"Dispersion-Controlled Excited-State Dynamics in Azobenzene Photoisomerization" in Journal of the American Chemical Society
Weak interactions, like London dispersion forces, are cumulative in nature and have been thought to be essential for only the structure and stability of large molecular systems. Only recently has their relevance for chemical reactivity been recognized. Until today, their role in photoreactions and subsequent ultrafast excited-state processes has remained elusive. Here, we show the impact of London dispersion on the excited-state behavior and the outcome of the photoreaction of the iconic photoswitch azobenzene as a representative example. Increased dispersion interactions between substituents decisively prolong the excited-state lifetimes by preventing direct passage through the conical intersection. This significantly alters the dynamics of the Z to E photoisomerization. We expect our findings to lead to increased research interest in such “dispersion-controlled excited-state dynamics” relevant for the steering of ultrafast processes.
"Trapped Together: How a Second Phase Induces Extrinsic Self-Trapping Leading to Enhanced Emitting States in 2D Perovskites" in Chemistry of Materials
Two-dimensional (2D) lead halide (R2PbX4, with X = Br, Cl, and R = organic ammonium cation) perovskites (PSK) are promising materials for the generation of white light due to self-trapping events leading to broadband photoluminescence (PL). In the mixed-halide series NEA2Pb(Br1−nCln)4 (NEA = 2-naphthyl 2-ethylammonium), we systematically investigated defect effects on optical properties. We focused on Cl-rich samples, which showed enhanced broad emission, and compared the chloride-only component with two mixtures, n = 0.7, which exhibits one phase, and n = 0.5, including a small quantity of an additional Br-rich phase. We combined X-ray diffraction with steady-state and time-resolved optical studies and found prolonged PL and recombination lifetimes for n = 0.5 > 0.7 > 1, demonstrating the enhancing effect of defect-induced extrinsic self-trapping on emitting states.
