CASPT2 calculation of the excited states of symmetric polyenyl cations

Abstract

Ab initio quantum mechanical calculations have been performed to study the excited state properties of the symmetric all-E-configurated polyenyl cations, [R₂C–(CH–CH)_n–CH–CR₂]⁺, with R = H and CH₃ and n from 1 to 5. All geometries were fully optimised within the C_2v space group using the DFT method and the Becke LYP3 exchange correlation functional in conjunction with the 6-31G** basis set. Excited state energies were calculated using multi-configurational second-order perturbation theory (CASPT2) within the framework of CASSCF and a 4s3p1d/2s ANO basis set. The vinylene shift of 76 nm which is experimentally observed for these compounds is reproduced with remarkable accuracy; the CASPT2 corrected vertical energies of the lowest excited state (1 ¹B₂) are within 0.07 eV of the experimental values. There is a steady increase of the bathochromic shift induced by the four terminal methyl groups, from 75 nm for the shortest cation 1 (which has five conjugated π-orbitals) to 115 nm in 5 (with 13 π-orbitals). The symmetry forbidden 2 ¹A₁ state is slowly decreasing with respect to the 1 ¹B₂ state as the length of the chromophore increases, with the energy gap going from 1.53 eV for 1 to 0.54 eV in 5. However, it appears that the 2 ¹A₁ state stays above the 1 ¹B₂ state even for very long chromophore lengths. There is a close correlation between the electronic states of symmetric polyenyl cations and streptocyanine dyes.