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Yuichi Kitagawa, Toranosuke Tomikawa, Kota Aikawa, Shiori Miyazaki, Tomoko Akama, Masato Kobayashi, Mengfei Wang, Sunao Shoji, Koji Fushimi, Kiyoshi Miyata, [Yuichi Hirai](https://orcid.org/0000-0002-0252-1243), [Takayuki Nakanishi](https://orcid.org/0000-0003-3412-2842), Ken Onda, Tetsuya Taketsugu, Yasuchika Hasegawa

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[Charge transfer emission between π- and 4f-orbitals in a trivalent europium complex](https://mdr.nims.go.jp/datasets/2153ef15-ec8f-4f9f-abba-2f19a5048bb6)

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Charge transfer emission between Ï€- and 4f-orbitals in a trivalent europium complexcommunications chemistry Articlehttps://doi.org/10.1038/s42004-025-01420-6Charge transfer emission between π- and4f-orbitals in a trivalent europiumcomplexCheck for updatesYuichi Kitagawa 1,2,8 , Toranosuke Tomikawa3,8, Kota Aikawa3, Shiori Miyazaki4, Tomoko Akama2,MasatoKobayashi2,5,MengfeiWang2, SunaoShoji 6, Koji Fushimi 1, KiyoshiMiyata 4, YuichiHirai 7,Takayuki Nakanishi7, Ken Onda 4, Tetsuya Taketsugu 2,5 & Yasuchika Hasegawa 1,2Photoinduced metal-to-ligand (or ligand-to-metal) charge-transfer (CT) states in metal complexeshave been extensively studied toward the development of luminescent materials. However, previousstudies have mainly focused on CT transitions between d- and π-orbitals. Herein, we report thedemonstration of CT emission from 4f- to π-orbitals using a trivalent europium (Eu(III)) complex,supported by both experimental and theoretical analyses. The Eu(III) complex exhibits an eight-coordination structure, comprising three anionic nitrates and two neutral electron-donating ligandscontaining a carbazole unit. The diffuse reflectance spectrum of the complex displays an absorptionband at 440 nmand time-resolved emission analyses reveal a characteristic emission band at 550 nm.Comparative studies employing a trivalent gadolinium (Gd(III)) complex, alongside quantum chemicalanalyses, confirm that theobserved absorption andemissionbandsare associatedwithCT transitionsbetween π- and 4f-orbitals. The observation of CT emission based on the 4f-orbital offers novelinsights into the field of molecular luminescence science and technology.Metal complexes are organic–inorganic hybrid compounds that have beenwidely used in the fabrication of molecular photofunctional materials, suchas photosensitizers1,2, luminophores3,4, andphotocatalysts5,6. Controlling thephotoinduced charge transfer (CT) between metals and organic ligands iskey to their photofunctionality. Numerous transition metal complexesreportedly exhibit prominent photofunctionality due to ligand-to-metalcharge transfer (LMCT) or metal-to-ligand charge transfer (MLCT)7–9.Kinoshita successfully constructedawide-band (∼1000 nm) sensitized solarcell based on the MLCT excited states of ruthenium complexes7. Recently,Bauer suggested dual emission from 3d-orbital-based LMCT and MLCTstates using an Fe(III) complex8. Zhang successfully constructed a Zr(IV)complex with long-lived LMCT excited states, featuring strong emission,and verified its applicability as an effective photoredox catalyst9. Previousresearch has expanded the application scope of CT excitons in variousmetal complexes; however, the investigationofCT excitons has been limitedto the d-orbital. The properties of CT excitons are closely linked to thecharacteristics of the transition orbitals. Investigating unexplored CT exci-tons is essential for the development of new photo-functional metalcomplexes.Herein, we focused on the CT excited state between the 4f- and π-orbitals in trivalent lanthanide (Ln(III)) complexes (Fig. 1a). The first studyon the CT excited states of lanthanide complexeswas reported by Jørgensenin 196210. He observed a broad absorption band in the visible region for atrivalent europium (Eu(III)) complex with dithiocarbamate ligands, whichwas assigned to the CT transition from the π- to 4f-orbital. This Eu(III)complex exhibited the lowest redox potential ([Eu(III)/Eu(II) =−0.35 V vs.NHE])11 among trivalent lanthanides and formed a low-energy CT levelbased on the 4f-orbital. Following this groundbreaking observation, sub-sequent studies have been conducted on CT excited states between the 4f-and π-orbitals for exploring phenomena, such as absorption and 4f-4femission quenching, the photosensitization effect, and investigating per-turbation effects on the chiroptical properties of Eu(III) complexes12–26. Thedemonstration of CT emission based on a combined experimental andtheoretical study will open new scientific avenues for the development ofphotofunctional metal complexes.To demonstrate CT emission from 4f- to π-orbitals, we designed aneight-coordinate Eu(III) complex (Fig. 1b, [Eu(NO3)3(MCPO)2],Eu-MCPO). An electron-donating neutral ligand, (3,6-dimethoxy-9H-1Faculty of Engineering, Hokkaido University, Kita 13, Nishi 8, Kita-ku, Sapporo, Hokkaido, 060–8628, Japan. 