Vibronic characteristics and spin-density distributions in the core bacteriochlorin macrocycle were revealed by spectroscopic and theoretical studies of 16 isotopologues. The vibrational modes in copper bacteriochlorin isotopologues were examined via resonance Raman and Fourier-transform infrared spectroscopy. The resonance Raman spectra exhibit an exceptional sparcity of vibronically active modes of the core macrocycle, in contrast with the rich spectra of the natural bacteriochlorophylls. The Q-excitation resonance Raman spectrum is dominated by a single mode at 727 cm, which calculations suggest is due to a symmetrical accordion-like deformation of the five-atom C(CNC)C portion of the ring core. This deformation also dominates the vibronic features in the absorption and fluorescence spectra. The spin-density distributions in the π-cation radical of the zinc bacteriochlorin isotopologues were studied by electron paramagnetic resonance spectroscopy. The spectra indicate a significant electron/spin density (ρ ∼ 0.1) on each meso-carbon atom. This observation contradicts the predictions of early calculations that have been assumed to be correct for nearly four decades. Collectively, these findings have implications for how the structural features that characterize natural bacteriochlorophylls might influence energy- and electron-transfer processes in photosynthesis and alter the thinking on the design of synthetic, bacteriochlorin-based arrays for solar-energy conversion.

Understanding hole/electron-transfer processes among interacting constituents of multicomponent molecular architectures is central to the fields of artificial photosynthesis and molecular electronics. Herein, we utilize a recently demonstrated (203)Tl/(205)Tl hyperfine "clocking" strategy to probe the rate of hole/electron transfer in the monocations of a series of three thallium-chelated porphyrin dyads, designated Tl(2)-U, Tl(2)-M, and Tl(2)-B, that are linked via diarylethynes wherein the number of ortho-dimethyl substituents on the aryl group of the linker systematically increases (none, one, and two, respectively). Variable-temperature (160-340 K) EPR studies on the monocations of the three dyads were used to examine the thermal activation behavior of the hole/electron-transfer process and reveal the following: (1) Hole/electron transfer at room temperature (295 K) slows as torsional constraints are added to the diarylethyne linker [k(Tl(2)-U) > k(Tl(2)-M) > k(Tl(2)-B)], with rate constants that correspond to time constants in the 2-5 ns regime. (2) As the temperature decreases, the hole/electron-transfer rates for the monocations of the three types of dyads converge and then cross over. At the lowest temperatures examined (160-170 K), the trend in the hole/electron-transfer rates is essentially reversed [k(Tl(2)-B) > k(Tl(2)-M) ~ k(Tl(2)-U)]. The trends in the temperature dependence of hole/electron-transfer among the three dyads are consistent with torsional motions of the aryl rings of the linker providing for thermal activation of the process at higher temperatures in the case of the less torsionally constrained dyads, Tl(2)-U and Tl(2)-M. In the case of the most torsionally constrained dyad, Tl(2)-B, the hole/electron-transfer process is activationless at all temperatures studied. The reversal in rates of hole/electron transfer among the three dyads at low temperature is qualitatively explained by the results of density functional theory calculations, which predict that static electronic factors could dominate the hole/electron-transfer process when torsional dynamics are thermally diminished.