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Gary J. Richards, Kazushi Nakada, Keita Aoki, Tomoki Jitsukata, Kana Hashimoto, Toshiki Tajima, Ryusuke Mizoguchi, Ayumi Ishii, [Jonathan P. Hill](https://orcid.org/0000-0002-4229-5842), Akiko Hori

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[Redox‐Activated Near Infrared/Shortwave Infrared Emissive Chromophores: Synthesis of Triphenylamine‐Appended Pyrazinacenes](https://mdr.nims.go.jp/datasets/cd46c0a3-d643-4dd6-8be3-d4c44825170e)

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Redox‐Activated Near Infrared/Shortwave Infrared Emissive Chromophores: Synthesis of Triphenylamine‐Appended PyrazinacenesResearch ArticleHow to cite: Angew. Chem. Int. Ed. 2025, 64, e202504564doi.org/10.1002/anie.202504564Fluorescent ProbesRedox-Activated Near Infrared/Shortwave Infrared EmissiveChromophores: Synthesis of Triphenylamine-Appended PyrazinacenesGary J. Richards,* Kazushi Nakada, Keita Aoki, Tomoki Jitsukata, Kana Hashimoto,Toshiki Tajima, Ryusuke Mizoguchi, Ayumi Ishii, Jonathan P. Hill,* and Akiko Hori*Abstract: Organic dyes showing absorbance and fluorescence in the near infrared and short-wave infrared regions areattractive for a variety of applications. Redox-coupled reversible switching of absorbance or fluorescence implies enhancedfunctionality of such dyes, especially where large changes in photophysical properties across the redox process can berealized. Here, the synthesis of two new pyrazinacenes containing four and five fused pyrazine units, appended withelectron-donor triphenylamine groups, and redox-coupled switching of their photophysical properties is reported. In theirreduced state, the compounds show absorbance and fluorescence in the visible region. Reversible shifts in absorbance andfluorescence from the visible to the near infrared or even short-wave infrared regions are observed upon chemical andelectrochemical oxidations. Such large redox-coupled shifts in photophysical properties are unprecedented for a redoxprocess that affects only a single, six-membered ring in which both reduced and oxidized states consist of neutral, closed-shell species. The compounds show high fluorescence quantum yields in their reduced states, and oxidized species showfluorescence quantum yields that compare well with existing near infrared and short-wave infrared active fluorescent dyes.IntroductionNIR and short-wave infrared (SWIR) dyes have attractedsignificant interest due to potential applications in awide variety of nascent technologies. Strong absorptionand fluorescence in the NIR region are both attrac-tive properties, where NIR emission is important in bio-imaging,[1–4] NIR light emitting diodes,[5–7] or for microscopystaining.[8–10] Strong NIR absorption is required for dye-sensitized solar cells,[11–13] sensitizers, and contrast agents forphotoacoustic tomography,[14–16] laser-protecting glasses,[17,18][*] G. J. Richards, K. Nakada, K. Aoki, T. Jitsukata, A. HoriDepartment of Applied Chemistry, Graduate School of Engineeringand Science, Shibaura Institute of Technology, Fukasaku 307,Minuma-ku, Saitama 337-8570, JapanE-mail: richards@shibaura-it.ac.jpahori@shibaura-it.ac.jpK. Hashimoto, T. TajimaDepartment of Applied Chemistry, Graduate School of Engineeringand Science, Shibaura Institute of Technology, Toyosu 3-7-5, Koto-ku,Tokyo 135-8548, JapanR. Mizoguchi, A. IshiiDepartment of Chemistry and Biochemistry, School of AdvancedScience and Engineering, Waseda University, Okubo 3-4-1,Shinjuku-ku, Tokyo 169-8555, JapanJ. P. HillInternational Center for Materials Nanoarchitechtonics, NationalInstitute for Materials Science, Namiki 1-1, Tsukuba, Ibaraki305-0044, JapanE-mail: jonathan.hill@nims.go.jpAdditional supporting information can be found online in theSupporting Information sectionor for NIR absorbing paints and windows.[19,20] Redox-coupled reversible switching of absorbance or fluorescencealso implies enhanced functionality in optical switch-ing devices[21–23] or as probes[24,25] of biological redoxprocesses.