|
|
||||||||
The physical and photophysical properties of three classic transition metal complexes, namely [Fe(bpy)3]2+, [Ru(bpy)3]2+, and [Co(bpy)3]2+, can be tuned by doping them into a variety of inert crystalline host lattices. The underlying guest-host interactions are discussed in terms of a chemical pressure. | ||||||||
|
||||||||
The light-induced high-spin → low-spin relaxation for the Fe(II) spin-crossover compounds [Fe(btpa)](PF6)2 and [Fe(b(bdpa))](PF6)2 in solution, where btpa is the potentially octadentate ligand N,N,N‘,N‘-tetrakis(2-pyridylmethyl)-6,6‘-bis(aminomethyl)-2,2‘-bipyridine and b(bdpa) is the analogous hexadentate ligand N,N‘-bis(benzyl)-N,N‘-bis(2-pyridylmethyl)-6,6‘-bis(aminomethyl)-2,2‘-bipyridine, respectively, has been studied by temperature-dependent laser flash photolysis. [Fe(b(bdpa))](PF6)2 shows single-exponential 5T2 → 1A1 relaxation kinetics, whereas [Fe(btpa)](PF6)2 exhibits solvent-independent biphasic relaxation kinetics. The fast process of [Fe(btpa)](PF6)2 with a rate constant, kHL, of 2.5 × 107 s-1 at 295 K and an activation energy, Ea, of 1294(26) cm-1 in methanol can be assigned to the 5T2 → 1A1relaxation as well. The slow process with a kHL(295 K) of 3.7 × 105 s-1 and a Ea of 2297(32) cm-1 in methanol - which is the slowest light-induced relaxation process observed so far for an Fe(II) spin-crossover complex in solution - is assigned to a coupling of the 5T2 → 1A1relaxation process to a geometrical rearrangement within the pendent pyridyl arms. | ||||||||
|
||||||||
Temperature-dependent laser flash photolysis experiments on the low-spin iron(II) systems [M1−xFex(bpy)3](PF6)2 (M=Cd, Mn and Zn,x≈0.01, bpy=2,2′-bipyridine) under external pressure are presented. Below 50 K the high-spin→low-spin relaxation is an almost temperature-independent tunnelling process. Above that temperature it tends towards a thermally activated behaviour. A change of the host from cadmium to zinc results in an increase of the low-temperature tunnelling rate constant by two orders of magnitude. An external pressure of 1 kbar accelerates the low-temperature tunnelling process by a factor of 2. [Mn1−xFex(bpy)3](PF6)2 and [Zn1−xFex(bpy)3](PF6)2show a phase transition at ≈1.1 kbar, which increases the tunnelling rate by a factor of about 6. | ||||||||
|
||||||||
The thermal spin transition in the diluted mixed crystal [Zn1−xFex(6-mepy)3tren](PF6)2 (x = 0.00025, (6-mepy)3tren = tris{4-[(6-methyl)-2-pyridyl]-3-aza-3-butenyl}amine) is studied at 1 bar and 1 kbar by temperature-dependent absorption spectroscopy. From thermodynamic analysis of the high-spin (HS) fractions, values for ΔHHL0 and ΔSHL0 of 1551(50) cm−1 and 7.5(5) cm−1/K and a molecular volume of reaction, ΔVHL0, of 22(2) Å3result. Reconsideration of the cooperative effects in the neat [Fe(6-mepy)3tren](PF6)2from Adler et al. [Hyperfine Interact. 47, 343 (1989)] result in a lattice shift, Δ, of 208(15) cm−1 and an interaction constant, Γ, of 109(15) cm−1. Temperature-dependent laser flash photolysis experiments in the spin-crossover system [Zn1−xFex(6-mepy)3tren](PF6)2 and the LS system [Zn1−xFex(py)3tren](PF6)2 in the pressure range between 1 bar and 1 kbar are presented. Above ≈100 K the HS→LS (low-spin) relaxations behave classically, whereas they become almost temperature independent below 50 K. At ambient pressure, the low-temperature tunneling rate constant in[Zn1−xFex(py)3tren](PF6)2 is more than three orders of magnitude larger than the one in[Zn1−xFex(6-mepy)3tren](PF6)2. External pressure of 27 kbar accelerates the low-temperature tunneling process by almost nine orders of magnitude. The kinetic results are discussed within the theory of nonadiabatic multiphonon relaxation. | ||||||||
|
||||||||
Laser flash photolysis experiments were performed on the mixed crystal [Zn1−xFex(6-mepy)3tren](PF6)2 (x=0.00025) at 10 K in the pressure range between 1 bar and 20 kbar. An external pressure of 20 kbar accelerates the low-temperature tunneling process by almost eight orders of magnitude. | ||||||||
|
||||||||
The high-spin → low-spin relaxation dynamics of the Fe(III) spin-crossover complexes [Fe(Sal2tr)]PF6 (H2Sal2tr = Bis(salicylaldimino)triethylenetetramine) and [Fe(acpa)2]PF6(Hacpa = N-(1-acetyl-2-propylidene)-2-pyridylmethylamine) are discussed within the theory of nonadiabatic multiphonon relaxation. A Huang−Rhys factor S of ≈25, estimated on the basis of average metal−ligand bond length differences ΔrHL of ≈ 0.12 Å, explains the observed low-temperature tunneling rate constants kHL(T→0) of ≈ 102 s-1 as well as the thermally activated process at T > ≈100 K semiquantitatively. The results obtained for the Fe(III) compounds are compared to those for Fe(II) spin-crossover compounds. | ||||||||
|
