  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.
