EFP1 study

Accepted Manuscript Excited state hydrogen atom transfer in micro-solvated dicoumarol: A TDDFT/EFP1 study

Kandigowda Jagadeesh, Yelechakanahalli Lingaraju Ramu, Mariyappa Ramegowda, Neratur K. Lokanath PII: DOI: Reference:

S1386-1425(18)30942-9 doi:10.1016/j.saa.2018.10.015 SAA 16521

To appear in:

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy

Received date: Revised date: Accepted date:

22 August 2018 1 October 2018 11 October 2018

Please cite this article as: Kandigowda Jagadeesh, Yelechakanahalli Lingaraju Ramu, Mariyappa Ramegowda, Neratur K. Lokanath , Excited state hydrogen atom transfer in micro-solvated dicoumarol: A TDDFT/EFP1 study. Saa (2018), doi:10.1016/ j.saa.2018.10.015

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ACCEPTED MANUSCRIPT Excited state hydrogen atom transfer in micro-solvated dicoumarol: A TDDFT/EFP1 study Kandigowda Jagadeesha, Yelechakanahalli Lingaraju Ramua, Mariyappa Ramegowdaa,* Neratur K Lokanathb

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[a] PG Department of Physics, Govt. College (Autonomous), Mandya - 571401, INDIA.

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[b] DOS in Physics, University Mysore, Mysore – 570006, INDIA.

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* E-mail: [email protected].

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Highlights:

1. In dicoumarol molecule each 4-hydroxycoumarin moieties lie in a plane, twisted by an

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angle of 180º with respect to each other and stabilized with the formation of two intramolecular hydrogen bonds C22-O32····H17-O11-C6 (1.737 Å) and C2O12····H37O31

group of the other.

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(1.651 Å) between the carbonyl group of one 4-hydroxycoumarin moiety to the hydroxyl

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2. Four inter-molecular hydrogen bonds established in the micro-solvated dicoumarol molecule with three water molecules; two HBs by C2O12····H37O31 group with one

molecules.

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water molecule and two hydrogen bonds by C22O32····H17O11 group with two water

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3. The intra-molecular charge transfer state of dicoumarol and its water complex is well stabilized by transfer of the hydrogen atom H17 with the contrariwise modification of carbonyl oxygen to hydroxyl oxygen and the transformation of intra-molecular hydrogen bond from C22=O32····H17-O11-C6 to C22-O32-H17····O11=C6 with the increase of 0.174 Å and 0.650 Å in dicoumarol and its water complex respectively.

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ACCEPTED MANUSCRIPT Abstract Ground (S0) and excited (S1) state properties of dicoumarol (DC) are investigated by applying density functional theory (DFT) and time dependent DFT (TDDFT) interfacing with the effective fragment potential (EFP) method of solvation. Benzene and pyrone rings of the each 4-hydroxy coumarin (4HC) moiety are in a plane and these planes are twisted by 180º with respect to each

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other. Two intra-molecular hydrogen bonds (HB) C=O∙∙∙H-O exist between the carbonyl (C=O)

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and hydroxyl (O-H) groups of different 4HC moieties (4HC-1 and 4HC-2). DC-(H2O)3 complex is formed using the original EFP model (EFP1). Four inter-molecular HBs are established by the

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carbonyl and hydroxyl oxygen atoms of 4HC-1 and 4HC-2 moieties; two HBs with two solvent molecules on one side of the complex and other two HBs with one solvent molecule at the other

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side. In S1 state, the hydrogen atom transfer takes place only from the hydroxyl group of 4HC-1 to the carbonyl group of 4HC-2. The natural charge analysis and the modification of HBs manifest

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the intra-molecular charge transfer (ICT) from one 4HC moiety to another. Theoretical and experimental studies of the absorption spectra, and the theoretical study of potential energy curves

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of O-H bonds at both S0 and S1 states affirm the hydrogen atom transfer from the hydroxyl group

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of 4HC-1 to the carbonyl group of 4HC-2 moiety.

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ESIHT.

