THALLIUM(III) TRIFLUOROACETATE TRIFLUOROACETIC-ACID IN THE CHEMISTRY OF POLYTHIOPHENES .1. POLYMERS FROM 2,2'-BITHIOPHENE AND 2,2',5',2''-TERTHIOPHENE - MECHANISTIC ASPECTS

A new chemical method of simultaneous polymerization and doping of 2,2'-bithiophene (1) and 2,2',5',2''-terthiophene (2) using thallium(III) trifluoroacetate (TTFA) in trifluoroacetic acid (TFA) is reported: These reactions have been carried out at 20 degrees C and at 72 degrees C observing no remarkable differences in the quality of the polymers, which are undoped or compensated and purified to their neutral state by extraction with different solvents, and redoped in I-2 atmosphere. Infrared spectra and conductivity measurements of the polymers, either as grown from the reaction mixture or after being redoped with I-2, qualify them as good semiconductors, and their electron paramagnetic resonance (EPR) spectra show characteristic single lines with Lorentzian shapes and narrow line widths attributed to unpaired electrons delocalized along the carbon backbone chains. 2,2',5',2'',5'',2''',5''',2'''',5'''',2'''''-Sexithiophene (alpha-sexithiophene; 3) has been isolated and identified from one of the polymers during the purification process. EPR of the radical cation of 3 in CH2Cl2-TFA shows a symmetric multiplet which consists of a quintet (alpha(4H) = 2.18 G) of septets (alpha(6H) = 1.10 G).: Variable-temperature EPR studies of 3 revealed the formation of its diamagnetic pi-dimer at low temperature in-equilibrium with the radical cation. EPR and electronic spectra of reaction mixtures of 1 and 2 with TTFA/TFA show that 3 is the shortest oligomer whose radical cation is stable enough; to be detected. 2 is protonated in TFA and it is suggested that; 2 dimerizes to 3 in the presence of TTFA, through an oxidative coupling involving its conjugate acid.