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

Light-induced 9,10-phenanthrenequinone-electron-rich
alkene (PQ-ERA) 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 PQ ortho-phenyl position as a powerful, unprecedented means to drastically
modulate kinetics. Introducing steric groups (e.g., 2,6-dimethyl in PQ-o2CH3) 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 (k2 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 PQ-ERA photoclick efficiency provides a foundation for designing
orthogonal ultrafast conjugation systems.