4′-Hydroxystyryldiazines: Synthesis and fluorescence properties

In order to investigate the effect that additional nitrogen substitution in the 4′-hydroxystyrylpyridine system would have on the photophysical properties of this molecule, we have synthesized a series of four isomeric 4′-hydroxystyryldiazines: the 2,4-diaza (3e), 2,5-diaza (3f), 2,3-diaza (3g), and 3,4-diaza (3h) isomers [the positions of the nitrogens relative to the site of styryl attachment are given as italicized numbers], and we have studied their UV absorbance and fluorescence emission in a variety of solvents and under different pH conditions. These compounds are prepared readily by the condensation of the appropriate methyldiazine with 4-methoxybenzaldehyde; deprotection with boron trifluoride-dimethylsulfide complex results in partial rehydration of the styrene double bond, but dehydration is readily effected by treatment with acid. The UV spectra of compounds 3e-h in ethanol show under neutral conditions a long wavelength absorbance at 330-350 nm, which shifts sharply to the red in both acid and base (390-420 nm). The most marked shift is seen with the 2,4- and 3,4-diaza isomers (3e and 3h). These four styryl diazines also show long wavelength fluorescence that is highly solvatochromic and sensitive to pH. Emission in acetonitrile is at 400-435 nm, but shifts in acid or base to 500-560 nm with the 2,4- and 3,4-diaza isomers (3e and 3h) and to 590-640 nm with the 2,5- and 2,3-diaza isomers (3f and 3g). The high environmental and pH sensitivity of these 4-hydroxystyryldiazine fluorophores makes them potentially useful as probes for biological systems.