Acetylene Derivatives of Cationic Diazaoxatriangulenes and Diaza [4]Helicenes - Access to Red Emitters and Planar Chiral Stereochemical Traits

Cationic triangulenes, and related helicenes, constitute a rich class of dyes and fluorophores, usually absorbing and emitting light at low energy, in the orange to red domains. Recently, to broaden the scope of applications, regioselective late-stage functionalizations on these core moieties have been developed. For instance, with the introduction of electron-donating groups (EDGs), important bathochromic shifts are observed pushing absorptions towards or in the near-infrared (NIR) spectral domain while emissive properties disappear essentially completely. Herein, to upset this drawback, acetylene derivatives of cationic diazaoxa triangulenes (DAOTA) and [4]helicenes are prepared (16 examples). Contrary to other EDG-functionalized derivatives, C equivalent to C- functionalized products remain broadly fluorescent, with red-shifted absorptions (Delta lambda(abs) up to 25 nm) and emissions (Delta lambda(em) up to 73 nm, phi(PL) up to 51 %). Quite interestingly, a general dynamic stereoisomerism phenomenon is evidenced for the compounds derived from achiral DAOTA cores. At low temperature in H-1 NMR spectroscopy (218 K), N-CH2 protons become diastereotopic with chemical shifts differences (Delta delta) as high as +1.64 ppm. The signal coalescence occurs around 273 K with a barrier of similar to 12 kcal mol(-1). This phenomenon is due to planar chiral conformations (S-p and R-p configurations), induced by the geometry of the alkyl (n-propyl) side-chains next to the acetylenic substituents. Ion pairing studies with Delta-TRISPHAT anion not only confirm the occurrence of the chiral conformations but evidence a moderate but definite asymmetric induction from the chiral anion onto the cations. Finally, DFT calculations offer a valuable insight on the geometries, the corresponding stereodynamics and also on the very large difference in NMR for some of the diastereotopic protons.