Reversible Spectral Changes in Chlorophyll 

 Solutions, Following Flash Illumination* 



HENRY LINSCHITZ and EDWIN W. ABRAHAMSON, The Departments 



of Chemistry, Syracuse University, Syracuse, New York, and Brookhaven 



National Laboratory, Upton, Long Island, New York 



Theories of the mechanism of dye-photosensitized reactions, in 

 particular those catalyzed by chlorophyll, have long postulated the 

 intermediacy of metastable excited states of the dye. This assumption 

 was based originally on indirect evidence, such as the maintenance of 

 high photochemical quantum yields even at low substrate concentra- 

 tions, and the lack of fluorescence quenching by photochemically 

 efficient substrates (1). More recently, direct verification of the exist- 

 ence of such long-lived states has been provided by study of reversible 

 spectral changes in chlorophyll solutions, either following intense 

 flash irradiation (2-4) or under steady cross illumination in rigid sol- 

 vents, under conditions precluding bimolecular solute reactions (5). 

 In this report we present observations and remarks on flash-bleach- 

 ing of chlorophyll in fluid solvents. The chief results to date are the 

 verification of the general shape of the transient spectra, observation 

 of a new far red absorption band in the "metastable state," and 

 demonstration of kinetic effects of polar and nonpolar solvents. 



APPARATUS 



This is shown schematically in Fig. 1. A beam of light from source 

 "S" was collimated by lens " L," passed through a shutter, the 

 dye solution, thence through a monochromator and onto a photo- 

 multiplier tube. Light transmissions at selected wavelengths were 

 recorded on the oscillograph, immediately preceding and following 

 flash excitation. 



A major problem in using this technique is to prevent swamping of 

 the measuring light by scattered light from the flash. Sample fluores- 

 cence may also interfere. Toward this end, the monochromator was 



* Research performed under the auspices of the U. S. Atomic Energy Com- 

 mission. 



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