Quantum Chemical Studies, Electronic & Optical properties and

Light Harvesting Efficiency of 4-Methoxybenzylchloride as -linker

with Donor-Acceptor variations effect for DSSCs performance

V. Sivagami1, *M. Karnan2 and M. Anuradha3

1,2,3PG & Research Department of Physics, Srimad Andavan Arts & Science College (Autonomous),

Trichy-620 005, Tamil Nadu, India

1siva.mallika@rediffmail.com 2karnanphysics@gmail.com*

ABSTRACT:

The optimized molecular structure of 4-Methoxybenzylchloride (4-MBC) has been

investigated using density functional theory (DFT) method. The HOMO - LUMO energies

and their band gap are calculated from DFT using the B3LYP/ 6311G++d,p basis set. The

local and global reactivity descriptors of 4-MBC are studied. Many approaches have

recently been proposed to extend the efficiency of solar cells greater than the theoretical

limit. In solar cell technology the application of methoxybenzene is used for improving sun-

light harvesting efficiency (LHE). LHE of the titled molecule and new designed dyes are

investigated using TD-DFT by using series of organic sensitizers including donors, acceptors

and binary linker conjugated bridges. The free energy change for electron injection, exciton

binding energy and open circuit voltage of 4-MBC were also obtained.

Keywords: 4-Methoxybenzylchloride; HOMO – LUMO; Local and global reactivity

descriptor; LHE; Exciton binding energy

1. Introduction

Methoxybenzene has lot of electron rich than aromatic hydrocarbon as results of the

resonance impact of methoxy cluster upon the aromatic ring and it reacts with electrophiles

within the electrophilic aromatic susbstitution reaction more quickly than benzene [1]. It may

be recognized as a monosubstituted aromatic hydrocarbon by-product that has associate in

uneven substituent connected to the phenyl ring. It’s of sizeable interest due to the

environmental concern and conjointly as a model compound for an excellent deal with

chemicals and biologically interesting system [2]. The titled compound 4-

Methoxybenzylchloride was worn to benzylate the aniline nitrogen [3]. 4-MBC is employed

as a chemical agent within the preparation of pyridone conjugated monobactam antibiotics

with gram-negative activity from Pseudomonas aeruginosa, Klebsiella pneumonia and

Escherichia coli [4]. It has been additionally utilized in the composite of 1-napthamide [5]

and diarymethanes via Suzuki cross-coupling potassium aryltrifluoroborates [6].

Dye-sensitized solar cells (DSSCs) have attracted lot of interest for the conversion of

solar power into electricity as a result of their high efficiency and low price [7]. The design

and synthesis of functional dyes became focus on current analysis in sight of their potential

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applications as sensitizers in dye-sensitized solar cells (DSSCs) technologies [8]. Many

analysts are investigating the event of latest dyes (design and synthesis) to reinforce the

photovoltaic performance of the DSSCs. During this context, theoretical studies plays a vital

role as they will offer semi-quantitative information on the effectiveness of sensitizers in dye

sensitized solar cells even before their synthesis. Many structures are developed for the

sensitizer like D-D--A [9], D-A--A [10], D--A-A [11], (D--A)2 [12].

The foremost extensively studied organic dyes sometimes adopt the donor-pi-spacer-

acceptor (D--A) structural motif so as to enhance the efficiency. The photovoltaic

properties of such dyes could also be clearly tuned by selecting appropriate groups inside the

D--A structure. The density functional theory (DFT) has emerged as a reliable standard tool

for the theoretical treatment of structures as well as electronic and absorption spectra. It’s

time-dependent extension, referred to as time-dependent DFT (TD-DFT), will give reliable

values for the valence excitation energies with standard exchange correlation functional. The

computational cost of TD-DFT calculation has maintains an uniform accuracy for open-shell

and closed-shell systems.

This paper presents the analysis of quantum chemical parameters, first order

hyperpolarizability and excitation energy of 4-Methoxybenzylchloride. The functional

groups present in methoxybenzene ends up in the variation of charge distribution within the

molecule and consequently have an effect on the structural, vibrational and electronic

parameters. The electronic property, optical property and LHE of the titled molecule and new

designed dyes are obtained theoretically. The reactive description of the compound 4-MBC

is understood by calculating local and global reactivity descriptors.

2. Computational Details

The molecular geometry optimization of 4-MBC were carried out with Gaussian 0W

software packages developed by Frisch and co-workers [13]. The global and local reactivity

descriptors of the titled compound with standard 6-31+G (d) basis sets was calculated by

using B3LYP, i.e. Becke’s three hybrid functional parameter with Lee-Yang-Parr correlation

method [14, 15].

