Strong Jahn-Teller Distortion Research Articles

Deciphering the electrocatalysis essence of cobalt diselenide in lithium-sulfur electrochemistry from crystal-phase engineering

Polysulfide shuttle effect and sluggish sulfur reaction kinetics severely impede the cycling stability and sulfur utilization of lithium-sulfur (Li-S) batteries. Precisely designing effective electrocatalysts to accelerate sulfur conversion kinetics and suppress polysulfide migration is urgently needed. Generally, electrocatalytic activity is highly dependent on the electronic structures of catalysts, while the crystal phases play a pivotal role in governing the electronic structures. However, the “crystal phase - electronic structure - catalytic capability” relation chain of Li-S batteries catalysts usually lacks clear elucidation. Herein, crystal-phase engineering is reported by regulating phases of CoSe2 to enhance the electrochemical performance of Li-S batteries. Experimental investigations and atomic level analyses reveal that the orthorhombic CoSe2 can trap and accelerate the conversion of sulfur species more effectively than cubic one. Electronic structure analysis further demonstrates that the superior electrocatalytic activity is originated from the strong Jahn-Teller distortion. As a result, the Li-S batteries with orthorhombic CoSe2 modified separator exhibit a high initial capacity of 1390.7 mAh g−1 at 0.1C and outstanding rate performance with a specific capacity of 738.4 mAh g−1 at 3C. Moreover, even at a high sulfur loading of 10.09 mg cm−2, a favorable initial areal capacity of 12.0 mAh cm−2 is achieved at 0.05C, and the battery still maintains at 7.9 mAh cm−2 over 50 cycles under 0.1C. This work may provide a new strategy for effective catalyst design in Li-S batteries.

Peculiarities of the local structure in new medium- and high-entropy, low-symmetry tungstates

New monoclinic (P2/c) tungstates – a medium-entropy tungstate, (MnNiCuZn)WO4, and a high-entropy tungstate, (MnCoNiCuZn)WO4 – were synthesized and characterized. Their phase purity and solid solution nature were confirmed by powder X-ray diffraction and Raman spectroscopy. X-ray absorption spectroscopy was used to probe the local structure around metal cations. The atomic structures based on the ideal solid solution model were optimized by a simultaneous analysis of the extended X-ray absorption fine structure spectra at multiple metal absorption edges – five for (MnNiCuZn)WO4 and six for (MnCoNiCuZn)WO4 – by means of reverse Monte Carlo simulations. In both compounds, Ni2+ ions have the strongest tendency to organize their local environment and form slightly distorted [NiO6] octahedra, whereas Mn2+, Co2+, and Zn2+ ions have a strongly distorted octahedral coordination. The most intriguing result is that the shape of [CuO6] octahedra in (MnNiCuZn)WO4 and (MnCoNiCuZn)WO4 differs from that found in pure CuWO4, where a strong Jahn–Teller distortion is present: [CuO6] octahedra become more regular with increasing degree of dilution.

Ground-state structure, orbital ordering and metal-insulator transition in double-perovskite PrBaMn2O6

In recent years, A-site ordered half-doped double-perovskite manganites $\rm RBaMn_2O_6$ (R=rare earth) have attracted much attention due to their remarkable physical properties and a prospect of application as magnetoresistance, multiferroic, and oxygen storage materials. The nature of the ground state in ${\rm RBaMn_2O_6}$ as well as sequence of phase transitions taking place at cooling are not yet well understood due to complexity in both experimental and theoretical studies. Here we address the origin of the ground-state structure in PrBaMn$_2$O$_6$ as well as its electronic and magnetic properties. Utilizing GGA+U approach and specially designed strategy to perform structural optimization, we show that the system has two competing AFM-A and AFM-CE magnetic structures with very close energies. The AFM-A structure is a metal, while AFM-CE is an insulator and the transition to the insulating state is accompanied by the charge Mn$^{3+}$/Mn$^{4+}$, and orbital $3x^2-r^2$/$3y^2-r^2$ orderings. This orbital ordering results in strong cooperative Jahn-Teller (JT) distortions, which lower the crystal symmetry. Our findings give a key to understanding contradictions in available experimental data on ${\rm PrBaMn_2O_6}$ and opens up the prospects to theoretical refinements of ground-state structures in other ${\rm RBaMn_2O_6}$ compounds.

