Cyanine dyes attract considerable attention of researchers from all over the world due to their rather unique optical properties, such as very narrow absorption and luminescence spectral bands, rather high values of extinction coefficients and quantum yields, which in turn are significantly dependent on the surrounding environment, as well as their ability to aggregation. Such supramolecular complexes, depending on the conditions, can be formed in the form of non-luminescent aggregates with an additional hypsochromic band (H-band), so-called H-aggregates, or in the form of luminescent aggregates with an additional bathochromic band (J-band), so-called J-aggregates. And in some cases, both H- and J- bands may appear in the spectra.
A significant contribution to the study of the nature of cyanine dyes was the discovery that the electronic excitations of aggregates of these dyes are Frenkel excitons, which in turn gives insight into the fact that the H- and J-bands have an excitonic nature due to the delocalization of electronic excitations of individual molecules along an ordered molecular chain. And the shift of the exciton band relative to the band of monomers to a certain position, or the presence of two bands in the spectrum at the same time, depends only on the geometry of the packing of the molecules in the aggregate. Also, it should be noted that for cyanine dyes the aggregation process is typical in the ground state, but the formation of excimers is not a characteristic feature for them.
From the point of view of applied use, luminescent J-aggregates, which are often referred to as supramolecular systems or molecular nanoclusters, are more interesting. Thanks to their unique optical properties, they have found wide application as light-harvesting complexes and photosensitizers in photonics and optoelectronics, as effective luminescent probes for biology and medicine etc. Also, efficient non-radiative energy transfer can be achieved between cyanine dyes, which will be very widely used in applied tasks, such as the creation of solar cells of the "Grätzel cell"-type.
Very easy manipulation of the spectral properties of J-aggregates is possible due to their nature of aggregation, which occurs due to non-covalent interaction, which means that it guarantees the so-called metastability, that is, a strong dependence of their structure and optical properties on the external influence, microenvironment. But in turn, this limits the wide practical application of J-aggregates due to the possibility of uncontrolled deterioration of their optical properties.
Therefore, the question of the formation of J-aggregates in a «harder» environment, such as thin polymer films and nanoporous matrices, where it is possible to preserve the useful unique properties of the aggregates, ensure their greater photostability, and maximally reduce the probability of their damage or destruction, is quite relevant from a fundamental point of view sight, as well as from applied use.
The dissertation is devoted to the study of the mechanisms of formation and interaction of J-aggregates of several cyanine dyes in various nanostructured media, such as thin polymer films and porous TiO2 matrices.
The dissertation investigated cyanine dyes whose molecules can form J-aggregates: anionic dyes TDBC (1,1'-disulfobutyl-3,3'-diethyl-5,5',6,6'-tetrachlorobenzimidazolo-carbo-cyanine sodium) and TCC (3,3'-disulfobutyl-5,5'-dichlorothiacarbocyanine triethyl-ammonium), as well as cationic dye PIC (1,1'-diethyl-2,2'-cyanine iodide, pseudoisocyanine).
Through a detailed spectroscopic study of the aggregation of TCC cyanine dye, it was found that, unlike typical J-aggregates with a "herringbone" structure, the optical interaction between H- and J-bands in TCC cyanine dye is completely absent. As a result, it becomes possible to control the processes of predominant J-aggregation of this dye under certain conditions of the microenvironment.
The obtained experimental results on controlling the spectral characteristics of J-aggregates can be used in the purposeful development of new materials with controlled optical properties based on organic compounds.
For the first time, dilution of TCC aggregates has been shown to result in the presence of the H-band even at very low dye concentrations, while the J-band disappears. It can be assumed that J-aggregates of TCC are formed from H-dimers, which are initially formed in aqueous solutions.
It was established that the structure of TСC J-aggregates can change to a two-dimensional island-like one during their formation in nanostructured materials, in contrast to the quasi-one-dimensional rod-like morphology characteristic of these J-aggregates in solutions.
