Chiroptical Properties of Helicenes

This chapter describes the historical and current state of affairs of optical and chiroptical properties of helicenes. Hexahelicene, composed of six benzene rings, has been an exemplar for experimental (synthetic) and theoretical (computational) challenges that affords and explains the exceptional chiroptical responses such as very large optical rotation (OR) value, which is briefly described at the beginning of the chapter. Then, all of the fundamental chiroptical properties such as optical rotatory dispersion (ORD), electronic circular dichroism (ECD), vibrational circular dichroism (VCD), Raman optical activity (ROA), and circularly polarized luminescence (CPL) of hexahelicenes are summarized. Because of the favorable chiroptical properties of helicenes, structure–chiroptical property relationships have been extensively examined for helicene derivatives through the structural modifications, for instance, by an elongation of helices. Molecular symmetry wields more substantial impact on the chiroptical responses of helicenes that are experimentally obtained by the fusion of multiple helicene moieties in double, triple, quintuple, and higher multiple helicenes. Finally, the effect of substitution of skeletal carbon and peripheral hydrogen atoms of helicenes is discussed with emphasis on vibronic effect in the ECD and CPL spectra.