A new class of giant tetrads for studying aspects of long-range intramolecular electron transfer processes: Synthesis and computational studies

A modular approach for synthesising the giant multichromophoric systems 19, 20 and 5 . 2PF(6) is presented which involves the sequential Diels-Alder r eactions of tetraene 7 with the chromophore-based dienophiles 6, 8 and 23. The pi-facial stereoselectivity of the Diels-Alder reactions between such b uilding blocks enables the isolation of two major stereoisomers, namely syn ,-syn-5 . 2PF(6) and anti,syn-5 . 2PF(6), which differ substantially in the ir shape. This geometrical difference offers a unique opportunity for the d elineation of two mechanisms of photoinduced electron transfer (ET): solven t-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 H-1 NMR and photophysical studies, namely the observation of NOE effects a nd strong upfield chemical shifts of the bipyridine proton resonances in sy n,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 (AM 1) and ab initio (HF/3-21G) MO calculations also lend support to the interpre tation of the NMR studies and to the stereochemical assignments for the two isolated 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 conformer s for syn,syn-19, and the formation of a closed form for syn,syn-5(2+) in w hich the terminal chromophores are separated by a distance of only approxim ate to 4 Angstrom in the gas phase.