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Keywords: DC, DC-(H2O)3 complex, DFT, TDDFT, EFP1, B3LYP, cc-pVDZ, 6-31G(d,p),

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ACCEPTED MANUSCRIPT Introduction Quantum theory of atoms in molecules (QTAIM) is one of the powerful tools in studies of intramolecular and inter-molecular interaction in HB complexes at the ground and excited states [1-3]. Many organic and biological molecules with donor-acceptor nature can form HB with protic solvent molecules, plays an important role in the photo-physical and biological activities at S0 and

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S1 states [4-14]. The HB interaction of solvent molecules with many organic and biological

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chromophores at S0 and S1 states was investigated extensively, both theoretically and

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experimentally by a number of researchers [15-27]. The structure and photo-physical properties of chromophores are also influenced by HBs [28-38]. The EFP method is a useful method for the micro-solvation of organic and biological molecules. In two EFP methods EFP1 and EFP2, EFP1

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was developed explicitly for solvation with a discrete number of water molecules [39-42]. EFP1 model was implemented with the polarisable continuum model (PCM) and TDDFT to study the

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effect of solvents on photo-physical properties of organic and biological molecules in both ground and excited states [43-45]. The irradiation of organic and biological molecules induces intra-

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molecular proton transfer/ hydrogen atom transfer at excited states (ESIPT/ESIHT). Solvation of organic and biological molecules with the polar solvents may strongly affect the ESIPT/ESIHT

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process, resulting in the modification of the hydrogen bond structure. The intra-molecular rearrangement of atoms along with the ESIHT has been comprehensively studied, especially in the

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photo-induced reactions [15,46-52].

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Coumarin derivatives present a variety of biological and medical activities [53-55] having excellent thermal stability, outstanding optical properties and superior photo-stability. These

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derivatives are used extensively as laser dyes, fluorescent probes, nonlinear optical chromophores, fluorescent whiteners and solar energy collectors [56-61]. Hydroxy coumarins have been exploring for their optical applications; laser dyes, fluorophores, chromophores, colorants [68-71] and biological applications; anticarcinogens, antifungals, spasmolytic and sun-screening additives, and enzyme inhibitors [72-75]. Dicoumarol is a 4HC derived naturally occurring anticoagulant, acts as a potent inhibitor of NAD(P)H: quinone oxidoreductase 1 (NQO1), binds to the oxidized form of the enzymes and affects the growth of tumor cells [62-64]. Dicoumarol treatment to human myeloid leukemia HL-60 cells increases the superoxide levels, essential for inhibition of G0/1 3

ACCEPTED MANUSCRIPT blockade [65]. The conformation of dicoumarol in various tautomeric forms and ionisation states in the gas phase and aqueous solutions were studied [66-75]. The benzene and pyrone rings of the each 4HC moiety are in a plane twisted by 180º with respect to each other planes of 4HC moieties, stabilized with the formation of two intra-molecular HBs CO····HO between the CO group of one 4HC

moiety to the OH group of the other. Dicoumarol can soluble in aqueous alcohols, pyridine, pyridine water mixture and can form hydrogen bonds only with water molecules as shown in Fig.

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1(a) and Fig. 1(b). Dicoumarol can form four inter-molecular HBs with solvent molecules; each

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from oxygen atom of carbonyl and hydroxyl groups of 4HC moieties as shown in Fig. 2. Computational methods

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The Avogadro package [76] is used to model the DC molecule with two pyridine molecules at the HB sites (C=O, O-H) and optimized using the MMFF94s force field. The pyridine molecules, as

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they are aprotic, were not established any HB with DC molecule. Then, few water molecules were added at the hydrogen bonding sites and optimized in the same force field. Four inter-molecular

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HBs were formed by the native chromophore with the three water molecules; two HBs by C2O12····H37O31 group with one water molecule and two HBs by C22O32····H17O11

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group with two water molecules. Further, the number of water molecules were added and optimized. In the cluster of two pyridine and water molecules, only four inter-molecular HBs were

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constituted by the DC chromophore with three water molecules. DC-(H2O)3 complex is separated from the cluster, and EFP1 method is applied to treat the water molecules for micro-solvation. The

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DC-(H2O)3 complex is optimized at the level of DFT/B3LYP [77-82] using the cc-pVDZ basis set [83]. Starting with the ground state geometries excited state optimization had been carried out by

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implementing the LR-TDDFT/B3LYP [84-91] method. The vertical transition energies were calculated using SS-TDDFT/CPCM/EFP1/PBE0/6-31G(d,p) method [92-96]. The variation of potential energy across the O-H bonds is also studied at the level of DFT/CAMB3LYP [97] in both S0 and S1 states. The NBO 6.0 [98] integrated GAMESS-US software [99,100] is used to perform all calculations.

