Exploiting singlet fission in para-azaquinodimethane nanoaggregates with high energy triplets to trigger phototoxic reactive oxygen species

In this study, aggregation-induced singlet fission (SF) was unveiled in water-dispersed organic nanoaggregates of push–pull para-azaquinodimethane (pAQM) derivatives exhibiting either asymmetrical (AsOMe and AsNMe2) or symmetrical (TPh and TPhOMe) molecular structures. Our ultrafast spectroscopic experiments revealed that SF occurs in a few to tens of picoseconds in these organic nanoparticles. The nanosecond and femtosecond transient absorption results showed quantitative SF for aggregates of three of the pAQMs, with triplet quantum yields of ca. 200% in water. In most cases, spherical nanoparticles with diameters of tens of nanometers were produced in an aqueous environment, and they could be successfully internalized within prostatic cancer and melanoma cells. The outstanding phototoxicity exhibited toward these cancer cells by the AsOMe nanoaggregates, with the highest production of ROS among the investigated compounds, was demonstrated to be due to their highest triplet energy (1.3 eV), which favours the energy transfer pathway to produce singlet oxygen. All the investigated molecules were also found to exhibit significant two-photon excited fluorescence, with their two-photon absorption cross sections being in trend with their symmetry and intramolecular charge transfer features. To the best of our knowledge, this is the first study highlighting the use of unconventional SF chromophores as new effective photosensitizers for two-photon triggered photodynamic therapy.