Enantioselective Protonation of Silyl Enol Ether Using Excited State Proton Transfer Dyes

Enantiopure excited state proton transfer (ESPT) dyes were used for the asymmetric protonation of silyl enol ether. Under 365 nm irradiation, with 3,3′-dibromo-VANOL as the ESPT dye, up to 49% enantioselectivity with a 68% yield of product was observed at room temperature. The reaction is effective with a range of silyl enol ethers and can also be achieved with visible light upon the addition of triplet sensitizer. The relatively low ee of the protonated product is due to the racemization/decomposition of the ESPT dye in the excited state as indicated by circular dichroism, HPLC, and UV–vis spectroscopy

Inhibiting Interfacial Recombination Events in Dye-Sensitized Solar Cells using Self-Assembled Bilayers

The rate and efficiency of electron transfer events at the semiconductor–dye–electrolyte interface is of critical importance to the overall performance of dye-sensitized solar cells (DSSCs) and dye-sensitized photoelectrosynthesis cells. In this work, we introduce self-assembled bilayers composed of a metal oxide electrode, bridging molecules, linking ions, and dye as an effective strategy to manipulate interfacial electron transfer events at the photoanode of DSSCs. Spectroelectrochemical measurements including current–voltage, incident photon-to-current efficiency, and electrochemical impedance spectroscopy are used to quantify interfacial electron transfer and transport events with respect to the length of the bridging molecules. The general trend in increased lifetime and diffusion length in TiO₂ as well as an increase in open circuit voltage with bridge length indicate that the bilayer is an effective strategy in inhibiting the TiO₂(e^–) to redox mediator recombination events. However, the increased separation between the dye and the semiconductor also reduces the electron injection rate resulting in a decrease in photocurrent as the bridge length increases. The observed enhancement in open circuit voltages are far outweighed by the significant decrease in photocurrent and thus overall device performance decreases with increasing bridge length

Increasing the Open-Circuit Voltage of Dye-Sensitized Solar Cells via Metal-Ion Coordination

Considerable efforts are dedicated to increasing the open-circuit voltage (<i>V</i>_oc) of dye-sensitized solar cells (DSSCs) by slowing charge recombination dynamics using atomic layer deposition, alkyl-substituted dyes, coadsorbents, and other strategies. In this report, we introduce metal-ion coordination to a metal oxide bound dye as an alternative means of increasing <i>V</i>_oc. Metal-ion coordination has minimal influence on the photophysical and electrochemical properties of the N3 dye, but presumably because of increased steric hindrance at the interface, it slows charge recombination kinetics and increases <i>V</i>_oc by upwards of 130 mV relative to the parent N3 DSSC. With respect to the nature of the metal ion, the trend in decreasing short-circuit current (<i>J</i>_sc) and increasing <i>V</i>_oc correlates with the charge of the coordinated metal ion (M^IV → M^III → M^II). We attribute this trend to electrostatic interactions between the metal cation and I^– or I₃^–, with the more highly charged cations maintaining a higher concentration of mediator anions in proximity to the surface and, as a result, increasing the regeneration and recombination rates

Modulating Electron Transfer Dynamics at Dye–Semiconductor Interfaces via Self-Assembled Bilayers

Forward and back electron transfer at dye–semiconductor interfaces are pivotal events in dye-sensitized solar cells and dye-sensitized photoelectrosynthesis cells. Here we introduce self-assembled bilayers as a strategy for manipulating electron transfer dynamics at these interfaces. The bilayer films are achieved by stepwise layering of bridging molecules, linking ions, and dye molecules on the metal oxide surface. The formation of the proposed architecture is supported by ATR-IR and UV–vis spectroscopy. By using time-resolved emission and transient absorption, we establish that the films exhibit an exponential decrease in electron transfer rate with increasing bridge length. The findings indicate that self-assembled bilayers offer a simple, straightforward, and modular method for manipulating electron transfer dynamics at dye–semiconductor interfaces

Integrated Photon Upconversion Solar Cell via Molecular Self-Assembled Bilayers

Molecular photon upconversion, by way of triplet–triplet annihilation (TTA-UC), is an intriguing strategy to increase solar cell efficiencies beyond the Shockley–Queisser limit. Here we introduce self-assembled bilayers of acceptor and sensitizer molecules on high surface area electrodes as a means of generating an integrated TTA-UC dye-sensitized solar cell. Intensity dependence and IPCE measurements indicate that bilayer films effectively generate photocurrent by two different mechanisms: (1) direct excitation and electron injection from the acceptor molecule and (2) low-energy light absorption by the sensitizer molecule followed by TTA-UC and electron injection from the upconverted state. The power conversion efficiency from the upconverted photons is the highest yet reported for an integrated TTA-UC solar cell. Energy transfer and photocurrent generation efficiency of the bilayer device is also directly compared to the previously reported heterogeneous UC scheme

Enhanced Diastereocontrol via Strong Light–Matter Interactions in an Optical Cavity

