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 d_z²-based LUMOβ orbital, whereas functionalized tetramethylated derivatives, 2-6, have ²A″ ground states with d_x²-y²-based LUMOβ orbitals due to their lower molecular symmetries (C_5vvs. C_s, 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 HOMOβ-LUMOβ gaps and excitation into the d_x²-y² orbital for 2-6 rather than d_z² 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.