(N-7-Azaindolyl)oligothiophenes: synthesis, characterization, and photophysical properties

The synthesis of a new series of mono- and oligothiophenes capped by 7-azaindoles such as 2-(N-azaindolyl)thiophene (1), 2-(N-azaindolyl)-5′-(bromo)oligothiophenes (2a–4a), and 2,5′-bis(N-azaindolyl)oligothiophenes (2b–4b) has been investigated. The reaction of 7-azaindole with 2-bromothiophene under the modified Ullmann condensation conditions led to the formation of 1. Simple extension of the same method to the reaction of 2,5′-dibromooligothiophenes in the presence of 4–5 M excess of 7-azaindole led to the formation of 2a–4a and 2b–4b in moderate overall yields (40–55%). All compounds were fully characterized by analytical and various spectroscopic techniques. The structures of 2b, 3b, and 4b were determined by X-ray diffraction analyses. All three compounds show several intermolecular C(π)⋯H interactions leading to the formation of herringbone packing in the solid-state structure. The UV absorption spectra of 1–4 consist of three characteristic electronic transitions corresponding to n→π∗ and π→π∗ transitions arising out of the π-conjugation of the entire molecule as well as local aromatic units. The emission spectra of the same compounds show intense fluorescence bands at the wavelengths between 422 and 495 nm. The length of the thiophene chain and the presence of bromine atom influence the band position of both absorption and emission spectra. While the extension in π-conjugation causes the reduction in the band gap, the bromine atom shifts the electronic transition energy to the blue region. The cyclic voltammetric measurements were performed with 1–4, which show that the compounds exhibit a typical pseudo-reversible redox wave with Eox in the range 0.6–1.2 V.