An effective way of generating student interest in chemical processes is the simple, visual demonstration of unusual chemical phenomena.1 A splendid example is a chemiluminescence reaction. The observation of "cold liquid light" streaming out of a flask is a delight to behold and a fascinating challenge to understand. In nature, many charming examples of chemiluminescence have been noted in the biolumi-nescence2 of the firefly and in luminous bacteria, fungi, deep sea fish, and other species.Chemiluminescence occurs when a sizable fraction of the exothermicity (AH) of a chemical reaction is converted into electronic excitation energy (*) of a reaction product,3 which then emits a photon of light (hv). We can describe chemiluminescence as a AH -* * -*• hv sequence. The heart of the latter is the chemiexcitation (AH -*) step. The * hv process is an ordinary and well-understood luminescence step which serves to announce the presence of an electronically excited state. If we consider photochemistry as a hv -* * -» ' AH sequence, we can imagine that a conceptual link exists between chemiluminescence Nicholas Turro received a B.A. degree from Wesleyan University In 1960, took his Ph.D. from Caltech, did a year's postdoctoral work at Harvard, and then joined the faculty at Columbia, where he is now Professor of Chemistry. He has research interests In many areas, Including work on cyclopropanes and on molecular photochemistry of alkanones In solution, which have been the subjects of previous Accounts. He Is recipient of the 1974 ACS Award in Pure Chemistry sponsored by Alpha Chi Sigma Fraternity and of the 1973 Fresenius Award In Chemistry sponsored by the Psi Lambda Upsilon Fraternity.Peter Lechtken was awarded the Ph.D. by Universitat Erlangen and returned there to join the faculty on completion of a NATO Postdoctoral Fellowship at Columbia In 1972-1973 Schore, who is a postdoctoral fellow at Caltech, received his Ph.D. at Columbia working with Dr. Turro. Gary Schuster's Ph.D. work was done at the University of Rochester. Following a hitch in the Army, Dr. Schuster is doing postdoctoral research at Columbia. Hans-Christian Steinmetzer received the Ph.D. from Universitat Würzburg, did postdoctoral work at Columbia, and is now at Universitat Frankfurt. Ahmad Yekta, who is now on the faculty of Ayra-Mehr University in Tehran, obtained the Ph.D. at Columbia in 1973. phenomena (photochemistry in reverse!) and photochemical reactions.
In this paper some examples of reactions which yield electronically excited products are presented. In particular, the 1 ,2-dioxetanes are discussed. These molecules cleave cleanly into two carbonyl fragments when heated or irradiated. It will be shown that these simple, high energy, four atom arrays can efficiently generate electronically excited carbonyl fragments when they decompose. Surprisingly, tetramethyl-1,2-dioxetane (V) yields acetone triplet selectively upon thermolysis or photolysis.A study of the kinetics of thermal decomposition of V as a function of solvent, and the mechanistic implications of these observations will be discussed. Based on summation of available evidence, we propose that the thermolyses of 1 ,2-dioxetanes require specific vibrational motions which enhance spin-orbit coupling as the molecule fragments, and allow efficient decomposition into triplet states.In the photochemistry of V an exceptional 'anti-Stokes' sensitization is demonstrated, which suggests the possibility of efficient execution of 'blue light' photochemistry with 'red light', and a 'quantum chain' reaction.It is demonstrated that, at 77°K, naphthvalene undergoes efficient photochemical conversion to naphthaJene triplets.Finally, the relationship of radiationless electronic relaxation and primary photochemical processes is discussed in the framework of our results. INTRODUCTIONIn this paper we shall be concerned with organic reactions in solution that generate electronically excited products by either thermal (equation 1) or photochemical (equation 2) activation of reactants.
17) (a) It is interesting to note that benzene triplet is believed to possess Dih symmetry, i.e., belong to the same point group as Dewar benzene: M. S. de Groot and J. H. van der Waals, Mol. Phys., 6, 545 (1963). (b) See S. K. Lowen and M. A. El-Sayed, Chem. Rev., 66, 199 (1966), for a discussion of spin-orbit interactions in organic molecules.(18) Nearly all reported chemiluminescent reactions involving organic molecules in solution are oxidative fragmentations or electron transfers.
An adiabatic photoreaction is a chemical process that occurs entirely on a single excited electronic energy surface. As a rule, most photoreactions of organic molecules start on an excited electronic surface but "jump" to a lower surface somewhere along the reaction coordinate. There are, however, exceptions to this general rule. For example, photoreactions involving small structural changes and minor alterations in covalent bonding (e. g., proton transfers and complex formation) are commonly found to occur adiabatically. The purpose of this review is to survey examples of more complicated adiabatic photoreactions such as fragmentations, electrocyclic rearrangements, and geometrical isomerizations. The concepts employed are presented in an introductory discussion.
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