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 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.
(© 2018 The American Society of Photobiology.)