# Comparison of Near Infrared Spectra of Three Lichen Substances and Several Common Synthetic Organic Sunscreens

## TL;DR

This study compares the near-infrared absorption of lichen substances and synthetic sunscreens, finding that less-substituted compounds offer better protection.

## Contribution

The paper provides the first NIR spectra of common organic sunscreens and lichen compounds, offering insights into their photoprotective potential.

## Key findings

- Less-substituted aromatic compounds like methyl orsellinate show better NIR absorption above 2100 nm.
- Compounds with hydroxyl and carbonyl groups may offer effective NIR photoprotection.
- NIR spectroscopy can be used for sunscreen quality control due to high concentrations in products.

## Abstract

Near‐infrared (NIR) and UV radiation have been reported to alter skin physiology, causing oxidative stress, mitochondrial genetic damage, and potentially leading to cancer and photoageing. NIR screening is recommended, but only inorganic sunscreens such as zinc and titanium oxide are known to absorb, reflect, and scatter NIR. Additionally, NIR spectra of common organic sunscreens are neither available in the literature nor in technical datasheets.

This study characterizes NIR absorption of several commercial sunscreens and alternative lichen compounds.

To measure NIR absorbance and reflectance spectra, solid organic sunscreens were impregnated with minimal chloroform onto pure dry potassium bromide (KBr) crystals and analyzed using a Varian Cary 5000 spectrophotometer. Liquid compounds were tested directly in standard 1 cm quartz cells. Reflectance spectra were processed using the Kubelka‐Monk equation.

Sunscreens and lichen compounds effectively absorb radiation beyond 1500 nm, especially above 2100 nm, with less‐substituted aromatic compounds such as sunscreens and methyl orsellinate performing better than more substituted ones like pseudocypherellin A and methyl 2,4‐dihydroxy‐3,5,6‐trimethylbenzoate. Additionally, less‐substituted compounds exhibited minor absorption bands between 1000 and 1500 nm.

Compounds with hydroxyl, carbonyl groups, medium‐length aliphatic chains, and low degree of aromatic substitution may provide NIR photoprotection. Owing to their antioxidant properties, lichen compounds are better alternatives. NIR spectroscopy is also suitable for quantification and quality control of sunscreens due to their high concentrations in finished products. However, there is still a need to develop or discover new organic sunscreens with strong NIR absorption.