optoelectronic properties of four azobenzene based iminopyridine ligands for photovoltaic application

E-mail: elalamyaziz@gmail.com, Tel.: +21200773252 Abstract Thanks to their optoelectronic properties and potential applications in a wide range of electronic and optoelectronic devices

Title: Optoelectronic properties of four azobenzene based

iminopyridine ligands for photovoltaic application

Authors: Aziz El alamy, Abdelkrim El-Ghayoury, Amina

Amine, Mohammed Bouachrine

DOI: http://dx.doi.org/doi:10.1016/j.jtusci.2016.10.008

To appear in:

Received date: 5-6-2016

Accepted date: 22-10-2016

Please cite this article as: Aziz El alamy, Abdelkrim El-Ghayoury, Amina Amine, Mohammed Bouachrine, Optoelectronic properties of four azobenzene based iminopyridine ligands for photovoltaic application, http://dx.doi.org/10.1016/j.jtusci.2016.10.008

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Optoelectronic properties of four azobenzene based iminopyridine ligands

for photovoltaic application

Aziz El alamy* 1,3 , Abdelkrim El-Ghayoury 2 , Amina Amine 1 , Mohammed Bouachrine3

1

LCBAE/CMMBA, Faculty of Science, Moulay Ismail University, Meknes, Morocco

2Laboratoire MOLTECH Anjou, Université d’Angers, UFR Sciences, UMR 6200, CNRS, Bât K, 2

Bd Lavoisier, 49045 Angers Cedex, France

3ESTM, Moulay Ismail University, Meknes, Morocco

* Corresponding author E-mail: elalamyaziz@gmail.com, Tel.: +21200773252

Abstract

Thanks to their optoelectronic properties and potential applications in a wide range of electronic

and optoelectronic devices such as organic solar cells, the research in the organic -conjugated

materials encompassing both polymers and oligomers have been widely studied over the last

years In this work, a theoretical study using the DFT method on four azobenzene based

iminopyridine is reported The theoretical ground-state geometry, electronic structure and the

optoelectronic parameters (the highest occupied molecular orbital (HOMO), lowest unoccupied

molecular orbital (LUMO) energy levels, the open-circuit voltage (Voc) and the oscillator

strengths (O.S)) of the studied molecules were obtained by density functional theory (DFT) and

time-dependent (TDDFT) approaches The effects of the structure length and the substituents

on the geometries and optoelectronic properties of these materials are discussed to investigate

the relationship between molecular structure and optoelectronic properties The results of this

study are in agreement with the experimental ones and suggest these materials as good

candidates for use in photovoltaic devices

HOMO-LUMO gap

1 Introduction

In the last decade, organic electronic devices represent an important part of the electronic research; these

electronic devices need special polymers and molecules with specific and adapted properties Π

conjugated materials have much attention for optoelectronic and photovoltaic applications such as

batteries [1,2], electroluminescent devices like light-emitting diodes (LEDs) [3], organic field-effect

transistors [4] and organic photovoltaic cells (OPCs) [5-16]

Small pi-conjugated materials short-chain have attracted much attention by many researchers the organic electronic field because they are not amorphous, can be synthesized as well-defined structures [17], to their unique electronic properties, to their high photoluminescence quantum efficiency and thermal stability [18].

Azobenzene and its derivatives [19-20] have attracted attention as materials for organic electronic and photovoltaic applications [21-26] because of their optoelectronic, photochromic and absorption properties [27] and were easily synthesized

In this work, theoretical study by using density functional theory (DFT) and time-dependent (TD-DFT) methods on four conjugated compounds of azobenzene based iminopyridine ligands M1, M2, M3 and M4 (Fig 1), which were easily synthesized by a condensation reaction between N,N-Dimethyl-4,4’azodianiline or 4-(4-nitrophenylazo)aniline and 2-pyridinecarboxaldéhyde or 2,6pyridinedicarboxaldehyde [28].The geometry structures of neutral and doped forms, electronic properties and spectroscopic characteristics of these compounds have been predicted using DFT method with B3LYP/6-31G(d) calculation, the HOMO, and LUMO level energies were exanimated and the gap energy is evaluated as the difference between the HOMO and LUMO energies (Egap = EHOMO – ELUMO) The calculations were carried out using the Gaussian09 program Thus, and based on the optimized molecule structures; the ground state energies and oscillator strengths were investigated using the TD-DFT/ B3LYP/6-31G(d,p) calculations The effects of the substituents and chain length on the geometries and optoelectronic properties of these materials were investigated and discussed The computed results are in good agreement with the experiment ones and the TD-CAM-B3LYP method gives a good prediction and describes well the optical properties of these compounds in their excited states

