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Kateřina Teichmanová, Stanislav Luňák, Karel Pauk, Lukáš Střižík, Zdeňka Růžičková, Tomáš Mikysek, Klára Melánová, Aleš Imramovský, [Kazuhiko Nagura](https://orcid.org/0000-0003-3910-1610)

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[Tuning NIR Absorption and Emission of Diphenyl‐Dihydrophenazine‐Based Merocyanines with Ultra Narrow Band Gap](https://mdr.nims.go.jp/datasets/884ca39e-c9c1-4c02-b1d4-2b86dd4bd76e)

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Tuning NIR Absorption and Emission of Diphenyl‐Dihydrophenazine‐Based Merocyanines with Ultra Narrow Band GapChemEurJResearch Articledoi.org/10.1002/chem.202501864www.chemeurj.orgTuningNIRAbsorptionandEmissionofDiphenyl-Dihydrophenazine-BasedMerocyanineswithUltraNarrowBandGapKateřina Teichmanová,[a, b] Stanislav Luňák Jr.,[c] Karel Pauk,[b] Lukáš Střižík,[d]Zdeňka Růžičková,[d] Tomáš Mikysek,[e] Klára Melánová,[f] Aleš Imramovský,*[b]and Kazuhiko Nagura*[a]Five merocyanine derivatives with a 5,10-diphenyl-dihydrophenazine donor and various indanone-and indandionebased-acceptors with one or two dicyanovinylene groups wereprepared by Knoevenagel condensation for tuning absorptionand fluorescence in the near-infrared region. Molecular confor-mation, bond length alternation, and molecular packing in thesolid state were studied by X-ray diffraction of single crystals incombination with density functional theory (DFT) calculations.By enhancing electron-accepting ability, a considerable decreaseof lowest unoccupied molecular orbitals (LUMO) energy by 1.01eV and retained highest occupied molecular orbitals (HOMO)energy within 0.13 eV were estimated by cyclic and rotatingdisc electrode voltammetry, relating semi-quantitatively to DFTprediction. Optical properties in solutions with various polarity,neat amorphous films, and crystalline powder states were stud-ied. The absorption maxima of the neat films evolved from 545nm to 931 nm. An ultranarrow optical band gap of DPPZ-IDD(1.09 eV) was found from the onset of thin film absorption andwell agreed with the electrochemical gap of 0.93 eV. Detectablefluorescence in the NIR region was observed in the film andpolycrystalline powder states.1. IntroductionThe solar light falling on the Earth’s surface covers a wide spec-tral range from ultraviolet (UV, 250 nm–380 nm) over visible (Vis,380 nm–740 nm) to infrared (IR, 740 nm–2500 nm).[1] The absorp-tion threshold of materials is given by an optical band gap (Eg),that is, the difference between the highest occupied (HOMO)and lowest unoccupied molecular orbitals (LUMO) energy lev-els. Besides other physical properties, such as high absorptivity,exciton dynamics, and charge transfer ability, materials utiliz-ing infrared radiation effectively (e.g., in organic photovoltaics(OPV)) must have at least a narrow band gap (Eg = 1.6 eV–1.3 eV)[2] or even an ultra-narrow band gap (Eg < 1.3 eV).[3]On the other hand, the materials for biological imaging andtherapeutics (theranostics) emitting over 1000 nm are of greatimportance,[4,5] because biological tissues are more transparentand scatter less in so-called NIR I (650–1000 nm) and especiallyNIR II (1000 nm–1700 nm) spectral windows.[6] Besides the above-mentioned utilities, the applications like infrared detectors,[7] NIROrganic Light-Emitting Diodes (OLEDs),[8] NIR solid-state lasers[9]and the materials for photoacoustic bioimaging[10] are hot top-ics of scientific research. Thus, the development of new-infraredabsorbing and emitting organic dyes is of general importance.Except for exotic chromophores with an open electronicshell,[11] typical organic NIR chromophores and fluorophores canbe classified as ionic (e.g., cyanines, rhodamines), intraionic[a] K. Teichmanová, K. NaguraResearch Center for Materials Nanoarchitectonics (MANA), National Institutefor Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 304-0044, JapanE-mail: nagura.kazuhiko@nims.go.jp[b] K. Teichmanová, K. Pauk, A. ImramovskýInstitute of Organic Chemistry and Technology, Faculty of ChemicalTechnology, University of Pardubice, Studentská 95, Pardubice 53009, CzechRepublicE-mail: ales.imramovsky@upce.cz[c] S. Luňák Jr.Materials Research Centre, Faculty of Chemistry, University of Technology,Purkyňova 464/118, Brno 61200, Czech Republic[d] L. Střižík, Z. RůžičkováDepartment of General and Inorganic Chemistry, Faculty of ChemicalTechnology, University of Pardubice, Studentská 95, Pardubice 53009, CzechRepublic[e] T. MikysekDepartment of Analytical Chemistry, Faculty of Chemical Technology,University of Pardubice, Studentská 95, Pardubice 53009, Czech Republic[f ] K. MelánováCenter of Materials and Nanotechnologies, Faculty of Chemical Technology,University of Pardubice, Studentská 95, Pardubice 53009, Czech RepublicSupporting information for this article is available on the WWW underhttps://doi.org/10.1002/chem.202501864© 2025 The Author(s). Chemistry – A European Journal published byWiley-VCH GmbH. This is an open access article under the terms of theCreative Commons Attribution-NonCommercial-NoDerivs License, whichpermits use and distribution in any medium, provided the original work isproperly cited, the use is non-commercial and no modifications oradaptations are made.Chem. Eur. J. 2025, 31, e202501864 (1 of 16) © 2025 The Author(s). Chemistry – A European Journal published by Wiley-VCH GmbHwww.chemeurj.orghttps://orcid.org/0000-0003-3210-1255https://orcid.org/0000-0001-8690-7858https://orcid.org/0000-0003-0259-5557https://orcid.org/0000-0003-1793-9367https://orcid.org/0000-0001-7163-2476https://orcid.org/0000-0001-9476-9627https://orcid.org/0000-0003-3910-1610mailto:nagura.kazuhiko@nims.go.jpmailto:ales.imramovsky@upce.czhttps://doi.org/10.1002/chem.202501864http://creativecommons.org/licenses/by-nc-nd/4.0/http://crossmark.crossref.org/dialog/?doi=10.1002%2Fchem.202501864&domain=pdf&date_stamp=2025-07-09ChemEurJResearch Articledoi.org/10.1002/chem.202501864Figure 1. A) Previous and current synthetic approaches to DPPZ-EWG derivatives, together with the nomenclature of the compounds in this study. B)Graphical representation and bond length alternation tendency of DPPZ-EWG for A) neutral form, B) cyanine limit as an intermediate, and (c) zwitterionicform.(e.g., squaraines, BODIPY), or coordination complex (e.g.,phthalocyanines).[12] Widely employed molecular design strate-gies for neutral molecules to achieve a bathochromic shift inabsorption and fluorescence are conjugation extension[13] andintramolecular charge transfer (ICT) between electron-donating(D) and electron-accepting (A) groups through a conjugatedbridge (π ).[14] If considering all three molecular components (D,π , A) separately, their changes enable tuning of the energiesof both frontier orbitals. Typically, an increasing strength ofa donor part mainly destabilizes HOMO of a molecule, rais-ing the strength of an acceptor moiety stabilizes LUMO, andconjugation extension may support both trends. Furthermore,the absolute HOMO and LUMO energy levels of materials inelectronic devices like OPV cells or OLEDs must be harmo-nized with the energy levels of other components of a device,which complicates achieving the required narrow band gap.[3b]Besides the simplest D-π -A arrangement, quadrupolar A-π -D-π -A architectures are popular in organic photovoltaic materialsas various modifications of ITIC and Y6 electron acceptors.[15]The 3-dicyanomethylene-indanone (IDO in our nomenclature inFigure 1) unit is typically used as a terminal electron-withdrawinggroup (EWG) in these nonfullerene electron acceptors.[2] On theother hand, compounds with D-π -A-π -D architecture are morefrequently used in the OLEDs area, because of their ability toemit in the NIR region in solid state.[8]Merocyanines are a special class of D-π -A chromophoreswith usually a polymethine chain, typically an amine donor anda carbonyl acceptor.[16] Polymethine dyes are characterized byan odd number of methine (sp2 hybridized) centers and an evennumber of π -electrons.[17] Optical and electronic properties ofmerocyanines are usually interpreted via the contribution ofneutral and zwitterionic resonance structures. The formationof centrosymmetric antiparallel stacks by strong electrostaticinteractions is frequently observed in the crystalline state and isoften considered as a main reason for the absence of solid-statefluorescence.