BENTLEY GLASS 829 



nioleciiles, and chain molecules such as cis- and />Y/»,v-still)ene and 

 various cya nines. He suggested speculatively that in the case of a 

 cyanine molecule the attachment of a donor molecule at one end 

 and an acceptor molecule at the other might j^ermit the chain to 

 serve as a conductor in a charge-transfer type of reaction. 



The Triplet State in Fluid Solution 



Two electronic excited states are of primary significance. The 

 lowest excited singlet state determines the long wavelength spectrum 

 and is the first state to be attained when light energy is absorbed. 

 The lowest triplet state, reached only indirectly via the singlet 

 state, is the excited state of lowest energy, has a relatively long life- 

 time, and consequently a greater probability of reacting chemically. 

 George Porter has considered the three spectroscopic methods available 

 for studying the triplet state: (1) direct absorption from the ground 

 state; (2) the spectrum emitted in transition from the triplet to the 

 groimd state; and (3) transition from the lowest triplet state to a 

 higher triplet level, as a result of a further absorption of energy. 

 The first method yields the energy differences between the lowest 

 triplet and the ground state, and also the extinction coefficient re- 

 lated to the probability of the transition and the lifetime of the triplet 

 state. The second method, involving study of the phosphorescence of 

 the system, yields the energy of the triplet level and its lifetime. Be- 

 cause phosphorescence is observed only in solution in very rigid 

 glasses or extremely viscous solutions, while workers are more in- 

 terested in gaseous and fluid phases, the method is not very useful 

 for the study of the influence of conditions upon the lifetime of the 

 triplet state, and the occurrence of reactions involving it. Method 

 three informs us directly of the energy of the higher triplet levels, 

 and makes possible the direct measurement of triplet concentration 

 as a function of time in all phases, gas, liquid, and solid. 



In fluid solutions the lifetime of the triplet state is less than one 

 millisecond; hence flash photolysis must be used to study it. In this 

 procedure, first adapted for the study of triplet states by Porter and 

 Windsor in 1952, the system is exposed to an electronic flash of a few 

 microseconds duration and several hiuidred Joides of energy; and 

 the transient absorj)tion spectra are recorded by photographing with a 

 second electronic flash or by continuous photoelectric recording of 

 optical density at a chosen wavelength. The first experiments showed 

 that the general absence of phosphorescence in gases and liquid 

 solutions is owing to the occurrence imder those conditions of radia- 



