The Role of Hydrogen‐Bonding Interactions in the Ultrafast Relaxation Dynamics of the Excited States of 3‐ and 4‐Aminofluoren‐9‐ones

The dynamics of the excited states of 3‐ and 4‐aminofluoren‐9‐ones (3AF and 4AF, respectively) are investigated in different kinds of solvents by using a subpicosecond time‐resolved absorption spectroscopic technique. They undergo hydrogen‐bonding interaction with protic solvents in both the ground and excited states. However, this interaction is more significant in the lowest excited singlet (S₁) state because of its substantial intramolecular charge‐transfer character. Significant differences in the spectroscopic characteristics and temporal dynamics of the S₁ states of 3AF and 4AF in aprotic and protic solvents reveal that the intermolecular hydrogen‐bonding interaction between the S₁ state and protic solvents plays an important role in its relaxation process. Perfect linear correlation between the relaxation times of the S₁ state and the longitudinal relaxation times (τ_L) of alcoholic solvents confirms the prediction regarding the solvation process via hydrogen‐bond reorganization. In the case of weakly interacting systems, the relaxation process can be well described by a dipolar solvation‐like process involving rotation of the OH groups of the alcoholic solvents, whereas in solvents having a strong hydrogen‐bond‐donating ability, for example, methanol and trifluoroethanol, it involves the conversion of the non‐hydrogen‐bonded form to the hydrogen‐bonded complex of the S₁ state. Efficient radiationless deactivation of the S₁ state of the aminofluorenones by protic solvents is successfully explained by the energy‐gap law, by using the energy of the fully solvated S₁ state determined from the time‐resolved spectroscopic data.     

Ultrafast Relaxation Dynamics of the Excited States of 1‐Amino‐ and 1‐(N,N‐Dimethylamino)‐fluoren‐9‐ones

The dynamics of the excited states of 1‐aminofluoren‐9‐one (1AF) and 1‐(N,N‐dimethylamino)‐fluoren‐9‐one (1DMAF) are investigated by using steady‐state absorption and fluorescence as well as subpicosecond time‐resolved absorption spectroscopic techniques. Following photoexcitation of 1AF, which exists in the intramolecular hydrogen‐bonded form in aprotic solvents, the excited‐state intramolecular proton‐transfer reaction is the only relaxation process observed in the excited singlet (S₁) state. However, in protic solvents, the intramolecular hydrogen bond is disrupted in the excited state and an intermolecular hydrogen bond is formed with the solvent leading to reorganization of the hydrogen‐bond network structure of the solvent. The latter takes place in the timescale of the process of solvation dynamics. In the case of 1DMAF, the main relaxation pathway for the locally excited singlet, S₁(LE), or S₁(ICT) state is the configurational relaxation, via nearly barrierless twisting of the dimethylamino group to form the twisted intramolecular charge‐transfer, S₁(TICT), state. A crossing between the excited‐state and ground‐state potential energy curves is responsible for the fast, radiationless deactivation and nonemissive character of the S₁(TICT) state in polar solvents, both aprotic and protic. However, in viscous but strong hydrogen‐bond‐donating solvents, such as ethylene glycol and glycerol, crossing between the potential energy surfaces for the ground electronic state and the hydrogen‐bonded complex formed between the S₁(TICT) state and the solvent is possibly avoided and the hydrogen‐bonded complex is weakly emissive.     

Towards Magnetic Carbo‐meric Molecular Materials

Numerous studies have underlined the putative diradical character of π‐conjugated molecules that can be described by closed‐shell Lewis structures, for instance, p‐dimethylene p–n phenylenes, or long polyacenes. In the latter compounds, the only way to save the aromaticity of the six‐membered rings is to give up the Lewis electron pairing in the singlet biradical ground state. The present work considers the possibility of doing the same by using the basic C₂ units of carbo‐meric architectures. A series of acyclic and cyclic carbo‐meric architectures is studied by using UB3LYP DFT broken‐symmetry calculations, including spin decontaminations and subsequent geometry optimization of the singlet diradical. The C₂ units are shown to stabilize the singlet biradical by spin delocalization, two of them playing approximately the same role as one radical‐insulating 1,4 phenylene moiety. The results are generalized to the investigation of open‐shell polyradical singlet states of rigid hydrocarbon structures, the symmetry and rigidity of which can assist cooperativity and self spin polarization effect. Several synthesis targets with challenging magnetic/spin properties are suggested in the carbo‐mer series.     

