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1. Mimicking Natural Photosynthesis: Designing Ultrafast Photosensitized Electron Transfer into Multiheme Cytochrome Protein Nanowires

   https://doi.org/10.3390/nano10112143  

   Marzolf, Daniel R. ; McKenzie, Aidan M. ; O’Malley, Matthew C. ; Ponomarenko, Nina S. ; Swaim, Coleman M. ; Brittain, Tyler J. ; Simmons, Natalie L. ; Pokkuluri, Phani Raj ; Mulfort, Karen L. ; Tiede, David M. ; et al ( November 2020 , Nanomaterials)

   Efficient nanomaterials for artificial photosynthesis require fast and robust unidirectional electron transfer (ET) from photosensitizers through charge-separation and accumulation units to redox-active catalytic sites. We explored the ultrafast time-scale limits of photo-induced charge transfer between a Ru(II)tris(bipyridine) derivative photosensitizer and PpcA, a 3-heme c-type cytochrome serving as a nanoscale biological wire. Four covalent attachment sites (K28C, K29C, K52C, and G53C) were engineered in PpcA enabling site-specific covalent labeling with expected donor-acceptor (DA) distances of 4–8 Å. X-ray scattering results demonstrated that mutations and chemical labeling did not disrupt the structure of the proteins. Time-resolved spectroscopy revealed three orders of magnitude difference in charge transfer rates for the systems with otherwise similar DA distances and the same number of covalent bonds separating donors and acceptors. All-atom molecular dynamics simulations provided additional insight into the structure-function requirements for ultrafast charge transfer and the requirement of van der Waals contact between aromatic atoms of photosensitizers and hemes in order to observe sub-nanosecond ET. This work demonstrates opportunities to utilize multi-heme c-cytochromes as frameworks for designing ultrafast light-driven ET into charge-accumulating biohybrid model systems, and ultimately for mimicking the photosynthetic paradigm of efficiently coupling ultrafast, light-driven electron transfer chemistry to multi-step catalysis within small,more »experimentally versatile photosynthetic biohybrid assemblies.« less
2. Symmetry controlled photo-selection and charge separation in butadiyne-bridged donor–bridge–acceptor compounds

   https://doi.org/10.1039/D0CP01235A  

   Li, Xiao ; Valdiviezo, Jesús ; Banziger, Susannah D. ; Zhang, Peng ; Ren, Tong ; Beratan, David N. ; Rubtsov, Igor V. ( May 2020 , Physical Chemistry Chemical Physics)

   Electron transfer (ET) in donor–bridge–acceptor (DBA) compounds depends strongly on the structural and electronic properties of the bridge. Among the bridges that support donor–acceptor conjugation, alkyne bridges have attractive and unique properties: they are compact, possess linear structure permitting access to high symmetry DBA molecules, and allow torsional motion of D and A, especially for longer bridges. We report conformation dependent electron transfer dynamics in a set of novel DBA compounds featuring butadiyne (C4) bridge, N -isopropyl-1,8-napthalimide (NAP) acceptors, and donors that span a range of reduction potentials (trimethyl silane (Si-C4-NAP), phenyl (Ph-C4-NAP), and dimethyl aniline (D-C4-NAP)). Transient mid-IR absorption spectra of the CC bridge stretching modes, transient spectra in the visible range, and TD-DFT calculations were used to decipher the ET mechanisms. We found that the electronic excited state energies and, especially, the transition dipoles (S 0 → S n ) depend strongly on the dihedral angle ( θ ) between D and A and the frontier orbital symmetry, offering an opportunity to photo-select particular excited states with specific ranges of dihedral angles by exciting at chosen wavelengths. For example, excitation of D-C4-NAP at 400 nm predominantly prepares an S 1 excited state in the planar conformations ( θmore »∼ 0) but selects an S 2 state with θ ∼ 90°, indicating the dominant role of the molecular symmetry in the photophysics. Moreover, the symmetry of the frontier orbitals of such DBA compounds not only defines the photo-selection outcome, but also determines the rate of the S 2 → S 1 charge separation reaction. Unprecedented variation of the S 2 –S 1 electronic coupling with θ by over four orders of magnitude results in slow ET at θ ca. 0° and 90° but extremely fast ET at θ of 20–60°. The unique features of high-symmetry alkyne bridged DBA structures enable frequency dependent ET rate selection and make this family of compounds promising targets for the vibrational excitation control of ET kinetics.« less
3. Ultrafast photo-driven charge transfer exciton dynamics in mixed-stack pyrene-perylenediimide single co-crystals

   https://doi.org/10.1039/d1tc04313g  

   Myong, Michele S. ; Qi, Yue ; Stern, Charlotte ; Wasielewski, Michael R. ( December 2021 , Journal of Materials Chemistry C)

