Polymorph of N,N′-di-n-butylperylene-3,4:9,10-bis(dicarboximide) and their electronic structure

Perylene pigments are known to exhibit a variety of shades in the solid state from vivid red, via maroon to black, although their solution spectra are quite similar. A second phase of the title compound (phase II) has newly been found when the single crystals were grown from the vapor phase. Phase II bears a black color while the previous phase (phase I) exhibits red-maroon. For this reason, the electronic structure of both phases has been investigated on the basis of the crystal structure as well as intermolecular interactions. In both phases, the absorption spectrum is composed of two bands: one is due to individual molecules (i.e. around 450–550 nm) and the second band arises from excitonic interactions between transition dipoles (i.e. around 600 nm in phase I and 625 nm in phase II). The intermolecular interactions along the stacking axis are primarily responsible for the appearance of the second band in both phases. Additionally, the interaction on the molecular plane is found to be slightly hypsochromic in phase I while bathochromic in phase II. This induces a difference in absorption maximum of the second band, leading to two different colors (red-maroon and black) in phases I and II.