[18] Among merocyanines derivatives, styryl dyes(streptomerocyanines) are categorized as a distinct subclass witha styryl unit and an even number of π -electrons distributed overan odd number of π -centers between D and A.[17b] Typically,they are prepared by Knoevenagel condensation of 4-amino-arylaldehyde with active methylene compounds. Regioisomericstyryl dyes with an amino donor at meta position fall outsideChem. Eur. J. 2025, 31, e202501864 (2 of 16) © 2025 The Author(s). Chemistry – A European Journal published by Wiley-VCH GmbH 15213765, 2025, 42, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/chem.202501864 by Kazuhiko Nagura - National Institute For , Wiley Online Library on [27/07/2025]. 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 LicenseChemEurJResearch Articledoi.org/10.1002/chem.202501864the definition of streptomerocyanines because of an evennumber of π -centers between D and A. Therefore, these metachromophores show a bathochromic shift of the longest absorp-tion maximum with a smaller molar coefficient and red-shiftedfluorescence compared to the para chromophores.[19]Free-rotating propeller-like triarylamines and partially fixed9-aryl-carbazoles are frequently used as terminal electron-donating groups. For emitting DA derivatives, the combinationof these donor groups with a sufficiently strong acceptor caninduce solid-state fluorescence (SSF) in the NIR I region.[20]Furthermore, the bridging of the triphenylamine core by het-eroatoms including oxygen, sulphur, and nitrogen atoms canenhance the donating ability and cause a red shift in absorptionand fluorescence spectra.[21] Among them, symmetrical 5,10-diphenyl-dihydrophenazine (DPPZ) is an electron-rich unit witha high HOMO level.[22] Symmetrically modified DPPZ derivativeswith aryl substitutions at positions 5 and 10 have demon-strated enhanced solid-state emission[23] and thermally acti-vated delayed fluorescence[24] as well as stable radical cationsformation,[25] singlet oxygen generation[24] and efficient pho-tocatalytic activity.[26] An absorption of the reported DPPZderivatives always falls into the visible region and their fluores-cence sometimes achieves between Vis and NIR borderline.[24]Direct introduction of strong EWG on dihydrophenazine core toform asymmetrical DA compounds is limited to dicyanovinylene(DCV)[27] and cyanopyridine-vinylene[28] (Figure 1). However, thisstrategy has the potential to shift fluorescence maximum further,as observed for DPPZ-DCV nanoparticles (700 nm).Previously we had developed a series of D-π -A-type stilbenederivatives for SSF. We succeeded in tuning fluorescence colorsfrom visible to NIR regions by introducing diphenylamino donorand various EWG and by controlling the polymorphism in thecrystalline states.[29] To develop novel chromophores with nar-row band gap and fluorophore in the NIR regions, we designeda series of merocyanine derivatives with strong DPPZ donor andindanone-based acceptors (Figure 1A). We systematically investi-gated the LUMO energy levels of the indanone acceptor unitsby modifying the number of terminal EWG with the expectedsequence from IO ˂ IOO <ID < IDO ˂ IDD.[30] Although theeffect of these units comes from a complicated influence of(branched) conjugation extension and heterosubstitutions, con-sidering them as the complex acceptors on styryl conjugatedbridge appeared sufficient for the interpretation of experimentalresults in most cases. We envisioned that this acceptor mod-ification affects the balance between neutral and zwitterionicelectronic structure of the merocyanine skeleton as well as theiroptical and electronic properties. Here, to reveal these structure–property relationships and the intrinsic effect of meta nitrogenatom of DPPZ toward the merocyanine skeleton, we have estab-lished synthetic routes for this series. Their molecular geometryand packing structure in crystalline state were characterizedby X-ray diffraction (XRD). Their electrochemical properties aswell as HOMO and LUMO energy levels were estimated bycyclic voltammetry (CV) and rotating disk electrode voltammetry(RDEV). Furthermore, their optical properties, including absorp-tion and fluorescence spectra in various environments, wereinvestigated in conjugation with quantum chemical calculationsbased on density functional theory (DFT) and time-dependent(TD) DFT calculations.2. Results and DiscussionThe preparation of the target molecules DPPZ-EWG began withthe synthesis of the key intermediate DPPZ-CHO. Previously,DPPZ-CHO was obtained through a two-step process startingfrom a condensation of N,N-diphenylbenzene-1,2-diamine and3,4-difluorobenzonitrile in the presence of sodium hydridefollowed by reduction of nitrile with diisopropylaluminiumhydride.[27] A direct method for preparing the monoaldehydederived from the N,N-dimethyl-dihydrophenazine has beenonly partially described.[31] In contrast, our novel approachto DPPZ-CHO was carried out according to Scheme 1 fromcommonly available, safe, and inexpensive starting compounds.Phenazine was first reduced by sodium dithionite with quan-titative yield, according to literature.[25c] Arylation of obtained5,10-dihydrophenazine using Buchwald-Hartwig aminationin the presence of a Pd catalyst afforded N,N-diphenyl-5,10-dihydrophenazine (DPPZ) in a moderate yield of 52%.[32] As adirect formylation approach, a straight and industrially applica-ble method of Vilsmeier-Haack formylation provided DPPZ-CHOas the major product (63% isolated yield). DPPZ-CHO was furtherconverted into six DPPZ-EWG derivatives containing differentacceptor groups with increasing electron-withdrawing ability. Toimprove the electron-accepting properties, one or both carbonylgroups in 1-indanone (IO) and indane-1,3-dione (IOO) were selec-tively functionalized by malononitrile in the presence of sodiumacetate.[33] Knoevenagel condensations between the aldehydegroup of DPPZ-CHO and the activated methylene group ofacceptors were performed in the presence of pyridine or piperi-dine as a base, except for compounds DPPZ-IO and DPPZ-IDD.For DPPZ-IDD, an optimized acidic media in acetic anhydridewas necessary because of the high stability of the deprotonatedintermediate IDD− without enough nucleophilicity under basicconditions.[33] All target compounds were purified by columnchromatography, recycling high-performance liquid chromatog-raphy (HPLC), and subsequent recrystallization. Their molecularstructures were confirmed by 1H, 13C NMR, and high-resolutionmass spectrometry (HRMS).Most of the compounds in this study formed single crystalssuitable for X-ray crystallographic analysis (Figure S1). Crystals ofDPPZ-CHO were formed as twins, which led to the static disordertreated by standard operations and attributing electron max-ima to pairs of atoms with occupancy of 66/34. Two polymorphsof DPPZ-DCV were obtained: the reported polymorph wasobtained from ethyl acetate,[27] while the new polymorph wascrystallized from either CHCl3 or CH2Cl2. The solvent molecules(CHCl3) were present within the crystal structures of DPPZ-DCV(parallel), DPPZ-IO, and DPPZ-IOO. Merocyanines with only car-bonyl acceptor often tend to accommodate solvents into crystallattice.[34] Except major prismatic crystals of DPPZ-IOO includedCHCl3 via the weak hydrogen bond between chloroform C─Hand O═C of acceptors, further needle crystals could be manuallyseparated from the crystallization crop. The conformation andChem. Eur. J. 2025, 31, e202501864 (3 of 16) © 2025 The Author(s). Chemistry – A European Journal published by Wiley-VCH GmbH 15213765, 2025, 42, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/chem.202501864 by Kazuhiko Nagura - National Institute For , Wiley Online Library on [27/07/2025]. 