Electronic and magnetic properties of quasi-one-dimensional osmium halide OsCl4

Using ab initio density functional theory, we study the electronic and magnetic properties of the van der Waals chain material OsCl4. In the nonmagnetic state, a strongly anisotropic band structure was observed, in agreement with its anticipated one-dimensional crystal geometry. Based on Wannier functions, we found that the four electrons of the 5d Os atom form a low-spin S = 1 state, with a large crystal field between the dxz/yz and dxy orbitals, corresponding to a strong Jahn–Teller distortion (Q3< 0). As a consequence, the magnetic properties are mainly contributed by the dxz/yz states. Furthermore, when a Mott gap develops after the introduction of the Hubbard U and Hund coupling J, we found that the staggered spin order is the most likely magnetic state, namely, spins arranged as (↑-↓-↑-↓) with π wavevector along the chain. In addition, the energy differences between various spin states are small, suggesting a weak magnetic exchange coupling along the chain. Our results provide guidance to experimentalists and theorists working on quasi-one-dimensional osmium halides chain materials.

Effective tuning of magnetic anisotropy in distorted pentagonal bipyramidal Ni(II) complexes via substitution of axial coligands.

A series of Ni(II) complexes with pyridine-based macrocyclic ligand L (3,12,18-triaza-6,9-dioxabicyclo[12.3.1]octadeca-1(18),14,16-triene) with general formula [Ni(L)(X)2]0/2+ (X = Br- (1), I- (2), CH3CN (3), NCS- (4), imidazole (5)) was prepared and thoroughly investigated. X-ray molecular structures confirmed pentagonal bipyramidal geometry for all studied complexes with a strong Jahn-Teller distortion in the pentagonal equatorial plane and significantly elongated Ni-O distance(s) with a decrease of this distortion by varying axial coligands (CH3CN > Br- > I- > NCS- > imidazole). Direct current magnetic measurements revealed the easy-axis type of magnetic anisotropy with negative as well as positive axial zero-field-splitting parameter D ranging from +6.8 to -14.5 cm-1, which remains not affected in the halogenido series Cl- → Br- → I-, but which increases in the series with N-axial ligands in order CH3CN → NCS- → imidazole. Theoretical calculations helped to elucidate (i) the final coordination numbers 6 + 1 for 1 and 2, and 5 + 2 for 2-5, (ii) the pattern of splitting of d-orbitals, contributions of excited states to the final D-values and their final signs, and (iii) the complexity in the variation of the D and E parameters with elongation of axial bond distances in such strongly distorted systems. The studied complexes did not show any alternating magnetic susceptibility signal, but it was clearly documented that the magnetic anisotropy of the pentagonal bipyramidal Ni(II) complexes can be modulated/tuned by variation of axial coligands. Nevertheless, great care has to be taken for symmetry of the equatorial ligand field.

In celebration of the 70thbirthday of Peter Klüfers

In celebration of the 70^thbirthday of Peter Klüfers

Octahedral distortion enhances exceptional oxygen catalytic activity of calcium manganite for advanced Zn-Air batteries

Calcium manganite (CMO) holds great potential for OER/ORR catalysis. But, the weak interaction between the oxygen intermediates and reaction sites leads to sluggish catalytic kinetics. Herein, to trigger the exceptional catalytic activity of CMO, we propose a thermal-induced MnO 6 octahedral distortion strategy to advance the absorption capability of intermediate reactants. Refined structural analysis and theoretical calculation reveal that the strong Jahn-Teller distortion of MnO 6 is capable of optimizing the surface electron redistribution and accelerating the electron transfer between the oxygen and Mn sites, which significantly boosts the intrinsic oxygen catalytic activities of the distorted CMO (D-CMO). Notably, the D-CMO functions as excellent bifunctional oxygen electrocatalysts and enables a high-performance solid-state Zn-air battery with satisfactory mechanical strength. Specifically, the as-assembled battery displays a remarkable open circuit voltage of 1.46 V and high peak power density of 149 mW cm −2 , even outstripping noble-metal-based counterparts. Modulating the octahedral units to ignite the intrinsic oxygen catalytic activity provides enlightening clues to design perovskite-type air cathodes. • A thermal-induced MnO 6 octahedral distortion strategy is proposed. • The octahedral distortion can optimize electron density and e g electron-filling states. • The octahedron-distortion CaMnO 3 (D-CMO) owns excellent oxygen catalytic activity. • The solid-state ZAB exhibits a high peak power density of 149 mW cm −2 .

