Bridge-Mediated Charge Separation in Isomeric N-Annulated Perylene Diimide Dimers

The possibility and rate of charge separation (CS) in donor-bridge-acceptor molecules mainly depend on two factors: electronic coupling and solvent effects. The question of how CS occurred in two identical chromophores is fundamental, as it is particularly interesting for potential molecular electronics applications and the photosynthetic reaction centers (RCs). Conjugated bridge definitely plays a crucial role in electronic coupling. To determine the bridge-mediated charge separation dynamics between the two identical chromophores, the isomeric N-annulated perylene diimide dimers (-BDNP and -BDNP) with different conjugated bridge structures have been comparatively investigated in different solvents using femtosecond transient absorption spectra (fs-TA). It is found that the charge separation is disfavored in weak polar solvent, whereas direct spectroscopic signatures of radicals are observed in polar solvents, and the rate of charge separation increases as the solvent polarity increasing. To our surprise, the rate of charge separation in -BDNP is more than an order of magnitude slower than that in -BDNP, although there is a larger negative ΔG_CS in -BDNP. The slow CS rate that occurred in -BDNP mainly results from the intrinsic destructive interference of the wave function through the meta-substituted bridge. The roles of solvent effects in free energy and electronic coupling for charge separation are further identified with quantum calculations.