SPECTRAL CHANGES IN CHLOROPHYLL SOLUTIONS 35 



noise ratio and more nearly monochromatic scanning light than was 

 used by Livingston and Ryan in their original flash experiments (2). 

 In addition, the present technique permits determination of the zero- 

 time of the flash and thereby observation of the development as well 

 as decay of the spectral changes. Despite these improvements, defini- 

 tive kinetics are still difficult to obtain, essentially because the flash 

 duration is comparable to the lifetime of the excited species. This 

 limits the experimental accuracy that can be achieved (transmission 

 changes are obtained by diff"erence) and greatly complicates the 

 theoretical treatment. Our apparatus is now being modified to provide 

 much shorter flashes. 



It has been suggested that a dye-solvent redox reaction might be 

 the mechanism of formation of a metastable species (1,5), and the 

 strong solvation reactions of chlorophyll (8-10) are consistent with 

 this view. To clarify the question, flash experiments were carried out 

 in purified methylcyclohexane, exhaustively dried by distilling over 

 calcium hydride, and then pumping away a large fraction of the sol- 

 vent at low pressure. The extent of dehydration was indicated by the 

 drop in fluorescence of the resulting chlorophyll b solution to 8% of 

 its original A'alue in the undried soh'ent (8) and the appearance in the 

 dry solution of a new band at 665 m^, about eciual in intensity to the 

 usual "wet" band at 655 m^ (9). On flashing, the "dry" band at 665 

 m^ was found to bleach very markedly. The strong bleaching in com- 

 pletel}' nonpolar solvent, as well as the appearance of a transient 

 spectrum rather similar to that observed in methanol or pyridine, 

 implies that at least one of the metastable states is reached from the 

 excited singlet by an intramolecular process alone. Pheophytin and 

 zinc tetraphenyl porphine also show marked spectral changes after 

 flash illumination. 



In pure hydrocarbon solvent, the appearance and decay of ab- 

 sorption at 525 mn occurs at the same rate as the bleaching out and 

 recovery of the 665-mM band, the situation apparently being simpler 

 than in polar solvents (4). The reactions in the polar case thus may 

 involve desolvations or keto-enol shifts in the excited molecule, or 

 excited dye-solvent reactions following formation of the first meta- 

 stable product. In this connection, it is of interest that in pyridine, the 

 half-life of the 525 absorption is considerably increased, and the re- 

 versibility of the reaction is much better than in carefully purified 

 95% methanol. 



