Synthesis and solvatochromism of a NIR II emissive amphiphilic aza-BODIPY dye and its application as a colorimetric and fluorometric probe in sequential detection of Cu ²⁺ and PO ₄ ³⁻

The amphiphilic aza-BODIPY 1 bearing two <i>N</i>,<i>N</i>-dialkyl-amino groups at the <i>para</i>-positions of the 1,7-phenyl and two amino groups modified by a pair of hydrophilic chains at the <i>para</i>-positions at the 3,5-phenyl of the boron-azadipyrromethene core was synthesized and characterized. UV/Vis/NIR spectroscopic measurements indicated that the S₀-S₁ absorption band 1 reached up to 900 nm while its emission band reached up to 1200 nm. The solvatochromic properties of 1 was investigated by absorption and fluorescence spectroscopy and further analysed by the Lippert-Mataga equation. The results suggested a strong intramolecular charge transfer (ICT) effect of dye 1 stemming from the electron-donating amino groups, which was further confirmed by DFT calculation studies. Such ICT could be disrupted upon addition of Cu²⁺ owing to the metal-ligand interaction between Cu²⁺ and the amino moieties, leading to a hypsochromic shift of the absorption band concomitant with a significant decrease in the fluorescence intensity of 1. Moreover, the spectroscopic properties of 1 were recovered upon subsequent addition of PO₄ ^3- in [1 + Cu²⁺] solution. Based on these observations, the dye 1 was applied successfully as a NIR colorimetric and fluorometric "turn-off" probe for selective detection of Cu²⁺ (LOD = 2.1 μM). Subsequent fluorometric "turn-on" detection of PO₄ ^3- by [1 + Cu²⁺] (LOD = 3.0 μM) was accomplished in further studies.