Nanosecond Laser Flash Photolysis and Steady-State Photolysis Studies of Benzyltrimethylsilane and Trimethylsilyldiphenylmethane

Abstract

Photophysical and photochemical processes of benzyltrimethylsilane and trimethylsilyldiphenylmethane have been studied by fluorescence measurements and by steady-state and nanosecond laser flash photolyses. A remarkable solvent effect was observed for the laser flash photolysis of benzyltrimethylsilane; in methanol, absorption bands due to benzyl-type radicals and the triplet were observed, while in cyclohexane, only T−T absorption was observed. On the other hand, no solvent effect was observed for the laser flash photolysis of trimethylsilyldiphenylmethane; a transient absorption attributable to the 1,3-trimethylsilyl-shifted intermediate was observed with a lifetime of ca. 27 ms in both solvents, and no absorption band ascribable to diphenylmethyl radicals or the triplet was observed. GC-MS spectra of photoproducts resulting from trimethylsilyldiphenylmethane were consistent with the assignment of the transient absorption. These results were explained in terms of the calculated molecular structure for the excited singlet states of these molecules by the PM3 method.

Triplet- vs. singlet-state imposed photochemistry. The role of substituent effects on the photo-Fries and photodissociation reaction of triphenylmethyl silanes

“…3b), which obviously corresponds to a species insensitive to oxygen. This residual spectrum resembles that obtained by Hiratsuka et al 31 by the photolysis of of the structurally similar compound Ph 2 CH-SiMe 3 and which they attributed to the 5-methylidene-1,3-cyclohexadienyl intermediate 5 resulted from a 1,3-shift of the Me 3 Si-group in a photo-Fries type rearrangement. By analogy, we assign the persistent absorption between 300-450 nm (Fig.…”

Evidence for the Specific Species of Benzyltriethoxysilane Derivatives with a High-Coordination Silicon Atom

“…Transient absorption spectra were measured by using a UNISOKU TSP601H nanosecond laser photolysis system with the fourth-harmonic light pulse from a Nd 3+ :YAG laser (Tokyo Instruments Lotis TII, LS-2137U, 266 nm, pulse width 6 ns, 1 mJ/pulse, repetition rate 10 Hz, or Quanta-Ray, GCR-130, Spectra-Physics, pulse width 8 ns, 3 mJ/pulse, repetition rate 10 Hz), a monitoring light from a Xenon lamp (500 W, USHIO UI-502Q) in the perpendicular direction to the pump light, and an oscilloscope (Tektronix TDS380P) controlled by a personal computer. 11 All of the sample solutions were prepared such that their absorbance was ca. 1.0 at the excitation wavelength (266 nm) and were saturated with high-purity Ar gas (99.999%).…”

“…We tried to observe this specific species by studying nanosecond laser flash photolysis of benzyltrimethylsilane and trimethylsilyldiphenylmethane in MeOH and in cyclohexane (c-Hex). 11 For benzyltrimethylsilane, the absorption bands of the benzyl radical and the triplet state of benzyltrimethylsilane were observed in MeOH, whereas, in c-Hex, only the absorption band of the triplet state was observed. For trimethylsilyldiphenylmethane, an absorption band ascribable to the 1,3-silyl rearranged intermediate was exclusively observed upon the 266 nm photolysis.…”

“…We also studied the photochemical reaction of trimethylsilyldiphenylmethane by examining reaction intermediates and found that diphenylmethyl radicals predominantly formed monophotonically in MeOH or EtOH glass via the excited singlet state, whereas, in 3MP glass, α-trimethylsilyldiphenylmethyl radicals and unstable reaction intermediate(s) were formed via the lowest triplet state. 11 To explain these photochemical reactions, we proposed a unique excited state, which would react with alcohol to give benzyl-type radicals in MeOH or EtOH. 11 On the basis of the calculated results, the silicon atom has been considered to locate at an intermediate position between benzylic carbon and ortho-carbon atoms in this unique excited state.…”

“…11 To explain these photochemical reactions, we proposed a unique excited state, which would react with alcohol to give benzyl-type radicals in MeOH or EtOH. 11 On the basis of the calculated results, the silicon atom has been considered to locate at an intermediate position between benzylic carbon and ortho-carbon atoms in this unique excited state. Thus, the calculated unique excited state can be related to the specific excited state with a high-coordination silicon atom.…”