Functional Luminescent Materials for Two-Photon Applications

Abstract

Novel organic materials with luminescent properties and non-linear response are studied in this PhD work. Organic nanoparticles, easily obtained via the reprecipitation method, and thin films have been developed and characterized for application based on Two-Photon Absorption.

At the beginning of the thesis, the reprecipitation method was optimized. A number of aspects that can affect and modify the process, such as the chemical composition of the starting material, the influence of concentration and the solvent effects were studied. Our results indicate that the molecular geometry and the way of packing of molecules within the particles are important, and the optimal final concentration of the chromophore in suspension is 0.1 μM in order to present some fluorescence. The optimized conditions were followed for the preparation of all the successive suspensions.

Core@Shell@Shell and Composite ternary organic nanoparticles were designed, prepared and investigated. In the multicomponent nanoparticles an excitation energy-transfer cascade between the different molecular components occurs, with an enhancement of luminescence in the red spectral region. The two-photon brightness of the ternary nanoparticles is greatly enhanced with respect to that of the single-component nanoparticles. In particular, they can be two-photon excited over a broad spectral range (from 600 nm to 1200 nm) inside the biological transparency window. This property, together with good luminescence in red spectral region and good colloidal stability in aqueous suspension, suggests that these fully organic multicomponent nanosystems can be potential nanoprobes for Two-Photon bioimaging.

Moreover, the luminescence properties of new materials based on open-shell molecular systems were studied. Organic nanoparticles and polymeric films doped with carbon-centered radicals, namely polychlorotriphenylmethyl radicals, were prepared and optically characterized. The luminescence properties of these binary systems are improved with respect to molecule in solution or in the solid state. Organic nanoparticles and films with low radical doping exhibit up to 10 times higher luminescence quantum yield than the radical in solution. Increasing the radical doping leads to a progressive decrease of the luminescence quantum yield and the appearance of a new broad excimeric band at longer wavelengths. The formation of excimers from stable and persistent supramolecular radical-pairs was observed here for the first time. The good stability and luminescence properties with emission in the red-NIR region (650-800 nm), together with the open-shell nature of the emitter, make these free-radical excimer-forming materials candidates for optoelectronics and bioimaging applications.

A part of the thesis is devoted to the work I performed during my secondments in University of Sloupsk (Poland) and in University of Bordeaux (France), and it is related to Two-Photon Polymerization. Particularly, fluorescence is applied to monitor the photo-polymerization process and the organic synthesis of an asymmetric aldehyde as Two-Photon initiator it is described.

The final part of the thesis summarizes the outreach activities performed during my training period in the framework of the Nano2Fun ITN project and of the International Year of Light 2015.