Redox-active -Conjugated Systems

The hybrid complexes composed of transition metals and redox-active 7C-conjugat-ed polymers (synthetic metals) or molecules provide novel multi-redox systems, with much potential. The structural control in the complexation leads to structurally defined hybrid systems. These systems are promising for the development of efficient catalysts and functionalized materials. 

The present work was done by researchers in our laboratory whose names are cited in the references. This work is partly supported by a Grant-in-Aid for Scientific Research on Priority Areas from the Ministry of Education, Culture, Sports, Science and Technology, Japan. 

---

Violene is constructed by a conjugated chain with two redox active chromophores. It is classified as two types (see Figure 2). As noted from the position of the end groups, one type is separated by an aromatic core and the other is located inside a cyclic 7c-system. In both cases, the colored species in this system are the radical ionic states. Thus, in general, the violene system does not show high redox stability (2). 

Another way of arranging the intramolecular transmembrane electron transfer is to use the so called molecular wires, i.e. molecules with the electron conduction chain of conjugated bonds, redox active polar terminal groups and the length sufficient to span across the membrane. Such molecules can in principle provide for electron transfer from the externally added or photogenerated reductant across the membrane to the oxidant. This mechanism was suggested [41, 94] to explain the action of carotene-containing System 1 and 38 of Table 1. However, as it was shown later, the transmembrane PET in these systems proceeded also without carotene. 

Using a different approach, the research gronps of Fabre and Freund have synthesized boronate-functionalized conjugated polymers, which serve as electrochemical sensors. For example, a conjugated redox-active film of polypyrrole (174) was electrodeposited onto a platinum electrode from acetonitrile solution. Addition of fluoride anions led to a new redox system that showed an anodic shift relative to polypyrrole itself, which was attributed to fluoride binding to the boronate group. A related poly(aniline boronic acid) (175) was also reported and studied for saccharide detection. " ... 

The main interest with porphyrins, however, lies not in the effects of variations in the conjugated n system but in the influence of various metal ions in the porphyrin cavity on the redox activity of the porphyrin ligands. With transition-metal ions both conjugative and inductive effects can be expected, and regularities in the oxidation and reduction potentials, as well as the differences between them, should yield direct information on the nature of the metal-porphyrin interactions. 

The TT-conjugated polymers or oligomers are able to serve as redox-active 7C-conju-gated ligands with coordination capability towards transition metals as described above. Few papers have been published on the complexation behavior of the redox-active Tt-conjugated polymers. Multi-coordination to -conjugated polymers possessing relevant redox function is likely to provide multi-redox systems. 

The present conjugated system appears to be of potential from the viewpoint of redox-active molecular recognition systems. 

Electron transfer is an appealing process allowing the same transformation as in photochemistry, i.e., cyclization-reopening. In this context, the DTE unit has proven to be useful for the elaboration of photo and/or electrochromic metal-based systems specially devoted to the control of the electronic communication in bimetallic complexes. Organometallic complexes crmsisting of Jt-conjugated system bridging two redox active metal centers have proven to be efficient molecular wires (Fig. 11). 

---