Oxidative Dimerization of Thiophene-Substituted Indenone Derivatives

Push-pull molecular entities are a special type of organic compounds that attract great interest in chemistry because of their structural organization. In fact, within the same molecule, electron-donating groups are directly connected to electron-accepting groups, and this unusual arrangement significantly influences the molecular features. Thus, the electrochemical features of some push-pull indenone derivatives bearing bithiophene (BT, as in T2-InD) or terthiophene (TT, as in T3-InD) moieties directly attached to the indenone nucleus were evaluated by cyclic voltammetry. Interestingly, the presence of unsubstituted terminal thiophene rings allowed T2-InD and T3-InD to be dimerized by electrochemical procedures into the corresponding dimers InD-T4-InD and InD-T6-InD showing quadrupolar A-π-D-π-A structures. Our results reveal a clear and predictable relationship between their structural features and their electrochemical and optical behaviors. To support our hypotheses, we also conducted semiempirical quantum mechanical calculations. Moreover, the dimerization of these dipolar indenone derivatives was also performed by standard chemical oxidation with FeCl₃ to obtain substantial amounts of dimers InD-T4-InD or InD-T6-InD to be used in (NMR) spectroscopy studies. Experimental evidence was collected about the clear-cut tendency of these quadrupolar A-π-D-π-A structures to aggregate with the possible formation of species showing different dimension scales.