Helicene‐Azulene Conjugates: Synthesis, Photophysical Properties and Proton‐Induced Chiroptical Switching

ABSTRACT The synthesis of 4,13‐dimethoxy‐carbo[6]helicene derivatives substituted at positions 2,15 by either 2‐azulene ( H‐2‐azu ) or 6‐azulene ( H‐6‐azu ) units is presented. The photophysical and chiroptical properties of these new helicene‐azulene derivatives have been examined in details. Both helical constitutional isomers H‐2‐azu and H‐6‐azu exhibit strong electronic circular dichroic responses in the azulene absorption region and reversible chiroptical switching upon protonation/deprotonation. Furthermore, although very weak, emission was also observed and was analyzed as originating from the S 1 excited state, i.e., obeying the Kasha rule, which is not common in azulene derivatives. Theoretical calculations helped confirm these observations. Finally, these systems revealed efficient chemical acid‐base triggered chiroptical switching activity, showing notably a strong red shift upon protonation.