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Tatsushige Izumi, [Ryoma Hayakawa](https://orcid.org/0000-0002-1442-8230), Momoka Isobe, [Ryosuke Ohnuki](https://orcid.org/0000-0001-5024-3025), [Yutaka Wakayama](https://orcid.org/0000-0002-0801-8884), Shinya Yoshioka, [Kaname Kanai](https://orcid.org/0000-0002-3952-5491)

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[Atmospherically Stable Poly(Heptazine Imide) Composites](https://mdr.nims.go.jp/datasets/168af3ae-e0eb-410f-95c7-d2ff5977e216)

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Atmospherically Stable Poly(Heptazine Imide) CompositesAtmospherically Stable Poly(Heptazine Imide) CompositesTatsushige Izumi, Ryoma Hayakawa, Momoka Isobe, Ryosuke Ohnuki, Yutaka Wakayama,Shinya Yoshioka, and Kaname Kanai*Cite This: ACS Omega 2026, 11, 16835−16843 Read OnlineACCESS Metrics & More Article Recommendations *sı Supporting InformationABSTRACT: Metal poly(heptazine imide) (MPHI), a two-dimensional carbon nitride polymer containing monovalent metalions (M+), has recently attracted attention as a novel visible-light-driven photocatalyst. It exhibits photochromism, changing fromyellow to blue-green upon light irradiation, regardless of the metalspecies, and is known to enhance ionic conductivity. Consequently,it has the potential to serve as a novel photoresponsive ionicconductor. However, the excited (color-changed) state that exhibitsionic conductivity is easily deactivated by atmospheric or dissolvedoxygen in solution, making its application in actual deviceschallenging. Therefore, in this study, we developed a composite,protonated poly(heptazine imide) (HPHI):poly(vinyl alcohol)(PVA), by dispersing HPHI prepared by the acid treatment of potassium poly(heptazine imide) into a matrix of the insulatingpolymer PVA, which possesses high oxygen-blocking properties. HPHI:PVA can maintain a color-changed state for extendedperiods, even in air, while sustaining a low electrical resistance state. The time constant derived from the decay curve of HPHI:PVA’sabsorbance over time is six times longer than that reported for HPHI composites using poly(methyl methacrylate) in previousstudie. The duration of this color-changed state can be controlled by varying the degree of PVA saponification or temperature.Furthermore, a detailed investigation of the dependence of the electrical properties of HPHI:PVA on the percentage of HPHIrevealed that proton conduction in HPHI:PVA arises from the percolation of poly(heptazine imide) particles within the composite.This finding also provides fundamental information regarding the ion-conduction mechanism in other MPHI composites. This studyserves as an important guideline for the future development of new MPHI composites and applied research.1. INTRODUCTIONMetal poly(heptazine imide) (MPHI), a two-dimensionalcarbon nitride (CN) polymer containing monovalent metalions (M+), has recently attracted attention because of its highphotocatalytic activity compared with those of other CNpolymers and its ability to exhibit dark photocatalytic activityby consuming the accumulated charge under visible-lightirradiation.2−6 Figure 1a shows the molecular structure ofpotassium poly(heptazine imide) (KPHI, M�K), while Figure1b shows the corresponding protonated poly(heptazine imide)(HPHI) obtained by protonating MPHI. Most MPHImaterials exhibit photochromism under light irradiation,transitioning in color from yellow to blue-green regardless ofthe incorporated metal.2,4−7 Several studies have explored howthis photochromic behavior contributes to the dark photo-catalytic activity of MPHI.1,7−9 We previously reported thatlight irradiation induces the desorption of metal ions fromMPHI, resulting in a sharp increase in the ionic conductivity.This ion desorption from the poly(heptazine imide) (PHI)framework alters the electronic state of PHI, giving rise tophotochromism.1,9,10 It is understood that when ions desorbedrecombine onto the PHI framework, MPHI reverts to yellow.To date, MPHI has been studied not only as a photocatalystbut also for its potential in photochromism-based colorswitching and light-responsive ion conductivity. The estimatedion conductivity of KPHI under light irradiation, as measuredusing KPHI nanosheets, reaches a maximum of approximately7.0 × 10−8 S/cm at room temperature.1 In contrast, the ionconductivity of PEO:Ga-LLZO, a representative solid polymerelectrolyte, reaches a maximum of 7.2 × 10−5 S/cm at 30 °C.11Thus, while the ionic conductivity of KPHI is extremely lowcompared to solid polymer electrolytes like PEO, it possessesthe unique functionality of light-responsive ion conductivity.For example, MPHI has been used to develop anticounterfeit-ing films and oxygen colorimetric sensors for foodapplications.12−14 In addition, research on the photo-Seebeckeffect of MPHI has also been reported.15 However, because theexcited state of MPHI is easily deactivated by oxygen,7,8,12,13challenges remain for its practical use as a light-responsive, ion-Received: January 2, 2026Revised: January 27, 2026Accepted: February 23, 2026Published: March 4, 2026Articlehttp://pubs.acs.org/journal/acsodf© 2026 The Authors. Published byAmerican Chemical Society16835https://doi.org/10.1021/acsomega.6c00037ACS Omega 2026, 11, 16835−16843This article is licensed under CC-BY 4.0Downloaded via 126.206.245.124 on March 17, 2026 at 11:12:51 (UTC).See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.https://pubs.acs.org/action/doSearch?field1=Contrib&text1="Tatsushige+Izumi"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Ryoma+Hayakawa"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Momoka+Isobe"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Ryosuke+Ohnuki"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Yutaka+Wakayama"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Shinya+Yoshioka"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Shinya+Yoshioka"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Kaname+Kanai"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/showCitFormats?doi=10.1021/acsomega.6c00037&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsomega.6c00037?ref=pdfhttps://pubs.acs.org/doi/10.1021/acsomega.6c00037?goto=articleMetrics&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsomega.6c00037?goto=recommendations&?ref=pdfhttps://pubs.acs.org/doi/10.1021/acsomega.6c00037?goto=supporting-info&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsomega.6c00037?fig=abs1&ref=pdfhttps://pubs.acs.org/toc/acsodf/11/10?ref=pdfhttps://pubs.acs.org/toc/acsodf/11/10?ref=pdfhttps://pubs.acs.org/toc/acsodf/11/10?ref=pdfhttps://pubs.acs.org/toc/acsodf/11/10?ref=pdfhttp://pubs.acs.org/journal/acsodf?ref=pdfhttps://pubs.acs.org?ref=pdfhttps://pubs.acs.org?ref=pdfhttps://doi.org/10.1021/acsomega.6c00037?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ashttps://http://pubs.acs.org/journal/acsodf?ref=pdfhttps://http://pubs.acs.org/journal/acsodf?ref=pdfhttps://creativecommons.org/licenses/by/4.0/conductive, and photochromic material. Therefore, compositeshave been developed in which MPHI is dispersed in ionicliquids, poly(methyl methacrylate) (PMMA) and poly(vinylacetate) (PVAc).9,10,13,16 These composites exhibit relativelystable MPHI-derived photochromism and light-responsive ionconductivity because the MPHI encapsulated within the matrixdoes not come into direct contact with atmospheric oxygen.However, even in these composites, the influence of ambientoxygen cannot be fully eliminated, and the photochromiccoloration produced by light irradiation typically persists foronly a few hours at most.10,13,16 Therefore, developing MPHIcomposites with enhanced resistance to oxygen is essential.Furthermore, the ion-conduction mechanisms operating inthese composites remain insufficiently understood. Under-standing the ion-conduction mechanism of MPHI compositesis essential for developing new materials with superior ionconductivity.In this study, we developed a novel MPHI composite bydispersing MPHI in a poly(vinyl alcohol) (PVA) matrix. PVAhas high chemical resistance and is water-soluble, making iteasy to handle. In addition, it possesses excellent oxygen-blocking properties.17−19 Furthermore, PVA is an ionconductor and widely used in applications such as polarizingfilms, solid polymer electrolytes, pharmaceuticals, andadhesives.20−23 HPHI was used as the MPHI dispersed inPVA (hereinafter, the HPHI composite with PVA as the matrixis referred to as HPHI:PVA). HPHI was prepared by the ionexchange of K+ in KPHI with H+ through acid treatment.16HPHI:PVA was found to maintain its color-changed state,which is indicative of the excited state of HPHI, for a longerperiod than other MPHI composites. Furthermore, it wasrevealed that the duration of the excited state of HPHI can becontrolled by adjusting parameters such as the degree ofsaponification (SD), which governs the free volume in PVA, aswell as the temperature. In addition, electrical propertymeasurements were performed to investigate the electricalconductivity of HPHI:PVA. Because HPHI does not containmetal ions and PVA exhibits proton conductivity,24−26HPHI:PVA is expected to exhibit proton conductivity.Electrical conductivity measurements conducted while varyingthe amount of dispersed HPHI within the composite revealedthat, contrary to expectations, protons released from the PHIframework under light irradiation were conducted not throughthe PVA matrix but through the dispersed HPHI particles inthe composite. In other words, it was found that protonconduction in HPHI:PVA exhibits so-called “percolationconduction.” The fundamental properties of HPHI:PVAclarified in this study provide important guidelines for futureresearch on MPHI composites.2. EXPERIMENTAL SECTION2.1. Preparation of KPHIMelon was prepared as the KPHI precursor. A quartz test tube andquartz tube for calcination were first heated at 700 °C for 45 min in atube furnace (JTEKT THERMO SYSTEMS Co., Ltd., KTF035N1).Melamine (3.0 g, 99.0%, Wako Pure Chemicals Co., Ltd., 139-00945)was then placed in a quartz test tube, covered with aluminum foilcontaining a single pinhole at the center (≈0.6 mm in diameter), andsecured with a tungsten wire. The test tube was inserted into thequartz tube. Synthesis was performed under a nitrogen atmosphere(purity: 99.99995%) using the following temperature program:heating at 1 °C min−1 to 550 °C, holding for 5 h, and subsequentlycooling at 2 °C min−1 to room temperature.To synthesize KPHI, a quartz test tube and quartz tube were firstheated at 700 °C for 45 min. Then, the synthesized melon (0.3 g) andKSCN (0.15 g, purity: 98.0%, FUJIFILM Wako Pure Chemicals Co.,Ltd.; 164-04555) were mixed and placed in a calcination boat. Themixture was covered with aluminum foil, placed in a quartz test tube,and secured with a tungsten wire. Synthesis was performed under anitrogen atmosphere (purity: 99.99995%) using the followingtemperature program: heating at 30 °C min−1 to 400 °C and holdingfor 1 h, followed by heating at 30 °C min−1 to 500 °C and holding for30 min, and then cooling at 2 °C min−1 to room temperature. Theproduct was washed four times with pure water (FUJIFILM WakoPure Chemical