State-of-the-art electrochromic materials based on metallo-supramolecular polymers

Metal ion induced self-assembly of iron(II)-acetate with the rigid ditopic ligand 1,4-bis(2,2′:6′,2′′-terpyridin-4′-yl)benzene results in a metallo-supramolecular coordination polyelectrolyte (Fe-MEPE). Fe-MEPE shows a strong absorption band in the visible region around 590 nm, attributed to a metal-to-ligand-charge-transfer (MLCT) transition, which is responsible for the deep blue colour. Large area thin films of high optical quality can be readily fabricated by a dip coating process on transparent conducting electrodes. The Fe-MEPE films have a temperature stability up to 80 °C, measured by optical spectroscopy and XAFS (x-ray absorption fine structure). The cathodically coloured Fe-MEPE shows outstanding electrochromic properties and can be reversibly switched from Fe(II) (blue) to Fe(III) (colourless) by applying a potential of 4.1 V vs. Li/Li+. A very high optical contrast ΔT of 71% at a wavelength of 590 nm and a colouration efficiency of around 525 cm2 C−1 can be realized. The devices show a long-term stability about 10,000 cycles. Thus, Fe-MEPE is a very promising electrochromic material for future applications of smart windows