at nanosecond time scale. The structural change in SA due to intramolecular proton transfer process between the O and N atoms leading to an enol-imine (E) ↔ keto-amine equilibrium (K) is displayed in Scheme 1. 2

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Scheme 1: Representation of enol-imine (E) ↔ keto-amine (K) tautomerisation in salicylideneaniline molecule

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The similar tautomerisation process though enol-imine (E) ↔ keto-amine (K) equilibrium by GSIPT process has not been studied much. Most of the theoretical calculations also highlight the ESIPT process. Information on GSIPT process in similar systems is sparse. The main aim of the present work is to investigate both GSIPT and ESIPT process and the effect of electronic substitution on the PT process using experimental and theoretical methods. Molecules containing two ortho hydroxyl groups at two different aromatic rings in salicylideneaniline Schiff base is a good model system to investigate both GSIPT and ESIPT process. Various derivatives of salicylideneaniline Schiff base molecules can be synthesised very easily. Herein, salicylideneaniline derivatives have been used to understand the effect of electronic substitution on GSIPT and ESIPT processes. Among such type of Schiff bases, 2(2-hydroxybenzylideneamino)phenol (SB1) has received quite interest due to its utilisation as chemosensor, photo luminescent material, bio activities. Photophysical properties of ESIPT process for SB1 has been studied using experimental methods in combination with quantum chemical calculations [35]. Two transitions have been observed in the absorption spectrum of acetonitrile solution in the wavelength range of 300 − 450 nm. The transition around 350 nm (A1) is assigned to the (π-π ∗ ) transition of aromatic electrons and the other at 425 nm (A2) to the CH=N linkage of SB1. In contrast, a single transition around 340 nm is reported earlier for SB1 in acetonitrile solution [27, 34]. This transition is assigned as the (π-π ∗ ) transition of SB1 which does primarily remain in the E form. Dual emission is observed at 420 nm (Em1) and 547 nm (Em2) while exciting at 350 nm (A1 transition) in acetonitrile solution (pH=7). The large Stokes shift of about 8000 cm−1 is attributed due to the ESIPT process. It is concluded that SB1 exists predominantly in the E form in the solutions at pH6 7, whereas in the keto form in solutions at pH> 7. In fluorescence emission spectra, the lower wavelength transition, Em1, is assigned to enol tautomer while the emission at longer wavelength (Em2) is assigned to keto tautomer. The presence of two species for SB1 in the electronic ground state is confirmed from the observed change in absorption spectra at various pH. It has been concluded that the keto anion is formed in the basic medium. The formation of keto conformer is based on the transfer of one of the hydroxyl protons to the solvent in basic medium and not due to the intramolecular proton transfer process. The origin of the A2 transition around 450 nm in highly polar solvents has been described as the delocalisation of the electron density of the aromatic system due to the interaction between the solvent and phenol group of SB1. 3

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The long wavelength transition (A2) in aromatic Schiff base molecules is common. The origin of the transition can be attributed to: (i) Weak S1 ←S0 (n-π ∗ ) transition of the enol form (E) [10, 36]; (ii) Self-aggeragation of the solute molecules [13, 15]; (iii) Ion formation in the solution [12, 16]; (iv) Hydrogen bonding interaction with solvent network [37, 38]; (v) S1 ← S0 (π − π ∗ ) transition of the keto form (K) [11, 14, 39, 40]. Crystal structure of SB1 is reported and it has been concluded that SB1 exclusively remain in the enol form [41, 42]. The SB1 molecule has been studied quite extensively using various photophysical methods to investigate its ability as ion sensor and to understand the mechanism of ESIPT process. The A2 transition in polar protic solvent of SB1 molecule is an indication of the formation of the keto (K) form in the ground state through GSIPT process. Although, it has not been concluded [35] in the recent work. Moreover, detail investigation to understand GSIPT process, change in electronic state due to GSIPT process, effect of electron density on the GSIPT process, and role of the solvent molecules in GSIPT are not yet addressed.

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Scheme 2: Representation of Schiff base molecules, SB(1 − 3), investigated herein

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Herein, GSIPT process have been investigated in detail for model Schiff base molecule, SB1, by photophysical characterisation using various experimental methods and quantum chemical calculations. Explicit solvent effect has been investigated by considering microsolvated clusters to understand the GSIPT process at the molecular level. Moreover, the effect of electron density on the GSIPT and ESIPT processes has been explored. SB1 is having two proton transfer reaction sites. Proton transfer is a local event as it happens along the coordinate of intramolecular hydrogen bonding. Proton transfer in SB1 may happen either in the aminophenol subunit (N· · ·Hb -Ob ) or in the salicylidine subunit (N· · ·Ha -Oa ) and are presented in Scheme 2. It is reported that the proton transfer in the salicylidine subunit is more favourable. Thus, the 2-hydroxyl groups is responsible for the keto-enol tautomerisation [39, 40]. It is expected that introduction of a selective electron donor/acceptor group in relative position with respect to the proton transfer reaction site may influence the proton transfer process even in the ground electronic state. Thus, the ground state population distribution between E and K conformer would be altered by the enol-imine (E) ↔ ketoamine (K) tautomerisation. In this paper, two derivatives of SB1 have been investigated, those are positional isomers of each other: 2-(2-hydroxy-4-methoxybenzylideneamino)-44

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Scheme 3: General synthesis route for Schiff base molecules using condensation reaction

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methylphenol (SB2) and 2-(2-hydroxy-5-methoxybenzylideneamino)-4-methylphenol (SB3). Electron donating functional group -OCH3 is present in meta and para position with respect to 2-hydroxy group of salicylaldehyde ring in SB2 and SB3, respectively. Moreover, a −CH3 group is present at the para position with respect to the hydroxy group of amine ring. Chemical structure of the compounds those have been investigated in this work are presented in Scheme 2. The main focus of this paper is the photophysical investigation of SB(1−3). Photophyical properties of SB(1−3) have been characterised in different solvents, such as, chloroform (less polar), acetonitrile (polar), methanol (polar protic having specific interaction) to investigate proton transfer process. Experimental results have been corroborated with the aid of density functional theory (DFT) and time-dependent density functional theory (TDDFT) studies. The proton migration barrier in terms of substituent’s position in aromatic ring has been investigated. Theoretical calculations of the energy and structure of different conformers, transition state analysis, stability of micro solvated clusters (n = 1 − 3), natural bond orbital (NBO) analysis [43], and quantum theory of atoms-in-molecules (QTAIM) [44] have been employed for better understanding of the GSIPT process at the molecular level. Experimental and computational details

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All the reagents and solvents have been purchased from Sigma Aldrich and Spectrochem, respectively. All the chemicals have been used without any further purification. Nuclear Magnetic Resonance (NMR) analyses have been carried out in a Bruker spectrometer with 400 MHz resonance for 1 H NMR and 100 MHz resonance for 13 C NMR in DMSO-d6 solvent. Tetramethylsilane (TMS) is used as an internal reference. Fourier Transform-Infrared (FTIR) analysis have been performed on IR Affinity-1S Schimadzu with transmittance mode in the range between 4000 to 1000 cm−1 . Syntheses and characterisation Schiff base molecules used in the current study have been easily synthesised through one-step procedure via. condensation of aldehyde derivatives and substituted amines in appropriate solvent [45]. The general syntheses route is explained in Scheme 3. Syntheses procedure and characterisation of SB(1 − 3) are provided below: Synthesis of SB1: 2-(2-hydroxybenzylideneamino)phenol has been prepared by condensation reaction of 2aminophenol (1g; 9.2 mmol) with salicylaldehyde (1.12 g; 9.2 mmol) in 25 ml of ethanol. 5

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This mixture is then refluxed with stirring for 3.5 h at 80◦ C and the solution is kept at the room temperature to crystallise the product. The final product is purified by repeated recrystallisations. FTIR (KBr, ν, cm−1 ): 3440, 1620 (C=N), 1527, 1465, 1218, 1141. 1 H NMR (400 MHz, DMSO-d6 ) δ 13.77 (s, 1H), 9.71 (s, 1H), 8.95 (s, 1H), 7.59 (dd, J = 8.0, 1.6 Hz, 1H), 7.41 – 7.30 (m, 2H), 7.11 (t, J = 7.7 Hz, 1H), 7.00 – 6.82 (m, 4H). 13 C NMR (100 MHz, DMSO-d6 ) δ 162.16, 161.20, 151.58, 135.44, 133.29, 132.77, 128.52, 120.07, 119.98, 119.20, 117.16, 116.99.

