Synthesis, characterization and optical properties of novel oligothiophenes bearing pyrene units attached via well defined oligo(ethylene glycol) spacers

Abstract

Two new thiophene monomers bearing pyrene units attached via di(ethylene glycol) and tetra(ethylene glycol) spacers were synthesized, 3-methyl-4-(diethoxy) thiophene (M2) and 3-methyl-4-(tetraethoxy) thiophene (M4). These monomers were linked to thiophene and bithiophene via a Suzuki coupling reaction to give the corresponding terthiophenes and quaterthiophenes: [3,3-(di (diethoxypyrene)), 4,4′′′dimethyl–2,2: 5#14−15#-terthiophene (TT2)], [3,3-(di(tetraethoxypyrene)), 4,4′′′dimethyl–2,2:5#14−15#-terthiophene (TT4)], [3,3′′′-di(diethoxypyrene), 4,4′′′methyl-2,2′:5′: 2′′:5′′,2′′′-quaterthiophene (QT2)], and [3,3′′′-di(tetraethoxypyrene), 4,4′′′methyl -2,2′:5′:2′′:5′′,2′′′-quaterthiophene (QT4)]. The obtained oligothiophenes were characterized by 1H, 13C NMR spectroscopies and MALDI-TOF mass spectrometry. The optical properties of these compounds were studied by absorption and fluorescence spectroscopy. The absorption spectra of these compounds exhibited a broad absorption band at λmax=350nm arising from the S0→S2 transition of the pyrene group. This broadening is an indication of the presence of pyrene–pyrene interactions in the ground state. A discrete band at ca λ=385 due to the S0→S1 transition (n–π*) of the oligothiophene backbone was also observed. The emission spectra of oligomers TT2, TT4, QT2 and QT4 showed a “monomer emission” band at λM=379–450nm followed by an intense excimer emission band at λE=570nm due to intramolecular pyrene–pyrene interactions. The effect of the flexible spacer length as well as that of the oligomer backbone influences significantly the formation of pyrene–pyrene complexes.