Theoretical analysis of photochemical and electrochemical properties of substituted cobalt bipyridine complexes: redox mediators in dye-sensitized solar cells as a case study

This study presents a comprehensive theoretical investigation into the photophysical and electrochemical properties of substituted cobalt bipyridine complexes, evaluated as potential redox mediators in dye-sensitized solar cells. Utilizing a variety of computational methods, including density functional theory and multireference electron correlation techniques, we examine the influence of various substituents on the electronic structure and stability of cobalt complexes. Our findings reveal that strong electron-withdrawing groups enhance the formal redox potential, while electron-donating groups decrease it, illustrating the delicate balance between electronic effects on the complex stability. The simulated properties were compared with the available experimental measurements, demonstrating good correlation and validating the computational approaches used. Analysis of the spin states indicates that cobalt(II) complexes predominantly exist in a high-spin state with low-spin configurations favored in cobalt(III). The reorganization energy for these complexes was calculated, highlighting the role of the inner and outer-shell contributions in determining electron transfer kinetics.