Time-resolved optical magnetic-circular-dichroism study of ligand-driven light-induced spin-change complexes

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In this project, we intend to apply our recently developed time-resolved magnetic circular dichroism (MCD) techniques to the study of metal complexes exhibiting the photomagnetic effect – light-induced excited-spin-state trapping (LIESST), where below a critical temperature one can reversibly switch the metal ion between a low-spin (LS) and high-spin (HS) state via irradiation. The target compound for the study is the well-established spin-crossover (SCO) complex Fe(II)phen2(NCS-)2 (phen = 1,10-phenanthroline). Previous investigations of related LS-Fe(II) complexes in room temperature solutions indicate that upon excitation, the LS compound reaches the HS state on a sub-picosecond time scale. This remarkable result demonstrates that the classical rules concerning spin dynamics (intersystem crossing rates) in organic systems need not apply in such metal-organic compounds. The precise excited-state dynamics dictates the quantum efficiency for magnetic switching and hence the performance of a given LIESST compound. Here we propose experiments for solid-state samples at low temperature (where the LIESST effect is operative), using paramagnetic UV-vis MCD as a sensitive probe of the spin state. We will determine the time scale for formation of the HS state using our femtosecond optical-pump MCD-probe system (in addition to transient absorption measurements), as well as addressing the role of intermediate triplet states during the LS-HS conversion. We will also further evaluate the use of continuous-wave MCD for characterising the SCO vs. temperature and LIESST effect during irradiation, and compare the results to corresponding SQUID magnetometry measurements.