Luminescent Perhalofluoro Trityl Radicals

Abstract

In this proof-of-concept study, we show that polyfluorinated trityl radicals with the, to this date, highest fluorination grade can be accessed in quantitative yields in a straightforward manner starting from the perfluorinated trityl cation. The trityl skeleton is functionalized with trimethylsilyl halides to yield perhalofluoro trityl cations, which are subsequently reduced using commercial zinc powder. In this way, we prepare three perhalofluoro trityl radicals and analyze the impact of the fluorine ligands on their electro-optical properties, revealing some interesting trends. In comparison to literature-known polychlorinated trityl radicals, the new polyfluorinated derivatives exhibit substantially higher fluorescence quantum yields, longer luminescence lifetimes, and an expanded emission range that extends into the yellow spectral region. They further display enhanced photostability under light irradiation. In radical-stained polystyrene nanoparticles, an additional broad emission band in the red-NIR wavelength region is observed, which is attributed to excimer formation. Finally, the stability of the new radicals is investigated under ambient conditions, showing the slow conversion with atmospheric oxygen yielding the respective peroxides, which are characterized by single-crystal X-ray diffraction. All in all, our study extends the present scope of luminescent trityl radicals, as the functionalization of the perfluorinated cationic precursor unlocks the path toward a vast variety of polyfluorinated trityl radicals.

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Top: Gomberg’s radical, its “dimer” and per- and polychlorinated trityl radicals by Ballester. − , Bottom: The only perhalofluoro trityl radical currently known and the radicals presented in this work.

1. Synthesis of 3X12F^• via Reduction of the Cationic Species p-3X12F⁺ Using Zinc Powder (X = Cl, Br)

2. Synthesis of 9Cl6F⁺ via Chlorodefluorination of the Perfluorinated Analogue 15F⁺ Using an Excess of TMSCl Followed by Reduction Using Zinc Powder Leading to 9Cl6F^•

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Molecular structures of all three perhalofluoro trityl radicals in the solid state. Thermal ellipsoids are set at 50% probability. (a) 3Cl12F^• . Selected bond lengths (pm) and angles [deg]: C1–C14 146.7(6), C1–C2 144.5(4), C1–C8 146.3(5), C14–C1–C8 118.6(3), C2–C1–C8 121.9(3), C14–C1–C2 119.5(3) (b) 3Br12F^• . Selected bond lengths [pm] and angles [deg]: C1–C3 145.6(3), C1–C4 145.9(6), C3–C1–C4 119.0(2), C3′–C1-C4 119.0(2), C3–C1–C3′ 122.0(4) (c) 9Cl6F^• . Selected bond lengths [pm] and angles [deg]: C3–C1 145.9(1), C1–C4 146.6, C3–C1–C4 120.3(6), and C3–C1–C3′ 119.4(4). (d) Experimental dihedral angles φ of 3X12F^• and 9Cl6F^• .

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(a) Isotropic X-band EPR spectra of 3X12F^• and 9Cl6F^• in toluene at 298 K. (b) Calculated spin density of 3Cl12F^• and 9Cl6F^• at the B3LYP-D3­(BJ)/def2-TZVPP level of theory (iso = ±0.005). − (c) Cyclic voltammograms of 3X12F^• and 9Cl6F^• in CH₂Cl₂ at 298 K (0.1 M NBu₄PF₆, 100 mV s^–1). (d) Normalized UV–vis absorption (solid lines) and fluorescence emission (dashed lines) spectra of 3X12F^• and 9Cl6F^• in deaerated CHCl₃ at 298 K. The fluorescence emission spectra were excited at 350 nm (3X12F^• ) and 370 nm (9Cl6F^• ). (e) Normalized fluorescence spectra of 3Cl12F^• in various deaerated organic solvents at 298 K. All spectra have been normalized at the respective absorption maximum.

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Emission spectra of 3Cl12F^• in 200 nm PS-NPs with an increasing loading concentration.

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(a) Space-filling model of 3Cl12F^• and 9Cl6F^• (B3LYP-D3­(BJ)/def2-TZVPP). The central carbon atom is marked in red. (b) Molecular structures of both perhalofluoro trityl peroxides in the solid state. Thermal ellipsoids set at 50% probability (3Cl12F-O) ₂ . Selected bond lengths [pm] and angles [deg]: C1–C2 153.6(4), C1–C14 155.1(4), C1–C8 154.3(4), C1–O2 144.5(3), O1–O2 148.3(3), C2–C1–C14 115.9(2), C14–C1–C8 104.9(2), C8–C1–O2 113.6(2), O2–C1–C2 105.6(2), C1–O2–O1-C20 175.7(2). (3Br12F-O) ₂ . Selected bond lengths [pm] and angles [deg]: C1–C2 155.7(1), C1–C3 154.5(1), C1–C4 154.9(1), C1–O2 143.8(1), O1–O2 147.8(1), C3–C1–C2 104.9(4), C2–C1–C4 114.7(5), C4–C1–O2 108.0(6), O2–C1–C3 114.4(5), and C1–O2–O1-C20 178.0(5). Photographs show the discoloration of the n-heptane solutions of both 3X12F^• over the course of 1 week. (c) Time-resolved UV–vis absorption spectra of 3Cl12F^• in n-heptane, recorded over 7 days to follow its reaction with atmospheric oxygen (see Supporting Information for more information).