3. Result and Discussions

3.1. Molecular Geometry

The photovoltaic performance of the titled compound can be understood by having

necessary data of an optimized molecular geometry and electron density distribution of the

organic dyes. This knowledge supports to learn the electronic and spectroscopic behaviour of

the -conjugated spacers [16]. The geometrical parameters of the titled compound 4-MBC

were optimized with Becke3-Lee-Yang-Parr (B3LYP) at 6-31G+(d) and 6-311G++(d,p) basis

sets using GAUSSIAN 09 [17]. The molecule does not possess any rotational, inversion or

reflection symmetry, the molecule is considered under C1 point group symmetry. The

optimized molecular structure of the titled molecule in accordance with the atom numbering

scheme is shown in Fig. 1. The bond length, bond angle and dihedral angle are also

calculated for the titled compound and it is appeared in Table 1. The computed bond lengths

and bond angles at B3LYP with 6-31G+(d) and 6-311G++(d,p) are compared with the

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experimental values. Both the values (calculated and experimental) are more or less

same. The theoretical calculations are executed upon isolated molecule in the gaseous phase

but the experimental results are executed to the molecules in solid state [18].

The optimized molecular geometry shows that O-CH3 is substituted in the first position and

benzyl chloride in the fourth position of the ring. The bond length between C4-C5 has

highest value with other C-C bonds in the ring. The calculated bond length of C1-O7 is

1.362Å which is 0.008 Å lower than the reported experimental value of 1.370 Å [19]. The

optimized bond angle of C3-C4-C5 has lowest value than the other C-C-C bonds in the ring.

The bond angle of C2-C1-C6 is 1.4938 Å compressed than the bond angle of C4-C5-C6, due

to the effect O-CH3 at C1 [20]. From this result it is clear that the phenyl ring appears to be

little distorted and the angles are slightly out of the perfect hexagonal structure due to the

substitution of methoxy group and benzyl chloride instead of H atom.

3.2 Designed Dyes

This was carried out to design new sensitizers for dye sensitized solar cell (DSSC).

The new designed dye has donor (D), pi-linker (), and acceptor (A) as shown in Fig. 2(a)

New dyes were designed by the structural modification of 4-Methoxybenzylchloride.

Structure of 4-MBC dyes is given in Fig. 2(b). In this structure, Benzene and Thiophene were

used as an electron-donor and Carboxyl, Cyano and Nitro groups (-COOH, -CN and -NO2)

were as an electron acceptor because of their high ability of electron- withdrawing and

bonding to semiconductor while the titled compound 4-Methoxybenzylchloride as -linker to

the donor-acceptor systems.

Fig. 1 Optimized molecular structure of 4-Methoxybenzylchloride

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Table 1: The calculated geometric parameters of 4-Methoxybenzylchloride using DFT/B3LYP/631G+d and 6311G++d,p basis set

Parameters

Bond Length (A˚)

Exp. Parameters

Bond Angle (A˚)

Exp. Parameters

Dihedral angles(A˚)

6-31G+(d) 6-311G++(d,p) 6-31G+(d) 6-311G++(d,p) 6-31G+(d) 6-311G++(d,p)

C1-C2 1.4012 1.3978 1.362 C2-C1-C6 119.6845 119.6308 120.7 C6-C1-C2-C3 0.0901 0.1383

C1-C6 1.4047 1.4015 1.384 C2-C1-O7 124.5804 124.589 C6-C1-C2-H12 -179.7804 -179.7455

C1-O7 1.3646 1.362 1.370 C6-C1-O7 115.735 115.78 O7-C1-C2-C3 179.944 179.9771

C2-C3 1.3982 1.3947 1.427 C1-C2-C3 119.4588 119.499 120.8 O7-C1-C2-H12 0.0736 0.0933

C2-H12 1.0842 1.0815 C1-C2-H12 121.201 121.1556 C2-C1-C6-C5 -0.0746 -0.1594

C3-C4 1.3984 1.3946 1.385 C3-C2-H12 119.34 119.3453 C2-C1-C6-H19 179.8536 179.8268

C3-H13 1.0879 1.0852 1.08 C2-C3-C4 121.543 121.5012 119.9 O7-C1-C6-C5 -179.9411 179.9879

C4-C5 1.4062 1.4028 1..363 C2-C3-H13 118.8758 118.922 O7-C1-C6-H19 -0.0129 -0.0258

C4-C14 1.4947 1.4917 C4-C3-H13 119.5807 119.5766 C2-C1-O7-C8 0.8987 0.8798

C5-C6 1.3886 1.3846 1.440 C3-C4-C5 118.1332 118.1827 119.3 C6-C1-O7-C8 -179.2422 -179.2758

C5-H18 1.0878 1.0851 C3-C4-C14 120.9567 120.939 C1-C2-C3-C4 -0.0748 -0.0866

C6-H19 1.0859 1.0832 C5-C4-C14 120.91 120.8782 C1-C2-C3-H13 179.6737 179.7373

O7-C8 1.4225 1.4222 1.422 C4-C5-C6 121.1594 121.1246 121.4 H12-C2-C3-C4 179.798 179.7993

C8-H9 1.0977 1.0951 1.09 C4-C5-H18 119.5831 119.5837 H12-C2-C3-H13 -0.4534 -0.3768

C8-H10 1.0912 1.0886 1.09 C6-C5-H18 119.2567 119.2914 C2-C3-C4-C5 0.0418 0.0527

C8-H11 1.0978 1.0953 1.09 C1-C6-C5 120.0209 120.0616 118.5 C2-C3-C4-C14 179.9342 -179.8224

C14-H15 1.0908 1.0875 C1-C6-H19 118.6872 118.6072 H13-C3-C4-C5 -179.7051 -179.77

C14-H16 1.0906 1.0874 C5-C6-H19 121.2918 121.3312 H13-C3-C4-C14 0.1874 0.3549

C14-Cl17 1.8488 1.849 C1-O7-C8 118.7109 118.7223 C3-C4-C5-C6 -0.0253 -0.0733

* Ref. [20]