Studies on the Magnetic Ground State of a Spin Möbius Strip

Here we report the synthesis, structure and detailed characterisation of three n‐membered oxovanadium rings, Na_n[(V=O)_nNa_n(H₂O)_n(α, β, or γ‐CD)₂]?m H₂O (n=6, 7, or 8), prepared by the reactions of (V=O)SO₄?x H₂O with α, β, or γ‐cyclodextrins (CDs) and NaOH in water. Their alternating heterometallic vanadium/sodium cyclic core structures were sandwiched between two CD moieties such that O‐Na‐O groups separated the neighbouring vanadyl ions. Antiferromagnetic interactions between the S=1/2 vanadyl ions led to S=0 ground states for the even‐membered rings, but to two quasi‐degenerate S=1/2 states for the spin‐frustrated heptanuclear cluster.     

Dramatic Influence of an Anionic Donor on the Oxygen‐Atom Transfer Reactivity of a MnV–Oxo Complex

Addition of an anionic donor to an Mn^V(O) porphyrinoid complex causes a dramatic increase in 2‐electron oxygen‐atom‐transfer (OAT) chemistry. The 6‐coordinate [Mn^V(O)(TBP₈Cz)(CN)]^? was generated from addition of Bu₄N⁺CN^? to the 5‐coordinate Mn^V(O) precursor. The cyanide‐ligated complex was characterized for the first time by Mn K‐edge X‐ray absorption spectroscopy (XAS) and gives Mn?O=1.53 Å, Mn?CN=2.21 Å. In combination with computational studies these distances were shown to correlate with a singlet ground state. Reaction of the CN^? complex with thioethers results in OAT to give the corresponding sulfoxide and a 2e^?‐reduced Mn^III(CN)^? complex. Kinetic measurements reveal a dramatic rate enhancement for OAT of approximately 24 000‐fold versus the same reaction for the parent 5‐coordinate complex. An Eyring analysis gives ΔH^≠=14 kcal mol^?1, ΔS^≠=?10 cal mol^?1 K^?1. Computational studies fully support the structures, spin states, and relative reactivity of the 5‐ and 6‐coordinate Mn^V(O) complexes.     

[CrF(O2CtBu)2]9: Synthesis and Characterization of a Regular Homometallic Ring with an Odd Number of Metal Centers and Electrons

The first regular homometallic ring containing an odd number of metal centers is reported. The ring was synthesized by means of amine‐templated self‐assembly. Extensive physical characterization studies, including magnetic measurements, powder inelastic neutron scattering (INS), and DFT calculations, show that the molecule has a near perfect match to the expected behavior for a frustrated system with the lowest energy pair of S=1/2 spin states separated by only 0.1 meV (0.8 cm^?1).     

Dioxygen Activation by Non‐Adiabatic Oxidative Addition to a Single Metal Center

A chromium(I) dinitrogen complex reacts rapidly with O₂ to form the mononuclear dioxo complex [Tp^tBu,MeCr^V(O)₂] (Tp^tBu,Me=hydrotris(3‐tert‐butyl‐5‐methylpyrazolyl)borate), whereas the analogous reaction with sulfur stops at the persulfido complex [Tp^tBu,MeCr^III(S₂)]. The transformation of the putative peroxo intermediate [Tp^tBu,MeCr^III(O₂)] (S=³/₂) into [Tp^tBu,MeCr^V(O)₂] (S=¹/₂) is spin‐forbidden. The minimum‐energy crossing point for the two potential energy surfaces has been identified. Although the dinuclear complex [(Tp^tBu,MeCr)₂(μ‐O)₂] exists, mechanistic experiments suggest that O₂ activation occurs on a single metal center, by an oxidative addition on the quartet surface followed by crossover to the doublet surface.     

