CH2Cl2 Vapor Research Articles

Vaporchromic Domino Transformation and Polarized Photonic Heterojunctions of Organoplatinum Microrods.

Photonic heterostructures with codable properties have shown great values as versatile information carriers at the micro- and nanoscale. These heterostructures are typically prepared by a step-by-step growth or post-functionalization method to achieve varied emission colors with different building blocks. In order to realize high-throughput and multivariate information loading, we report here a strategy to integrate polarization signals into photonic heterojunctions. A U-shaped di-Pt(II) complex has been assembled into highly polarized yellow-phosphorescent crystalline microrods (Y-rod) by strong intermolecular Pt⋅⋅⋅Pt interaction. Upon end-initiated desorption of the incorporated CH2Cl2 solvents, the Y-rod is transformed in a domino fashion into tri-block polarized photonic heterojunctions (PPHs) with alternate red-yellow-red emissions or red-phosphorescent microrods (R-rods). The red emissions of these structures are also highly polarized; however, their polarization directions are just orthogonal to those of the yellow phosphorescence of the Y-rod. With the aid of a patterned mask, the R-rod can be further programmed into multi-block PPHs with precisely controlled block sizes by side-allowed adsorption of CH2Cl2 vapor. X-ray diffraction analysis and theoretical calculations suggest that the solvent-regulated modulation of the crystal packing and excited-state property is critical for the construction of these PPHs.

Ultra-Strong Janus Covalent Organic Framework Membrane with Smart Response to Organic Vapor.

Vapor-driven smart Janus materials have made significant advancements in intelligent monitoring, control, and interaction, etc. Nevertheless, the development of ultrafast response single-layer Janus membrane, along with a deep exploration of the smart response mechanisms, remains a long-term endeavor. Here, the successful synthesis of a high-crystallinity single-layer Covalent organic framework (COF) Janus membrane is reported by morphology control. This kind of membrane displays superior mechanical properties and specific surface area, along with excellent responsiveness to CH2Cl2 vapor. The analysis of the underlying mechanisms reveals that the vapor-induced breathing effect of the COF and the stress mismatch of the Janus structure play a crucial role in its smart deformation performance. It is believed that this COF Janus membrane holds promise for complex tasks in various fields.

Synthesis, structure and tri-stimuli-responsive luminescent switching properties of a bis(σ-acetylide) platinum(II) complex

A square-planar bis(σ-acetylide) platinum(II) complex, Pt(PhenCCTES)(CCC6H42Cl)2 (1) (PhenCCTES = 3-(2-triethylsilylethynyl)-1,10-phenanthroline) was synthesized. Three species with different stacking structures, including two solvated yellow-green samples 1·Cl2CHCHCl2 and 1·ClCH2CH2Cl, and one red solvent-free species 1, were obtained by exposure 1 into the different VOC vapors. 1·Cl2CHCHCl2 and 1·ClCH2CH2Cl emit a green luminescence peaked at 513 (548, sh) nm and 545 (517, sh) nm, respectively and exhibit the vapor-, thermo-, and mechanical grinding triggered tri-stimuli-responsive luminescent switching properties. Once exposure to CH2Cl2 vapor or heating at 110 °C, both 1·Cl2CHCHCl2 and 1·ClCH2CH2Cl will lose their solvated molecule and converted to the red solvent-free species 1. Upon mechanical grinding, 1·Cl2CHCHCl2 and 1·ClCH2CH2Cl will be changed to an amorphous species 1G with red color and red luminescence peaked at 675 nm. Meanwhile, the heating and ground samples could be converted back to the original crystalline 1·Cl2CHCHCl2 and 1·ClCH2CH2Cl by absorbing Cl2CHCHCl2 or ClCH2CH2Cl vapor. Systematic studies on the crystal structure, luminescence spectrum, PXRD patterns as well as TD-DFT calculations demonstrate that the mechanism of the reversible tri-stimuli-responsive luminescent switching properties of complex 1 is due to the interconversion of the lowest-energy excited states of metastable species between ligand to ligand/metal to ligand charge transfer (LLCT/MLCT) and ligand to ligand/metal-metal to ligand charge transfer (LLCT/MMLCT) caused by the formation/destruction of intermolecular Pt-Pt interactions and/or aromatic π-π stacking interactions under external stimuli.

