LONG-LIVED ACTIVE STATES AND AFTERGLOW 793 



state (at least in one case — that of fluorescein in a rigid solvent). The ob- 

 served paramagnetic moment corresponds to that of the spin of two un- 

 paired electrons, as expected for a triplet spectroscopic state, and it was 

 argued that this provides a clinching argument for the Lewis-Kasha theory. 

 However, according to what we said above, paramagnetism does not prove 

 that the biradical is of a purely electronic nature, and not a tautomer of the 

 normal molecule. (Perhaps, one should call the metastablc molecules 

 envisaged by Lewis and Kasha "electronic tautomers" and contrast them 

 with ordinary or "atomic tautomers"; the term "mesomers" usually is 

 applied only to electronic structures of equal, or nearly equal, energy.) 



Life-time calculations of metastable organic molecules have been made 

 in three ways — theoretically, on the basis of spin-orbit interaction alone 

 (this should give a high upper limit for actual life times in condensed sys- 

 tems!), and experimentally, either from the duration of phosphorescence, 

 or from the intensity of the (weak) absorption bands which have been 

 found to correspond to the phosphorescence bands in some organic com- 

 pounds. In general, the actual life-times of phosphorescence were not 

 shorter, but longer (by factors of the order of 10, 10-, or even 10^) than the 

 theoretical life-times, particularly in the case of aromatic compounds. The 

 life-times derived from the intensity of the absorption bands also often 

 were shorter than those observed by the phosphoroscopic method. 

 Whether these results indicate that in some molecules, at least, the metas- 

 table state corresponds to an atomic, rather than an electronic, tautomer 

 remains to be seen. A possible alternative explanation is that the life- 

 time calculations on the basis of the triplet-singlet exclusion rule alone give 

 too small values because this exclusion rule is reinforced, particularly in 

 aromatic systems, by additional symmetry considerations. 



In the present chapter, we are concerned particularly with one aspect 

 of the problem of long-lived active states — that of the "afterglow." The 

 photochemically produced tautomeric products or the metastable triplet 

 molecules may have such high energy that, with the help of thermal energy 

 fluctuations, they can return, after a certain interval of time, into the orig- 

 inal electronically excited state and cause the emission of "delayed fluores- 

 cence" (also designated as "afterglow" or "phosphorescence"). This cycle 

 (c/. schemes 23.1 A and B) provides the most plausible explanation of phos- 

 phoresence of many dyestuff solutions. (Solutions of eosin, erythrosin, 

 rose bengal and many other dyes all show an afterglow lasting for 10"* 

 to 10~^ sec.) According to Kautsky, Hirsch and Flesch (1935), who 

 studied this effect in numerous dyes, the afterglow is extremely sensitive 

 to oxygen; a few millimeters pressure of this gas suffice to suppress it. 



