Gating Charge Recombination Rates through Dynamic Bridges in Tetrathiafulvalene–Fullerene Architectures

Abstract

Conversion of light into chemical energy is a process that nature has optimized over eons in photosynthetic organisms, such as bacteria, plants, and algae. However, the search for non-natural systems that mimic the complex overall process of photosynthesis has remained a challenge. In particular, the key step of the initial light-induced charge separation between electron donors and acceptors is hampered by its inherent microscopic reversibility, that is, competing charge recombination. Well-defined molecular model systems typically comprise a donor (D) and an acceptor (A) covalently linked by a bridge (B). In the resulting D–B–A structures, the role of the bridge is ideally to facilitate the desired initial photoinduced charge separation, yet, to slow down the undesired charge recombination. Among the many combinations of donors and acceptors that have been explored, those consisting of proaromatic tetrathiafulvalene (TTF) and fullerene derivatives, such as C60, have shown outstanding results. The exceptional electron donating and accepting properties originate from the aromatic stabilization of the formed TTF radical cation and from C60 s unique three dimensional delocalized p-electron system, respectively. This last feature leads to low reorganization energies upon the reduction to the C60 radical anion and allows for the uptake of up to six additional electrons. The photophysical properties of various TTF–C60 conjugates featuring different p-conjugated molecular bridges have been investigated and charge-separated states with lifetimes ranging from a few nanoseconds up to hundreds of microseconds have been realized. Of particular interest are conjugates with p-extended TTF derivatives, in which a conjugated p-quinoid anthracene moiety is placed between the TTF s two 1,3-dithiole rings. Nevertheless, the design of such D–B–A architectures features inherent drawbacks. For example, even with optimized donors and acceptors, the bridge needs to play two opposing roles. On the one hand, it should enhance the coupling between D and A to facilitate the initial charge separation. On the other hand, once the charge-separated state has been formed it should prevent charge recombination by decoupling DC and AC . Clearly, conventional, static bridges have to be a compromise of these two demands. However, if D and A are connected by a dynamic bridge, which can be switched between a coupled and a decoupled form, prolonged charge-separated state lifetimes could potentially be attained without compromising the initial charge separation. Such improved molecular design requires a switch entity that adopts two electronically distinct forms and allows for precise timing of the switching, that is, when the bridge is being coupled or decoupled. Dithienylethenes (DTE) are ideal candidates as switchable bridges as they reversibly interconvert between their ring-open (decoupled) and ring-closed (p-conjugated) forms upon irradiation with light of specific wavelengths. Adopting this new strategy, we prepared four novel D–DTE–A structures connecting either TTF or exTTF acting as D and with C60 functioning as A by photochromic dithienylperhydrocyclopentene or perfluorocyclopentene bridges (Scheme 1). Some researchers have used photochromic units as lightresponsive electronic traps that allow or prevent intramolecular electron transfer from D to A depending on the adopted isomeric form. There are also some examples, in which the electron transfer kinetics are clearly altered by the structural modification of the bridging units by chemical inputs (chelation) or, in mechanically interlocked D and A units, by topological changes. However, herein, we show for the first time that in (ex)TTF–DTE–C60 architectures, the lifetime of the charge-separated state can be significantly shortened or prolonged by performing light-induced structural changes in the bridging unit. [*] Dr. S. Castellanos, Dr. J. Moreno, Prof. Dr. S. Hecht Department of Chemistry Humboldt-Universit t zu Berlin Brook-Taylor-Strasse 2, 12489 Berlin (Germany) E-mail: sh@chemie.hu-berlin.de