A Physical Organic Odyssey: From the Ultrafast Dynamics of Carbenes and Nitrenes to Contemporary Chemical Biology and Drug Discovery

The field of photoaffinity labeling (PAL) stands as a testament to the enduring power of physical organic chemistry in driving biological discovery. For nearly five decades, Professor Matthew S. Platz has occupied a singular position at the intersection of reactive intermediate chemistry, ultrafast spectroscopy, and chemical biology—transforming our mechanistic understanding of carbenes, nitrenes, oxenium ions, and their excited‐state precursors into actionable design principles for molecular probes. This review, dedicated to Professor Platz on the occasion of his 75th birthday, traces the arc of his scientific legacy: from foundational matrix‐isolation and laser flash photolysis studies of phenylcarbene and phenylnitrene, through the transformative discovery of the fluorine effect in aryl azide photochemistry, to the rigorous mechanistic revision of widely used nucleotide‐based PAL probes. A central section surveys the remarkable breadth of Platz's ultrafast spectroscopic program—encompassing diarylcarbenes, acyl and sulfonyl azides, Wolff rearrangements, oxenium ions, and heteroatom‐containing reactive intermediates—establishing the quantitative kinetic framework that underpins rational probe design. We then connect these foundational insights to the contemporary chemoproteomics landscape of 2020–2025: the diazo problem in diazirine labeling, the rational design of PALBOX probes, the emergence of acyl silane photophores, photocatalytic proximity labeling platforms, and quantitative mass spectrometry workflows. Throughout, we argue that the recurring motif of Platz's career—that rigorous kinetic and mechanistic characterization of fleeting intermediates is the prerequisite for rational reagent design—remains as productive and necessary today as when he first turned a laser onto a solution of phenyl azide.