[26–28] For many applications, large redox-coupledshifts in absorption or fluorescence are advantageous. Forexample, optical switching and photonic devices often relyon well-separated absorption or fluorescence bands betweenswitched states. Detection of redox imbalance in biologicalsamples using redox-active fluorescent probes is facilitatedwhen the oxidized and reduced species have very differ-ent absorption and fluorescence characteristics. Fluorescentredox sensors can operate in the preferred ratiometric modewhen the fluorescence spectra of reduced and oxidized speciesare well separated. Various redox-active dyes have beendeveloped[27–29] with most examples having activity in theNIR/SWIR regions being cyanine dyes, which can suffer frompoor photostability or low fluorescence quantum yields.[30,31]Thus, the development of new types of redox-active dyes withactivity in the NIR region remains an important goal.Pyrazinacenes[32–34] (Scheme 1a,b) consist of rectilinearlyfused pyrazine units and are highly nitrogenated counterpartsof C─H acenes (fused benzenes). In contrast to therelatively electron-rich acenes, which are increasingly proneto oxidation with increasing length, higher analogs ofpyrazinacenes are increasingly electron-deficient so they tendto exist containing a reduced dihydropyrazine unit.[35] Despitethis, pyrazinacenes with 4 or 5 pyrazine units can undergoreversible redox processes between a highly electron-deficientpyrazine-only oxidized state and a reduced species with asingle dihydropyrazine unit.[36] However, before this work,redox-active pyrazinacenes only showed small redox-coupledshifts (typically < 100 nm) in absorption and fluorescenceAngew. Chem. Int. Ed. 2025, 64, e202504564 (1 of 8) © 2025 Wiley-VCH GmbHmailto:richards@shibaura-it.ac.jpmailto:ahori@shibaura-it.ac.jpmailto:jonathan.hill@nims.go.jphttp://crossmark.crossref.org/dialog/?doi=10.1002%2Fanie.202504564&domain=pdf&date_stamp=2025-03-20Research ArticleScheme 1. Structures of unsubstituted a) reduced pyrazinacenes and b)pyrazinacenes in their fully oxidized state with no dihydropyrazine units.These core structures of 1 and 2 were used for DFT studies anddemonstrated variation in electron-accepting capacity during the redoxprocess. c) Structures of reduced species 1-red and 2-red and redoxinterconversion between the reduced and fully oxidized species 1-ox and2-ox (i) PbO2, CHCl3; (ii) sodium dithionite (aq), CHCl3.bands well within the visible region. Higher pyrazinaceneshaving 6 or 7 pyrazine units, while showing fluorescence bandsapproaching the NIR region, always contain a dihydropy-razine unit and are stable against oxidation.[32,35] To createredox-active chromophores with absorption and fluorescencebands well into the NIR region, another strategy is needed.Intramolecular charge transfer (ICT) has been widelyinvestigated as a means to reduce the optical energy gap inorganic conjugated materials. The typical strategy involveslinking electron donor (D) and acceptor (A) groups through aconjugated π -bridge which results in a lowering of the opticalenergy gap through ICT interactions. It is known that pyrazi-nacenes consisting of three or more fused pyrazine rings all intheir fully oxidized state are extremely electron-deficient[32,34]and therefore should act as strong acceptors when combinedwith appropriate donor groups. Here, we describe a new seriesof compounds based on the redox-active octaazatetracene(1) and decaazapentacene (2) chromophores appended withelectron-donating triphenylamine (TPA) groups (Scheme 1c)and investigate their redox-coupled photophysical properties.Results and DiscussionSynthesis and CharacterizationSynthesis of the D-A-D type compounds 1 and 2 wasachieved through the condensation of a TPA-appendedpyrazinopyrazine-2,3-dicarbonitrile with a TPA-appendeddiaminopyrazine (S2) for 1-red as shown in Scheme 2 ordiamino-pyrazinopyrazine (S4) for 2-red as shown in SchemeS1. Compounds were isolated in the expected reduced formwith a single dihydropyrazine unit occupying the centralpyrazine unit of the pyrazinacene backbone. Typically, a reac-tion mixture of 5,6-bis(4-diphenylamino)phenylpyrazine-2,3-diamine and 6,7-bis(4-(diphenylamino)phenyl)pyrazino[2,3-b]pyrazine-2,3-dicarbonitrile in DMF in the presence ofsodium bicarbonate was stirred for 2 h at 120 °C to yieldthe reduced compound 1-red, which was purified by columnchromatography (Yield: 48 %). 