Nanosecond laser flash photolysis kinetics

The merocyanine form of numerous BIPS compounds in solution complex with many transition and rare-earth metal ions. The complexation between 6-nitro-8-methoxyBIPS and several ions was studied by spectrophotometric, luminescent, stopped-flow, and nanosecond laser flash photolysis techniques. The absorption maximum of the dye, 580 nm, is shifted to the 480-500 nm region, and the relatively weak fluorescence shows a similar hypsochromic shift. The kinetics of the complexation involved a fast reaction between the components, followed by a slow equilibrium of the complex to its most stable isomer. The photoreactions of the complexes include formation of a short-lived triplet state (lifetime about 2 x 10 5 s,... 

This review summarizes the generation and spectroscopic characterization of alkene radical cations and kinetic and mechanistic studies of their reactions with nucleophiles and cycloaddition chemistry. Most of the data have been obtained using laser flash photolysis techniques, but comparisons with kinetic data obtained using other methods and with steady-state experiments are presented where appropriate. To date most kinetic measurements using laser Hash photolysis techniques have focused on arylalkene radical cations since these are relatively easy to generate and have spectroscopic and kinetic behavior that is commensurate with nanosecond laser flash photolysis techniques. 

The reaction kinetics of the electron transfer from the catalyst to the photogenerated oxidant [Ru(bpy)3] + has been studied by nanosecond laser flash photolysis experiment. The kinetics of the hole scavenging, are pseudo first order when [P u(bpy)3] +] [Ru4(POM)], and are characterized by a bimolecular rate con-... 

Kinetics of the reactions of singlet species 32b in solution at room temperature were studied using time-resolved IR spectroscopy (TRIR) " and nanosecond laser flash photolysis. The absolute rate constants of bimolecular reactions of 32b with... 

Phenyltrimethyldisilene (15) and (E)- and (Z)-l,2-dimethyl-l,2-diphenyldisilene (16) were also observed as transient absorption spectra by laser flash photolysis of the precursors in methylcyclohexanes28. The absorption band at 380 nm, assigned to the disilene 15, reached maximum intensity at ca 10 ns after the excitation and then started to decrease. The half-life assigned to 15 was 700 ns. The logarithm of the decay profile of the transient absorption at 380 nm versus time shows a very good linear relationship, indicating that the decay of the transient absorption fits first-order kinetics. This result shows that intramolecular isomerization or proton abstraction from the solvent is the origin for the decay of the disilene 15, which survives in solution only for several nanoseconds. 

All of the molecules in this study have triplet states which are easily detectable by the technique of nanosecond transmission laser flash photolysis. (11) The triplet state of acetoveratrone has a lifetime in excess of 15 ps in ethanol (Figure 2) under conditions of laser excitation the decay involves a mixture of first and second order kinetics, with the latter dominating at high laser powers. This second order decay demonstrates that the triplet state is decaying at least partly by triplet-triplet annihilation. 

Recent laser flash photolysis studies of the kinetics of the process 7 —> 9 suggest that following photoexcitation with time-resolved spectroscopy, an isomer 8 (E form) is formed in the nanosecond or microsecond time domain which undergoes a first-order conversion to the Z form 9.42 43 Details will be published elsewhere. 

Smface modification with ruthenium complexes has proven valuable in studies of both interprotein and intraprotein electron transfer in systems that are difflcult to stndy by traditional kinetic tools. The choice of ruthenium complexes in these investigations stems from an extensive photochemistry as well as exceptional thermal stability. The photochemistry provides a means of examining reactions over a time range of nanoseconds to seconds by laser-flash photolysis and the thermal stability allows researchers to covalently bind a wide variety of complexes to proteins with... 

Figure 2.18. (a) Transient absorption spectra observed at 40 ps before and 20 and 150 ps after the laser flash during two-color two-laser flash photolysis of 4T in toluene employing a nanosecond YAG laser (355 nm, FWHM 5 ns, 7 mJ pulse-1) and a picosecond YAG laser (532 nm, FWHM 30 ps, 21 mJ pulse-1), (b) Difference spectra of transient absorption spectra at 20 and 150 ps. (c) Kinetic traces of AO.D. at 650 and 600 nm. Thick lines are fitted curves. 

In 1976, Closs and Rabinow made the first measurement of rate constants for the reaction of a carbene. They used a flash photolysis technique. A brief flash of radiation generated carbene intermediates, and their decay was then monitored spectrophotometri-cally. In this way the rate constant for reaction of diphenylcarbene with 1,1 -diphenylethylene was found to be 4.8 x 10 s h Using this technique, measurements were possible on a microsecond time scale. With the advent of laser flash photolysis techniques, the resolution time was reduced to nanoseconds, causing a resurgence of interest in the kinetics of carbene reactions. The early results of use of this technique have been discussed by Griller and coworkers. ... 

A number of alkene radical cations have been generated in matrices at low temperature and have also been studied by ESR, CIDNP, and electrochemical methods. However, until recently very little absolute kinetic data have been available for the reactions of these important reactive intermediates in solution under conditions comparable to those used in mechanistic or synthetic studies. In a few cases, competitive kinetic techniques have been used to estimate rates for nucleophilic additions or radical cation/alkene cycloaddition reactions. In addition, pulse radiolysis has been used to provide rate constants for some of the radical cation chemistry relevant to the pho-topolymerization of styrenes. More recently, wc and others have used laser flash photolysis to generate and characterize a variety of alkene radical cations. This method has been extensively applied to the study of other reactive intermediates such as radicals, carbenes, and carbenium ions and is particularly well-suited for kinetic measurements of species that have lifetimes in the tens of nanoseconds range and up and that have at least moderate extinction coeffleients in the UV-visible region. 

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