Effect of nitrogen substitution and π-conjugation on photophysical properties and excited state intramolecular proton transfer reactions of methyl salicylate derivatives: Theoretical investigation

Journal Pre-proof Effect of nitrogen substitution and ␲-conjugation on photophysical properties and excited state intramolecular proton transfer reactions of methyl salicylate derivatives: Theoretical investigation Athis Watwiangkham, Thantip Roongcharoen, Nawee Kungwan

PII:

S1010-6030(19)31654-5

DOI:

https://doi.org/10.1016/j.jphotochem.2019.112267

Reference:

JPC 112267

To appear in:

Journal of Photochemistry & Photobiology, A: Chemistry

Received Date:

26 September 2019

Revised Date:

8 November 2019

Accepted Date:

26 November 2019

Please cite this article as: Watwiangkham A, Roongcharoen T, Kungwan N, Effect of nitrogen substitution and ␲-conjugation on photophysical properties and excited state intramolecular proton transfer reactions of methyl salicylate derivatives: Theoretical investigation, Journal of Photochemistry and amp; Photobiology, A: Chemistry (2019), doi: https://doi.org/10.1016/j.jphotochem.2019.112267

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Effect of nitrogen substitution and π-conjugation on photophysical properties and excited state intramolecular proton transfer reactions of methyl salicylate derivatives: Theoretical investigation Athis Watwiangkham, †,‡ Thantip Roongcharoen, † Nawee Kungwan†,§, †

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Department of Chemistry, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand The Graduate School, Chiang Mai University, Chiang Mai 50200, Thailand

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Corresponding author E-mail : [email protected]

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Center of Excellence in Materials Science and Technology, Chiang Mai University, Chiang Mai 50200, Thailand

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Phone: +66-53-943341 ext 126. Fax: +66-53-892277

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Graphical Abstract

Highlights  

HOMO-LUMO energy gaps of compounds with π-extended conjugation are significantly decreased, leading to the red-shift of electronic spectra. Compounds with π-extended conjugation (M3HN, M2HQC, and M3HQC) have remarkably large Stokes shifts of about 1000 cm-1.

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ESIPT reactions of studied compounds are observed in an ultrafast timescale with the remarkably high PT probability.

Abstract Effects of nitrogen substitution and π-conjugation on the strength of intramolecular hydrogen bond (H-bond), photophysical properties, and excited-state intramolecular proton transfer (ESIPT)

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reactions of MS derivatives have been systematically investigated using DFT and TD-DFT

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methods at the B3LYP/TZVP level. The H-bond strength of all studied compounds becomes stronger in the S1 than in the S0. This result is confirmed by vibrational IR spectra and topological

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analysis. The H-bond strength of nitrogen substituted compounds is lower than those of

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compounds with the π-extended conjugation. For photophysical properties, the HOMO-LUMO energy gaps in normal and tautomer forms of nitrogen substituted compounds are insignificantly

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increased, whereas those of compounds with the π-extended conjugation are notably decreased. The slight widening of the energy gaps results in the slight blue-shifts of the absorption and

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emission peaks of nitrogen substituted compounds compared to that of MS, whereas the significant narrowing of the energy gaps makes the incredible red-shifts of both electronic spectra for compounds with the π-extended conjugation. Thus, remarkably large Stokes shifts of about 1000 cm-1 are observed for compounds with the π-extended conjugation (M3HN, M2HQC, and

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M3HQC). In addition, potential energy curves along PT reaction indicate that the ESIPT of all studied compounds should occurred due to low PT barriers in the exothermic reaction. Dynamic simulations reveal that the ESIPT reactions of studied compounds are observed in an ultrafast time scale (38-130 fs). Most of them have remarkably high PT probabilities (0.84-1.00) except M3HP (0.56), which are in good agreement of the results of H-bond strength. Therefore, the photophysical

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properties of studied compounds can be significantly tuned by controlling the effect of πconjugation.

Keywords: Excited State Intramolecular Proton Transfer (ESIPT); Methyl Salicylate; Nitrogen Substitution Effect; π-Conjugation Effect; TD-DFT

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1. Introduction

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Fluorescent organic molecules having the strong intramolecular hydrogen bond (H-bond) connected by proton donating and accepting groups have gained considerable attention in recent

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years [1], due to their unique properties from the excited state intramolecular proton transfer

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(ESIPT) mechanism. This reaction can be described by the characteristic four-level photocycle as illustrated in Figure 1. Initially, the molecule in a normal form (N) absorbs light in the short

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wavelength region and resulting in the photoexcitation process from the ground state (S0) into the excited state (S1). The H-bond is strengthened in the S1 due to the charge redistribution, leading to

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the transferring of proton from the donor (D:−NH2, −OH) to the acceptor (A:C=O, −N=), which changes the normal form (N*) to the tautomer form (T*) in the S 1. After that, the T* molecule emits the fluorescence in the remarkably longer wavelength than the absorption and relaxes to the S0, resulting in the notably large Stokes shift (the difference between positions of absorption and

