# Substitution Effects on the Optoelectronic Properties of Coumarin Derivatives
## 3 Results and Discussion
### Solvent Effects on the Absorption Spectra for the Coumarin Derivatives  
In order not to utterly neglect possible solvent effects on equilibrium conformations of the coumarin derivatives, and therefore, indirectly, on their optical properties, we performed classical MD simulations for the molecules in water. The representative conformation of the four compounds in water was extracted and the spectra was calculated for these geometries and compared with those of the vacuum-optimized one (Figure 8). For all the representative molecular structures in solvent, the optical onset is red-shifted with respect to the vacuum ones. Only in the case of molecule **D** (which belongs to heteroarylcoumarin family) the main absorption peak is nearly unchanged in the position with respect to the vacuum optimized structure. Furthermore, the oscillator strength of the main peak is lower only for molecule **D** for the TD-DFT calculations performed on the representative molecular structures obtained from classical MD simulations in water. Thus, a clear difference with respect to impact of solvent on the absorption characteristics is noted between the heteroarylcoumarin (**D**) and 3-phenylcoumarin (**A**, **B**, **C**) derivatives.  
It is evident from the absorption spectrum that molecular structures occur in the UV region (Figure 8). Therefore, one could expect the main absorption peak of the coumarin derivatives to be located in the UV region. Regarding the solvent optimized structures of **A**, **B** and **D**, the optical onset and the main absorption peak are the same, and the transitions involve an excitation of the HOMO-LUMO states (Table 6). This optical feature is conserved only for the optimized structures of **A** and **D** in vacuum (Table 4). In detail, with the solvent treatment, the main absorption peak of molecules **A**, **B** and **D** represent excitation from the HOMO\(\rightarrow\)LUMO level, while for **C** from HOMO-1\(\rightarrow\)LUMO level (Table 6). Overall, among the molecules, the optical features display same trend between the vacuum and solvent treatment.  
\begin{table}
\begin{tabular}{l c c c c c} \hline \hline
**Molecule** & **B3 LYP/6-31G* & **B3 LYP/6-311+G* & **PBE/6-311+G* \\ \hline
**(eV)** & **E\({}_{\text{OPT}}\)** & **E\({}_{\text{BIND}}\)** & **E\({}_{\text{OPT}}\)** & **E\({}_{\text{BIND}}\)** & **E\({}_{\text{OPT}}\)** & **E\({}_{\text{BIND}}\)** \\ \hline
**A** & 3.80 & 3.49 & 3.77\(\downarrow\) & 3.36 \(\downarrow\) (3.4\%) & 3.37 \(\downarrow\) & 3.59 \(\uparrow\) (2.9\%) \\
**B** & 3.59 & 3.46 & 3.58 \(\downarrow\) & 3.31 \(\downarrow\) (4.3\%) & 2.92 \(\downarrow\) & 3.70 \(\uparrow\) (6.9\%) \\
**C** & 3.50 & 3.46 & 3.47 \(\downarrow\) & 3.33 \(\downarrow\) (3.8\%) & 2.74 \(\downarrow\) & 3.83 \(\uparrow\) (10.7\%) \\
**D** & 3.35 & 3.31 & 3.24 \(\downarrow\) & 3.11 \(\downarrow\) (6\%) & 2.98 \(\downarrow\) & 3.41 \(\uparrow\) (3\%) \\ \hline \hline \end{tabular}
\end{table}
Table 5: Optical onset and exciton binding (E\({}_{\text{BIND}}\)) energy values are reported here. The symbol (\(\downarrow\)) is denotes a decrease and (\(\uparrow\)) denotes an increase in the values with respect to B3 LYP/6-31G* level of description.