Fluorogenic NIR-probes based on 1,2,4,5-tetrazine substituted BF₂-azadipyrromethenes

A series of 1,2,4,5-tetrazine integrated near infrared (NIR) fluorophores based on the BF₂ azadipyrromethene (NIR-AZA) class has been synthesised and their ability to modulate emission from low to high in response to Diels-Alder cycloaditions has been assessed. Substituents on the tetrazine component of the probe (Cl, OMe, p-NO₂C₆H₄O) were seen to strongly influence quantum yields, fluorescence enhancement factors, and rates of cycloadditions. Cycloadditions between tetrazine-NIR-AZA constructs and a strained alkyne substrate were seen to be highly efficient in organic or aqueous solutions and in gels with high fluorescence enhancements of up to 48-fold observed. Real-time demonstration of the cycloaddition mediated fluorogenic property was achieved by imaging the "turn-on" reaction within a continous flow micro-reactor. Preliminary evidence indicates that excited state quenching involves a photoinduced electron transfer.