Exciton Delocalization in a DNA-Templated Organic Semiconductor Dimer Assembly

Abstract

A chiral dimer of an organic semiconductor was assembled from octaniline (octamer of polyaniline) conjugated to DNA. Facile reconfiguration between the monomer and dimer of octaniline-DNA was achieved. The geometry of the dimer and the exciton coupling between octaniline molecules in the assembly was studied both experimentally and theoretically. The octaniline dimer was readily switched between different electronic states by protonic doping and exhibited a Davydov splitting comparable to those previously reported for DNA-dye systems employing dyes with strong transition dipoles. This approach provides a possible platform for studying the fundamental properties of organic semiconductors with DNA-templated assemblies, which serve as candidates for artificial light-harvesting systems and excitonic devices.

Keywords: DNA conjugation; circular dichroism; exciton delocalization; organic semiconductor; proton doping.

Figure 1

Octaniline–DNA assemblies and the different states in response to environmental change. Three different octaniline–DNA constructs were synthesized and prepared (top): IS, I0, and II. Construct I0 spontaneously assembles into a chiral dimer in aqueous solution, and the dimeric state is switched between two different monomer constructs by adding a triggering DNA strand or a surfactant (middle). The octaniline molecules can exist in different electronic states, emeraldine base and emeraldine salt, which both show strong exciton exchange as a result of electronic coupling in the dimer (bottom).

Figure 2

Characterization of octaniline–DNA chiral dimer and two different monomers. (a) Schematic depiction of octaniline–DNA chiral dimer and two monomers. (b) Circular dichroism of dimeric and monomeric octaniline–DNA constructs. (c) Ferguson plots used to reveal the shape of different constructs, in comparison to similarly sized linear DNA duplexes.

Figure 3

Coupling of octaniline chromophores. (a) Two aggregated octaniline moieties interact, resulting in a couplet and affording two broad, unresolved peaks in the electronic absorption spectrum (b). The coupled peaks are out of phase in the CD spectrum (c), affording a bisignate curve.

Figure 4

Octaniline–DNA dimer construct generated by strand displacement and the theoretical analysis. (a) Octaniline–DNA compositional monomer spontaneously assembled into the chiral dimer by adding triggering DNA strand. (b) Absorbance and CD spectral theoretical fits. Experimental data sets are shown as black lines, while theoretical fits are given as red curves.

Figure 5

Molecular models of the octaniline dimer. Top view shows the oblique angle, in degrees, as an angle between vectors of the two octaniline molecules. Side view shows the intermolecule distance from center to center. The position and orientation of the long axes of the octaniline molecules are from the theoretical fitting, but the rotation around the long axis of both molecules was arbitrarily chosen.

Figure 6

Proton doping of octaniline–DNA dimer construct. Octaniline chiral dimer undergoes proton doping at a lower pH. The emeraldine base (blue) and emeraldine salt (green) states of octaniline dimer were confirmed by circular dichroism spectra.