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ACCEPTED MANUSCRIPT Results and discussion Electronic structure in S0 and S1 state The lowest energy state optimized structure of the dicoumarol-water complex is presented in Fig. 2 and the optimized parameters of DC and DC-(H2O)3 complex at S0 and S1 states are presented in Table 1. Each 4HC moiety of DC-(H2O)3 complex are lying in a plane, twisted by an angle of

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180º with respect to the planes of each other moieties and stabilized with the formation of two

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intra-molecular HBs C22-O32····H17-O11-C6 (1.737 Å) and C2O12····H37O31 (1.651 Å)

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between the carbonyl group of one 4HC moiety to the hydroxyl group of other 4HC moiety. In DC molecule the planes of 4HC moieties are bent at the CH2 group by an angle 57.19º and increased

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by 1.53º in DC-(H2O)3 complex due to the formation of inter-molecular HBs. In DC-(H2O)3 complex the intra-molecular HB C22-O32····H17-O11 increased by 0.070 Å

and

C2=O12····H37-O31 bond length decreased by 0.011 Å as compared to DC molecule. Four inter-

C2O12····H37O31

group

with

one

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molecular HBs were formed by the DC with three water molecules; two HBs by water

molecule

and

two

other

HBs

by

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C22O32····H17O11 group with two water molecules. Intra-molecular and inter-molecular HBs

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length of DC and DC-(H2O)3 complex at S0 and S1 states are given in Table 2. In S1 state, angle between the planes of 4HC moieties of DC molecule and DC-(H2O)3 complex

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decreased by 0.53º and 1.64º respectively, and structure of the coumarin moieties does not change appreciably. The hydrogen atom H17 transfer from one coumarin moiety to another by changing

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the CO and intra-molecular HB structure from C22=O32····H17-O11-C6 to C22-O32H17····O11=C6 with the increase of HB by 0.174 Å and 0.650 Å in DC molecule and DC-(H2O)3

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complex respectively. The excited state hydrogen atom transfer in the DC-(H2O)3 complex modified the inter-molecular HB from C6-O11-H17····O41 to C22-O32-H17····O41 with the decrease of bond length by 0.794 Å. All other intra-molecular and inter-molecular HBs increase in the S1 state. Intra-molecular charge transfer nature Natural charge analysis has been carried out at S0 and S1 states using NBO 6.0 package integrated with GAMESS-US software at the level of DFT/TDDFT/B3LYP/cc-pVDZ along with the 5

ACCEPTED MANUSCRIPT molecular electrostatic potential (MEP). The natural charges on various atoms were presented in Table 3 and MEP along with the charges on various atoms and groups/rings were depicted in Fig. 3. The positive potential regions were confined on pyrone rings of the coumarin moieties, and negative potential regions were confined to the hydroxyl and carbonyl groups of DC molecule indicating the reactive sites for electrophilic attack. Upon excitation of the DC molecule the natural charge on O11, H17 reduced by -0.117e, 0.007e respectively and increased by -0.055e on O32,

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whereas the natural charge on O12, H37 decreased by -0.097e, 0.017e respectively, and increased

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by -0.011e on O31. In the S1 state, the natural charge on hydroxyl oxygen (O11) of the

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O11/H17/O32 group decreases and on the carbonyl oxygen (O32) increases, whereas in the O12/H37/O31 group the natural charge on hydroxyl oxygen (O31) increases and on the carbonyl

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oxygen (O12) decreases. This indicates that the possibility of hydrogen atom transfer only in the O11/H17/O32 group with the modification of hydrogen bond structure from C22=O32····H17-

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O11-C6 to C22-O32-H17····O11=C6. The charge on pyrone rings shows positive nature and on the benzene ring shows negative nature at both S0 and S1 states; the net effect on the pyrone and

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benzene ring shows a positive potential region. The highest occupied molecular orbitals (HOMO), lowest un-occupied molecular orbitals (LUMO) and the difference charge density () due to

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excitation of the chromophore were calculated and displayed in Fig. 4. These molecular orbital plots indicating the charge transfer from coumarin moiety 2 to 1 as a result of molecular excitation.

