Polycyclic aromatic hydrocarbon (PAH) molecules are considered among the most likely candidates to explain the Unidentified Infrared Bands (UIR). Up until now, only room temperature absorption properties of some neutral PAH species have been used to simulate the observed emission spectra of a number of quite different celestial sources and to infer the physical and chemical properties of the interstellar PAH species. This was done under the implicit assumption that optical properties of PAHs do not depend on temperature. To test this hypothesis we have measured, in the temperature range 300-520 K, the infrared transmittance of three different PAH molecules, coronene (C24H12), chrysene (C18H12) and 1-methylcoronene (C25H14), embedded in KBr and CSI matrices. The results clearly show that all the bands, whose intensities are considerably affected by temperature, are commonly assigned to out-of-plane bending vibrations. The only exception is the 7.7 μm band whose assignment, widely discussed in § III, is in our opinion doubtful. Such behavior is interpreted as arising from solid-state effects since the interaction between the out-of-plane bending groups and the surrounding molecular environment should be temperature dependent. Although our results cannot be extrapolated to situations where PAHs are in the gas phase, they will be of valuable help in understanding some open problems connected with the UIR. This is because the idea that PAHs can be present in the interstellar and circumstellar environments in solid phase attached and/or embedded in solid carbonaceous grains is becoming more and more accepted.
