Time-resolved optical magnetic-circular-dichroism study of ligand-driven light-induced spin-change complexes
Final Report Abstract
This project aimed to apply optical magnetic circular dichroism (MCD) to study a photomagnetic effect in metal complexes known as ligand-driven light-induced spin-change (LD-LISC). The LD-LISC effect is based on the use of ligands that can be optically switched between two stable configurations, which favour either a high-spin (HS) or low-spin (LS) state for the metal ion, respectively. The specific compounds studied here were Fe(II) complexes with multiple cis-trans photoisomer ligands (4-styrlypyridine), synthesised by a collaborating inorganic chemistry group. The key open issues to address were (i) the time-scale of the magnetic switching process following the ultra-fast photoisomerisation (PI) process, and (ii) the role of multiple photoisomer ligands and their interaction in the complex. As the cis-trans PI involves a large structural rearrangement, it occurs readily only in dilute samples, i.e. solutions and doped polymer thin films, but is hindered in the solid-state. Paramagnetic-MCD in the UV-vis spectral range offers a unique tool in this context, as it is very sensitive (compared e.g. to conventional SQUID magnetometry), provides spectral selectivity, and can be extended to ultra-fast measurements in the excited-state. In order to first determine the precise time-scale of the PI process vs. intra- and intermolecular environment, we carried out extensive two-colour transient absorption experiments of both the free ligands and corresponding complexes in solution, including transient anisotropy to reveal details of the molecular reorientation driven by the PI. From such measurements in various solvents, it became clear that fresh solution samples prepared from the free ligand and complex produced essentially the same results, indicating that the majority of the ligands do not remain intact in the complex in solution, even before any UV irradiation. This unexpected result raised serious concerns for the project work, as even the doped polymer films are prepared from precursor solutions. After numerous tests of different solvents and preparation conditions, conclusive evidence for the ligand decomplexation was provided by our tests with filtered solutions prepared from the complexes. Here the absorption bands associated with the Fe(II) centres were absent following filtration, while the π-π* bands of the stpy ligands were still present. A second LD-LISC derivative (whose stability may have been superior) was unfortunately not provided by our synthetic collaborators during the project. While this situation prevented us from carrying out the target MCD study, we did succeed in developing the proposed (M)CD experimental methodologies of the project, i.e. (i) a highsensitivity continuous-wave (cw) UV-vis CD spectrometer, and, more notably, (ii) a system allowing femtosecond (fs) transient (M)CD experiments with our 1-kHz laser amplifier system. We demonstrated the performance of each of these systems using established chiral CD compounds in solution. The fs-(M)CD system uses a novel approach with asynchronous polarisation modulation and real-time extraction of the CD signal contribution, and achieves a sensitivity approaching that of the optimised cw system. While we do not currently have access to reliable LD-LISC compounds for on-going studies, we are working to apply these time-resolved MCD methods to study the ultrafast magnetisation changes in compounds demonstrating the photomagnetic LIESST effect (light-induced excited-spin-state trapping), beginning with the established compound Fe(phen)2(NCS)2.
Publications
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Synthesis, structure, photoluminescence and photoreactivity of 2,3-diphenyl-4-neopentyl-1-silacyclobut-2-enes. Chem. Eur. J. 15, 8625-8645 (2009)
D. Yan, M. D. Thomson, M. Backer, M. Bolte, R. Hahn, R. Berger, W. Fann, H. G. Roskos, N. Auner