Stimuli-Responsive Properties on a Bisbenzofuropyrazine Core: Mechanochromism and Concentration-Controlled Vapochromism.

Stimulus-responsive organic materials with luminescence switching properties have attracted considerable attention for their practical applications in sensing, security, and display devices. In this paper, bent-type bisbenzofuropyrazine derivatives, Bent-H and Bent-sBu, with good solubilities were synthesized, and their physical and optical properties were investigated in detail. Bent-H gave three crystalline polymorphs, and they showed different luminescence properties depending on their crystal packing structures. In addition, Bent-H exhibited mechanochromic luminescence in spite of its rigid skeleton. Bent-sBu exhibited unique concentration-dependent vapochromic luminescence. Ground Bent-sBu was converted to blue-emissive, green-emissive, and green-emissive high-viscosity solution states at low, moderate, and high concentrations of CHCl3 vapor, respectively. This finding represents a concentration-dependent multi-phase transition with an organic solvent, which is of potent interest for application in sensing systems.

Multistimuli-Responsive Squaraine Dyad Exhibiting Concentration-Controlled Vapochromic Luminescence.

Stimuli-responsive organic materials with controllable luminescence are of enormous importance because of their potential applications in sensing, data security, and display devices. In this study, a multistimuli-responsive squaraine dyad (SQ-d) composed of two rigid squaraine moieties and a flexible ethylene linker was rationally designed and synthesized. SQ-d exhibits polymorphic luminescence, which can be reversibly switched by various external stimuli, including solvent vapor exposure, heat, and shear force. Unexpectedly, the weakly luminescent phase (O1) of SQ-d exhibits concentration-controlled vapochromic behavior. Film O1 can convert to a highly green-emissive phase (G1) under a low concentration of CHCl3 vapor and convert to a highly yellow-emissive phase (Y) under a high concentration of CHCl3 vapor; these originate from two distinct crystallization-induced emission enhancement processes. To the best of our knowledge, this is the first investigation of the effect of vapor concentration on the phase transitions of organic vapochromic luminophores. By analyzing the single-crystal structures and photophysical properties of SQ-d, we concluded that the green and yellow emissions probably originated from a zigzag stacking mode and an H-type π-π stacking mode, respectively. Finally, two prototypes based on SQ-d for applications in information encryption and vapor sensing were successfully demonstrated.

Mechnofluorochromic properties of N-alkyl amide anthracene derivatives

Several types of N,N′-dialkylanthracene-1,8-dicarboxamides 1 and N-alkylanthracene-1-carboxamides 2 were prepared, and their photophysical properties in the solid state and solution state were observed. N,N′-Di-tert-butylanthracene-1,8-dicarboxamide 1e and N-tert-butylanthracene-1-carboxamide 2e showed slightly high fluorescence quantum yields (φf = 0.63–0.67) in the solid state due to their bulky tert-butyl group. The mechanofluorochromic phenomenon of the N,N′-dialkylanthracene-1,8-dicarboxamide 1 (1,8-bis(N-alkylamide)anthracene) was observed due to the presence of the two amide groups at the 1,8-position of the anthracene ring, because the mechanofluorochromic behavior was not observed for the N-alkylanthracene-1-carboxamide 2 (mono(N-alkylamide)anthracene). Interestingly, changes in the luminous color of the ground samples were observed upon heating and upon fuming with CH2Cl2 vapor, while the pristine samples did not show any change in their fluorescence color under the same conditions.

A selective and stable vapochromic system constructed by pillar[5]arene-based host–guest interactions

The development of new strategies for the construction of highly selective and stable vapochromic system is still in great demand. Here, we present a highly selective and stable vapochromic system constructed by pillar[5]arene-based host–guest interactions between a pillar[5]arene (EtP5) and a phenazine derivative. The present material in this work showed specific selective responsiveness to chloroform vapor. The resultant dark orange solid sample was highly stable, which can maintain the color in air for at least two months at room temperature. Moreover, CHCl3 vapor responsive thin film and light-emitting diode (LED) was successfully prepared by coating the EtP5⊃1 complex on a glass sheet and a commercially available UV-LED. This work provided a new approach for the design and construction of selective and stable vapochromic systems with obvious color changes using pillar[5]arene-based host–guest interactions.

