Investigation of the primary electron transfer processes in some rigidly bridged dyads and triads by (sub)picosecond pump-probe experiments

The electron transfer kinetics of several completely rigid dyads and triads which contain N,N-dimethylaniline (DMA) and dimethoxynaphthalene (DMN) as possible donors and the dicyanovinyl group (DCV) as acceptor were studied b y means of (sub)picosecond time-resolved transient absorption spectroscopy. In the dyads DMN[n]DCV, the rate of charge separation decreases exponentia lly with the number of sigma-bonds n in the bridge, the "damping factor'' b eing 0.8 per bond in tetrahydrofuran solvent. In the triads DMA[4]DMN[8]DCV , the primary electron transfer from DMN to DCV occurs in solvents of low a nd medium polarity within 10 ps in both isomers (syn and anti). The rates o f the secondary electron transfer step (formation of the fully charge separ ated state, DMA(+)[4]DMN[8]DCV-) and the following deactivation processes d epend, however, strongly on the conformation. In acetonitrile, the primary electron transfer involves the two donor groups yielding preferentially DMA (+)[4]DMN-[8]DCV. In the anti-conformer this state is fairly long-lived; in the syn-conformer, however, it decays rapidly, in part to locally excited triplet states.