Ultrafast Dynamics of Spin-Crossover Complexes with Photochromic Ligands Investigated by Time-Resolved Electronic Absorption Spectroscopy

Light-induced spin crossover (SCO) in transition-metal complexes is a phenomenon bearing huge application potential as it enables for the manipulation of the spin state and, consequently, the magnetism of individual atoms. For the design of molecular devices that exploit the SCO phenomenon, the stability of these (two) spin states has to be ensured. However, as the photo-produced spin state is usually metastable, large molecule intrinsic barriers are required. The compounds investigated in this Thesis, namely an nickel(II) azoporphyrine (azoNiPor) and an iron azo-tetrapyridylamino (Fe3AzoN4Py) complex have been functionalized with azopyridines as molecular photoswitches in order to achieve control over and bi stability of the two spin states. Since light-induced SCO in transition metal complexes as well as the reactions of photochromic molecular switches occur on ultrafast timescales, femtosecond time-resolved spectroscopic techniques were employed to monitor the dynamical processes that drive the reactivity of these compounds.

For azoNiPor it was demonstrated that the stable SCO product is only formed upon direct exctiation and isomerization of the attached azopyridine according to the proposed concept of light-driven coordination-induced spin state switching (LDCISSS). However, this process is accompanied by vanishingly small quantum yields as most photons are absorbed by the porphyrine macrocyle, which only leads to the formation of  metastable SCO species.

The novel complex Fe(3AzoN4Py) was designed based on the concept of ligand-driven light-induced spin change (LD-LISC). It was shown that besides the insufficient change in ligand field strength upon trans-cis-isomerization, the isomerization process itself is furhter impeded by competing energy transfer from the excited azo-ligand to a metastable SCO state of the iron, pointing out an intrinsic dilemma of the LD-LISC concept.


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