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Thus, the molecular excitation of the dicoumarol chromophore is due to the intra-molecular charge transfer from the one coumarin moiety to other.

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Absorption spectral study

The pure dicoumarol sample was purchased from Sigma-Aldrich, dissolved in pyridine, diluted

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with water. The absorption spectral study was carried out using Labtronics LT-291 UV-Vis spectrometer with pyridine and pyridine-water mixtures as solvents. The dielectric constants () of the solvent mixtures were determined using the BI-870 dielectric constant meter. The simulation of absorption spectra of DC and DC-(H2O)3 complex, in the gas phase, water, pyridine, methanol and pyridine-water mixture (1:115ml) were performed at the level of TDDFT/PBE0/6-31G(d,p). The custom solvent model for 1:115ml pyridine-water mixture was created by employing the measured static dielectric constant, =77.1 and dielectric constant at microwave frequency, ∞=4. The theoretical absorption wavelengths, in the gas phase, water, pyridine, methanol and pyridine6

ACCEPTED MANUSCRIPT water mixture with corresponding oscillator strengths are presented in Table 4, the experimental and simulated spectra are displayed in Fig. 5. The experimental study with pyridine solvent reveals that the absorption energy above 3.50 eV becomes maximum and the spectrum turn into flat. Hence, the experiment was re-conducted with pyridine-water mixtures of 1:50 ml and 1:115 ml, and the absorption peaks were observed at 3.76 and 3.87 eV respectively. From these experimental data, it can be observed that the increasing dilution with water, absorption peak produces a blue shift.

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The calculated values of ab (4.20 and 3.96 eV) in methanol and pyridine-water mixture (1:115 ml)

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are in good agreement with the experimental values, 4.27 eV [101] and 3.87 eV respectively. Excited state hydrogen transfer

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The S1 state optimization of DC and DC-(H2O)3 complex in the gas phase at the level of LRTDDFT/B3LYP using cc-pVDZ reveals the transfer of hydrogen atom H17 from 4HC moiety 1 to

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2. The ICT state of the DC molecule and DC-(H2O)3 complex is well stabilized by the transfer of hydrogen atom H17 by changing the CO and intra-molecular HB structure from C22=O32····H17-

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O11-C6 to C22-O32-H17····O11=C6 with the increase of bond length by 0.174 Å and 0.650 Å in DC molecule and DC-(H2O)3 complex respectively. To substantiate the hydrogen atom transfer,

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the potential energy profiles of the OH bond at S0 and S1 (hydrogen transfer) state of the DC(H2O)3 complex have been carried out using the LR-TDDFT/CAM-B3LYP/cc-pVDZ method and,

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the relevant potential energy curves are depicted in the Fig. 6. The DC molecule excited to S1* state by the absorption of 4.00 eV, the OH bond length increases to 1.34 Å, where the energy

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crossing occurs. At this conical intersection, the hydrogen atom transferred from 4HC moiety 1 to 2, and strongly stabilized in the ICT state (S1) of the DC-(H2O)3 complex at 3.34 eV above the

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ground state. The potential energy curve also shows that the S0* state of the molecule lies above 2.00 eV from the ground state. The experimental value of em in methanol is found to be 3.30 eV [93], indicates the probable downward transition may be from S1 to S0 (~ 3.34 eV in the gas phase). Conclusion The computational studies at S0 and S1 states of DC molecule and its water complex using TDDFT/EFP1 method reveal the structure of dicoumarol, especially the arrangement of 4HC moieties, intra-molecular HB and inter-molecular HB with water molecules. The NBO analysis and absorption spectral studies both theoretical and experimental support the existence of ICT state 7

ACCEPTED MANUSCRIPT of the DC molecule and DC-(H2O)3 complex. The ICT state of both DC molecule and DC-(H2O)3 complex is well stabilized by the transfer of hydrogen atom from one coumarin moiety to other. These spectral and structure property studies support the researchers for the investigation of dicoumarol as one of the potent inhibitor of NAD(P)H. Acknowledgement

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This work was supported by the University Grants Commission, India under the research project

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No. 1502-MRP/14-15/KAMY022/UGC-SWRO.

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R. Polacek, P. Majek, K. Hrobonova, J. Sadecka, J. Fluoresc., 2015, 25, 297.

