study of electronic structure, light harvesting efficiency and...

Study of electronic structure, Light Harvesting

Efficiency and electron injection efficiencies

of 2, 4-Diflouroanisole dye Sensitizer for

Solar Cells. M.Anuradha*1, Dr.M. Karnan*2, M. Karunanidhi*3, V. Sivagami*4

*Assistant Professor, PG & Research Department of Physics,

Srimad Andavan Arts & Science College (Autonomous), Trichy-620005, Tamil Nadu, India

ABSTRACT

Functional density hypothesis was employed to establish anisole ground state

geometries and newly design theoretical dyes. In comparison to 2, 4-Difluoroanisole (DFA),

HOMO-LUMO energy differences for new design dyes were lower. Quantum chemical

calculations for the title compound and its dyes were calculated. Improved effectiveness in

photo collection (LHE) and free power shift in electron injection by the newly designed

sensitizers demonstrated the excellence of these products. This theoretical design will provide

experimentalists with the ability to synthesize efficient cell sensitizers. The role of Fukui and

the molecular electrostatic potential of the name compound have been researched as reactivity

indicators.

Keywords: Light harvesting efficiency, sensitizer, 2, 4-Difluoroanisole, HOMO-LUMO

*Corresponding author: [email protected]

I Introduction

Anisole may be reflected as more electron rich than benzene derivative or as an electron

depleted ether owing the resonance effect of methoxy group of the aromatic ring. Anisole reacts

with electrophiles in the electrophilic aromatic substitution reaction more quickly than benzene

[1, 2]. Anisole typically used as a preliminary material for several pharmaceutical/ Flavonoid

products and as solvent in organic synthesis and physical studies [3]. Anisole is a feebly polar

aprotic solvent with lesser electric permittivity (εs = 4.33) which permits its usability in cyclic

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mailto:[email protected]

voltammetry studies [4]. The use of lethal and toxic solvent in synthesis and characterization

is viewed as a very significant point for the welfare of Lab-workers and pollution. Now a days,

a rationally green solvent, anisole have been positively applied to process organic/polymer

solar cells [5].

Among the halogen – substituted benzenes, the fluoro substitutions have special spectroscopic

interest .The relative high freezing point of anisole given extra difficulties to separate the

isotopes. To solve this introducing electronegative group like fluorine atom on the benzene

ring of anisole, reduced the electron density of benzene [6-8]. In which 2,4 Difluoroanisole

(DFA) revealed worthy properties compared with anisole. DFA showed better result than

anisole in separation coefficient, freezing point and stability and it is a best complexing agent

for Boron isotopes [9].

The conversion of photo energy into electrical energy is generally considered as the

most potential way to resolve the world energy crisis, due to its huge reserves and pollution

free character. The direct conversion of sunlight into electrical energy by solar cells is of

particular interest because it has many advantages over most presently used electrical power

generation methods. In recent years dye-sensitized solar cells (DSSCs), as a novel technology

for the conversion of solar energy into electricity, have attracted tremendous and continuous

research interest because of easy fabrication, lower cost and relatively higher efficiency

compared to other photo voltaic technology.

In the present work we report a structure analysis of 2,4 Difluoroanisole using Gaussian 09W

software. To realize the reactive nature of the title compound we calculated the global reactivity

descriptors ie., hardness, chemical potential, and electrophilicity index. Besides, the dipole

moment, first hyper polarizability, thermodynamical parameters (heat capacity, entropy,

enthalpy) at different temperatures and natural Bond analysis also carried out Moreover, this

work focuses on a thorough explanation of isotropic chemical shifts, excitation energy and

HOMO-LUMO energies of 2, 4 DFA and its derivatives.

II Experimental Details

2,4- Difluoroanisole was obtained from Sigma Aldrich chemical suppliers and are of

analar grade. FTIR spectra (KBr pellets) were recorded on Bruker IFS 66V vacuum FTIR

spectrophotometer recorded in the region 4000 – 400 cm-1. FT – Raman spectra have been

recorded in the region 4000 – 0 cm-1 on a same instrument with FRA 106 Raman accessories.

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III Computational method

All computational calculations were performed in vacuum, Gaussian 09 W software

package, at the B3LYP 6-31+G(d,p) and cc-pVTZ basis sets to predict the molecular structure

and vibrational wave numbers. Parameters corresponding to optimized geometry (B3LYP) of

the title compound with experimental data (Fig.2) are given in Table 1. The absence of

imaginary wavenumbers on the calculated vibrational spectrum confirms that the structure

deduced corresponds to minimum energy. The assignments of the calculated wavenumbers are

aided by the animation option of Gaussview program, which gives a visual presentation of the

vibrational modes [10].

IV Results and Discussion

4.1 Molecular geometry

The optimization geometrical parameters of DFA obtained by the ab initio DFT/

B3LYP methods with 6-31+ G(d,p) and cc-pVTZ as basis sets are listed in Table 1. Their

geometries are considered to possess C1 point group symmetry is shown in Fig 1. The

B3LYP/6-31+G(d,p) and B3LYP/cc-pVTZ calculation gives the optimized molecular structure

of DFA with Cs symmetry, which is in good agreement with those reported in the literature

[11-13].

The benzene ring can be constructed through the use of five C-C bond distances and four C-C-

C bond angles. One bond (C1-C6) is not specially defined and small variations in the C-C-C

angles lead to quite large changes in the C1- C6 distances. From table 1, it can be seen that

there are some variations in the computed geometrical parameters from those reported [14].

Fig.1. Optimized Structure of 2,4 Difluoroanisole

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The bond lengths of C4-F14 is decreased from 1.367 to 1.356 A˚ and the bond length of C5-

C6 is increased from 1.373 to 1.388 A˚ in their respective basic sets. The decrease of C4- F14

bond length and the increase of bond length C5-C6 are probably caused by stronger interaction

of fluorine atom. The stronger interaction can lead to larger changes of the electron density

distribution on the ring. There is strong correlation between C-O and C-F bonds, C-C-O and

C-O-C angles, and C-C-F [14]. As is seen from the table the experimental bond distances, bond

angles agree with theoretical ones within the experimental values.

4.2 Vibrational spectra

The title compound consists of 16 atoms and its 42 normal modes of vibrations. All the

vibrations are active both in the Raman scattering and infrared absorption. The detailed

vibrational assignment of fundamental modes of DFA along with the calculated IR and Raman

frequencies and normal mode of descriptions are presented in Table 2. The FT-IR and FT-

Raman spectra of the title compound are shown in Figs. 2(a) and 2(b), respectively.

4.2.1 Methyl group vibrations

DFA under consideration possesses a CH3 group in the side substituted chain. For O-

CH3 compound, the C-H asymmetric and symmetric stretching vibrations appear in the range

2860-2935 cm-1 and 2825-2870 cm-1, respectively [15,16]. The FT-Raman bands are assigned

at 2833 cm-1 and FT-IR bands are assigned at 2926, 2805 cm-1 for asymmetric and symmetric

CH3 stretching vibrations of the methyl group in DFA. The theoretically computed frequencies

by B3LYP and cc-pVTZ shows an excellent agreement with experimental and literature values.

