15956
A series of polyacenes shows the similar behavior of the optically allowed singlet excitation energies obtained by time - dependent spin - flip density functional theory calculations and experiments , which bears testimony to the singlet open - shell character in long polyacenes .
 Chemical: polyacenes
Please list all entity words in the text that fit the category.Output format is "type1: word1; type2: word2". 
Option: Chemical 
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Answer:
 Chemical: polyacenes