Computational studies of the electronic structure of copper-doped ZnO quantum dots

Copper-doped ZnO quantum dots (QDs) have attracted substantial interest. The electronic structure and optical and magnetic properties of Cu3+(d8)-, Cu2+(d9)-, and Cu+(d10)-doped ZnO QDs with sizes up to 1.5 nm are investigated using the GGA+U approximation, with the +U corrections applied to d (Zn), p(O), and d(Cu) orbitals. Taking +Us parameters, as optimized in previous bulk calculations, we obtain the correct band structure of ZnO QDs. Both the description of electronic structure and thermodynamic charge state transitions of Cu in ZnO QDs agree with the results of bulk calculations due to the strong localization of Cu defect energy levels. Atomic displacements around Cu are induced by strong Jahn-Teller distortion and affect Kohn-Sham energies and thermodynamic transition levels. The average bond length of Cu-O and the defect structure are crucial factors influencing the electronic properties of Cu in ZnO QDs. The analysis of the optical properties of Cu in ZnO QDs is reported. The GGA+U results, compared with the available experimental data, support Dingle's model [Phys. Rev. Lett. 23, 579 (1969)], in which the structured green luminescence observed in bulk and nanocrystals originates from the [(Cu+, hole) → Cu2+] transition. We also examine the magnetic interaction between the copper pair for two charge states: 0 and +2, and four positions relative to the center of QDs. Ferromagnetic interaction between ions is obtained for every investigated configuration. The magnitude of ferromagnetism increases for positive charge defects due to the strong hybridization of the d(Cu) and p(O) states.

Pressure-induced transition from Jeff=1/2 to S=1/2 states in CuAl2O4

The spin-orbit entangled (SOE) Jeff-state has been a fertile ground to study novel quantum phenomena. Contrary to the conventional weakly correlated Jeff=1/2 state of 4d and 5d transition metal compounds, the ground state of CuAl2O4 hosts a Jeff=1/2 state with a strong correlation of Coulomb U. Here, we report that surprisingly Cu2+ ions of CuAl2O4 overcome the otherwise usually strong Jahn-Teller distortion and instead stabilize the SOE state, although the cuprate has relatively small spin-orbit coupling. From the x-ray absorption spectroscopy and high-pressure x-ray diffraction studies, we obtained definite evidence of the Jeff=1/2 state with a cubic lattice at ambient pressure. We also found the pressure-induced structural transition to a compressed tetragonal lattice consisting of the spin-only S=1/2 state for pressure higher than Pc=8 GPa. This phase transition from the Mott insulating Jeff=1/2 to the S=1/2 states is a unique phenomenon and has not been reported before. Our study offers a rare example of the SOE Jeff-state under strong electron correlation and its pressure-induced transition to the S=1/2 state.

Local polarization in oxygen-deficient LaMnO3 induced by charge localization in the Jahn-Teller distorted structure

The functional properties of transition metal perovskite oxides are known to result from a complex interplay of magnetism, polarization, strain, and stoichiometry. Here, we show that for materials with a cooperative Jahn-Teller distortion, such as LaMnO$_3$ (LMO), the orbital order can also couple to the defect chemistry and induce novel material properties. At low temperatures, LMO exhibits a strong Jahn-Teller distortion that splits the $e_g$ orbitals of the high-spin Mn$^{3+}$ ions and leads to alternating long, short, and intermediate Mn--O bonds. Our DFT+$U$ calculations show that, as a result of this orbital order, the charge localization in LMO upon oxygen vacancy formation differs from other manganites, like SrMnO$_3$, where the two extra electrons reduce the two Mn sites adjacent to the vacancy. In LMO, relaxations around the defect depend on which type of Mn--O bond is broken, affecting the $d$-orbital energies and leading to asymmetric and hence polar excess-electron localization with respect to the vacancy. Moreover, we show that the Mn--O bond lengths, orbital order and consequently the charge localization and polarity are tunable via strain.

