[Ru(bipy)₃]²⁺ absorbs visible light and produces a long-lived excited state (τ ≈ 600 ns). This excited state is both a better oxidant AND a better reductant than the ground state. How is this possible?
AThe excited state has more electrons available for donation and acceptance simultaneously
BLight absorption creates an MLCT excited state where an electron has been promoted from a metal-based orbital to a ligand-based orbital — the metal center is now effectively Ru(III) (a better oxidant) while the ligand carries an extra electron (a better reductant)
CThe excited state simply has more energy, which makes all reactions more favorable
DSpin-orbit coupling in the excited state changes the selection rules for redox reactions
The MLCT (metal-to-ligand charge transfer) excited state of [Ru(bipy)₃]²⁺ is best described as [Ru³⁺(bipy)₂(bipy⁻)]²⁺* — one electron has moved from a metal d-orbital to a bipy π* orbital. The metal center, now effectively Ru(III), is electron-poor and a good oxidant (it wants to gain an electron to return to Ru(II)). The reduced bipyridine ligand is electron-rich and a good reductant (it wants to lose its extra electron). The excited state stores ~2.1 eV of energy, which provides the thermodynamic driving force for redox reactions that the ground state cannot achieve.
Question 2 True / False
The Marcus inverted region is particularly relevant in inorganic photochemistry because excited-state electron transfer reactions often have very large driving forces.
TTrue
FFalse
Answer: True
Excited states store substantial energy (1-3 eV for visible-light absorbers). This energy is available as thermodynamic driving force for electron transfer. When the driving force |ΔG°| exceeds the reorganization energy λ, Marcus theory predicts the reaction enters the inverted region — the rate actually decreases with increasing driving force. In photochemical systems, where |ΔG°| can easily reach 1-2 eV, the inverted region is frequently encountered. This has practical consequences for solar energy conversion: the rate of wasteful charge recombination (which has a large driving force) can be slowed by designing systems where recombination falls in the Marcus inverted region.
Question 3 True / False
Phosphorescence in inorganic complexes involves emission from a triplet excited state and is typically longer-lived than fluorescence because the transition to the singlet ground state is spin-forbidden.
TTrue
FFalse
Answer: True
In heavy-metal complexes like Ru²⁺, Ir³⁺, and Os²⁺, strong spin-orbit coupling facilitates intersystem crossing from the initially formed singlet excited state to a lower-energy triplet state. Emission from this triplet state (phosphorescence) is formally spin-forbidden (triplet → singlet), making it slow (microsecond to millisecond lifetimes) compared to fluorescence (nanosecond). However, the same spin-orbit coupling that enables intersystem crossing also partially relaxes the spin-selection rule for emission, making phosphorescence observable (though still slower than fluorescence). The long lifetime of the triplet state is advantageous for photochemistry because it provides time for bimolecular reactions to occur.
Question 4 Short Answer
Explain how [Ru(bipy)₃]²⁺ can be used as a photocatalyst for water splitting, describing the role of sacrificial reagents and the connection to artificial photosynthesis.
Think about your answer, then reveal below.
Model answer: Upon visible light absorption, [Ru(bipy)₃]²⁺ enters the ³MLCT excited state with E° ≈ −0.86 V (as a reductant) and +0.84 V (as an oxidant). For water oxidation: the excited Ru²⁺* oxidizes water (with a catalyst like IrO₂ to lower the kinetic barrier), generating O₂ and H⁺ while being reduced to Ru⁺. A sacrificial oxidant regenerates Ru²⁺. For hydrogen evolution: the excited Ru²⁺* transfers an electron to a proton-reduction catalyst (like colloidal Pt), generating H₂, while a sacrificial reductant regenerates Ru²⁺. In a complete artificial photosynthesis system, both half-reactions run simultaneously without sacrificial reagents — but this requires solving the challenging problem of coupling the oxidative and reductive cycles through a common intermediate.
This approach mimics natural photosynthesis, where chlorophyll absorbs light and drives charge separation that powers water oxidation (O₂ evolution) and CO₂ reduction. Replacing chlorophyll with more robust synthetic photosensitizers like Ru and Ir complexes is a major research direction in sustainable energy.