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emission peaks). Then the molecule in tautomer form (T) changes to the N form through ground state intramolecular proton transfer (GSIPT) spontaneously due to the endothermic reaction. Their photophysical properties can be easily modulated using many strategies [2, 3] such as introducing electron-donating and withdrawing substituents, the heteroatom substitution, and the πconjugation, to give the desirable absorption and emission spectra as well as large Stokes shift. ESIPT molecules such as derivatives of salicylates [4-6], salicylideneanilines [7-9], flavones [103

13], benzazoles [14-18], and chalcones [19-21] with tuning photophysical properties have been developed and widely used in various applications [22-27] including chemical sensing, cell

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imaging, laser dyes, and light-emitting diodes.

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Figure 1. Four-level photocycle of ESIPT process

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As one of the first studied ESIPT molecules, methyl salicylate (MS) has been used as the

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prototype to investigate the fundamental ESIPT reaction in many experimental and theoretical studies [28-40] because of its simple structure and facile structural modification. For substituted derivatives of MS, Balamurali and Dogra [41] reported that the MS derivative with the nitrogen

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substitution called methyl 2-hydroxynicotinate (MEHNA) exhibited the blue-shift of both absorption and emission spectra. It showed smaller Stokes shift than MS. In addition, Law and Shoham [42] revealed photophysical properties of methyl 2-hydroxy-3-naphthoate (M3HN)

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which is the MS derivative with π-extended conjugation. It exhibited the red-shift of both absorption and tautomer emission spectra, resulting in the remarkable larger Stokes shift than that of MS. Moreover, Dogra [43] revealed that methyl 3-hydroxy-2-quinoxalinate (M3HQ) which has both nitrogen substitution and π-extended conjugation showed the red-shift of both absorption and tautomer emission spectra as well as the larger Stokes shift compared to MS. Apart from the photophysical studies, the ESIPT reaction of MS was further investigated using various 5

experimental and theoretical techniques to obtain the PT dynamic information [33, 36, 44-47]. Smith and Kaufmann [46] studied the kinetic behavior of ESIPT for MS in picosecond timescale and suggested that this process was ultrafast. Then, Herek et al. [47] revealed that isolated MS undergoes ESIPT within 60 fs using femtosecond depletion techniques. Upon the improvement of experimental facilities, recently, Ling et al. [36] have used femtosecond spectroscopy to observe the real-time ESIPT process of MS and found that the ESIPT reaction was completed within 100

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fs which is in accordance with the PT time (100 fs) from ab initio dynamical calculation by Coe

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and Martinez [33] with the PT probability of 1.00. For M3HN, McCarthy and Ruth [48] studied photophysical properties of M3HN in a supersonic jet and found that its ESIPT lifetime was in

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picosecond timescale. However, there is no real-time femtosecond report of M3HN. For the other

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MS derivatives, to the best of our knowledge, their PT dynamic information have not been reported. It might be due to the lack of facilities and high cost of instrument to probe the PT

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dynamics information.

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Figure 2. MS and its derivatives

From literature reviews, nitrogen substitution and π-conjugation cause the significant

difference of photophysical properties of the derivatives compared to those of MS. However, there is no systematic study into details for the effect of nitrogen substitution and π-conjugation on photophysical properties of MS derivatives yet. Moreover, their ESIPT reactions are still less understood, especially for the PT dynamics information. Therefore, in this work, we will 6

systematically investigate the effect of nitrogen substitution and π-conjugation on photophysical properties and ESIPT reactions of MS derivatives using theoretical studies. Studied compounds will be divided into two groups: a phenyl skeleton group which is the derivative with the same skeleton as MS, and a naphthalene skeleton group which is the derivative with the π-extended conjugation. Important parameters from optimized structures and simulated infrared (IR) vibrational spectra as well as the topology analysis were used to assess the H-bond strength of

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studied compounds. The frontier molecular orbitals (MOs) was used to analyze the charge

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distribution while the absorption and emission spectra were calculated to reveal the effect of nitrogen substitution and π-conjugation on photophysical properties. In addition, the potential

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energy curves along PT reactions and the reaction energy in S 0 and S1 states were performed to

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obtain the PT barriers and thermodynamic information. Moreover, PT time constants and probabilities of all studied compounds would be further investigated by using molecular dynamics

2. Computational details

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2.1 Static calculations

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simulations.

