Anti-Kasha Photophysics and Photochemistry in Pt-dithiolene Complexes Investigated with Ultrafast Transient Absorption

Metal-dithiolene complexes with d8-square planar structure and non-innocent ligands show outstanding hyperpolarizability and nonlinear optical behavior due to highly delocalized frontier electrons, making them interesting for photovoltaics, visual and sensing technologies. They can potentially act as charge transfer (CT) relays for artificial photocatalyzers, controllable by external stimuli as light, temperature, solvent polarity, etc. For instance complexes containing quinoxdt ligands show proton-switchable properties in acidic solutions. Surprisingly, quinoxdt complexes have additionally a photophysical response that markedly dependents on the excitation wavelength (anti-Kasha behavior). Systems with such an uncommon behavior have raised more and more interest because of the possibility to conceive multi-response molecular devices or to explore novel photochemical routes. Such a behavior originates from a competition between the functional process in the upper photo-excited state and internal conversion (IC) towards the lowest excited state. Therefore femtosecond transient absorption is ideal to characterize the anti-Kasha process with the perspective to optimize its performance. We elucidated the origin of the anti-Kasha behavior both in homoleptic and heteroleptic quinoxdt complexes, which stems from a neat spatial separation between the molecular orbitals of the excited states localized on the quinoxdt moiety. This electronic arrangement gives a CT character to the IC slowing down the process. We also discuss the role of such an unusually long lifetime of the photo-excited higher state on the functional processes characterizing the two investigated complexes, namely photocatalytic H2 production and photo-induced chemiluminescence, respectively.