94 LIGHT AND LIFE 



plot ol the reciprocal of the polarization against the concentration 

 and can therefore have significance to a fraction of an A vmit. 



Observation of the polarization spectrum at different concentra- 

 tions indicates that the depolarization of the fluorescence is inde- 

 pendent of the exciting wavelength in the case of phenol, though not 

 in the case of indole (Fig. 9) . In the latter instance, excitation with 

 wavelengths shorter than about 290 niyu, results in transfer with 

 Ro =^ 14 A; but transfer is not conspicuously observed upon excita- 

 tion with wavelengths longer than 305 ni/x. From these measure- 

 ments, it appears that once the molecule has reached the zero level 

 of the excited state of Si, transfer to another indole molecule is im- 

 probable, and that it occurs only through the momentary thermal 

 excitation to Sg (improbable at — 70°C) or during the thermal 

 equilibration of the vibrational energy when the molecule is excited 

 to S2 or higher levels. 



Excited States Belonging to Specifically Bound Groups 



Since these are discussed in detail in the symposium by Dr. Velick, 

 I will not attempt to deal with them here. 



Excited States of Artificially Coupled Moieties 



Of these, the only one that has been studied from the points of view 

 of interest to us are the conjugates of proteins with dimethylamino 

 naphthalene sullochloride (DNS) (25, 22). The lifetime of the ex- 

 cited state of DNS-conjugates with different proteins ranges from 

 12 to 15 m/i,sec. The fluorescence spectrum of simple DNS deriva- 

 tives, e.g., the sulfonamide, shows very considerable displacement 

 toward longer wavelengths in media of high dielectric constant. Fig. 

 10 shows this fact for mixtures of dioxane and water. A Lippert 

 plot of the wave number difference between the maxima of absorp- 

 tion va, and fluorescence v/ against the A/ value shows the predicted 

 straight-line relationship, from which it may be concluded that the 

 dipole moment in the excited state is higher than that of the ground 

 state by 10-12 D units and corresponds to a contribution of the ionic 

 forms to the excited state of over 50 per cent. The absorption maxi- 

 mum of the last band is found in dioxane at 340 m^x and in 80% 

 water — 20% dioxane at 332 niyu,. Such changes can ])e correlated 

 with the proton acceptor character of the NH2 in hydrogen bonding, 

 and the large contribution of the ionic form 



