Design, synthesis and photoinduced processes in molecular interlocked photosynthetic [60]fullerene systems

Abstract

In natural photosynthesis, the protein backbone directs and positions primary and secondary electron donor and acceptor moieties in the reaction center to control the yield and kinetics of the sequential electron transfer reactions that transform light energy into chemical potential. Organization of the active cofactors is mainly driven by noncovalent interactions between the protein scaffold and the chromophores. Based on the natural system blueprint, several research efforts have investigated π–π stacking, ionic interactions as well as formation of hydrogen and coordinative bonds as noncovalent organizing principles for the assembly of electron donors and acceptors in artificial reaction centers. Introduction of supramolecular concepts into the organization of electron donor–acceptor in artificial photosynthetic models raises the possibility of applying template-directed synthesis techniques to assemble interlocked systems in which the photo-redox components are mechanically rather than covalently linked. Rotaxanes and catenanes are the leading examples of interlocked molecules, whose recent developments in synthetic chemistry have allowed their efficient preparation. Introduction of mechanical bonds into electron donor–acceptor systems allows the study of the interlocked components’ submolecular motions and conformational changes, which are triggered by external stimuli, on the thermodynamic and kinetic parameters of photoinduced processes in artificial reaction centers. This Tutorial discusses our efforts in the synthesis and photophysical investigation of rotaxanes and catenanes decorated with peripheral electron donors and [60]fullerene as the acceptor. The assembly of our rotaxanes and catenanes is based on the classic 1,10-phenanthroline-copper(I) metal template strategy in conjunction with the virtues of the Cu(I)-catalyzed-1,3-dipolar cycloaddition of azides and alkynes (the CuAAC or “click” reaction) as the protocol for the final macrocyclization or stoppering reactions of the entwined precursors. Time-resolved emission and transient absorption experiments revealed that upon excitation, our multichromophoric rotaxanes and catenanes undergo a cascade of sequential energy and electron transfer reactions that ultimately yield charge separated states with lifetimes as long as 61 microseconds, thereby mimicking the functions of the natural systems. The importance of the Cu(I) ion (used to assemble the interlocked molecules) as an electronic relay in the photoinduced processes is also highlighted. The results of this research demonstrate the importance of the distinct molecular conformations adopted by rotaxanes and catenanes in the electron transfer dynamics and illustrate the versatility of interlocked molecules as scaffolds for the organization of donor–acceptor moieties in the design of artificial photosynthetic reaction centers.