456 



S. S. Brody and M. Brody 



The latter workers noted (as had Brody (4 )) that at room temperature the 

 emission spectrum from concentrated solutions differed from dilute solutions, 

 but they attributed the difference to deformation resulting from self-absorp- 

 tion of fluorescence, rather than to the presence of the aggregate. While the 

 shift of the main emission band from 676 to 680 m|j. - in going from dilute to 

 concentrated solutions, respectively - can probably be attributed in large part 

 to self-absorption, the shift in the "secondary peak" from 728 to 734 m|jL can- 

 not be. It cannot be, because at room temperature there is no significant 

 absorption at wavelengths longer than 710 m|j.; the shift has to be attributed to 

 the fornnation of another fluorescent species. 



It remains to be seen if the difference in position of the low temperature 

 emission maximum in concentrated solutions - reported at 734 mfj. by Stensby 

 and Rosenberg (52), and at 720 m^JL by Brody (4, 9) - arises simply from an 

 instrumental artifact or from a more subtle source. 



The lifetime of emission of the aggregate, at 77 K, has recently been 

 redetermined by Butler (17); he reports it to be several nanoseconds, in 

 which case, emission from the aggregate probably arises from a singlet state. 



2. Evidence Based on Absorption Spectra : Trurnit and Colmano (53) have 

 found that by compressing monolayers of chlorophyll it is possible to shift the 

 postion of the red absorption maximum to 675 mfjL - to correspond with the 

 position of the maximum in Chlorella . The red shift is accompanied by in- 

 crease in band width and change in ratio of red/blue absorption maxima - 

 demonstrating that the in vivo absorption spectrum can be simulated solely 

 by chlorophyll-chlorophyll systems. 



In ethanolic solutions in which the chlorophyll concentration is greater than 

 lO'^M, broadening of absorption spectra can be observed at room tempera- 

 ture ( 6 ). A study restricted to the Soret band has indicated that similar 

 broadening (24) occurs in chlorophyll dissolved in coUodian when the concen- 

 tration is increased from 10"^ to 5 x 10" M; these experiments could not be 

 continued to higher concentrations because of high optical density. 



Stensby and Rosenberg (52) have reported, however, that at room temper- 

 ature they could not detect a change in the red absorption band of chlorophyll 

 in ethanol - even though they worked with concentrations up to 8 x 10"^ M. 

 They reported that at higher concentrations undissolved pigments were 

 observed. We have recently repeated our measurements and agree with 

 Stensby and Rosenberg that there is no appreciable change in the red region 

 of the spectrum at concentrations of 8 x 10'^ M. However, at concentrations 

 above 10"^ M, we find the same general features as we reported previously 

 (6 ) (see Fig. 1 for our recently determined absorption spectrum. ) 



Stensby and Rosenberg do find evidence for the aggregate at low tempera- 

 tures. Their absorption spectrum of an 8 x lO'^ m ethanolic solution of 

 chlorophyll clearly shows an absorption band with a maximum at 705 mfi at 



