Asymmetrical aza-boron-dipyridomethene derivatives with large Stokes shifts: synthesis and spectroscopic properties

The synthesis and characterization of three asymmetrical aza-boron-dipyridomethene derivatives 3–5 are reported. Single crystal X-ray structures are described and the spectroscopic properties of the dyes have been investigated in depth based on an analysis of the optical spectroscopy and the results of theoretical calculations. In contrast with what is normally observed with symmetrical aza-boron-dipyridomethene dyes, very large apparent Stokes shifts are observed for 3–5 and the emission spectra are strongly solvent dependent. DFT calculations have been carried out to explore the structure/property relationships and substituent effects.