Linear and Nonlinear Optical Spectroscopy of Fluoroalkylated BODIPY Dyes

A series of fluoroalkyl-labeled BODIPY dyes have been synthesized with different substituents on the 2- and 6-positions and investigated in terms of their spectroscopic properties. The dyes were decorated with aryl, alkenyl, and alkynyl substituents, respectively. Those substituents are able to expend the dyes’ delocalized π-electron system to different extents. Detailed linear and nonlinear optical spectroscopy methods as well as quantum-chemical calculations have been employed to verify the influence of those different substituents on the electronic distribution and, ultimately, on the molecules’ optical properties. From the nonlinear optical measurements, we find that the substituents highly influence the excited-state absorption properties, which could be tuned in wavelength by ∼100 nm and in magnitude by over 1 order of magnitude, reaching peak values close to 10^–15 cm² for one of the alkynyl-containing dyes

Efficient Biexciton Interaction in Perovskite Quantum Dots Under Weak and Strong Confinement

Cesium lead halide perovskite quantum dots (PQDs) have emerged as a promising new platform for lighting applications. However, to date, light emitting diodes (LED) based on these materials exhibit limited efficiencies. One hypothesized limiting factor is fast nonradiative multiexciton Auger recombination. Using ultrafast spectroscopic techniques, we investigate multicarrier interaction and recombination mechanisms in cesium lead halide PQDs. By mapping the dependence of the biexciton Auger lifetime and the biexciton binding energy on nanomaterial size and composition, we find unusually strong Coulomb interactions among multiexcitons in PQDs. This results in weakly emissive biexcitons and trions, and accounts for low light emission efficiencies. We observe that, for strong confinement, the biexciton lifetime depends linearly on the PQD volume. This dependence becomes sublinear in the weak confinement regime as the PQD size increases beyond the Bohr radius. We demonstrate that Auger recombination is faster in PQDs compared to CdSe nanoparticles having the same volume, suggesting a stronger Coulombic interaction in the PQDs. We confirm this by demonstrating an increased biexciton binding energy, which reaches a maximum of about 100 meV, fully three times larger than in CdSe quantum dots. The biexciton shift can lead to low-threshold optical gain in these materials. These findings also suggest that materials engineering to reduce Coulombic interaction in cesium lead halide PQDs could improve prospects for high efficiency optoelectronic devices. Core–shell structures, in particular type-II nanostructures, which are known to reduce the bandedge Coulomb interaction in CdSe/CdS, could beneficially be applied to PQDs with the goal of increasing their potential in lighting applications