The Role of Charge Transfer Induced Spin Crossover Complexes on Charge-Separated State Lifetime in Photoelectrochemical Devices

Abstract

In sustainable fuel production, the conversion of solar energy into chemical energy is one of the great interests of storing high-density energy. The relatively low observed quantum yields (~19%) for such process compared to theoretical efficiencies (in the range of 30-40%, depending on target reaction) result in part from the prevalent short-lived photo-induced charge-separated states. In our previous studies, manganese (II/III) poly(pyrazolyl)borate (Mn(pzb)2) complexes demonstrated exceptional charge-separated-state lifetimes in the dye-sensitized photoanode constructs. Mn(pzb)2 undergoes a spin transition from a high-spin, sextet state for Mn(II), to a low-spin, triplet state for Mn(III). The spin change is induced upon a pseudo-octahedral-to-octahedral structural change that modifies the orbital overlap between the pyrazolyl lone pair and central Mn. The large reorganization energy associated with the structural and spin transitions increased the lifetime of charge-separated states. Inspired from the results, we sought to determine the degree of molecular reorganization/spin transition that results in the greatest modulation in back electron transfer rates. Two methods were explored 1) the use of zwitterionic ligands, 2) the formation of a multi-component molecularly sensitized interface with [M(pzb)2] (M= Mn, Fe, and Co). In the first approach, the Lewis base of the zwitterion ligand affects redox potential and the magnetic properties. By tuning the pKa of the Lewis base and the functional group on pyrazole, the extent of the spin transition can be modified. Moreover, the zwitterion ligand is charge-neutral and a counterion is needed to balance the Mn positive charge. Thus, the overall charge neutral zwitterionic [Mn(pzb)2] complex has improved its solubility in organic solvents, a downfall of our first public [M(pzb)2] complex derivation in quantum dot solar cell. The second approach is to decorate the [M(pzb)2] (M= Mn, Fe and Co) with phosphoric acid group (-PO3H2) to form a self-assembly layer with a cognizant chromophore. [M(pzb)2] (M= Mn, Fe and Co) exhibit charge transfer induced spin crossover (CTISC) but the change of spin multiplicity is different depending on the central metal. The spin multiplicity difference were hypothesized to manipulate the extent spin transition/reorganization. Furthermore, the self-assembled layer forms the molecular layer in the photoanode construct, which bypasses the aforementioned solubility issue.