Microsolvation, hydrogen bond dynamics and excited state hydrogen atom transfer mechanism of 2′,4′-dihydroxychalcone

Journal Pre-proofs Research paper Microsolvation, hydrogen bond dynamics and excited state hydrogen atom transfer mechanism of 2′,4′-dihydroxychalcone Yelechakanahalli Lingaraju Ramu, Kandigowda Jagadeesha, Tavarekere Shivalinga Swamy, Mariyappa Ramegowda PII: DOI: Reference:

S0009-2614(19)31011-5 https://doi.org/10.1016/j.cplett.2019.137030 CPLETT 137030

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Chemical Physics Letters

Received Date: Accepted Date:

11 November 2019 7 December 2019

Please cite this article as: Y.L. Ramu, K. Jagadeesha, T.S. Swamy, M. Ramegowda, Microsolvation, hydrogen bond dynamics and excited state hydrogen atom transfer mechanism of 2′,4′-dihydroxychalcone, Chemical Physics Letters (2019), doi: https://doi.org/10.1016/j.cplett.2019.137030

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Microsolvation, hydrogen bond dynamics and excited state hydrogen atom transfer mechanism of 20 ,40 -dihydroxychalcone Yelechakanahalli Lingaraju Ramu1 , Kandigowda Jagadeesha1 , Tavarekere Shivalinga Swamy1 , Mariyappa Ramegowda1, PG Department of Physics, Govt. College (Autonomous), Mandya - 571401, INDIA

Abstract Electronic structure of 20 ,40 -dihydroxychalcone (DHC) and its water complex DHC-(H2 O)4 (DHCH) at ground and first excited states were investigated by implementing the DFT and SS-TDDFT with EFP method for microsolvation and the NBO method for charge analysis. One intramolecular and four intermolecular hydrogen bonds (HB) exist in the DHCH molecule reinforce the excited state hydrogen atom transfer from the 20 -hydroxyl group to the carbonyl group. The natural charge analysis and potential energy profiles along intramolecular HB path at ground and excited states establish the excited state hydrogen atom transfer mechanism in DHC and DHCH molecules. Keywords: DHC, DHCH, DFT, TDDFT, ESIHT

1. Introduction Chalcones (1,3-diaryl-2-propen-1-ones) belong to the flavonoid, a vital class of natural products with widespread distribution in vegetables, fruits, spices, tea and soy based foodstuff having interesting pharmacological activities [1] such as 5

anti-inflammatory, antimicrobial, antifungal, antioxidant, cytotoxic, antitumor, anticancer and antileishmaniasis activities.[2, 3] A number of chalcone derivatives are used as inhibitors of enzymes in the cellular system. [4, 5] Chalcones

Email address: [email protected] (Mariyappa Ramegowda)

Preprint submitted to Chemical Physics Letters

December 6, 2019

also find applications in agrochemicals and artificial sweeteners.[6] Chalcones having electron-donor and electron-acceptor groups with a π10

conjugated spacer (D-π-A system), classified into symmetrical and asymmetrical D-A-D systems on the basis of electron-donating substituents of the aromatic rings. [7, 8] Hence, these molecules are useful in the development of nonlinear optical systems, ultrafast optical nonlinear materials and as absorption filters due to their intramolecular charge transfer (ICT) nature.[9, 10]

15

Hydroxychalcones are a major part of the naturally occurring chalcones, used as food preservatives due to their radical quenching properties of the phenolic groups present in their aryl rings.[11] Butein is used for the treatment of parasitic infections, cancer and gastritis, as well as a food preservative.[12, 13] Isoliquiritigenin, a liquorice chalcone is used for the treatment of cardiovascular

20

diseases.[14] 20 -hydroxychalcones are suitable for quantitative analysis of metal ions due to their good complexing ability.[15] Although, hydroxychalcones have applications in various fields, studies on their structural properties have been rarely reported. A few theoretical investigations have been performed on radical scavenging activity,[16] redox, confor-

