Imino Nitroxyl Research Articles

Reaching Optimal Light‐Induced Intramolecular Spin Alignment within Photomagnetic Molecular Device Prototypes

Ground-state (GS) and excited-state (ES) properties of novel photomagnetic molecular devices (PMMDs) are investigated by means of density functional theory. These organic PMMDs undergo a ferromagnetic alignment of their intramolecular spins in the lowest ES. They are comprised of: 1) an anthracene unit (An) as both the photosensitizer (P) and a transient spin carrier (SC) in the triplet ES ((3)An*); 2) imino-nitroxyl (IN) or oxoverdazyl (OV) stable radical(s) as the dangling SC(s); and 3) bridge(s) (B) connecting peripheral SC(s) to the An core at positions 9 and 10. Improving the efficiency of the PMMDs involves strengthening the ES intramolecular exchange coupling (J(ES)) between transient and persistent SCs, hence the choice of 2-pyrimidinyl (pm) as B elements to replace the original p-phenylene (ph). Dissymmetry of the pm connectors leads to [SC-B-P-B-SC] regio-isomers int. and ext., depending on whether the pyrimidinic nitrogen atoms point towards the An core or the peripheral SCs, respectively. For the int. regio-isomers we show that the photoinduced spin alignment is significantly improved because the J(ES)/k(B) value is increased by a factor of more than two compared with the ph-based analogue (J(ES)/k(B)>+400 K). Most importantly, we show that the optimal J(ES)/k(B) value ( approximately +600 K) could be reached in the event of an unexpected saddle-shaped structural distortion of the lowest ES. Accounting for this intriguing behavior requires dissection of the combined effects of 1) borderline intramolecular steric hindrance about key An-pm linkages, which translates into the flatness of the potential energy surface; 2) spin density disruption due to the presence of radicals; and 3) possibly intervening photochemistry, with An acting as a light-triggered electron donor while pm, IN, and OV behave as electron acceptors. Finally, potentialities attached to the [(SC)-pm-An-pm](int) pattern are disclosed.

Peculiarity in the Electronic Structure of Cu(II) Complex Ferromagnetically Coupled with Bisimino Nitroxides

By means of the electron spin resonance (ESR) technique, we have investigated the electronic structures of the tridentate imino nitroxyl diradical complex with copper(II) (Cu-bisimpy), which has a square planar structure and a ground quartet state with an extremely strong ferromagnetic exchange interaction, and its related compounds (bisimpy = 2,6-bis(1'-oxyl-4',4',5',5'-tetramethyl-4',5'-dihydro-1' H-imidazol-2'-yl)pyridine). It was clarified that Cu-bisimpy had unique magnetic orbitals, compared with the biradical ligand (bisimpy), a zinc(II) biradical complex (Zn-bisimpy) and a copper(II) terpyridine complex (Cu-tpy) (tpy = 2,2';6',2''-terpyridine). Multifrequency ESR spectroscopy provided a reliable set of magnetic parameters of Cu-bisimpy, which has a small g anisotropy ( g x = 2.02, g y = 2.01, g z = 2.08) and small hyperfine coupling with Cu (|A x| = 42.0 MHz, |A y|<or= 14 MHz, |A z| = 153 MHz) but huge zero-field splitting (D = +17.4 GHz, E = -1.0 GHz). The maximum principal axis of zero-field interaction ( z ZF) is perpendicular to the z axis for the g and A tensors, which is normal to the molecular plane. These characteristics of the magnetic properties prove that the substantial spin transfer from the d x (2) - y (2) orbital of copper to the n-orbitals of the ligand is caused by a sigma-type covalent bonding effect between the central metal and the ligand nitrogens. The covalent bonding effect produces carbene configurations on the nitrogen atoms of the imino nitroxyl radicals. The carbene configuration was concluded to be the main reason for the strong ferromagnetic coupling in Cu-bisimpy. Multifrequency electron spin resonance spectroscopy clarified the unique electronic structure of a square planar copper(II) complex with an imino nitroxyl diradical, which undergoes a strong ferromagnetic interaction caused by a covalent bonding effect.

Intramolecular Spin Alignment in Photomagnetic Molecular Devices: A Theoretical Study

Ground- and excited-state magnetic properties of recently characterized pi-conjugated photomagnetic organic molecules are analyzed by the means of density functional theory (DFT). The systems under investigation are made up of an anthracene (An) unit primarily acting as a photosensitizer (P), one or two iminonitroxyl (IN) or oxoverdazyl (OV) stable organic radical(s) as the dangling spin carrier(s) (SC), and intervening phenylene connector(s) (B). The magnetic behavior of these multicomponent systems, represented here by the Heisenberg-Dirac magnetic exchange coupling (J), as well as the EPR observables (g tensors and isotropic A values), are accurately modeled and rationalized by using our DFT approach. As the capability to quantitatively assess intramolecular exchange coupling J in the excited state makes it possible to undertake rational optimization of photomagnetic systems, DFT was subsequently used to model new compounds exhibiting different connection schemes for their functional components (P, B, SC). We show in the present work that it is worthwhile considering the triplet state of anthracene, that is, P when promoted in its lowest photoexcited state, as a full magnetic site in the same capacity as the remote SCs. This framework allows us to accurately account for the interplay between transient ((3)An) and persistent (IN, OV) spin carriers, which magnetically couple according to a sole polarization mechanism essentially supported by phenyl connector(s). From our theoretical investigations of photoinduced spin alignment, some general rules are proposed and validated. Relying on the analysis of spin-density maps, they allow us to predict the magnetic behavior of purely organic magnets in both the ground and the excited states. Finally, the notion of photomagnetic molecular devices (PMMDs) is derived and potential application towards molecular spintronics disclosed.

