Distance-Dependent Symmetry-Breaking Charge Transfer in 9,10- Bis(phenylethynyl)anthracene Dimers

To investigate the effect of the through-bond coupling strength on the symmetry-breaking charge separation (SB-CS) dynamics and mechanism, three 9,10-bis((4-hexylphenyl)ethynyl)anthracene dimers with varying distances, viz., a single-bond linked dimer (0-dimer), a phenylene linked dimer (1-dimer) and a para-biphenylene linked dimer (2-dimer), were synthesized and studied systematically using steady-state and transient spectroscopy. Steady-state absorption spectra revealed that the electronic coupling strength decreased gradually with the increase of the inter-chromophore distance, and the transition of S₀→S₁ includes the dark charge transfer (CT) excitation. fs-TA spectra demonstrated that SB-CS could be conducted in both weakly and highly polar solvents for 0-dimer, but the SB-CS dynamics has a significant difference. In weakly polar solvents, SB-CS only produces the partial CT (PCT) state, but it could generate the CT state via the PCT state in highly polar solvent. In comparison, SB-CS is only proceeded in highly polar solvents in 1-dimer and 2-dimer to produce the CT state directly. These results demonstrate that the SB-CS dynamics is strongly dependent on the inter-chromophore electronic coupling, and the relatively strong electronic coupling is crucial for the occurrence of SB-CS in weakly polar environment that is commonly presented in photoelectric devices.