Hierarchical Microphase Behaviors of Chiral Block Copolymers under Kinetic and Thermodynamic Control

Hierarchical Microphase Behaviors of Chiral Block Copolymers under Kinetic and Thermodynamic Control

A tetraphenylethylene derivative with reversible luminous colours applied in data security

Exploration of the relationship between molecular structure and properties of materials, particularly the multi-stimuli-responsive types, remain a challenge. Herein, we designed, synthesized and tested the photo-physical properties of a multi-stimuli-responsive TPE derivative (TPE-(C3Br)4). The TPE-(C3Br)4 exhibits piezochromic emitting deep-blue to green-blue light under external grinding. Interestingly, both non-ground and ground TPE-(C3Br)4 exhibits vapochromic and thermochromic properties. When exposed to CH2Cl2 vapor for 3 min and thereafter heated at 90 °C for 5 min or heat at 140 °C for 3 min and exposed to CH2Cl2 vapor for 2 s, TPE-(C3Br)4 reversibly glows between deep-blue and green-blue. The ground sample glows between green-blue and deep-blue under only one stimulus for a few min (e.g. heating or fuming alone). The multi-stimuli-responsive properties of TPE-(C3Br)4 are attributed to variation in intermolecular stacking and intramolecular twisting. The TPE-(C3Br)4 was used to develop a new encryption-decryption method. These findings demonstrate the utility of smart responsive TPE-(C3Br)4 material in developing data protection systems.

Reversible Mechanochromic Luminescence of Tetranuclear Cuprous Complexes.

Mechanochromic luminescence materials have attracted rapidly growing interest. Nevertheless, the designed synthesis of such materials remains a challenge, and there have been few examples based on weak intramolecular interactions. Herein, we report a new approach for preparing mechanochromic luminescence materials of Cu(I) complexes, i.e., constructing a photoluminescence system that bears a large coplanar multinuclear Cu(I) unit showing weak intramolecular π···π interactions with the planar rings of the coordinated ligands in the molecule. Using it, a series of novel mechanochromic luminescent tetranuclear Cu(I) complexes have been successfully designed and synthesized. As revealed by single-crystal X-ray crystallography, these Cu(I) complexes share an identical {Cu4[μ3-η2(N,N),η1(N),η1(N)-pyridyltetrazole]2}2+ planar fragment whose coplanar pyridyl rings exhibit weak intramolecular π···π interactions with the phenyl rings of the coordinated phosphine ligands in the molecule. All of these Cu(I) complexes exhibit reversible mechanochromic luminescence, which can be attributed to the change in the rigidity of the molecular structure resulting from the disruption and restoration of intramolecular π···π interactions between the pyridyl and phenyl rings triggered by grinding and CH2Cl2 vapor, as supported by powder X-ray diffraction and Fourier transform infrared spectrometry. In addition, the results might provide a new route for developing mechanochromic luminescence materials of Cu(I) complexes for intelligent responsive luminescent devices.

Breaking the lignin conversion bottleneck for multiple products: Co-production of aryl monomers and carbon nanospheres using one-step catalyst-free depolymerization

Lignin is considered as a renewable and sustainable resource for producing value-added aromatic chemicals and functional carbon materials. Herein, we develop a one-step catalyst-free depolymerization strategy to convert lignin into aryl monomers and carbon nanospheres simultaneously. Importantly, microwave-assisted depolymerization (MAD) in conjunction with dichloromethane (CH2Cl2) vapors is developed. The total mass yield of guaiacols reached the highest amount of 225.1 mg/g at 600 °C, and the highest yields of phenols (49.0 mg/g) and aromatic hydrocarbons (155.1 mg/g) were obtained at 700 °C. Hydrogen radicals and hydrogen chloride (HCl) are in-situ formed from CH2Cl2, significantly decreasing the activation barrier and reforming pyrolysis vapors to promote the formation of aryl monomers. Interestingly, uniform carbon nanospheres with an average size of 140 nm were produced as co-products at 700 °C. The microwave “hot-spots”, allied with the continuous surface erosion and the decrease in surface energy of lignin-derived carbon precursors by CH2Cl2 vapor, can be considered the driving force for the ultimate formation of carbon nanospheres. The CH2Cl2/MAD system produces aryl monomers (26.8 wt% yield) and carbon nanospheres (36.6 wt% yield) at 700 °C. We provide a facile, intriguing and scalable approach to convert lignin to valuable aryl monomers and sustainable carbon materials that can be applied in the chemistry, energy and environmental fields.

