Electropolymerizable 2,2′-Carboranyldithiophenes. Structure−Property Investigations of the Corresponding Conducting Polymer Films by Electrochemistry, UV−Visible Spectroscopy and Conducting Probe Atomic Force Microscopy

Abstract

Carborane-functionalized conducting polymer films have been electrogenerated in dichloromethane from the anodic oxidation of ortho-(1), meta-(3) and para-carborane (4) isomers linked to two 2-thienyl units. The corresponding electrochemical response was characterized by a broad reversible redox system corresponding to the p-doping/undoping of the polythiophene backbone, the formal potential of which increased in the order poly(1) < poly(3) < poly(4), from ca. 0.50 to 1.15 V vs Ag/Ag + 10 −2 M. From further UV-visible spectroscopy analysis, the optical band gap was estimated at 1.8, 2.0 and 2.2 eV for poly(1), poly(3) and poly(4), respectively. The more conjugated and electroconductive character of poly(1) is ascribed to a more planar conformation of the conjugated backbone resulting from an intramolecular β-β′ cyclization reaction in the monomer, consequently yielding a fused conjugated polymer. Molecular modeling calculations using the DFT method support this hypothesis. The surface topography and maps of the conductive domains of the electropolymerized films were evaluated by conducting probe AFM. The three polymers exhibit fairly similar morphological characteristics and a surface roughness of ~2 nm. Current-voltage (I-V) characteristics of conducting AFM tipcarborane polymer-ITO junctions showed that poly(1) had the highest conductivity.

Icosahedral boron clusters: a perfect tool for the enhancement of polymer features

“…13 The ratio of monomers incorporated into the PPy backbone of the material was evaluated by UV/vis spectroscopy and turned out to be roughly 1 : 1, indicating that the rate of polymerization of both monomers is practically the same. These synthons (see Chart 5) are as follows: (i) neutral mono-substituted (19), 14 (20), 14 (21), 15 (22); 15 (ii) neutral di-substituted (23), 16 (24) 15 and (iii) anionic mono-substituted compounds (25), 14 (26). This value represents the best performance ever obtained in a heterogeneous phase using a chemical oxidant.…”

Synthesis and electropolymerization of a series of 2,2′-(ortho-carboranyl)bisthiophenes

“…7) [21] with anion (6), followed by electropolymerization to form the self-doped polymers under suitable conditions; (iii) monomers were modified by using neutral carboranes (o-, m-, p-) at the 2-position [22,23] and at the 3 and/or 4-positions (e.g. 8) [24][25][26][27] followed by polymerization; and (iv) copolymerization between carborane-containing monomers with α-free monomers (pyrroles or thiophenes) using both chemical [28,29] and electrochemical [21,30] methods, for the introduction of carborane groups into the main chain ( Figure 2).…”

“…Furthermore, some carborane-containing molecules even fail to be successfully electropolymerized when the carborane groups are too close to the aromatic rings. [23,24,26,27] On the other hand, a wide range of structurally-modified π-conjugated oligomers have been designed, synthesized and polymerized to afford materials with specific properties (e.g. photovoltaic properties and controlled length of the main chain).…”

“…The key intermediate 2,2'-o-carboranyldithiophene 9 was obtained using a slightly modified procedure from that previously published by us, [22,23] in 30% overall yield. The Sonogashira cross-coupling reaction of commercially available 2-iodothiophene and 2-[(trimethylsilyl)ethynyl]-thiophene, deprotected in situ in the presence of 1,8-diazabicycloundec-7-ene (DBU), using bis(triphenylphosphine)-palladium(II) dichloride and copper(I) iodide as catalysts, produced 2,2'-ethylenedithiophene in 87% yield.…”

“…[50] Reaction of 2,2'-ethylenedithiophene with decaborane and diethyl sulfide gave the key precursor 2,2'-o-carboranyldithiophene 9 in 35% yield. [22,23] Bromination of the two α-positions of 2,2'-carboranyldithiophene 9 was accomplished using 6 equiv of N-bromosuccinimide (NBS) in chloroform and acetic acid (1:1), at 130 o C overnight, producing the dibrominated product 10a in 76% yield, [51] as shown in Scheme 1. The two α-positions of 2,2'-carboranyldithiophene 9 are deactivated toward bromination due to the electronwithdrawing character of the ortho-carborane group, therefore requiring high temperature to proceed.…”