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 LicenseChemEurJResearch Articledoi.org/10.1002/chem.202501864Scheme 1. Synthetic route of DPPZ-EWG. Reaction conditions: A) malononitrile, piperidine, CH2Cl2, RT, 20 minutes; B) 1-indanone, 6M NaOH, CHCl3, rt, 18hours; C) indan-1,3-dione, piperidine, ethanol, reflux, 18 hours; D) ID, piperidine, toluene, reflux, 18 hours; E) IDO, pyridine, ethanol, reflux, 18 hours; F) IDD,Ac2O, reflux, 5 minutes.packing arrangement of the needle crystals of DPPZ-IOO weresimilar to those of prismatic crystals of DPPZ-IO where the chlo-roform C─H directed toward the electron-rich dihydrophenazinering A (Figure 2). Solvent-free crystals of DPPZ-ID and DPPZ-IDOwere obtained from ethyl acetate. Unfortunately, the attempts toobtain single crystals of DPPZ-IDD suitable for single-crystal XRDfailed. The crystallographic parameters are summarized in TablesS1–S7, data are stored within the CSD (Nos. 2434164–2434170).To obtain structural insights about conformation and bondlength alternation (BLA) in solution state, we conducted DFTcalculations of DPPZ-EWG. The optimized structures in solu-tion state were estimated at the CAM-B3LYP/6-311G(d,p) withCHCl3 involved through polarizable continuum models (PCM)(Figure S3). Monomethine merocyanines DPPZ-EWG may exist ineither s-trans or s-cis conformation with respect to the exocyclicsingle bond in neutral form.[35] The differences in the calcu-lated energies of both conformers are comparable (Table 1), s-cisconformations are thermodynamically a bit more stable thans-trans conformations for all derivatives. The calculated energybarriers with respect to ϕ1 rotation are slightly higher (e.g.,∼9.0 kcal·mol−1 for DPPZ-IOO and ∼7.4 kcal·mol−1 for DPPZ-IDO) than that of a conventional single bond but are not highenough to prevent a thermal equilibrization of both conformersin the ground state. Thus 1H NMR spectrum of DPPZ-EWG indi-cated averaged structures of rotamers at room temperature. DFTcalculations imply perfectly coplanar dihydrophenazine donor,methine bridge, and acceptor, and exactly perpendicular sidephenyls C and D for DPPZ-CHO, DPPZ-DCV, DPPZ-IO, and DPPZ-IOO in both conformations. For s-cis conformations of DPPZ-IDO, DPPZ-ID, and DPPZ-IDD, insertion of sterically demandingdicyanovinyl groups into indan(di)one twists a molecule mainlyaround the methine single bond with dihedral angles of ϕ1 =6°, 13°, and 20°, respectively. Theoretically, the exocyclic methinebonds of DPPZ-IH without an electron acceptor considerablyalternate, that is, bond lengths c and d are 1.468 Å and 1.334 Å(Table 1). DPPZ-IO with the weakest acceptor has the shorter sin-gle bond (1.453 Å) and the longer double bond (1.341 Å) thanDPPZ-IH. This trend continues with the rising strength of theacceptor; for example, for DPPZ-IDO, these bond lengths are1.433 Å and 1.369 Å.We compared the molecular structures of DPPZ-EWG,obtained from crystals, with DFT-optimized structures to revealthe effect of intermolecular interactions on structural parameters(Figure 2). In crystal structures of DPPZ-IO and both polymorphsof DPPZ-DCV, DPPZ-ID, and DPPZ-IDO, the s-cis conformerswere observed in accordance with the calculation. The onlyexception was found in prismatic crystals of DPPZ-IOO, whichwas ascribed to the blocking of a position of IOO by CHCl3,fixed through a C─H…O═C interaction instead of a CH−πinteraction for side phenyls C and D. Intermolecular interactionsled to significant deviations from the calculated structure indihedral angles around the side-phenyl groups and the methinedouble bond (ϕ2). For instance, in the most distorted DPPZ-IDO,both side phenyls C and D were rotated only about 75° fromdihydrophenazine plane, and the twists around both methineexocyclic bonds (ϕ1 and ϕ2) were close to 20°. Such a huge twistaround the formally double bond in a neutral form relates wellwith the evolution of bond lengths, when going from weaker tostronger acceptors (Table 1). The experimental bond lengths cand d semi-quantitatively agree with calculations, being 1.453(2)Chem. Eur. J. 2025, 31, e202501864 (4 of 16) © 2025 The Author(s). Chemistry – A European Journal published by Wiley-VCH GmbH 15213765, 2025, 42, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/chem.202501864 by Kazuhiko Nagura - National Institute For , Wiley Online Library on [27/07/2025]. 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 LicenseChemEurJResearch Articledoi.org/10.1002/chem.202501864Figure 2. Molecular structures of DPPZ-EWG in ORTEP view, drawn at the 50% probability level.Å and 1.344(2) Å for DPPZ-IO and 1.428(2) Å and 1.376(2) Å forDPPZ-IDO. Furthermore, the bond lengths a between paranitrogen atoms and styryl bridges were considerably shortenedfrom 1.398(2) Å (DPPZ-IO) to 1.380(2) Å (DPPZ-IDO) with increas-ing acceptor strength. Therefore, these trends in BLA apparentlyindicate the increasing weight of zwitterionic resonance formin the electronic structure both in solution and crystal states(Figure 1Bc). Nevertheless, all compounds fall into the polyene-like interval, not the betaine-like one.[18b] The cyanine limitwith bond length equalization has equal contribution of bothneutral and zwitterionic forms and is usually achieved for mero-cyanines with either a pure polymethine-type bridge[34,36] orfive-membered heterocycles.[18b,37] The aromaticity of a benzenering in the styryl bridge probably suppresses overcoming thecyanine limit leading to quinoidization even for a combinationof a strong donor and a strong acceptor.Specific nearest-neighbor intermolecular interactions inmolecular packing were highlighted (Figure 3). Especially,Chem. Eur. J. 2025, 31, e202501864 (5 of 16) © 2025 The Author(s). Chemistry – A European Journal published by Wiley-VCH GmbH 15213765, 2025, 42, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/chem.202501864 by Kazuhiko Nagura - National Institute For , Wiley Online Library on [27/07/2025]. 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 LicenseChemEurJResearch Articledoi.org/10.1002/chem.202501864Table 1. Structural parameters of DPPZ-EWG in optimized structure (DFT: CAM-B3LYP/6-311G(d,p) with CHCl3 involved through PCM) and crystal structure.ϕ1[b] ϕ2[b] a[c] b[c] c[c] d[c]DPPZ-EWG �Ect[a] Exp Exp Theor Exp Theor Exp Theor Exp Theor ExpIH[d] cis 0.146 1.403 1.406 1.468 1.334trans 1.403 1.405 1.469 1.333IO cis 0.535 11.08 −1.27 1.394 1.398(2) 1.404 1.405(2) 1.453 1.453(2) 1.341 1.344(3)trans 1.395 1.404 1.454 1.340IOO cis 0.290 1.384 1.405 1.437 1.356trans 177.65 −179.30 1.385 1.382(2) 1.404 1.409(2) 1.437 1.433(2) 1.357 1.365(2)ID cis 0.499 5.52 1.97 1.389 1.384(3) 1.403 1.406(3) 1.449 1.443(4) 1.352 1.357(4)trans 1.390 1.402 1.451 1.350IDO cis 0.304 18.92 25.76 1.378 1.380(2) 1.404 1.409(2) 1.433 1.428(2) 1.369 1.376(2)trans 1.379 1.404 1.433 1.368IDD cis 1.122 1.380 1.400 1.436 1.364trans 1.383 1.400 1.442 1.360[a] Energy difference between s-cis (left) and s-trans (right) conformers [kcal·mol−1].[b] Dihedral angles [°].[c] Selected bond lengths [Å].[d] Hypothetical indan compound with X = Y = H2.π -stacking motifs are at the center of our interest, as they con-siderably affect the photophysical properties in solid state.[29,38]Except for DPPZ-CHO, all compounds formed π -stacked dimericstructures in the crystal packing. DPPZ-IOO (needle crystals)formed nearly the same type of π -stacking as DPPZ-IO. Thepacking of the rest of the compounds is shown in Figure 3.As for other reported monomethine merocyanines, the mostimportant packing motif is the formation of columns withalternating dimers in an antiparallel arrangement.[39] The onlyexception within our set was the new polymorph of DPPZ-DCV.The infinite columns with parallel arrangement of the moleculeswere formed by the π -π stacks with an overlap between Band A of adjacent molecules and were interconnected withadjacent columns by CH–π interaction between side phenyls Dand unsubstituted phenazine benzene rings A. In the antipar-allel polymorph of DPPZ-DCV, molecules in the π -π stackeddimer interacted through a wider overlap of DCV vinyl withunsubstituted A rings of dihydrophenazine with an interplanedistance of 3.929 Å. In addition, another molecule stacked withthe B ring of dihydrophenazine with narrower overlap andcloser interplane distance (3.300 Å). Close (proximal) π -π stacksin DPPZ-IO and DPPZ-IOO solvates were formed through anoverlap between the indan(di)one benzene ring with one of thephenazine rings, while distant (distal) dimers were formed bya mutual indan(di)one overlap. Both closer and distant dimersof DPPZ-ID were formed by similar interactions between DCVand either unsubstituted benzene ring A or substituted benzenering B of dihydrophenazine, respectively, as in the antiparal-lel polymorph of DPPZ-DCV. In DPPZ-IDO, both alternatingπ -π stacked dimers arose from an overlap of acceptor parts,either in a benzene-vinylene (closer) or benzene-benzene (dis-tant) fashion. Such dimers with specific π -stacking are criticalbecause they significantly impact exciton coupling and relatedphotophysical properties.Electrochemical measurements of DPPZ-EWG were carriedout primarily using CV and RDEV to investigate the trend ofelectron donating and accepting strength. The cyclic voltam-mograms of DPPZ-EWG derivatives showed one-electron irre-versible reduction process and two reversible oxidation pro-cesses (Figures 4 and S2). These results imply stability of radicalcation and dication species and instability of radical anions.