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

• Oligothiophenes bearing pyrene units.
• Excimer formation.
• Pyrene–pyrene interactions.

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 = 350 nm 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 = 379–450 nm followed by an intense excimer emission band at λE = 570 nm 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.