Semiaromatic polyimides (PIs) and imide compounds containing heavy halogens (Br, I) in pyromellitic moieties were designed and synthesized to examine their photoluminescence properties. Solutions of imides and PI films exhibited reddish-color room-temperature phosphorescence (RTP) with very large Stokes shifts (ca. 10000 cm −1 ). In addition, the PI films showed small-Stokes-shifted fluorescence emissions at around 540 nm with absorption bands arising from aggregated PI chains at 400−500 nm. Enhanced phosphorescence observed for the PI films under vacuum indicates that the RTP lifetime is significantly influenced by the triplet−triplet energy transfer to atmospheric oxygen. These PIs with very-large-Stokes-shifted RTP are applicable as spectral conversion materials in displays, photovoltaic devices, and crop cultivators, as well as to oxygen/air sensors.

A novel polyimide (PI) and imide compound emitting prominent reddish-orange fluorescence under excitation by UV light were prepared based on 3-hydroxypyromellitic dianhydride (PHDA), and their fluorescence properties were examined. The steady-state fluorescence spectrum of a PI film displayed an emission band at 590 nm with a very large Stokes shift (ν = 10 448 cm −1 ) via the excited-state intramolecular proton transfer (ESIPT), while the time-resolved fluorescence spectrum showed a rapid decay of the emission band of the enol form at around 400 nm within 15 ps. Transient absorption measurements showed an induced absorption and stimulated emission of the keto form with a time constant of ca. 3.0 ps, implying that ESIPT occurs on this time scale. Consequently, introduction of a hydroxy group into the pyromellitic moiety of PIs and imide compounds led to the long-wavelength ESIPT emission applicable to spectral converters having high thermal, mechanical, and environmental stabilities.

A novel polyimide (PI) emitting a prominent red fluorescence was prepared based on 3,6-dihydroxypyromellitic dianhydride (P2HDA) and 4,4′-diaminocyclohexylmethane (DCHM). In order to investigate the fluorescence properties of the PI, an imide model compound, N,N′-dicyclohexyl-3,6-dihydroxypyromellitimide (P2H-Ch), corresponding to one repeating unit of the PI, was also synthesized. The UV−vis absorption and fluorescence spectra of P2H-Ch in CHCl 3 and the solid PI film demonstrated intense red fluorescence at around 640 nm with a very large Stokes shift (ν) of 7655 and 8994 cm −1 , respectively, via the excited-state intramolecular proton transfer (ESIPT). Moreover, the corresponding monoanion and dianion species were formed in basic conditions with an organic base (DBU) and basic salt (NaOH), which were characterized by highly visible halochromism. The introduction of −OH groups into the pyromellitic moiety of imide compounds or PIs at their para positions led to the long-wavelength fluorescent emission as well as to the high pH sensitivity.

A colorless and thermally stable polyimide film exhibiting dual fluorescence and phosphorescence emission promising for solar spectral down-converters.

Due to their high thermal and environmental stability, polyimides (PIs) are one of the most attractive candidates for novel highly fluorescent polymers, though photophysical studies of PIs are challenging owing to their poor solubility in common solvents. To overcome these problems, we have synthesized and examined a series of low molecular weight model imide compounds: substituted N-cyclohexylphthalimides with alicyclic amino groups at the 3 or 4-positions of the benzene rings (x-NHPIs). Their photophysical properties were systematically investigated by steady-state UV/Visible absorption, fluorescence, and time-resolved fluorescence techniques. In solution, unsubstituted N-cyclohexylphthalimide (NHPI) showed almost no emission, while x-NHPIs exhibited enhanced fluorescence emission depending on the solvent polarity. Analysis of the solvatochromism of the x-NHPIs via Lippert-Mataga plots indicated the generation of large dipole moments in the excited singlet states originating from the intramolecular charge-transfer (ICT) states. The significant difference in the fluorescence quantum yields (Φ) between the 3-substituted (3Pi and 3Pyr) and 4-substituted NHPIs (4Pi and 4Pyr) strongly suggests that the former form a twisted ICT (TICT) state, whereas the latter form a planar ICT (PICT) state when excited. 4-Substituted NHPIs also show high fluorescence yields in the crystalline state. A particularly large Φ value was obtained for the 4Pi crystal, which we explain by the large intermolecular distances and the arrangement of molecules minimizing intermolecular interactions as well as the small non-radiative deactivation rate. These facts clearly demonstrate that the introduction of an alicyclic amino group into NHPI at the 4-position enhances the fluorescence quantum yields significantly, which suggests a new pathway for the development of novel, highly fluorescent PIs.