2Institute for Chemical Reaction Design andDiscovery (WPI-ICReDD), Hokkaido University, Kita 21, Nishi 10, Kita-ku, Sapporo, Hokkaido, 001-0021, Japan. 3Graduate School of Chemical Sciences andEngineering, Hokkaido University, Kita 13, Nishi 8, Sapporo, Hokkaido, 060-8628, Japan. 4Department of Chemistry, Kyushu University, 744 Motooka, Nishi,Fukuoka, 829-0395, Japan. 5Faculty of Science, HokkaidoUniversity, Kita 10, Nishi 8, Kita-ku, Sapporo, Hokkaido, 060-0810, Japan. 6Faculty of Engineering, NaraWomen’s University, Kitauoya-Nishimachi, Nara, 630-8506, Japan. 7National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki, 305-0044, Japan.8These authors contributed equally: Yuichi Kitagawa, Toranosuke Tomikawa. e-mail: y-kitagawa@eng.hokudai.ac.jp; hasegaway@eng.hokudai.ac.jpCommunications Chemistry |            (2025) 8:24 11234567890():,;1234567890():,;http://crossmark.crossref.org/dialog/?doi=10.1038/s42004-025-01420-6&domain=pdfhttp://crossmark.crossref.org/dialog/?doi=10.1038/s42004-025-01420-6&domain=pdfhttp://crossmark.crossref.org/dialog/?doi=10.1038/s42004-025-01420-6&domain=pdfhttp://orcid.org/0000-0003-1487-2531http://orcid.org/0000-0003-1487-2531http://orcid.org/0000-0003-1487-2531http://orcid.org/0000-0003-1487-2531http://orcid.org/0000-0003-1487-2531http://orcid.org/0000-0002-0329-1136http://orcid.org/0000-0002-0329-1136http://orcid.org/0000-0002-0329-1136http://orcid.org/0000-0002-0329-1136http://orcid.org/0000-0002-0329-1136http://orcid.org/0000-0002-8945-0969http://orcid.org/0000-0002-8945-0969http://orcid.org/0000-0002-8945-0969http://orcid.org/0000-0002-8945-0969http://orcid.org/0000-0002-8945-0969http://orcid.org/0000-0001-6748-1337http://orcid.org/0000-0001-6748-1337http://orcid.org/0000-0001-6748-1337http://orcid.org/0000-0001-6748-1337http://orcid.org/0000-0001-6748-1337http://orcid.org/0000-0002-0252-1243http://orcid.org/0000-0002-0252-1243http://orcid.org/0000-0002-0252-1243http://orcid.org/0000-0002-0252-1243http://orcid.org/0000-0002-0252-1243http://orcid.org/0000-0003-1724-2009http://orcid.org/0000-0003-1724-2009http://orcid.org/0000-0003-1724-2009http://orcid.org/0000-0003-1724-2009http://orcid.org/0000-0003-1724-2009http://orcid.org/0000-0002-1337-6694http://orcid.org/0000-0002-1337-6694http://orcid.org/0000-0002-1337-6694http://orcid.org/0000-0002-1337-6694http://orcid.org/0000-0002-1337-6694http://orcid.org/0000-0002-6622-8011http://orcid.org/0000-0002-6622-8011http://orcid.org/0000-0002-6622-8011http://orcid.org/0000-0002-6622-8011http://orcid.org/0000-0002-6622-8011mailto:y-kitagawa@eng.hokudai.ac.jpmailto:hasegaway@eng.hokudai.ac.jpwww.nature.com/commschemcarbazol-9-yl)diphenyl phosphine oxide (MCPO), was designed to form aCT excited energy level based on the 4f-orbital lower than the localizedπ-π*excited energy level. The formation of a low-energy level is essential forsuppressing the relaxation process from the CT excited state to the ligand-localized excited state. An anionic nitrate ligand with low steric hindrancewas selected for the formation of stable coordination structures. The pho-tophysical properties of the eight-coordinate Gd(III) complex([Gd(NO3)3(MCPO)2], Gd-MCPO, Fig. 1b) and nine-coordinate Eu(III)complex ([Eu(NO3)3(MCPO)2(H2O)], Eu-MCPO-H2O, Fig. S1) were alsoevaluated for the comparison of the CT transition properties. The CTemission properties were analyzed by steady and time-resolved emissionspectroscopy. Quantum chemical calculations were performed to investi-gate the electronic structures of the CT excited states.Results and discussionStructural analysisSteric structures of Ln-MCPO and Ln-MCPO-H2O (Ln = Eu or Gd) wereanalyzed by X-ray crystallography and quantum chemical calculations. Eu-MCPO-H2O was found to possess a triclinic crystal system, with spacegroup P-1 (for crystallographic data, see Fig. S6 and Table S1).We observedthat the coordination site in the Eu(III) complex comprises three bidentatenitric ligands, two phosphine oxide ligands, and one water molecule. Thebidentate nitric acid and MCPO ligands have average Eu−O (NO3) andEu–O (MCPO) distances of 2.498 and 2.368 Å, respectively. We preparedEu-MCPO by the elimination of H2O from Eu-MCPO-H2O toward theformationof the stabilizedCTexcited