Knowledge Bank

Simple calculations of singlet electronic state energies in linear polyenes predict an $Ag$ ground state with alternating excited states of $Bu$ and $Ag$ symmetry respectively (a review of linear polyenes can be found in reference 1). However, Hudson and $Kohler2$ in 1972 showed that the lowest lying excited singlet state in 1,4-diphenyloctatetraene was of $Ag$ symmetry and Schulten and $Karplus3$ showed that this result could be predicted theoretically if doubly excited configurations were included in their calculations. This result has also been shown to be general in many $polyenes.3,4$ The question can now be asked if these results carry over to the polyynes. What are the state orderings in polyynes? Does simple theory adequately explain the state ordering of must configuration interaction be included? In this work the above questions will be addressed with results obtained from 1,4-diphenylbutadiyne and 1,4-dimethylbutadiyne (2,4-hexadiyne).