Fluorescence of molecular composites that consist of nematic liquid crystal and merocyanins

Abstract

Background: Considered merocyanine molecules have donor D and acceptor A moieties connected by polymer chain. The conjugated donor D and acceptor A parts of merocyanine molecules are planar in the ground state. The D-π-A molecular complexes can change their conformations in an excited state due to cis-trans-conformational transitions. The viscosity of the solvent affects conformational changes, photoluminescence (PL) decay and PL lifetime. Therefore the PL of merocyanine molecules strongly depends on medium in excited and ground states. A nematic liquid crystal was utilized as a solvent for merocyanine molecules since it is characterized by long range order, orientates merocyanine molecules along its axis and is sensitive to external electro-magnetic field. The molecules with conjugated donor D and acceptor A parts are promising for molecular electronics (instruments for recording and processing information), optoelectronics (laser technologies) and biological research (fluorescent probes like prominent thioflavin T), since their PL essentially depends on the nature of the environment. Objectives: The main objective of this paper is to investigate and explain impact of liquid crystal medium on considered molecules. Also influence of molecular conformational changes on their spectral properties is considered. Materials and Methods: Two types of merocyanine molecules M-1 and M-2 were investigated. The spectral properties of molecules in different organic solvents such as acetonitrile, toluene, glycerol, and in 4-pentyl-4′-cyanobiphenyl (5CB) liquid crystal were compared. Stationary and time-resolved emission spectra of molecular merocyanine solutions were used to investigate conformational changes of molecules. To determine a lifetime of molecular excited states, a technique of Time Correlated Single Photon Counting with picosecond resolution in time was used. Results: The results indicate that optical properties strongly depend on conformation of conjugated donor-π-acceptor compounds. A relaxation of dye molecules to the ground state is accompanied by conformational changes. The quantum yield and lifetime of PL increase in more viscous solvents. The liquid crystal made conformational changes of considered molecules in ground and excited states. Conclusions: Analysis of the results serves as a basis for constructing a theory that explains properties of D-π-A molecular compounds under optical excitation. The molecular pairs are formed by merocyanine and liquid crystal molecules. The liquid crystal molecule defines conformation of merocyanine molecule in a pair. Moreover, both types of merocyanine molecules in liquid crystal have the same conformation. This hypothesis explains similarities of optical properties of M-1 and M-2 molecules in liquid crystal.