Local polarity effects on TADF dynamics as probed by single molecule fluorescence

Molecules that undergo thermally activated delayed fluorescence (TADF) represent an important class of systems for the design of efficient organic lighting (OLEDS) because they utilize both singlet and triplet excitons for electrically-generated light emission. Most molecules of this type have considerable charge transfer (CT) character as this is known to result in nearly degenerate singlet and triplet energies. Another important consequence of this CT character is that the TADF efficiency as well as the emission wavelength and color purity are highly sensitive to the polarity of the local environment. Here we present data demonstrating the effect of local polarity on the fluorescence intermittency (blinking) of single TADF molecules isolated in a series of host matrices of varying dielectric constants. The on and off times of the fluorescence of single chromophores are shown to be highly sensitive to local polarity and are used to model the dynamics of singlet-triplet crossing.