Probing the rate of hole transfer in oxidized synthetic chlorin dyads via site-specific ¹³C-Labeling

Understanding electronic communication among interacting constituents of multicomponent molecular architectures is important for rational design in diverse fields including artificial photosynthesis and molecular electronics. One strategy for examining ground-state hole/electron transfer in an oxidized tetrapyrrolic array relies on analysis of the hyperfine interactions observed in the EPR spectrum of the φ-cation radical. This strategy has been previously employed to probe the hole/electron-transfer process in oxidized multiporphyrin arrays of normal isotopic composition, wherein ¹H and ¹⁴N serve as the hyperfine "clocks", and in arrays containing site-specific ¹³C-labels, which serve as additional hyperfine clocks. Herein, the hyperfine-clock strategy is applied to dyads of dihydroporphyrins (chlorins). Chlorins are more closely related structurally to chlorophylls than are porphyrins. A de novo synthetic strategy has been employed to introduce a ¹³C label at the 19-position of the chlorin macrocycle, which is a site of large electron/hole density and is accessible synthetically beginning with ¹³C-nitromethane. The resulting singly ¹³C-labeled chlorin was coupled with an unlabeled chlorin to give a dyad wherein a diphenylethyne linker spans the 10-positions of the two zinc chlorins. EPR studies of the monocations of both the natural abundance and ¹³C-labeled zinc chlorin dyads and benchmark zinc chlorin monomers reveal that the time scale for hole/electron transfer is in the 4-7 ns range, which is 5-10-fold longer than that in analogous porphyrin arrays. The slower hole/electron transfer rate observed for the chlorin versus porphyrin dyads is attributed to the fact that the HOMO is a_1u-like for the chlorins versus a_2u-like for the porphyrins; the a_1u-like orbital exhibits little (or no) electron/hole density at the site of linker attachment whereas the a_2u-like orbital exhibits significant electron/hole density at this site. Collectively, the studies of the chlorin and porphyrin dyads provide insights into the structural features that influence the hole/electron-transfer process.