ISSN print edition: 0366-6352 ISSN electronic edition: 1336-9075 Registr. No.: MK SR 9/7 Published monthly

Melatonin photoreactivity: phosphorescence formation and quenching processes

Kristján Einar Guðmundsson, Guðrún Marteinsdóttir, Kristberg Kristbergsson, and Ágúst Kvaran

Faculty of Food Science and Nutrition, University of Iceland, Reykjavík, Iceland

E-mail: keg9@hi.is

Received: 14 February 2022  Accepted: 9 April 2022

Abstract:

Studies of melatonin photoreactivity in water solutions: An effect of an external heavy atom I⁻ on UV/Vis absorption, fluorescence and phosphorescence spectra is explored. The data allowed determination of relevant energetics for the system.The heavy atom effect (HAE) of I⁻ on melatonin is clearly found to induce an intersystem crossing from the lowest energy singlet state to the lowest energy triplet state (T₁) by a state mixing. Lifetime for the first excited triplet states of melatonin in association with I⁻ and quenching rates for halomethanes (CH₂X₂, CHX₃, CY₄, X = Cl, Br, Y = Cl) are determined from Time-Correlated Single-Photon Counting decay curves for the phosphorescence. Dramatic alterations in quenching rate constants with quenchers as CH₂X₂ < CHX₃ < CX₄ and Cl < Br are attributed to energy transfer from an I^−…Me*(T₁) complex to low-lying electronic states of the halomethanes followed by dissociation to form R and X fragments. Relevance of the melatonin photoreactivity to photosensitizer properties in organic media is discussed.

Graphical abstract

Keywords: Phosphorescence; Quenching; Melatonin; Intersystem crossing; Photosensitizer; TCSPC

Full paper is available at www.springerlink.com.

DOI: 10.1007/s11696-022-02222-z

Chemical Papers 76 (8) 5253–5265 (2022)