Excitation Of High-spin States Research Articles

Energy conversion and storage via photoinduced polarization change in non-ferroelectric molecular [CoGa] crystals

To alleviate the energy and environmental crisis, in the last decades, energy harvesting by utilizing optical control has emerged as a promising solution. Here we report a polar crystal that exhibits photoenergy conversion and energy storage upon light irradiation. The polar crystal consists of dinuclear [CoGa] molecules, which are oriented in a uniform direction inside the crystal lattice. Irradiation with green light induces a directional intramolecular electron transfer from the ligand to a low-spin CoIII centre, and the resultant light-induced high-spin CoII excited state is trapped at low temperature, realizing energy storage. Additionally, electric current release is observed during relaxation from the trapped light-induced metastable state to the ground state, because the intramolecular electron transfer in the relaxation process is accompanied with macroscopic polarization switching at the single-crystal level. It demonstrates that energy storage and conversion to electrical energy is realized in the [CoGa] crystals, which is different from typical polar pyroelectric compounds that exhibit the conversion of thermal energy into electricity.

Near-Infrared Absorption Features of Triplet-Pair States Assigned by Photoinduced-Absorption-Detected Magnetic Resonance.

Singlet fission proceeds through a manifold of triplet-pair states that are exceedingly difficult to distinguish spectroscopically. Here, we introduce a new implementation of photoinduced-absorption-detected magnetic resonance (PADMR) and use it to understand the excited-state absorption spectrum of a tri-2-pentylsilylethynyl pentadithiophene (TSPS-PDT) film. These experiments allow us to directly correlate magnetic transitions driven by RF with electronic transitions in the visible and near-infrared spectrum with high sensitivity. We find that the new near-infrared excited-state transitions that arise in thin films of TSPS-PDT are correlated with the magnetic transitions of T1, not 5TT. Thus, we assign these features to the excited-state absorption of 1TT, which is depleted when T1 states are driven to a spin configuration that forbids subsequent fusion. These results clarify the disputed origin of triplet-associated near-infrared absorption features in singlet-fission materials and demonstrate an incisive general purpose tool for studying the evolution of high-spin excited states.

Defying the inverse energy gap law: a vacuum-evaporable Fe(ii) low-spin complex with a long-lived LIESST state.

The novel vacuum-evaporable complex [Fe(pypypyr)2] (pypypyr = bipyridyl pyrrolide) was synthesised and analysed as bulk material and as a thin film. In both cases, the compound is in its low-spin state up to temperatures of at least 510 K. Thus, it is conventionally considered a pure low-spin compound. According to the inverse energy gap law, the half time of the light-induced excited high-spin state of such compounds at temperatures approaching 0 K is expected to be in the regime of micro- or nanoseconds. In contrast to these expectations, the light-induced high-spin state of the title compound has a half time of several hours. We attribute this behaviour to a large structural difference between the two spin states along with four distinct distortion coordinates associated with the spin transition. This leads to a breakdown of single-mode behaviour and thus drastically decreases the relaxation rate of the metastable high-spin state. These unprecedented properties open up new strategies for the development of compounds showing light-induced excited spin state trapping (LIESST) at high temperatures, potentially around room temperature, which is relevant for applications in molecular spintronics, sensors, displays and the like.

π-Pimer, π-Dimer, π-Trimer, and 1D π-Stacks in a Series of Benzene Triimide Radical Anions: Substituent-Modulated π Interactions and Physical Properties in Crystalline State

π-Pimer, π-Dimer, π-Trimer, and 1D π-Stacks in a Series of Benzene Triimide Radical Anions: Substituent-Modulated π Interactions and Physical Properties in Crystalline State

Solid-state NMR studies of metal ion and solvent influences upon the flexible metal-organic framework DUT-8

Within the present contribution, we describe solid-state NMR spectroscopic studies of the paddle wheel unit in the prototypic flexible MOF compound DUT-8(M) (M = Ni, Co, Zn). The 13C NMR chemical shift of these carboxylates shows a remarkable behavior. The pure 2,6-H2ndc linker carboxylates as well as DUT-8(Zn) exhibit a13C chemical shift of only about 170 ppm. In contrast, much higher values are observed for DUT-8(Ni) and especially DUT-8(Co). In the open pore state, the shift strongly depends on the solvent polarity in these two latter cases. The present contribution elucidates the reason for this solvent influence. It is concluded that the solvent mainly modifies the isotropic Fermi contact coupling constant for the excited high-spin states in DUT-8(Ni) and DUT-8(Co).

