Color Change Effect in an Organic-Inorganic Hybrid Material Based on a   Porphyrin Diacid
  Porphyrinic materials show a range of interesting and useful optical and electrical properties. The less well-known sub-class of porphyrin diacids has been used in this work to construct an ionic hybrid organic-inorganic material in combination with a halogenidometalate anion. The resulting compound, $[H_6TPyP][BiCl_6]_2$ (1) (TPyP = tetra(4-pyridyl)porphyrin) has been obtained via a facile solution based synthesis in single crystalline form. The material exhibits a broad photoluminescence emission band between 650 and 850 nm at room temperature. Single crystals of $[H_6TPyP][BiCl_6]_2$ show a photocurrent in the fA and a much higher dark current in the nA range. They also display an unexpected reversible color change upon wetting with different liquids. This phenomenon has been investigated with optical spectroscopy, SEM, XPS and NEXAFS techniques, showing that a surface-based structural coloration effect is the source of the color change. This stands in contrast to other materials where structural coloration typically has to be introduced through elaborate, multi-step processes or the use of natural templates. Additionally, it underscores the potential of self-assembly of porphyrinic hybrid compounds in the fabrication of materials with unusual optical properties. 
Bettina Wagner, Natalie Dehnhardt, Martin Schmid, Benedikt P. Klein,   Lukas Ruppenthal, Philipp M\"uller, Malte Zugermeier,...
cond-mat.mtrl-sci