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1.
Dalton Trans ; 49(27): 9516-9528, 2020 Jul 21.
Artigo em Inglês | MEDLINE | ID: mdl-32608402

RESUMO

Two novel mixed valence CoII-CoIII complexes, namely [CoIICoIII(L1)(ab)(mb)2(H2O)]·dmf (1) and [CoCoII(L2)4(H2O)4]·2H2O (2) [H2L1 = (E)-2-((1-hydroxybutan-2-ylimino)methyl)-6-methoxyphenol, ab = 2-amino-butan-1-ol anion, mb = p-methyl benzoate, H2L2 = 3-((2-hydroxy-3-methoxy-benzylidene)-amino)-propionic acid, and dmf = N,N-dimethyl-formamide], were synthesized and characterized by single crystal X-ray diffraction and magnetic studies at low temperature. The structure determination reveals that both complexes belong to the monoclinic system with P21/c (1) and I2/a (2) space groups. Complex 1 is a dinuclear CoIIICoII compound with distorted octahedral cobalt centers showing different coordination environments. In 2, a bent trinuclear CoCoII complex, the coordination environments around the two terminal CoIII sites are alike, whereas they are different in the central CoII ion. Alternating current/direct current (ac/dc) magnetic studies revealed that both complexes show field-induced slow magnetic relaxation. The dc magnetic susceptibility and magnetization data were analyzed with the following Hamiltonianwhere D and E are the axial and rhombic zero-field splitting (zfs) parameters, respectively, and a good agreement between experimental and simulated results was found using the parameters g⊥ = 2.585, g∥ = 2.437, D = +98.1 cm-1, E/D = 0.008 and F = 8.2× 10-5 for 1 and g⊥ = 2.580, g∥ = 2.580, D = +55.4 cm-1, and E/D = 0.000 for 2.

2.
Chemistry ; 2020 Jul 10.
Artigo em Inglês | MEDLINE | ID: mdl-32649799

RESUMO

We report a mononuclear iron(III) porphyrin compound exhibiting unexpectedly slow magnetic relaxation which is a characteristic of single-ion magnet behaviour. This behaviour originates from the close proximity (≈ 550 cm -1 ) of the intermediate-spin S = 3/2 excited states to the high-spin S = 5/2 ground state. More quantitatively, although the ground state is mostly S = 5/2, a spin-admixture model evidences a sizable contribution (≈ 15%) of S = 3/2 to the ground state, which as a consequence experiences large and positive axial anisotropy ( D = +19.2 cm -1 ). Frequency-domain EPR allowed us to directly access the m S = |±1/2〉 → |±3/2〉 transitions, thus unambiguously measuring the very large zero-field splitting (ZFS) in this 3d 5 system. Other experimental results including magnetization, Mössbauer, and field-domain EPR (HFEPR) studies are consistent with this model, which is also supported by theoretical calculations.

3.
Dalton Trans ; 49(19): 6280-6294, 2020 May 19.
Artigo em Inglês | MEDLINE | ID: mdl-32329759

RESUMO

Three mononuclear complexes [M(hfac)x(ATEMPO)y], where M = Cu (11) and Co (12), x = y = 2; M = Nd (13), x = 4, y = 1, and two polynuclear complexes [{Cu(hfac)2(ATEMPO)}n], where n = 2 (14) and 4 (15), were obtained by the reaction of M(hfac)x (M = CuII, CoII, NdIII; x = 2, 3) with 4-amino-TEMPO (4-amino-2,2,6,6-tetramethylpiperidin-N-oxyl) in good yields and their structural, electrochemical and magnetic properties were examined. In all cases, the radical is coordinated to the metal through the amino group, except 15, and the metal ions have an octahedral geometry, except 13. Different coordination architectures of the copper complexes were obtained as a function of the stoichiometry and solvents used. In complexes 11 and 12 the radicals show an equatorial-equatorial and axial-equatorial arrangement, respectively, giving rise to two distinct 2D supramolecular systems through intermolecular interactions. Compound 13 is the first example of a lanthanide complex of the ATEMPO radical. The NdIII ion adopts a rare nine-coordination via binding to four hfac ligands and the radical. The dinuclear complex 14 shows a (Cu-O)2 core in which the CuII ions are bridged by the oxygen atoms from the hfac ligands. In compound 15 the ATEMPO radical acts as a bidentate ligand through the amino and nitroxyl groups leading to an unprecedented tetranuclear square-shaped framework. Cyclic voltammetry showed redox processes associated with the copper and TEMPO moieties. Electrochemical impedance spectroscopy revealed the temperature dependence of the conductivity for compound 15 with a maximum of 2.09 × 10-5 S cm-1 at 408 K. The magnetic behavior of complexes 11-15 is determined by metal-radical interactions. Ferromagnetic interaction has been observed for complex 11 due to the existence of two different exchange pathways arising from the conformational arrangement of the radicals around the metal center, whereas the single conformation of the radical in complex 14 resulted in a weak antiferromagnetic coupling. In complex 15 both O-Cu and N-Cu contacts are present giving rise to ferromagnetic and antiferromagnetic interactions, respectively.

