Synthesis, characterization and applications of imine derivatives with near-infrared (NİR) absorption and/or emission properties

Abstract

The use of organic dyes in a wide range of applications, including high-contrast bioimaging, solar panels, phototherapy, molecular probe design, and biosensors, has led to a significant increase in interest in these dyes (particulary, λmax = 700–1000 nm) in recent years. Many dye compounds with absorption in the near infrared have been created and developed up to this point. Nevertheless, the majority of these compounds exhibit poor thermal and photostability characteristics. Organoboron complexes, especially BODIPY (4,4-difluoro-4-bora-3a,4a-diaza-s-indacene) derivatives, exhibit remarkable absorption values, strong fluorescence characteristics, higher photostability, and are structurally adjustable, making them extremely attractive structures. Due to these characteristics, BODIPYs have a broad range of potential uses in a variety of research fields, such as fluorescence imaging and sensor applications where they can be used as indicator dyes or laser dyes. To cause the absorption and emission bands of BODIPY compounds to shift to higher wavelengths, a few synthesis techniques and structural modification strategies have been developed. Performing an aza-substitution at the meso-position, wherein the C8-carbon is substituted with an aza-nitrogen atom to form borondifluoride-azadipyrromethene (aza-BODIPY), can also yield higher wavelength values and emissions. Considering that Schiff bases will increase the conjugation of the structure, starting from the highly conjugated tetra-phenyl aza-BODIPY compound, Schiff bases were synthesized as tri- and tetra-dentate ligands utilizing imine and amine nitrogen atoms. Contrary to our expectations, adding imine functionality to the structure did not improve conjugation of the tetra-phenyl substituted aza-BODIPY system. Moreover, instead of increasing absorption and emission values, we obtained decreased results on the contrary. On the other hand, in the UV/Vis spectrum results, it was observed that the highest redshift in both of the designed ligands occurred in the sample containing copper (II) metal solution. Subsequently, the second and third highest interactions were achieved in zinc (II) and cobalt (II) solutions, respectively. Although the interaction time of the mercury (II) metal with the ligands is shorter than that of other metals, the maximum shift in wavelength of the mercury (II) complex had the lowest value among all metals. At the same time, although the maximum wavelength at which the cobalt (II) complex shifts is high, it has been determined that the interaction time is longer than that of other metals. The measurements also allowed us to make comments about tri- and tetra-dentated ligands (synthesized from DETA and TETA, respectively). It was observed that the interactions of the TETA-based ligand with the metal were higher than the interactions of the DETA-based ligand, although by a small difference. However, it has also been noted that the DETA-based ligand reacts faster with metals in terms of interaction time. This situation can be explained by the shorter distances between the complexation centres of the tridentate ligand. On the other hand, the higher difference between the binding constant and the dissociation constant of the short-distance tridentate ligand compared to the tetradentate ligand provides an advantage in binding rate. In light of these results and considering all these factors, it is thought that the tetradentate ligand, which is the TETA product providing a wide complexation area, remains kinetically slow in its interaction with the metal due to the bond length.