Chiral control of spin-crossover dynamics in Fe(II) complexes.

Iron-based spin-crossover complexes hold tremendous promise as multifunctional switches in molecular devices. However, real-world technological applications require the excited high-spin state to be kinetically stable-a feature that has been achieved only at cryogenic temperatures. Here we demonstrate high-spin-state trapping by controlling the chiral configuration of the prototypical iron(II)tris(4,4'-dimethyl-2,2'-bipyridine) in solution, associated for stereocontrol with the enantiopure Δ- or Λ-enantiomer of tris(3,4,5,6-tetrachlorobenzene-1,2-diolato-κ2O1,O2)phosphorus(V) (P(O2C6Cl4)3- or TRISPHAT) anions. We characterize the high-spin-state relaxation using broadband ultrafast circular dichroism spectroscopy in the deep ultraviolet in combination with transient absorption and anisotropy measurements. We find that the high-spin-state decay is accompanied by ultrafast changes of its optical activity, reflecting the coupling to a symmetry-breaking torsional twisting mode, contrary to the commonly assumed picture. The diastereoselective ion pairing suppresses the vibrational population of the identified reaction coordinate, thereby achieving a fourfold increase of the high-spin-state lifetime. More generally, our results motivate the synthetic control of the torsional modes of iron(II) complexes as a complementary route to manipulate their spin-crossover dynamics.

Weak Antiferromagnetic Exchange and Ferromagnetic Alignment of FeII (S=2) Spins in Differently Charged {HAT ⋅ (FeII Cl2 )3 }n (n=2- and 3-) Assemblies of Hexaazatriphenylenes (HAT).

Hexaazatrianthracene (HATA) and hexaazatriphenylenehexacarbonitrile {HAT(CN)6 } are reduced by metallic iron in the presence of crystal violet (CV+ )(Cl- ). Anionic ligands are produced, which simultaneously coordinate three FeII Cl2 to form (CV+ )2 {HATA ⋅ (FeII Cl2 )3 }2- ⋅ 3 C6 H4 Cl2 (1) and (CV+ )3 {HAT(CN)6. (FeII Cl2 )3 }3- ⋅ 0.5CVCl ⋅ 2.5 C6 H4 Cl2 (2). High-spin (S=2) FeII atoms in both structures are arranged in equilateral triangles at a distance of 7 Å. An antiferromagnetic exchange is observed between FeII in {HATA ⋅ (FeII Cl2 )3 }2- (1) with a Weiss temperature (Θ) of -80 K, the PHI estimated exchange interaction (J) is -4.7 cm-1 . The {HAT(CN)6 ⋅ (FeII Cl2 )3 }3- assembly is obtained in 2. The formation of HAT(CN)6 .3- is supported by the appearance of an intense EPR signal with g=2.0037. The magnetic behavior of 2 is described by a strong antiferromagnetic coupling between the FeII and HAT(CN)6 .3- spins with J1 =-164 cm-1 (-2 J formalism) and by a weaker antiferromagnetic coupling between the FeII spins with J2 =-15.4 cm-1 . The stronger coupling results in the spins of the three FeII Cl2 units to be aligned parallel to each other in the assembly. As a result, an increase of the χM T values is observed with the decrease of temperature from 9.82 at 300 K up to 15.06 emu ⋅ K/mol at 6 K, and the Weiss temperature is also positive being at +23 K. Thus, a change in the charge and spin state of the HAT-type ligand to ⋅3- results in ferromagnetic alignment of the FeII spins, yielding a high-spin (S=11/2) system. DFT calculations showed that, due to the high symmetry and nearly degenerated LUMO of both HATA and HAT(CN)6 , their complexes with FeII Cl2 have a variety of closely lying excited high-spin states with multiplicity up to S=15/2.

