Iron-Catalyzed Synthesis, Structure, and Photophysical Properties of Tetraarylnaphthidines

We describe the synthesis and photophysical properties of tetraarylnaphthidines. Our synthetic approach is based on an iron-catalyzed oxidative C-C coupling reaction as the key step using a hexadecafluorinated iron-phthalocyanine complex as a catalyst and air as the sole oxidant. The N,N,N',N'-tetraarylnaphthidines proved to be highly fluorescent with quantum yields of up to 68%.

Single-crystal-to-single-crystal post-synthetic modifications of three-dimensional LOFs (Ln = Gd, Eu): a way to modulate their luminescence and thermometric properties

Single-crystal-to-single-crystal post-synthetic modifications of {[Ln₂(H₂L)₃(DMF)₄]·2DMF}_n LOFs (Ln = Gd, Eu) to modulate their luminescence and thermometric properties.

Increase of network hydrophilicity from sql to lvt supramolecular isomers of Cu-MOFs with the bifunctional 4-(3,5-dimethyl-1H-pyrazol-4-yl)benzoate linker

A slight difference in the H-bonding of the linker pyrazole-NH group changes the framework hydrophilicity drastically.

Methylene spacer regulated variation in supramolecular assembly of zinc(ii) dicyanamide complexes with reduced Schiff base ligands: synthesis, structure and DFT study

Three tetranuclear zinc dicyanamide complexes with three potential hexadentate reduced Schiff base ligands have been synthesized and characterized. The DFT study is devoted to the analysis of the competition between C–H⋯π and π⋯π interactions.

Emission solvatochromic, solid-state and aggregation-induced emissive α-pyrones and emission-tuneable 1H-pyridines by Michael addition-cyclocondensation sequences

Starting from substituted alkynones, α-pyrones and/or 1H-pyridines were generated in a Michael addition-cyclocondensation with ethyl cyanoacetate. The peculiar product formation depends on the reaction conditions as well as on the electronic substitution pattern of the alkynone. While electron-donating groups furnish α-pyrones as main products, electron-withdrawing groups predominantly give the corresponding 1H-pyridines. Both heterocycle classes fluoresce in solution and in the solid state. In particular, dimethylamino-substituted α-pyrones, as donor-acceptor systems, display remarkable photophysical properties, such as strongly red-shifted absorption and emission maxima with daylight fluorescence and fluorescence quantum yields up to 99% in solution and around 11% in the solid state, as well as pronounced emission solvatochromism. Also a donor-substituted α-pyrone shows pronounced aggregation-induced emission enhancement.

The Origin of p-Xylene Selectivity in a DABCO Pillar-Layered Metal-Organic Framework: A Combined Experimental and Computational Investigation

We report high experimental p-xylene (pX) selectivity in a pillar-layered metal-organic framework, DUT-8(Cu). Vapor- and liquid-phase adsorption experiments were carried out to confirm high pX selectivity and large pX uptakes in DUT-8(Cu). Grand canonical Monte Carlo simulation results show that the presence of DABCO ligands allows for the packing of pX molecules and is responsible for the pX selective nature of the material. The simulation also suggests that the presence of isooctane solvents in the liquid-phase experiments plays an essential role by lowering the adsorption of other xylene isomers, and leads to increased pX selectivity in the liquid-phase as compared to the vapor phase. Density functional theory simulations show that the preferential arrangement is due to the preferential adsorption of pX on the DABCO ligand and the preferential adsorption of isooctane over other xylene isomers.

Influence of ligand positional isomerism on the molecular and supramolecular structures of cobalt(II)-phenylimidazole complexes