The asymmetric deformation of CH3 group is usually observed at around 1450 cm-1 for methyl

substituted benzenes [17]. As expected, this band appeared at 1451 cm-1 in FT-Raman

spectrum.

4.2.2 C-F vibrations

Aromatic fluorine compounds give stretching bands in the region 1270-1100 cm-1 [18].

The vibrations are easily affected by the adjacent atoms or groups. The very strong FT-IR and

FT-Raman bands appeared at 955, 986, and 957 cm-1 in DFA is assigned to C-F stretching

vibration. The predicted band corresponding to C-F bending mode as 567, 562 cm-1 in the

calculated values, the observed FT-Raman band at 593 cm-1 is assigned to C-F out-of-plane

vibration.

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Table 1: The calculated geometric parameters of 2,4-Difluoroanisole.

Parameters Bond Length (A˚) Parameters Bond Angle (A˚) Parameters Dihedral angles(A˚)

6-31+G(d) cc-pVTZ Exp.a 6-31+G(d) cc-pVTZ Exp.a

6-31+g(d) cc-pVTZ

C1-C2 1.400 1.403 1.362 C2-C1-C6 116.82 116.84 120.7 C6-C1-C2-F12 -179.99 -180.00

C1-C6 1.402 1.405 1384 C2-C1-O7 127.71 127.77 O7-C1-C2-C3 179.99 179.99

C1-O7 1.360 1.363 1.370 C6-C1-O7 115.47 115.39 C2-C1-C6-H16 180.00 180.00

C2-C3 1.389 1.392 1.427 C1-C2-C3 122.34 122.41 120.8 O7-C1-C6-C5 -179.99 -180.00

C2-F12 1.360 1.364 - C1-C2-F12 120.78 120.81 C6-C1-O7-C8 -179.98 -179.99

C3-C4 1.382 1.386 1.385 C3-C2-F12 116.88 116.77 C1-C2-C3-H13 180.00 180.00

C3-H13 1.082 1.084 - C2-C3-C4 118.48 118.34 119.9 F12-C2-C3-C4 179.99 180.00

C4-C5 1.386 1.390 - C2-C3-H13 119.96 120.06 C2-C3-C4-F14 -180.00 -180.00

C4-F14 1.356 1.360 - C4-C3-H13 121.55 121.59 H13-C3-C4-C5 179.99 180.00

C5-C6 1.388 1.393 - C3-C4-C5 121.68 121.81 C3-C4-C5-H15 -180.00 -180.00

C5-H15 1.082 1.085 - C3-C4-F14 118.84 118.78 F14-C4-C5-C6 -179.99 -180.00

C6-H16 1.083 1.086 - C5-C4-F14 119.47 119.41 C4-C5-C6-H16 180.00 180.00

O7-C8 1.432 1.432 - C4-C5-C6 118.56 118.47 H15-C5-C6-C1 180.00 -180.00

C8-H9 1.092 1.094 - C4-C5-H15 120.14 120.20 C1-O7-C8-H9 61.46 61.51

C8-H10 1.089 1.091 1.080 C6-C5-H15 121.29 121.32 C1-O7-C8-H10 179.99 179.997

C8-H11 1.092 1.094 - C1-C6-C5 122.12 122.11 C1-O7-C8-H11 -61.47 -61.51

C1-C6-H16 117.25 117.32

C5-C6-H16 120.62 120.57

C1-O7-C8 122.09 122.14

O7-C8-H9 111.47 111.42

O7-C8-H10 104.78 104.65

O7-C8-H11 111.47 111.42

H9-C8-H10 109.65 109.71

H9-C8-H11 109.68 109.81

H10-C8-H11 109.65 109.71

*Ref. [14].

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Table 2: Experimental FT-IR, FT-Raman and Calculated DFT-B3LYP/6-31G(d,p), DFT-B3LYP/ cc-pVTZ levels

of vibrational frequencies (cm1), IR intensity (kmmol1) and Raman intensity (kmmol1) of 2,4 – difluoroanisole

No Observed

frequencies

(cm1)

Calculated frequencies (cm1)

IR Intensity (kmmol1) Raman Intensity

(kmmol1)

Vibrational

assignments /PED

(≥ 10%) B3LYP/6-31+ G(d) B3LYP/cc-pVTZ

IR Raman Unscaled Scaled Unscaled Scaled B3LYP/6-31

G+(d)

B3LYP/ cc-

pVTZ

B3LYP/6-

31+ G(d)

B3LYP/ cc-

pVTZ )

1 3076 3081 3214 3079 3236 3078 0.766 0.290 2.908 5.260 νCH (99)

2 3011 3008 3210 3016 3230 3013 0.589 1.457 5.109 9.468 νCH (99)

3 2951 2947 3195 2957 3214 2955 1.847 2.812 1.911 3.772 νCH (99)

4 2926 3139 2931 3163 2928 17.070 19.047 3.778 7.266 νCH3ass (97)

5 2833 3109 2837 3134 2834 29.555 31.638 2.222 4.166 νCH3ass (96)

6 2805 3037 2808 3060 2807 56.623 61.305 7.350 12.110 νCH3ss (96)

7 1609 1615 1658 1611 1668 1609 1.183 1.065 7.528 1.887 νCC (77), δCH(22)

8 1597 1635 1591 1644 1589 48.610 47.687 1.564 0.388 νCC (74), δCH(19)

9 1525 1536 1529 1549 1526 352.913 343.758 0.128 0.041 νCC (54), νCO(22),

δCH(19),

10 1451 1501 1458 1522 1457 22.421 16.528 1.731 0.502 CH3 (94)

11 1438 1431 1494 1440 1518 1435 9.230 8.182 4.932 1.293 CH3 (92)

12 1305 1482 1308 1496 1306 30.600 41.505 2.068 0.677 CH3 (97),

13 1292 1449 1301 1460 1300 12.863 12.838 2.044 0.377 νCC (70), δCH(15), CH3

(14)

14 1281 1278 1349 1289 1363 1285 108.810 107.664 81.725 19.417 νCO (63), δCH(19)

15 1250 1250 1297 1252 1311 1249 32.877 38.900 51.686 14.076 νCO (54), νCC(19), νCF

(15)

16 1213 1273 1214 1285 1211 88.633 85.783 9.262 2.247 δCH(65), νCC(18), δCO

(54)

17 1208 1235 1208 1247 1206 102.507 101.887 18.193 4.749 δCH(63), νCC(11),

νCO(13)

18 1172 1167 1194 1176 1204 1174 21.386 21.107 2.815 0.821 δCH(61), νCC(13), νCO

(17)

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-stretching, δ -in-plane bending, γ-out-of-plane bending, -scissoring, -rocking, -twisting, -wagging, t-torsion,

R- ring, ipr-in plane rocking, opr-out of plane rocking.