Double polarisation observable [formula omitted] for single pion photoproduction from the proton

We report measurements of π+ and π0 meson photoproduction from longitudinally spin-polarised protons by an energy tagged (0.73-2.3 GeV) and linearly polarised photon beam. A close to complete solid angle coverage for the reaction products was provided by the CEBAF Large Acceptance Spectrometer at Jefferson Laboratory. The double-polarisation observable G is extracted from Maximum Likelihood fits to the data, enabling the first accurate determination for the reaction γ→p→→π+n, while also significantly extending the kinematic coverage for γ→p→→π0p. This large data set provides an important constraint on the properties and spectrum of excited nucleon states decaying to Nπ in the mass range from 1.4 to 2.2 GeV, as well as for background (non-resonant) photoproduction processes. The considerable improvement achieved in the description of the observable G within the SAID and Bonn-Gatchina approaches after implementation of our data, illustrates that the partial-wave analyses now significantly extend the knowledge on Nπ photoproduction amplitudes at W>1.8 GeV. A partial-wave analysis using the new high-precision data set has a large impact on the extracted properties of high-spin nucleon excited states.

Influence of the heme distal pocket on nitrite binding orientation and reactivity in Sperm Whale myoglobin.

Nitrite binding to recombinant wild-type Sperm Whale myoglobin (SWMb) was studied using a combination of spectroscopic methods including room-temperature magnetic circular dichroism. These revealed that the reactive species is free nitrous acid and the product of the reaction contains a nitrite ion bound to the ferric heme iron in the nitrito- (O-bound) orientation. This exists in a thermal equilibrium with a low-spin ground state and a high-spin excited state and is spectroscopically distinct from the purely low-spin nitro- (N-bound) species observed in the H64V SWMb variant. Substitution of the proximal heme ligand, histidine-93, with lysine yields a novel form of myoglobin (H93K) with enhanced reactivity towards nitrite. The nitrito-mode of binding to the ferric heme iron is retained in the H93K variant again as a thermal equilibrium of spin-states. This proximal substitution influences the heme distal pocket causing the pKa of the alkaline transition to be lowered relative to wild-type SWMb. This change in the environment of the distal pocket coupled with nitrito-binding is the most likely explanation for the 8-fold increase in the rate of nitrite reduction by H93K relative to WT SWMb.

Interesting spin state properties of iron(II) polypyridine complex substituted by fluorine: A theoretical study

The special iron(II) polypyridyl complex substituted by fluorine ([Fe(dftpy)2]2+, dftpy = 6,6″-difluoro-2,2’; 6′2″-terpyridine) with uncommon mixed ground states has been detected recently, but the related explanation and characterization are not enough, especially for the influence of substituents. On the basis of previous studies, the key problem of mixed spin states is expected to be described very well by means of theoretical calculations. In this work, density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations have been carried out to study the characteristics of the excited states in detail. Comparing with the parent complex [Fe(tpy)2]2+ (tpy = 2,2′:6′,2″-terpyridine) and corresponding bromine or chlorine substituted derivates, the ground states of [Fe(dftpy)2]2+ stay around the mixture of singlet state and quintet state, but also there is rare high spin excited state lifetime. Among the halogen substituted complexes, [Fe(dftpy)2]2+ has the shortest MLCT state lifetime of 14.0 ps which has been much longer than subpicosecond lifetime of [Fe(tpy)2]2+. The reason is explored by the combination of electronic structures, absorption properties, extended transition state coupled with natural orbitals for chemical valence (ETS-NOCV) and potential energy curves (PECs). We can find that the bond lengths of Fe-Nt play a significant role on the change of metal centered (MC) ground states. With Fe-Nt extended by fluorine atoms, the quintet state becomes lower than the singlet state. Due to the deformation of structures, the interactions between metal and ligands diminish and give rise to weaker d orbital splitting than that of [Fe(tpy)2]2+, but slightly impact the pairwise orbital deformation density characteristics. And the PEC of 5MC intersects with 1,3MC which renders faster non-radiative deactivation through low-lying energy crossing points.

(Invited) Luminescent Organic Solids with Unusual Exciton Multiplicity

High quantum yield luminescent organic solids are required for a number of optoelectronic applications, including organic light-emitting diodes (OLEDs) and transistors (OLETs), lasers, etc. The absolute majority of organic materials can only emit efficiently from their singlet excited states. Involving high-multiplicity states (such as triplets) in exciton dynamics has several established and potential benefits; it is required to overcome 25% theoretical efficiency in OLEDs and might enable various magneto-optical devices. We will report our recent findings of (i) purely organic (C, H, N, O only) room temperature phosphors[1] and (ii) stable free radical fluorophores with record-breaking efficiency of triplet and singlet emission, respectively. The lecture will discuss the molecular and supramolecular mechanisms in modulating the emission from high-spin excited states in organic solids.[1] E. Hamzehpoor, D. F. Perepichka, Crystal Engineering of Room Temperature Phosphorescence in Organic Solids, Angew. Chem. Int. Ed. 2019, DOI: 10.1002/anie.201913393

Observation of Proton Transfer Coupled Spin Transition and Trapping of Photoinduced Metastable Proton Transfer State in an Fe(II) Complex.