25

mational and antioxidant properties. The influence of hydrogen atom transfer mechanism on radical quenching activity were also demonstarted.[17, 18] The excited state intarmolecular hydrogen atom transfer/proton transfer (ESIHT/ESIPT) reaction is one of the transpire reaction in chemistry and biology.[19, 20, 21, 22] ESIHT/ESIPT is always associated with the intramolec-

30

ular HB that exists between electron donor and acceptor groups. These groups can also form HB with protic solvent molecules. The experimental and theoretical study on ESIHT/ESIPT along with the excited state intra/inter-molecular HB dynamics of many organic and biomolecules via microsolvation have been investigated by various researchers. [23, 24, 25, 26, 27] The solvent polarity de-

35

pendent ESIPT reaction, common and unusual HB dynamics, and mechanism of excited-state double proton transfer (ESDPT) have been studied.[28, 29, 30] These studies reveal the influence of HBs and ESIHT/ESIPT on structure related photophysical properties of the chromophores in the ground and excited 2

states. 40

The aqueous calculations including the models in which few water molecules are treated explicitly, either without or with a surrounding continuum of bulk solvent. The effective fragment potential (EFP) method is one of the reliable method for the explicit treatment of solvent molecules.[31, 32, 33] Two EFP models were developed, namely EFP1 and EFP2. EFP1 method is used to

45

treat water molecules explicitly at the level of DFT, and it can also be implemented along with the TDDFT/polarizable continuum model (PCM) to study the influence of solvent molecules on photophysical properties of biological and organic chromophores at ground and excited states.[34, 35, 36, 24, 25, 26] DHC is one of the promising candidates of hydroxychalcone which is suit-

50

able for chemical optimization to generate a new antileishmanial lead.[37] This molecule contains open-chain flavonoid as shown in the Fig.1, in which the two aromatic rings are connected by a three-carbon α, β-unsaturated carbonyl chain. In the present work, we carry out the computational study on microsolvation, hydrogen atom transfer and effect of microsolvation on H-atom transfer in DHC

55

molecule.

2. Computational methods DHC and its water complex, DHCH were modeled in Avogadro - a free cross platform molecular editor and optimized using MMFF94s force field. The ground state (S0 ) optimizations have been carried out at the level of dendity 60

functional theory (DFT)[38, 39, 40, 41, 42, 43] using Becke’s three-parameter hybrid exchange function, the Lee-Yang-Parr gradient-corrected correlation functional (B3LYP)[44, 45] with Pople’s split-valence double-zeta basis set added with d-polarization functions on non-hydrogen and p-polarization functions for hydrogen atoms [6-31G(d,p)].[46] Using the S0 state optimized coordinates,

65

DHC and DHCH molecules were optimized at excited state (S1 ) by implementing the state specific time dependent DFT (SS-TDDFT) with B3LYP/631G(d,p) method.

The optimization tolerance is set with maximum gradient

3

of 3 × 10−5 Hartee/Bohr and rms gradient < 1 × 10−5 Hartee/Bohr.

All

geometries of the S0 and S1 states were optimized without imposing constraint 70

to any parameter. The frequency calculation has been carried out for both molecules using the optimized geometries of the S0 and S1 states. These calculations produce the frequency spectrum -8.71 cm−1 - 10.01 cm−1 corresponding to translations and rotations of the molecules. Natural charges of the molecules at both ground and excited states were computed using the natural bong or-

75

bitals (NBO)[47] program. All calculations were performed by employing the NBO 6.0 integrated GAMESS-US software suit.[48, 49]

3. Results and discussion 3.1. Electronic structure Optimized structures of the molecules, DHC and its water complex DHCH at 80

ground and excited states calculated using 6-31G(d,p)/B3LYP method are presented in Fig. 1. In DHC molecule, an intramolecular HB, O17 −H29 ···O16 −C9 exist between hydrogen of the 20 -hydroxyl group and oxygen of the carbonyl group. In addition to this intramolecular HB, 20 -hydroxyl and carbonyl groups constitute two HBs with water molecules; one from carbonyl oxygen and other

85

from 20 -hydroxyl oxygen. The 40 -hydroxyl group also forms two HBs with water molecules; one of its hydrogen and other from its oxygen. On observing the structural parameters (tables are added in supplementary information (SI)) of DHC and DHCH at ground state, no appreciable changes occur due to microsolvation except the intramolecular HB, which contracts by 0.027 ˚ A.

