  Context. In the framework of the interstellar polycyclic aromatic
hydrocarbons (PAHs) hypothesis, far-IR skeletal bands are expected to be a
fingerprint of single species in this class. Aims. We address the question of
detectability of low energy PAH vibrational bands, with respect to spectral
contrast and intensity ratio with ``classical'' Aromatic Infrared Bands (AIBs).
Methods. We extend our extablished Monte-Carlo model of the photophysics of
specific PAHs in astronomical environments, to include rotational and
anharmonic band structure. The required molecular parameters were calculated in
the framework of the Density Functional Theory. Results. We calculate the
detailed spectral profiles of three low-energy vibrational bands of neutral
naphthalene, and four low-energy vibrational bands of neutral anthracene. They
are used to establish detectability constraints based on intensity ratios with
``classical'' AIBs. A general procedure is suggested to select promising
diagnostics, and tested on available Infrared Space Observatory data for the
Red Rectangle nebula. Conclusions. The search for single, specific PAHs in the
far-IR is a challenging, but promising task, especially in view of the
forthcoming launch of the Herschel Space Observatory.
