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.