Molecular structure–optical property relationships of 1,3-bis (4-methoxyphenyl) prop-2-en-1-one: A DFT and TD-DFT investigation

Author’s Accepted Manuscript Molecular structure-optical property relationships of 1,3-BIS (4-methoxyphenyl) prop-2-en-1-one: A DFT and TD-DFT investigation S. Ghomrasni, I. Aribi, S. Ayachi, A. Haj Said, K. Alimi www.elsevier.com/locate/jpcs

PII: DOI: Reference:

S0022-3697(15)00092-X http://dx.doi.org/10.1016/j.jpcs.2015.04.005 PCS7516

To appear in: Journal of Physical and Chemistry of Solids Received date: 16 December 2014 Revised date: 6 March 2015 Accepted date: 9 April 2015 Cite this article as: S. Ghomrasni, I. Aribi, S. Ayachi, A. Haj Said and K. Alimi, Molecular structure-optical property relationships of 1,3-BIS (4-methoxyphenyl) prop-2-en-1-one: A DFT and TD-DFT investigation, Journal of Physical and Chemistry of Solids, http://dx.doi.org/10.1016/j.jpcs.2015.04.005 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Molecular structure-optical property relationships of 1,3-bis (4-methoxyphenyl) prop-2-en-1-one: A DFT and TD-DFT Investigation S. Ghomrasnia, I. Aribib, S. Ayachia, A. Haj Saidb and K. Alimia,* a

Unité de Recherche (UR 11ES55), Matériaux Nouveaux et Dispositifs Electroniques Organiques, Faculté des Sciences de Monastir, Université de Monastir, 5000, Tunisie. b

Laboratoire Interfaces et Matériaux Avancés, Faculté des Sciences de Monastir, Université de Monastir, 5000, Tunisie. *Corresponding author: E-mail: [email protected] Tel: 216.73.500.274, Fax: 216.73.500.278

Abstract Some fundamental properties of the 1,3-bis (4-methoxyphenyl) prop-2-en-1-one, as functional monomer, are measured as well as calculated. The combined results are used for modeling and predicting monomer structure-property relationships. Thus, theoretical calculations based on Density Functional Theory (DFT) and its Time-Dependent counterpart (TD-DFT) are performed to evaluate the vibrational frequencies [IR and Raman], magnetic shielding for nuclear magnetic resonance [1H and 13C-NMR], electronic and optical properties of the studied material, respectively. The DFT/TD-DFT at B3LYP with 6-31G(d,p), 6-31G(d) and 3-21G(d) were employed to choose appropriate basis set that provides a more accurate molecular-property description. The simulated spectra are found to agree well, in shape, position, and relative intensity of peaks, with the available experimental measurements. In addition, frontier molecular orbitals, Mullikan charge and electron spin density distributions are carried out. Our results highlight the use of predictive calculations to provide an indepth understanding evidence of the electrochemically-initiated monomer reactivity. Keywords: A. Organic compound; C. Raman spectroscopy; C. Ab-initio calculations; D. Optical properties; D. Electronic structure

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1. Introduction The structure-property relationships for the great variety of polymers are poorly understood. They might be due to the conformational changes regarding reaction process and mechanisms [1]. In many cases, calculations based on Density Functional Theory (DFT) are able to provide accurate values of some properties that cannot be interrogated directly by current experimental methods [2-4]. In fact, they provide further information on configuration and conformation and give insight into intra-molecular interactions that may be ultimate sources for the experimental trends. Unfortunately, the structural and photo-physical properties are extremely sensitive to changes in basis sets implemented in the DFT code [5-8]. Then, for an accurate calculation of molecular properties, the choice of the basis set and methods are very important task, varies depending on the type of molecules of interest. Several examples of the application of computational methods to the interpretation and analysis of experimentally obtained results are reported in an effort to understand the structure-property relationship of polymers [9,10]. Then, there is a considerable opportunity for exploring this correlation between monomer structure and polymer properties, since polymerization process is not always fully understood [11]. Knowledge of the relationships between the monomer chemical structures and the polymer properties is an important prerequisite for the successful development of organic electronic devices. In our previous work [12] and due to their interesting electro-optical properties, we have analyzed new conjugated systems alternating acceptor–donor–acceptor (A–D–A) type copolymers based on chalcone derivatives for their hosting PCBM into a single layer BHJ solar cell. In this work, we are particularly interested in studying the 1, 3-bis (4-methoxyphenyl) prop-2-en-1-one (Fig.1). Although some experimental and theoretical properties have already been reported, unfortunately no information regarding the first steps of coupling reaction 2

mechanisms was developed. Our investigation provides additional support to facilitate the understanding of the radical coupling mechanism and predicting some opto-electronic properties of the compound. Theoretical calculations based on DFT and its time-dependent counterpart (TD-DFT) and the B3LYP level of theory in conjunction with 3-21G(d), 6-31G(d) and 6-31G(d,p) basis functions are used to evaluate the vibrational frequencies, magnetic shielding for nuclear magnetic resonance (NMR), electronic and optical properties of the studied material, respectively. These calculations will be compared to experimental results from a variety of spectroscopic analyses based on Infrared/scattering Raman, 1H-/13C-NMR and UV-visible optical absorption to study and optimize the photo-physical processes of interest.

2. Experimental and computational details

2.1. Monomer synthesis The chalcone was prepared by the base-catalyzed Claisen–Schmidt condensation. The substituted chalcone was prepared by addition of a small amount of aqueous sodium hydroxide to an ethanolic equimolar solution of p-anisaldehyde and p-methoxy-acetophenone. The chalcone precipitated from the reaction medium after refluxing for two hours. The completion of the reaction was monitored by thin layer chromatography TLC. The mixture was chilled in ice for 10–15 min, filtered on a Buchner funnel, and the precipitate was washed with water until the washings were neutral. After drying, the product was re-crystallized from 95% ethanol to afford the chalcone as a yellowish powder (80% yield).

