A Near Infrared Emission Feature from Visible Fluorescence Tails and its Correlation with Singlet Oxygen

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 photonics. Here, a persistent NIR tail and a feature at 923 nm 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. The emission is present under all photochemical conditions (photosensitizers, solvents, excitation sources and optical setups) but it is not observed for chemically generated singlet oxygen. Its intensity correlates with photosensitizer concentration (1 µM – 1 mM), excitation intensity, and the visible fluorescence intensity, but does not require the presence of oxygen. A simulation shows that this feature arises from the NIR‐tail of (visible) photosensitizer emission and the non‐linear increase of detector sensitivities in the range between 900– 950 nm. At the same time, this signal correlates with photochemically generated singlet oxygen concentration. Thus, this NIR signal contains valuable information, but careful interpretation is required.