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Spin crossover (SCO) is one of the most widely studied phenomena in transition metal complexes, in part due to the possible technological applications of such species in molecular electronics, memory storage, electrochromics, and display devices. Any transition metal complex with a d⁴–d⁷electron configuration, such as 2+ and 3+ ions of Cr, Mn, Fe, and Co, can theoretically exhibit SCO effects which depend highly on the coordination environment and ligand field strength. Redox-coupled SCO processes provide a mechanism to enhance the molecular bistability and trigger transitions between low- and high-spin states. Similar reaction mechanisms often occur in electrocatalytic and enzymatic reactions. The electrochemical response of redox-coupled SCO processes can be modeled as a square scheme containing two one-electron reactions and two chemical steps, resulting in a separation of anodic and cathodic waves. Herein we describe the influence of ligand coordination and structural changes on the redox-coupled SCO properties of several Co complexes. This structural-functional analysis allows us to understand the fundamental properties that control redox-coupled SCO processes and optimize redox bistability for a range of applications. 

The synthesis, separation, and characterization of 1,6-, 1,7-, and 1,6,7- derivatives of dodecylthio–N,N’– (2,4–diisopropylphenyl)-3,4,9,10-perylenetetracarboxylic diimide are reported. The three derivatives display noticeable differences in their electronic absorption and emission properties, with a bathochromic shift in spectral maxima from 1,6–Thio–PDI, to 1,7–Thio–PDI, to 1,6,7–Thio–PDI. The 1,7–Thio–PDI derivative had the highest fluorescence quantum yield and longest fluorescence lifetime in toluene (ΦF = 0.85, τF = 8.3 ns), followed by 1,6–Thio–PDI (ΦF = 0.54, τF = 6.6 ns) and 1,6,7–Thio–PDI (ΦF = 0.16, τF = 4.4 ns). The difference in dipole moment between the ground and excited states were estimated by the Lippert-Mataga analysis of solvent dependent absorption and emission properties and ranged from Δμ = 7.1 Debye for 1,6,7–Thio–PDI to 8.3 Debye for 1,6–Thio–PDI. These measurements were supported by time–dependent density functional theory calculations, which helped interpret the differences in absorption spectra. Cyclic voltammetry showed minor differences in the reduction potentials for the singly and doubly reduced states. The absorption spectra of the singly and doubly reduced states are also reported.