title: Carbonyl-functionalized quaterthiophenes: a study of the vibrational Raman and electronic absorption/emission properties guided by theoretical calculations.
abstract: 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 S(0)→S(1) electronic transition and evidence the cross-conjugated character that the central carbonyl group confers on 3.
authors.affiliation: Instituto de Ciencia Molecular, Universidad de Valencia, 46980 Valencia, Spain.
authors.forename: Juan
authors.initials: J
authors.lastname: Aragó
authors.forename: Rocío
authors.initials: R
authors.lastname: Ponce Ortiz
authors.forename: Belén
authors.initials: B
authors.lastname: Nieto-Ortega
authors.forename: Víctor
authors.initials: V
authors.lastname: Hernández
authors.forename: Juan
authors.initials: J
authors.lastname: Casado
authors.forename: Antonio
authors.initials: A
authors.lastname: Facchetti
authors.forename: Tobin J
authors.initials: TJ
authors.lastname: Marks
authors.forename: Pedro M
authors.initials: PM
authors.lastname: Viruela
authors.forename: Enrique
authors.initials: E
authors.lastname: Ortí
authors.forename: Juan T
authors.initials: JT
authors.lastname: López Navarrete