Photosensitization of TiO₂ and SnO₂ by Artificial Self-Assembling Mimics of the Natural Chlorosomal Bacteriochlorophylls

Abstract

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.

The supramolecular organization of self-assembling chlorosomal bacteriochlorophyll c , d , or e mimics

“…Artificial self-assembling chromophores, which mimic the chlorosomal BChls, do not have this disadvantage and are good candidates for incorporation into hybrid solar cells (23,24).…”

“…The self-assembly algorithm programmed within chlorosomes could be reproduced with completely artificial pigments (18)(19)(20)(21)(22), that lends hope that it can be used in solid-state devices such as hybrid solar cells (23,24). This would enable efficient light capturing and operation also under lowlight illumination conditions.…”

Away from TiO2: A critical minireview on the developing of new photocatalysts for degradation of contaminants in water

“…The only works that we could found were performed of TiO 2 -containing hybrid systems such as TiO 2 -SiO 2 -POM [72] and g-C 3 N 4 -Mn þ /CeO 2 -TiO 2 [73]. This was accompanied by works involving photosensitization of oxides such as WO 3 and (surprisingly) SiO 2 [74] as well as SnO 2 [75]. The only exception we found (an exception that hopefully is the first sign of a change) was a manuscript on the origin of slow carrier transport in thin films photoanodes made of BiVO 4 [76].…”

TiO2-based building materials: Above and beyond traditional applications

“…Many challenges exist in enhancing photoinduced activity of TiO 2 , or increasing spectral sensitivity of TiO 2 to visible light. Ion doping [60] , ion implantation [61] , composite semiconductor [62] , noble metal deposition [63] , photosensitization [64] and other modification methods have been used for reducing the combination of electron-hole pairs or extending the threshold of exciting wavelength. Recently, several researchers found that the morphology of TiO 2 had a strong relationship with its photoactivity, and some work had been done for the synthesis of TiO 2 with 1D structure [65] , nanospheres structure [66] , nanoflowers structure [67] etc.…”