4'-HYDROXYSTYRYLDIAZINES - SYNTHESIS AND FLUORESCENCE PROPERTIES

In order to investigate the effect that additional nitrogen substituti on in the 4'-hydroxystyrylpyridine system would have on the photophysi cal 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 nitr ogens relative to the site of styryl attachment are given as italicize d numbers], and we have studied their UV absorbance and fluorescence e mission in a variety of solvents and under different pH conditions. Th ese compounds are prepared readily by the condensation of the appropri ate 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 treatm ent with acid. The UV spectra of compounds 3e-h in ethanol show under neutral conditions a long wavelength absorbance at 330-350 nm, which s hifts 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 acetonitril e 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 sens itivity of these 4-hydroxystyryldiazine fluorophores makes them potent ially useful as probes for biological systems.