Structure–property relationships of azulene-fused tryptanthrin π-systems: impact of fusion mode and dicyanomethylidene substitution

Structurally distinct azulene-fused tryptanthrin analogs were synthesized to investigate how the fusion mode of the azulene unit affects the electronic structure and physicochemical properties of tryptanthrin-based π-conjugated systems. The corresponding malononitrile derivatives were also prepared as more strongly electron-accepting analogs of the fused heterocyclic framework. UV–Vis absorption spectroscopy, voltammetric analysis, and theoretical calculations revealed that both the optical properties and the redox behavior are dependent on the mode of azulene fusion. Introduction of the azulene unit and the dicyanomethylidene group reduced the HOMO–LUMO gaps of the fused heterocyclic systems and induced pronounced bathochromic shifts, although the extent of these effects varied considerably with the fusion pattern. These results indicate that the electronic interaction between the electron-rich azulene moiety and the electron-deficient heterocyclic core is strongly influenced by the mode of azulene fusion, resulting in substantial differences in conjugation and donor–acceptor communication across the π-framework. The present findings further demonstrate that precise control over the azulene fusion mode is an effective means of tuning donor–acceptor interactions in the fused heterocyclic systems. This insight offers a useful molecular design guideline for the development of long-wavelength-absorbing organic chromophores based on tryptanthrin-type π-systems.