A new mineral species zincobradaczekite, NaCuCuZn2(AsO4)3, and a new isomorphous series bradaczekite–zincobradaczekite in the alluaudite group

A new alluaudite-group mineral zincobradaczekite, a Zn–Cu ordered highly zincian analogue of bradaczekite, was discovered in sublimates of the Yadovitaya fumarole at the Second scoria cone of the Northern Breakthrough of the Great Tolbachik Fissure Eruption, Tolbachik volcano, Kamchatka, Russia. It is associated with bradaczekite, lammerite, lammerite-β, borisenkoite, mcbirneyite, sanidine, hematite, tenorite, pseudolyonsite, lyonsite, starovaite, rutile, tripuhyite, pseudobrookite, piypite, langbeinite, calciolangbeinite, aphthitalite, alumoklyuchevskite, and palmierite. Zincobradaczekite forms oblique-angled prismatic crystals up to 0.02 × 0.02 × 0.08 mm3 and equant or tabular crystals up to 0.05 mm. It is transparent, blue, greenish-blue, grey-blue or bluish-grey. The lustre is vitreous. Dcalc is 4.71 g cm−3. Zincobradaczekite is optically biaxial (–), α 1.786(5), β 1.846(8), γ 1.90(1), 2Vmeas. 80(5)o. Chemical composition (wt%, electron microprobe) is Na2O 4.07, K2O 0.53, CaO 0.01, MgO 0.71, MnO 0.01, CuO 19.89, ZnO 24.21, Al2O3 0.02, Fe2O3 0.90, TiO2 0.03, P2O5 0.41, V2O5 1.05, As2O5 47.10, SO3 1.01, and total 99.95. The empirical formula based on 12 O apfu is Na0.90K0.08Cu1.70Mg0.12Zn2.03Fe3+0.08(As2.79S0.09V0.08P0.04)Σ3.00O12. The simplified chemical formula is NaCu2Zn2(AsO4)3. Zincobradaczekite is monoclinic, C2/c, a 12.0375(13), b 12.4500(13), c 7.2213(8) A, β 117.506(7)o, V 959.90(18) A3 and Z 4. The strongest reflections of the powder XRD pattern [d,A(I)(hkl)] are 6.21(31)(020), 3.416(70)(310, − 112), 3.200(17)(002), 2.779(23)(− 402), 2.691(100)(− 132, 240), 1.841(20)(152) and 1.680(14)(− 642). The crystal structure was solved (single-crystal XRD, R = 4.38%). The simplified crystal chemical formula is A(1)□A(1)ʹCuA(2)□A(2)ʹNaM(1)CuM(2)Zn2(AsO4)3 (□—vacancy) and the end-member formula is NaCuCuZn2(AsO4)3. Zincobradaczekite and bradaczekite, ideally NaCuCuCu2(AsO4)3, are isostructural. In the Yadovitaya fumarole, Mg-poor bradaczekite forms an isomorphous series with zincobradaczekite, whilst in the neighbouring Arsenatnaya fumarole only a Zn-depleted and Mg-enriched variety of bradaczekite occurs. Zincobradaczekite and bradaczekite differ from other anhydrous alluaudite-group arsenates by increased values of the c and β unit-cell parameters due to the strong Jahn–Teller distortion of M(1)O6 octahedron predominantly occupied with Cu2+.

Lead-Free Perovskite Variant Solid Solutions Cs2 Sn1- x Tex Cl6 : Bright Luminescence and High Anti-Water Stability.

Underwater lighting is important for the exploration of the underwater world in different areas. It is of great significance for developing underwater emitters with high penetrability, high luminous efficiency, good anti-water stability, and environmental friendliness. Stable lead-free perovskite luminescent materials, represented by vacancy-ordered double perovskites, are worthy of research because they can almost meet the above requirements. Here, lead-free perovskite variant solid solutions with the formula of Cs2 Sn1- x Tex Cl6 are reported. Upon the exchange of Sn/Te ions, strong Jahn-Teller distortion of octahedra occurs in the lattice structure. The combination of Te luminescent center and Jahn-Teller-like self-trapped excitons gives this material yellow-green luminescence with a wavelength of 580 nm and a high photoluminescence quantum yield of 95.4%. Moreover, these solid solutions can withstand the extreme conditions of immersion in water probably due to the formation of amorphous alteration phase. Such good anti-water stability is also supported by the molecule dynamics simulation result that no reaction occurs on the water/Cs2 SnCl6 interface. The high luminous, suitable wavelength, and good anti-water stability enable the solid solutions suitable for the application for underwater lighting.

