Photolysis of ortho-Nitrobenzyl Esters: Kinetics and Substituent Effects

ortho-Nitrobenzyl (oNB) esters are widely employed as photolabile protecting groups and linkers in diverse applications ranging from materials science to chemical biology. While their photochemical decomposition mechanism has been well studied, a comprehensive understanding of the structure-property relationships governing their photolysis kinetics remains lacking. In this study, the synthesis and kinetic analysis of 11 novel oNB esters with varying ester and aromatic substituents are reported. Using time-resolved NMR spectroscopy, we determined first-order rate constants for the photochemical cleavage of each compound under UV irradiation. A clear correlation between the reaction rate and the acidity (p<i>K</i> _a) of the corresponding acid anion, with more acidic leaving groups yielding faster decomposition, was found. In contrast, the nature of the substituents on the aromatic ring shows no strong correlation with Hammett parameters or steric hindrance, quantified by Sterimol parameters. A multilinear model incorporating both steric and electronic parameters confirms this observation. Our findings highlight the critical role of the ester moiety in tuning photolysis rates while suggesting that aromatic substitution can be strategically employed for functionalization (e.g., polymer cross-linking) without substantially affecting reactivity. These insights lay the foundation for the rational design of photoresponsive oNB-based linkers for advanced materials and chemical applications.