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Acid-base strength and acido(fluoro)chromism of three push-pull derivatives of 2,6-distyrylpyridine

The acidochromism and acid-base properties of 2, 6- distyrylpyridine (2, 6-DStP) derivatives bearing on the sides push/pull substituents (namely two dimethylamino, one nitro, and one methoxy and two nitro groups in the case of 2, 6- bis[(E)-2-(4- dimetylaminophenyl)ethenyl]pyridine, 2- [(E)-2-(4- nitrophenyl)ethenyl], 6- [(E)-2’-(4’- metossiphenyl)ethenyl]pyridine and 2, 6- bis[(E)-2- (4- nitrophenyl)ethenyl]pyridine, respectively) were investigated by stationary and time-resolved spectroscopies. The sensitivity of the absorption and emission spectrum to the medium acidity was found to enhance in the dimethylamino-derivative relative to the unsubstituted 2, 6-DStP, also because of the second protonation by the N(CH3)2 group. Spectrophotometric titrations, also processed by a global fitting approach, gave pKa values, for the protonation of the central pyridine, higher in the derivatives with electron- donor unities and lower in compounds bearing electron- acceptor groups. A fluorometric titration was performed in the case of the dimethylamino- derivative thanks to non- negligible emission efficiencies for both neutral and protonated species, unveiling an attractive naked-eye acido(fluoro)chromism from green to yellow upon pyridine protonation, and then to purple with the second protonation involving the lateral N(CH3)2 substituent. Due to the extremely short excited- state lifetimes, as resulted from femtosecond transient absorption experiments, the pKa values for the excited state (pKa*) were estimated through the Förster cycle, revealing that the monoprotonated species of the dimethylamino- derivative would become upon excitation the only stable form in a wide range of pH.

acid-base properties ; acido(fluoro)chromism ; push-pull systems ; 2, 6-distyrylpyridine derivatives ; spectroscopic titrations, Förster cycle

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