Biomimetic Electron Transfer Using Low Energy Excited States: A Green Perylene-Based Analogue of Chlorophyll a

Abstract

We have prepared a green chromophore, 1,7-bis(pyrrolidin-1-yl)-3,4:9,10-perylene-bis(dicarboximide) (5PDI), that exhibits photophysical and redox properties similar to those of chlorophyll a (Chl a ), yet unlike Chl a, it can be easily functionalized and incorporated into a wide variety of biomimetic electron donor−acceptor systems. The N,N ‘ -dicyclohexyl derivative (5PDI) absorbs strongly (ε = 46 000 M -1 cm -1 ) at 686 nm in toluene and fluoresces at 721 nm with a 35% quantum yield. Additionally, 5PDI is both oxidized and reduced in CH 2 Cl 2 at 0.57 V and −0.76 V vs SCE, respectively, making it a facile electron donor or acceptor. Rodlike covalent electron donor−acceptor molecules 5PDI−PI, 5PDI−NI, and 5PDI−PDI were prepared by linking the imide group of the 5PDI donor to pyromellitimide (PI), 1,8:4,5-naphthalenebis(dicarboximide) (NI), and 1,7-bis(3,5-di- tert -butylphenoxy)-3,4:9,10-perylene-bis(dicarboximide) (PDI) acceptors via an N−N bond. Following femtosecond laser excitation of 5PDI, 5PDI−PI, 5PDI−NI, and 5PDI−PDI in both toluene and 2-methyltetrahydrofuran, the formation and decay of their excited and radical ion pair states were monitored directly by transient absorption spectroscopy. We also report steady state emission and spectro-electrochemistry data for these molecules, which aid in elucidation of the transient spectra and the mechanisms of photoinduced charge separation. In toluene, charge separation occurs with high yield only in 5PDI−NI and 5PDI−PDI, whereas for 5PDI−PI charge separation is slow relative to excited-state decay of 1* 5PDI−PI indicating that Δ G CS ≅ 0. This fact provides a means of estimating the ionic radii of the photogenerated ions, which for perylene chromophores 5PDI and PDI are 7.6 ± 0.5 Å, whereas those of the PI and NI electron acceptors are 5.6 ± 0.5 Å. These ionic radii are used in turn to determine the free energies of reaction of the remaining molecules with the series. Electroabsorption measurements are used to show that the change in dipole moment, Δμ that occurs upon formation of 1* 5PDI is 3.5 D. The rates of charge separation in 5PDI−NI, 5PDI−PI, and 5PDI−PDI are compared to those of related donor−acceptor molecules having a 9-(pyrrolidin-1-yl)-perylene-3,4-dicarboximide (5PMI) donor. The 5PMI donor with Δμ = 15.4 D has a lowest excited singlet state with significantly higher charge-transfer character than does 5PDI, and has greater electron density near the imide group to which the acceptor is attached. The rate constants for charge separation from 1* 5PMI are greater than those from 1* 5PDI, which suggests that the rates of electron transfer from donors with CT excited states to an attached acceptor depend on the charge distribution in the CT excited state.