Structure and redox properties of polymethine dyes: Electrochemical and DFT/TD-DFT study

The electrochemical properties of 48 cationic, anionic and merocyanine polymethine dyes have been studied using cyclic voltammetry. The effect of structural parameters, viz. the chromophore charge, the donor-acceptor properties of the terminal groups, the polymethine chain length, and the molecular symmetry on the redox-potentials are analyzed. The DFT calculations of all the studied dyes have been performed with three different exchange-correlation functionals (B3LYP, CAM-B3LYP, and B97D3) in both vacuum and acetonitrile using the PCM solvation model; additionally, for selected species the BP86, PBE0 and M06-2X functionals were employed. The HOMO and LUMO energies of the dyes obtained from voltammetry data are compared with those from the DFT calculations to select the best performing theoretical approach for evaluation of the frontier orbital energy levels and energy gap of these π-conjugated compounds. Linear correlation equations obtained from the data for 48 dyes may be employed for credible prediction of the HOMO and LUMO energies in the search for designed molecules with required redox properties. TD-DFT calculations have demonstrated that the pure (non-hybrid) BP86 and B97D3 functionals give a relatively low mean absolute error for excitation energies, proving to perform better than the hybrid functionals applied, especially CAM-B3LYP.