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

Pressure-Induced Enhancement of Room-Temperature Phosphorescence in Heavy Halogen-Containing Imide and Polyimide

To investigate the correlation between the aggregated state and photoluminescence (PL) mechanism of dual fluorescent (FL) and phosphorescent (PH) polyimides (PIs), the photophysical processes of FL-type BP-PI, PH-type DBrBP-PI, and their corresponding imide model compounds (BP-MC and DBrBP-MC) dispersed in poly(methyl methacrylate) (PMMA) films were analyzed at elevated pressures up to 8 GPa using a diamond anvil cell. Dibromo-substituted DBrBP-MC demonstrated a shorter wavelength absorption than BP-MC owing to the larger dihedral angle in the biphenyl moiety. Both MCs exhibited red-shifts in their absorption spectra with increasing pressure, indicating planarization occurred at the biphenyl moieties associated with the compression of the free volume in PMMA. The PL intensity of BP-MC increased with increasing pressure, while its quantum yield (ΦPL) decreased sharply due to the enhanced energy transfer via the Förster mechanism. In contrast, the PH quantum yield (ΦPH) of DBrBP-MC monotonically increased at lower pressures, while it showed excitation wavelength-dependent behaviors at higher pressures: ΦPH remained unchanged under excitation at 340 nm but gradually increased under excitation at 365 nm. This fact suggests that, at higher pressures, 365 nm excitation promoted intersystem crossing (ISC) from excited singlet states at higher energy levels. Using this phenomenon, a significant pressure-induced PH enhancement (PIPE) was observed for DBrBP-PI up to 0.9 GPa upon excitation at 365 nm, which is a rare phenomenon for organic polymers. This study indicates that even in colorless and optically transparent amorphous polymers, an enhancement of PH due to restricted molecular motion and intensified ISC outweighs the deactivation due to intermolecular energy transfer under certain pressures, leading to an increase in ΦPH.

Photophysical analysis of dual fluorescence and phosphorescence emissions observed for semi-aliphatic polyimides at lower temperatures

The photoluminescence (PL) properties of four types of blue fluorescent semi-aliphatic polyimides (PIs) derived from aromatic dianhydrides (ODPA, BPDA, HQDEA, and BPADA) and an alicyclic diamine (DCHM) were investigated at temperatures ranging from room temperature (RT, 298 K) to 30 K to analyse the origins of their non-radiative relaxation (NR) processes. These PIs exhibited significant increases in fluorescence (FL) intensity and lifetimes when lowering the temperature, stabilising below 100 K. The PIs containing ether (-O-) linkages showed a shoulder peak at around 500 nm below 150 K, which is attributable to phosphorescence (PH). These results show that the NR deactivation at RT includes three processes: intersystem crossing (ISC) from the excited singlet (S1) to the triplet (T1) state, temperature-dependent NR from the S1 state, which becomes suppressed below around 100 K, and temperature-independent NR. Based on the analyses of the temperature dependences, polymer structures, and quantum chemical analysis of molecular orbitals, we contemplate that the temperature-dependent NR is attributable to the excitation quenching by defect states mediated by excitation migration, and the temperature-independent NR may be caused by the deactivation of the excited state induced by molecular vibrations.

Characteristic changes in the structures and properties of polyimides induced by very high pressures up to 8 GPa

Various in situ measurement techniques have been applied to investigate changes in the three-dimensional structures and the properties of fully aromatic polymers (mainly aromatic polyimides: PIs) generated at very high pressures up to 8 GPa. In particular, significant changes occurred in the ordered structures, aggregation states, electronic structures, and intermolecular interactions in the repeating units of the PI molecular chains and were observed by applying pressure with a high-pressure optical cell (up to 0.4 GPa, ca. 4000 atm) or a diamond anvil cell (DAC, up to 8.0 GPa, ca. 80,000 atm). In addition, the structural changes in the PI molecular chain repeating units and interchain distances induced by the ultrahigh pressures were observed with wide-angle X-ray diffraction, and they were compared and contrasted with optical absorption, fluorescent and phosphorescent emission spectra, infrared absorption spectra, and refractive indexes observed under the same conditions. These findings obtained at very high pressures provide molecular design guidelines for new PI materials with novel optical, electronic, and thermal functionalities that are not easy to achieve under ambient conditions.