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4. Global and Local reactivity descriptors

4.1 Global reactivity descriptor

The global reactivity descriptors such as hardness, chemical potential, softness,

electronegativity and electrophilicity index are widely used to understand the global nature of

the molecules in terms of their stability and it is possible to obtain knowledge about reactivity

of molecules based on density functional theory. From Koopman’s theorem, the ionization

potential (I) and electron affinity (A) are the Eigen value of HOMO and LUMO with the

change of sign [21]

Ionization potential (I) = - E HOMO

Electron affinity (A) = - E LUMO

Electron affinity refers to the capability of a ligand to accept precisely one electron

from a donor. Ionization potential is characterized as the amount of energy required to extract

an electron from the atom. High ionization energy demonstrates high stability and chemical

inertness and small ionization energy shows high reactivity of the atoms and molecules [22].

Absolute hardness and softness are the most important property to measure the

molecular stability and reactivity. Using Koopman’s theorem for closed shell compounds,

hardness (), softness (S) and chemical potential () can be defined as,

----------1; ----------2 and ----------3

Chemical hardness is useful in studying the stability and reactivity of compounds

in terms of HOMO and LUMO energies [23]. It measures the resistance to change the

electron distribution in a collection of nuclei and atoms. The large chemical hardness have

large excitation energies or their electron densities are difficult to alter, while the small

chemical hardness has small excitation energies ie., the electron densities are easily altered.

Fig. 2 (a): Different parts of Donor- spacer-Acceptor system

2 (b): Chemical structure of 4-Methoxybenzylchloride

R1-Benzene, Thiophene; R2 – CN, COOH, NO2

R1

R2

Fig. 2 (b)

e

D

A

h

Fig. 2 (a)

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The electrochemical potential measures an escaping tendency of an electron from an

equilibrium system and it is very well related with the molecular electronegativity in a

compound [24].

Parr et al. [25] have characterized another descriptor to compute the global

electrophilic power of the compound as electrophilicity index () as a measure energy

lowering due to maximal electron flow among donor and acceptor. It is a combined

descriptor contains electronic chemical potential and chemical hardness which explicit

tendency of nature to accept electron. They characterized electrophilicity index () as

follows:

----------4

The maximum charge transfer Nmax measures the tendency of molecule to acquire

additional electronic charge from the environment in the direction of the electrophile. It is

calculated by the following equation.

-------(5)

The two new reactivity indices nucleofugality (En) and electrofugality(Ee) are

proposed by Ayers and co-workers to quantify the nucleophilic and electrophilic capabilities

of leaving group. They can be defined as follows,

---------(6)

---------(7)

Gomez et al. proposed an uncomplicated charge transfer exemplary for donation and

back-donation of charges [26]. An electronic back-donation mechanism is an interaction

between the inhibitor molecule and the metal surface. This idea builds up that if both the

progresses occur, namely charge transfer to the molecule and back-donation from the

molecule. Thus the energy change is preciously proportional to the hardness of the molecule.

It can be denoted by the subsequent equation,

----------(8)

The E back-donation signifies that, when >0 and E back-donation <0 the charge

exchange to a molecule pursue by a back-donation from the molecule is energetically

favoured. In this practice, it is feasible to equity the stabilization among inhibiting

molecules.

The values of chemical hardness, electrochemical potential, electrophilicity index,

maximum charge transfer and E back-donation are listed in Table 2. It is shown that the value of

4-MBC has largest chemical hardness compared with the new designed dyes. The value is

decreased by substituting various donor and acceptor to the titled compound as pi-conjugated

molecule. From the results, the designed dye 4-MBC6 Thiophene as donor and NO2 as

acceptor have the lowest hardness value compared with all those dyes. If the value of

chemical potential is progressively negative, it is increasingly hard to lose an electron but

easier to increase one. It shows that the titled compound 4-MBC is less stable and more

reactive among all other compounds. It is noticeable that 4-MBC6 has the high value of

electrophilicity index which indicates that it have strong electrophiles than the others. The

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maximum charge transfer is obtained in 4-MBC6 (whose energy gap is very low) compared

to the others. The calculated value of E back-donation shows that the dye 4-MBC6 is the best

inhibitor (ie. it has the highest value) than other dyes.

Table 2: Quantum Chemical Parameters: HOMO-LUMO energies, Energy gap, Global

reactivity descriptors and Back donation of 4-Methoxybenzylchloride calculated at

B3LYP/6-311++G(d,p) basis set.

