## 3 Results and Discussion
### Frontier Molecular Orbitals (FMOs) and Ultraviolet Spectral Analysis  
The excitation energies obtained using the TD-DFT/B3 LYP method were computed for five excited electronic states. To enable a more relevant comparison between the theoretical and experimental data, the calculated transition energies were uniformly blue shifted such that the calculated and measured energies of the most intense transitions matched. The UV calculations were performed for a single molecule in the gas phase, DMSO, and acetonitrile without interactions with the environment, and the computed results were directly compared to the measured UV data for (\(2E\))-HIPC in acetonitrile solution. The major energy contribution was approximately 0.98 in HOMO to LUMO for all three UV analyses. The 100-400 nm wavelength regions of the theoretical and experimental UV spectra of (\(2E\))-HIPC are illustrated in Figure 4. The observed electronic absorption wavelengths at 250-320 nm were assigned to the \(\pi\)-to-\(\pi\)* transitions. The computed electronic absorption wavelengths were  
Figure 4: Theoretical (B3 LYP) and experimental UV-Vis spectra of (\(2E\))-HIPC. calculated to be 347.13, 304.34, 283.91, 274.33, and 267.55 nm in the gas phase; 318.40, 286.17, 278.08, 262.57, and 252.13 nm in DMSO; and 319.90, 287.47, 279.77, 265.00, and 254.05 nm in acetonitrile. The corresponding dipole moment values were 6.6951, 8.4925, and 8.6364 Debye. Based on a comparison of the dipole moments of DMSO and acetonitrile, more electronic transitions occur in acetonitrile due to its value. It is worth mentioning that the experimental and calculated absorption wavelengths are in good agreement, as shown in Figure 4.  
HOMO-LUMO or frontier molecular orbitals (FMOs) are an important measure of the nonlinear optical and electric properties, which can be obtained from quantum chemistry, UV-Vis, and NMR spectra [20]. The HOMO has the ability to donate an electron, and the LUMO, which acts as an electron acceptor, has the ability to receive an electron. GaussSum 2.2 software [21] was used to calculate the minor and major energy contributions to the HOMOs and LUMOs. The energy gap between the FMOs determines the kinetic stability, chemical reactivity, optical polarizability, and chemical hardness/softness of a molecule. Chemical hardness and softness can be used as an additive tool to describe the thermodynamic aspects of chemical reactivity. To assess the trends in the transition energies of (2\(E\))-HIPC, we carried out calculations for (2\(E\))-HIPC in acetonitrile, DMSO, and the gas phase. The energies of the FMOs for (2\(E\))-HIPC (_i.e._, HOMO, HOMO-1, LUMO and LUMO +1) were calculated at the B3 LYP/6-311++G(d,p) level, and the results are listed in Table S4. The values of the HOMO-LUMO energy gap were 4.3772, 4.3994, and 4.0621 eV for (2\(E\))-HIPC in acetonitrile, DMSO, and the gas phase, respectively. The energy gap of the FMOs explains the ultimate charge transfer and interactions of atoms within the molecule. This activity influences the biological functions of the molecule. Three-dimensional plots of the HOMO and LUMO orbitals were computed at the B3 LYP/6-311++G(d,p) level. Positive charge is shown in violet, and negative charge is shown in green. Based on the results shown in Figure 5, although the HOMO was localized on one benzene ring, the LUMO was localized on a different benzene ring as well as on the chain linking the two rings. Figure 5: 3D plots of the HOMO and LUMO of (2\(E\))-HIPC at the B3 LYP level. The values of electronegativity, chemical hardness, softness, and electrophilicity index for (2_E_)-HIPC in acetonitrile were 3.9125, 2.1886, 1.0943, and 5.2418 eV, respectively. These values are between the corresponding values for (2_E_)-HIPC in DMSO and in the gas phase. The lowest value of the dipole moment was calculated for the gas phase. The stronger intermolecular interactions in acetonitrile are due to the dipole moment value being larger than those in DMSO and the gas phase.