Fluorescence of Helical Molecular Springs under High Pressure.

Although the unique structure of helicenes resembles molecular springs, the effects of their extension-contraction cycles on their properties have rarely been explored. Here, we investigated the fluorescence of two π-extended [n]helicenes with different helical lengths n, named [7] and [9], under high pressures in a diamond anvil cell. The experimental results showed that compound [9], with a longer helical length, exhibited a more sensitive fluorescence response than [7] in both crystalline and solvated states upon compression. Theoretical calculations reveal that π-π overlapping at their helices in these molecular springs provides an additional contribution to their fluorescence properties under compression when the overlap becomes sufficiently strong. Our results provide insights into structure-property relationships of helical molecules under high-pressure conditions and verify the potential of helicenes as molecular springs for future applications in molecular machines.