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Synthesis of SB2: The 2-(2-hydroxy-4methoxybenzylideneamino)-4-methyl phenol has been synthesised by condensation reaction of 2-amino-4-methylphenol (0.81g; 6.6 mmol) with 2-hydroxy-4- methoxybenzaldehyde (1g; 6.6 mmol) in 25 mL of ethanol. This mixture is then refluxed with stirring for 6 h at 80◦ C and the solution is allowed to cool at the room temperature for crystallisation of the product. The product was purified by repeated recrystallisation and are washed with cooled ethanol. Finally, purified product is dried in air. FTIR (KBr, ν, cm−1 ): 3371, 1589 (C=N), 1473, 1211, 1110. 1 H NMR (400 MHz, DMSO-d6 ) δ 14.49 (s, 1H), 9.55 (s, 1H), 8.85 (s, 1H), 7.44 (d, J = 8.7 Hz, 1H), 7.19 (d, J = 2.3 Hz, 1H), 6.89 (dd, J = 8.2, 1.5 Hz, 1H), 6.83 (d, J = 8.1 Hz, 1H), 6.46 (dd, J = 8.7, 2.4 Hz, 1H), 6.38 (d, J = 2.4 Hz, 1H), 3.79 (s, 3H), 2.25 (s, 3H). 13 C NMR (100 MHz, DMSO-d6 ) δ 166.44, 164.27, 159.89, 148.55, 134.30, 133.75, 128.79, 128.19, 119.67, 116.65, 113.43, 106.97, 55.81, 20.70.

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Synthesis of SB3: The 2-(2-hydroxy-5-methoxybenzylideneamino)-4-methyl phenol has been prepared by condensation reaction of 2-amino-4-methylphenol (0.081g; 0.66 mmol) with 2-hydroxy-5methoxybenzaldehyde (0.082 ml; 0.66 mmol) in 25 mL of ethanol. This mixture is then refluxed with stirring for 6 h at 80◦ C and the solution is allowed to cool at the room temperature for crystallisation of the product. The product was purified by repeated recrystallisation and are washed with cooled ethanol. Finally, purified product is dried in air. FTIR (KBr, ν, cm−1 ): 1605 (C=N), 1489, 1219, 1141. 1 H NMR (400 MHz, DMSO-d6 ) δ 13.14 (s, 1H), 9.48 (s, 1H), 8.93 (s, 1H), 7.23 (d, J = 3.0 Hz, 1H), 7.16 (s, 1H), 7.05 – 6.81 (m, 4H), 3.75 (s, 3H), 3.36 (s, 3H).13 C NMR (100 MHz, DMSO-d6 ) δ 161.43, 155.13, 152.13, 149.33, 135.19, 128.93, 128.74, 120.42, 120.23, 119.84, 117.89, 116.82, 115.47, 55.98, 20.65. Steady state measurement All the spectral measurements have been carried out with ∼ 10−5 M concentration of solution in order to avoid aggregation and self-quenching. The steady-state absorption spectra of the Schiff base molecules are recorded in V-650 Jasco spectrophotometer. Fluorescence emission spectra are recorded with the Fluoromax-4 Horiba Jobin Yvon Fluorescence Spectrophotometer with 5 nm slit width. The individual solvent background has been subtracted in all the fluorescence experiments during the steady state spectral measurement. The effect of acid and base on the designed Schiff base molecules have been performed using trifluoroacetic acid (TFA) and sodium hydroxide (NaOH). Solution of acid and base have been prepared in MeOH to avoid any hydrolysis of the Schiff base molecules [46, 47]. Effect of 6

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viscous medium on the absorption spectra have been investigated using polyethylene glycol (PEG-600) solution.

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Figure 1: Absorption spectra of SB1, SB2, and SB3 recorded in chloroform solution

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Fluorescence up-conversion measurement In femtosecond up-conversion set-up (FOG-100), the samples have been excited at 375 nm using second harmonic generation from 750 nm radiation (pulse width < 100 fs) from a mode-locked Ti:Sapphire oscillator (MaiTai-HP) operating at 80 MHz. Second harmonic of fundamental was generated by 0.5 mm thick nonlinear β-barium borate (BBO) crystal (θ = 38◦ and φ = 90◦ ). About 18 mW of power from second harmonic beam (375 nm) is used to excite the samples and the residual fundamental beam (∼ 250 mW) is used as gate-pulse. The gate beam is directed by an automated optical delay-line to introduce delay between fluorescence and gate-pulse. All fluorescence decays are collected at magic angle polarisation. Detail of the fluorescence up-conversion set-up is available elsewhere [48]. In brief, fluorescence up-conversion is based on the sum-frequency generation by cross-correlating fluorescence signal and gate pulse. At time t = 0, second harmonic pump pulse (ωp ) excite the sample and incoherent fluorescence (ω2 ) was collected and mixed with the gate-pulse (ω1 ) which arrives at a delayed time τ to another 0.5 mm thick nonlinear BBO crystal (θ = 38◦ and φ = 90◦ ). The mixing of fluorescence (ω2 ) and gate-pulse (ω1 ) provides sum frequency, (ωsum = ω1 + ω2 ). The intensity of the sum frequency signal, Isum at a given delay, τ , is proportional to the correlation function of the fluorescence intensity I2 with the intensity of gate pulse, I1 as Z ∞

Isum ∝

I2 (t) I1 (t) (t − τ ) dt −∞

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(1)

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The time delay between fluorescence (ω 2 ) and gate pulse (ω 1 ) is controlled by the optical delay (≥ 0.78 fs). The up-converted signal (Isum ) is collected through a double monochromator and finally detected by photomultiplier tube (PMT). Time-resolution (∼ 270 fs; FWHM) of up-conversion set-up is determined by cross-correlation of water Raman (at ∼ 428 nm with excitation at 375 nm). Solvent λabs (nm)

λem (nm)

357 349 348

442

440 440 436

528 532 521

SB2

CHL ACN MOH

357 351 346

425 422

420 421 412

512 513 500

2.9×10−5

3.5×10−5 7.1×10−5 3.1×10−4

SB3

CHL ACN MOH

382 372 371

481

452 452 465

569 572 560

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1.2×10−4 1.5×10−4 3.6×10−4

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SB1

CHL ACN MOH

Quantum yield E form K form 3.6×10−5 6.9×10−5 4.2×10−5 2.5×10−4

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Table 1: The spectroscopic parameters of SB1, SB2, and SB3 in chloroform (CHL), acetonitrile (ACN), and methanol (MOH): absorption maxima (λabs ), fluorescence emission maximum (λem ), and quantum yield in methanol solution.

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Fluorescence quantum yield measurement: The fluorescence quantum yield (φf ) of SB(1 − 3) have been measured using 0.001 M quinine sullfate in 0.1 M sulfuric acid as the standard (φf = 0.54) and are calculated using the following equation: R 2 0 n A If (λf ) dλf φf = φ0f 2 R 0 (2) n0 A If (λf ) dλf where, n0 is the refractive index of the 0.1 M sulfuric acid solution and n is the refractive index of various solvents used in the experiment, such as, chloroform, acetonitrile, and methanol. A0 and A are the absorbance, and φ0f and φf are the fluorescence quantum yield, If and I0f fluorescence intensity for quinine sulfate and various samples, respectively. Theoretical calculation: All the quantum chemical calculations have been carried out using Gaussian09 program package [49] at the DFT level. Geometry optimisation of SB(1 − 3) are carried out using B3LYP functionals in conjunction with cc-pVDZ basis set. All the coordinates are relaxed 8

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Figure 2: Absorption spectra of SB1 (a), SB2 (b), and SB3 (c), recorded in chloroform (CHL), acetonitrile (ACN), and methanol (MOH) solutions. Concentration of individual Schiff base molecule is kept constant in all the solutions.

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while optimising the geometry of the molecules. Harmonic vibrational frequency analysis of the optimised structures has also been performed to identify whether the optimised geometry is a local minimum, transition state or higher order saddle point. Time Dependent Density Functional Theory (TDDFT) method has been employed to calculate excited state properties using B3LYP functional and cc-pVDZ basis set. Integral equation formalism polarisable continuum model (IEFPCM) with methanol as solvent has been used to explain experimental observations. Transition state analysis has also been carried out to explore the potential energy surface for enol-to-keto tautomerisation process. The nature of the stationary points is confirmed by means of a vibrational analysis. Energy of different conformers, i.e. E and K, in micro-solvated clusters are also investigated to establish the GSIPT process. Microsolvation study of individual enol and keto conformers (where number of solvent molecules, n=1 − 3) have been carried out together with corresponding transition state calculation to investigate solvent mediated GSIPT process. NBO analysis has been carried out to estimate the extent of charge transfer from hydrogen bond donor site to the hydrogen bond acceptor site in SB(1-3) for the enol-keto tautomerism. Moreover, QTAIM analysis has been performed to understand the nature of intramolecular hydrogen bonding interaction and the effect of electronic substitution on the GSIPT process. 9

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Results and discussion

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Figure 3: Absorption spectra of SB1 (a), SB2 (b), and SB3 (c), recorded in mixed solvents of chloroform (CHL) and methanol (MOH). Experiments have been performed using pure chloroform solution (100% CHL) and gradually MOH concentration has been increased to 100% methanol. Concentration of individual Schiff base molecule is kept constant.