2-red is obtained using asimilar procedure with minor modifications to increase yield.Although oxidation of the compounds to 1-ox and 2-ox canbe performed using different oxidants (e.g., 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ), sodium hypochlorite), forconvenience, lead (IV) oxide (PbO2) suspended in CHCl3 wasselected as the oxidizing system because the oxidant can beseparated by simple filtration. PbO2 was added to a solutionof 1-red in CHCl3. After completion, PbO2 was filtered andthe solvent was removed by evaporation. Purification bycolumn chromatography yielded a dark green powder of 1-ox in quantitative yield. The molecular structures of 1 and2 were fully characterized using NMR, FTIR, and HRMSstudies (Figures S1–S32); the dihydropyrazine protons wereobserved at δ = 7.77 and 8.70 for 1-red and 2-red, respectively,and ionization peaks were found at 1211.4974 m/z for 1-red(calcd. as [C82H59N12]+ [M + H]+: 1211.4986 m/z), 1263.5022m/z for 2-red (calcd. as [C84H59N14]+ [M + H]+ 1263.5047m/z), respectively. Electron-donating TPA substituents areexpected to stabilize the oxidized states by transfer of electrondensity to the highly electron-deficient pyrazinacene (i.e., 1-ox/2-ox). The bulky TPA groups impart good solubility tothese compounds such that they are readily soluble in low tomoderately polar solvents such as toluene, tetrahydrofuran,and CHCl3. Also, the attachment of TPA groups at theterminal carbon atoms of the pyrazinacene rather than as N-substituents of the dihydropyrazine unit allows the study ofprotic processes such as deprotonation, and importantly here,redox properties.Green prismatic crystals of 1-ox·4CHCl3 suitable for X-raycrystallographic studies were obtained by slow evaporationof a CHCl3 solution (Figure 1) revealing several interestingproperties of the compound in the solid state. No electrondensities, suggestive of hydrogen atoms characteristic of thereduced form, were observed around the pyrazine nitrogenatoms.The TPA groups do not appear to be affected by theoxidation process as they possess the classic neutral TPApropeller shape. Oxidized TPAs are typically planarized byoxidation and are usually only stable when appropriatelysubstituted.[37] Thus, the oxidation process only appears toAngew. Chem. Int. Ed. 2025, 64, e202504564 (2 of 8) © 2025 Wiley-VCH GmbH 15213773, 2025, 19, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/anie.202504564 by National Institute For, Wiley Online Library on [28/10/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons LicenseResearch ArticleScheme 2. Synthesis of 1-red. (i) p-Toluenesulfonic acid, toluene, reflux; (ii) sodium dithionite, AcOH, 100 °C; (iii) sodium bicarbonate, DMF, 120 °C.Figure 1. a) Appearances of solid samples of 1 and 2 with and withoutdispersion in CHCl3. Crystal structure of 1-ox·4CHCl3 (CCDC2381770): b)ORTEP view of 1-ox with 50 % thermal ellipsoids, c) intermolecularassociation showing π ···π -hole interactions between the electron donorTPA units and electron acceptor pyrazinacene units, and d) packingstructure viewed along the a-axis; color schemes: 1-ox, pink; CHCl3,green.affect the pyrazinacene cores. The dihedral angle betweenrings D and E is 8.66°, with the torsion angle subtendedbetween planes of the core (ring-D) and the two TPAsubstituents at 15.9(5)°, indicating that the bulky TPAsubstituents are responsible for a slightly twisted form of thepyrazinacene unit. Pyrazinacene and TPA sites are stackedin the structure indicating a π -hole···π interaction with theelectron-deficient pyrazinacene moiety