2.2. Experimental part 1

H and 13C-NMR experiments were performed with a Bruker 300 MHz in the solvent

CDCl3. Infrared absorption measurement was recorded by using a Nicolet IR interferometer

3

20 SXC with a wave-number resolution of 4 cm-1. The sample was prepared in pellet of KBr mixed with the organic compound under study and the band positions are expressed in wavenumber (cm-1) from 400 to 1750. Raman scattering spectrum was recorded by using an excitation laser (He-Ne) wavelength of 636.8 nm (1.96 eV) on Raman spectrometer HORIBA Jobin-Yvon LabRAM, HR 800. Optical absorption spectrum was recorded by using a Varian cary 5 UV-visible-near-infrared spectrometer. The wavelengths, expressed in nanometers, vary from 200 nm (6.2 eV) to 450 nm (2.75 eV).

2.3. Theoretical part All calculations for the studied molecule have been performed using Gaussian 09 software package [13]. The molecule under consideration was explored at Density Functional Theory (DFT), and the Becke three-parameter exchange functions in combination with the LYP correlation function of the Lee, Yang and Parr (B3LYP) method. The geometry optimization and vibrational frequency determinations of the molecule have been developed using DFT/B3LYP/3-21G (d), 6-31G (d) and 6-31G(d,p) methods for the sake of comparison. This later basis set augmented by “d” polarization functions on heavy atoms and ‘p’ polarization functions on hydrogen atoms were used for better description of polar bonds of molecules [14,15]. The nuclear magnetic resonance (NMR) chemical shift calculations were performed using Gauge-Invariant Atomic Orbital (GIAO) method [16] at B3LYP level with the same basis sets. The

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C isotropic chemical shifts were referenced to the corresponding

values for TMS, which was calculated at the same level of theory. The effect of CDCl3 solvent on the theoretical NMR parameters was included using default PCM model provided by Gaussian 09 program. Simulated UV–Vis optical absorption spectra, electronic transitions, vertical excitation energies and oscillator strengths of the studied compound were computed with the TimeDependent DFT (TD-DFT) method using the same basis sets. Since the experimental 4

absorption spectrum was taken in chloroform solution, the inclusion of the solvent effect in theoretical calculations was important when seeking to reproduce or predict the experimental one with a reasonable accuracy. Polarizable continuum model (PCM) [17,18] is used as one of the most effective tools treating bulk solvent effects for the ground state. The density of states (DOS) was computed using GaussSum program [19]. Electronic properties of compound were performed using the Argus Lab 4.0.1 program. This shows the surface of the map on a scale of colors, each color represents the different electrostatic potential of the molecule: red represents the regions of the most negative electrostatic potential, white represents the regions of the most positive electrostatic potential and blue represents the region of zero potential. Regions relatively rich in electrons have a negative electrostatic potential. 3. Results and discussion 3.1.Molecular Structure, Energy, and Dipole Moment The molecular structure of the compound was examined. Based on the optimized geometrical parameters extracted from DFT, the results of bond lengths at the three different methods of calculation were displayed in Table 1S (See Supplementary Information). The atomic numbering scheme is illustrated in Fig. 1. According to this table, it can be seen that a part a slight difference, the considered bond lengths are approximately equal independently to the use of basis set. As referred to the un-substituted backbone of molecule, the bond lengths have approximately the same values and thus the largest differences are less than 0.01 Ǻ. However, the methoxy substituent groups on the para-benzene rings cause some changes in dihedral angles that twisted about 3.5°. This is due to the extended electron conjugated system in this molecule [20]. Hence, the torsion angles are not very much different, indicating that substitution does not basically distort the molecular skeleton.

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The calculated total energy and dipole moment for un-substituted and substituted chalcone monomer with different basis sets are summarized in Table 1. For each molecule, the lower value of the total energy is obtained at B3LYP/6-31G(d,p) level of theory. The electron donating of methoxy groups increase the stability of the molecule, whatever the basis set is. Moreover, the calculation of the dipole moment of each molecule (see Table 1) revealed that the methoxy substituent groups alter the delocalization of electron density and affect the dipole moment [21]. It is worthy to note that the intra-molecular H---O distances are 2.36 and 2.40 Å for the substituted and the un-substituted forms, respectively. This distance is significantly shorter than that of the Van der Waals separation between the oxygen and hydrogen atoms (2.72 Å) [22]. This result shows the existence of a non-covalent C-H---O interaction in both compounds (see Fig.1). Then, the low torsion angles between the aromatic rings arise from the rigidification of the molecular structures through these non-covalent C-H---O interactions. 3.2.Vibrational spectral analysis 3.2.1. IR analysis As mentioned above, for IR/Raman vibrational properties of the studied molecule, we have used the same three different methods of calculation in order to select the most appropriate one for computed vibrational frequencies analysis. Thus, the experimental IR spectrum was depicted in Fig. 2a (red colored) and others calculated spectra in Fig. 2b-d. First of all, resolved fine structure of vibrational spectra is a signature of high organized monomer structure. The main peaks and their corresponding assignments for the experimental and the theoretical spectra are shown in Table 2. The strongest absorption band from experimental IR spectrum, centered at 1602 cm-1, corresponds to C=C stretching of aromatic ring. This can be observed at 1612 cm-1 using both 6-31G(d,p) and 3-21G(d) and 1620 cm-1 for 6-31G(d) basis sets. 6

