52 G. OSTER 



LONG-LIVED EXCITED STATES 



We have found that very small amounts of certain substances will 

 retard the photoreduction of dyes. For example 2.5 X lO"* mole 

 per liter of paraphenylenediamine will decrease the rate of photoreduc- 

 tion of eosin (carried out in the presence of allyl thiourea, oxygen 

 being excluded) to one-half its normal value (8). For aqueous solu- 

 tions at room temperature there are 6.6 X 10' encounters per second 

 in a liter of molar solution. Hence, we calculate that the lifetime of 

 the photoreactive dye molecules is at least 10 ~^ second. This is about 

 ten thousand times greater than the lifetime of the first excited sing- 

 let state. Our studies with se^•eral other of the dye-reducing agent 

 combinations show that in all cases the electron donor reacts \v\th 

 dye only when the latter is in the long-lived metastable excited state. 

 Detailed kinetic studies on the photoreduction of fluorescein and its 

 halogenated derivatives (9) show that the steps of the reaction are 

 similar to those proposed for the ethyl chlorophylHde-sensitized oxi- 

 dation of allyl thiourea reaction by Gaffron (10) and the photobleach- 

 ing of chlorophyll in methanol by Livingston (11). Our studies show 

 that not only is the quantum yield of photoreduction decreased by 

 the addition of the dye itself but also a wide variety of other dyes 

 (with or without spectra which overlap the spectra of the original 

 dye) will, even in very small concentration, markedly decrease the 

 quantum yield. 



The hmiting quantum yield obtained by extrapolating the rates 

 to infinite reducing agent concentration varies markedly even with 

 dyes of the same family. For example, fluorescein has a hmiting 

 quantum yield of 2.0 X 10 "^ whereas for dibromofluorescein the 

 hmiting quantum yield is 12.4 X IQ-^. The limiting quantum yield 

 is determined by the relative number of quanta which are used to 

 obtain the long-lived reactive species to those wasted by fluorescence 

 and internal conversion. Results for eosin, for example, show that 

 on an average about 1 of 11 molecules which are in the first excited 

 singlet state undergoes transition to the metastable state. Of these 

 remaining 10 excited molecules about 1.5 (on an average) revert to 

 the ground state with, the emission of fluorescence, since the quantum 

 yield of eosin is approximately 0.15, and the remainder fall to the 

 ground state by a radiationless transition. 



