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.

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