88 



LIGHT AND LIFE 



spectively. Measurements of the polarization of the fluorescence in 

 viscous solvents, and the use of Perrin's equation (17) , are in quali- 

 tative agreement with these figures; but since rotational relaxation 

 times cannot be accurately assigned to the molecules in the viscous 

 solvents used, the method cannot be made to yield precise values. 



Polarization of the Emitted Fluorescence 

 Polarization measurements in viscous solvents can be used to ob- 

 tain information on two important points: measurements of change 

 in polarization with concentration at a fixed wavelength can be used 

 to give information upon the transfer of electronic energy among 

 like molecules (16, 18), while measurements at vanishing concentra- 

 tion and different exciting wavelengths can yield information about 

 the relative directions of the transition moments of the different elec- 

 tronic bands to supplement and often clarify the absorption spectrum. 

 To begin with the latter, this so-called polarization spectrum is best 

 studied in rigid solutions at low temperature. We have used propy- 

 lene glycol solutions at — 70°C. Phenol solutions, and also those of 

 tyiosine and cresol, show a polarization spectrum consisting of two 

 regions, one positive at longer wavelength and one negative at shorter 

 wavelength (Fig. 4) . Calculation of the angle between the two transi- 

 tions from the fiuorescence data (28) with the aid of the equations 

 of Jablonski, and on the assumption that the two main transitions 

 are contained in the plane of the benzene ring, gives a value of 



Polarization of Cresol and Tyrosine 



220 230 240 250 260 ^270 280 290 300 3IO 



Fig. 4. The polari/atioii spectra of tyrosine and cresol. P, polarization. 



X. wavelengtli. 



