Molecular Design, Synthesis, and Properties of Highly Fluorescent Polyimides

Abstract

To develop a molecular design concept for highly fluorescent polyimides (PIs), ultraviolet-visible optical absorption spectra and fluorescent excitation/emission spectra of PI thin films were extensively investigated with the aid of density functional theory calculations. It was clarified that fully aromatic PIs (Ar-PIs) have strong charge transfer (CT) interactions, and their lowest excited states are attributable to CT(pi-pi*) states which emit weak fluorescence. The CT interactions in PIs are effectively suppressed by using alicyclic diamines as the source material, regardless of the dianhydride structure. This is due to the nonaromatic nature and weak electron-donating property of alicyclic diamines. The lowest excited states of Al-PIs derived from aromatic dianhydrides having delocalized pi conjugation and an alicyclic diamine are attributable to LE(pi-pi*) states which emit strong fluorescence. In particular, Al-PIs having ether (-O-) linkages in the dianhydride moiety demonstrated the strongest blue photoluminescence with a quantum efficiency of 11%. This is explainable by the fact that the bent and rotatable diphenyl ether structure prevents dense chain packing which effectively weakens the fluorescence. However, the lowest excited states of Al-PIs derived from dianhydrides with localized pi conjugation and an alicyclic diamine are attributable to LE(n-pi*) states which emit quite weak fluorescence. These results indicate that the Al-PIs synthesized from alicyclic diamines and aromatic dianhydrides having delocalized conjugation with nonplanar structures are potential candidates for thermally stable polymers which emit strong fluorescence in the visible region, i.e., "highly fluorescent polyimides".

Synthesis and properties of high‐performance functional polyimides containing rigid nonplanar conjugated tetraphenylethylene moieties

“…The different values of T g measured by DSC and DMA may be attributed to the different heating stories and the distinct natures of these two testing methods. The thermal stability and T g of the synthesized PI are much higher than those of the reported Al‐PIs ( T d5% : 425–457 °C, T g : 220–322 °C),48 because of the aromatic nature of TetraPEDA. Moreover, all the polymers showed no clear melting endothermic peaks up to the decomposition temperatures on their DSC thermograms (not shown here), and this is in agreement with the wide‐angle X‐ray diffraction (WAXD) results (see Supporting Information Fig.…”

Effect of Casting Solvent on Interfacial Molecular Structure and Proton Transport Characteristics of Sulfonated Polyimide Thin Films

“…14, [45][46][47] The peak position and intensity of vibrational modes for both IP and OP components are taken into account for estimating the molecular orientation by the variation of solvent systems. The spectra absorbance between IP and OP spectra were different in the two SPI thin films.…”

Polyimide and Imide Compound Exhibiting Bright Red Fluorescence with Very Large Stokes Shifts via Excited-State Intramolecular Proton Transfer

“…Recently, we have proposed and confirmed a novel molecular design concept for "highly fluorescent PIs" based on the DFT calculations and optical measurements of imide model compounds. 16 The principles for designing highly fluorescent PIs can be summarized as follows: (1) use alicyclic diamines and (2) use aromatic dianhydrides that have flexible linkages with extended π-conjugation. According to this concept, we have successfully synthesized a strongly blue fluorescent Al-PI derived from 1,4-bis(3,4-dicarboxyphenoxy)benzene dianhydride (HQDEA) and DCHM, which has a Φ value of 0.11.…”