Cross-Linkable Fluorene-Diphenylamine Derivatives for Electrochromic Applications

Multicolor
electrochromic systems based on heat cross-linkable arylamine-substituted
fluorene derivatives, FD and FDOMe, are
reported. These derivatives with pendant vinyl groups have been synthesized
by the Buchwald–Hartwig amination reaction and were well-characterized
using various analytical and spectroscopic techniques such as NMR,
ESI-MS, and single-crystal X-ray diffraction analysis. FD and FDOMe exhibited thermally activated cross-linking
above their melting temperatures, which was confirmed through absorption,
differential scanning calorimetry (DSC), FT-IR, and wide-angle X-ray
diffraction (WAXD) techniques. Cross-linked FD films
(FD-X) on ITO showed two reversible redox peaks at 0.74
and 0.91 V (versus Ag/AgCl) that correspond to the formation of radical
cations and dications, respectively. The corresponding redox peaks
were observed at 0.6 and 0.8 V for cross-linked FDOMe films (FDOMe-X). Spectroelectrochemical studies of
the electrochromic films on ITO revealed multicolor electrochromism
of FD-X (colorless–yellow–dark cyan) and FDOMe-X (colorless–brick red–blue) with a color
contrast of ∼44% at 485 nm for FD-X and ∼63%
at 500 nm for FDOMe-X and good switching stability between
the neutral and oxidized states (>300 cycles) with low switching
voltages (<0.9 V for the first oxidation and <1.3 V for the
second oxidation). Furthermore, fabrication of electrochromic devices
using FD-X and FDOMe-X on FTO substrate
with PMMA-based solid electrolyte was demonstrated, where the devices
exhibited reasonably low switching time between the redox states (<30
s) with good optical contrast.