[n]-Cyclo-9,9-dibutyl-2,7-fluorene (n=4,5): Nanoring size influence in carbon bridged cyclo-para-phenylenes.

For the last ten years, ring-shaped pi-conjugated macrocycles possessing radially directed pi-orbitals have been subject to intense research. The electronic properties of these rings are deeply dependent on their size. However, most studies involve the flagship family of nanorings: the cyclo- para -phenylenes. We report herein the synthesis and study of the first examples of cyclofluorenes possessing five constituting fluorene units. The structural, optical and electrochemical properties were elucidated by X-ray crystallography, UV-vis absorption and fluorescence spectroscopy, and cyclic voltammetry. By comparison with a shorter analogue, [4]-cyclofluorene, we show how the electronic properties of [5]-cyclofluorenes are drastically different from those of [4]-cyclofluorenes, highlighting the key role played by the ring size in the cyclofluorene family.