Effect of Imino Nitroxyl and Nitronyl Nitroxyl Groups on the Photochromic Reactivity of Diarylethenes

Diarylethene derivatives having imino nitroxide and nitronyl nitroxide have been prepared to examine the effect of the radical substituents on the photochromic reactivity of 1,2-bis(2-methyl-1-benzothiophen-3-yl)perfluorocyclopentene. These radical substituents reduce the quantum yields of both cyclization and cycloreversion reactions. The nitronyl nitroxyl moiety is more effective to suppress the reactivity in comparison with the imino nitroxide moiety. [reaction: see text]

Experimental and theoretical studies on ferromagnetically coupled metal complexes with imino nitroxides.

Copper(II), zinc(II), and nickel(II) complexes with tridentate imino nitroxyl diradicals, [CuCl(bisimpy)(MeOH)](PF(6)) (1), [ZnCl(2)(bisimpy)] (2), and [NiCl(bisimpy)(H(2)O)(2)]Cl x 2H(2)O (3) (bisimpy = 2,6-bis(1'-oxyl-4',4',5',5'-tetramethyl-4',5'-dihydro-1'H-imidazol-2'-yl)pyridine), were prepared, and their magnetic properties were studied. In 1, the Cu(II) ion has a square pyramidal coordination geometry, of which the equatorial coordination sites are occupied by three nitrogen atoms from the bisimpy and a chloride ion. The coordination geometry of the Zn(II) ion in 2 can be described as a trigonal bipyramid, with two chloride ions and a bisimpy. In 3, the Ni(II) ion has a distorted octahedral coordination geometry, of which four coordination sites are coordinated by the bisimpy and chloride ion, and two water molecules occupy the remaining cis positions. Magnetic susceptibility and EPR measurements revealed that in 1 and 3 the Cu(II) and Ni(II) ions with imino nitroxyl diradicals were ferromagnetically coupled, with the coupling constants J (H = -2J(ij) summation operator S(i)S(j)) of +165(1) and 109(2) cm(-1), respectively, and the intraligand ferromagnetic interactions in 1-3 were very weak. DFT molecular orbital calculations were performed on the diradical ligand, 1, and 2 to study the spin density distribution before and after coordination to the metal ions.

Metal Complexes with an Imino Nitroxyl Diradical

Copper(II), nickel(II), and zinc(II) complexes with tridentate imino nitroxyl diradicals were prepared and their magnetic properties were studied. The copper(II) and nickel(II) complexes showed substantial ferromagnetic interactions being operative between metal centers and coordinated radical ligands.

Ferromagnetic interactions in metal complexes with imino nitroxides

Copper(II), nickel(II), and zinc(II) complexes with tridentate imino nitroxyl diradicals, [CuCl(bisimpy)(MeOH)](PF 6) ( 1), [NiCl(bisimpy)(H 2O) 2]Cl·2H 2O ( 2), and [ZnCl 2(bisimpy)] ( 3) (bisimpy=2,6-bis(1′-oxyl-4′,4′,5′,5′-tetramethyl-4′,5′-dihydro-1′ H-imidazol-2′-yl)pyridine), were prepared and their magnetic properties studied. In 1–3 the diradicals are ferromagnetically coupled with the inter-radical coupling constant of J = + 6.5 cm - 1 . In 1 and 2 the ferromagnetic interactions between the paramagnetic metal ions and the coordinated imino nitroxides are operative with the J values of +165 and +120 cm −1 ( H=−2 J ij ∑ S i S j ), respectively. Magnetic properties of a Cu(II) complex with a mono radical ligand, [CuCl 2(impy)] ( 4) (impy=2-(1′-oxyl-4′,4′,5′,5′-tetramethyl-4′,5′-dihydro-1′ H-imidazol-2′-yl)pyridine), were also studied; a ferromagnetic coupling constant of J = + 50 ( 2 ) cm - 1 was obtained.