Moreover, the C-H in plan bending was observed at 1116, 1220 cm-1 for C-H aliphatic and 1174, 1309 cm-1 for C-H aromatic. It should be noted that the same peaks are theoretically observed at 1076, 1204, 1212 cm-1 for the first mode and 1148, 1156, 1196, 1292, 1308 cm-1 for the second one, obtained by different basis sets. The signal at 1259 cm-1 assigned to C-O methoxy groups stretching in the experimental IR spectrum [23], was calculated at 1244, 1260 cm-1 using the both 6-31G(d,p) and 6-31G(d) and 3-21G(d) basis sets, respectively. The band detected at 1513 cm-1 and attributed to the C-C stretching of phenyl ring in experimental data appeared theoretically at 1500 and 1516 cm-1. The C=O stretching modes for the α,βunsaturated ketone group of 1662 cm-1 was founded theoretically at 1630, 1660 cm-1 and 1652, 1644 cm-1. For an easier understanding of the IR analysis, we have plotted, in Fig. 3, the correlation graphics between the experimental and calculated IR frequencies for the three adopted methods of calculation. We note that the experimental values have a better correlation with those obtained with B3LYP/6-31G(d,p) (Fig. 3-c). The relation between the calculated and experimental frequencies is linear (see the Fit equation in the same figure). Furthermore, selected IR vibrational modes are displayed in Fig. 3-d. 3.2.2. Raman analysis The Raman spectrum of the molecule is given in Fig. 4.a. Since the carbon-oxygen double bond carbonyl group (C=O) is highly polar, it gives rise to a strong absorption band in IR spectrum (1662 cm-1) and a weak band in Raman spectrum (1661 cm-1). The calculated values of the C=O stretching mode were found as 1700, 1652 and 1644 cm-1 by means of DFT/B3LYP with 3-21G(d), 6-31G(d) and 6-31G(d,p) basis sets, respectively (See Fig. 4b-d). The ring C=C and C-C stretching vibrations, called skeletal vibration, usually occur in the region 1400-1625 cm-1 [24]. Then, in our case, the two peaks at 1576 and 1594 cm-1 arise

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from the symmetric and anti-symmetric stretching modes of phenyl rings, respectively. Likewise, the scissoring or asymmetric bending mode of methyl groups is observed at 1475 cm-1 and its symmetric bending mode is observed at 1422 cm-1. The calculated values are detected at 1468/1478 cm-1 and 1388/1404 cm-1, in that order. In addition, the C-O stretching band is observed at 1266 cm-1 and its calculated values were found to be around 1244-1260 cm-1. The other weak bands in the range of 1039-1172 cm-1 are attributed to the C-C stretching vibrations in aliphatic chain. Based on geometry optimization structures of the studied molecule, it is relevant to note that vibrational spectrum calculated with the DFT//B3LYP/6-31G(d,p) methodology satisfactorily agrees with experimental spectrum both in relative intensities and peak positions. 3.3. NMR spectral analysis Experimental 1H-NMR and

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C-NMR spectra, in CDCl3 solution as a solvent, of the

studied molecule are shown in Fig. 5 and Fig. 6, respectively. A complete assignment of the signals was collected in Table 3. It is recognized that accurate predictions of molecular geometries are essential for reliable calculations of magnetic properties. Firstly, full geometry optimization of the title compound was performed at the gradient corrected DFT/B3LYP. Then, gauge-invariant atomic orbital (GIAO) for

13

C chemical shift calculations of the

compound were made by the same methods used for vibrational analysis part. The experimental 1H-NMR spectrum shows that the ethylenic double-bond hydrogens of the α,β unsaturated system are observed at 7.58, 7.61 ppm and 8.02, 8.04 ppm for H(A) and H(B), respectively. Their calculated chemical shifts obtained by 6-31G(d,p) appear at 7.25,7.64 ppm (Fig. 7c). While, the computed values by 3-21G(d) and 6-31G(d) basis sets which are observed at 8.12 and 8.38 ppm (Fig. 7a,b), correspond to the trans double-bond configuration which is commonly found in naturally occurring chalcones. On the other hand, 8

a previous report on the substituent effect on the 1H-NMR spectra of chalcones shows that the effect of a 4-methoxy group on the A-ring of several chalcones causes a slight shift on the signals for H-α and H-β [25]. This effect can be attributed to the influence of the oxygen lone pair. The influence of a 4-methoxy substituent on the B-ring does not show a large difference compared with a non-substituted ring indicating that the influence of the 4’-methoxy is decreased probably by the carbonyl group. As our chalcone contains two oxygenated substituents in both aromatic rings, the observed signals are shifted in both H-α and H-β due to the electron richness of this system. It can also be seen that a structured overlapped signals, for the methoxy groups, appear between 3.85 and 3.88 ppm. Their corresponding calculated values by 6-31G(d,p) are of 3.88 and 4.32 ppm. The values are detected at 4.00, 4.46 ppm and 4.56, 4.57 ppm when calculations are performed with 6-31G(d) and 3-21G(d), respectively (Table 3). The signals corresponding to H(c), H(c'), H(f) and H(f') are found at 6.96, 6.99, 6.91 and 6.94 ppm, in that order. The 6-31 G(d,p) simulated spectrum, shows that these signals are detected at 7.46, 8.52, 7.55 and 8.78 ppm. While the two others computed values are obtained at7.16, 8.32 and 7.38, 9.58 ppm by 6-31G(d) and 7.05, 7.21, 6.98 and 7.81 by 3-21G(d) levels of theory. The signals appeared at 7.75 H(d), 7.80 H(d'), 7.40 H(e) and 7.45 ppm H(e') in the experimental spectrum are located at 7.83, 8.73, 7.98, 8.03 ppm in the simulated spectrum by 6-31G(d, p). Whereas, the calculated data close to 7.96, 8.28, 8.06, 8.00 ppm by 6-31G(d) and 8.88, 8.28, 7.81, 8.44 ppm, by 3-21G(d). Moreover, the experimental

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C-NMR data (Fig. 6 and Table 3) shows that the

methoxy pronounced signals of substituted compound were observed at 55.54 and 55.46 ppm [25]. The computed values by 6-31G(d, p) are of 58.10 and 56.57 ppm (Fig. 8c). However, the other calculated values still have large deviations from the experimental data (Fig. 8a-b and Table 3). The aromatic carbons give signals in overlapped areas of the spectrum with