4.
Dalton Trans ; 49(19): 6328-6340, 2020 May 19.
Artigo em Inglês | MEDLINE | ID: mdl-32342075

RESUMO

Construction of efficient multifunctional materials is one of the greatest challenges of our time. We herein report the magnetic and catalytic characterization of dinuclear [CoIIICoII(HL1)2(EtOH)(H2O)]Cl·2H2O (1) and trinuclear [CoIIICoII2(HL2)2(L2)Cl2]·3H2O (2) mixed valence complexes. Relevant structural features of the complexes have been mentioned to correlate with their magnetic and catalytic properties. Unique structural features, especially in terms of significant distortions around the CoII centre(s), prompted us to test both spin-orbit coupling (SOC) and zero field splitting (ZFS) methodologies for the systems. The positive sign of D values has been established from X-band EPR spectra recorded in the 5-40 K temperature range and reaffirmed by CAS/NEVPT2 calculations. ZFS tensors are also extracted for the compounds along with CoIIGaIII and CoIIZnIICoIII model species. Interestingly, 1 shows slow relaxation of magnetization below 6.5 K in the presence of a 1000 Oe external dc field with two relaxation processes (Ueff = 37.0 K with τ0 = 1.57 × 10-8 s for the SR process and Ueff = 7 K with τ0 = 1.66 × 10-6 s for the FR process). As mixed valence cobalt complexes with various nuclearities are central to the quest for water oxidation catalysts, we were prompted to explore their features and to our surprise, water oxidation ability has been realized for both 1 and 2 with significant nuclearity control.

5.
J Am Chem Soc ; 2020 Feb 06.
Artigo em Inglês | MEDLINE | ID: mdl-31967803

RESUMO

Polyoxometalates (POMs) provide rigid and highly symmetric coordination sites and can be used as a strategy for the stabilization of magnetic ions. Herein, we report a new member of the Keggin archetype, the Cr-centered Keggin anion [α-CrW12O40]5- (CrW12), with the unusual tetrahedral coordination of CrIII reported for the first time in POMs conferring unattended magnetic properties. POM chemistry has recently presented excellent examples of single-molecule and single-ion magnets (SMMs and SIMs) as well as molecular spin qubits; however, the majority of POM-based SIMs reported to date contain lanthanoid ions. CrW12, as the first example of a chromium(III) SIM, exhibits slow relaxation of magnetization and quantum tunneling with a single-ion magnetic behavior even above 10 K with an energy barrier for the reversal of the magnetization of 3.0 K. The first 3d-metal SIM based on a nonlacunary Keggin anion is the foundation for a new research area in POM chemistry.

