State-selective electron transfer in an unsymmetric acceptor-Zn(II)porphyrin-acceptor triad: toward a controlled directionality of electron transfer from the porphyrin S2 and S1 states as a basis for a molecular switch.

A series of Zn(II) porphyrin (ZnP) compounds covalently linked to different electron acceptor units, naphthaleneimide (NI) and naphthalenediimide (NDI), are reported. The aim was to demonstrate a state-selective direction of electron transfer, where excitation to the lowest excited S(1) state of the porphyrin (Q-band excitation) would give electron transfer to the NDI unit, while excitation to the higher S(2) state (Soret-band 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 S(1) state to NDI occurred in solvents of both high and low polarity, whereas no electron transfer to NDI was observed from the S(2) state. With NI as acceptor instead, very rapid (tau = 200-400 fs) electron transfer from the S(2) state occurred in all solvents. This was followed by an ultrafast (tau approximately 100 fs) recombination to populate the porphyrin S(1) 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 relaxed ZnP(+)NI(-) state lies at lower energy than the S(1) state so that further charge separation occurred from S(1) 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: S(2) --> ZnP(+)NI(-)("hot") --> S(1) --> ZnP(+)NI(-) --> S(0). The electron transfer dynamics and its solvent dependence are discussed, as well as the function of the present molecules as molecular switches.