Ultrasonic treatment maintains the flavor of the melon juice

Thermal treatment usually leads to the flavor deterioration of melon juice. This study was initiated to evaluate the retention effect of ultrasonic (US) and ultra-high pressure (UHP) on volatile components of melon juice by gas chromatography-mass spectrometer (GC-MS) and gas chromatography-ion mobility spectrometry (GC-IMS). The electronic nose, electronic tongue, and GC-IMS analysis showed that US was much better way to contain the flavor of melon juice than UHP was does. The correlation coefficient between the US and the control was as high as 0.99. The concentration of characteristic aroma components in melon juice after ultrasonic treatment was 2.77 times and 3.02 times higher than that in the control and UHP, respectively. Moreover, the US treatment gave no significant difference in the total soluble solids, pH, and color of the juice. And it dramatically enhanced the flavor profile of melon juice.

High-pressure treatment enhanced aromatic compound concentrations of melon juice and its mechanism

Introduction

The flavor deterioration blocks the development of melon juice.

Methods

The effects of ultra-high temperature (UHT) and high pressure (HP) treatments on the aromatic compound concentrations of melon juice and their mechanisms were explored with fresh juice as the control.

Results

A total of 57 volatile compounds were identified by gas chromatography-tandem mass spectrometry analysis. β-ionone was shown to be the major aromatic component of melon juice for the first time. The HP at 200 MPa for 20 min increased the total volatile concentration of melon juice by 1.54 and 3.77 times the control and UHT, respectively. Moreover, the sum concentration of a major aromatic component in the HP treatment was 1.49 and 5.94 times higher than that of the control and UHT, respectively.

Discussion

The HP treatment raised the concentration of volatile and aromatic components of melon juice by reducing their surface tension.

Synthesis of a Novel Rigid Semi-Alicyclic Dianhydride and Its Copolymerized Transparent Polyimide Films' Properties

A new series of colorless polyimides (CPIs) with outstanding thermal properties and mechanical properties were fabricated by the copolymerization of a novel dianhydride and 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) with 2,2′-bistrifluoromethyl benzidine (TFDB). The novel dianhydride, 10-oxo-9-phenyl-9-(trifluoromethyl)-9,10-dihydroanthracene-2,3,6,7-tetraacid dianhydride (3FPODA), possessed a rigid semi-alicyclic structure, –CF₃ and phenyl side groups, and an active carbonyl group. Benefitting from the special structure of 3FPODA, the glass transition temperatures (T_g) of the new CPIs improved from 330 °C to 377 °C, the coefficient of thermal expansion (CTE) decreased from 46 ppm/K to 24 ppm/K, and the tensile strength (T_S), tensile modulus (T_M), and elongation at break (E_B) increased from 84 MPa to 136 MPa, 3.2 GPa to 4.4 GPa, and 2.94% to 4.13% with the increasing amount of 3FPODA, respectively. Moreover, the active carbonyl group of the 3FPODA could enhance the CPI’s adhesive properties. These results render the new dianhydride 3FPODA an ideal candidate monomer for the fabrication of high-performance CPIs.

Synthesis and Characterization of White-Light Luminescent End-Capped Polyimides Based on FRET and Excited State Intramolecular Proton Transfer

N-cyclohexylphthalimide-substituted trifluoroacetylamino (CF₃CONH-) group (3TfAPI), which forms an intramolecular hydrogen bond, was synthesized, and it exhibited a bright yellow fluorescence owing to the excited-state intramolecular proton transfer (ESIPT) in the solution and crystalline states. In addition, CF₃CONH-substituted phthalic anhydride (3TfAPA) was synthesized, which was attached to the termini of a blue-fluorescent semi-aromatic polyimide (PI) chain. Owing to the efficient Förster resonance energy transfer (FRET) occurring from the main chain to the termini and the suppression of deprotonation (anion formation) at the 3TfAPA moiety by H₂SO₄ doping, the resulting PI films display bright white fluorescence. Moreover, the enhancement of the chain rigidity by substituting the diamine moiety results in an increase in the quantum yield of white fluorescence (Φ) by a factor of 1.7, due to the suppression of local molecular motion. This material design strategy is promising for preparing thermally stable white-light fluorescent PIs applicable to solar spectral convertors, displays, and ICT devices.