[23]The first half-wave redox potentials from RDEV and estimatedHOMO and LUMO energy levels are summarized in Table S8,together with the results of DFT calculations based on theenergy differences between adiabatic neutral species in the sin-glet ground state and charged radical ions in the doublet stateinvolving the solvent effect of acetonitrile.[40,41] The first andsecond oxidation potentials of DPPZ-EWG with indan(di)oneacceptors were negatively shifted compared to DPPZ-CHO andDPPZ-DCV, probably due to the extended conjugation, evenChem. Eur. J. 2025, 31, e202501864 (6 of 16) © 2025 The Author(s). Chemistry – A European Journal published by Wiley-VCH GmbH 15213765, 2025, 42, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/chem.202501864 by Kazuhiko Nagura - National Institute For , Wiley Online Library on [27/07/2025]. 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 LicenseChemEurJResearch Articledoi.org/10.1002/chem.202501864Figure 3. Columnar arrangement of DPPZ-EWG with alternating close and distant dimers. For DPPZ-DCV (parallel), highly disordered solvent (CHCl3)density was removed by the SQUEEZE program.though stronger electron acceptor groups were introduced. Onthe contrary, lower reduction potentials are presumably dom-inated by the intrinsic electron-accepting ability of acceptorunits. These experimental results agree well with the estima-tion from DFT calculations. Results show the marginal differencesin HOMO energy within the set and dramatic differences inLUMO energies, for example, more than 1 eV between DPPZ-IO and DPPZ-IDD. DPPZ-ID, DPPZ-IDO, and DPPZ-IDD showultranarrow electrochemical gap (Figure 5). Not surprisingly, spindensities of radical cation and anion are mainly localized ondihydrophenazine donor and indan(di)one acceptor, respectively(Figure 6).The main task of the molecular design, that is, tuningabsorption and emission bands toward the infrared regionthrough the strength of an acceptor, was accomplished. InFigure 7A, the absorption spectra in chloroform followed thetrend of a bathochromic shift with decreasing LUMO energyand achieved the NIR region for DPPZ-ID (695 nm), DPPZ-IDO(778 nm), and DPPZ-IDD (880 nm). These absorption max-ima of DPPZ-EWG were significantly bathochromically shiftedcompared to their analogues with triphenylamine or carbazoledonors in dichloromethane.[20,30a] Especially, the bathochromicshift of DPPZ-IDD reached up to 240 nm by introducing meta-nitrogen atom. PMMA film of these compounds showed a similartrend with respect to the acceptor strength (Figure S8), butthey are always slightly blue shifted with respect to the CHCl3solutions. Absorption of amorphous thin films showed furtherbathochromic shift (Figure 7B). Their longest wavelength onsetsimplied narrow band gaps for DPPZ-IOO (1.51 eV) and DPPZ-ID(1.30 eV) and even ultranarrow optical band gaps for DPPZ-IDO (1.16 eV) and DPPZ-IDD (1.09 eV). The exact values of allabsorption maxima are summarized in Table S10. In addition tothis straightforward dependence of the absorption maxima onthe acceptor strength, several other optical properties requirefurther discussion. These include the nature of the electronictransitions between the ground and excited states, the vibronicstructure of the spectra, solvatofluorochromism, fluorescenceintensity, and the emission behavior in thin films and crystals.Chem. Eur. J. 2025, 31, e202501864 (7 of 16) © 2025 The Author(s). Chemistry – A European Journal published by Wiley-VCH GmbH 15213765, 2025, 42, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/chem.202501864 by Kazuhiko Nagura - National Institute For , Wiley Online Library on [27/07/2025]. 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 LicenseChemEurJResearch Articledoi.org/10.1002/chem.202501864Figure 4. Representative cyclic voltammograms of A) DPPZ-IO and B)DPPZ-IDO measured in CH3CN with 0.1 M Bu4NPF6. at the scan rate of100 mV/s.Figure 5. Graphical presentation of HOMO and LUMO energies andelectrochemical gaps of DPPZ-EWG.Figure 6. SCF spin densities of the A) C) radical anion and B) D) radicalcation of DPPZ-IOO (left) and DPPZ-IDO (right) calculated at theUB3LYP/6-311G(d,p). Isovalue = 0.002.Figure 7. Absorption spectra of DPPZ-EWG in A) CHCl3 solution and B)neat film.The character of the electronic transitions in Vis-NIR regionswas studied using TD DFT calculation with both B3LYP and CAM-B3LYP[42] xc functionals (see Table 2 and S6 in Supporting Infor-mation). B3LYP excitation energies of S0→S1 transition of s-cisconformations are close to the experimental longest absorptionmaxima and enable the interpretation of the whole absorptionspectrum, as shown in Figure 8 for DPPZ-IDO as an example. Theabsorption bands in Vis–NIR regions attribute to mainly threetransitions of S0→Sn (n = 1–3), each well defined by a dominantmonoexcited configuration HOMO→LUMO, HOMO→LUMO+1,and HOMO–1→LUMO, respectively. The first transition of DPPZ-IDO, named CT1, is the same for all other compounds exceptDPPZ-IDD calculated by B3LYP (Table 2). The S0→S3 transition ofDPPZ-IDO, named CT2, relates to S0→S2 of DPPZ-ID (Figure S4).Chem. Eur. J. 2025, 31, e202501864 (8 of 16) © 2025 The Author(s). Chemistry – A European Journal published by Wiley-VCH GmbH 15213765, 2025, 42, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/chem.202501864 by Kazuhiko Nagura - National Institute For , Wiley Online Library on [27/07/2025]. 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 LicenseChemEurJResearch Articledoi.org/10.1002/chem.202501864Table 2. Summary of electronic transitions of DPPZ-EWG with s-cis conformations calculated with TD DFT at the B3LYP and CAM-B3LYP/6-311G(d,p) withPCM (CHCl3).B3LYP CAM-B3LYP Exp.S0→S1 S0→S2 S0→S3 S0→S1 S0→S2 S0→S3 S0→S1Compounds E [eV] fosc[a] E [eV] fosc[a] E [eV] fosc[a] E [eV] fosc[a] E [eV] fosc[a] E [eV] fosc[a] E [eV][b]DPPZ-CHO 2.56 0.12 3.27 0.08 3.28 0.00 3.07 0.13 3.86 0.19 4.2 0.00 2.62DPPZ-IO 2.12 0.40 3.11 0.05 3.18 0.00 2.84 0.48 3.70 0.29 3.72 0.00 2.29DPPZ-DCV 2.05 0.34 3.25 0.00 3.4 0.00 2.53 0.45 3.73 0.08 3.99 0.82 2.08DPPZ-IOO 1.90 0.44 2.34 0.01 3.15 0.00 2.46 0.56 3.57 0.01 3.62 0.00 1.89DPPZ-ID 1.66 0.55 2.87 0.49 3.04 0.06 2.32 0.68 3.49 0.42 3.61 0.02 1.78DPPZ-IDO 1.60 0.45 1.98 0.10 2.83 0.48 2.17 0.71 2.96 0.02 3.46 0.52 1.59DPPZ-IDD 1.44 0.03 1.58 0.50 2.60 0.18 1.99 0.49 2.38 0.11 3.23 0.16 1.41[a] Oscillator strength.[b] The longest absorption maximum in CHCl3 solution.Figure 8. The experimental absorption spectrum of DPPZ-IDO in CHCl3 andcalculated excitation energy bars computed using s-cis geometry with TDDFT at the B3LYP/6-311G(d,p) with PCM (CHCl3). The characters ofcharge-transfer (CT) and charge-separation (CS) transitions aredemonstrated in Figure 9.Both CT1 and CT2 transitions show nearly the same “particle”natural transition orbitals (NTOs) delocalized on methine andacceptor moieties and similar “hole” NTOs mainly delocalized ondihydrophenazine with a bit different molecular orbital shapebetween HOMO and HOMO–1 (Figure 9). The crucial charac-teristics of both these charge transfer transitions is a moreeffective depletion of an electron density from meta nitrogenatom, as compared to para nitrogen, which is typical for sim-ple meta chromophores, as compared to regioisomeric parachromophores.