   Electron donor–acceptor co-crystals are receiving increasing interest because of their many useful optoelectronic properties. While the steady-state properties of many different co-crystals have been characterized, very few studies have addressed how crystal morphology affects the dynamics of charge transfer (CT) exciton formation, migration, and decay, which are often critical to their performance in device structures. Here we show that co-crystallization of a pyrene (Pyr) electron donor with either N , N ′-bis(2,6-diisopropylphenyl)- or N , N ′-bis(3′-pentyl)-perylene-3,4:9,10-bis(dicarboximide) (diisoPDI or C 5 PDI) electron acceptors, respectively, yields mixed π-stacked Pyr–diisoPDI or Pyr–C 5 PDI donor–acceptor co-crystals. Femtosecond transient absorption microscopy is used to determine the CT exciton dynamics in these single crystals. Fitting the data to a one-dimensional charge transfer CT exciton diffusion model reveals a diffusion constant that is two orders of magnitude higher in the Pyr–diisoPDI co-crystal compared to the Pyr–C 5 PDI co-crystal. By correlating the co-crystal structures to their distinct excited-state dynamics, the effects of each mixed stacked structure on the exciton dynamics and the mechanisms of CT exciton diffusion are elucidated.
4. Engineering opposite electronic polarization of singlet and triplet states increases the yield of high-energy photoproducts

   https://doi.org/10.1073/pnas.1901752116  

   Polizzi, Nicholas F. ; Jiang, Ting ; Beratan, David N. ; Therien, Michael J. ( June 2019 , Proceedings of the National Academy of Sciences)

   Efficient photosynthetic energy conversion requires quantitative, light-driven formation of high-energy, charge-separated states. However, energies of high-lying excited states are rarely extracted, in part because the congested density of states in the excited-state manifold leads to rapid deactivation. Conventional photosystem designs promote electron transfer (ET) by polarizing excited donor electron density toward the acceptor (“one-way” ET), a form of positive design. Curiously, negative design strategies that explicitly avoid unwanted side reactions have been underexplored. We report here that electronic polarization of a molecular chromophore can be used as both a positive and negative design element in a light-driven reaction. Intriguingly, prudent engineering of polarized excited states can steer a “U-turn” ET—where the excited electron density of the donor is initially pushed away from the acceptor—to outcompete a conventional one-way ET scheme. We directly compare one-way vs. U-turn ET strategies via a linked donor–acceptor (DA) assembly in which selective optical excitation produces donor excited states polarized either toward or away from the acceptor. Ultrafast spectroscopy of DA pinpoints the importance of realizing donor singlet and triplet excited states that have opposite electronic polarizations to shut down intersystem crossing. These results demonstrate that oppositely polarized electronically excited states can be employed to steermore »photoexcited states toward useful, high-energy products by routing these excited states away from states that are photosynthetic dead ends.« less
5. Inelastic Charge‐Transfer Dynamics in Donor–Bridge–Acceptor Systems Using Optimal Modes

   Whaley, K. B. ; Yang, X. ; Pereverzev, A. ; Bittner, E. R. ( January 2018 , Advances in chemical physics)

   A time‐convolutionless master equation approach for computing state‐to‐state rates was developed in which the coupling between states depends on the nuclear coordinates. This approach incorporates a fully quantum‐mechanical treatment of both the nuclear and electronic degrees of freedom and recovers the well‐known Marcus expression in the semiclassical limit. A significant breakthrough was made in using this approach by tying it to a fully ab initio quantum chemical approach for determining the diabatic states and electron‐phonon coupling terms, allowing unprecedented accuracy and utility for computing state‐to‐state electronic transition rates. The Weinstein group at the University of Sheffield reported recently upon a series of donor‐bridge‐acceptor (DBA) molecular triads whose electron‐transfer (ET) pathways can be radically changed by infrared light excitation of specific intramolecular vibrations. Once the diabatic states and couplings are determined, the TCLME approach is used to compute the time‐correlation functions and state‐to‐state golden‐rule rates.