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chemical shift values from 100 to 150 ppm [24,26]. The experimental chemical shift values of aromatic carbons except C2 and C2' are in the range of 113.83–131.40 ppm. At the same time, as calculated by the three basis sets, they are in the range of 112.85-129.64 ppm. The chemical shift of olefinic carbon attached to the aromatic ring C6 was detected at 143.86 ppm while the signal of the carbon C7 was detected at 119.59 ppm. The chemical shifts of these carbons were observed at 148.94 and 119.06 ppm, respectively, in the NMR simulated spectra by 6-31G(d,p). Similar results are obtained with 6-31G(d) basis set. However, findings strongly deviate when using 3-21G(d); they are close to 100.47 and 125.87 ppm. The carbonyl group which is an electronegative functional group polarizes the electron distribution. Then, the 13C-NMR chemical shift value of C8 from carbonyl group is too high and detected at 188.8 ppm [21]. It appeared theoretically at around 186 ppm computed by 6-31G(d, p) as well as by 6-31G(d) basis sets. Nevertheless, it appeared at 166.9 ppm employing 3-21G(d). 3.4. UV-Vis spectral analysis Optical absorption spectra analyses of the studied monomer were computed in the framework of TD-DFT calculations by using the small basis set 3-21G(d) and relatively moderate-size basis sets 6-31G(d) and 6-31G(d,p) for isolated gas-phase molecule (Fig. 9A) as well as in chloroform solution (Fig. 9B).The experimental spectrum recorded in the same solvent was given for comparison. The measured wavelength of maximum absorption and calculated electronic parameters including excitation energies (E), oscillator strengths (f) and the main molecular orbitals which contributed to electronic transitions are tabulated in Table 4. First of all, the presence of chloroform as a solvent lowers the HOMO and LUMO energy levels, increases the energy gap and then decreases the lowest excitation energy, as shown in Table 5 and Table 4. In the following section, the computational trends are compared with the experimental ones whenever possible. The Exp of 232 and 335 nm are attributed to n→* and Abs

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→*, respectively. In this regard, all the simulated profiles consist of two distinct bands, whatever the state of studied molecule (Fig. 9A and Fig. 9B). The Gaussian peaks fitting parameters containing

peak

positions,

full widths at

half

maximum

and integrated

intensities for each individual spectrum are given in Table 6. Surprisingly enough, the results showed that the use 3-21 G(d) was the most suitable to reproduce accurately the experimental data in spectral shape, peaks position as well as relative bandwidth. Since, the integrated intensity obtained by integrating the area under the absorption line is proportional to the amount of the absorbing substance present; calculations for isolated molecules are performed. Interestingly, the first observed electronic transition (232 nm) was attributed to the H→L+1 and the second one (335 nm) to H→L and H-1→L (see Table 4).On the other hand, good general agreement was observed between the experimental and theoretical results, although no intermolecular interactions are considered for calculations based on single molecules in the gas phase. It is obvious to note that to use of TD-DFT approach to predict the electronic absorption spectra is a quite reasonable method [27,28]. Through an extrapolation of the linear trend observed in the optical spectra (Fig. 9A), the estimated energy band gap and the slightly shifts of the maximum absorption peaks make the B3LYP with 3-21G(d) small basis set as an accurate level of theory to predict the optical properties for isolated gas-phase molecule. Conversely, B3LYP with 6-31G(d,p), as relatively large basis set, will provide more accurate vibrational and structural properties of the studied molecule, but will also require more computer resources. 3.5. Frontier molecular orbital analysis The HOMO and LUMO energy levels are known to be directly related to the ionization potential and the electron affinity [29-30], respectively. -conjugated molecules are characterized by the HOMO-LUMO energy separation, which is the result of a significant degree of intra-molecular charge transfer (ICT) from the end-capping electron-donor to the

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efficient electron acceptor group through -conjugated path [31]. The strong ICT through conjugated bridge results in substantial ground state donor-acceptor mixing and the appearance of a charge transfer band in the electronic absorption spectrum [32]. The inspection of density of states (DOS) shows high delocalization of energy levels for the studied molecule (Fig. 10) resulting from the simultaneous effect of acceptor and donor group and to spacer effect on the electron delocalization. These results are confirmed by the observed HOMO/LUMO frontier molecular orbitals, computed at B3LYP/6-31g (d,p) level of theory. Firstly, we have examined and found that the presence of methoxy-substituent groups does not have a significant effect on the molecular orbital distribution. As recapitulated in Table 4, we show that electronic transition process is mainly due to the HOMOLUMO configuration (~70%). Likewise, it can be seen from this figure that the HOMO is delocalized over the benzene rings and C=C bonds. The HOMO→LUMO transition implies an electron density transfer to carbonyl group through C-C bonds from benzene rings and C=C bonds. Moreover, the energy difference between HOMO and LUMO orbitals which is called the energy gap is a critical parameter in determining molecular electrical transport properties because it is a measure of electron conductivity. The computed energy values of HOMO and LUMO are -5.71 eV (-5.79 eV) and -1.75 eV (-1.94 eV) for isolated gas-phase molecule (in chloroform solution). Their associated band gap values are around 3.96 eV and 3.85 eV, respectively. Lower value of energy band gap explains the eventual ICT taking place within the molecule. For instance, we have separately examined the total Mulliken atomic charges of the sub-units constituting the monomer molecule (See Fig. 1) predicting the established ICT. In addition, we have calculated the values of the total energy (Etot), ionization potential (IP) and electronic affinity (EA) and results are given in Table 5. The obtained results demonstrated that the monomer molecule is indeed more stable in solvent medium than in the gas phase with a slight difference between the two states.