Optimizations of both normal and tautomer forms for S0 and S1 states were performed using DFT and TDDFT methods, respectively, with the polarization functions (TZVP) combined with

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the correlation hybrid functional (B3LYP) [49-52]. The S0 optimization of normal forms of studied compounds were performed without the constraint of bonds, angles and torsion angles and optimized structures were confirmed to be local minima by vibrational frequency calculation that there is no imaginary frequency. The H-bond strength was determined by important structural parameters including covalent O1−H bond (R1), H-bond (R2), and distance between O1 ̶ O2 heavy atoms as well as torsion angle (C1C2C3O2). These parameters were involved in the ESIPT 7

reaction. The H-bond strength was further ensured by the simulated infrared (IR) spectra of the R1 bond in both S0 and S1 states, together with the topology analysis at bond critical point (BCP) from quantum theory of atoms in molecules (QTAIM) performed by Multiwfn [53], which was reported in previous studies [54-56]. Furthermore, vertical S 0 to S1 excitation energies were carried out from S0 optimized structures in their normal forms using TD-B3LYP with ten low-lying singlet states absorbing transition. simulated tautomer emission spectra were done with S 1 optimized tautomer

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structures at the same level of theory. Moreover, the frontier molecular orbitals (MOs) were

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performed to understand the distribution of charges in the studied molecules. Additionally, the potential energy curve was scanned as a function of fixed distances along the R1 bond in S0 and S1

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states by constraint optimization. This methodology was employed in previous studies [57, 58].

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the relative energy difference was calculated to obtain the thermodynamic information of the

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ESIPT reaction. All calculations were carried out by using Gaussian 09 package [59].

2.2 Dynamics simulations

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For excited state dynamics simulation, on-the-fly twenty-five trajectories were generated at TD-B3LYP/TZVP level of theory. All trajectories were simulated on the first excited state (S 1) using a harmonic-oscillator Wigner distribution for each normal mode, with the time step of 1 fs

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with a maximum duration of 300 fs covering the pre- and post-PT ultrafast reactions. These simulations were carried out using the NEWTON-X program [60] interfaced with the TURBOMOLE 5.10 program [61]. Newton’s equation of nuclear motion for all atom was solved by Velocity-Verlet algorithm [62]. In addition, each simulated trajectory was defined into two types for ESIPT reaction: (1) ‘Yes’ when PT process could be observed and (2) ‘No’ when PT process could not be observed within given simulation time. The information from dynamics 8

simulation including the PT probability, the time evolution (defined as the time at the intersection point between average breaking and forming bond distances), the energy difference between S0 and S1 state (S1-S0), and the torsion angle (C1C2C3O2) was analyzed to explain the ESIPT reaction. These approaches have been used to investigate the ESIPT reactions of other systems in previous studies [56, 63-66].

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3. Results and discussion

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3.1 Structural optimizations

S0 and S1 structures of studied compounds in phenyl skeleton and naphthalene skeleton

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groups (normal form) are shown in Figure 3. Important atoms, bonds and torsion angles related to

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the H-bonds of S0 structures in normal form, have been labeled. S 0 structures in tautomer forms are displayed in Figure S1 in the electronic supporting information (ESI). Obtained parameters

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are listed in Table 1.

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including the length of R1 and R2 bonds, the distance between heavy atoms, and the torsion angle

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Figure 3. Optimized geometries of normal forms for studied compounds in the S 0 state; phenyl

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skeleton group (a) MS (b) MEHNA (c) M3HP (d) M3HPC, and naphthalene skeleton group (e) M3HN (f) M2HQC (g) M3HQC (h) M3HQ. All structures were computed in gas phase at

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B3LYP/TD-TZVP level. Intramolecular hydrogen-bonded distances are shown in dashed line.

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Table 1 lengths of covalent bonds (R1), lengths of H-bonds (R2), distances between O1 ̶ O2 heavy atoms (Å), and torsion angles of C1C2C3O2 (๐) of studied compounds (normal form) in gas phase computed at B3LYP/TD-B3LYP with TZVP basis set

O1 ̶ O2 2.613 2.626 2.604 2.616 2.621 2.632 2.611 2.621

C1C2C3O2 0.04 0.01 0.06 0.01 0.02 0.00 0.00 0.01

S1 state R1 R2 1.032 1.511 1.030 1.529 1.004 1.645 1.021 1.548 1.051 1.469 1.041 1.495 1.045 1.480 1.014 1.571

O1-O2 2.486 2.492 2.569 2.504 2.464 2.478 2.465 2.514

C1C2C3O2 0.00 0.01 0.00 0.00 0.01 0.01 0.00 0.01

ΔR1* ΔR2** 0.049 0.047 0.019 0.036 0.070 0.059 0.063 0.031

-0.230 -0.221 -0.087 -0.194 -0.280 -0.259 -0.260 -0.173

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MS MEHNA M3HP M3HPC M3HN M2HQC M3HQC M3HQ

S0 state R1 R2 0.983 1.741 0.983 1.750 0.985 1.732 0.985 1.742 0.981 1.749 0.982 1.754 0.982 1.740 0.983 1.744

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Compound

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* ΔR1 = R1 of the S1 state − R1 of the S0 state, positive values indicate the increasing of length

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** ΔR2 = R2 of the S1 state – R2 of the S0 state, negative values indicate the decreasing of length From Table 1, R1 lengths of the phenyl skeleton group; MS, MEHNA, M3HP, and

