Abstract

Polyacene molecules such as pentacene and tetracene as well as their dimers and oligomers have considerable potential in molecular electronic devices and to improve the efficiency of photovoltaic cells. Recent experimental research has found that bonding monomers via a carbon at the ‘end’ of a polyacene, rather than one along the ‘top’ or ‘bottom’ leads to the second peak in the electronic spectrum to be hugely enlarged and redshifted – enough to change the molecule’s colour to the naked eye. Furthermore, Pople-Parr-Pariser (PPP) theory, a more sophisticated version of Huckel theory commonly used to investigate alkene spectra, predicts this intense transition to be forbidden. Using tools ranging from simple molecular orbital theory to multireference electronic structure calculations and quantum mechanical group theory, we show that this transition is to a highly multiconfigurational excited state whose intense transition dipole moment results from a breakdown of the neglect of differential overlap assumption in PPP Theory. These results illustrate the need for highly multireference electronic structure calculations, elucidate the limits of Huckel Theory and provide widely-applicable design rules for polyacene chromophores.