Carbonyl-functionalized quaterthiophenes: A study of the vibrational raman and electronic absorption/emission properties guided by theoretical calculations

This work investigates the evolution of the molecular, vibrational, and optical properties within a family of carbonyl-functionalized quaterthiophenes: 5,5″-diheptanoyl-2,2:5,2″:5″,2″-quaterthiophene (1), 5,5″-diperfluorohexylcarbonyl-2,2:5,2″:5″,2″- quaterthiophene (2), and 2,7-[bis(5-perfluorohexylcarbonylthien-2-yl)]-4H- cyclopenta[2,1-b:3,4-b]-dithiophene-4-one (3). The analysis is performed by Raman and UV/Vis absorption/excitation/fluorescence spectroscopy in combination with density functional calculations. Theoretical calculations show that substitution with carbonyl groups and perfluorohexyl chains induces progressive quinoidization of the π-conjugated backbone in comparison to the carbonyl-free compound 5,5″-dimethyl-2,2:5,2″:5″,2″- quaterthiophene (DM-4T) used as reference. Raman spectra are dominated by a strong Raman line which mainly corresponds to a combination of C-C/C=C stretching vibrations spreading over the whole thiophene core. This band undergoes a remarkable downshift as a consequence of the structural changes induced by the electron-withdrawing groups on the π-conjugated backbone. The band splitting on incorporation of a central carbonyl bridge evidences the formation of two structural domains in the molecule. The excitation and fluorescence spectra recorded at low temperature show well-resolved vibronic structures associated with the most intense collective C-C/C=C stretching mode. Optical absorption and fluorescence bands exhibit remarkable bathochromic dispersion on carbonyl functionalization, indicative of extension of π conjugation. TDDFT calculations enable a detailed description of the trends observed in the absorption spectra. Resonance Raman spectra reflect the structural changes predicted for the S₀S₁ electronic transition and evidence the cross-conjugated character that the central carbonyl group confers on 3.