No abstract

Pulsed lasers have been used in the study of photochemical reactions for close to a quarter of a century.2 It is well-recognized that under certain conditions the consequences of laser excitation can differ from those of conventional (e.g., lamps or ambient light) irradiation at the same wavelength.Laser and conventional excitation of organic materials can lead to different consequences for a variety of reasons. Three of the major origins for these differences are the following:(i) This Account deals with the case where laser excitation can generate a sufficiently high concentration of intermediates ( t y p i d y 10-100 p M ) to compete with the ground-state precursor for the exciting photons. Under these conditions, photolysis of the intermediates (excited states, free radicals, etc.) can lead to new chemical processes, quite different from those induced by conventional excitation. We avoid the use of the term "biphotonic" since these two-photon processes involve stepwise photon absorption in which the second photon is normally absorbed by a chemical species quite distinct from the original ground-state precursor.The other two common examples leading to "lamp vs laser" differences are as follows:(ii) Simultaneous two-photon absorption can lead to the population of high-energy excited states, not usually accessible by one-photon excitation. This interesting area will not be covered in this Account.(iii) When the reaction intermediates (e.g., free radicals) produced by photolysis of their precursor decay by a mechanism that involves a competition of first-and second-order processes, laser excitation will favor the product of the latter. For example, photolysis of 6-heptenoyl peroxide in benzene leads to radical I, which can isomerize to II.3 Under 308-nm laser excitation the main product is 1,ll-dodecadiene (>go%, 1111, while under lamp irradiation I11 is not observed and the main J. C. ('fito) Scalano was born in Buenos Aires. Argentina, in 1945. He recelved his BSc. from the University of Buenos Alres and his Ph.D. from the University of CMle. He is currently Principal Research Officer and Head of the Reaction Intermediates Grwp at the DMslon of Chemistry of the National Research Council of Canada. His interests include the study of reactive intermediates in soiutlon and In heterogeneous systems and laser-induced twophoton processes. Linde Johnston was bom in Charlottetown, Prince Edward Island, in 1956. She recelved her B.Sc. (1978) from Acadla Universtty, Wolhrii, Nova Scotia, and a Ph.D. (1983) from the University of Western Ontario, London, Ontario, and has been at NRC since 1983. Research interests include the photochemistry of reactive intermedlates and photochemistry in heterogeneous systems. Grant McOimpsey was born in St. Catharines, Ontario, in 1956. He received his B.Sc. (1978) and M.Sc. (1981) degrees from Brock Unhrerstty. st. Catharines, and his PhD. degree (1985) from Queen's Universtty, Kingston. His research interests include the study of multiphoton processes and upper excked state energy trans...

The photochemistry of α‐terthienyl (αT) and its mono‐ and dodo derivatives has been examined using nanosecond laser Hash photolysis techniques. The triplet states of these intermediates have been characterized, and show strong triplet‐triplet absorptions with maxima in the 450 to 490 nm region. The triplet lifetimes are normally reduced by efficient triplet‐triplet annihilation and self‐quenching both of which approach diffusion control. Triplet lifetimes in methanol obtained by extrapolation to zero laser dose and zero concentration are 30, 12.5 and 9.4 μs for αT and its mono‐ and dodo derivatives, respectively; the effect of iodo substitution on the lifetimes is attributed to heavy atom effects. The triplet states are efficiently quenched by oxygen and the electron acceptor methyl viologen, while amines tire very poor triplet quenchers. The iodo derivatives are photolabile. undergoing C‐I bond cleavage from the singlet state, a process that was studied in benzene solvent, where the complex between iodine atoms and benzene can be readily characterized. Modification of αT by replacement of the central thiophene ring by an aromatic ring (i.e. DTB) causes drastic changes in the triplet and singlet state kinetic and spectroscopic characteristics.

No abstract

The singlet oxygen generating efficiency of several thiophenic compounds, including oc‐terthienyl (αT) and some of its synthetic analogues, has been determined using laser flash photolysis techniques. The method, which is based on the time‐resolved bleaching of 1,3‐diphenylisobenzofuran, led to relatively high efficiencies for all the substrates examined; for example the values were 0.86, 0.69 and 0.63 for αT, DBrαT and TNP, respectively in chloroform‐d at 293K. Our results also indicate that high intersystem crossing efficiencies must be a common characteristic in these compounds. The variations in singlet oxygen yields were found to be modest in comparison with changes in the biological activity of these compounds, therefore suggesting that other factors are also quite important in determining the latter.