Aggregation Effects on Pigment Coatings: Pigment Red 179 as a Case Study

Abstract

Here, we have studied, with a combined experimental and computational approach, the effect of the crystal environment and aggregation on the electronic properties of Pigment Red 179, which affect both its color and optical energy gap. Spectra acquired in the near-infrared and visible range of energies suggest that this molecule is indeed a “cool” dye, which can be employed as a red pigment that provides effective color coverage to different substrates without contributing to their heating during light irradiation. Spectra acquired on different polymer mixtures at different pigment concentrations (i.e., 2.5–10 wt %) suggest that absorption features depend on chromophoric arrangements promoted by the strong intermolecular π–π interactions. Calculations, performed at the time-dependent density functional theory level, allowed to both attribute the nature of the electronic transitions causing the observed spectra involved and understand the effect of the environment. Indeed, the visible spectra of the pigment is dominated by two localized transitions, with negligible charge transfer for both a dye monomer and dimer either in vacuum or acetonitrile solution. Instead, models including the crystal environment of the pigment show the presence of a high-wavelength S1 ← S0 charge transfer transition between two adjacent molecules, in quantitative agreement with the experimental absorption energy of the crystal pigment.

Facile Synthetic Route for Direct Access of Perylenediimide Single Crystals in High Yield through In Situ Crystallization

“…Thus, the ordered molecular arrangement with strong aromatic cycle stacking in the single crystals leads to NIR absorption (Figure ). It has been recently shown that the increasing concentration of PDI dye in a polymer matrix leads to strong intermolecular aromatic stacking that strongly influences on the absorption . In contrast, solid state fluorescence studies showed clear red fluorescence for PDI‐80, PDI‐120 and PDI‐130, however, fluorescence has been completely quenched for PDI‐180 and PDI‐250 (Figure ).…”

“…It has been recently shown that the increasing concentration of PDI dye in a polymer matrix leads to strong intermolecular aromatic stacking that strongly influences on the absorption. [47] In contrast, solid state fluorescence studies showed clear red fluorescence for PDI-80, PDI-120 and PDI-130, however, fluorescence has been completely quenched for PDI-180 and PDI-250 ( Figure 3). The digital images also showed strong quenching of fluorescence for PDI-180 and PDI-250 whereas PDI-80, PDI-120 and PDI-130 showed red fluorescence (Figure 4).…”

Mechanochromic LLDPE Films Doped with NIR Reflective Paliogen Black

“…The model expected to reproduce the absorption behavior in an aggregate phase, referred to as crystal‐embedded and described in the computational details in the crystal embedded calculation section, yields instead a less structured spectrum, with significant absorption in the 300–700 nm range of wavelengths, thus spanning a large portion of the visible range (as expected from a black material). The nature of the first excitation (I* in Figure 1) is also different, with a more pronounced intermolecular charge‐transfer character, a behavior found also in the TD‐DFT study of so‐called Paliogen Red molecule (or Pigment Red 179) [ 39 ] when simulated embedded into a crystal. The D CT [ 45 ] quantum descriptor for charge transfer suggests an hole‐electron distance of ≈3.15 Å, to be compared with an intermolecular distance between P‐black molecules in the crystal of ≈3.4 Å.…”

“…The molecular structure of P-black molecules was relaxed in the ground electronic state (GS) using the Gaussian 16 package, [48] adopting the B3LYP exchange and correlation functional [49,50] in conjunction with 6-31+G(d,p) basis set and Grimme's D3-BJ dispersion correction: [51] this level of theory proved to be suitable in a previous investigation on a similar system (dye Paliogen Red). [39] All relaxed structures are potential energy minima, as all normal modes have a positive frequency. Calculations of electronic excited states were performed with the timedependent density functional theory (TDDFT) approach using the rangeseparated exchange and correlation functional CAM-B3LYP, [52] to better describe charge-transfer transitions and the spectral shape.…”

“…[36] P-black mimics the transparent and NIR reflective features of certain green plants, [37] thanks to the high symmetry of PBI that favors the formation of crystalline supramolecular assemblies promoted by the intermolecular π-π interactions among perylene nuclei. [32,34,[38][39][40][41] Its supramolecular structure preserved within the polymer coatings allowed NIR reflectances close to 50% and consistent cooling effects. [31,42] Being the NIR reflective features strongly depending on the supramolecular packing, polymer drawing may promote the exfoliation of the dispersed PBIs aggregates, thus causing the loss of NIR reflection that in turn substantially reduces the cooling effect of film surfaces.…”