A Strategy for Constructing Photosynthetic Models: Porphyrin-containing Modules Assembled Around Transition Metals

摘要

We have indicated that photoinduced ET between terminal por-phyrins can occur across an interspersed metal complex, the latter species appearing as a virtual or real redox intermediate, over distances relevant to the RC complex (Figure 14). Our model systems developed to date, although capable of high rates of ET, do not possess optimal thermodynamic properties with respect to the mutual relationship between ΔG° and λ which is an integral feature of the RC. Future model systems will need to take better advantage of this relationship in order to maximize rates of CS and, at the same time, minimize rates of CR. It is also apparent that close attention must be given to the preferred orientation of any bridging phenyl rings since this can influence the extent of electronic coupling between appended redox-active subunits. Indeed, the inadvertent positioning of alkyl substituents on the tetrapyrrolic macrocycle may make a major contribution to this effect; we have, however, demonstrated the necessity for such bridges. Even so, triads such as 9 can be considered as belonging to a family of molecules that represents a genuine modular approach to construction of multi-component molecular arrays having a logical and well-defined arrangement of the subunits, as in the RC, and which allows for a cascade of ET events. The availability of a gradient of redox centres is an important, if not essential, aspect of designing molecular systems capable of circumventing CR and, thereby, realizing long-lived, widely-spaced CS states. The modular approach advocated here represents a viable and highly versatile alternative to the common, and successful, practice of using only covalent bonding to assemble giant arrays. It provides new opportunities for the construction of elaborate molecular architectures that closely resemble the RC complex. In particular, the methodology can be readily adapted to synthesize higher-order oligomers possessing quinoid and/or haem-like redox centres that could serve to extend the ET sequence so as to cover many redox partners.