The calculated frontier orbitals HOMO LUMO and band gaps by using B3LYP6 31Gdp

The calculated frontier orbitals homo lumo and band

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The calculated frontier orbitals HOMO, LUMO and band gaps by using B3LYP/6-31G(d,p) level of six compounds (P1, P2, P3, P4, P5and P6) are listed in Table  2 . The values of HOMO/LUMO energies are 5.025/ 3.057  eV for P1, 5.276/ 3.293  eV for P2, 5.091/ 3.099  eV for P3, 5.139/ 3.124  eV for P4, 5.155/ 3.140 eV for P5 and 3.140/ 3.159 for P6 and corresponding values of energy gaps are 1.968 eV for P1, 1.983 eV for P2, 1.992 eV for P3, 2.015 eV for P4, 2.015 eV for P5 and 2.171 eV for P6. The calculated band gap Eg of the studied model compounds increases in the follow- ing order P1 < P2 < P3 < P4  =  P5 < P6. The much lower Eg of P1, P2 and P3 compared to that of P6 indicates a significant effect of intramolecular charge transfer, which would make the absorption spectra red shifted. However, the Eg values of P1, P2 and P3 are smaller than that of P6. This is clearly due to the effect of the electron-donor unit which is strong of P1, P2, and P3 than that of other compounds. All molecules present low energy gap are expected to have the most outstanding photophysical properties especially P1. Quantum chemical parameters Generally, the molecules having a large dipole moment, possesses a strong asymmetry in the distribution of elec- tronic charge, therefore can be more reactive and be sen- sitive to change its electronic structure and its electronic properties under an external electric field. Through the Table  2 , we can observe that the dipole moment (ρ) of compounds P1 and P4 are greater than others com- pounds, therefore we can say that these compound are more reactive that other compound, indeed, these com- pounds are more favorite to liberate the electrons to PCBM. On another side, we note that the PCBM has the small- est value of the chemical potential (μ  =   4.9) com- pared to six compounds (P1, P2, P3, P4, P5, and P6) (see Table  2 ), this is a tendency to view the electrons to escape from compound Pi has a high chemical potential to PCBM which has a small chemical potential, there- fore PCBM behaves as an acceptor of electrons and oth- ers compounds Pi behave as a donor of electrons. For the electronegativity, we remark that the PCBM has a high value of electronegativity than other compounds (P1, P2, P3, P4, P5, and P6) (Table  2 ), thus the PCBM is the com- pound that is able to attract to him the electrons from others compounds. In another hand, we remark that the PCBM compound has a high value of chemical hardness (η) in comparison with other six compounds, this indi- cates that the PCBM is very difficult to liberate the elec- trons, while the other compounds are good candidates to give electrons to the PCBM (see Table  2 ). Figure  3 shows the frontier molecular orbitals for all the Six compounds (computed at B3LYP/6-31G(d,p) level). The FMOs of all six models have analogous distribution characteristics. All HOMOs show the typical aromatic features with electron delocalization for the whole con- jugated molecule and are mainly localized at the donor parts and conjugated spacer, whereas the LUMOs are concentrated on the π-spacer and at the acceptor moie- ties (cyano acrylic unit). In another hand, the HOMO
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