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-(1H-pyrazol-1-yl)pyrazine (I) and 2,5-di(1H-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 10⁴ 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.