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 <i>D</i>_e<i>D</i>_h, where <i>D</i>_e (<i>D</i>_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 <i>D</i>_e<i>D</i>_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

Contrasting Photophysical Properties of Star-Shaped vs Linear Perylene Diimide Complexes

The absorption line shapes of a series of linear and star-shaped perylene diimide (PDI) complexes are evaluated theoretically and compared to experiment. The cyclic trimer and tetrahedral complexes are part of the symmetric series, characterized by a single interchromophoric coupling, <i>J</i>₀, between any two PDI chromophores. The measured spectra of all complexes show pronounced vibronic progressions based on the symmetric ring stretching mode at ∼1400 cm^–1. The spectral line shapes are accurately reproduced using a Holstein Hamiltonian parametrized with electronic couplings calculated using time-dependent density functional transition charge densities. Although the “head-to-tail” linear complexes display classic J-aggregate behavior, the star-shaped complexes display a unique photophysical response, which is neither J- nor H-like. In the symmetric <i>N</i>-mers (<i>N</i> = 2–4), absorption and emission are polarized along <i>N</i> – 1 directions in contrast to linear complexes where absorption and emission remain polarized along the long molecular axis. In the symmetric complexes the red-shift of the 0–0 peak with increasing |<i>J</i>₀|, as well as the initial linear rise of the 0–0/1–0 oscillator strength ratio with increasing |<i>J</i>₀|, are independent of the number of PDI chromophores, <i>N</i>, and are markedly smaller than what is found in the linear series, where the shifts and ratios depend on <i>N</i>. Moreover, whereas the radiative decay rate, γ_r, scales with <i>N</i> and is therefore superradiant in linear complexes, γ_r scales with <i>N</i>/(<i>N</i> – 1) in the symmetric complexes. Vibronic/vibrational pair states (two-particle states) are found to profoundly affect the absorption line shapes of both linear and symmetric complexes for sufficiently large coupling