A molecular probe of the electric field produced by a photogenerated ion pair

The electric field photogenerated by an electron donor-acceptor pair is probed with a covalently attached dye molecule using transient absorption spectroscopy. The donor molecule, 4-X-naphthalene-1,8-dicarboximide (Dx) (where X is (1) piperidine or (2) pyrrolidine), is directly attached to the 1,4,5,8-naphthalenetetracarboxydiimide acceptor (A), which is attached through a benzene spacer to the probe molecule, 4,4-difluoro-3,5-dimethyl-4-bora-3a, 4a-diaza-s-indacene (P). A ~150fs laser pulse at 412nm selectively excites Dx. The transition dipole of P is approximately perpendicular to the direction of the electric field generated by DX-A~, minimizing the rate of excitation energy transfer from DJ to P, whereas the maximum change in the state dipole upon excitation is parallel to the electric field, maximizing Stark-shift effects. The groundstate absorption spectrum of P in 1,4-dioxane (Ama)( = 512 nm) blue-shifts 5 + 1 nm and bleaches as the nearby radical pair (DX-A~) forms. The band-shift effect diminishes in the higher polarity solvents tetrahydrofuran and butyronitrile but does not disappear completely. Solvatochromism measurements performed at room temperature and conventional electrochromism measurements performed at 77 K on P alone, indicate that the state dipole of P decreases by 2.0+0.2 D upon excitation to the first excited singlet state. From this change in dipole moment and the observed band-shift in DX-A-P, the effective electric field due to the charge-separated pair, experienced by P in 1,4-dioxane, is found to be 6±1 MV/cm. '