4.2 Local reactivity descriptor

Fukui function (FF) is one of the broadly used local reactivity descriptor to display

chemical reactivity and site selectivity [23]. The atom with highest Fukui function is highly

reactive compared to other atoms in the molecule. Fukui function is characterized as the

subordinate of electron density (r) as for the complete number of electrons N in the system,

at uniform external potential v(r) following on an electron due to all the nuclei in the system,

where is chemical potential of the system.

In chemical reaction, a change in the number of electrons involves the addition or

subtraction of at least one electron in the frontier orbital. During the reaction, behaviour of

electrophilic and nucleophilic attack depends on the local behaviour of molecule. Thus, the

calculated Fukui function helps us to determine active sites of a molecule, based on the

electron density changes experience by it.

However, for studying the local reactivity at the atomic level, the more convenient

way of calculating Fukui function is through the condensed forms of the Fukui functions for

an atom k in a molecule which are expressed as follows: [27]

, for nuclephilic attack,

, for electrophilic attack,

, for radical attack

Parameters

Values (eV)

4-MBC 4-MBC1 4-MBC2 4-MBC3 4-MBC4 4-MBC5 4-MBC6

HOMO energy -6.3669 -6.3312 -6.1881 -6.5293 -6.0997 -5.9590 -6.2923

LUMO energy -0.8941 -1.6650 -1.7847 -2.6645 -1.7115 -1.8179 -2.6784

Energy gap 5.4728 4.6662 4.4033 3.8648 4.3881 4.1410 3.6139

Hardness(η) 2.7364 2.3331 2.2017 1.9324 2.1941 2.0705 1.8070

Softness(S) 0.1827 0.2143 0.2271 0.2587 0.2279 0.2415 0.2767

Chemical potential(μ) -3.6305 -3.9982 -3.9865 -4.5970 -3.9057 -3.8885 -4.4854

Electrophilicity index (ω) 2.4084 3.4258 3.6091 5.4678 3.4762 3.6514 5.5670

Charge Transfer (ΔNmax) 1.3268 1.7137 1.8107 2.3789 1.7801 1.8780 2.4823

Nucleofugality (ΔEn) 0.1461 0.5942 0.7234 1.8370 0.6676 0.7981 1.9851

Electrofugality (ΔEe) 7.4072 8.5905 8.6964 11.0309 8.4789 8.5752 10.9558

Back donation (ΔEback-donation) -0.6841 -0.5833 -0.5504 -0.4831 -0.5485 -0.5176 -0.4517

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Where qk(N) is the charge on the kth atom for neutral molecule while qk(N+1) and

qk(N-1) are the same for its anionic and cationic species respectively. These calculations are

performed at the equilibrium geometries of the neutral charge state of the molecule. Fukui

functions and will give the regions at which the molecule is most able to accommodate

the addition and removal of an electron, respectively. The large values of will point out

the molecular regions most susceptible to nucleophilic attacks, while large values will

couple with regions susceptible to electrophilic attacks. In molecular system, the atomic site

which posses highest condense Fukui function favours the higher reactivity. Lee et al. [28]

have studied the condensed Fukui function and concluded that the most reactive site during

the chemical reaction has the higher value of fk.

Morel et al., [29] have recently proposed a dual descriptor (f(r)), which is defined as

the difference between nucleophilic and electrophilic fukui function. It is given by the

equation,

]

According to dual descriptor fr provides the difference between nucleophilic and

electrophilic attack at a particular site with their sign. If fr > 0, the site is favoured for a

nucleophilic attack, whereas if fr < 0, then the site is favoured for an electrophilic attack.

The values of local reactivity descriptors are calculated at B3LYP/6-311G++(d,p) method

from Mulliken atomic charges in molecules which are presented in Table 3. According to

the condition for dual descriptor, the nucleophilic site for titled compound 4-MBC Positive

i.e. fr>0 is C1, C2, C4, C5, C6, O7, C8, H9, H10, H11, H12, H13, H16, H18 and H19.

Similarly the electrophilic site Negative i.e. fr<0 is C3, C14, H15 and Cl17.

Table 3: Condensed Fukui functions for 4-Methoxybenzylchloride calculated at B3LYP/

