Femtosecond Optical Switching of Electron Transport Direction in Branched Donor−Acceptor Arrays

Abstract

The branched donor−acceptor triad 4 and tetrad 5 were synthesized to study the possibility of controlling the direction of electron transfer in a divergent array of electron acceptors using ultrafast laser pulses. Compounds 4 and 5 employ 1,3,5-triaminobenzene as the central branch point. In 4, a 4-(N-piperidinyl)-1,8-naphthaleneimide electron donor (ANI) was attached to the 1 position and two electron acceptors, 1,8:4,5-naphthalenediimide, NI, and pyromellitimide, PI, were attached to the 3 and 5 positions of the central benzene ring. In 5, the terminal end of the PI acceptor is functionalized with an additional NI molecule. Selective excitation of ANI in 4 and 5 with 400 nm, 130 fs laser pulses results in exclusive formation of NI-−ANI+−PI and NI-−ANI+−PI−NI, respectively, with τ = 115 ps, and a quantum yield of 0.99. Excitation of NI- with 480 nm, 130 fs laser pulses produces the excited doublet state *NI-, which transfers an electron to PI on the second branch of the benzene ring with a time constant of τ = 600 fs in 4 and τ = 750 fs in 5. The overall quantum yields for the two step process are 0.44 and 0.36 in 4 and 5, respectively. The resultant state NI−ANI+−PI- in 4 undergoes a charge shift reaction returning to the initial ion pair state NI-−ANI+−PI with τ = 400 ps, while the corresponding state NI−ANI+−PI-−NI in 5 undergoes a charge shift with τ = 200 ps to yield NI−ANI+−PI−NI-, which in turn exhibits a 2000 ps lifetime. These results show that once the electron has been switched to the branch containing the thermodynamically uphill PI acceptor, the electron will cascade down that branch to other acceptors that are more easily reduced. Model compounds 1−3, which were synthesized to aid in characterization of the switching dynamics, are also discussed. Photochemical control of directional charge transport may make it possible to design networks of organic molecules for information processing.