Design, synthesis, and characterization of intrinsic black polyimide with complete absorption of visible light and low coefficient of thermal expansion

Black polyimides (BPIs) show increasing demand in the microelectronics and optoelectronics fields. However, the existing BPIs have drawbacks of poor masking ability, dimensional stability, thermal and electrical properties. 3,6-Bis(thiophen-2-yl)diketopyrrolopyrroles (TDPP) is a typical dye that possesses a high molar extinction coefficient. In the work, a new diamine (DPPTOMePDA) bearing TDPP connected with methoxyl-substituted benzene rings was synthesized. Based on such diamine and pyromellitic dianhydride (PMDA), a highperformance intrinsic BPI (DPPTOMePPI) was obtained. The resulting DPPTOMePPI exhibited a red-shifted and broadened absorption spectrum, thus achieving complete absorption of visible light and highly black color. Its cut-off wavelength (lambda cut) was up to 734 nm, and the Commission International de l'Eclairage (CIE)-Lab coordinate L* decreased to 0.53. Furthermore, DPPTOMePPI exhibited excellent thermal and electrical performances with a low coefficient of thermal expansion (CTE). Density functional theory calculation was executed to analyze the underlying electron transitions of DPPTOMePPI. The results showed that the strong light absorption of DPPTOMePPI was primarily ascribed to the electronic transition of HOMO to LUMO+1 that occurred in the chromophores units. The flexible copper clad laminates (FCCLs) derived from DPPTOMePPI film showed high peel strength and resistance to solder heat. The work offers a theoretical guide for the construction of intrinsic black PI possessing complete visible absorption and low CTE.