[43] Thus, the transition to the S1 state is going onpre-polarized electronic structure with the effect of para nitro-gen donor in the ground state in contrast to conventional metachromophores. Not surprisingly, the energies of CT1 and CT2transitions of DPPZ-IDO are closer to DPPZ-ID than to DPPZ-IOO(Figures 7, 8, and S4), as the local DCV group is a consider-ably stronger acceptor than carbonyl in complex IDO acceptor(Figure 9). Altogether, considering two nitrogen atoms and twoacceptor terminals separately is crucial to understand these twoCT transitions. On the other hand, the S0→S2 transition of DPPZ-IDO, named CS, is significantly different from CT transitions andre lates to S0→S2 of DPPZ-IOO (Figure S4). It can be character-ized as an enhanced electron transfer from dihydrophenazinedonor to an indan(di)one acceptor, leading to partial (DPPZ-IDOon Figure 9) or even full (DPPZ-IOO on Figure S4a) charge sep-aration. On the contrary to both CT states, the CS one showsa tendency to adopt an ortho-quinodimethane-like electronicstructure on indan(di)one benzene ring in the S2 state. WhileCT1 and CT2 transitions can be easily detected as the distinctbands with moderate oscillator strengths (fosc) in the absorp-tion spectrum, CS transition is too weak due to small fosc andmanifests itself only as a shoulder on short-wavelength regionof CT1 band (Figure 8). Superposition of an intense CT1 bandat a shorter wavelength and a weaker CS band at a longerwavelength would cause an asymmetrical shape of the longestabsorption band of DPPZ-IDD at 880 nm compared to all othercompounds (Figure 7).While the absorption spectra in chloroform are blurred(Figure 7A), well-structured absorption bands with vibronicpattern can be observed in cyclohexane (Figure 10). Thewavenumber differences between 0–0 and 0–1 vibronic bandsin DPPZ-IOO and DPPZ-DCV were around 1200 cm−1, whichcorresponds well with the C─C bond stretching vibrations ofmerocyanines.[44] Except for a twisted DPPZ-IDD, there is a clearrelation between the decreasing excitation energy (given by anacceptor strength) and the raising ratio of 0–0 and 0–1 vibronicband intensity. The closer the merocyanines are in polyene-likeregion to the cyanine limit, the more suppressed is the rela-tive intensity of 0–1 transition.[12a,45] Therefore, the evolution of avibronic structure in Figure 10A is a spectral manifestation of thetrend, estimated from BLA (Table 1). Even in weakly polar toluenesolution, the absorption spectra of all compounds showed broadbands without vibronic structure (Figure S7). While DPPZ-IO andDPPZ-ID showed positive solvatochromism, DPPZ-IOO, DPPZ-IDO, and DPPZ-IDD showed inverse solvatochromism, that is,positive up to chloroform and then negative toward acetonitrile(Figure 11 for DPPZ-IOO and Table S10). Inverse solvatochromismoften implies electronic structure near the cyanine limit,[34]but that was not observed in the more polar DPPZ-ID thanDPPZ-IOO. Furthermore, this inverse solvatochromism led tobroadened absorption bands without characteristic evolution,suppressing a 0–1 relative intensity.[35] We speculate that thesmall bathochromic shift of all three indandione derivatives inChem. Eur. J. 2025, 31, e202501864 (9 of 16) © 2025 The Author(s). Chemistry – A European Journal published by Wiley-VCH GmbH 15213765, 2025, 42, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/chem.202501864 by Kazuhiko Nagura - National Institute For , Wiley Online Library on [27/07/2025]. 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 LicenseChemEurJResearch Articledoi.org/10.1002/chem.202501864Figure 9. Hole and particle pairs of NTOs of DPPZ-IDO for S0→S1 (HOMO→LUMO), S0→S2 (HOMO→LUMO+1), and S0→S3 (HOMO–1→LUMO) transitions,calculated with TD DFT at the B3LYP/6-31G(d,p) with PCM (CHCl3), isovalue 0.02.Figure 10. A) Absorption and B) fluorescence spectra of DPPZ-EWG exceptDPPZ-IDD in cyclohexane.Table 3. Calculated bond lengths a─d [Å] in the S0 and S1 states (DFT:CAM-B3LYP/6-311G(d,p) with cyclohexane involved through PCM).Compounds States a b c dDPPZ-IO S0 1.395 1.405 1.454 1.340S1 1.390 1.389 1.422 1.369DPPZ-IOO S0 1.386 1.405 1.438 1.355S1 1.396 1.390 1.419 1.386DPPZ-ID S0 1.391 1.404 1.450 1.351S1 1.388 1.391 1.430 1.379DPPZ-IDO S0 1.380 1.405 1.435 1.368S1 1.393 1.390 1.428 1.392halogenated solvents may be caused either by an increasedpopulation of s-trans conformers by specific intermolecular inter-actions with solvents, as observed in crystal structure, or the roleof the higher refractive index.Fluorescence spectra and photoluminescence quantumyields (PLQY) of DPPZ-EWG derivatives depended stronglyboth on structure and environment (Table S10 and Figure S7).The simplest situation was found for fluorescence in nonpolarcyclohexane (Figure 10B). Except for DPPZ-IDD with a verylow signal-to-noise ratio (Figure S9), all compounds showedwell-resolved fluorescence bands from visible (493 nm for DPPZ-CHO) to NIR regions (832 nm for DPPZ-IDO) following the samesequence as their absorption maxima. The vibronic structurewith about a 1200 cm−1 interval consistently showed a 0–0 abso-lute maximum and gradual suppression of 0–1 relative intensity(Figure 10B). As the prominent vibronic pattern relates to C─Cbond stretching, the vibronic relaxation in the excited statemainly causes a dramatic extension of d bond and shorteningof exocyclic c bond according to the geometry optimization inthe S1 state (Table 3). During relaxation from the Franck-CondonChem. Eur. J. 2025, 31, e202501864 (10 of 16) © 2025 The Author(s). Chemistry – A European Journal published by Wiley-VCH GmbH 15213765, 2025, 42, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/chem.202501864 by Kazuhiko Nagura - National Institute For , Wiley Online Library on [27/07/2025]. 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 LicenseChemEurJResearch Articledoi.org/10.1002/chem.202501864Figure 11. Solvatochromism (left) and solvatofluorochromism (right) of A) B) DPPZ-IO and C) D) DPPZ-IOO.state with S0 geometry to relaxed S1 geometry, while thedifferences in d bond length �d(S1−S0) of DPPZ-IO, DPPZ-IOO,DPPZ-ID, and DPPZ-IDO are relatively comparable (+0.029,+0.031, +0.028, and +0.024 Å, respectively), the corresponding�c(S1−S0) values depend dramatically on an acceptor strength(−0.032, −0.019, −0.020, and −0.007 Å, respectively). Thesetendencies are consequently responsible for the decrease of therelative intensity of 0–1 vibronic band, relating to stretching of cbond according to the Frank-Condon principle.[46] We note onlya considerable shortening of b bonds, which are consistent witha CT excitation originating mainly from meta-nitrogen atoms.The shapes of both the absorption and fluorescence bands incyclohexane imply that the larger is a zwitterionic character inthe S0 state; that is, the larger the proximity of a compoundto a cyanine limit, the lower the difference between S0 andS1 hypersurface minima with respect to prominent c bondlength coordinate and, consequently, the lower is the structuralrelaxation energy upon excitation.All compounds without any exception showed positive sol-vatofluorochromism (Figure 11 and S7). The vibronic structures offluorescence spectra were poorly resolved even in weakly polartoluene solution and completely disappeared with raising polar-ity of the solvent, due to the inhomogeneous broadening.[47]Sufficiently strong fluorescence intensities for PLQY measure-ments were found only in a few cases (Table S10). In toluenesolutions, raising acceptor strength clearly decreased PLQYs fromDPPZ-CHO (60.0%) over DPPZ-IO (28.7%) to DPPZ-DCV (9.0%)and DPPZ-IOO (3.0%). A further decrease of PLQY was observedin polar solvents; for example, in chloroform for DPPZ-CHO(3.0%), DPPZ-DCV (2.2%), and DPPZ-IOO (0.3%). But the depen-dence of solvatofluorochromism on the molecular structure wasnot as straightforward as, for instance, for PLQY. For nearlyisomorphic DPPZ-IO, DPPZ-IOO, DPPZ-ID, and DPPZ-IDO, thedifferences between fluorescence maxima in nonpolar cyclohex-ane and moderately polar dichloromethane (often considered astwice the solvation energy[48]) were 4690, 2690, 3570, and ∼1680cm−1 respectively. We leave open the question of to what extentsolvatofluorochromism, that is, energies of fluorescence maximain each solvent, is driven by intramolecular structural relax-ation (dominant in nonpolar cyclohexane) and intermolecularsolute–solvent, mainly dipole–dipole, interactions.Monomethine merocyanines are well known to undergo flu-orescence quenching via twisting in the S1 state toward a conicalintersection, followed by a hot back-twist to the ground stateminimum.