Axially- and Meso-Substituted Aza-Crown-Ether-Incorporated B^III Subporphyrins: Control of Electron-Donating Ability by Metal Ion Chelation

A series of subporphyrin-based fluorescent probes bearing 1-aza-15-crown-5 or 1-aza-18-crown-6 moieties at the meso or axial positions were prepared by Pd-catalyzed Buchwald-Hartwig amination reaction of the corresponding bromosubporphyrins. Both types of aza-crown-ether-incorporated subporphyrins were fluorescent in solution and exhibited cation-dependent absorption and fluorescence changes. In fluorescence titration experiments, opposite responses were observed for the two types of subporphyrins. Namely, fluorescence quenching occurred for the meso-substituted subporphyrins while fluorescence enhancement was observed for the axially-substituted subporphyrins. These results demonstrate the advantage of subporphyrins being viable to serve as turn-off-type or turn-on-type fluorescence probes, depending upon substitution pattern.