Solvent-polarity (In)dependent Deactivation of BODIPY Molecular Rotors∗

Molecular rotors based on meso-substituted boron-dipyrromethane (BODIPY) are widely recognized fluorescent viscosity sensors. The viscosity dependence of their fluorescence arises from an efficient excited-state deactivation process that can only occur when molecularscale motion is not hindered. Here, we use visible and IR pump-probe spectroscopies combined with TD-DFT calculations to show that this fluorescence deactivation takes place through a fast and irreversible process which does not involve intermediate electronic states. Our data indicate that nonradiative excited-state deactivation of BODIPY molecular rotors is practically independent of solvent polarity, but strongly governed by viscoelastic/free volume properties of the local environment in both lowand high-viscosity regimes. ∗ This chapter is adapted from: Suhina, T; Amirjalayer, S.; Woutersen, S.; Bonn, D.; Brouwer, A. M., submitted.