# Substitution Effects on the Optoelectronic Properties of Coumarin Derivatives
## 3 Results and Discussion
### Optical Properties  
Time-dependent density function theory method was employed at B3 LYP/6-31G\({}^{*}\) and B3 LYP/6-311+G\({}^{*}\) levels to calculate the absorption spectra of the compounds optimized ground state structure in the gaseous phase. The peaks in the absorption spectrum reported with a Lorentzian broadening allowed characterizing the behavior of the molecules when irradiated. The first absorption peak corresponds to the optical onset of the compound and the main peak corresponds to the one with the maximum value of transition intensity (Figure 6). Figure 5: Electrostatic potentials mapped on electron density isosurfaces for coumarin and molecules **A–D** calculated by DFT (contour plot 0.002). The colors are mapped in accordance with the EP at the point on the isosurfaces. Figure 6: Calculated absorption spectra of coumarin derivatives using B3 LYP XC functional with 6-31G\({}^{*}\) (black) and 6-311+G\({}^{*}\) (red) basis sets, respectively. All the four derivatives display optical onset in the near ultra-violet (UV) region, ranging between [3.26-3.94] eV. In general, using 6-311+G* basis set, the absorption profile was found to be red-shifted with respect to 6-31G* basis set, and with a reasonable overlap in the first peak of absorption. Significant difference in the absorption profile between the basis sets in found in the 5 to 7 eV range, which corresponds to deep UV (UV-C) region. Among the derivatives, compounds **A B** and **C** displayed a good overlap in the UV-B [3.5-4.0 eV] and UV-C [5.5-6.2 eV] regions, thus suggesting an optical signature of a phenylcoumarin derivative. On the other hand, for compound **D** (belonging to heteroarylcoumarin family) the optical onset value was red-shifted with respect to the other derivatives. To obtain the electronic absorption characteristics of the derivatives TD-DFT calculations were repeated with PBE/6-31G* level of description (Figure 7). Consistent with our previous findings, we note a general red-shifted absorption profile with respect to the B3 LYP XC functional (Table 4). Interestingly, we note for compound **B** (2.92 eV), **C** (2.75 eV) and **D** (2.98 eV) the first signs of optical transition to occur in the visible region (between blue and violet). For the optical structure one can obtain information about the transition intensity, the transition weight (indicating the dominant transition), and also the type of the transition itself, which will be discussed in the next section. In Table 4 is summarized the transition energy and relative transition intensity (given as oscillator strength) for the main peak (MP) absorption and the E\({}_{\text{OPT}}\) (first signs of optical transition) value employing two XC functionals and basis sets. \begin{table}
\begin{tabular}{l c c c c c c c c c c c c} \hline \hline  & \multicolumn{3}{c}{**B3 LYP/6-31G*} & \multicolumn{3}{c}{**B3 LYP/6-311+G*} & \multicolumn{3}{c}{**PBE/6-31+G*} \\ \cline{2-13}
**Molecule** & \multicolumn{3}{c}{**Main Peak**} & \multicolumn{3}{c}{**E\({}_{\text{OPT}}\)**} & \multicolumn{3}{c}{**Main Peak**} & \multicolumn{3}{c}{**E\({}_{\text{OPT}}\)**} & \multicolumn{3}{c}{**Main Peak**} & \multicolumn{3}{c}{**E\({}_{\text{OPT}}\)**} \\ \cline{2-13}  & **eV** & **OS** & **eV** & **OS** & **eV** & **OS** & **eV** & **OS** & **eV** & **OS** & **eV** & **OS** \\ \hline
**A** & 3.80 & 0.50 & 3.80 & 0.50 & 3.77 & 0.49 & 3.77 & 0.49 & 6.47 & 0.38 & 3.37 & 0.35 \\
**B** & 3.72 & 0.55 & 3.59 & 0.02 & 3.66 & 0.48 & 3.58 & 0.06 & 3.16 & 0.34 & 2.92 & 0.02 \\
**C** & 3.82 & 0.36 & 3.50 & 0.21 & 5.89 & 0.38 & 3.47 & 0.23 & 6.32 & 0.14 & 2.74 & 0.04 \\
**D** & 3.35 & 0.58 & 3.35 & 0.58 & 3.24 & 0.55 & 3.24 & 0.55 & 2.98 & 0.42 & 2.98 & 0.42 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Energy and oscillator strength (OS) for the main peak (MP) and optical onset (E\({}_{\text{OPT}}\)) extracted from the absorption spectra. Figure 7: Calculated absorption spectra of coumarin derivatives using PBE XC functional (red) and B3 LYP (black) and with 6-31G* basis set, respectively.