19 1138 1163 1139 1174 1137 0.924 1.518 14.925 2.975 δO-CH3(75)

20 1128 1154 1127 1167 1126 59.856 57.747 2.650 0.748 CH3ipr(64), δCH(20)

21 1100 1097 1109 1101 1120 1108 28.677 25.054 24.298 6.636 CH3opr(91)

22 1038 1039 1038 1049 1036 74.639 75.055 20.347 4.519 νOCH3(35), δCH(16)

23 986 975 987 980 978 67.694 63.469 50.318 13.211 νCF(80)

24 955 957 940 956 949 952 1.402 1.005 0.066 0.006 νCF(75)

25 900 859 902 868 898 53.726 51.732 0.594 0.081 γCH(65)

26 820 817 823 823 820 16.663 23.174 0.141 0.044 γCH(61)

27 759 750 752 754 755 752 27.294 24.486 149.967 40.212 δC0CH3(63), δCC(15)

28 715 714 730 714 730 713 31.286 29.869 77.875 18.949 γCH (62), δC0(23)

29 681 685 678 684 676 681 0.567 0.367 7.836 2.229 γ CC(45), γ CF(26)

30 593 601 595 601 594 6.229 0.747 9.055 23.871 γ CF(46), γ CC(27)

31 600 567 598 562 0.727 4.831 100.000 2.799 δ CF(63), δ COCH3(28)

32 505 499 525 506 524 504 3.062 2.778 62.510 15.933 γ CC(40), δCF(15)

33 444 471 444 471 442 4.055 3.537 0.540 0.183 γ CH(56), γCC(34)

34 430 442 432 441 430 2.335 2.239 22.752 5.712 γ CC(33), γ CO(27), γ

CF(23)

35 368 370 369 371 367 0.002 0.010 36.011 11.400 δ CO(35), δCF(26)

36 362 360 362 358 4.050 4.079 58.339 14.477 δCF(45), γCC(25), γ(16)

37 292 317 296 317 264 2.506 2.498 15.755 50.317 Twisting CH3(56)

38 236 238 237 244 235 3.094 0.747 12.720 80.365 δCC(26), δCF(22)

39 231 229 237 227 0.294 3.020 31.317 32.413 tOCH3(85)

40 224 223 226 220 0.698 0.219 19.537 61.234 tCH3(36),

41 136 135 138 132 1.752 1.852 9.747 34.192 tRingasy(24), OC(24),

δCF(12)

42 60 59 55 53 5.419 5.346 0.126 100.000 γOC(60), δCO(16),

δOC(14)

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4.2.3 C-H vibrations

The aromatic structure shows the presence of C-H stretching vibrations in the region

3100-3000 cm-1 which is the characteristic region for the ready identification of the C-H

stretching vibrations [19]. In this region, the bands are not affected by the nature of the

substituent. Hence, in the present investigation, the FT-Raman band is observed at 3081, 3008,

2947 cm-1 and FT-IR band observed at 3076,311, 2951 cm-1, respectively in DFA have bee to

C-H stretching vibrations. In general, the aromatic C-H vibrations calculated theoretically are

found to be in good agreement with the experimentally reported values [20] for trisubstituted

benzene. The in-plane aromatic C-H deformation vibrations occur in the region 1450-990 cm-

Tra

nsm

itta

nce

(%

)

1500 4000 3500 3000 2500 2000 1000 500

Observed

Wavenumber (cm-1)

B3LYP/cc-pVTZ

B3LYP/6-31+G(d,p)

Fig. 2(a). Observed and simulated FT-IR spectra of 2,4-Difluoroanisole

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1 [21]. The bands are sharp but are weak to medium intensity. The C-H in-plane bending

vibrations computed at 1213, 1172 cm-1 and 1208, 1167 cm-1 in FT-IR and FT-Raman

spectrum, respectively and C-H out-of–plane bending vibrations are strongly coupled

vibrations and occur in the region 900-667 cm-1. In this current work, the FT-IR and FT-Raman

peaks observed at 820, 715 and 900, 714 cm-1 are assigned to C-H out-of-plane bending

vibrations, respectively.

4.2.4 C-O vibrations

R

am

an

in

ten

sity

Fig. 2(b) Observed and simulated FT-Raman spectra of 2,4-Difluoroanisole

Wavenumber (cm-1)

1500 4000 3500 3000 2500 2000 1000 500

B3LYP/ 6-31+G(d,p)

B3LYP/ cc-pVTZ

Observed

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The interaction of carbonyl groups present in the system did not produce such a drastic

and characteristic changes in the frequency of C-O stretch as did by N-H stretch. The absorption

caused by C-O stretching occurs in the region 1260-1000 cm-1 [22]. In the present case the FT-

IR and FT-Raman bands are 1281, 1250 and 1278, 1250 cm-1 is assigned to C-O stretching

vibrations, respectively. The in-plane and out-of-pane bending vibrations of C-O group are

represented in Table 2.

4.2.5 C-C vibrations

The bands 1430-1650 cm-1 in benzene derivatives are assigned to C-C stretching modes

[23]. Socrates mentioned that the presence of conjugate substituent such as C=C causes a heavy

doublet formation around the region 1625-1575 cm-1. The six ring carbon atoms undergo

coupled vibrations, called skeletal vibrations and give a maximum of four bands with region

1660-1220 cm-1. In this title compound, the FTIR and FT-Raman peaks observed at 1609, 1525

cm-1 and 1615, 1597, 1292 cm-1are assigned to C-C stretching vibrations, respectively. The in-

plane and out-of-plane bending vibrations are summarized in Table 2.

4.3 Mulliken Charge distribution of DFA

The calculation of atomic charges plays an important role in the application of quantum

mechanical calculations to molecular systems. Mulliken charges are calculated by determining

the electron population of each atom as defined in the basis functions. Chlorine and fluorine

are highly electronegative and tries to obtain additional electron density, it attempts to draw it

from the neighbouring atoms which moves closer together in order to the electrons more easily

as a result. The charge distributions calculated by the Mulliken [24] for the equilibrium

geometry of DFA are given in Table 3 and depicted in Fig. 3. The charge distribution on the

molecule has an important influence on the vibrational spectra. In 2, 4 DFA, the Mulliken

atomic charge of the carbon atoms in the neighbourhood of C1, C2, C4, H9, H10, H11, H13,

H15, H16 become more positive, shows the delocalization to the charges in the molecular

structure in the Mulliken’s net charge. While C3, C5, C6, O7, C8, F12, F14 atoms exhibit

negative charge because of halogen derivatives.