Microfluidic ratio metering devices fabricated in PMMA by CO2 laser

We describe microfluidic fabrication results achieved using a 10.6 μm CO2 engraving laser on cast PMMA, in both raster and vector mode, with a 1.5″ lens and a High Power Density Focussing Optics lens. Raster written channels show a flatter base and are more U-shaped, while vector written channels are V shaped. Cross-sectional images, and, where possible, stylus profilometry results are presented. The sides of V-grooves become increasing steep with laser power, but broader shallower channels may be produced in vector mode by laser defocus, as illustrated. Smoothing of raster engraved channels by heated IPA etch, and transparency enhancement by CHCl3 vapour treatment are briefly discussed. An asymmetric Y meter is discussed as one method of diluting acid into seawater for dissolved CO2 analysis. Alternatively, microfluidic snake channel restrictors of different lengths in 2 channels may achieve the same result. Samples are fabricated with bases bonded by CHCl3 vapour treatment, and the devices are flow tested with either dilute food dye or DI water. Microfluidics fabricated in this manner have applications in ocean sensing of dissolved CO2 and other analytes, as well as broader sensing measurements, including biomedical sensors.

Reversible Solid-State Phase Transitions between Au-P Complexes Accompanied by Switchable Fluorescence.

Six complexes {(3-bdppmapy)(AuCl)2}n (1-6; 3-bdppmapy = N,N'-bis(diphenylphosphanylmethyl)-3-aminopyridine and tht = tetrahydrothiophene) were simultaneously formed by the reaction of Au(tht)Cl and 3-bdppmapy in CH2Cl2 followed by infusion with hexane. Complexes 4-6 could be produced independently by volatilizing solvent in air, solid-state heating, or solvothermal reaction. The PPh2-Au-Cl moieties extended in different directions, forming Au-Au and Au-Au-Au interactions. Complex 4 could be converted to 5 by heating to 130 °C, with the cleavage of one Au-Au bond, while 5 reverted back to 4 upon exposure to CH2Cl2 vapor over 11 h. This solid-state phase transition could be recycled and was accompanied by a change in solid-state fluorescence, without obvious intensity decay over five cycles. The reason for both the phase transition and difference in photoluminescence is related to the different numbers and strengths of aurophilic interactions in each complex that could be modeled by density functional theory calculations.

Cruciform Molecules Bearing Bis(phenylsulfonyl)benzene Moieties for High‐Efficiency Solution Processable OLEDs: When Thermally Activated Delayed Fluorescence Meets Mechanochromic Luminescence

AbstractFour crucifix‐shaped molecules, named TPA‐BPSB, DMAc‐BPSB, MTPA‐BPSB and MDMAc‐BPSB, bearing the same acceptor fragment of bis(phenylsulfonyl)benzene (BPSB) and different donor segments (TPA and MTPA are the diphenylamine derivatives while DMAc and MDMAc are the 9,9‐dimethylacridine derivatives) are synthesized and characterized by NMR, mass spectra, and single crystal X‐ray crystallography. The molecular structure–property relationship of these crucifix‐shaped molecules is systematically explored. All compounds display thermally activated delayed fluorescence (TADF) in the region of 500–550 nm. In addition, charming mechanochromic luminescence properties are observed for all these TADF molecules under external stimuli, such as grinding and exposure to CH2Cl2 vapor. Four TADF molecules are used as the emitters in organic light‐emitting diodes (OLEDs) fabricated via solution process. MTPA‐BPSB‐based device presents a best performance with a highest external quantum efficiency of ≈21%, which is among the highest efficiencies for reported BPSB‐based solution‐processable OLEDs so far. This research has an important significance in designing high‐efficiency multifunctional TADF molecules.

