Chiral carbon nanohoops: topology, luminescence and aromaticity

Materials that exhibit circularly polarized luminescence (CPL) are desirable for the development of optoelectronic devices such as circularly polarized light-emitting diodes and photodetectors. Furthermore, CPL has the potential to provide new insights into the mechanism of photoluminescence and to develop new materials for advanced technologies. Cycloparaphenylenes ([n]CPPs), are excellent candidates for CPL materials due to their favorable luminescence properties, such as visible-light fluorescence with a high quantum yield and large Stokes shifts, and good processability. However, CPPs are achiral, which prevents them to display CPL. This could be circumvented by introducing a stereogenic unit to the structure of CPPs.
In this dissertation, we have explored the synthesis and properties of structurally diverse carbon nanohoops. We have gained insights about the chiroptical properties of chiral carbon nanohoops, their topology and its connection to aromaticity. In the first part, theoretical description of spiral para-phenylenes incorporating [2.2]paracyclophane moieties is provided in order to establish a clear structure–property relationship. It was discovered that the CPL dissymmetry factor drops markedly due to planarization of the most strained segment of the nanohoop. Additionally, consequences of preventing the planarization on resulting dissymmetry factor was explored.
In the second part, we developed the synthesis of chiral helicene carbon nanohoops by incorporating [5]- and [6]-helicenes into [n]CPP scaffolds. The structure of the nanohoops was studied by single crystal X-ray diffraction, 1D and 2D nuclear magnetic resonance, and mass spectrometry and the photophysical properties investigated by absorption and emission spectroscopies including CPL. Helicene carbon nanohoops were established as a new class of CPL emitters that possess relatively high dissymmetry factors and excellent luminescence quantum yields at the same time. Therefore, they combine the chiroptical properties and configurational stability of helicenes with the favorable optoelectronic properties of CPPs. In addition, helicene carbon nanohoops are topologically equivalent to that of a Möbius strip in the solid state and in solution. We investigated the topological aromaticity of these molecules and their doubly oxidized and reduced forms.