66 LIGHT AND LIFE 



Dr. Calvin: I don't mean that they arc the same but can such a transition 

 occur? 



Dr. Kasha: Oh, aljsolutely, and your other Cjuestion was then why should 

 the fluorescence and phosphorescence difl;er? Well, it turns out that when 

 one examines singlet-triplet mixing for nr electron states in a planar molecule, 

 the singlet-triplet mixing vanishes for single-center interactions as McClure 

 showed, and one has to consider very weak three-centered integrals in order 

 to calculate any magnitude of single-triplet mixing. But in the n — > tt case 

 you get one-center interaction, that is, the spin orbital coupling involves a 

 .single atomic center. So you get a large factor of gain in the rate constant 

 for a single-triplet interaction. 



Dr. Calvin: I wasn't thinking of another triplet. 1 was thinking of another 

 singlet TT state which you achieve by an ?? — > tt* transition which is different 

 from a it state which you achieve by a tt -» -tt* transition. There are two 

 different excited singlets, and you can transfer from one excited singlet tt* 

 to another. The one that you come to from the n state is achievable through 

 the excited "tt ^> tt* transition. 



Dr. Kasha: I.et me simply state that the major source of the difficulty 

 in my understanding your question is that it is based on an orbital diagram. 

 If we go to a state diagram, we get the following picture. If we go from the 

 ground state to the singlet tt, tt* state, we can easily get to the singlet n, tt* 

 state. Then we necessarily get down to the lowest triplet state, the tt, tt* 

 state. If the singlet //, tt* state were not lowest, the molecule could easily 

 re-emit from the singlet tt, tt* state. The reason for this behavior lies in 

 the kinetics of excitation, and is largely due to enhanced singlet-triplet mixing 

 in n . TT* states. 



Dr. Franc;k: IVIay I say a word. I believe thai I ha\e some understanding 

 of the difficulty that Dr. Calvin has. Namely, that the excited states that 

 you arrive at by tt — > tt* transitions and by ?i — > -* transitions are different. 

 In the one case you get a positive hole by removing an electron from the 

 lone pair of the nitrogen atom, in the other case vou get a positi\e hole 

 in tiie highest occupied le\el of the tt orbital. 



Dr. Calvin: But what I was asking is, can you get a transition between 

 these excited states? 



Dr. Franck: Yes, you can Init you can't get it by radiative transitions, 

 only by radial ionless transitions, i.e., l)y internal conversion or by imjjacts. 



Dr. Wkblr: Dr. Kasha has spoken of some energy transfers in DNA, and 

 I think it would be an appropriate time to describe an experiment that I 

 iiave clone. This is an actual experiment showing that the singlet state can 

 be transferred in DN.\. In this experiment we stiulied the excitation fluores- 

 cence spectrum of solutions of DNA to which aurann'n had been added. It 

 had been sliown a numl)er of years ago that aurann'n adsorbs on DN.A with 

 the appearance of (hiorescence. .Amamin itself does not llucjresce in water 

 solutions. Therefore, in this experiment the fluorescence excitation spectrum 