An important technique to realize novel electron- and/or proton-based functionalities is to use a proton-electron coupling mechanism. When either a proton or electron is excited, the other one is modulated, producing synergistic functions. However, although compounds with proton-coupled electron transfer have been synthesized, crystalline molecular compounds that exhibit proton-transfer-coupled spin-transition (PCST) behavior have not been reported. Here, we report the first example of a PCST Fe(II) complex, wherein the proton lies on the N of hydrazone and pyridine moieties in the ligand at high-spin and low-spin Fe(II), respectively. When the Fe(II) complex is irradiated with light, intramolecular proton transfer occurs from pyridine to hydrazone in conjunction with the photoinduced spin transition via the PCST mechanism. Because the light-induced excited high-spin state is trapped at low temperatures in the Fe(II) complex-a phenomenon known as the light-induced excited-spin-state trapping effect-the light-induced proton-transfer state, wherein the proton lies on the N of hydrazone, is also trapped as a metastable state. The proton transfer was accomplished within 50 ps at 190 K. The bistable nature of the proton position, where the position can be switched by light irradiation, is useful for modulating proton-based functionalities in molecular devices.

Theoretical study on the excited state decay properties of iron(ii) polypyridine complexes substituted by bromine and chlorine.

Transition metal iron(ii) polypyridyl complexes with quintet ground states were deeply investigated by density functional theory (DFT) and time-dependent density functional theory (TDDFT). Compared with the parent complex [Fe(tpy)2]2+ (tpy = 2,2′:6′,2′′-terpyridine), the ground states of the complexes substituted by halogen atoms changed from singlet states to quintet states with rare high spin excited state lifetimes. The substituted complex [Fe(dbtpy)2]2+ (1) results in a high spin metal–ligand charge transfer lifetime of 17.4 ps, which is 1.4 ps longer than that of [Fe(dctpy)2]2+ (2) with the substitution of chlorine atoms. The reason for this is explored by a combination of electronic structures, absorption spectra, extended transition state coupled with natural orbitals for chemical valence (ETS-NOCV) studies and potential energy curves (PECs). The distortion of 1 in the angles and dihedrals of the ligands is slightly larger than that in 2, although the average metal–ligand bond lengths of the latter are larger. The twisted octahedron decreases the interactions between the d orbitals of iron(ii) and the n/π orbitals of the ligands. Compared with 2, the enlarged energy gaps among the different PECs of 1 and the increased energy crossing points caused by the larger distortion result in the increase of its excited state lifetime. The different pairwise orbital interaction contributions between the metal center and the ligands in their singlet states are qualitatively estimated by ETS-NOCV. The results show that the substitution of bromine atoms will decrease the electrostatic attraction between the metal and ligands but not significantly impact the orbital interactions.

Slow spin crossover in bis-meridional Fe2+ complexes through spin-state auto-adaptive N6/N8 coordination.

