Solution-Processable Heat-Resistant Polymers with Extremely Intense Pure-Blue Photoluminescence Functionality and Impact of Casting Solvents

This study aimed to develop unique heat-resistant polymers with very high-intensity blue photoluminescence (PL), particularly by maximizing the PL efficiencies (ΦPL) of fluorophore-incorporated polyimides (PIs) while maintaining their PL color. A bifunctional amide-linked fluorophore (HTA-BAPA) was synthesized from a hydrogenated trimellitic anhydride (HTA) derivative and 9,10-bis(4-aminophenyl)anthracene (BAPA) to covalently incorporate into the main chains of PIs. The dependence of the ΦPL on the HTA-BAPA content was investigated using a wholly cycloaliphatic PI matrix, derived from 1,2,3,4-cyclobutanetetracarboxylic dianhydride (CBDA) with 4,4′-methylenebis(cyclohexylamine) (MBCHA), without charge-transfer (CT) interactions, which mask the desired PL. The ΦPL in the PI precursor film significantly decreased after thermal imidization while maintaining the PL spectral profile (spectral shape and position = PL color). This is likely related to low-level (two-molecular) fluorophore aggregation during thermal imidization, which is responsible for concentration quenching (CQ). Then, the effect of the chemical imidization process (Route-C) on ΦPL was investigated. Route-C provided PI films via a simple solution coating and drying process without thermal imidization. However, when CBDA/MBCHA was used as the PI matrix, gelation/precipitation occurred during chemical imidization, which inhibited subsequent solution casting. To solve this problem, an alternative cycloaliphatic tetracarboxylic dianhydride, derived from the HTA derivative and 4,4′-biphenol (44′BP), was used to combine with 2,2′-bis(trifluoromethyl)benzidine. This matrix PI exhibited Route-C compatibility, excellent solubility, and high heat resistance (Tg = 263 °C), while maintaining a CT-inhibiting function. The PI cast film incorporating HTA-BAPA (2 mol%) exhibited a highly intense blue PL with an exceedingly high ΦPL of 0.68 (68%) and a color coordinate, CIE (x = 0.151, y = 0.074), corresponding to deep-blue PL. The impact of casting solvent type on the ΦPL was also investigated. A clear correlation between the boiling points (Tb) of the casting solvents and ΦPL of the resulting PI cast films was observed, where ΦPL monotonically increased with decreasing Tb. These results probably suggest that faster evaporation (solidification) related to the lower Tb during the first soft-drying step at 60 °C kinetically overcame the two-molecular fluorophore aggregation responsible for CQ. Solution casting from tetrahydrofuran (with the lowest Tb) afforded a maximum ΦPL of 0.84 (84%). Thus, unique polymeric materials with excellent solubility, high Tg, relatively high thermal stability and acceptable film ductility along with highly intense pure-blue PL were obtained, broadening the scope of photoluminescent PI applications, such as heat-resistant fluorescent QR codes.