Molecular design, synthesis, and properties of highly fluorescent polyimides

To develop a molecular design concept for highly fluorescent polyimides (PIs), ultraviolet-visible optical absorption spectra and fluorescent excitation/emission spectra of PI thin films were extensively investigated with the aid of density functional theory calculations. It was clarified that fully aromatic PIs (Ar-PIs) have strong charge transfer (CT) interactions, and their lowest excited states are attributable to CT(pi-pi*) states which emit weak fluorescence. The CT interactions in PIs are effectively suppressed by using alicyclic diamines as the source material, regardless of the dianhydride structure. This is due to the nonaromatic nature and weak electron-donating property of alicyclic diamines. The lowest excited states of Al-PIs derived from aromatic dianhydrides having delocalized pi conjugation and an alicyclic diamine are attributable to LE(pi-pi*) states which emit strong fluorescence. In particular, Al-PIs having ether (-O-) linkages in the dianhydride moiety demonstrated the strongest blue photoluminescence with a quantum efficiency of 11%. This is explainable by the fact that the bent and rotatable diphenyl ether structure prevents dense chain packing which effectively weakens the fluorescence. However, the lowest excited states of Al-PIs derived from dianhydrides with localized pi conjugation and an alicyclic diamine are attributable to LE(n-pi*) states which emit quite weak fluorescence. These results indicate that the Al-PIs synthesized from alicyclic diamines and aromatic dianhydrides having delocalized conjugation with nonplanar structures are potential candidates for thermally stable polymers which emit strong fluorescence in the visible region, i.e., "highly fluorescent polyimides".