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1. Insights into the charge-transfer character of electronic transitions in ^R Cp ₂ Ti(C ₂ Fc) ₂ complexes using solvatochromism, resonance Raman spectroscopy, and TDDFT

   https://doi.org/10.1039/d0dt04282j  

   Carlton, Elizabeth S.; Sutton, Joshua J.; Gale, Ariel G.; Shields, George C.; Gordon, Keith C.; Wagenknecht, Paul S. (February 2021, Dalton Transactions)

   null (Ed.)

   A series of complexes with low-energy Fe II to Ti IV metal-to-metal charge-transfer (MMCT) transitions, Cp 2 Ti(C 2 Fc) 2 , Cp* 2 Ti(C 2 Fc) 2 , and MeOOC Cp 2 Ti(C 2 Fc) 2 , was investigated using solvatochromism and resonance Raman spectroscopy (RRS) augmented with time-dependent density functional theory (TDDFT) calculations in order to interrogate the nature of the CT transitions. Computational models were benchmarked against the experimental UV-Vis spectra and B3LYP/6-31G(d) was found to most faithfully represent the spectra. The energy of the MMCT transition was measured in 15 different solvents and a multivariate fit to the Catalán solvent parameters – solvent polarizability (SP), solvent dipolarity (SdP), solvent basicity (SB), and solvent acidity (SA) – was performed. The effect of SP indicates a greater degree of electron delocalization in the excited state (ES) than the ground state (GS). The small negative solvatochromism with respect to SdP indicates a smaller dipole moment in the ES than the GS. The effect of SB is consistent with charge-transfer to Ti. Upon excitation into the MMCT absorption band, the RRS data show enhancement of the alkyne stretching modes and of the out-of-plane bending modes of the cyclopentadienyl ring connected to Fe and the alkyne bridge. This is consistent with changes in the oxidation states of Ti and Fe, respectively. The higher-energy transitions (350–450 nm) show enhancement of vibrational modes consistent with ethnylcyclopentadienyl to Ti ligand-to-metal charge transfer (LMCT). The RRS data is consistent with the TDDFT predicted character of these transitions. TDDFT suggests that the lowest-energy transition in Cp 2 Ti(C 2 Fc) 2 CuI, where CuI is coordinated between the alkynes, retains its Fe II to Ti IV MMCT character, in agreement with the RRS data, but that the lowest-energy transitions have significant CuI to Ti character. For Cp 2 Ti(C 2 Fc) 2 CuI, excitation into the low-energy MMCT absorption band results in selective enhancement of the symmetric alkynyl stretching mode. 

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2. A new era of LMCT: leveraging ligand-to-metal charge transfer excited states for photochemical reactions

   https://doi.org/10.1039/D3SC05268K  

   May, Ann Marie; Dempsey, Jillian L (May 2024, Chemical Science)

   Ligand-to-metal charge transfer (LMCT) excited states are capable of undergoing a wide array of photochemical reactions, yet receive minimal attention compared to other charge transfer excited states. This work provides general criteria for designing transition metal complexes that exhibit low energy LMCT excited states and routes to drive photochemistry from these excited states. General design principles regarding metal identity, oxidation state, geometry, and ligand sets are summarized. Fundamental photoreactions from these states including visible light-induced homolysis, excited state electron transfer, and other photoinduced chemical transformations are discussed and key design principles for enabling these photochemical reactions are further highlighted. Guided by these fundamentals, this review outlines critical considerations for the future design and application of coordination complexes with LMCT excited states. 

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3. Electronic absorption of ferricenium derivatives: an unexpected twist

   https://doi.org/10.1080/00958972.2025.2453078  

   Khan, Firoz_Shah Tuglak; Das, Sandip; Hematian, Shabnam (February 2025, Journal of Coordination Chemistry)

   Our results revealed that the introduction of either electron-donating or electron-withdrawing group(s) on the cyclopentadienyl rings leads to a substantial red shift of the lowest energy ligand-to-metal charge transfer (LMCT) band, generally known as 2E2g → 2E1u, relative to that of the parent ferricenium complex. The observed stabilization of the lowest unoccupied molecular orbitals (LUMO), i.e. e2g orbitals, in the electron-deficient system, was ascribed to the δ back donation from the iron atom into the antibonding molecular orbitals of the ligand rings. A series of DFT calculations reproduce the experimental trend highlighting the shift in energies of the molecular orbitals involved in the charge transfer transition. The direct relationship between the location of the LMCT band and reduction potential for specific class of substituents is also presented which allows for more informed structural modifications in the development of these complexes. 