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On the other hand, it is well known that the chemical hardness and softness are good indicator of the chemical stability of a molecule. These features can be related to the HOMO and LUMO energy levels. In fact, a soft molecule is more polarizable than a hard one because this needs small energy to excitation. The hardness value of a molecule can be determined by the flowing formula [33]:



( HOMO   LUMO ) 2

Then, values of  in the title molecule are around 1.98 and 1.92 in the gas phase and in the solvent medium, respectively. Consequently, we conclude that the molecule taken for investigation belongs to soft material. 3.6.Molecular electrostatic potential analysis To give some information about chemical reactivity of the studied molecule, we have determined their Molecular electrostatic potential (MEP) maps. As shown in Fig. 11, the colors in mapped density surface are the values at the points on the electron density surface in the ground state of molecule. In this way, from the different values of the electrostatic potential given by different colors in the plot (as described in the theoretical part), it can be seen that the negative regions are mainly over the oxygen atom. This can relate to electrophilic reactivity and could correspond to an attraction of the proton by the aggregate electron density in the molecule (shades of red). While the positive MEP (white color), related to nucleophilic reactivity and corresponds to the repulsion of the proton by the atomic nuclei (shades of white). These results give information about sites ability to involve metallic bondings and intermolecular interactions. It can be noted that a region of zero potential envelopes the -system of the aromatic rings, leaving a more electrophilic region in the plane of hydrogen atoms in molecule.

3.7.Ground-state of oxidized monomer 13

In this section, we are interested in C-C bond lengths and Mulliken charges distribution in the monomer skeleton in their neutral and oxidized states. Then, the basis set, 6-31g (d, p) is used to calculate the bond lengths and Mulliken atomic charges for the studied monomer. The results are illustrated in Table 1S and Table 2S (See Supplementary Information), respectively. It was found that the C-C bond lengths in the monomer are distorted in the oxidized state. This is due to the partial loss of ring aromaticity which forces some bond lengths to shorten or lengthen. This may represents the formation of quinoid-like structure in the monomer. A schematic representation for the intra-molecular charge transfer (ICT) at the ground neutral and oxidized states of the monomer molecule, calculated as the average of the summation of Mulliken charges distribution of the sub-units, is displayed in Fig. 12. In general, intra-molecular charge transfer is generated through the alternating donor-acceptor conjugated systems [34]. From this figure, we conclude that the alternating sub-units of the model compound, positively and negatively charged, are considered as donor and acceptor, respectively. In addition, in order to predict the reactivity of the radical cation through radical coupling reactions, DFT was used to calculate the unpaired electron -spin density distribution of the monomeric radical cations. The obtained results show that the carbon C8 of the vinylene group has the highest spin density (see Table 2S, Supplementary Information). A high spin density at a given site suggests that the site is likely to be more reactive. Methoxy group having an electron donating effect affects the distribution of electron spin density. Particularly, in the case of symmetric homo-coupling reaction, the structures of the three possible conformers of dimer (See Fig. 13): Cis-transoid (C-T), the Trans-cisoid (T-C) and the Trans-transoid (T-T) were proposed. These results will direct our next experimental investigations.

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4. Conclusion We reported on an experimental investigation and theoretical analysis on molecule 1,3-bis (4-methoxyphenyl) prop-2-en-1-one, as a functional monomer. Then, various spectroscopic techniques such as IR, Raman scattering, 1H and 13C-NMR and UV-visible absorption spectroscopies were used to elucidate the structure of the compound. Based on the DFT calculations, the results are analyzed, discussed and compared with the experimental values. The combination of both experimental and theoretical investigations provides a powerful approach to the understanding of the structure-property relationship of the studied compound. In fact, large basis set will provide more accurate vibrational and structural properties. In contrast, we found that the 3-21G(d) small basis set efficiently reproduces the observed optical

properties.

In

addition,

the calculated

spin

density distribution

of

the

unpaired electron in the monomer radical cation permits the prediction of the radical coupling sites. In the case of a homo-coupling reaction, three conformers are possible for this molecule. To this regard, current ongoing efforts are focused on the dimerization mechanisms and related structure-properties relationship at the molecular level from appropriate method of calculations. 5. References [1] I. Botiz, N. Stingelin, Materials 7 (2014) 2273-2300. [2] S. Ayachi, A. Mabrouk, M. Bouachrine, K. Alimi, Photophysical properties of two new donor–acceptor conjugated copolymers and their model compounds: applications in polymer light emitting diodes (PLEDs) and polymer photovoltaic cells (PPCs), Jai Singh (Ed.), Organic Light Emitting Diode, In Tech, Croatia (2012). [3] I. Ben Khalifa, S. Bargaoui, A. Haj Said, S. Ayachi, B. Zaidi, J. Wéry, K. Alimi, J. Mol. Struct. 997 (2011) 37-45.

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[4] S. Ayachi, S. Bergaoui, I. Ben Khalifa, A. Haj Said, M. Chemek, F. Massuyeau, J. Wéry, E. Faulques and K. Alimi, Synth.Met.166 (2013) 22-32. [5] S. Ayachi, S. Ghomrasni, K. Alimi, J. App. Polym. Sci. 123 (2012) 2684-2696. [6] N. Cotelle, P. Hapiot, J. Pinson, C. Rolando, H. Vézin, J. Phys. Chem. B 109 (2005) 23720-23729. [7] G. Moro, G. Scalmani, U. Cosentino, D. Pitea, Synth. Met. 108 (2008) 165-172. [8] Y. Ye, M. Zhang, H. Liu, X. Liu, J. Zhao, J. Phys. Chem. Sol. 69 (2008) 2615-2621. [9] J. Jancar, J.F. Douglas, F.W. Starr, S.K. Kumar, P. Cassagnau, A.J.

Lesser, S.S.

