Colorless Partially Alicyclic Polyimides Based on Tröger’s Base Exhibiting Good Solubility and Dual Fluorescence/Phosphorescence Emission

Abstract

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.

Highly Refractive Polyimides Derived from Efficient Catalyst-Free Thiol–Yne Click Polymerization

“…17 As is known to all, conventional aromatic PI films always present deep colorations as a result of the intra-and intermolecular charge− transfer complexes. 51,52 Figure 6 shows the transmittance curves of the resulting PI films with a thickness of ca. 10 μm.…”

Synthetic strategies for highly transparent and colorless polyimide film

“…Tröger's base (TB), a rigid bridging unit with V‐shaped and nonplanar heterobicyclic structure, was introduced into PI system together with alicyclic structure by Zhuang et al 71 to prepare three types of Tröger's Base semi‐alicyclic PIs (Figure 7d). Due to the fact that low electronic affinity of alicyclic dianhydride and the presence of V‐shaped Tröger's base effectively destroy the conjugated structure, all of the three PIs are highly‐transparent colorless films (~25 μm).…”

“…Also, it is important to tune the aggregation state of polymers during film production and post‐treatment. The structural design principle is to reduce the electron accepting ability of dianhydride or the electron donating ability of diamine through the selection of functional monomers and the combination of different monomers, for example, introducing strong electronegative groups such as CF 3 , 61–64 alicyclic structure, 65–72 and inorganic particles such as SiO 2 , 6,73,74 asymmetric and unconjugated structure 75–77 into PI.…”

Colorless and transparent semi‐alicyclic polyimide films with intrinsic flame retardancy based on alicyclic dianhydrides and aromatic phosphorous‐containing diamine: Preparation and properties

“…Among the up‐to‐date efforts, incorporation of substituents or structures with less polarity, less conjugated skeleton, high electronegativity, and high molar volumes into the molecular structures of the conventional high‐temperature polymers seems to be most promising modification procedures. For example, a series of colorless and transparent PI (CPI) films with great potential applications in optoelectronic fabrication have been developed in the past decades via incorporation of either alicyclic units, 8‐10 fluoro‐containing substituents, 11‐13 or asymmetrical molecular structures 14 into the molecular chains. Semi‐alicyclic CPI and fluoro‐containing CPI films represent two of the most important commercially available optical PI films up to now.…”