The electronic spectra of symmetric cyanine dyes: A CASPT2 study

Abstract

Ab initio quantum mechanical calculations were performed to study the electronic spectra of symmetric all-E configurated streptocyanine dyes, [R₂N–(CH–CH)_n–CH–NR₂]⁺, R = H and CH₃, from n = 0 (monomethine) to n = 4 (nonamethine). Ground state geometries were optimized based on density functional theory. Excitation energies were calculated using multi-configurational second-order perturbation theory (CASPT2). CASPT2 corrected vertical excitation energies for the long wavelength absorptions were within 0.08 eV of the experiment. Also, these calculations reproduced the 100 nm vinylene shift which is characteristic for these dyes. The agreement between calculated and experimental oscillator strengths was satisfactory. The results were compared with calculations based on a single determinant ab initio approach and on density functional linear response theory. These methods systematically overestimated experimental transition energies.