EXCITATION OF POLYENES AND PORPHYRINS 



81 



Each configuration gives rise to a group of states, which have shghtly 

 different energies, because of the interactions of the electrons with each 

 other which have been neglected up to now. For polyenes, which are 

 especially simple, each configuration has two states: a singlet in which 

 all electron spins are paired and a triplet in which two are unpaired. An 

 exception is the ground state and others like it in which the electrons in 

 each shell are paired and there can be no triplet. For convenience, we 

 may label the states A (ground), B (first excited), C (second excited), 

 and so on. The configurations and states for any polyene are listed in 

 Table 2-1. A left-hand superscript on a state shows whether it is singlet 



With hetero replacements 



