Influence of push–pull configuration on the electro-optical and charge transport properties of novel naphtho-difuran derivatives: a DFT study

Abstract

We present a density functional theory (DFT) study pertaining to electro-optical and charge transport properties of two novel derivatives of diphenyl-naphtho[2,1-b:6,5-b′]difuran (DPNDF) as investigated based on push-pull configuration. Both molecular structures of the designed derivatives were optimized, in ground state (S₀) as well as excited state (S₁), using DFT and time-dependent DFT (TD-DFT) respectively. The push-pull configuration effect was studied meticulously for different electro-optical properties including adiabatic/vertical electron affinity (EAa/EAv), adiabatic/vertical ionization potential (IPa/IPv) and hole/electron reorganization energies (λ_h/λ_e), hole/electron transfer integrals (V_h/V_e), hole/electron mobility and photostability. We observed smaller λ_e, improved V_e and higher electron mobility for compound 1 compared with the parent molecule. Our calculated value of the electron mobility for compound 1 (2.43 cm² V⁻¹ s⁻¹) revealed it to be an efficient electron transport material. Moreover, the influence of the push-pull on the electronic structure was also investigated by calculating the total and partial density of states (DOS). Taking advantage of the strong push-pull configurations effect on other properties, the study of the designed chemical systems was extended to their nonlinear optical (NLO) properties. Our designed novel derivatives (1 & 2) exhibited significantly larger amplitude values for first hyperpolarizability with β_tot equal to 209.420 × 10⁻³⁰ esu for compound 1 and 333.830 × 10⁻³⁰ esu for compound 2. It was found that the first hyperpolarizability and HOMO–LUMO energy gap are in an inverse relationship for compounds 1 and 2.

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1. Introduction

Push–pull is an important strategy widely used in organic semiconductor materials (OSMs) to tune the photophysical properties.^1–4 More recently, the push–pull strategy has been a good approach for enhancing the electronic and charge transport properties.^5–8 A stronger push–pull effect represents a significant charge separation on molecular geometry and dipole moment.⁶ A push–pull configuration usually consists of an electron donating group (EDG), a π conjugation bridge and an electron withdrawing group (EWG), which is expressed as EDGs-π-EWGs. Such a type of push–pull configuration with strong EDGs and EWG results in lowering the HOMO–LUMO energy gap E_g, which leads to efficient intramolecular charge transfer⁹ and material performance that is beneficial for designing excellent OSMs. OSMs with push–pull configuration^10–12 are widely studied at both levels (theoretical and experimental) due to their light weight and their low fabrication cost on flexible substrates and large area bendy displays. These advantages of OSMs give them an edge over the silicon-based traditional inorganic semiconductors and have attracted massive interest of academic researchers as well as their industrial partners because of their potential applications in photonics and electronic devices, such as organic field effect transistors (OFETs),^13–15 organic light emitting diodes (OLEDs),^16,17 organic photo-voltaics (OPVs)^17,18 and organic light emitting transistors (OLETs).¹⁹

OFETs are fabricated and produced by adding an electron system with π-conjugation,²⁰ or an aromatic compound²¹ which helps in the orbital wave functions delocalization²² and establishes a good relationship between the geometric and electronic structure.^23–28 Several experimental and theoretical research reports are available on thiophene containing materials for use in OFETs and OLEDs.^29–38 The push–pull effect on the electronic, optical and charge transport properties of the benzo[2,3-b]thiophene derivatives has been studied theoretically.⁶ However, in the literature only a very small number of investigations has been reported about the furan containing OFETs and OLEDs materials.^39–42 Considerable attention is currently being given to furan as a basic building block for organic π-conjugated materials that are more stable and that have given an indication of their potential applications, especially in OFETs and OLEDs.^19,43–48 Binaphtha-furanyl has been reported as an OLET.¹⁹ Recently, diphenyl-naphtho[2,1-b:6,5-b′]difuran (DPNDF) has been experimentally synthesized and reported as a good hole transport OSM for OFETs.⁴⁹ In our previous study,⁵⁰ it was also found that the furan ring is one of the best electron transport materials because it demonstrates very low reorganization energy for electron λ_e.

No study on DPNDF with push–pull strategy has been found in the literature so far. In our present work, as a starting point, the experimental crystal of DPNDF⁴⁹ has been used as the parent molecule and two new structures were derived by employing a push–pull approach where EDGs were attached on one side of the naphtho-difuran (NDF) ring and EWGs on the other side (EDGs-π-EWGs). In these structures, three CH₃/OCH₃ groups were attached as EDGs at position X, whereas three CN groups were attached as EWGs at position Y for structures 1/2, respectively (named compound 1 and compound 2) (see Scheme 1 and Fig. S1 of the ESI†).

Scheme 1 Labeled diagram of DPNDF showing the positions of the attached groups.

The derived geometries were optimized in ground state (S₀) and first excited states (S₁) at the level of DFT and TD-DFT, respectively. Also, the other properties such as highest occupied molecular orbitals (HOMOs), lowest unoccupied molecular orbitals (LUMOs), HOMO–LUMO energy gap (E_g), adiabatic and vertical electron affinities (EAa/EAv), reorganization energies for hole (λ_h)/electron (λ_e), adiabatic/vertical ionization potentials (IPa/IPv), total/partial density of states (TDOS/PDOS), nonlinear optical properties (NLO), hole extraction potential (HEP), electron extraction potential (EEP) and electronegativity (χ) were computed and are discussed in detail. In addition, the transfer integrals, mobility and photostability of these compounds were evaluated. Moreover, the push–pull effect was investigated on the above mentioned properties. A comparison of our obtained results with experimental data is also made where available.

2. Computational methodology

DFT was used to optimize the initial molecular structures for S₀ by applying the hybrid exchange correlation functional B3LYP^51–53 with 6-31G** basis sets.^{30,39–42,54–56} For S₁ TD-DFT,^56–59 the hybrid functional TD-B3LYP^60–63 with the same basis set was used to optimize the geometries of the analogues. The electronic and photophysical properties including absorption (λ_abs), and emission (λ_emis) wavelengths were calculated at the same level of theory. Reorganization energy (λ) represents the geometric relaxation energy of a molecule from the charged (cation/anion) to the neutral state and from neutral to the charged (cation/anion) state (for details see the computational method expressed in the ESI†). The reorganization energy for hole (λ_h) and electron (λ_e) was evaluated as:

λ_h = λ₊ + λ₁ and λ_e = λ₋ + λ₂ (1)

where the energy of geometry relaxation from neutral to charged (cation/anion) state is λ₊ and λ₋, and the relaxation energy of a molecule from charged (cation/anion) state to neutral is λ₁ and λ₂, respectively.^64,65 These two terms were calculated directly from the adiabatic potential energy surfaces for λ_h and λ_e.^1,66,67 In the next step, the calculations related to term transfer integrals were performed. To calculate the transfer integrals, inter-molecular nearest-neighboring hopping pathways were generated using the single-crystal structure. There are two widely employed approaches to obtain transfer integrals; one is a Koopmans' theorem based method⁶⁸ and the other one is a direct evaluation method for the frontier molecular orbitals (FMOs).^69,70 We used the direct approach^69,70 to investigate the charge transport properties in this study. The hole/electron transfer integrals in this approach can be expressed as:

t_h/e = 〈ϕ^0,site1_LUMO/HOMO|F⁰|ϕ^0,site2_LUMO/HOMO〉 (2)

where t_h/e is the hole/electron transfer integrals, ϕ^0,site1_LUMO/HOMO and ϕ^0,site2_LUMO/HOMO correspond to the HOMOs and LUMOs of the two consecutive molecules when there is no contact between the adjacent molecules, and F⁰ is the Fock operator with unperturbed molecular orbitals for the dimer of a fixed pathway.