[18b] Such twisted intramolecular charge transfer (TICT)of merocyanine often occurs around a single bond in neutralform or around the double bond in zwitterionic form.[49] Asexpected from TD DFT calculations,[50] B3LYP, and CAM-B3LYP xcChem. Eur. J. 2025, 31, e202501864 (11 of 16) © 2025 The Author(s). Chemistry – A European Journal published by Wiley-VCH GmbH 15213765, 2025, 42, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/chem.202501864 by Kazuhiko Nagura - National Institute For , Wiley Online Library on [27/07/2025]. 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 LicenseChemEurJResearch Articledoi.org/10.1002/chem.202501864Figure 12. Energy dependence on the twist angle of DPPZ-IOO in theground and excited states, calculated at the B3LYP andCAM-B3LYP/6-311G(d,p) with PCM (CHCl3).functionals show a dramatic difference in the dependence of anoptimized S1 state energy on the twist angle ϕ1, as shown forDPPZ-IOO as an example (Figure 12, and see detailed analysis inS6 Supporting Information). Regardless of the different solventeffect models, the results revealed similarities with a benchmarkstudy about the simpler DA compounds.[50b] The energy profilesfor the S1 state computed with B3LYP and CAM-B3LYP begin todiverge significantly at ϕ1 ≈ 45°. While B3LYP suggests the TICTstate as a global minimum and planar intramolecular chargetransfer (PICTc and PICTt) states as local minima, CAM-B3LYP con-verged to PICTc and PICTt states with rotational barriers in theexcited state. And B3LYP computed transition energies relatingthe fluorescence at the planar geometry (PICTt) are closer tothe experimental fluorescence energies (Table S9). As CAM-B3LYPshowed similar trends as more sophisticated QCH methods,including EOM-CCSD[50a] and LCC2,[50b] we conclude that theweak fluorescence observed for the four compounds in polar sol-vents in Figure 12 originates from planar geometries in the S1state mainly, and the twist geometry leading to the nonradiativedeactivation is limited within the small twist angles region andprevents the TICT geometry with full charge separation.[51]Solid-state fluorescence of polycrystalline powders wasdetected for all compounds except DPPZ-IDD (Figure 13), whilean emission of DPPZ-IDO around 1100 nm was extremely weakwith a low signal-to-noise ratio (Figure S9). Among these emis-sive derivatives, similar trends in fluorescence wavelength werefound between the solution and the amorphous films (FigureS8). Neat film’s SSF is generally red shifted with respect to PMMAfilms (Table S10). Powder X-ray diffraction of the emissive poly-crystalline samples suggested a similar packing structure withsingle crystals except for DPPZ-IOO (Figure S10). The neat filmof DPPZ-DCV showed a fluorescence maximum at 761 nm, whilePMMA films exhibited maximum at a 706 nm, which is compa-rable to the reported nanoparticles (700 nm).[27] This significantred shift of neat film is presumably due to the intermolecularFigure 13. Fluorescence spectra of DPPZ-EWG in polycrystalline states.interaction with surrounding adjacent molecules with randomarrangement. The SSF maximum of the antiparallel arrangedpolymorph was observed at a longer wavelength of 785 nmthan that of the parallel one at 731 nm, which we attribute tothe 0–1 vibronic maximum of the former and the 0–0 maximumof the latter.[52] As an antiparallel arrangement generally prevailsfor all compounds according to single-crystal XRD (Figure 3), 0–1vibronic maximum may be quite common, but without a doubtit is observed only for DPPZ-IO (Figure 13). Weak solid-state emis-sion with structureless symmetrical bands of DPPZ-ID (975 nm)and DPPZ-IDO (∼1100 nm) is tentatively attributed to excimeremission from isolated π -stacked dimers (Figure 3).[29a] PLQYsof polycrystalline samples are typically too low to quantify, asan aggregation accelerates fluorescence quenching processes,including dissipation of excitation energy through energy trans-fer and nonradiative decay.[53] Nevertheless, a considerableincrease in PLQY of DPPZ-IO upon solidification was observedfrom CHCl3 solution (PLQY ˂˂1%) to crystal (PLQY = 4.0%) anddoped PMMA film (PLQY = 8.4%). Similar but less pronouncedenhancements were seen for DPPZ-IOO (0.3% in CHCl3 and 1.1%both in powder and PMMA film). Such an enhancement of PLQYin the solid state may imply suppression of twist and rotationalmotions leading to nonradiative decay in the solution state.[54]We note only that all DPPZ-EWG derivatives were sufficientlystable with respect to the above-mentioned experiments, espe-cially in the solid state under visible light. We have observedtwo types of reactivity. First, upon irradiation by UV radia-tion in solution, the compounds degraded with color changes(Figures S11, S12). We ascribe this to the photooxidation in higherexcited states (Figure S13).[26]. Second, compounds with low-lyingLUMO, like DPPZ-IDO and DPPZ-IDD, show irregular changes ofabsorption in polar solvents with high donor numbers (DMSO,methanol, pyridine). Thus, these solvents were excluded fromsolvatochromism and solvatofluorochromism studies.3. Experimental3.1. Synthesis and Characterization3.1.1. DPPZ-CHOIn a 250 mL round-bottomed flask, 5,10-diphenyldihydrophenazine(DPPZ, 2.00 g, 5.98 mmol, 1.00 eq.) was suspended in anhydrousChem. Eur. J. 2025, 31, e202501864 (12 of 16) © 2025 The Author(s). Chemistry – A European Journal published by Wiley-VCH GmbH 15213765, 2025, 42, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/chem.202501864 by Kazuhiko Nagura - National Institute For , Wiley Online Library on [27/07/2025]. 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 LicenseChemEurJResearch Articledoi.org/10.1002/chem.202501864DMF (100 mL) and cooled to 0 °C with an ice bath. After addingPOCl3 (1.12 mL, 11.9 mmol, 2.00 eq.) dropwise, the resulting mixturewas heated to 60 °C and stirred for 3 hours under a nitrogen atmo-sphere. After cooling down to room temperature, the mixture waspoured on ice, followed by precipitation of an orange solid. Crudeproduct was purified by column chromatography (mobile phasechloroform/n-hexane 1/4) and crystallized from a mixture of ethylacetate and n-hexane. It was obtained 1.47 g of DPPZ-CHO as orangecrystals (63%).Rf: 0.22 (n-hexane/EtOAc = 7/1, v/v), m.p. 239.5–240.0 °C. 1H NMR(400 MHz, C6D6): δ 9.29 (s, 1H), 7.14–7.07 (m, 6H), 7.05–6.97 (m, 4H),6.54 (d, J = 9.2 Hz, 1H), 6.43 (s, 1H), 6.24–6.21 (m, 1H), 6.19–7.16 (m,1H), 5.73 (d, J = 7.7 Hz, 1H), 5.69 (d, J = 7.7 Hz, 1H), 5.61 (d, J = 8.1 Hz,1H). 13C NMR (101 MHz, C6D6): δ 188.9, 142.6, 139.5, 139.4, 137.7, 136.7,135.5, 131.8, 131.6, 131.3, 130.9, 130.9, 128.7, 128.4, 127.5, 122.9, 121.5, 113.8,113.4, 111.9, 110.3. HRMS: [M]+ Calcd. for C25H18N2O 362.14136; found362.14100.3.1.2. DPPZ-IOIn a 100 mL round-bottomed flask, a mixture of DPPZ-CHO (600 mg,1.65 mmol, 1.00 eq.) and 1-indanone (328 mg, 2.48 mmol, 1.50 eq.) inCHCl3 (50 mL) was treated with 6M NaOH (1.10 mL, 6.60 mmol, 4.00eq.) and stirred at room temperature for 18 hours. The reaction wasquenched by adding water (20 mL). After the aqueous phase wasextracted with ethyl acetate (3 × 50 mL). After drying over Na2SO4,the organic phase was evaporated under reduced pressure. Crudeproduct was purified by column chromatography (mobile phaseCHCl3/n-hexane 1/3) and crystallized from a mixture of CHCl3 andn-hexane. It was obtained 400 mg of molecule DPPZ-IO as purplecrystals (51%).Rf: 0.25 (n-hexane/EtOAc = 3/1, v/v), m.p. 211.4–212.0 °C. 1H NMR(500 MHz, DMSO-d6): δ 7.81 (t, J = 7.5 Hz, 2H), 7.73 (t, J = 7.5 Hz, 2H),7.69–7.61 (m, 3H), 7.58 (t, J = 7.3 Hz, 1H), 7.53 (d, J = 7.5 Hz, 2H), 7.50–7.40 (m, 4H), 6.98 (s, 1H), 6.73 (d, J = 8.1 Hz, 1H), 6.35 (t, J = 7.5 Hz, 1H),6.30 (t, J = 7.4 Hz, 1H), 5.83 (s, 1H), 5.62–5.61 (m, 1H), 5.54–5.51 (m, 2H),3.33 (overlap with signal of H2O in DMSO, 2H). 