Corporation, 161-08247) and separated bycentrifugation. Finally, the samples were dried in a desiccator toobtain KPHI as a yellow powder.2.2. Preparation of HPHISulfuric acid (2 mL, sulfuric acid content: 97%; FUJIFILM WakoPure Chemical Corporation, 190-04675) was added to a graduatedcylinder containing pure water (35 mL). Additional pure water wasadded to bring the total volume to 40 mL, yielding a dilute sulfuricacid solution. This solution was transferred to a beaker, and KPHI(300 mg) was added. The mixture was stirred at 500 rpm for 30 min,followed by ultrasonic treatment for 10 min. The mixture was thentransferred to a centrifuge tube and centrifuged for 2 min. Thesupernatant in the centrifuge tube was discarded, pure water wasadded, and the mixture was centrifuged again for 2 min; thisprocedure was repeated twice. After discarding the supernatant, purewater was added, and the mixture was centrifuged for 10 min. Finally,Figure 1. (a) Molecular structure of potassium poly(heptazine imide) (KPHI). The diagram shows three K+ ions inside the pore; however, inreality, KPHI may contain an average of one K+ per unit.1 In that case, two nitrogen atoms are protonated on average. (b) Molecular structure ofprotonated poly(heptazine imide) (HPHI).ACS Omega http://pubs.acs.org/journal/acsodf Articlehttps://doi.org/10.1021/acsomega.6c00037ACS Omega 2026, 11, 16835−1684316836https://pubs.acs.org/doi/10.1021/acsomega.6c00037?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsomega.6c00037?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsomega.6c00037?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsomega.6c00037?fig=fig1&ref=pdfhttp://pubs.acs.org/journal/acsodf?ref=pdfhttps://doi.org/10.1021/acsomega.6c00037?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asthe sample was dried in a desiccator to obtain HPHI as a whitepowder.2.3. Preparation of HPHI:PVAPVA (average polymerization degree ≈900−1100; partially saponi-fied, saponification degree 86%−90%; fully saponified, saponificationdegree ≥96%; FUJIFILM Wako Pure Chemical Corporation, 9002-89-5) was added to pure water and stirred at 85−100 °C whileheating until the PVA completely dissolved, at approximately 300rpm. HPHI was then added, and the mixture was heated at 85 °Cwhile stirring at 300 rpm for several hours. For electrical measure-ments, the samples were prepared by dispensing the above aqueoussolution (40 μL) between Au electrodes spaced 2 mm apart, followedby heating and drying in the dark at 50 °C for 3 days. The Auelectrodes were fabricated by sputter deposition; first, 5 nm of Cr wasdeposited onto a glass substrate, followed by 50 nm of Au.2.4. CharacterizationAn LED (HAYASHI-REPIC, LA-HDF 100NA) was used as thevisible light source for the PHI photochromism. The light spectrum ofthis source is shown in Figure S1 in Supporting Information.Ultraviolet−visible (UV−vis) absorption spectroscopy was performedFigure 2. (a) Molecular structure of the copolymer of poly(vinyl alcohol) (PVA) and polyvinyl acetate (PVAc). The degree of polymerization ofthe PVAc unit is denoted by m, and that of the PVA unit by n. (b) Left: KPHI:PVA#1 before light irradiation; right: KPHI:PVA#1 after lightirradiation. Light was irradiated on the upper half of the sample. PVA#1 refers to partially saponified PVA with an SD in the range of 86%−90%. (c)Left: HPHI:PVA#1 before light irradiation; right: HPHI:PVA#1 after light irradiation. Light was irradiated on the upper half of the sample.Figure 3. (a) Time-dependent ultraviolet−visible (UV−vis) spectra of a sample containing 30% HPHI relative to the mass of PVA#1 (PVA#1).The sample was prepared from a 15 wt % aqueous solution of PVA#1. “Yellow” denotes the spectrum before light irradiation; “0 min” denotes thespectrum recorded immediately after 5 min of light irradiation; “60 min” and “240 min” denote the spectra measured 60 and 240 min, respectively,after irradiation was stopped. All measurements were performed at room temperature. (b) Time-dependent UV−vis spectra of a sample containing30% HPHI relative to the mass of PVA#2 (PVA#2). The sample was prepared from a 15 wt % aqueous solution of PVA#2. The legend is the sameas in panel (a). Measurements were performed at room temperature. (c) Time-dependent UV−vis spectra of PVA#1. The legend is the same as inpanel (a). Measurements were performed at 56 °C. (d) Time-dependent changes in the absorbance at λ = 668 nm in the UV−vis spectra shown inpanels (a−c) after light irradiation. Absorbance values are normalized to the absorbance immediately after irradiation (0 min).ACS Omega http://pubs.acs.org/journal/acsodf Articlehttps://doi.org/10.1021/acsomega.6c00037ACS Omega 2026, 11, 16835−1684316837https://pubs.acs.org/doi/suppl/10.1021/acsomega.6c00037/suppl_file/ao6c00037_si_001.pdfhttps://pubs.acs.org/doi/10.1021/acsomega.6c00037?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsomega.6c00037?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsomega.6c00037?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsomega.6c00037?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsomega.6c00037?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsomega.6c00037?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsomega.6c00037?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsomega.6c00037?fig=fig3&ref=pdfhttp://pubs.acs.org/journal/acsodf?ref=pdfhttps://doi.org/10.1021/acsomega.6c00037?