The carrier mobility μ can be evaluated with the help of the Einstein relation as:

μ = eD/k_BT (3)

where D represents a charge diffusion constant, e is the electronic charge, T is the temperature and k_B denotes the Boltzmann constant. Further details related to transfer integrals and mobility can be seen in the computational method expressed in the ESI.† All these first-principles calculations were carried out using the GAUSSIAN 09 package.⁷¹

For investigation of NLO response, we calculated the static first hyperpolarizability (β_tot) and its components by a finite field (FF) method. The FF method is broadly applied to investigate NLO because this methodology can be used with the electronic structure method to compute β values.^72–81 In some very recent reports, β_tot calculated by this method was found to be substantiated with the experimental structure–property relationship.^10,82 In the FF method, a molecule is subjected to a static electric field (F) and the energy (E) of the molecule is expressed as:

where E⁽⁰⁾ is the energy of the molecule in the absence of an electronic field, μ is the component of the dipole moment vector, α is the linear polarizability tensor, β and γ are the first and second hyperpolarizability tensors respectively, and i, j and k label the x, y and z components respectively. It is clear from eqn (4) that the values of μ, α, β, and γ can be obtained by differentiating E with respect to F. In our present investigation, we calculated the molecular first hyperpolarizability. For a molecule, the components of the first hyperpolarizability can be calculated using the following:

Using the x, y and z components, the magnitude of first hyperpolarizability (β_tot) can be calculated by the following:

where

β_x = (β_xxx + β_xxy + β_xyy),

β_y = (β_yyy + β_xxz + β_yyz),

β_z = (β_xzz + β_yzz + β_zzz)

Therefore, the complete equation for calculating the magnitude of the total first static hyperpolarizability from GAUSSIAN 09 outputs is given by:

β_tot = [(β_xxx + β_xxy + β_xyy)² + (β_yyy + β_xxz + β_yyz)² + (β_xzz + β_yzz + β_zzz)²]^1/2 (7)

Since these β_tot values of the GAUSSIAN 09 output files are reported in atomic units (a.u.), the calculated β_tot values were converted into electrostatic units (esu) (1 a.u. = 8.6393 × 10⁻³³ esu). First hyperpolarizability is a third rank tensor that can be described by a 3 × 3 × 3 matrix. The 27 components of the 3D matrix can be reduced to 10 components due to the Kleinman symmetry (β_xyy = β_yxy = β_yyx, β_yyz = β_yzy = β_zyy,… likewise other permutations also take same value).

3. Results and discussion

3.1. Ground and excited state geometries

The optimized values of bond lengths and bond/dihedral angles for the neutral, cation and anion structures are tabulated in Table S1 of the ESI.† For compound 1/2, the cation and anion structures differ from the neutral one. For compound 1, alterations in the bond lengths C₁₉–C₂₀, C₂₀–C₂₁, C₂₁–C₂₂, C₂₂–C₂₃, C₂₃–C₂₄ and C₂₃–C₂₈ were found; for the cation as 0.021 Å, −0.032 Å, 0.37 Å, −0.033 Å, 0.026 Å and 0.023 Å; for the anion as −0.003 Å, 0.00 Å, 0.001 Å, −0.001 Å, 0.004 Å and 0.002 Å, respectively; the bond angles C₁₀–C₉–C₁₄, O₁₈–C₂₂–C₂₁ and O₁₈–C₂₂–C₂₃ were altered as 1.02°, −1.08° and 1.14° for the cation and as −1.86°, −0.24° and −0.04° for the anion, respectively. Similarly the distortion in the dihedral angles C₂₀–C₂₁–C₂₂–C₂₃, O₁₈–C₂₂–C₂₃–C₂₄, O₁₈–C₂₂–C₂₃–C₂₈, C₂₁–C₂₂–C₂₃–C₂₄ and C₂₁–C₂₂–C₂₃–C₂₈ was found as −1.13°, 23.23°, −23.53°, −22.09° and 22.40° for the cation and as −0.69°, 4.25°, −4.34°, −3.64° and 3.74° for the anion, respectively, as compared with the neutral ones. A graphical representation of bond lengths in angstrom (Å) for compound 1 (left) and compound 2 (right) is shown in Fig. 1(a) for a more clear understanding of the bond length alteration. The optimized coordinates of the neutral, cation and anion structures for both the compounds are tabulated in Tables S3–S5, respectively, in the ESI.†

Fig. 1 (a) Selected optimized bond lengths in angstrom (Å) for compound 1 (left) and compound 2 (right) of ground state (neutral, cation and anion) optimized at B3LYP/6-31G** level of theory. (b) Selected optimized bond angles (degree) for compound 1 (left) and compound 2 (right) of ground state (neutral, cation and anion) optimized at B3LYP/6-31G** level of theory.

For compound 2, the bond lengths C₉–C₁₀, C₉–C₁₄ and C₁₁–C₁₂, varied; for the cation as −0.007 Å, −0.007 Å and 0.00 Å; for the anion as −0.034 Å, 0.031 Å and 0.013 Å, respectively; the bond angles O₁–C₈–C₇, C₈–C₉–C₁₄, C₁₀–C₉–C₁₄, C₁₀–C₁₁–C₁₂, C₁₁–C₁₂–C₁₃ and C₁₂–C₁₃–C₁₄ were altered as 0.67°, −0.34°, 1.02°, −0.37°, 0.58° and −0.47° for the cation and as −1.19°, 1.08°, −1.83°, 1.32°, −1.50° and 1.09° for the anion, respectively; similarly the distortions in the dihedral angles C₅–C₁–C₈–C₉, C₆–C₇–C₈–C₉, C₁–C₈–C₉–C₁₀, C₁–C₈–C₉–C₁₄, C₇–C₈–C₉–C₁₀ and C₇–C₈–C₉–C₁₄ were found as 0.37°, 0.66°, −10.12°, 10.12°, 9.39° and −9.40° for the cation and as 1.33°, 1.65°, 17.01°, −16.75°, −18.69° and 18.43° for the anion, respectively, as compared with the neutral ones.

