Phosphorescence emission of 2,6-diacetylpyridine: Role of N-heteroatom and effect of hydrogen-bonding.

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The absorption, excitation, and luminescence spectra of 2,6-diacetylpyridine (DAP) and acetophenone (ACP) are investigated in nonpolar methylcyclohexane (MCH) and polar ethanol (EtOH) matrices at 300 K and 77 K. The emitting state of DAP is found to be the lowest triplet state of essentially nπ* nature. In the emitting lowest triplet nπ* states of DAP and ACP the carbonyl group is observed to be the chromophore. The comparative studies on the photophysical properties of DAP and ACP reveal that the introduction of a pyridinic nitrogen atom in the phenyl nucleus of ACP leads to large nonradiative transitions from the lowest 3noπ* due to vibronic interaction between this state and the pyridinic nNπ* state. In the case of ACP a weak ππ* fluorescence, in addition to phosphorescence is observed in EtOH glass matrix at 77 K. On the other hand in EtOH no such fluorescence is detected for DAP. The phosphorescence decay profile of DAP in presence of EtOH is found to be of single exponential indicating emission from a particular excited electronic state. It is suggested that due to comparatively much weaker hydrogen-bonding interaction between DAP molecule and EtOH solvent both intersystem crossing and internal conversion rates are little affected.
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