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Table 3: Mulliken population analysis of 24DFA performed at B3LYP/(6-31+G(d,p) & cc-

pVTZ)

5

4.4 Molecular Electrostatic Potential (MEP)

MEP is connected to electron density and is a highly appropriate descriptor for

electrophilic and nucleophilic responses in permissive locations along with hydrogen bonding

interactions [25, 26]. The electrostatic potential V(r) is well adapted for analysing procedures

focused on the "acceptance" of one molecule by another, as in drug-receptor and enzyme-

substrate relationships, since the two species first "see" each other through their potential [

27,28]. The calculation of MEP at the B3LYP / cc-pVTZ optimized geometry was used to

Atoms

Atomic charges (from basis set)

Atoms

Atomic charges (from basis set)

B3LYP/6-

31+g(d,p)

B3LYP/cc-

pVTZ

B3LYP/6-

31+g(d,p)

B3LYP/cc-

pVTZ

C1 0.13320 0.019427 H9 0.21329 0.168813

C2 0.72986 0.577891 H0 0.20654 0.167703

C3 -0.18143 -0.18787 H11 0.21330 0.16882

C4 0.12788 0.302099 F12 -0.36686 -0.179351

C5 -0.02626 -0.657911 H13 0.21938 0.223159

C6 -0.57924 -0.344505 F14 -0.35761 -0.179582

O7 -0.40168 -0.204136 H15 0.20296 0.199502

C8 -0.33825 -0.287357 H16 0.20491 0.213298

Atom lists

Mu

llike

n a

tom

ic c

har

ges

(a.u

)

Fig.3 Mulliken atomic charges plot of 2,4 - Difluoroanisole

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forecast reactive locations of electrophilic and nucleophilic interactions for the studied

molecule. Different colours represent the distinct dimensions of the electrostatic potential on

the MEP layer. The regions with the most negative, positive and zero electrostatic potential are

red, blue and green respectively. In the order red < orange < yellow < green < blue, potential

increases. Blue region shows the greatest attraction and yellow shows the greatest repulsion.

MEP's adverse (red and yellow) area was associated with electrophilic reactivity and MEP's

favourable (blue) areas were associated with nucleophilic reactivity (Fig.4) it is obvious that

the adverse force includes the carbonyl group and the favourable region is above the residual

groups.

4.5 Magnetic susceptibility

Magnetic susceptibility is caused by electronic orbit reaction or otherwise unpaired

electrons to the field applied. Magnetism occurs only in compounds containing unpaired

electrons, as when the electrons exist in pairs, the spin and orbital angular moments are

cancelled out. A magnetic susceptibility vs. 1/temperature is referred to as the Curie curve.

Ideally, if the Curie-Weiss law is followed, it should be predictable. The susceptibility vs 1/T

for DFA is shown in Fig. 5. It is linear and therefore Curie–Weiss law is followed. From such

a graph, the Curie value can then be obtained from the x-intercept from the slope and the Weiss

constant [29]. For DFA, Table 4(a) indicates the temperature and figure variation of

Fig.4 Molecular Electrostatic Potential for 2,4, Difluoroanisole

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susceptibility. Figure shows the storyline of the Curie. The story demonstrates that the essence

of DFA is paramagnetic.

Y= -1.04515 E-10 + 9.90742 E-5 X R =1

Curie constant = 2.90E-03 Weiss constant = 1 x 10-04 (BM). The Weiss constant is almost zero.

Hence the plot passes through the origin which proves the paramagnetic nature of DFA. Then

the magnetic moment of DFA is also calculated and tabulated in Table 4(b).

Table 4(a) Magnetic susceptibility of 2,4 - Difluoroanisole at various temperatures.

Table 4(b) Magnetic moment of 24DFA

S.NO Temp. (kelvin) Susceptibility(m)

mole per m3

1/Temp.

(Kelvin-1)

1 50 1.9814E-06 0.02000

2 100 9.9068E-07 0.01000

3 150 6.6045E-07 0.00667

4 200 4.9534E-07 0.00500

5 250 3.9627E-07 0.00400

6 298.5 3.3188E-07 0.00335

7 350 2.8305E-07 0.00286

8 400 2.4767E-07 0.00250

9 450 2.2015E-07 0.00222

10 500 1.9814E-07 0.00200

11 550 1.8012E-07 0.00182

12 600 1.6511E-07 0.00167

Ions No. of lone pairs Magnetic moment( bohr magnetron)

O7 2 2.828

F(2) 6 6.928

Total magnetic moment 9.756

1/Temperature (K-1)

Susc

epti

bil

ity (

χ m)

mole

per

m3

0.0 5.0x10-3

1.0x10-2

1.5x10-2

2.0x10-2

0.0

5.0x10-7

1.0x10-6

1.5x10-6

2.0x10-6

Fig. 5. Magnetic susceptibility plot of 2, 4 - Difluoroanisole

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4.6 Thermodynamical properties

Theoretical information on DFA's molecular structure acquired in this study to enable

us to accurately predict the molecule's thermodynamic properties. There are no laboratory

information on building temperature, thermal ability and enthalpy. However, calorimetric tests

were used to obtain the values for thermal capacity and enthalpy of anisole structure [30, 31].

These thermodynamic amounts are raised from 50 K to 500 K as the intensities for molecular

vibration rise with temperature rise. The main contributions to thermodynamic functions at low

temperatures are the molecule's translation and rotational motion. The results with different

temperatures is shown in Table 5. The amplitudes of vibrations are reinforced in opposition

molecules at the higher temperature when thermodynamic functions increase. The graph was

diagrammed between the temperature and the thermodynamic parameter. The curve equipped

by parabolic equations for the parameter. It is shown in Fig 6.

Cp = 3.60237 + 0.24133 X - 2.13995 X 10-4 X2 R2 = 0.9997

H = 50.83625 + 0.23551 X + 3.47479 X 10-5 X2 R2 = 0.9999

S = 4.50301 + 0.06955 X + 1.19212 X 10-4 X2 R2 = 0.9995

G =-46.33741 - 0.16589 X + 8.42839 X 10-5 X2 R2 = 0.9994

Table 5 Statistical thermodynamic parameters of 2,4 - Difluoroanisole at various temperatures,

performed at B3LYP/cc-PVTZ

Thermodynamic parameters (k cal mol–1)

Temp.

