Emission of ProtonatedRiboflavin Induced by Inhibitionof Out-of-plane Vibration in a Rigid Polymer Network

Chromophores incorporated into rigid polymer matrices may exhibit novel photophysical properties distinct from those in liquid solutions. In this work, we explored the decay path of the second ππ* state (2ππ*) of riboflavin in poly(vinyl alcohol) (PVA) solutions and films with various acidities. Highly efficient internal conversion from 2ππ* to the lowest ππ* state (1ππ*) induced by slight in-plane motion is demonstrated in all PVA solutions and films, irrespective of environmental acidity and rigidification. Ground-state protonation of riboflavin occurs in the highly acidic PVA film, and this cationic species emits fluorescence around 460 nm, in contrast to the ultrafast nonradiative relaxation in aqueous solution. The emission is demonstrated to originate from the 2ππ* state rather than the 1ππ* state. The 2ππ*-1ππ* internal conversion of protonated riboflavin is demonstrated to be induced by out-of-plane motion and impeded in a rigid PVA network, resulting in the anti-Kasha fluorescence.