as an acceptor and theelectron-rich TPA as a donor unit.Redox-Coupled Photophysical PropertiesEssential absorption properties of compounds 1 and 2 aresummarized in Figure 2, Figures S33, S34. Compound 1-redhas absorption maxima (λmax) in the range of 500–520 nm inFigure 2. a) UV–vis–NIR absorption spectra of both states of 1 and 2 inCCl4 solution with extinction coefficients, εmax /M−1 cm−1. b) Reversiblecolor changes observed under constant current electrolysis, c) repeatedCV (scan rate = 50 mV s−1), and d) UV–vis–NIR absorption spectraunder constant current electrolysis of 1-ox/1-red in CH2Cl2. Constantcurrent electrolysis was performed on a 2 × 10−4 m solution of 1-ox withaliquots diluted to 1 × 10−5 M for UV–vis–NIR absorptionmeasurements.most solvents (Figure 2a and Table 1). In CCl4, its spectrumshows an acene-like vibronic structure with broadened peaks.Compound 2-red behaves similarly but with λmax under-going a bathochromic shift to 566 nm in CCl4 due to theadditional fused pyrazine unit. For 1-ox, λmax shifts from527 nm (εmax = 54000 M−1 cm−1, red solid line in Figure 2a) toAngew. Chem. Int. Ed. 2025, 64, e202504564 (3 of 8) © 2025 Wiley-VCH GmbH 15213773, 2025, 19, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/anie.202504564 by National Institute For, Wiley Online Library on [28/10/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons LicenseResearch ArticleTable 1: Photophysical properties of 1 and 2. Maximum absorption wavelengths (λabs) in cyclohexane (C6H12), CCl4, toluene, CH2Cl2, CHCl3, andCH3CN along with maximum emission wavelengths (λem) in the exemplary solvent CCl4 are shown. Excitation wavelengths (λex) are given inparentheses.Compoundλabs/nmC6H12λabs/nmCCl4λabs/nmtolueneλabs/nmCH2Cl2λabs/nmCHCl3λabs/nmCH3CNλem/nm (λex/nm)CCl4�PLCCl41-red 507 497, 527 495, 523 503, 523 512, 533 493, 515 560 (496) 0.581-ox 795 793 731, 793 829 879 802 847 (700)a) 0.162-red 538, 572 536, 566 525, 560 715 586 548 599 (535) 0.432-ox 907 836, 937 840b), 927 1042 1051 965 1012 (700)a) 0.014a) Compounds were excited at the maximum absorption wavelength except for oxidized species 1-ox and 2-ox which were excited at 700 nm due to aninstrumental limitation. b) Shoulder absorption.793 nm (εmax = 80400 M−1 cm−1, green solid line in Figure 2a)– a 266 nm bathochromic shift. For 2-ox a bathochromic shiftof 371 nm is observed in CCl4 with λmax shifting from 566 nm(εmax = 80200 M−1 cm−1, violet dashed line in Figure 2a)to 937 nm (εmax = 130000 M−1 cm−1, black dashed line inFigure 2a).We conducted electrochemical analyses to further investi-gate the reversibility of the redox processes of the compounds(Figure 2b–d). A green solution of 1-ox (2 × 10−4 mCH2Cl2/CH3OH and electrolyte Bu4NBF4) was used toperform cyclic voltammetry (CV) and constant currentelectrolysis. The first and second reversible reduction waveswere found respectively at −0.28 V (�E = 70 mV) and−0.66 V (�E = 60 mV) with good reversibility even afterseveral cycles, as shown in Figure 2c. The first reductionwave consists of two overlapping peaks suggesting, togetherwith results of previous reports and DFT calculations,that the central pyrazine (ring-E) is reduced. Attempts toisolate reduction products using constant potential elec-trolysis proved unsuccessful, possibly due to low currentissues. However, we note that the electrochemical reduc-tion of structurally similar azaacenes in the presence of aproton source, including quinoxalino[2,3-b]quinoxaline andpyrazino[2,3-b]quinoxaline, typically results in successive 2e−,2H+ processes corresponding to reduction of pyrazine unitsdirectly to 1,4-dihydropyrazine units.