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 πpyrazine→π*pyrazine transitions, including an additional intraligand charge transfer transition for I (πpyrazol→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 πpyrazine→π*pyrazine and πpyrazol→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.
Áreas temáticas de ASJC Scopus
- Química física y teórica