No abstract

Porphyrin arrays consisting of three peripheral Zinc porphyrins (ZnPs) and a central free base porphyrin (HP)-all rigidly linked to each other-serve as light-harvesting antennas as well as electron donors and are flexibly coupled to an electron-accepting C to realize the unidirectional flow of (i) excited-state energy from the ZnPs at the periphery to the HP, (ii) electrons to C, and (iii) holes to HP 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 HP and ZnP to convert the (ZnP)-HP-C charge-separated state into (ZnP)-HP-C 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.

We report the synthesis of a full-fledged family of covalent electron donor-acceptor-acceptor 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, C and C. 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 C toward C. This strategy helps minimize energy losses in the reductive, short-range charge shift from C to C. 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.

The lifetime of a charge separated state is enhanced by the effects of solvent polarity and the coordination controlled shuttling of a dumbbell in a porphyrin/fullerene rotaxane.

Here, we present a novel conjugate based on a weakly electron-donating Zn(II)-meso-tetrakis(pentafluorophenyl)porphyrin linked to a strongly electron-accepting C 60 . From spectroscopic and electrochemical investigations a 1.9 eV high charge-separated state is derived. Transient absorption spectroscopy measurements analyzed by global target analysis confirmed the successful charge separation with a quantum yield of 72% despite an overall small driving force of only −0.2 eV. The chargeseparated state lifetime is in the nanosecond range. Considering that 90% of the singlet excited state energy is transiently stored in the charge-separated state, we conclude a charge separation energy efficiency of 65%.