Abstract

The vibronic structure observed in absorption spectra of conjugated polymers can hide important information regarding the electronic transition and it is tightly connected to the conformation of the polymer. Vibronic intensities are associated with the transition dipole moment and the change in the excited state structure relative to that of the ground state. In the solid state, vibronic intensities have been related to polymer aggregation and order. In this presentation, we will address the influence of molecular weight on the vibronic structure of a diketopyrrolopyrrole polymer in solution using Resonance Raman Spectroscopy (RRS). The intensities of RR bands are associated with structural changes upon electronic excitation and are thus directly related to the displacement between the ground and excited state potential energy surface minima along specific normal coordinates, determining in turn the shape of absorption spectra. Quantification of the intensities of the RR bands and analysis in combination with the absorption spectrum (a method called resonance Raman intensity analysis) can provide valuable information for the excited state potential energy surface landscape in the Frank-Condon region, a difficult region to probe with other methods. We use this method to analyze the different vibronic structure exemplified by different molecular weight fractions of a conjugate polymer, offering in addition an explanation for the high absorption exemplified by this polymer.