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(π−π*) 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 π conjugation and an alicyclic diamine are attributable to LE(π−π*) states which emit strong fluorescence. In particular, Al-PIs having ether (−O−) linkages in the dianhydride moiety demonstrated the strongest blue pho...