(Kelvin) Heat capacity(CP) Entropy(S) Enthalpy (H)

Gibbs free

energy (G)

50 15.223 62.681 8.998 -53.683

100 25.292 74.918 12.296 -62.622

150 34.854 86.880 17.027 -69.853

200 43.574 99.134 22.755 -76.379

250 50.919 111.702 29.229 -82.473

298.5 56.846 124.476 36.292 -88.185

350 61.596 137.536 43.835 -93.704

400 65.437 150.677 51.774 -98.909

450 68.587 163.900 60.042 -103.870

500 71.202 177.172 68.587 -108.604

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4.7 Fukui function, local softness, local philicity index

Three different types of local softness associated with the corresponding Fukui function

can be defined as follows: 𝑠𝛼(𝑟) = 𝑓𝛼(𝑟)𝑆. Considering the existence of a local

electrophilicity index ω(r)) that varies from point to point in an atom, molecule, ion, or solid,

it can define as = ∫ 𝜔(𝑟)𝑑𝑟 , where𝜔(𝑟) = 𝜔𝑓(𝑟). Three different types of 𝜔(𝑟) (henceforth,

it may called as local philicity index because it takes care of all types of reactions) can be easily

defined as 𝜔𝑎(𝑟) = 𝜔𝑓𝛼(𝑟)

Where 𝑎 = +, −, 𝑎𝑛𝑑 0 refer to nucleophilic, electrophilic, and radial attacks, respectively.

This inturn highlights the strength of the Fukui function [32] and the frontier orbital theory [33,

34]. The calculated value of local softness is listed in Table 6 It also shows the values of

electrophilicity index for each atom. It is clear from the trends that although the sum of any

type of philicity (𝜔𝑎) over all atoms is constant and equal to the global electrophilicity index

(𝜔). Local quantities such as Fukui function and local softness describe the

reactivity/selectivity of a definite site in a molecule. The Fukui function [32] or the frontier

function forms the background of local reactivity/selectivity theories in the spirit of Fukui’s

frontier orbital theory [33]. Then three different types of Fukui functions [32], namely,

ƒ+(𝑟) = 𝜌𝑁+1(𝑟) - 𝜌𝑁(𝑟) (for nucleophilic attack)

0 100 200 300 400 500

-100

-50

0

50

100

150

200

Heat capacity

Entropy

Enthalpy

Gibbs free energy

Temperature (k)

Th

erm

od

yn

amic

Par

amet

ers

(kca

lmo

l-1)

Fig. 6. Thermodynamical properties plot of 2,4 - Difluoroanisole

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Table. 6: Condensed Fukui functions for 2,4-difluoroanisole calculated at B3LYP/cc-pVTZ method

Atom qk(N+1) qk(N) qk(N-1) fkn fk

e fkr sk

n

ske sk

r ωk+

ωk- ωk°

C1 0.019

0.260 -0.119 -0.138 -0.240 -0.189 -0.053 -0.093 -0.073 -0.452 -0.786 -0.619

C2 -0.188 0.367 0.169 0.357 -0.555 -0.099 0.138 -0.214 -0.038 1.168 -1.813 -0.323

C3 0.578 -0.162 0.434 -0.144 0.740 0.298 -0.055 0.285 0.115 -0.471 2.421 0.975

C4 -0.658 0.371 -0.087 0.571 -1.029 -0.229 0.220 -0.396 -0.088 1.866 -3.365 -0.749

C5 0.302 -0.121 0.163 -0.139 0.424 0.142 -0.054 0.163 0.055 -0.454 1.385 0.465

C6 -0.345 -0.098 0.513 0.857 -0.247 0.305 0.330 -0.095 0.118 2.804 -0.806 0.999

O7 -0.204 -0.411 -0.003 0.201 0.207 0.204 0.077 0.080 0.079 0.657 0.677 0.667

C8 -0.287 -0.286 0.000 0.287 -0.001 0.143 0.111 0.020 0.055 0.940 -0.004 0.468

H9 0.169 0.253 0.000 -0.169 -0.084 -0.127 -0.065 -0.032 -0.049 -0.553 -0.275 -0.414

H10 0.168 0.263 0.000 -0.168 -0.096 -0.132 -0.065 -0.037 -0.051 -0.548 -0.312 -0.430

H11 0.169 0.253 0.000 -0.169 -0.084 -0.127 -0.065 -0.032 -0.049 -0.553 -0.275 -0.414

F12 -0.179 -0.262 0.001 0.181 0.082 0.131 0.070 0.032 0.051 0.591 0.269 0.430

H13 0.223 0.281 -0.026 -0.249 -0.058 -0.154 -0.096 -0.022 -0.059 -0.815 -0.189 -0.502

F14 -0.180 -0.230 -0.001 0.178 0.051 0.114 0.069 0.019 0.044 0.583 0.165 0.374

H15 0.200 0.262 -0.012 -0.212 -0.063 -0.137 -0.081 -0.024 -0.053 -0.692 -0.205 -0.449

H16 0.213 0.261 -0.031 -0.244 -0.048 -0.146 -0.094 -0.018 -0.056 -0.799 -0.157 -0.478

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ƒ−(𝑟) = 𝜌𝑁(𝑟) - 𝜌𝑁−1(𝑟) (for electrophilic attack)

ƒ0(𝑟) =1

2(𝜌𝑁+1(𝑟) - 𝜌𝑁−1(𝑟) ) (for Radical attack)

Where 𝜌𝑁+1, 𝜌𝑁, 𝜌𝑁−1 are the electronic densities of anionic, neutral and cationic species

respectively. The local softness is related to Fukui function as 𝑠(𝑟) = 𝑓(𝑟)S, Where S is the

global softness given as [34], namely, 𝑆 =1

2𝜂= ∫ 𝑠(𝑟) 𝑑𝑟

In this case the 𝑆𝑘𝑒 𝑎𝑛𝑑 𝑆𝑘

𝑛 values are highest for position 14 (ie F14), and C atom

(i.e., position 8), indicating the most electrophilic and most nucleophilic nature of these two

centres, respectively. The local softness contains the same information as Fukui function (i.e.,

sensitivity of the chemical potential of a system to a local external perturbation [35]) plus

additional information about the total molecular softness. Therefore either the Fukui function

or local softness can be used in studies of intramolecular reactivity sequences. But only S(r)

should be a better descriptor of the global reactivity with respect to a reaction partner with a

given hardness (or softness), as stated in hard soft acid base theory principle.

4.8 Nonlinear optical properties

To investigate and characterize the structural features responsible for the properties,

vibrational spectroscopy is an important tool [36, 37]. Earlier it was usually assumed that the

hyperpolarizability of organic material is almost of pure electronic origin with a very weak

acoustic contribution. However, it has been established through theoretical quantum chemical

treatment that molecular vibration modes considerably contribute to non-linear response [38-

39]. This vibrational component arises due to the coupling between electronic and nuclear

motion. Urea is taken as the reference material whose first order hyperpolarizability is 0.3728

X 10-30 esu and dipole moment is 1.732 Debye calculated at the DFT/cc-pVTZ level. Total

static dipole moment (µ), the anisotropy of the polarizabiity (∆α) and the mean first order hyper

polarizability (β) for title compound and its dyes has been given in Table 7.

The B3LYP/6-31+G(d,p) gas phase dipole moments of 6.4203 Debye for DFA dye 6

is in fair agreement with those compared with other dyes, which range from 0.36684 to 6.4203

Debye. The greater dipole moments DFA dye 6 suggest a larger charge separation in the

former. This larger separation of charge is perhaps due to the presence of electron donor

triphenylamine in addition to the electron rich anisole group in the para position of ring.

