Circularly Polarized Light Probes Excited‐State Delocalization in Rectangular Ladder‐type Pentaphenyl Helices

Abstract

Ladder-type pentaphenyl chromophores have a rigid, planar π-system and show bright fluorescence featuring pronounced vibrational structure. Such moieties are ideal for studying interchromophoric interactions and delocalization of electronic excitations. We report the synthesis of helical polymers with a rigid square structure based on spiro-linked ladder-type pentaphenyl units. The variation of circular dichroism with increasing chain length provides direct evidence for delocalization of electronic excitations over at least 10 monomeric units. The change in the degree of circular polarization of the fluorescence across the vibronic side bands shows that vibrational motion can localize the excitation dynamically to almost one single unit through breakdown of the Born-Oppenheimer approximation. The dynamic conversion between delocalized and localized excited states provides a new paradigm for interpreting circular dichroism in helical polymers such as proteins and polynucleic acids.

One‐Handed Helical Tubular Ladder Polymers for Chromatographic Enantioseparation**

“…The onehanded helical spiro-conjugated ladder polymers have a rigid, but extended helical ribbon-like ladder structure with no internal cavity (Figure 1a). [16b] The synthesis of opticallyactive helical ladder polymers containing spirobifluorene frameworks as chiral sources with and/or without a helical cavity has also been reported by Takata et al [18] and recently by Meskers et al, [19] but their structural integrities were not completely elucidated.…”

Tightly-Twisted One-Handed Helical Tubular Ladder Polymers with π-Electron-Rich Cylindrical Helical Cavities for Chromatographic Enantioseparation

“…The one-handed helical spiroconjugated ladder polymers have a rigid, but extended helical ribbon-like ladder structure with no internal cavity (Figure 1a). [15b] The synthesis of optically-active helical ladder polymers containing spirobifluorene frameworks as chiral sources with and/or without a helical cavity has also been reported by Takata et al [17] and recently by Meskers et al, [18] but their structural integrities were not completely elucidated.…”

The Exciton Model for Molecular Materials: Past, Present and Future?

“…In particular, the role of molecular vibrations in the electronic transitions and electronic circular dichroism has come into focus. We discuss here an oligomeric model compound [13f] of the newly developed square helical conjugated ladder polymers, [13] see Figures 3,4. This molecule contains two identical chromophoric groups, namely planarized, ladder type tetra para ‐phenylene.…”

“… (left) α‐helical peptide segment showing also the chemical structure of the HNC=0 amide chromophores. (right) Square helical poly(ladder para‐ pentaphenylene) polymer [13f] …”