A Redox‐Active Bridging Ligand to Promote Spin Delocalization,High‐Spin Complexes,and Magnetic Multi‐Switchability

A dinuclear Co^II complex, [Co₂(tphz)(tpy)₂]ⁿ⁺ (n=4, 3 or 2; tphz: tetrapyridophenazine; tpy: terpyridine), has been assembled using the redox‐active and strongly complexing tphz bridging ligand. The magnetic properties of this complex can be tuned from spin‐crossover with T_1/2≈470 K for the pristine compound (n=4) to single‐molecule magnet with an S_T=5/2 spin ground state when once reduced (n=3) to finally a diamagnetic species when twice reduced (n=2). The two successive and reversible reductions are concomitant with an increase of the spin delocalization within the complex, promoting remarkably large magnetic exchange couplings and high‐spin species even at room temperature.     

Long‐Lived Spin States for Low‐Field Hyperpolarized Gas MRI

Parahydrogen induced polarization was employed to prepare a relatively long‐lived correlated nuclear spin state between methylene and methyl protons in propane gas. Conventionally, such states are converted into a strong NMR signal enhancement by transferring the reaction product to a high magnetic field in an adiabatic longitudinal transport after dissociation engenders net alignment (ALTADENA) experiment. However, the relaxation time T₁ of ～0.6 s of the resulting hyperpolarized propane is too short for potential biomedical applications. The presented alternative approach employs low‐field MRI to preserve the initial correlated state with a much longer decay time T_LLSS=(4.7±0.5) s. While the direct detection at low‐magnetic fields (e.g. 0.0475 T) is challenging, we demonstrate here that spin‐lock induced crossing (SLIC) at this low magnetic field transforms the long‐lived correlated state into an observable nuclear magnetization suitable for MRI with sub‐millimeter and sub‐second spatial and temporal resolution, respectively. Propane is a non‐toxic gas, and therefore, these results potentially enable low‐cost high‐resolution high‐speed MRI of gases for functional imaging of lungs and other applications.     

Quantum Monte Carlo Simulations and High‐Field Magnetization Studies of Antiferromagnetic Interactions in a Giant Hetero‐Spin Ring

Chromium lanthanide heterometallic wheel complexes {Cr₈Ln₈} (Ln=Gd, Dy and Y) with alternating metal centres are presented. Quantum Monte Carlo simulations reveal antiferromagnetic exchange‐coupling constants with an average of 2.1 K within the {Cr₈Gd₈} wheel, which leads to a large ground spin state (S _T=16) that is confirmed by magnetization studies up to 20 Tesla. The {Cr₈Dy₈} wheel is a single‐molecule magnet.     

Electronic Structure Modulation in an Exceptionally Stable Non‐Heme Nitrosyl Iron(II) Spin‐Crossover Complex

The highly stable nitrosyl iron(II) mononuclear complex [Fe(bztpen)(NO)](PF₆)₂ (bztpen=N‐benzyl‐N,N′,N′‐tris(2‐pyridylmethyl)ethylenediamine) displays an S=1/2?S=3/2 spin crossover (SCO) behavior (T_1/2=370 K, ΔH=12.48 kJ mol^?1, ΔS=33 J K^?1 mol^?1) stemming from strong magnetic coupling between the NO radical (S=1/2) and thermally interconverted (S=0?S=2) ferrous spin states. The crystal structure of this robust complex has been investigated in the temperature range 120–420 K affording a detailed picture of how the electronic distribution of the t_2g–e_g orbitals modulates the structure of the {FeNO}⁷ bond, providing valuable magneto–structural and spectroscopic correlations and DFT analysis.     

