A Comparative DFT Study of Metalloporphyrins and a Manganese Tetracarbene Complex for Energy Storage Applications

The research studies the density functional theory (DFT), which evaluates quantum chemical properties that affect the performance of traditional metalloporphyrins (M-Porph, M = Mn, Fe, Cu, Zn) and the new manganese tetracarbene complex for energy storage applications. The photoredox applications of metalloporphyrins remain limited because earth-abundant first-row transition metals produce short excited-state lifetimes. The manganese tetracarbene complex maintains a 190 ns metal-to-ligand charge transfer (MLCT) excited-state lifetime because its Mn(I) center exists in a rigid tetradentate carbene ligand environment.

The central metal ion determines essential properties according to our computational study. The Zn-porphyrin system maintains the greatest stability and shows the widest HOMO-LUMO energy difference, while Fe- and Mn-porphyrins display reduced energy gaps and lower chemical hardness, which supports electron transfer processes. The excited state of this complex remains stable for an extended period because of its reductant properties. The Mn(I) center of this complex shows high reactivity because it can easily undergo oxidation, which drives photoredox reactions.

Mn and Fe porphyrins exhibit low chemical hardness and medium-high to high electron affinity, indicating high reactivity and a strong tendency to accept electrons. In contrast, Cu and Zn porphyrins exhibit high chemical hardness and low electron affinity, indicating greater stability and reduced reducibility.

The tetracarbene framework serves as an example of how ligand design enables manganese complexes to achieve exceptional photophysical properties using earth-abundant metals. The research proves [Mn(pbmi)_2][OTf] functions as a design model which demonstrates how specific electronic structure parameters, including high ligand field strength, HOMO destabilization, and structural inflexibility, create superior molecular candidates for photocatalytic energy storage systems that replace precious-metal-based photosensitizers.