Scientists in India have discovered an organic dye which upon binding with Ca2+ self-assembles to form a spherical micellar assembly that reorganises into a cyclindrical fiber-like shape. The supramolecular structure was found to be of nanometer size with high visible light absorption ability. 

Electronically and photonically active molecular architectures are important in the field of advanced materials for optoelectronic applications and nanodevices. The team led by Ayyappanpillai Ajayaghosh at the National Institute for Interdisciplinary Science and Technology, Trivandrum, India found that the molar absorptivity of the calcium-complexed bis-squaraine dye is increased about threefold in the fiber structures. This observation was attributed to morphology changes. 

Using microscopic analyses, Ajayaghosh showed that in the presence of Ca2+, the bis-squaraine dye self-assembles quickly to a spherical micellar structure which over time reorganises to the more thermodynamically stable fiber-like structure. A possible explanation for this process is the difference in the mode of complexation and the stability of the resulting dye:Ca2+ complex. It is also believed that the presence of the aromatic groups of the catechol linker in the bis-squaraine dye plays a key role in the cation controlled self-assembly process. This is because in the absence of the linker no stable self-assembly of hyperchromism occurs. 

In the future, Ajayaghosh hopes to develop functional dye structures with controlled size and high light absorption ability with potential application in biology and advanced materials. 

Kathleen Too