Tryptanthrin derivatives as efficient singlet oxygen sensitizers

Halogenated tryptanthrin and aminotryptanthrin were synthesized from indigo or isatin precursors. Dibromo- and tetrabromotryptanthrin 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-1 and T-1, were obtained in different 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-NH2), leads a dominance of the radiative decay channel. Moreover, with the sole exception of TRYP-NH2, S-1 similar to similar to > 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 agreement with tryptanthrin, emission of the triplet state of tryptanthrin derivatives (with exception of TRYP-NH2), 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. [GRAPHICS] .

Automated summary

Halogenated tryptanthrin and aminotryptanthrin were synthesized from indigo or isatin precursors, with spectral and photophysical properties including the dominant role of the triplet state (T-1) and the enhancement by halogens at the fused-phenyl rings. Substitution with an amino group led to a dominance of the radiative decay channel. The emission of the triplet state of tryptanthrin derivatives was observed, with an enhancement of the singlet oxygen sensitization quantum yield, showing a contrast with indigo and its derivatives in terms of singlet oxygen sensitization efficiency.