Ultrafast photophysics of high-spin metal complexes

Abstract

Transition metal complexes (TMCs) show unusual optical, magnetic and chemical properties in comparison to purely organic molecules. These properties make them suitable for several applications. Organic light emitting diodes (OLEDs), catalysis, anti-cancer drugs, molecular switches and new magnetic materials are all areas where TMCs are in current use or within the scope of future applications. It is important to study their photophysical properties since they are of great importance in all these applications. To study the photophysical dynamics of TMCs, an ultrafast broadband transient absorption (TA) spectrometer was set up. For that, two spectrometers were build using charge coupled device(CCD) cameras as detector. Further, a white light continuum was required in order to measure broadband spectra. White light generation was induced in CaF2, which needs continuous movement in order to avoid damage on the crystalline CaF2 disk itself. In this thesis, the photophysics of three groups of TMCs were studied by several optical techniques including ultrafast TA spectroscopy. First, three manganese complexes - two of them showing single molecule magnet (SMM) behaviour - were studied. Manganese complexes are known to have fascinating magnetic and catalytic properties. Their usage as SMMs and catalytic centre in chlorophyll makes them an important class of TMCs. To study the photophysical properties of Mn SMMs, a comparison of Mn complexes with one, three and six Mn ions in one molecule was performed. Two out of the three samples were Mn(III)-based SMMs, which are [Mn(III)3O(Et − sao)3(β−pic)3(ClO4)] or Mn3 and [Mn(III)6O2(Et−sao)6(O2CP h(Me)2)2(EtOH)6] or Mn6. The third one was the Mn(acac)3 (acac ≡ acetylacetonate anion) complex. As a result from the ultrafast TA, it was found that a vibrational wave packet is formed upon photoexcitation of all three complexes. The results show new possibilities for non-thermal control of the magnetisation in SMMs and open up new molecular design challenges to control the wave packet motion in the excited state. Iron complexes, especially iron in oxidation state +2 with six electrons in the d-orbitals, are well-studied due to a spin-crossover phenomenon that can be triggered by optical stimulation. In this thesis, the focus is on the complex [F e3O(Ac)6(H2O)3] (F e3) with three iron ions in mixed oxidation states. One ion is in oxidation state +2 while the other two are in oxidation state +3 (Fe(III)Fe(III)Fe(II)). This molecule is of interest due to possible long-lived charge transfer states and spin dynamics due to the mixed oxidation states. Ultrafast transient absorption spectroscopy was performed of the Fe complex in solution at room-temperature exciting either at 400 nm or 520 nm and a long-lived excited-state absorption (ESA) signal was observed. The broad ESA band is comprised of several un-resolved bands, showing shoulders at 405 nm, 440 nm and 530 nm. From the transient absorption results, three decay constants of τ1 = 360 ± 30 fs, τ2 = 5.3 ± 0.6 ps, τ3 = 65 ± 5 ps and a long-lived state (τ4 > 500 ps) were extracted by a global multi-exponential fit over the full wavelength range (340 nm to 690 nm). The comparison of the TA results with two pump wavelengths indicates that the lowest excited state is populated on a sub 120 fs time scale. Calculations on the coupled cluster level of theory, performed by collaborators, showed that this state has a mixture of both charge-transfer and ligand-field/d-orbital character. Third, the ultrafast photophysics of a terbium phthalocyanine double-decker complex (TbPc2) were investigated. The interaction of the ligands via the terbium linker and the luminescence properties are studied. Static UV/Vis absorption and luminescence spectroscopy were performed. The typical Q- and B-bands associated with excitations in the first and second excited singlet states of the phthalocyanine ligands were found. In the luminescence spectrum after excitation in the UV, signatures of the typical emission for a Tb(III) ion in the green part of the spectrum were found, overlapped with emission from the ligands. The ultrafast TA difference spectra are dominated by ground state bleach bands, which overlap with ESA. The dynamics show a long-lived state, which is important for the luminescence of the Tb3+ ion. The energy transfer pathway was tentatively assigned to a direct excitation of the emissive Tb3+ via the B-band levels of the radical phthalocyanine.