The enantiopurification of racemic mixtures of chiral molecules is important for a range of applications. Recent work has shown that chiral group-directed photoisomerization is a promising approach to enantioenrich racemic mixtures of BINOL, but increased control of the diasteriomeric excess (de) is necessary for its broad utility. Here we develop a cavity quantum electrodynamics (QED) generalization of time-dependent density functional theory and demonstrate computationally that strong light–matter coupling can alter the de of the chiral group-directed photoisomerization of BINOL. The relative orientation of the cavity mode polarization and the molecules in the cavity dictates the nature of the cavity interactions, which either enhance the de of the (R)-BINOL diasteriomer (from 17% to ≈40%) or invert the favorability to the (S)-BINOL derivative (to ≈34% de). The latter outcome is particularly remarkable because it indicates that the preference in diasteriomer can be influenced via orientational control, without changing the chirality of the directing group. We demonstrate that the observed effect stems from cavity-induced changes to the Kohn–Sham orbitals of the ground state

Photon Upconversion and Photocurrent Generation via Self-Assembly at Organic–Inorganic Interfaces

Molecular photon upconversion via triplet–triplet annihilation (TTA-UC), combining two or more low energy photons to generate a higher energy excited state, is an intriguing strategy to surpass the maximum efficiency for a single junction solar cell (<34%). Here, we introduce self-assembled bilayers on metal oxide surfaces as a strategy to facilitate TTA-UC emission and demonstrate direct charge separation of the upconverted state. A 3-fold enhancement in transient photocurrent is achieved at light intensities as low as two equivalent suns. This strategy is simple, modular and offers unprecedented geometric and spatial control of the donor–acceptor interactions at an interface. These results are a key stepping stone toward the realization of an efficient TTA-UC solar cell that can circumvent the Shockley–Queisser limit

Elucidating the Role of the Metal Linking Ion on the Excited State Dynamics of Self-Assembled Bilayers

Metal ion-linked, self-assembled multilayers on nanocrystalline metal oxide surfaces have recently emerged as an effective strategy for manipulating energy and electron transfer dynamics at organic–inorganic interfaces. The choice of metal ion can have a large impact on the stability, loading concentration, and other properties of the films. Here we report our investigation into the role of the linking ion on the subnanosecond excited state dynamics in the bilayer films (TiO₂–B–M–RuP). While metal linkers like Cd^II, La^III, Sn^IV, Zn^II, and Zr^IV are photochemically inert, paramagnetic linking ions such as Cu^II, Fe^II, and Mn^II quench the excited state of the dye with a rate constant on the order of 10⁸ s^–1. The absence of new spectral features in the transient absorption spectrum suggests that energy transfer, and not electron transfer, is responsible for the excited state quenching. On TiO₂, the electron injection rate for TiO₂–B–M–RuP is an order of magnitude slower (∼1 × 10⁹ s^–1) than for the dye directly on TiO₂ (∼3 × 10¹⁰ s^–1) due to increased spatial separation and reduced electronic coupling between the dye and the surface. In dye-sensitized solar cells, the TiO₂–B–M–RuP devices exhibit a notably lower <i>J</i>_sc but higher <i>V</i>_oc compared to TiO₂–RuP with even lower photocurrents for Cu^II, Fe^II, and Mn^II bilayers presumably at least in part due to competitive quenching of the excited state by the metal ion. The increases in <i>V</i>_oc are offset by the decrease in <i>J</i>_sc; thus, the overall efficiency of the bilayer devices is lower than the that of the parent, monolayer device

Molecular Orientation and Energy Transfer Dynamics of a Metal Oxide Bound Self-Assembled Trilayer

Self-assembly of molecular multilayers via metal ion linkages has become an important strategy for interfacial engineering of metalloid and metal oxide (MOx) substrates, with applications in numerous areas, including energy harvesting, catalysis, and chemical sensing. An important aspect for the rational design of these multilayers is knowledge of the molecular structure–function relationships. For example, in a multilayer composed of different chromophores in each layer, the molecular orientation of each layer, both relative to the adjacent layers and the substrate, influences the efficiency of vectorial energy and electron transfer. Here, we describe an approach using UV–vis attenuated total reflection (ATR) spectroscopy to determine the mean dipole tilt angle of chromophores in each layer in a metal ion-linked trilayer self-assembled on indium-tin oxide. To our knowledge, this is the first report demonstrating the measurement of the orientation of three different chromophores in a single assembly. The ATR approach allows the adsorption of each layer to be monitored in real-time, and any changes in the orientation of an underlying layer arising from the adsorption of an overlying layer can be detected. We also performed transient absorption spectroscopy to monitor interlayer energy transfer dynamics in order to relate structure to function. We found that near unity efficiency, sub-nanosecond energy transfer between the third and second layer was primarily dictated by the distance between the chromophores. Thus, in this case, the orientation had minimal impact at such proximity

Elucidating the Energy- and Electron-Transfer Dynamics of Photon Upconversion in Self-Assembled Bilayers

Self-assembled bilayers of acceptor (A) and sensitizer (S) molecules on a metal oxide surface is a promising strategy to facilitate photon upconversion via triplet–triplet annihilation (TTA-UC) and extract charge from the upconverted state. The hypothesized mechanism for TTA-UC in a bilayer film includes low energy light absorption, triplet energy transfer, cross-surface energy migration, triplet–triplet annihilation, and electron injection into TiO₂. Nonproductive processes can also occur including sensitizer-sensitizer TTA, radiative/nonradiative decay, back-electron transfer, and others. Steady-state and time-resolved emission/absorption spectroscopy were used to determine the rate constants of these processes. The rate constants indicate that S to A triplet energy transfer as well as S and A nonradiative rates are the primary efficiency-limiting processes for TTA-UC at the interface. This information is necessary to guide the design of new self-assembled UC films and is a critical stepping stone toward the long-term goal of generating a practical UC solar cell