Synthesis, structure and dual-stimulus-responsive luminescence switching of a new platinum(II) complex based on 3-trimethylsilylethynyl-1,10-phenanthroline

A new diimine-platinum(II) complex, Pt(PhenC≡CSiMe3)(C≡C-C6H4-3-F)2 (1) based on 3-trimethylsilylethynyl-1,10-phenanthroline (PhenC≡CSiMe3), was synthesized and characterized. Two solvated crystals, 1·1/2(ClCH2CH2Cl) and 1·CHCl3 exhibited dual-stimulus-responsive color and luminescence switching behaviors triggered by heating or mechanical grinding. Upon heating, the color and luminescence of both complexes were changed to red with luminescence red-shifting to 656 for 1·1/2(ClCH2CH2Cl) and 691 nm for 1·CHCl3, corresponding to the thermo-triggered luminescence red-shifts of 112–142 nm and 156–189 nm, respectively. The different heated samples confirm that the original stacking structure can accurately affect the luminescence switching properties of luminescent switching materials. Under the mechanical grinding, both complexes were converted to the same red amorphous product with red emission peaked at 702 nm. Interestingly, once exposure to ClCH2CH2Cl or CHCl3 vapor, the heated and ground samples could be thoroughly reverted to crystalline 1·1/2(ClCH2CH2Cl) and 1·CHCl3. Such reversible thermo- and mechanical-grinding-triggered luminescence switching properties of the complexes are attributed to the interconversions of the lowest energy excited states between metal-to-ligand charge transfer (MLCT) and metal-metal-to-ligand charge transfer (MMLCT) transitions during the heating/grinding-vapor absorbing processes. Furthermore, 1·CHCl3 was successfully used for rewritable data recording.

Sensitive conductive polymer nanocomposites from multiwalled carbon nanotube coated with polypyrrole and hydroxyl-terminated poly(butadiene-co-acrylonitile) polyurethane for detection of chloroform vapor

Sensitive conductive polymer nanocomposites based on noncovalent interactions were prepared via combination of an in situ oxidative and stepwise polymerization, which consisted of multiwalled carbon nanotubes coated with polypyrrole and hydroxyl-terminated poly (butadiene-co-acrylonitile) polyurethane (MWCNTs@PPY/HTBN PU). The nanocomposites were fabricated into thin film sensors to detect CHCl3 vapor in environments. The experimental results indicated that the covering of PPY contributed to the improvement of the compatibility and conductivity of the composite systems, whereas the incorporation of the HTBN PU made for improving the dispersion behavior, the structural stability, the film-forming properties and vapor responsivity mainly triggered by the swelling of PU cross-linking networks. The vapor sensors could detect CHCl3 vapor as low as 50 ppm, and offered a wide vapor concentration range, good restorability and high vapor sensitivity of 0.055 at the CHCl3 concentration of 500 ppm. The cyclic sensing measurements showed good repeatability, stability and thus a great practical application prospect in detection of environmental gaseous pollutants.

Preparation, film fabrication and gas-sensitive responsive properties of MWCNTs@PS-b-HTPB5-b-PS conductive polymer nanocomposites.

Novel nanocomposites consisting of polystyrene-block-polybutadienyl polyhexamethylene dicarbamate-block-polystyrene (PS-b-HTPB5-b-PS) and multiwalled carbon nanotubes (MWCNTs) were designed and prepared via noncovalent interactions. Scanning electron microscopy and transmission electron microscopy observations showed that segregated networks of MWCNTs were formed due to the cladding of PS-b-HTPB5-b-PS, presenting a parallel-arranged topology of the MWCNTs in a continuous PS-b-HTPB5-b-PS phase, which improved the dispersibility of the MWCNTs. The nanocomposites were fabricated into vapor sensing elements to detect CH2Cl2 vapor in the environment, exhibiting excellent responsive sensitivity, reproducibility and a low limit of detection (LOD) of 1 ppm when exposed to CH2Cl2 vapor. The chain extension of HTPB overcame the fragility and improved the tenacity of the thin films, and the responsivity was optimized by adjusting the content of the MWCNTs and the length of the PS chains. The newly developed conductive composites can be applied as a promising vapor sensor to accurately monitor CH2Cl2 vapor in the environment.

