The energy transfer (ET) from carotenoids (Cars) to chlorophylls (Chls) in photosynthetic complexes occurs with almost unitary efficiency thanks to the synergistic action of multiple finely tuned channels whose photophysics and dynamics are not fully elucidated yet. We investigated the energy flow from the Car peridinin (Per) to Chl a in the peridinin chlorophyll a protein (PCP) from marine algae Amphidinium carterae by using two-dimensional electronic spectroscopy (2DES) with a 10 fs temporal resolution. Recently debated hypotheses regarding the S2-to-S1 relaxation of the Car via a conical intersection and the involvement of possible intermediate states in the ET were examined. The comparison with an N89L mutant carrying the Per donor in a lower-polarity environment helped us unveil relevant details on the mechanisms through which excitation was transferred: the ET yield was conserved even when a mutation perturbed the optimization of the system thanks to the coexistence of multiple channels exploited during the process.
Dye‐sensitized photoanodes for C−H activation in organic substrates are assembled by vacuum sublimation of a commercially available quinacridone (QNC) dye in the form of nanosized rods onto fluorine‐doped tin oxide (FTO), TiO2, and SnO2 slides. The photoanodes display extended absorption in the visible range (450–600 nm) and ultrafast photoinduced electron injection (<1 ps, as revealed by transient absorption spectroscopy) of the QNC dye into the semiconductor. The proton‐coupled electron‐transfer reactivity of QNC is exploited for generating a nitrogen‐based radical as its oxidized form, which is competent in C−H bond activation. The key reactivity parameter is the bond‐dissociation free energy (BDFE) associated with the N⋅/N−H couple in QNC of 80.5±2.3 kcal mol−1, which enables hydrogen atom abstraction from allylic or benzylic C−H moieties. A photoelectrochemical response is indeed observed for organic substrates characterized by C−H bonds with BDFE below the 80.5 kcal mol−1 threshold, such as γ‐terpinene, xanthene, or dihydroanthracene. This work provides a rational, mechanistically oriented route to the design of dye‐sensitized photoelectrodes for selective organic transformations.
The Cover Feature shows the photoelectrochemical activation of C−H bonds with mesoporous SnO2 electrodes sensitized with a quinacridone dye (pigment violet 19, used in paints). The key feature of quinacridone is the possibility to operate through a proton‐coupled electron transfer mechanism, generating a nitrogen‐based radical characterized by a bond dissociation free energy (BDFE) of 80.5 kcal mol−1 and capable of hydrogen atom abstraction from allylic and benzylic C−H bonds. More information can be found in the Research Article by Y. Yang, G. A. Volpato, E. Rossin et al.
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