Symmetry Breaking Charge Separation in Zinc Phthalocyanine Dimers Self-Assembled via Hydrogen Bonding

Photoinduced symmetry-breaking charge separation (SB-CS) in a symmetrical pair of dimers is known to maximize energy conversion during the light-to-chemical energy conversion process. Although this has been shown in a few covalently linked dimers, the demonstration of such an event in self-assembled chromophore dimers has been very rare, especially in dimers formed by hydrogen bonding interactions. Here, we present a classic example of hydrogen bonding paired self-assembled zinc phthalocyanine dimers ((ZnPc-COOH)₂ and (ZnPc-Ph-COOH)₂) of different chromophore distances and demonstrate SB-CS resulting in ZnPc•+-ZnPc•- electron transfer products upon far-red light illumination. The occurrence of SB-CS in the ZnPc-dimers was confirmed by multiple studies, including fluorescence emission, which showed significant quenching in the dimer. Estimation of energy stored in the charge-separated states from electrochemical studies (redox gap of ∼1.55 eV) revealed the energy to be close to the pumping energy (E₀,₀ ∼ 1.80 eV), thus minimizing the energy loss during light-to-energy conversion. While DFT studies were able to assess geometry and carboxylic acid pairing strength, the TD-DFT studies were able to point out the excited states responsible for promoting excited-state electron transfer within the ZnPc-dimers. Finally, femtosecond pump-probe studies provided definitive evidence of SB-CS, with electron transfer rate constants of 5 × 10¹¹ and 0.7 × 10¹¹ s⁻¹, respectively, for (ZnPc-COOH)₂ and (ZnPc-Ph-COOH)₂, revealing the significance of H-bond-pairing in promoting efficient charge separation in the far-red capturing ZnPc dimers.