Strong Jahn-Teller effect at NiO4 tetrahedron in NiCo2O4 spinel

The local structural distortion and electronic structure of the NiCo2O4 spinel are studied from first-principles calculations. It is found that very strong Jahn-Teller (J-T) distortion occurs to the NiO4 tetrahedron and a large energy gain is obtained, leading to a substantial elongation of the lattice and a cubic to tetragonal phase transition. Further analysis shows that the Ni-3d states are split by not only the broken Td symmetry of the NiO4, chemical bonding interactions also exist between half-occupied dyz and dxz orbitals, which further lower the energy levels of the occupied dyz and dxz states in the spin-up channel. As a result, the energy gains from the J-T distortion are obtained to be 0.53 eV, 0.84 eV, and 0.83 eV per NiCo2O4 formula unit for using LDA, PBE and PW91 functionals, respectively.

Preparation and magnetic properties of o-LiMnO2

Pure o-LiMnO2 prepared by solid state reaction under Ar flow at 850 °C crystallizes in an orthorhombic symmetry (SG: Pmnm). Mn3+ with four unpaired electrons is subjected to a strong Jahn-Teller (J-T) distortion, where the MnO6 octahedron is significantly elongated (1.17%) along the c axis. In air, the oxide converts to spinel LiMn2O4 at ∼320 °C. The magnetic susceptibility of LiMnO2, measured between 4.2 and 900 K, indicates a long range order. The negative θP value (−790 K) reveals strong antiferromagnetic interactions between half-filled dz21 orbital. The predominant interaction is negative and occurs between Mn3+ and its next-nearest neighbors via O2− ions by super exchange with Mn3+-O: 2p-Mn3+ angle of 178°. Above 300 K, the susceptibility follows a Curie Weiss law with a magnetic moment of 5.26 μB consistent with the high spin state Mn3+ (t2g3 eg1) but slightly larger than the spin only contribution for the ground state 5Do. The susceptibility is field dependent down to 4.2 K and a spin canting has been detected at low temperature. As magnetic chains are separated by non-magnetic Li ones, the interactions are confined along the [1 1 0] direction and one can expect a good realization of one dimensional model. Therefore, the magnetic data have been fitted on the base of the isotropic model for finite linear chain in terms of intra-chain antiferromagnetic interactions using the Fisher model. The best fitting is obtained for a value of J/k = −60 K. The results are compared with those of the crednerite CuMnO2 with smaller J/k value (−33 K).

Electrochemical Detection of Chloride Ions by Copper (II) Complex with Mixed Ligand of Oxindole Derivative and Dithiocarbamates Moiety

The present work describes the synthesis of a new copper (II) complex with bidentate ligands based on oxindole (indolin-2-one) derivatives, namely: 1H,1′H,1″H-[2,3′:2′,3″-terbenzo[b]pyrrol]-2″(3″H)-one (L1) and [sodium diethyldithiocarbamate (DTC)] (L2) as a second bidentate ligand. The ligand L1 was prepared by the cyclization reaction of oxindole (2-indolone) with phosphorus oxychloride. A mixed-ligand was synthesized using L1 and L2 ligands with copper (Cu (II)) via a simple reflux process. The synthesized mixed Cu (II) complex [C53H44CuN7O4S2 and [Cu(L1)2(L2)]2H2O] exhibited superior solubility in organic solvents like dichloromethane, chloroform, ethanol, methanol, DMF and DMSO. The optical characterizations revealed that the synthesized Cu (II) complex displayed a broad band (2Eg→2T2g) with the absorption at ~420 nm, suggesting a distorted octahedral geometry due to the strong Jahn-Teller distortion of the Cu2+ ion. The elemental analysis confirmed the existence of Cu, C, S, N, and other elements in the synthesized mixed Cu (II) complex. The physicochemical studies of the organic ligand and Cu(II) complex were investigated by TG analysis, NMR, FTIR, SEM, EDX, electronic spectra and cyclic voltammetry measurements. The detection of chloride ions with the prepared mixed Cu(II) complex was studied by cyclic voltammetry measurements at different scan rates.

The new ordered double perovskite SrLaCuIrO6

The new fully rock-salt ordered Ir5+ double perovskite SrLaCuIrO6 was synthesized, and its structural, magnetic and electronic properties were investigated. The compound crystallizes tetragonally (space group I4/m). The Cu2+ cation leads to strong Jahn-Teller distortion of the CuO6 octahedra. The compound shows semiconducting behavior with an estimated band gap of approximately 0.2 eV. At around 10 K a magnetic transition is observed.