Square‐Planar Ruthenium(II) Complexes: Control of Spin State by Pincer Ligand Functionalization

Functionalization of the PNP pincer ligand backbone allows for a comparison of the dialkyl amido, vinyl alkyl amido, and divinyl amido ruthenium(II) pincer complex series [RuCl{N(CH₂CH₂PtBu₂)₂}], [RuCl{N(CHCHPtBu₂)(CH₂CH₂PtBu₂)}], and [RuCl{N(CHCHPtBu₂)₂}], in which the ruthenium(II) ions are in the extremely rare square‐planar coordination geometry. Whereas the dialkylamido complex adopts an electronic singlet (S=0) ground state and energetically low‐lying triplet (S=1) state, the vinyl alkyl amido and the divinyl amido complexes exhibit unusual triplet (S=1) ground states as confirmed by experimental and computational examination. However, essentially non‐magnetic ground states arise for the two intermediate‐spin complexes owing to unusually large zero‐field splitting (D>+200 cm^?1). The change in ground state electronic configuration is attributed to tailored pincer ligand‐to‐metal π‐donation within the PNP ligand series.     

Synthesis and Magnetic Properties of (Pyrrolidin‐1‐oxyl)–(Nitronyl Nitroxide)/(Iminonitroxide)‐Dyads

Unlike extensively studied diradicals linked by π‐conjugated systems, only a few studies have investigated weakly coupled diradicals linked by an sp³ carbon atom. Herein, we prepared pyrrolidin‐1‐oxyl–(nitronyl nitroxide)‐dyad 5 and pyrrolidin‐1‐oxyl–iminonitroxide‐dyad 6 . From the observed temperature dependence of the magnetic susceptibility, 5 and 6 were determined to be in singlet ground states with 2J_intra/k_B=?35.2 K and ?13.6 K, respectively. From these results and theoretical calculations of related diradicals, the spin‐polarization model counting the small spin density of the sp³ carbon atom could be used as a spin‐prediction model.     

Intramolecular Spin Alignment within Mono‐Oxidized and Photoexcited Anthracene‐Based π Radicals as Prototypical Photomagnetic Molecular Devices: Relationships Between Electrochemical,Photophysical, and Photochemical Control Pathways

AnOV is a π‐conjugated radical built from an anthracene (An) unit linked by a p‐phenylene to an oxoverdazyl (OV) moiety. The mono‐oxidized (cationic) form of AnOV was generated both electrochemically and photochemically (in the presence of an electron acceptor). The triplet nature (S=1) of the electronic ground state of AnOV ⁺ was demonstrated by combining spectroelectrochemistry, electron‐spin resonance (ESR) experiments, and ab initio molecular orbital (MO) calculations. The intramolecular spin alignment (ISA) within AnOV ⁺ results from the ferromagnetic coupling (J_electrochem>0) of the two unpaired electrons located on the oxidized electron donor (An⁺) and on the pendant OV radical. The spin‐density distribution pattern of AnOV ⁺ is akin to that of AnOV when photopromoted ( AnOV *) to its high‐spin (HS) lowest excited quartet (S=3/2) state. This high‐spin state results from the ferromagnetic coupling (J_photophys>0) of the triplet locally excited state of An (³An*) with the doublet ground state of OV. As a shared salient feature, AnOV ⁺ and AnOV * (HS) show a spin delocalization within the domain of activated An in either An⁺ or ³An* (nexus states) forms. The present study essentially contributes to establish and clarify relationships between electrochemical, photophysical, and photochemical pathways to achieve ISA processes within AnOV . In particular, we discuss the impact of the spin polarization of the unpaired electron of OV on electronic features of the An electron‐donating subunit. Close analysis of this polarizing interplay allows one to derive a novel functional paradigm to manipulate electron spins at the intramolecular level with light and under an external magnetic field. Indeed, two original functional elements are identified: light‐triggered donors of spin‐polarized electrons and spin‐selective electron acceptors, which are of potential interest for molecular spintronics.     

