Deuterium Isotope Effect in Single Molecule Photophysics and Photochemistry of Hypericin.

The peripherical protons of the dye molecule hypericin can undergo structural interconversion (tautomerization) between different isomers separated by a low energy barrier with rates that depends sensitively on the interaction with local chemical environment defined by the nature of host material. We investigate the deuterium (D) isotope effect of hypericin tautomerism at the single-molecule level to avoid ensemble averaging in different polymer matrices by a combined spectroscopic and computational approach. In the 'innocent' PMMA matrix only intramolecular isotope effects on the internal conversion channel and tautomerization are observed; while PVA specifically interacts with the probe via H- and D-bonding. This establishes a single molecular picture on intra- and intermolecular nano-environment effects to control chromophore photophysics and -chemistry.