6-311G++(d,p) method

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

e fkr Δfr sk

n ske sk

r ωk+ ωk

- ωk°

C1 -0.124 -0.163 -0.159 0.039 -0.005 0.017 0.044 0.015 -0.002 0.007 0.128 -0.015 0.056

C2 0.141 0.190 0.295 -0.049 -0.105 -0.077 0.056 -0.019 -0.040 -0.030 -0.160 -0.344 -0.252

C3 -0.681 -0.346 -0.180 -0.335 -0.166 -0.251 -0.169 -0.129 -0.064 -0.097 -1.097 -0.543 -0.820

C4 0.773 0.320 0.156 0.452 0.164 0.308 0.288 0.174 0.063 0.119 1.479 0.537 1.008

C5 -0.057 -0.099 -0.093 0.042 -0.006 0.018 0.048 0.016 -0.002 0.007 0.136 -0.020 0.058

C6 -0.450 -0.571 -0.491 0.121 -0.080 0.020 0.202 0.047 -0.031 0.008 0.397 -0.263 0.067

O7 -0.362 -0.345 -0.238 -0.016 -0.108 -0.062 0.092 -0.006 -0.042 -0.024 -0.053 -0.353 -0.203

C8 -0.420 -0.410 -0.382 -0.010 -0.028 -0.019 0.017 -0.004 -0.011 -0.007 -0.033 -0.090 -0.062

H9 0.224 0.230 0.271 -0.006 -0.041 -0.023 0.034 -0.002 -0.016 -0.009 -0.021 -0.133 -0.077

H10 0.213 0.234 0.281 -0.021 -0.048 -0.034 0.027 -0.008 -0.018 -0.013 -0.067 -0.156 -0.111

H11 0.224 0.227 0.269 -0.003 -0.042 -0.022 0.038 -0.001 -0.016 -0.009 -0.010 -0.136 -0.073

H12 0.227 0.212 0.264 0.015 -0.051 -0.018 0.066 0.006 -0.020 -0.007 0.049 -0.168 -0.060

H13 0.188 0.200 0.262 -0.012 -0.063 -0.037 0.051 -0.004 -0.024 -0.014 -0.038 -0.206 -0.122

C14 -0.801 -0.604 -0.528 -0.197 -0.076 -0.136 -0.121 -0.076 -0.029 -0.053 -0.643 -0.249 -0.446

H15 0.163 0.280 0.309 -0.116 -0.030 -0.073 -0.087 -0.045 -0.011 -0.028 -0.380 -0.097 -0.239

H16 0.271 0.276 0.310 -0.004 -0.035 -0.019 0.030 -0.002 -0.013 -0.007 -0.014 -0.113 -0.064

Cl17 -0.916 -0.066 0.097 -0.850 -0.163 -0.506 -0.687 -0.327 -0.063 -0.195 -2.779 -0.532 -1.656

H18 0.180 0.205 0.262 -0.024 -0.057 -0.041 0.033 -0.009 -0.022 -0.016 -0.080 -0.188 -0.134

H19 0.207 0.233 0.295 -0.025 -0.062 -0.044 0.037 -0.010 -0.024 -0.017 -0.083 -0.203 -0.143

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5. Electronic and UV-Vis spectral properties

5.1 Electronic Properties

Inhibition efficiency of the molecule is closely related to frontier molecular orbital

[30]. The HOMO and LUMO are combined to form the frontier molecular orbital (FMO).

Among electronic applications of these materials as an organic solar cell, the theoretical

knowledge about the energy levels (HOMO and LUMO) of the components is mandatory

[31]. HOMO means the Highest Occupied Molecular Orbital which is the outer most orbital

acts as an electron donor, similarly LUMO means Lowest Unoccupied Molecular Orbital

which is the inner most orbital contains free places to accept electrons [32]. The HOMO and

LUMO energy levels of the donor and the acceptor segments for photovoltaic devices are an

appropriate essential element to decide if the effective charge transfer will occur among

donor and acceptor [33].

The calculated frontier orbital such as HOMO energy, LUMO energy and band gaps

by using B3LYP/6-311G++(d,p) basis set of the titled compound and for six newly designed

dyes are listed in Table.2. The corresponding HOMO-LUMO images for 4-MBC and

designed dyes are shown in Fig. 3(a) & (b). The calculated energy gap (Eg) of the studied

compounds decreases in the following order 4-MBC > 4-MBC1 > 4-MBC2 > 4-MBC4 > 4-

MBC5 > 4-MBC3 > 4-MBC6. However, the dye 4-MBC6 has the most outstanding photo

physical property than the others (ie. band gap value is smaller than that of other Dyes and

the titled compound). The much lower band gap of 4-MBC6 shows a powerful impact of

intramolecular charge transfer.

E HOMO =-6.367

eV E LUMO = -0.894 eV E =5.472 eV

Fig. 3(a) HOMO – LUMO plot of 4-Methoxybenzylchloride

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E LUMO =-1.665 eV EHOMO = -6.331eV E = 4.666eV

E LUMO = -1.784eV EHOMO = -6.188eV E = 4.403eV

E LUMO = -2.664eV EHOMO = -6.529eV E = 3.864eV

E LUMO = -1.711eV E = 4.388eV EHOMO = -6.099eV

E LUMO = -2.678eV

E LUMO = -1.817eV E = 4.141eV

E =3.613 eV

EHOMO = -5.959eV

EHOMO = -6.292eV

Fig. 3(b) HOMO – LUMO plot of different D--A

4-MBC1

4-MBC2

4-MBC3

4-MBC4

4-MBC5

4-MBC6

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5.2 Non-Linear Optical Properties

Dipole moment is another necessary electronic criterion that results from uneven

distribution of charges on different atoms in a molecule. It is primarily helps to investigate

the intermolecular interactions associate with Van-der Waal’s dipole-dipole forces etc., As a

result, if the dipole moment is large it which has stronger intermolecular attraction [34]. The

first order hyperpolarizability of titled compound 4-MBC is calculated using B3LYP/6-

31G+(d) and 6311G++(d,p) basis set, established on finite field approach. The components

of first order hyperpolarizability () are characterized as the coefficient in Taylor’s series

expansion of energy in an external electric field [35].