states between the 4f- andπ-orbitals24.The PXRD pattern of dehydrated Eu-MCPO is different from that of Eu-MCPO-H2O (Fig. S7). The Eu-MCPO structure optimized using quantumchemical calculations (Supplementary Note 1, Fig. S8a) demonstrates ashorter distance between the Eu(III) center and coordination oxygen atoms[average Eu−O (NO3): 2.474 Å, average Eu–O (MCPO): 2.321Å] than theEu-MCPO-H2O structure (For the optimized structure ofEu-MCPO-H2O(Fig. S8b), the bidentate nitric acid and MCPO ligands have average Eu–O(NO3) and Eu–O (MCPO) distances of 2.482 and 2.386 Å, respectively.).The PXRDpattern ofGd-MCPOwas almost the same as that ofEu-MCPO(Fig. S7), indicating thepresenceof a similar crystal structure independentofthe type of lanthanide ions. Thus, a comparative photophysical study usingGd-MCPO is reasonable for investigating the CT excited state between the4f- and π-orbitals in Eu-MCPO.Photophysical propertiesThe diffuse reflectance spectra of Eu-MCPO andGd-MCPO are shown inFig. 2. The absorption bands at 340 nm (29,400 cm−1) for Eu-MCPO wereassigned to singlet π−π* transitions of theMCPO ligands, which agree withthe absorption spectrum of Gd-MCPO. We also observed additionalabsorption bands at approximately 440 nm (22,700 cm−1) for Eu-MCPO,which were not observed forGd-MCPO (Fig. 2, inset). Quantum chemicalcalculations were performed to determine the origin of the characteristicabsorption band observed in the Eu-MCPO spectrum. According to thecalculations, in theEu-MCPO spectrum, the absorption bands in the visibleregion originated in the CT transition from theMCPO ligand to the Eu(III)ion (Table S2, Fig. S9). The analysis of atomic contributions to molecularorbitals indicatesminimalmixingbetween theπ-orbital and the 4f-orbital ofEu(III) (Table S3). In addition, the occupied π- and unoccupied 4f-orbitalsexhibit slight mixing with the NO3 orbitals. These findings suggest that theNO3 orbitals may also contribute to the CT transition. The absorptioncoefficient at the CT absorption band is relatively low (ε < 10 cm−1 M−1,Fig. S10), which is correlatedwith the calculated small oscillator strength forthe CT transition (f = 0.0019, Table S2). This low transition probability islikely due to the minimal orbital overlap between the 4f- and π-orbitalsmediated by the phosphine oxide spacer.To gain a more detailed understanding of the CT states in Eu-MCPO,cyclic voltammetry measurements were conducted (Fig. S11). The firstoxidation potential was determined to be 0.97 V, closely matching that ofthe free MCPO ligand (Fig. S12, 1.00V). Quantum chemical calculationsalso indicate that the observed oxidation potential corresponds to theMCPO cation (Table S4). Distinct redox waves were not observed in thevoltammograms, and the reduction potential of the Eu(III) ion could not bedetermined; however, quantum chemical calculations suggest that the firstreduction potentialmainly corresponds to Eu(II) (Table S4). The π−π* andCT absorption bands of the eight-coordinate Eu-MCPO were found to beblue-shifted and red-shifted, respectively, compared to those of the nine-coordinateEu-MCPO-H2O (Tables S5 andS6, Figs. S13 and S14). This shiftindicates a change inMCPO ligand orientation and the formation of stableCT excited states upon the elimination of H2O. The stabilization of the CTexcited state by reducing the coordination number is consistent with pre-viously reported phenomena24, according to which this phenomenon isascribed to enhanced interaction between the Eu(III) ion and the organicligand due to the reduced coordination bonding distance. The results ofquantum chemical calculation with the solvent effect suggest that the stableCT excited states of Eu-MCPO were also formed in the solution state(Figs. S15 and S16,Tables S7 and S8) (Unfortunately, emission properties ofEu-MCPO in solution state cannot be estimated experimentally because ofpoor photo-stability (Supplementary Note 3–4, Figs. S25–S32).).Eu-MCPO shows sharp emission bands at approximately 580, 590,610, 635, and 700 nm, which correspond to the 5D0→7F0,5D0→7F1,Fig. 1 | Charge transfer state and chemical structures. a Schematic image of photo-induced charge transfer state between the 4f- and π-orbitals. bMolecular structuresof Eu-MCPO and Gd-MCPO.Fig. 2 | Diffuse reflectance spectra. Diffuse reflectance spectra of solid state Eu-MCPO (black solid line) and Gd-MCPO (black broken line). The weak absorptionpeak at approximately 465 nm corresponds to the 7F0→5D2 transition for Eu(III).https://doi.org/10.1038/s42004-025-01420-6 ArticleCommunications Chemistry |            (2025) 8:24 