Sjöqvist, Jonas2; Linares, Mathieu2; Mikkelsen, Kurt Valentin3; Norman, Patrick2
1 Administration, Department of Chemistry, Faculty of Science, Københavns Universitet2 Linköping University3 Administration, Department of Chemistry, Faculty of Science, Københavns Universitet
a study of luminescent conjugated oligothiophenes for use as biophysical probes
A methodological development is reported for the study of luminescence properties of conjugated polyelectrolytes, encompassing systems in which dihedral rotational barriers are easily overcome at room temperature. The components of the model include (i) a molecular mechanics (MM) force field description of the solvent in its electronic ground state as well as the chromophore in its electronic ground and excited states, (ii) a conformational sampling by means of classical molecular dynamics (MD) in the respective electronic states, and (iii) spectral response calculations by means of the quantum mechanics/molecular mechanics QM/MM approach. A detailed analysis of the combined polarization effects of the ionic moiety and the polar water solvent is presented. At an increased computational cost of 30% compared to a calculation excluding the solvent, the error in the transition wavelength of the dominant absorption band is kept as small as 1 nm as compared to the high-quality benchmark result, based largely on a QM description of the solvent. At a reduced computational cost the error of the same quantity is kept as small as 6 nm, with the cost reduction being the result of an effective description of the effects of the solvent by means of replacing the carboxylate ions with neutral hydrogens. In absorption spectroscopy, the obtained best theoretical results are in excellent agreement with the experimental benchmark measurement, regarding excitation energies as well as band intensities and profiles. In fluorescence spectroscopy, the experimental spectrum shows a vibrational progression that is not addressed by theory, but the theoretical band position is in excellent agreement with experiment, with a highly accurate description of the Stokes shift as a result.
Journal of Physical Chemistry Part A: Molecules, Spectroscopy, Kinetics, Environment and General Theory, 2014, Vol 118, Issue 19, p. 3419-3428