EXCITATION OF POLYENES AND PORPHYRINS 85 



The singlet-singlet radiationless process is much faster than photon 

 emission or singlet-triplet radiationless transitions. As a result, the mole- 

 cule cascades directly down from the higher excited singlets to the first 

 one in about 10~^'^ sec or less without light emission. This cascade inter- 

 nally quenches the fluorescence that n;iight otherwise have been expected 

 from these higher states. 



The first excited state is sufficiently far above the ground state in most 

 cases so that the radiationless process becomes unimportant. The mole- 

 cule may then go from this state to the ground state by fluorescence 

 {F in Fig. 2-7) in 10^^ sec. Or it may occasionally go to the lowest 

 triplet in 10~^ sec and from there to the ground state by phosphorescence 

 (P) in 10~^ sec (Lewis and Kasha, 1944). 



Effect of Heavy Atoms and Oxygen. The time to go to the lowest triplet 

 and the time to phosphoresce are materially shortened by the presence of 

 heavy atoms such as iodine, bromine, or heavy metals in the molecule 

 or in the surrounding medium (McClure, 1949; Kasha, 1952). This is 

 due to the large spin-orbit coupling in heavy atoms which makes the 

 singlet and triplet states interact more strongly; the coupling is pro- 

 portional to the square of the nuclear charge. [Some complexing agents 

 have effects like those of heavy atoms, but the explanation of this effect 

 is not yet clear (Reid, 1952).] The result may be that the fluorescence is 

 almost completely converted to phosphorescence, which then becomes 

 very strong but has a short lifetime. 



The presence of paramagnetic substances such as dissolved molecular 

 oxygen or the presence of a metal atom with an odd electron, as in the 

 copper-porphyrin complexes, may have much the same effect (Calvin 

 and Dorough, 1948). 



Thermal Quenching. The "intrinsic lifetimes" of long-lived lumines- 

 cences can be measured accurately only at low temperatures or in solids 

 where another competing process, "external quenching," is negligible. 

 In solutions at room temperature the random interaction with such mole- 

 cules as solvent molecules or dissolved oxygen produces a crossing of 

 potential curves even between the first excited states and the ground 

 state, so that the fluorescence and phosphorescence energy is dissipated 

 into vibrations and thermal energy in times of the order of perhaps 10~^ 

 sec. This quenches the phosphorescence, though not the fluorescence. 



Energy Transfer. Another type of external quenching which has 

 received much attention lately is the radiationless transfer of energy 

 from one excited molecule to another that has a lower-energy excited 

 configuration (Bowen, 1938; Kallmann and Furst, 1950; Franck and 

 Livingston, 1949; Forster, 1951; Moodie and Reid, 1952). Li mixtures 

 the light emission may come almost exclusively from the molecular species 

 that has the lowest fluorescent or phosphorescent state, apparently even 

 when the concentration of this species is only a fraction of a per cent, and 



