Flexibility-driven 1D-structural preference in a bis-terpyridine-Fe(ii)-metallo-supramolecular polymer possessing potential tricolor electrochromism

The recent advancement of metallo-supramolecular polymeric (MSP) complexes using terpyridine (TPY) chelating ligands with a linear or angular spacer is appealing. Still, architectural (linear/cross or cyclic) tuning is challenging. Ditopic angular TPY ligands with a carbazole or triphenylamine spacer creating metallocycles with Fe2+ have been identified earlier. This paper introduces a more flexible and electron-rich N-heptyldiphenylamine as a spacer for an angular ditopic TPY ligand that contrarily affords a 1-D linear polymer, as revealed from SEM and STM experiments along with partial support of H-1-NMR. Notably, the formation of such a thermodynamically unfavored linear polymer is rare. Based on the single crystal X-ray and DFT-optimized ligand structure, TPY units are widely separated (similar to 67 degrees) due to conformational and electronic diversity, leading to the control of the structural variation of the MSP. The absorption spectrum indicated a metal-ligand charge transfer, but emissions were mainly centered on the ligand part. However, the presence of this redox-active diphenylamine and Fe2+ centers enables this 1D MSP to be a potential candidate for electrochromism within a possible window of +2.8 V to -1.5 V. A tricolor electrochromic device was prepared with this MSP that exhibited rapid color switching with a bleaching and coloration time (s) of 2.3 and 1.8, respectively, with a coloration efficiency of 180 cm(2) C-1. The unchanged 50% optical contrast of this device, even after 500 cycles, is commendable. The involvement of the ligand along with Fe2+ in the overall redox process was identified. Hence, this result would recognize a new approach for varying the architectural motifs of MSPs using a more flexible electron-rich spacer. Finally, an efficient tri-color electrochromic device was invented for real-life applications.