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4. Light- and Chemical-Doping-Induced Magnetic Behavior of Eu Molecular Systems

   https://doi.org/10.1021/acs.inorgchem.3c01154  

   Rajh, Tijana; Masson, Eric; Latt, Kyaw Zin; Smith, Ashton; Brugh, Alexander M.; Dandu, Naveen; Trainer, Daniel; Curtiss, Larry A.; Ngo, Anh T.; Hla, Saw-Wai (August 2023, Inorganic Chemistry)

   Variable temperature electron paramagnetic resonance (VT-EPR) was used to investigate the role of the environment and oxidation states of several coordinated Eu compounds. We find that while Eu(III) chelating complexes are diamagnetic, simple chemical reduction results in the formation of paramagnetic species. In agreement with the distorted D3h symmetry of Eu molecular complexes investigated in this study, the EPR spectrum of reduced complexes showed axially symmetric signals (g⊥ = 2.001 and g∥ = 1.994) that were successfully simulated with two Eu isotopes with nuclear spin 5/2 (151Eu and 153Eu with 48% and 52% natural abundance, respectively) and nuclear g-factors 151Eu/153Eu = 2.27. Illumination of water-soluble complex Eu(dipic)3 at 4 K led to the ligand-to-metal charge transfer (LMCT) that resulted in the formation of Eu(II) in a rhombic environment (gx = 2.006, gy = 1.995, gz = 1.988). The existence of LMCT affects the luminescence of Eu(dipic)3, and pre-reduction of the complex to Eu(II)(dipic)3 reversibly reduces red luminescence with the appearance of a weak CT blue luminescence. Furthermore, encapsulation of a large portion of the dipic ligand with Cucurbit[7]uril, a pumpkin-shaped macrocycle, inhibited ligand-to-metal charge transfer, preventing the formation of Eu(II) upon illumination. 

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5. Delocalization tunable by ligand substitution in [L ₂ Al] ⁿ⁻ complexes highlights a mechanism for strong electronic coupling

   https://doi.org/10.1039/D0SC02812F  

   Arnold, Amela; Sherbow, Tobias J.; Bohanon, Amanda M.; Sayler, Richard I.; Britt, R. David; Smith, Allison M.; Fettinger, James C.; Berben, Louise A. (January 2021, Chemical Science)

   null (Ed.)

   Ligand-based mixed valent (MV) complexes of Al( iii ) incorporating electron donating (ED) and electron withdrawing (EW) substituents on bis(imino)pyridine ligands (I 2 P) have been prepared. The MV states containing EW groups are both assigned as Class II/III, and those with ED functional groups are Class III and Class II/III in the (I 2 P − )(I 2 P 2− )Al and [(I 2 P 2− )(I 2 P 3− )Al] 2− charge states, respectively. No abrupt changes in delocalization are observed with ED and EW groups and from this we infer that ligand and metal valence p-orbitals are well-matched in energy and the absence of LMCT and MLCT bands supports the delocalized electronic structures. The MV ligand charge states (I 2 P − )(I 2 P 2− )Al and [(I 2 P 2− )(I 2 P 3− )Al] 2− show intervalence charge transfer (IVCT) transitions in the regions 6850–7740 and 7410–9780 cm −1 , respectively. Alkali metal cations in solution had no effect on the IVCT bands of [(I 2 P 2− )(I 2 P 3− )Al] 2− complexes containing –PhNMe 2 or –PhF 5 substituents. Minor localization of charge in [(I 2 P 2− )(I 2 P 3− )Al] 2− was observed when –PhOMe substituents are included. 

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