*3.2. Theoretical Calculations*

Theoretical studies were realized in order to investigate the energy levels as well as the molecular orbitals (M.O.) compositions of the different dyes. DFT calculations of all synthetized compounds were performed by the B3LYP/6-311G(d,p) level of theory using Gaussian 09 programs. Dichloromethane as the solvent and the polarizable continuum model (PCM) as the solvent model were used for the TD-DFT calculations. The optimized geometries as well as the highest occupied molecular orbitals (HOMO) and the lowest unoccupied molecular orbital (LUMO), i.e., the frontier orbitals' electronic distributions of selected compounds (**PP6** and **PP16**) are given in Figure 5. The data for all compounds are supplied in the Supplementary Materials. The simulated absorption spectra are shown in the Figure 6 while the Table 2 summarizes all HOMO and LUMO energy levels and electronic transitions associated with the intramolecular charge transfer (ITC) absorption peaks. The assignment of the electronic transitions for λmax reported in the Table 2 has been determined with GaussSum 3.0 software, and especially, the contribution of the different transitions.

**Figure 5.** Optimized geometries and HOMO/LUMO electronic distributions of **PP6** and **PP16**.

As can be observed for all compounds, clear common trends are well observed for the two series:


The absorption peaks corresponded to the ICT are well observed in the Figure 6 and the values of maximal ICT absorption wavelengths are given in the Table 2. A clear trend is observed in both series: The ICT transition is mainly associated with the HOMO-LUMO transition. When the electron donor properties of the donor part increase, a clear redshift is obtained for the ICT transition. When we compare the two molecules in the two series with the same donor part, we observe that the ICT of the **EA4**-based molecule is redshift compared to that of **EA1**-based counterpart (for instance: λmax (nm) = 518 and 490 for **PP6** and **PP16**, respectively, both compounds have triphenylamine as donor part).


Within the same series, the values of EHOMO importantly vary as a function of the electron donating capacity of the donor part while the values of ELUMO slightly change. When we compare cross the two series: two molecules with the same donor part, we observe that they have nearly the same EHOMO while the ELUMO of the **EA4**-based molecule is deeper compared to that of **EA1**-based counterpart, which is in good accordance with the more electron deficient nature of **EA4** (for instance: EHOMO (eV) = −5.633 and −5.635 for **PP6** and **PP16**, respectively, while ELUMO (eV) = −2.836 and −2.694, respectively. Both compounds have triphenylamine as donor part).


The HOMO orbitals are well developed over the electron donor part while the LUMO orbitals are on the electron deficient moiety. Only a small overlap between the HOMO and LUMO orbitals is detected (see Figure 5).

(**b**)

**Figure 6.** Simulated absorption spectra in dilute dichloromethane (5 <sup>×</sup> 10−<sup>3</sup> M) of synthetized compounds **PP1**–**PP10** (**a**) and **PP11**–**PP20** (**b**).

**Table 2.** Summary of simulated absorption characteristics in dilute dichloromethane of synthetized compounds. Data were obtained in dichloromethane solution.



**Table 2.** *Cont.*
