Mehrdimensionale Fluoreszenzspektroskopie an borbasierten Donor-Akzeptor-Farbstoffen: Zusammenhang von Struktur und photophysikalischer Funktionalität

Böhling, Lena

Fluorescence spectroscopy is an indispensable analytical method in modern research. The complexity of luminescent systems usually requires the combination of several techniques.
In this work, novel donor-acceptor fluorescence dyes with potential applications in electro-optical devices, materials with non-linear optical properties and fluorescence sensors were characterized spectroscopically. For the targeted synthesis of these fluorophores
with specific fluorescence properties, a detailed understanding of their optical characteristics is essential.
An important part of this work was the optimization of dynamic and static fluorescence techniques for the spectroscopic characterization of donor-acceptor dyes. In particular, an excitation-emission-spectrometer has been supplemented by an integration sphere for the detection of solid state fluorescence. Furthermore, a newly designed TCSPC-experiment enables the analysis of excited states in solution as well as in the solid state.
A low-temperature sample holder was integrated in both experiments for static and dynamic fluorescence spectroscopy at temperatures down to 77 K.
In the second part of this work, a number of boron-based donor-acceptor-dyes, containing a 1,3,2-benzodiazaborolyl or a dicarba-closo-dodecarborane unit, were investigated
spectroscopically. For a comprehensive understanding of the relationship between the structure of dyes and their fluorescence properties, the spectroscopic studies were
supplemented by the results of TD-DFT calculations and X-ray structure analysis. Benzodiazaborolyl-based donor-acceptor dyes exhibit intense luminescence accompanied
by large Stokes shifts. The distinct solvatochromism of the emission confirms a strong change in the dipole moment between the ground state and the excited state which is
characteristic for donor-acceptor dyes. By the variation of the substituents at the nitrogen atoms of the benzodiazaborole, the originally electron donating character can be changed to an electron accepting behaviour. As a result, the benzodiazaborolyl moiety
offers the unique opportunity to create donor-acceptor dyes in which both functionalities originate from the same group of substances.
Systematic measurements revealed that ortho-carboranebased donor-acceptor dyes possess unexpected and complex emission properties which include dual fluorescence. UVemissions are due to local transitions while the low-energy emissions could be traced back to a charge-transfer-transition to an excited state with a geometry significantly different to the ground state. Benzodiazaborolyl-substituted ortho-carboranes exhibit
yellow to red emission in the solid state. Because of the high quantum yields of up to 70 %, these compounds are a promising candidate for technical applications.

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