Thermally stable copolyimide (CoPI) films exhibiting high optical transparency and room-temperature phosphorescence (RTP) were prepared by copolymerizing fluorescent dianhydride and brominated phosphorescent dianhydride with an alicyclic diamine. The CoPI films underwent a 5 wt % degradation at a temperature higher than 349 °C and exhibited dual fluorescent and phosphorescent emissions owing to their efficient Förster resonance energy transfer from the fluorescent to phosphorescent dianhydride moieties in the main chains, followed by an intersystem crossing from the singlet to triplet state of the latter moiety atoms. The CoPIs displayed bright RTP under a vacuum with various colors produced when adjusting the copolymerization ratio. CoPI with 5 mol % phosphorescent moiety (CoPI-05) emitted white light with high optical transparency owing to the suppression of the PI chain aggregation that causes a yellowish coloration. The copolymerization of fluorescent and phosphorescent PI moieties can control the photoluminescent properties of PI films and is applicable to color-tunable solid-state emitters, ratiometric oxygen sensors, and solar-spectrum converters.

A series of semi-aliphatic polyimide (PI) copolymers (CoPIs) were prepared through the copolymerization of two dianhydrides, 1-hydroxy pyromellitic dianhydride (PHDA) and 4,4′-oxydiphthalic anhydride (ODPA), with 4,4′-diaminodicyclohexylmethane, in which the PHDA molar ratio was controlled at less than 5% to suppress aggregate formation in a solid state. Upon increasing the molar ratio of the PHDA unit, the fluorescence color of the CoPI films continuously changed from pink to yellow through an orange color owing to the enhanced emission from the anionic form of the PHDA unit, and the efficiency of the energy transfer from the locally excited S1 state to the anionic state simultaneously increased. Moreover, the CoPI film with a PHDA content of 3 mol % that formed on a silica substrate was colorless and transparent and showed bright orange fluorescence. However, this CoPI film formed on a soda-lime-silica (soda) glass substrate showed a pale-yellow color as well as yellowish fluorescence originating from the anionic form owing to the high basicity of the soda glass. To suppress the absorption and emission from the anions, a small amount of sulfuric acid was doped into the CoPI film, and a colorless and transparent film exhibiting large Stokes-shifted orange fluorescence was successfully obtained on the soda glass substrate. The wavelength-converting spectrum of the CoPI demonstrated that UV irradiation in solar light was efficiently absorbed, and its energy was converted into visible light between 500 and 700 nm with a quantum efficiency of 20%. These CoPI films exhibiting large Stokes-shifted ESIPT fluorescence on both silica and the soda glass substrates are promising materials for solar spectral conversion applications.

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Excited state intramolecular proton transfer (ESIPT) is one of the distinctive photophysical processes of fluorescent compounds and polymers, which induces enhanced fluorescence with a very large Stokes shift. In this work, 6FDA/DCHM/3HPA polyimides (PIs) having bulky-CF 3 groups were synthesized, in which 3HPA is an end-group which exhibits ESIPT with enhanced green emission when irradiated by UV light at 365 nm. Introduction of bulky side chains can improve fluorescent properties via suppression of aggregation induced quenching as well as intermolecular interactions. All the 6FDA-based PI films were colorless and transparent in the visible region, and the PIs having 3HPA end-groups exhibited obvious green emission at 530 nm, when excited at 340 nm. Compared with the corresponding ODPA-based PIs which have no bulky side groups, the 6FDA-based PIs demonstrated significantly higher quantum yields in the solid state (Φ = 0.14−0.25), which is attributable to the looser aggregation structures owing to the bulky and less polarizable-CF3 group and the absence of energy transfer to/from the non-fluorescent main chain units. Accordingly, the experimental results proved that the introduction of bulky-CF 3 groups using 6FDA moiety is a promising way to create highly fluorescent end-capped PIs.