  1,3-bis(phenylethynyl)benzene is the primary chromophore of light-harvesting
polyphenylene ethynylene (PPE) dendrimers. It is experimentally known to share
the same absorption spectrum as its pair of diphenylacetylene (aka. tolane)
meta-substituted branches, yet exhibits an unusual Stokes shift of about 2000
cm$^{-1}$ with respect to its band origin (corresponding to the loss of one
vibrational quantum within the antisymmetric acetylenic stretching) in its
emission spectrum. We suggest in the present work the unusual but plausible
involvement of molecular symmetry selection rules in a situation where the
Born-Oppenheimer approximation is far to be valid. Our hypothesis is comforted
with quantum dynamics (MCTDH) simulations of absorption and emission UV-visible
spectra based on quantum chemistry (TD-DFT) data and a diabatic vibronic
coupling Hamiltonian model.