Sternstein, M.J. Buehler, Polym. 51 (2010) 3321-3343. [10] S.Ö. Kart, A.E. Tanboğa, H.C. Soyleyici, M. Ak, H. H. Kart, Spectrochim. Acta Part A Mol. Biomol. Spectrosc. 137 (2015) 1174-1183. [11] S. Ayachi, S. Bouzakraoui, M. Hamidi, M. Bouachrine, P. Molinié, K. Alimi, J. App. Polym. Sci. 100 (2006) 57-64. [12] S. Ghomrasni, S. Ayachi, k. alimi, J. Phys. Chem. Sol 76 (2015) 105-111. [13] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, et al. Gaussian 09, Revision B.01 Gaussian, Inc., Walling ford, CT (2009). [14] G.A. Petersson, M.A. Allaham, J. Chem. Phys. 94 (1991)6081-6090. [15] G.A. Petersson, A. Bennett, T.G. Tensfeldt, M.A. Allaham, W.A. Shirley, J. Mantzaris, J. Chem. Phys. 89 (1988) 2193-2218. [16] K. Wolinski, J.F. Hinton, P. Pulay, J. Am. Chem. Soc. 112 (1990) 8251-8260. [17] S. Bhattacharya, T.K. Pradhan, A. De, S.R. Chaudhury, A.K. De, T. Ganguly, J. Phys. Chem. A. 110 (2006) 5665-5673. [18] E. Cances, B. Mennucci, J. Tomasi, J. Chem. Phys 107 (1997) 3032-3041. [19] N.M. O’Boyle, A.L. Tenderholt, K.M. Langner. J. Comp. Chem. 29 (2008) 839-845.

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[20]Y. Xue, J.Mou, Y. Liu, X. Gong, Y. Yang, L.An, Cent. Eur. J. Chem. 8(2010) 928-936. [21] R. Nithya, N. Santhanamoorthi, P. Kolandaivel, K. Senthilkumar, J. Phys. Chem. A. 115 (2011) 6594-6602. [22] X. H. Li, R. Z. Zhang and X. Z. Zhang, Can. J. Chem. 91 (2013) 1225-1232. [23] J.C. Espinoza-Hicks, L.M. Rodríguez-Valdez, G.V. Nevárez-Moorillón, A. Camacho Dávila, J. Mol. Struct. 1020 (2012) 88-95. [24] S. Sudha, N. Sundaraganesan, K. Vanchinathan, K. Muthu, S.P. Meenakshisundaram, J. Mol. Struct. 1030 (2012) 191-203. [25] K. Pihlaja, E. Kleinpeter (Eds.), Carbon-13 Chemical Shifts in Structural and, Stereochemical Analysis, VCH Publishers, Deerfield Beach, 1994. [26] H.O. Kalinowski, S. Berger, S. Braun, Carbon-13 NMR Spectroscopy, John Wiley & Sons, Chichester, 1988. [27] Y. Xue, Y. Liu, L. An, L. Zhang, Y. Yuan, J. Mou, L. Liu, Y. Zheng,Computational and Theoretical Chemistry. 965 (2011) 146-153. [28] S. Gunasekaran, R.A. Balaji, S. Kumeresan, G. Anand, S. Srinivasan, Can. J. Anal. Sci. Spectrosc. 53 (2008) 149-162. [29] C.H. Choi, M. Kertesz, J. Phys. Chem. 101 A (1997) 3823-3831. [30] A. Kumar, V. Deval, P. Tandon , A. Gupta, E. D.D’silva, Spectrochim. Acta Part A. 130 (2014) 41-53. [31] J.S. Murray, K. Sen, Molecular electrostatic potentials, Concepts and Applications, Elsevier, Amsterdam, 1996. [32] T. Tibaoui, S. Ayachi, M. Chemek, K. Alimi, Spectrochim. Acta Part A Mol. Biomol. Spectrosc. 142 (2015) 25-33. [33] W. Loued, J. Wéry, A. Dorlando, K. Alimi, J. Mol. Struct. 1081 (2015) 486-493. 17

[34] S. Ayachi, S. Ghomrasni, M. Bouachrine, M. Hamidi and K. Alimi, J. Mol. Struct. 1036 (2013) 7-18. Figure Captions Fig. 1: Molecular structure and atomic numbering scheme of studied monomer molecule. Fig. 2: The experimental (a) and DFT-calculated IR spectra of the monomer molecule by the use of: (b) 3-21G(d), (c) 6-31G(d) and (d) 6-31G(d,p) basis sets. Fig. 3: Correlation graphics of experimental and calculated Infrared vibrational frequencies for the monomer by the use of: (a) 3-21G(d), (b) 6-31G(d) and (c) 6-31G(d,p) basis sets.

(d)

displayed the three selected infrared vibrational modes. Fig. 4: The experimental (a) and DFT-calculated Raman spectra of the monomer molecule by the use of: (b) 3-21G(d), (c) 6-31G(d) and (d) 6-31G(d,p) basis sets. Fig. 5: Experimental 1H-NMR spectrum in CDCl3 solution of the monomer molecule. Fig. 6: Experimental 13C-NMR spectrum in CDCl3 solution of the monomer molecule. Fig. 7: Simulated 1H-NMR spectra (GIAO/DFT approach) by the use of (a) 3-21G(d), (b) 631G(d) and (c) 6-31G(d,p). Fig. 8: Simulated 13C-NMR spectra (GIAO/DFT approach) by the use of (a) 3-21G(d), (b) 631G(d) and (c) 6-31G(d,p). Fig. 9: Experimental (a) and theoretical UV–Vis spectra of monomer: (A) in gas phase and (B) in chloroform solution: (b) 3-21G(d), (c) 6-31G(d) and (d) 6-31G(d,p) basis sets. Fig. 10: Density of States (DOS) spectrum, electronic structure as well as ground state frontier molecular orbitals significantly contributing to the electronic transitions of the studied monomer molecule from DFT//B3LYP/6-31G(d,p) level of theory. Fig. 11: Molecular electrostatic potential (MEP) maps for monomer molecule in its ground state. 18