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The total hyperpolarizabilities, the isotropic polarizability, polarizability anisotropy are

also calculated. The calculated anisotropy of DFA is -56.771 esu. However, the calculated

anisotropy polarizability of DFA dye 1 to dye 6 is shown in Table 7. The above data indicate

that the donor-conjugate π bridge-acceptor (D-π-A) chain like dyes have stronger response for

external electric field. Whereas, for dye sensitizers on the basis of the photo-to-current

conversion efficiencies, the similarity and the difference of geometries, and the calculated

polarizabilities, it is found that he longer the length of the conjugated bridge in similar dyes.

This may be due to the fact that the longer conjugate - π – bridge enlarged the delocalization

of electrons, thus it enhanced the response of the external field, but the enlarged delocalization

may be not favourable to generate charge separate state effectively. So it induces the lower

photo-to-current conversion efficiency.

4.9 Electronic structure and Properties of DFA dyes:

The structure and labels of DFA series of dyes is shown in Fig. 7(a) and 7(b). The light

harvesting efficiency, electron injection efficiency, and open circuit voltage of the six designed

dyes, has been theoretically assessed using a simple approach by 6-31+G(d,p) basis set.

The distribution patterns of highest occupied molecular orbitals (HOMO) and

lowest unoccupied molecular orbitals (LUMO) are used to study the efficiency of sensitizers.

Fig. 8(a) and 8(b) shows the frontier molecular orbital energies and corresponding density of

states spectrum of 2, 4 - Difluoroanisole. The distribution pattern of highest occupied molecular

orbitals (HOMO) and lowest unoccupied molecular orbitals (LUMO) of new sensitizers are

shown in Fig. 9(a) and 9(b). From figure it is clear that HOMOs are present on benzene unit

while LUMOs are present on acceptor group. Such electron density distributions are beneficial

for efficient charge separation and electron injection. This indicates the charge transfer from

donor to acceptor through π – spacer. Significant charge transfer from donor to acceptor side

proved that these dyes would be vivid sensitizers. Highest occupied molecular orbitals energy

(EHOMO), lowest unoccupied molecular orbitals (ELUMO) and HOMO-LUMO energy gap (Egap)

are given in Table 8.

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Table 7: Nonlinear optical properties of 2,4-Difluoroanisole.

Parameters B3LYP/6-31+G(d,p)

B3LYP/cc-VTZ

Dye1 Dye 2 Dye 3 Dye 4 Dye 5 Dye 6

x -0.3473 1.6814 -2.9097 -2.4249 -3.993 3.9445 3.4824 -4.6334

y -0.1227 -0.4344 3.9838 0.2479 3.5751 -4.4004 -0.973 4.3791

z 0.001 0.0002 0.0048 0.0013 0.0052 0.7647 0.9061 0.7588

tot 0.3684 1.7367 4.9333 2.4376 5.3596 5.9588 3.7275 6.4203

xx -59.1243 -56.5271 -106.39 -113.8691 -115.2825 -194.284 -202.658 -204.1225

yy -54.093 -55.0601 -108.3098 -95.4153 -108.1456 -175.8006 -160.1736 -176.3975

zz -57.114 -58.4208 -103.3074 -108.7671 -107.2862 -171.2752 -176.8316 -175.2044

xy 4.6773 2.0178 11.2741 -6.2469 7.0885 20.9965 -1.9217 17.0941

xz -0.0002 0.0008 0.0111 -0.0029 0.0099 5.8019 6.124 -6.4443

yz -0.0067 -0.0002 -0.0113 0.0138 -0.0072 -2.5844 -0.7997 1.863

Δ(esu) -56.7771 -56.6693 -106.0024 -106.0171 -110.2381 -180.4532 -179.8877 -192.6670

xxx -29.0711 47.2844 -82.1534 -81.2637 -85.8737 127.3701 150.5807 -139.128

yyy -6.4841 4.2084 99.5286 -19.0828 49.2852 -107.1009 35.3097 58.6739

zzz -0.0024 0.0002 -0.0397 -0.0746 -0.084 10.9902 13.7823 14.8569

xyy 15.8609 -15.1843 -67.7962 12.5271 -40.8269 136.5565 -11.6947 -84.6304

xxy -4.0692 0.611 39.3221 -25.746 15.659 -144.488 -21.422 112.1777

xxz 0.0117 0.0015 0.1057 0.0736 0.1155 -9.3743 -12.0381 -11.1203

xzz -0.3903 1.3162 11.2653 20.0211 23.5934 10.8728 -3.8982 10.5977

yzz -6.3958 6.6464 4.9251 1.1614 -1.5405 -6.1458 2.7516 -4.7001

YYZ -0.0042 0.0001 0.0966 -0.0457 0.0501 -13.1362 -1.7435 -7.194

XYZ -0.0083 -0.0003 -0.0973 0.0152 -0.0655 8.9183 -0.6692 4.319

tot(esu) 1.6226E-30 2.63788E-30 1.4915E-29 4.884E-30 9.037E-30 2.814E-29 1.01554E-29 2.87982E-29

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The energy gaps of DFA and newly designed dyes are in following order

DFA Dye 3< DFA Dye 1< DFA Dye 2< DFA. This order can be explained on the basis of

donor moiety, and acceptor group

D

A

PI

Fig. 7(a) Different parts of Donor – Pi spacer – Acceptor.

R2 R1

Fig. 7(b) Chemical structures of 2,4-Difluoroanisole for newly designed dyes.

R1 = Benzene, Tiphenylamine, R2 = CN, COOH, NO2

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Table 8 Frontier molecular orbital analysis of 2,4- Difluoroanisole.

DYES LUMO Energy (eV) HOMO Energy (eV) Energy gap (eV)

24DFA -0.848 -6.369 5.521

1 -1.820 -6.694 4.875

2 -1.864 -6.458 4.593

3 -2.739 -6.773 4.034

4 -1.651 -5.800 4.149

5 -1.734 -5.662 3.928

6 -2.614 -5.845 3.231

HOMO

LUMO

Fig. 8 (b). Density of states (DOS) diagrams for 24DFA

Fig. 8 (a). Frontier molecular orbital analysis of 2, 4-difluoroanisole

HOMO = -6.369 eV LUMO =-0.848 eV ΔE = 5.521 eV

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E = 4.593 eV

ELUMO = - 1.820 eV

E = 4.875 eV

EHOMO = - 6.458 eV

EHOMO = - 6.694 eV

ELUMO = - 1.864 eV

E = 4.034 eV

ELUMO = - 2.739 eV

EHOMO = - 6.773 eV

Fig. 9 (a).HOMO-LUMO plot of 2,4-Difluoroanisole with Benzene as donor and acceptors (CN,COOH,NO2)

