Biomedical subjects

Epiphane Codjovi

Publications and source records attributed to Epiphane Codjovi.

3 recordsLinked to original sources

PubMed2006

Photoexcitation and relaxation properties of a spin-crossover solid in the case of a stable high-spin state.

The molecular solid [Fe(II)L(2)](ClO(4))(2).CH(3)CN where L is 2,6-bis(3,5-dimethylpyrazol-1-ylmethyl)pyridine provides a stable high-spin (HS) state at low temperature. Photoexcitation and subsequent relaxation have been studied using light-induced excited state spin trapping [LIESST(H --> L)] in the 700-850 nm range, determination of T(LIESST), relaxation curves at different temperatures, and temperature dependence of the light-induced spin equilibrium under constant irradiation. The measured photoinduced population of the metastable low-spin (LS) state (<30%) was drastically limited by the concomitant L --> H photoprocess. The absence of static light-induced thermal hysteresis and the stretched exponential shape of the relaxation curves respectively revealed the absence of sizable interactions and a large spreading of the activation energies attributed to the ligand flexibility. The whole data set has been simulated using a linear rate equation, with a simplified correction for the bulk extinction of light in the powder sample.

Journal Article↗

PubMed2005

Molecular and multidimensional polyoxotungstates functionalized by {Cu(bpy)}2+ groups.

Five new materials built from polyoxotungstates and Cu(ii) ions as linkers have been synthesized by hydrothermal reactions from a mixture of sodium tungstate, copper chloride and bipyridine. The value of the initial pH, the nature of the heteroelement (P or Si) and of the ligand (2,2'- and/or 4,4'-bipyridine) permit the control of the nature of the polyoxotungstate clusters and their connectivity via the copper ions, and hence the dimensionality of the framework. A single phase has been obtained with silicon as heteroelement at an initial pH of 5, namely the 2D material [SiW(12)O(40)][Cu(2,2'-bpy)(2)](2).10H(2)O (1) with saturated Keggin polyoxotungstates linked by {Cu(2,2'-bpy)(2)}(2+) groups. With phosphorous as heteroelement and at the same initial pH, three different structures have been isolated according to the nature of the ligand. Indeed, the two 1D materials [{Cu(5)(2,2'-bpy)(5)(H(2)O)(HPO(4))(PO(4))}PW(11)CuO(39)].6H(2)O (2) with 2,2'-bpy and [4,4'-Hbpy][{Cu(2)(2,2'-bpy)(2)(4,4'-bpy)(2.5)}PW(11)CuO(39)].16H(2)O (3) with a mixture of 2,2'- and 4,4'-bpy have been characterized, and a coordination polymer with polyoxometalate guests Na(3)[4,4'-Hbpy]{Cu(4)(4,4'-bpy)(8)(H(2)O)(8)}[PW(11)CuO(39)(H(2)O)][PW(10)Cu(2)O(38)(H(2)O)(2)].38H(2)O (4) with 4,4'-bpy has been obtained. Finally, in basic medium (pH = 10) the unprecedented molecular cluster Na(2)[{Cu(8)(2,2'-bpy)(8)}(PW(8)O(31))(2)].15H(2)O (5) has been evidenced. Magnetic studies of compound 2 revealed that the predominant interactions involve only 4 paramagnetic centers, which are interacting within pairs, among the 6 Cu(ii) centers. The chi(M)T=f(T) curve can be fitted using the dinuclear expression appropriate to the HDVV isotropic exchange Hamiltonian H=-JS(1)xS(2), with S(1)=S(2)=(1/2) and J=-105.4 cm(-1), showing strong antiferromagnetic interactions within the two Cu(ii) pairs.

2,2'-Dipyridyl↗

PubMed2004

Nonexponential relaxation of the metastable state of the spin-crossover system [Fe(L)2](ClO4)2.H2O [L = 2,6-Bis(pyrazol-1-ylmethyl)pyridine].

The relaxation of the metastable state of the spin-crossover compound [Fe(L)(2)](ClO(4))(2).H(2)O, with L = 2,6-bis(pyrazol-1-ylmethyl)pyridine, populated by the LIESST (light induced excited spin state trapping) effect, has been investigated by magnetic measurements. The time dependence of the relaxation curve at several temperatures, starting from different initial states, is in the shape of stretched exponentials, and the thermal variation of the photostationary state under constant photoexcitation is progressive and reversible. These features are satisfactorily modeled by considering noninteracting two-level systems with a distribution of activation energies. A suggested origin for the distribution is the conformational flexibility of the nonplanar heterocyclic ligands. The effect of the intensity distribution during the LIESST process is also accounted for in a simple way.

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