Luminescence Properties of Self-Aggregating TbIII-DOTA-Functionalized Calix[4]arenes.

Florian Mayer,Daniel T Schühle,Éva Tóth,Svetlana V Eliseeva,Stéphane Petoud,Sriram Tiruvadi Krishnan,Kristina Djanashvili

doi:10.3389/fchem.2018.00001

Abstract

Self-aggregating calix[4]arenes carrying four DOTA ligands on the upper rim for stable complexation of paramagnetic GdIII-ions have already been proposed as MRI probes. In this work, we investigate the luminescence properties of TbIII-DOTA-calix[4]arene-4OPr containing four propyl-groups and compare them with those of the analog substituted with a phthalimide chromophore (TbIII-DOTA-calix[4]arene-3OPr-OPhth). We show that, given its four aromatic rings, the calix[4]arene core acts as an effective sensitizer of Tb-centered luminescence. Substituents on the lower rim can modulate the aggregation behavior, which in turn determines the luminescence properties of the compounds. In solid state, the quantum yield of the phthalimide derivative is almost three times as high as that of the propyl-functionalized analog demonstrating a beneficial role of the chromophore on Tb-luminescence. In solution, however, the effect of the phthalimide group vanishes, which we attribute to the large distance between the chromophore and the lanthanide, situated on the opposite rims of the calix[4]arene. Both quantum yields and luminescence lifetimes show clear concentration dependence in solution, related to the strong impact of aggregation on the luminescence behavior. We also evidence the variability in the values of the critical micelle concentration depending on the experimental technique. Such luminescent calix[4]arene platforms accommodating stable lanthanide complexes can be considered valuable building blocks for the design of dual MR/optical imaging probes.

Highlights

Calix[4]arenes were initially proposed as artificial enzyme mimics in the late 70s (Gutsche and Muthukrishnan, 1978) and today represent versatile building blocks with potential for application in industrial, technical and biomedical fields, ranging from wastewater treatment (Konczyk et al, 2016) to medical imaging (Schühle et al, 2011; Sreenivasu Mummidivarapu et al, 2015)
The lower rim of the calix[4]arene backbone was modified in accordance to the described procedures for the propylation (7a,b) of the hydroxylic groups (Gutsche and Lin, 1986), as well as condensation of one of the hydroxyls (8a) with N-(3-bromo)propylphthalimide (Lalor et al, 2007)
The hydroxyl groups on the lower rim of calix[4]arene 1 are alkylated to yield either three (7a) or four (7b) O-propyl functionalized calix[4]arenes. In the former case, the remaining OH-group was used for conjugation with propylphthalimide (8a)

Summary

Introduction

Calix[4]arenes were initially proposed as artificial enzyme mimics in the late 70s (Gutsche and Muthukrishnan, 1978) and today represent versatile building blocks with potential for application in industrial, technical and biomedical fields, ranging from wastewater treatment (Konczyk et al, 2016) to medical imaging (Schühle et al, 2011; Sreenivasu Mummidivarapu et al, 2015). Enzyme mimics are thought to benefit from this basket that resembles hydrophobic pockets in enzymes and serves for positioning of the substrate (Blanchard et al, 1998; Baldini et al, 2012). Despite those exciting examples, most often calix[4]arenes are used as platforms to attach the groups of interest in a predetermined spatial arrangement (Modi et al, 2016), which downgrades the core to a mere steric support without further function. Calix[4]arene derivatives intended for medical applications often lack active participation of the core structure itself

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Conclusion