Spider‐Like Oligothiophenes

Abstract

Careful analysis and comparison of optical and electrochemical data available in recent literature for multi-thiophene molecular assemblies suggested a few basic rules for the design of structurally simple and easily accessible oligothiophenes endowed with properties not far from those exhibited by much more complex and synthetically demanding architectures. The synthesis and computational investigation of three examples of a class of oligothiophenes (spider-like) tailored according to these indications are reported together with their exhaustive optical and electrochemical characterization. The new compounds (T9 5, T14 6, T19 7) are characterized by a thiophene, a 2,2'-bithiophene and a 2,2',5',2''-terthiophene unit (the spider body) fully substituted with 5-(2,2'-bithiophen)yl pendants (the spider legs). Absorption and electrochemical data are in good agreement and point to a high pi-conjugation level, comparable to those displayed by much larger assemblies. Electrode potential cycling in proximity of the first oxidation peak affords fast and reproducible formation of conducting, highly stable [TXn]m films, mainly consisting of dimers (m=2). Electrooxidation kinetic experiments on deuterium-labelled T9 5, coupled to laser-desorption-ionization mass spectroscopy on the resulting dimer demonstrated that the coupling process is extremely regioselective in the alpha positions of the more conjugated pentathiophene chain. The optical and the electrochemical properties of the films are reported and discussed. A peculiar feature is their impressive charge-trapping ability. Spider-like oligothiophenes are promising materials for applications as active layers in multifunctional organic devices.

Towards Molecular Design Rationalization in Branched Multi‐Thiophene Semiconductors: The 2‐Thienyl‐Persubstituted α‐Oligothiophenes

“…The situation is very similar to that calculated for T9 5 in which the five rings belonging to the a-conjugated main chain lie on the same plane, whereas the two dithiophene pendant groups are orthogonal to the latter. [1] These observations suggest that the electrochemical oxidative polymerization of T'5 3 can reasonably be regioselective, as previously demonstrated in the case of T9 5 , since the electron abstraction involved in the first step of the process is expected to occur at the termini of the a-conjugated system. In the case of T'8 4 we were also able to prepare crystals suitable for XRD analysis (Figure 1 b).…”

“…In particular, both the onset and the maximum absorption wavelengths exhibit a significant blueshift when passing from T14 6 to T'14 6 ; this nicely accounts for the reduced conjugation in the main, six-membered a-conjugated backbone with increasing node density, that is, (n a À3) instead of (n a À5). In the T'14 6 absorption spectrum we observe the disappearance of the high-energy peak (which is related in T14 6 to the decorating dithienyl units b-linked to the central backbone [1] ) together with the appearance of a shoulder at about 292 nm, the absorbance of which is nearly proportional to the number of b-linked thiophene pendant groups (see Figure 6). These features confirm that b-linked thiophene or dithiophene units should be regarded as nearly independent, and hardly participating in the lower-energy absorption processes involving the main chain.…”

“…[6] The most consistent sacrifice in conjugation occurs at the level of the two inner rings, which display a remarkable deviation from coplanarity, as already suggested by DFT calculations in the case of T14 6 . [1] The torsional angle is about 658, which corresponds to an unusual synclinal arrangement. The two inner rings are, instead, nearly coplanar with the outer thiophene units (the torsional angle is about 1578).…”

“…The thiophene rings connected to the a-conjugated backbone through a-b junctions can also contribute to the spreading and stabilization of the charges injected into the conjugated backbone. [1] Herein, we report the results of an investigation into the synthesis and the electrochemical and spectroscopic properties of five members of a new series of oligothiophenes, which are structurally related to the above spiderlike systems but are much more easily accessible. These compounds are characterized by an internal core constituted by a single thiophene (T'5 3 ), a 2,2'-dithiophene (T'8 4 ), a 2,2',5',2''-trithiophene (T'11 5 ), a 2,2',5',2'',5'',2'''-tetrathiophene (T'14 6 ), and a 2,2',5',2'',5'',2''',5''',2''''-pentathiophene (T'17 7 ) moiety, which are fully substituted by 2-thienyl pendant groups (Scheme 1).…”

“…We have recently reported the results of an investigation on the synthesis and electro-optical properties of 5-(2,2'-dithiophene)yl-persubstituted thiophene (T9 5 ), dithiophene (T14 6 ), and trithiophene (T19 7 ) (spiderlike oligothiophenes) and on the characterization of the materials resulting from their electrochemical oxidation [1] (Scheme 1). The design of these molecules was based on the results of an extensive literature examination demonstrating that, in the case of multi-thiophene molecules, the number of units constituting the longest a-conjugated chain (n a ) is much more important than the overall number of thiophene units constituting the molecule (n) for determining the electronic and optical…”