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M3HPC are 0.983, 0.993, 0.985 and 0.985 Å in the S0 state, then these bond lengths increase to 1.032, 1.030, 1.004 and 1.021 Å in the S1 state. The S1-S0 deviations of R1 (ΔR1) are 0.049, 0.047,

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0.019 and 0.036 Å, respectively. Correspondingly, the lengths of R2 (ΔR2) for these compounds of 1.741, 1.750, 1.732, and 1.742 Å in the S0 state are shortened to 1.511, 1.529, 1.645, and 1.548 Å in the S1 state, with decreasing values of 0.230, 0.221, 0.087, and 0.194 Å, respectively. It is noted that M3HP has the least deviation in both R1 and R2 lengths for compounds in this group.

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Their O1 ̶ O2 distances of 2.613, 2.626, 2.604, and 2.616 Å in the S 0 state are decreased to 2.486, 2.492, 2.569, and 2.504 Å in the S1 state, respectively. Torsion angles of C1C2C3O2 for these compounds do not significantly alter in the S1 state compared to the S0. For the naphthalene skeleton group including M3HN, M2HQC, M3HQC, and M3HQ, R1 lengths of 0.981, 0.982, 0.982, and 0.983 Å in the S0 state are enlarged to 1.051, 1.041, 1.045,

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and 1.014 Å in the S1 state, with increments of 0.070, 0.059, 0.063, 0.031 Å , respectively. R2 lengths are shortened from 1.749, 1.754, 1.740, and 1.744 Å in the S0 state to 1.469, 1.495, 1.480, and 1.571 Å in the S1 state with decrements of 0.280, 0.259, 0.260, and 0.173 Å, respectively. These compounds have O1 ̶ O2 distances of 2.621, 2.632, 2.611 and 2.621 Å in the S0 state and these distances are shortened in the S1 state to 2.464, 2.478, 2.465, and 2.514 Å , respectively.

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C1C2C3O2 torsion angles do not change from S0 to S1 states. Overall, the R1 length is decreased while R2 length and O1 ̶ O2 distance are increased from

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S0 to S1 states for all studied compounds. Consequently, their H-bonds (R2) could be strengthened

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in the S1 state, providing the ESIPT possibility. Moreover, their torsion angles (C1C2C3O2) involved the ESIPT reaction are insignificantly different after the photoexcitation into S 1 states. In

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addition, the strength of H-bonds driving the ESIPT reaction will be further determined in the next

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section.

3.2 Simulated infrared (IR) vibrational spectra and topology analysis

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Simulated IR spectra of R1 for selected compounds are shown in Figure 4, while the others are displayed in Figure S2 of the ESI. Obtained vibrational frequencies are listed in Table 2. It should be noted that the lowering of the R1 wavenumber indicates the weakening of R1 bond,

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resulting in the strengthening of H-bond.

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Table 2 Vibrational frequencies of O‒H stretching modes involving PT process both in the S 0 and S1 states

Δν 887 801 364 662 1148 1015 1041 572

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MS MEHNA M3HP M3HPC M3HN M2HQC M3HQC M3HQ

O‒H frequency (cm-1) S0 S1 3425 2538 3429 2628 3396 3032 3399 2737 3463 2315 3458 2443 3439 2398 3432 2860

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Compound

Figure 4. Simulated IR spectra of selected compounds in S 0 and S1 states; (a) MS, (b) MEHNA,

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(c) M3HP, (d) M3HPC, and (e) M3HQ

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From Table 2, for phenyl skeleton compounds, the vibrational frequency of R1 for MS is strongly red-shifted from the wavenumber of 3425 cm-1 in the S0 state to 2538 cm-1 in the S1 state by 887 cm-1. This shift of the O-H stretching frequency from S0 to S1 states is quite in good agreement with the shift of 700 cm-1 estimated by Abd El-Hakam Abou El-Nasr et. al [67]. While vibrational frequencies of MEHNA, M3HP and M3HPC are 3429, 3396, and 3399 cm-1 in the S0 state, decreasing 801, 364, and 662 cm-1 to 2628, 3032, and 2737 cm-1 in the S1 state, respectively.

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Noted that M3HP shows the least deviation of 364 cm-1 in this group. For naphthalene skeleton

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compounds, M3HN, M2HQC, M3HQC, and M3HQ, vibrational frequencies in the S0 state are 3463, 3458, 3439, and 3432 cm-1, then these frequencies are decreased to 2315, 2443, 2398, and

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2860 cm-1 in the S1 state with deviations (Δν) of 1148, 1015, 1041, and 572 cm-1, respectively. It

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should be denoted that M3HN, M2HQC, and M3HQC show the remarkable large red-shifts (> 1000 cm-1).

