Syntheses of 3-(N,N-dimethylamino)tryptanthrin (T3NMe2) and 9-(N,N-dimethylamino) tryptanthrin (T9NMe2) were performed, and their absorption and fluorescence properties were evaluated. T3NMe2 and T9NMe2 were both expected to undergo intramolecular charge transfer (ICT) between their conjugated electron-donating amino groups and electron-withdrawing carbonyl groups as did the previously reported 2-(N,N-dimethylamino) tryptanthrin (T2NMe2). However, the characteristic fluorescence solvatochromism resulting from ICT was not observed in T3NMe2. Density functional theory (DFT) calculations indicated that the highest occupied molecular orbitals (HOMOs) of both T2NMe2 and T9NMe2 were localized on the amino group side, while the lowest unoccupied molecular orbitals (LUMOs) were localized on the side of the conjugated carbonyl group. However, the HOMO of T3NMe2 was localized on the amino group side, while its LUMO was localized on the unconjugated carbonyl group side. It was thus concluded that only T3NMe2 exhibited no ICT behavior.

2-Hydroxytryptanthrin analog (2-(2-hydroxy-12-oxoindolo[2,1-b]quinazolin-6(12H)ylidene)malononitrile, DCM-T2OH) was synthesized as a near-infrared (NIR) dye for fluorescent imaging. In high-polarity aprotic solvents (acetonitrile, N,N-dimethylformamide, and dimethyl sulfoxide), protic solvents (methanol and ethanol), and an aqueous solution containing 20% acetonitrile, both the absorption and fluorescence bands of DCM-T2OH were observed at the NIR region (650-900 nm), which is referred to as the "optical window" of cells and tissues.