In a study to evaluate the impact of flexible positional isomeric ligands on the coordination geometry and self-assembly process of 3d metal complexes, the synthesis of eight new cobalt(II) complexes with the 2-phenylimidazole (LH) and 5-phenylimidazole (L'H) ligands has been carried out. A variety of parameters/conditions have been probed using the general Co^II/X^-/LH or L'H (X^- = Cl^-, Br^-, I^-, NO₃^-, NCS^-, ClO₄^-, SO₄^2-) reaction system. Interestingly, X-ray analyses reveal two distinct groups of complexes: reactions with LH only lead to tetrahedral or quasi-tetrahedral complexes {i.e. [CoCl₂(LH)₂] (1), [CoI₂(LH)₂] (2), [Co(NO₃)₂(LH)₂] (3), [Co(NCS)₂(LH)₂] (4)}, whereas L'H favours octahedral coordination {i.e. [Co(L'H)₄(MeCN)(H₂O)]I₂ (5), [Co(L'H)₄(MeCN)(H₂O)](NO₃)₂ (6) and [Co(NCS)₂(L'H)₄)]·2MeOH (7·2MeOH)}. A tetrahedral [Co(NCS)₂(L'H)₂)] (8) complex was also concurrently isolated with complex 7. The effects of the positional isomeric ligands LH and L'H and of the coordinated inorganic anions on the stoichiometry and packing arrangements of the complexes are thoroughly discussed. The supramolecular assembly is firmly directed, in all types of complexes, by robust N-H...X (X = Cl, I, O or S) motifs, leading to varying dimensionalities (1D, 2D or 3D) and packing arrangements. The formation of these motifs has been activated by choosing appropriate anions X, acting as terminal ligands or counterions. At a second level of organization, additional subordinate C-H...X (X = Cl, I, O or S), C-H...π and π...π intermolecular interactions complement the rigidity of the complexes' packing towards compact 3D assemblies. Hirshfeld surface analyses provided insight into the intermolecular interactions, allowed quantification of the individual contact types and comparison between the complexes.

A Million Crystal Structures: The Whole Is Greater than the Sum of Its Parts

The founding in 1965 of what is now called the Cambridge Structural Database (CSD) has reaped dividends in numerous and diverse areas of chemical research. Each of the million or so crystal structures in the database was solved for its own particular reason, but collected together, the structures can be reused to address a multitude of new problems. In this Review, which is focused mainly on the last 10 years, we chronicle the contribution of the CSD to research into molecular geometries, molecular interactions, and molecular assemblies and demonstrate its value in the design of biologically active molecules and the solid forms in which they are delivered. Its potential in other commercially relevant areas is described, including gas storage and delivery, thin films, and (opto)electronics. The CSD also aids the solution of new crystal structures. Because no scientific instrument is without shortcomings, the limitations of CSD research are assessed. We emphasize the importance of maintaining database quality: notwithstanding the arrival of big data and machine learning, it remains perilous to ignore the principle of garbage in, garbage out. Finally, we explain why the CSD must evolve with the world around it to ensure it remains fit for purpose in the years ahead.

Emissions and the application of a series of twisted fluorophores with intramolecular weak hydrogen bonds

A series of twisted fluorophores of CEOCH (((2,5-dimethoxy-1,4-phenylene)bis(ethene-2,1,1-triyl))-tetra-benzene) derivatives with intramolecular weak hydrogen bonds (IMWHBs) were synthesized to investigate how different substituents on outer benzenes influence their emissive properties. Because of the twisted structure and weak intermolecular interactions, the emissions of the CEOCH derivatives were intense in the aggregated state but as the flexibility and electronic effect of the substituents changed, their quantum yields (QYs) changed from over 40% to 1% in solution. Based on the adjustable QYs of CEOCHs with different substituents in solutions, a fluorescent sensor for hydrazine with an extremely strong light and dark contrast was obtained via the conversion of dicyanovinyl groups to hydrazone groups.

C-H/π and C-H-O Interactions in Concert: A Study of the Anisole-Methane Complex using Resonant Ionization and Velocity Mapped Ion Imaging

Noncovalent forces such as hydrogen bonding, halogen bonding, π-π stacking, and C-H/π and C-H/O interactions hold the key to such chemical processes as protein folding, molecular self-assembly, and drug-substrate interactions. Invaluable insight into the nature and strength of these forces continues to come from the study of isolated molecular clusters. In this work, we report on a study of the isolated anisole-methane complex, where both C-H/π and C-H/O interactions are possible, using a combination of theory and experiments that include mass-selected two-color resonant two-photon ionization spectroscopy, two-color appearance potential (2CAP) measurements, and velocity mapped ion imaging (VMI). Using 2CAP and VMI, we derive the binding energies of the complex in ground, excited, and cation radical states. The experimental values from the two methods are in excellent agreement, and they are compared with selected theoretical values calculated using density functional theory and ab initio methods. The optimized ground-state cluster geometry, which is consistent with the experimental observations, shows methane sitting above the ring, interacting with anisole via both C-H/π and C-H/O interactions, and this dual mode of interaction is reflected in a larger ground-state binding energy as compared with the prototypical benzene-methane system.

