Effect of synthesis temperature and alkoxy side chain length on molecular structure and photoelectrochemical properties of terthiophenes

J. Braz. Chem. Soc., Vol. 20, No. 2, 229-235, 2009. Printed in Brazil - ©2009 Sociedade Brasileira de Química 0103 - 5053 $6.00+0.00 Marcos J. L. Santos and Emerson M. Girotto* Departamento de Química, Universidade Estadual de Maringá, Av. Colombo 5790, 87020-900 Maringá-PR, Brazil As propriedades fotoeletroquímicas dos tertiofenos alcóxi substituídos poli(4,4’’dimetóxi-3’metil-2,2’:5’,2’’ tertiofeno) (PDM) e poli(4,4’’dipentóxi-3’-metil-2,2’:5’,2’’ tertiofeno) (PDP) foram investigadas em termos da influência causada pelo comprimento da cadeia lateral. Experimentos de voltametria cíclica e espectroscopia UV-Vis sugeriram que a temperatura de síntese afeta de modo diferenciado a organização molecular dos materiais e, desse modo, não deve ser usada como regra geral. A maior eficiência de conversão apresentada pelo PDM resulta de seu melhor empacotamento molecular tipo-π. Devido à sua cadeia lateral curta o PDM apresenta uma grande interação entre cadeias, a qual favorece o movimento eletrônico e a condutividade do polímero. The photoelectrochemical properties of the alkoxy-substituted terthiophenes poly(4,4’’dimethoxy3’-methyl-2,2’:5’,2’’ terthiophene) (PDM) and poly(4,4’’dipentoxy-3’-methyl-2,2’:5’,2’’ terthiophene) (PDP) have been investigated in terms of the influence of side chain length. Cyclic voltammetry and UV-Vis experiments have suggested that the synthesis temperature affects the molecular organization in different ways and, thus it should not be used as a general rule. The more efficient light harvesting of PDM, results from its improved molecular π-stacking. Due to its short side chain, PDM presents a large chain interaction, which favors electron hopping and the polymer conductivity. Keywords: polyterthiophene, photoelectrochemical properties, alkoxy chain length Introduction Since the discovery of the conductivity of organic conjugated polymers,1 a number of research groups have devoted their efforts to understanding and enhancing their properties aiming at applications in electrochemical and optical devices.2‑6 In the last 15 years, the results of several studies have encouraged the investment in the field of photovoltaic and photoelectrochemical cells using organic materials.7‑12 Polythiophenes have been widely studied. This family of conductive polymer is known for its simple functionalization, p-type semiconductor characteristic, and relatively good stability in air in both the neutral and the oxidized states.13 However, the efficiency of photovoltaic and photoelectrochemical cells still need improving and as well as further understanding of the mechanisms around photo‑driven phenomena. The electronic and optical properties of organic conjugated polymers are controlled both by the primary *e-mail: molecular structure (intramolecular functionality: π-conjugation) and by long-range organization (intermolecular interactions: π-stacking). The control of intermolecular interactions has been attained mainly through the structural, chemical, and electronic properties of side chains.14 The alkoxy substituents on the polymer chain induce a low oxidation potential, and if at the α position (involved in the polymerization reaction), they provide fast electropolymerization kinetics.15-18 Theoretical studies have been devoted to understanding interchain interactions and their effect on the optical properties of thiophene oligomers.19-23 The effect of both the planar and the interchain distances on the gap energy (Egap) of oligothiophene systems are well known, and so are the effect of chain torsions on π-conjugation length.24 In all the previous cases, differences presumably due to the preparation method and conditions and the resulting structures have observed between the various materials. These differences are substantial in some cases and subtler in others. Thus, it seemed worthwhile preparing and investigating a series of conjugated systems as thoroughly Article Effect of Synthesis Temperature and Alkoxy Side Chain Length on Molecular Structure and Photoelectrochemical Properties of Terthiophenes 230 Effect of Synthesis Temperature and Alkoxy Side Chain Length as possible. The present contribution is one of a series of such studies. Aiming at contributing to the understanding of how the synthesis temperature and the side chain length affect the photoelectrochemical properties of substituted terthiophenes, the present work reports some structural studies on the photoelectrochemical characterization of polymer films obtained from terthiophene derivatives25 with substituents with different lengths at positions 4,4”: poly(4,4”-dimethoxy-3’-methyl-2,2’:5’,2”terthiophene) and poly(4,4”-dipentoxy-3’-methyl2,2’:5’,2”-terthiophene). The systems were studied in solution by mono- and polychromatic irradiation. The photoelectrochemical results are discussed in terms of the possible structural organization taking into consideration the influence of the alkoxy side group and the effect of the synthesis temperature. Experimental The preparation procedure of monomers 4,4’’dimethoxy-3’-methyl-2,2’:5’,2’’ terthiophene DMM, and 4,4’’dipentoxy-3’-methyl-2,2’:5’,2’’ terthiophene DPM (Figure 1) is reported elsewhere.26 All reagents were reagent-grade quality and used without further purification. Acetonitrile (AN Merck) was stored under argon pressure and manipulated under argon flow. Dichloromethane (DM Merck) was dehydrated with CaCl 2 for 12 h, successively distilled in the presence of P2O5 under argon flow, and stored in the dark under argon pressure. Tetra-nbutyl ammonium perchlorate (TBAP, Fluka) was purified by crystallization in methanol. Figure 1. Structures of monomer DMM and DPM. Film preparation PDM and PDP films were deposited onto an ITO substrate (Delta Technologies, 20 Ω/cm2) and a Pt sheet by cyclic voltammetry (CV) of 3 mmol L-1 of monomer in 3:1 v/v AN/DM + 0.1 mol L-1 TBAP at controlled temperature (thermobath Tecnal TE184) at scan rate of 20 mV s-1. A Pt wire was used as a counter‑electrode and the reference was an Ag|AgCl electrode. The electrosynthesis and electrochemical characterizations were carried out with an Autolab PGSTAT 30 potentiostat/galvanostat apparatus. J. Braz. Chem. Soc. Spectroelectrochemical characterization Spectroelectrochemical measurements in the UV‑Vis region were carried out using a 0.1 mol L-1 solution of LiClO4 in acetonitrile, an Ag|AgCl electrode as reference, ITO-glass sheets (area of 1.0 cm 2, surface resistivity 20 Ω/cm2) as working electrodes, and a Pt wire as a counter electrode. The in situ spectroelectrochemical measurements were carried out by placing the ITO‑modified electrodes in the sample compartment of a Shimadzu spectrophotometer (UV mini 1240) and applying the potential by using an Autolab PGSTAT 30 potentiostat/ galvanostat. The absorbance spectra of the films were recorded at polarization potentials of –0.2 (vs. Ag|AgCl). X-ray diffraction X‑ray scattering films were synthesized onto mirrorpolished (...truncated)