Effect of Iodination on the Photophysics of the Laser Borane -BH: Generation of Efficient Photosensitizers of Oxygen.

Treatment of the laser borane -BH (compound ) with iodine in ethanol gives the monoiodinated derivative 7-I--BH (compound ) in 67% yield, or, by reaction with iodine or ICl in the presence of AlCl in dichloromethane, the diiodinated derivative 4,4'-I--BH (compound ) in 85% yield. On excitation with 360 nm light, both compounds and give strong green phosphorescent emissions (λ = 525 nm, Φ = 0.41 and λ = 545 nm, Φ = 0.71 respectively) that are quenched by dioxygen to produce O(Δ) singlet oxygen with quantum yields of Φ = 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 and promising O(Δ) photosensitizers. The molecular structures of compounds and 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.