The bond/dihedral angles (degree) are represented graphically for compound 1 (left) and compound 2 (right) in Fig. 1(b), for a more clear understanding of the bond/dihedral angle distortion. The relaxation in the geometric parameters of the compound 1 cation structure was found to be more than that of the anion, whereas for compound 2 the anion had more distortion as compared with the cation. This high distortion especially in bond/dihedral angles⁸³ might increase the reorganization energy of the compounds due to the increased polarization caused by this distortion. Generally, it has been stated that more relaxation in geometric parameters from neutral to anion/cation can increase the reorganization energy.^2,83

3.2. Electronic properties

3.2.1 Frontier molecular orbitals (ground and excited states). HOMO and LUMO formation patterns for both the compounds at S₀ and S₁ were formed at isosurface values of 0.02 and are presented in Fig. 2(a) and (b). In compound 1, for HOMO formation, it was found that the charge is delocalized on C₇–C₈, C₅–C₆, C₃–C₄, C₂–C₂₀–C₁₉, C₁₆–C₁₇, C₂₁–C₂₂, C₂₃–C₂₄ and C₂₅–C₂₆ while on C₁₀, C₁₂ and C₁₄ the charge is localized (lone-pair). Neither of the O atoms take part in the formation of the HOMO, whereas a lone-pair is formed on all N atoms. For LUMO formation, the delocalization of charge was found on C₂–C₃, C₅–C₆–C₁₅, C₇–C₈, C₈–C₉, C₁₀–C₁₁, and C₁₃–C₁₄. The charge is localized (lone-pair) on C₁₇, C₂₀ and on all N atoms. Similar patterns of HOMO and LUMO formation were found for compound 2 at S₀. Charge delocalization and localization behavior followed the same trend for compound 1/2 at S₁. It is clear from Fig. 2(a) that in the formation of HOMOs all the charge density is distributed on EDGs and the central core; for LUMOs the charge density is shifted on EWGs, revealing good intramolecular charge transfer for both the compounds.

Fig. 2 (a) HOMOs and LUMOs formation patterns at ground state. (b) HOMOs and LUMOs spreading patterns at excited state.

The HOMO energies (E_HOMO), LUMO energies (E_LUMO) and HOMO–LUMO energy gaps (E_g) at S₀ and S₁ (in the brackets) states for both the compounds are tabulated in Table 1. A graphical representation of E_HOMO and E_LUMO for S₀/S₁ is shown in Fig. 3 left and right, respectively, for a more clear understanding of the E_g. E_HOMO of compound 1 and 2 are −5.65 eV and −5.18 eV, respectively, and are in good agreement with the experimental E_HOMO (−5.48 eV)⁴⁹ and the computational E_HOMO (−5.10 eV)⁸⁴ of the parent molecule DPNDF. The trend of E_HOMO and E_LUMO, respectively, is compound 1 (−5.65 eV, −2.93 eV) > compound 2 (−5.18 eV, −2.78 eV). A similar trend of E_HOMO and E_LUMO was found at S₁ for both the compounds.

Table 1 The E_HOMO^a,b, E_LUMO and E_g for S₀ and S₁ states (in the brackets) at the B3LYP/6-31G** and TD-B3LYP/6-31G** levels of theory

Molecule	EHOMO	ELUMO	Eg
a Experimental data (EHOMO = −5.48 eV) from ref. 49.b Computed value (EHOMO = −5.10 eV) from ref. 84.
Compound 1	−5.65	−2.93	2.72
	(−5.38)	(−3.05)	(2.33)
Compound 2	−5.18	−2.78	2.40
	(−4.86)	(−2.86)	(2.00)

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Fig. 3 Comparison of E_HOMO and E_LUMO for ground (S₀) state (left) and excited (S₁) state (right) at the B3LYP/6-31G** and TD-B3LYP/6-31G** levels of theory.

The energy gap is theoretically expressed as the difference of the orbital energies between HOMO and LUMO whereas experimentally it is the lowest energy transition from the S₀ to the S₁ state, and termed as the band gap, which can be obtained from the absorption spectra. When electron promotion takes place from HOMO to LUMO, quantitatively E_g can be approximately the same as the optical band gap.^1,85–87 The trend in the E_g is as compound 1 (2.72 eV) > compound 2 (2.40 eV) for S₀ and for S₁ it is compound 1 (2.33 eV) > compound 2 (2.00 eV). The smaller E_g of compound 2 reveals the red shift in the absorption and emission wavelengths in comparison with compound 1. Similarly, the lower E_g of compound 2 illuminates the high charge transfer interaction^9,88 within the compound. A high amplitude of first hyperpolarizability (β) correlates^9,88 with a lower E_g of a compound. We anticipated that compound 2 might show larger first hyperpolarizability (β) as compared with compound 1. It has been reported earlier that an electronic system with a smaller E_g might be more reactive^83,84 than one with a larger E_g, so compound 2 may be more reactive than compound 1, revealing that the latter one would be more stable.

It is expected that compounds having a low-lying LUMO energy level might be thermodynamically more stable and charge transfer could not be quenched by electron loss. Moreover, according to Koopman's theorem the LUMO energy is directly proportional to the EA. The higher LUMO energy level is illuminating that the electron injection barrier would be small resulting in the improvement of charge injection ability. It can be seen from Table 1 that the value of E_LUMO is increased in compound 1 and 2 as compared with the computed value of E_LUMO (−2.17 eV)⁸⁴ for the parent molecule DPNDF, which would decrease the electron injection barrier resulting in an improvement of the electron injection. Therefore it is expected that the new compounds might be better materials as electron transporters.

3.3. Photophysical properties

The calculated absorption (λ_abs) and emission wavelengths (λ_emis), oscillator strengths (f) and HOMO–LUMO (H → L) contribution were evaluated and are tabulated in Table 2. The λ_abs and λ_emis against f are represented graphically in Fig. 4(a) and (b), respectively. Table 2 shows the maximum H → L contribution at the S₀, which is 99% from H → L for both compounds 1 and 2. The maximum contribution of H → L for S₁ is H → L (99%) and H → L (100%) for compounds 1 and 2, respectively. The λ_abs/λ_emis have a red shift of 131/180 nm for compound 1 and 207/294 nm for compound 2, respectively, as compared with the parent molecule of DPNDF (λ_abs/λ_emis 381/427 nm) as evaluated computationally,⁸⁴ whereas compound 2 has a red shift of 76/114 nm as compared with compound 1 for λ_abs/λ_emis, respectively. This might be due to the strong EDGs and EWGs attached to compound 2. The structure–property relationship revealed that by substituting the EDGs and EWGs, the λ_abs and the λ_emis have shown red shifted behavior. Compound 1 would be an orange light emitter while compound 2 would be a red light emitter.