Fe2+ spin crossover (SCO) complexes with long-lived excited high-spin (HS) states are promising molecular switches. An enhanced kinetic stability of spin-state isomers can be expected to foster applications beyond the limits of cooperative SCO. In this study, we describe a new approach to slow down the spin-state exchange by simple commutation of a phenyl substituent by a pyridyl substituent. To this end, N4 ligand 6-(3-(pyridin-2-yl)-1H-pyrazol-1-yl)-2,2'-bipyridine (3b) is synthesized as an N4 homologue of the well-established meridional N3 ligands motif. Phenyl-substituted 6-(3-phenyl-1H-pyrazol-1-yl)-2,2'-bipyridine (3a) serves as an intrinsic N3 reference throughout. 3b offers variable coordination numbers, N3 versus N3(+1) and N4, reflecting the preferences of the metal center. As is shown herein through an extended solid-state structure-chemical and solution-state NMR study, which is augmented by density-functional theory modeling, both the coordination geometry and its structural dynamics are indeed highly sensitive towards the expansion of the nominal donor number. The additional donors in 3b introduced through the phenyl-pyridine commutation actually give rise to a rich and diverse stereochemistry of the derived Zn2+ and Fe2+ complexes. Notably, even within a single complex unit coordination of 3b ranges from strongly distorted N3 coordination with a long assisting additional contact (Zn2+ and Fe2+) to a more symmetric N2(+2) or N4 situation in Fe2+. DFT modeling unravels that the additional donors are hemi-labile and coordinate to the Fe2+ only in HS state, leaving the elusive low-spin (LS) state in a fairly undisturbed octahedral environment with 3b being N3 coordinate. That is, the coordination number of the complex autogeneously responds to the altered spin-state. Necessarily this switch in coordination number requires strong structural changes upon SCO. This leads to increased activation barriers for SCO as could be deduced from a temperature-dependent analysis of the dynamic 1H NMR-line broadening and corroborated by accompanying theoretical analysis of the SCO reaction coordinate. For [Fe(3b)2]2+ long spin-state lifetimes τ > 1 ms prevail below the characteristic temperature T (1 ms) = 235 K; this value should be compared with a lifetime of only 150 ns derived for the close analogue [Fe(3a)2]2+. The principle applied herein is general and allows transferring of LS Fe2+ complexes with suitably placed phenyl substituents into SCO complexes with spin-state adaptive coordination number and hence long-lived HS excited states.

Fluorenyl Based Macrocyclic Polyradicaloids.

Synthesis of stable open-shell polyradicaloids including control of intramolecular spin-spin interactions is a challenging topic in organic chemistry and materials science. Herein, we report the synthesis and physical characterization of two series of fluorenyl based macrocyclic polyradicaloids. In one series (FR-MCn, n = 4-6), the fluorenyl radicals are directly linked at 3,6-positions; whereas in the other series (MC-FnAn, n = 3-5), an additional ethynylene moiety is inserted between the neighboring fluorenyl units. To access stable macrocyclic polyradicaloids, three synthetic methods were developed. All of these stable macrocycles can be purified by normal silica gel column chromatography under ambient conditions. In all cases, moderate polyradical characters were calculated by restricted active space spin-flip method due to the moderate intramolecular antiferromagnetic spin-spin interactions. The excitation energies from the low-spin ground state to the lowest high-spin excited state were evaluated by superconducting quantum interference device measurements. Their physical properties were also compared with the respective linear fluorenyl radical oligomers (FR-n, n = 3-6). It is found that the geometry, i.e., the distortional angle and spacer (w or w/o ethynylene) between the neighboring fluorenyl units, has significant effect on their polyradical character, excitation energy, one-photon absorption, two-photon absorption and electrochemical properties. In addition, the macrocyclic tetramers FR-MC4 and MC-F4A4 showed global antiaromatic character due to cyclic π-conjugation with 36 and 44 π-electrons, respectively.

Photoluminescence properties of the “bent sandwich-like” compounds [Eu(TpiPr2)2] and [Yb(TpiPr2)2] – Intermediates between nitride-based phosphors and metallocenes

The photoluminescence properties of the homoleptic lanthanide(II) coordination compounds [Eu(TpiPr2)2] and [Yb(TpiPr2)2] (TpiPr2=hydro-tris(3,5-diisopropylpyrazolyl)borate) at 10K and room temperature are presented and their relationship to the structure is discussed in detail. Both compounds are characterized by broad-banded luminescence which is typical for the 5d↔4f transitions of divalent lanthanide ions. [Eu(TpiPr2)2] shows a remarkably bright greenish emission located at around 550nm (18,180cm-1) with a very small effective full width at half maximum (FWHM) (54nm or 1860cm-1 at 10K; 70nm or 1910cm-1 at 300K) and a correspondingly small Stokes shift (2150cm-1 at 10K; 2830cm-1 at 300K). These results are related to the huge rigidity of the complex within the crystal structure. The decay time 0f 0.86µs is also typical for Eu2+-based coordination compounds. [Yb(TpiPr2)2] shows red emission with an asymmetric band peaking at around 642nm (15,580cm-1) and a similarly small Stokes shift (1540cm-1 at 10K; 2260cm-1 at 300K) and effective FWHM (50nm or 1240cm-1 at 10K; 162nm or 3010cm-1 at 300K). The long decay time of 0.12ms at 10K indicates the sole presence of a spin-forbidden emission, which is explained by interactions of the excited electronic states with highly energetic vibrations of the TpiPr2 ligands. This enhances non-radiative relaxation from the excited low spin to the lowest excited high spin state of Yb2+. Moreover, the effect of the vibronic coupling on the luminescence quenching in [Yb(TpiPr2)2] is discussed. The optical properties nicely illustrate that the presented scorpionate complexes are an intermediate case between metallocenes and lanthanide-activated nitrides that are also in current use for LED applications.

