Vol.:(0123456789) 1 3 Photochemical & Photobiological Sciences https://doi.org/10.1007/s43630-021-00117-8 ORIGINAL PAPERS Tryptanthrin derivatives as efcient singlet oxygen sensitizers Daniela Pinheiro 1  · Marta Pineiro 1  · J. Sérgio Seixas de Melo 1 Received: 24 June 2021 / Accepted: 14 October 2021 © The Author(s), under exclusive licence to European Photochemistry Association, European Society for Photobiology 2021 Abstract Halogenated tryptanthrin and aminotryptanthrin were synthesized from indigo or isatin precursors. Dibromo- and tetrabromo- tryptanthrin were obtained from indigo dyes following green chemistry procedures, through microwave-assisted synthesis in mild oxidation conditions. Spectral and photophysical properties of the compounds, including quantitative determination of all the diferent deactivation pathways of S 1 and T 1 , were obtained in diferent solvents and temperatures. The triplet state (T 1 ) has a dominant role on the photophysical properties of these compounds, which is further enhanced by the halogens at the fused-phenyl rings. Substitution with an amino group, 2-aminotryptanthrin (TRYP-NH 2 ), leads a dominance of the radiative decay channel. Moreover, with the sole exception of TRYP-NH 2 , S 1 ~ ~ > T 1 intersystem crossing constitutes the dominant route, with internal conversion playing a minor role in the deactivation of S 1 in all the studied derivatives. In agree- ment with tryptanthrin, emission of the triplet state of tryptanthrin derivatives (with exception of TRYP-NH 2 ), was observed together with an enhancement of the singlet oxygen sensitization quantum yield: from 70% in tryptanthrin to 92% in the iodine derivative. This strongly contrasts with indigo and its derivatives, where singlet oxygen sensitization is found inefcient. Graphic abstract Keywords Tryptanthrin · Tryptanthrin derivatives · Fluorescence · Singlet oxygen sensitization * J. Sérgio Seixas de Melo sseixas@ci.uc.pt 1 Department of Chemistry, CQC, University of Coimbra, Rua Larga, 3004-535 Coimbra, Portugal