Table 2 Calculated absorption (λ_abs) and emission (λ_emis) wavelengths^a (nm), oscillator strength (f) and HOMO–LUMO contribution for S₀ and S₁ states at the TD-DFT level of theories

Molecule	λabs	f	Contribution	λemis	f	Contribution
a Computed values (λabs = 381 nm; λemis = 427 nm) for comparison with ref. 84.
Compound 1	512	0.467	H → L (99%)	607	0.373	H → L (99%)
Compound 2	588	0.342	H → L (99%)	721	0.278	H → L (100%)

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Fig. 4 (a) Computed absorption spectra and (b) computed emission spectra of compounds 1 and 2 at TD-B3LYP/6-31G** levels of theory.

3.4. Density of states

As remarkable electro-optical properties are attributed to a push–pull configuration in designed chemical systems, we also calculated explicit contributions for the individual parts in the form of their PDOS as shown in Fig. 5. We define three fragments for each compound: fragment one contains the phenyl ring with EWGs; fragment two is the central core (CC); and fragment three consists of the phenyl ring with EDGs. The individual fragment represents its contribution to the TDOS of the whole molecule as shown in Fig. 5 with different curves. As shown in Fig. 5, for compound 1, the peaks from −15.0 to −8.0 eV for the valence band and from 0.0 to 7.0 eV for the conduction band are due to the similar contributions from EWGs, CC and EDGs. At the HOMO between −6.0 and −4.0 eV, the major contribution is from CC, whereas EDGs have a minor contribution. On the other hand, the EWGs take maximum part in the conduction band (−3.0 and −2.0 eV) while the CC has minimum contribution. EDGs have no contribution in the lower region of the conduction band while they have significant contribution between −1.0 to 0.0 eV and 2.0 to 7.0 eV. In TDOS the EDG contribution dominates in the lower valence bands from −13.0 to −9.0 eV and the higher conduction bands from 2.0 to 7.0 eV. The contribution of EDGs is more in the lower energy bonding molecular orbitals (−5.65 to −7.0 eV) while the EWG contribution is larger in the higher energy anti-bonding molecular orbitals (−2.93 to −3.5 eV), which facilitates as easy charge transfer during the transition process of this push–pull configuration. This contribution of TDOS/PDOS from valence and conduction bands revealed good intramolecular charge transport from EDGs to EWGs. The high intramolecular charge transport from EDGs to EWGs leads to a very large value of β. A similar trend for TDOS and PDOS was found in compound 2.

Fig. 5 Graphical representation of TDOS and PDOS for compound 1 (left) and compound 2 (right) computed at the B3LYP/6-31G** level of theory.

3.5. Charge transfer properties

The EA and IP are the most essential properties to calculate the charge transport barriers, and these were evaluated at the DFT/B3LYP/6-31G** level of theory. In OSMs, a lower IP and higher EA is very crucial to enhance the charge transport ability for electron and hole, respectively. The adiabatic/vertical IP (IPa/IPv) and adiabatic/vertical EA (EAa/EAv) of all derivatives were calculated and are tabulated in Table 3. A graphical comparison of the IPv, electronegativity and EAv is shown in Fig. 6 (left) to represent the results more clearly. In OFETs, the OSMs having high EAv and small IPv might be better for n-type and p-type charge injection, respectively.⁸⁹ From Table 3, it can be seen that compounds 1 and 2 have EAv of 1.63 and 1.49 eV, respectively, which are higher than that of the parent molecule DPNDF (0.29 eV).⁸⁴ Thus it is expected that the new designed compounds might be much better electron transport materials as compared with DPNDF. The EAv follow the same trend as E_LUMO for both compounds as it has been observed that the molecule with high E_LUMO has the higher EAv. It can be seen from Tables 1 and 3 that compound 1 has the highest E_LUMO (−2.93 eV) among the two compounds and hence has the highest EAv (1.63 eV).

Table 3 The vertical/adiabatic ionization potential and electron affinity, hole extraction potential, electron extraction potential, electronegativity and reorganization energy for hole^a λ_h/electron λ_e at the B3LYP/6-31G** level of theory. All values are in eV

	Compound 1	Compound 2
a Data for comparison (λh = 0.17 eV) from ref. 49.
IP (vertical)	6.85	6.37
IP (adiabatic)	6.68	6.15
HEP	6.53	5.95
EA (vertical)	1.63	1.49
EA (adiabatic)	1.76	1.61
EEP	1.87	3.36
χ	4.24	3.93
λh	0.32	0.42
λe	0.24	1.87

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Fig. 6 Graphical representation of IPv, electronegativity and EAv (left) and of λ_h and λ_e (right) calculated at the B3LYP/6-31G** level of theory.

Fig. 7 The definition of the Cartesian axis for optimized structures of compounds 1 and 2.

The reorganization energy is a quantity which is very important for estimation of the ability to carry the charge in a solid.^90,91 The reorganization energies at the B3LYP/6-31G** level of theory for electron λ_e/hole λ_h are given in Table 3. A graphical representation of hole λ_h and λ_e is shown in Fig. 6 (right) to represent the trend for further clarity. The calculated λ_h of the DPNDF is 0.17 eV (ref. 84) at the same level of theory and is in good agreement with the already computed value.⁴⁹ From Table 3, it can be seen that compounds 1 and 2 have λ_h of 0.32 and 0.42 eV, respectively, and λ_e of 0.24 and 1.87 eV, respectively. For compound 1 λ_h is higher than λ_e, whereas for compound 2 λ_h is lower than λ_e. The alteration and distortion in the bond/dihedral angles of the cation is more than that of the anion for compound 1, resulting in more polarization^2,83 so λ_e for compound 1 is less than λ_h. On the other hand, for compound 2, the bond/dihedral angle distortion in the anion is higher than in the cation; it might be due to this that the λ_e of compound 2 is much higher than λ_h. From this trend it is predicted that compound 1 would be good as an electron-transport material; and compound 2 might be good as a hole-transport material. The value of λ_e for compound 1 is smaller than those of diphenyl-naphtho-dithiophene (0.34 eV)⁹² and oligofuran (0.40 eV),⁹³ which indicates that the new designed compound 1 might be more efficient as an electron transport material.

The electronegativities (χ) of the two compounds are given in Table 3. The electronegativity is the power of an atom in a molecule to attract electrons towards itself. A molecule with high electronegativity might be more efficient as an electron transport material because it can pull more electrons towards itself, resulting in high electron charge transfer.^{57,71–73,94} The trend of electronegativities in compound 1 > compound 2 revealed that the former might be better as an electron transport material as compared with the latter. The reorganization energies decreased with the increase in electronegativities of the compounds. It is the same trend as for E_LUMO and EAv of the two compounds.