Ultrafast Spectroscopic Analysis of Pressure-Induced Variations of Excited-State Energy and Intramolecular Proton Transfer in Semi-Aliphatic Polyimide Films

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 -CH₃ and -CF₃ 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 Förster 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.

Effective Reduction of Volumetric Thermal Expansion of Aromatic Polyimide Films by Incorporating Interchain Crosslinking

To develop a facile method for reducing the coefficient of volumetric thermal expansion (CVE) of polymer films, the thermal expansion behaviors of thermally cross-linkable polyimide (PI) films with isomeric diamine structures were investigated via thermal mechanical analyses and optical interferometry measurements. The degree of crosslinking of the PI films containing the diphenylethynylene (Ph–C≡C–Ph) structure in the main chain was characterized by far-infrared (far-IR) spectra and density functional theory (DFT) calculations, and variations in the CVE induced by thermal crosslinking were quantitatively estimated. The crosslinking reactions effectively reduced the CVEs of the PI films by suppressing intermolecular free volume expansion and local molecular motions promoted at elevated temperatures. The lowest CVE value observed for a crosslinked PI cured at 400 °C (+98 ppm/K at 80–280 °C) was one of the smallest values reported to date in polymers. Incorporating interchain crosslinking into the main chain is an effective method for reducing the CVE of aromatic polymers.

Flexible Multifunctional Aromatic Polyimide Film: Highly Efficient Photoluminescence, Resistive Switching Characteristic, and Electroluminescence

We report a flexible multifunctional aromatic polyimide (BTDBPI) that shows yellow-green fluorescence with high photoluminescence quantum yield (PLQY) of 30% in the film state. The nonvolatile "write once-read many" (WORM) characteristic in a memory device with the configuration of ITO/BTDBPI/Au indicates that BTDBPI possesses organic semiconductor behavior. Moreover, polymer light-emitting diodes (PLEDs) with the structure of ITO/PEDOT:PSS/BTDBPI/TPBI/Mg-Ag exhibits an interesting dual-emission phenomenon that originates from the electroluminescence (EL) of the BTDBPI nanometer film (yellow-green, 525 nm) and TPBI (deep blue, 380 nm), demonstrating that BTDBPI shows both the charge-transporting and EL properties.

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.

Blue emitting organic semiconductors under high pressure: status and outlook

This review describes essential optical and emerging structural experiments that use high GPa range hydrostatic pressure to probe physical phenomena in blue-emitting organic semiconductors including π-conjugated polyfluorene and related compounds. The work emphasizes molecular structure and intermolecular self-organization that typically determine transport and optical emission in π-conjugated oligomers and polymers. In this context, hydrostatic pressure through diamond anvil cells has proven to be an elegant tool to control structure and interactions without chemical intervention. This has been highlighted by high pressure optical spectroscopy whilst analogous x-ray diffraction experiments remain less frequent. By focusing on a class of blue-emitting π-conjugated polymers, polyfluorenes, this article reviews optical spectroscopic studies under hydrostatic pressure, addressing the impact of molecular and intermolecular interactions on optical excitations, electron-phonon interaction, and changes in backbone conformations. This picture is connected to the optical high pressure studies of other π-conjugated systems and emerging x-ray scattering experiments from polyfluorenes which provides a structure-property map of pressure-driven intra- and interchain interactions. Key obstacles to obtain further advances are identified and experimental methods to resolve them are suggested.

Thiolactone-maleimide: a functional monomer to synthesize fluorescent aliphatic poly(amide-imide) with excellent solubility via in situ PEGylation

Combining aminolysis of thiolactones and amine-maleimide Michael addition contributed to the synthesis of fluorescent aliphatic poly(amide-imide)s, accomplishing excellent solubility via in situ PEGylation.

High-efficiency fluorescent polyimides based on locally excited triarylamine-containing dianhydride moieties

Three fluorescent polyimides (PIs) were readily synthesized from the polycondensation of triarylamine-based tetracarboxylic dianhydride monomers with aliphatic diamine. The PL intensity of solid films and nanofibers revealed high PL quantum yields of up to 32% and 35%, respectively.

Fluorescence emissions of imide compounds and end-capped polyimides enhanced by intramolecular double hydrogen bonds

Enhancement of the fluorescence quantum yield of imide compounds by introducing intramolecular double hydrogen bonds.