90

In the excited state, the structural parameters of the both DHC and DHCH molecules slightly vary. The bond lengths C1-C6, C5-C6, C7-C8, C9-C10, C9O16, C11-C12, C12-C13, C14-C15 elongates (∼ 10−2 ˚ A), C6-C7, C8-C9, C10C15, C11-O17, C13-C14 contracts (∼ 10−2 ˚ A) and the other bond lengths vary ∼ 10−3 ˚ A. The HB, O17 −H29 ···O16 −C9 modified as O16 −H29 ···O17 −C11 due

95

to tranfer of the hydrogen atom H29 from 20 -hydroxyl group to the carbonyl group, contracted by 0.083˚ A in DHC and 0.029˚ A in DHCH molecule. The bond

4

angles do not change appreciably except the bond angles associated with the 20 -hydroxyl group, carbonyl group and its neighbouring atoms, which were alter in the range 0 − 7◦ . 100

3.2. Hydrogen bond dynamics The intramolecular and inter-molecular HBs in DHC and DHCH molecules at the ground and excited states were calculated at DFT/TDDFT level by implementing 6-31G(d,p)/B3LYP method. In S1 state of DHC and DHCH molecules, negative charge on O16 upsurge by -0.032e, whereas on O17 it is downsurge

105

by -0.045e and -0.028e respectively. As the consequence HB O17 −H29 ···O16 −C9 is modified as O16 −H29 ···O17 −C11 due to the transfer of H29 atom from 20 hydroxyl group to the carbonyl group, contracted by 0.086˚ A in DHC and 0.020˚ A in DHCH molecules respectively. The contraction of HB in the DHCH molecule is considerably lesser as compared with the DHC molecule, and it indicates the

110

influence of intermolecular HBs on the carbonyl and hydroxyl groups at the S1 state. In the ICT state of the molecules, the hydrogen atom transfer from 20 hydroxyl group to carbonyl group, results in the deportation of the O16 atom to hydroxyl and O17 atom to carbonyl nature. The intermolecular HB C11 −O17 ···H32 −O31

115

gets contracted by 0.049˚ A and C9 −O16 ···H35 −O34 elongated by 0.051˚ A. The charge on O18 is decreased by -0.008e/-0.013e and on H30 increased by 0.006e/0.017e in DHC/DHCH molecule, which causes to increase the HB C13 −O18 ···H38 −O37 by 0.088˚ A and decrease the HB O18 −H30 ···O40 by 0.046˚ A. Among the two HBs exist between water molecules; one of HB O34 −H36 ···O31 is decreased by 0.016˚ A

120

and the other O40 −H42 ···O37 is increased by 0.002˚ A. 3.3. Natural charge analysis and ESIHT For both molecules, DHC and DHCH molecular electrostatic potential (MEP), molecular orbitals (HOMO and LUMO), difference charge density (ρ) were computed by implementing 6-31G(d,p)/B3LYP method, which were found to be

125

similar and the plots of DHCH are depicted in Fig. 2 and 3. The red and blue

5

regions of the orbitals correspond to ρ− and ρ+ , respectively. In both molecules, upon excitation, the charge density increment zones (blue) are established on the C1, C5, C9, C10 and O18 atoms, whereas the regions of density depletion (red) are localized on the C3, C7, C14, C15, O16 and O17 atoms, and O16 atom 130

exibit more negative charge than the O17 atom. The natural charges on various groups and atoms have been reckoned and presented in the Fig. 2 and Table 1. The plot of electrostatic potential at ground and excited states of both the molecules indicate the electrophilic reaction nature of the hydroxyl and carbonyl groups. The benzene rings and alkyl chain shows the tendency to nucleophillic

135

reaction. In the ground state, natural charges on both rings exhibit positive nature and alkyl chain shows negative nature. On excitation of the molecules the benzene ring with the hydroxyl and carbonyl groups turns into negative, the other benzene ring shows slightly negative and the alkyl chain becomes positive due to the transfer of hydrogen atom from phenyl ring to alkyl chain.

