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Understanding the landscape of molecular photocatalysis is vital to enable efficient conversion of feedstock molecules to targeted products and inhibit off-cycle reactivity. In this study, the light-promoted reactivity of [RuCp*2]+ was explored via electronic structure, photophysical, and photostability studies and the reactivity of [RuCp*2]+ within a photocatalytic hydrogen evolution cycle was assessed. TD-DFT calculations support the assignment of a low-energy ligand-to-metal charge transfer transition (LMCT) centered at 500 nm, where an electron from a ligand-based orbital delocalized across both Cp* ligands is promoted to a dx2–y2-based β-LUMO orbital. Upon irradiating the LMCT absorption feature, ultrafast transient absorption spectroscopy measurements show that an initial excited state (τ1 = 1.3 ± 0.1 ps) is populated, which undergoes fast relaxation to a longer-lived state (τ2 = 12.0 ± 0.9 ps), either via internal conversion or vibrational relaxation. Despite the short-lived nature of these excited states, bulk photolysis of [RuCp*2]+ demonstrates that photochemical conversion to decomposition products is possible upon prolonged illumination. Collectively, these studies reveal that [RuCp*2]+ undergoes light-driven decomposition, highlighting the necessity to construct molecular photocatalytic systems resistant to off-cycle reactivity in both the ground and excited states.
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This content will become publicly available on January 20, 2027
Symmetry and substituent electronics dictate electronic structure of low spin, mixed-ring rhenocene complexes
Identifying the impact of molecular structure and symmetry on excited state character and energetics is vital to enable and control photochemical reactions. In this work, density functional theory (DFT) and time-dependent DFT (TD-DFT) were used to determine the electronic structure and excited state reduction potentials of six heteroleptic rhenocene complexes, each bearing a cyclopentadienyl ligand and a functionalized tetramethylated cyclopentadienyl ligand (ReCp(CpMe4R), where Me = CH3 and R = Me, CF3, tBu, CHCH2, CHO, or OMe). Calculations were performed using the B3LYP and BP86 functionals, utilizing SDD+f effective core potential and its associated basis set for Re, and 6-311G* basis set for all other atoms. All six complexes exhibit eclipsed geometries with similar Re-ring bond distances and angles. Despite their structural similarities, the electronic structure of these complexes varies with ligand functionalization. ReCp(Cp*), 1, (where Cp* = pentamethylcyclopentadienyl) has a 2A1 ground state with a dz2-based LUMOβ orbital, whereas functionalized tetramethylated derivatives, 2-6, have 2A′′ ground states with dx2-y2-based LUMOβ orbitals due to their lower molecular symmetries (C5v vs. Cs, respectively). Regardless, complementary TD-DFT and fragment orbital analyses show that low-energy LMCT excited states are retained across all six complexes (where LMCT character > 90%). Furthermore, hypsochromic shifts and higher oscillator strengths are observed for 2-6 compared to 1, resulting from the changes in HOMO–LUMO gaps and excitation into the dx2-y2 orbital for 2-6 rather than dz2 orbital for 1, which increases the orbital overlap between the hole-particle pairs that describe the lowest-energy LMCT excitations. Appending electron donating and withdrawing groups to these mixed-ring rhenocene derivatives also tunes ground state and excited state reduction potentials over a 400 mV and 700 mV range, respectively, enabling access to more oxidizing LMCT excited states. Collectively, these results showcase design strategies to control acceptor orbital character, orbital energetics, excited state energies, and reduction potentials, while simultaneously retaining low-energy LMCT excited states across a series of rhenocene derivatives. In result, this work establishes approaches to design and tailor next-generation mixed-ring rhenocenes with low-energy LMCT excited states for photochemical applications.
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- PAR ID:
- 10692794
- Publisher / Repository:
- Royal Society of Chemistry
- Date Published:
- Journal Name:
- Dalton Transactions
- Volume:
- 55
- Issue:
- 3
- ISSN:
- 1477-9226
- Page Range / eLocation ID:
- 1490 to 1497
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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