Effect of Iodination on the Photophysics of the Laser Borane anti-B₁₈H₂₂: Generation of Efficient Photosensitizers of Oxygen

Treatment of the laser borane anti -B 18 H 22 (compound 1 ) with iodine in ethanol gives the monoiodinated derivative 7-I- anti -B 18 H 21 (compound 2 ) in 67% yield, or, by reaction with iodine or ICl in the presence of AlCl 3 in dichloromethane, the diiodinated derivative 4,4′-I 2 - anti -B 18 H 20 (compound 3 ) in 85% yield. On excitation with 360 nm light, both compounds 2 and 3 give strong green phosphorescent emissions (λ max = 525 nm, Φ L = 0.41 and λ max = 545 nm, Φ L = 0.71 respectively) that are quenched by dioxygen to produce O 2 ( 1 Δ g ) singlet oxygen with quantum yields of Φ Δ = 0.52 and 0.36 respectively. Similarly strong emissions can be stimulated via the nonlinear process of two-photon absorption when exciting with 720 or 800 nm light. The high quantum yields of singlet-oxygen production, coupled with the option of two-photon excitation, make compounds 2 and 3 promising O 2 ( 1 Δ g ) photosensitizers. The molecular structures of compounds 2 and 3 were determined by single-crystal X-ray crystallographic studies as well as multinuclear NMR spectroscopy and mass spectrometry. Time-resolved UV–vis spectroscopy was used to delineate their photophysical properties, and the electronic-structure properties of the emitting species were determined by means of multiconfigurational quantum-chemistry computations.