This study proposes a molecular design and synthetic route to novel colorless, transparent polyimides that exhibit dual fluorescence/phosphorescence emission at long wavelengths applicable to solar energy conversion. Partially alicyclic polyimides (Ac-PI-TBs) based on Tröger’s base (TB) and denoted as Ac-PI-TB-1, Ac-PI-TB-2, and Ac-PI-TB-3 were synthesized by in situ TB formation. The resulting Ac-PI-TBs are readily soluble in common organic solvents and have good mechanical properties with tensile strengths of 72.5–102.3 MPa, elongations at breaks of 12.5–75.0%, low dielectric constants (∼2.66) and low thermal diffusivities (D ⊥ ≤ 7.7 × 10–8 m2/s), and good thermal stability. The films are totally colorless and transparent with transmittances above 77% at 400 nm. The films also show dual fluorescence and phosphorescence emissions with Stokes shifts as large as 11 421 cm–1 at low temperatures. The results highlight the possible application of these films in the spectral conversion of unused UV solar radiation to useful visible light.

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.

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.

The relationship between the photoexcitation dynamics and the structures of semi-aliphatic polyimides (3H-PIs) was investigated using ultrafast fluorescent emission spectroscopy at atmospheric and increased pressures of up to 4 GPa. The 3H-PI films exhibited prominent fluorescence with extremely large Stokes shifts (Δν > 10 000 cm −1 ) through an excited-state intramolecular proton transfer (ESIPT) induced by keto−enol tautomerism at the isolated dianhydride moiety. The incorporation of bulky −CH 3 and −CF 3 side groups at the diamine moiety of the PIs increased the quantum yields of the ESIPT fluorescence owing to an enhanced interchain free volume. In addition, 3H-PI films emitted another fluorescence at shorter wavelengths originating from closely packed polyimide (PI) chains (in aggregated forms), which was mediated through a Forster resonance energy transfer (FRET) from an isolated enol form into aggregated forms. The FRET process became more dominant than the ESIPT process at higher pressures owing to an enhancement of the FRET efficiency caused by the increased dipole−dipole interactions associated with a densification of the PI chain packing. The efficiency of the FRET rapidly increased by applying pressure up to 1 GPa owing to an effective compression of the interchain free volume and additionally gradually increased at higher pressures owing to structural and/or conformational changes in the main chains.

Fluorescent polymers are one of the most desired materials for making novel flexible electronic components or spectral converters. Favourable properties of polyimides (PIs) make them a potentially attractive class of polymers but, unfortunately, they generally show a very weak fluorescence. As a result, there has been a large amount of work directed towards finding the reasons behind the lack of fluorescence of PIs and making their new fluorescent variants. Amine-substituted phtalimides are heavily used as model compounds for fluorescent PIs. In this work we have examined the photophysical relaxation mechanism of such two phtalimides (3Pi and 3Pyr) that were previously discovered to have low quantum yields of fluorescence. Our aim was to find the reason behind this lack of fluorescence and to suggest ways to make these properties more attractive. We have investigated the photophysics of phtalimides by time resolved fluorescence and ultrafast transient absorption techniques in solvent mixtures of different viscosities. Our results demonstrate that intramolecular rotation is a crucial process that opens up an alternative relaxation pathway to fluorescence in phtalimides, which has to be suppressed for designing more fluorescent materials.