REVERSIBLE PHOTOCHEMICAL PROPERTIES OF DYES 5 



significance without some knowledge of the state of binding of the 

 pigments in their natural state. 



SPECTRAL SHIFTS IN METASTABLE SYSTEMS 



It has been known for more than 50 years that certain dyes in very 

 rigid media (e.g., glycerol at — 80°C or boric acid glass at room 

 temperature) exhibit a phosphorescence. The phosphorescence may be 

 of the same color as the normal fluorescence or, more usually, of 

 longer wavelength; these are referred to as alpha and beta phos- 

 phorescence, respectively (for review, see Pringsheim, 1949). It is 

 generally agreed that a metastable state of the dye is involved which 

 lies intermediate between the first excited singlet state and the ground 

 state, and that alpha phosphorescence arises from thermal excitation 

 of the molecules in the metastable state to the ground state via the 

 singlet excited state (Jablonski, 1935). On the other hand, passage 

 directly from the metastable state to the ground state is accompanied 

 by beta phosphorescence. This latter transition is "forbidden" by 

 ordinary spectroscopic rules, and the metastable state is long-lived. In 

 fact, G. N. Lewis and his co-workers have shown that the metastable 

 state is the triplet excitation state (for review, see Kasha, 1947). 



Since the triplet state for an excited dye in very rapid media may 

 have a lifetime of several seconds, it is possible to populate this state 

 with high-intensity illumination (such as is obtainable from a super- 

 high-pressure mercury lamp of the AH-6 type, for example). If then 

 the absorption spectra of the dye are determined under intense cross- 

 illumination, the spectra of the metastable species are obtained. Thus, 

 it was found that fluorescein in boric acid glass which normally ab- 

 sorbs maximally at 490 mt^ exhibits its maximum absorption at 660 

 niiit when cross-illuminated (Lewis et ai, 1941). 



It is unlikely, however, that light-induced spectral shifts of this kind 

 are of direct importance in photoperiodism. The probability of transi- 

 tion to the triplet state is generally low, and the lifetime of this state 

 under normal viscosity conditions is only about one-tenth of a milli- 

 second (Adelman and Oster, 1956). Of more relevance, perhaps, is 

 the fact that many aromatic substances form dimers under the action 

 of light. In most cases ultraviolet light is required, and the dimer is 