140

In the S1 state of DHC and DHCH molecules, the negative charge on carbonyl oxygen is raised by -0.032e, whereas negative charge on hydroxyl oxygen is lowered by -0.045e and -0.028e respectively. The charge on hydroxyl hydrogen is abated by +0.002e on both the molecules. On the combined effect, the intramolecular re-arrangement of the charges occurs, the hydroxyl hydrogen

145

transfers to the carbonyl oxygen. 3.4. UV/Vis Electronic spectra The absorption energies of the DHC and DHCH molecules were assessed in gas phase, and with water and ethanol as solvents using SS-TDDFT/631G(d,p)/B3LYP/CPCM/EFP1 method. The absorption energies, oscillator

150

strength and probable wave functions are presented in Table 2. The plot of absorption wavelength with oscillator strength is displayed in Fig. 4. The absorption wavelengths were found to be 370.6 nm, 370.0 nm, 370.5 nm for DHC and 362.7 nm, 371.5 nm, 371.8 nm for DHCH in gas phase, water and ethanol respectively. The computed absorption wavelength in ethanol found to be in

155

good agreement with the experimental value 349.0 nm [50]. The blue shift of 6

7.9 nm in gas phase of DHCH indicate the strengthening of HBs in the excited state. 3.5. ESIHT mechanism The charge transfer state of DHC and DHCH is strongly stabilised by the 160

transfer of hydrogen atom from 20 -hydroxyl group to the carbonyl group with the modification of HB O17 −H29 ···O16 −C9 to O16 −H29 ···O17 −C11 . The S0 and S1 state electronic potential energy surface scans of the hydrogen atom transferred path in gas phase have been done with SS-TDDFT/6-31G(d,p)/B3LYP/EFP1 method. The plots of variation of potential energy along the O-H bond in

165

DHC and DHCH molecules are presented in the Fig. 5. The DHC and DHCH molecules can exist in unrelaxed first excited state (S∗1 ) by irradiating with the radiation of 3.35 eV and 3.42 eV, the O17 −H29 bond stretches to 1.62˚ A and 1.59˚ A respectively, the H17 atom detached from O17 and bonded to O16. This is the point at which ESHT occurs, the DHC and DHCH molecules stabilises

170

in the relaxed first excited state, S1 with O16 −H29 bond length of 1.040˚ A and 1.023˚ A respectively. From the Fig.5, we can interpret theoretically that the fluorescence may occur with the emission of 1.82eV and 1.75eV energy, the molecules de-excited to unrelaxed ground state, S∗0 . At this state the O16 −H29 bond length in DHC and DHCH molecules again stretches to 1.44˚ A and 1.52

175

˚ A respectively, where the deportation of H29 atom from O16 to O17 occurs and the molecules return to their ground state. The observation can be made that the OH bond stretching at HT point is 0.017˚ A lesser at S1 state and 0.007˚ A greater at S0 state in DHCH as compared to the DHC molecule due to the influence of intermolecular HBs.

180

4. Conclusion The electronic structure studies of DHC and its water complex DHCH molecules in both ground and ICT state reveal the structure of DHC, inter and intra molecular HBs, and their dynamics due to the molecular excitation. The studies of

7

molecular orbitals along with the natural charges in both S0 and S1 states sup185

ports the tendency of hydrogen atom transfer from 20 -hydroxyl group to the carbonyl group. The detailed examination of the the absorption spectra and energy profile along the HT path in S0 and S1 states affirm the ESIHT mechanism in both DHC and DHCH molecules and the effect of intermolecular HBs on OH bond stretching. This study contributes to the ongoing research on the

190

chemical and biological activity of DHC molecule.

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280

UDC547.824.03.04.

11

(a)

(b)

(c)

12 (d) Figure 1: Optimized molecular structure of DHC and DHCH molecules at S0 ((a) and(b)) and at S1 ((c) and (d)).

DHCH at S1

DHCH at S0 Figure 2: Molecular electrostatic potential with charges on various atoms and groups for DHCH at S0 and S1 states.

