Comparative analysis between solution-phase and thin films of cobalt(II) spin crossover (SCO) complexes with 8, 10, 12-carbon alkyl chains based on structural, optical and electrical properties

Spin crossover (SCO) materials are compounds capable of switching between high-spin and low-spin states in response to external stimuli such as temperature. In this work, a series of cobalt(II) SCO complexes [Co₂(CH₃COO)₄(LC_n)₂] (L = ligand, n = 8, 10, 12) were explored as potential materials for optoelectronic and thermo-electrochemical (TEC) devices. Previous efforts focused on solution-phase SCO complexes for TEC applications due to their high conductivity values but faced challenges such as solvent evaporation and instability. This study presents: (i) a comparative analysis of the physical properties between solution and thin film forms of these complexes, and (ii) optimisation of these properties by understanding the correlation between the molecular structure of the SCO complexes and their physical characteristics. Compared to their solution counterparts, the complexes in thin film formats demonstrated enhanced structural and optical stability. The thin films exhibited higher bandgap values (2.75–2.83 eV), making them suitable for optoelectronic applications. These films also showed more stable spin transitions, enhancing the overall system stability. The complex with the longest alkyl chain (12-carbon) showed higher solubility in solvents, allowing for more uniform and higher quality film. The longer alkyl chain in thin films showed a decrease in conductivity, suggesting enhanced charge trapping, making them promising for storage and memory devices. Conversely, in the solution phase, the longer alkyl chain showed an increase in ionic conductivity, beneficial for TEC applications. This study provides a systematic approach to designing SCO complexes for optimal performance in various electronic and electrochemical applications.