48 



PHOTOCHEMICAL PRINCIPLES 



may be degraded into heat, concomitant with an unpairing of electron 

 spins, resulting in an intersystem crossing into the lowest excited triplet 

 state (process J) ; the quantum of excitation energy may be transferred 

 to another molecule. Molecules in the lowest excited triplet state may. 



LOWEST EXCITED 

 SINGLET STATE ~f 



GROUND 

 STATE 



LOWEST EXCITED 

 TRIPLET STATE 



Fig. 1. Lowest electronic energy levels of an isolated molecule. Straight 

 lines represent radiative processes; zigzag lines represent radiationless 

 (thermal) processes, a, absorption of light; h, fluorescence; c, thermal 

 degradation; d, intersystem crossing; e, thermal degradation; /, phospho- 

 rescence. 



similarly, undergo one of three processes: phosphorescence (process 

 /), thermal degradation (process e), or energy transfer to another 

 molecule. 



Theoretical Aspects of Energy Transfer 



There are three mechanisms by which electronic excitation energy 

 may be transferred from one molecule to another in unordered systems. 



These are: 



1. The emission of a quantum of radiation by the excited molecule 

 followed by the reabsorption of this quantum by an unexcited mole- 

 cule. This may be repeated many times. The probability of this process 

 is determined simply by the Beer-Lambert law and by the geometry 

 of the system. In general, the lifetime of the excited state of a particu- 

 lar molecule will remain the same, but the lifetime of the emission in 

 a finite system may be increased by the "imprisonment of radiation" 

 (Pringsheim, 1949, pp. 60-63). This mechanism has been shown to 

 be of relatively minor importance in energy transfer in solution (Furst 

 and Kallman, 1954; Kallman et al., 1956). 



2. The transfer of electronic excitation energy through close col- 



