Rigid, Branched Porphyrin Antennas: Control over Cascades of Unidirectional Energy Funneling and Charge Transfer

Porphyrin arrays consisting of three peripheral Zinc porphyrins (ZnPs) and a central free base porphyrin (H₂P)all rigidly linked to each otherserve as light-harvesting antennas as well as electron donors and are flexibly coupled to an electron-accepting C₆₀ to realize the unidirectional flow of (i) excited-state energy from the ZnPs at the periphery to the H₂P, (ii) electrons to C₆₀, and (iii) holes to H₂P and, subsequently, to ZnP. Dynamics following photoexcitation are elucidated by time-resolved transient absorption measurements on the femto-, pico-, nano-, and microsecond time scales and are examined by multiwavelength as well as target analyses. Hereby, full control over the charge shift between H₂P and ZnP to convert the (ZnP)₃–H₂P^•+–C₆₀^•– charge-separated state into (ZnP)₃^•+–H₂P–C₆₀^•– charge-separated state is enabled by the solvent polarity: It is deactivated/switched-off in apolar toluene, while in polar benzonitrile it is activated/switched-on. Activating/switching impacts the recovery of the ground state via charge recombination rates, which differ by up to 2 orders of magnitude. All charge-separated states lead to the repopulation of the ground state with dynamics that are placed in the inverted region of the Marcus parabola

Coordinating Electron Transport Chains to an Electron Donor

Two electron transport chains (<b>2</b> and <b>3</b>) featuring two fullerenes with different electron acceptor strengths have been synthesized, characterized, and coordinated to a light harvesting/electron donating zinc porphyrin. Electrochemical assays corroborate the redox gradients along the designed electron transport chains, and complementary absorption and fluorescence titrations prove the assembly of ZnP-<b>2</b> and ZnP-<b>3</b> hybrids

Mediating Reductive Charge Shift Reactions in Electron Transport Chains

We report the synthesis of a full-fledged family of covalent electron donor–acceptor₁–acceptor₂ conjugates and their charge-transfer characterization by means of advanced photophysical assays. By virtue of variable excited state energies and electron donor strengths, either Zn­(II)­Porphyrins or Zn­(II)­Phthalocyanines were linked to different electron-transport chains featuring pairs of electron accepting fullerenes, that is, C₆₀ and C₇₀. In this way, a fine-tuned redox gradient is established to power a unidirectional, long-range charge transport from the excited-state electron donor via a transient C₆₀^•– toward C₇₀^•–. This strategy helps minimize energy losses in the reductive, short-range charge shift from C₆₀ to C₇₀. At the forefront of our investigations are excited-state dynamics deduced from femtosecond transient absorption spectroscopic measurements and subsequent computational deconvolution of the transient absorption spectra. These provide evidence for cascades of short-range charge-transfer processes, including reductive charge shift reactions between the two electron-accepting fullerenes, and for kinetics that are influenced by the nature and length of the respective spacer. Of key importance is the postulate of a mediating state in the charge-shift reaction at weak electronic couplings. Our results point to an intimate relationship between triplet–triplet energy transfer and charge transfer