Study of the Light‐Induced Spin Crossover Process of the [Fe^II(bpy)₃]²⁺ Complex

Abstract

Ab initio calculations have been performed on [Fe(II)(bpy)3](2+) (bpy = bipyridine) to establish the variation of the energy of the electronic states relevant to light-induced excited-state spin trapping as a function of the Fe-ligand distance. Light-induced spin crossover takes place after excitation into the singlet metal-to-ligand charge-transfer (MLCT) band. We found that the corresponding electronic states have their energy minimum in the same region as the low-spin (LS) state and that the energy dependence of the triplet MLCT states are nearly identical to the (1) MLCT states. The high-spin (HS) state is found to cross the MLCT band near the equilibrium geometry of the MLCT states. These findings give additional support to the hypothesis of a fast singlet-triplet interconversion in the MLCT manifold, followed by a (3)MLCT-HS ((5)T2) conversion accompanied by an elongation of the Fe-N distance.

Computational approach to the study of thermal spin crossover phenomena

“…[4][5][6] This phenomenon, called Light-Induced Excited Spin State Trapping (LIESST), initially found in Fe(II) complexes [7][8][9][10][11] and later also observed in systems containing Fe(III), [12][13][14][15] and Ni(II) [16][17][18] has been intensively studied in the last years in order to unravel its mechanism both with experimental techniques 11,[19][20][21][22][23][24][25] and by means of theoretical calculations. [26][27][28][29][30][31][32][33] The most numerous and most studied family of SCO systems involves octahedral Fe(II) complexes in the solid state or in solution. The LS-HS transition in Fe(II) complexes is accompanied by an enlargement of the iron-ligand distances due to the occupation of antibonding e orbitals in the HS state.…”

“…[45][46][47][48][49][50][51][52][53][54][55][56][57] Multiconfigurational wave function based methods, on the other hand, have proved to be capable of giving accurate values for the LS-HS energy difference but they are computationally much more expensive, particularly in the calculation of optimized geometries and vibrational frequencies. 26,28,29,51,52,[58][59][60][61][62] The aim of this work is to determine through calculations the key parameters of the thermal SCO process, i.e., the zeropoint corrected energy difference between the LS and HS states, ?H HL , the entropy change associated to the spin transition, ?S HL , and an estimation of the transition temperature, T 1/2 , for a set of Fe(II) compounds with ligands of different nature. In order to do that, we have combined DFT calculations on the geometries and vibrational frequencies for the LS and HS states, with multiconfigurational wave function calculations that allow us to compute accurate electronic energy differences.…”

From Mononuclear to Dinuclear: Magnetic Properties of Transition Metal Complexes

“…This is one of the reasons why more than twenty years after its discovery, LIESST e↵ect is still at the very center of the SCO research. The advances in short pulsed lasers enables researchers to extract information on the deactivation mechanism [15][16][17][18][19][20][21][22][23] and theoretical studies also contributed to the understanding of LIESST in octhedral Fe(II) complexes [60][61][62]. For Fe(III) complexes, the light irradiation induces ligand-tometal charge transfer transition (LMCT).…”

“…From previous studies, it is well-known that the optimal CASPT2 Fe-N distance does not necessarily coincide with the one of the optimized DFT geometry [60,175,185,186]. Since the excitation energies are rather sensitive to this parameter (especially the MC excitations), we have manually determined the optimal CASPT2 distance in the field of DFT relaxed ligand geometries by generating a set of DFT optimized geometries with di↵erent Fe-N distances and calculated the CASPT2 energy of the LS state at all points with a CAS(10,12) reference wave function.…”

“…Info. of ioChem-bd, DOI:10.19061/iochem-bd-2-3), which was previously used in the study of the deactivation mechanism in [Fe(bpy) 3 ] 2+ [60]. The adiabatic energy di↵erences and vertical excitation energies are compared to experimental data when available and to results of multiconfiguration wave function calculations, specifically complete active space self-consistent field (CASSCF) followed by CAS second-order perturbation theory (CASPT2).…”