A "Multi-Heavy-Atom" Approach toward Biphotonic Photosensitizers with Improved Singlet-Oxygen Generation Properties

Abstract

Two trispicolinate 1,4,7-triazacyclonane (TACN)-based ligands bearing three picolinate biphotonic antennae were synthetized and their Yb³⁺ and Gd³⁺ complexes isolated. One series differs from the other by the absence (L¹ )/presence (L² ) of bromine atoms on the antenna backbone, offering respectively improved optical and singlet-oxygen generation properties. Photophysical properties of the ligands, complexes and micellar Pluronic suspensions were investigated. Complexes exhibit high two-photon absorption cross-section combined either with NIR emission (Yb) or excellent ¹ O₂ generation (Gd). The very large intersystem crossing efficiency induced by the combination of bromine atom and heavy rare-earth element was corroborated with theoretical calculations. The ¹ O₂ generation properties of L² Gd micellar suspension under two-photon activation leads to tumour cell death, suggesting the potential of such structures for theranostic applications.

Keywords: gadolinium; intersystem crossing; lanthanides; photodynamic therapy; two-photon microscopy.