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From results of IR spectra, the strength of H-bonds (R2) of all studied compounds is stronger in the S1 state than the S0 state since the R1 frequency in the S1 state is large red-shifted

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from the S0 state. Hence, the PT process might occur more easily in the S 1 state than the S0 state. The topology analysis of the electron density was used to further determine the strength of H-bonds in S1 structures (normal form) of studied compounds. Following parameters at bond

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critical points (BCPs) were analyzed. Electron density ρ(r), potential energy density V(r), Laplacian of the electron density ∇2ρ(r), Lagrangian kinetic energy G(r), Hamiltonian kinetic energy density H(r), and electron delocalization index (DI) between the proton acceptor and transferred proton (H…O2). In addition, the H-bond energy (EHB) can be calculated by using the Espinosa's equation: EHB 

1 V (rBCP ) . Results are summarized in Table 3. 2 14

Table 3 Electron density ρ(r), potential energy density V(r), Laplacian of the electron density ∇2ρ(r), Lagrangian kinetic energy G(r), Hamiltonian kinetic energy density H(r), and electron delocalization index (DI), and H-bond energy (EHB) at selected bond critical point parameters (BCPs) in the S1 state (a.u. unit) for studied compounds G(r) 0.0729 0.0605 0.0465 0.0580 0.0688 0.0654 0.0677 0.0557

V(r) -0.1125 -0.0852 -0.0596 -0.0805 -0.1016 -0.0944 -0.0990 -0.0754

H(r) -0.0396 -0.0247 -0.0131 -0.0225 -0.0328 -0.0290 -0.0313 -0.0197

∇2ρ(r) 0.1332 0.1430 0.1334 0.1422 0.1442 0.1460 0.1455 0.1440

DI 0.0000 0.0211 0.0000 0.0001 0.0560 0.0007 0.0007 0.0002

EHB 0.0562 0.0426 0.0298 0.0402 0.0508 0.0472 0.0495 0.0377

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ρ(r) 0.0939 0.0738 0.0554 0.0703 0.0855 0.0802 0.0835 0.0663

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Compound MS MEHNA M3HP M3HPC M3HN M2HQC M3HQC M3HQ

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From Table 3, the energy of H-bond (EHB) of MS (0.0562 a.u.) is the highest among studied compounds. Then, the EHB is slightly dropped for M3HN (0.0508), M3HQC (0.0495 a.u.),

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M2HQC (0.0472 a.u.), MEHNA (0.0426 a.u.), and M3HPC (0.0402 a.u.), and it is obviously

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decreased for M3HQ (0.0377 a.u.), and M3HP (0.0298 a.u.). Thus, the strength of H-bond follows this order: MS > M3HN > M3HQC > M2HQC > MEHNA > M3HPC > M3HQ > M3HP.

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Overall, it can be observed in the phenyl skeleton group that the H-bond strength is reduced in nitrogen substituted compounds compared to MS. For the naphthalene skeleton group consisting of compounds with the π-extended conjugation, their H-bonds are also decreased from MS and, but they are stronger than their analogues. Therefore, it could be stated that the π-conjugation has

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more effect on the H-bond strength of studied compounds than nitrogen substitution.

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3.3 Frontier molecular orbitals (MOs) analysis and electronic spectra To further investigate the behaviors of the charge distribution and charge transfer in the excited-state, highest occupied molecular orbitals (HOMO) energy levels, lowest unoccupied molecular orbitals (LUMO) energy levels, HOMO-LUMO energy gaps, together with frontier molecular orbitals (MOs) of studied compounds (both normal and tautomer forms) were analyzed

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and illustrated in Figure 5. Moreover, their important parameters were compiled in Table S2 of the

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ESI.

For nitrogen substituted compounds, the diagram in Figure 5(a) shows that the HOMO

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energy levels of MEHNA, M3HP, and M3HPC are slightly stabilized to be lower than that of MS while their LUMO energy levels are moderately shifted to be lower. Consequently, the energy

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gap is found to be slightly larger in MEHNA (4.92 eV), M3HP (4.89 eV), and M3HPC (4.83 eV)

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than that of MS (4.81 eV). For compounds with the π-extended conjugation, considering M3HN, its HOMO energy is significantly increased while its LUMO energy is reduced, resulting in the narrower energy gap of M3HN (3.88) than that of MS (4.81 eV). For M2HQC, M3HQC, and

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M3HQ, their HOMO energies are not significant difference from that of MS, whereas their LUMO energies are obviously decreased. So, energy gaps of M2HQC (4.12 eV), M3HQC (3.95 eV), and M3HQ (4.01 eV), are found to be smaller than that of MS (4.81 eV). Energy gaps of studied

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compounds in the normal form follow this order: MEHNA > M3HP > M3HPC > MS > M2HQC > M3HQ > M3HQC > M3HN

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Figure 5. Diagram of calculated HOMO and LUMO energy levels, HOMO-LUMO energy gaps (eV), and frontier molecular orbitals (MOs) at B3LYP/TZVP level of (a) normal form, and (b) tautomer form of all compounds