13C NMR (126 MHz,DMSO-d6): δ 192.6, 149.0, 139.0, 138.5, 138.3, 137.6, 136.4, 135.6, 134.9,134.4, 132.7, 131.9, 131.8, 130.9, 130.5, 129.0, 128.9, 127.7, 127.5, 126.7, 126.2,123.4, 122.0, 121.0, 112.8, 112.5, 112.4, 111.9, 55.0, 31.7. HRMS: [M]+ Calcd.for C34H24N2O 476.18831; found 476.18887.3.1.3. DPPZ-DCVIn a 50 mL round-bottomed flask, a mixture of DPPZ-CHO (500 mg,1.38 mmol, 1.00 eq.) and malononitrile (0.230 mL, 4.14 mmol, 3.00eq.) in CH2Cl2 (15 mL) was treated with two drops of piperidineand stirred at room temperature for 20 minutes. The reaction wasquenched by adding water (10 mL). After separation, the aqueousphase was extracted with ethyl acetate (3 × 30 mL). After dry-ing over Na2SO4, the organic phase was evaporated under reducedpressure. Crude product was purified by column chromatography(mobile phase ethyl acetate/n-hexane 1/3) and crystallized from amixture of ethyl acetate and n-hexane. It was obtained 450 mg ofDPPZ-DCV as purple powder (78%).Rf: 0.35 (n-hexane/EtOAc = 7/1, v/v), m.p. 219.5–220.0 °C. 1H NMR(400 MHz, C6D6): δ 7.34–7.32 (m, 2H), 7.19–7.18 (m, overlap with C6D6signal, 1H), 7.13–7.08 (m, 4H), 7.05–7.01 (m, 1H), 7.90–6.86 (m, 2H), 6.27(d, J = 2.1 Hz, 1H), 6.24–6.18 (m, 2H), 6.15 (dt, J = 7.7, 1.5 Hz, 1H), 6.04(s, 1H), 5.71 (dd, J = 7.8, 1.4 Hz, 1H), 5.63 (dd, J = 7.9, 1.5 Hz, 1H), 5.42(d, J = 8.5 Hz, 1H). 13C NMR (101 MHz, C6D6): δ 156.0, 143.0, 138.9,138.6, 137.4, 136.6, 134.7, 132.3, 131.7, 130.7, 130.4, 129.4, 129.2, 129.1, 125.2,123.6, 121.7, 115.5, 114.3, 114.1, 113.7, 111.8, 111.1, 75.6. HRMS: [M]+ Calcd.for C28H18N4 410.15260; found 410.15337.3.1.4. DPPZ-IOOIn a 50 mL round-bottomed flask, a mixture of DPPZ-CHO (250 mg,0.689 mmol, 1.00 eq.) and 1,3-indandione (206 mg, 1.38 mmol, 2.00eq.) in ethanol (20 mL) was treated with two drops of piperidine andrefluxed for 18 hours. After cooling down to room temperature, adark blue solid precipitated in the freezer. After filtration and wash-ing with chilled ethanol, the crude product was purified by columnchromatography (mobile phase CHCl3/n-hexane 1/4) and recyclingHPLC (mobile phase CHCl3). After final crystallization from a mixtureof CHCl3 and n-hexane, 252 mg of DPPZ-IOO was obtained as a darkblue powder (75%).Rf: 0.29 (n-hexane/EtOAc = 3/1, v/v), m.p. 262.5–263.0 °C. 1H NMR(400 MHz, C6D6): δ 7.68–7.64 (m, 1H), 7.64–7.59 (m, 2H), 7.48–7.45 (m,2H), 7.39–7.37 (m, 2H), 7.32–7.30 (m, 2H), 7.26–7.24 (m, 1H), 7.13–7.10(m, 2H), 7.04–7.00 (m, 1H), 6.97–6.89 (m, 4H), 6.25 (dt, J = 7.7, 1.4 Hz,1H), 6.17 (dt, J = 7.3, 1.4 Hz, 1H), 5.81 (dd, J = 7.9, 1.3 Hz, 1H), 5.68(dd, J = 7.9, 1.3 Hz, 1H), 5.64 (d, J = 8.6 Hz, 1H). 13C NMR (101 MHz,C6D6): δ 190.3, 189.0, 145.8, 142.9, 142.7, 140.4, 139.8, 139.0, 137.2, 137.0,135.0, 133.9, 133.9, 133.2, 131.9, 131.6, 131.3, 130.6, 128.8, 128.7, 128.2, 127.9,125.7, 123.4, 122.6, 121.3, 116.7, 114.2, 113.6, 112.5. HRMS: [M]+ Calcd. forC34H22N2O2 490.16758; found 490.17148.3.1.5. DPPZ-IDIn a 100 mL round-bottomed flask, a mixture of DPPZ-CHO (600 mg,1.65 mmol, 1.00 eq.) and ID (298 mL, 1.65 mmol, 1.00 eq.) in toluene(50 mL) was treated with two drops of piperidine and refluxed for18 hours. The reaction was quenched by adding water (20 mL). Afterseparation, the aqueous phase was extracted with ethyl acetate (3 ×30 mL). After drying over Na2SO4, the organic phase was evaporatedunder reduced pressure. Crude product was purified by columnchromatography (mobile phase ethyl acetate/n-hexane 1/39) andcrystallized from a mixture of CH2Cl2 and n-hexane. It was obtained738 mg of DPPZ-ID as green crystals (73%).Rf: 0.48 (n-hexane/EtOAc = 3/1, v/v), m.p. 282.5–283.4 °C. 1H NMR(500 MHz, C6D6): δ 8.64 (d, J = 8.1 Hz, 1H), 7.89 (s, 1H), 7.22–7.18 (m,overlap with C6D6 signal, 4H), 7.12–7.10 (m, 2H), 7.08–7.05 (m, 4H),6.99 (t, J = 7.2 Hz, 1H), 6.88 (t, J = 7.6 Hz, 1H), 6.80 (d, J = 7.5 Hz, 1H),6.32–6.27 (m, 2H), 6.24 (dt, J = 7.6, 1.2 Hz, 1H), 5.84–5.83 (m, 2H), 5.78(dd, J = 7.8, 1.2 Hz, 1H), 5.61 (d, J = 8.3 Hz, 1H), 2.82 (s, 2H). 13C NMR(126 MHz, C6D6): δ 165.5, 146.7, 140.0, 139.7, 139.3, 137.5, 137.4, 137.0,136.5, 135.5, 132.9, 132.8, 131.7, 131.6, 131.4, 130.9, 129.0, 128.3, 128.6, 128.3,127.5, 125.8, 124.6, 122.8, 121.7, 116.4, 116.2, 114.0, 113.3, 113.1, 112.8, 67.6,37.1. HRMS: [M]+ Calcd. for C37H24N4 524.20010; found 524.20036.3.1.6. DPPZ-IDOIn a 100 mL round-bottomed flask, a mixture of DPPZ-CHO (200 mg,0.551 mmol, 1.00 eq.) and IDO (161 mg, 0.837 mmol, 1.50 eq.) in CHCl3(20 mL) was treated with two drops of pyridine and refluxed for 18hours. After cooling to room temperature, solvent was evaporatedby reduced pressure. 10 mL of ethanol was added and sonicated for1 minute. After filtration and washing with chilled ethanol, the crudeproduct was purified by column chromatography (mobile phaseCHCl3/n-hexane 4/1) and recycling HPLC (mobile phase CHCl3). Afterfinal crystallization from a mixture of CHCl3 and n-hexane, 217 mg ofDPPZ-IDO was obtained as a dark green powder (73%).Rf: 0.31 (n-hexane/EtOAc = 3/1, v/v), m.p. 245.6–246.2 °C. 1H NMR(400 MHz, C6D6): δ 7.41–7.40 (m, 1H), 7.83 (s, 1H), 7.46–7.44 (m, 1H),7.41–7.37 (m, 2H), 7.32–7.31 (m, 2H), 7.25–7.21 (m, 2H), 7.20–7.19 (m, 1H),7.14–7.12 (m, 2H), 7.05–7.03 (m, 1H), 6.94–7.92 (m, 2H), 6.85–6.82 (m,2H), 6.25 (dt, J = 7.6, 1.4 Hz, 1H), 6.16 (td, J = 7.2, 1.8 Hz, 1H), 5.80 (dd,J = 8.0, 1.3 Hz, 1H), 5.70 (dd, J = 7.9, 1.3 Hz, 1H), 5.55 (d, J = 8.4 Hz,Chem. Eur. J. 2025, 31, e202501864 (13 of 16) © 2025 The Author(s). Chemistry – A European Journal published by Wiley-VCH GmbH 15213765, 2025, 42, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/chem.202501864 by Kazuhiko Nagura - National Institute For , Wiley Online Library on [27/07/2025]. 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 LicenseChemEurJResearch Articledoi.org/10.1002/chem.2025018641H). 13C NMR (126 MHz, C6D6): δ 186.5, 162.1,146.0, 144.6, 143.5, 139.9,139.5, 138.5, 137.7, 137.1, 136.8, 135.0, 134.4, 134.0, 133.2, 132.0, 131.6, 131.2,130.4, 129.0, 128.9, 125.1, 124.6, 124.0, 123.1, 121.4, 116.5, 115.6, 115.2, 114.6,113.6, 112.5, 69.4. HRMS: [M]+ Calcd. for C37H22N4O 538.17881; found538.17963. Minor isomer (not separable): 1H NMR (400 MHz, C6D6)δ 8.20 (d, J = 7.8 Hz, 1H), 7.50–6.77 (overlap with aromatic peaks ofmajor isomer), 6.76 (dd, J = 8.4, 1.4 Hz, 2H), 5.96 (d, J = 2.0 Hz, 1H),5.89 (s, 1H), 5.73 (d, J = 1.3 Hz, 1H), 5.64 (dd, J = 7.8, 1.3 Hz, 1H), 5.50(d, J = 8.3 Hz, 1H).3.1.7. DPPZ-IDDIn a 50 mL two-necked flask, a mixture of DPPZ-CHO (150 mg, 0.414mmol, 1.00 eq.) and IDD (120 mg, 0.497 mmol, 1.20 eq.) in aceticanhydride (10 mL) was refluxed under an argon atmosphere for5 minutes. After cooling down to room temperature, solvent wasevaporated under reduced pressure. 10 mL of ethanol was addedand sonicated for 1 minute. After filtration and washing with chilledethanol, the crude product was purified by column chromatogra-phy (mobile phase CHCl3/n-hexane 1/7) and recycling HPLC (mobilephase CHCl3). After final crystallization from a mixture of CHCl3 andn-hexane, 50 mg of DPPZ-IDD was obtained as black powder (21%).Rf: 0.24 (n-hexane/EtOAc = 3/1, v/v), m.p. 240.0 °C decomposed.1H NMR (400 MHz, C6D6): δ 8.14–8.10 (m, 2H), 8.10 (s, 1H), 7.43 (t, J =7.7 Hz, 2H), 7.21 (tt, J = 7.6, 1.0 Hz, 1H), 7.11–7.09 (m, 2H), 7.06–7.04 (m,2H), 7.02–6.98 (m, 1H), 6.79–6.77 (m, 2H), 6.75–6.72 (m, 2H), 6.35 (dd,J = 8.5, 2.0 Hz, 1H), 6.24 (dt, J = 7.6, 1.2 Hz, 1H), 6.15 (dt, J = 7.6, 1.3Hz, 1H), 5.76 (d, J = 1.3 Hz, 1H), 5.67 (td, J = 8.0, 1.2 Hz, 2H), 5.41 (d,J = 8.4 Hz, 1H). 13C NMR (101 MHz, C6D6): δ 160.0, 143.4, 143.0, 138.0,137.7, 137.6, 136.4, 133.8, 133.1, 133.0, 131.6, 130.6, 130.0, 123.0, 129.2, 129.1,125.7, 124.6, 124.3, 121.8, 115.0, 114.9, 114.3, 113.7, 113.4, 110.7, 67.8. HRMS:[M+H]+ Calcd. for C40H23N6 587.1979; found 587.1962 (APCI).Single-crystal X-ray diffraction: Full sets of diffraction data werecollected at 150(2) K with a Bruker D8-Venture diffractometerequipped with Cu (Cu Kα radiation; λ = 1.54178 Å) or Mo (MoKα radiation; λ = 0.71073 Å) microfocus X-ray (IμS) sources, Pho-ton CMOS detector, and Oxford-Cryosystems cooling device. Theframes were integrated with the Bruker SAINT software packageusing a narrow-frame algorithm. Data were corrected for absorp-tion effects using the Multi-Scan method (SADABS). Obtained dataare treated by XT-version 2017/1 and SHELXL-2014/7 software imple-mented in APEX3 v2017.1-0 (Bruker AXS) system.