RESEARCH
Excited states energize reactions Chemiluminescent reactions generate energy for photochemical reactions without light A method for producing photochemical reactions without using light has been developed by three chemists at Johns Hopkins University, Baltimore, Md. The principle involved in the method is the use of singlet and triplet excited states generated in chemical reactions, Dr. Emil II. White explains. He and his coworkers, Dr. Jacek Wiecko, and Dr. David Roswell (with some important preliminary work done by Dr. Harold Doshan) find that chemiluminescent reactions can be used as "electronic energy generators." The results will be described this month in the Journal of the American Chemical Society (Vol. 91, page 5194). The Johns Hopkins work is an outcome of the current widespread interest in photochemistry coupled with Dr. White's research interest in chemiluminescence.
Chemiluminescence is a phenomenon in which chemical energy is converted into electronic excitation energy, Dr. White explains. In the reaction B + C —> D*, for example, the excited intermediate or product molecule D* normally is fluorescent, and light emission occurs as an end result of the chemical reaction. The excitation can also be transferred to another molecule or intramolecularly to a different part of the molecule. In the past these processes have been determined by light emission. However, the Johns Hopkins group finds that the electronic excitation energy that resides in D* can be used to do work. Energy sources. In principle, Dr. White points out, any chemiluminescent reaction can be used as an "energy generator." As energy sources, he and his coworkers have used a phthalic
hydrazide and trimethyl-2-oxaoxetane. Oxaoxetanes appear to be critical intermediates in a large fraction of the know7n organic chemiluminescent reactions in solution, according to earlier work by Dr. White and others in the field. Recently, Dr. K. R. Kopecky and his coworkers at the University of Alberta, Edmonton, Alta., isolated trimethyl-2-oxaoxetane and showed that it generates electronically excited states when decomposed thermally. Intermolecular energy transfer from the excited states was shown by the fluorescence of added anthracene (implying a singlet state) and the phosphorescence of added biacetyl (implying a triplet state). Photoisomerism. The Johns Hopkins group finds that the excitation energy from the excited states can be transferred to acceptors, which then
Two energy sources produce photochemical reactions Intermolecular energy transfer-. CM*
CH
V CII H *
H CHs
v
o Trimethyl-2-oxaoxetane
Excited state
Energy from excited state i
C*H5
C*H5 6,6-Diphenyl-[3,l,0]-bicyclohexene-l-one
4,4-Diphenylcyclohexadienone
ffntramoSecufar ersergy utilization:
NH
Hemin
SOURCE. Dr. Emil H. White (left) and Dr. Jacek Wiecko of Johns Hopkins University examine fluorescent spectrum of a compound that is a potential energy source for a photochemical reaction produced without light
C6H5(H)0»CH Excited state
Afrans-diacyihydrazine MMMOttMMMUfaW
Methyl esters 97% trans 3% cis
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undergo a "photochemical change." For example, when a solution of the oxaoxetane and transstilbene in benzene is heated to about 80° C. for seven minutes, 20% of the trans-stilbene is converted to ds-stilbene. The Johns Hopkins team has also applied the "photochemistry without light" principle to the photoisomerism of 4,4-diphenylcyclohexadienone. The photoisomerism has been studied by Dr. Howard E. Zimmerman and his coworkers at the University of Wisconsin. They found that the principal photoproduct, a bicyclic ketone, is formed from the triplet state of the cyclohexadienone. Using the new method, Dr. White and his group heated a benzene solution of trimethyl-2-oxaoxetane and 4,4-diphenylcyclohexadienone on a steam bath for 10 minutes and obtained about a 259v yield of the bicyclic ketone. Therefore it seems that the new method, with the proper energy source, gives results normally obtained in photo-sensitized reactions energized by light. Dr. White's approach differs from earlier "photochemistry without light" work of Dr. Zimmerman's group at Wisconsin University. The Wisconsin workers generated photochemical intermediates formed from the excited state and did not generate the excited state itself. Using the oxidation of a diacylhydrazide as the energy generator, the Johns Hopkins chemists also noted isomerism in a reaction. They oxidized a frans-hydrazide to give a mixture of stryrylphthalic acids that contained 37c of the cis isomer (determined by gas chromatography of the methyl esters). This approach could be extended to the intramolecular energy transfer from any of the energy generators (E.G.) used in chemiluminescence to an acceptor molecule (A) (to be acted on) attached by a labile bond to the energy generator: A + (E.G.)-KA - E.G.)-> (product - E.G. product) —» product -f E.G. product. One of the advantages of the "photochemistry without light" method is that all of the energy is localized in the donor moiety. Another advantage is that only simple equipment is required: Beakers, not monochromators. Dr. White believes that he and his coworkers have just examined a few of the possibilities of running photochemical reactions without light. Now that the technique is feasible, they want to make it practical. So far the yields have been relatively small, but no effort has been made to optimize them. One goal of the group is to develop the approach into a useful one. Other future work includes measuring the efficiency of the energy generation.