The energy of an uncharged molecule under a weak, general electric field can be

expressed by Buckingham type expansion [36].

E= E0 - F - FF - FFF+.....

where, E is the energy of a molecule under the electric field F, E0 is the energy of an

unperturbed free molecule, F is the field at the origin, , and are the components of

dipole moment, polarizability and the first order hyperpolarizabilities respectively. The total

static dipole moment (), the mean polarizability (0), the anisotropy of the polarizability

() and the mean first order hyperpolarizabilty (0), are defined by using the x, y and z

components. It is as follows,

The total static dipole moment is,

The isotropic polarizability is,

The anisotropic polarizability is,

The mean first order hyperpolarizability is,

Where,

Since the output values of polarizabilities () and first order hyperpolarizability ()

using Gaussian 09 are reported in atomic units (a.u.). The calculated values are converted

into electrostatic units (e.s.u.) (Note: 1 a.u. = 8.639 10-33 e.s.u.)

The values of total dipole moment () and first order hyperpolarizability () of 4-

MBC for monosubstituted molecule and the new designed dyes are calculated and listed in

Table 4. Total dipole moment and first order hyperpolarizability for 4-MBC is 3.3156 Debye

(2 times greater than those of Urea) and 5.362210-30 e.s.u. (14 times greater than those of

Urea) respectively for the basis set B3LYP/6-31+G(d) (Note: and of urea are 1.3732

Debye and 0.372810-30 e.s.u). The better results were obtained by using different donors and

acceptors and the titled compound is as - linker (D--A).

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Table 4: The DFT/ B3LYP/6-31G+(d) and B3LYP/6311G++(d,p) calculated dipole moments (Debye), polarizability (a.u), β components

and βtot (esu) value of 4-Methoxybenzylchloride and its derivatives (NON LINEAR OPTICAL PROPERTIES)

Parameters B3LYP/

6-31+G(d)

B3LYP/

6-311++G(d,p)

631G+d

(Ben+CN)

631G+d

(Ben+COOH)

631G+d

(Ben+NO2)

631G+d

(Thio+CN)

631G+d

(Thio+COOH)

631G+d

(Thio+NO2)

x 2.7138 2.693 2.7538 0.2252 3.2416 2.9161 -0.2174 -3.5147

y -1.0032 -1.0089 1.8874 -4.1404 2.2186 0.954 -3.4845 -1.2707

z 1.6193 1.5768 -0.0002 0.0033 0.0003 -0.0016 0.002 -0.0011

tot 3.3156 3.2796 3.3385 4.1465 3.9281 3.0682 3.4913 3.7374

xx -66.5115 -66.5003 -127.2485 -112.3741 -128.196 -125.791 -113.222 -127.1023

yy -62.157 -62.1015 -100.6668 -107.1696 -109.771 -104.451 -108.6588 -113.1657

zz -69.0163 -68.9277 -114.0532 -119.4689 -117.943 -114.622 -120.04 -118.5047

xy -6.1297 -6.1046 5.6076 -10.8488 2.7142 5.3201 -12.6338 1.5725

xz -6.3957 -6.212 0.0193 0.0018 0.0172 0.0142 0.0069 -0.0112

yz -0.4115 -0.4124 -0.0073 0.0001 -0.0108 -0.0049 -0.0036 0.0083

Δ(esu) -65.894933 -65.843167 -113.9895 -113.0042 -118.63667 -114.95467 -113.9736 -119.5909

xxx 73.5557 72.6741 157.1295 10.9559 97.6919 156.508 -21.0129 -108.9558

yyy 2.9578 2.8828 68.5268 -86.2186 76.5731 52.3436 -82.5801 -64.1085

zzz -6.4244 -6.4153 -0.0112 0.011 0.0152 -0.0053 0.0166 0.021

xyy 0.8919 0.9673 42.0855 0.3818 35.1522 54.6303 3.3881 -41.9025

xxy -15.336 -15.1788 -58.6036 -42.6494 -34.0679 -69.4425 -36.7703 41.0374

xxz 8.9992 8.6291 -0.077 0.0537 -0.0442 -0.0803 0.032 -0.0549

xzz -4.1443 -3.93 -15.1825 28.4926 -31.8744 -10.8873 23.7622 26.9589

yzz -1.8406 -1.9025 1.8109 -3.8961 5.6823 3.3028 -6.3605 -8.2553

YYZ 0.9769 0.9818 0.0362 0.0689 0.044 0.0076 0.0551 0.0195

XYZ 1.4543 1.41 0.0548 -0.0109 0.052 0.0789 0.0078 0.0755

tot(esu) 5.3622E-30 5.3174E-30 1.3769E-29 1.035E-29 8.3537E-30 1.4988E-29 9.3976E-30 9.5423E-30

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From the results the new designed dye 4-MBC4 (ie. Thiophene and Cynate are

substituted as donor and acceptor) has the total dipole moment 3.0682 Debye (2 times greater

than those of Urea) and first order hyperpolarizability 1.498810-29 e.s.u., (40 times greater

than those of Urea) respectively. The large value of hyperpolarizability is a measure of non-

linear optical activity of the sub-atomic system is related with the intermolecular charge

transfer, resulting from the electron cloud movement through pi-conjugated system from

electron benefactor to electron acceptor gatherings.

