Structure effect on transition mechanism of UV–visible absorption spectrum in polyimides: A density functional theory study

Understanding of the nature of the transition mechanism in UV-visible absorption spectrum may contribute to the designing of highly transparent polyimide (PI) materials. In the present work, structure effect (alicyclic ring, flexible -O- linkage, and -CF3) on the intra- and inter-molecular transition mechanism of optical absorptions in PI model molecules is investigated by the Density functional theory (DFT) in combination with the time dependent DFT calculations. Six pyromelliticdianhydride (PMDA) derived aromatic and six 1,2,4,5-cyclohexanetetracarboxylic dianhydride (CHDA) derived semi-aromatic polyimide (PI) species are considered and their intermolecular transitions are analyzed by designing typical dimers with classical face-to-face π-π stacking orientation. The natural bond orbital method (NBO) is employed to analyze the delocalization degree in the PI species. By calculating the energies of the HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) as well as the transition characters between them, the electron excitation nature concerning on the structural variations in the monomers and the dimers are clarified. These theoretical results agree well with the related experimental observations and provide a deep understanding into the transition mechanism of optical absorptions in the PI species.