Confocal ultrafast pump-probe spectroscopy: a new technique to explore nanoscale composites

This article is devoted to the exploration of the benefits of a newultrafast confocal pump-probe technique, able to study the photophysics of different structuredmaterials with nanoscale resolution. This tool offers many advantages over standard stationary microscopy techniques because it directly interrogates excited state dynamics inmolecules, providing access to both radiativeandnon-radiative deactivation processes at a local scale. In this paper we present a few different examples of its application to organic semiconductor systems. The first two are focussed on the study of the photophysics of phase-separated polymer blends: (i) a blue-emitting polyfluorene (PFO) in an inertmatrix of PMMAand (ii) an electron donor polythiophene (P3HT) mixed with an electron acceptor fullerene derivative (PCBM). The experimental results on these samples demonstrate the capability of the technique to unveil peculiar interfacial dynamics at the border region between phase-segregated domains, which would be otherwise averaged out using conventional pump-probe spectroscopy. The third example is the study of the photophysics of isolated mesoscopic crystals of the PCBMmolecule. Our ultrafast microscope could evidence the presence of two distinctive regions within the crystals. In particular, we could pinpoint for the first time areas within the crystals showing photobleaching/stimulated emission signals from a charge-transfer state.