13

LUMO+1

LUMO

HOMO

HOMO-1

∆ρ 14

Figure 3: Molecular orbitals and difference electron density map of DHCH

0.90

DHC-Gas Phase 0.80

0.70

0.60

DHC-Water DHC-EtOH DHCH-Gas Phase DHCH-Water DHCH-EtOH

f

0.50

0.40

0.30

0.20

0.10

0.00 350

355

360

365

370

375

380

385

390

λ (nm) Figure 4: Simulated absorption spectra of DHC and DHCH molecules in gas phase, water and ethanol.

15

5

4 S*1 S1 3

E (eV)

HT:O17−>O16

1.82 2

3.35

*

1

S0 S0 HT:O16−>O17

0

0.8

1

1.2

1.4

1.6

1.8

2

rO−H (Å)

(a) 5

4 *

S1 S1

E (eV)

3

HT: O17−>O16

1.75 2 3.42

1

S*0 S0 HT: O16−>O17

0

0.8

1

1.2

1.4

1.6

1.8

2

rO−H 16 (Å)

(b) Figure 5: Gas phase energy profiles of the O-H bond (O17/16 −H29 ) in (a) DHC and (b) DHCH molecules at ground and excited states.

Table 1: Natural charges on various atoms of DHC and DHCH molecules at S0 and S1 states

DHC Atom

DHCH

S0

S1

S0

S1

C1

-0.200

-0.203

-0.195

-0.201

C2

-0.237

-0.243

-0.235

-0.241

C3

-0.224

-0.236

-0.222

-0.238

C4

-0.236

-0.240

-0.237

-0.241

C5

-0.201

-0.212

-0.201

-0.212

C6

-0.089

-0.104

-0.090

-0.100

C7

-0.137

-0.141

-0.121

-0.144

C8

-0.304

-0.379

-0.309

-0.374

C9

0.516

0.485

0.525

0.468

C10

-0.244

-0.280

-0.238

-0.293

C11

0.416

0.400

0.402

0.403

C12

-0.386

-0.227

-0.365

-0.162

C13

0.372

0.339

0.379

0.359

C14

-0.323

-0.336

-0.345

-0.385

C15

-0.172

-0.078

-0.168

-0.052

O16

-0.633

-0.665

-0.666

-0.698

O17

-0.693

-0.648

-0.738

-0.710

O18

-0.681

-0.672

-0.755

-0.742

H19

0.244

0.237

0.248

0.239

H20

0.246

0.241

0.246

0.242

H21

0.244

0.239

0.243

0.238

H22

0.245

0.240

0.244

0.238

H23

0.237

0.231

0.237

0.229

H24

0.255

0.245

0.253

0.241

H25

0.224

0.229

0.226

0.225

H26

0.248

0.245

0.257

0.255

H27

0.255

0.259

0.256

0.261

H28

0.238

0.249

0.238

0.247

H29

0.524

0.522

0.526

0.524

H30

0.498

0.504

0.605

0.623

17

Table 2: Absorption spectral wave length (S0 → S1 ), λa (nm), oscillator strength, f and assignment of the electronic excitations of DHC and DHCH molecules in gas phase, water and ethanol. The transitions with considerable f around the main peak are presented.

Molecule DHC

DHCH

Solvent/Gas phase

λa (nm)

f

Gas phase

370.6

0.306

H→ L(0.974), H-1→ L(-0.163), H-2→ L(0.141)

Water

370.0

0.561

H→ L(0.981), H-1→ L(0.123), H-2→ L(-0.128)

Ethanol

370.5

0.566

H→ L(0.983), H-1→ L(-0.115), H-2→ L(-0.126)

Gas phase

362.7

0.414

H→ L(-0.936), H-1→ L(-0.303), H-2→ L(0.157)

Water

371.5

0.670

H→ L(-0.966), H-1→ L(0.207), H-2→ L(0.139)

Ethanol

371.8

0.679

H→ L(0.968), H-1→ L(-0.199), H-2→ L(-0.138)

18

Wave function (excitation amplitude)



One intra and 4 inter-molecular HBs exist in microsolvated 2’,4’-dihydroxychalcone.



ESIHT occurs along the intramolecular HB.



Potential energy profiles can be used as a useful tool for studying ESIHT mechanism.