Aggregation Effects on Pigment Coatings: Pigment Red 179 as a Case Study
Résumé
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 S 1 ← S 0 charge transfer transition between two adjacent molecules, in quantitative agreement with the experimental absorption energy of the crystal pigment. ■ INTRODUCTION Nowadays, it is widely accepted that human activities strongly contribute to the urban heat island (UHI) effect, with metropolitan areas significantly warmer than surrounding country areas. 1−3 Urban areas are densely populated, thus contributing to average and peak energy demands especially during summer and straining energy resources mainly based on fossil fuels still today. In EU countries, buildings account for 40% of final energy demand and 32% of CO 2 emission, including residential, industrial, and service sectors. 4 With the stringent need for increasing energy efficiency and reducing greenhouse gas emission, improving the energy performance of Europe's building stock is mandatory. Nevertheless, the EU renovation rate (1.2% in EU) is slower than expected and Kyoto and Paris agreement targets reasonably appear out of reach as also mentioned in the 2010/31/EU Directive, which states that each new building should be made "nearly zero energy" from 2020 onward. An accessible solution to comply with the EU regulation and to mitigate the UHI effect is mimicking the NIR transparent and reflective features of plants and leaves by coating the building surfaces with cool pigments. 5,6 When the sunlight shines onto a material, part of the radiation is absorbed, thus heating its surface. This is caused by 52% of the electromagnetic spectrum of light composed by the near-infrared (NIR) window (i.e., from 700 to 2500 nm) that promotes chemical bond vibration, thus provoking temperature increase. 7 Heat is then transported by conduction into the material, elevating the temperature of the internal side by convection. It is already reported that cool pigments and coatings are able to reduce the heating effect of sunlight with corresponding temperature differences > 10°C, therefore reducing the electricity demand and saving energy required to downgrade the UHI phenomenon. 8,9 Cool surfaces are measured considering the amount of NIR radiation they reflect (solar reflectance) and how rapidly and efficiently they dissipate heat (thermal emittance).
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