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

Abstract

Sensitized triplet−triplet annihilation upconversion is a promising strategy to use visible light for chemical reactions requiring the energy input of UV photons. This strategy avoids unsafe ultraviolet light sources and can mitigate photo-damage and provide access to reactions, for which filter effects hamper direct UV excitation. Here, we report a new approach to make blue-to-UV upconversion more amenable to photochemical applications. The tethering of a naphthalene unit to a cyclometalated iridium(III) complex yields a bichromophore with a high triplet energy (2.68 eV) and a naphthalenebased triplet reservoir featuring a lifetime of 72.1 μs, roughly a factor of 20 longer than the photoactive excited state of the parent iridium(III) complex. In combination with three different annihilators, consistently lower thresholds for the blue-to-UV upconversion to crossover from a quadratic into a linear excitation power dependence regime were observed with the bichromophore compared to the parent iridium(III) complex. The upconversion system composed of the bichromophore and the 2,5-diphenyloxazole annihilator is sufficiently robust under long-term blue irradiation to continuously provide a high-energy singlet-excited state that can drive chemical reactions normally requiring UV light. Both photoredox and energy transfer catalyses were feasible using this concept, including the reductive N−O bond cleavage of Weinreb amides, a C−C coupling reaction based on reductive aryl debromination, and two Paterno−Buchi [2 + 2] cycloaddition reactions. Our work seems relevant in the context of developing new strategies for driving energetically demanding photochemistry with low-energy input light.

Coulomb interactions for mediator-enhanced sensitized triplet–triplet annihilation upconversion in solution

“…As a consequence, decreased annihilator concentrations are usable and/or lower energy transfer rate constants are sufficient for obtaining similar or even enhanced upconversion quantum yields. This general concept was realized with nanocrystals, 15–20 quantum dots, 21–24 and molecular dyads 25–31 in solution. Additionally, similar sensitizer-mediator-annihilator combinations were explored in solid state perovskite solar cells.…”

“…This is particularly relevant for annihilators with very small Stokes shifts and high extinction coefficients, 34,57–60 which holds true for many annihilators especially in blue-to-UV upconversion systems, a field that has recently received increased attention. 5,26,61–64 Also in systems where the comparably short lifetimes of the sensitizers call for high concentrations of the annihilators, filter effects can significantly limit the output of high-energy photons. 21,65–70 These performance improvements are important for applications in which the upconversion system is used as light source to drive secondary reactions.…”

Bulky Substituents Promote Triplet–Triplet Annihilation Over Triplet Excimer Formation in Naphthalene Derivatives

“…Triplet–triplet annihilation (TTA) is a process in which two dark triplet excitons interact in a spin-allowed process to form a bright and highly energetic singlet state . While it has been known to occur in many organic molecules since the 60s, research on this topic has soared in the last few decades due to its ability to partake in photochemical upconversion (TTA-UC). − Low-energy, incident light is then transformed to light of higher energy, with potential applications in, e.g., biomedicine, − photovoltaics, − and photocatalysis. ,− TTA-UC from visible to ultraviolet (UV) light is of specific interest for photocatalysis and has been proven useful for driving demanding photoreactions under mild conditions. ,, …”

“…The efficiency of UV-emitting TTA-UC systems has improved drastically in only a few years’ time, in part due to the emergence of new materials. ,− Naphthalene is well-known to undergo TTA , but has rendered little success in the context of TTA-UC. Derivatives thereof have been used to some extent, with reports of TTA-UC quantum yields (Φ UC , here defined with a 50% theoretical maximum) ranging from below 1% (refs ,, ) to well above 10% (refs ,,, ). These differences are to some degree explained by differences between the sensitizers of choice and substitution-induced differences in Φ F , but little to no effort has been made to understand why the TTA-UC performance of naphthalene derivatives is so drastically affected by the choice of substituents.…”

Amplified Triplet Emission of Organic Periphery Groups by Exothermic Triplet–Triplet Energy Transfer from the 3MLCT State of an Ir(pmi)3 Core Complex to the 3LC State of Geometrically Confined Carbazole/Naphthyl Tethers

“…TTET is an important type of energy transfer in chemical and biochemical processes. − The mechanism for triplet energy transfer is usually described by Dexter electron-exchange interaction and can be visualized in terms of a two-electron transfer process . The common line of TTET studies in the previous organometallic hybrid systems was mainly based on the mechanism of energy transfer from organic chromophore to metal core, − whereas the presented research is focused on the organic phosphorescence resulting from the TTET process from the metal core to organic units, which is less common and underexploited. − …”