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asusing a spectrometer (JASCO Corporation, V-670 equipped with anintegrating sphere). The film samples were prepared on quartz glassplates, and the reflectance of the light was measured. A film heater(HET-ON4; KYOHRITSU ELECTRONIC INDUSTRY Co., Ltd.)was used to heat the samples. Electrical measurements wereconducted using a source measurement unit B2912B (KeysightTechnology). A visible light source (LED head unit CL-H1-405-9-1-Bwith controller CL-1503; ASAHI SPECTRA Co., Ltd.) was used forlight irradiation during the electrical measurements. The relativehumidity, recorded using a precision thermo-hygrometer (HD-120,Crecer Co., Ltd.), ranged from 40%RH to 47%RH during themeasurements. Electrochemical impedance spectroscopy (EIS) wasconducted using an LCR meter (Keysight Technologies, E4980A). Allmeasurements were performed using a four-probe system at roomtemperature under atmospheric conditions. The pyZwx software wasemployed to obtain the impedance spectrum and fit it usingappropriate equivalent circuit models.27 These electrical measure-ments were performed on HPHI:PVA films formed on Au electrodes.The Au electrode had a spacing of 2 mm and a thickness of 50 nm.3. RESULTS AND DISCUSSIONFigure 2a shows the structure of PVA. In general, PVA issynthesized from its precursor, PVAc, through a process calledsaponification. During this process, the acetic acid groups inthe side chains of PVAc are hydrolyzed to form hydroxylgroups in the side chains of PVA. The extent of this hydrolysisis referred to as the SD. When the degree of polymerization ofPVAc is denoted as m and that of PVA as n, the SD [%] isdefined by the following equationnn mSD 100=+×(1)in this paper, the copolymer of PVA and PVAc shown inFigure 2a is referred to simply as “PVA.” In addition, PVA withan SD in the range of 86%−90% is referred to as “partiallysaponified PVA” (PVA#1), and PVA with an SD exceeding orequal to 96% is referred to as “fully saponified PVA” (PVA#2).As the SD increases, the steric hindrance from the acetategroup of PVAc decreases, facilitating the aggregation of PVAmolecules and formation of hydrogen bonds. As a result, infully saponified PVA, the glass transition temperature increasesand the segment motion of the polymer chains is suppressed.Consequently, oxygen intrusion and diffusion into PVA can beeffectively prevented. Therefore, the SD is an importantparameter that determines the oxygen permeability of PVA.28Figure 2b,c show photographs of the KPHI:PVA#1 andHPHI:PVA#1 films, respectively. These films were prepared bydispensing a KPHI:PVA#1 or HPHI:PVA#1 solution (50 μL)onto a glass substrate, spreading it evenly, and drying it in thedark at 50 °C for 3 days. As shown in the photographs, whenlight is irradiated on the upper half of each sample,photochromism causes KPHI:PVA#1 to turn blue-green andHPHI:PVA#1 to turn dark blue (hereinafter, the state in whichthe color has changed due to photochromism is referred to asthe “color-changed state”). As shown in Figure S2 inSupporting Information, KPHI:PVA#2 and HPHI:PVA#2also exhibited color changes in response to light. Thedifference in color between the KPHI and HPHI compositesafter light irradiation is attributed to differences in theirelectronic states. Previous studies have shown that HPHI hasan energy gap approximately 0.3 eV larger than that of KPHI.16The color-changed states of these composites gradually revertto their original yellow states over several days. This indicatesthat composites using PVA as a matrix maintain their color-changed states for a longer period than other PHI composites.In addition, PVA is water-soluble, facilitating the formation offlexible composite films, and exhibits excellent adhesion tosubstrates such as glass, thereby significantly improvinghandling properties.Figure 3 presents the time-dependent UV−vis spectra ofHPHI:PVA#1 and HPHI:PVA#2. Figure 3a shows the UV−visspectra of HPHI:PVA#1. Immediately after 5 min of lightirradiation, an absorption band with a peak wavelength ofapproximately 668 nm appeared owing to the photochromismof HPHI. Over time, the intensity of this absorption bandgradually decreased; however, as shown in the figure, itretained substantial intensity even after 240 min. This durationis significantly longer than that reported for composites such asKPHI:PMMA or KPHI:PVAc in previous studies.13,16 Ingeneral, the free volume influences the gas permeability ofpolymeric materials. This quantity is determined by factorssuch as the cohesive forces within the polymer and the extentof steric hindrance arising from its side chains. For example, itis known that bulky side chains on polymers inhibit the densepacking of polymer chains, thereby increasing the freevolume.29 In general, a larger free volume allows gas moleculesto permeate and diffuse more easily through the polymer film.Therefore, when the steric hindrance of the side chains is large,gas permeability tends to increase.29 PVA exhibits a low freevolume owing to its high crystallinity, which results fromstrong aggregation driven by hydrogen bonding and its shortside chains. Furthermore, a higher SD reduces the presence ofside chains with significant steric hindrance originating fromthe acetate groups derived from PVAc. This promotes strongeraggregation of the PVA molecules, enhances the crystallinity,and further decreases the free volume. As a result, the oxygenpermeability decreases.28,30,31 Figure 3b shows the UV−vismeasurement results for the HPHI:PVA#2 composite, whichwas prepared using fully saponified PVA (PVA#2). Comparedwith HPHI:PVA#1 shown in Figure 3a, the rate of decrease inthe intensity of the 668 nm absorption band was slower. Thisresult suggests that using PVA with a high SD and low