Harnessing the reversible isomerization of spiropyran to merocyanine in conjugated polymers for broadband ultra-violet to near-infrared electrochromic switching

Abstract

Organic electrochromic device (ECD) technologies have undergone rapid development leading to the commercialization of large-scale smart windows and the emergence of new applications for thermal energy management, energy storage, and sensing. Here, a new approach for achieving broadband ultraviolet (UV) to near-infrared (NIR) electrochromic switching is described by incorporating spiropyran into electrochromic conjugated polymers. Spiropyran has restricted π-conjugation, limiting optical absorption to the UV wavelength range, but upon electrochemical oxidation, it undergoes reversible ring-opening to yield merocyanine, which has extended π-conjugation and broad UV-visible-NIR optical absorption. To probe the utility of this reversible isomerization in electrochromic conjugated polymers, a series of spiropyran–thiophene copolymers (PSP2T, PSP4T, PSPr2T, and PSPr4T) are synthesized where the identity of the π-spacer (2-thienyl or 2,2′-bithienyl) and functionalization patterns are varied. PSP2T, PSP4T, PSPr2T, and PSPr4T provide optical absorption in the UV or near-UV wavelength range (λ_max = 393–444 nm and E_g = 2.15–2.55 eV) in the neutral state and achieve UV-visible-NIR (λ_max = 930–1100 nm) optical absorption upon electrochemical oxidation. It is found that incorporation of the 2,2′-bithienyl π-spacer affords a >4× increase in coloration efficiency (CE), a >3.5× reduction in switching time (t₉₅), and uncompromised optical contrast in the NIR-wavelength range (ΔT%: ∼50%). Correlation of DFT and TD-DFT calculations using model compounds with polymer spectroelectrochemistry uncovers a plausible mechanism for the electrochemical isomerization and electrochromic switching where spiropyran is first oxidized followed by ring-opening to yield oxidized merocyanine. These results establish reversible isomerization as a new mechanism for achieving broadband UV-visible-NIR electrochromic switching in conjugated polymers and semi-transparent-to-black coloration.