Ultrafast charge separation due to excited state symmetry breaking in dimers of push-pull perylenes

Photoexcitation of chromophoric dimers constrained to a symmetric π-stacked geometry by their molecular structure usually produce excimers independent of solvent polarity, while dimers with edge-to-edge perpendicular π systems undergo excited-state symmetry breaking in highly polar solvents leading to intradimer charge separation. We present direct evidence for symmetry breaking in the lowest excited singlet state of a symmetric cofacial dimer of 9-(N-pyrrolidinyl)-1,6-bis(3,5-di-tert-butylphenoxy)perylene-3,4-dicarboximide (5PMI) in the low-polarity solvent toluene to produce a radical ion pair quantitatively. This dimer, cof-5PMI₂, was synthesized by attaching two 5PMI chromophores via imide groups to a xanthene spacer. For comparison, a linear symmetric dimer, lin-5PMI₂, was prepared in which the 5PMI chromophores are linked end-to-end via a N-N single bond between their imides. The edge-to-edge π systems of the 5PMI chromophores within lin-5PMI ₂ are perpendicular to one another. Ground-state absorption spectra of both 5PMI dimers show exciton coupling, which is consistent with the orientation of the 5PMI chromophores relative to one another. Ultrafast transient absorption spectroscopy following excitation of the dimers with 400-nm, 80-fs laser pulses shows that quantitative intradimer electron transfer occurs in cof-5PMI₂ in toluene with τ =.9 ps, followed by charge recombination to ground state with τ = 780 ps. Similar measurements on lin-5PMI₂ reveal that photoinduced electron transfer does not occur in toluene, but occurs in more polar solvents such as 2-methyltetrahydrofuran, wherein τ = 4.5 ps for charge separation and τ = 660 ps for charge recombination. Excited-state symmetry breaking in 5PMI dimers provides new routes to biomimetic charge separation and storage assemblies that can be more easily prepared and modified than those based on multiple tetrapyrrole macrocycles.