3.7.3. Theoretical Electronic Absorption Spectra

The predicted electronic spectra of the resultant complexes of PA with ChA and DDQ after being calculated using the first six single-point calculations in the acetonitrile solvent system at the TD-DFT/wB97XD/6-31+G(d,2p) basis set level of theory are shown in Figure 14. The spectra were plotted by applying a Gaussian broadening of 0.333 eV half-width at half height. The theoretical electronic absorption spectra of the donor (PA) and acceptors (ChA/DDQ) as well as synthesized CT complexes (PA-ChA/PA-DDQ) are shown in Figure 14. A strong absorption band at 250 nm was observed for PA, while weak absorption maxima for ChA or DDQ were observed at 291 and 276 nm, respectively. On the other hand, two broad bands at 426 nm (excitation energy of 2.91 eV and oscillator strength of 0.041) and 622 nm (excitation energy of 1.99 eV and oscillator strength of 0.142) were observed for the two new CT complexes PA-ChA and PA-DDQ, respectively, in the acetonitrile solvent system at 298 K.

**Figure 14.** DFT calculated UV-Vis spectra of PA, ChA, DDQ, and their complexes: (**A**) PA; (**B**) ChA; (**C**) DDQ; (**D**) face-to-face I fashion complex of PA with ChA; and (**E**) face-to-face I fashion complex of PA complex with DDQ at the ωB97XD/6-311++G(2d,p) level of theory in acetonitrile solvent system.

#### **4. Conclusions**

Two new CT complexes (PA-ChA and PA-DDQ) have been synthesized from an electron donor PA and an electron acceptor ChA/DDQ using a simple, easy, and economically inexpensive synthetic method. The formation of the complexes was confirmed by various spectroscopic analysis techniques. The DFT-computed calculation strongly supports our experimental results.

**Supplementary Materials:** The following data are available online at https://www.mdpi.com/ article/10.3390/pr11030711/s1, DFT/TD-DFT calculation files.

**Author Contributions:** Conceptualization, A.F.M.M.R. and G.A.E.M.; methodology, A.F.M.M.R. and G.A.E.M.; validation, A.F.M.M.R. and G.A.E.M.; formal analysis, A.F.M.M.R., S.R., A.H.B. and G.A.E.M.; investigation, A.F.M.M.R. and G.A.E.M.; resources; A.F.M.M.R., H.A. and G.A.E.M.; Software, S.R. and A.H.B.; data curation, A.F.M.M.R., S.R., A.H.B. and G.A.E.M.; writing—original draft preparation, A.F.M.M.R., S.R., A.H.B. and G.A.E.M.; writing—review and editing, A.F.M.M.R., S.R., A.H.B., H.A. and G.A.E.M.; supervision, A.F.M.M.R. and G.A.E.M.; project administration G.A.E.M.; funding acquisition, H.A. and G.A.E.M. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Researchers Supporting Project, King Saud University, through grant No. RSP-2023R501.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** All the Supporting Data for DFT/TD-DFT calculation can be obtained upon request.

**Acknowledgments:** The authors extend their appreciation to the Researchers Supporting Project, King Saud University.

**Conflicts of Interest:** The authors clarify that this study has no competing interest.
