π-Expanded 1,3-diketones – synthesis, optical properties and application in two-photon polymerization

Abstract

Four π-expanded α,β-unsaturated 1,3-diketones have been prepared via attaching strong electron-donating and electron-withdrawing groups at positions 9 and 10 of the anthracene scaffold. The strategic incorporation of (C₁₂H₂₅)₂N groups at the periphery of these D–π–A molecules resulted in dyes with excellent solubility in most organic solvents. These non-planar diketones possess very broad absorption of light and negligible fluorescence. The two-photon absorption cross-section was measured via Z-scan as well as by two-photon excited fluorescence. The results clearly confirm that, in the case of compounds possessing such small fluorescence quantum yields, the Z-scan was a more reliable approach. Depending on the chemical structure, these compounds exhibited two-photon absorption cross-sections (σ₂) up to 2500 GM at 725 nm. For the first time an α,β-unsaturated ketone derived from a proton sponge has been synthesized and was shown to possess optical features distinct from its simpler analogs, such as weak emission. All studied ketones possessed two-photon absorption cross-sections ∼200 GM at wavelength of two-photon polymerization (i.e. 800 nm) and broad fabrication windows have been achieved using all these dyes as photoinitiators.