Ultrafast Deactivation Mechanism of the Excited Singlet in the Light‐Induced Spin Crossover of [Fe(2,2′‐bipyridine)₃]²⁺

Abstract

The mechanism of the light‐induced spin crossover of the [Fe(bpy)3]2+ complex (bpy=2,2′‐bipyridine) has been studied by combining accurate electronic‐structure calculations and time‐dependent approaches to calculate intersystem‐crossing rates. We investigate how the initially excited metal‐to‐ligand charge transfer (MLCT) singlet state deactivates to the final metastable high‐spin state. Although ultrafast X‐ray free‐electron spectroscopy has established that the total timescale of this process is on the order of a few tenths of a picosecond, the details of the mechanisms still remain unclear. We determine all the intermediate electronic states along the pathway from low spin to high spin and give estimates for the deactivation times of the different stages. The calculations result in a total deactivation time on the same order of magnitude as the experimentally determined rate and indicate that the complex can reach the final high‐spin state by means of different deactivation channels. The optically populated excited singlet state rapidly decays to a triplet state with an Fe d6() configuration either directly or by means of a triplet MLCT state. This triplet ligand‐field state could in principle decay directly to the final quintet state, but a much faster channel is provided by internal conversion to a lower‐lying triplet state and subsequent intersystem crossing to the high‐spin state. The deactivation rate to the low‐spin ground state is much smaller, which is in line with the large quantum yield reported for the process.

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.…”

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).…”

“…In analogy to the study of the LIESST phenomenon in Fe(II) complexes [60,61,185,186], we constructed the potential energy curves of the ground and excited states along the approximate minimal energy path that connects the LS and HS minima shown in Fig. 4.2 as a thick black line.…”

“…To give support for this deactivation path we estimate the intersystem crossing rates using the time-dependent formulation of Fermi's golden rule [205] and successfully applied to describe the photocycle in [Fe(bpy) 3 ] 2+ [61]. Vibrational frequencies and normal modes were determined for the LS, IS, and HS states in an ordinary DFT treatment, while the 2 LMCT state was addressed with TD-DFT.…”

“…The first intersystem crossing from singlet to triplet takes place in the MLCT manifold but it is not clear whether this state directly couples with the final HS state or that the deactivation goes via the triplet ligand field states. Arguments have been given for both scenarios [19,61,201,202] and it is obvious that more research is needed to clarify this issue.…”