2www.nature.com/commschem5D0→7F2,5D0→7F3, and5D0→7F4 transitions, respectively (Fig. S17,100 K). The time-resolved emission spectra were also measured for Eu-MCPO and Gd-MCPO (Fig. 3). The Eu-MCPO spectrum showed fouremission bands at approximately 430 nm, 490 nm (shoulder), 550 nm, and610 nm on a nanosecond timescale after light excitation (Fig. 3a). Twoemission bands were also observed at 430 and 490 nm in the Gd-MCPOspectrum (Fig. 3b); these bands were assigned to the fluorescence andphosphorescence exhibited by the localized MCPO excited states,respectively (Fig. S18). These results confirm that the additional emissionbands at 550 nm and 610 nm for Eu-MCPO are associated with specificEu(III) orbitals (Fig. 3a). The emission band at 610 nm corresponds to the5D0→7F2 transition. Time-resolved emission spectra (Fig. S19) revealed anemission lifetime (τ) of 4.1 ns in the transition at 550 nm, characterized byfluorescence from the septet state (The characteristic emission band at550 nm is also observed for Eu-MCPO excited by 440 nm (SupplementaryNote 5, Fig. S33).). Thisfluorescence bandwas not observed forEu-MCPO-H2O (Fig. S20). To determine the origin of the fluorescence band at 550 nmin the spectrumofEu-MCPO, we investigated its structural relaxation in thelowest CT excited state between the 4f- and π-orbitals and associatedchanges in the electronic transition property using quantum chemical cal-culations. The calculations indicated that the CT excited state of Eu-MCPOwouldbe strongly stabilizedby structural relaxationafter the transition fromthe ground state (Fig. S21). Transition orbital analysis of the structure withlarge oscillator strength during structural relaxation confirmed that theemissive transition originated from the CT excited states between the 4f-and π-orbitals (Fig. 3c). To reveal the excited state dynamics related to theCT excited state between the 4f and π-orbitals, we used kinetic analysis toestimate the energy transfer rate constants (kEnT) from 4f-4f to the CTexcited state. The temperature dependence of kEnTwas expected to followanArrhenius-type equation:ln1τobs� 1τconst� �¼ ln kEnT� � ¼ lnA� EaR×T�1where A and Ea denote the frequency factor and activation energy,respectively. According to the temperature-dependent emission lifetime(Fig. S23), the activation energy is estimated to be 740 cm−1. The sum of the5D0 energy level and Ea is 17,990 cm−1, which is similar in energy to the CTexcited state. Arrhenius analyses suggested an energy migration channelbetween 5D0 and theCTexcited state (depicted in the energydiagramshownin Fig. 4). Based on the experimental findings and theoretical analyses, theobserved emission at around 550 nm is conclusively attributed to transitionfrom the 4f- to π-orbital in a Eu(III) complex.To obtain amore detailed understanding of excited-state dynamics forEu-MCPO, we calculated the energy transfer rate constants from theMCPO to the Eu(III) ion using LUMPAC software (the detail procedure issummarized in Supplementary Note 2)27. The calculated energy transferrates from T1 to5D1 and from T1 to5D0 are 6.5 × 108 and 5.9 × 108s−1,respectively. The T1→ S0 deactivation rate was experimentally estimated tobe 2.2 s−1 at 100 K usingGd-MCPO (Fig. S24), which is significantly lowerthan the calculated energy transfer rate. The observed phosphorescence at490 nm in the nanosecond range forEu-MCPO (Fig. 3a) is extremely weak,which reveals that the CT emission is also extremely weak. This relativelylow emission intensity is caused by small radiative and large non-radiativerate constants. The small radiative rate constant induced by the phosphineoxide spacerwas also confirmed in the CT excited state (Fig. S22). The largenon-radiative rate constant is related to the drastic excited energy levelchanges in the CT excited states. Based on these photophysical findings andtheoretical considerations, it is evident that designing an Eu(III) complexwith donor-type ligands that strongly overlap with the 4f-orbital and pro-vide a rigid structure is crucial for achieving strongCT emissions from4f- toπ-orbitals.ConclusionIn summary, theCT emission from4f- toπ-orbitals in the solid-state Eu(III)complexwasdemonstrated using time-resolved emission spectroscopy.Theobserved CT emission underwent a nanosecond-scale fast decay, indicatingthe occurrence of fluorescence from the septet states. Quantum chemicalcalculations revealed the contribution of CT transitions, energy variabilitywithin the CT excited states, and slight mixing between the π- and 4f-orbitals. The small radiative rate constant and large non-radiative rateconstant, as predicted by electronic absorption and quantum chemicalcalculations, resulted in weak emission intensity. Based on theseFig. 