M1 M2

M3

M4 Fig.1 Chemical structures of the studied molecules

2 Computational studies

All computations were performed using the Gaussian09 package [29] The density functional theory (DFT) with Becke’s three-parameter functional and Lee–Yang–Parr functional (B3LYP) [30] and 6-31G (d, p) basis set [31] was used to investigate the geometry and electronic properties (optimization structures, HOMO, LUMO, and gap (HOMO – LUMO) energies) of all compounds To obtain the charged (doped) structures of the studied molecules, we start from the optimized structures of the neutral form The study of the electronic transitions (Vertical electronic excitation spectra, including wavelengths, oscillators strengths (OS), and main configuration assignment) was carried out by means

of dependent density functional theory (TD-DFT) [32-35] calculations with B3LYP and CAM-B3LYP functional on the corresponding DFT-optimized structure of the ground state and employing the 6-31G(d,p) basis set In fact, these calculation methods have been successfully applied to other conjugated materials [36]

3.1 Geometric properties

The optimized structures of the four organic compounds in their neutral forms in a vacuum without symmetry constraints at B3LYP/6-31G(d,p) level are illustrated in Fig 2

M1 M2

M3

M4

Fig.2: optimized geometries obtained by B3LYP/6-31G(d, p) of the studied molecules

Fig 3: The scheme of the bond di (i=1-14) lengths and dihedral angles θi (i=1-8).

The selected dihedral angle θi (i=1-8) and bond distance parameters di (i=1-14) in neutral and doped forms are collected in Table 1,2 and Fig 3 On the one hand, the dihedral angles θ1, θ2, θ4, θ5, θ7 and θ8

of all molecules are similar and are 180° except θ3 and θ6 have a slight torsion and are in the range of 142-147°, which can due to the repulsion interaction between the hydrogen atom related to the carbon

of the fragment C=N and the hydrogen atom of the adjacent phenyl ring On the other hand, the simple bond lengths C-C; d1, d2, d4, d5, d7, d8, d10, d11, d13 and d14 for all compounds in neutral forms are all within 1.37 –1.47 Å, which is shorter by ~ 0.1 Å than that of ethane (1.54 Å) This is partly caused

by the p-bonding interaction and results in the partial double-bond character of the bridge bond, thereby strengthening and shortening the bridge bond Moreover, and going from the neutral structures to the excited ones and from M1 to M8, we found that the simple bond lengths (d1, d2, d4, d5, d7, d8, d10, d11, d12, d13 and d14) in the neutral form decrease passing from (M2 ≈ M4) to (M1 ≈ M3) and going from the neutral structures to the excited ones While, the double bond lengths (d3, d6, d9 and d12) become longer than that of the ethylene (1.34 Å), moreover, during the doping process the double bond lengths (d3, d6, d9 and d12) become longer and increase in the following order: (M2 ≈ M4) < (M1 ≈ M3) This can be explaining by the introduction of the substituents N,dimethyl (for M1 and M3) and nitro (for M2 and M4) The results of the geometric properties improve the intermolecular charge transfer (ICT) within the studied molecules

