Fabrication of cross-linked thiophene-coumarin-fluorene donor–acceptor pentamer thin films for electrochromic devices application

Functionalized p-conjugated polymers have emerged as promising electrochromic materials due to their well-defined molecular structures, facile synthesis, and ability to form smooth and homogeneous thin films. Furthermore, surface cross-linking enhances the mechanical and environmental stability of these films while improving their durability during electrochemical operation. In this study, a unique band gap-tunable pentamer (FBP) was developed featuring a D-A-D-A-D configuration, which includes thiophene, coumarin, and fluorene. Cross-linked films (FBPX) were obtained using both spray and electrospray methods in the presence of tetrathiol-based crosslinkers and 366 nm light, thanks to the functionalizable allyl groups present in the FBP structure. UV–Vis analysis revealed absorption maxima at 410 nm and 330 nm for FBP and FBPX films, respectively, with corresponding optical band gap (Eg,opt) values of 2.88 and 3.30 eV. Moreover, electrochemical measurements indicated HOMO–LUMO energy levels of eV for FBP and eV for FBPX, demonstrating the influence of cross-linking on the electronic structure of the films. Atomic force microscopy analysis revealed that the electrospray deposition method significantly increased the surface roughness, with RMS values rising from 6.2 nm for the spin-coated film to 84.6 nm for the electrospray-coated film, resulting in a highly textured and porous morphology. Furthermore, electrochromic devices based on FBPX films were fabricated using different coating and surface cross-linking techniques. These results demonstrate that surface cross-linking and coating methodology significantly influence the electrochromic performance, enabling enhanced color modulation, durability, and response characteristics of FBPX-based electrochromic devices.