Fig. 12: Illustration of the monomer structure with Mulliken charges distributions for subunits at the ground neutral and oxidized states. Fig. 13: Molecular structure of the three most dimer conformers. Table 1: The DFT/B3LYP calculated total energy and dipole moment of un-substituted and studied monomer molecule. Table 2: The experimental and computed IR vibrational frequencies of the studied monomer molecule. Table 3: Experimental and theoretical NMR chemical shifts of the studied molecule. Table 4: The vertical excited energies and their oscillator strengths for the ground state (S0 → S1) of the monomer molecule calculations using TD//B3LYP with different basis sets. Table 5: DFT//B3LYP/6-31G(d,p) calculated electronic parameters of the monomer molecule. Table 6: Results of the Gaussian fit of experimental and TD-DFT/B3LYP simulated optical absorption spectra using three different basis sets: (a) 3-21G(d), (b) 6-31G(d) and (c) 631G(d,p). Table 1S: Calculated bond lengths (Ǻ) for neutral monomer (NS) and its radical cation (RC) at DFT/B3LYP with the different basis sets. Table 2S : The charge and the spin density distribution over the monomer molecule and its radical cation (RC). aThe Mulliken atomic charge (with hydrogen summed into heavy atom) for selected atoms and bthe atomic spin density. First, second and third lines denote the calculted values by DFT/B3LYP with the 6-31G(d,p), 6-31G(d) and 3-21G(d), respectively.

Total energy (Hartree) Compound Un-substituted form

6-31G(d,p)

6-31G(d)

3-21G(d)

654.0569131

654.0379382

650.4431045 19

Dipole moment (Debye) 66331G(d,p) 31G(d) 21G(d) 3.030

3.050

2.882

Monomer molecule

883.1108212

883.0869696

878.2374986

1.762

1.774

Table 1 Exp (cm-1) 526 607 821 991 1020 1116 1174 1220 1259 1309 1336 1425 1469 1513 1602 1662

3-21g(d) 548 612 876 --1020 1156 1212 1260 1292 1324 1396 1468 1500 1612 1636

Th (cm-1) 6-31g(d) 6-31g (d, p) 564 564 612 612 844 844 ----1028 1020 1076 1076 1148 1196 1204 1244 1244 1308 1308 1332 1348 1388 1476 1468 1500 1516 1620 1612 1660 1660

Assignments Methyl torsion C-C bending of Phenyl ring C-H aromatic out-of-plane bending Ring breathing Ring in plane bending of phenyl C-H aliphatic in plane bending C-H aromatic in plane bending C-H aliphatic in plane bending C–O stretching C-H aromatic in plan bending Methyl symmetric stretching C-C stretching of phenyl ring Methyl asymmetric bending C-C stretching of phenyl ring C=C stretching of aromatic ring α, β-C=C stretching + C=O Stretching

Table 2

20

1.308

Carbon atoms

Experimental 13C-NMR chemical shifts

1 1’ 2 2’ 3 3’ 4 4’ 5 5’ 6 7 8

55.54 55.46 161.55 163.31 113.83 114.43 130.15 130.76 131.40 127.85 143.86 119.59 188.8

Hydrogen atoms Experimental 1H-NMR chemical shifts A B a a’ c c’ d d’ e e’ f f’

7.58-7.61 8.02-8.04 3.85 3.88 6.96 6.99 7.75 7.80 7.40 7.45 6.91 6.94

Table 3

21

DFT/B3LYP/CPCM (CDCl3) (ppm) 3-21g(d) 49.9 49.89 145.11 144.18 101.97 103.97 110.68 109.8 113.2 110.48 125.87 100.47 166.9 3-21g(d) 8.12 8.38 4.57 4.56 7.05 7.21 8.88 8.28 7.81 8.44 6.98 7.81

6-31g(d) 60.87 59.56 164.30 163.94 113.64 113.58 139.06 132.00 129.83 129.53 149.56 119.54 186.04 6-31g(d) 6.98 7.05 4.00 4.46 7.16 8.32 7.96 8.28 8.06 8.00 7.38 9.58

6-31g (d, p) 58.10 56.57 164.63 164.31 112.85 113.20 138.17 134.57 129.64 129.54 148.94 119.06 185.90 6-31g (d, p) 7.25 7.64 3.88 4.32 7.46 8.52 7.83 8.73 7.98 8.03 7.55 8.78

Method of calculation

B3LYP/3-21g(d)

B3LYP/6-31g(d)

B3LYP/6-31g (d,p)

Experimental values

Optical absorption properties of ground state (S0S1) Oscillator strength States MO/Character max, nm/eV (f) HL 333.5/3.69 0.996 Gas H-1L Phase 240.3/5.16 0.164 H→L+1 HL 341.1/3.63 0.118 CHCl3 H-1L Solution 241.9/5.12 0.026 H→L+1 H→L 338.5/3.66 0.998 Gas H-1L Phase H-2→L+1 236/5.25 0.097 H→L+2 HL 347.1/3.57 0.116 CHCl3 H-1L Solution 246.9/5.02 0.014 H→L+1 H→L 338.8/3.65 0.996 Gas H-1L Phase 245/5.06 0.088 H→L+1 HL 347.3/3.57 0.116 CHCl3 H-1L Solution 247.3/5.01 0.014 H→L+1 335/3.70 CHCl3 232/5.34 ----Solution

Table 4

22

Coefficient (%) 71 16 65 65 10 59 87 12 37 20 67 7 59 74 12 65 67 7 59 ---

Gas phase Chloroform Solution Etotal (Hartree) -883.1108212 -883.1150014 EHOMO (eV) -5.71 -5.79 ELUMO (eV) -1.75 -1.94 3.96 3.85 H-L (eV) EHOMO-1 (eV) -6.04 -6.19 ELUMO+1 (eV) -0.26 -0.37 IPad (eV) 7.01 6.61 EAad (eV) -0.44 1.93 1.98 1.92 Global hardness () Table 5

23

(1) Param eters

Band  (nm)

width (nm)