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Absorption spectra: The steady state absorption spectra of SB2 and SB3 in CHCl3 solution are presented in Figure 1 along with SB1 absorption spectra in the same solvent for an easy comparison. The relevant data of absorption spectra in various solvents are displayed in Table 1. A broad intense absorption band is observed for SB2 and SB3 at 357 and 382 nm (A1), respectively, and are assigned as the S1 ← S0 (π-π ∗ ) transition of the enol form (E) [17]. The corresponding S1 ← S0 (π − π ∗ ) transition of SB1 in CHCl3 is observed at 357 nm (A1) and is considered as the reference to determine the substituent induced changes in the π − π ∗ transition. Thus, π − π ∗ transition of the basic aromatic moiety remain unaffected in SB2 whereas red-shifted to 382 nm in SB3 [17, 26]. Absorption spectra of SB(1 − 3) in two other solvents, viz. less polar chloroform (CHL) and polar protic methanol (MOH), have been recorded and are presented in Figure 2. A new transition (A2) is observed in methanol solution of SB(1 − 3) in the range of 400 to 525 nm [35]. The A2 transition is observed at 442 nm, 422, and 481 nm for SB1, SB2 and SB3, respectively. The A2 transition is about 20 nm blue-shifted in SB2 and 39 nm red-shifted in SB3 compared to SB1. In SB2, the A2 transition at 425 nm is even appeared in acetonitrile solution with low intensity (Figure 2b). 10

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SB1 0.167 0.120 0.104 0.106 0.085

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α 0.93 0.83 0.79 0.78 0.76

SB2 1.323 1.156 1.116 1.139 1.017

SB3 0.142 0.103 0.099 0.104 0.094

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Solvent Methanol Ethanol 1-Butanol n-Propanol 2-Propanol

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The intensity of the A2 transition of SB2 in methanol solution is higher compared to that of the π-π ∗ transition (A1) at 357 nm . The A2 transition is absent in acetonitrile solution of SB1 and SB3. The appearance of A2 transition does indicate the presence of another species in polar protic medium for both compounds, i.e., SB2 and SB3, as it has been observed in case of SB1 and other related hydroxy Schiff bases [11, 14, 35, 39, 40]. The presence of the new species is even prominent for SB2 in polar aprotic solvent with low population as the intensity of the A2 transition around 425 nm is very weak in acetonitrile solution. Experiments have been performed with an increase in the concentration (0.05 mM to 3.0 mM) of Schiff base molecules in methanol solution to investigate the formation of higher cluster of the Schiff base molecules (ESI Figure 1). A gradual increase in the absorbance has been observed for both the transitions, i.e. A1 and A2, without any change in the peak position for SB(1 − 3). A linear correlation has been observed in between the absorbance and concentration of the Schiff base molecules. The absorbance maxima does not change with an increase in the concentration of the Schiff base molecules. Thus, the A2 transition is not due to the formation of higher clusters of the molecules.

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Table 2: The ratio of the absorbance of keto and enol (keto:enol) conformers of SB(1 − 3) obtained from the absorption spectrum recorded with various polar protic solvents with different hydrogen donating ability (α)

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Controlled experiments have been performed to confirm the formation of new species in the polar protic solvent. In mixed solvents (CHCl3 + CH3 OH), it is observed that the A2 transition becomes pronounced with gradual addition of CH3 OH. Intensity of the A2 transition increases with an enhancement in the concentration of CH3 OH (Figure 3). A well-defined isobestic point is evidenced in case of SB1 and SB3, i.e., equilibrium does exist between two species those are responsible for A1 and A2 transitions (Figure 3a and 3c). Absorption spectra of SB2 in mixed solvents (CHCl3 + CH3 OH) does not exhibit any isobestic point (Figure 3b). Although, population responsible for the A2 transition increases with an increase in the methanol concentration. Several interpretations are available in literature to explain the origin of the long wavelength transition in ortho-hydroxy Schiff bases [10]. The most widely accepted explanation about the origin of long wavelength transition (A2) is S1 ← S0 (π-π ∗ ) excitation of the K conformer. It has been assumed that in nonpolar solvents the E form is predominantly being populated in the electronic ground state of the molecule. In contrast, the K form is more stable compared to the E form in polar protic solvents. Thus, population from E form can be converted to the K form in the electronic ground state with an increase in the polarity and proton donating ability of the medium, 11

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Figure 4: Absorption spectra of SB1 (a), SB2 (b), and SB3 (c), recorded with increase in trifluroacetic acid (TFA) concentration. Concentration of individual Schiff base molecule is kept constant.

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i.e., solvent molecules. The ratio of absorbance of the keto form (AK ) and enol form (AE ) for SB(1 − 3) in selected protic solvents, such as, methanol, ethanol, 1-butanol, n-propanol, and 2-propanol, are provided in Table 2. A linear correlation is obtained (ESI, Figure 2)  AK vs. α parameter for SB1 (R = 0.9644), SB2 (R = 0.9220), between the plot of log AE and SB3 (R = 0.9434). The A2 transition in SB2 is even observed in polar aprotic  solvent,  AK such as, acetonitrile. Linear correlation is not being observed from the plot of log vs. AE solvent polarity parameter (f parameter). This indicates that the solvent-solute interaction does not solely control the ground electronic state keto-enol equilibrium in case of SB2 (ESI, Figure 3). Thus, electronic substitution effect plays crucial role to govern the ground state intramolecular proton transfer (GSIPT) process. Ground state equilibrium between the K and E conformers of SB1 and SB3 is mainly governed by the proton donating ability of the solvent. Effect of acid on GSIPT process of SB(1 − 3) has been investigated by controlled experiments using trifluoroacetic acid (TFA). All solutions have been prepared in methanol solvent as hydrolysis is a prominent channel in aqueous medium [46, 47]. The effect of addition of TFA on the absorption spectra of SB(1 − 3) have been shown in Figure 4. The effect of acid has been investigated by addition of small amount of TFA to the solution of Schiff 12

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Figure 5: Absorption spectra of SB1 (a), SB2 (b), and SB3 (c), recorded in methanol solution of sodium hydroxide (NaOH) with various concentrations. Concentration of individual Schiff base molecule is kept contant.

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bases in methanol solution. Methanol solution is used as the K form is already populated in the electronic ground state of SB(1 − 3). A decrease in the absorbance of the A1 and A2 transitions of SB1 has been observed along with emergence of a new transition at 406 nm. A gradual increase in the absorbance of this new transition is observed. Two isobestic points are observed at 377 nm and 440 nm. The effect of acid addition to SB3 is very similar to that observed in case of SB1. A decrease in the absorbance of the A1 and A2 transitions of SB3 has been observed along with the emergence of a new transition at 440 nm. A gradual increase in the intensity of the absorbance of the new transition is observed. In case of SB2, initially a gradual decrease in the absorbance of the A1 and A2 transitions has been observed. At high TFA concentration (∼ 12mM) the A1 transition is completely disappeared in SB2 whereas the A2 transition is blue-shifted to 410 nm. Thus, it is clear that the mechanism of protonation for SB1 and SB3 is similar whereas the protonation process in SB2 is different. The effect of strong base on GSIPT process of SB(1 − 3) has been investigated by controlled experiments using NaOH in methanol solution and are presented in Figure 5. A decrease in the intensity along with a red-shift of the A1 transition of SB1 is observed upon addition of NaOH (1.5 mM) solution. The absorbance of the A2 transition is increased along with a new transition at 464 nm. A gradual increase in the absorbance of the A2 and 464 nm transitions have been observed due to the addition of more NaOH. A similar effect 13

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Figure 6: Absorption spectra of SB1 (a), SB2 (b), and SB3 (c), recorded in viscous medium of polyethylene glycol along with methanol and chloroform.

