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

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 S₁ 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.