Rationalization of excited-state tuning through ultrafast transient absorption and vibrational coherence spectroscopy

Photophysical and-chemical processes make use of light as strongly quantized energy source, rendering mechanisms possible, which involve excited states that are thermally unavailable. This puts them at the heart of many exciting and promising technologies from photovoltaics to photocatalysis and photodynamic therapy. In this work, several strategies to tuning these excited states are rationalized by ultrafast transient absorption and impulsive vibrational spectroscopy, applied to two different classes of samples. Firstly, the excited-state dynamics of two iron(II) complexes are investigated for the tuning effect of solvent choice and ligand design. They toggle on and off the involvement of metal-centered (MC) excited states acting as loss channels for desired metal-to-ligand charge transfer (MLCT) states. Impulsive vibrational spectroscopy is established as suitable method for identifying MLCT-MC transitions in [Fe(bpy)(CN)4]2-, a well-known reference sample. The method is then applied to an iron(II)N-heterocyclic carbene complex and identifies an ultrafast MLCT-MC branching in this promising dye-sensitizer candidate. Secondly, the photophysics and -chemistry of triphenylamine is thoroughly investigated for the influences of solvent, the oxygen content therein and enforced planarity. In n-hexane, triphenylamine is converted to N-phenylcarbazole, with oxygen playing an intricate double role. The conversion is stopped completely by planarization due to the cancellation of p-orbital preorientation. In chloroform, ultrafast electron transfer to the solvent dominates the photochemistry, producing the radical cation leading to chromophore dimerization