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Fig. 1. S0 state optimized structure of (a) DC with pyridine molecules and (b) DC with water and pyridine molecules.

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(a)

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Fig. 2. Optimized structure of DC-(H2O)3 complex at (a) S0 and (b) S1 states.

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(a)

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(b) Fig. 3. Molecular electrostatic potential of DC at (a) S0 and (b) S1 states in complex with natural charges on various atoms and groups.

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Fig. 4. Frontier molecular orbitals of DC-(H2O)3 complex.

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(c)

Fig. 5. Absorption spectra of DC and DC-(H2O)3 complex in gas phase and in various solvents (a) Experimental, (b) and (c) Simulated. 19

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Fig. 6. Potential energy profiles of the hydrogen atom transfer from C22-O32-H17····O11=C6 (4HC-1) to C22=O32····H17-O11-C6 (4HC-2).

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ACCEPTED MANUSCRIPT Table 1. Selected bond lengths, r (Å ) and bond angles, A(º) in the S0 and S1 state

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S1 1.459 1.453 1.509 1.365 1.221 1.377 1.402 1.398 1.476 1.406 1.241 1.390 1.405 1.393 1.527 1.370 1.435 1.361 1.334 1.388 1.421 1.384 1.417 1.422 1.366 1.393 1.399 1.409 0.974 0.977 120.5 119.5 116.2 121.3 121.8 122.4 121.2

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S0 1.438 1.379 1.514 1.370 1.238 1.369 1.406 1.397 1.452 1.410 1.332 1.390 1.406 1.392 0.999 1.514 1.437 1.378 1.372 1.237 1.370 1.407 1.399 1.451 1.409 1.334 1.389 1.407 1.393 1.006 119.8 119.5 119.7 124.4 121.2 121.6 121.3

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S1 1.457 1.454 1.506 1.369 1.222 1.377 1.403 1.398 1.471 1.408 1.243 1.389 1.406 1.393 1.525 1.370 1.435 1.360 1.343 1.389 1.421 1.385 1.418 1.421 1.360 1.393 1.400 1.408 0.973 0.981 120.5 119.5 116.4 120.9 121.5 122.6 121.1

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S0 1.440 1.379 1.512 1.374 1.233 1.369 1.407 1.399 1.453 1.409 1.330 1.390 1.406 1.392 1.004 1.513 1.439 1.380 1.375 1.233 1.368 1.407 1.398 1.454 1.409 1.330 1.390 1.406 1.392 1.004 120.4 119.1 119.2 124.6 121.2 121.8 121.1

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r/A R(1-2) R(1-6) R(1-18) R(2-3) R(2-12) R(3-4) R(4-5) R(4-10) R(5-6) R(5-7) R(6-11) R(7-8) R(8-9) R(9-10) R(11-17) R(18-21) R(21-22) R(21-26) R(22-23) R(22-32) R(23-24) R(24-25) R(24-30) R(25-26) R(25-27) R(26-31) R(27-28) R(28-29) R(29-30) R(31-37) R(17-32) A(2-1-6) A(1-2-3) A(1-6-5) A(1-6-11) A(2-3-4) A(3-4-5) A(5-4-10)

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119.6 119.0 119.0 120.5 119.7 120.8 123.4 128.6 119.9 119.7 112.3 120.7 122.4 118.1 117.1 119.4 120.6 120.9 119.6

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119.4 119.1 118.9 120.3 119.7 120.9 123.2 127.6 119.2 119.4 109.9 121.1 122.2 118.3 117.3 119.4 120.5 120.9 119.7

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A(4-5-6) A(4-5-7) A(4-10-9) A(5-7-8) A(6-11-17) A(7-8-9) A(8-9-10) A(21-22-23) A(21-22-32) A(21-26-25) A(22-23-24) A(22-32-17) A(23-24-25) A(25-24-30) A(24-25-26) A(24-25-27) A(24-30-29) A(25-27-28) A(27-28-29) A(28-29-30)

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Table 2. Intra-molecular and inter-molecular HB lengths of DC and DC–(H2O)3 complex at S0 and S1 States r(Å)

