Iodide as a Triplet State Promoter and Quencher – Mechanisms and Possible Implications

* corresponding author. Phone: +46855378030 Email: [email protected] In this work, Fluorescence Correlation Spectroscopy (FCS) was used to investigate the effects of potassium iodide (KI) on the electronic state population kinetics of a range of organic dyes in the visible wavelength range. Apart from a heavy atom effect promoting intersystem crossing to the triplet states in all dyes, KI was also found to enhance the triplet state decay by a charge-coupled deactivation. This deactivation was only found for dyes with excitation maximum in the blue range, not for those with excitation maxima at wavelengths in the green range or longer. Consequently, under excitation conditions sufficient for triplet state formation, KI can promote the triplet state build-up of one dye and reduce it for another, red-shifted dye. The anticorrelated, spectrally separable responses of two dyes to the presence of one and the same agent are likely to provide a useful readout for biomolecular interaction and micro-environmental monitoring studies. In contrast to the typical notion of KI as a fluorescence quencher, the FCS measurements also revealed that when added in micromolar concentrations KI can act as an anti-oxidant, promoting the recovery of photo-oxidized fluorophores. However, in millimolar concentrations KI also reduces intact, fluorescently viable fluorophores to a considerable extent. In aqueous solutions, an optimal concentration of KI of approximately 5 mM can be defined at which the fluorescence signal is maximized. This concentration is not high enough to allow full triplet state quenching. Therefore, as a fluorescence enhancement agent, it is primarily the anti-oxidative properties of KI that play a role.