Functionalized pentacenes: A combined theoretical, Raman and UV-Vis spectroscopic study

This work presents an analysis of the evolution of the molecular, vibrational and optical properties of a family of functionalized pentacenes. The analysis is performed on the basis of DFT quantum-chemical calculations in combination with spectroscopic techniques (Raman and UV-Vis). Theoretical calculations show that the bond length C-C/C=C alternation along the peripheral oligoenic ribbons increases with electron-releasing dioxolane substituents and diminishes with electron-withdrawing chlorine atoms and cyano groups. The attachment of triisopropylsilylethynyl groups increases the complexity of the Raman spectra. The spectra present many intense features of similar intensities in the 1,200-1,600 cm^-1 range which are described by a combination of C-C/C=C stretching vibrations and in-plane C-H deformations spreading over the whole pentacene backbone. The absorption spectra display absorption bands in three different energy regions of the UV-Vis electromagnetic range. The spectra are dominated by a strong absorption band measured in the 300-350 nm region, which undergoes a sizeable red-shift with the substitution pattern, and a low-intensity, three-peak band in the 500-700 nm region, which undergoes a blue- or a red-shift depending on the electronic nature of the substituents. TDDFT calculations enable a detailed description of the trends observed in the absorption spectra.