7.
Dalton Trans ; 48(46): 17266-17280, 2019 Nov 26.
Artigo em Inglês | MEDLINE | ID: mdl-31713552

RESUMO

Substituted pyrazines were successfully used to prepare two new coordination polymers of formulas {[Co(dca)2(NH2pyz)2]·H2O}n (1) and [Co3(dca)6(HOpyz)5(H2O)2]n (2) [dca = dicyanamide, NH2pyz = 2-aminopyrazine and HOpyz = 2-hydroxypyrazine] whose structures were determined by single-crystal X-ray crystallography. The structure of 1 consists of a two-dimensional rhombus grid of cobalt(ii) ions where the dca ligand adopts the µ1,5 bridging mode with trans-positioned monodentate NH2pyz molecules completing the six-coordination around each metal ion. Compound 2 exhibits a stair-like two-dimensional structure where the intralayer connections are performed by the dca and HOpyz groups exhibiting µ1,5 and bis-monodentate coordination modes, respectively. The values of the cobalt-cobalt separation through the dca bridges are 8.2107(3) (1) and 8.4746(4) and 8.5249(4) Å (2) whereas the value through the hydroxypyrazine is 7.2052(6) Å (2). Solid-state direct-current magnetic susceptibility analyses in the temperature range of 1.9-300 K for 1 and 2 reveal the occurrence of magnetically isolated high-spin cobalt(ii) ions with a significant contribution to the magnetic moment (1 and 2), D = +95.4 (1) and +76.5 cm-1 (2) and the antiferromagnetically coupled pairs of cobalt(ii) centres through the bis-monodentate 2-hydroxypyrazine, J = -0.3 cm-1 (2). Both compounds exhibit frequency dependence of the out-of-phase alternating current (ac) magnetic susceptibility (χ''M) under non-zero applied dc fields, a feature which is characteristic of single-ion magnet behaviour (SIM). Q-band EPR studies on the polycrystalline samples of 1 and 2 at low temperatures confirm the positive sign of D and reveal the occurrence of a strong asymmetry in the g-tensors. Theoretical calculations by CASSCF/NEVPT2 support these results. An analysis of the dynamic behaviour of 1 and 2 suggests that the relaxation of the magnetization occurs in the ground state under applied fields through two Orbach processes possibly bound to low-lying vibrational modes in the high temperature range, and to the slowing down of the fast interconversion between the two contributions of the ground Kramers doublet at lower temperatures induced by the applied dc field.

8.
Inorg Chem ; 58(23): 15726-15740, 2019 Dec 02.
Artigo em Inglês | MEDLINE | ID: mdl-31738531

RESUMO

Two mononuclear cobalt(II) compounds of formula [Co(dmphen)2(OOCPh)]ClO4·1/2H2O·1/2CH3OH (1) and [Co(dmbipy)2(OOCPh)]ClO4 (2) (dmphen = 2,9-dimethyl-1,10-phenanthroline, dmbipy = 6,6'-dimethyl-2,2'-bipyridine and HOOCPh = benzoic acid) are prepared and magnetostructurally investigated. Each cobalt(II) ion is six-coordinate with a distorted octahedral CoN4O2 environment. The complex cations are interlinked leading to supramolecular chains (1) and pairs (2) that grow along the crystallographic c-axis with racemic mixtures of (Δ,Λ)-Co units. FIRMS allowed us to directly measure the zero-field splitting between the two lowest Kramers doublets, which led to axial anisotropy values of 58.3 cm-1 ≤ D < 60.7 cm-1 (1) and 63.8 cm-1 ≤ D < 64.1 cm-1 (2). HFEPR spectra of polycrystalline samples of 1 and 2 at low temperatures confirm the positive sign of D and provide an estimate of the E/D quotient [0.147/0.187 (1) and 0.052 (2)]. Detailed ac and dc magnetic studies reveal that 1 and 2 are new examples of field-induced single-ion magnets (SIMs) with small transversal anisotropy. CASSCF/NEVPT2 calculations support these results. Two Orbach processes or one Orbach plus a direct relaxation mechanism provide similar agreements with the nonlinear experimental Arrhenius plots at Hdc = 500 and 2500 G for 1. Two independent relaxation processes occur in 2, but in contrast to 1, an observed linear dependence of ln(τ) vs 1/T substantiates Orbach processes against the most widely proposed Raman and direct mechanisms. The analysis of each relaxation process in 2 provided values for Ea and τ0 that are very close to those found for 1, validating the predominant role of the Orbach relaxations in both compounds and, probably, also in other cobalt(II) SIMs. A mechanism based on a spin-phonon coupling is proposed to account for the SIM behavior in 1 and 2 with any Raman or direct processes being discarded.