DC

S0

S1

1.737

1.666

1.892

C6-O11-H17····O41

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C22-O32-H17····O41

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C22=O32····H45-O44

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O44····H43

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C6=O11····H42-O41

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C26-O31····H40-O38

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C2=O12····H37-O31

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2.387

1.651

1.890

2.481

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1.687 1.988

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1.798

1.841

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2.243

2.283

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2.062

2.078

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2.607

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1.841

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S1

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1.667

C22-O32-H17····O11=C6 C26-O31····H37-O12=C2

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C22=O32····H17-O11-C6

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S1 0.026 0.806 -0.510 0.344 -0.188 0.460 -0.163 -0.240 -0.190 -0.251 -0.556 0.596 -0.743 0.251 0.238 0.239 0.250 -0.518 0.302 0.260 -0.245 0.700 -0.514 0.334 -0.154 0.327 -0.241 -0.229 -0.246 -0.280 0.239 0.229 0.229 0.377 -0.721 0.512 -0.590

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S0 -0.229 0.811 -0.497 0.351 -0.189 0.440 -0.182 -0.236 -0.201 -0.250 -0.691 0.530 -0.698 0.250 0.237 0.238 0.251 -0.499 0.321 0.255 -0.232 0.812 0.501 0.354 -0.156 0.439 -0.183 -0.232 -0.198 -0.256 0.249 0.239 0.239 0.249 -0.707 0.525 -0.683

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S1 0.020 0.761 -0.507 0.345 -0.194 0.474 -0.156 -0.237 -0.181 -0.251 -0.571 0.518 -0.709 0.257 0.243 0.242 0.255 -0.508 0.272 0.268 -0.252 0.690 -0.505 0.331 -0.152 0.339 -0.233 -0.230 -0.244 -0.278 0.242 0.229 0.229 0.240 -0.700 0.508 -0.556

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C1 C2 O3 C4 C5 C6 C7 C8 C9 C10 O11 H17 O32 H13 H14 H15 H16 C18 H19 H20 C21 C22 O23 C24 C25 C26 C27 C28 C29 C30 H33 H34 H35 H36 O31 H37 O12

S0 -0.242 0.805 -0.499 0.350 -0.161 0.440 -0.179 -0.238 -0.202 -0.254 -0.688 0.525 -0.654 0.253 0.237 0.237 0.249 -0.478 0.259 0.259 -0.242 0.806 -0.501 0.353 -0.159 0.438 -0.181 -0.237 -0.201 -0.256 0.254 0.238 0.238 0.249 -0.689 0.525 -0.653

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ACCEPTED MANUSCRIPT Table:4 Absorption energy (S0S1), Ea (ev), oscillator strength, f and assignment of the electronic excitations of DC-(H2O)3 molecule in gas phase, water and pyridine. Selected transitions with sufficient oscillator strength around the main peak are included.

f

Wave function (excitation amplitude)

4.19

0.267 H→L (0.978), H-1→L+1 (0.132)

Water

4.20

0.050 H→L (0.971), H-1→L+1 (0.133)

Pyridine

4.17

0.417 H→L (0.643), H→L+1 (0.750)

Methanol

4.20

0.053 H→ L (0.969), H→L+1 (0.145)

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3.96

Gas Phase

4.17

Water

4.20

Pyridine

4.18

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0.394 H→ L (0.440), H→L+1 (0.882)

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0.123 H→L (0.971), H-1→L (0.165)

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Ea (eV)

0.131 H→L (0.966), H→L+1 (0.169) 0.264 H→L (0.938), H→ L+1 (0.298)

4.20

0.134 H→L (0.965), H→ L+1 (0.17)

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0.310

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Solvent / gas phase

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H→ L (0.805), H→L+1 (0.562) H-1 →L+1 (0.115)

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Graphical Abstract

The S0 and S1 state properties of the dicoumarol and its water complex have been studied using the

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DFT/TDDFT combined with the EFP1 method. Four inter-molecular HBs are formed by the dicoumarol molecule with three water molecules; two HBs by C2O12····H37O31 group with one water molecule

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and other two HBs by C22O32····H17O11 group with two water molecules. The DC molecule excited

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to the S1* state by the absorption of 4.00 eV radiation, the OH bond length increases to 1.34 Å, where the energy crossing occurs (conical intersection), and the hydrogen atom transferred from coumarin moiety

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1 to 2, and is strongly stabilized in the ICT state (S1) of the DC-(H2O)3 complex, while S0 energy raises by

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3.34 eV.

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