Tryptanthrin derivatives as efficient singlet oxygen sensitizers.

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 different deactivation pathways of S and T, were obtained in different solvents and temperatures. The triplet state (T) 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), leads a dominance of the radiative decay channel. Moreover, with the sole exception of TRYP-NH, S ~  ~  > T intersystem crossing constitutes the dominant route, with internal conversion playing a minor role in the deactivation of S in all the studied derivatives. In agreement with tryptanthrin, emission of the triplet state of tryptanthrin derivatives (with exception of TRYP-NH), 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 inefficient.