Excited-State Dynamics of Graphitic Carbon Nitride Photocatalyst and Ultrafast Electron Injection to a Ru(II) Mononuclear Complex for Carbon Dioxide Reduction

We have previously developed photocatalytic CO₂ reduction systems using graphitic carbon nitride (g-C₃N₄) and a Ru(II) mononuclear complex (e.g., trans(Cl)-[Ru^II{4,4′-(H₂PO₃)₂bpy}₂(CO)₂Cl₂] bpy = 2,2′-bipyridine, abbreviated as RuP) hybrids and demonstrated its high activities under visible light (λ > 400 nm). To understand the excited-state dynamics of C₃N₄ and electron-transfer process to RuP, here we examined the photophysical properties of g-C₃N₄ as well as mesoporous g-C₃N₄ (mpg-C₃N₄) by means of time-resolved emission and/or time-resolved infrared absorption (TR-IR) spectroscopy. The emission decay measurements showed that g-C₃N₄ (as well as mpg-C₃N₄) has at least three emissive excited states with different lifetimes (g-C₃N₄; 1.3 ± 0.4, 3.9 ± 0.9, and 15 ± 4 ns at 269 nm photoexcitation) in aqueous suspension. These excited states were not quenched upon addition of a hole scavenger (e.g., disodium dihydrogen ethylenediamine tetraacetate dehydrate) and/or an electron acceptor (RuP), even though photochemical electron-transfer processes from/to g-C₃N₄ has been experimentally confirmed by photocatalytic reactions. On the other hand, TR-IR spectroscopy clearly indicated that mobile electrons photogenerated in mpg-C₃N₄, which are shallowly trapped and/or free electron in the conduction band, are able to move into RuP with a timescale of a few picoseconds. These results suggest that main emission centers and reaction sites (including charge-transfer interfaces) are separately located in the C₃N₄ materials, and that electron transfer from C₃N₄ to RuP progresses through less- or non-luminescent sites, in which mobile electrons exist with a certain lifetime.