Dipicolylamine styryldiazine derivatives: Synthesis and photophysical studies

• D–π–A molecules with dipicolylamine group and diazines rings were synthesized.
• All the compounds showed yellow-green light emission.
• All the compounds exhibit significant emission solvatochromism.
• Coordination of metal cations leads to blue or red shift in the fluorescence spectra.

Different D–π–A push–pull molecules in which dipicolylamine (DPA) is used as the donor group, different diazines as the acceptor groups, and styryl as the π-conjugated spacer have been synthesized in a straightforward manner by aldol condensation of 4-(di-2-picolylamino)benzaldehyde and the appropriate methyldiazine. All of the compounds showed π–π* transitions in the UV or visible region and the emission of yellow-green light upon irradiation. Significant red shifts were observed in the fluorescence emission maxima of these compounds on increasing the solvent polarity, a finding that suggests the formation of an intramolecular charge-separated emitting state that is also supported by semi-empirical calculations. In some cases, protonation led to marked color changes, thus showing the ability of these molecules to function as colorimetric pH sensors. The DPA and diazine units can act as coordination sites for metal cations such as Zn2+, Cd2+ or Hg2+, leading to a blue or red shift in the fluorescence spectra due to the change in the intramolecular charge transfer properties. This phenomenon could become a powerful tool for the creation of multiple emission colors with a single molecule after suitable design.

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