Steric Engineering of Phenanthrenequinone for Ultrafast and Tunable Visible Light-Induced Photoclick Reaction

Light-induced 9,10-phenanthrenequinone-electron-rich alkene (<b>PQ-ERA</b>) photocycloadditions are an attractive new type of photoclick reaction featuring fast conversions. However, the tunability of the reaction has hardly been investigated up to now. To this end, we explored steric engineering at the <b>PQ</b> <i>ortho</i>-phenyl position as a powerful, unprecedented means to drastically modulate kinetics. Introducing steric groups (<i>e.g.,</i> 2,6-dimethyl in <b>PQ-o2CH</b>_<b>3</b>) enforced large dihedral angles (69.5°) as a result of which the ¹nπ* state became the lowest electronically excited singlet state S₁ instead of the ¹ππ* state, while retaining T₁ as ³ππ*, the triplet lifetime was boosted and an ultrafast direct [4 + 2] photoclick reaction was enabled. Critically, steric groups accelerated rates by >21-fold (<i>k</i>₂ up to 11,300 M^-1 s^-1), spanning 2 orders of magnitude. Leveraging this novel steric-kinetic relationship, we demonstrated a "burn-after-reading" encryption concept via spatially controlled photoclick and rapid erasure by uniform isoconversion. This demonstration of steric control over <b>PQ-ERA</b> photoclick efficiency provides a foundation for designing orthogonal ultrafast conjugation systems.