Influence of Dispersion in Liquid Crystal on Optical Properties of Carbocyanine Dye J-Aggregates

In recent years, liquid crystals have been used more and more widely as host matrices for the introduction and dispersion of various inorganic and organic nanoparticles, both in fundamental studies and to obtain new composite nanomaterials. In particular, due to the unique combination of optical and anisotropic properties, luminescent liquid crystals are of great interest from the point of view of optoelectronic applications. In condensed phases, they can organize with crystalline order, leading to, for example, attractive charge transport properties, while they retain fluidity, which provides the self-healing ability and dynamic properties. Furthermore, their anisotropic organization is particularly interesting for applications with polarised light.

However, traditional luminescent liquid crystals often suffer from fluorescence quenching caused by aggregation, which significantly limits their further use. One of the approaches to overcome this problem is the use of aggregation-induced emission. An important kind of luminescent aggregate is J-aggregates, which are low-dimensional molecular crystals of some types of organic dyes, such as cyanines or perylene derivatives. Due to the excitonic nature of their electronic excitations and 1D or 2D structure, the optical properties of J-aggregates differ significantly from the properties of individual molecules or bulk crystals.

We report the formation of J-aggregates of the anionic cyanine dye TDBC in a nematic liquid crystal 5CB matrix with analysis of optical-luminescent and electro-optical properties. Unlike water solution, the TDBC J-aggregates show a rather long lifetime and high photostability in the nematic matrix. The electro-optical characteristics of the LC matrix are substantially modified, with the Fredericks transition threshold slightly increased, which is, on the other hand, accompanied by the improvement of the optical contrast. Only a minor effect of the forming J-aggregates on the molecular order of the LC structure could be noted.