Cyanine dyes are a class of organic chromophores well known for their strong light absorption and emission in the visible and near-infrared range. In the past, cyanine dyes have been used as light sensitizer for silver halide photography, in recording media technology or as contrast agent for biological applications. They are currently being investigated in the form of amorphous thin films as active layers for photovoltaic solar cells or light-emitting devices. The optical properties of cyanine molecules are highly anisotropic. They depend on the orientation of the transition dipole moment and the dipole-dipole interactions with neighboring molecules. However, in amorphous films the macroscopic optical properties become isotropic due to the random orientation of individual chromophores. Therefore, controlling the molecular orientation in cyanine thin films by fabricating single crystals could be an important strategy to tune the opto-electronic properties of cyanine-based devices.
In this work we fabricate single crystals of the dye 1,1’-diethyl-3,3,3’,3’-tetramethylcarbocyanine perchlorate directly on substrates through a solution-based process. The single crystals show a platelet morphology making them potentially interesting for use in thin film devices. Using X-ray crystallography, we show that the structure is made of strongly interacting molecular layers, each displaying a different herringbone-type arrangement. This leads to the splitting and polarization of the optical absorption bands through excitonic coupling of the transition dipole moments of the chromophores. Additionally we also measure the photoluminescence of single crystals which can also be related to the crystal structure. We show that compared to amorphous film, the crystals exhibit an additional intense emission peak. The experimental results are compared to theoretical calculations and a good correlation is found. In addition, the theoretical approach allows pinpointing specific molecular exciton contribution to the different spectroscopic bands.
With this work we demonstrate a simple method to fabricate thin film crystals and provide new insights in the photo-physics of cyanine dyes which opens the way for applications in optoelectronic devices.