Direct Magnetization-Polarization Coupling in BaCuF_{4}.

Herewith, first-principles calculations based on density functional theory are used to describe the ideal magnetization reversal through polarization switching in BaCuF_{4} which, according to our results, could be accomplished close to room temperature. We also show that this ideal coupling is driven by a single soft mode that combines both polarization, and octahedral rotation. The later being directly coupled to the weak ferromagnetism of BaCuF_{4}. This, added to its strong Jahn-Teller distortion and its orbital ordering, makes this material a very appealing prototype for crystals in the ABX_{4} family for multifunctional applications. The described mechanism behaves ideally as it couples the ferroelectric and the magnetic properties naturally and it has not been reported previously.

Vibronic Superexchange in Double Perovskite Electrocatalyst for Efficient Electrocatalytic Oxygen Evolution.

Perovskites are prototype electrocatalyts benefiting from their high terrestrial availability and high stability. Electronic state regulation plays a key role in promising higher electrocatalytic efficiencies. Herein, we highlighted a vibronic superexchange in double perovskite to synergistically optimize eg electron filling state and increase the formation of active species on the surface of catalysts. Vibronic superexchange of Ni3+-O-Mn3+ in La2NiMnO6 nanoparticles brings the optimal eg electron filling state of Mn and Ni ions toward unity. Moreover, a vibronic superexchange interaction of Ni3+-O-Mn3+ induces strong Jahn-Teller distortion of MnO6 and NiO6 octahedra, elongating metal-O bonds, which helps to form the active species of Mn/Ni hydroxide/oxide on the surface of catalysts. Surprisingly, La2NiMnO6 nanoparticles exhibit superior oxygen evolution reaction (OER) catalytic performance with higher current density and lower Tafel slope than its bulk counterpart. Our finding will be a promising pathway to develop advanced precious-metal-free catalysts.

A series of oxyfluoride chains containing asymmetric basic building units of both early- and late-transition metal cations

Four new transition metal oxyfluorides revealing chain structures composed of asymmetric basic building units of [MO2F4]2− (M = Mo and W) and [M’(pz/mpz)4]2+ (M′ = Cu and Ni) polyhedra, i.e., [Cu(mpz)4][MoO2F4] (1), [Cu(pz)4]2[WO2F4]2 (2), [Ni(pz)4]2[MoO2F4]2 (3), and [Ni(pz)4][WO2F4] (4) (pz = pyrazole; mpz = 3-methyl pyrazole) have been synthesized via hydrothermal reactions. The linear chain structures found from the title compounds are attributable to the trans-directing [MO2F4]2- octahedral moiety and hydrogen bonding interactions. Strong Jahn–Teller distortions are observed for Cu2+ cations with d9 configuration in compounds 1 and 2. All the compounds show both lower-energy and higher-energy absorption band gaps, which originate from the d–d transitions and the distortion of octahedral geometry of M2+ cations, respectively. The magnetism of [Cu(mpz)4][MoO2F4] turns out to be described by noninteracting Cu2+ spins rather than a spin chain due to long exchange path mediated via the nonmagnetic Mo6+ ions. The title compounds have been also thoroughly characterized using spectroscopic and thermal analyses along with calculations of local dipole moments and the extent of out-of-center distortions.

Indirect Intersystem Crossing (S1 → T3/T2 → T1) Promoted by the Jahn-Teller Effect in Cycloparaphenylenes.

Vibronic-coupling effects play a key role for excited-state charge- and energy-transfer processes in organic molecular systems. Here, we demonstrate how the Jahn-Teller effect in triplet excited states of highly symmetric cycloparaphenylenes triggers an indirect intersystem crossing deactivation pathway. Strong Jahn-Teller distortion in the doubly degenerate second excited triplet state (T2) brings the molecular system energetically close to the lowest triplet state (T1), thereby opening the possibility for an extremely rapid internal conversion. Quantum dynamics simulations reveal an initial T2 → T1 population decay within 50 fs. Experimental observation of size-dependent intersystem crossing rates of cycloparaphenylenes is explained based on the proposed S1 → T3/T2 → T1 mechanism.