Jochen Autschbach

Organic helicenes have attracted increasing attention owing to their unique topology, tunable chiroptical properties, and potential applications in organic electronics, photonics, and spintronics. Direct access to excited‐state interactions and their fine control through molecular design are essential for these applications. Here, we show that the combination of magnetic and magneto‐optical techniques with electronic structure analysis provides unique insight into both diamagnetic (singlet, S = 0) and paramagnetic (triplet, S = 1) excited states, offering detailed information on the relative orientation of transition dipole moments of singlet excited states and their energies, as well as wavefunction geometry and population pathways for the triplet state. This integrated approach applies broadly to both chiral and achiral materials and reveals excited‐state fine structures that are key to designing next‐generation spin‐ and optoelectronics.

In this work, uphill photocatalysis allowed stereoconvergent access to the less stable geometric isomer of 1,2,3-trisubstituted 1,3-dienes.

In crystallo synthetic and analytical methods are remarkably effective for stabilizing and structurally characterizing otherwise reactive compounds, and elucidating transient intermediates in catalytic cycles. In this work, we investigate the mechanism of the ring-opening isomerization of a CeIV-cyclopropenyl complex, CeIV(3,3-diphenylcyclopropenyl)(TriNOx) (Ce-1), to the corresponding CeIV-allenyl isomer, CeIV(CH═C═CPh2)(TriNOx) (Ce-2), where TriNOx3–═[((2-tBuNOH)C6H4CH2)3N]3–. The isomerization occurs by a single-crystal-to-single-crystal (SCSC) transformation under thermal and photochemical conditions, allowing for the direct extraction of kinetic parameters from sequential single-crystal X-ray diffraction (SC-XRD) measurements. In crystallo kinetic isotope effect (KIE) experiments using a deuterium-labeled isotopologue of Ce-1 revealed a modest KIE (1.01) for the thermal reaction and an inverse KIE (0.74) for the photochemical reaction. Additionally, variable-temperature studies demonstrated that the photochemical SCSC reaction occurs without a detectable thermal activation energy barrier over a temperature range of 163 K. An investigation of the solution-phase thermal and photochemical decomposition products yielded results that were consistent with a multistep reaction initiated by the ring-opening isomerization reaction of Ce-1 to Ce-2. Complementary solution-phase KIE studies tracking the consumption of the two isotopologues of the CeIV-cyclopropenyl complex revealed, qualitatively, an inverse KIE for the photochemical process, consistent with the results obtained from the photocrystallographic studies. The combined results from the in crystallo and solution-phase studies provide experimental support for the previously computed thermal mechanism, clarify the distinct photochemical mechanism, and illustrate how single-crystal X-ray diffractometry can be used to obtain detailed kinetic and thermodynamic insights into SCSC transformations for reactions on appropriate time scales.

Actinide–carbon multiple bonds are synthetically challenging to access, primarily due to high bond polarization and poor energetic matching between metal and ligand orbitals. This challenge is especially pronounced for thorium, such that thorium–carbon multiple bonds are exceptionally rare. Herein, we report the synthesis of a novel actinide endohedral fullerene, UCTh@D3h(5)-C78, which encapsulates a unique heterobimetallic actinide carbide cluster. Single-crystal X-ray diffraction analysis reveals a carbon-bridged U–C–Th cluster stabilized within a D3h(5)-C78 cage, featuring remarkably short U–C and Th–C distances of 1.962(8) Å and 2.121(8) Å, respectively. Quantum chemical studies further indicate that the U–C bond possesses a rare triple-bond character, while the Th–C bond can be described as a partial triple bond─a bonding mode previously unreported for thorium. This work uncovers a unique bonding motif involving multiple bonds for both uranium and thorium, most notably a rare Th–C interaction with triple-bond character. These findings provide important perspectives toward the design and synthesis of compounds containing thorium-based triple bonds.

Metal–ligand bonding in [CeX ₆ ] ^2– complexes (X = F, Cl, Br) is investigated using density functional theory (DFT) and pre-bonding analysis. The analysis quantifies atomic orbital (AO) energies and overlaps in the complexes, with a focus on the mixing between Ce 4f AOs with halide 𝑛p donor AOs. Results indicate that decreasing Ce–ligand AO overlap from X = F to Br is compensated by improved AO energy matching, leading to increased 4f-based ligand-to-metal donation across the series. Population analysis derived from pre-bonding data rationalizes recent experimental X-ray absorption near-edge structure (XANES) findings. Effective orbital overlaps extracted from XANES data using a simplified AO bonding model based on the Wolfsberg–Helmholtz (WH) relation are compared directly with pre-bonding overlaps. The consequences of using ionization potential (IP) based AO energies in the experimental data workup and other assumptions inherent in the bonding model are also scrutinized. Using consistent pre-bonding AO energies in the model requires a WH constant of 𝐾 ≃ 3 to reproduce the calculated overlaps for 4f-based interactions. Combining IP-based AO energies with a conventional value 𝐾 = 2, as done in prior studies, is shown to yield comparable overlaps due to a cancellation of errors. Overall, pre-bonding analysis provides a framework rooted in first-principles theory for connecting AO interactions in the molecule with experimental bonding trends.