Ultralow-dielectric, nanoporous poly(methyl silsesquioxanes) films templated by a self-assembled block copolymer upon solvent annealing

A homogenous, nanoporous poly(methyl silsesquixoxane) (PMSSQ) with a uniform pore size distribution was prepared by templating with a poly(styrene-b-4-vinylpyridine) (PS-b-P4VP) block copolymer. The self-assembly of the PS-b-P4VP and PMSSQ precursor (PMSSQ-P) blends thin films was achieved by solvent annealing firstly; then the self-assembled thin films were thermally cured to prepare nanoporous PMSSQ films after removal of the PS-b-P4VP porogen. The influence of annealing solvent and PS-b-P4VP loading on the self-assembly behavior of the PS-b-P4VP/PMSSQ-P blends films were studied. By exposing to CHCl3 vapor, fingerprint-type microphase-separated structures were achieved for the PS-b-P4VP/PMSSQ-P blends films with PS-b-P4VP loading ranging from 30% to 60%. The formation of the microphase-separated structures is attributed to the substantial mobility of the PS blocks and the P4VP/PMSSQ-P complexes, and segregated repulsion between them under the CHCl3 vapor. Atomic force microscopy (AFM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) results demonstrated the prepared nanoporous PMSSQ films had a homogenous spherical pore morphology, with closed nanopores and a uniform size distribution over large areas. The porosity and dielectric properties of the homogenous, nanoporous PMSSQ could be adjusted by the content of PS-b-P4VP in the PS-b-P4VP/PMSSQ-P blends. As the PS-b-P4VP loading increased to 60 wt%, an ultralow dielectric constant of 1.41 was obtained, making the nanoporous PMSSQ films have potential applications in microelectronic devices.

Synthesis, luminescent and multiple stimuli-responsive properties of π-extended BF2 β-diketone complexes containing an acridone unit

A class of π-extended BF2 β-diketone chromophores, ADBF2-a and ADBF2-b, that containing electron donating N-butyl acridone units have been synthesized. ADBF2-a and ADBF2-b are highly fluorescent with quantum efficiencies (ΦF) close to unity in toluene. The fluorescence of ADBF2-b displays more pronounced solvatochromism and shows a twisted intramolecular charge transfer (TICT) character in polar solvent and aggregation induced emission enhancement (AIEE) behavior. Furthermore, ADBF2-b shows high contrast and sensitive acid/base induced emission “On/Off” switching that result from the reversible protonation/deprotonation of the nitrogen atom. The compounds are also highly luminescent in solid state with a ΦF value of 0.63 for ADBF2-a and 0.28 for ADBF2-b. Single crystal analysis reveals the highly rigid and planar backbone of ADBF2-a. The emission stimuli-responsive properties of the compounds in solid state have been investigated. ADBF2-a shows yellow emission (λem = 564 nm) in crystalline state while turned to be orange (λem = 586 nm) after grinding. Intriguingly, CH2Cl2 vapor fuming of the ground solid led to red emissive (λem = 624 nm) solid, which turned to original yellow color with further CH2Cl2 vapor fuming. The solid of ADBF2-b emits red (λem = 645 nm) fluorescence in crystalline state and red-shifted to λem = 680 nm for amorphous solid after mechanical grinding. The crystalline state was recovered by CH2Cl2 vapor fuming. We believe that this BF2 system will inspire the development of high luminescent solid-state materials, and provide important insights into smart stimuli-responsive properties.

β-Diketone difluoroboron complexes-based luminescent π-gelators and mechanofluorochromic dyes with low-lying excited states

New tert-butylcarbazole modified β-diketone difluoroboron complexes CVB, PCVB, MPCVB and BPCVB have been synthesized. It was found that BPCVB without any traditional gelation group formed organogel in toluene, xylene, 1,3,5-triethylbenzene and 1,4-dioxane. The gained organogel and xerogel based on BPCVB gave a strong red fluorescence. The emission for the xerogel-based film was quenched by aniline vapor selectively, and the detection limit was as low as 2.0 ppb. It is interesting that the solid emission of the synthesized complexes was in the range of 600–850 nm, and the highest solid fluorescent quantum yield reached 0.37. These NIR dyes showed MFC behavior. For example, the as-synthesized sample of PCVB gave an emission band at 647 nm, and emitted red light under UV irradiation. The grinding of the as-synthesized sample resulted in the emitting color to turn to dark purple, accompanied with the red-shift of the emission to ca. 700 nm. The emission could recover to initial states by fuming the ground powders with CH2Cl2 vapor. Based on the UV–vis spectra and XRD patterns in different solid-state for PCVB suggested that π-aggregates were formed in the as-synthesized crystals, and the grinding could destroy the aggregates and yield amorphous state. The transformation between crystalline and amorphous states led to MFC behavior.