# Synthesis and Electrical Properties of a New Bipolar Material Using Spacer Moiety
## 3 Results and Discussion
### Optical Properties and Theoretical Calculations  
The ultraviolet-visible (UV-Vis) absorption and PL properties of the materials are summarized in Figure 2 and Table 1. The strong absorption bands at approximately 290 nm were assigned to the \(\pi\)-\(\pi^{\star}\) transition of the Cz ring; the weak bands at 310-350 nm correspond to n-\(\pi^{\star}\) transitions of the Cz units. DTPCZ revealed weak and broad absorption bands in the wavelength range of 350-370 nm, which were attributed to intramolecular charge transfer from the Cz donor to the Tz acceptor [21]. The observed behavior was similar to the behavior of the film state. Additionally, through time-dependent density functional theory (TD-DFT), the range of absorption spectra for DTPCZ was investigated. The results showed the largest oscillator strength at 294 nm, indicating the primary absorption peak where the transition from the highest occupied molecular orbital (HOMO)-2 to the lowest unoccupied molecular orbital (LUMO) occurs (Table S1). At this transition, the electron density in HOMO-2 was mainly located in the \(p\)-xylene moiety and partially in the Cz ring (Figure S1). Also, significant absorptions were observed at 303 and 409 nm, corresponding to transitions from HOMO to LUMO + 2 and from HOMO to LUMO, respectively. In these transitions, the electron density in HOMO was primarily distributed in the Cz ring, with partial localization in the \(p\)-xylene moiety. Thus, the results confirmed similar absorption characteristics to those observed previously. The PLmax peaks of DTPCZ in the solution  
Figure 1: Chemical structures of the newly synthesized compounds. and film states occurred at 421 and 425 nm, respectively. Generally, during the change from a solution state to a film state, the PLmax wavelength was red-shifted and the full width at half maximum (FWHM) was broadened because of the intermolecular interaction caused by the close intermolecular distance. However, FWHM values of 54 and 56 nm were maintained for the solution and film (Figure 2a). The methyl group of the spacer between the donor and the acceptor induced a twist, leading to a dihedral angle of 75.3\({}^{\circ}\) between Cz and the spacer (Figure 3). This angle prevented the intramolecular conjugation length from increasing and inhibited intermolecular packing; thus, deep-blue emission was achieved, similar to the emission of the solution, even in the film state [22]. Poly DTPCZ showed broad absorption in the 300-400-nm range. In the solution state, poly DTPCZ exhibited a PLmax value of 461 nm, while in the film state, it showed a PLmax \begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \hline \multirow{3}{*}{**Compound**} & \multicolumn{3}{c}{**Solution \({}^{\textbf{a}}\)**} & \multicolumn{3}{c}{**Film \({}^{\textbf{b}}\)**} & \multirow{3}{*}{**HO**} & \multirow{3}{*}{**LU**} \\  & \multirow{3}{*}{\(\mathbf{\lambda_{\textbf{Abs}}}\) (nm)} & \(\mathbf{\lambda_{\textbf{PL}}}\) & & & \(\mathbf{\lambda_{\textbf{Hb}}}\) & & \(\mathbf{\phi_{\textbf{F}}}\) & \(\mathbf{\phi_{\textbf{F}}}\) & **MO \({}^{\textbf{c}}\)** & **MO \({}^{\textbf{c}}\)** & **Band Gap \({}^{\textbf{d}}\)** \\  & & **(FWHM)** & \(\mathbf{\phi_{\textbf{F}}}\) **e (\%)** & \(\mathbf{\lambda_{\textbf{Abs}}}\) (nm) & & **(FWHM)** & \(\mathbf{\phi_{\textbf{F}}}\) **e (\%)** & **(eV)** & **(eV)** & **(eV)** \\ \hline DTPCZ & 340 & 421 (54) & 43.2 & 341 & 425 (56) & 18.9 & \(-\)5.9 & \(-\)2.8 & 3.1 \\ Poly DTPCZ & 380 & 461 (68) & 32.4 & 387 & 475 (84) & 9.1 & \(-\)5.5 & \(-\)2.6 & 2.9 \\ \hline \hline \end{tabular}  
* Toluence solution (0.5 wt\(\%\)), \({}^{\textbf{b}}\) in thin film state (thickness: 50 nm) for DTPCZ, spin coating film (1 wt\(\%\) in toluene) for poly DTPCZ; \({}^{\textbf{c}}\) HOMO values derived from ultraviolet photoelectron spectra (Riken-Keiki, AC-2); \({}^{\textbf{d}}\) optical energy band gap (E\({}_{\textbf{g}}\)) estimated from the onset of the absorption spectra in a neat film at room temperature; \({}^{\textbf{e}}\) absolute photoluminescence quantum yield.  
\end{table}