Effect of methyl trifluoride substitution on colorless transparency of polyimide: A DFT/TD-DFT study

The rapid advancement of flexible electronic devices has rendered high-performance colorless transparency polyimide (PI) films essential for energy storage and devices. However, there still remains controversy regarding the charge-transfer complexes in relation to the optical properties of PIs. Herein, we employed a combination of DFT and TD-DFT methods to elucidate the hole and electron characteristics of PIs. The results revealed that the incorporation of a CF3 group significantly enhances the dipole moment. Notably, the incorporation of CF3 groups onto both the anhydride and diamine units impedes charge transfer, resulting in a reduction of the HOMO energy level and an increse of the LUMO energy. The calculation results also indicate that the introduction of CF3 groups markedly hinders charge transfer within P4 in its excited state, leading to the minimum values for three characteristic parameters (D = 2.60 Å, CT = 0.51 e, t = 0.53 Å). Therefore, P4 exhibits a shortest charge transfer pathway resulting in minimal electron transfer and obvious blue-shifted UV–vis spectra, thus contributing to its excellent colorless transparency. This work provides a theoretical foundation and guidance for targeted design and development of colorless transparent PI films.