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Metal-ion-linked molecular multilayers on metal oxide surfaces are promising for applications ranging from solar energy conversion to sensing. Most of these applications rely on energy and electron transfer between layers/molecules which can be envisioned to occur via intra-assembly (IA; between metal-ion-linked molecules) and interlayer (IL; between separate layers of nonlinked molecules) processes. Here, we describe our effort to differentiate between IL and IA energy transfer using a bilayer composed of ZrO2, a phosphonated anthracene derivative (A), a zinc(II) linking ion, and a Pt(II)porphyrin (P). Both time-resolved emission and transient absorption measurements show no impact of diluting the anthracene layer with a spectroscopically inert spacer on the rate of 1A* to P and 3P* to A, singlet, and triplet energy transfer, respectively. These results indicate that energy transfer within the metal-ion-linked assembly (i.e., ZrO2-A–Zn-P) is more rapid than with an adjacent, nonlinked A molecule, even for a P derivative capable of laying down on the surface. These insights are an important step toward structural design principles maximizing the efficiency/rate of energy transfer in multilayer assemblies. 

Metal ion linked multilayers offer a means of controlling interfacial energy and electron transfer for a range of applications including solar energy conversion, catalysis, sensing, and more. Despite the importance of structure to these interlayer transfer processes, little is known about the distance and orientation between the molecules/surface of these multilayer films. Here we gain structural insights into these assemblies using a combination of UV-Vis polarized visible attenuated total reflectance (p-ATR) and Förster Resonance Energy Transfer (FRET) measurements. The bilayer of interest is composed of a metal oxide surface, phosphonated anthracene molecule, Zn(II) linking ion, and a platinum porphyrin with one (P1), two (P2), or three (P3) phenylene spacers between the chromophoric core and the metal ion binding carboxylate group. As observed by both time-resolved emission and transient absorption, the FRET rate and efficiency decreases with an increasing number of phenylene spacers (P1 > P2 > P3). However, from p-ATR measurements we observe a change in orientation of porphyrins in the bilayer, which inhibits a uniform determination of the orientation factor (κ2) across the series. Instead, we narrow the scope of viable structures by determining the best agreement between experimental and calculated FRET efficiencies. Additionally, we provide evidence that suggests, for the first time, that the bilayer structure is similar on both planar and mesoporous substrates. 

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. 

Electron imaging of biological samples stained with heavy metals has enabled visualization of nanoscale subcellular structures critical in chemical-, structural-, and neuro-biology. In particular, osmium tetroxide has been widely adopted for selective lipid imaging. Despite the ubiquity of its use, the osmium speciation in lipid membranes and the mechanism for image contrast in electron microscopy (EM) have continued to be open questions, limiting efforts to improve staining protocols and improve high-resolution imaging of biological samples. Following our recent success using photoemission electron microscopy (PEEM) to image mouse brain tissues with a subcellular resolution of 15 nm, we have used PEEM to determine the chemical contrast mechanism of Os staining in lipid membranes. Os (IV), in the form of OsO2, generates aggregates in lipid membranes, leading to a strong spatial variation in the electronic structure and electron density of states. OsO2 has a metallic electronic structure that drastically increases the electron density of states near the Fermi level. Depositing metallic OsO2 in lipid membranes allows for strongly enhanced EM signals of biological materials. This understanding of the membrane contrast mechanism of Os-stained biological specimens provides a new opportunity for the exploration and development of staining protocols for high-resolution, high-contrast EM imaging. 

Metal ion-linked multilayers have emerged as a simple and modular means of assembling molecular components on an inorganic substrate. The choice of molecules, based on their excited state energies and redox potentials, has enabled the directional control of energy and electron transfer events for application in electrochromics, solar energy harvesting, molecular rectifiers, photocatalysis, and more. Here, we recount the more than 35-year journey of metal ion-linked multilayers and their transition from planar to mesoporous substrates and from homogeneous to heterogeneous multilayers. This includes showcasing the vast range of components (i.e., substrates, metal ions, and molecules), structural insights, and applications. We also highlight current limitations in our knowledge of and ability to control these systems which must be overcome to realize the full potential of metal ion-linked multilayers. 

Intermolecular interactions on inorganic substrates can have a critical impact on the electrochemical and photophysical properties of the materials and subsequent performance in hybrid electronics. Critical to the intentional formation or inhibition of these processes is controlling interactions between molecules on a surface. In this report, we investigated the impact of surface loading and atomic-layer-deposited Al2O3 overlayers on the intermolecular interactions of a ZrO2-bound anthracene derivative as probed by the photophysical properties of the interface. While surface loading density had no impact on the absorption spectra of the films, there was an increase in excimer features with surface loading as observed by both emission and transient absorption. The addition of ALD overlayers of Al2O3 resulted in a decrease in excimer formation, but the emission and transient absorption spectra were still dominated by excimer features. These results suggest that ALD may provide a post-surface loading means of influencing such intermolecular interactions. 

Integrating molecular photon upconversion via triplet–triplet annihilation (TTA-UC) directly into a solar cell offers a means of harnessing sub-bandgap, near infrared (NIR) photons and surpassing the Shockley–Queisser limit. However, all integrated TTA-UC solar cells to date only harness visible light. Here, we incorporate an osmium polypyridal complex (Os) as the triplet sensitizer in a metal ion linked multilayer photoanode that is capable of harnessing NIR light via S 0 to T 1 * excitation, triple energy transfer to a phosphonated bis(9,10-diphenylethynyl)anthracene annihilator (A), TTA-UC, and electron injection into TiO 2 from the upcoverted state. The TiO 2 -A-Zn-Os devices have five-fold higher photocurrent (∼3.5 μA cm −2 ) than the sum of their parts. IPCE data and excitation intensity dependent measurements indicate that the NIR photons are harvested through a TTA-UC mechanism. Transient absorption spectroscopy is used to show that the low photocurrent, as compared to visible light harnessing TTA-UC solar cells, can be atributed to: (1) slow sensitizer to annihilator triplet energy transfer, (2) a low injection yield for the annihilator, and (3) fast back energy transfer from the upconverted state to the sensitizer. Regardless, these results serve as a proof-of-concept that NIR photons can be harnessed via an S 0 to T 1 * sensitizer excited, integrated TTA-UC solar cell and that further improvements can readily be made by remedying the performance limiting processes noted above.