Direct one-pot access to fluorescent benzimidazole-purine nucleosides: selective metal cation/pH sensing and...

A sustainable one-pot sequential strategy has been developed for the synthesis of novel unprotected benzimidazole-purine nucleosides using K₂S₂O₈ as a green oxidant. This atom- and step-economical protocol proceeds under mild conditions and exhibits broad functional group tolerance. Among the synthesized derivatives, the pyridine-tethered nucleoside 5h displayed notable fluorescence properties with quantum yields of Φ _F = 0.6-17.6% and larger stokes shifts (Δv̄ = 8278-12554 cm^-1) across solvents of varying polarity. Importantly, probe 5h functions as a highly sensitive and selective "Turn-Off" fluorescent chemosensor for Cu²⁺ and Fe³⁺ ions, displaying nanomolar limits of detection 5.49 nM and 3.58 nM respectively, along with distinct pH-responsive behaviour. The pK _a values of the conjugate acids of nucleoside 5h were found to be 2.50 and 4.51. Job's plot analysis confirmed a 1 : 1 probe 5h to metal binding stoichiometry, which was further supported by HRMS studies, and frontier molecular orbital (FMO) calculations provided a plausible selective chemosensing mechanism through a strong CHEQ (Chelation-Enhanced Quenching) effect. In addition, the pyrene-tethered nucleoside 5l exhibited dual emission behaviour (λ _{em (Solution)} = 480 nm and λ _{em (Solid)} = 510 nm), attributed to its rigid structural framework. The practical applicability of these fluorescent nucleosides was demonstrated through the accurate detection of Cu²⁺/Fe³⁺ ions in real-world water samples and their utilization as encryption ink for forensic science and data security. This work establishes a versatile, environmentally benign, and pioneering approach to the design of novel functional modified nucleosides, with promising applications in fluorescence chemosensing and dual-state emissive materials.