Effect of molecular weight on electronic, electrochemical and spectroelectrochemical properties of poly(3,3 ''-dioctyl-2,2 ': 5 ',2 ''-terthiophene)

Poly(3,3 ''-dioctyl- 2,2': 5',2 ''-terthiophene), obtained from its corresponding monomer by oxidative polymerization with FeCl3, has been fractionated into five fractions of reduced polydispersity, covering the M-n range from 1.50 kDa to 10.50 kDa ( measured vs. polystyrene standards). The effect of Mn on spectroscopic, electrochemical, spectroelectrochemical and electrical transport properties has been investigated. Fractions of growing Mn show an increasing bathochromic shift of the band originating from the pi - pi* transition in the neutral polymer with the appearance of a clear vibrational structure for the two highest molecular fractions. The onset of oxidative doping determined from the cyclic voltammogram shifts towards lower potentials with increasing molecular weight. A similar trend is observed for doping induced near infrared bands, which shift towards lower energies ( higher wavelengths) with increasing molecular weight and appear at lower potentials in spectroelectrochemical experiments. Finally, a comparison of the FET mobility in two transistors fabricated under identical conditions from polymer fractions differing in their molecular weight shows that a ca. fourfold increase of M-n (from 2.40 kDa to 10.50 kDa) results in a two orders of magnitude increase in the carriers' mobility ( from mu(sat) = 4 x 10(-5) cm(2) V-1 s(-1) to mu(sat) = 2 x 10(-3) cm(2) V-1 s(-1)). The obtained results underline the importance of the control of the macromolecular parameters in the preparation of electronic and electrochemical devices from poly(3,3 ''-dioctyl-2,2': 5',2 ''- terthiophene).