Nanooptics and Photonics

Effect of J-aggregate formation in liquid crystal matrix on their optical properties
Sorokin A.V., Grankina I.I., Kasian N.A., Lisetski L.N., Yefimova S.L.
Yu.V. Malyukin Nanostructured Materials Department, Institute for Scintillation Materials of NAS of Ukraine, Nauky Ave. 60, Kharkiv, 61072, Ukraine
E-mail: sorokin@isma/kharkov.ua

Ordered luminescent molecular aggregates of dyes (so-called J-aggregates), primarily cyanine, merocyanine and porphyrin, form a wide class of luminescent nanoclusters, which are characterized by ultrahigh extinction coefficients, efficient energy migration and electron excitation, extremely high coefficients of cubic susceptibility etc. The unique optical properties of J-aggregates are related to the exciton nature of their electronic excitations of the Frenkel type. This phenomenon is possible only due to the high degree of ordering of molecules in chains, in fact translational symmetry, which is achieved due to the flat structure of molecules from which J-aggregates are formed. In fact, J-aggregates can be attributed to low-dimensional molecular crystals. Unlike bulk crystals, the exciton properties of J-aggregates are highly dependent on the degree of molecules disorientation in the chains or static disorder. Thus, the structure, and hence the optical properties of J-aggregates are largely determined by their microenvironment, which can be used to create J-aggregates with specified optical properties. At the same time, a number of studies have shown that in nanostructured media the J-aggregates luminescence can be significantly quenched due to the growth of static disorder.
In this work, the J-aggregate formation of anionic cyanine dye TDBC in the liquid crystal (LC) matrix 5CB has been studied using optical spectroscopy. The absorption spectra of the TDBC dye formed in LCs shown that the J-band is more intense and narrower. Simultaneously, lifetime increasing has been found for that case. The spectral changes can be associated with static disorder decreasing caused by the spatial restriction in the matrix. Photostability of this matrix is of major importance for a variety of applications, such as material sensing and fluorescence imaging. Herein, we demonstrated that TDBC J-aggregates formed in LCs are much more photostable comparing with ones in solution.