Donor-Acceptor π-Conjugated Enamines: Functional Group-Compatible Synthesis from Amides and Their Photoabsorption and Photoluminescence Properties

High functional group compatibility of iridium-catalyzed synthesis of enamines from amides and 1,1,3,3-tetramethyldisiloxane (TMDS) realized facile access of a series of donor (D)-π-acceptor (A)-conjugated enamines, in which enamine behaves as a donor functional group. The amide precursors containing reducible functional groups, such as halogen, carbonyl, and nitro groups, underwent reaction with TMDS to give the corresponding enamines in high yields. In most cases, chemoselective hydrosilane reduction of the amide group occurred while other reducible groups remained intact. Absorption and emission properties including solvatochromic behavior for the resulting D-π-A-conjugated enamines were determined using UV-visible and fluorescent spectra, which provided an understanding of the donor properties of the CH=CHNPh₂ group and photofunctional properties of the D-π-A conjugated enamines as a fluorescent dye. Maximum absorption wavelength (λ_abs) of p-ZC₆H₄CH=CHNPh₂ was predictable from λ_abs of p-ZC₆H₄NPh₂, which was supported by density functional theory calculations. Some of the D-π-A-conjugated enamines showed fluorescence with moderate fluorescence quantum yields (φ_fl). Of interest are unusually emissive π-conjugated enamines containing a nitro group, which generally behaves as strong quenchers of fluorescence. The additive effect of B(C₆F₅)₃ resulted in significant red shifts of λ_abs and λ_fl. In some cases, high φ_fl was observed in the solution state.