The absorption and emission spectra in solution of oligothiophene-based push-pull biomarkers: a PCM/TD-DFT vibronic study

We here report a thorough quantum mechanical study of the optical properties of several N-succinimidyl-ester and methyl-amide derivatives of bi- and terthiophene, which have shown promising performances as biomarkers. We study in particular the dependence of the absorption and emission spectra on the substituents and on the nature of the embedding medium, from the gas phase to polar solvents. Our approach is based on time-dependent density functional theory calculations, by comparing the performances of different functionals and, finally, using long-range-corrected CAM-B3LYP as reference functional. Solvent effects are included by the Polarizable Continuum Model, exploiting both its linear response and state-specific implementations. In order to simulate the absorption spectra at room temperature, a hybrid quantum/classical approach is adopted where the broadening effects due to the torsional flexibility of the system are taken into account within the classical Franck-Condon principle, starting from a relaxed three-dimensional potential energy surface, while the quantum vibronic contribution of the remaining degrees of freedom is described in harmonic approximation according to a time-dependent approach. The theoretical predictions are in good agreement with experiments, providing interesting indications on the accuracy of different functionals and on the main chemical-physical effects modulating the excited state properties of these compounds.