By reducing at least one characteristic dimension to the nanometer scale, quantum effects and increased surface-to-volume ratio become increasingly important and lead to a dramatic change of the optical, electronic, mechanical and thermal properties. By using the structure and size as a tunable parameter, novel nanohybrid materials can be designed in which the functionalities of the individual building blocks couple and yield new, fundamentally different properties. This enables a wide range of applications in diverse fields as chemistry, physics, biology, medicine and engineering. Particularly interesting in this perspective are single-wall carbon nanotubes (SWCNTs), that allow to tune the dimensions of such supramolecular assemblies into one dimension. The filling of SWCNTs with various molecules and their external (covalent or non-covalent) functionalisation has therefore become a novel path to add and create new functionalities, through the mutual interaction between the encapsulated or attached molecules and the host SWCNTs.

In this paper, I will first discuss a few examples of this endohedral functionalisation of the CNTs. In particular cases, the encapsulated molecules form strongly interacting molecular arrays that result in severely altered optical properties.[1-4] For example, when encapsulating dipolar dye molecules inside the SWCNT hollow core, they automatically align head to tail with all dipoles pointing in the same sense, resulting in a hybrid structure with enhanced nonlinear optical properties.[1] As a second example, when encapsulating squarilium dyes inside CNTs, the specific molecular arrangement of the dyes inside SWCNTs with different diameters results in strongly correlated J-aggregates that show efficient energy transfer to the surrounding SWCNTs.[3-4]

Also the CNTs themselves have unique properties. Their quasi one-dimensional character results in the formation of strongly bound electron-hole pairs (excitons) that can even be observed at room temperature (i.e. binding energies of the order of several hundred meV). The exciton fine structure of SWCNTs is quite complex, with multiple singlet and triplet excitonic states, of which only one is optically allowed, thereby resulting in extremely low fluorescence (PL) quantum yields. While the singlet excitons have already been investigated thoroughly, little is known about the longer-living triplet excitons. As a second part of the talk, I will therefore focus on the characterization of these triplet excitons by means of spin-sensitive optically-detected magnetic resonance spectroscopy.[5]

References:

1. S. Cambré et al. Nature Nanotechnol. 10, 248–252 2015.
2. E. Gaufrès et al. Nature Photonics 8, 72–78 2014.
3. S. Van Bezouw et al. ACS Nano 12, 6881–6894 2018.
4. S. Forel et al, Nanoscale 14, 8385 (2022)
5. I. Sudakov et al, in revision