Influence of Isotope Substitution on Lattice and Spin‐Peierls‐Type Transition Features in One‐Dimensional Nickel Bis‐dithiolene Spin Systems

Four new 1D spin‐Peierls‐type compounds, [D₅]1‐(4′‐R‐benzyl)pyridinium bis(maleonitriledithiolato)nickelate ([D₅]R‐Py; R=F, I, CH₃, and NO₂), were synthesized and characterized structurally and magnetically. These 1D compounds are isostructural with the corresponding non‐deuterated compounds, 1‐(4′‐R‐benzyl)pyridinium bis(maleonitriledithiolato)nickelate (R‐Py; R=F, I, CH₃, and NO₂). Compounds [D₅]R‐Py and R‐Py (R=F, I, CH₃, and NO₂) crystallize in the monoclinic space group P2₁/c with uniform stacks of anions and cations in the high‐temperature phase and triclinic space group P$\bar 1$ with dimerized stacks of anions and cations in the low‐temperature phase. Similar to the non‐deuterated R‐Py compounds, a spin‐Peierls‐type transition occurs at a critical temperature for each [D₅]R‐Py compound; the magnetic character of the 1D S=1/2 ferromagnetic chain for [D₅]F‐Py and the 1D S=1/2 Heisenberg antiferromagnetic chain for others appear above the transition temperature. Spin‐gap magnetic behavior was observed for all of these compounds below the transition temperature. In comparison to the corresponding R‐Py compound, the cell volume is almost unchanged for [D₅]F‐Py and shows slight expansion for [D₅]R‐Py (R=I, CH₃, and NO₂) as well as an increase in the spin‐Peierls‐type transition temperature for all of these 1D compounds in the order of F>I≈CH₃≈NO₂. The large isotopic effect of nonmagnetic countercations on the spin‐Peierls‐type transition critical temperature, T_C, can be attributed to the change in ω₀ with isotope substitution.     

trans‐Bis(cyano‐κC)(1,4,8,11‐tetraazacyclotetradecane‐κ4N)manganese(III) perchlorate,a low‐spin manganese(III) complex

The crystal structure of the low‐spin (S = 1) Mn^III complex [Mn(CN)₂(C₁₀H₂₄N₄)]ClO₄, or trans‐[Mn(CN)₂(cyclam)](ClO₄) (cyclam is the tetradentate amine ligand 1,4,8,11‐tetra­aza­cyclo­tetra­decane), is reported. The structural parameters in the Mn(cyclam) moiety are found to be insensitive to both the spin and the oxidation state of the Mn ion. The difference between high‐ and low‐spin Mn^III complexes is that a pronounced tetragonal elongation of the coordination octahedron occurs in high‐spin complexes and a slight tetragonal compression is seen in low‐spin complexes, as in the title complex.     

Revisiting the Intriguing Electronic Features of the BeOBeC Carbyne and Some Isomers: A Quantum‐Chemical Assessment

Extensive high‐level quantum‐chemical calculations reveal that the rod‐shaped molecule BeOBeC, which was recently generated in matrix experiments, exists in two nearly isoenergetic states, the ⁵Σ quintet (⁵ 6 ) and the ³Σ triplet (³ 6 ). Their IR features are hardly distinguishable at finite temperature. The major difference concerns the mode of spin coupling between the terminal beryllium and carbon atoms. Further, the ground‐state potential‐energy surface of the [2Be,C,O] system at 4 K is presented and differences between the photochemical and thermal behaviors are highlighted. Finally, a previously not considered, so far unknown C_2v‐symmetric rhombus‐like four‐membered ring ³[Be(O)(C)Be] (³ 5 ) is predicted to represent the global minimum on the potential‐energy surface.     

