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.

emission color of an organic light-emitting diode (OLED), for example, arises primarily from the electronic structure of the individual molecular building block making up the thin film of the device, implying that single-molecule fluorescence techniques are ideal probes of the underlying intrinsic electronic structure. [1] Intermolecular interactions can, conceivably, arise between polymer chains, inducing H-and J-type aggregation effects, [2] but such coupling is usually effectively suppressed by resorting to molecules with bulky sidechains. [3] A π-conjugated polymer material that has proven particularly interesting in this regard is ladder-type poly(para-phenylene) (LPPP). [4] This compound not only displays a remarkable structural rigidity, minimizing excited-state relaxation and therefore making absorption and luminescence spectra near-perfect mirror images of each other. [5] It also shows well-resolved vibronic transitions, attesting to the low degree of intermolecular disorder. Interchain electronic aggregation effects are virtually absent, so that ensemble absorption and emission spectra are almost identical in the dissolved form and in bulk films. [5] In fact, intermolecular interactions are so weak that the sum of single-molecule luminescence spectra almost perfectly replicates the bulk-film ensemble spectrum. [6] At the same time, LPPP-based materials have a high fluorescence quantum yield and exhibit substantial photostability, making them ideal for applications involving large excitation densities. [5] Feldmann and Lemmer pioneered the use of LPPP in low-threshold mechanically flexible laser structures, [7] a feat that received widespread attention culminating in the award of the Philip-Morris Research Prize. [8] To this day, a photograph of the far-field intensity pattern generated by such a plastic laser [9] decorates the cover of the journal "Organic Electronics". [10] More recently, the unique characteristics of LPPP have been exploited in optical microcavities to create exciton-polariton condensates by strong light-matter coupling, [11] which can even enable optical transistor-like action [12] and single-photon optical nonlinearities at room temperature. [13] Inferring polymer aggregation effects from spectral signatures alone can be challenging and requires a detailed understanding A fundamental question relating to the nature of light emission and absorption in organic semiconductors is the dimension of the domain within a bulk material responsible for the interaction of light and matter. How large can a nanoparticle become to retain the quantized nature of light emission? Excitons are only a few nanometers in size, but because they diffuse in space, they probe a much larger volume than the single molecule. When excitons meet, they may decay non-radiatively by singlet-singlet or singlet-triplet annihilation (SSA or STA). Fluorescence photon statistics reveal whether single photons are emitted (photon antibunching) or arrive in randomly spaced packets (photon bunching), offering direct insight ...

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.