Systematic Investigation of indole-based BODIPYs: Synthesis, Dual-State Emission, singlet oxygen formation

Dual-state emission (DSE) dyes that exhibit intense emission in both dilute solution and solid state have obtained significant attention due to their wide applications. Herein, two series of indole-based BODIPYs (IB-BODIPYs) with DSE properties are synthesized through two routes. Those dyes display yellow to red fluorescence emission in dilute solution and red to near-infrared (NIR) fluorescence emission in the solid state. This work mainly focuses on the relationship between the structures and their optical properties in dual states. Methyl group at the meso-position of IB-BODIPY is beneficial for enhancing the solid emission intensity (8 (78.2%), 13 (11.5%), 15 (33.1%)) and shifting to the red region of emission wavelength in non-polar solvents. With methyl group substituted at the meso-position of IB-BODIPY, the mono-bromo substitution and enlargement of π-conjugated system are beneficial for the red-shift of the emission spectra in dual states. Furthermore, the luminous mechanism is illustrated by single crystals, electrochemical date and DFT calculation. Finally, the ability to produce singlet oxygen is discussed. This work provide a convenient method to obtain dual-state emission BODIPYs in the NIR region with singlet oxygen formation properties.