Probing ultrafast photochemical mechanisms of molecular and heterogenized rhodium bipyridine photocatalyst

Molecular and heterogenized rhodium bipyridine (Bpy) complexes are highly active and selective for the carbon dioxide photoreduction into formic acid using visible light as sole energy source. The excited state of the molecular 5,5’-di(pyren-1-yl)-2,2’-bipyridine Pyr2Bpy and of the corresponding conjugated microporous polymer PyrBpy-CMP, envisioned as macroligand, as well as of their organometallic complexes with pentamethylcyclopentadienyl (Cp*) rhodium [Pyr2Bpy]Cp*RhCl2 and Cp*Rh@PyrBpy-CMP have been investigated by femtosecond UV-vis transient absorption spectroscopy. In both polymers PyrBpy-CMP and Cp*Rh@PyrBpy-CMP the fs measurements reveal the formation of a broad excited state absorptions bands decaying in the sub-ns time scale. For Cp*Rh@PyrBpy-CMP , the ultrafast energy transfer from the framework to the catalytic centres is demonstrated. Pyr2Bpy and [Pyr2Bpy]Cp*RhCl2 have been studied as model molecular building blocks of the CMP. The results show the participation of a mesomeric intramolecular charge transfer (MICT) state and of a twisted intramolecular charge transfer state (TICT) stabilized by the torsion of the pyrene and bipyridine moiety, that are then converted into ligand to metal charge transfer states (LMCT) in [Pyr2Bpy]Cp*RhCl2. The photophysical parameters determined for the molecular compounds were applied to calculate the Förster Resonance Energy Transfer rate from the light-harvesting organic units to the heterogenized Rh metal centres. Finally, the role of the triethanolamine, a common sacrificial electron donor (SED) employed for the CO2 reduction, as an efficient quencher of the excited states of the Pyr2Bpy is demonstrated. This quenching reaction is expected to occur for a wide range of organic and organo-metallic photocatalysts, and its consequences on the reduction of the photoconversion yield are certainly underestimated for most of the applications.