Circularly polarized luminescence (CPL) spectroscopy is the emission analogue to circular dichroism (CD) spectroscopy. 1 It is common to report the degree of CPL in terms of the luminescence dissymmetry factor, g lum (λ), which is defined as follows: g lum (λ) = 2ΔI/I = 2 (I L -I R )/(I L + I R ), where I L and I R refer respectively to the intensity of left and right circularly polarized emissions. A value of 0 for g lum corresponds to no circular polarization, while the absolute maximum value is 2. Although the development of useful correlation between CPL spectrum and chiral structure is still limited, 1 a study by Bruce et al. 2 conducted on welldefined DOTA-based macrocyclic Eu(III) complexes led to the following conclusion. The sign and magnitude of CPL are affected by the degree of helical twist of the complex, the nature of the ligand field, and the axial donor group solvation. Of special importance is that the CPL will reflect the time-averaged local helicity around the lanthanide(III) ion (i.e. the magnitude of g lum values increases with an increase in the degree of conformational rigidity of the complex).To date, the largest g lum value reported in the literature was for the commercially available NMR shift reagent tris(3-trifluoroacetyl-(+)-camphorato)europium(III), [Eu((+)-facam) 3 ], in dry DMSO (g lum value of -0.78 at 588.2 nm), 3,4 whereas lanthanidecontaining systems with chiral 2-hydroxyisophthalamide-, pyridyl diamide-,1-hydroxy-2-pyridinone-, or DOTA-based ligand derivatives exhibited g lum values as high as 0.5. 1,4-8 Recently, we reported on the isolation and structural characterization of sodium or cesium tetrakis(3-heptafluorobutylryl-(+)-camphorato) lanthanide(III) complexes, M I [Ln((+)-hfbc) 4 ], by X-ray analysis and/or CD spectroscopy. The solution structure is supposed to take a square antiprism eight coordination (SAPR-8) with Δ-configurational chirality on the basis of the exciton CD spectra. 9,10 In this communication, the CPL as well as CD of M I [Eu((+)-hfbc) 4 ] complexes (M I = Cs and Na) in CHCl 3 and EtOH were examined in order to reveal the detailed chiral configuration in solution. Of special interest is the importance of using CPL for selectively studying only luminescent chromophores present in the systems of interest, in contrast to CD, which is affected by most chromophores and/or equilibrium mixtures in an additive manner. The NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author ManuscriptThe CPL spectra of 2 mM solutions of M I [Eu((+)-hfbc) 4 ] (M I = Cs and Na) in CHCl 3 are plotted in Figure 1 in the spectral range of the 5 D 0 → 7 F 1 transition, which is particularly wellsuited for CPL measurements since it satisfies the magnetic-dipole selection rule, ΔJ = 0, ±1 (except 0↔0), respectively. As shown in Figure 1, the detection of a CPL signal confirmed the presence of stable chiral emitting species on the luminescence time scale. The g lum values of the M I [Eu((+)-hfbc) 4 ] (M I = Cs and Na) complexes amounted to +1.38 and +0.15 at 595 nm, respecti...

The luminescence and circularly polarized luminescence (CPL) spectra of MI[Eu((+)–hfbc)4] show a similar behavior to the exciton CD in the intraligand π–π* transitions when the alkali metal ions and solvents are manipulated. There is a difference in susceptibility in solvation toward the alkali metal ions but not toward the Eu(III) ion, as in the case of axially symmetric DOTA–type compounds. The remarkable CPL in the 4f–4f transitions provide much more information on stereospecific formation of chiral Eu(III) complexes, since CPL spectroscopy is limited to luminescent species and reflects selectively toward helicity of the local structural environment around the lanthanide(III). While in comparison, exciton CD reveals the chiral structural information from the helical arrangement of the four bladed chelates. Of special importance, the observation of the highest CPL activities measured to date for lanthanide(III)–containing compounds (i.e. Eu and Sm) in solution supports that the chirality of Lanthanide(III) in the excited state corresponds to that in the ground state, which was derived from the exciton CD.