9.
Inorg Chem ; 58(22): 15359-15370, 2019 Nov 18.
Artigo em Inglês | MEDLINE | ID: mdl-31657914

RESUMO

Electrocrystallization of the bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) organic donor in the presence of the [Fe(ClCNAn)3]3- tris(chlorocyananilato)ferrate(III) paramagnetic anion in different stoichiometric ratios and solvent mixtures afforded two different hybrid systems formulated as [BEDT-TTF]4[Fe(ClCNAn)3]·3H2O (1) and [BEDT-TTF]5[Fe(ClCNAn)3]2·2CH3CN (2) (An = anilato). Compounds 1 and 2 present unusual structures without the typical segregated organic and inorganic layers, where layers of 1 are formed by Λ and Δ enantiomers of the anionic paramagnetic complex together with mixed-valence BEDT-TTF tetramers, while layers of 2 are formed by Λ and Δ enantiomers of the paramagnetic complex together with dicationic BEDT-TTF dimers and monomers. Compounds 1 and 2 show semiconducting behaviors with room-temperature conductivities of ca. 6 × 10-3 S cm-1 (ambient pressure) and 1 × 10-3 S cm-1 (under applied pressure of 12.1 GPa), respectively, due to strong dimerization between the donors. Magnetic measurements performed on compound 1 indicate weak antiferromagnetic coupling between high-spin FeIII (SFe = 5/2) and mixed-valence radical cation diyads (BEDT-TTF)2+ (Srad = 1/2) mediated by the anilate ligands, together with an important Pauli paramagnetism typical for conducting systems.

10.
Dalton Trans ; 48(31): 11862-11871, 2019 Aug 21.
Artigo em Inglês | MEDLINE | ID: mdl-31305843

RESUMO

The successful utilization of the "direct synthesis" approach yielded the unprecedented hexanuclear complex of formula [Co2MnMn(L1)4Cl2(µ3-O)2(dmf)4]·2dmf (1) (H3L is the Schiff base derived from the condensation of salicylaldehyde and 3-aminopropane-1,2-diol). Single crystal X-ray analysis revealed that 1 crystallizes in the monoclinic system P21/c and it contains a rare mixed-valence {CoMnMn(µ2-O)8(µ3-O)2} core where all metal ions are linked through the phenolato and alkoxo groups of the L3- ligand. Besides the charge balance resulting from the X-ray structure, the oxidation state of the metal ions has been confirmed by XPS spectroscopy. Cryomagnetic studies indicate the coexistence of ferro- (MnIV-MnII, J2 = +1.10(3) cm-1, J3 = +2.19(3) cm-1; MnII-MnII, j = +0.283(3) cm-1) and antiferromagnetic interactions (MnIV-MnIV, J1 = -17.31(4) cm-1), with the six-coordinate CoIII ions being diamagnetic. DFT type calculations were carried out to substantiate these values. The energy diagram for the different spin states using the best-fit parameters shows the occurrence of six low-lying spin states (S = 0-5) which are close in energy but clearly separated from the remaining ones, with the ground spin state being S = 5. Complex 1 is found to be the first example where weak ferromagnetic exchange between MnII ions through the long -O-MnIV-O- pathway takes place.

