Recognition of steric factor in external association of xanthenocrown-5 and bis-napthalenocrown-6 with bis(benzimidazolium)propane borontetrafluoride

• Strong Charge Transfer band appeared in UV region due to H-bonding of crown-axel adducts.
• Least crowded axel forms most stable adducts.
• Sterically hindered crown forms only one H-bond with axel.
• No. of H-bonding determine the stability of adducts.
• Both isosbestic and isoemissive are formed.

This is the first report of charge transfer band for crown-axel H-bonding interaction in acetonitrile medium. Monte Carlo simulation established external association of small cavity crown with BBIM-propane dication axels. Resonance energy, binding constant and ratiometric detection of association of xanthenocrown-5 (1) and bis-napthalenocrown-6 (2) with bis-(benzimidazolium)propane borontetrafluoride (3a–3d) in acetonitrile determined the effect of steric factor towards association.

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