3.6. First hyperpolarizability

It is well known that push–pull chemical configurations usually show remarkable NLO responses. In the present investigation taking advantage of strong push–pull configurations, we have also spotlighted the NLO responses of our designed chemical systems by calculating their static first hyperpolarizabilities. The calculated values of hyperpolarizability (β) along with their individual tensor components are shown in Table 4. A well-established electronic communication of two different parts of a push–pull molecule usually accompanies a larger amplitude of its first hyperpolarizability, which is perhaps the case in our present designed compounds 1 and 2 Fig. 7. In Table 4, it can be seen that the calculated amplitudes of first hyperpolarizability (β_tot) for compound 1 and 2 are significantly larger with β_tot values of 209.420 × 10⁻³⁰ esu and 333.830 × 10⁻³⁰ esu, respectively. These values of first hyperpolarizability of compound 1 and 2 are much larger than that of proto-type urea molecule [β for urea is 0.3728 × 10⁻³⁰ esu]. These total hyperpolarizability values are dominated by their diagonal components (components along the dipole moment axis) of β_xxx. This is because there is significant charge transfer from EDGs to EWGs along the x-axis. It can be seen from Tables 1 and 4 that the first hyperpolarizability and E_g are in inverse relationship for both compounds, which supports our prediction on the basis of E_g. Thus our designed systems have significant potential for NLO applications with good stability and large first hyperpolarizability amplitudes.

Table 4 The calculated values of polarizability (α) and hyperpolarizability (β) along their individual tensor components

Compound 1			Compound 2
Component	a.u.	(×10^−30) esu	Component	a.u.	(×10^−30) esu
βxxx	−24 305	200.100	βxxx	−38 827	−335.500
βxxy	−676	−5.841	βxxy	−881	−7.613
βxyy	−59	−0.509	βxyy	119	1.028
βyyy	7	0.0604	βyyy	17	0.146
βxxz	102	0.881	βxxz	−362	−3.128
βxyz	−90	−0.777	βxyz	−103	−0.890
βyyz	−30	−0.259	βyyz	−8	−0.069
βxzz	137	1.183	βxzz	86	0.743
βyzz	11	0.095	βyzz	−3	−0.025
βzzz	0	0.000	βzzz	18	0.155
βtot	24 236	209.420	βtot	38 633	333.830

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3.7. Molecular simulation

In our previous study,⁹⁵ we optimized the initial geometry of the parent molecule at S₀ by a hybrid functional B3LYP along with the 6-31G** basis set using the GAUSSIAN 09 package. The crystal structure was simulated using facilities provided within the Materials Studio (MS) package using the same lattice parameters as used for the experimental crystal of DPNDF.⁴⁹ The crystal was simulated using a Molecular Mechanics (MM) simulation approach, and the energy of the crystal was minimized by the FORCITE module⁹⁶ with the P21/c space group as available in the MS package, which is considered to be a good tool for this purpose. The DREIDING force field⁹⁷ was used, which is suitable for these kinds of OMs with C, H, O, and N atoms. The simulated crystal structure was found to be in good agreement with the experimentally synthesized structure (see Fig. 8). In that study we described the four pathways to compute the transfer integrals and mobility. Previously, the main computed transfer integral for the parent molecule DPNDF was 36.9 meV (ref. 95) using the GAUSSIAN package at B3LYP/6-31G** level and was closer to the available data of the same crystal 39.9 meV evaluated by the ADF program.⁴⁹ Similarly, the computed mobility was (1.1 cm² V⁻¹ s⁻¹), which shows good agreement with the experimentally measured value (1.30 cm² V⁻¹ s⁻¹). These results revealed that our adopted approach was reliable to build the crystal, and to compute the transfer integrals and mobility. In the current study, the same approach has been used to simulate the crystal structures for the new designed compounds.

Fig. 8 Experimental (a) and simulated (b) crystal structure of DPNDF along (aoc) direction.

3.8. Transfer integrals

We have also evaluated four discrete nearest neighboring hopping pathways for the two compounds. Transfer integrals for electron and hole have been evaluated using the method expressed in eqn (2) and presented in Table 5. A graphical comparison of hole and electron transfer integrals is shown in Fig. 9(a) (left) for a more clear representation. It can be seen that some transfer integrals have negligibly small values so they are not discussed further here. The strongest hole/electron transfer integrals for compound 1 are 65.1/118.4 meV and for compound 2 are 12.9/−92 meV, respectively. Compound 1 has higher electron transfer integrals than compound 2, revealing that compound 1 is a better electron transport material than compound 2.

Table 5 The transfer integrals (meV), mass centers (Å) and mobilities (cm² V⁻¹ s⁻¹) for hole and electron for compound 1 and 2, computed with DFT

Molecules	Pathways	Transfer integrals^c		Mass centers	Mobility^c
		Vh^a	Ve		Hole^b	Electron
a Vh (39.9 meV) by other method from ref. 49.b Experimental hole mobility (1.30 cm^2 V^−1 s^−1) from ref. 49.c Computed values from ref. 95.
Compound 1	i	65.1	114.1	5.071	0.49	2.09
	ii	−20.6	118.4	5.078	4.89 × 10^−3	2.43
	iii	−2.9	6.2	7.661	4.37 × 10^−6	4.15 × 10^−5
	iv	−15.9	5.7	19.106	2.46 × 10^−2	1.85 × 10^−4
Compound 2	i	12.9	67	5.078	5.89 × 10^−3	2.66 × 10^−8
	ii	−6.5	−92	7.661	8.64 × 10^−4	2.15 × 10^−7
	iii	2.65 × 10^−3	6.64 × 10^−3	11.969	5.84 × 10^−17	1.43 × 10^−23
	iv	−0.15738	0.21216	19.108	1.85 × 10^−9	3.79 × 10^−17

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Fig. 9 (a) Graphical representation hole/electron transfer integrals (left) and mobilities (right) computed with DFT. (b) The dimers investigated in the present study to calculate the transfer integrals and mobilities.

