40 



LUMINESCENCE SPECTROSCOPY OF MOLECULES 



torsional mode of vibration, probably by introduction of a bamer 

 (dotted curve, Fig. 3) on the potential curve (we disregard the simi- 

 lar barrier in the ground state; it introduces no new feature). Conse- 

 quently, absorption is to a nearly equilibrium configuration, with re- 

 emission of fluorescence directly observable. 



90 



AN&LE 



180 



Fig. 3. The variation of potential energy with twist of the double bond in ethyl- 

 enic molecules. The dotted curve represents a barrier to torsion introduced 

 by inhibition of twisting motion in rigid solvents by substituted groups on 

 the ethylene. A, fluid; A', rigid. 



The vinyl group — CH = CH2 in the chlorophylls is conjugated 

 with the main 7r-electron system of the molecule. If this vinyl group 

 is free to undergo torsion upon excitation, it is possible that in the 

 isolated molecule this could serve as the chief dissipator of electronic 

 excitation energy. Its efficiency as a dissipator would depend on the 

 magnitude of the interaction of the main 7r-electron system with the 

 torsional potential. The low quantum yield of fluorescence in alcohol 

 solution in both chlorophyll a and chlorophyll h may be due to such 

 an interaction. The absence of phosphorescence in chlorophyll a and 



