Synthesis, characterization and photophysical studies of dual-emissive base-modified fluorescent nucleosides

A straightforward and efficient methodology has been employed for the synthesis of a diverse set of base-modified fluorescent nucleoside conjugates via Cu(I)-catalysed cycloaddition reaction of 5-ethynyl-2',3',5'-tri-O-acetyluridine/3',5'-di-O-acetyl-2'-deoxyuridine with 4-(azidomethyl)-N⁹-(4'-aryl)-9,10-dihydro-2H,8H-chromeno[8,7-e][1,3]oxazin-2-ones in BuOH to afford the desired 1,2,3-triazoles in 92-95% yields. Treatment with NaOMe/MeOH resulted in the final deprotected nucleoside analogues. The synthesized 1,2,3-triazoles demonstrated a significant emission spectrum, featuring two robust bands in the region from 350-500 nm (with excitation at 300 nm) in fluorescence studies. Photophysical investigations revealed a dual-emissive band with high fluorescence intensity, excellent Stokes shift (140-164 nm) and superior quantum yields (0.068-0.350). Furthermore, the electronic structures of the synthesized triazoles have been further verified by DFT studies. Structural characterization of all synthesized compounds was carried out using various analytical techniques, including IR, ¹H-NMR, ¹³C-NMR, ¹H-¹H COSY, ¹H-¹³C HETCOR experiments, and HRMS measurements. The dual-emissive nature of these nucleosides would be a significant contribution to nucleoside chemistry as there are limited literature reports on the same.