freevolume enhances oxygen blocking, thereby allowing longer-lasting color-change retention.Heating polymers can increase the thermal motion of thepolymer chains, potentially increasing their free volume.Therefore, as the temperature rises, the free volume of thepolymer expands, leading to a higher oxygen permeability.Figure 3c shows the time-dependent UV−vis spectrum ofHPHI:PVA#1 measured at 56 °C. Compared with the resultsin Figure 3a, the decrease in the intensity of the 668 nmabsorption band in Figure 3c occurs more rapidly. This resultindicates that the color-changed state of HPHI:PVA#1disappeared more rapidly because the free volume of PVAincreased at high temperatures, facilitating oxygen perme-ation.31−33 In fact, the glass transition temperature of PVA#1has been reported to be around 53 °C.28In general, moisture-absorbing polymer films such as PVAexhibit a rapid increase in free volume under highhumidity.34−37 As water molecules are absorbed into thepolymer film, they break the hydrogen bonds between PVAmolecules while increasing hydrogen bonding between PVAand water molecules, as well as between the water moleculesthemselves. Consequently, the intermolecular bonds betweenthe PVA molecules weaken, causing the PVA chains toseparate. This separation allows the chains to move morefreely, thereby increasing the free volume. Figure S3 inSupporting Information shows the time-dependent colorACS Omega http://pubs.acs.org/journal/acsodf Articlehttps://doi.org/10.1021/acsomega.6c00037ACS Omega 2026, 11, 16835−1684316838https://pubs.acs.org/doi/suppl/10.1021/acsomega.6c00037/suppl_file/ao6c00037_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acsomega.6c00037/suppl_file/ao6c00037_si_001.pdfhttp://pubs.acs.org/journal/acsodf?ref=pdfhttps://doi.org/10.1021/acsomega.6c00037?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-aschange of HPHI:PVA at 60 °C after light irradiation. Samplesheated under high-humidity conditions exhibited more rapidcolor recovery than those heated under ambient conditions.This clearly demonstrates that the retention of the color-changed state in HPHI:PVA strongly depends on the oxygenpermeability of PVA. Figure 3d shows the time dependence ofthe absorbance at λ = 668 nm in the UV−vis spectra of Figure3a−c after light irradiation. The time constants τ obtainedfrom fitting these decay curves using eq 2 are summarized inTable 1.A A A e A et t0 1( / )2( / )1 2= + + (2)where A0 corresponds to the absorbance of HPHI:PVA in theyellow state prior to light irradiation.As shown in Table 1, the time constant τ1 of the transientcomponent shows little difference between HPHI:PVA#1 andHPHI:PVA#2. In contrast, the time constant τ2 of the delayedcomponent for HPHI:PVA#2 is more than twice that ofHPHI:PVA#1. The τ2 of PVA#2 is approximately six times thatof the τ2 of HPHI:PMMA (τ2 = (2.75 ± 0.35) × 102 min)reported in the previous work.16 Furthermore, the τ2 of PVA#1heated to 56 °C is less than half that of HPHI:PVA#1measured at room temperature.Figure 4 shows the effects of light irradiation on the protonconductivities of HPHI:PVA#1 and HPHI:PVA#2 (ITcharacteristics, Figure 4a), as well as their current−voltagecharacteristics (Figure 4b). During the IT measurements, avoltage of 5 V was applied to the samples.From the IT characteristics shown in Figure 4a, bothHPHI:PVA#1 and HPHI:PVA#2 exhibited negligible currentsbefore light irradiation. Since the HPHI:PVA compositeconsists of HPHI dispersed within the insulating polymerPVA, electrons or holes cannot be injected as carriers from theelectrodes, and electrical conduction occurs via protonconduction. Immediately after light irradiation, the currentdue to proton conduction increased sharply in both samplesbut did not saturate. During light irradiation, the photocurrentgradually increased for HPHI:PVA#1 but gradually decreasedfor HPHI:PVA#2. HPHI:PVA#1 exhibited an approximately 4-fold greater current increase under light irradiation comparedwith that of HPHI:PVA#2. This difference is attributed to thevariation in the amount of moisture absorbed by the PVA.Yamada et al. reported that, within the SD range of 99% to77%, PVA with a higher SD exhibits a lower rate of moistureabsorption.38 This is because PVA#2 has higher crystallinitythan PVA#1, resulting in less free volume for water moleculesto penetrate. Consequently, PVA#1 contains more moisturethan PVA#2. As discussed in detail below, the water moleculeswithin the composite play a crucial role in proton conduction.This likely explains why PVA#1 exhibits a higher photocurrentowing to its greater proton conductivity, as shown in Figure 4a.For both samples, after light irradiation was stopped, thecurrent did not decrease immediately, exhibiting persistentphotoconductivity (PPC). PPC has also been observed incomposites such as KPHI with ionic liquids or KPHI withPMMA.10,16 In general, proton conduction occurs primarily viathe Grotthuss mechanism, which involves a series of protonjumps along a hydrogen-bond network, and the vehiclemechanism, where the oxonium ion itself, attached to awater molecule, moves through the medium. Based solely onthe electrical properties shown in Figure 4a, it is impossible todetermine which mechanism governs proton conduction inHPHI:PVA; however, the observation of PPC provides animportant clue. To understand the origins of PPC, two pointsTable 1. Parameter Values Obtained by Fitting the Absorbance at λ = 668 nm in Figure 3d as a Function of Time after LightIrradiation Using eq 2A0 A1 τ1 (min) A2 τ2 (min)PVA#1 (RT) 0.288 ± 0.039 (9.8 ± 3.4) × 10−2 29 ± 4 0.605 ± 0.006 (8.1 ± 