3 | Time-resolved emission spectra and transition orbital. Time-resolvedemission spectra (λex = 368 nm) of solid-state a Eu-MCPO and b Gd-MCPO at100 K (a, black line: 0–0.5 ns, red line: 0.5–1.0 ns, blue line: 2.0–3.0 ns, b black line:0–0.5 ns, red line: 0.5–1.0 ns, blue line: 5.0–44 ns). Normalized by intensity maxima.c Results of the transition orbital analysis from the CT excited to ground state at thestructure with large oscillator strength appeared during the structural relaxationusing quantum chemical calculations (Fig. S21). Gray spheres represent carbon; redspheres, oxygen; blue spheres, nitrogen; orange spheres, phosphorus; and greenspheres, europium.Fig. 4 | Energy diagram. Energy diagram and proposed emission mechanism forEu-MCPO.https://doi.org/10.1038/s42004-025-01420-6 ArticleCommunications Chemistry |            (2025) 8:24 3www.nature.com/commschemphotophysical and theoretical findings, it is evident that designing donor-type ligands that strongly overlap with the 4f-orbital and provide a rigidstructure is crucial for achieving strong CT emissions from 4f- to π-orbitals.The effective use of CT excitons based on the 4f-orbital provides newinsights into the design of next-generation nanomaterials and promotes thedevelopment of a research domain within the field of light science andtechnology.MethodsGeneral methodsNMR spectra were recorded on a JEOL ECS-400 spectrometer (1H:400MHz, 13C: 100MHz, 31P: 162MHz). Tetramethylsilane (δ = 0.00 ppmfor 1H NMR), CDCl3 (δ = 77.0 ppm for 13C NMR), and H3PO4 (δ = 0.00ppm for 31P NMR) were employed as internal standards. Electrosprayionization (ESI) mass spectrometry was performed using JEOL JMS-T100LP instrument. Elemental analyses were performed using MICRO COR-DER JM10. Fourier transform infrared (FT-IR) spectra were recorded on aJASCO FT/IR-4600 spectrometer. Thermogravimetry (TG) analysis wasperformed on a Seiko Instruments Inc. EXSTAR 6000 (TG/DTA6300)under a nitrogen atmosphere at a heating rate of 5 °C min–1. Cyclic vol-tammetry experiments were conducted using a BAS Model 2325 electro-chemical apparatus.Optical measurementsDiffuse reflection spectra were obtained using a JASCO V‐670 spectro-photometer with an ISN‐723 integrating sphere unit. Steady-state emissionspectroscopy for solid-state Eu(III) and Gd(III) complexes was conductedusing a Horiba Fluorolog-3 spectrofluorometer equipped with a cryostat(Thermal Block Company, SA-SB1905HA). The delayed emission spec-trum for the solid-state Gd(III) complex was recorded on an FP-6300spectrofluorometer with a cryostat (Thermal Block Company, SA-SB1905HA). Time-resolved emission spectroscopy for the evaluation ofsolid-state Eu(III) andGd(III) complexeswasperformedon anoptical setupconsisting of Ti : Sapphire solid-state laser with an optical parametricoscillator (OPO), a 3D Raman confocal microscope system, a high-resolution monochromator, and a streak camera. The particle was directlydispersed on a copper substrate. It was excited with an OPO (ChameleonVision-S, Coherent, Inc.) pumped by a pulsed Ti:Sapphire laser with anemission wavelength at 365 nm, which is able to generate the SHG laserbeam at 730 nm with 75 fs pulse width and 2MHz repetition rate. The 3DRaman confocal microscope system (Nanofinder 30, Tokyo Instruments,Inc.) functions as a real-time imaging spectrometer tomonitor the emissionfrom single-particle phosphors under the OPO laser excitation. Thereflected optical signals, collected through a fluorescence microscope(BX51M, Olympus Co., Ltd.) was directed to a high-resolution mono-chromator (SpectraProHRS-300, Princeton Instruments, Inc.) and a streakcamera (C14831-110, Hamamatsu Photonics Co., Ltd.). The emissionspectral range was monitored between 406 nm and 730 nm with a40 gmm−1 grating. The measurement timescale was set to 50 ns, and lowtemperature experiments (100 K) were performed in a microscopy cryostat(Janis ST-500, Lake Shore Cryotronics, Inc.). Temperature-dependent 4f-4femission lifetimes for Eu-MCPOwere measured using the third harmonic(λex = 355 nm) of a Q-switched Nd:YAG laser (Spectra Physics, INDI-50,fwhm= 5 ns, λ = 1064 nm) and a photomultiplier (Hamamatsu Photonics,R5108, response time ≤1.1 ns) with a cryostat (Thermal Block CompanySA-SB245T) and a temperature controller (Scientific InstrumentsModel 9700).MaterialsEuropium(III) nitrate hexahydrate, 3N5, gadolinium nitrate hexahydrate,3N5, n-butyllithium (n-BuLi) in n-hexane (1.6mol/L), and chloroform-d(99.8%) were purchased from Kanto Chemical Co., Inc. Dichloromethane(for organic synthesis), methanol, super dehydrated (for organic synthesis),tetrahydrofuran, super dehydrated, with a stabilizer (for organic synthesis),hydrogen peroxide (30%), and sodium sulfate, anhydrous were purchasedfrom Wako Pure Chemical Industries, Ltd. 3,6-Dimethoxy-9H-carbazole(>98.0%) and chlorodiphenylphosphine (>97.0%) were purchased fromTokyo Chemical Industry Co., Ltd.Preparation of (3,6-dimethoxy-9H-carbazol-9-yl)diphenylphosphineoxide (MCPO). MCPO was synthesized by a previously reportedmethod. 