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

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 ''' dimethy1-2,2: 5'2 ''-terthiophene (TT2)], [3,3-(di(tetraethoxypyrene)), 4,4 ''' dimethyl-2,2:5'2 ''-terthiophene (114)], [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 H-1, C-13 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 lambda(max) = 350 nm arising from the S-0 -> S-2 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 lambda = 385 due to the S-0 -> S-1 transition (n-pi*) of the oligothiophene backbone was also observed. The emission spectra of oligomers TT2, TT4, QT2 and QT4 showed a "monomer emission" band at lambda(M) = 379-450 nm followed by an intense excimer emission band at lambda(E) = 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. (C) 2015 Elsevier B.V. All rights reserved.