Polarization Analysis and Humidity Dependence of Dielectric Properties of Aromatic and Semialicyclic Polyimides Measured at 10 GHz

In this study, dielectric constants (Dk) and dissipation factors (Df) of 15 types of polyimides (PIs), including wholly aromatic, semialicyclic, partially fluorinated, and perfluorinated PIs, were measured in the transverse electric (TE011) mode at a frequency of 10 GHz and analyzed based on their chemical structures and the dominant factors of dielectric polarization per unit volume (Pt). The relationships among the dipolar orientational (Pd) and electronic (Pe) polarizations and dielectric properties (Dk and Df) were quantitatively investigated. Although a systematic correlation between the structure and Df of PIs has not yet been established, we found that the Df of PIs is anisotropic and linearly proportional to Pd in the in-plane direction, which can be estimated from the Dk and in-plane refractive index (nTE) at 1310 nm. This could be related to the bidirectional relationship between the real (Dk) and imaginary (Df) parts of the relative permittivity. Furthermore, the Dk and Df of all PIs increased linearly with respect to the relative humidity (RH) of the measurement environment, and their slopes (hDk and hDf), i.e., the RH sensitivities of Dk and Df, showed a proportional relationship with a high correlation coefficient (R2 = 0.986). The weight fraction of polar imide and ester groups (Polar %) and the hydrophobicity of PIs, which are closely related to the fluorine content (F %), are essential factors in determining the values of Dk, Df, hDk, and hDf. These relationships are crucial for developing low-dielectric PI materials exhibiting low Dk and Df values, as well as reduced sensitivity to humidity

Polyimides with Heavy Halogens Exhibiting Room-Temperature Phosphorescence with Very Large Stokes Shifts

Semiaromatic polyimides (PIs) and imide compounds containing heavy halogens (Br, I) in pyromellitic moieties were designed and synthesized to examine their photoluminescence properties. Solutions of imides and PI films exhibited reddish-color room-temperature phosphorescence (RTP) with very large Stokes shifts (ca. 10000 cm^–1). In addition, the PI films showed small-Stokes-shifted fluorescence emissions at around 540 nm with absorption bands arising from aggregated PI chains at 400–500 nm. Enhanced phosphorescence observed for the PI films under vacuum indicates that the RTP lifetime is significantly influenced by the triplet–triplet energy transfer to atmospheric oxygen. These PIs with very-large-Stokes-shifted RTP are applicable as spectral conversion materials in displays, photovoltaic devices, and crop cultivators, as well as to oxygen/air sensors

Discrete Self-Assembly and Functionality of Guest Molecules in an Organic Framework

In this study, the fundamental issue of “how various functional molecules can be homogeneously and densely arrayed in a solid material” is addressed using discrete self-assembly of guest molecules with an orthogonal architecture (<b>1</b>) comprising hydrogen bonds; this method has become a new paradigm for constructing functional materials. The serendipitous finding of cocrystallization in <b>1</b> was prompted by its unexpectedly tight but transmutable inclusion of guest molecules. Notably, the self-assembly of <b>1</b> with imidazole molecules produced a water-durable, heat-resistant, one-dimensional (1D) anhydrous proton-conducting channel with proton-transfer sites in very close proximity (N–N distance, 2.481 Å). The close sites lead to faster proton transfer when compared with pure imidazole; this advantage via the close sites is attributable to wobbling rather than rotational motion. These results suggest that discrete self-assembly can enable the development of new design concepts for functional materials

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

A novel polyimide (PI) emitting a prominent red fluorescence was prepared based on 3,6-dihydroxy­pyromellitic dianhydride (P2HDA) and 4,4′-diaminocyclohexylmethane (DCHM). In order to investigate the fluorescence properties of the PI, an imide model compound, <i>N</i>,<i>N</i>′-dicyclohexyl-3,6-dihydroxy­pyromellitimide (P2H-Ch), corresponding to one repeating unit of the PI, was also synthesized. The UV–vis absorption and fluorescence spectra of P2H-Ch in CHCl₃ and the solid PI film demonstrated intense red fluorescence at around 640 nm with a very large Stokes shift (ν) of 7655 and 8994 cm^–1, respectively, via the excited-state intramolecular proton transfer (ESIPT). Moreover, the corresponding monoanion and dianion species were formed in basic conditions with an organic base (DBU) and basic salt (NaOH), which were characterized by highly visible halochromism. The introduction of −OH groups into the pyromellitic moiety of imide compounds or PIs at their para positions led to the long-wavelength fluorescent emission as well as to the high pH sensitivity

