During solvent extraction, amphiphilic extractants assist the transport of metal ions across the liquid-liquid interface between an aqueous ionic solution and an organic solvent. Investigations of the role of the interface in ion transport challenge our ability to probe fast molecular processes at liquid-liquid interfaces on nanometer-length scales. Recent development of a thermal switch for solvent extraction has addressed this challenge, which has led to the characterization by X-ray surface scattering of interfacial intermediate states in the extraction process. Here, we review and extend these earlier results. We find that trivalent rare earth ions, Y(III) and Er(III), combine with bis(hexadecyl) phosphoric acid (DHDP) extractants to form inverted bilayer structures at the interface; these appear to be condensed phases of small ion-extractant complexes. The stability of this unconventional interfacial structure is verified by molecular dynamics simulations. The ion-extractant complexes at the interface are an intermediate state in the extraction process, characterizing the moment at which ions have been transported across the aqueous-organic interface, but have not yet been dispersed in the organic phase. In contrast, divalent Sr(II) forms an ion-extractant complex with DHDP that leaves it exposed to the water phase; this result implies that a second process that transports Sr(II) across the interface has yet to be observed. Calculations demonstrate that the budding of reverse micelles formed from interfacial Sr(II) ion-extractant complexes could transport Sr(II) across the interface. Our results suggest a connection between the observed interfacial structures and the extraction mechanism, which ultimately affects the extraction selectivity and kinetics.

Spin-forbidden triplet excited states of conjugated polymers have important ramifications for material performance and stability, yet triplet processes are difficult to understand and control at the bulk material level. We investigate the effect of a heavy heteroatom and chain conformation on triplet-mediated oxygen photochemistry events in poly(3-hexylthiophene) (P3HT) and poly(3-hexylselenophene) (P3HS) systems using high throughput single molecule spectroscopic imaging. Fluorescence intensity transients of both polymers exhibit discrete intermittency behavior (blinking) characteristic of collapsed conformations and efficient energy funneling. Although both systems have similar molecular weights (∼30 kDa), P3HS transients show unexpectedly longer average "on" times and larger average intensities that we attribute to shorter-lived triplets and faster ground electronic state recycling than in P3HT counterparts. This lowers the probability of sensitizing reactive oxygen species and residence times in "off" states. We use detailed statistical modeling incorporating a hidden two-state Markov chain with a transient bleach state to simulate irreversible photobleaching. Statistical distributions of "on" and "off" time distributions from simulated fluorescence intensity transients are in excellent agreement with experiment, consistent with lower average triplet occupancies in P3HS. Our findings offer new molecular-level insights of heavy atom effects on triplet occupancies and discrete photochemistry events that are difficult to resolve at the ensemble level because of averaging over all conformations and packing arrangements.

Charge transport and collection in organic solar cells are heavily influenced by traps which ultimately limit the ability to harvest all photogenerated carriers. We investigate photocurrent responses of organic solar cells subjected to varying degrees of aging from time-and frequency-domain perspectives. Intensitymodulated photocurrent spectroscopy (IMPS) is primarily used here to resolve the effect of trap-assisted nongeminate charge recombination over a broad frequency range (e.g., ∼1 mHz−1 MHz). We use a combination of IMPS and time-dependent photocurrent transients to understand characteristic degradation signatures (i.e., positive, low-frequency imaginary component and "gain peak" where the real photocurrent exhibits a characteristic maximum, I max , at high frequencies) unique to organic solar cells. As trap densities and occupation increase with aging and light intensity, the photocurrent contrast (i.e., maximum/steady-state photocurrent, I max /I DC ) and the size of the low-frequency imaginary contribution increase. Substantial harmonic content underlies this trend which becomes more prominent as modulation frequencies and trap levels increase. We then use drift-diffusion simulations to describe IMPS responses and photocurrent transient signals over the entire frequency sampling window for aged devices that show excellent agreement with experiment. The results provide deeper insights into trap-related phenomena over a larger frequency bandwidth and further demonstrate the effectiveness of IMPS in its ability to identify mechanistic and kinetic details of degradation.

Resolving the population dynamics of multiple triplet excitons on time scales comparable to their lifetimes is a key challenge for multiexciton harvesting strategies, such as singlet fission. We show that this information can be obtained from fluorescence quenching dynamics and stochastic kinetic modeling simulations of single nanoparticles comprising self-assembled aggregated chains of poly(3-hexylthiophene) (P3HT). These multichromophoric structures exhibit the elusive J-aggregate type excitonic coupling leading to delocalized intrachain excitons that undergo facile triplet formation mediated by interchain charge transfer states. We propose that P3HT J-aggregates can serve as a useful testbed for elucidating the presence of multiple triplets and understanding factors governing their interactions over a broad range of time scales. Stochastic kinetic modeling is then used to simulate discrete population dynamics and estimate higher order rate constants associated with triplet-triplet and singlet-triplet annihilation. Together with the quasi-CW nature of the experiment, the model reveals the expected amounts of triplets at equilibrium per molecule. Our approach is also amenable to a variety of other systems, e.g., singlet fission active molecular arrays, and can potentially inform design and optimization strategies to improve triplet harvesting yields.

We investigate the effect of molecular geometry and conformational flexibility on electronic coupling and charge transfer interactions within propeller-shaped perylene diimide (PDI) tetramer arrays differing by the number of covalent linkages to a central spirobifluorene core. Electronic spectra of tetramers with one (“floppy”) or two (“rigid”) bay covalent linkages display evidence of charge transfer character in either ground or excited states. Floppy tetramers exhibit marked red-shifted and broadened absorption features that we assign as overlapping inter-PDI charge transfer and PDI-centered π–π* transitions, whereas rigid tetramers retain features similar to single PDI molecules, albeit with broader line widths. Interestingly, both tetramers exhibit charge transfer character in their fluorescence emission, but this is most prominent in the rigid tetramer, which displays dominant long-lived excimer behavior in addition to a minority component resembling single PDI-like emission. We then use single-molecule spectroscopy and imaging to understand how conformational-dependent charge transfer properties influence tetramer photophysics. Over 90% of single rigid tetramers display telegraphic (i.e., two-level) blinking behavior with relatively short “on” times compared to ∼60% of single floppy tetramer transients, which tend to exhibit emission from multiple levels. Electronic structure simulations were next performed to aid in the assignment of electronic transitions and photophysical behavior. Floppy tetramer canonical and natural transition orbitals reveal remarkable similarities with significant charge transfer character in the lowest energy excited states involving transverse PDI units and appreciable spirobifluorene contributions in the ground electronic state. Rigid tetramers exhibit greater electronic delocalization, and calculated absorption transition energies show good agreement with experiment, although excited-state interactions are less straightforward to discern from simulations. Raman spectroscopy and polarization-dependent single-molecule spectroscopy were also performed, supporting assignments based on theoretical predictions and electronic spectroscopy results. Overall, we demonstrate the importance of molecular geometry and conformational flexibility of multichromophore arrays in determining the nature of electronic interactions in ground and excited states, which can eventually be harnessed to improve performance attributes at the materials level.