Quadrupolar, emission-tunable π-expanded 1,4-dihydropyrrolo[3,2-b]pyrroles – synthesis and optical properties

The synthesis and optical characterization of six novel heteroaromatic-based chromophores is described. The new dyes present mostly an A–D–A general framework, where A is an electron-deficient aromatic ring and D is an electron-rich pyrrolo[3,2-b]pyrrole moiety, linked via triple bonds. It was demonstrated that the increase in the molecular length of the chromophore effectively extends π-conjugation. The effect of structural variations on photophysical properties was studied in detail for these compounds and the relationship between the structure and photophysical properties was thoroughly elucidated by comparison with simpler tetraaryl-analogues. The strong charge-transfer characteristic of these functional dyes can be illustrated by large Stokes shifts (4100–7100 cm−1) for A–D–A architectures. The replacement of phenyl rings at positions 2 and 5 with the arylethynylaryl substituents bathochromically shifts both absorption and emission at ca. 50–150 nm. The clear dependence of fluorescence maxima on the electron-accepting property of the peripheral arylethynyl substituent emphasizes strong π-conjugation in these molecules. The donor–acceptor interactions were also found to influence the two-photon absorption properties.