Symmetry-breaking charge separation in weakly coupled anthracene dimers

Abstract

Symmetry-breaking charge separation (SB-CS), which could convert a singlet exciton into a radical anion and cation pair, has the potential to be applied in organic photoelectric devices; however, it is critical to achieve a fast CS rate and slow charge recombination (CR) simultaneously for its application. Thus, we planned to optimize the CS and CR process via molecular engineering and synthesized three isomeric 9,10-bis((4-hexylphenyl)ethynyl) anthracene (BPEA) dimers linked by the phenylene spacer in ortho-(o-dimer), meta-(m-dimer), and para-arrangements (p-dimer) and systematically studied their SB-CS dynamics using ultrafast transient absorption (fs-TA) spectroscopy. The fs-TA experiments confirmed that these three dimers all underwent the SB-CS process in polar solvents, but with different SB-CS dynamics. The o-dimer displayed the fastest CS rate (∼8 ps), which was attributed to its relatively strong electronic coupling (V) and large CS driving force. Meanwhile, it also showed the slowest CR rate (∼20 ns) due to the small CR driving force. Hence, the ratio of its rates of CS over CR reached an unprecedently high value of ∼2500 in DMF. The result of this study provides some insights into developing novel SB-CS materials with a fast CS rate and slow CR rate for their application.