3,6-Dimethoxy-9H-carbazole (1.0 g, 4.4 mmol) was dissolved inanhydrous THF (30mL) under argon. n-BuLi (5.0 mL, 7.8 mmol) wasadded slowly dropwise to the solution at−78 °C and stirred for 3 h. ThenPPh2Cl (1.5 mL, 8.4 mmol) was added at−78 °C and stirred for 18 h. Theproduct was extracted using CH2Cl2, and the extract was washed withwater. The extract was dried over anhydrous Na2SO4 and concentratedby a rotary evaporator. The reaction mixture was dissolved in CH2Cl2(10 mL) and the solution was cooled at 0 °C. A 30% hydrogen peroxideaqueous solution (7.5 mL) was added to the solution, and the reactionmixture was stirred for 4 h. The product was extracted using CH2Cl2, andthe extract was washed with water. The extract was dried over anhydrousNa2SO4 and the solution was evaporated. The product was purified bysilica gel chromatography (3% CH2Cl2-MeOH). A white powder wasobtained by reprecipitation using CH2Cl2-hexane.Yield: 879.6 mg (47%). 1H NMR (400MHz, CDCl3, Fig. S2) δ/ppm =7.75–7.68 (m, 4H), 7.61 (td, J = 7.3, 1.4 Hz, 2H), 7.47 (td, J = 7.5, 3.4 Hz, 4H),7.42 (d, J = 2.4 Hz, 1H), 7.15 (d, J = 9.1 Hz, 2H), 6.80 (dd, J = 9.1, 2.4 Hz,2H), 3.88 (s, 3H). 13CNMR(100MHz,CDCl3, Fig. S3) δ/ppm=155.1, 136.7,133.1, 132.0, 130.3, 128.9, 127.2, 115.8, 114.8, 102.7, 55.8. 31P NMR(162MHz, CDCl3, Fig. S4) d/ppm = 25.9 (s). FT-IR �ν/cm−1 = 1196 (s,P = O). MS (ESI): m/z calcd for C26H23NO3P [M+H]+ = 428.14; found:428.14. Elemental analysis calc. (%) for [C26H22NO3P]: C 73.06, H 5.19, N3.28; found: C 72.70, H 5.13, N 3.22.Preparation of Eu(NO3)3(MCPO)(H2O) (Eu-MCPO-H2O). MeOH solu-tion (5mL) containing Eu(NO3)3･6H2O (625.0 mg, 1.401mmol) andMCPO (298.4 mg, 0.6981mmol) was stirred for 2 h. The solution wasevaporated, and recrystallization in CH3OH gave colorless crystals. Yield:270.5 mg (32%).FT-IR �ν/cm−1 = 3116-3681 (st, O–H), 1158 (st, P=O). Elementalanalysis calc. (%) for [C52H46EuN5O16P2]: C 51.58, H 3.83, N 5.78; found: C51.28, H 3.66, N 5.64.PreparationofEu(NO3)3(MCPO) (Eu-MCPO).Eu-MCPOwas preparedfrom Eu-MCPO-H2O by heat treatment (400 K, 1 h) under vacuumcondition. The elimination of the coordination water was confirmed bythermogravimetric analysis (Fig. S5).FT-IR �ν/cm−1 = 1149 (st, P=O). Elemental analysis calc. (%) for[C52H44EuN5O15P2]:C52.36,H3.72,N5.87; found:C51.90,H3.63,N5.74.Preparation of Gd(NO3)3(MCPO)(H2O) (Gd-MCPO-H2O). MeOH solu-tion (4mL) containing Gd(NO3)3･6H2O (430.0 mg, 0.9527mmol) andMCPO (199.8 mg, 0.4674mmol) was stirred for 2 h. The solution wasevaporated, and recrystallization in CH3OH gave colorless crystals. Yield:164.7mg (29%).FT-IR �ν/cm−1 = 3122–3696 (st, O–H), 1162 (st, P=O). Elementalanalysis calc. (%) for [C52H46GdN5O16P2]: C 51.36,H 3.81, N 5.76; found: C50.84, H 3.63, N 5.63.Preparation of Gd(NO3)3(MCPO) (Gd-MCPO). Gd-MCPO was pre-pared from Gd-MCPO-H2O according to the same procedure of pre-paration of Eu-MCPO.FT-IR �ν/cm−1 = 1152 (st, P=O). Elemental analysis calc. (%) for[C52H44GdN5O15P2]: C 52.13, H 3.70, N 5.85; found: C 51.65, H3.61, N 5.69.Crystal structure determinations. The X-ray crystal structures of Eu-MCPO-H2O is shown in Fig. S6. The crystallographic data are shown inTable S1. Single crystal X-ray diffraction data were obtained using ahttps://doi.org/10.1038/s42004-025-01420-6 ArticleCommunications Chemistry |            (2025) 8:24 4www.nature.com/commschemRigaku XtaLAB Synergy-DW equipped with a HyPix-6000HE detector(MoKα radiation, λ = 0.71073 Å). Non-hydrogen atoms were refinedanisotropically using the SHELX system. Hydrogen atoms were refinedusing the riding model. All calculations were performed using the crystalstructure crystallographic and Olex 2 software package. The CIF datawere confirmed by the check CIF/PLATON service. CCDC-2324703 (forEu-MCPO-H2O) contain the supplementary crystallographic data forthis paper. These data can be obtained free of charge fromTheCambridgeCrystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.Powder X-ray diffractometry (PXRD) was performed on a RigakuSmartLab using Cu Kα radiation (λ = 1.5418 Å).Data availabilityThe