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In addition, for the HOMO-LUMO energy levels in tautomer form, results from Figure 5(b) reveal that HOMO energies of all compounds are significantly increased, whereas those of LUMO energies are remarkably decreased, which causes the narrowing of energy gaps of studied compounds. Moreover, HOMO and LUMO energy levels of nitrogen substituted derivatives (MEHNA, M3HP, and M3HPC) are shifted to be lower than those of MS, thus making their energy gaps to be larger than that of MS. For the naphthalene skeleton group, the HOMO energy

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of M3HN was stabilized to be higher than that of MS whereas its LUMO energy was lower. So,

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the energy gap of M3HN (2.46 eV) is smaller than MS (3.36 eV). For M2HQC and M3HQC, their HOMO energies are not significantly different from that of MS, but their LUMO energies

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are clearly lower. These results lead to narrower energy gaps of M2HQC (2.65 eV), and M3HQC

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(2.62 eV) than MS (3.36 eV). In the case of M3HQ, both its HOMO and LUMO energy levels are lower than those of MS. Thus, the energy gap of M3HQ (3.36 eV) is smaller than that of MS (3.36

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eV). Overall, the energy gap for the tautomer form follows this order: MEHNA > M3HP > MS > M3HPC > M3HQ > M2HQC > M3HQC > M3HN.

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From Figure 5, the main transition of all compounds could be mainly attributed to π to π*. The HOMO orbital shows the π character while the LUMO orbital owns the π* character. Additionally, the slight shift of electron density from the proton donating unit (the hydroxyl oxygen atom of phenolic group) to the proton accepting unit (the carbonyl oxygen atom of ester group) in

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studied compounds from HOMO to LUMO is partly attributed to the intramolecular charge transfer (ICT) character.

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For electronic spectra of studied compounds, the absorption band maxima of normal form (λads), emission band maxima of tautomer form (λ emiss), oscillator strength (f) and major contribution (%) of the absorption band, as well as the Stokes shift for all compounds are reported in Table 4. The absorption of the normal form and emission spectra of the tautomer form for each compound are plotted in Figure 6.

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From the information in Table 4, The simulated absorption peaks of MS, MEHNA, M3HN, and M3HQ are in good agreement with the experimental results. For nitrogen substituted

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compounds, the calculated absorption maxima peaks of MEHNA (283 nm), M3HP (283 nm), and

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M3HPC (285 nm) are slightly blue-shifted from MS (292 nm), while their emission peaks of 370, 382, and 373 nm are also moderately blue-shifted from 420 nm of MS. It is observed that Stokes

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shifts of MEHNA (831 cm-1), M3HP (916 cm-1), and M3HPC (828 cm-1) are smaller than that of MS (1044 cm-1). For the naphthalene skeleton group, M3HN, M2HQC, M3HQC, and M3HQ

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show the absorption maxima peaks at 369, 349, 359, and 358 nm, respectively, which are significantly red-shifted from that of MS. For emission spectra, their peaks located at 619, 587,

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561, and 525 nm are also remarkable red-shifted from that of MS, leading to large Stokes shifts of 1095, 1162, 1003, and 889 cm-1. These shifts of absorption and emission spectra for studied compounds (i.e. blue-shifts for nitrogen substituted compounds, and red-shifts for compounds with

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the π-extended conjugation) are accordance with their HOMO-LUMO energy gaps.

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Table 4 Absorption band maxima of normal form (λads), emission band maxima of tautomer form (λemiss), oscillator strength (f), major contribution (%), and Stokes shift of all compounds calculated at TD-B3LYP/TZVP level Absorption*

Stokes shift (cm-1)

Emission

Compounds 292 (330[8]) 283 (306[41]) 283 285 369 (370[42]) 349 359 358 (353[43])

MOs (%contribution)

λemiss (nm)

f

0.0939 0.1487 0.1544 0.1867 0.0425 0.0420 0.0568 0.0513

HOMO->LUMO (94%) HOMO->LUMO (94%) HOMO->LUMO (95%) HOMO->LUMO (96%) HOMO->LUMO (96%) HOMO->LUMO (96%) HOMO->LUMO (96%) HOMO->LUMO (96%)

420 370 382 373 619 587 561 525

0.0948 0.1421 0.1708 0.1925 0.0407 0.0428 0.0636 0.0642

1044 831 916 828 1095 1162 1003 889

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MS MEHNA M3HP M3HPC M3HN M2HQC M3HQC M3HQ

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λads (nm)

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* The absorption peak from experimental study is shown in the bracket.

Figure 6. Simulated absorption (solid line) and emission spectra (dash line) of all compounds computed at the B3LYP/TZVP level 20

3.3 Potential-energy curves (PECs) analysis In this section, PECs were used to reveal ESIPT occurrence for all compounds in term of PT barriers. The PECs of the R1 bond in S0 and S1 states for selected compounds are shown in Figure 7. Those of the others are illustrated in Figure S3. The results of relative PT barriers and calculated energy differences between the normal and tautomer forms in S0 and S1 states of studied

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compounds are summarized in Table 5.