We investigated by optical microscopy the thermal transition of the spin-crossover dinuclear iron(II) compound [(Fe(NCSe)(py)(2))(2)(m-bpypz)]. In a high-quality crystal the high-spin (HS) low-spin (LS) thermal transition took place with a sizable hysteresis, at ~108 K and ~116 K on cooling and heating, respectively, through the growth of a single macroscopic domain with a straight LS and HS interface. The interface orientation was almost constant and its propagation velocity was close to ~6 and 26 μ m s(-1) for the on-cooling and on-heating processes, respectively. We found that the motion of the interface was sensitive to the intensity of the irradiation beam of the microscope, through a photothermal effect. By fine-tuning the intensity we could stop and even reverse the interface motion. This way we stabilized a biphasic state of the crystal, and we followed the spontaneous motion of the interface at different temperatures inside the thermal hysteresis loop. This experiment gives access for the first time to an accurate determination of the equilibrium temperature in the case of thermal hysteresis--which was not accessible by the usual quasistatic investigations. The temperature dependence of the propagation velocity inside the hysteretic interval was revealed to be highly nonlinear, and it was quantitatively reproduced by a dynamical mean-field theory, which made possible an estimate of the macroscopic energy barrier.

The variable temperature magnetic susceptibility, X-ray crystallography, and IR and Raman spectra of a new dinuclear complex [{Fe(II)(NCBH(3))(4-phpy)}(2) mu-bpypz)(2)] demonstrated the first two-step spin-crossover associated with a 1 : 1 mixture of high-spin pair [HS-HS] and low-spin pair [LS-LS] at the plateau.

The reaction of Ln(III) ions with a tripodal ligand HBpz(3)(-) (hydrotris(pyrazol-1-yl)borate) and a "complex ligand" [Cr(acac)(2)(ox)](-) (acac(-) = acetylacetonate, ox(2)(-) = oxalate) in aqueous solution afforded a series of the novel 3d-4f heterodinuclear complexes [(acac)(2)Cr(ox)Ln(HBpz(3))(2)] (Ln = Eu (1), Gd (2), Tb (3), Yb (4), Lu (5)). The crystal structure of 4 has been determined by X-ray diffraction. Complex 4 crystallizes in monoclinic space group P2/n, of which the cell parameters are a = 8.594(3) Å, b = 18.538(4) Å, c = 12.093(2) Å, beta = 93.71(2) degrees, and Z = 2. The Yb atom has an eight-coordinate distorted square antiprismatic coordination geometry. The intramolecular Cr.Yb distance is 5.631(1) Å. The magnetic susceptibility data for complex 2 showed that the Cr(III)-Gd(III) interaction is weakly antiferromagnetic with an exchange coupling constant J(CrGd) = -0.09 cm(-)(1). The luminescence measurements demonstrated the energy transfers for both Ln(III) --> Cr(III) and Cr(III) --> Ln(III), of which the degree of emission quenching depends on the energy gap of the excited levels in two metal centers. These results reveal that the metal-metal interactions between Cr(III) and Ln(III) are very weak in magnetic interaction but are strong from the viewpoint of energy transfer.

The synthesis and characterization of new spin-crossover pyrazolato bridged dinuclear [(FeII(NCS)(py))2(mu-bpypz)2] and the corresponding cyanotrihydroborato-kappaN complexes are described together with the X-ray crystal analysis of the latter in both the high-spin and low-spin states as well as the variable temperature magnetic susceptibility and/or Mossbauer spectra demonstrating the spin-crossover which exhibits an one-step process, but not a two-step one characteristic of the known bpym bridged dinuclear complexes.

Two series of pyrazolato bridged dinuclear NCS and NCBH3 diiron(II) complexes with various types of 3- or 4-substituted pyridines, [{Fe(NCS or NCBH3)(X-py)}2(mu-bpypz)2], were prepared and their variable-temperature magnetic susceptibilities were measured. There were found linear correlations of the spin-crossover temperatures Tc not only between the NCS and NCBH3 complexes with the corresponding substituted pyridines, but also between the Tc and the Hammett constants, supporting the electronic substituent effect of the coordinated pyridine rather than a steric effect. The ligand field and the interelectronic repulsion parameters together with the thermodynamic data and/or cooperativity factor were discussed in relation with their spin-crossover behavior.

The phenomenon of spin-crossover between the low-spin (LS) and high-spin (HS) states, [1] which is observed in some octahedral 3d n (4 n 7) metal complexes and is induced by an external perturbation, such as temperature, pressure, or light irradiation, is one of the most spectacular examples of molecular bistability. [2, 3] Such phenomena can be utilized in new electronic devices, such as molecular memories and switches that are controlled by many different physical channels. [4,5] It is now established that cooperative spincrossover, the interaction between spin-crossover sites, is the predominant factor governing bistability. [6,7] We now report the first homochiral mixed-valence spin-crossover compound, in which three independent properties are united, namely, mixed valence states of iron metal centers, spin-crossover of the Fe II