Ultrafast Triplet Formation in N-Substituted Pentacene Derivatives

Higher members of the acene molecule class, like the benchmark compound pentacene, are known to undergo singlet fission (SF). This extraordinary process is attractive regarding its ability of boosting quantum effciencies \nin photovoltaic cells and overcoming the Shockley-Queisser limit of single junction devices. There is consensus that in pentacene SF proceeds on an ultrafast timescale (<100 fs), but the details of the process are still subject \nof debate. Within this work, pump-probe as well as pump-depletion-probe techniques were used in combination with rate model simulations to disentangle the initial steps of SF. In particular, the effect of nitrogen atoms, \nwhich were incorporated into the pentacene backbone, on the photoinduced dynamics of TIPS-pentacene was studied. These solution-processable azaderivatives, called Diaza-TIPS- pentacene and Tetraaza-TIPS-pentacene, are great building blocks for the design of new architectures with optimized electronic properties and represent promising candidates for their use in photovoltaic devices. Measurements in the visible as well as in the near-infrared spectral region were performed in order to get a complete picture of the triplet manifold. This required the design and implementation of a new experimental setup. By applying global target analyses and numerical simulations, a \ndetailed kinetic model of the excited state dynamics of all investigated materials could be established. The experimental observations indicate a participation of the intermediate coupled triplet pair state 1TT, which has become a hot topic regarding its role in mediating SF. The nitrogen substitution in the heteroacenes not only accelerates the formation of the 1TT state compared \nto TIPS-pentacene, but also the population of the final triplet state T1 via the 1TT state, implying higher quantum efficiencies. The absorption of all relevant exited states were successfully assigned with regard to their \nspectral occurence.