5.3 UV-Visible Absorption Spectra

The absorption wavelength (), vertical excitation energy (E) and oscillator strength

(f) of 4-MBC and therefore the new designed dyes are calculated by using TD-DFT/

B3LYP/6-31G+(d) level. The calculated values are listed in Table 5 that demonstrates the

Fig. 4 (a), (b), (c) & (d) Theoretical UV-Vis spectrum of 4-Methoxybenzylchloride and designed dyes

Fig. 4 (a) Fig. 4 (b)

Fig. 4 (c) Fig. 4 (d)

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lowest singlet electronic excitation is characterized as a typical - transition. They are

because of electron motions between the frontier molecular orbital. All transitions from

ground state to excited state (i.e. HOMO to LUMO), the most probable transition is which has

the largest oscillator strength [33]. The theoretically calculated absorption spectra of 4-MBC

and their new designed dyes are shown in Fig. 4. The dye 4-MBC6 has maximum absorption

wavelength whose value is 369.58 nm. From the results, it is clear that the maximum

absorption wavelength shows a red-shift with respect to 4-MBC.

6. Photovoltaic Properties

6.1 Light Harvesting Efficiency for DSSCs

The Light Harvesting Efficiency (LHE) is incredibly vital factor for the organic dyes

considering the role of dyes within the DSSC, i.e. absorbing photons and injecting photo

excited electrons to the conduction band of semiconductor [37]. It often be expressed as,

LHE = 1 – 10-A = 1 – 10-f

Where, f is that the oscillator strength of dye associate to the wavelength max. We

have a tendency to determine that the larger value of f, obtained the higher LHE value. For an

efficient photocurrent response, the LHE of the dye molecule ought to be high. The D--A

structure scheme is shown in Fig. 9a and chemical structure for designing new dyes is shown

in Fig. 9b. All these theoretically calculated LHE values are performed in gas phase and it is

shown in Table 5. The dye 4-MBC1 has the largest light harvesting efficiency value

compared with other dyes and also the titled compound.

Table 5: Calculated light harvesting efficiency (LHE) of 4-Methoxybenzylchloride and

the designed dyes using TD-DFT/B3LYP/6-31G+(d) basis set.

System Wavelength

(λ) nm

Excitation

energy (E) eV

Oscillator

Strength (f) LHE

4-MBC 215.65 5.7492 0.2335 0.41588

4-MBC1 ( Ben+CN) 285.91 4.3365 0.4953 0.68033

4-MBC 2 (Ben+COOH) 304.15 4.0765 0.4665 0.65684

4-MBC3 (Ben+NO2) 348.01 3.5626 0.3622 0.56569

4-MBC4 ( Thio+CN) 302.37 4.1004 0.4926 0.67834

4-MBC 5 (Thio+COOH) 321.34 3.8583 0.4688 0.66022

4-MBC 6 (Thio+NO2) 369.58 3.3548 0.3572 0.56066

6.2 Electron injection

Preat et al. [38-40] theoretically proposed a method to quantify the electron injection

from an excited state of the molecule to the conduction band. The efficiency of solar cell is

highly depends on the amount of electron injection from excited state of the molecule to the

conduction band of the semiconductor (TiO2).

The free energy change (in eV) for the electron injection can be expressed as,

--------(1)

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Where is the oxidation potential of the dye in the excited state and is the

reduction potential of the semiconductor conduction band. Two models can be used for the

evaluation of [41].

In the first method, it is assumed that the electron injection occurs from an unrelaxed

excited state of the dye molecule and the excited state oxidation potential is calculated from

the redox potential of the ground state ( ) and the absorption energy associated with the

photoinduced intramolecular charge transfer ( )

------(2)

------(3)

In the second method, it is assumed that the electron injection occurs from the relaxed

excited state of the dye molecule and is calculated as,

------(4)

The 0-0 transition energy is calculated using max, total energy of first excited

state in ground state symmetry Es1(Qs0) and the total energy of first excited geometry Es1(Qs1)

as,

The and values for 4-MBC and new designed dyes are calculated

using eqn. (1) and (2) and it is shown in Table 6. For the effective functioning of solar cell,

the amount of electron injection from dye molecule to the conduction band of the

semiconductor should be high. If the calculated value of is negative which implies

that they are exergonic injection reaction which is favourable for electron transfer [42]. This

means that the excited state of dyes lie above the conduction band of TiO2, thus favouring the

electron injection from the excited state dyes to the conduction band of TiO2 (-4.0 eV). This

shows a good electron injection from these dyes to the acceptor TiO2, indicating that these

dyes may be good candidates for application in photovoltaic devices.

Table 6: The calculated redox potential of the ground state (EOXdye), oxidation potential

of the dye ( ), absorption energy ( ), free energy change for electron injection

(Ginject) of 4-Methoxybenzylchloride and the designed dyes using B3LYP/631G+d basis

set.