Exp. (CHC l3 )

(2) Integr

Band

ated

 (nm)

Intens

width (nm)

ity

Integr ated Intens ity

(2

293.56

38.35

17.11

231.17

28.92

17.77

)

0.65

0.93

0.68

0.09

0.26

0.13

(2

337.23

40.26

49.35

’)

0.26

0.30

0.64

Relati ve Band width

Relative in tegrated intensities

 2.15

 3.74

2.14

9.50

2.20

17.27

2.21

17.76

2.14

12.85

1.81

4.25

2.72

25.28

R^ 2

0.9 99

In chloroform solution (a)

(b)

(c)

242.62

25.64

7.18

343.53

0.40

54.88

0.81

0.19

0.13

0.26

0.28

247.36

25.81

4.10

349.66

56.99

70.83

0.56

1.13

0.15

0.10

0.21

0.23

247.70

25.71

3.99

349.96

57.03

70.89

0.74

1.50

0.20

0.13

0.28

68.25

0.29

0.9 94 0.9 95 0.9 94

Isolated gas-phase molecule (a)

(b)

(c)

240.73

24.41

5.07

335.75

0.57

1.16

0.20

0.14

0.28

0.30

236.07

29.81

15.79

340.95

54.00

67.18

0.65

0.53

0.23

0.14

0.28

0.30

245.29

23.73

2.66

341.21

54.05

67.26

1.08

2.17

0.21

0.14 Table 6

24

52.38

0.29

65.18

0.31

0.9 93 0.9 92 0.9 93

Bond Length (Å) C1-C2 C2-C3 C3-C4 C4-C5 C5-C6 C6-C1 C5-C7 C7-C8 C8-C9 C9-O10 C9-C11 C11-C12 C12-C13 C13-C14 C14-C15 C15-C16 C16-C11 C14-O19 O19-C20 C2-O17 O17-C18

B3LYP/6-31 G(d,p) B3LYP/6-31 G(d) B3LYP/3-21G(d) NS RC NS RC NS RC 1.406 1.425 1.406 1.425 1.406 1.425 1.399 1.418 1.399 1.418 1.398 1.416 1.393 1.376 1.394 1.376 1.392 1.375 1.404 1.426 1.404 1.427 1.405 1.427 1.412 1.429 1.412 1.429 1.413 1.429 1.382 1.368 1.382 1.369 1.381 1.368 1.457 1.428 1.457 1.428 1.457 1.428 1.349 1.370 1.349 1.370 1.347 1.366 1.482 1.490 1.482 1.490 1.477 1.485 1.232 1.228 1.232 1.228 1.251 1.248 1.496 1.486 1.497 1.486 1.492 1.482 1.402 1.414 1.402 1.414 1.401 1.413 1.390 1.380 1.391 1.380 1.388 1.379 1.402 1.415 1.402 1.415 1.401 1.413 1.403 1.415 1.403 1.415 1.404 1.416 1.386 1.378 1.387 1.379 1.386 1.378 1.407 1.415 1.407 1.415 1.405 1.413 1.360 1.334 1.360 1.334 1.380 1.353 1.421 1.436 1.420 1.436 1.461 1.478 1.360 1.323 1.360 1.323 1.380 1.342 1.420 1.442 1.420 1.442 1.461 1.484 Table 1S

25

St ate

a

N

R Ca

D Sb

Ring A C1 0.0 19 0.0 30

C2 0.3 55 0.3 81

-C=C-

C3

C4

C5

0.0 45 0.0 60 0.0 22

0.0 44 0.0 57 0.0 14

0.0 28

0.0 03

0.0 95

0.1 26 0.1 72

C6 0.0 24 0.0 40

C7

C8

0.0 26 0.0 07

0.0 72 0.0 78 0.0 77

-C=O O1 C9 0

0.3 72 0.3 77

0.0 19

0.3 22

0.0 54

0.3 86

0.0 27

0.0 43

0.1 32

0.0 43

0.0 76

0.0 11

0.3 78

0.0 44

0.4 10

0.0 13

0.0 32

0.1 76

0.0 28

0.0 57

0.0 06

0.3 82

0.0 95

0.3 76

0.0 46

0.0 92

0.0 14

0.0 58

0.1 44

0.0 01

0.4 04

0.0 42

0.1 15

0.0 36

0.0 20

0.1 84

0.0 42

0.1 16

0.0 36

0.0 20

0.1 83

0.0 49

0.0 93

0.0 41

0.0 13

0.1 86

0.0 11 0.0 11 0.0 19

0.0 61 0.0 61 0.0 68

0.3 07 0.3 09 0.2 93

0.3 95

0.0 42 0.0 42 0.0 43

0.5 09 0.5 07 0.4 98 0.4 81 0.4 78 0.4 68

Ring B C11 C12 C13 C14 C15 C16 0.0 39

0.0 09

0.0 65

0.0 03

0.0 71

0.0 59

0.0 45

0.0 64

0.0 70 0.0 55

---

0.1 36

---

0.1 36

---

0.1 40

0.0 47 0.0 61 0.0 25

0.3 60 0.3 86

0.0 32 0.0 42

0.0 25 0.0 41

0.3 25

0.0 14

0.0 12

0.0 10

0.3 81

0.0 23

0.0 07

0.0 52

0.0 02

0.4 06

0.0 13

0.0 06

0.1 08

0.0 32

0.3 61

0.0 74

0.0 14

0.0 48

0.0 59

0.0 43

0.0 48

0.0 59

0.0 43

0.0 53

0.0 44

0.0 53

0.0 15 0.0 14 0.0 20

0.0 25 0.0 25 0.0 33

Table 2S

highlights  Chalcone derivative with non-covalent C-H---O interaction.  A combined modeling and experimental analysis is applied in this study.  Theoretical prediction of vibrational, structural and optical properties of the studied molecule.

26

27

28

29

30

31

32

33

34

35

36

37

38

39