ENERGY EXCHANGE IN PHOTOREACTIONS 17 



in molecular orbitals. In the higher excited states of benzene, a Rydberg- 

 like series of levels is observed owing to the entire benzene molecule acting 

 like a single atom for highly excited electrons (Sponer and Teller, 1941). 

 Atoms with electrons over and above those reciuired for the <j single bonds 

 can substitute for double bonds to make the picture of conjugation quite 

 general. Pyrrole, for instance, behaves somewhat like benzene, and the 

 electronic correlation between thiophene and benzene is very close. 



2-3. CONJUGATED MOLECULES 



By far the largest number of molecules of importance in biological 

 photochemistry are of the conjugated type. The list includes, among 

 others, all the pigments of photosynthesis and flower coloration; ribo- 

 flavin, which photosensitizes auxin destruction in the control of plant 

 growth (Galston and Hand, 1949) ; and the pigments of all types of vision. 

 Their distinct role can be attributed to five characteristics: 



1. Low-lying excited states, frequently in the visible and hence useful 

 for vision, photosynthesis, and the like, and at the same time not effec- 

 tive in destroying the molecule. 



2. Stable excited states with poor crossing to the ground-state potential 

 surface 



3. High electrical polarizabilities, i.e., loosely held electrons that favor 

 interaction with other molecules. 



4. Low ionization potentials. , 



5. Metallike conduction of electrons throughout the conjugated bond 

 system. 



These properties of conjugated molecules are worthy of further con- 

 sideration: 



1. The overlap of electron clouds, or, more accurately, of wave func- 

 tions describing the positions for 7r-electrons, allows greater movement of 

 the electrons in space and hence lower frequencies of sympathetic oscil- 

 lation with the pulsating field of the incident radiation. Benzene has a 

 long-wave-length cutoff at 2800 A, but, as additional aryl rings are added, 

 this is extended into the near ultraviolet and finally, with naphthacene, 

 into the visible at 4500 A (Sponer and Teller, 1941). The strong absorp- 

 tion of red light by chlorophyll is the result of extensive coupling of 

 aromatic rings (pyrrole) conjugated to each other and to a magnesium 

 atom introduced into their center. Addition of two hydrogen atoms in 

 one pyrrole, strangely enough, improves conjugation, since bacterio- 

 chlorophyll, a photosynthetic pigment of bacteria, absorbs in the infrared 

 at 8000 A. Absorption in the visible and near infrared allows best use of 

 that fraction of the sun's radiation which reaches the earth. It is, for 

 instance, very important that the pigments of plants or animals not be 

 destroyed in each primary process, since, in addition to the problem of 



