Geometry-Governed Symmetry-Breaking Charge Separation and Photocatalysis of Benzoperylenetriimide Dimers

An in-depth understanding of the structure-property relationships in multichromophoric systems undergoing excited electron transfer is essential for designing reliable photocatalytic and optoelectronic devices. However, creating architectures to govern electron-transfer events in a controlled manner remains challenging due to synthetic challenges and suboptimal conditions. Here, we report two novel benzoperylenetriimide (PTI) dimers, PTI-PTI and PTI-Ph-PTI, utilizing an easily reducible PTI. The former dimer features a direct connection through the bay positions of the PTI, whereas the latter has a phenyl ring spacer. Their ability to undergo symmetry-breaking charge separation (SB-CS) and subsequent performance in photocatalytic electron pooling is probed. For the PTI-PTI dimer, which is rigid and orthogonal, SB-CS was not observed due to a lack of electronic communication between the PTI moieties, namely, a perfect "null-exciton" system. In contrast, the flexible PTI-Ph-PTI dimer showed clear evidence of SB-CS due to its ability to communicate between the macrocycles. Steady-state and time-resolved emission studies revealed significant quenching, especially in PTI-Ph-PTI compared to the monomer PTI, indicating initial signs of SB-CS. The redox gap obtained from electrochemical studies confirmed that the SB-CS state can be populated from the singlet excited state in both polar and nonpolar solvents without appreciable energy loss, a much-sought-after property. Additionally, DFT calculations supported experimental results, showing an asymmetric distribution of electron density in frontier orbitals. Meanwhile, TD-DFT studies identified the excited states responsible for the observed SB-CS. Subsequent femtosecond pump-probe studies provided conclusive evidence of charge separation in the PTI-Ph-PTI dimer, independent of the solvent polarity. The lifetimes of the charge-separated states were found to be 119 and 228 ps in benzonitrile and toluene, respectively, which were sufficient to engage them in photocatalytic studies. Consequently, the PTI-Ph-PTI dimer was used for photocatalytic electron-pooling experiments from an application perspective. Higher yields of electron-pooling were observed compared to the monomer, indicating the potential application of this class of systems in developing future photocatalytic systems that function at higher efficiency. Importantly, how "perfect orthogonal rigidity" prohibits SB-CS and its photocatalytic activity, helping future design of symmetric dimers to observe SB-CS, is borne out from this study.