Singlet fission, where an electronically excited singlet on one chromophore converts into a doubly excited state on two, has gone from a curiosity in organic photophysics to a potential pathway for increasing solar energy conversion efficiencies. Focusing on the role of solvent-induced energy level fluctuations that would be present in a dye-sensitized solar cell, we present a microscopic model for singlet fission. Starting from an electronic model Hamiltonian, we construct diabatic states in a manifold of single and double excitations with total singlet multiplicity and then develop a multilevel non-Markovian theory of dynamics for electronic populations in the presence of energy level fluctuations. Depending on the energy scales, energy gap fluctuations can either facilitate or hinder interconversion steps that lead to singlet fission. We critically assess the Markovian approximation that leads to golden rule rates and study the role of intramolecular solvation dynamics and electron transfer.

Deciphering the significance of length, sequence, and stereochemistry in block copolymer self-assembly remains an ongoing challenge. A dearth of methods to access uniform block co-oligomers/polymers with precise stereochemical sequences has precluded such studies. Here, we develop iterative exponential growth (IEG) methods for the synthesis of a small library of unimolecular stereoisomeric diblock 32-mers. X-ray scattering reveals that stereochemistry modulates the phase behavior of these polymers, which we rationalize based on simulations carried out on a theoretical model system. This work demonstrates that stereochemical sequence can play a crucial role in unimolecular polymer self-assembly.

Polymer gels are often very soft due to their low branch functionality ( f) and the inevitable presence of defects (e.g., primary loops or dangling chains). Polymer metal−organic cage (polyMOC) gels are a relatively new class of supramolecular gels with precisely defined junction structures made possible by subcomponent assembly of nanoscale MOCs connected by polymer chains. Herein, we report that variation of the molecular weight and architecture of the polymer component of polyMOCs provides an entry into gels with ultra-high f. For example, materials with f ∼ 9− 12, i.e., ∼ 9−12 polymer chains connect each MOC within the gel network, are realized. As a consequence of their increased f, these gels display exemplary mechanical properties at low concentrations (down to 240 μM) of metal ions and only 5.4−5.9 wt % of polymer. Furthermore, X-ray and neutron scattering pointed to an additional level of structural hierarchy that arises from the assembly of M 12 L 24 MOCs into clusters. The relationships between polymer and polyMOC network structure revealed here will facilitate the design of high-performance polyMOCs.

We present a model to describe collective features of singlet fission in molecular crystals and analyze it using many-body theory. The model we develop allows excitonic states to delocalize over several chromophores which is consistent with the character of the excited states in many molecular crystals, such as the acenes, where singlet fission occurs. As singlet states become more delocalized and triplet states more localized, the rate of singlet fission increases. We also determine the conditions under which the two triplets resulting from fission are correlated. Using the Bethe Ansatz and an entanglement measure for indistinguishable bipartite systems, we calculate the triplet-triplet entanglement as a function of the biexciton interaction strength. The biexciton interaction can produce bound biexciton states and provides a source of entanglement between the two triplets even when the triplets are spatially well separated. Significant entanglement between the triplet pair occurs well below the threshold for bound pair formation. Our results paint a dynamical picture that helps to explain why fission has been observed to be more efficient in molecular crystals than in their covalent dimer analogues and have consequences for photovoltaic efficiency models that assume that the two triplets can be extracted independently.

Vibrational spectroscopy can provide information about structure, coupling, and dynamics underlying the properties of complex molecular systems. While measurements of spectral line broadening can probe local chemical environments, the spatial averaging in conventional spectroscopies limits insight into underlying heterogeneity, in particular in disordered molecular solids. Here, using femtosecond infrared scattering scanning near-field optical microscopy (IR s-SNOM), we resolve in vibrational free-induction decay (FID) measurements a high degree of spatial heterogeneity in polytetrafluoroethylene (PTFE) as a dense molecular model system. In nanoscopic probe volumes as small as 10(3) vibrational oscillators, we approach the homogeneous response limit, with extended vibrational dephasing times of several picoseconds, that is, up to 10 times the inhomogeneous lifetime, and spatial average converging to the bulk ensemble response. We simulate the dynamics of relaxation with a finite set of local vibrational transitions subject to random modulations in frequency. The combined results suggest that the observed heterogeneity arises due to static and dynamic variations in the local molecular environment. This approach thus provides real-space and real-time visualization of the subensemble dynamics that define the properties of many functional materials.