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.

We investigate the ability of dynamic fluorescence probes to accurately track populations of multi-excitonic states in molecular dyads based on conjugated acenes capable of intramolecular singlet fission (iSF). Stochastic simulations of reported photophysical models from time-resolved spectroscopic studies of iSF dyads based on large acenes (e.g., tetracene and pentacene) are used to extrapolate population and fluorescence yield dynamics. The approach entails the use of repetitive rectangular-shaped excitation waveforms as a stimulus, with durations comparable to triplet lifetimes. We observe unique dynamics signatures that can be directly related to relaxation of multi-exciton states involved over the entire effective time of singlet fission in the presence and absence of an excitation light stimulus. In particular, time-dependent fluorescence yields display an abrupt decay followed by slower rise dynamics appearing as a prominent “dip” feature in responses. The initial fast decrease in the fluorescence yield arises from the formation of triplet pairs and separated triplets that do not produce emission resembling a complete ground state bleach effect. However, relaxation of one separated triplet allows the system to absorb, and in some cases, this increases the fluorescence yield, causing rise dynamics in the emissive response. Our approach also permits extrapolation of all multi-exciton state population dynamics up to steady state conditions in addition to the ability to explore consequences of alternative relaxation channels. The results demonstrate that it is possible to resolve unique signatures of singlet fission events from dynamic fluorescence studies, which can augment detection capabilities and extend sensitivity limits and accessible time scales.

The formation of long-lived triplet excited electronic states has important ramifications for conjugated organic materials used in optoelectronic devices. In the case of polymers, unravelling various structural factors mediating triplet processes is difficult because of heterogeneity effects due to intrinsic molecular weight polydispersity and large conformational degrees of freedom. Conformation-dependent electronic coupling between chromophore segments also modulates relaxation branching ratios that may vary substantially from molecule to molecule. However, ensemble-level spectroscopy experiments typically average over distinct responses, which disguises important qualities of the overall material photophysical landscape. Suppression of heterogeneity by diluting polymers into inert solid hosts permits single molecule level investigations of conformation-dependent triplet dynamics thereby avoiding the most insidious consequences of ensemble averaging. Interestingly, the multichromophoric nature of polymers can lead to significant likelihoods of multiple coexisting triplets, where population dynamics are revealed from fluorescence quenching dynamics on time scales comparable to triplet lifetimes (i.e., μs to ms). Stochastic photodynamic models are then used to extract key kinetic constants, including bimolecular triplet−triplet annihilation, that tend to exhibit pronounced dependences on polymer conformational ordering. Furthermore, simple processing strategies to selectively control chain conformation and packing order in hierarchical polymer assemblies can be combined with experiment and modeling to uncover the evolution in triplet processes from single molecule to bulk material levels. We posit that molecular-level control can be harnessed to more accurately manage triplet yields and interactions over a large range of time scales, which has potential uses in multiexciton harvesting schemes, such as singlet fission.

Photoinduced oxidation (doping) of conjugated polymers by complexation with oxygen can have a significant impact on electronic properties and performance in device environments. Nanofiber model forms of poly(3‐hexylthiophene) (P3HT) are investigated using single molecule spectroscopy that possess similar morphological qualities as their bulk thin film counterparts yet, heterogeneity is confined to the spatial dimensions of these particles. Specifically, P3HT nanofibers assembled in anisole solutions contain both aggregated and nonaggregated (amorphous) chains with distinct electronic properties. Excitation intensity dependent photoluminescence (PL) emission imaging is then used to expose differences in oxygen affinity and reactivity upon photoexcitation. Nanofiber regions with low PL yields tend to show faster PL intensity saturation that also degrade much faster following periods of high excitation intensity soaking. Conversely, other regions show gains in PL intensity and virtually no saturation. These PL “gainer” and “loser” behaviors are assigned as originating from amorphous and aggregated P3HT chains, respectively. The apparent propensity of aggregated chains to undergo latent oxygen doping indicates a greater affinity probably due to a larger extent of electronic delocalization in these structures. The results shed new light on degradation factors studied frequently at the bulk material level, which often lacks sufficient sensitivity to specific structural forms.