# Molecular Controlling the Transport Properties for Benzothiadiazole-Based Hole Transport Materials
## 3 Results and Discussion
### Optical Absorption Properties  
Based on the optimized geometry structures of the six molecules, we employed the TD-DFT/ CAM-B3 LYP/6-31G(d) to study the excited state properties and optical absorption properties of these molecules. The calculated transition energies, absorption peaks \(\lambda\), major transition molecular orbitals and oscillator strength\(f\) of three experimental molecules Jy5-Jy7 have been listed in Table 4, and the three designed molecules Jy8-Jy10 have been listed in Table 5. As shown in Table 4, the absorption peaks of Jy5-Jy7 are 404.04 nm, 405.84 nm, 400.73 nm, respectively; and their corresponding oscillator strength\(f\) are 0.9012, 0.8423 and 0.7346, respectively. The absorption peaks of Jy5-Jy7 are adjacent, and the oscillator strengths of them are in the following order: Jy5 \(>\) Jy6 \(>\) Jy7. Introduction of fluorine has no obvious effect on absorption peaks, but decreases the oscillator strength of the experimental molecules. From Table 5, for designed molecules Jy8, Jy9 and Jy10, the absorption peaks of them are 343.01 nm, 349.88 nm and 355.46 nm, respectively; and their corresponding oscillator strength are 1.4740, 0.8324 and 1.3031. The absorption peaks of the designed molecules are obviously blue-shifted, and the oscillator strengths of them are significantly increased when compared with experimental molecules. It indicates that the absorption peak will make blue-shifted, and the oscillator strength will be significantly boosted by replacing the core structures of HTM molecule from electron-withdrawing group to electron-donating group. Among the six molecules, the Jy8 has the largest oscillator strength. Tables 4 and 5 also list the transition molecule orbitals for the each excited state of each molecule. For S1 of each molecule, except for Jy9, the transition MO of S1 for other molecule is from HOMO to LUMO. The transition MO of S1 for Jy9 is from HOMO-4 to LUMO.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline
**Molecules** & **Jy5** & **Jy6** & **Jy7** & **Jy8** & **Jy9** & **Jy10** \\ \hline \(\lambda_{h}\) & 0.14 & 0.15 & 0.15 & 0.11 & 0.17 & 0.11 \\ \(\lambda_{e}\) & 0.34 & 0.35 & 0.36 & 0.39 & 0.39 & 0.42 \\ \hline \end{tabular}
\end{table}
Table 3: The hole reorganization energies \(\lambda_{h}\) (eV) and electron reorganization energies \(\lambda_{e}\) (eV) of six investigated molecules.  
The simulated optical absorption spectra of the six studied molecules are shown in Figure 6. As shown in Figure 6, the optical absorption ranges of experimental molecules Jy5-Jy7 are all approximately from 300 nm to 550 nm, and the designed molecules Jy8-Jy10 are all about from 250 nm to 450 nm. It is obvious that the absorption ranges of experimental molecules are more red-shifted and broader than those of designed molecules. In addition, Figure 6 shows the absorption peaks of Jy5-Jy7 are more red-shifted than those of Jy8-Jy10. But the absorption strengths of designed molecules are clearly higher than those of the experimental molecules, which coincide with the trend of oscillator strengths for HTM molecules.