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From Figure 7 and Table 5, the energy of PT reaction rises incredibly with the elongation of the R1 bond in the S0 state for all compounds, resulting in the large barrier in the range of 14.48

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– 20.52 kcal/mol. Moreover, reaction energies on the S0 surface are greatly endothermic. Thus, the PT reaction is mostly like not to occur in the S 0 state for all compounds. For the phenyl skeleton

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group, in the S1 state, the barrierless process of PT reaction for MS is observed with the barrier of

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0.03 kcal/mol. Then, barriers of MEHNA (0.88 kcal/mol), M3HP (2.71 kcal/mol), and M3HPC (1.08 kcal/mol) are moderately increased from that of MS. For the naphthalene skeleton group, M3HN, M2HQC, M3HQC, and M3HQ, their PT barriers of 0.46, 0.66, 0.73, and 1.71 kcal/mol,

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respectively, are slightly larger than that of MS. Moreover, reaction energies on the S1 surface of MS, MEHNA, M3HPC, M3HN, and M2HQC are exothermic while those of M3HP, M3HQC, and M3HQ are slightly endothermic. The PT barrier in the S1 state follows this order: MS <

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M3HN < M2HQC < M3HQC < MEHNA < M3HPC < M3HQ < M3HP. This result is mostly in agreement with the trend of the H-bond strength for studied compounds in the section 3.2. Overall, the PT processes of studied compounds are quite difficult to occur in the S 0 state

because of their high PT barriers (14.48 – 20.52 kcal/mol) and highly endothermic reaction but should be possible in the S1 state due to their low barriers (0.03 – 2.71 kcal/mol) and exothermic

21

or lowly endothermic reaction. In addition, the ESIPT possibility will be confirmed by dynamics

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simulations in the next section.

Figure 7. Calculated potential energy curves in the S0 and S1 states of (a) MS (b) MEHNA (c) M3HN (d) M3HPC (e) M3HQ at B3LYP/TZVP level

22

Table 5 Relative PT barriers and calculated energy differences between the normal and tautomer forms (ΔE = ΔE tautomer - ΔE normal) in S0 and S1 states for all compounds, in kcal/mol.

S1 -0.62 -0.84 0.58 -1.22 -0.16 -0.28 0.49 4.09

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MS MEHNA M3HP M3HPC M3HN M2HQC M3HQC M3HQ

ΔE S0 15.64 17.11 15.66 16.65 20.70 19.27 19.01 22.92

PT barrier S0 S1 14.48 0.03 17.07 0.88 14.94 2.71 15.67 1.08 20.40 0.46 20.52 0.66 16.16 0.73 19.34 1.71

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Compound

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3.5 Dynamics simulations

PT probability (the ratio between the number of ESIPT trajectories and the total number of

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trajectories), and average PT time for each compound are summarized in Table 6. Changes of

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important bond length for the breaking of O−H bond and the forming of O…H bond, energy differences of S0 and S1 states, and changes of torsion angle (C1C2C3O2) for selected compounds

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during PT process were illustrated in Figure 8. For other compounds, their dynamics analyses are

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exhibited in Figure S4.

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Table 6 Summary of excited-state dynamics simulations performed at the TD-B3LYP/TZVP level Probability

39 57 110 38 48 51 130 57

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1.00 0.84 0.56 0.84 1.00 1.00 0.96 0.84

PT time (fs)

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Compound ESIPT Reaction Yes No 25 MS 21 4 MEHNA 14 11 M3HP 21 4 M3HPC 25 M3HN 25 M2HQC 24 1 M3HQC 21 4 M3HQ

From Table 6 and Figure 8, the average PT time of MS is 39 fs with the probability of 1.00.

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This result is in well agreement with the time scale of 100 fs investigated by the femtosecond realtime investigation from Li et al. [36]. The average S1-S0 energy difference is suddenly dropped

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within 50 fs and remains at about 3 eV after the PT process (see the middle panel of Figure 8),

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indicating that there is no additional relaxation process after the ESIPT reaction within simulation time. The average torsion angle changes insignificantly; it is not twisted over 10°, as shown in the

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right panel of Figure 8. Snapshots of the ESIPT reaction from dynamics simulations for MS are illustrated in Figure 9 while the others are in Figure S5 of ESI. For nitrogen substituted compounds, MEHNA, M3HP, and M3HPC show the average PT times of 57, 110, and 38 fs, respectively. Their probabilities of 0.84, 0.56, and 0.84 are decreased from that MS. Their S1-S0 energies are

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dropped to be at around 2 eV. Their average torsion angle does not change after PT reaction. For compounds with the π-extended conjugation, the average PT times of M3HN, M2HQC, M3HQC and M3HQ are 48, 51, 130, and 57 fs, respectively. The PT probability of both M3HN and M2HQC are 1.00, as same as that of MS. Whereas those of M3HQC, and M3HQ are 0.96 and 0.84, respectively, which are slightly lower than that of MS. Average torsion angle and average S1-S0 energy difference are not significant different compared to the other compounds. Overall, 24

the PT probability follows this order: MS = M3HN = M2HQC > M3HQC > MEHNA = M3HPC = M3HQ > M3HP, which is mostly in the accordance with the strength of H-bond of the studied

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compounds. The greater strength of H-bond results in the higher probability of the ESIPT reaction.