Table 1: Optimized selected bond lengths (Å) of the studied molecules in neutral and

doped forms obtained by B3LYP/6-31G (d, p) level

di

d 1 1.3788 1.3527 1.4698 1.4837 1.3788 1.3643 1.4704 1.4790

d 2 1.4028 1.3688 1.4172 1.3896 1.4027 1.3877 1.4173 1.4024

d 3 1.2676 1.2960 1.2639 1.2511 1.2676 1.2787 1.2634 1.2626

d 4 1.4124 1.3713 1.4082 1.3566 1.4123 1.3946 1.4092 1.3913

d 5 1.4027 1.3688 1.3997 1.3496 1.4027 1.3920 1.4005 1.3827

d 6 1.2811 1.2852 1.2810 1.2836 1.2809 1.2828 1.2804 1.2851

d 7 1.4726 1.4634 1.4722 1.4596 1.4728 1.4697 1.4729 1.4660

Table 2: Dihedral angle (°) values in neutral forms obtained by DFT/B3YP/6-31G (d,p)

calculation

3.2 Frontier molecular orbital and electronic properties

3.2.1 Frontier molecular orbital

The study of the frontier molecular orbitals (FMO) leads to give us an indication about the intramolecular charge transfer (ICT) in the pi-conjugated organic molecule and can inform us about excitation properties Figure 4 illustrates the frontier orbital density surfaces of HOMO and LUMO orbitals of the studied compounds As shown in Fig.4, the HOMO and LUMO are localized on the entire studied molecules, with much localization on the donor part for HOMO, and on the acceptor part for LUMO Moreover, the HOMO in the neutral form of all compounds possesses a p-anti-bonding character between two adjacent fragments and p-bonding character within subunit, whereas the LUMO exhibits a p-bonding character between the subunits

HOMO LUMO

M1

M2

M3

M4

Fig.4: The contour plots of HOMO and LUMO orbital’s of the studied compounds

3.2.2 HOMO–LUMO energy gap

The conjugated molecules are characterized by a highest occupied molecular (HOMO) orbital and a lowest unoccupied molecular orbital (LUMO) , these parameters are particularly very interesting since

in which the photoinduced electron transfers from the excited-state compound to the acceptor PCBM The energy gap (Eg) for M1, M2, M3 and M4 was obtained by the differences of HOMO and LUMO energy levels (ΔHOMO-LUMO) using B3LYP/6-31G(d,p) and the results are listed in Table 3 The calculated HOMO/LUMO (in eV) energy values of M1, M2, M3 and M4 are -5.02/-2.06 eV, -6.27/-3.03eV, -5.01/-2.13eV and -6.37/-3.17eV respectively We found a destabilization of the HOMO and LUMO energies passing from M1 to M3 and from M2 to M4, this can be explained by a π-conjugated length in M3 and M4 compared with M1 and M2 The HOMO–LUMO energy gap values are 2.96 eV (M1), 3.23 eV (M2), 2.88 eV (M3) and 3.20 eV (M4) , these values are increased in the following order M3 < M1 < M4 < M2 This may be attributed to the increase of mesomeric effect in the NO2 acceptor group (comparing M1 with M2 and M3 with M4) and to the conjugated length system in these molecules (comparing M1/M2 with M3/M4) Therefore, the obtained band gap values (2.88 - 3.23 eV) is sufficient

to consider applications of this oligomer in optoelectronic and photovoltaic devices

In addition, the higher dipole moments are beneficial to facilitate efficient intramolecular photoinduced electron transfer The calculated dipole moment of M1-M4 in their neutral and doped forms, as shown

in Table 2 the dipole moment values of these compounds are in the range 3.18-8.53 D/neutral forms and 6.96-10.78D/doped forms We can remark that the dipole moment increases in the following order M1

< M3 < M4 < M2 and this factor increases passing from the neutral to the doped forms

Table 3: Theoretical electronic properties parameters (HOMO, LUMO, Gap) by (eV)

obtained by B3LYP/6-31G(d,p) of the studied molecules Compounds EHOMO (eV) ELUMO (eV) Egap (eV) µ (Debye)

Neutral Doped Neutral Doped

Fig.5 Data of the absolute energy of the frontier orbitals HOMO and LUMO for the studied molecules

and ITO, PCBM A, PCBM and the aluminum (Al)

3.3 Absorption properties

To investigate the UV-vis absorption properties of these compounds, the vertical excitation energies (eV), wavelength absorption (λabs /nm), oscillator strengths (OS /eV) for electronic excitations and main transition contribution were carried out through The absorption spectra max of the compounds Mi

by IEF-PCM/TD-CAM-B3LYP/6-31G(d, p) in dichloromethane The calculated results are summarized

in Table 5, and the simulated absorption spectra are shown inFig 6 The calculated wavelength values were compared with experiment ones