Variations in Aggregation Structures and Fluorescence Properties of a Semialiphatic Fluorinated Polyimide Induced by Very High Pressure

Variations in the molecular aggregation structures and optical properties of a semialiphatic fluorinated 10FEDA/DCHM (poly­(4,4′-diaminocyclohexylmethane 1,4-bis­(3,4-dicarboxytrifluorophenoxy) tetrafluorobenzeneimide) polyimide (PI), which exhibits strong cyan fluorescence, were examined under very high pressure up to 8 GPa using synchrotron wide-angle X-ray diffraction (WAXD) and fluorescence spectroscopy. The fluorescence intensity of a PI film imidized at 220 °C was significantly reduced by applying pressure up to 1 GPa, which should be due to an appreciable reduction in interchain free volume, as indicated by a decrease in the <i>d</i>-spacing values of WAXD peaks which correspond to intermolecular ordering. In contrast, a PI film imidized at 300 °C, which exhibited weaker fluorescence than that imidized at 220 °C at atmospheric pressure, demonstrated a much smaller reduction in fluorescence intensity below 1 GPa. Such differences in pressure dependence clearly reflect the degrees of PI chain packing formed at different imidization temperatures. These phenomena induced by high pressure were almost reversible between pre- and postpressurization states with small hysteresis in the WAXD patterns and fluorescence spectra

Anisotropic Thermal Diffusivity of Hexagonal Boron Nitride-Filled Polyimide Films: Effects of Filler Particle Size, Aggregation, Orientation, and Polymer Chain Rigidity

A series of inorganic/organic composite films exhibiting high thermal stability and high thermal diffusivity was prepared from five different grades of flake-shaped hexagonal boron nitride (hBN) and aromatic polyimides (PIs). Thermal diffusivities along the out-of-plane (<i>D</i>_⊥) and in-plane (<i>D</i>_//) directions of hBN/PI films were separately measured and analyzed in terms of particle size, shape, concentration, and orientation, as well as molecular structures of rigid and flexible PI matrices. hBN/PI films filled with large flake-shaped particles exhibited a large anisotropy in <i>D</i>_⊥ and <i>D</i>_// due to the strong in-plane orientation of heat-conducting basal plane of hBN, while smaller anisotropy was observed in composites with small flakes and aggregates which tend to orient less in the in-plane direction during film processing. The anisotropic thermal diffusion property observed in hBN/PI films exhibited strong correlation with the orientation of hBN particles estimated using scanning electron micrographs (SEM) and wide-angle X-ray diffraction. Moreover, composites of hBN with a rigid-rod PI matrix exhibited much larger anisotropy in <i>D</i>_⊥ and <i>D</i>_// than flexible PI-composites, reflecting the effect of the rigid and densely packed PI chains preferentially orienting parallel to the film plane. The thermal conductivities of the hBN/rigid-rod PI films were estimated as 5.4 and 17.5 W/m·K along the out-of-plane and in-plane directions, respectively, which is one of the largest values ever reported

Nonstoichiometric Stille Coupling Polycondensation for Synthesizing Naphthalene-Diimide-Based π‑Conjugated Polymers

