Unveiling Aggregation‐Induced Singlet Fission in Push–Pull Diketopyrrolopyrrole Water Dispersed Nanoaggregates and Thin Films

In this work, the optical properties of a set of carbazole functionalized diketopyrrolopyrrole derivatives, characterized by dipolar/quadrupolar structures and by meta/para substitutions, are investigated by laser spectroscopies with nanosecond and femtosecond resolution. In particular, their absorption, emission, charge transfer, and triplet production abilities are studied in depth in view of their use in organic photovoltaics and photodynamic therapy. To this aim, their photobehavior is explored not only as monomers in dilute solution but also as aggregates produced in solid‐state thin films and as organic nanoparticles in water. The bright fluorescence exhibited in solution is largely quenched upon aggregation, with the singlet excited state lifetimes significantly shortened. The femtosecond transient absorption experiments unveil the non−radiative decay pathway competitive to the emission to be aggregation‐induced singlet fission, occurring in a few picoseconds and with triplet yields exceeding 100% in both the organic nanoparticles and the films. The largest intramolecular charge transfer degree of the para derivatives interestingly enables the fastest intermolecular singlet fission in the aggregates. The study sets the stage for the singlet fission enhanced use of these strongly visible light‐absorbing push–pull diketopyrrolopyrrole‐based organic materials for solar and biomedical applications, also triggered by two photon excitation in the biological transparency window.