The Taming of Redox-Labile Phosphidotitanocene Cations

Tame d⁰ phosphidotitanocene cations stabilized with a pendant tertiary phosphane arm are reported. These compounds were obtained by one-electron oxidation of d¹ precursors with [Cp₂ Fe][BPh₄ ]. The electronic structure of these compounds was studied experimentally (EPR, UV/Vis, and NMR spectroscopy, X-ray diffraction analysis) and through DFT calculations. The theoretical analysis of the bonding situation by using the electron localization function (ELF) shows the presence of π-interactions between the phosphido ligand and Ti in the d⁰ complexes, whereas dπ-pπ repulsion prevents such interactions in the d¹ complexes. In addition, CH-π interactions were observed in several complexes, both in solution and in the solid state, between the phosphido ligand and the phosphane arm. The d⁰ complexes were found to be light sensitive, and decompose through Ti-P bond homolysis to give Ti^III species. A naked d⁰ phosphidotitanocene cation has been trapped by reaction with diphenylacetylene, yielding a Ti/P frustrated Lewis pair (FLP), which was found to be less reactive than a previously reported Zr analog.

Structural insights into methyl- or methoxy-substituted 1-(α-aminobenzyl)-2-naphthol structures: the role of C-H...π interactions

Aminobenzylnaphthols are a class of compounds containing a large aromatic molecular surface which makes them suitable candidates to study the role of C-H...π interactions. We have investigated the effect of methyl or methoxy substituents on the assembling of aromatic units by preparing and determining the crystal structures of (S,S)-1-{(4-methylphenyl)[(1-phenylethyl)amino]methyl}naphthalen-2-ol, C₂₆H₂₅NO, and (S,S)-1-{(4-methoxyphenyl)[(1-phenylethyl)amino]methyl}naphthalen-2-ol, C₂₆H₂₅NO₂. The methyl group influenced the overall crystal packing even if the H atoms of the methyl group did not participate directly either in hydrogen bonding or C-H...π interactions. The introduction of the methoxy moiety caused the formation of new hydrogen bonds, in which the O atom of the methoxy group was directly involved. Moreover, the methoxy group promoted the formation of an interesting C-H...π interaction which altered the orientation of an aromatic unit.

Molecular superoxide radical photogeneration in cancer cells by dipyridophenazine iridium(iii) complexes

The switch from Type II to Type I photochemical mechanism by new Ir(iii) complexes for improved PDT of cancer under hypoxia is demonstrated.

Hydrophobicity and CH/π-interaction-driven self-assembly of amphiphilic aromatic hydrocarbons into nanosheets

The hydrophobicity and CH/π-interaction-driven self-assembly of an amphiphile that contains a biphenylanthracene group furnishes micrometer-scale nanosheets with a windmill-shaped molecular packing structure in dilute aqueous solution.

Copper(i) complexes of functionalized sulfur-containing ligands: structural and theoretical insights into chalcogen bonding

Four new copper(i) thiocyanate complexes were studied using geometrical parameters and the lump–hole approach for justification of the strength and nature of chalcogen bonding.

Halogen and Hydrogen Bonding Interplay in the Crystal Packing of Halometallocenes

This paper focuses in the influence of halogen atoms in the design and structural control of the crystal packing of Group VIII halogenated metallocenes. The study is based on the present knowledge on new types of intermolecular contacts such as halogen (X⋯X, C-X⋯H, C-X⋯π), π⋯π, and C-H⋯π interactions. The presence of novel C-H⋯M interactions is also discussed. Crystal packings are analysed after database search on this family of compounds. Results are supported by ab initio calculations on electrostatic charge distributions; Hirshfeld analysis is also used to predict the types of contacts to be expected in the molecules. Special attention is given to the competition among hydrogen and halogen interactions, mainly its influence on the nature and geometric orientations of the different supramolecular motifs. Supramolecular arrangements of halogenated metallocenes and Group IV di-halogenated bent metallocenes are also compared and discussed. Analysis supports halogen bonds as the predominant interactions in defining the crystal packing of bromine and iodine 1,1′-halometallocenes.