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has been observed in case of SB3 upon addition of NaOH solution. The A1 transition is observed to be red-shifted along with decrease in intensity. The absorbance of A2 transition is gradually increases with an increase in the concentration of NaOH solution. At a high concentration of NaOH (∼ 3 mM), a new absorption band is appeared at 496 nm along with some intensity component for the A2 transition. The effect of NaOH addition to SB2 is different compared to that observed in SB1 and SB3. A decrease in the absorbance of the A1 transition is observed along with a red-shift in the A2 transition. At a high concentration of NaOH solution (∼ 3 mM), the A1 transition is completely disappeared and the A2 transition is red-shifted to 448 nm. Thus, the proton abstraction mechanism in SB1 and SB3 is similar whereas that in SB2 is different. The effect of viscosity on the absorption spectra of SB(1 − 3) has been investigated by addition of polyethylene glycol (PEG) to chloroform solution of individual species. Methanol has also been used as solvent having dielectric constant of 33.0 compared to 12.7 for PEG. The effect of addition of PEG in chloroform solution of all the three Schiff base molecules is similar. The A2 transition is observed when PEG is added to chloroform solution. Absorbance of the A2 transition is evidenced in the (1 : 1) solution of chloroform and PEG (Figure 6). In the solution of methanol, similar transition is observed with almost twice the intensity compared to the PEG solution. Thus, the lowering of K population in PEG medium is attributed to the viscosity of the medium which in turn indicates that the GSIPT process is governed by some internal motion of the molecule.

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Figure 7: Fluorescence emission spectra of SB1 (a), SB2 (b), and SB3 (c), recorded in chloroform (CHL), acetonitrile (ACN), and methanol (MOH) solution. Excitation wavelengths are ∼ 350 nm for SB1, SB2 and ∼ 370 nm for SB3.

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Emission/excitation spectra: The steady state emission spectra of SB(1 − 3) have been recorded in different solvents by exciting at the respective absorption maxima (∼ 350 nm for SB1 and SB2 and ∼ 370 nm for SB3) and are presented in Figure 7. The spectral transitions of SB(1 − 3) in different solvents are presented in Table 1. Dual emission is observed in SB1 and SB3 molecules with dominant signal of emission from the K conformer, i.e., from the long wavelength emissive species having the characteristic Stokes shift (Em2). The short wavelength transition (Em1) in the wavelength range of 400 nm to 500 nm is due to the emission from the excited state of the E conformer. It is expected that after photoexcitation, E conformer undergoes ESIPT process and most of the emission occurs from excited K conformer. Emission spectra have also been recorded by excitation at the A2 transition, i.e., ∼442 nm form SB1, ∼420 nm for SB2, and ∼480 nm for SB3 in methanol solution. Emission profile corresponding to the A2 excitation resembles with the emission from the K conformer only (ESI Figure 4). Dual emission is not observed for SB2 in methanol solution. Emission is only obtained from the K conformer at around ∼500 nm. This is due to the fact that the E conformer population in the ground electronic state is less compared K conformer. Moreover, the ESIPT process is much efficient in SB2 compared to SB1 and SB3. Similar emission profiles have been obtained by excitation at ∼ 350 nm (A1 transition) and ∼ 420 nm (A2 transition). It is clear that the K tautomer formed in the ground electronic state by GSIPT process and formed in the electronic excited state by ESIPT process are similar for SB(1-3). Excitation spectra of SB(1 − 3) are presented in Figure 8. Excitation spectra of all the 15

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Figure 8: Fluorescence excitation spectra of SB1 (a), SB2 (b), and SB3 (c), recorded in chloroform (CHL), acetonitrile (ACN), and methanol (MOH) solutions. Emission is recorded at the corresponding Em2 transition of the individual species.

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three molecules are different from their respective absorption spectra. Existence of more than one species in the electronic excited state is evidenced from the nature of the electronic excitation spectra. In case of SB1, the excitation peak for the E conformer is slightly redshifted compared to the A1 transition in the absorption spectra. Transition corresponding to the K conformer is broad and further red-shifted compared to the A2 transition. A blue-shifted transition is observed for the E form in the excitation spectrum of SB2 along with a broad feature corresponding to the K form. The broad feature might be due to the presence of more than one species in the excited state. The nature of excitation spectra of SB3 is completely different from the absorption spectra. Presence of three different species in the excited state is confirmed from the nature of the excitation spectra. For all the three Schiff base molecules, the difference in spectral signature along with shift in band positions compared to the absorption spectra indicates structural reorientation and formation of new species in the excited state. Controlled fluorescence experiments have been performed to record the effect of acid on the emission spectra of SB(1 − 3) in methanol solution and the spectra are presented in ESI Figure 5. The molecules have been excited on corresponding absorption maxima, (λabs ∼ 350 nm for SB1, λabs ∼ 350 nm for SB2 and λabs ∼370 nm for SB3). The spectral behaviour is completely different from that obtained at acidic pH using phosphate buffer in water solution [35]. For SB1 both Em1 and Em2 peaks are present in acidic medium with an enhancement in intensity and blue-shift for Em2 transition (∼ 10 nm). For SB3, both Em1 and Em2 peaks merge and shift in peak position is observed. The fluorescence spectra 16

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Figure 9: Fluorescence decay profile of SB1 (a), SB2 (b), and SB3 (c), recorded in methanol solvent using fluorescence up-conversion technique.

of cation is totally different from K tautomer, when excited on the corresponding absorption maxima of cationic species (406 nm for SB1 and 440 nm for SB3) and are presented in ESI Figure 6. The emission maxima is observed at 509 nm and 540 nm for SB1 and SB3, respectively. In both cases the quantum yield of the new species is also high [10]. In case of SB2, with addition of acid quenching takes place, that correlates well with its absorption spectra. For SB2 no change in peak position is found, wherever the excitation is. Fluorescence quantum yield measurement: The fluorescence quantum yield of SB(1 − 3) at room temperature with variation in polarity and hydrogen bonding ability of the solvents are presented in Table 1. The quantum yield of E form (∼ 10−5 ) is less compared to the K form (∼ 10−4 ) due to efficient ESIPT process and is further justified from the lifetime data in methanol solution.

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540 nm τ1 = 300 fs (22%) τ2 = 9.3 ps (23%) τ3 = 29.2 ps (65%) <τ > = 21.2 ps

SB2

480 nm τ1 = 400 fs (35%) τ2 = 6.9 ps (54%) τ3 = 23.0 ps (27%) <τ > = 10.1 ps

500 nm τ1 = 300 fs (26%) τ2 = 6.1 ps (52%) τ3 = 22.0 ps (38%) <τ > = 11.6 ps

520 nm τ1 = 361 fs (24%) τ2 = 6.4 ps (53%) τ3 = 22.1 ps (36%) <τ > = 11.5 ps

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530 nm τ1 = 290 fs (83%) τ2 = 6.2 ps (21%) τ3 = 27.6 ps (3%) <τ > = 2.4 ps

550 nm τ1 = 300 fs (45%) τ2 = 4.9 ps (23%) τ3 = 25.3 ps (54%) <τ > = 14.9 ps

570 nm τ1 = 300 fs (31%) τ2 = 8.3 ps (40%) τ3 = 37.7 ps (47%) <τ > = 21.1 ps

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500 nm τ1 = 300 fs (77%) τ2 = 26.2 ps (56%) <τ > = 14.9 ps

Lifetime (τ ) 520 nm τ1 = 300 fs (40%) τ2 = 6.2 ps (8%) τ3 = 27.3 ps (73%) <τ > = 20.6 ps

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Table 3: The fluorescence lifetime (τ ) of SB1, SB2, and SB3 in methanol solution at different wavelengths

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Lifetime measurement: Fluorescence up-conversion measurements of SB(1−3) in methanol solution has been carried out in different spectral region using an excitation wavelength of 375 nm. Several decay profiles are obtained from frequency up-conversion experiments in different wavelengths and are presented in Figure 9. Multiple time scales (fs to ps) have been observed in the emission decay profile of the three Schiff base molecules and are presented in Table 3. The general mechanism of photochromisim of common salicylideneaniline (SA) or its other derivative is explained as: (a) ground state E conformer is excited to a π-π ∗ state, (b) then quickly ESIPT takes place and Kc conformer is formed, and (c) Kc form undergoes rotation along C=N bond (cis-trans isomerisation) to form Kt conformer. The results of multi-exponential decay analysis for SB(1 − 3) exhibits a similar trend and are reported in Table 3. Three emission decays with time constants of τ1 ∼ 300 fs, τ2 ∼ 6.2 ps, and τ3 ∼ 27.3 ps are obtained for SB1 when the emission signal is monitored at 520 nm. The contribution of 300 fs component is most prominent in shorter wavelength range 500 nm (∼ 70%) and the decay profile is biexponential with a contribution of slower time component, τ2 ∼26 ps. Very fast decay component is from the S1 (π − π ∗ ) state of enol form [50]. The 6 ps component is having medium proportion (upto 20%) in the 520-540 nm region which is assigned to the Kc form produced by the ESIPT process [51]. Higher proportion of a long lived component (∼ 26 ps) is also observed at the long wavelength range and is attributed to the photochromic form 18

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of the Kt conformer [34, 51]. Three emission decays have been observed for SB2 with time constants of τ1 ∼ 300 fs (∼ 26%), τ2 ∼ 6 ps (∼ 52%), and τ3 ∼ 22 ps (∼ 38%) with the emission decay at 520 nm. The contribution of ∼ 300 fs component is prominent in shorter wavelength range of 480 nm (up to 35%). Contribution of the slower components are more at higher wavelength range. Similar decay profile have been observed in case of SB3. The shortest component of ∼ 300 fs is maximum (∼ 80%) when the signal has been monitored at 530 nm and contribution of slower component increases when the signal has been monitored at 550 and 570 nm.

