EXCITATION OF POLYENES AND PORPHYRINS 



97 



2. Chlorins (including rhodins), which are derivatives of dihydro- 

 porphin; and 



3. Bacteriochlorins, which are derivatives of tetrahydroporphin (Fig. 

 2-14) (Rabinowitch, 1944; Aronoff, 1950). 



Typical spectra of these compounds are shown in Figs. 2-15, 16, and 17. 



PORPHIN 



One-electron Orbitals. The orbitals and transition energies were com- 

 puted for porphin and tetrahydroporphin by Longuet-Higgins et al. (1950) 

 using the LCAO molecular-orbital approximation.^ The highest filled 

 and lowest empty orbitals are shown in Fig. 2-18, and the centers of 



^lU -^lU ^2U -D2U -c/g 



-0.8 - 



-0.4 - 



0.0 - 



0.4 - 



0.8 



PORPHIN TETRAHYDROPORPHIM 



Fig. 2-18. Orbital energies according to symmetry classes, and lowest allowed transi- 

 tions in porphin and tetrahydroporphin. The double symbol J_ , 1| indicates double 

 degeneracy. The single symbols indicate polarization with respect to the longest axis 

 of the conjugated system in tetrahydroporphin. 



gravity of the first excited configurations are shown after them in Fig. 

 2-19 for comparison with the observed excited singlet states. The 

 orbitals in the columns marked y were computed assuming the central 

 nitrogen atoms to be equivalent to carbon atoms. Those marked y' were 

 computed with a higher electron affinity at the nitrogen positions, all 

 nitrogen atoms still being assumed equivalent to each other. The pre- 

 dicted configuration energies from either set of orbitals fit the observed 

 singlet levels about as well as predictions fit in other molecules when made 

 by the same methods (Piatt, 1950). The symbols at the top of Fig. 2-18 



^ Including consideration of the overlap integral. 