single-crystal data generated in this study have been deposited in TheCambridge Crystallographic Data Center under accession code CCDC-2324703 (Supplementary Data 1). The data can be obtained free of chargefrom The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/structures. The atomic coordinates for calculated structures (Figs. S8and S15) can be obtained from Supplementary Data 2. All of the other datasupporting the findings of this study are available from the correspondingauthor upon reasonable request.AbbreviationsCT charge transfer.LMCT ligand-to-metal charge transfer.MLCT metal-to-ligand charge transfer.Received: 2 May 2024; Accepted: 20 January 2025;References1. Kinoshita, T. Highly efficient wideband solar energy conversionemploying singlet-triplet transitions. Bull. Chem. Soc. Jpn. 95,341–352 (2022).2. Yamazaki, Y., Takeda, H. & Ishitani, O. Photocatalytic reduction ofCO2 using metal complexes. J. Photochem. Photobiol., C 25,106–137 (2015).3. Lo, K.W., Tong,G. S.M., Cheng,G., Low,K. H. &Che,C.M.DinuclearPtII complexes with strong blue phosphorescence for operationallystable organic light-emitting diodes with EQE up to 23% at1000 cdm–2. Angew. Chem., Int. Ed. 61, e202115515 (2022).4. Wegeberg, C., Häussinger, D. &Wenger, O. S. Pyrene-decoration of aChromium(0) tris(diisocyanide) enhances excited state delocalization:A strategy to improve the photoluminescence of 3d6 metalcomplexes. J. Am. Chem. Soc. 143, 15800–15811 (2021).5. Bürgin, T. H., Glaser, F. & Wenger, O. S. Shedding light on theoxidizing properties of spin-flip excited states in a Cr(III) polypyridinecomplex and their use in photoredox catalysis. J. Am. Chem. Soc.144, 14181–14194 (2022).6. Woodhouse, M. D. & McCusker, J. K. Mechanistic origin ofphotoredox catalysis involving Iron(II) polypyridyl chromophores. J.Am. Chem. Soc. 142, 16229–16233 (2020).7. Kinoshita, T. et al. Enhancement of near-IR photoelectric conversionin dye-sensitized solar cells using an osmium sensitizer with strongspin-forbidden transition. J. Phys. Chem. Lett. 3, 394–398 (2012).8. Steube, J. et al. Janus-type emission from a cyclometalated iron(III)complex. Nat. Chem. 15, 468–474 (2023).9. Zhang, Y. et al. Delayed fluorescence from a zirconium(IV)photosensitizer with ligand-to-metal charge-transfer excited states.Nat. Chem. 12, 345–352 (2020).10. Jørgensen, C. K. Electron transfer spectra of lanthanide complexes.Mol. Phys. 5, 271–277 (1962).11. Bünzli, J.-C. G. Benefiting from the unique properties of lanthanideions. Acc. Chem. Res. 39, 53–61 (2006).12. Kitagawa, Y., da Rosa, P. P. F. & Hasegawa, Y. Charge-transferexcited states of π- and 4f-orbitals for development of luminescent.Dalton Trans. 50, 14978–14984 (2021).13. Tsaryuk, V. I., Zhuravlev, K. P. & Gawryszewska, P. Processes ofluminescence quenching in europium aromatic carboxylates with theparticipation of LMCT states: a brief review. Coord. Chem. Rev. 489,215206 (2022).14. Napier, G. D. R., Neilson, R. J. D. & Shepherd, T. M. Charge-transferexcited state in tris(acetylacetonato) europium(III). Chem. Phys. Lett.31, 328–330 (1975).15. Berry, M. T., May, P. S. & Xu, H. Temperature dependence of the Eu3+5D0 lifetime in europium tris(2,2,6,6-tetramethyl-3,5-heptanedionato).J. Phys. Chem. 100, 9216–9222 (1996).16. Villata, L. S., Wolcan, E., Féliz, M. R. & Capparelli, A. L. Competitionbetween intraligand triplet excited state and LMCT on the thermalquenching in β-diketonate complexes of Europium(III). J. Phys.Chem.A 103, 5661–5666 (1999).17. Gawryszewska, P. et al. Experimental and theoretical study of thephotophysics and structures of europium cryptates incorporating3,3’-bi-isoquinoline-2,2’-dioxide. ChemPhysChem 5, 1577–1584(2004).18. Faustino, W. M., Malta, O. L. & de Sá, G. F. Intramolecular energytransfer through charge transfer state in lanthanide compounds: atheoretical approach. J. Chem. Phys. 122, 054109 (2005).19. Fu, L. M. et al. Role of ligand-to-metal charge transfer state innontriplet photosensitization of luminescent europium complex. J.Phys. Chem. A 114, 4494–4500 (2010).20. Räsänen, M. et al. Study on photophysical properties of Eu(III)complexes with aromatic β-diketones—role of charge transfer statesin the energy migration. J. Lumin. 146, 211–217 (2014).21. Miranda, Y. C. et al. The role of the ligand-to-metal charge-transferstate in the dipivaloylmethanate-lanthanide intramolecular energytransfer process. Eur. J. Inorg. Chem. 18, 3019–3027 (2015).22. Nonat, A. et al. The role of ligand tometal charge-transfer stateson theluminescence of Europium complexes with 18-memberedmacrocyclic ligands. Dalton Trans. 