  Recent synthetic studies on the organic molecules tetracene and pentacene
have found certain dimers and oligomers to exhibit an intense absorption in the
visible region of the spectrum which is not present in the monomer or many
previously-studied dimers. In this article we combine experimental synthesis
with electronic structure theory and spectral computation to show that this
absorption arises from an otherwise dark charge-transfer excitation 'borrowing
intensity' from an intense UV excitation. Further, by characterizing the role
of relevant monomer molecular orbitals, we arrive at a design principle that
allows us to predict the presence or absence of an additional absorption based
on the bonding geometry of the dimer. We find this rule correctly explains the
spectra of a wide range of acene derivatives and solves an unexplained
structure-spectrum phenomenon first observed seventy years ago. These results
pave the way for the design of highly absorbent chromophores with applications
ranging from photovoltaics to liquid crystals.