0.5) × 102PVA#2 (RT) 0.294 ± 0.033 (4.5 ± 0.6) × 10−2 27 ± 5 0.659 ± 0.004 (16.5 ± 0.8) × 102PVA#1 (56 °C) 0.260 ± 0.008 0.278 ± 0.020 26.2 ± 3.1 0.464 ± 0.020 (3.40 ± 0.36) × 102Figure 4. (a) Electric current as a function of time for HPHI:PVA#1 (PVA#1) and HPHI:PVA#2 (PVA#2). The samples measured here wereprepared using a 6 wt % aqueous solution of PVA. Measurements were performed by applying 5 V to the samples. The light yellow area in thegraph indicates the period during which light was irradiated onto the yellow sample surface. (b) Electric current as a function of applied voltage forHPHI:PVA#1 (PVA#1_Before irra. and PVA#1_After irra.) and HPHI:PVA#2 (PVA#2_Before irra. and PVA#2_After irra.) before and after 5min of light irradiation.ACS Omega http://pubs.acs.org/journal/acsodf Articlehttps://doi.org/10.1021/acsomega.6c00037ACS Omega 2026, 11, 16835−1684316839https://pubs.acs.org/doi/10.1021/acsomega.6c00037?fig=fig4&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsomega.6c00037?fig=fig4&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsomega.6c00037?fig=fig4&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsomega.6c00037?fig=fig4&ref=pdfhttp://pubs.acs.org/journal/acsodf?ref=pdfhttps://doi.org/10.1021/acsomega.6c00037?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asmust be considered. First, when the light irradiation ofHPHI:PVA was stopped, the color of HPHI:PVA graduallychanged from blue back to yellow, and the current decreased insynchrony with this change.1,10 This indicates that protonsreleased from the PHI framework by light irradiation do notrecombine with the PHI framework immediately after theirradiation is stopped, but rather do so gradually. Second, if theproton conduction mechanism in HPHI:PVA were dominatedby the vehicle mechanism, it would suggest that the mobility ofoxonium ions (H+/H3O+) within the composite is likely verylow. For example, partial proton detachment, which ionizes thePHI framework upon light irradiation, may distort the PHIstructure and inhibit H+/H3O+ movement. Therefore, H+/H3O+ moving within HPHI must experience significantscattering and trapping by the negatively charged PHIframework. Combining the two points above, the followingscenario based on the vehicle mechanism can be proposed:First, light irradiation causes a sharp increase in the number ofprotons within HPHI:PVA, leading to a rapid rise inconductivity. During this process, protons are conductedwithin the composite in the form of oxonium ions. Thishypothesis is plausible because in MPHI materials, includingKPHI, ion conduction originates from the hydration of metalions,39 which explains why PVA#1 with a high water contentexhibits higher photoconductivity. However, the mobility ofthe oxonium ions is extremely low because they are scatteredand trapped by the charged and distorted PHI framework.Upon cessation of light irradiation, protons slowly recombinedwith the PHI framework, causing the proton number togradually decrease and thereby inducing PPC. Because it isunlikely that the oxonium ion releases a single proton directly,the proton may instead be transferred between watermolecules and ultimately transferred to the PHI framework,leading to its reincorporation into the PHI framework.Figure 4b shows the changes in the current−voltagecharacteristics of HPHI:PVA#1 and HPHI:PVA#2 beforeand after light irradiation. For both samples, only a very smallcurrent flowed before light irradiation; however, the currentincreased significantly after light irradiation. The currentobserved after light irradiation increases linearly over a widerange of applied voltages. This indicates that only protonconduction occurred in the composites without the injection ofelectrons or holes from the electrodes into the HPHI:PVA. Ifcarriers were injected into the composites from the electrodes,the current would not increase linearly from approximately 0V. Instead, a threshold voltage that produces distinct currentincreases should be observed.1Figure 5. (a) Increase in current (ΔI) before and after light irradiation plotted against the HPHI percentage (x) in HPHI:PVA#1. The ITmeasurement was performed by applying 1 V to the sample. Measurements were taken three times within a relative humidity range of 40%RH to47%RH, and the average values of ΔI are plotted. (b) IV measurement results for HPHI:PVA#1 (x = 50%). For reference, the IV measurementresult for PVA#1 (x = 0%) is also shown. Measurements for HPHI:PVA#1 were taken before light irradiation, under light irradiation, 10 min afterlight irradiation, and 2 h after light irradiation. (c) Electrochemical impedance spectroscopy results for HPHI:PVA#1 samples. Nyquist plotsmeasured as a function of x. The horizontal axis Z′ shows the real part of the impedance, and the vertical axis Z′′ shows the imaginary part of theimpedance. (d) Ion resistances R of the samples calculated from the Nyquist plots plotted against x (logarithmic axis).ACS Omega http://pubs.acs.org/journal/acsodf Articlehttps://doi.org/10.1021/acsomega.6c00037ACS Omega 2026, 11, 16835−1684316840https://pubs.acs.org/doi/10.1021/acsomega.6c00037?fig=fig5&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsomega.6c00037?fig=fig5&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsomega.6c00037?fig=fig5&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsomega.6c00037?fig=fig5&ref=pdfhttp://pubs.acs.org/journal/acsodf?ref=pdfhttps://doi.org/10.1021/acsomega.6c00037?