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Synthetic chemists use photochemistry to achieve challenging or unusual chemical transformations, but not all compounds are photoactive. Photosensitization is a process by which a molecule that is incapable of efficiently absorbing a particular wavelength of light directly is promoted to its triplet excited state (T 1 ) by an intermolecular triplet energy transfer from a photosensitizer, which is a compound that ideally has a large extinction coefficient, rapid rate of intersystem crossing, and a long-lived T 1 . A particular advantage of photosensitization is that distinctive reactivity profiles not accessible through ground states become facile from corresponding excited states ( 1 ). The use of photosensitizers in chemical synthesis has paralleled the rise in popularity and use of various photoredox catalysts ( 2 ). On page 1338 of this issue, Ma et al. ( 3 ) report a photosensitized dearomative [4 + 2] cycloaddition that converts simple, unsaturated building blocks into products of increased molecular complexity using visible light.
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Photophysical Characterization and Excited State Dynamics of Decamethylruthenocenium
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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- PAR ID:
- 10621099
- Publisher / Repository:
- American Chemical Society
- Date Published:
- Journal Name:
- The Journal of Physical Chemistry A
- Volume:
- 129
- Issue:
- 6
- ISSN:
- 1089-5639
- Page Range / eLocation ID:
- 1558 to 1565
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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