Light-Induced Excited-State Spin Trapping in Tetrazole-Based Spin Crossover Systems

Abstract

Ab initio calculations have been performed on Fe (II) (tz) 6 (tz = 1- H-tetrazole) to establish the variation of the energy of the electronic states relevant to (reverse) light-induced excited-state spin trapping (LIESST) as function of the Fe-ligand distance. Equilibrium distances and absorption energies are correctly reproduced. The deactivation of the excited singlet is proposed to occur in the Franck-Condon region through overlap of vibrational states with an intermediate triplet state or an intersystem crossing along an asymmetric vibrational mode. This is followed by an intersystem crossing with the quintet state. Reverse LIESST involves a quintet-triplet and a triplet-singlet intersystem crossing around the equilibrium distance of the high-spin state. The influence of the transition metal is studied by changing Fe (II) for Co (II), Co (III), and Fe (III). The calculated curves for Fe (III) show remarkable similarity with Fe (II), indicating that the LIESST mechanism is based on the same electronic conversions in both systems.

Femtosecond X-ray Absorption Spectroscopy at a Hard X-ray Free Electron Laser: Application to Spin Crossover Dynamics

“…17 The stability of the LS state depends critically on the distance of the bpy ligands, where expansion of the Fe−N coordination bonds by ∼10% makes the high-spin (HS) quintet state the lowest energy configuration. 18 Due to the rather large energy difference between LS and HS states (0.6 eV for the compound presented here), only photoexcitation can lead to an efficient conversion to the HS states. 19 Ultrafast optical transient absorption spectroscopy has been used to investigate the dynamics of iron-based SCO complexes by photoexciting their metal-to-ligand charge transfer (MLCT) absorption bands centered around 520 nm, which results in shifting the electron density from Fe(II) center to the ligands.…”

Computational approach to the study of thermal spin crossover phenomena

“…81,82 The contracted Gaussian basis sets applied are: (7s, 6p, 5d, 4f, 3g, 2h) for Fe, (4s, 3p, 1d) for the N atoms bonded to the Fe, (3s, 2p) for the remainder N atoms, for O and C, (4s, 3p) for S, and (2s) for H. The active space used to construct the CASSCF wave functions for the LS and HS states contains ten electrons distributed among 12 orbitals, the five 3d Fe orbitals, two e g -like σ -bonding ligand orbitals, and a second set of diffuse Fe-3d orbitals to account for the large electron correlation effects in the 3d-shell. This active space has been used in previous studies 26,29,51 and aims at a proper description of the different Fe d-states and the ligand to metal charge transfer effects. CASPT2 accounts for the remaining electron correlation by correlating all the electrons except the deep core electrons (1s 2 for N and C and 1s 2 , 2s 2 , 2p 6 for Fe and S).…”

“…[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

“…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.…”