NO–hemoglobin may be a light-sensitive source of nitric oxide both in solution and in red blood cells

Hemoglobin in solution and inside red blood cells forms a complex with nitric oxide exhibiting a specific EPR signal both at room and liquid nitrogen temperatures. In the present paper it was shown that the nitrosyl complex of hemoglobin (NO–Hb) is photochemically sensitive and hence may serve as a source of free NO under He–Cd laser irradiation (441 nm). It was found that at laser light radiant power of 3.9 mW, room temperature and in the presence of oxygen, 50% decrease of NO–Hb EPR signal occurred at doses of 54, 30, and 18 kJ/m 2 for NO–hemoglobin solution, hemolysed and intact erythrocytes, respectively. The detection of free NO produced as a result of NO–Hb photolysis was performed by means of a spin trap, nitronyl nitroxyl radical NNR, which in the presence of NO is transformed into imino nitroxyl radical (INR) showing different EPR signal. In isolated hemoglobin solution, 20 mM INR was accumulated under irradiation with the maximal dose of 700 kJ/m 2. In intact cells the HbFe 2+–NO photolysis and NO release occur with essentially higher efficacy. To produce 100 mM INR, a dose of 290 kJ/m 2 was needed in erythrocyte lysates and 100 kJ/m 2 in intact red blood cell suspension. Measurements of absorption spectra showed that in all systems studied (NO–Hb in solution, intact erythrocytes and hemolysed erythrocytes) NO–Hb concentration decreased after irradiation by 14–22% with simultaneous formation of methemoglobin. These observations show that NO–Hb may serve as a store of nitric oxide from which free NO can be released by intensive illumination.

Assembly of Metal Complexes

The reactions of silver(I) hexafuluorophosphate with imino nitroxyl diradicals yield complexes of [Ag2(pyrd-im2)2](PF6)2·2CH3OH (1) and [Ag2(bpy-im2)2](PF6)2·CH3OH (2) (pyrd-im2 = 3,6-bis(l'-oxyl-4′,4′,5′,5′-tetramethyl-4′,5′-dihydro-1′H-imidazol-2′-yl)pyridazine) and bpy-im2 = 2,2′-bis(1′-oxyl-4′,4′,5′,5′-tetramethyl-4′,5′-dihydro-1′H-imidazol-2′-yl)bipyridine)). Compelxes 1 and 2 have four radicals being double-helically assembled by the coordination to the silver ions. Magnetic susceptibility measurements revealed a strong antiferromagnetic interaction (-70.8(7) cm with H=2JS 1·S2) in 1 being operative through the pyridazine group, which was understood by the spin polarization mechanism. The reaction of MnSO4·4H2O with K2CrO4 in water yielded highly crystalline black 3. X-ray crystal-lographic study revealed that 3 has the chemical formula K[Mn2(μ-CrO4)2(μ-OH)2], which implies that the manganese ions are mixed valent Mn(II,III). Magnetic susceptibility measurement reveals that the antiferromagnetic interactions are operative without antiferromagnetic ordering and the exchange coupling constant was estimated to be −1.4 cm−1

Double helical assembly of imino nitroxyl diradicals by silver(I) ions

The co-ordination of imino nitroxyl diradicals to diamagnetic silver(I) ions led to the self-assembled double helical dimer [Ag2(bisimpy)2][PF6]2 1 (diradical bisimpy = 2,6-bis(4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-2-yl 1-oxyl)pyridine). The diradical has two imino nitroxyl groups linked by the pyridine group at its meta positions and acts as a tridentate ligand. In the double helicate two bisimpy bridge two silver(I) ions in a {4 + 2} connectivity, and the silver(I) ions have a very short contact of 2.8943(5) Å. Magnetic susceptibility measurements and crystal structure analysis revealed that the two radical moieties in bisimpy are ferromagnetically coupled (J = 17.4(8) cm–1; H = –2JΣSi·Sj) through the pyridine group, and imino nitroxyl overlaps between adjacent molecules result in an intermolecular antiferromagnetic interaction (–52.3(7) cm–1).

ESR and 1H NMR studies of a new class of nitroxyl, nitronylnitroxyl and iminonitroxyl radicals

AbstractA new synthesis of nitronylnitroxyl (NNR), iminonitroxyl (INR) and nitroxyl radicals (NR) is suggested, involving oxidation of 4H‐imidazol di‐N‐oxides with O2 or PbO2 in the presence of nucleophilic reagents. This method allowed the preparation of radicals which could not be synthesized by known procedures. These radicals were studied by ESR and 1H NMR spectroscopy. The hyperfine interaction constants of the NNR were measured and assigned (in some cases radicals containing 15N were used). The influence of protonation and deprotonation of functional groups (OH, NH2) in the NNR on their ESR spectra was investigated and the pK values of these radicals were measured by ESR.Differences between the ESR spectra of two isomeric INR were found, and the structures of these radicals were established by 1H NMR. Changes in spin density delocalization in 3‐imidazoline 3‐oxide nitroxyl radicals were studied, and the spin density was found to be ‘pushed out’ to the peripheral fragments of these radicals on increasing the number of methoxy groups in the α‐position to the N—O. fragment. Hindered rotation around the σ‐bond of a bulky substituent in the α‐position to the N—O. fragment was detected by ESR. The activation energy of this hindered rotation was ΔE = 30±1.7 kJ mol−1.