Interpreting the near-infrared reflectance of a series of perylene pigments

Near-infrared (NIR) reflecting pigments offer the potential to incorporate thermal insulating qualities into paint coatings. Experimental reflectance measurements are presented of a series of perylene based pigments, revealing significant variations in the NIR region as a function of derivatisation and substitution. Theoretical methods are applied to account for the observed trends. Derivatisation of perylene leads to an extension of the π-aromatic system, resulting in a bathochromic shift of UV/Vis absorption maxima towards the NIR region. However, this is insufficient to account for the observed variations in NIR reflectance. In comparison, overtones and combination modes of fundamental IR vibrations appear to be more important in explaining the observed variations in NIR reflectance/absorbance. Aromatic groups lead to a small number of intense NIR bands, while alkyl groups generally lead to a greater number of less intense NIR bands. However, neighbouring group effects and/or a molecular dipole moment can substantially alter the intensity of these bands.