Spreading Electron Density Thin: Increasing the Chromophore Size in Polyaromatic Wires Decreases Interchromophoric Electronic Coupling

The development of novel polychromophoric materials using extended polycyclic aromatic hydrocarbons as a single large chromophore holds promise for long-range charge-transfer applications in photovoltaic devices and molecular electronics. However, it is not well-understood how the interchromophoric electronic coupling varies with the chromophore size in linearly connected molecular wires. Here, we show with the aid of electrochemistry, electronic spectroscopy, density functional theory calculations, and theoretical modeling that as the number of aromatic moieties in a single chromophore increases, the interchromophoric electronic coupling decreases and may reach negligible values if the chromophore is sufficiently large. The origin of this initially surprising result becomes clear when one considers this problem with the aid of Hückel molecular orbital theory, as at the polymeric limit energies of the molecular orbitals cluster to form bands and thus the energy spacing between orbitals, and thereby the electronic coupling must decrease with the chromophore expansion.