Singlet fission is the spin-conserving process by which a singlet exciton splits into two triplet excitons. Singlet fission occurs via a correlated triplet pair intermediate, but direct evidence of this state has been scant, and in films of TIPS-pentacene, a small molecule organic semiconductor, even the rate of fission has been unclear. We use polarization-resolved transient absorption microscopy on individual crystalline domains of TIPS-pentacene to establish the fission rate and demonstrate that the initially created triplets remain bound for a surprisingly long time, hundreds of picoseconds, before separating. Furthermore, using a broadband probe, we show that it is possible to determine absorbance spectra of individual excited species in a crystalline solid. We find that triplet interactions perturb the absorbance, and provide evidence that triplet interaction and binding could be caused by the π-stacked geometry. Elucidating the relationship between the lattice structure and the electronic structure and dynamics has important implications for the creation of photovoltaic devices that aim to boost efficiency via singlet fission.

We investigate the dynamics of rigid bodies (hollow 'pyramids') placed within a background airflow, oscillating with zero mean. The asymmetry of the body introduces a net upward force. We find that when the amplitude of the airflow is above a threshold, the net lift exceeds the weight and the object starts to hover. Our results show that the objects hover at far smaller air amplitudes than would be required by a quasi-steady theory, although this theory accounts qualitatively for the behaviour of the system as the body mass becomes small. †

Inorganic lead halide perovskite nanostructures show promise as the active layers in photovoltaics, light emitting diodes, and other optoelectronic devices. They are robust in the presence of oxygen and water, and the electronic structure and dynamics of these nanostructures can be tuned through quantum confinement. Here we create aligned bundles of CsPbBr 3 nanowires with widths resulting in quantum confinement of the electronic wavefunctions and subject them to ultrafast microscopy. We directly image rapid one-dimensional exciton diffusion along the nanowires, and we measure an exciton trap density of roughly one per nanowire. Using transient absorption microscopy, we observe a polarization-dependent splitting of the band edge exciton line, and from the polarized fluorescence of nanowires in solution we determine that the exciton transition dipole moments are anisotropic in strength. Our observations are consistent with a model in which splitting is driven by shape anisotropy in conjunction with long-range exchange.

By spatially resolving the polarized ultrafast optical transient absorption within several tens of individual domains in solution-processed polycrystalline smallmolecule organic semiconducting films, we infer the domains' extents of structural and orientational heterogeneity. As metrics, we observe variations in the time scales of ultrafast excited state dynamics and in the relative strength of competing resonant probe transitions. We find that films of 2,8-difluoro-5,11-bis(triethylsilylethynyl)anthradithiophene (diF-TES-ADT) exhibit a much higher degree of both structural and orientational heterogeneity among their domains than do films of 6,13-bis(triisopropylsilylethynyl)pentacene (TIPSPn), despite the apparent structural similarity between these two small molecules. Since both molecules feature prominently in solution-processed organic transistors, correlating the extent of heterogeneity to bulk transport using our approach will be highly valuable toward determining the underlying design principles for creating high-performing devices. Furthermore, our ability to characterize such variation in heterogeneity will enable fundamental studies of the interplay between molecular dynamics and driving forces in controlling emergent unequilibrated structures. S mall-molecule organic semiconducting thin films serve many applications in solution-processed electronics. In many cases, they form polycrystalline films with a wide range in the size and shape of individual domains. The high degree of intermolecular order in these films predisposes them to applications in solution-processed transistors, where comparatively high charge carrier mobilities can be engineered due to regular π-stacking or other forms of orbital overlap along a particular in-plane direction.

The design of chemical formulations is a challenging, high-dimensional problem. In typical formulations, tens of thousands of ingredients are available for use, yet only a tiny fraction end up in a given formulation. Deformulation, the problem of reverse engineering the precise amounts of each ingredient starting from just a list of ingredients, is similarly challenging but is a key capability for staying up-to-date with industry competitors. Here, we take advantage of a large, curated formulations dataset from CAS, a division of the American Chemical Society, which offers a consistent and highly structured representation of the formulations and the chemical identities of their components to show that a variational autoencoder neural network learns meaningful representations of formulations in various product classes such as antiperspirants and oral care. Furthermore, it can be used in conjunction with a two-step sampling algorithm to generate accurate ingredient amount suggestions for deformulation. Deformulation using a variational autoencoder produces estimates that are significantly more accurate than nearest neighbor methods, extrapolates better to formulations that are significantly different than previously seen formulations, and provides a way to leverage large datasets for industrially relevant capabilities.