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

Treatment of the laser borane anti-B18H22 (compound 1) with iodine in ethanol gives the monoiodinated derivative 7-I-anti-B18H21 (compound 2) in 67% yield, or, by reaction with iodine or ICl in the presence of AlCl3 in dichloromethane, the diiodinated derivative 4,4'-I-2-anti-B18H20 (compound 3) in 85% yield. On excitation with 360 nm light, both compounds 2 and 3 give strong green phosphorescent emissions (lambda(max) = 525 nm, Phi(L) = 0.41 and lambda(max)= 545 nm, Phi(L) = 0.71 respectively) that are quenched by dioxygen to produce O-2((1)Delta(g)) singlet oxygen with quantum yields of Phi(Delta) = 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)Delta(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.