[38,39] This is consistentwith our findings using constant current electrolysis, duringwhich the solution color and spectra (with a clear isosbesticpoint–Figure 2d) indicate conversion of 1-ox directly to itsreduced state 1-red without the involvement of any stableintermediate species.The reversibility of the chemical redox process wasconfirmed through successive chemical redox transformationsbetween oxidized and reduced species. PbO2 was added toa solution of as-synthesized 1-red in tetrachloroethane-d2 or2-red in CDCl3 and after stirring for 3 min, the mixturewas filtered before 1H NMR analysis of the filtrate, whichconfirmed conversion to the oxidized species as evidenced bythe distinctive large downfield shift of the aromatic hydrogenresonances immediately adjacent to the pyrazinacene cores.Reduction was achieved by adding an aqueous solution ofsodium dithionite and the biphasic mixture was stirred for3 min before solvent partitioning and washing the organiclayer with brine before 1H NMR analysis which confirmedconversion back to the reduced species. The same oxidationand reduction processes were repeated several times, and theFigure 3. 1H NMR spectra, starting with as-synthesized 1-red showingsuccessive chemical redox transformations between 1-red and 1-ox.Remarkably, all redox steps were performed without the necessity offurther purification beyond filtration of the PbO2 oxidant and solventpartitioning of the aqueous dithionite reducing agent demonstratingsmooth, quantitative chemical conversions between the two species.results are shown in Figure 3 and Figure S35 demonstratingthe repeatability of the chemical redox process.The reduced species exhibit fluorescence in solution withmoderate to high photoluminescence quantum yields (PLQY,�PL) as shown in Figure 4 and Table 1. Compound 1-red inCCl4 exhibits fluorescence at λem = 560 nm with a PLQY�PL = 0.58. Compound 2-red shows similar fluorescenceproperties but with a �PL = 0.43 and a bathochromicallyshifted λem = 599 nm. 1-red and 2-red exhibit vivid yellowand red luminescence, respectively, as shown in Figure 4a. Inlow-polarity solvents, such as CCl4, cyclohexane, or toluene,NIR fluorescence is also observed for both oxidized species.For 1-ox, λem = 847 nm in CCl4 while for compound 2-ox,fluorescence extends to the SWIR region (1000–1700 nm)with λem = 1012 nm. Note that the excitation wavelength λexin both cases was 700 nm due to an instrumental limitation,although values of PLQY obtained are sufficiently high toconsider applications of the materials.As expected for NIR emitters, fluorescence quantumyields of the oxidized species are significantly diminishedcompared to their reduced counterparts due to increasedAngew. Chem. Int. Ed. 2025, 64, e202504564 (4 of 8) © 2025 Wiley-VCH GmbH 15213773, 2025, 19, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/anie.202504564 by National Institute For, Wiley Online Library on [28/10/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons LicenseResearch ArticleFigure 4. a) Appearance of CCl4 solutions of 1 and 2 under visible lightand UV illumination (365 nm) and b) emission spectra of 1 and 2 inCCl4 showing the extreme shift in fluorescence wavelengths betweenreduced and oxidized species.rates of non-radiative processes, which for NIR and SWIRemitters are typically dominated by non-adiabatic couplingbetween excited and ground electronic states.[40] Nonetheless,the fluorescence quantum yields compare well with someof the highest reported values for organic NIR and SWIRfluorophores[41] with compound 1-ox showing a PLQY of16.4 % with a brightness of 13200 M−1 cm−1 (SWIR brightness= 1690 M−1 cm−1) and 2-ox showing a PLQY of 1.4 %with a brightness of 1810 M−1 cm−1 (SWIR brightness =1370 M−1 cm−1) in CCl4.Computational Studies and DiscussionElectronic absorption spectra of the compounds were pre-dicted using Time-Dependant Density Functional Theory(TDDFT B3LYP def2-TZVP) including a conductor-likepolarizable continuum solvent model (CPCM) for CCl4 asimplemented in the Orca 6.0.0 software package[42] and areshown in Figure 5.The large redox-coupled shifts in absorption peaks werepredicted remarkably well, albeit with peaks somewhatbathochromically shifted compared to experimental values.For 1-red the longest wavelength absorption peak wascalculated at λabs = 608 nm (c.f. measured λabs = 527 nm).For 1-ox the longest