Symmetry breaking and the turn-on fluorescence of small, highly strained carbon nanohoopsElectronic supplementary information (ESI) available. CCDC 1883238. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c9sc00169g

• Main Authors: Lovell, Terri C, Colwell, Curtis E, Zakharov, Lev N, Jasti, Ramesh
• Format: Article
• Language: English
• ISSN: 2041-6520; 2041-6539
• DOI: 10.1039/c9sc00169g

[ n ]Cycloparaphenylenes, or "carbon nanohoops," are unique conjugated macrocycles with radially oriented π-systems similar to those in carbon nanotubes. The centrosymmetric nature and conformational rigidity of these molecules lead to unusual size-dependent photophysical characteristics. To investigate these effects further and expand the family of possible structures, a new class of related carbon nanohoops with broken symmetry is disclosed. In these structures, referred to as meta [ n ]cycloparaphenylenes, a single carbon-carbon bond is shifted by one position in order to break the centrosymmetric nature of the parent [ n ]cycloparaphenylenes. Advantageously, the symmetry breaking leads to bright emission in the smaller nanohoops, which are typically non-fluorescent due to optical selection rules. Moreover, this simple structural manipulation retains one of the most unique features of the nanohoop structures-size dependent emissive properties with relatively large extinction coefficients and quantum yields. Inspired by earlier theoretical work by Tretiak and co-workers, this joint synthetic, photophysical, and theoretical study provides further design principles to manipulate the optical properties of this growing class of molecules with radially oriented π-systems. By breaking the centrosymmetry of the structure, the most strained carbon nanohoops become fluorescent as predicted by optical selection rules.