A New Class of Giant Tetrads for Studying Aspects of Long-Range Intramolecular Electron Transfer Processes: Synthesis and Computational Studies

摘要

A modular approach for synthesising the giant multichromophoric systems 19, 20 and 5.2PF_6 is presented which involves the sequential Diels-Alder reactions of tetraene 7 with the chromophore-based dienophiles 6,8 nad 23. The #pi#-facial stereoselectivity of the Diels-Alder reactions between such building blocks enables the isolation of two major stereoisomers, namely synsyn-5.2PF_6 and anti,syn-5.2PF_6, which differ substantially in their shape. This geometrical difference offers a unique opportunity for the delineation of two mechanisms of photoinduced electron transfer (ET): solvent-mediated ET and through-bridge-mediated ET. The stereochemical assignments of syn-syn-5.2PF_6 and anti,syn-5.2PF_6 were secured on the basis of ~1H NMR and photophysical studies, namely the oservation of NOE effects and strong upfield chemical shifts of the bipyridine proton resonances in syn,syn-19, and the observation of photoinduced ET in syn,syn-5.2PF_6, but not in anti,syn-5.2PF_6. The results of gas-phase semi-empirical (AM1) and ab initio (HF.3-21G) MO calculations also lend support to the interpretation of the NMR studies and to the stereochemical assignments for the two dsolated stereoisomers of 5.2PF_6. Importantly, 19 and 5~2+ are not as rigid as might be expected and give rise to the existence of two conformers for syn,syn-19, and the formation of a closed form for syn,syn-5~2+ in which the terminal chromophores are separated by a distance of only≈4 A in the gas phase.