Photophysics and photochemistry of carminic acid and related natural pigments

• Published in: Physical chemistry chemical physics : PCCP Vol. 22; no. 17; pp. 9534 - 9542
• Main Authors: Zucchelli, Margherita, Villarruel, Fernando D, David-Gara, Pedro, Costante, Mariana R, Tascon, Marcos, Marte, Fernando, García Einschlag, Fernando S, Cabrerizo, Franco M
• Format: Journal Article
• Language: English
• Published: England Royal Society of Chemistry 06.05.2020
• Subjects: Absorption spectra; Aqueous solutions; Chromophores; Cosmetics; Cultural heritage; Cultural resources; Data analysis; Deactivation; Emission analysis; Emission spectra; Fluorescence; Food processing industry; Hydrogen peroxide; Irradiation; Partial pressure; Photochemistry; Photodegradation; Photoexcitation; Pigments; Singlet oxygen; Sodium azides; Spectrum analysis
• ISSN: 1463-9076; 1463-9084; 1463-9084
• DOI: 10.1039/d0cp01312a

Carminic acid (CA) and other related compounds have been widely used as dyes in cultural heritage, cosmetics and the food industry. Therefore, the study of their properties upon photoexcitation is particularly important. In this work, the photophysical and photochemical properties of CA, carminic lake and other related pigments in aqueous solutions are revisited. Novel quantitative information regarding the fate of the photoexcited states is provided including the efficiency of reactive oxygen species (ROS) photosensitized production ( i.e. , singlet oxygen and hydrogen peroxide) as well as the efficiency of nonradiative deactivation pathways. Laser-induced optoacoustic spectroscopy (LIOAS) data revealed that for all the investigated compounds, almost all the absorbed energy is released as prompt heat to the media. This is in agreement with the fact that other deactivation pathways, including fluorescence ( Φ F ∼ 10 −3 -10 −5 ), photochemical degradation ( Φ R ∼ 10 −4 ) and/or photosensitized ROS formation ( Φ H 2 O 2 < 10 −5 and Φ Δ ∼ 0), are negligible or null. In addition, a comprehensive investigation of the photodegradation of CA and lake is herein reported. The influence of different experimental parameters such as irradiation wavelength and oxygen partial pressure was evaluated. UV-vis absorption and fluorescence emission spectroscopy in combination with chemometric data analysis were used to elucidate the relevant aspects of the photodegradation mechanism involved and the spectroscopic features of the photoproducts generated. In aqueous media, CA follows an O 2 -dependent photochemical degradation when subject to elapsed photoexcitation in the UVB, UVA and visible regions. The photoproduct profile depends on the excitation wavelength giving rise to quite distinctive spectroscopic profiles. With respect to lake, our data suggest that upon photoexcitation, this pigment releases a CA-like chromophore that follows a similar fate to CA. Carminic acid (CA) and other related compounds have been widely used as dyes in cultural heritage, cosmetics and the food industry.