The Effect of Pyrazolyl Substituents on the Photophysical and Photochemical Properties of Pyrazine Derivatives

The reaction of 2,5‐dibromopyrazine with N‐Lithium pyrazolate in a 1:2 ratio leads to a mixture of 2‐bromo‐5‐(1 H ‐pyrazol‐1‐yl)pyrazine ( I ) and 2,5‐di(1 H ‐pyrazol‐1‐yl)pyrazine ( II ). The structures of I and II are highly planar. Two absorption bands can be observed for the compounds in the UV ‐Vis region, having ε in the order of 10 4   m −1  cm −1 . TD ‐ DFT computed results support the nature of the lower energy absorptions as π pyrazine → π * pyrazine transitions, including an additional intraligand charge transfer transition for I ( π pyrazol → π * pyrazine ). Upon excitation at 280 or 320 nm, the emission of both compounds is almost not affected by solvent polarity or oxygen presence, showing two bands for I and one for II in the 350–450 nm region. Emission of II follows a mono‐exponential decay, while I decays following a bi‐exponential law, hypothesized from π pyrazine → π * pyrazine and π pyrazol → π * pyrazine transitions. Photodegradation of I and II follows a first‐order kinetic with constants of 1.18 × 10 −2  min −1 and 0.13 × 10 −2  min −1 , respectively. Results suggest that photodegradation of I starts with the loose of bromide followed by intermolecular pyrazolyl subtraction and ring opening. This path is not available for II , which is reflected in its enhanced photostability.