Magnetism in pristine and chemically reduced graphene oxide

The evolution of magnetism for graphene oxide (GO) before and after chemical reduction was investigated by means of static magnetization and electron spin resonance (ESR) spectroscopy. Strong paramagnetism with a saturation magnetization of ∼1.2 emu/g and weak antiferromagnetic interactions were identified in pristine GO. Apart from spin-half defect centers, ESR spectroscopy indicated the excitation of high spin states, consistently with the high spin (S = 2) magnetic moments derived from the magnetization analysis, corroborating the formation of spatially “isolated” magnetic clusters in GO. A marked reduction of GO's magnetization (∼0.17 emu/g) along with an appreciable rise of diamagnetism (−2.4 × 10−6 emu/g Oe) was detected after chemical reduction by sodium borohydride, reflecting the drastic removal of paramagnetic defects and the concomitant growth of sp2 domains in reduced graphene oxide (rGO). ESR revealed a large drop of the spin susceptibility for rGO, which, in addition to the main paramagnetic Curie component, showed an appreciable Pauli contribution. The latter together with the g-factor shift and the broadening of the ESR line indicated the coupling of localized spins with conduction electrons. The rGO ESR signal presented a metallic line shape, which could be analyzed in terms of two separate spectral components, a broad one that may be related to defect states strongly coupled with itinerant spins within the sp2 clusters and a narrow one due to edge/vacancy defect spins, indicative of rGO's persistent structural inhomogeneity.

Anomalies of the electronic structure and physical properties of rare-earth cobaltites near spin crossover

The features of the characteristics of LnCoO3 cobaltites, where Ln is a rare-earth element, are discussed. Both experiment and theory demonstrate that their essentials are related to the low-spin ground state of cobalt ions. The thermally induced occupation of the excited high-spin state gives rise to peaks in the magnetic susceptibility, specific heat, and thermal expansion, as well as to a smooth insulator–metal transition. The analysis is based both on the data from the current literature concerning LaCoO3 and in many aspects on our own studies of GdCoO3 and La1−x Gd x CoO3 solid solutions.

The mutable nature of particle-core excitations with spin in the one-valence-proton nucleus 133 Sb

The gamma-ray decay of excited states of the one-valence-proton nucleus 133Sb has been studied using cold-neutron induced fission of 235U and 241Pu targets, during the EXILL campaign at the ILL reactor in Grenoble. By using a highly efficient HPGe array, coincidences between gamma-rays prompt with the fission event and those delayed up to several tens of microseconds were investigated, allowing to observe, for the first time, high-spin excited states above the 16.6 micros isomer. Lifetimes analysis, performed by fast-timing techniques with LaBr3(Ce) scintillators, reveals a difference of almost two orders of magnitude in B(M1) strength for transitions between positive-parity medium-spin yrast states. The data are interpreted by a newly developed microscopic model which takes into account couplings between core excitations (both collective and non-collective) of the doubly magic nucleus 132Sn and the valence proton, using the Skyrme effective interaction in a consistent way. The results point to a fast change in the nature of particle-core excitations with increasing spin.

Fully Fused Quinoidal/Aromatic Carbazole Macrocycles with Poly-radical Characters.

While the chemistry of open-shell singlet diradicaloids has been successfully developed in recent years, the synthesis of π-conjugated systems with poly-radical characters (i.e., beyond diradical) in the singlet ground state has been mostly unsuccessful. In this study, we report the synthesis and isolation of two fully fused macrocycles containing four (4MC) and six (6MC) alternatingly arranged quinoidal/aromatic carbazole units. Ab initio electronic structure calculations and various experimental measurements indicate that both 4MC and 6MC have an open-shell singlet ground state with moderate tetraradical and hexaradical characters, respectively. Both compounds can be thermally populated to high-spin excited states, resulting in weak magnetization at room temperature. Our study represents the first demonstration of singlet π-conjugated molecules with poly-radical characters and also gives some insights into molecular magnetism in neutral π-conjugated polycyclic heteroarenes.