  Employing high accuracy fixed node diffusion Monte Carlo (DMC) method we
calculated the lowest triplet collective excitation (spin gap), as well as an
upper bound for the singlet excitations (charge gap) in a series of charge
neutral planar non-ladder aromatic compounds. Both excitation energies lie
below the continuum of particle-hole excitation energies obtained from
Hartree-Fock orbitals. Hence they can be interpreted as genuine bound states in
the particle-hole channel. Assuming a resonating valence bond (RVB) ground
state which has been recently suggested for $sp^2$ bonded systems [ M. Marchi,
{\em et. al.}, Phys. Rev. Lett. {\bf 107}, 086807 (2011)], offers a unified
description of both excited states as two-spinon and doublon-holon bound
states. We corroborate our interpretation, by Exact diagonalization study of a
minimal model on finite honeycomb clusters.
