Thus, it is proposed herein to design and synthesize a series of organic photocatalysts (PCs) based on the cationic azatriangulenium motif for two-photon excited photoredox transformation with CO2 and O2.  

To reduce the carbon footprint of the chemical industry, much efforts have been paid on studying the promotion of chemical reactions by solar energy. Photoredox organic transformation was widely explored in the recent decade as an effective means of performing preparative-scale reactions, which are commonly inaccessible via conventional synthetic procedures. In particular, organic transformation occurring via photoactivation of earth-abundant small molecules, e.g. carbon dioxide (CO2) and oxygen (O2), are drawing much attentions. Related reactions were performed mainly using precious-metal and more recently cheaper organic photocatalysts (PCs) by single-photon excitation with visible light. Since UV/visible photons are substantially absorbed by the reaction matrix, i.e. solvent and substrates, and could lead to catalyst degradation, the usable range of solar radiation and the access to reactive higher excited states will be limited. Photon upconversion (PUC), via the absorption of multiple photons, were widely explored in optoelectronics, bioimaging and photodynamic therapy. However, two-photon excited photocatalysis was confined mainly to reductive transformation, such as hydrodehalogenation, reductive coupling, hydrogenation and bond-breaking reactions, but related examples in photocatalytic functionalization with small molecules are rare. Thus, it is proposed herein to design and synthesize a series of organic photocatalysts (PCs) based on the cationic azatriangulenium motif for two-photon excited photoredox transformation with CO2 and O2. Photophysical/chemical properties of the excited states in two-photon absorption will be studied as a function of varied structural features. Photoredox transformation with CO2 and O2 promoted by two-photon excitation will then be examined with visible (vis) and near infrared (NIR) radiation. Detailed mechanistic studies will also be performed. The photophysical/chemical studies will be supplemented with DFT calculations. In preliminary studies, the cationic organic photocatalysts (PC+ ) of the azatriangulenium motif, namely tri-n-butyl-triazatriangulenium (1 + ) and di-n-butyl-diazaoxatriangulenium (2 + ) have been prepared. Our results suggest the catalysts exhibit stepwise two-photon absorption with a photoreduced triplet 3*PC+ (PC·) intermediary, which on secondary excitation gives rise to an emissive *PC· state (lem = 507 nm (1 + ) and 497 nm (2 + )). The triplet state (3*PC+ ) of 2 + is also found to be populated by simultaneous two-photon absorption (TPA) of NIR (800 nm). Besides, 1 + promote the carboxylation of alkenes with CO2 via consecutive two-photon excitation (lmax = 525 nm, 12 W LED). The catalysts also mediate the oxidative coupling of benzylamine and anilines with O2, when excited via TPA of lower-energy radiation (lmax = 635 nm, 12 W LED and lmax = 970–980 nm, 5W NIR LED).