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Figure 10: Optimised geometry of SB1. (a) φ represents the dihedral angle between the two planes of the moleculs, (b) enol conformer of SB1(SB1En), (c) cis-keto conformer of SB1(SB1Kc), and (d) trans-keto conformer of SB1(SB1Kt)

Conclusion from experimental measurement: Three Schiff base molecules, SB(1 − 3) have been investigated using absorption spectroscopy, fluorescence emission and excitation spectroscopy, and time-resolved spectroscopy. Absorption spectra in non-polar solvent exhibit only π−π ∗ transition (A1) in the wavelength range of 350 nm to 400 nm. A new transition (A2) is observed at lower energy (∼ 450 nm) in polar protic solvent for all the three molecules in addition to the π − π ∗ (A1) transition. The A2 transition is observed in case of SB2 even in polar aprotic solvent, such as, acetonitrile. Further experiments have been performed to identify the species responsible for the A2 transition and the mechanism of formation of the species. Controlled experiments have been performed with mixed solvents of chloroform and methanol. It has been concluded that the A2 transition is due to the formation of a new species in the electronic ground state. 19

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The new species is certainly not the hydrogen bonded cluster with the solvent molecules. Spectral shift is not being observed in the recorded absorption spectra of the molecules with an increase in the methanol concentration (Figure 3). Concentration dependent absorption spectra have been recorded in methanol. Spectral shift is expected if the origin of the transition is due to the formation of higher cluster of the molecules. No such observation is evidenced from the absorption spectra recorded at various concentration of the Schiff base molecules (ESI Figure 1). Rather, gradual increase in the absorbance has been observed for both transitions, i.e., A1 (∼ 350 nm) and A2 (∼ 450 nm) , with an increase in concentration. A linear correlation is observed between the absorbance and concentration of the Schiff base molecules. Thus, the A2 transition is not due to the formation of higher order cluster of the Schiff base molecule.

Figure 11: Optimised geometry of methanol cluster of SB1 with cluster size up to n = 3 obtained at the B3LYP/cc-pVDZ level of theory

The intensity of A1 and A2 transitions has been reduced with the addition of TFA in 20

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Bond distance Bond angle Oa -Ha N-Ha C-Oa (∠Oa − Ha − N , in (◦ )) 0.9964 1.7330 1.3422 148.8 1.4890 1.0856 1.2813 149.1 4.7316 1.0157 1.2387 46.2 0.9974 1.7304 1.3417 149.2 1.4774 1.0902 1.2838 149.9 4.7295 1.0155 1.2405 46.0 0.9938 1.7472 1.3478 148.5 1.4645 1.0928 1.2868 150.1 4.7370 1.0157 1.2398 46.7

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Dipole moment (Debye) 2.62 3.41 6.79 4.62 4.88 7.55 3.12 3.09 5.37

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SB1En SB1Kc SB1Kt SB2En SB2Kc SB2Kt SB3En SB3Kc SB3Kt

Relative energy (kcal mol−1 ) 0.0 6.8 16.5 0.0 6.6 17.2 0.0 7.6 16.7

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methanol solution. Drastic change is expected in the absorption profile for a n-π ∗ transition as the lone pair of nitrogen gets blocked by protonation. Rather, gradual decrease in absorbance is most likely due to the formation of cationic species, i.e., protonated species. The cationic species of Schiff base molecules in presence of acid can be formed by protonation at nitrogen atom of E form and at the oxygen atom of the K form. Thus, the A2 transition is not originated from the n-π ∗ transition of the E form. Even in presence of strong base, NaOH, the intensity of the A1 and A2 transitions decreases steadily. Proton abstraction is expected in presence of strong base. Formation of a ionic species is confirmed from the appearance of the red-shifted transition compared to the A2 transition in all the three Schiff base molecules, i.e., 464 nm, 448 nm, 498 nm in SB1, SB2, and SB3, respectively. Origin of the A2 transition is certainly not due to the formation of a anionic species as the absorption peak is observed even with low intensity.

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Table 4: Selective structural parameters, energies, dipole moment of ground state enol and keto conformers of SB1, SB2, adn SB3 molecules obtained at the B3LYP level of calculation using cc-pVDZ basis set.

Thus, the A2 transition appeared around ∼ 450 nm is due to the formation of K conformer of the Schiff base molecules in the ground state mediated by GSIPT process. Mechanism of GSIPT process could be either solvent mediated or direct proton transfer along the intramolecular hydrogen bond coordinate that is governed by solvent properties. GSIPT process is observed even in presence of viscous medium of PEG-600 solution. Thus, it is expected that the PT does happen directly from oxygen centre to the nitrogen centre along the intramolecular hydrogen bond coordinate. Electronic structure calculation have been performed to investigate the mechanism of GSIPT process and to corroborate experimental results. Theoretical calculations: Ground state geometry: All the calculations at the electronic ground state for SB(1− 3) have been carried out at the density functional theory (DFT) level using B3LYP functional 21

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Gas phase (kcal mol−1 ) 6.8 6.9 6.6 6.7 7.6 7.6

n=1 1.4 3.4 0.9 3.0 2.6 4.1

Methanol cluster (kcal mol−1 ) n=2 -1.3 1.3 -3.7 1.0 0.8 1.9

n=3 -5.3 -2.4 -5.5 -2.5 -3.9 -1.7

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SB1Kc SB1TS SB2Kc SB2TS SB3Kc SB3TS

PCM model (kcal mol−1 ) in MeOH as solvent 5.4 5.9 5.0 5.6 6.4 6.7

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Table 5: Relative stability of various microsolvated clusters obtained at the B3LYP level using cc-pVDZ basis set. All the energies are with respect to the enol conformer.

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in conjunction with cc-pVDZ basis set. Conformational analysis of the three molecules have been performed at the electronic ground state. Structure and relative stability of various conformers are almost similar for the three Schiff base molecules. Several conformers are expected for SB1 type of molecules. Most of those conformers are having similar energy and low barrier height under consideration of room temperature investigation in the solution phase. Harmonic vibrational frequency analysis have been performed to ensure that all those are minima in the potential energy surface. Only three conformers have been considered to explain the experimental observations and are presented in Figure 10 for SB1. The structure of the individual conformers of SB2 and SB3 are almost similar with regard to the proton transfer site. The E form (SBmEn, with m = 1 − 3 for Schiff base molecules) is found to be the lowest energy conformer. The SB1En conformer is characterised by O-H· · ·N intramolecular hydrogen bonding interaction. The cis-keto (Kc) conformer (SBmKc, with m = 1 − 3 for Schiff base molecules) is formed by the proton transfer process. The SB1Kc is less stable compared to SB1En conformer by 6.8 kcal mol−1 . The SB1Kc conformer is characterised by N-H· · ·O intramolecular hydrogen bonding interaction. The next less stable conformer is the trans-keto (Kt) conformer (SBmKt, with m = 1 − 3 for Schiff base molecules) which is characterised by a rotation of 180◦ with respect to C=N of the Kc conformer. The SB1Kt is 16.5 kcal mol−1 less stable compared to the global minimum structure (SB1En). Intramolecular hydrogen bonding interaction is absent in the SB1Kt conformer. Conformational analysis of SB2 and SB3 yield similar result. SB2En is most stable and is characterised by O-H· · ·N intramolecular hydrogen bonding interaction. The SB2Kc is characterised by N-H· · ·O intramolecular hydrogen bond and is 6.6 kcal mol−1 less stable compared to SB2En. The SB2Kt is less stable compared to the SB2En conformer by 17.2 kcal mol−1 . Similarly, E form of SB3 (SB3En) is most stable. The SB3Kc conformer is less stable compared to the SB3En by 7.6 kcal mol−1 ). The SB3Kt is less stable compared to SB3En by 16.7 kcal mol−1 .

