Long triplet lifetimes of excited photosensitizers are essential for efficient energy transfer reactions in water, given that the concentrations of dissolved oxygen and suitable acceptors in aqueous media are typically...
Incorporation of sulfur dioxide into organic compounds is achieved by a photocatalytic approach using sensitizers made from earth-abundant chromium(III) ions and visible light leading to sulfones and sulfonamides. We employed three different chromium(III) sensitizers [Cr(ddpd) 2 ] 3 + , [Cr(bpmp) 2 ] 3 + and [Cr-(tpe) 2 ] 3 + with long excited state lifetimes and different ground and excited state redox potentials as well as varying stability under the reaction conditions (ddpd = N,N'-dimethyl-N,N'-dipyridin-2-yl-pyridine-2,6-diamine; bpmp = 2,6-bis(2-pyridylmethyl)pyridine; tpe = 1,1,1-tris(pyrid-2-yl)ethane). Key reaction steps of the catalytic cycles are identified by electrochemical, luminescence quenching, photolysis, laser flash photolysis and catalytic experiments delivering a detailed picture of the challenges in these transformations. The reactivity of the reduced chromium complex was identified as a key property to explain the reaction outcomes. Initial cage escape yield determinations with [Cr(tpe) 2 ] 3 + revealed that desired photoreactions occur with unusually high quantum efficiencies, whereas side reactions are almost unproductive.
Substituted diphenylthioureas (DPTUs) are efficient hydrogen-bonding organo-catalysts, and substitution of DPTUs has been shown to greatly affect catalytic activity. Yet, both the conformation of DPTUs in solution and the conformation and hydrogen-bonded motifs within catalytically active intermediates, pertinent to their mode of activation, have remained elusive. By combining linear and ultrafast vibrational spectroscopy with spectroscopic simulations and calculations, we show that different conformational states of thioureas give rise to distinctively different N−H stretching bands in the infrared spectra. In the absence of hydrogen-bond-accepting substrates, we show that vibrational structure and dynamics are highly sensitive to the substitution of DPTUs with CF 3 groups and to the interaction with the solvent environment, allowing for disentangling the different conformational states. In contrast to bare diphenylthiourea (0CF-DPTU), we find the catalytically superior CF 3 -substituted DPTU (4CF-DPTU) to favor the trans−trans conformation in solution, allowing for donating two hydrogen bonds to the reactive substrate. In the presence of a prototypical substrate, DPTUs in trans−trans conformation hydrogen bond to the substrate's C�O group, as evidenced by a red-shift of the N−H vibration. Yet, our time-resolved infrared experiments indicate that only one N−H group forms a strong hydrogen bond to the carbonyl moiety, while thiourea's second N−H group only weakly interacts with the substrate. Our data indicate that hydrogen-bond exchange between these N−H groups occurs on the timescale of a few picoseconds for 0CF-DPTU and is significantly accelerated upon CF 3 substitution. Our results highlight the subtle interplay between conformational equilibria, bonding states, and bonding lifetimes in reactive intermediates in thiourea catalysis, which help rationalize their catalytic activity.
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