# Synthesis and Electrical Properties of a New Bipolar Material Using Spacer Moiety
## 3 Results and Discussion
### Optical Properties and Theoretical Calculations

Table 1: Optical properties of the synthesized materials. Figure 3: Electron density distributions and dihedral angle of the HOMO and LUMO orbits of DTPCZ (calculated using the B3 LYP/6-31G(d) method). Figure 2: UV-Vis absorption spectra and PL spectra of the synthesized materials: (**a**) DTPCZ and (**b**) poly DTPCZ. value of 475 nm, resulting in red shifts of approximately 40 and 50 nm compared to DTPCZ, respectively. Generally, as the solution/film transitions to a polymer state, the molecular weight increases and the conjugation length becomes longer, resulting in a greater red shift compared with the monomer [23]. However, the difference between the solution and film states of poly DTPCZ itself was observed to be 14 nm, and this characteristic is presumably related to the suppression of intermolecular aggregation via steric hindrance between neighboring molecules in the polymer chain to maintain a stable state [24]. DTPCZ and poly DTPCZ are bipolar materials with a donor-acceptor structure. These materials have distinct optical properties depending on the solvent polarity. There is a slight blue shift of this band with increasing solvent polarity in absorption, which is a typical behavior of n-\(\pi^{4}\) transitions [25; 26]. Also, they exhibited similar absorption spectra and shapes overall (Figure S2). However, the PL spectra were red-shifted according to increases in solvent polarity (Figure 4). As shown in Figure 4 and Table 2, the peak of the PL spectra of DTPCZ varied from 397 nm for hexane to 522 nm for dimethyl sulfoxide (DMSO); the FWHM broadened from 44 to 111 nm, respectively. Notably, hexane and DMSO have dielectric constants of 1.9 and 46.7, respectively. Poly DTPCZ was not soluble in hexane; thus, the PL spectrum could not be measured. The PLmax shifted from 466 nm for toluene (dielectric constant: 2.38) to 559 nm for DMSO, respectively; the FWHM broadened from 71 to 125 nm. The PL wavelength of DTPCZ and poly DTPCZ exhibited a red shift in polar solvents because bipolar molecules are influenced by the solvation effect, which stabilizes with increasing solvent polarity in the excited state. From toluene to DMSO, DTPCZ shifted by 101 nm, and poly DTPCZ shifted by 89 nm. Poly DTPCZ was expected to exhibit a greater shift; however, as described above, steric hindrance with neighboring molecules in the polymer may have prevented large shifts in the PL spectra. As the polarity of the solvent increases, the PL emission shifts to longer wavelengths. This is because, in the ground state, the molecule's dipole moment is not influenced by the solvent polarity, resulting in similar absorption peaks. However, in the excited state, the molecule is affected by both the generated charge within the molecule and the solvent polarity. As a result, as the dielectric constant of the solvent increases, the molecule interacts more with the dipole of the solvent, stabilizing the excited state and reducing the band gap, leading to a gradual red shift in the PL max value [27]. The photoluminescence quantum yields (PLQY) values of DTPCZ were 43.2% in the solution state and 18.9% in the film state. The PLQY values of poly DTPCZ were 32.4% in the solution state and 9.1% in the film state. As shown in Table 1, there is a difference of about 4 nm in the PL wavelength between the solution and the film of DTPCZ. This could be due to the minimization of intermolecular packing with the introduction of the spacer. If intermolecular packing had not been prevented, the PL value in the film would have shown a further red shift or exhibited peaks in the excime  
Figure 4: PL spectra of synthesized bipolar materials in different solvents at 0.5 wt%: (**a**) DTPCZ, (**b**) poly DTPCZ. decrease in PLQY value in the film is a common phenomenon observed when all materials are converted from solution to film state, and it is interpreted that the effect of preventing intermolecular packing by the methyl group of DTPCZ did not fully resolve this issue. In the solution state, the decrease in the PLQY value between DTPCZ and poly DTPCZ was small.