Which molecules for photoinduced symmetry-breaking charge separation? A thermodynamic perspective

Photoinduced symmetry-breaking charge separation (SB-CS) is an electron-transfer between two identical molecules (M) following photoexcitation of one of them (M* + M → M˙⁺ + M˙^-). Compared to conventional photoinduced electron transfer with distinct donor and acceptor moieties, SB-CS offers several advantages for light-driven applications such as organic photovoltaics and photoredox catalysis. Despite its renewed interest, the molecular properties governing SB-CS remain poorly understood. Herein, we systematically evaluate the free energy for SB-CS (ΔG0SB-CS) for nearly 40 polycyclic aromatic hydrocarbons using experimental gas-phase properties combined with quantum-chemically computed solvation energies. We identify several promising aromatic frameworks for rational SB-CS chromophore design (at least from a thermodynamic perspective) and elucidate the role of molecular structure, redox properties, solvation energy and excited-state character. Most notably, we find a correlation between the singlet-triplet energy gap and ΔG0SB-CS, which we attribute to differences in the exchange integrals of the different chromophores. This relationship explains, for example, why anthracene exhibits a large thermodynamic driving force for SB-CS, whereas azulene does not, despite their essentially identical redox and solvation properties.