Abstract

Despite considerable advances in constructing molecules which absorb at particular frequencies (colours) there is a need for chromophores with greater intensity of absorption (oscillator strength) for use in applications such as photovoltaics. Recent synthetic studies on the organic molecules tetracene and pentacene have found some dimers and oligomers to have dramatically enhanced absorption in the visible region of the spectrum, whereas other dimers have spectra identical to the monomer. By combining electronic structure theory with intensity borrowing perturbation theory, here we show how this extra absorption arises from an otherwise dark charge-transfer state borrowing intensity from an intense UV absorption, and by examining the relevant molecular orbitals we explain its dependence on bonding geometry. With this reasoning we construct a design rule for highly absorbent acenes and find this solves a structure-spectrum phenomenon first observed seventy years ago.