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2.80 (methanol)

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3.47 (chloroform) 3.58 (methanol)

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Experimental absorption (eV) 3.47 (chloroform) 3.56 (methanol)

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Oscillator strength 0.35 0.47 0.26 0.26 0.28 0.28 0.37 0.28 0.33 0.35 0.40 0.56 0.31 0.31 0.34 0.24 0.36 0.30 0.47 0.36 0.28 0.27 0.27 0.27 0.29 0.22 0.22 0.22 0.35 0.27

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Absorption (eV) 3.32 3.35 3.16 3.19 3.20 3.04 3.05 2.98 2.94 2.93 3.28 3.32 3.13 3.17 3.17 3.08 3.09 3.01 3.06 2.96 3.08 3.08 3.02 3.07 3.07 2.63 2.61 2.58 2.79 2.56

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Species SB1En (gas) SB1En (PCM) SB1EnL1 SB1EnL2 SB1EnL3 SB1Kc SB1Kc SB1KcL1 SB1KcL2 SB1KcL3 SB2En SB2En SB2EnL1 SB2EnL2 SB2EnL3 SB2Kc SB2Kc SB2KcL1 SB2KcL2 SB2KcL3 SB3En SB3En SB3EnL1 SB3EnL2 SB3EnL3 SB3Kc SB3Kc SB3KcL1 SB3KcL2 SB3KcL3

2.94 (methanol)

3.24 (chloroform) 3.34 (methanol)

2.57 (methanol)

Table 6: Absorption energy and oscillator strength obtained using TDDFT calculation at the B3LYP/ccpVDZ level. Absorption energy is calculated in the gas phase (gas), in solution using PCM in methanol (PCM), and under microsolvation using methanol as solvent.

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The GSIPT process leads to structural rearrangement in the phenolic ring, amino ring, and C=N group [52–54]. Aromaticity of the phenolic ring is lost due to the formation of the K conformer. A significant difference in the bond length of the amino ring between the E and K form have been observed. The O-H bond length in SB1En is 1.00 Å and changes to 1.49 Å in SB1Kc. Similar effect in the O-H bond length has been observed due to GSIPT process in SB2 (∆O-H = 0.48 Å) and SB3 (∆O-H = 0.47 Å). Significant effect has also been observed in the N-H bond length. In all the three compounds, the N· · ·H distance in E form is ∼ 1.73 Åand changes to ∼ 1.09 Å in the Kc form. The molecule is slightly non-planar in the ground state and the dihedral angle (φ, Figure 10) varies in the range of 174◦ to 176◦ . Relative energies, dipole moment, and selective structural parameters of various conformers of SB1, SB2, and SB3 are presented in Table 4. Transition state calculation has been carried out to examine the process of single proton transfer followed by enol-imine (E) ↔ keto-amine (K) tautomerisation in electronic ground state. Nature of the stationary points have been ascertained by harmonic vibrational frequency analysis. Tautomerisation in the electronic ground state is governed by GSIPT process. The first step in the GSIPT process is the formation of the transition state from the E form. Followed by the proton transferred K state. Relative energy for the ground state E ↔ K tautomerisation of SB(1 − 3) are presented in Table 5. Energy of the K conformer in the gas phase is closer to the transition state, i.e., structurally the K state does resemble with the transition state. The energy difference between the K form and the transition state is within 0.1 kcal mol−1 . The E to K energy barrier in all the three compounds is higher than 6.0 kcal mol−1 in the gas phase. The energy barrier is 6.9 kcal mol−1 for SB1 which reduces to 5.9 kcal mol−1 in methanol solution using PCM. Energy barrier for SB2 is 6.7 kcal mol−1 in the gas phase and reduces to 5.6 kcal mol−1 in methanol solution. The energy barrier for SB3 is 7.6 and 6.7 kcal mol−1 in the gas phase and methanol solution, respectively. The E to K energy barrier is lowest for SB2 among the three compounds. Results obtained from the PCM model calculation does not provide any indication of GSIPT process as the energy barrier is significant and comparable to that of the gas phase energy barrier. PCM model is found to be inadequate to explain the GSIPT process in the model Schiff base molecules. The effect of solvation on the GSIPT process has been further investigated using microsolvation of the Schiff base molecules with methanol, SBmLn , with m and n ≤3. Microsolvation studies have been performed with both E form (SBmEnLn ) and cis-keto form (SBmKcLn ). Optimised structures of microsolvated clusters of SB1 with methanol are presented in Figure 11 and are used to describe the microsolvated structures of SB2 and SB3 as the geometry of the microsolvated structures are almost similar. Main aim of the microsolvation studies is to investigate the explicit solvent effect on the GSIPT process. The effect of microsolvation on the relative energies of the E form, the transitions state, and the K form have been investigated. Vibrational frequency analysis have been performed for each structures to ensure that microsolvated E and K geometries are real minima and transitions state geometry is a first order saddle point. In the gas phase the E form is more stable compared to the K form by 6.8 kcal mol−1 , 6.7 kcal mol−1 , and 7.6 kcal mol−1 for SB1, SB2, and SB3, respectively. The E to K barrier energy is more than 6 kcal mol−1 . The 24

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Atoms SB1En

SB1Kc

SB2En

SB2Kc

SB3En

SB3Kc

−0.700 0.503 −0.594 −0.682 0.482

−0.709 0.460 −0.593 −0.684 0.489

−0.701 0.503 −0.602 −0.686 0.481

−0.720 0.460 −0.596 −0.687 0.490

−0.707 0.501 −0.589 −0.684 0.482

−0.719 0.461 −0.589 −0.687 0.490

Oa Ha N13 Ob Hb

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Table 7: Atomic charge (e) on selective atoms of enol and keto conformers of SB1, SB2, and SB3 molecules obtained using natural population analysis at the B3LYP level of calculation in conjunction with cc-pVDZ basis set.

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relative stability of the E tautomer decreases due to the addition of one methanol molecule compared to the gas phase relative stability. The relative stability between SB2EnL1 and SB2KcL1 is maximum (0.9 kcal mol−1 ) followed by SB1 with energy difference of 1.4 kcal mol−1 between SB1EnL1 and SB1KcL1 . The relative stability between SB3En and SB3Kc is 4.1 kcal mol−1 . Similar effect has also been observed for the transition state. The E to K energy barrier reduces to 3.0, 3.4, and 4.1 kcal mol−1 in SB2, SB1, and SB3, respectively. The K conformer becomes more stable with the addition of two methanol solvents in SB1 and SB2 compared to the E conformer by 1.3 kcal mol−1 and 3.7 kcal mol−1 , respectively. The SB3EnL2 is almost equally stable compared to SB3KcL2 (∆ = 0.8 kcal mol−1 ). E to K barrier energy has been reduced significantly by addition of two methanol molecules. Barrier height is 1.3 kcal mol−1 , 1.0 kcal mol−1 , and 1.9 kcal mol−1 in SB1, SB2, and SB3, respectively. Relative stability of the E and K conformers is reversed in presence of three methanol molecules. The K conformer is more stable compared to the E conformer by 5.3, 5.5, and 3.9 kcal mol−1 for SB1, SB2, and SB3, respectively. The transition state energy becomes lower compared to the E form which makes the E to K tautomerisation as a barrier less process in presence of three methanol molecules. Theoretical absorption spectra: UV-VIS absorption spectra have been simulated at the TD-DFT level using B3LYP functional in conjunction with cc-pVDZ basis set and relevant data are presented in Table 6. The simulated UV-VIS spectra show the same trend as observed in the experimental spectra. The A2 transition is blue-shifted in SB2 and redshifted in SB3 compared to SB1. In the gas phase, absorption transition for SB1 is calculated to be 3.32 eV for the E conformer. A red-shift of 0.04 eV and 0.24 eV has been observed for E conformer of SB2 and SB3, respectively compared to SB1. Clear spectral shift for the K conformer have been observed in the three Schiff base molecules. In vacuum the calculated absorption energy value for SB1En is 3.32 eV, whereas experimentally in CHCl3 solvent the value is 3.47 eV. For SB2En and SB3En, the theoretical absorption energy value in vacuum is 3.28 eV and 3.08 eV, respectively, whereas the experimental value in CHCl3 solvent is 3.4 eV and 3.2 eV, respectively. Calculated oscillator strength of this band is ∼ 0.3 indicates the transition is of π − π ∗ type. In methanol solvent using PCM model, the calculated absorption maximum of K conformer of SB1, SB2, and SB3 are 3.05 eV, 3.09 eV, and 2.61 eV, respectively. The experimental absorption maximum in methanol is 2.80 25

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Occupancy (φi ) 1.8454 1.8433 1.8482 1.8269 1.8224 1.8205

SB1En SB2En SB3En SB1Kc SB2Kc SB3Kc

(2)

Occupancy ∆Eij (φj ) (kcal mol−1 ) 0.0750 27.09 0.0759 27.54 0.0714 25.62 0.1363 55.03 0.1407 57.90 0.1440 60.34

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Table 8: Stabilisation energies (∆Eij in kcal mol−1 ) for selected NBO pairs as obtained by second-order perturbation theory analysis of the Fock matrix in the NBO basis for enol and keto conformers of SB1, SB2, and SB3 molecules. Occupation (e) in each of the φi and φj orbitals are mentioned as well. φi and φj designates the donor and acceptor NBO. φi is the lone pair on N atom and O atom for enol and keto conformers, respectively. φj is the antibonding O-H orbital (φ∗O−H ) and antibonding N-H orbital (φ∗N −H ) in enol and keto conformers, respectively.