wavelength absorption was calculatedat λabs = 938 nm (c.f. measured λabs = 793 nm). For 2-red the longest wavelength absorption was calculated atλabs = 648 nm (c.f. measured λabs = 566 nm). For 2-oxthe longest wavelength absorption was calculated at λabs =1078 nm (c.f. measured λabs = 937 nm). All calculated longestwavelength absorptions were due to simple highest occupiedto lowest unoccupied molecular orbital (HOMO-LUMO)singlet (S0→S1) transitions.Oxidation of the pyrazinacene cores results in a significantdeepening in the energy of the LUMO of the core units.Figure 5. Calculated UV–vis–NIR absorption spectra for compounds 1and 2 using the TDDFT B3LYP def2-TZVP method with CPCM (CCl4)solvent model. Artificial spectra are created by broadening calculatedtransitions by 500 cm−1 at full-width-half-maximum (FWHM) andscaling vertically according to the oscillator strength. The maincalculated transitions are shown by vertical solid lines.DFT modeling (B3LYP 6–311 + G**) shows a LUMO energylevel, ELUMO = −2.68 eV for 1-red-core which decreasesto −4.56 eV upon oxidation to 1-ox-core (Figures S40,S41).Similarly, 2-red-core has a calculated ELUMO = −3.24 eVwhich decreases to −4.99 eV in the fully oxidized species 2-ox-core (Figures S42,S43). This latter value is comparable withthat of the strongly electron-accepting 2,5-difluoro-7,7,8,8-tetracyanoquinodimethane (ELUMO = −4.6 eV).[43] Thus, thedriving force for the large shift in photophysical propertiesobserved after oxidation is due to a change from D-A-D typestructures having weak-to-moderate acceptor units in 1-redand 2-red to D-A-D type structures containing exceptionallystrong electron acceptor units in 1-ox and 2-ox. Nitrogenatoms can act as either electron donors or acceptors whenincorporated into a π -system depending on the orientation ofthe nitrogen lone pair electrons. When the nitrogen lone pairelectrons are oriented orthogonally to the π -system such asthe sp2 hybridized nitrogen atoms of pyrazine, the lone pairelectrons cannot overlap with and donate into the π -systemefficiently and the strongly electronegative nitrogen atomswithdraw electron density from the π -system inductively.Conversely, the lone pair electrons of sp3 hybridized nitrogenatoms pendant to a π -system, such as those in TPA, canoverlap with and donate to the π -system and the nitrogenatoms act as electron donors. The nitrogen atoms of thereduced 1,4-dihydropyrazine ring can be regarded as a specialcase of the latter in which the lone pair electrons are fullydonated into the π -system to give a planarized, electron-rich anti-aromatic system in which the anti-aromaticity isstabilized through delocalization across the π -system.[44,45]The molecules described in this manuscript take advantageof all three of these effects to achieve extreme redox-coupledshifts in absorption and fluorescence. To gain further insightinto the changes in donor and acceptor strength of the TPAgroups and pyrazinacene cores across the redox process, theelectrostatic potential (ESP) energy surfaces were simulated(Figures S44–S47). Strongly negative ESP values were foundat the centers of TPA phenyl groups (−42.5 to −65.4 kJ mol−1)indicating the electron-rich character of the donor groups.Angew. Chem. Int. Ed. 2025, 64, e202504564 (5 of 8) © 2025 Wiley-VCH GmbH 15213773, 2025, 19, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/anie.202504564 by National Institute For, Wiley Online Library on [28/10/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons LicenseResearch ArticleStrongly positive ESP values (+24.9 to +121.6 kJ mol−1) werefound at the centers of the pyrazinacene rings indicating theelectron-deficient character of the acceptor units. An increasein the ESP values at the centers of the TPA phenyl groupsoccurs upon oxidation of the compounds suggesting there issome transfer of electron density from the TPA groups tothe more electron-deficient pyrazinacene cores in the oxidizedspecies. Changes to the ESP values on the pyrazinacene