Structure and dynamics of perylene bisimide pigments for "cool" organic coatings by solid-state NMR: A combined experimental and DFT study

In this work, a combination of high-resolution solid-state nuclear magnetic resonance (SSNMR) experiments and density functional theory calculations has been exploited for obtaining a detailed characterization of the structural and dynamic properties of the perylene bisimide derivative (PBI) Paliogen Black L0086 (P-black). This "cool"organic pigment is characterized by unusual transparent, reflective, and cooling features in the near-infrared (NIR) spectrum. A full assignment of the 13C SSNMR spectrum was achieved, highlighting correlations between 13C isotropic chemical shifts and specific supramolecular features. The π-flip motion of the phenyl rings of the methoxybenzyl side groups bonded to the imide nitrogen atoms was characterized in the 310-386 K temperature range, determining activation energy and correlation times. This motion showed a strong dependence on the supramolecular order of P-black and the length of the alkyl linker between the side group and the perylene bisimide core. The results obtained in this work open the way to investigations on similar PBI pigments, aiming at achieving a collection of molecular, supramolecular, and NIR data that could allow the identification of the main factors determining the NIR performance.