The reaction of [Fe₃O(O₂CPh)₆(H₂O)₃](O₂CPh) with lanthanide/rare earth nitrate salts in the presence of triethanolamine (H₃tea) in acetonitrile/methanol solution yields a series of compounds with isostructural tetranuclear core motifs [Fe^III ₂Ln^III ₂(μ₃-OH)₂(teaH)₂(O₂CCPh)₆]·3MeCN (Ln = Ce (1), Pr (2), Nd (3), Sm (4), Eu (5), Gd (6), Tb (7), Dy (8), Ho (9), Er (10), Tm (11), Yb (12), Y (13)). In all cases the core topology is a defect-dicubane planar or “butterfly” Fe₂Ln₂ motif. Compounds 1–13 were investigated using a combination of experimental techniques and theoretical studies. Magnetic susceptibility measurements were carried out on all compounds. The magnetic coupling between the two Fe^III centres is antiferromagnetic, with J_FeFeca. −6.71(4) cm⁻¹, while the Fe–Ln couplings are much weaker, e.g. J_FeGd = 0.18(1) cm⁻¹. Compounds 6, 7, 8 and 13 were selected for Mössbauer studies in order to investigate the influence of isotropic (Gd^III), highly anisotropic non-Kramers and Kramers (Tb^III and Dy^III) and diamagnetic (Y^III) rare earth ions on the local environment of the Fe^III centres. Compounds 3, 6, 8 and 13 were also studied using X-Band EPR spectroscopy. For 13, with the diamagnetic Y^III ion, this made it possible to obtain the D, E, J_FeFe and g parameters for the iron centres. It is shown that the low-temperature spectra of compounds 3, 6 and 8 are determined by magnetic properties of rare-earth ions and the dipole–dipole interactions between the Ln^III ions. The Fe–Ln interactions were confirmed as very weak and dipolar in nature by the temperature dependence of EPR spectra at T > 20 K.

We observed a long‐lived charge transfer (CT) state in a novel orthogonal compact electron donor–acceptor dyads, with closed form of rhodamine (Rho) as electron donor and pyromellitimide (PI),or thionated PI, as electron acceptor. The two parts in the dyads are connected via a spiro quaternary carbon atom, thus the torsion between the donor and acceptor is completely inhibited, which is beneficial to reduce the reorganization energy and to exploit the Marcus inverted region effect to prolong the CT state lifetime. Femtosecond transient absorption spectra show that the charge separation is rather fast, while nanosecond transient absorption spectra confirmed the formation of long‐lived CT state (2.6 μs). Time‐resolved electron paramagnetic resonance (TREPR) spectra determined the spin multiplicity of the long living state and assigned it to a ³CT state. Replacement of an oxygen atom in the PI part with a sulfur atom favoring classical intersystem crossing processes, causes a consistently shortening of the lifetime of the ³CT state (0.29 μs).

A twisted π-conjugated framework does not necessarily promote efficacious ISC, energy matching of S ₁ /T _n excited states offers a more promising route to triplet enhancement.

Electron spin dynamics are crucial to photochemical and photophysical processes. However, to a large extent, they are neglected in routine photochemistry studies. Herein, we summarized the recent developments of electron spin dynamics in organic molecular systems. The electron-spin selective intersystem crossing (ISC) as well as charge separation (CS) and charge recombination (CR) of the organic molecular system are discussed, including ISC of the compounds with twisted π-conjugation frameworks and CR-induced ISC in compact orthogonal electron donor–acceptor dyads. We found that the electron spin polarization (ESP) of the triplet state formed in these systems is highly dependent on the molecular structure and geometry. The zero-field-splitting (ZFS) D and E parameters of the triplet state of series chromophores determined with time-resolved electron paramagnetic resonance (TREPR) spectroscopy are presented. Some unanswered questions in related areas are raised, which may inspire further theoretical investigations. The examples demonstrate that the study of electron spin dynamics is not only important in fundamental photochemistry to attain in-depth understanding of the ISC and the charge transfer processes, but is also useful for designing new efficient organic molecular materials for applications including photodynamic therapy, organic light-emitting diodes, and photon upconversion.