A series of Zn(II) poiphyrin (ZnP) compounds covalently linked to different electron acceptor units, naphthaleneimide (NI) and naphthalenediimide (NDI), are reported. The aim was to demonstrate a stateselective direction of electron transfer, where excitation to the lowest excited S1 state of the porphyrin (Qband excitation) would give electron transfer to the NDI unit, while excitation to the higher S2 state (Soretband excitation) would give electron transfer to the NI unit. This would constitute a basis for an opto-electronic switch in which the direction of electron transfer and the resulting dipole moment can be controlled by using light input of different color. Indeed, electron transfer from the S1state to NDI occurred in solvents of both high and low polarity, whereas no electron transfer to NDI was observed from the S2 state. With NI as acceptor instead, very rapid (τ= 200-400 fs) electron transfer from the S2 state occurred in all solvents. This was followed by an ultrafast (τ≈100 fs) recombination to populate the porphyrin Si state in nearly quantitative yield. The charge-separated state ZnP-NI-was spectroscopically observed, and evidence was obtained that recombination occurred from a vibrationally excited ("hot") ZnP+NI-state in the more polar solvents. In these solvents, the thermally relaxedZnP -NI- state lies at lower energy than the S1 state so that further charge separation occurred from S1 to form ZnP-NI-. This resulted in a highly unusual "ping-pong" sequence where the reaction went back and forth between locally excited ZnP states and charge-separated states: S2 ZnP -NI-«hot»→S1=→ ZnP +NI-→ S0. The electron transfer dynamics and its solvent dependence are discussed, as well as the function of the present molecules as molecular switches.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry