Symmetry-breaking charge separation in a null-excitonic 3-dimensional rigid nonconjugated trimer

Abstract

Photoinduced symmetry-breaking charge separation (SB-CS) has been extensively observed in various oligomers and aggregates, which holds great potential for robust artificial solar energy conversion systems. It attaches great importance to the precise manipulation of interchromophore electronic coupling in realizing efficient SB-CS. The emerging studies on SB-CS suggested that it could be realized in null-excitonic aggregates, and a long-lived SB-CS state was observed, which offers an advanced platform and has gathered immense attention in the SB-CS field. Here, we unveiled the null-exciton coupling induced ultrafast SB-CS in a rigid polycyclic aromatic hydrocarbon framework, triperyleno[3,3,3]propellane triimides (TPPTI), in which three chromophores were attached through a nonconjugated bridge. Through a combination of theoretical calculations and steady-state absorption results, we demonstrated that this nonconjugated TPPTI possesses negligible exciton coupling. Increased solvent polarity was found to significantly enhance state mixing between local excited and charge transfer states. Using transient absorption spectroscopy, ultrafast SB-CS was observed in highly polar dimethylformamide, facilitated by a selective hole-transfer coupling and a favorable charge separation free energy (ΔGCS). Additionally, the rate ratio between SB-CS and charge recombination was at least high to 1800 in dimethylformamide. This investigation provides profound insights into the role of null-exciton coupling in dominating ultrafast SB-CS in multichromophoric systems.