Control of Light Energy Converting Processes into Langmuir Monolayers and Langmuir-Blodgett Films of Conjugated Organic Compounds

This study is devoted to the revelation of new possibilities to control aggregation structure and structurally dependent photophysical characteristics of organized ultrathin films based on amphiphilic π-conjugated organic dyes.

It is shown, that alteration in subphase cationic composition can directly affect the type and the degree of molecular aggregation in crown-substituted hemicyanine monolayers. The phenomenon of reversible lateral pressure induced excimer formation in such monolayers formed on cation-containing subphases upon the monolayer cyclic compression-expansion is found out. According to UV-Vis spectroscopy and X-Ray reflectivity data, on pure water, two dye molecules arrange themselves in comparatively stable pairs with head-to-tail orientation due to the electrostatic interactions. Such a structure causes rather uniform electron density distribution with long monotonously falling tail. Instead, the presence of barium ions in the subphase leads to the organization of neighboring molecules in head-to-head manner due to the formation of crown-barium-crown sandwich structures at the initial state of monolayer spreading. The structure of the layer manifests itself in a clear electron density distribution with two maxima of crown-ether groups and ClO₄^- ions. Such a structure is responsible for the excimer formation.

Possibility to manage the donor-acceptor excitation energy transfer in mixed monolayers of two naphthalimide type fluorophores is discussed in terms or Forster theory. The extreme character of the dependence of irradiative energy transfer efficiency on the distance between donor and acceptor monolayers is revealed. It is found experimentally, that the optimal efficiency can be achieved when these monolayers are spaced by an inert layer thickness of 5-7 nm.

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