Supramolecular Packing of a Series of N-Phenylamides and the Role of NH···O=C Interactions

A series of seven N-phenylamides [R-C(O)NHPh, in which R: CH₃, C(CH₃)₃, Ph, CF₃, CCl₃, CBr₃, and H] were used as models in this study. Molecular packing and intermolecular interactions were evaluated by theoretical calculations, solution NMR, and quantum theory of atoms in molecules analyses. Crystallization mechanisms were proposed based on the energetic and topological parameters using the supramolecular cluster as demarcation. Concentration-dependent ¹H NMR experiments corroborated the proposed interactions between molecules. For all compounds (except for R: H, which initially formed tetramers), layers (two-dimensional) or chains (one-dimensional) were formed in the first stage of the proposed crystallization mechanisms. The presence of strong intermolecular NH···O=C interactions promoted the first stages. The study in solution provided different values of association constant (K _ass) governed by the hydrogen bond NH···O=C, showing that the stronger interactions are directly influenced by the substituent steric hindrance. A correlation between K _{ass(NH···O=C)} from the solution and the NH···O=C interaction energy in the crystal showed a good trend.

Self-assembly of lattices with high structural complexity from a geometrically simple molecule

Here we report an anomalous porous molecular crystal built of C-H···N-bonded double-layered roof-floor components and wall components of a segregatively interdigitated architecture. This complicated porous structure consists of only one type of fully aromatic multijoint molecule carrying three identical dipyridylphenyl wedges. Despite its high symmetry, this molecule accomplishes difficult tasks by using two of its three wedges for roof-floor formation and using its other wedge for wall formation. Although a C-H···N bond is extremely labile, the porous crystal maintains its porosity until thermal breakdown of the C-H···N bonds at 202°C occurs, affording a nonporous polymorph. Though this nonporous crystal survives even at 325°C, it can retrieve the parent porosity under acetonitrile vapor. These findings show how one can translate simplicity into ultrahigh complexity.

Strong and insusceptible photo-emissions from an intramolecular weak hydrogen bond strengthened twisted fluorophore

Intramolecular weak hydrogen bonds of CHO and CH/Pi were introduced into a twisted fluorophore backbone of 1,4-bis(2,2-diphenylvinyl)benzene, which enables the fluorophore to emit violently and stably in both solubilized and aggregated states, and be inert to solvent environments and preserve over 10% quantum yield at temperature as high as 90 °C in solution.

Making an order: the concerted alignment of [MOF5]2- (M = Nb and Ta) dipolar anions in one-dimensional coordination chains sustained by tris(3,4,5-trimethyl-1H-pyrazole)copper(II)

Electronic distortions, which are inherent in the oxide fluoride anions [MOF₅]^2- (M = Nb and Ta), provide an origin of polar molecular arrangements for the development of new polar second-harmonic-generating, piezo-, pyro- and ferroelectric materials. It is still a challenge to expand this approach to the realm of metal-organic polymers, while insufficient control over the environment of the [MOF₅]^2- units results in their orientational disorder and loss of polarity. The structures of catena-poly[[tris(3,4,5-trimethyl-1H-pyrazole-κN²)copper(II)]-μ-oxido-[tetrafluoridoniobium(V)]-μ-fluorido], [CuNbF₅O(C₆H₁₀N₂)₃]_n, (I), and its isostructural pentafluoridooxidotantalate(V) analogue, catena-poly[[tris(3,4,5-trimethyl-1H-pyrazole-κN²)copper(II)]-μ-oxido-[tetrafluoridotantalum(V)]-μ-fluorido], [CuTaF₅O(C₆H₁₀N₂)₃]_n, (II), are the first examples of the strict orientational order of [MOF₅]^2- (M = Nb and Ta) in one-dimensional coordination chains. A primary factor for the exact discrimination of one orientation of the anion over the other is strong and shape-selective multiple interactions of [MOF₅]^2- with the inherently acentric CuL₃²⁺ platform, with a set of two coordination and three N-H...F hydrogen bonds. In (I) and (II), the Cu²⁺ ions exhibit distorted square-pyramidal fivefold coordination formed by three pyrazole N atoms and the oxide O atom, defining the equatorial plane, and the anionic bridging F atom (which is trans with respect to the M-O bond) residing in the apical position. The inorganic bridges connect CuL₃²⁺ moieties into polar zigzag chains; the bulk polarity of the structure is eliminated by an antiparallel alignment of the individual chains. These chains are further connected through C-H...F hydrogen bonding and very weak C-H...π interactions of the organic ligands.

