Fluorescence Emission Mechanism of Three N,N-dimethylaminotryptanthrins by Density Functional Theory Calculations

Abstract

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.