A hydrophobic nanocapsule controls the photophysics of aromatic molecules by suppressing their favored solution pathways.

A combination of hydrophobic forces and guest templation drive the assembly of cavitands into molecular capsules. Remarkably, anthracene that dimerizes with unit efficiency in solution does not dimerize within the capsule despite forming 2:2 complex. The capsule allows an unprecedented examination of the anthracene excimer.