A nonstoichiometric Stille coupling polycondensation was first succeeded between 2,5-bis­(trimethylstannyl)­thiophene (<b>1</b>) and 4,9-dibromo-2,7-bis­(2-decyltetradecyl)­benzo­[<i>lmn</i>]­[3,8]-phenanthroline-1,3,6,8-tetraone (<b>2</b>) with ratios ranging from 1:1 to 1:10. The model reaction using 2-(tributylstannyl)­thiophene (<b>3</b>) and 4,9-dibromo-2,7-bis­(2-hexyl)­benzo­[<i>lmn</i>]­[3,8]-phenanthroline-1,3,6,8-tetraone (<b>4</b>) at a 1:1 molar ratio in the presence of catalytic Pd₂(dba)₃/P­(<i>o</i>-tolyl)₃ indicated that the rate constant of the second substitution reaction (<i>k</i>₂) is 15 times higher than that of the first one (<i>k</i>₁). It was found that the selective intramolecular catalyst transfer was promoted by the naphthalene-diimide (NDI) skeleton. The results also provided a new one-pot symmetrical end-functionalization method to synthesize an NDI-based n-type polymer with NDI groups at both α,ω-chain ends

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

A novel polyimide (PI) emitting a prominent red fluorescence was prepared based on 3,6-dihydroxy­pyromellitic dianhydride (P2HDA) and 4,4′-diaminocyclohexylmethane (DCHM). In order to investigate the fluorescence properties of the PI, an imide model compound, <i>N</i>,<i>N</i>′-dicyclohexyl-3,6-dihydroxy­pyromellitimide (P2H-Ch), corresponding to one repeating unit of the PI, was also synthesized. The UV–vis absorption and fluorescence spectra of P2H-Ch in CHCl₃ and the solid PI film demonstrated intense red fluorescence at around 640 nm with a very large Stokes shift (ν) of 7655 and 8994 cm^–1, respectively, via the excited-state intramolecular proton transfer (ESIPT). Moreover, the corresponding monoanion and dianion species were formed in basic conditions with an organic base (DBU) and basic salt (NaOH), which were characterized by highly visible halochromism. The introduction of −OH groups into the pyromellitic moiety of imide compounds or PIs at their para positions led to the long-wavelength fluorescent emission as well as to the high pH sensitivity

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

A novel polyimide (PI) emitting a prominent red fluorescence was prepared based on 3,6-dihydroxy­pyromellitic dianhydride (P2HDA) and 4,4′-diaminocyclohexylmethane (DCHM). In order to investigate the fluorescence properties of the PI, an imide model compound, <i>N</i>,<i>N</i>′-dicyclohexyl-3,6-dihydroxy­pyromellitimide (P2H-Ch), corresponding to one repeating unit of the PI, was also synthesized. The UV–vis absorption and fluorescence spectra of P2H-Ch in CHCl₃ and the solid PI film demonstrated intense red fluorescence at around 640 nm with a very large Stokes shift (ν) of 7655 and 8994 cm^–1, respectively, via the excited-state intramolecular proton transfer (ESIPT). Moreover, the corresponding monoanion and dianion species were formed in basic conditions with an organic base (DBU) and basic salt (NaOH), which were characterized by highly visible halochromism. The introduction of −OH groups into the pyromellitic moiety of imide compounds or PIs at their para positions led to the long-wavelength fluorescent emission as well as to the high pH sensitivity

Pressure-Induced Changes in Crystalline Structures of Polyimides Analyzed by Wide-Angle X‑ray Diffraction at High Pressures

Variations in the crystalline structures of polyimides (PIs) were analyzed under high pressures up to 8 GPa using wide-angle X-ray diffraction. The compressibilities along the polymer chain axis (<i>c</i>-axis) of rigid-rod PIs increased with an increase in the number of phenyl rings in the diamine moiety (PMDA/PPD < PMDA/BZ < PMDA/DATP). This could be due to an increased shrinkage of the C–C bond lengths between the phenyl rings and/or a pressure-induced deformation of the periodic structure associated with changes in bond angles and dihedral angles. In contrast, PMDA/ODA, having an ether linkage, showed an increase in the lattice parameter along the <i>c</i>-axis up to 0.8 GPa, which could be due to a widening of the ether bond angle. Moreover, PMDA/PPD showed isotropic compression along interchain directions, whereas PMDA/DATP and PMDA/ODA showed anisotropic compression along the cofacial stacking direction, which resulted in the larger volumetric shrinkages of the latter PIs