π‐Expanded Thioxanthones – Engineering the Triplet Level of Thioxanthone Sensitizers for Lanthanide‐Based Luminescent Probes with Visible Excitation

Abstract

Bright lanthanide based probes for optical bioimaging must rely on the antenna principle, where the lanthanide‐centred excited state is formed by a complex sensitization process. Efficient sensitization of lanthanide‐centred emission occurs via triplet states centred on the sensitizing chromophore. Here, the triplet state of thioxanthone chromophores is modulated by extending the π‐system. Three thioxanthone chromophores‐thioxanthone, benzo[c]thioxanthone, and naphtho[2,3‐c]thioxanthone were synthesised and characterised. The triplet state energies and lifetimes is found to change as expected, and two dyes are found to be suitable sensitizers for europium(iii) luminescence. Reactive derivatives of thioxanthone and benzo[c]thioxanthone were prepared and coupled to a 1,4,7,10‐tetraazacyclododecane‐1,4,7‐triacetic acid (DO3A) lanthanide binding pocket. The photophysics and the performance in optical bioimaging of the resulting europium(iii) complexes were investigated. It is concluded that while the energetics favour efficient sensitization, the solution structure does not. While it was found that the complexes are too lipophilic to be efficient luminescent probes for optical bioimaging, we successfully demonstrated bioimaging using europium(iii) luminescence following 405 nm excitation.

Recent Advances in Luminescence Imaging of Biological Systems Using Lanthanide(III) Luminescent Complexes

“…Shifting the excitation towards longer wavelengths is another strategy to minimize interference from the background emission. However, the shift towards longer wavelengths results in decrease of the triplet level energy and inefficient sensitization of visible emitting Ln III [148]. Two-photon showing migration of the Yb III complex from the stomach (pH 1-3) to the intestine (pH 6-7) [17].…”

“…Shifting the excitation towards longer wavelengths is another strategy to minimize interference from the background emission. However, the shift towards longer wavelengths results in decrease of the triplet level energy and inefficient sensitization of visible emitting Ln III [148]. Two-photon absorption (2PA) [149][150][151][152][153][154] is a nonlinear process where two photons with half the energy required by the one-photon excitation (1PA) are absorbed simultaneously (Figure 14) [155,156].…”

Electronic Energy Levels and Optical Transitions in Samarium(III) Solvates

“…Lanthanide luminescence remains an interesting topic in molecular photophysics, as it embodies the two most fascinating traits of the field: colorful emission and unexplained light matter interactions. , While the red-emitting europium­(III) and the green-emitting terbium­(III) have found use in bioassays, − most applications of lanthanides are limited to the solid state and alloys. , Thus, the mapping of photophysical properties of most trivalent ions of the lanthanide series has been focused on solid materials. ,− …”

Electronic Energy Levels and Optical Transitions in Samarium(III) Solvates

“…Lanthanide luminescence remains an interesting topic in molecular photophysics, as it embodies the two most fascinating traits of the field: colorful emission, and unexplained light matter interactions. [1][2] While the red emitting europium(III), and the green emitting terbium(III) have found use in bioassays, [3][4][5][6][7][8][9] most applications of lanthanides are limited to the solid state and alloys. [10][11] Thus, the mapping of photophysical properties of most trivalent ions of the lanthanide series have been focused on solid materials.…”