3.9. Mobility

Hole and electron mobilities of both compounds for four pathways were calculated and are tabulated in Table 5. A graphical representation of hole and electron mobilities is shown in Fig. 9(a) (right) for a more clear comparison. In our previous study⁹⁵ we computed the mobility of the parent crystal DPNDF using the direct method and found the hole mobility to be 1.1 cm² V⁻¹ s⁻¹, which is in good agreement with the experimentally calculated mobility of 1.30 cm² V⁻¹ s⁻¹ of the DPNDF.⁴⁹ We have used the same method for our current study for the four nearest neighboring molecules and found that some of the pathways have very low mobility for hole and electron; hence not discussed in the text; only the highest hole/electron mobilities for two pathways of each compound are discussed in detail here. The hole mobilities of the two pathways were found as 0.49 and 4.89 × 10⁻³ cm² V⁻¹ s⁻¹ for compound 1 and as 5.89 × 10⁻³ and 8.64 × 10⁻⁴ cm² V⁻¹ s⁻¹ for compound 2, whereas the electron mobilities of the two pathways were found as 2.09 and 2.43 cm² V⁻¹ s⁻¹, for compound 1 and as 2.66 × 10⁻⁸ and 2.15 × 10⁻⁷ cm² V⁻¹ s⁻¹ for compound 2. It can be seen that compound 1 exhibits the highest electron mobility of 2.43 cm² V⁻¹ s⁻¹ for specific 2^nd pathway, which is higher than the already computed electron mobility of 1.10 cm² V⁻¹ s⁻¹ of the parent molecule DPNDF.

The four pathways are shown in Fig. 9(b) for a more clear understanding of dimers packing which may affect the mobilities.^30,49 These highest electron mobility values of the 1^st and 2^nd pathways are in the stacking direction and might be due to the smallest distance between the two molecules of the dimers. This packing and distance allowed maximum overlapping of the orbitals ensuring enhanced mobilities, whereas the 3^rd and 4^th pathways have side to side packing and greater distance between two molecules, which might be the reason for the lowest mobilities.

The highest electron mobilities for compound 1 are several times higher than DPNDF⁴⁹ and α-oligofuran⁹³ 0.0134 cm² V⁻¹ s⁻¹, hence the former is predicted to be a good electron transport material in comparison with DPNDF and α-oligofuran. It might be due to the attached EWGs (-CN), the push–pull effect and the comprehensive intramolecular charge transfer from donor to acceptor moieties. From the highest average electron/hole intrinsic mobilities (1.13/0.13) and (6.08 × 10⁻⁸/1.69 × 10⁻³) cm² V⁻¹ s⁻¹ for compound 1 and 2, respectively, we anticipate that compound 1 is a good electron transport material; compound 2 may be a hole transport material. These results support our prediction about the same materials in terms of E_LUMO, IPv, EAv and reorganization energies for hole and electron.

3.10. Photostability

Molecular electrostatic surface potentials of all compounds were mapped onto a total electron density surface as shown in Fig. 10. High electron density regions and low electron density regions are shown in indigo and green, respectively. High electron density is distributed on O and N atoms in the studied systems. Previously, the photostability of organic materials has been explained on the basis of molecular electrostatic surface potentials.⁵⁶ Recently, we pointed out that more electron density distributed on the system would favor enhanced photostability.⁹⁸ In our last study we observed that in the parent molecule DPNDF the high electron density is distributed on oxygen atoms⁹⁵ (see Fig. S2 of the ESI†). By substituting the EDGs and EWGs, the photostability was augmented in our designed molecules. The substitution of –CH₃ and –CN at the outer phenyl rings in compound 1 improved the photostability compared with the parent DPNDF molecule. High electron density distribution in compound 2 covered more area due to the –OCH₃ and –CN than in compound 1, revealing that the former would be more photostable than the latter, that is due to strong EDGs (–OCH₃) attached with compound 2. We noticed that by increasing the size and strength of EDGs, the photostability might be enhanced. Higher electron density in compound 2 would decrease the oxidation resulting in improved photostability, which is in good agreement with our previous study.⁹⁹

Fig. 10 Molecular electrostatic potential surfaces of the two compounds.

4. Conclusions

Push–pull configurations have shown interesting effects for tuning the electro-optical properties of compounds 1 and 2. In the light of our present DFT investigation, we can draw the following interesting conclusions:

1. The two designed novel compounds have higher EAv values as compared with their parent molecule DPNDF.

2. The absorption and emission spectra of designed compounds 1 and 2 have a red shift as compared with their parent molecule. This is because of well-established communication between donor and acceptor parts. The HOMOs and LUMOs in both the studied compounds are delocalized as well as localized on the central core and EWGs, respectively. The EWGs take part in the establishment of LUMOs only. The HOMO energies are in good agreement with the experimentally estimated HOMO of the parent molecule DPNDF.

3. The influence of the push–pull parts has been investigated by calculating their total and partial density of states (DOS).

4. Taking advantage of strong push–pull configurations, our designed chemical systems have also been rationalized as efficient NLO materials with significantly larger amplitudes of first hyperpolarizability for compounds 1 and 2.

5. The first hyperpolarizability and HOMO–LUMO energy gap are in inverse relationship for compounds 1 and 2.

6. The electron transfer integrals and electron mobility have been enhanced significantly in compound 1 by introducing the –CH₃ and –CN groups, respectively. So it is predicted that compound 1 would be a good electron transport material as compared to compound 2 and the parent molecule DPNDF.

7. The photostability has been enhanced significantly in compound 2 by introducing the –OCH₃ and –CN groups, because of enriched electron density distributed on these groups. As a result it is predicted that compound 2 would be more stable than compound 1.

Hence, we expect that these compounds would serve as excellent candidates for OFET, OLET and OLED applications with enhanced photostability.

Acknowledgements

The authors are grateful to the Ministry of Education/Universiti Teknologi Malaysia (UTM) for providing funding via project Q.J130000.2526.06H15 for the successful execution of this project and to King Khalid University (KKU) for providing the support and facilities to complete the research study.

