Photochemistry and photophysics of organosilicon compounds

Photochemical and photophysical processes of organosilicon compounds have been studied. Dual (local and CT) emission has been found in aromatic disilanes. The intramolecular CT fluorescence has a broad and structureless band with a large Stokes shift. The CT process in the excited state occurs very rapidly with a time constant less than 10 ps even in rigid glass at 77 K This finding shows that the CT mechanism is quite different from TICT (or OICT) which needs twisting or internal rotation during the lifetime in the excited state. The CT emission originates from the ¹(2pσ,3dσ) state having an in-plane long-axis polarization, which is produced by the 2pσ* (aromatic ring) vacant 3dσ (Si-Si bond) intramolecular charge transfer. The CT state plays an important role in the photochemical and photophysical properties of phenyldisilanes. At room temperature a long-lived 425 nm transient (silene) is produced with a time constant of 30 ps from the CT state. The photolysis of cyclotetrasilanes is remarkably dependent on their molecular structures: two molecules of the corresponding disilene are produced from the S₁ state of planar cyclotetrasilanes, while silylene is generated by ring contraction in the S₁ state of bent cyclotetrasilanes. Remarkably large Stokes shifts are observed in these cyclotetrasilanes. Dimethylsilylene with a transient peak at 470 nm is observed by laser photolysis of cyclohexasilanes. The dynamic behaviours of the intermediates have been studied by nanosecond laser photolysis. The phenylsilyl radical is generated by photolysis of phenylsilanes in rigid glass at 77 K, which gives a structured emission similar to that of benzyl radical.     

Structure and bioactivity of a trisnorditerpenoid and a diterpenoid from an Okinawan soft coral, Cespitularia sp.

An Okinawan soft coral Cespitularia sp. has proven to be a good source of cytotoxic metabolites having a carbon skeleton of the seco-type variety of xenicin. This soft coral yielded alcyonolide, the major constituent, and other related compounds, all of which have proven to be cytotoxic. Reinvestigation of the cytotoxic ethyl acetate extracts of the coral yielded two new alcyonolide congeners, trisnorditerpenoid 1 and diterpenoid 2, possessing the same carbon skeleton. Their structures were elucidated by a detailed analysis of spectroscopic data (1D, 2D NMR, and MS). Metabolites 1 and 2 showed cytotoxicity against HCT116 cells (IC₅₀ 6.04 and 47.0 μM, respectively) and a dose dependent, anti-inflammatory effect in LPS/IFN-γ-stimulated inflammatory RAW 264.7 macrophage cells.     

High-energy quasi-phase-matched optical parametric oscillation in a periodically poled MgO:LiNbO3 device with a 5 mm x 5 mm aperture

Fabrication of a 5 mm thick periodically poled MgO-doped LiNbO3 device with a 32.1 microm period for mid-infrared generation was demonstrated. The periodic structure was evaluated by measurement of second-harmonic generation with the d31 coefficient. Optical parametric oscillation using this device with an uncoated 5 mm x 5 mm aperture and a 36 mm effective length realized a high-energy output of 77 mJ for both signal (wavelength 1.83 microm) and idler (2.54 microm) waves with a 72% slope efficiency at 110 mJ pumping of a Q-switched Nd:YAG laser with a 12 ns pulse duration.     

High-energy quasi-phase-matched optical parametric oscillation in a 3-mm-thick periodically poled MgO:LiNbO3 device

We have demonstrated high-energy quasi-phase-matched optical parametric oscillation in a 3-mm-thick periodically poled 5-mol. % MgO-doped LiNbO3 device with a 32.1-microm grating period and a 30-mm length. With a large-spot-size pump laser of 2.2-mm diameter, we obtained a total output pulse energy of 22 mJ for both the signal (wavelength 1.82 microm) and the idler (2.56 microm) waves at an input pump energy of 46 mJ.     

Highly efficient 1063-nm continuous-wave laser emission in Nd:GdVO4

Highly efficient 1-microm continuous-wave laser emission in 3-mm-thick, 0.5- and 1.0-at. % Nd:GdVO4 crystals longitudinally pumped at 879 nm into the laser emitting level is reported. Under Ti:sapphire pumping, the slope efficiency in absorbed power is approximately 80% for both crystals, while the slope efficiency, the optical-to-optical efficiency (at 1700-mW pump power), and the laser threshold in incident power are 79%, 78%, and 31 mW for 0.5-at. % Nd and 80%, 77%, and 40 mW for 1.0-at. % Nd, respectively. The slope efficiency is close to the quantum defect limit, the difference being fully accounted for by the residual optical losses. Under 879-nm diode laser pumping, the slope efficiency and the optical-to-optical efficiency in absorbed power of the 0.5-at. % Nd:GdVO4 crystal are 60% and 53%, owing to poorer superposition of the pumped and the laser mode volumes.     

Group-velocity-matched cascaded quadratic nonlinearities of femtosecond pulses in periodically poled MgO:LiNbO3

Large nonlinear phase shifts were generated with femtosecond pulses at 1560 nm through cascaded quadratic interactions in periodically poled MgO-doped LiNbO3. The off-diagonal component of the nonlinear coefficient was utilized for simultaneous quasi phase matching and group-velocity matching. The effective nonlinear refractive index was varied from -2.9 x 10(-14) to +3.3 x 10(-14) cm2/W by tuning the phase-mismatch conditions.