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of ro -p re lP ur na Jo Figure 8. Analyses of all ESIPT trajectories for selected compounds; (a) time evolution of average breaking and forming bonds, (b) average torsion angle (C1C2C3O2), and (c) average S1 and S0 energies and average energy diff erence between S 1 and S0 states (S1–S0) 26

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Figure 9. Snapshots of dynamics simulations along with the ESIPT reaction of MS at a different

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time (left: front view, right: side view).

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4 Conclusion Effects of nitrogen substitution and π-conjugation on the strength of H-bond, ESIPT reactions, and photophysical properties of MS derivatives have been investigated using DFT and TD-DFT methods at B3LYP/TZVP level both static and dynamics studies. From the static results, the H-bond strength of all studied compounds is increased upon photoexcitation into the S 1 state,

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confirmed by changes of important bond lengths (the increasing of the covalent O−H bond length, together with decreasing of the H-bond (O…H) and distance between heavy atoms connecting

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proton donor and acceptor), red-shift of the O-H stretching mode, and bond energy from the

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topology analysis. The nitrogen substitution makes the strength of H-bond decrease, while the πconjugation makes it increase. In addition, frontier MOs confirm that the main MOs contribution

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for vertical S0 to S1 transition is attributed to the excitation from HOMO (π) to LUMO (π*). Both HOMO and LUMO energy levels of nitrogen substituted compounds are shifted down leading to

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the slight rising of the HOMO-LUMO energy gap compared to that of MS. Whereas, the energy of the HOMO is increased, and that of LUMO is decreased for compounds with π-extended

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conjugation, resulting in the lowering of the energy gap. Thus, the π-conjugation could modulate HOMO and LUMO energy levels more than nitrogen substitution. For photophysical properties, absorption peaks of nitrogen substituted compounds (MEHNA, M3HP, and M3HPC) show slightly blue-shifted from that of MS, whereas those of compounds with the π-extended

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conjugation exhibit the red-shift compared to their analogs. For emission spectra, the nitrogen substitution leads to the moderate blue-shift in emission peaks of the studied compounds, while the π-conjugation causes more red shifts. The blue-shift of the electronic spectra (both absorption and emission) affected by nitrogen substitution and the red-shift of electronic spectra caused by πconjugation are in accordance with results of HOMO-LUMO energy gap. Moreover, compounds

28

with the π-extended conjugation including M3HN, M2HQC, and M3HQC have large Stokes shifts of about 1000 cm-1, which is beneficial for uses as fluorescent probes. Moreover, potential energy curves along the PT coordinate show that ESIPT processes of all studied compounds easily occur with low PT barriers. Results of PT barriers suggest that nitrogen substitution induces the increasing of PT barriers, but the π-conjugation does not significantly affect PT barriers. From dynamics results, ESIPT reactions of studied compounds are observed in an ultrafast time scale

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(38-130 fs). Moreover, the PT probability of MS is 1.00 whereas probabilities are decreased with

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the small degree for MEHNA (0.84), and M3HPC (0.84), and considerably dropped for M3HP (0.56). For compounds with the π-extended conjugation, probabilities are remarkably high for

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M3HN (1.00), M2HQC (1.00), M3HQC (0.96), and M3HQ (0.84). The decreasing of PT

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probabilities in nitrogen substituted compounds, and the increasing of PT probabilities in compounds with the π-extended conjugation are in a correspondence with results of the H-bond

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strength. If the EHB is larger than 0.0377 a.u., the probability will higher than 0.84. Therefore, the obtained information from this investigation especially for the calculation of photophysical

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properties of compounds with the π-extended conjugation of MS will be very useful as guidance for a design of fluorescent probes with large Stokes shift.

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Conflicts of interest

Authors would like to declare that there are no conflicts of interest related to this work.

Acknowledgements

29

N. Kungwan would like to thank Thailand Research Fund (Grant No. RSA6180044). The authors wish to thank the Center of Excellence in Materials Science and Technology, Chiang Mai University for financial support. A. Watwiangkham would like to thank The Science Achievement Scholarship of Thailand (SAST) for the fellowship. Computational Chemistry Laboratory Chiang Mai University (CCL-CMU), Department of Chemistry, Faculty of Science, and Graduate School,

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Chiang Mai University, Chiang Mai, Thailand are also acknowledged.

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