THERMOCHEMICAL CONSIDERATIONS 1971 



In the absence of spectroscopic evidence we do not know how deep 

 under the 41 kcal level the metastable state hes. Franck considers 5 

 kcal/mole a conservative estimate. Therefore, if the chlorophyll molecule 

 is transferred into the metastable state before participating in photosyn- 

 thesis, it has only 37 kcal/mole (or less) to contribute. However, it is not 

 certain that sensitization occurs only after the chlorophyll molecule has 

 been transferred into the metastable state; in a condensed system, sensiti- 

 zation may successfully compete with this transfer. If it does, all 42 kcal 

 are available for photosynthesis. 



Livingston and Ryan (c/. chapter 35, page 1498) found that chloro- 

 phyll solutions illuminated with a strong flash of light, acquire a transient 

 absorption band at 515 m/x, which could be attributed to a metastable 

 chlorophyll molecule. Witt (1955) noted the same band in plants exposed 

 to a sudden, intense flash (in the same way as chlorophyll solutions in 

 Ryan's experiments). These observations add circumstantial evidence 

 for the existence of the (theoretically anticipated) metastable chlorophyll 

 molecule, and its formation in light, both in vitro and in vivo, but give no 

 clue to what the energy of this state is. 



We do not know by what specific mechanism the electronic excitation 

 energy of chlorophyll — be it 37 or 42 kcal — is first converted into potential 

 energy of atoms, by displacing their nuclei. What we know, however, is 

 that such transformation never occurs without some energy being converted 

 into kinetic energy of the nuclei and ultimately lost as heat. (To prevent 

 this loss, the whole process would have to be conducted "adiabatically," 

 i. e., infinitely slowly — while electronic excitation or deactivation by ab- 

 sorption or emission of a photon is practically instantaneous, and electronic 

 energy transfers between two molecules occur very quickly compared to the 

 speed with which nuclei can re-arrange themselves into a new stable pat- 

 tern.) Consequently, in accordance with the so-called "Franck-Condon 

 principle," we can expect the sensitizer (chlorophyll molecule), after its 

 electronic energy has been transferred to the sensitization substrate (e. g., 

 an appropriately bound HOH or ROH molecule), to be left behind in a 

 configuration that was stable while the electronic system was in the excited 

 state, but appears distorted after the electronic system has reverted into 

 the ground state. The nuclei will therefore begin to vibrate, Uke a re- 

 leased spring. This means that a part of the excitation energy of chloro- 

 phyll will be "left behind," after sensitization, as vibrational energy of the 

 chlorophyll molecule. By the same token, the products of the sensitized 

 reaction (e. g., the radicals H and OH, or R and OH attached to appropri- 

 ate "acceptors," such as X and Z in chapter 7) will be created not in the 

 state of rest, but endowed with more or less violent motion, and some energy 

 wiU be dissipated in the medium in stopping them. Incidentally, since a 



