Tuning Electron-Transfer Driving Force in Photosynthetic Special Pair Models.

Visible-light excitation of a family of bimetallic ruthenium polypyridines with the formula [Ru(tpy)(bpy)(-CN)Ru(py)L] (RuRuL), where L=Cl, NCS, DMAP and ACN, was used to prepare photoinduced mixed-valence (PI-MV) MLCT states as models of the photosynthetic reaction center. Ultrafast transient absorption spectroscopy allowed to monitor photoinduced IVCT bands between 6000 and 11000 cm. Mulliken spin densities resulting from DFT and (TD)DFT computations revealed the modulation of the charge density distribution depending on the ligand substitution pattern. Results are consistent with PI-MV systems ranging from non-degenerate Class II to degenerate Class III or II/III, with electronic couplings between 1000 and 3500 cm. These findings guide the control electron localization-delocalization in charge-transfer/charge-separated excited states, like those involved in the photosynthetic reaction center.