Electrochromic properties of novel octa-pinene substituted double-decker Ln(iii) (Ln = Eu, Er, Lu) phthalocyanines with distinctive near-IR absorption

Abstract

Octa-pinene substituted double-decker lanthanide(III) phthalocyanines LnPc*₂ (Ln = Eu, Er, Lu) were prepared and their spectral, electrochemical, spectroelectrochemical and electrochromatic properties were studied. The introduction of the bulky and rigid pinene groups into phthalocyanines provides several advantages for the resulting double-decker lanthanide(III) phthalocyanine complex. First of all, the intermolecular interaction in the new LnPc*₂ complex was weakened, leading to excellent solubility. Secondly, intramolecular distances between the macrocycles in LnPc*₂ molecules were also increased, resulting in a significant red shift of the ring-to-ring intervalence charge transfer bands. Particularly, the intervalence bands of EuPc*₂, ErPc*₂ and LuPc*₂ were observed at 1944, 1693 and 1620 nm, respectively, which appear to be the most red-shifted absorption in comparison with the literature values of mononuclear double-decker Eu(III), Er(III) and Lu(III) phthalocyanines. Thirdly, given the red-shifted near-IR (NIR) absorption and the various colorful oxidation states, the solution of LnPc*₂ exhibits electrochromic behavior both in the UV-Vis and near-IR regions. Notably, spectral change of EuPc*₂ covers almost the whole range of the NIR region. Finally, the pinene groups also enhance the film-forming ability. Therefore, we were able to fabricate solid state electrochromic devices through a solution processable method. The as-fabricated devices show reversible electrochromic behavior with high color efficiency and good stability.