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 ^tBuOH 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.