PHOTOCHEMISTRY 7 



the amount of energy available, internal convn'rsion may lead to the dis- 

 sociation of a molecule either into two stable molecules or into radicals. 



PHOSPHORESCENCE AND LONG-LIVED FLUORESCENCE 



Metastable states appear to play an important part in the photo- 

 chemistry of complex molecules. The existence of these states has been 

 demonstrated indirectly by the analysis of photochemical data (Shpol'skii 

 and Sheremet'ev, 1936) and directly by a study of the "phosphorescence" 

 and "long-lived fluorescence" of these molecules (Pringsheim, 1949; 

 Forster, 1951). Practically all complex molecules (at least those which 

 contain a double bond) are either fluorescent or phosphorescent (or both) 

 when they are dissolved in glassy media or adsorbed on suitable solids. 

 One of the first examples to be studied quantitatively was the dye trypa- 

 flavin adsorbed on siUca gel (Pringsheim and Vogels, 1936). At ordinary 

 temperatures this system emits a strong fluorescent green band and a 

 separate weak orange band. The green band is made up of ordinary 

 short-lived fluorescence and a relatively long-lived emission, having the 

 same wave-length distribution. The half life Ty^ corresponding to the 

 latter process is an exponential function, ry, = ke^'^'^, of temperature. 

 There is no short-lived fluorescence corresponding to the orange band. 

 The half life corresponding to this latter transition is independent of tem- 

 perature and equal to 1.2 sec. As the temperature is reduced, the life 

 corresponding to the green phosphorescence eventually becomes longer 

 than that pertaining to the orange, long-lived fluorescence, and the slow 

 emission becomes predominantly orange. This general behavior is 

 exhibited by a wide variet}^ of substances (Kasha, 1947). Many measure- 

 ments of this type (Lewis and Kasha, 1944, 1945) have been made with 

 absorbing substance dissolved in a solvent, such as a mixture of ether, 

 pentane, and alcohol, which is fluid at ordinary temperatures and becomes 

 rigid at low temperatures. Under these conditions, only ordinary 

 fluorescence is observed in the fluid solvent, the temperature-dependent 

 phosphorescence appears when the solvent becomes very viscous, and the 

 temperature-independent, long-lived fluorescence becomes noticeable at 

 still lower temperatures. 



A reasonable explanation of these phenomena, which was first proposed 

 by Jablonski (1935), is illustrated by the simplified energy diagram of Fig. 

 1-2. The several electronic-energy levels are represented by horizontal 

 lines, capped by a bundle of horizontal lines which indicate the overlap- 

 ping generalized oscillational levels. For an ordinary stable molecule, the 

 ground level N and the two excited levels F and F' are singlet states. The 

 metastable level M is, presumably, a triplet level. The transitions which 

 correspond to arrows 1 and 2 represent the absorption of photons, which 

 raise the molecule into its first or second excited (singlet) state. In its 

 initial state the molecule will be in thermal equilibrium with its surround- 