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EHOMO = - 5.800 eV

ELUMO = - 1.651 eV

E = 4.149 eV

EHOMO = - 5.662 eV

E = 3.928 eV

ELUMO = - 1.734 eV

ELUMO = - 2.614 eV

EHOMO = - 5.845 eV

E = 3.231 eV

Fig. 9 (b). HOMO–LUMO plot of 2,4 – Difluoroanisole with TPA as donor and acceptors(CN, COOH, NO2)

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4.9.1 Energy level of HOMO and LUMO

The energy level of HOMOs and LUMOs of the dye sensitizer should match with

iodine/iodide redox potential and the conduction band edge level of the TiO2 semiconductor

[40]. For all dyes considered here, the simulated LUMOs lie above the TiO2 conduction band

edge (-4.000eV in vacuum) [41], providing he thermodynamic driving force for favourable

electron injection from the excited state dye o the TiO2 conduction band edge. Meanwhile the

HOMOs of all dyes lie below the iodide redox potential (-4.80eV in vacuum) [42], leading to

fast dye regeneration and avoiding the geminate charge recombination between oxidized dye

molecules and photo-injected electrons in the nano crystalline TiO2 film.

From, Table 8 observed a significant change in the energy of molecular orbitals of DFA

dyes on the modification of pi-bridge unit. The addition of highly electron- deficient DFA unit

in dye 3 effectively lowered the LUMO of dye 3 to -2.739 eV compared with that of dye 6 (-

2.614 eV). Due to the significant decrease of the LUMO level, the HOMO-LUMO gap of DFA

dye 6 is 3.231 eV smaller than that of DFA dye 3. TPA is electron rich than benzene ring.

Therefore in TPA attached with the DFA gives the more result than benzene donor.

4.9.2 Global reactivity descriptors

The global reactivity descriptors used to analyse the stability of the molecules and to

acquire the information about reactivity of molecules. The values obtained for global reactivity

descriptors are listed in Table 9.

According to the frontier molecular orbital theory (FMO), the realization of a transition

state is due to an interaction between frontier orbitals (HOMO and LUMO) of reacting species

[43]. HOMO is frequently associated with the electron donating ability of a molecule.

According to Koopman’s theorem [44] the ionization potential (IP) and electron affinity (EA)

of the title compound are calculated.

Negative energy of HOMO is ionization potential (IP). In this high values of HOMO

energy (EHOMO) or low energy values of IP, DFA dye 5 indicate that the molecule has a

tendency to donate electrons to appropriate acceptor molecules with low energy empty

molecular orbital. Increasing values of EHOMO facilitate adsorption by influencing the transport

process through the adsorbed layer [45] and therefore applicable to DSSCs and also the

inhibition efficiency. Negative energy value of LUMO (ELUMO) is stated as electron affinity

(EA).

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Table 9: Quantum Chemical Parameters of 2,4 -Difluoroanisole calculated B3LYP/ 6-31+G(d, p) basis set.

ELUMO indicates the ability of the molecules to accept electrons. The lower values of ELUMO or

higher values of EA, for the DFA dye 6 has the more probable it is that the molecule would

accept electron [45]. Low values of band gap gives to make good dye sensitizer in DSSCs and

also gives good inhibition efficiencies, because the energy to remove an electron from the last

occupied orbital will be low. For the designed dyes DFA dye 6 (3.231eV) has lowest value

among its sister dyes.

The higher HOMO energy of DFA-dye 5 molecule corresponds to the more reactive in

the reactions with electrophiles, while lower energy for DFA- dye 1 is important for molecular

reactions with nucleophiles [46]. Chemical softness of DFA-dye 6 (0.309 eV) has the highest

value, it describes the capacity of an atoms or molecules to receive electrons large in nature.

Among quantum chemical methods also used for evaluation of corrosion inhibitors, density

functional theory, DFT has shown significant promise [47] and appears to be adequate for

pointing out the charges in electronic structure responsible for inhibitory action. Using B3LYP

the description of the inhibitor is recommended for the study of chemical reactivity and

selectivity of molecule [48].

Electronegativity and chemical hardness measures the resistance of an atom to a charge

transfer.

Parameters

Values (eV)

DFA DFA- DYE

1

DFA-

DYE2

DFA-

DYE3

DFA-

DYE4

DFA-

DYE5 DFA- DYE6

Ionization

potential 6.369 6.694 6.458 6.773 5.800 5.662 5.845

Electron affinity 0.848 1.820 1.864 2.739 1.651 1.734 2.614

Energy gap 5.521 4.875 4.593 4.034 4.149 3.928 3.231

Hardness(η) 2.760 2.437 2.297 2.017 2.075 1.964 1.616

Softness(S) 0.181 0.205 0.218 0.248 0.241 0.255 0.309

Chemical

potential(μ) -3.61 -4.257 -4.161 -4.756 -3.725 -3.698 -4.229

Electrophilicity

index 2.359 3.718 3.769 5.607 3.345 3.481 5.535

Charge Transfer 1.307 1.747 1.812 2.358 1.796 1.883 2.618

Nucleofugality

(ΔEn) 3.207 5.537 5.633 8.346 4.996 5.215 8.149

Electrofugality

(ΔEe) 8.727 10.412 10.227 12.380 9.145 9.143 11.380

Back

donation(ΔE) -0.69 -0.609 -0.574 -0.504 -0.519 -0.491 -0.404

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Electronegativity χ =𝐼+𝐴

2

Chemical hardness η =𝐼−𝐴

2

The global electrophilicity index (ω), introduced by Parr [49], and calculated using the

electronic chemical potential and chemical hardness is given by ω =𝜇2

2η . The eletrophilicity

index measures the propensity of chemical species to accept electrons [50]. A good, more

reactive, nucleophile DFA (2.359 eV) the title compound characterized by lower value of ω,

and conversely DFA dye 3 (5.607 eV) and DFA dye 6 (5.535 eV) a good electrophile.

In Table 9 the calculated values of the number of electrons transferred (∆N) is tabulated.

Values of this parameter show that the inhibition efficiency resulting from electron donation

of DFA agrees with Lukovit’s study [51]. If ∆N < 3.6, the inhibition efficiency increases by

increasing electron donating ability of the inhibitors to donate electrons to the metal surface.

The highest fraction of electron transferred DFA dye3 (2.358 eV) and DFA dye 6 (2.618 eV)

are associated with the best inhibitor.

The adsorption of the molecule on metal surface recognised to the inhibition effect.

The energy of the highest occupied molecular orbital (EHOMO) measures the tendency towards

the donation of electron by a molecule [52].