Group 2 metal bis(arenecarbochalcogenoate)(crown ether) complexes: isolation and structural analysis

A series of Group 2 metal bis(arenecarbochalcogenoato)(crown ether) complexes M(EE'CAr)2(L)(L')x (M = Mg, Ca, Sr, Ba; Ar = aryl; E = S, Se; E' = O, S; L = H2O or THF; L' = 15-crown-5, 18-crown-6) were synthesized and their structures were revealed by X-ray analyses. The two carbothioato ligands in Mg, Ca and Sr 15-crown-5 complexes are located on the same side of the crown ether plane, while those in Mg, Ca, Sr, and Ba 18-crown-6 compounds are on both sides of the 18-crown-6 plane (trans relative to the plane). For the Ca 15-crown-5 complex, both carbothioato ligands are connected to the central Ca ion through an oxygen atom in a monodentate manner, and the two hydrogen atoms of the coordinated water molecule are intramolecularly hydrogen-bonded with thiocarbonyl sulfur atoms. One of the two carbothioate groups in the Ca 18-crown-6 complex is connected in a bidentate manner to the central metal, while the other is connected in a monodentate manner through an oxygen atom. The two thiocarboxylato ligands in the Sr 15-crown-5 congener are connected in a bidentate fashion to the same side of the crown ether plane. The Ca and Sr compounds are the first examples of alkali earth metal carbochalcogenoate complexes in which carbochalcogenolato ligands are connected in a monodentate manner through an oxygen atom. Both chalcogenoato ligands in Ba(SOCC6H4Me-4)2(18-crown-6) and Sr(SeOCC6H4Me-4)2(18-crown-6) are coordinated in a bidentate manner to the central metal ion.

A comparative experimental and theoretical investigation of hydrogen-bond, halogen-bond and π–π interactions in the solid-state supramolecular assembly of 2- and 4-formylphenyl arylsulfonates

To explore the operational role of noncovalent interactions in supramolecular architectures with designed topologies, a series of solid-state structures of 2- and 4-formylphenyl 4-substituted benzenesulfonates was investigated. The compounds are 2-formylphenyl 4-methylbenzenesulfonate, C14H12O4S, 3a, 2-formylphenyl 4-chlorobenzenesulfonate, C13H9ClO4S, 3b, 2-formylphenyl 4-bromobenzenesulfonate, C13H9BrO4S, 3c, 4-formylphenyl 4-methylbenzenesulfonate, C14H12O4S, 4a, 4-formylphenyl 4-chlorobenzenesulfonate, 4b, C13H9ClO4S, and 4-formylphenyl 4-bromobenzenesulfonate, C13H9BrO4S, 4c. The title compounds were synthesized under basic conditions from salicylaldehyde/4-hydroxybenzaldehydes and various aryl sulfonyl chlorides. Remarkably, halogen-bonding interactions are found to be important to rationalize the solid-state crystal structures. In particular, the formation of O...X (X = Cl and Br) and type I X...X halogen-bonding interactions have been analyzed by means of density functional theory (DFT) calculations and characterized using Bader's theory of `atoms in molecules' and molecular electrostatic potential (MEP) surfaces, confirming the relevance and stabilizing nature of these interactions. They have been compared to antiparallel π-stacking interactions that are formed between the arylsulfonates.

Model molecules to classify CHO hydrogen-bonds

We developed a set of conformationally locked molecules each of which makes a single CHO H-bond/short contact and has different electron density at the acceptor oxygen atom. The downfield shift of the 1H NMR signals due to the hydrogen involved in the CHO H-bond varied from 0.93-1.6 ppm, and the magnitude of Δδ is in correlation with the hybridization state of the acceptor oxygen and with the CHO H-bond strengths quantified using a computational method.

1-Methoxy-1,1-diphenylbut-2-yne

In the title compound, C17H16O, the phenyl rings are twisted relative to each other at an angle of 85.9 (1)°. The crystal structure features weak C—H...π interactions, which connect the molecules into a three-dimensional supramolecular network.

3,4-Dibromo-2,2,5,5-tetraphenyl-2,5-dihydrofuran

The crystal structure of the title compound, C28H20Br2O, was solved in the orthorhombic space group P212121 with one molecule in the asymmetric unit. The phenyl rings are nearly planar and inclined at angles of 67.7 (1), 68.8 (1), 79.3 (1) and 62.3 (1)° to the plane of the 2,5-dihydrofuran ring. The crystal structure features C—H...π and Br...Br interactions, which connect the molecules to a three-dimensional supramolecular network.