coresbetween oxidized and reduced species are less consistent,possibly due to the structural changes involved as well assignificant contributions from the nitrogen lone pair electrons.Inspection of the frontier orbital locations and changes intheir energy levels during the redox process provides moredetailed insight into the redox-coupled changes in photophys-ical properties observed in the present compounds. In theseD-A-D type structures, the HOMO lies predominantly on theTPA donor units (Figures S36–S39) and is largely unperturbedby the redox process (1-red EHOMO = −5.2 eV, 1-ox EHOMO =−5.3 eV; 2-red EHOMO = −5.2 eV, 2-ox EHOMO = −5.4 eV).The LUMOs in all species are located predominantly on thepyrazinacene cores and are strongly perturbed by the redoxprocess (1-red ELUMO = −2.8 eV, 1-ox ELUMO = −3.6 eV; 2-redELUMO = −2.8 eV, 2-ox ELUMO = −3.9 eV). The stabilizationof the LUMO energy levels together with the only minorvariations in the HOMO energy levels upon oxidation leadsto a significant narrowing of the energy gap and so a largebathochromic shift in absorption/fluorescence is realized. Thiscontrasts significantly with our previously reported redox-active pyrazinacenes[34,36] whose HOMOs and LUMOs areboth located on the pyrazinacene cores and are thus bothperturbed to a similar extent by the redox process so that largeshifts in the optical energy gaps are not observed.For many organic NIR emitters, high-frequency C─Hvibrational modes make a significant contribution to thenon-radiative relaxation rate due to a large overlap integralbetween ground and excited states.[46] A strategy to improveNIR fluorescence quantum yields has involved the substitu-tion of hydrogen atoms with heavier deuterium or fluorineatoms to reduce the frequency of vibrational modes.[46,47]This strategy, however, can be synthetically challenging andfor more complex molecules, not economically viable dueto the high cost of deuterated synthesis intermediates. Ourmolecular design strategy incorporates the strong acceptorsoctaazatetracene and decaazapentacene as major componentsof their π -conjugated systems and eliminates adjacent C─Hbonds, which is expected to reduce the non-radiative decayrates. As the molecules reported here are the first examplesusing this strategy, we anticipate further improvements inNIR fluorescence quantum yields could be realized, especiallythrough tuning of the donor structures.ConclusionIn conclusion, we present a strategy for the synthesis oforganic chromophores based on the octaazatetracene anddecaazapentacene pyrazinacene cores appended with TPAelectron donor groups. To achieve efficient fluorescence inthe NIR and SWIR regions, these compounds incorporatefully oxidized pyrazinacenes as a significant component ofthe chromophore. This component contains no C─H orN─H bonds whose high-frequency vibrational modes wouldstrongly contribute to non-radiative decay above 850 nm,as is found in most organic compounds.[46] Replacement ofC─H units with nitrogen atoms demonstrates an alternativeto existing strategies relying on deuterium or fluorine sub-stitution to achieve higher NIR fluorescence quantum yields.The molecules described herein have a D-A-D quadrupolartype motif with the pyrazinacene forming a strong acceptorand a large part of the π -conjugated system. However, thereis no reason why fully oxidized pyrazinacenes cannot beused in other structures commonly used in long wavelengthemitters, such as D-A dipolar, anionic, cationic, or zwitterionicmotifs. The chromophores described herein have fluorescencequantum yields that compare well with some of the best-known chromophores at these wavelengths and we anticipatethat by tuning the donor structures, it will be possible tofurther increase fluorescence quantum yields. Additionally,these compounds can be easily switched using a simple,reversible redox process between highly fluorescent reducedstates with absorption and emission in the visible region (λem= 560 nm for 1-red and λem = 599 nm for 2-red in