LONG-LIVED ACTIVE STATE AND AFTERGLOW 791 



states {i. e., states with total electronic spin zero). Triplet states, with 

 a total electronic spin of one unit, can be obtained by reversing the spin of 

 one electron; but to do this in a closed shell, it is necessary to change also 

 its orbital eigenfunction, thus converting it from a bonding into an anti- 

 bonding electron— in other words, dissociating or weakening a chemical 

 bond (e. g., breaking the second bond in a C=C, C=0 or C=N double 

 bond) . The molecule in the triplet state thus partakes of the character of a 

 biradical.* The radiative return of such a molecule into the ground state, 

 with the emission of fluorescence, is "prohibited," because of the require- 

 ment that the spin must be conserved; the light quantum has no mecha- 

 nism for carrying the spin away. This makes "activated" molecules of the 

 triplet biradical type metastable— at least as far as termination of activa- 

 tion by fluorescence is concerned. The singlet-triplet prohibition apphes, 

 however, strictly only when the electron spin does not interact with other 

 modes of motion of the electrons in the system, since such interactions give 

 the chance of disposing of the spin momentum by converting it into other 

 rotational momenta. One interaction, which is always there, is the cou- 

 pling of spin momentum with the rotational momentum of the electron 

 movement around the nucleus (orbital momentum). This interaction is 

 weak in light elements and increases with increasing atomic number. Other 

 interactions arise when the triplet molecule is exposed to external fields of 

 force (electric or magnetic). This is the case in condensed phases, where 

 each molecule finds itself in the fields of force of the adjoining molecules. 

 Thus, the theoretical life-time of the metastable triplet molecules, as cal- 

 culated for an isolated molecule by considering only the coupling with the 

 orbital momentum, must be considerably reduced in condensed systems 

 in consequence of coupling with the medium. Furthermore, in addition 

 to radiative transfer into the ground state (fluorescence), metastable 

 molecules are exposed also to energy dissipation by internal conversion 

 into vibrational energy — made irreparable, in a condensed phase, by loss 

 of vibrational quanta to the surrounding molecules. This is the same kind 

 of process by which metastable molecules are produced from the excited 

 molecules in the fluorescent singlet state (transition A -^ T in schemes 

 23. lA and 23. IB). The latter transition occurs within <10-^ sec. (since 

 it successfully interferes with fluorescences); could it be that a similar 

 transition, T -^ X, is delayed for a second or even a minute? (This is the 

 lifetime of long-lived activation in some dyestuffs, as derived from the 

 decay curves of phosphorescence.) 



These were the considerations which have caused us in Volume I to 

 consider the hypothesis of metastable tautomeric states as more likely 

 to explain long-lived activation than the hypothesis of metastable elec- 



* We mean by this term a radical with two free valencies, not a combination of 

 two radicals. 



