Application of ultrafast spectroscopic techniques to single-molecule magnets

Abstract

The photochemical and photophysical properties of transition metal complexes are rich and varied due to the complex interplay between various electronic, vibrational and spin degrees of freedom. The intricate coupling between these leads to complicated dynamic behaviour from the moment of photoexcitation to the final electronic state or photochemical product. This leads to difficulty in tracking the photophysical processes that occur but it is also the source of many features which makes them suitable for light capture and conversion. In particular, the dynamics occurring at the earliest moments after light absorption can often dictate the final outcome of a light-induced process. In this thesis, a range of femtosecond absorption spectroscopies are used to study the electronic, nuclear and spin dynamics within transition metal complexes with a particular focus on single-molecule magnets. Single-molecule magnets are one of the smallest realisable units of magnetic recording media, and therefore combining these with excitation from femtosecond laser pulses could lead to ultrafast and ultradense data storage.

Manganese(III)-based single-molecule magnets were studied as their magnetic properties are strongly coupled to the complex’s nuclear structure. Optical transient absorption was used to track coherent vibrational dynamics in manganese(III) complexes. The vibrational motion could be observed as oscillations in light absorption after photoexcitation as a result of a switch in the Jahn-Teller distortion. Three complexes of the form [Mn(2,2’;6’,2”-terpyridine)X₃] where X = fluoride, chloride, and azide were studied. Upon photoexcitation of a metal-centred transition, a vibrational wavepacket is formed corresponding to a Jahn-Teller pincer-like motion of the terpyridine ligand. The vibrational dephasing times decrease from 620 to 370 fs as the number of vibrational modes with frequencies below the pincer mode increases, suggesting that low-frequency modes are an effective bath to dissipate excess energy.

Despite the utility of optical transient absorption and its ability to identify normal modes that are activated upon excitation, this technique does not provide the direction or magnitude of the change in nuclear coordinates. Therefore, K-edge X-ray transient absorption spectroscopy was performed to study the single-molecule magnet [Mn(III)₃O(Et sao)₃(β pic)₃(ClO₄)], where saoH₂ and β-pic are salicylaldoxime and 3-methylpyridine, respectively. K-edge X-ray spectroscopy is particularly use- 1 ful, as it is an element-specific technique and carries more structural information than optical spectroscopies. These measurements were carried out at the Spring-8 Angstrom Compact Free-Electron Laser (SACLA). Coherent vibrational motion of a Jahn-Teller mode was observed with a frequency of 180 cm⁻¹ in agreement with the previous optical studies. The K-edge X-ray spectra were simulated at different geometries along this Jahn-Teller mode. Good agreement between the calculated and experimental spectrum was found for bond length changes of only 0.01 ˚A. This shows the potential capabilities of X-ray absorption to track nuclear motion in large transition metal complexes, perfect for the emerging area of ultrafast molecular magnetism.

Ruthenium(II) polypyridyl complexes display a variety of light-induced functions, ranging from luminescence to photochemical ligand loss. It is well-known that triplet metal-centred states act to quench emission but promote photochemistry. Despite the importance of triplet metal-centred states in these excited state processes, detecting and following the dynamics that occur within these remains a challenge because of their short lifetimes. If the polypyridyl ligands are replaced with triazolyl ligands, this results in a destabilisation of the luminescent metal-toligand charge transfer state, which leads to the metal-centred state being the lowest energy excited state. Using femtosecond optical transient absorption, it is shown that triplet metal-centred states are populated within 100 fs, which launches a vibrational wavepacket. This vibrational coherence arises from a Jahn-Teller normal mode that is activated upon population of one of the e*ᶢ orbitals, similar to manganese( III) complexes. This coherent vibrational motion provides a clear signature of the triplet metal-centred states. In addition, small modifications to the ligand framework have a significant effect on the observed wavepacket dynamics, which suggests that synthetic control of coherent nuclear dynamics could be achieved. In addition to tracking nuclear motion in single-molecule magnets, it is important to be able to measure spin dynamics to gain insight into changes in the magnetisation. Recently, a broadband time-resolved magnetic circular dichroism setup was developed within the lab. Magnetic circular dichroism describes the differential absorption of left and right circularly polarised light under the application of a magnetic field. Zinc(II) tetraphenylporphyrin has been studied as its static magnetic circular dichroism spectrum and photoinduced dynamics are well understood. After excitation of the porphyrin B-band, there is a fast decay to the lowest excited singlet state, which shows an excited state magnetic circular dichroism spectrum. This is one of the first examples of a femtosecond magnetic circular dichroism spectrum of a molecule and opens up the possibility of studying femtosecond spin dynamics in a range of different systems.