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asAssuming that protons are conducted through HPHI:PVAvia a vehicle mechanism, the next question is how theconduction path is formed. To address this, Figure 5a showsthe measured electrical current for samples with different massfractions of HPHI in HPHI:PVA. The ΔI in Figure 5arepresents the difference between the maximum currentobserved during 5 min of light irradiation of HPHI:PVA#1and the current before irradiation. In Figure 5a, the ΔI isplotted against the mass ratio x of HPHI relative to the totalsample mass.In the region where the HPHI ratio is low (x ≤ 10%), the ΔIis very small. The ΔI gradually increases from approximately x= 20% HPHI, reaches high values at x = 40% and 50%, andthen decreases when x is increased to 60%. These resultssuggest the following possibilities regarding the protonconduction mechanism in the composite. When x is low, aslight increase in the ΔI is observed upon light irradiation. Thisindicates that HPHI functions as a proton source for thecomposite.40 The IV characteristics of PVA#1 shown in Figure5b indicate that the current for PVA alone is very small, on theorder of 10−1 to 10−2 nA. This means that proton conductiondue to protons supplied by water or other componentscontained in PVA is essentially absent. Figure 5a shows thatphotoconduction occurs when x reaches 20%. Protonconduction arises when protons are released from the PHIframework upon irradiation with light. The results shown inFigure 5a provide insight into the formation of protonconduction paths within the composite. A key point is that,within a small x range, photoconduction does not increaseproportionally with x. If the protons supplied by HPHI uponlight irradiation were conducted directly through PVA, the ΔIwould be expected to increase proportionally with x. The factthat the ΔI increases when x exceeds a threshold value of 20%indicates that protons supplied by HPHI cannot be conductedbelow this value. In other words, when the percentage of HPHIin the composite exceeds 20%, it can be considered thatpercolation conduction occurs because the HPHI domainsbecome interconnected within the composite. Protonsgenerated within the HPHI under light irradiation are thoughtto hydrate into oxonium ions and migrate through channels inthe PHI framework without conducting through PVA.Therefore, for the composite to exhibit photoconductivity,the HPHI particles must form connected pathways that bridgethe gap between the negative and positive electrodes.The remaining question is why, after the ΔI reaches itsmaximum at 40%, further increasing x to 60% leads to adecrease in the ΔI. As the percentage of HPHI in thecomposite increases, HPHI particles near the complex surfacecan no longer be fully encapsulated by PVA and becomereadily exposed to oxygen. Furthermore, these surface HPHIdomains absorb light, reducing the amount that reaches thebulk of the composite. HPHI in the surface region, even whenabsorbing light and undergoing photochromism, is rapidly de-excited by oxygen, thereby suppressing efficient protongeneration. Therefore, as x increases, although the connectivityof the HPHI particles establishes sufficient proton-conductionpathways within the composite, but the actual number ofprotons that pass through these pathways might decrease.However, since this is currently only a hypothesis, we need todevise a method to experimentally verify it in the future.Experimental results supporting the hypothesis that protonconduction in HPHI:PVA occurs through percolation werealso obtained from the EIS measurements. Figure 5c presentsthe EIS data for HPHI:PVA#1 with varying HPHI percentages.The left and bottom axes correspond to the imaginary and realcomponents of the impedance, respectively, and the measure-ment frequency ranged from 20 Hz to 2 MHz. The legend inthe figure indicates the percentages of HPHI added to theHPHI:PVA mixtures. Results for HPHI contents ranging from0% to 20% are very similar; the impedance is almost entirelydominated by the imaginary component, resulting in a Nyquistplot that does not form a circle. This behavior corresponds tothe capacitive response of the insulating PVA film, as HPHIdoes not form connected pathways for proton conductionwithin the composite. Next, increasing the HPHI content to30%−60% significantly reduces the imaginary component ofthe impedance, leaving almost exclusively the real component,and the HPHI exhibits a response similar to that of a resistor.Generally, when no external voltage is applied, most organicmaterials exhibit capacitor-like behavior because they lackcharge carriers capable of responding to the applied ACvoltage. However, when carriers capable of responding toexternal electric fields, such as mobile ions, are present within amaterial, it exhibits resistor-like behavior. Similar to manyother materials, HPHI:PVA exhibited a capacitor-like responsebefore light irradiation. However, upon light irradiation,protons are released from the heptazine skeleton and becomemobile, causing the material to respond in a resistor-likemanner. Figure 5c shows an equivalent circuit consisting of theion resistance (R) of HPHI and a constant phase element. Inthe equivalent circuit, the diameters of the semicircles in theNyquist plots represent the R values of the samples. Thediameter of the semicircle changes dramatically with the HPHIpercentage, indicating that R exhibits a strong dependence onthe HPHI percentage. Figure 5d plots R as a function of x.These results show that HPHI:PVA#1 exhibits protonconduction at x ≥ 30%, reaches its lowest resistance at x =40%, and then shows increasing resistance as x furtherincreases. The dependence of R on x is consistent with thatof the ΔI in