Theoretical investigation of the relaxed geometry in the first singlet B_u excited state of polyacetylene oligomers

We present the results of calculations carried out at both Pariser-Parr-Pople and ab initio split-valence basis set levels, aimed at finding the nature of the relaxed geometry configuration in S₂ which is the first singlet one-photon optically allowed B_u excited state of three polyene molecules: hexatriene, decapentaene, and tetradecaheptaene. We investigate as a function of chain length the relative stabilities of two types of geometry configuration in the excited state: one configuration derives from the use of traditional bond order-bond length relationships; the other configuration comes from the explicit consideration of the electron-lattice coupling and leads to the formation of a soliton-antisoliton pair. The latter configuration is found to correspond to a stable situation in polyenes containing over ten carbon atoms and provides a smooth evolution to the long chain (polyacetylene) properties.