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.