The molecular structure of high‐spin (S = ) manganese(II) phthalocyanine in tetrabutylammonium bromido(phthalocyaninato)manganese(II)

The title complex salt, (C₁₆H₃₆N)[MnBr(C₃₂H₁₆N₈)] or (TBA)[Mn^IIBr(Pc)] (TBA is tetrabutylammonium and Pc is phthalocyaninate), has been obtained as single crystals by the diffusion technique and its crystal structure was determined using X‐ray diffraction. The high‐spin (S = ) [Mn^IIBr(Pc)]⁻ macrocycle has a concave conformation, with an average equatorial Mn—N(Pc) bond length of 2.1187 (19) Å, an axial Mn—Br bond length of 2.5493 (7) Å and with the Mn^II cation displaced out of the 24‐atom Pc plane by 0.894 (2) Å. The geometry of the Mn^IIN₄ fragment in [Mn^IIBr(Pc)]⁻ is similar to that of the high‐spin (S = ) manganese(II) tetraphenylporphyrin (TPP) in [Mn^II(1‐MeIm)(TPP)] (1‐MeIm is 1‐methylimidazole).     

Synthesis,Structures, and Magnetic Properties of Chiral One‐dimensional CrIII‐MnIII Heterobimetallic Complexes Based on [(Tp)Cr(CN)3]–

Two Cr^III‐Mn^III heterobimetallic compounds, [Mn((R,R)‐5‐MeOSalcy)Cr(Tp)(CN)₃ · 2CH₃CN]_n ( 1‐RR ) and [Mn((S,S)‐5‐MeOSalcy)Cr(Tp)(CN)₃·2CH₃CN]_n ( 1‐SS ) [Salcy = N,N′‐(1,2‐cyclohexanediylethylene)bis(salicylideneiminato) dianion], were synthesized by using the tricyanometalate building block, [(Tp)Cr(CN)₃]^– [Tp = tris(pyrazolyl) hydroborate] and chiral Mn^III Schiff base precursors. Structural analyses and circular dichroism (CD) spectra revealed that 1‐RR and 1‐SS are a pair of enantiomers containing a neutral cyano‐bridged zigzag chain with (–Cr–C≡N–Mn–N≡C–)_n as the repeating unit. Magnetic studies show that antiferromagnetic couplings between Cr^III and Mn^III ions occur by cyanide bridges. 1‐RR and 1‐SS present metamagnetic, spin‐canting, and antiferromagnetic order behaviors at low temperatures.     

EPR Studies on Branched High‐Spin Arylnitrenes

The UV (λ>305 nm) photolysis of triazide 3 in 2‐methyl‐tetrahydrofuran glass at 7 K selectively produces triplet mononitrene 4 (g=2.003, D_T=0.92 cm^?1, E_T=0 cm^?1), quintet dinitrene 6 (g=2.003, D_Q=0.204 cm^?1, E_Q=0.035 cm^?1), and septet trinitrene 8 (g=2.003, D_S=?0.0904 cm^?1, E_S=?0.0102 cm^?1). After 45 min of irradiation, the major products are dinitrene 6 and trinitrene 8 in a ratio of ～1:2, respectively. These nitrenes are formed as mixtures of rotational isomers each of which has slightly different magnetic parameters D and E. The best agreement between the line‐shape spectral simulations and the experimental electron paramagnetic resonance (EPR) spectrum is obtained with the line‐broadening parameters Γ(E_Q)=180 MHz for dinitrene 6 and Γ(E_S)=330 MHz for trinitrene 8 . According to these line‐broadening parameters, the variations of the angles Θ in rotational isomers of 6 and 8 are expected to be about ±1 and ±3°, respectively. Theoretical estimations of the magnetic parameters obtained from PBE/DZ(COSMO)//UB3LYP/6‐311+G(d,p) calculations overestimate the E and D values by 1 and 8 %, respectively. Despite the large distances between the nitrene units and the extended π systems, the zero field splitting (zfs) parameters D are found to be close to those in quintet dinitrenes and septet trinitrenes, where the nitrene centers are attached to the same aryl ring. The large D values of branched septet nitrenes are due to strong negative one‐center spin–spin interactions in combination with weak positive two‐center spin–spin interactions, as predicted by theoretical considerations.