48, 4035–4045 (2019).23. Kitagawa, Y., Kumagai,M., daRosa,P. P. F., Fushimi, K. &Hasegawa,Y. Long-range LMCT coupling in EuIII coordination polymers for aneffective molecular luminescent thermometer. Chem. – Eur. J. 27,264–269 (2021).24. da Rosa, P. P. F. et al. Coordination geometrical effect on ligand-to-metal charge transfer-dependent energy transfer processes ofluminescent Eu(III) complexes. J. Phys. Chem. A 125, 209–217 (2021).25. Tsurui, M. et al. Asymmetric lumino-transformer: circularly polarizedluminescence of chiral Eu(III) coordination polymer with phase-transition behavior. J. Phys. Chem. B 126, 3799–3807 (2022).26. Faustino, W. M. et al. Photoluminescence of europium(III)dithiocarbamate complexes: electronic structure, charge transfer andenergy transfer. J. Phys. Chem. A 110, 2510–2516 (2006).27. Dutra, J. D. L., Bispo, T. D. & Freire, R. O. LUMPAC Lanthanideluminescence software: efficient and user friendly. J. Comput. Chem.35, 772–775 (2014).AcknowledgementsThis work was partially supported by a grant-in-aid from JSPS KAKENHI(Grant Numbers JP20H02748, JP21K18969, JP22H02152,JP22H04516, JP23K17925, JP23H01977, JP23H04631, andJP23K20039). It was also supported by the Adaptable and SeamlessTechnology Transfer Program through Target-driven R&D (A-STEP) ofthe Japan Science and Technology Agency (JST), Japan (Grant NumberJPMJTR23T5) and by the Institute for Chemical Reaction Design andDiscovery (ICReDD) established by the World Premier InternationalResearch Initiative (WPI) of MEXT, Japan.https://doi.org/10.1038/s42004-025-01420-6 ArticleCommunications Chemistry |            (2025) 8:24 5http://www.ccdc.cam.ac.uk/data_request/cifhttp://www.ccdc.cam.ac.uk/structureshttp://www.ccdc.cam.ac.uk/structureswww.nature.com/commschemAuthor contributionsY.K. designed the study and wrote the paper. K.A. and T.To. performedsynthesis of lanthanide complexes and their measurements. Y.K., T.To.,K.A., M.W., K.F., S.S., and Y.Ha. discussed the research. T.N., Y.Hi., S.M.,K.M., and K.O. supported the time-resolved emission measurements. T.A.,M.K., and T.Ta. performed the theoretical calculations of lanthanide com-plexes. All authors reviewed the manuscript.Competing interestsThe authors declare no competing interests.Additional informationSupplementary information The online version containssupplementary material available athttps://doi.org/10.1038/s42004-025-01420-6.Correspondence and requests for materials should be addressed toYuichi Kitagawa or Yasuchika Hasegawa.Peer review information Communications Chemistry thanks Jean-ClaudeBünzli and the other, anonymous, reviewers for their contribution to the peerreview of this work. Peer reviewer reports are available.Reprints and permissions information is available athttp://www.nature.com/reprintsPublisher’s note Springer Nature remains neutral with regard tojurisdictional claims in published maps and institutional affiliations.Open Access This article is licensed under a Creative CommonsAttribution-NonCommercial-NoDerivatives4.0 International License,whichpermits any non-commercial use, sharing, distribution and reproduction inany medium or format, as long as you give appropriate credit to the originalauthor(s) and the source, provide a link to the Creative Commons licence,and indicate if you modified the licensed material. You do not havepermission under this licence to share adapted material derived from thisarticle or parts of it. The images or other third partymaterial in this article areincluded in the article’s Creative Commons licence, unless indicatedotherwise in a credit line to the material. 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To viewa copyof this licence,visit http://creativecommons.org/licenses/by-nc-nd/4.0/.© The Author(s) 2025https://doi.org/10.1038/s42004-025-01420-6 ArticleCommunications Chemistry |            (2025) 8:24 6https://doi.org/10.1038/s42004-025-01420-6http://www.nature.com/reprintshttp://creativecommons.org/licenses/by-nc-nd/4.0/www.nature.com/commschem Charge transfer emission between π- and 4f-orbitals in a trivalent europium complex Results and discussion Structural analysis Photophysical properties Conclusion Methods General methods Optical measurements Materials Preparation of (3,6-dimethoxy-9H-carbazol-9-yl)diphenylphosphine oxide (MCPO) Preparation of Eu(NO3)3(MCPO)(H2O) (Eu-MCPO-H2O) Preparation of Eu(NO3)3(MCPO) (Eu-MCPO) Preparation of Gd(NO3)3(MCPO)(H2O) (Gd-MCPO-H2O) Preparation of Gd(NO3)3(MCPO) (Gd-MCPO) Crystal structure determinations Data availability Abbreviations References Acknowledgements Author contributions Competing interests Additional information