CCl4) tooxidized states having absorption and emission bands in theNIR (λem = 847 nm for 1-ox) or even SWIR (λem = 1012 nmfor 2-ox) regions. Such large shifts in photophysical propertiesare quite remarkable and unprecedented for a redox processthat affects only a single, six-membered ring in which bothreduced and oxidized states consist of neutral, closed-shellspecies. Redox switching can be achieved either electrochem-ically or by using different oxidizing and reducing agentsincluding the biologically relevant oxidant hypochlorite andHantzsch ester, a mimic of the biological reductant 1,4-dihydronicotinamide adenine dinucleotide (NADH). Whilethese compounds currently lack water solubility for biologicaluse, their redox properties suggest that water-compatibleanalogs could lead to a novel category of fluorescent probesfor tracking redox processes in biological and cellular envi-ronments. The molecular design strategy showcased hereshould not only apply to a wide variety of electron-donorappended pyrazinacenes, but also to any ICT conjugatedorganic systems in which the acceptor units can be modifiedto incorporate a redox-active pyrazine/dihydropyrazine unit.Based on the results of this work, such materials can beexpected to show similarly large redox-coupled changes intheir photophysical properties that should be beneficial to awide variety of applications.Supporting InformationDetails of synthetic procedures, crystal structural information,additional electronic absorption spectra, and DFTcalculations are provided in the supporting information.Deposition number 2381770 contains the supplementarycrystallographic data for this paper. These data are providedfree of charge by the joint Cambridge CrystallographicData Center and Fachinformationszentrum KarlsruheAngew. Chem. Int. Ed. 2025, 64, e202504564 (6 of 8) © 2025 Wiley-VCH GmbH 15213773, 2025, 19, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/anie.202504564 by National Institute For, Wiley Online Library on [28/10/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons LicenseResearch ArticleAccess Structures service. The authors have cited additionalreferences within the Supporting Information.[48–51]AcknowledgementsThis work was supported by Grants-in-Aid for ScientificResearch C, no. 21K05044 and 24K08401 (G.J.R.) and partsof this work was supported by Grant-in-Aids for ScientificResearch B, no. 21H01955 and 23K21122 (A.H.) of JSPS KA-KENHI and by World Premier International Research CenterInitiative (WPI Initiative), MEXT, Japan (J.P.H.).Conflict of InterestsThe authors declare no conflict of interest.Data Availability StatementThe data that support the findings of this study are availablein the supplementary material of this article.Keywords: Fluorescent probes • NIR absorption • NIRfluorescence • Pyrazinacenes • Redox-active dyes[1] Z. Guo, S. Park, J. Yoon, I. Shin, Chem. Soc. Rev. 2014, 43, 16–29.[2] C. Liu, S. Zhang, J. Li, J. Wei, K. Müllen, M. Yin, Angew. Chem.,Int. Ed. 2019, 58, 1638–1642.[3] Y. Chen, S. Wang, F. Zhang, Nat. Rev. Bioeng. 2023, 1, 60–78.[4] J. Mu, M. Xiao, Y. Shi, X. 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Tong, Dow Chemical Co., US3987044 A, 1976.Manuscript received: February 25, 2025Revised manuscript received: March 01, 2025Accepted manuscript online: March 03, 2025Version of record online: March 20, 2025Angew. Chem. Int. Ed. 2025, 64, e202504564 (8 of 8) © 2025 Wiley-VCH GmbH 15213773, 2025, 19, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/anie.202504564 by National Institute For, Wiley Online Library on [28/10/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License Redox-Activated Near Infrared/Shortwave Infrared Emissive Chromophores: Synthesis of Triphenylamine-Appended Pyrazinacenes  Introduction  Results and Discussion  Synthesis and Characterization  Redox-Coupled Photophysical Properties  Computational Studies and Discussion  Conclusion  Supporting Information  Acknowledgements  Conflict of Interests  Data Availability Statement