4.10 Efficiency of Dye sensitized solar cell

4.10.1 Light harvesting efficiency (LHE) and Absorption spectra

Light harvesting efficiency plays a vital role in improving the power conversion

efficiency (PCE) of DSSCs. The electron donating ability of the donor group determining the

energetic positions of Frontier Molecular Orbitals (FMO) of design dyes, which is correlated

with the LHE of dyes and the charge transfer kinetics at the titanium/dye/ electrolyte interface,

such as dye regeneration and charge recombination.

The light harvesting efficiency (LHE) of the dye has to be as high as the potential to increase

the photocurrent response [53]. LHE is represented as

LHE=1-10-A=1-10-f

where A(f) is the absorption (oscillator strength) of the dye corresponds to λmax.

LHE is the efficiency of dye that responds to light which indicates out the efficiency of

DSSC also. Table 10 inclines the calculated excitation wavelengths along with their oscillator

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strengths. Fig. 10 denotes the calculated absorption spectra of the designed dyes exhibit wide-

ranging absorption in the UV region. This may eventually lead to a wide spectral response with

high LHE value for DFA dye 6. In this case, the DFA dye 6 functionalized by triphenylamine

based donor group seems to be the good molecule with the maximum wavelength 299nm.

Table 10: Light harvesting efficiency (LHE) of 2,4- Difluoroanisole and its derivatives

calculated using TD-DFT/B3LYP/6-31+G(d,p) basis set.

The values of the wavelength, the strength of the selected dyes calculated using the TD-

DFT/B3LYP 6-31+G(d,p) method are presented in Table 10. It can be seen that the lowest-

energy absorption spectrum for the DFA dye 6 (299 nm) > DFA dye 4 (285 nm) > DFA dye 5

(285 nm) > DFA dye 3 (251 nm) > DFA dye 2 (244 nm) > DFA dye 1 (242 nm) > DFA (162

nm). This almost reflects a decreasing trend in the HOMO-LUMO energy levels because the

HOMO to LUMO transition is the main factor influencing the spectrum. The maximum

absorption absorbed in Ultra-Violet Visible region covers the entire region as calculated spectra

shown in Fig. 10. Furthermore, it is clear that the oscillator strength of the peak corresponding

to the maximum absorption (0.6071) is much higher for DFA dye 6 than that for the other dyes.

This finding implies a direct relation between the light-harvesting properties of the dye and the

improvement of DSSCs efficiency.

4.10.2 Electron injection efficiency

Further improvement of DSSCs, the expansion of novel sensitized dye is essential. Such

dyes must have wide absorption spectra and high electron injection efficiency (ϕinj). It is

accompanying with Jsc can be indirectly characterized by the electron injection driving force

(∆Ginject) from excited dyes to the conduction band of the semiconductor. The Table 11 listed

the values. It can be seen that ∆Ginject of DFA dye 1, DFA dye 2, DFA dye 4 and DFA dye 5

get almost in the same range of electron injection rate, that is -1.628, -1.586, -1.945, -1.843

respectively.

Dyes Wavelength

(λmax)(nm)

Excitation energy (E)

(eV)

Oscillator

Strength(f)

LHE

24DFA 162 7.646 0.4048 0.6063

1 242 5.126 0.2789 0.4739

2 244 5.077 0.2831 0.4789

3 251 4.947 0.2959 0.4941

4 292 4.251 0.2899 0.4870

5 285 4.354 0.3582 0.5617

6 299 4.150 0.6071 0.7529

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The positions DFA dye 3 and DFA dye 6 at benzene and triphenylamine as donors and NO2 as

acceptor would be favourable to enhance the electron injection from donor moiety of the dye

to conduction band of TiO2. By modification, the conjugation improves the electron injection

but dye 1 to dye 5 not augment the LHE as compared to DFA dye 6.

Moreover, the effect of TPA and NO2 in these designed sensitizers is very suitable

towards the enhancement of electron injection.

4.10.3 Exciton binding energy (EBE)

To realize high energy-conversion efficiency, the excited electron and hole pairs should

be separated into positive and negative charges to escape from recombination due to the

columbic attraction. To realise the process, the binding energy has to be overcome. That, the

dye molecule should possess less exciton binding energy for high energy conversion. At this

point, exciton binding energy was calculated using the formula [54, 55]

𝐸𝐵𝐸 = ΔE𝐿−𝐻− 𝜆𝑚𝑎𝑥𝐴𝑏𝑠

Where ΔE𝐿−𝐻 is the band gap and it is energy difference between HOMO and LUMO energies.

𝜆𝑚𝑎𝑥𝐴𝑏𝑠 is the first excitation energy. The calculated exciton binding energies of the designed

Wavelength (nm)

Epsi

lon

Fig. 10. Simulated absorption spectra of DFA and its dyes calculated in gas phase

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dyes are listed in Table 11. The exciton binding energy of the order of maximum is -0.102, -

0.252, -0.426, -0.483, -0.913, -0.919, -2.125 for DFA dye 4, DFA dye 1, DFA dye 5, DFA dye

2, DFA dye 3, DFA dye 6, DFA respectively. The above results shows that DFA dye 4

sensitizer is more suitable for DSSCs applications.

4.10.4 Evaluating open circuit voltage (Voc)

Besides the JSC, a potential cell should also possess a high open-circuit photo voltage

(VOC) value to reach high power conversion efficiency (η). For DSSCs the VOC is controlled

mainly by the recombination process occurring in TiO2/electrolyte interface. The VOC is

directly related to the CB energy (ECB). It is identified that the adsorption of a dye molecule on

TiO2 surface can induce a shift of the conduction band profile of TiO2 (QCB).Then smaller

values were found from DFA dye 3 and DFA dye 6 than for the other dyes is labelled in Table

11. Consequently, the relatively similar ΔGinject and Voc values and comparing DFA dye 3 &

DFA dye 6 the larger LHE value and smaller open circuit voltage for DFA dye 6 would lead

to higher efficiency conversion efficiencies.

V Conclusion

The vibrational frequencies of the 2,4-Difluoroanisole, fundamental modes of

the compound are precisely assigned and analyzed and the theoretical results are compared

with the experimental frequencies. Mulliken charge calculated by B3LYP/6-31G(d,p) basis set

compared with B3LYP/cc-pVTZ basis set. The molecular electrostatic potential and Fukui

function analysis pointed out the nucleophilic and electrophilic sites on the title molecule. The

thermodynamic properties and magnetic properties calculated at varying temperatures and the

relevant correlation graphs are presented. The energies of important Molecular Orbitlals,

absorption wavelength, oscillator strength and excitation energies of the compound are also

determined from the TD-DFT method. The HOMO-LUMO gap extensively decreases from

5.521 eV to 3.231 eV for DFA as the donor and acceptor varies. Comparatively, the first

hyperpolarizabilities of DFA dye 6 has the value of 2.87982E-×10−29 esu, is greater than other

DFA dyes which shows that triphenylamine as donor is suitable for nonlinear optical studies..

The best DSSC performance is observed in DFA derivatives with triphenylamine donor group

compared to other donor groups.

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