  We report on the absorption spectra of the polycyclic aromatic hydrocarbon
(PAH) molecules anthracene, phenanthrene, and pyrene carrying either an ethynyl
(-C2H) or a butadiynyl (-C4H) group. Measurements were carried out in the mid
infrared at room temperature on grains embedded in CsI pellets and in the near
ultraviolet at cryogenic temperature on molecules isolated in Ne matrices. The
infrared measurements show that interstellar populations of
polyynyl-substituted PAHs would give rise to collective features in the same
way non-substituted PAHs give rise to the aromatic infrared bands. The main
features characteristic of the substituted molecules correspond to the
acetylenic CH stretching mode near 3.05 mum and to the almost isoenergetic
acetylenic CCH in- and out-of-plane bending modes near 15.9 mum.
Sub-populations defined by the length of the polyynyl side group cause
collective features which correspond to the various acetylenic CC stretching
modes. The ultraviolet spectra reveal that the addition of an ethynyl group to
a non-substituted PAH molecule results in all its electronic transitions being
redshifted. Due to fast internal energy conversion, the bands at shorter
wavelengths are significantly broadened. Those at longer wavelengths are only
barely affected in this respect. As a consequence, their relative peak
absorption increases. The substitution with the longer butadiynyl chain causes
the same effects with a larger magnitude, resulting in the spectra to show a
prominent if not dominating pi-pi* transition at long wavelength. After
discussing the relevance of polyynyl-substituted PAHs to astrophysics, we
conclude that this class of highly conjugated, unsaturated molecules are valid
candidates for the carriers of the diffuse interstellar bands.
