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1. Excited-state dynamics of deuterated indigo

   https://doi.org/10.1140/epjd/s10053-023-00744-z  

   Cohen, Trevor; Svadlenak, Nathan; Smith, Charles; Vo, Krystal; Lee, Si-Young; Parejo-Vidal, Ana; Kincaid, Joseph R. A.; Sobolewski, Andrzej L.; Rode, Michal F.; de Vries, Mattanjah S. (September 2023, The European Physical Journal D)

   Abstract Indigo, a rich blue dye, is an incredibly photostable molecule that has survived in ancient art for centuries. It is also unique in that it can undergo both an excited-state hydrogen and proton transfer on the picosecond timescale followed by a ground-state back transfer. Previously, we performed gas phase excited-state lifetime studies on indigo to study these processes in a solvent-free environment, combined with excited-state calculations. We found two decay pathways, a fast sub-nanosecond decay and a slow decay on the order of 10 ns. Calculations of the excited-state potential energy surface found that both hydrogen and proton transfer are nearly isoenergetic separated by a 0.1 eV barrier. To further elucidate these dynamics, we now report a study with deuterated indigo, using resonance-enhanced multi-photon ionization and pump-probe spectroscopy with mass spectrometric isotopomer selection. From new calculations of the excited-state potential energy surface, we find sequential double-proton or hydrogen transfer, whereby the trajectory to the second transfer passes a second barrier and then encounters a conical intersection that leads back to the ground state. We find that deuteration only increases the excited-state lifetimes of the fast decay channel, suggesting tunneling through the first barrier, while the slower channel is not affected and may involve a different intermediate state. Graphical abstract 

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2. Photophysical Characterization and Excited State Dynamics of Decamethylruthenocenium

   https://doi.org/10.1021/acs.jpca.4c06564  

   May, Ann Marie; Deegbey, Mawuli; Fosu, Emmanuel Adu; Danilov, Evgeny O; Castellano, Felix N; Jakubikova, Elena; Dempsey, Jillian L (February 2025, The Journal of Physical Chemistry A)

   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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3. Triplet Excited State Energetics and Dynamics in Molecular “Roller Wheels”

   https://doi.org/10.1021/acs.jpcc.9b03934  

   Livshits, Maksim Y.; He, Wenhan; Zhang, Zhen; Qin, Yang; Rack, Jeffrey J. (June 2019, The Journal of Physical Chemistry C)
4. Excited State Dynamics of 7-Deazaguanosine and Guanosine 5’-Monophosphate

   Krul, S. E.; Hoehn, S. J.; Feierabend, K.; Crespo-Hernández, C. E. (January 2021, ChemRxiv)

   null (Ed.)

   Minor structural modifications to the DNA and RNA nucleobases have a significant effect on their excited state dynamics and electronic relaxation pathways. In this study, the excited state dynamics of 7-deazaguanosine and guanosine 5′-monophosphate are investigated in aqueous solution and in a mixture of methanol and water using femtosecond broadband transient absorption spectroscopy following excitation at 267 nm. The transient spectra are collected using photon densities that ensure no parasitic multiphoton-induced signal from solvated electrons. The data can be fit satisfactorily using a two- or three-component kinetic model. By analyzing the results from steady-state, time-resolved, computational calculations, and the methanol–water mixture, the following general relaxation mechanism is proposed for both molecules, Lb → La → 1πσ*(ICT) → S0, where the 1πσ*(ICT) stands for an intramolecular charge transfer excited singlet state with significant πσ* character. In general, longer lifetimes for internal conversion are obtained for 7-deazaguanosine compared to guanosine 5′-monophosphate. Internal conversion of the 1πσ*(ICT) state to the ground state occurs on a similar time scale of a few picoseconds in both molecules. Collectively, the results demonstrate that substitution of a single nitrogen atom for a methine (C–H) group at position seven of the guanine moiety stabilizes the 1ππ* Lb and La states and alters the topology of their potential energy surfaces in such a way that the relaxation dynamics in 7-deazaguanosine are slowed down compared to those in guanosine 5′-monophosphate but not for the internal conversion of 1πσ*(ICT) state to the ground state. 

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5. Excited state dynamics of 7-deazaguanosine and guanosine 5′-monophosphate

   https://doi.org/10.1063/5.0038123  

   Krul, Sarah E.; Hoehn, Sean J.; Feierabend, Karl J.; Crespo-Hernández, Carlos E. (February 2021, The Journal of Chemical Physics)