Steady state and time-resolved spectroscopic studies of the photochemistryof 1-arylsilacyclobutanes and the chemistry of 1-arylsilenes

Direct photolysis of 1-phenylsilacyclobutane and 1-phenyl-, 1-(2-phenylethy nyl)-, and 1-(4'-biphenylyl)-1-methylsilacyclobutane in hexane solution lea ds to the formation of ethylene and the corresponding 1-arylsilenes, which have been trapped by photolysis in the presence of methanol. Quantum yields for photolysis of the three methyl-substituted compounds have been determi ned to be 0.04, 0.26, and 0.29, respectively, using the photolysis of 1,1-d iphenylsilacyclobutane (Phi(silene) = 0.21) as the actinometer. The corresp onding silenes have been detected by laser flash photolysis; they have Life times of several microseconds, exhibit UV absorption maxima ranging from 31 5 to 330 nm, and react with methanol with rate constants on the order of (2 -5) x 10(9) M-1 s(-1) in hexane. Absolute rate constants for reaction of 1- phenylsilene and 1-methyl-1-phenylsilene with water, methanol, tert-butanol , and acetic acid in acetonitrile solution have been determined, and are co mpared to those of 1,1-diphenylsilene under the same conditions. With the p henylethynyl- and biphenyl-substituted methylsilacyclobutanes, the triplet states can also be detected by laser flash photolysis, and are shown to not be involved in silene formation on the basis of triplet sensitization and (or) quenching experiments. Fluorescence emission spectra and singlet lifet imes have been determined for the three 1-aryl-1-methylsilacyclobutanes, 1, 1-diphenylsilacyclobutane, and a series of acyclic arylmethylsilane model c ompounds. These data, along with the reaction quantum yields, allow estimat es to be made of the rate constants for the excited singlet state reaction responsible for silene formation. 1-Methyl-1-phenylsilacyclobutane undergoe s reaction from its lowest excited singlet state with a rate constant 10-80 times lower than those of the other three derivatives. The results are con sistent with a stepwise mechanism for silene formation, involving a 1,4-bir adicaloid intermediate that partitions between product and starting materia l.