Photosensitization of TiO2 and SnO2 by artificial self-assembling mimics of the natural chlorosomal bacteriochlorophylls

Of all known photosynthetic organisms, the green sulfur bacteria are able to survive under the lowest illumination conditions due to highly efficient photon management and exciton transport enabled by their special organelles, the chlorosomes, which consist mainly of self-assembled bacteriochlorophyll c, d, or e molecules. A challenging task is to mimic the principle of self-assembling chromophores in artificial light-harvesting devices. In the present work we have studied exciton transport and dissociation in a bilayer of an electron-accepting semiconductor and an artificial self-assembling zinc porphyrin that mimics natural chlorosomal bacteriochlorophylls using time-resolved microwave conductivity (TRMC). Scanning electron microscopy (SEM) reveals the presence of large domains with dimensions up to several micrometers that consist of self-assembled stacks. In addition to these large self-assembled stacks, absorption and fluorescence spectra reveal the presence of monomers. The fluorescence in the solid state, just as in the chlorosomes, is only partially quenched and its decay shows two components, one with lifetimes of 40 ps stemming from the aggregates and a longer one with 2.5 ns lifetime ascribed to monomeric zinc porphyrins. Predominantly those photons that are absorbed by the monomers lead to the formation of charge-separated states. The rather low contribution of self-assembled stacks to the formation of charge-separated states, most likely, results from their interaction with the semiconductor, combined with the presence of monomers at the semiconductor surface and the energetically unfavorable exciton transfer from a stack to a monomer. However, we prove herein that biomimetic self-assembling porphyrins can be used to photosenzitize wide band gap semiconductors as a 2.2% incident photon to charge separation efficiency could be measured. Realizing an ordered structure of stacks in proper contact with the electron-accepting semiconductor will probably improve their contribution to the formation of charge-separated states. This might pave the way to cost-efficient hybrid solar cells using artificial chlorosome-like antenna architectures, allowing them to work also under dim or diffuse light.