Designing J- and H-aggregates through wave function overlap engineering: applications to poly(3-hexylthiophene).

A novel mechanism for J- and H-aggregate formation is presented on the basis of wave function overlap (WFO) coupling between neighboring chromophores which supplements the usual through-space (Coulombic) coupling. In cases where the latter is relatively small compared to the former, as might arise for excitons based on molecular transitions with low oscillator strengths, J- vs H-aggregation is determined by the sign of the product D(e)D(h), where D(e) (D(h)) is the coupling between a neutral Frenkel exciton and the charge transfer exciton created through the transfer of an electron (hole) to a neighboring chromophore. Adapting a sign convention based on translational symmetry in a linear array of chromophores, a positive (negative) sign for D(e)D(h) places the bright exciton on the bottom (top) of the exciton band, consistent with J- (H-) aggregation. The J- (H-) aggregates so formed behave as direct (indirect) bandgap semiconductors with vibronic signatures in absorption and photoluminescence that are identical to those displayed by conventional Coulomb coupled aggregates. WFO coupling leading to the mixing of intrachain Frenkel excitons and polaron pairs may be important in conjugated polymer aggregates where the Coulomb coupling practically vanishes with the (conjugation) length of the polymer. Calculations based on octathiophene (8T) dimers show that the eclipsed geometry yields a WFO coupling favoring H-aggregate behavior, although a longitudinal (long-axis) displacement by only 1.5 Å is enough to change the sign of the coupling, leading to J-aggregate behavior. Hence, it should be possible to design thiophene-based polymers which act as J-aggregates with respect to the interchain coupling.

Heads and tails: simultaneous exposed and buried interface imaging of monolayers.

We have simultaneously imaged the chemically bound head groups and exposed tail groups in bicomponent alkanethiolate self-assembled monolayers on Au{111} with molecular resolution. This has enabled us to resolve the controversy of scanning tunneling microscopy image interpretation and to measure the molecular polar tilt and azimuthal angles. Our local measurements demonstrate that ordered domains with different superstructures also have varied buried sulfur head group structures.