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.