[55] Hydrogen atomswere mostly localized on a different Fourier map; however, to ensureuniformity of treatment of the crystal, all hydrogen atoms wererecalculated into idealized positions (riding model) and assignedtemperature factors Hiso(H) = 1.2 (1.5 for methyl) Ueq (pivot atom).H atoms in methyl and vinylidene moieties and hydrogen atoms inaromatic rings were placed with C-H distances of 0.98 and 0.94 Å,respectively.Rint = �Fo2 – Fo,mean2|/�Fo2, S = [�(w(Fo2 – Fc2)2)/(Ndiffrs –Nparams)]12 for all data, R(F) = �||Fo – |Fc|/�|Fo|for observed data,wR(F2) = [�(w(Fo2 – Fc2)2)/(�w(Fo2)2)]12 for all data.Crystallographic data for structural analysis have been depositedwith the Cambridge Crystallographic Data Centre, CCDC nos.2434164–2434170. Copies of this information may be obtained free ofcharge from The Director, CCDC, 12 Union Road, Cambridge CB2 1EY,UK (fax: +44-1223-336033; e-mail: deposit@ccdc.cam.ac.uk or w w w:http://www.ccdc.cam.ac.uk). The structure of DPPZ-CHO is twinned,producing static disorder of all atoms, which was treated by splittingby standard methods into two parts with occupancies of 66:34. Sol-vent disorders (CHCl3) are treated in DPPZ-IO and DPPZ-IOO (prism).For DPPZ-DCV (parallel), the PLATON/Squeeze program was used toeliminate disordered unassignable solvent (CHCl3) densities.[56]Powder X-ray diffraction: Powder X-ray diffraction data (Cu Kα, λ= 1.5418 Å) of powdered samples were collected on a DiffractometerD8 ADVANCE.DAVINCI (Bruker AXS, Germany) with Bragg-Brentano -  goniometer (radius 280 mm) equipped with a LynxEye XE-T detector. The generator was operated at 40 kV and 30 mA.The scan was performed at room temperature from 2 to 50° (2 )in 0.01° steps with a counting time of 1 second (total step time192 seconds).Deposition of thin films: Neat films were prepared by spin coat-ing (1000 rpm for 20 seconds, followed by 2000 rpm for 20 seconds)using 10 mM solution in chloroform on quartz plate. PMMA filmswere prepared by dissolving 140 mg PMMA an d 2.50 mg of tar-get compounds in 2.60 mL dichloromethane, and spin-coated (1000rpm for 10 seconds, followed by 2000 rpm for 30 seconds) on quartzplate.Electrochemical measurements: Electrochemical measurementswere carried out in acetonitrile containing 0.1 M Bu4NPF6 in athree-electrode cell by CV and RDEV with rotation frequency f =500 min−1. The scan rate was 100 mV/s. The working electrodewas a glassy carbon disk (3 mm in diameter) for CV and RDEVexperiments. A saturated calomel electrode (SCE) separated by abridge filled with supporting electrolyte and Pt wire was used asthe reference and auxiliary electrodes. All potentials are given vs.SCE. Voltammetric measurements were performed using a poten-tiostat PGSTAT 128N (AUTOLAB, Metrohm Autolab B.V., Utrecht, TheNetherlands) operated via NOVA 1.11 software.Absorption and fluorescence: UV-Vis-NIR spectroscopy has beenemployed for measurements of optical transmittance in the spec-tral region of 250–1200 nm with a step 0.5 nm by using a JASCOV-770 and a Jasco V-570 spectrometers. Photoluminescence emis-sion spectra were measured by spectrophotometer JASCO FP-8300in the spectral region of 400–800 nm. Emission in the NIR regionwas recorded using a Fluorolog-3 spectrophotometer (HORIBAJOVIN IVON INC.) equipped with a Hamamatsu-photonics InGaAsNIR photomultiplier tube detector R5509-43 (550–1300 nm) anda liquid nitrogen cooler C9940. Additionally, an FLS1000 (Edin-burgh Instruments) using the Xe lamp (450 W) as an excitationsource and a liquid-nitrogen-cooled InGaAs photomultiplier tubeas a detector in the spectral region of 400–1350 nm was used.All emission spectra have been corrected to monochromator anddetector system response. Photoluminescence quantum yield ofsamples has been determined by using the integrating spherecoated with BaSO4 on a Quantaurus-QY® C11347(Hamamatsu Pho-tonics) and FLS1000 (Edinburgh Instruments). The reference forliquid samples was quartz cuvettes with pure solvent, and thosefor solids, BaSO4 powder of spectral quality covered with a quartzlid.Quantum chemical calculations: Monomer geometries in theground (neutral, radical cation, and radical anion) and the low-est excited state were optimized by DFT and TD DFT, respectively.B3LYP or CAM-B3LYP[42] XC functionals with 6–311G(d,p) basis setswere used. Vibrational analysis was carried out for all optimizedgeometries, and only the minima with no imaginary frequency wereconsidered. Solvent effect was introduced by the polarized contin-uum model (PCM). All calculations were carried out with Gaussian09 software.[57]Chem. Eur. J. 2025, 31, e202501864 (14 of 16) © 2025 The Author(s). Chemistry – A European Journal published by Wiley-VCH GmbH 15213765, 2025, 42, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/chem.202501864 by Kazuhiko Nagura - National Institute For , Wiley Online Library on [27/07/2025]. 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 Licensehttps://www.ccdc.cam.ac.uk/services/structures?id=doi:10.1002/chem.202501864mailto:deposit@ccdc.cam.ac.ukhttp://www.ccdc.cam.ac.ukChemEurJResearch Articledoi.org/10.1002/chem.2025018644. ConclusionThe study presents a novel strategy for chromophores exhibit-ing long-wavelength absorption and NIR fluorescence, that is,streptomerocyanines with dihydrophenazine and strong accep-tors. The key design principle is based on a different and specificrole of both dihydrophenazine nitrogen donors. The resonanceeffect of para-nitrogen atom in DPPZ to the acceptor enabledto tune a zwitterionic contribution by a strength of an accep-tor, forming the pre-polarized electronic structure within thepolyene-like region toward the cyanine limit in the groundstate. On the other hand, meta-nitrogen atom in DPPZ playeda key role in intramolecular charge transfer to form a highlypolarized electronic structure in the first excited state. Suchstreptomerocyanines realized considerably narrow optical andelectrochemical band gaps compared to any reported DPPZ andIDD derivatives, to the best of our knowledge. Especially, a thinfilm of DPPZ-IDD with the lowest LUMO energy level exhibitedan ultranarrow optical band gap close to 1 eV. Their generatedexcited state emitted fluorescence in visible to NIR regions in var-ious environments. Although further improvements for higherPLQY are required, enhanced fluorescence in the aggregationstate would have potential for use in theranostics.Supporting InformationThe authors have cited additional references within the Support-ing Information.[S1–S9]AcknowledgmentsThis work was supported by the International Cooperative Grad-uate Program (ICGP) provided by National Institute for MaterialsScience (NIMS). This work was supported by the World Pre-mier International Research Center Initiative (WPI), MEXT, Japan.The authors thank the Czech Science Foundation Grant No.24–10479S for the financial support. Computational resourceswere supplied by the project “e-Infrastruktura CZ” (e-INFRACZLM2018140) supported by the Ministry of Education, Youth,and Sports of the Czech Republic. The authors thank the NIMSopen facility for access to NIR spectrophotometer.Open access publishing facilitated by Univerzita Pardubice, aspart of the Wiley - CzechELib agreement.Conflict of InterestsThe authors declare no conflict of interest.Data Availability StatementThe data that support the findings of this study are availablefrom the corresponding author upon reasonable request.Keywords: chromophore • dihydrophenazines • merocyanine •narrow bandgap • NIR emission[1] a) N. Zhao, J. Wang, Renewable Sustainable Energy Rev. 2024, 196, 114368;b) Y. Zhang, Z. Xie, Y. Ma, CCS Chem. 2025, 7, 10.[2] W. Liu, X. Xu, J. Yuan, M. Leclerc, Y. Zou, Y. Li, ACS Energy Lett. 2021, 6,598.[3] a) J. Lee, S. Song, J. Huang, Z. Du, H. Lee, Z. Zhu, S.-J. Ko, T.-Q. Nguyen,J. Y. Kim, K. Cho, G. C. 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elax Tuning NIR Absorption and Emission of Diphenyl-Dihydrophenazine-Based Merocyanines with Ultra Narrow Band Gap 1. Introduction 2. Results and Discussion 3. Experimental 3.1. Synthesis and Characterization 4. Conclusion Supporting Information Acknowledgments Conflict of Interests Data Availability Statement