Inverse detection applied to the 1H and 13C assignment and to the regiochemical characterization of beta-substituted alpha,alpha' coupled oligo- and polythiophenes

beta-Substituted alpha-alpha'-coupled oligo- (OTs) and polythiophenes (PTs) are materials currently investigated for their potential applications in optical and electronic devices. beta-Substituents modify both processability and electronic properties with respect to the unsubstituted analogues and these modifications are function not only of the type of substituents but also of their relative positions. When two beta-substituted thiophenes are alpha-alpha' coupled three regiochemically different bithiophenes can be obtained from the three different possible coupling paths: head-to-head, head-to-tail and tail-to-tail. The number of regioisomers enhances in longer oligomers, whereas the interpretation of 1-H and 13-C of PTs is usually based on a four configurational triad model. When the complete 1-H and 13-C assignment and the regiochemical characterisation of OTs and PTs are required, 1-H and 13-C inverse detection proves to be a very powerful tool of investigation. Heteronuclear inverse-detected correlations over one and more bonds are studied and the proton and carbon assignments are often contextual. One-bond correlations are utilised to distinguish between alpha and beta(C,H) pairs, thanks to the different values of alpha and beta 1J(H,C) coupling constantsof thiophene. As far as PTs are concerned, the absence of alpha(C,H) pairs confirms that only alpha,alpha' coupling occurred. Intra-residue correlations over two or three bonds are found running experiments with diffferent evolution delays. When long evolution delays are employed inter-residue correlations are also detected. These last correlations are extremely useful in establishing the thiophene backbone connectivities. Similarly, the association of each beta-substituent to the corresponding ring is found through long-range correlations.