Probing the rate of hole transfer in oxidized porphyrin dyads using thallium hyperfine clocks

Understanding hole/electron-transfer processes among interacting constituents of multicomponent molecular architectures is central to the fields of artificial photosynthesis and molecular electronics. One strategy for examining ground-state hole/electron transfer in oxidized tetrapyrrolic arrays entails analysis of the hyperfine interactions observed in the electron paramagnetic resonance (EPR) spectrum of the π-cation radical. Herein, it is demonstrated that ²⁰³Tl/²⁰⁵Tl hyperfine "clocks" are greatly superior to those provided by ¹H, ¹⁴N, or ¹³C owing to the fact that the ²⁰³Tl/²⁰⁵Tl hyperfine couplings are much larger (15-25 G) than those of the ¹H, ¹⁴N, or ¹³C nuclei (1-6 G). The large ²⁰³Tl/ ²⁰⁵Tl hyperfine interactions permit accurate simulations of the EPR spectra and the extraction of specific rates of hole/electron transfer. The ²⁰³Tl/²⁰⁵Tl hyperfine clock strategy is applied to a series of seven porphyrin dyads. All of the dyads are joined at a meso position of the porphyrin macrocycle via linkers of a range of lengths and composition (diphenylethyne, diphenylbutadiyne, and (p-phenylene)_n, where n = 1-4); substituents such as mesityl at the nonlinking meso positions are employed to provide organic solubility. The hole/electron-transfer time constants are in the hundreds of picoseconds to sub-10 ns regime, depending on the specific porphyrin and/or linker. Density functional theory calculations on the constituents of the dyads are consistent with the view that the relative energies of the porphyrin versus linker highest occupied molecular orbitals strongly influence the hole/electron-transfer rates. Variable-temperature EPR studies further demonstrate that the hole/electron-transfer process is at best weakly activated (12-15 kJ mol^-1) at room temperature and somewhat below. At lower temperatures, the process is essentially activationless. The weak activation is attributed to restricted torsional motions of the phenyl rings of the linker. Collectively, the studies provide the physical basis for the rational design of multicomponent architectures for efficient hole/electron transfer.