System EOXdye EOX dye* max max

ICT G inject

4-MBC 6.3669 1.19 239.49 5.1769 -2.81

4-MBC1 ( Ben+CN) 6.3313 1.9948 285.91 4.3365 -2.0052

4-MBC 2 (Ben+COOH) 6.1881 2.1116 304.15 4.0765 -1.8884

4-MBC3 (Ben+NO2) 6.5294 2.9668 348.01 3.5626 -1.0332

4-MBC4 ( Thio+CN) 6.0997 1.9993 302.37 4.1004 -2.0007

4-MBC 5 (Thio+COOH) 5.9590 2.1007 321.34 3.8583 -1.8993

4-MBC 6 (Thio+NO2) 6.2924 2.9376 369.58 3.3548 -1.0624

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6.3 Exciton binding energy and Open circuit voltage

Excitons arise from an electron – hole attraction in gapped periodic systems like bulk

insulators, semiconductors as well as in many varietites of nano-scale systems, polymers and

biomolecules [43, 44]. Certain excitons seem in optical spectra of extended systems as

distinct absorption peaks below the quantum gap, whereas continuum excitons enhance the

band-edge absorption [45]. Excitons play a vital role in photovolatics, wherever photoexcited

excitons propagate to hetero-junctions and dissociate to yield currents.

Table 7: Exciton binding energy (Eb) and open circuit voltage (VOC) of 4-

Methoxybenzylchloride and designed dyes are calculated using B3LYP/631G+d basis

set.

System EHOMO

(eV)

ELUMO

(eV) Eg (eV) Ex (eV) Eb (eV) VOC (eV)

4-MBC -6.3669 -0.8942 5.4728 5.1769 0.2959 3.1058

4-MBC1 ( Ben+CN) -6.3313 -1.6651 4.6662 4.3365 0.3297 2.3349

4-MBC 2 (Ben+COOH) -6.1881 -1.7848 4.4033 4.0765 0.3268 2.2152

4-MBC3 (Ben+NO2) -6.5294 -2.6645 3.8648 3.5626 0.3022 1.3355

4-MBC4 ( Thio+CN) -6.0997 -1.7116 4.3881 4.1004 0.2877 2.2884

4-MBC 5 (Thio+COOH) -5.9590 -1.8180 4.141 3.8583 0.2827 2.182

4-MBC 6 (Thio+NO2) -6.2924 -2.6784 3.6139 3.3548 0.2591 1.3216

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

dissociate to separate positive and negative charges to escape from recombination due to the

coulombic attraction. To achieve this process, the binding energy has to be overcome i.e. the

dye molecule should posses less exciton binding energy for high energy conversion. The

exciton binding energy was calculated using the following formula [44, 46].

where Eg is the band gap (i.e. energy difference between HOMO-LUMO) and EX is

the optical gap and is defined as the first singlet excitation energy max. The maximum open

circuit voltage (VOC) is an important photovoltaic parameter that can be determined

theoretically by the difference between HOMO of the dye and LUMO of an electron acceptor

TiO2. The theoretically calculated data of VOC is determined from the following equation

[47]:

--------- (5)

While in DSSCs, VOC can be approximately estimated as the difference energy

between LUMO of the dye and conduction band (CB) of the semiconductor TiO2 (ECB = - 4.0

eV):

---------(6)

The calculated exciton binding energy and open circuit voltage of 4-MBC and the

designed dyes are listed in Table 7. The result shows that the dye 4-MBC6 is most suitable

for DSSC application by the substitution of Thiophene as donor and NO2 as acceptor. If the

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values of VOC are positive, suggests that the electron transfer will be easy from the dyes to

TiO2. Further, these values are sufficient to obtain the high efficient electron injection.

Besides, these compounds can be utilized as sensitizers by virtue of an electron injection

process from an excited molecule to the conduction band of semiconductor (TiO2).

7. Conclusion

In this present work, the active sites for an electrophilic and nucleophilic

reactions are also observed by the local reactivity descriptors (Fukui function) of 4-

methoxybenzylchloride. The theoretical analysis on organic sensitizers including various

donor and acceptors of 4-MBC and the designed dyes 4-MBC1 to 4-MBC6 have been studied

by TD-DFT / B3LYP / 6-31G+(d) method. The Non-linear optical property of the titled

compound is calculated theoretically by the determination of first order hyperpolarizability.

From the results, it have been seen that 4-MBC4 has the greater value than those of Urea, and

then the titled compound is good candidature for NLO study. Light Harvesting Efficiency

(LHE), free energy change for electron injection ( ), exciton energy (Eb) and open

circuit voltage (VOC) for the titled compound and new designed dyes are calculated. The

global reactivity descriptors (such as chemical hardness, electrochemical potential,

electrophilicity index) UV-Visible absorption spectra were also obtained theoretically. All

the new designed dyes are red shifted as compared with the titled compound. From the above

results, it is clear that the compound 4-MBC based dyes having the best photovoltaic

properties for the dye sensitized solar cells (DSSCs).

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Journal of Information and Computational Science

Volume 9 Issue 8 - 2019

ISSN: 1548-7741

www.joics.org709