The continued development of solar energy as a renewable resource necessitates the design and study of new approaches to sustaining photodriven charge separation (CS). To this end, we present a bioinspired approach in which triggered conformational changes are used to control electron transfer (ET) events in Cu^I coordination complexes. Herein, photo-induced conformational rearrangements of a ligand are translated into changes in the coordination geometry and environment about a bound metal ion. Taking advantage of the differential coordination properties of Cu^I and Cu^II, these dynamics facilitate intramolecular ET from Cu^I to the ligand to create a CS state. The synthesis and photophysical characterization of CuCl(dpaa^R) (dpaa = dipicolylaminoacetophenone, with R = H and OMe) is presented. These ligands incorporate a fluorophore into their framework that gives rise to a twisted intramolecular charge transfer (TICT) excited state. Excited state ligand twisting provides a tetragonal coordination geometry capable of capturing Cu^II in the CS state when an internal ortho-OMe binding site is present (as in dpaa^OMe). We employ NMR, IR, EPR, and optical spectroscopies, X-ray diffraction, and electrochemical methods to establish the ground state properties of the Cu^I and Cu^II complexes. We then investigate the photophysical dynamics of these Cu^I complexes via time-resolved photoluminescence (TR-PL), and optical transient absorption (OTA) spectroscopies. We show that relative to controls lacking a TICT-active ligand, the lifetime of the CS state is enhanced ~1000-fold. We also show that the presence of the ortho-OMe substituent on the TICT ligand critically enhances both the yield and lifetime of the CS state, presumably by locking the coordination environment of Cu^II in place and imposing a large reorganization energy for charge recombination. These systems represent a first demonstration of our approach to solar energy conversion in which light-driven conformational changes are applied to manipulate the reorganization energy of ET steps.

Josh Vura-Weis

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