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The mechanistic insights afforded by investigations of photochemicallygenerated stable fluorescent compounds are examined. Examples of the types of such photofluorescent processes found in the literature are reviewed. The photochemical self-quenching rearrangement of furyl chromones, wavelengthdependent rearrangement of azirines, halogen-dependent cycloelimination from halophenylnaphthalens and decarboxylation-decarbonylation of 1,2dicarboxylic anhydrides are examined as photofluorescent systems. The efficient photoelimination of carbon monoxide and carbon dioxide from thermally stable 1,4-dicarboxylic anhydrides is reported and mechanistic information obtained from the formation of fluorescent products in this process is described. The photochemical generation of stable fluorescent products from nonfluorescent precursors, a process which we have termed photofluorescence, has been casually noted in the literature for many years. There appears to have been no previous deliberate effort to create and examine this phenomenon, although there are very good reasons for doing so. I would first like to mention briefly some of the more cogent reasons and then proceed to examine the photochemistry that our efforts in this area have led to. The most fundamental reason for photogenerating fluorescent materials is to use them as a probe for examining the potential energy surface of the photochemical transformation. Professor Forster described his studies on diabatic and adiabatic processes in photochemistry1 to the Third IUPAC Symposium on Photochernistry. He raised the question as to how molecule A is transformed photochemically to molecule B (Figure 1). Does deactivation of the electronic excitation take place in the reactant (as in 1) or in the product (as in 2) or in between (as in 3). Observation of characteristic emission from the fluorescent product excited state achieved through absorption by the precursor is good evidence for process 2 deactivation. We will explore the experimental requirements for such observations and report our results further on, but before that I'd like to go on to the applied areas and mention the photo-imaging possibilities with stable photofluorescent systems. We are weil aware that under most circumstances a fluorescent test can 389 A. ZWEIG A + hv A* Excitation A* AVIbT B Deactivation (1) A* .B* B (2) A* (3) Figure 1. Alternative pathways in the photochemical conversion of A to B detect orders of magnitude less material than a colour test and this is why fluorescent tags and tracers are widely used in trace analytical applications2. Indeed it is a common laboratory practice to examine a chromatographed reaction mixture with an ultra-violet lamp to detect separating fluorescent products. It should be evident that an efficient photochemical process that generates a stable and efficient fluorescent product from a non-fluorescent precursor will have an effective speed orders of magnitude greater than an equally efficient photochernical process that yields even a deeply-coloured product only detectable by differences in absorption. It should not be surprising, therefore, that as seen in Figure 2, the first photofluorescent imaging material3 exhibited practical sensitivities comparable to that of photopolymer systems and much faster effective speed than that of non-polymerizing organic imaging materials, but, of course, still five orders of magnitudes lower than that of high speed silver photographic film. System Practical exposure microjoules/cm2 Relative sensitivity Silver halide, Tri-X 0.00 1 3 x 1O' Photofluorescer 100 300 Photoplastic, 216 film 150 200 Dry silver 800 43 UVI film 5000 6 Photochromic 30000 1 Kalvar 4 0.075 Diazo 8 0.033 Photofluorescer data from A. Zweig, Symposium 111 on Unconventional Photographic Systems (1971). Other data from J. E. Bigelow, 9th National Symposium on Information Display, May 1968. Figure 2. Relative sensitivity of various films Practical exposure as used in Figure 2 refers to the exposure to radiation per unit area required to differentiate between exposed areas and background after suitable development. The high speed of silver halide film as compared to organic systems is due to the development step where the effect of only a few quanta makes a micron size grain develop. Professor Bird has pointed out that because several photons are needed to make a grain developable and because of other losses, the detective quantum efficiency of even the most sensitive silver halide films is only of the order of one to two per cent4. He has suggested how suitable organic photofluorescent materials could be utilized to capture and display an image with greater sensitivity than that available from silver halide film4. In his concept, as illustrated in Figure 3, the photofluorescent material captures the image in a molecular transformation in the 390 PHOTOCHEMICAL GENERATION OF STABLE FLUORESCENT COMPOUNDS Imaging [1 [1 light surce Molecular Molecular fluorescence fluorescer precursor (latent image) hi Signal averaging fluorescence emission collection Figure 3. High speed photography with photofluorescent materials first step and then in a developing step the presence of fluorescing molecules is detected and amplified by a developing beam of light. The development is effected by employing the initially produced stable fluorescer as a sort of catalyst which reveals its presence through the performance of its function. Thus conceptually, if not yet in practice, photofluorescent systems offer the opportunity of even greater sensitivity than silver halide-based materials. One of the first descriptions of the photolytic generation of an identified fluorescent product was made by Lewis, Magel and Lipkin5, who observed development of a blue fluorescent product when non-fluorescent cis-stilbene (1) was photolysed. The photolytic change induced was shown to be the isomerization of I to trans-stilbene (2). The authors utilized this photolytic H H H o/c:cc cis-stilbene trans-stilbene non-fluorescent blue fluorescence generation of a fluorescent product to help clear up the existing confusion regarding this cis—trans isomerism. Cis to trans photoisomerisms are an important class of photofluorescers. In Figure 4 we have listed some familiar examples such as substituted stilbenes6, indigo dyes7, and f-styryl compounds8, as well as the remarkable photoconversion of a non-fluorescent all cis 1,2,4,7-tetraphenylcyclooctatetraene to a fluorescent trans containing isomer9, illustrating the apparent thoroughness of the relationship of cis—trans photoisomerization to photofluorescence. Despite this ubiquity, these are not the best reactions to choose for an initial detailed investigation of the photofluorescence process because : (1) there are relatively small differences in absorption spectra between product and precursor, (2) under some conditions the cis isomer becomes fluorescent9a, and, (3) photoreversibility, modest photoefficiency and occurrence of side reactions cloud attempts at quantitative evaluation. There are certainly many other types of reactions described in the literature 391 Qcfl X = S, NH or NR

Photochemical generation of stable fluorescent compounds (photofluorescence)