Investigating the CO2 uncaging mechanism of nitrophenylacetates by means of fs-IR spectroscopy and quantum chemical calculations

Abstract

Caged compounds are widely utilized for light-triggered control of biological and chemical reactions. In our study we investigated the photo-induced decarboxylation of all three constitutional isomers of nitrophenylacetate (NPA), which can be regarded as caged-CO₂. UV-pump/IR-probe spectroscopy was used to directly observe the nascent CO₂ in the region of 2340 cm⁻¹. Together with quantum chemical calculations the reaction models for all three components could be obtained. For meta- and para-NPA the main decarboxylation pathway proceeds via a triplet state with a lifetime of 0.2 ns. In the case of ortho-NPA the photodecarboxylation reaction is suppressed by an H⁺- or H˙-transfer reaction in the excited state as a result of the proximity of the nitro and acetate substituents. Nevertheless, the photodecarboxylation can be investigated due to the isolated spectral position of the CO₂ band. The analysis of the data reveals that a weak ultrafast release channel (<300 fs) represents the main photodecarboxylation reaction pathway for ortho-NPA. The detailed understanding of the molecular mechanisms of CO₂ uncaging should provide general guidelines for the design of systematically improved nitrobenzyl cages.