Figure 5a. Therefore, when x is 30% or higher,these results support a percolation conduction mechanism,wherein HPHI particles within the composite bond together toform proton-conduction pathways. Furthermore, when xexceeds 50%, the scenario in which the number of protonsactually conducted through these pathways decreases as thefraction of HPHI particles exposed on the composite surfaceincreases is supported by the observation that R decreases as xincreases beyond 50%.4. CONCLUSIONIn this study, we successfully developed a PHI:PVA compositeusing PVA, an insulating polymer, as the matrix. First, wedemonstrated that HPHI:PVA exhibited a significantlyimproved retention time for the light-irradiation-inducedcolor-changed state of HPHI compared with that of HPHIalone or previously reported HPHI:polymer composites. Next,we examined the factors contributing to the long-lasting color-changed state of PHI:PVA by analyzing changes in theretention time as a function of the SD of PVA andtemperature. UV−vis measurements of PHI:PVA revealedthat increasing the SD of PVA prolongs the lifetime of thecolor-changed state, whereas increasing the temperatureshortens it. These results indicate that changes in the freevolume, one of the parameters governing the oxygenpermeability of PVA films, significantly affect the retentiontime of the color-changed state.ACS Omega http://pubs.acs.org/journal/acsodf Articlehttps://doi.org/10.1021/acsomega.6c00037ACS Omega 2026, 11, 16835−1684316841http://pubs.acs.org/journal/acsodf?ref=pdfhttps://doi.org/10.1021/acsomega.6c00037?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asThe measured electrical properties of HPHI:PVA led toimportant conclusions regarding the conduction pathways ofprotons within the composite. The irradiation-induced increasein the proton conductivity of HPHI:PVA was investigated as afunction of the HPHI percentage in the composite. Theseresults clearly demonstrate that protons released from HPHIare conducted within the HPHI phase rather than throughPVA. This behavior corresponds to a percolation conductionmechanism, and the EIS measurement results support thisconclusion.The PHI:PVA developed in this study contributes to thedevelopment of novel MPHI composites. Furthermore,insights into the mechanism of proton conduction within thecomposite are important for materials design. In particular, thelong-lived color-changed state realized in HPHI:PVA exhibitedPPC in proton conduction, suggesting potential for the futuredevelopment of novel devices that utilize its photoresponsiveionic conductivity.■ ASSOCIATED CONTENT*sı Supporting InformationThe Supporting Information is available free of charge athttps://pubs.acs.org/doi/10.1021/acsomega.6c00037.The data includes the spectrum of the LED used forirradiating the samples, photographs of KPHI:PVA#2and HPHI:PVA#2 before and after light irradiation, andphotographs of HPHI:PVA#1 and HPHI:PVA#2 at 60°C under different humidity conditions after lightirradiation (PDF)■ AUTHOR INFORMATIONCorresponding AuthorKaname Kanai − Department of Physics and Astronomy,Faculty of Science and Technology, Tokyo University ofScience, Noda, Chiba 278-8510, Japan; orcid.org/0000-0002-3952-5491; Email: kaname@rs.tus.ac.jpAuthorsTatsushige Izumi − Department of Physics and Astronomy,Faculty of Science and Technology, Tokyo University ofScience, Noda, Chiba 278-8510, JapanRyoma Hayakawa − Department of Physics and Astronomy,Faculty of Science and Technology, Tokyo University ofScience, Noda, Chiba 278-8510, Japan; Research Center forMaterials Nanoarchitectonics (MANA), National Institutefor Materials Science (NIMS), Tsukuba, Ibaraki 305-0044,Japan; orcid.org/0000-0002-1442-8230Momoka Isobe − Department of Physics and Astronomy,Faculty of Science and Technology, Tokyo University ofScience, Noda, Chiba 278-8510, JapanRyosuke Ohnuki − Department of Physics and Astronomy,Faculty of Science and Technology, Tokyo University ofScience, Noda, Chiba 278-8510, Japan; orcid.org/0000-0001-5024-3025Yutaka Wakayama − Research Center for MaterialsNanoarchitectonics (MANA), National Institute forMaterials Science (NIMS), Tsukuba, Ibaraki 305-0044,Japan; orcid.org/0000-0002-0801-8884Shinya Yoshioka − Department of Physics and Astronomy,Faculty of Science and Technology, Tokyo University ofScience, Noda, Chiba 278-8510, JapanComplete contact information is available at:https://pubs.acs.org/10.1021/acsomega.6c00037Author ContributionsThe manuscript was written with contributions from allauthors, and all authors approved the final version. TatsushigeIzumi: investigation, methodology, and writing; RyomaHayakawa: methodology; Momoka Isobe: investigation;Ryosuke Ohnuki: methodology; Yutaka Wakayama: method-ology; Shinya Yoshioka: methodology; Kaname Kanai:conceptualization, funding acquisition, project administration,writing, review, and editing.NotesThe authors declare no competing financial interest.■ ACKNOWLEDGMENTSThis study was supported by a Grant-in-Aid for ScientificResearch (Grant No. 22K05259) from the Ministry ofEducation, Culture, Sports, Science and Technology of Japan.■ REFERENCES(1) Seo, G.; Hayakawa, R.; Wakayama, Y.; Ohnuki, R.; Yoshioka, S.;Kanai, K. Mechanism of Charge Accumulation in PotassiumPoly(Heptazine Imide). Phys. Chem. Chem. Phys. 2024, 26 (30),20585−20597.(2) Lau, V. W.; Moudrakovski, I.; Botari, T.; Weinberger, S.; Mesch,M. B.; Duppel, V.; Senker, J.; Blum, V.; Lotsch, B. V. Rational Designof Carbon Nitride Photocatalysts by Identification of CyanamideDefects as Catalytically Relevant Sites. 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