830 LIGHT AND LIFE 



tionless decay at much greater rates than those of the radiative 

 processes characterizing phosphorescence. The triplet states in Hquid 

 solvents have lifetimes of the order of 10-^ seconds. The kinetics of 

 the transitions in a number of systems aromatic hydrocarbons and 

 their derivatives have been studied by continuous photoelectric re- 

 cording of optical density. It seems that certain modes of behavior 

 are rather characteristic of the triplet states. In rigid media a first- 

 order decay to the ground state is radiative, with a lifetime determined 

 by the probability of transition between the two states and of the 

 order of 10-^ up to 10 seconds. In fluid or gas phases, the non- 

 radiative decay has a lifetime of 10-^ to 10-3 seconds, bearing little 

 relation to the radiative lifetime. This decay is partly second-order, 

 from collisions between two triplet molecules, and partly first-order, 

 depending upon triplet concentration. Both processes are affected 

 by the viscosity of the medium. The nature of the first-order process 

 is not yet understood. The second-order reaction is a special example 

 of the more general quenching of the triplet state by all paramagnetic 

 species. The first such quencher to be found was oxygen, and it is 

 so efficient that pains must be taken to degas the solutions before 

 triplet state absorption can be observed. Nitric oxide is about as 

 efficient a quencher as oxygen. Paramagnetic ions are quenchers and 

 diamagnetic ions are not; but the efficiency of the former bears 

 little relation to its magnetic susceptibility or number of unpaired 

 electrons. Porter consequently interprets the differences between such 

 quenchers in efficiency as arising from the varying overlap of the 

 orbitals of the unpaired electrons in the triplet and in the quencher, 

 rather than from magnetic field interactions. Quenching may arise 

 also directly from triplet-triplet energy transfer between molecules 

 when the quenching molecule is capable of a lower trij^let energy level 

 than the original molecule. Reactions of this sort conserve the elec- 

 tron spin and hence occur with high probability: 



Ar + Bs -> ^s + Br 



Chemical reaction of the triplet state will also lead to its disappear- 

 ance. Photochemical reactions may proceed from either the lowest 

 excited singlet or triplet state. In relatively few cases has the precise 

 state been identified. 



The excited triplet state of a molecule is "a species with its own 

 structure and its own physical and chemical properties . . . which may 

 be very different from the properties of the same molecule in its 

 ground state." Redox potential and pW are often entirely different. 