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eV, 2.94 eV, and 2.57 eV, respectively. In microsolvated clusters, the absorption maxima is shifted towards red side compared to the values obtained using PCM model. In case of SB1, when n = 3 for solvated cluster of methanol, the absorption maxima of K form is at 2.93 eV. For SB2 and SB3 the value is 2.96 eV and 2.56 eV, respectively, and matches well with experimental data. Oscillator strength value for keto band, i.e., A2 (∼ 0.3) also confirm the π − π ∗ nature of transition. The system behaves similar to the bulk phase with a cluster size of three methanol molecules. NBO Analysis: The atomic charge distribution obtained by NBO analysis is presented in Table 7 and stabilisation energy of selected NBO pairs are listed in Table 8. Difference in charge distribution due to the substitution effect is clear. NBO charge on nitrogen centre is −0.594 e, −0.602 e, and −0.589 e in SB1En, SB2En, and SB3En, respectively. Negative charge on nitrogen centre is directly correlated to the basicity of the nitrogen atom in the molecule. It is clear that the basicity of the nitrogen centre is maximum in SB2En, followed by SB1En, and least in SB3En. Lone pair occupancy on nitrogen atom is less than two in the E form of all the three Schiff base molecules. Lone pair occupancy is minimum in SB2En (1.8433 e) followed by SB1En (1.8454 e), and maximum in SB3En (1.8482 e). Electron ∗ density from nitrogen atom lone pair is transferred to the σO−H orbital of the hydrogen ∗ ∗ bond donor O-H group (N→ σO−H ). The N→ σO−H charge transfer is 0.0750 e, 0.0759 e, and 0.0714 e in SB1En, SB2En, and SB3En, respectively. Second order interaction energy ∗ (∆Eij ) due to charge transfer (N → σO−H ) is 27.09 kcal mol−1 , 27.54 kcal mol−1 , and 25.62 kcal mol−1 in SB1En, SB2En, and SB3En, respectively. The K form is stabilised by the formation of O-H· · ·N intramolecular hydrogen bonding interaction. Charge transfer from ∗ oxygen atom lone pair to the antibonding orbital of the N-H donor group (O → σN −H ) is −1 calculated and are presented in Table 8. The ∆Eij is 55.03 kcal mol , 57.90 kcal mol−1 , and 60.34 kcal mol−1 in SB1Kc, SB2Kc, and SB3Kc, respectively. QTAIM analysis: The quantum theory of atoms-in-molecules (QTAIM) is used to explore 26

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Figure 12: The moleucular graph of enol (a) and cis-keto (b) conformer of SB1 using wavefunctions obtained at the B3LYP level of calculation with cc-pvDZ basis set.

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the electron densities and to obtain further insight into the intramolecular hydrogen bonding interactions in E and K form of SB (1 − 3). The topological parameters of electron densities of the H· · ·N and O· · ·H (H = Ha , Hb ) are computed at the bond critical point (BCP) using the AIM2000 program package. The wavefunction obtained at the B3LYP level of theory using cc-pVDZ basis set have been used to calculate the electron density, ρ(r), and Laplacian of electron density, ∇2 ρ (r) at the bond critical point. The existence of O-H· · ·N and N-H· · ·O hydrogen bond are characterised by a bond path connecting H and N atoms and H and O atoms respectively with a (3, -1) BCP. A well-defined range has been proposed to confirm the existence of hydrogen bonds in terms of electron density at the BCP and the corresponding Laplacian [55]. The values of ρ(r)H···X and ∇2 ρ (r)H···X (X = N, O) are expected to be within the range of 0.002–0.04 a.u. and 0.02–0.15 a.u., respectively. The nature of interaction can be predicted from relative values of ρ(r)H···X and the sign of the Laplacian. The negative Laplacian values are an indicator for shared interaction that is characteristics of covalent bonds, and positive Laplacian values are the indicator of closedshell interactions typically observed in ionic bonds, hydrogen bonds, and van der Waals’ interactions. It appears from the molecular graph that there is a BCP between the H and N atoms in the E conformer. The topological structure shows that the intramolecular hydrogen bond exist in both E and K form of SB(1 − 3). Values of electron densities ρ and ∇2 ρ, at the hydrogen bond critical point, O-H bond critical point, and N-H bond critical point for the E and K conformers of SB(1 − 3) are presented in Table 9. The charge density at the BCP of Ha · ··N for SB1, SB2 and SB3 are 0.0502 au, 0.0505 au, and 0.0486 au, respectively. 27

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SB1En SB1Kc SB2En SB2Kc SB3En SB3Kc

ρHa ···N 0.0502 0.2696 0.0505 0.2661 0.0486 0.2642

∇2 ρHa ···N 0.0330 −0.2795 0.0329 −0.2716 0.0325 −0.2669

ρOa ···Ha 0.3165 0.0848 0.3154 0.0874 0.3193 0.0902

∇2 ρOa ···Ha −0.4725 0.0409 −0.4696 0.0394 −0.4797 0.0384

ρHb ···N ∇2 ρHb ···N 0.0210 0.0183 0.0164 0.0161 0.0210 0.0182 0.0163 0.0160 0.0209 0.0182 0.0165 0.0159

ρOb ···Hb 0.3440 0.3440 0.3440 0.3438 0.3439 0.3434

∇2 ρOb ···Hb −0.5229 −0.5257 −0.5219 −0.5259 −0.5226 −0.5251

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Table 9: Values of electron densities ρ and ∇2 ρ, at the hydrogen bond critical point, O-H bond critical point, and N-H bond critical point for the enol and keto conformers of SB1, SB2, and SB3 obtained using QTAIM calculations. Wavefunctions are obtained at the B3LYP level of calculations with cc-pVDZ basis set

V (rc ) Ha · · ·N −0.0405 −0.3925 −0.0407 −0.3860 −0.0387 −0.3833

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G(rc ) Ha · · ·N 0.0367 0.0565 0.0368 0.0572 0.0356 0.0582

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The charge density ρ(r)Ha ···N is higher in case of SB2. The magnitude of electron density at the BCP indicates the presence of strong hydrogen bond. The negative Laplacian of Oa -Ha suggests the covalent nature of the O-H bond in the E conformer. The charge density at the BCP of the Oa -Ha bond decreases with the proton transfer to N centre and Ha · · ·N charge density increases. The charge density at the BCP of Oa · ··Ha for SB1, SB2 and SB3 are 0.0848 au, 0.0874 au, and 0.0902 au, respectively. The negative Laplacian of Ha -N suggests the covalent nature of the N-H bond in the K conformer. Moreover, energy density, H(rc ), is also a meaningful descriptor to investigate hydrogen bonding interaction. The H(rc ) is the sum of the G(rc ) (kinetic energy) and V (rc ) (potential energy). A negative value of H(rc ) implies partial covalent character [56–59]. The values of G(rc ), V (rc ), and H(rc ) for the E and K conformers of SB-(1 − 3) are presented in Table 10. The H(rc ) value for Ha · · ·N in the E form and Oa · ··Ha in the K form are negative. Thus, the Ha · · ·N and Oa · ··Ha bonds are partial covalent in nature. H(rc ) Ha · · ·N −0.0038 −0.3360 −0.0039 −0.3288 −0.0031 −0.3251

