Theory and experiment examining electron transfer through molecules bound to electrodes are increasingly focused on quantities that are conceptually far removed from current chemical understanding. This presents challenges both for the design of interesting molecules for these devices and for the interpretation of experimental data by traditional chemical mechanisms. Here, the concept of electronic coupling from theories of intramolecular electron transfer is extended and applied in the scattering theory (Landauer) formalism. This yields a simple sum over independent channels, that is then used to interpret and explain the unusual features of junction transport through cross-conjugated molecules and the differences among benzene rings substituted at the ortho, meta, or para positions.

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We calculate that significant quantum interference effects can be observed in elastic electron transport through acyclic molecules. Interference features are evident in the transmission characteristics calculated for cross-conjugated molecules; significantly, these effects dominate the experimentally observable conduction range. The unusual transport characteristics of these molecules are highlighted through comparison with linearly conjugated and nonconjugated systems. The cross-conjugated molecules presented here show a large dynamic range in conductance. These findings represent a new motif for electron transfer through molecules that exhibit both very high and very low tunneling conductance states accessible at low bias without nuclear motion. In designing single molecule electronic components, a large dynamic range allows a high on/off ratio, a parameter of fundamental importance for switches, transistors, and sensors.

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The study of photoinitiated electron transfer in donor-bridgeacceptor molecules has helped elucidate the distance dependence of electron transfer rates and behavior of various electron transfer mechanisms. In all reported cases, the energies of the bridge electronic states involved in the electron transfer change dramatically as the length of the bridge is varied. We report here, in contrast, an instance in which the length of the bridge, and therefore the distance over which the electron is transferred, can be varied without significantly changing the energies of the relevant bridge states. A series of donor-bridge-acceptor molecules having phenothiazine (PTZ) donors, 2,7-oligofluorene (FLn) bridges, and perylene-3,4:9,10-bis(dicarboximide) (PDI) acceptors was studied. Photoexcitation of PDI to its lowest excited singlet state results in oxidation of PTZ via the FLn bridge. In toluene, the rate constants for both charge separation and recombination as well as the energy levels of the relevant FL n ؉• bridge states for n ‫؍‬ electron transfer ͉ hopping ͉ superexchange E fficient long-distance electron transfer is a prerequisite for molecular materials designed to serve as active components in solar cells and nanoscale devices. Being able to consistently achieve ''wire-like'' charge transport in synthetic systems requires a thorough understanding of the mechanisms involved. Recently progress has been made toward this goal (1, 2), but the complexity of even simple molecular systems poses a formidable challenge. In particular, the distance dependence of electron transfer has been shown to be a complex function of individual parameters, including molecular geometry and energetics (3,4). While the use of rigidly bound electron donor (D), bridge (B), electron acceptor (A) compounds has simplified the investigation of the distance dependence of electron transfer by keeping the donor-acceptor distance (r DA ) well defined, it is difficult to vary r DA by changing the length of the bridge without substantially altering the energy levels of the bridge.In this report we examine electron transfer in a system in which r DA can be changed considerably, whereas the oxidation potential of the intervening bridge changes very little. Approximate matching between the donor and bridge energy levels (2, 5, 6) has been shown to be critical for promoting incoherent, or ''wire-like,'' transport over coherent, or exponentially distance-dependent, superexchange transport (7). These earlier studies have shown how the energetic convergence of the relevant donor and bridge energy levels results in a minimal injection barrier for hole transfer to the bridge, leading to a striking change in mechanism as the bridge is lengthened.The relative importance of incoherent transport at larger values of r DA has been confirmed by independent measurements of the decreasing contribution of superexchange to the overall electron transfer rate with increasing length (8, 9). The indirect electronic coupling between the electron donor and acceptor via the br...

All-inorganic cesium lead iodide (CsPbI3) perovskite has improved thermal stability over the organic–inorganic hybrid perovskites and a suitable bandgap for optoelectronic and photovoltaic applications, but it is thermodynamically unstable at room temperature and has multiple structural polymorphs. Here, we show that the use of long-chain ammonium additives during thin film deposition as surface capping ligands results in the stabilization of metastable bulk CsPbI3 perovskite phases without alloying mixed cations or anions into the perovskite lattice. Moreover, two different metastable CsPbI3 perovskite polymorphs in the cubic (α-CsPbI3) and the much less common orthorhombic (β-CsPbI3) structures can be directly synthesized in a one-step spin coating film deposition by using oleylammonium or phenylethyl­ammonium additives, respectively, and both phases are stable at room temperature for months. Time-resolved photoluminescence and photoluminescence quenching experiments show that the photoexcited species in the stabilized orthorhombic CsPbI3 thin film are mainly free carriers under solar illumination with a carrier lifetime of ∼50 ns and carrier diffusion length on the order of ∼100 nm, which implies efficient carrier transport within the film despite the presence of surface ligands. Our results provide a new chemical strategy to synthesize metastable all-inorganic CsPbI3 perovskites, which, together with the good photophysical properties, will open them up for applications in photovoltaic and other optoelectronic devices.

Observing the dynamics of single biomolecules over prolonged time periods is difficult to achieve without significantly altering the molecule through immobilization. It can, however, be accomplished using the Anti-Brownian ELectrokinetic (ABEL) Trap, which allows extended investigation of solution-phase biomolecules - without immobilization -through real-time electrokinetic feedback. Here we apply the ABEL trap to study an important photosynthetic antenna protein, Allophycocyanin (APC). The technique allows the observation of single molecules of solution-phase APC for more than one second. We observe a complex relationship between fluorescence intensity and lifetime that cannot be explained by simple static kinetic models. Light-induced conformational changes are shown to occur and evidence is obtained for fluctuations in the spontaneous emission lifetime, which is typically assumed to be constant. Our methods provide a new window into the dynamics of fluorescent proteins and the observations are relevant for the interpretation of in vivo single-molecule imaging experiments, bacterial photosynthetic regulation, and biomaterials for solar energy harvesting.

Circular dichroism (CD) finds widespread application as an optical probe for the structure of molecules and supramolecular assemblies. Its underlying chiral light−matter interactions effectively couple between photonic spin states and select quantum-mechanical degrees of freedom in a sample, implying an intricate connection with photon-to-matter quantum transduction. However, effective transduction implementations likely require interactions that are antisymmetric with respect to the direction of light propagation through the sample, yielding an inversion of the chiroptical response upon sample flipping, which is uncommon for CD. Recent experiments on organic thin films have demonstrated such chiroptical behavior, which was attributed to "apparent CD" resulting from an interference between the sample's linear birefringence and linear dichroism. However, a theory connecting the underlying optical selection rules to the microscopic electronic structure of the constituent molecules remains to be formulated. Here, we present such a theory based on a combination of Mueller calculus and a Lorentz oscillator model. The theory reaches good agreement with experimental CD spectra and allows for establishing the (supra)molecular design rules for maximizing or minimizing this chiroptical effect. It furthermore highlights that, in addition to antisymmetrically, it can manifest symmetrically such that no chiroptical response inversion occurs, which is a consequence of a helical stacking of molecules in the light propagation direction.