References

1. Y. Li, L.-Y. Zou, A.-M. Ren and J.-K. Feng, Comput. Theor. Chem., 2012, 981, 14.
2. A. Irfan, R. Cui, J. Zhang and L. Hao, Chem. Phys., 2009, 364, 39.
3. A. É. Masunov and P. M. Zorkii, J. Struct. Chem., 1992, 33, 423.
4. S. Ananthavel and M. Manoharan, Chem. Phys., 2001, 269, 49.
5. A. Irfan, A. G. Al-Sehemi and S. Muhammad, Synth. Met., 2014, 190, 27.
6. A. Irfan, A. G. Al-Sehemi and M. S. Al-Assiri, Comput. Theor. Chem., 2014, 1031, 76.
7. A. Irfan, A. G. Al-Sehemi and M. S. Al-Assiri, J. Fluorine Chem., 2014, 157, 52.
8. A. Irfan, A. G. Al-Sehemi and M. S. Al-Assiri, J. Mol. Graphics Modell., 2013, 44, 168.
9. H. Alyar, Rev. Adv. Mater. Sci., 2013, 34, 79.
10. P. Srinivasan, T. Kanagasekaran and R. Gopalakrishnan, Cryst. Growth Des., 2008, 8, 2340.
11. M. Rao, R. Ponce Ortiz, A. Facchetti, T. J. Marks and K. S. Narayan, J. Phys. Chem. C, 2010, 114, 20609.
12. I. Osaka, T. Abe, M. Shimawaki, T. Koganezawa and K. Takimiya, ACS Macro Lett., 2012, 1, 437.
13. H. E. Katz, J. Mater. Chem., 1997, 7, 369.
14. G. Horowitz and M. E. Hajlaoui, Adv. Mater., 2000, 12, 1046.
15. C. R. Newman, C. D. Frisbie, D. A. da Silva Filho, J.-L. Brédas, P. C. Ewbank and K. R. Mann, Chem. Mater., 2004, 16, 4436.
16. C. W. Tang and S. A. VanSlyke, Appl. Phys. Lett., 1987, 51, 913.
17. P. K. H. Ho, J.-S. Kim, J. H. Burroughes, H. Becker, S. F. Y. Li, T. M. Brown, F. Cacialli and R. H. Friend, Nature, 2000, 404, 481.
18. F. Padinger, R. S. Rittberger and N. S. Sariciftci, Adv. Funct. Mater., 2003, 13, 85.
19. K. Niimi, H. Mori, E. Miyazaki, I. Osaka, H. Kakizoe, K. Takimiya and C. Adachi, Chem. Commun., 2012, 48, 5892.
20. A. W. Hofmann, Proc. R. Soc. London, 1856, 8, 1.
21. IUPAC, Compendium of Chemical Terminology (the Gold Book), ed. A. D. McNaught and A. Wilkinson, 2nd edn, 1997.
22. P. v. R. Schleyer, Chem. Rev., 2005, 105, 3433.
23. J.-L. Brédas, D. Beljonne, V. Coropceanu and J. Cornil, Chem. Rev., 2004, 104, 4971.
24. J.-L. Brédas, J. Cornil, D. Beljonne, D. A. dos Santos and Z. Shuai, Acc. Chem. Res., 1999, 32, 267.
25. J. Cornil, D. A. dos Santos, X. Crispin, R. Silbey and J. L. Brédas, J. Am. Chem. Soc., 1998, 120, 1289.
26. J. L. Bredas and G. B. Street, Acc. Chem. Res., 1985, 18, 309.
27. A. Warshel and M. Karplus, J. Am. Chem. Soc., 1974, 96, 5677.
28. S.-C. Yang, W. Graupner, S. Guha, P. Puschnig, C. Martin, H. R. Chandrasekhar, M. Chandrasekhar, G. Leising, C. Ambrosch-Draxl and U. Scherf, Phys. Rev. Lett., 2000, 85, 2388.
29. A. Irfan, M. Nadeem, M. Athar, F. Kanwal and J. Zhang, Comput. Theor. Chem., 2011, 968, 8.
30. A. Irfan, A. G. Al-Sehemi, S. Muhammad and J. Zhang, Aust. J. Chem., 2011, 64, 1587.
31. M. S. Wrackmeyer, M. Hein, A. Petrich, J. Meiss, M. Hummert, M. K. Riede and K. Leo, Sol. Energy Mater. Sol. Cells, 2011, 95, 3171.
32. S. Das, S. P. Senanayak, A. Bedi, K. S. Narayan and S. S. Zade, Polymer, 2011, 52, 5780.
33. J. A. Letizia, S. Cronin, R. P. Ortiz, A. Facchetti, M. A. Ratner and T. J. Marks, Chem.–Eur. J., 2010, 16, 1911.
34. F. Buonocore and A. Matteo, Theor. Chem. Acc., 2009, 124, 217.
35. Q.-X. Wu, Y. Geng, Y. Liao, X.-D. Tang, G.-C. Yang and Z.-M. Su, Theor. Chem. Acc., 2012, 131, 1.
36. K. N. N. Unni, S. Dabos-Seignon and J.-M. Nunzi, J. Mater. Sci., 2006, 41, 317.
37. P. Pingel, A. Zen, D. Neher, I. Lieberwirth, G. Wegner, S. Allard and U. Scherf, Appl. Phys. A, 2009, 95, 67.
38. H. Koezuka, A. Tsumura and T. Ando, Synth. Met., 1987, 18, 699.
39. Y. Miyata, T. Nishinaga and K. Komatsu, J. Org. Chem., 2005, 70, 1147.
40. Y. Miyata, M. Terayama, T. Minari, T. Nishinaga, T. Nemoto, S. Isoda and K. Komatsu, Chem.–Asian J., 2007, 2, 1492.
41. O. Gidron, A. Dadvand, Y. Sheynin, M. Bendikov and D. F. Perepichka, Chem. Commun., 2011, 47, 1976.
42. C.-C. Wu, W.-Y. Hung, T.-L. Liu, L.-Z. Zhang and T.-Y. Luh, J. Appl. Phys., 2003, 93, 5465.
43. K. Kadac, M. J. Bosiak and J. Nowaczyk, Synth. Met., 2012, 162, 1981.
44. M. Watanabe, W.-T. Su, Y. J. Chang, T.-H. Chao, Y.-S. Wen and T. J. Chow, Chem.–Asian J., 2013, 8, 60.
45. M. Nakano, S. Shinamura, Y. Houchin, I. Osaka, E. Miyazaki and K. Takimiya, Chem. Commun., 2012, 48, 5671.
46. M. Nakano, K. Niimi, E. Miyazaki, I. Osaka and K. Takimiya, J. Org. Chem., 2012, 77, 8099.
47. K. Mitsudo, J. Harada, Y. Tanaka, H. Mandai, C. Nishioka, H. Tanaka, A. Wakamiya, Y. Murata and S. Suga, J. Org. Chem., 2013, 78, 2763.
48. H. Chen, W. Delaunay, J. Li, Z. Wang, P.-A. Bouit, D. Tondelier, B. Geffroy, F. Mathey, Z. Duan, R. Réau and M. Hissler, Org. Lett., 2013, 15, 330.
49. C. Mitsui, J. Soeda, K. Miwa, H. Tsuji, J. Takeya and E. Nakamura, J. Am. Chem. Soc., 2012, 134, 5448.
50. A. R. Chaudhry, R. Ahmed, A. Irfan, A. Shaari and A. G. Al-Sehemi, Mater. Chem. Phys., 2013, 138, 468.
