Abstract
Herein, we present a new strategy for the development of efficient heavy-atom free singlet oxygen photosensitizers based on rigid borafluorene scaffolds. Physicochemical properties of borafluorene complexes can be easily tuned through the choice of ligand, thus allowing exploration of numerous organoboron structures as potent 1O2 sensitizers. The singlet oxygen generation quantum yields of studied complexes vary in the range of 0.55–0.78. Theoretical calculations reveal that the introduction of the borafluorene moiety is crucial for the stabilization of a singlet charge transfer state, while intersystem crossing to a local triplet state is facilitated by orthogonal donor–acceptor molecular architecture. Our study shows that quantitative oxidation of selected organic substrates can be achieved in 20–120 min of irradiation with only 0.05 mol % loading of a photocatalyst.
Highlights
Singlet oxygen plays an essential role in a wide range of applications including photocatalysis,[1−3] water and air purification,[4,5] photodynamic therapy (PDT),[6−8] in vivo oxygen sensing,[9] and bioimaging.[10]
Pischel et al demonstrated that (C,N) chelate dimesitylboron complexes based on arylisoquinoline ligand can be used for singlet oxygen production with a moderate singlet oxygen quantum yield value of 0.34.19 In addition, selected boron dipyrromethanes (BODIPYs) were proven as efficient triplet state PSs.[20−25] Filatov et al showed that the formation of a triplet state can be strongly accelerated in the compact donor−acceptor BODIPY dyads.[26−28] In such systems, intramolecular photoinduced electron transfer (PeT) facilitates the formation of charge transfer state (1CT), which is followed by its direct conversion to the lowest triplet excited state (3LE) via spin−orbit charge transfer intersystem crossing (SOCT-ISC)
The syntheses of BODIPY dyads were performed according to the known or modified literature protocols.[42−48] The borafluorene-based BODIPYs Bf-A1, Bf-A2, and Bf-A3 were obtained from the corresponding dipyrromethene ligand precursors (A1-H, A2-H, A3-H) and 9-chloroborafluorene in the presence of base (i-Pr)2NEt
Summary
Singlet oxygen plays an essential role in a wide range of applications including photocatalysis,[1−3] water and air purification,[4,5] photodynamic therapy (PDT),[6−8] in vivo oxygen sensing,[9] and bioimaging.[10]. Smallmolecule, heavy atom-free organic photosensitizers constitute interesting alternatives Due to their controlled structure−property tunability, modular structure, and feasible synthesis, organoboron complexes seem to be ideal candidates for application as singlet oxygen photosensitizers, yet most of them de-excite via fluorescence emission.[17,18] Exceptionally, Pischel et al demonstrated that (C,N) chelate dimesitylboron complexes based on arylisoquinoline ligand can be used for singlet oxygen production with a moderate singlet oxygen quantum yield value of 0.34.19 In addition, selected boron dipyrromethanes (BODIPYs) were proven as efficient triplet state PSs.[20−25] Filatov et al showed that the formation of a triplet state can be strongly accelerated in the compact donor−acceptor BODIPY dyads.[26−28] In such systems, intramolecular photoinduced electron transfer (PeT) facilitates the formation of charge transfer state (1CT), which is followed by its direct conversion to the lowest triplet excited state (3LE) via spin−orbit charge transfer intersystem crossing (SOCT-ISC). The photocatalytic properties were evaluated in singlet-oxygen mediated oxidations of model organic substrates
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