The first ab initio simulation of Magneto-Chiral Dichroism (MChD) for a lanthanide-based complex is reported. The previously experimentally studied [Yb(hfac)3(P–H6bpy)] complex is investigated with multireference wavefunction-based methods using complete active space self-consistent field (CASSCF) calculations, further refined by second-order perturbation theory (CASPT2) and state-interaction spin–orbit approach (RASSI-SO), in combination with the microscopic MChD theory developed by Barron and Vrbancich (BV). The simulations enable a detailed analysis of the 2F5/2 ← 2F7/2 electronic transitions. At 4 K, the MChD response is found to be dominated by the MChD Cterm contribution, arising from electric dipole-magnetic dipole interactions and population differences within the ground state, split by the magnetic field. Overall, the simulated spectrum shows good agreement with experiment in terms of both relative transition energies and band-sign assignments. Zeeman splitting was explicitly evaluated and found to be negligible under the experimental conditions, indicating that vibronic coupling is the most likely origin of the remaining discrepancies. By providing a quantitative and physically grounded description of the MChD spectrum of a lanthanide complex, this work goes beyond the current state of the art in theoretical simulations of MChD and fills a crucial gap, at a time when the growing body of experimental data on lanthanide complexes calls for direct theory–experiment comparisons to enable deeper interpretation.

Low‐energy electronic transitions in four actinide complexes are analyzed via complete active space wavefunction calculations including the spin–orbit (SO) interaction. Ground‐ and excited‐state wavefunctions are analyzed via SO natural orbitals. The transitions are characterized via SO natural transition orbitals as well as changes in the electronic entropy of the states involved, changes in the occupation number vectors, and the integrated modulus of the density change. For the complexes [U(N,N′‐bis[(4,4′‐dimethylamino)salicylidene]−1,2‐phenylenediamine)], [CmClMeH(2,2)IAM], and [Es(HOPO)], the calculated photophysical data are considered in comparison with available experimental data. The complex [U(N,N′‐bis[(4,4′‐dimethylamino)salicylidene]‐1,2‐phenylenediamine)] is analyzed with respect to its absorption spectrum. The circularly polarized luminescence (CPL) of the chiral complex [CmClMeH(2,2)IAM] is also modeled and agrees qualitatively with the available experimental CPL spectrum. The study highlights challenges for the computational study of transitions in actinide complexes with strong SO coupling as well as the potential utility of the electronic structure analysis tools applied in this work.

The reaction of [LaI₃(THF)₄] or [Ce(NO₃)₃(THF)₄] with 4 equiv of in situ generated LiFmes (FmesH = 1,3,5-(CF₃)₃C₆H₃) resulted in the formation of the novel homoleptic lanthanide-aryl complexes [Li(THF)₂(Et₂O)₂][La(Fmes)₄] ([Li][1]) and [Li(THF)_3.5(Et₂O)_0.5][Ce(Fmes)₄] ([Li][2]), respectively. Likewise, the reaction of [Lu(NO₃)₃(THF)₃] with 4 equiv of in situ generated LiFmes resulted in the formation of [Li(THF)₃][LuF(Fmes)₃] ([Li][3]). Complexes [Li][2] and [Li][3] were obtained as mixtures with unreacted LiFmes. Additionally, the reaction of [Lu(NO₃)₃(THF)₃] with 4 equiv of in situ generated LiC₆Cl₅ and 3 equiv of DME resulted in the formation of the homoleptic lutetium-aryl complex, [Li(DME)₃][Lu(C₆Cl₅)₄] ([Li][4]). All four complexes were characterized by X-ray crystallography and multinuclear NMR spectroscopy. In the solid state, all four complexes exhibit weak F/Cl→Ln dative interactions; however, these interactions do not apparently offer much stability, since all four complexes are thermally sensitive. DFT calculations reveal that the Ln-C_ipso and Lu-F bonds are modestly covalent, although the Lu-F bond exhibits less covalency than the Ln-C_ipso bonds. The calculations also indicate that the La-C_ipso bonds exhibit a small amount of 4f orbital participation, whereas the Lu-C_ipso bonds have (as expected) no 4f character. Despite this difference, the spin-orbit deshielding effects on the C_ipso ¹³C chemical shifts are comparable between complexes [1]^- and [3]^-.

A series of isostructural tetravalent cerium and thorium parent amide (NH₂^–) complexes were synthesized and characterized. The electronic structure of the M–N(amide) bond was investigated using ¹⁵N NMR spectroscopy and DFT calculations in a comparative study of the Na-capped complexes [Na(OEt₂)₂][M(NH₂)(TriNOx)][BAr^F₄], M = Ce (3-Ce), Th (3-Th). The ¹⁵N(amide) resonance in 3-Ce was observed at δ = 192 ppm, approximately 95 ppm downfield of the corresponding resonance in 3-Th (δ = 97 ppm). Computational analysis found that the spin–orbit (SO) heavy atom effect on the amide nitrogen NMR shift was relatively small, and comparable, at Δ_SO = 12/16 ppm for 3-Ce/3-Th. The observed large difference in the ¹⁵N chemical shift for 3-Ce/3-Th is primarily due to differences in paramagnetic shielding generated by the amide lone pairs.

Metal-ligand covalency is not fully understood for the actinides and studies comparing group IV transition metals with isostructural Th and U complexes are limited. Herein, we report the synthesis and characterization of [(C₅Me₅)₂(2,6-ⁱPr₂C₆H₃O)M(H)], M = Hf, Th, and U, which can be compared to the previously reported Zr analog, to explore the differences between transition metal and actinide bonding situations. The ¹H NMR chemical shifts of each complex, including the paramagnetic U(IV) compound, showed reasonable agreement with the computed shifts and benchmark the covalency of the metal-hydride bonds.