Figure 3. Neutral optimized structures of the AB1 and AB2.
The torsional angles between the phenyl and pyrazole rings of AB1 and AB2 are 46.12° and 56.99° in gas phase which is in good agreement with that of solid phases of the molecules (41.7° and 64.7° for AB1and AB2 ). When a molecule gains or loses charges, it will relax its molecular geometry for a new charge distribution[44, 59, 60]. Therefore, torsional angles in AB1 are 36.03° and 22.33° for the anionic state and cationic state, respectively while AB2has torsional angles of 56.82° and 66.97° for the anionic state and cationic state, respectively. The change of torsion angles in the cationic state for both molecules is more dominant than those in the anionic state compared with the neutral state, specifying that the hole reorganization energy is smaller than those of electron[61–63].
The bond length changes due to oxidation (electron lose from the neutral to the cation state) and reduction (electron gain form the neutral to the anion state) of AB1 and AB 2 are given in Table 4. The geometrical changes in AB2 are more conspicuous than those in AB1 , which can be taken in to account as the effect of introduction of nitro groups in the molecule. For the investigated molecules, the hole reorganization energies is smaller than those of electron since the bond-length changes upon reduction are significantly larger than those upon oxidation. In addition, the outcomes obtained from the calculation of the hole and electron reorganization energies of the molecules listed in Table 6 confirm this result. According to Marcus Electron Theory, low reorganization energy is desired property for high charge transport rate. As a result, it can be said that both compounds have hole transport property rather than electron.
Table 4 . Bond Length Alternation Values of the moleculeAB1 and AB2