The absorption spectra show similar profile for M1/M2 and M3/M4 which present a main intense band

at higher energies from 379 to 457 nm which was observed in the visible region and was assigned to the ICT transitions For the compounds, M1 and M2, the strongest absorption peaks arise from S0 to S2,

which corresponds to the dominant promotion of an electron from HOMO to LUMO While the strongest absorption peaks arise from S0 to S3 for M3 and M4, these transitions correspond to the dominant promotion of an electron from HOMO to LUMO+1 Moreover, the adsorption maximum of M3 is centered at 457 nm, which is 21, 76 and 78 nm red-shifted compared to the λmax of M2 (436 nm), M4 (381 nm) and M1 (379 nm) respectively This bathochromic shift due to the electron-withdrawing effect of the introduction of the NO2 group (comparing M1,M3 with M2,M4) and of the chain length effect passing from M1,M2 to M3,M4 Therefore, the CAM-B3LYP was the functional of choice for UV/Vis absorption spectra calculation for these dyes because is in excellent agreement with the experiment results (Table 4)

Table 4: Experimental (in dichloromethane 2.10-5 M) and theoretical absorption

maxima λmax (nm) for all molecules calculated by; (a) ZINDOs, (b)

TD-B3LYP/6-31G (d,p) and (c) TD CAM-B3LYP/6-TD-B3LYP/6-31G(d, p) in dichloromethane

Table 5: Absorption spectra data obtained by the IEF-PCM/TD-CAM-B3LYP/6-31G(d, p) methods for

the studied compounds in dichloromethane

transitions

Wavenumber

abs (nm)

OS

(eV) Transition (%)

exp (nm) [28] M1 S0 S1 S0 S2 S0 S3 S0 S4 S0 S5 S0 S6 22850.65 24763.81 32243.84 36779.13 36881.56 37217.90 437.62 436.81 310.13 271.89 271.13 268.68 0.0007 1.7598 0.0126 0.2165 0.0022 0.0117 HOMO-2LUMO (71%)

HOMOLUMO (84%)

HOMO-1LUMO (34%)

HOMO-1LUMO (35%)

HOMO LUMO+4 (24%)

HOMO-4 LUMO (36%)

438 M2 S0 S1 S0 S2 S0 S3 S0 S4 S0 S5 S0 S6 21185.10 26639.87 31915.57 33489.17 35394.27 35559.61 482.02 379.37 313.32 298.60 282.53 281.21 0.0002 1.6699 0.0000 0.0221 0.0212 0.0267 HOMO-1LUMO (77%)

HOMOLUMO (84%)

HOMO-9LUMO (48%)

HOMO LUMO+1 (32%)

HOMO-3LUMO (70%)

HOMO-5LUMO (79%)

380 M3 S0 S1 S0 S2 S0 S3 S0 S4 S0 S5 S0 S6 22821.61 22822.42 24311.33 25032.39 31961.55 32093.02 478.18 468.16 457.33 399.48 312.87 311.59 0.0050 0.0004 3.0397 0.6317 0.0197 0.0000 HOMO-4LUMO+1 (38%)

HOMO-5LUMO+1 (37%)

HOMOLUMO+1 (44%)

HOMO LUMO (43%)

HOMO-1LUMO+2 (17%)

HOMO-2 LUMO (23%)

457 M4 S0 S1 S0 S2 S0 S3 S0 S4 S0 S5 S0 S6 21168.97 21168.97 26238.20 27114.12 33316.57 33350.44 472.38 472.38 381.12 368.81 300.15 299.84 0.0002 0.0001 2.895 0.5920 0.0425 0.0191 HOMO-2LUMO+1 (38%)

HOMO-3LUMO+1 (37%)

HOMOLUMO+1 (45%)

HOMO LUMO (46%)

HOMO-1LUMO+2 (22%)

HOMO LUMO+2 (25%)

380