Oxidative Coupling of Anionic Abnormal N-Heterocyclic Carbenes: Efficient Access to Janus-Type 4,4'-Bis(2H-imidazol-2-ylidene)s

The oxidative coupling of anionic imidazol-4-ylidenes protected at the C2 position with [MnCp(CO)₂ ] or BH₃ led to the corresponding 4,4'-bis(2H-imidazol-2-ylidene) complexes or adducts, in which the two carbene moieties are connected through a single C-C bond. Subsequent acidic treatment of the later species led to the corresponding 4,4'-bis(imidazolium) salts in good yields. The overall procedure offers practical access to a novel class of Janus-type bis(NHC)s. Strikingly, the coplanarity of the two NHC rings within the mesityl derivative 4,4'-bis(IMes), favored by steric hindrance along with stabilizing intramolecular C-H⋅⋅⋅π aryl interactions, allows the alignment of the π-systems and, as a direct consequence, significant electron communication through the bis(carbene) scaffold.

Thermodynamic properties of selenoether-functionalized ionic liquids and their use for the synthesis of zinc selenide nanoparticles

Three selenoether-functionalized ionic liquids (ILs) of N-[(phenylseleno)methylene]pyridinium (1), N-(methyl)- (2) and N-(butyl)-N'-[(phenylseleno)methylene]imidazolium (3) with bis(trifluoromethanesulfonyl)imide anions ([NTf2]) were prepared from pyridine, N-methylimidazole and N-butylimidazole with in situ obtained phenylselenomethyl chloride, followed by ion exchange to give the desired compounds. The crystal structures of the bromide and tetraphenylborate salts of the above cations (1-Br, 2-BPh4 and 3-BPh4) confirm the formation of the desired cations and indicate a multitude of different supramolecular interactions besides the dominating Coulomb interactions between the cations and anions. The vaporization enthalpies of the synthesized [NTf2]-containing ILs were determined by means of a quartz-crystal microbalance method (QCM) and their densities were measured with an oscillating U-tube. These thermodynamic data have been used to develop a method for assessment of miscibility of conventional solvents in the selenium-containing ILs by using Hildebrandt solubility parameters, as well as for modeling with the electrolyte perturbed-chain statistical associating fluid theory (ePC-SAFT) method. Furthermore, structure-property relations between selenoether-functionalized and similarly shaped corresponding aryl-substituted imidazolium- and pyridinium-based ILs were analyzed and showed that the contribution of the selenium moiety to the enthalpy of vaporization of an IL is equal to the contribution of a methylene (CH2) group. An incremental approach to predict vaporization enthalpies of ILs by a group contribution method has been developed. The reaction of these ILs with zinc acetate dihydrate under microwave irradiation led to ZnSe nanoparticles of an average diameter between 4 and 10 nm, depending on the reaction conditions.

Intermolecular Interaction Tuning of Spin-Crossover Iron(III) Complexes with Aromatic Counteranions

Iron(III) spin-crossover (SCO) complexes [Fe(qsal)₂]BS·MeOH·H₂O (1), [Fe(qsal)₂](NS)·MeOH (2), [Fe(qnal)₂](NS) (3), and [Fe(qnal)₂]PS·MeOH·CH₂Cl₂ (4) (Hqsal, N-(8-quinolinyl)salicylaldimine; Hqnal, N-(8-quinolinyl)-2-hydroxy-1-naphthaldimine; BS, benzenesulfonate; NS, 1-naphthalenesulfonate; PS, 1-pyrenesulfonate) have been synthesized and characterized by X-ray structure determinations and temperature-dependent magnetic susceptibility measurements. The aromatic counteranions BS, NS, and PS can be used for the tuning of intermolecular coupling through a variety of weak interactions. All of the complexes show temperature-dependent SCO behavior. but the light-induced excited spin-state trapping (LIESST) effect was observed only for 1, 3, and 4 when the samples were illuminated (λ 808 nm) for 1 h at 5 K. In particular, 59% of the LS form of 1 was converted to the metastable HS state by illumination, equal to the highest degree of conversion yet known for LIESST in [Fe(qsal)₂]⁺ derivatives. The lack of a LIESST effect for 2 may be due to the relatively limited degree of interaction between the cations and anions in the lattice, reflected in a much longer minimum Fe···Fe separation in this complex in comparison to the others.