51. P. J. Stephens, F. J. Devlin, C. F. Chabalowski and M. J. Frisch, J. Phys. Chem., 1994, 98, 11623.
52. A. D. Becke, J. Chem. Phys., 1993, 98, 5648.
53. C. Lee, W. Yang and R. G. Parr, Phys. Rev. B: Condens. Matter Mater. Phys., 1988, 37, 785.
54. E. Cho, C. Risko, D. Kim, R. Gysel, N. Cates Miller, D. W. Breiby, M. D. McGehee, M. F. Toney, R. J. Kline and J.-L. Bredas, J. Am. Chem. Soc., 2012, 134, 6177.
55. T. Sajoto, S. P. Tiwari, H. Li, C. Risko, S. Barlow, Q. Zhang, J.-Y. Cho, J.-L. Brédas, B. Kippelen and S. R. Marder, Polymer, 2012, 53, 1072.
56. A. Irfan, J. Zhang and Y. Chang, Theor. Chem. Acc., 2010, 127, 587.
57. R. Bauernschmitt and R. Ahlrichs, Chem. Phys. Lett., 1996, 256, 454.
58. C. Van Caillie and R. D. Amos, Chem. Phys. Lett., 2000, 317, 159.
59. F. Furche and R. Ahlrichs, J. Chem. Phys., 2002, 117, 7433.
60. G. Scalmani, M. J. Frisch, B. Mennucci, J. Tomasi, R. Cammi and V. Barone, J. Chem. Phys., 2006, 124, 094107.
61. J. E. Lee, G. C. Choi, N. G. Park, Y. K. Ha and Y. S. Kim, Curr. Appl. Phys., 2004, 4, 675.
62. M. C. Zwier, J. M. Shorb and B. P. Krueger, J. Comput. Chem., 2007, 28, 1572.
63. E. Li, A. Kim, L. Zhang, Journal of Student Computational Chemistry, Spring, 2007, vol. 1, North Carolina School of Science and Mathematics.
64. N. E. Gruhn, D. A. da Silva Filho, T. G. Bill, M. Malagoli, V. Coropceanu, A. Kahn and J.-L. Brédas, J. Am. Chem. Soc., 2002, 124, 7918.
65. J. R. Reimers, J. Chem. Phys., 2001, 115, 9103.
66. A. Irfan, R. Cui and J. Zhang, Theor. Chem. Acc., 2009, 122, 275.
67. V. Coropceanu, T. Nakano, N. E. Gruhn, O. Kwon, T. Yade, K.-i. Katsukawa and J.-L. Brédas, J. Phys. Chem. B, 2006, 110, 9482.
68. B. C. Lin, C. P. Cheng, Z.-Q. You and C.-P. Hsu, J. Am. Chem. Soc., 2004, 127, 66.
69. A. Troisi and G. Orlandi, Chem. Phys. Lett., 2001, 344, 509.
70. S. Yin, Y. Yi, Q. Li, G. Yu, Y. Liu and Z. Shuai, J. Phys. Chem. A, 2006, 110, 7138.
71. M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven Jr, J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski and D. J. Fox, Gaussian 09 Revision A.02, Gaussian Inc., Wallingford, CT, 2009.
72. C. Dehu, F. Meyers and J. L. Bredas, J. Am. Chem. Soc., 1993, 115, 6198.
73. S. Muhammad, M. R. S. A. Janjua and Z. Su, J. Phys. Chem. C, 2009, 113, 12551.
74. S. Muhammad, C. Liu, L. Zhao, S. Wu and Z. Su, Theor. Chem. Acc., 2009, 122, 77.
75. S. Muhammad, H. Xu, Y. Liao, Y. Kan and Z. Su, J. Am. Chem. Soc., 2009, 131, 11833.
76. H.-L. Xu, Z.-R. Li, Z.-M. Su, S. Muhammad, F. L. Gu and K. Harigaya, J. Phys. Chem. C, 2009, 113, 15380.
77. S. Muhammad, H. Xu, M. R. S. A. Janjua, Z. Su and M. Nadeem, Phys. Chem. Chem. Phys., 2010, 12, 4791.
78. S. Muhammad, T. Minami, H. Fukui, K. Yoneda, R. Kishi, Y. Shigeta and M. Nakano, J. Phys. Chem. A, 2012, 116, 1417.
79. D. L. Wang, H. L. Xu, Z. M. Su, S. Muhammad and D. Y. Hou, ChemPhysChem, 2012, 13, 1232.
80. R.-L. Zhong, H.-L. Xu, S. Muhammad, J. Zhang and Z.-M. Su, J. Mater. Chem., 2012, 22, 2196.
81. S. Muhammad, H. Xu, Z. Su, K. Fukuda, R. Kishi, Y. Shigeta and M. Nakano, Dalton Trans., 2013, 15053.
82. A. B. Ahmed, H. Feki, Y. Abid, H. Boughzala and A. Mlayah, J. Mol. Struct., 2008, 888, 180.
83. Y. Xiaodi, L. Qikai and S. Zhigang, Nanotechnology, 2007, 18, 424029.
84. A. R. Chaudhry, R. Ahmed, A. Irfan, A. Shaari, H. Maarof and A. G. Al-Sehemi, Sains Malays., 2014, 43, 867.
85. P. J. Hay, J. Phys. Chem. A, 2002, 106, 1634.
86. A. Curioni, W. Andreoni, R. Treusch, F. J. Himpsel, E. Haskal, P. Seidler, C. Heske, S. Kakar, T. Van Buuren and L. J. Terminello, Appl. Phys. Lett., 1998, 72, 1575.
87. S. Y. Hong, D. Y. Kim, C. Y. Kim and R. Hoffmann, Macromolecules, 2001, 34, 6474.
88. V. Arjunan, P. S. Balamourougane, C. V. Mythili and S. Mohan, J. Mol. Struct., 2011, 1003, 92.
89. Y. Zhang, X. Cai, Y. Bian, X. Li and J. Jiang, J. Phys. Chem. C, 2008, 112, 5148.
90. R. A. Marcus, Rev. Mod. Phys., 1993, 65, 599.
91. J. L. Brédas, J. P. Calbert, D. A. da Silva Filho and J. Cornil, Proc. Natl. Acad. Sci. U. S. A., 2002, 99, 5804.
92. Y. Yi, L. Zhu and J.-L. Brédas, J. Phys. Chem. C, 2012, 116, 5215.
93. S. Mohakud, A. P. Alex and S. K. Pati, J. Phys. Chem. C, 2010, 114, 20436.
94. Y. Ie, Y. Umemoto, M. Okabe, T. Kusunoki, K.-i. Nakayama, Y.-J. Pu, J. Kido, H. Tada and Y. Aso, Org. Lett., 2008, 10, 833.
95. A. R. Chaudhry, R. Ahmed, A. Irfan, A. Shaari and A. G. Al-Sehemi, Sci. Adv. Mater., 2014, 6, 1727.
96. W. H. Baur and D. Kassner, Acta Crystallogr., Sect. B: Struct. Sci., 1992, 48, 356.
97. S. L. Mayo, B. D. Olafson and W. A. Goddard, J. Phys. Chem., 1990, 94, 8897.
98. A. Irfan and J. Zhang, Theor. Chem. Acc., 2009, 124, 339.
99. A. Irfan, R. Cui, J. Zhang and M. Nadeem, Aust. J. Chem., 2010, 63, 1283.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra05850j

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