11.
Inorg Chem ; 58(12): 8086-8099, 2019 Jun 17.
Artigo em Inglês | MEDLINE | ID: mdl-31136160

RESUMO

Supported by endogenous (part of the ligand, in-built) phenoxo bridges provided by the ligand 2,6-bis[{{(5-bromo-2-hydroxybenzyl)}{(2-(pyridylethyl)}amino}methyl]-4-methylphenol) (H3L), in its deprotonated form, exogenous (not part of the ligand, externally added or generated) oxo-/hydroxo- and acetato-bridged [FeII4FeIII2(O)2(O2CMe)4(L)2]·4Et2O (1) and [FeIII4(OH)2(O2CMe)3(L)2](ClO4)·3MeCN·2H2O (2) coordination clusters have been synthesized and structurally characterized. Complexes 1 and 2 have µ4-O and µ3-OH bridges, respectively. Magnetic studies on 1 reveal slow magnetic relaxation below 2 K. Both in-phase ( χ'M) and out-of-phase (χ″M) magnetic susceptibility were found to be frequency dependent. This is typical of a single-molecule magnet (SMM) with τ0 = 1.9(2) × 10-7 s-1 and Ea = 5.1(3) cm-1. Assuming that Ea corresponds to the energy splitting of the ground spin state ( S = 2) by the zero-filed-splitting (zfs), Ea = 4| D| ( D is the axial zfs parameter), D ≈ - 1.3 cm-1 could be estimated. For 2, three types of magnetic interactions are observed: JA = -56.5(3), JB = -71.6(4), and JC = +4.5(2) cm-1. Considering the observed structural parameters, the magnetic behavior for both of the coordination clusters 1 and 2 has been rationalized.

12.
Dalton Trans ; 48(26): 9765-9775, 2019 Jul 02.
Artigo em Inglês | MEDLINE | ID: mdl-31011739

RESUMO

This work describes the synthesis, and structural, spectroscopic, and theoretical studies of a mononuclear silver(i) complex with the formula [Ag(Xantphos)(4,4'-(MeO)2-2,2'-bipy)]BF4·DCM (1·BF4) [Xantphos: 4,5-bis(diphenylphosphino)-9,9'-dimethylxanthene]. We provide meaningful insights into the enhancement of the photoluminescence features of this silver(i) complex compared to its copper(i) analogue.

13.
Dalton Trans ; 48(18): 5909-5922, 2019 May 07.
Artigo em Inglês | MEDLINE | ID: mdl-30638234

RESUMO

Three dimanganese(iii) complexes have been synthesised and fully characterised by standard spectroscopic methods and spectroelectrochemistry. Each MnIII ion is chelated by a salen type ligand (H2L), but there is variation in the bridging group: LMn(OOCCH[double bond, length as m-dash]CHCOO)MnL, LMn(OOCC6H4COO)MnL, and LMn(OOCC6H4C6H4COO)MnL. X-ray diffraction revealed an axial compression of each six-coordinate high-spin d4 MnIII ion, which is a Jahn-Teller-active ion. Temperature dependent magnetic susceptibility and variable temperature-variable field (VTVH) magnetisation measurements, as well as high-frequency and -field EPR (HFEPR) spectroscopy were used to accurately describe the magnetic properties of the complexes, not only the single-ion spin Hamiltonian parameters: g-values and zero-field splitting (ZFS) parameters D and E, but also the exchange interaction constant J between the two ions, which has been seldom determined for a di-MnIII complex, particularly when there is more than a single bridging atom. Quantum chemical calculations reproduced well the electronic and geometric structure of these unusual complexes, and, in particular, their electronic absorption spectra along with the spin Hamiltonian and exchange parameters.

14.
Dalton Trans ; 48(4): 1404-1417, 2019 Jan 22.
Artigo em Inglês | MEDLINE | ID: mdl-30623959

RESUMO

The preparation, X-ray crystal structure, spectroscopic and variable-temperature dc and ac magnetic properties of two six-coordinate cobalt(ii) complexes of formula [Co(bim)4(tcm)2] (1) and [Co(bmim)4(tcm)2] (2) (bim = 1-benzylimidazole, bmim = 1-benzyl-2-methylbenzimidazole and tcm- = tricyanomethanide ion) are reported. 1 and 2 crystallize in the monoclinic P21/n and C2/c space groups with the asymmetric units composed of one tcm- ion and half the [Co(bim)4]2+ and [Co(bmim)4]2+ complex cations, respectively. Their cobalt atoms are in compressed (1)/rhombic (2) CoN6 octahedral environments, the axial positions being occupied by monodentate tricyanomethanide anions. The neutral molecules in 1 are linked through weak C-HN type interactions into supramolecular chains, which are further interconnected into supramolecular 2D motifs by C-Hπ stacking. No short intermolecular interactions occur in 2. The values of the shortest intermolecular cobalt-cobalt separation are 10.901(1) (1) and 10.577(3) Å (2). Detailed ac and dc magnetic studies indicate that 1 and 2 are field-induced single-ion magnets (SIMs) with D = +46.1 (1)/+80.1 cm-1 (2) thus presenting new examples of SIMs with transversal magnetic anisotropy. Theoretical calculations by CASSCF/NEVPT2 support these results and suggest that the relaxation of the magnetization occurs in the ground state under applied fields through two Orbach processes possibly bound to low-lying vibrational modes. Q-band EPR study for polycrystalline samples 1 and 2 at low temperatures confirms the positive sign of D, allows the rough estimation of the E/D ratio [0.144 (1) and 0.180 (2)] and reveals the occurrence of a strong asymmetry in the g-tensors. The values found for the spin-reversal barrier, Ea ≈ 28 and 11 cm-1 (1) and 20 and 9 cm-1 (2), are within the range of those found in other cobalt(ii) field-induced SIMs with a pseudooctahedral trans-CoN4N'2 chromophore.

