Organoboron compounds are highly valuable in synthetic chemistry. In particular, α-borylcarbonyl compounds have shown versatile synthetic applications, owing to fruitful chemistries of both the boryl and carbonyl moieties. However, the synthesis of these molecules still remains tedious and time-consuming. Here we report a straightforward and practical route to synthesize α-borylcarbonyl molecules based on a regioselective radical α-borylation of α,β-unsaturated carbonyl compounds. The reaction features unusual α-regioselectivity and high functional-group compatibility. Further synthetic applications of new α-borylated products were also demonstrated. DFT and kinetic studies implicated that the α-regioselectivity of β-aryl-α,β-unsaturated carbonyl compounds was determined by the thermodynamically more favorable radical α-addition step, whereas the formation of α-addition products from β-alkyl-α,β-unsaturated carbonyl compounds was driven by an energetically favored hydrogen atom transfer step. Given that α,β-unsaturated carbonyl compounds can be easily obtained in abundance and variety, this method enjoys great advantages in diverse and economical synthesis of valuable α-borylcarbonyl molecules.

Three Os(II) complexes were synthesized with ligands 2,2′-dipyridyl (dipy), 1,10-phenanthroline monohydrate (phen), and 4,7-diphenyl-1,10-phenanthroline (diphen), and applied as triplet photosensitizers for triplet–triplet annihilation (TTA) fluorescence upconversion. The strong spin-orbital coupling made direct spin-forbidden transition of S₀–T₁ feasible. Lifetimes of the lowest triplet state of these complexes were determined to be 107 ns, 373 ns, and 386 ns for Os-dipy, Os-phen, and Os-diphen, respectively, using nanosecond transient absorption spectra. From steady-state phosphorescence emission spectra, energies of the triplet states were derived to be 1.75 eV, 1.80 eV, and 1.74 eV for Os-dipy, Os-phen, and Os-diphen, respectively. Using these photosensitizers, strong upconverted fluorescence of the triplet acceptors, 9,10-diphenylanthracene (DPA), perylene, and 9,10-bis(phenethynyl) anthracene (BPEA), was observed in the visible to violet range. In particular, fluorescence emission with the largest anti-Stokes shift of 1.14 eV was observed for the Os-phen/DPA system, and the upconverted quantum yield was determined as 5.9% in deoxygenated dichloroethane. Additionally, upconversion was determined in air using mixtures of dichloroethane and DMSO solvents, and the maximal quantum yield was measured to be 4.5% for Os-phen/DPA.

Solvent effects play a very important role in photochemical reactions and energy transfer processes in solution; however, these effects are rarely mentioned in the triplet–triplet annihilation (TTA) upconversion fluorescence experiments. In a typical TTA upconversion system of a photosensitizer of diiodo-Bodipy (I₂-Bodipy) and a triplet acceptor of perylene, five common inert solvents, hexane, heptane, toluene, 1,4-dioxane, and dimethyl sulfoxide (DMSO), were used to investigate the solvent effects on the overall quantum yield of upconversion fluorescence. Femtosecond and nanosecond time-resolved transient difference absorption spectra were obtained to study the efficiencies of intersystem crossing (ISC) and triplet–triplet energy transfer (TTET). From the obtained upconversion fluorescence emission spectra, the overall TTA upconversion fluorescence quantum yield was derived. Among the five solvents, the upconversion quantum yield in dioxane is the highest at 19.16%, more than twice that that in toluene (8.75%). For the solvents hexane, heptane, toluene, and dioxane, the yields generally follow the sequences of polarity and viscosity. However, a very low upconversion quantum yield (1.51%) was observed in DMSO although the TTET process and fluorescence quantum yield of perylene in DMSO were almost as efficient as in dioxane. Based on density functional theory calculations, a reasonable explanation for these solvent effects was proposed.

Three new triplet photosensitizers consisting of a bodipy derivative and C ₆₀ moieties were synthesized for triplet–triplet annihilation upconversion of perylene.