# Substitution Effects on the Optoelectronic Properties of Coumarin Derivatives
## 4 Conclusions  
The aim of this work was to investigate the wide-range utility of coumarin derivatives used as enzyme inhibitors, three 3-phenylcournarin (**A**, **B**, **C**), and one 3-heteroarylcumarin (**D**) compounds. In particular, we examined the molecular structure, the electronic states (HOMO, LUMO), vertical ionization potential (IE\({}_{\rm V}\)) and electron affinity, quasi-particle gap, and optical properties of the compounds using DFT and TD-DFT methods. Coumarin was chosen as a reference molecule, considering the existence of experimental data. A good match between the calculated and experimental IE\({}_{\rm V}\) values supported the adopted computational approach. The reliability of the computed data was verified using two XC functionals (B3 LYP, PBE) and two basis sets (6-31G\({}^{\star}\), 6-311+G\({}^{\star}\)).  
Molecule **B** displayed the highest value of IE\({}_{\rm V}\) and EA\({}_{\rm V}\), while molecule **D** had the lowest HOMO-LUMO energy gap value. Concerning the optical properties, all the molecules showed first-signs of absorption (optical onset E\({}_{\rm OPT}\)) in the UV-region with molecule **D** displaying the lowest E\({}_{\rm OPT}\) and exciton binding (E\({}_{\rm BIND}\)) energy values. The exciton energy values found lie in the range of wide band gap semiconductor materials.  
To address solvent effects on the optical properties, TD-DFT calculation was performed on the extracted representative molecular structures from MD simulations in water. The first absorption peak was found to be red shifted with respect to calculations performed on geometries optimized in vacuum. Interestingly, same dominant HOMO-LUMO transition levels for the main absorption peak (and the optical onset) was prominent between the molecular structures in vacuum and their solvent counterparts.  
\begin{table}
\begin{tabular}{l l l l l l l} \hline \hline \multicolumn{1}{c}{**Molecule**} & \multicolumn{2}{c}{**B3 LYP/6-31G\({}^{\star}\) (Vacuum)**} & \multicolumn{2}{c}{**B3 LYP/6-31G\({}^{\star}\) (Water)**} \\ \hline  & **MP (eV)** & **OS** & **Transition** & **MP (eV)** & **OS** & **Transition** \\ \hline
**A** & 3.80 & 0.5 & H\(\rightarrow\)L (96.3\%) & 3.46 & 0.54 & H\(\rightarrow\)L (98\%) \\
**B** & 3.72 & 0.55 & H\(\rightarrow\)L (82.6\%) & 3.38 & 0.65 & H\(\rightarrow\)L (98\%) \\
**C** & 3.82 & 0.36 & H\(\rightarrow\)L (79.4\%) & 3.64 & 0.49 & H-1\(\rightarrow\)L (81\%) \\
**D** & 3.35 & 0.58 & H \(\rightarrow\)L (97. 8\%) & 3.31 & 0.44 & H\(\rightarrow\)L (97\%) \\ \hline \hline \end{tabular}
\end{table}
Table 6: Main absorption peak energy value, oscillator strength (OS) and their associated electronic excitations transition for the vacuum and solvent model systems, are included here. The transition weight is reported in %, and H stands for HOMO and L for LUMO.  
Figure 8: Absorption spectra of coumarin derivatives in vacuum (black) and solvent (red) calculated with B3 LYP/6-31G\({}^{\star}\) model.  
In conclusion, we provide valuable information that can inspire further research on coumarin-based derivatives as wide band gap semiconductor materials for applications in optoelectronic devices, and promote the use of DFT based computational methodology to identify and design novel molecules with useful and interesting applications.  
Conceptualization, A.K., A.F., L.P. and G.G.; Data curation, A.K.; Formal analysis, A.K., R.B., A.C., L.P. and G.G.; Funding acquisition, G.G.; Investigation, A.K., R.B., A.F. and L.P.; Methodology, A.K.; Resources, G.G.; Validation, A.K., A.C., L.P. and G.G.; Writing--original draft, A.K.; Writing--review & editing, A.K., R.B., A.F., A.C., L.P. and G.G. All authors have read and agreed to the published version of the manuscript.  
This work was partially supported by University of Cagliari.  
A.K. acknowledges the use of high performance computing facility resources of CRS4, with special thanks to [PERSON] for his constant help and assistance.  
The authors declare no conflict of interest.