15.
Dalton Trans ; 47(4): 1010-1013, 2018 Jan 23.
Artigo em Inglês | MEDLINE | ID: mdl-29265158

RESUMO

A one-pot synthesis of a 3d-3d'-3d'' heterotrimetallic coordination polymer with double diphenoxido, single cyanido and bis-bidentate oxalate as alternating bridges which exhibits an overall antiferromagnetic behaviour has been developed.

16.
Dalton Trans ; 46(46): 16025-16033, 2017 Nov 28.
Artigo em Inglês | MEDLINE | ID: mdl-28786445

RESUMO

Six novel one-dimensional chloro-bridged ReIVCuII complexes of formula {[Cu(L)4][ReCl6]}n, where L = imidazole (Imi, 1), 1-methylimidazole (Meim, 2), 1-vinylimidazole (Vim, 3), 1-butylimidazole (Buim, 4), 1-vinyl-1,2,4-triazole (Vtri, 5) and N,N'-dimethylformamide (DMF, 6) are characterised structurally, magnetically and theoretically. The structures exhibit significant differences in Cu-Cl bond lengths and Re-Cl-Cu bridging angles, resulting in large differences in the nature and magnitude of magnetic exchange interactions between the ReIV and CuII ions. Theoretical calculations reveal the coupling to be primarily ferromagnetic, increasing in magnitude as the bridging angle becomes smaller and the bond lengths shorten.

17.
Dalton Trans ; 46(35): 11890-11897, 2017 Sep 12.
Artigo em Inglês | MEDLINE | ID: mdl-28849845

RESUMO

The employment of pyrazine (pyz), pyrimidine (pym) and s-triazine (triz) ligands in ReIV chemistry leads to the isolation of a family of complexes of general formula (NBu4)2[(ReX5)2(µ-L)] (L = pyz, X = Cl (1) or Br (2); L = pym, X = Br (3); L = triz, X = Br (4)). 1-4 are dinuclear compounds where two pentahalorhenium(iv) fragments are connected by bidentate pyz, pym and triz ligands. Variable-temperature magnetic measurements, in combination with detailed theoretical studies, uncover the underlying magneto-structural correlation whereby the nature of the exchange between the metal ions is dictated by the number of intervening atoms. That is, the spin-polarization mechanism present dictates that odd and even numbers of atoms favour ferromagnetic (F) and antiferromagnetic (AF) exchange interactions, respectively. Hence, while the pyz ligand in 1 and 2 mediates AF coupling, the pym and triz ligands in 3 and 4 promote F interactions.

18.
Dalton Trans ; 46(35): 11817-11829, 2017 Sep 12.
Artigo em Inglês | MEDLINE | ID: mdl-28848942

RESUMO

A new high-spin d4 roughly trigonal-bipyramidal (TBP) manganese(iii) complex with a salen type ligand (H2L), namely MnL(NCS)·0.4H2O, has been synthesised and characterised by elemental analysis, ESI mass spectrometry, IR and UV-vis spectroscopy, and spectroelectrochemistry. X-ray diffraction analysis revealed an axial compression of the approximate TBP. Temperature dependent magnetic susceptibility and variable-temperature variable-field (VTVH) magnetisation measurements, as well as high-frequency and -field EPR (HFEPR) spectroscopy, were used to accurately describe the magnetic properties of this complex and, in particular, determine the spin Hamiltonian parameters: g-values and the zero-field splitting (ZFS) parameters D and E. The HFEPR spectra allowed the extraction of fourth order ZFS parameters. Quantum chemical calculations reproduced well the electronic and geometric structures of this unusual complex and, in particular, its electronic absorption spectrum along with the spin Hamiltonian parameters.

