Concentration and Solvent Effects on the Excited State Dynamics of the Solar Cell Dye D149: The Special Role of Protons

D149 is one of the best-performing metal-free, organic dyes for dye-sensitized solar cells. Excited state lifetimes strongly depend on the solvent used and have previously been reported to be between 100 and 700 ps, without any mechanistic explanation being given. We have earlier shown that photo-isomerization is one of several deactivation processes. Here, we report that lifetimes in certain solvents depend on concentration, even in very dilute (nanomolar) solutions. A detailed investigation of the concentration dependence enables us to assign a second, faster deactivation channel besides isomerization that reduces lifetimes further: a ground-state, hydrogen-bonded 1:1 complex of D149 with acids or interaction with protic solvents leads to excited state quenching, most probably through excited state proton transfer. This includes self-quenching caused by D149's own carboxylic group through intermolecular interaction, accounting for the concentration-dependent lifetimes. We are now able to dissect the complex excited state behavior into its components, allowing us to attribute rate constants to the isomerization and the excited-state proton transfer process. We are also able to explain the excited state of D149 in a wide range of environmental conditions, in the presence of acids/bases, at different concentrations as well as with varying temperatures. Furthermore, we determine the barrier for isomerization, a thermally activated process. The consequences of these effects on solar cells are discussed. Also we show that ultrafast techniques like femtosecond pump probe and upconversion inherently do not provide the required responsiveness for work with the concentration ranges required here, whereas single photon counting with its ultimate sensitivity is able to resolve the underlying processes.