A near infrared emission feature from visible fluorescence tails and its correlation with singlet oxygen

TL;DR

This study identifies a persistent near-infrared emission tail at 923 nm linked to singlet oxygen generation, revealing its origin from photosensitizer fluorescence and emphasizing its importance in fluorescence imaging and photonics.

Contribution

It uncovers a new NIR emission feature associated with singlet oxygen, clarifies its origin from photosensitizer tails, and highlights its potential as an indicator in photochemical processes.

Findings

NIR tail at 923 nm correlates with singlet oxygen production

Emission originates from photosensitizer fluorescence tails

The feature's intensity depends on photosensitizer concentration and excitation

Abstract

The near infrared (NIR, >800 nm) or short wave infrared (SWIR) range of the electromagnetic spectrum overlays with the tissue transparency window and the telecommunication window, which makes it important for fluorescence-based imaging and photonics in general. Additionally, there are NIR absorption features related to vibrational overtone or combination modes and autofluorescence in photosynthetic organisms such as plants. Here, we report a persistent NIR tail and feature at 923 nm that is observed during the photochemical generation of singlet oxygen using various photosensitizers. This feature initially appears to originate from singlet oxygen as its energy fits to known higher order optical and vibrational transitions, and it occurs simultaneously with the characteristic singlet oxygen phosphorescence at 1275 nm. To determine its origin, a systematic investigation is carried out using different photosensitizers (Rose Bengal, a ruthenium complex, Methylene Blue, chlorophyll), solvents, excitation sources and optical setups. The emission is present under all photochemical conditions, but it is not observed for chemically generated singlet oxygen. Its intensity correlates with photosensitizer concentration (1 uM - 1 mM), excitation intensity and the visible fluorescence intensity, but does not require the presence of oxygen as the 1275 nm phosphorescence of singlet oxygen. A simulation shows that this feature arises from the NIR-tail of (visible) photosensitizer emission and the non-linear increase of detector sensitivity in the range between 900 nm - 950 nm. At the same time this feature and the whole NIR tail correlates with photochemically generated singlet oxygen concentration under certain conditions. Thus, careful interpretation of NIR emission is crucial but it also contains valuable information.