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1. Symmetry and substituent electronics dictate electronic structure of low spin, mixed-ring rhenocene complexes

   https://doi.org/10.1039/D4DT03309D  

   Asempa, Eyram; Curtin, Gregory M; May, Ann Marie; Dempsey, Jillian L; Jakubikova, Elena (January 2026, Dalton Transactions)

   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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2. Luminescent Arylalkynyltitanocenes: Effect of Modifying the Electron Density at the Arylalkyne Ligand, or Adding Steric Bulk or Constraint to the Cyclopentadienyl Ligand

   https://doi.org/10.3390/cryst15080745  

   Barker, Matilda; Walter, Samantha C; McCallum, Elizabeth A; Golden, River S; Zimmerman, John H; McCarthy, Jackson S; McMillen, Colin D; Wagenknecht, Paul S (August 2025, Crystals)

   Photocatalysis using complexes of d0 metals with ligand-to-metal charge-transfer (LMCT) excited states is an active area of research. Because titanium is the second most abundant transition metal in the earth’s crust, d0 complexes of TiIV are an appropriate target for this research. Recently, our group has demonstrated that the arylethynyltitanocene Cp2Ti(C2Ph)2CuBr is not emissive in room-temperature fluid solution, whereas the corresponding Cp* complex, Cp*2Ti(C2Ph)2CuBr, is emissive. The Cp* ligand is hypothesized to provide steric constraint that inhibits excited-state structural rearrangement. However, modifying the structure also changes the orbital character of the excited state. To investigate the impact of the excited-state orbital character on the photophysics, herein we characterize complexes similar to Cp*2Ti(C2Ph)2CuBr—but one with a more electron-rich arylethynyl ligand, ethynyldimethylaniline (C2DMA), and one with a more electron-poor arylethynyl ligand, ethynyl-α,α,α-trifluorotoluene. We have also prepared complexes with the C2DMA ligand but with different Cp ligands that adjust the steric bulk and constraint around the Ti, by replacing the Cp* ligands with either indenyl ligands or an ansa-cyclopentadienyl ligand where the two Cp ligands are bridged by a dimethylsilylene. All four target complexes have been characterized crystallographically and structure activity relationships are highlighted. 

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3. 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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4. Electronic structure and photophysics of a supermolecular iron complex having a long MLCT-state lifetime and panchromatic absorption

   https://doi.org/10.1073/pnas.2009996117  

   Jiang, Ting; Bai, Yusong; Zhang, Peng; Han, Qiwei; Mitzi, David B.; Therien, Michael J. (August 2020, Proceedings of the National Academy of Sciences)

   null (Ed.)

   Exploiting earth-abundant iron-based metal complexes as high-performance photosensitizers demands long-lived electronically excited metal-to-ligand charge-transfer (MLCT) states, but these species suffer typically from femtosecond timescale charge-transfer (CT)-state quenching by low-lying nonreactive metal-centered (MC) states. Here, we engineer supermolecular Fe(II) chromophores based on the bis(tridentate-ligand)metal(II)-ethyne-(porphinato)zinc(II) conjugated framework, previously shown to give rise to highly delocalized low-lying 3 MLCT states for other Group VIII metal (Ru, Os) complexes. Electronic spectral, potentiometric, and ultrafast pump–probe transient dynamical data demonstrate that a combination of a strong σ-donating tridentate ligand and a (porphinato)zinc(II) moiety with low-lying π*-energy levels, sufficiently destabilize MC states and stabilize supermolecular MLCT states to realize Fe(II) complexes that express 3 MLCT state photophysics reminiscent of their heavy-metal analogs. The resulting Fe(II) chromophore archetype, FeNHCPZn, features a highly polarized CT state having a profoundly extended 3 MLCT lifetime (160 ps), 3 MLCT phosphorescence, and ambient environment stability. Density functional and domain-based local pair natural orbital coupled cluster [DLPNO-CCSD(T)] theory reveal triplet-state wavefunction spatial distributions consistent with electronic spectroscopic and excited-state dynamical data, further underscoring the dramatic Fe metal-to-extended ligand CT character of electronically excited FeNHCPZn. This design further prompts intense panchromatic absorptivity via redistributing high-energy absorptive oscillator strength throughout the visible spectral domain, while maintaining a substantial excited-state oxidation potential for wide-ranging photochemistry––highlighted by the ability of FeNHCPZn to photoinject charges into a SnO 2 /FTO electrode in a dye-sensitized solar cell (DSSC) architecture. Concepts enumerated herein afford opportunities for replacing traditional rare-metal–based emitters for solar-energy conversion and photoluminescence applications. 

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5. 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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