Effect of Hydrogen Bonding on Symmetry-Breaking Charge Transfer in Tetra(phenoxy)perylenediimide Single Crystals

Symmetry-breaking charge separation (SB-CS), the generation of electron–hole pairs between neighboring identical molecules, is a process of interest in solar energy conversion because it can potentially enhance the open-circuit voltage of organic photovoltaics with a minimal loss of photon energy. Previous work showed that thin films of 1,6,7,12-tetra(4′-t-butylphenoxy)perylene(3,4:9,10)bis(dicarboximide) (tpPDI) undergo SB-CS but did not determine how the interchromophore geometry facilitates this charge-transfer (CT) process. In this work, we show how hydrogen bonding determines the interchromophore geometry within tpPDI single crystals that in turn controls SB-CS. Using steady-state and time-resolved absorption and emission microscopies on single crystals having zero, one, or two unsubstituted NH groups at their imide positions, N,N-di(n-propyl)-tpPDI (C3-tpPDI), N-(n-propyl)-tpPDI (C3H-tpPDI), and H-tpPDI, respectively, we determine that the S0–S1 energy gap is related to the CT character present in the mixed S1–CT state, where H-tpPDI < C3-tpPDI < C3H-tpPDI. Modifying the number of hydrogen-bonding sites in tpPDI provides fine control of the CT character, enabling efficient SB-CS in the solid state.