G(rc ) Oa · ··Ha 0.0703 0.0671 0.0702 0.0685 0.0705 0.0708

V (rc ) Oa · ··Ha −0.6131 −0.0934 −0.6100 −0.0977 −0.6206 −0.1032

H(rc ) Oa · ··Ha −0.5428 −0.0263 −0.5398 −0.0292 −0.5501 −0.0324

Table 10: The G(rc ), V (rc ), and H(rc ) values for the O-H bond and N-H bond of the enol and keto conformers of SB1, SB2, and SB3 obtained using QTAIM calculations. Wavefunctions are obtained at the B3LYP level of calculations with cc-pVDZ basis set

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Further discussion:

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The origin of the long wavelength transition (A2) in the ortho-hydroxy aromatic Schiff base molecules can be explained as (i) weak S1 ←S0 (n-π ∗ ) transition of the E conformer; (ii) Self-aggeragation of the solute molecules; (iii) Ion formation in the solution; (iv) Hydrogen bonding interaction with solvent network; (v) S1 ← S0 (π−π ∗ ) transition of the K conformer. Herein, the origin of the A2 transition in SB(1 − 3) has been investigated using absorption and emission spectroscopy along with the electronic structure calculation. The A2 transition is observed in polar protic solvent for all the three Schiff base molecules whereas SB2 exhibits the same transition in acetonitrile solution as well. Intensity of the A2 transition gradually increases with an increase in the concentration of methanol in mixed solvent with chloroform. Thus, the formation of new species in the ground electronic state in presence of polar protic solvent is being confirmed. The A2 transition is weak compared to the π − π ∗ (A1) transition for SB1 and SB3. Relative intensity of the A2 transition in case of SB2 is higher compared to the A1 transition. Thus, the long wavelength transition is not n − π ∗ in nature. Further support is obtained by measuring the effect of acid on this transition. A sudden decrease in the absorbance of a n − π ∗ transition is expected in presence of acid. A gradual decrease in the absorbance is observed with an increase in acid concentration along with the appearance of a new transition. Thus, it is concluded that the A2 transition is not n − π ∗ in nature. Absorption spectra have been recorded at various concentration of the Schiff base molecules (0.05 mM to 0.30 mM) in methanol. Absorption maxima remains unaffected while the intensity of the transition increases. Thus, it is concluded that the A2 transition is not due to selfaggregation. Absorption spectra have been recorded in presence of strong base. Effect of base is supposed to be insignificant in case the A2 transition is due to the ion formation in solution. The A2 transition is further red-shifted in presence of base and is expected to be due to ion formation by abstraction of a proton. The proposition that the origin of the transition is due to the hydrogen bonding interaction of the E form with the solvent is discarded based on the observation obtained from controlled experiments in chloroform and methanol mixture. Moreover, this transition is also observed in acetonitrile solution for SB2. The present investigation confirms that the A2 transition is due to the π − π ∗ transition of the K conformer Fluorescence measurements confirms ESIPT process in all the three Schiff base molecules. Dual emission is observed in methanol solution of SB1 and SB3 with dominant signal of emission (Em2) from the Kc conformer. In case of SB2, emission is only obtained from the Kc conformer. Emission spectra recorded by excitation at the A2 transition yield only Em2 transition from K conformer. Thus, K conformer formed in the electronic ground state by GSIPT process is similar to that formed in the electronic excited state by ESIPT process. Experimental observation of GSIPT process is further supported by the electronic structure calculation. Effect of solvation using continuum model does not capture the alteration of relative energies of the E and K conformer. Thus, explicit solvation has been considered using microsolvation of Schiff base molecules with methanol. The relative stability of the E and K form is reversed in presence of three methanol molecules. The K conformer is more 29

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stable compared to the E conformer. Moreover, the transition state energy becomes lower compared to the E conformer which makes the E to K tautomerisation as a barrier less process. Moreover, simulation of absorption spectra confirms the assignment of A2 transition to π − π ∗ of K conformer. Conclusion:

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The effect of polar protic solvent on photophysical properties and E to K tautomerisation by GSIPT process of three Schiff base molecules, SB(1 − 3), have been investigated. The molecular structure and intramolecular proton transfer process have been studied using absorption spectroscopy, fluorescence spectroscopy, lifetime measurement along with electronic structure calculations using density functional theory (DFT) and time-dependent density functional theory (TDDFT). The enol-imine form is more stable in the gas phase and also in slightly polar solvents, such as, chloroform. The ground state population distribution in E and K tautomers have been altered by changing the substituents, polarity, and proton-donating ability of the solvent. Moreover, substitution on salicylaldehyde ring does effect enol-imine (E) ↔ keto-amine (K) equilibrium. It has been observed that an increase in the basicity at the nitrogen centre does favour the GSIPT process. Thus, the keto-amine form form is considerably stabilised in presence of polar protic solvents, such as, methanol. The short wavelength A1 transition in the absorption spectra of SB(1 − 3) has been assigned as the π − π ∗ transition of the E form. A long wavelength transition (A2) has been observed for SB(1 − 3) in polar protic solvents which has been assigned as the π − π ∗ transition of the K form. The GSIPT process in SB2 is very efficient. The relative intensity of A2 transition is higher compared the A1 transition in methanol. Moreover, GSIPT process even takes place in polar aprotic solvent, such as, acetonitrile. Substitution by methoxy group increases basicity at the nitrogen centre (−C = N ) in SB2 and decreases acidity of the ˘Oa Ha on salicyldehyde ring of SB3 compared to SB1. Consequently, relative stability of ground state K form and relative population in methanol is maximum for SB2 that is followed by SB1 and SB3. All the Schiff base molecules exhibit dual emission. The short (Em1) and long (Em2) wavelength transitions have been assigned as the emission from locally excited (E emission) and PT state (K emission), respectively. The large Stokes shift give a clear indication of a fast proton transfer process in the excited state, which shows the excited enol relaxes to excited keto form. In the femtosecond up-conversion measurements the emission of the K shows a multi-exponential decay because of the formation of transketo (Kt) form. Microsolvation of Schiff base molecules by methanol clearly indicates that the keto-amine (K) form is more stable compared to the enol-imine (E) form in presence of two methanol molecules for SB2 and SB1. In case of SB3, the K conformer becomes stable compared to the E conformer in presence of three methanol molecules. Moreover, the lowering of activation barrier in higher order microsolvated clusters for enol-imine (E) ↔ keto-amine (K) tautomerisation favours the GSIPT process in SB(1 − 3). The GSIPT via. enol-imine to keto-amine formation becomes barrierless process by the addition of three methanol molecules in the microsolvated clusters. The natural bond orbital analysis has been carried out to estimate the extent of charge transfer from the hydrogen bond acceptor 30

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to the anti-bonding orbital of hydrogen bond donor. Substitution plays a significant role in the GSIPT process which has been supported by theoretical calculations. NBO and QTAIM analysis support efficient GSIPT process in SB2 compared to other two molecules. Acknowledgement:

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This study is supported by the financial grant received from Science and Engineering Research Board (SERB), Department of Science and Technology (DST), Government of India [SB/F T /CS − 106/2013] and [EM R/2016/003756]. BD would like to acknowledge the SRF scholarship received from CSIR-India [09/719(0083)/2018/EMR-I]. AC would like to acknowledge financial support received under Women Scientist Scheme-A, Department of Science and Technology (DST), Government of India [SR/W OS − A/CS − 67/2017]. Authors thank computer centre at BITS-Pilani, Pilani Campus, where all calculations were performed, for providing time in High-Performance Computational Facility. Authors also thank Advanced Instrumentation Research Facility (AIRF) of Jawaharlal Nehru University (JNU), Delhi, India where life-time measurements have been performed using Femtosecond up-conversion (UPC) facility. Authors are thankful to Prof. Sobhan Sen and his team for their help to analyse femtosecond up-conversion data.

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molecule, The Journal of Chemical Physics 73 (6) (1980) 2871–2883. [57] S. Jenkins, I. Morrison, The chemical character of the intermolecular bonds of seven phases of ice as revealed by ab initio calculation of electron densities, Chemical Physics Letters 317 (1-2) (2000) 97–102. [58] G. Jana, S. Pan, G. Merino, P. K. Chattaraj, Mngcch (m= cu, ag, au; ng= xe, rn): the first set of compounds with m–ng–c bonding motif, The Journal of Physical Chemistry A 121 (34) (2017) 6491– 6499. [59] G. Jana, S. Pan, E. Osorio, L. Zhao, G. Merino, P. K. Chattaraj, Cyanide–isocyanide isomerization: stability and bonding in noble gas inserted metal cyanides (metal= cu, ag, au), Physical Chemistry Chemical Physics 20 (27) (2018) 18491–18502.

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Electronic sustitution effect on GSIPT Solvent mediated GSIPT Barrierless enol to keto tautomerisation in microlvated clusters Electronic substitution effect on ESIPT

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