Comparing the ultraviolet photostability of azole chromophores

Abstract

Ultrafast time-resolved velocity map imaging methods are used to interrogate the timescales for H-atom elimination in the azole isomers imidazole and pyrazole, the former of which is a prevalent moiety in biomolecules that exhibit a high degree of photostability following the absorption of ultraviolet (UV) radiation (e.g. DNA bases and aromatic amino acids). The results presented here, for the first time, draw focus on the statistical H-atom elimination dynamics in these two heteroaromatics, which result from vibrationally hot ground state (S₀) molecules that are formed following ultrafast internal conversion from an initially populated excited electronic state (¹ππ* or ¹πσ*) at 200 nm. Measurements on imidazole suggest that statistical H-atom elimination is minimal over the temporal window of these experiments (which extends to 600 ps) and occurs on a timescale of >270 ps. Conversely, pyrazole shows a significant statistical H-atom yield by 600 ps with a time constant of 165 ± 30 ps. This highlights statistical unimolecular dissociation dynamics which, on these timescales, cannot be interpreted with traditional RRKM theory. Additional experiments on deuterated isotopomers of the two species also reveal that in imidazole statistical H-atom generation is localized to N–H bond fission, while in pyrazole there is approximately a 1 : 1 ratio between statistical C–H and N–H cleavage, and the two processes have associated time constants of 151 ± 20 ps and 193 ± 35 ps, respectively. We postulate that the observed high fraction of rapid irreversible C–H fission in pyrazole, relative to imidazole, may lead to the formation of toxic free radicals within specific biological environments, whereas statistical dissociation, restricted to only the N–H coordinate, may hypothetically quench UV photodamage yields via H-atom ‘caging’ and ‘recombination’ dynamics in hydrogen bonded networks (e.g. secondary protein structures).