The triplet-sensitized photorearrangements of 2-phenylallyl phosphites 1, 3, and 4 to the corresponding 2-phenylallylphosphonates la, 3a, and 4a are shown to proceed with complete regioselectivity (5 --> 6 and Scheme 1). A mechanism is proposed in which the 1,2-biradical-like styryl triplet, 9, adds oxidatively to three-coordinate phosphorus to generate a cyclic, triplet, phosphoranyl 1,3-biradical, 10, that undergoes rapid subsequent beta scission to generate product 2-phenylallylphosphonate, 11. Phosphonate la is formed near-quantitatively from dimethyl 2-phenylallyl phosphite, 1. Phosphites 2-4, with phosphorus contained in a five-, six-, or seven-membered ring, respectively, photorearrange much more slowly, although yields of phosphonates 3a and 4a of 50-70% are generated. Quantum yields for the formation of phosphonates 1a, 3a, and 4a, on sensitization by triplet triphenylene, were determined to be 0.25, 0.003, and 0.005, respectively, in benzene. Similar values were found in acetonitrile and benzene using benzophenone as triplet sensitizer. Rate constants, k(q), for efficient quenching of the triplets of benzophenone (phosphorescence quenching) and triphenylene (photothermal lense measurements) were all in the range 4.0-5.4 x 10(9) M(-1) s(-1). The low quantum yields for phosphonate formation from 3 and 4 (and presumably 2), therefore, result from the relative inefficiencies of reactions of their 2-phenylallyl triplets toward phosphorus. This is ascribed to the large reduction in the rate of isomerization of the kinetically formed initial 1,3-biradical, 23, to the thermodynamically more stable species, 24, an effect found in spiro phosphoranyl monoradicals. A reduction in the rate of beta scission of biradical 23, caused by its spiro structure, also may play a role. The failure of the 3-phenylallyl phosphite 7 to undergo the same cyclic photorearrangement as 1 is ascribed to the inability of the benzyl radical-like terminus of triplet 7 to react with phosphorus. Phosphite homolog 8 also is inert toward cyclic photorearrangement via a six-membered ring 1,3-biradical (28) analogous to 10. The reactivity patterns of phosphites 1-4, 7, and 8 can be rationalized in terms of the 1,2-biradical nature of the triplet styryl moiety and factors known to govern the formation and permutational properties of phosphoranyl monoradicals.

Nongeminate recombination of photodissociated chlorine atoms in carbon tetrachloride initiated by 355 and 325 nm radiation is observed using photothermal grating and photothermal lens spectroscopy. From the amplitudes of the photothermal transients, the yield of photodissociation is determined to be 0.35 +/- 0.02 at 355 nm and 0.42 +/- 0.06 at 325 nm. The prompt amplitude of the photothermal transients, and its intensity dependence, provide evidence of transient absorption by an excited intermediate species. Based on previous reports, the source of the transient absorption is assigned to be the chlorine atom. A quantitative estimate of the absorption cross section for transient absorption of the chlorine atom is reported at two wavelengths; the cross section of chlorine atom absorption is determined to be sigma(2) = (3.4 +/- 0.4) x 10(-17) cm(2) at 355 nm and sigma(2) = (1.5 +/- 0.2) x 10(-16) cm(2) at 325 nm. From the kinetic signature of the photothermal transients, the second-order rate constant for nongeminate recombination of chlorine atom is determined to be (1.2 +/- 0.2) x 10(10) M(-1) s(-1).

Diffraction from photothermal gratings is monitored on a nanosecond time scale to determine the rates of a propagating free-radical chain reaction. The kinetics of photochlorination of chloroform, initiated by 355 nn laser radiation, are investigated. Monitoring these reactions on a nanosecond time scale avoids chain termination and enables the time-resolved diffraction transients to be interpreted using a simple kinetic model for the two propagation steps of the chain reaction. From the pseudo-first-order rate parameter of this simple kinetic model, individual rate constants for the steps that propagate the chain are determined from Stern-Volmer relationships. The rate constant for hydrogen abstraction from chloroform by a chlorine atom in carbon tetrachloride is determined, k(1) = (7.8 +/- 0.6) x 10(6) M(-1) s(-1), and the chlorine atom transfer rate constant is found, k(2) = (4.6 +/- 0.4) x 10(8) M(-1) s(-1). The reported rate constants do not depend on the concentration of radicals generated in the laser pulse. This direct approach is an attractive alternative to stationary-state techniques that generally only yield composite rate constants.

Interpreting rates and yields of photoinitiated reactions that follow second-order or mixed-order decay kinetics requires modeling the spatial distribution of excited states in the expressions for the reaction rates and yields. Under conditions of linear absorption, the spatial distribution of photoexcited intermediates is proportional to the incident excitation intensity pattern. A mixed-order kinetic model has been developed for interpreting the rates and yields of photoinitiated reactions under conditions of Gaussian radial profile excitation. From a numerical study, it was shown that the spatial distribution of excited states must be explicitly included in the kinetic model to acquire accurate rates and yields of second-order and mixed-order processes; it was also found that errors in fitted parameters can arise when no significant lack-of-fit can be detected. The Gaussian-weighted model is used to determine the triplet-triplet annihilation rate constant for benzophenone in acetonitrile at room temperature from its phosphorescence decay where k(TT)(OBS) = (1.0 +/- 0.1) X 10(10) M(-1) s(-1); the T-T annihilation rate constant for anthracene was determined under the same conditions by delayed fluorescence measurements where k(TT)(OBS) = (5.5 +/- 0.5) X 10(9) M(-1) s(-1).

The pulsed thermal lens experiment is useful for investigating the kinetics and energetics of intermediate species in photoinitiated reactions. We have adapted this technique to allow decay rates to be estimated under conditions where second-order processes make a significant contribution to the observed kinetics. The excited triplet state photophysics of benzophenone in acetonitrile are investigated using this method. Using a mixed-order kinetic model, the rate constant of triplet-triplet annihilation was found to be (1.9 +/- 0.2) x 10(10) M(-1) s(-1), indistinguishable from a diffusion-controlled value. A finite rate of self-quenching of the excited triplet state was also found. From the amplitudes of the photothermal transients, the triplet energy of the benzophenone triplet was found to be 67 +/- 4 kcal/mol in acetonitrile. We also use this method to investigate the kinetics and energetics of hydrogen abstraction from 2-propanol, where the rate constant of radical production was found to be (4.5 +/- 0.3) X 10(6) M(-1) s(-1) and the O-H bond energy of the ketyl radical in acetonitrile was found to be 103 +/- 3 kcal/mol.

The triplet-sensitized photorearrangement of 2-phenyllayl phosphite 4 is of the order 50–100 times less efficient than that for its acyclic counterpart, 1. This surprising effect can be understood in terms of the influence of the spiro ring system of 10 on the rate of its conversion to thermodymanically more stable 11 from which phosphonate 12 is formed.

We have used the scanning tunneling microscope in air to investigate the topographic and diffusion properties of the surfaces formed by melting gold and platinum wires into spheres. Nanolithographic parameters, including thresholds for surface modification of the gold and platinum surfaces, have been studied and coefficients for self-diffusion have been determined. The data suggest diffusion rates at room temperature 1-2 orders of magnitude lower on platinum than on gold.