19.
Dalton Trans ; 46(19): 6312-6323, 2017 May 16.
Artigo em Inglês | MEDLINE | ID: mdl-28452386

RESUMO

This work studies the effect of the σ-Hammett parameter (σp) - i.e., the σ-donation effect caused by substitution at the para position of a bipyridine ligand (4,4'-R2bipy, where R is MeO, Me, H, NO2) - on both the photo- and electro-luminescence features of a series of heteroleptic copper(i) complexes - i.e., [Cu(N^N)(P^P)]+ where N^N and P^P ligands are R2bipy and Xantphos, respectively. By virtue of a comprehensive photophysical, theoretical, and thin-film lighting device - i.e., light-emitting electrochemical cells (LECs) - investigation, we note a clear relationship between the σp and the photo- and electro-luminescence parameters, such as photoluminescence quantum yields, excited-state lifetimes, and emission maxima, as well as device brightness, stability, and efficacy, respectively. As the most relevant finding, the substitution with the group featuring the most negative σp - i.e., MeO - provides a ca. five-fold enhancement of all of the aforementioned figures-of-merit upon comparison within the series of complexes. As such, this work provides a new guideline for a device optimization through a rational ligand design for heteroleptic copper(i) complexes.

20.
Inorg Chem ; 56(4): 2108-2123, 2017 Feb 20.
Artigo em Inglês | MEDLINE | ID: mdl-28157308

RESUMO

In the series described in this work, the hydrothermal synthesis led to oxidation of the 5-methyl-pyrazinecarboxylate anion to the 2,5-pyrazinedicarboxylate dianion (2,5-pzdc) allowing the preparation of three-dimensional (3D) lanthanide(III) organic frameworks of formula {[Ln2(2,5-pzdc)3(H2O)4]·6H2O}n [Ln = Ce (1), Pr (2), Nd (3), and Eu (4)] and {[Er2(2,5-pzdc)3(H2O)4]·5H2O}n (5). Single-crystal X-ray diffraction on 1-5 reveals that they crystallize in the triclinic system, P1̅ space group with the series 1-4 being isostructural. The crystal structure of the five compounds are 3D with the lanthanide(III) ions linked through 2,5-pzdc2- dianions acting as two- and fourfold connectors, building a binodal 4,4-connected (4·648)(426282)-mog network. The photophysical properties of the Nd(III) (3) and Eu(III) (4) complexes exhibit sensitized photoluminescence in the near-infrared and visible regions, respectively. The photoluminescence intensity and lifetime of 4 were very sensitive due to the luminescence quenching of the 5D0 level by O-H oscillators of four water molecules in the first coordination sphere leading to a quantum efficiency of 11%. Variable-temperature magnetic susceptibility measurements for 1-5 reveal behaviors as expected for the ground terms of the magnetically isolated rare-earth ions [2F5/2, 2H4, 4I9/2, 7F0, and 4I15/2 for Ce(III), Pr(III), Nd(III), Eu(III), and Er(III), respectively] with MJ = 0 (2 and 4) and ±1/2 (1, 3, and 5). Q-band electron paramagnetic resonance measurements at low temperature corroborate these facts. Frequency-dependent alternating-current magnetic susceptibility signals under external direct-current fields in the range of 100-2500 G were observed for the Kramers ions of 1, 3, and 5, indicating slow magnetic relaxation (single-ion magnet) behavior. In these compounds, τ-1 decreases with decreasing temperature at any magnetic field, but no Arrhenius law can simulate such a dependence in all the temperature range. This dependence can be reproduced by the contributions of direct and Raman processes, the Raman exponent (n) reaching the expected value (n = 9) for a Kramers system.

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