**2. The formulation of CPI**

Conventional aromatic PIs showing deep color by absorbing visible light intensely is not applicable in microelectronic and optoelectronic engineering. This is well known due to the formation of charge transfer complexes (CTCs) from the interactions between electron-donating diamine and electron-accepting dianhydride. Recent development shows that the suppression of CTC can be achieved by incorporating structures such as fluorinated group, alicyclic monomers, noncoplanar structure, meta-substitute structure, or sulpone into the main chain of PIs thus increasing the transparency of PI films [20]. According to the composition of main chain, colorless polyimides (CPIs) will be classified into fluorinated CPIs, alicyclic CPIs, noncoplanar CPIs, and other CPIs. The key monomers are listed below in tables to show the studies clearly (**Tables 2**–**6**).
