756 FLUORESCENCE OF PIGMENTS IN VITRO CHAP. 23 



(a) Internal Conversion (Physical Dissipation of Excitation Energy) 



In complex molecules, electronic excitation states are likely to be con- 

 verted, before fluorescence has had time to occur, into strongly vibrating 

 states. The reason for this is that the nonvibrating (or weakly vibrating) 

 excited electronic state may have not only the same energy, but also the 

 same nuclear configuration as strongly vibrating, electronically nonexcited 

 states. (This is the many-dimensional equivalent of the "crossmg of po- 

 tential curv^es" of diatomic molecules.) It was stated above that, in an 

 isolated molecule, internal conversion is reversible, and therefore can only 

 delay but not prevent fluorescence. In condensed systems, on the other 

 hand, the vibrational quanta of the "converted" molecule can be lost, by 

 collisions, to the molecules of the medium. The loss of one or a few vibra- 

 tional quanta may be sufficient to make return into the original, electroni- 

 cally excited state impossible, and thus prevent fluorescence. The re- 

 maining vibrational quanta can then be lost, at leisure, one by one, to the 

 surrounding molecules. 



(b) Isomerization or Dissociation ("Monomolecular'' Chemical Quenching) 



The excitation energy can be used, within the absorbing molecule 

 (either directly, or after "internal conversion" into vibrational energy), for 

 a chemical change, e. g., isomerization, or dissociation. Here again, the 

 reversible character that the process has in vacuum may be lost in the 

 presence of foreign molecules. The photochemically formed isomer or 

 tautomer may, for example, lose one or several vibrational quanta by col- 

 lisions, and thus become incapable of reconversion into the original, elec- 

 tronically excited form. Similarly, the recombination energy of the disso- 

 ciation fragments may be lost to foreign molecules serving as "third bodies," 

 so that the original molecule will be formed directly in the nonexcited 

 ground state. (In this case, the dissociation remains chemically reversible, 

 but recombination is not the exact reversal of photochemical dissociation, 

 smce it occurs without chemiluminescence.) 



(c) Reaction with Foreign Molecules {" Bimolecular" Chemical Quenching) 



The presence of foreign molecules opens the possibility of "bimolecular" 

 chemical reactions of electronically excited molecules. The reaction part- 

 ners may be the molecules of the solvent, or molecules of an accidental im- 

 purity (e. g., dissolved oxygen) or specially provided "quenchers." In this 

 case again, even if the photochemical reaction is reversed afterward, the 

 reversal is likely to occur without chemiluminescence; in other words, the 

 light energy used for the photochemical forward reaction will not be avail- 

 able for re-emission in the back reaction. 



