Metal-Organic Bichromophore Lowers the Upconversion Excitation Power Threshold and Promotes UV Photoreactions

Sensitized triplet-triplet annihilation upconversionisa promising strategy to use visible light for chemical reactions requiringthe energy input of UV photons. This strategy avoids unsafe ultravioletlight sources and can mitigate photo-damage and provide access toreactions, for which filter effects hamper direct UV excitation. Here,we report a new approach to make blue-to-UV upconversion more amenableto photochemical applications. The tethering of a naphthalene unitto a cyclometalated iridium-(III) complex yields a bichromophore witha high triplet energy (2.68 eV) and a naphthalene-based triplet reservoirfeaturing a lifetime of 72.1 mu s, roughly a factor of 20 longerthan the photoactive excited state of the parent iridium-(III) complex.In combination with three different annihilators, consistently lowerthresholds for the blue-to-UV upconversion to crossover from a quadraticinto a linear excitation power dependence regime were observed withthe bichromophore compared to the parent iridium-(III) complex. Theupconversion system composed of the bichromophore and the 2,5-diphenyloxazoleannihilator is sufficiently robust under long-term blue irradiationto continuously provide a high-energy singlet-excited state that candrive chemical reactions normally requiring UV light. Both photoredoxand energy transfer catalyses were feasible using this concept, includingthe reductive N-O bond cleavage of Weinreb amides, a C-Ccoupling reaction based on reductive aryl debromination, and two Paterno-Bu'chi [2 + 2] cycloaddition reactions. Our work seems relevant in the contextof developing new strategies for driving energetically demanding photochemistrywith low-energy input light.

Automated summary

This study reports a new approach to achieve control over blue-to-UV upconversion by tethering a naphthalene unit to a cyclometalated iridium-(III) complex, providing a better option for photochemical applications. The bichromophore demonstrated a higher triplet energy and longer lifetime compared to the parent complex. The threshold for blue-to-UV upconversion to shift from quadratic to linear excitation power dependence was consistently lower with the bichromophore. Additionally, the bichromophore showed long-term stability under blue irradiation and successfully drove chemical reactions that normally require UV light.